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Topics in Catalysis

, Volume 60, Issue 15–16, pp 1226–1250 | Cite as

Glycerol Steam Reforming for Hydrogen Production over Nickel Supported on Alumina, Zirconia and Silica Catalysts

  • N. D. Charisiou
  • K. N. Papageridis
  • G. Siakavelas
  • L. Tzounis
  • K. Kousi
  • M. A. Baker
  • S. J. Hinder
  • V. Sebastian
  • K. Polychronopoulou
  • M. A. GoulaEmail author
Original Paper

Abstract

The aim of the work was to investigate the influence of support on the catalytic performance of Ni catalysts for the glycerol steam reforming reaction. Nickel catalysts (8 wt%) supported on Al2O3, ZrO2, SiO2 were prepared by the wet impregnation technique. The catalysts’ surface and bulk properties, at their calcined, reduced and used forms, were determined by ICP, BET, XRD, NH3-TPD, CO2-TPD, TPR, XPS, TEM, TPO, Raman, SEM techniques. The Ni/Si sample, even if it was less active for T <600 °C, produces more gaseous products and reveals higher H2 yield for the whole temperature range. Ni/Zr and Ni/Si catalysts facilitate the WGS reaction, producing a gas mixture with a high H2/CO molar ratio. Ni/Si after stability tests exhibits highest values for total (70%) and gaseous products (45%) glycerol conversion, YH2 (2.5), SH2 (80%), SCO2 (65%), H2/CO molar ratio (6.0) and lowest values for SCO (31%), SCH4 (3.1%), CO/CO2 molar ratio (0.48) among all samples. The contribution of the graphitized carbon formed on the catalysts follows the trend Ni/Si (I D /I G  = 1.34) < Ni/Zr (I D /I G = 1.08) < Ni/Al (I D /I G = 0.88) and indicates that the fraction of different carbon types depends on the catalyst’s support nature. It is suggested that the type of carbon is rather more important than the amount of carbon deposited in determining stability. It is confirmed that the nature of the support affects mainly the catalytic performance of the active phase and that Ni/SiO2 can be considered as a promising catalyst for the glycerol steam reforming reaction.

Keywords

Hydrogen production Glycerol steam reforming Nickel catalysts Alumina Zirconia Silica 

1 Introduction

Fossil based energy has helped built our civilization, created our wealth and enriched the lives of billions. However, the finite nature of the resource and the terrifying impacts of the effects of ‘climate change’ have pushed the drive for the development of renewable energy sources (RES). In the transport sector the only current realistic alternative to petro-oil is the use of biofuels and their development, especially that of biodiesel, has experienced a tremendous increase in the last 10–15 years. Biodiesel is commonly produced by the transesterification of vegetable oils and/or fats with alcohols, which yields the esters as the main product and glycerol as a by-product [1, 2]. However, the quantities of glycerol that result from this process are substantial, as every 1 Kg of oil undergoing transesterification produces 100 g of glycerol as byproduct. Thus, the use of glycerol for the production of hydrogen by catalytic reactions such as steam reforming (GSR), oxidative steam reforming (OSR), auto-thermal reforming (ATR), aqueous phase reforming (APR) and supercritical water (SCW) reforming is increasingly drawing attention.

GSR is attractive because, as can be deduced from Eq. (1), every mole of glycerol fed to the reactor can theoretically produce 7 moles of hydrogen. The overall reaction is endothermic (with ΔH° = 123 kJ mol−1) and may be viewed as a combination of glycerol decomposition (Eq. 2) and water–gas shift (WGS, Eq. 3). These may be accompanied by methanation (Eq. 4), methane dry reforming (Eq. 5) and a series of reactions for carbon formation (Eqs. 69) that depend on the operating conditions [3, 4, 5].
$${{\text{C}}_{\text{3}}}{{\text{H}}_{\text{8}}}{{\text{O}}_{\text{3}}}+{\text{ }}3{{\text{H}}_{\text{2}}}{\text{O }} \to {\text{ }}3{\text{C}}{{\text{O}}_{\text{2}}}+{\text{ }}7{{\text{H}}_2}$$
(1)
$${{\text{C}}_3}{{\text{H}}_8}{{\text{O}}_3} \to {\text{ }}3{\text{CO }}+{\text{ }}4{{\text{H}}_2}$$
(2)
$${\text{CO }}+{\text{ }}{{\text{H}}_{\text{2}}}{\text{O }} \leftrightarrow {\text{ C}}{{\text{O}}_2}+{\text{ }}{{\text{H}}_2}$$
(3)
$${\text{CO }}+{\text{ }}3{{\text{H}}_2} \leftrightarrow {\text{ C}}{{\text{H}}_4}+{\text{ }}{{\text{H}}_{\text{2}}}{\text{O}}$$
(4)
$${\text{C}}{{\text{H}}_4}+{\text{ C}}{{\text{O}}_2} \leftrightarrow {\text{ }}2{{\text{H}}_2}+{\text{ }}2{\text{CO}}$$
(5)
$$2{\text{CO }} \leftrightarrow {\text{ C}}{{\text{O}}_2}+{\text{ C}}$$
(6)
$${\text{C}}{{\text{H}}_4} \leftrightarrow {\text{ }}2{{\text{H}}_2}+{\text{ C}}$$
(7)
$${{\text{H}}_{\text{2}}}{\text{O }}+{\text{ C }} \leftrightarrow {\text{ CO }}+{{\text{H}}_{\text{2}}}$$
(8)
$${{\text{C}}_{\text{3}}}{{\text{H}}_8}{{\text{O}}_3} \to {\text{ }}{{\text{H}}_2}+3{{\text{H}}_2}{\text{O}}+3{\text{C}}$$
(9)

Thermodynamic studies predict that high temperatures, low pressures and high water/glycerol ratio favor hydrogen production. A number of researchers suggest that the ideal condition to obtain hydrogen is at temperatures between 627 and 700 °C, molar ratio of water to glycerol from 9:1 to 12:1, and atmospheric pressure. Under these conditions, methane production is minimized and carbon formation is thermodynamically inhibited [6, 7, 8, 9].

Hydrogen production through the GSR is facilitated if the catalyst used promotes the cleavage of C–C, O–H, and C–H bonds in the glycerol mole and not the cleavage of the C–O bonds (a cleavage of the C–O bond will lead to the production of alkanes). The catalyst should also allow the WGS reaction to take place, so that absorbed CO from the surface can be removed as CO2 [10, 11]. A number of noble metals have been tested in the GSR, such as Pt [11, 12, 13, 14], Pd [15], Ru [11, 16] and Rh [11, 17] however, their high cost virtually prohibits their use on an industrial scale. Nickel, which has good intrinsic activity when it is highly dispersed over the support and is widely available [18, 19], has been the most extensively tested metal in the GSR. Nickel has been used in monometallic systems based on Al2O3 [20, 21, 22, 23, 24, 25, 26, 27, 28] (or alumina modified with ZrO2 [29], CeO2 [29, 30], MgO [31] and alkaline promoters [32]), SiO2 [33, 34, 35, 36, 37, 38], CeO2 [39, 40, 41], TiO2 [35, 37, 39, 41], ZrO2 [18, 35, 37, 42], MgO [39, 41], natural clays [43, 44] and fly ash [45]. Nickel has also been used in bimetallic systems, usually in combination with a noble metal [11, 13, 46] or other metals such as, Cu [47, 48], Co [49, 50] and Sn [19].

However, there is clear evidence that the use of different supports for Ni-based systems result in different catalytic performances [35, 37, 39, 41]. The attraction of alumina-based supports lies with its high specific surface area (which facilitates metal dispersion) and its mechanical and chemical resistance under reaction conditions [51, 52]. However, it is also known that carbon deposition and catalyst sintering, both factors that result in the deactivation of the catalyst, are also promoted when alumina is used as support [53, 54]. Zirconia has a much smaller surface area in comparison to Al2O3, however it exhibits higher strength, toughness and thermal stability [55, 56]. ZrO2 is an acid-based bi-functional oxide (in the sense that it possesses both acidic and basic properties on its surface that work both independently and cooperatively), that has redox functions [57, 58]; it also has the capacity to inhibit catalyst sintering in the presence of water at high temperatures, and is resistant to coke formation [18]. Silica possesses a high surface area and is generally considered to be inert [59, 60]. There are a number of works that have investigated the performance of Ni/Al catalysts however the information in the literature concerning the performance of nickel catalysts based on silica or zirconia in glycerol steam reforming is still scant. Table 1 presents the results obtained by different research groups, in the GSR, for monometallic Ni systems that were based on unmodified alumina, zirconia or silica supports. To the best of our knowledge, no work exists in the literature that performs a comparative in depth investigation of nickel catalysts based on these supports with different acid-base characteristics. Furthermore, the specific influence that these properties have on catalytic performance, including catalyst’s stability and deactivation have not been studied until now. Moreover, to the best of our knowledge, a detailed liquid products’ distribution (through quantitative analysis) is also provided for the first time, that helps clarify the kind of carbonaceous intermediates formed during the reaction, closely related with the type of deposited carbon that leads to catalytic deactivation.

Table 1

Summary of selected works reporting on monometallic Ni catalysts based on unmodified alumina, zirconia or silica for the GSR

Active phase/support

Reaction T (°C)

WGFRa (molar, unless otherwise stated)

GHSVb (unless otherwise stated)

Gas products (% mol/mol, dry basis, unless otherwise stated)

Liquid products

Ref

Ni (2%)/α-Al2O3 (impregnation, calcination 750 °C)

450–600

6:1

3.9 × 104 cm3 h−1 gcat −1

H2 = 69.9, CH4 = 1.6, CO = 8.9, CO2 = 19.7 (T = 600 °C)

1-Hydroxy-2-propanone, acetic acid, 1–2 propane diol, propanol, 2-methyl-2-cyclopentenone

[29]

Ni (4%)/Al2O3 (incipient wetness impregnation, calcination: 500 °C)

300, 500, 700 (stability tests)

6:1

10 h−1 (WHSV3)

H2 ≈ 60, CH4 ≈ 1.4, CO ≈ 14, CO2 ≈ 22 (T = 700 °C, t < 5 h)

Not analyzed

[49]

Ni (5.1%)/Al2O3 (incipient wetness impregnation, calcination: 500 °C)

700 (deactivation cycles)

3:1

5 h−1 (WHSV)

H2 ≈ 78, CH4 ≈ 6, CO ≈ 30, CO2 = n/ad (T = 700 °C, t < 5 h)

Not analyzed

[25]

Ni(5.8%)/Al2O3 (incipient wetness technique, calcination : 500 °C)

600,650,700 (stability tests 4 h and 8 h)

20 wt% glycerol

3.4, 5.0, 10.0 h1 (WHSV)

H2 ≈ 98.3, (T = 600 °C, t < 4 h)

H2 ≈ 98.5, (T = 600 °C, t < 8 h)

Not analyzed

[26]

Ni(8%)/Al2O3 (wet impregnation, incipient wetness impregnation, equilibrium deposition filtration, calcination: 800 °C)

400–750

20/1

50,000 mL g−1 h−1

Ni(8%)/Al 2 O 3 (wet impregnation)

H2 ≈ 70, CO ≈ 45, CO2 ≈ 50, CH4 ≈ 5 (T = 750 °C)

Ni(8%)/Al 2 O 3 (incipient wetness impregnation)

H2 ≈ 60, CO ≈ 70, CO2 ≈ 22, CH4 ≈ 8 (T = 750 °C)

Ni(8%)/Al 2 O 3 (equilibrium deposition filtration)

H2 ≈ 65, CO ≈ 58, CO2 ≈ 34, CH4 ≈ 8 (T = 750 °C)

Acetaldehyde, acrolein, acetone, allyl alcohol, acetic acid, acetol, 2-cyclopenten-1-one, 2-cyclopenten-1-one, 2-methyl, phenol, glycerol, 2,3-butanedione, propylene glycol, 1,2-ethanediol, propanoic acid, 2-cyclohexen-1-one, 1,3 dioxan-5-ol, phenol, 2 methyl

[22]

Ni (10%)/Al2O3 (incipient wetness impregnation, calcination: 500 °C)

600

9:1

3.09 gcat h/molglycerol

H2 ≈ 4, CH4 ≈ 0, CO ≈ 0.5, CO2 ≈ 1.2 (mol)

Not analyzed

[31]

Ni (10%)/Al2O3 (wet impregnation, xerogel pretreated at 700, 800, 900, 1000 °C)

600

6:1

n/a

H2 ≈ 45, CH4 = n/a, CO = n/a, CO2 = n/a (xerogel pretreated at 800 °C)

Not analyzed

[24]

Ni (13%)/Al2O3 (wet impregnation, calcination: 500 °C)

500–600

10 wt% glycerol

7.7 h−1 (WHSV)

H2 ≈ 5, CH4 = n/a, CO = n/a, CO2 = n/a (mol)

Acetaldehyde, Acrolein, Propanal, Acetone, Acetic acid, Methanol, Ethanol, 1,2-Propylene glycol

[18, 30]

Ni (15%)/Al2O3 (wet impregnation, calcination: 600 °C)

450–550

60–30 wt%

5.0 × 104 ml gcat −1 h-1

H2 ≈ 92, CH4 = 18, CO = 22, CO2 = 60 (T = 550 °C)

Not analyzed

[20]

Ni (15%)/Al2O3 (dry impregnation, calcination: 750, 850, 950 °C)

600, 800

36 wt% glycerol

10,000 h−1

H2 ≈ 65, CH4 = 5.5, CO = n/a, CO2 = n/a

Not analyzed

[32]

Ni(5%)/Al2O3

Ni(10%)/Al2O3

Ni(15%)/Al2O3

Ni(20%)/Al2O3

(wet impregnation, calcination: 450 °C)

600–750

750 (Stability test)

3:1

6:1

9:1

35,000–62,500 mL g−1 h−1

Ni(5%)/Al 2 O 3

H2 ≈ 98, CO ≈ 16, CO2 ≈ 83, CH4 ≈ 0.75 (T = 750 °C)

Ni(10%)/Al 2 O 3

H2 ≈ 99, CO ≈ 12, CO2 ≈ 87, CH4 ≈ 0.7 (T = 750 °C)

Ni(15%)/Al 2 O 3

H2 ≈ 100, CO ≈ 11, CO2 ≈ 89, CH4 ≈ 0 (T = 750 °C)

Ni(20%)/Al 2 O 3

H2 ≈ 100, CO ≈ 8, CO2 ≈ 92, CH4 ≈ 0 (T = 750 °C)

Not analyzed

[27]

Ni (30%)/Al2O3 (incipient wetness impregnation, calcination: 400 °C)

550–800

3:1

5000–30,000 ml C3H8O3 h−1 mlcat −1

H2, CH4, CO, CO2 (H2 yield ≈ 1 mol of H2 mol−1 C3H8O3 converted, T = 800 °C, t < 5 h)

Acetone, acetaldehyde, ethanol, propanol, acetic acid, 2,3-dyhydroxylpropanal

[17]

Ni (33%)/Al2O3 (wet impregnation, calcination: 600 °C)

600 (Stability test)

9:1

12:1

n/a

H2 = 69, CO = 6, CO2 = 24, HC = 1 (T = 600 °C)

Acetol, pyruvaldehyde, pyruvic, acetaldehyde, othersa, minor amounts of formic acid, acetic acid and lactic acid

[28]

Ni(2%)/SiO2

Ni(5%)/SiO2

(ionic exchange)

450

10 wt% glycerol

0.44–1.66 min

Ni(2%)/SiO 2

H2 = 68.2, CH4 = 0.8, CO = 15.5, CO2 = 15.5, (T = 450 °C), τ = 0.88 min

Ni(5%)/SiO 2

H2 = 66, CH4 = 2, CO = 17, CO2 = 15, (T = 450 °C), τ = 0.88 min

Acetaldehyde, 2-oxopropanal, hydroxy-acetaldehyde, 1-hydroxy-2-propanone, 1,2-ethanediol, 1,3-dihydroxy-2-propanone, 2,3-dihydroxy-propanal

[14]

Ni (7%)/SBA-15 (incipient wetness impregnation, calcination: 550 °C)

600 (Stability test 5h)

6:1

22,500 h−1

H2 = 52,6, CO2 = 22,4, CH4 = 1.0, CO = 24, (t < 5 h)

Acrolein, propyleneglycol, hydroxyacetone, acetaldehyde, acetic acid and ethanol, in concentrations lower than 3.5 wt%

[38]

Ni(10%)/SiO2

(wet impregnation, calcination: 600 °C)

600 (Stability test)

3:1

1000 mL g−1 h−1

H2 = 2.2, CH4 = 0.2, CO = 0.6, CO2 = 0.8 (mol) (t < 5 h)

Propionic acid, acetic acid, 2-propanone, formic acid, ethanol, propylene glycol, glycidol, 1,3-dioxane, 1,2-propanediol and Sorbitol/not quantified

[33]

Ni(10%)/SiL (incipient wetness impregnation, calcination: 800 °C)

650 (Stability test 20 h)

10 wt% glycerol

1.8 g GLY/(g CAT*h)

H2 = 82, CO2 = 87.8, CH4 = 1.3, CO = 10.9, (t < 5 h)

H2 = 81, CO2 = 86.0, CH4 = 1.1, CO = 12.9, (t < 10 h)

H2 = 87, CO2 = 70.2, CH4 = 1.5, CO = 28.3, (t < 20 h)

Not analyzed

[37]

Ni(25%)/SiO2 (commercial)

500

550

600

(Stability test 10h)

9:1

12:1

15:1

18:1

n/a

H2 = 66, CH4 = 3.5, CO = 12.5, CO2 = 18, (T = 600 °C), (9:1)

Not analyzed

[36]

Ni(2.6%)/Zr (wet impregnation, calcination 500 °C)

500

600 (Stability test 8 h)

10 wt% glycerol

7.7 h−1

H2 ≈ 0.11, CO2 ≈ 0.02, CO ≈ 0.03, mol product/mol C3H8O3 (T = 500 °C, t < 8h)

H2 ≈ 0.9, CO2 ≈ 0.3, CO ≈ 0.25, CH4 ≈ 0.1 mol product/mol C3H8O3 (T = 600 °C, t < 8 h)

Traces of acetaldehyde, acrolein, propanal (T = 500, 600 °C, t < 8 h)

[18]

Ni(8.8%)/ZrL (incipient wetness

impregnation, calcination: 800 °C)

650 (Stability test 20 h)

10 wt% glycerol

1.8 g GLY/(g CAT*h)

H2 = 89, CO2 = 88.1, CH4 = 0, CO = 12, (t < 5 h)

H2 = 92, CO2 = 92.2, CH4 = 0, CO = 7.8, (t < 10 h)

H2 = 80, CO2 = 91.1, CH4 = 0, CO = 8.9, (t < 20 h)

Not analyzed

[37]

Ni(10%)/Zr (incipient wetness impregnation, calcination: 500 °C)

500, 650 (Stability test 20h)

10 wt% glycerol

n/a

H2 ≈ 70, CO2 ≈ 98, CO ≈ 2, (T = 500 °C, t < 20 h)

H2 ≈ 60, CO2 ≈ 92, CO ≈ 8,(T = 650 °C, t < 20 h)

Not analyzed

[27]

a WGFR water glycerol feed ratio, b GHSV gas hour space velocity, c WHSV weight hour space velocity, dn/a not available

Prompted from the above, Ni catalysts based on Al2O3, ZrO2 and SiO2 with nickel loadings of 8 wt% were prepared using the wet impregnation technique. The aim of the work was to investigate the influence of the support on catalytic performance for the glycerol steam reforming reaction. Towards this end, the final catalysts’ surface and bulk properties, at their calcined, reduced and used forms, were determined by applying several characterization techniques (ICP, BET, XRD, TPD, TPR, XPS, TEM, TPO, Raman, SEM). The performance of the catalysts was studied in order to investigate the effect of the reaction temperature on: (i) Glycerol total conversion, (ii) Glycerol conversion to gaseous products, (iii) Hydrogen selectivity and yield, (iv) Selectivity of gaseous products, (v) Selectivity of liquid products, and (vi) Molar ratio of H2/CO and CO/CO2 in the gaseous products mixture. The stability of all catalytic samples was also investigated through time on stream experiments and was correlated with carbon deposition.

2 Experimental

2.1 Catalyst Preparation

The alumina support was purchased from Akzo, while the ZrO2 (SZ31108) and SiO2 (SS61138) supports were kindly provided for free by Saint Gobain NorPro. Table 2 provides information of the properties of the untreated catalyst carriers. All supports were crashed and sieved to 350–500 μm before being calcined at 800 °C for 4 h.

Table 2

Properties of untreated catalyst carriers

Support

Al2O3

ZrO2

SiO2

Pellet size

5.2 mm

3.0 mm

3.0 mm

Packing density, Kgm− 3

570

n/a

n/a

Median pore diameter, m

7.8 × 10− 9

n/a

n/a

SSA (m2g−1)

281

77

250

Vp (ml g−1)

0.65

0.30

1.0

n/a not available

The catalysts were prepared via the wet impregnation technique using Ni(NO3)2·6H2O aqueous solutions with the required concentration (C = 0.17 M), in order to obtain final catalysts with Ni content of 8 wt%. The nickel nitrate for the catalyst preparation was obtained from Sigma Aldrich. All solutions for catalyst preparation throughout this study utilized distilled and de-ionized pure water generated by a NANOpure Diamond UV unit (Barnstead International). The resulting slurries were evaporated using a rotary evaporator at 75 °C for 5 h and dried at 120 °C for 12 h followed by calcination at 800 °C for 4 h; these samples will hereafter be denoted as “calcined” catalysts. Reduced counterparts were also produced by reduction for 1 h at 800 °C in pure H2 flow; these will be hereafter denoted as “reduced” catalysts. The catalysts are labeled as Ni/Al, Ni/Zr and Ni/Si.

2.2 Catalyst Characterization

The total metal loading (wt%) of the calcined catalysts was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a Perkin-Elmer Optima 4300DV apparatus. The methodology used has been described previously [61].

N2 adsorption/desorption isothermal curves at −196 °C were recorded manometrically up to 1 bar using the 3Flex (Micromeritics, USA) accelerated surface area and porosimetry analyzer. The 3Flex (Micromeritics) is equipped with high-vacuum system, and three 0.1 Torr pressure transducers. A liquid N2 bath was employed for keeping the system cooled at −196 °C, while nitrogen gas of ultra-high purity (99.9999%) was used for the sorption measurements. Prior to testing, the samples (≈120 mg) were degassed under strong vacuum (10−6 mbar) at 250 °C for 12 h. Total specific surface area (SSA) was calculated by the multi-point Brunauer-Emmet-Teller (BET) method in the relative pressure range 0.05 < P/P0 < 0.20. Pore size distribution (PSD) was estimated by the BJH Theory. The specific surface area and pore size distribution were determined for the calcined and reduced catalysts.

The X-ray diffraction (XRD) technique was used for the determination of the calcined and reduced catalysts’ crystalline structure. The equipment used was a ThermoAl diffractometer, operating at 40 kV and 30 mA, with CuKα radiation (λ = 0.154178 nm). Diffractograms were recorded in the 2θ = 2–70° range at a scanning rate of 0.04° over 1.2 min−1. The diffraction pattern was identified by comparison with those of known structures in the International Centre for Diffraction Data database.

CO2-TPD and NH3-TPD experiments were conducted using Autochem 2920, (Micromeritics, Atlanta, USA). In particular, a gas mixture (30 NmL min−1) of 5 vol% CO2/Ar and 1 vol% NH3/He respectively, was passed over ~0.15 g of the pre-calcined (20 vol% O2/He, 500 °C, 2 h) NF using a temperature ramp of 30 °C min−1, while the TCD signal was recorded continuously. The mass numbers (m/z) 15, 30, 44 and 46 were used for NH3, NO, N2O and NO2 during NH3-TPDs, while the (m/z) 28 and 44 were used for CO and CO2, respectively, during CO2-TPDs.

Temperature programmed reduction (H2-TPR) was performed by loading 100 mg of the calcined catalysts or supports in a U-type quartz tube adapted to a continuous flow TPR/TPD apparatus coupled with mass spectrometry, following the procedures described in detail elsewhere [62]. In short, a total flow of 16 mL min−1 was employed as feed, with a H2 content of 1% (v/v) in He. Sample temperature was varied from ambient temperature up to 950 °C, at a ramp rate of 10 °C min−1. The main m/z fragment registered was H2 = 2. Samples were pre-treated at 200 °C for 1 h under He flow and then cooled down to room temperature under the same atmosphere before the TPR spectra acquisition.

XPS analyses were performed on a ThermoFisher Scientific Instruments (East Grinstead, UK) K-Alpha + spectrometer. XPS spectra were acquired using a monochromated Al Kα X-ray source (hν = 1486.6 eV). An X-ray spot of ~400 μm radius was employed. Survey spectra were acquired employing a Pass Energy of 200 eV. High resolution, core level spectra for all elements were acquired with a Pass Energy of 50 eV. All spectra were charge referenced against the C1s peak at 285 eV to correct for charging effects during acquisition. Quantitative surface chemical analyses were calculated from the high resolution, core level spectra following the removal of a non-linear (Shirley) background. The manufacturers Avantage software was used which incorporates the appropriate sensitivity factors and corrects for the electron energy analyzer transmission function.

The carbonaceous deposits on the spent catalysts were measured by Temperature Programmed Oxidation (TPO), as described elsewhere [63]. Briefly, the catalyst sample was heated linearly (10 °C min−1) from RT to 750 °C under 20 v/v% O2/He flow. The signals of O2, CO and CO2 were continuously monitored by an MS detector (FL-9496 Balzers). Calibration of MS signals was performed with the use of self-prepared gas mixtures of known concentration.

Raman spectroscopy was used to characterize the coke deposited on spent catalyst samples. The equipment and methodology employed have been described in a previous publication [64]. Spectra were collected using a WITEC alpha300R micro-Raman system (RAMAN Imaging System WITEC alpha300R) with a 20 × long distance objective (0.35 numerical aperture) in the back-scattering geometry with an excitation wavelength of 532 nm from an Ar+ ion laser (laser power set at 2 mW calibrated against a silicon standard). For each sample, at least three Raman spectra were collected in different areas to assess the homogeneity of the investigated material. All spectra collected from the same samples showed similar features confirming the homogeneity of the carbon deposits.

Morphological examination of used catalysts was done using scanning electron microscopy (SEM) in a JEOL 6610LV. The elemental analysis, by means of energy dispersive spectroscopy (EDS), was carried out using a large area (80 mm2) silicon drift detector (X-Max 80 Oxford Instruments). Images, elements maps and spectra were acquired and analyzed with the AZtech Nanoanalysis software (Oxford Instruments) following the methodology described previously [64].

Transmission electron microscopy (TEM) observations were carried out using a 200 kV G2 20 S-Twin Tecnai microscope with a LaB6 electron source fitted with a “SuperTwin®” objective lens allowing a point to point resolution of 2.4 Å. Energy dispersive X-ray spectroscopy (EDS) analysis and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) were performed on a Tecnai G2-F30 Field Emission Gun microscope with a super-twin lens and 0.2 nm point-to-point resolution and 0.1 line resolution operated at 300 kV. HAADF detector enables the acquisition of HAADF-STEM images with atomic number contrast for high scattering angles of the electrons (Z-contrast). To prepare the samples for electron microscopy observation, catalyst powder was dispersed in milli-Q water. After 30 s in an ultrasonic bath, a drop of this suspension was applied to a copper grid (200 mesh) coated with carbon film, and allowed to dry in air.

2.3 Catalytic Tests

The glycerol steam reforming (GSR) reaction was carried out at atmospheric pressure, in a continuous flow, fixed-bed, single pass, tubular stainless steel reactor, with an inner diameter of 14 mm, at temperature ranging from 400 to 750 °C. The experimental set up used allowed the feeding of both liquid and gaseous streams, having two vaporizers and a pre-heater before the reactor and a condenser after it. The vaporizers, pre-heater and reactor were placed into electrical ovens and regulated with programmed-temperature controllers.

Prior to catalytic testing, 200 mg of undiluted catalyst (the catalyst bed was supported by quartz wool) was reduced in situ under a flow of high purity H2 (99.999%, 100 mL min−1) at 800 °C for 1 h. The catalyst was then purged with high purity He (99.999%) for 45 min, the temperature was lowered to 750 °C and the reaction feed was introduced into the catalyst bed. In order to ensure operation at steady state conditions, the catalyst was left for approximately 50 min at each step. Liquid products were obtained at the end of this 50 min period. The same procedure described above was also followed for the time on stream experiments. The difference was that temperature was reduced to 600 °C, catalytic stability was tested for 20 h, and measurements were taken every 1 h for the gaseous products and every 4 h for the liquid products.

The reaction feed consisted of the liquid stream—an aqueous solution of C3H8Ο3 and H2O with 9:1 steam/glycerol molar ratio, with a total liquid flow rate of 0.12 mL min−1, which was kept under continuous stirring at room temperature—and the gas stream, which consisted of high purity He (99.999%, 71 ml/min). The glycerol used had 99.5% purity and was obtained from Sigma Aldrich. The water/glycerol mixture was fed with a HPLC pump (Series I) into the evaporator and was first vaporized at 350 °C before it was mixed with helium. To prevent overpressure phenomena, pressure controllers were placed before and after the inlet and outlet gas, respectively. The gas feed at the reactor’s inlet consisted of a gas mixture of 63% H2O, 7% glycerol and 30% helium, corresponding to a Weight Hourly Space Velocity (WHSV) of 50,000 mL g−1 h−1. The reactor’s outlet gases passed through a cold trap for liquid products capture.

The gaseous products were analyzed on-line by a gas chromatograph (Agilent 7890 A), with two columns in parallel, HP-Plot-Q (19095-Q04, 30 m length, 0.530 mm I.D.) and HP-Molesieve (19095P-MSO, 30 m length, 0.530 mm I.D.), equipped with TCD and FID detectors. Liquid products were analyzed via a combined system of a Gas Chromatograph and a Mass Spectrometer (GC-MS, Agilent 5975 C). The analysis procedure followed for the liquid products involved the following reagents and standards: Glycerol (≥99%), acetol (90%), acetone (≥99.8%), acetaldehyde (≥99.5%), allyl alcohol (≥99%), acrolein (≥99%), acetic acid (99.0%) and 2-butanol (≥99%). These were obtained from Sigma–Aldrich (St. Louis, MO, USA) and were used to prepare standard solutions in order to create calibration curves for each compound (typically 4–5 standard solutions were prepared for the calibration of each compound). The determination of liquid products was carried out on a 7890A/5975C Triple—Axis Detector diffusion pump—based GC-MS equipped with split/splitless inlet (Agilent Technologies, Santa Clara, USA). Chromatographic separation was achieved by a 30 m × 250 μm HP-5MS (5% phenyl, 95% methylpolysiloxane) capillary column with film thickness of 0.25 μm. Helium 5.0 (99.999%) was used as carrier gas at 1 mL min−1 in a constant flow rate mode. The oven temperature program was 35 °C for 5 min, increased by 10 °C min−1 to 250 °C and held at 250 °C for 10 min. The temperature of the split/splitless injector was 280 °C and the volume of the samples injected were 1 μL with a split ratio of 100:1 using ultra inert liner with glass wool (Agilent Technologies). The temperature of the ion source, the quadrupole and the MS interface for both instruments were 230, 150 and 250 °C, respectively. Both full scan (40–160 m/z) and Selective Ion Monitoring (SIM) modes were performed under electron impact ionization mode at 70 eV in mass spectrometer. The quantification of compounds was done by internal standard method to monitor batch reproducibility and to correct for variations that occurred during sample preparation and analysis. 2-butanol was used as internal standard.

Catalytic performance is reported in terms of H2 yield, H2, CO, CH4 and CO2 selectivity, glycerol conversion into gaseous products, and total glycerol conversion. Moreover, the performance of the catalysts for the liquid phase products is reported in terms of acetol (C3H6O2), acetone [(CH3)2CO], allyl alcohol (CH2=CHCH2OH), acetaldehyde (C2H4O), acetic acid (C2H4O) and acrolein (C3H4O) selectivity. Performance parameters were calculated based on Equations (10)-(15):
$$\% \;glycero{l_{}}\;conversio{n_{\left( {globa{l_{}}\;conversion} \right)}}=\left( {\frac{{Glycero{l_{in}} - Glycero{l_{out}}}}{{Glycero{l_{in}}}}} \right) \times 100$$
(10)
$$\% \;glycero{l_{}}\;conversio{n_{\left( {gaseou{s_{}}\;products} \right)}}=\left( {\frac{{{C_{}}\;atom{s_{}}\;i{n_{}}\;th{e_{}}\;ga{s_{}}\;products}}{{tota{l_{}}\;{C_{}}\;atom{s_{}}\;i{n_{}}\;th{e_{}}\;feedstock}}} \right) \times 100$$
(11)
$$H_2 \; {yield}= \frac{{H_2}\;moles\;produced}{moles\;of\;glycerol\;in\;the\;feedstock}$$
(12)
$$\% \;{H_2\;}_{}selectivity=\left( {\frac{{{H_2\;}_{}mole{s_{}}\;produced}}{{C\;atom\;produced\;in\;the\;gas\;phase}}} \right) \times \left( {\frac{1}{{RR}}} \right) \times 100$$
(13)
where, RR is the reforming ratio (7/3), defined as the ratio of moles of Η2 to CO2 formed.
$$\% \;selectivity\;of\;i=\left( {\frac{{C\;atoms\;in\;species\;i}}{{C\;atom\;produced\;in\;the\;gas\;phase}}} \right) \times 100$$
(14)
where, species \(i\) refers to CO, CO2 and CH4.
$$\% \;selectivity\;of\;i'=\left( {\frac{{C\;atoms\;in\;species\;i'}}{{C\;atoms\;produced\;in\;the\;liquid\;phase}}} \right) \times 100$$
(15)

where, species \(i'\) refers to acetol, acetone, allyl alcohol, acetaldehyde, acrolein and acetic acid.

3 Results and Discussion

3.1 Characterization Results

3.1.1 Physicochemical, Structural and Textural Properties of Catalytic Samples

Table 3 presents the physicochemical, structural and textural properties of the calcined and reduced catalytic samples. The ICP measurements indicate that the desired metal loading was achieved for all samples. Notably, the specific surface area values of all catalysts after calcination are substantially lower than that of the original supporting materials used (Table 2). This can be attributed to the calcination preconditioning of the supports at 800 °C before catalyst preparation and also to a partial pore blockage by nickel particles formed during catalyst calcination [22, 52]. On the other hand, no significant change in the SSA is observed after the reduction step. BET values of the catalysts under study are also presented in Table 2. According to IUPAC classification [65], the Ni/Zr sample (Fig. 1) shows a IV-type isotherm with a H4-type hysteresis, which is linked to a rather mesoporous catalyst with a relatively high surface area of about 50 m2 g−1 and a sharp pores size distribution with a maximum at 6 nm, whereas Ni/Al and Ni/Si sample exhibit an isotherm more typical to mesoporous material with some macroporosity. The N2 adsorption–desorption isotherms for Ni/Si and Ni/Al were very similar in shape (isotherm Type II), that is, minor adsorption observed at the low pressure regime (P/P0 < 0.05) and major adsorption at the higher pressures (0.7 < P/P0 < 1.0). A small hysteresis loop was observed in all isotherms, because the desorption rate is lower than the adsorption rate of nitrogen (attributed to percolation effects on porous media) [66]. The pore size distribution in the case of Ni/Zr corroborates with the hysteresis loop shape, where the majority of the population of pores are in the meso-range. Some ordering in the porosity of Ni/Zr catalyst can be inferred based on the hysteresis shape. Ni/Al and Ni/Si pore size distribution curves are quite similar, in agreement with their N2 adsorption isotherms similarity.

Table 3

Physicochemical, structural and textural properties of catalytic samples

Catalyst

Calcined samples

Reduced samples

Metal loading

(Ni, wt%)

SSA

(m2g−1)

NiO mean crystallite size (nm)a

NiO dispersion

(%)b

SSA

(m2g−1)

Pore volume (cm3/g)

Av. pore width (nm)

Ni0 mean crystallite size (nm)a

Ni0 dispersion

(%)b

Ni/Al

7.14

158

18.1

3.6

136

0.32

20.1

16.8

3.9

Ni/Zr

7.65

50

15.7

4.2

44

0.05

19.9

23.0

2.9

Ni/Si

7.95

155

15.8

4.2

145

0.42

21.6

31.4

2.1

aCalculated by XRD measurements (Scherrer analysis), bcalculated by the Vannice method [67]

Fig. 1

N2 adsorption–desorption isotherms and pore size distribution (inset) of the reduced catalysts (Note y axis not of the same scale)

The degree of crystallinity and the identification of the phases of the prepared catalysts, at their calcined and reduced forms, were performed by applying the XRD analysis technique. For the alumina support and the calcined/reduced Ni/Al catalyst (Fig. 2a), the characteristic peaks of γ-Al2O3 were detected at 2θ = 37.7°, 45.9° and 67.0°. The formation of the spinel NiAl2O4 phase, observed at the diffraction lines 2θ = 19.0°, 32.0°, 37.0°, 46.0° and 59.6°, is caused by the reaction between NiO and Al2O3 due to the high calcination temperature [48], while the absence of the nickel oxide (NiO) is an indication that the structure is nearly amorphous [52]. Two major differences can be observed between the calcined and reduced samples; the first, is the decreasing intensities of Al2O3 and NiAl2O4, and the second difference, is the appearance of small peaks due to the presence of metallic nickel (Ni0) at 2θ = 44.0° and 51.2°. The low intensity of the Ni0 peaks is correlated to the small size of the metallic nickel species [68].

Fig. 2

XRD patterns of calcined supports and calcined and reduced catalytic samples: a Alumina and Ni/Al, b Zirconia and Ni/Zr and c Silica and Ni/Si

From the diffractogram of the calcined zirconia support (Fig. 2b) it can be seen that ZrO2 exhibits mainly monoclinic (2θ = 24.0°, 28.2°, 31.5°, 34.4°) and tetragonal polymorphs (2θ = 30.0°, 33.9°, 50.0°, 59.4° and 62.8°). However, after the introduction of the active species and the subsequent calcination and reduction procedures there is a noticeable transformation from the monoclinic to the cubic phase (2θ = 30.5°, 50.5° and 60.4°). As reported in the literature, the transformation from one form to another is sensitive to the existence of impurities (e.g., Hf, Si, Fe) [57, 69]; it is believed that such impurities were also present in the ZrO2 support used herein. Moreover, from the diffractograms of the calcined catalysts, peaks at 2θ = 37.2° and 43.2° corresponding to NiO can be observed. For the reduced samples, NiO disappears and a peak corresponding to metallic nickel (Ni0) at 2θ = 44.5° appears.

Figure 2c shows the XRD diffractograms of the calcined silica support and the calcined and reduced Ni/Si catalyst. The broad and low peak a 2θ = 22.0° corresponding to SiO2 indicates that the structure is amorphous, with a low degree of crystallinity [33]. For the calcined catalyst, clearly defined peaks at 2θ = 37.2°, 43.2° and 62.9°, corresponding to NiO can be observed. The major difference between the calcined and reduced catalyst is the total disappearance of nickel oxide, and the appearance of high, sharp peaks corresponding to metallic nickel at 2θ = 44.3° and 51.7°. The appearance of the Ni0 peaks is evidence of the reduction of the Ni species in the catalyst.

The Ni species mean particle size (Table 3), for both calcined and reduced samples, was determined from the XRD spectra using the Scherrer equation. Regarding the calcined catalysts, the nickel species particle size was calculated at 18.1, 15.7 and 15.8 nm for the Ni/Al, Ni/Zr and Ni/Si, respectively. Interestingly, the reduction procedure appears to cause noticeable particles agglomeration on the Ni/Zr and Ni/Si catalysts (23.0 and 31.4 nm, respectively), while the Ni/Al remains virtually unaffected (16.8 nm).

3.2 Surface Acidity–Basicity Estimation

Figure 3a shows the NH3-TPD profiles obtained for the Ni supported catalysts. It can be stated that all catalysts are dominated by two desorption regions at lower than 300 °C and higher than 300 °C associated with weak and medium/strong acid sites, respectively. For the Ni/Si catalyst there is a small desorption peak (Tmax = 175 °C) caused by Si–OH of the support surface. The Ni/Zr catalyst presented a higher population of weak acid sites (Tmax <150 °C) compared to the Ni/Si, suggesting that the weak acid strength is enhanced. In addition, another desorption peak also appears in the range of 170–280 °C, whereas the peak at around the 450 °C suggests the existence of medium and strong acid sites. Damyanova et al. [70] reported on the enhancement of the acidic character of the ZrO2 using XPS and FT-IR techniques. According to their work ZrO2 is dominated from Lewis acid sites primarily due to the higher ionic character of the Zr–O bond. Anderson et al. [71] reported on the surface acidity of silica–zirconia aerogel system with different Si/Zr ratios and they found an almost linear increase in Lewis acidity as a function of mol% ZrO2, in agreement with [72]. Alumina and silica supported Ni catalysts presented majorly medium to strong acid sites.

Fig. 3

a NH3-TPD and b CO2-TPD profiles on Ni/Al, Ni/Si and Ni/Zr catalysts

Figure 3b shows the CO2-TPD profiles on fresh Ni/Al, Ni/Si and Ni/Zr catalysts after pretreatment in oxygen atmosphere. CO2 was adsorbed on the catalyst at room temperature (25 °C). A broad CO2 desorption peak appeared in the case of the Ni/Zr catalyst. This exhibited that CO2 weakly adsorbed on the catalyst and only a kind of adsorption state of CO2 formed [73]. In the case of Ni/Si, the TPD profile shows at least three CO2 desorption peaks: the first peak centered at lower temperature (around 70–200 °C) can be assigned to low basic sites, while the second peak centered at 300 °C (range around 230–400 °C) can be ascribed to medium strength basic sites. The Ni/Al catalyst presents two CO2 desorption peaks, centered at 100 and 350 °C, respectively, linked to the presence of weak and medium strength basic sites.

3.2.1 Ni Species Reducibility

H2-TPR measurements were carried out in order to investigate the reducibility of both calcined catalysts and corresponding supports (Fig. 4) and to examine the interaction strength of Ni species with the surface of the supports. The Gaussian-type deconvolution of the profiles is also considered. For the Ni/Al catalyst, a small peak at low temperature attributed to the reduction of bulk nickel oxide phase (α-peak) and a broad reduction band corresponding to the nickel aluminate structures (β-peak and γ-peak), indicating the strong interaction between nickel species and alumina support, can be observed. It can be assumed that the contribution of β- and γ-peaks to the observed reduction peaks, as well as their position, could be related to the different Ni2+ coordination in the spinel framework [22, 52]. The estimated (from the deconvolution curves) contribution of the β-peak in the total Ni species present in the spinel structure was 67.8%. These results are consistent with the XRD results (Fig. 2a) showing the existence of the nickel aluminate phase; free α-NiO species were not observed.

Fig. 4

TPR profiles of catalytic samples

For the Ni/Zr catalyst, three broad peaks can be observed; the lowest peak extends from 450 to 480 °C, the middle peak centers around 620 °C and the highest around 730 °C. From the available literature data, all three reduction peaks can be attributed to species formed by interaction of Ni2+ and ZrO2. The low and middle temperature reduction peaks can be ascribed to the reduction of NiO interacting with ZrO2, while the peak detected at high reduction temperature probably corresponds to the reduction of NiO-ZrO2 solid solutions. As the deconvolution of the Ni/Zr profile shows, the reduction peak at the lowest temperature is formed by the sum of two peaks (with reduction temperatures at 442 and 495 °C) and can be associated to NiO species interacting with tetragonal and cubic zirconia, respectively. The reduction peak at the middle temperature is also formed by the sum of two peaks (with reduction temperatures at 567 and 609 °C) and can be ascribed to NiO species bound to monoclinic zirconia [18, 57].

For the Ni/Si catalyst, two broad peaks can be observed. The first peak at 540 °C can be assigned to the reduction of nickel species moderately interacting with the support. The peak at 630 °C can be ascribed to the reduction of nickel species interacting strongly with the support or alternatively, to hardly reducible nickel silicate which can be formed via the reaction of small nickel oxide particles with silica [14, 74]. It can be also observed that the interaction strength between active metal and support is lower for this catalyst in comparison with the Ni/Al and Ni/Zr samples.

Table 4 summarizes the amount of H2 consumed per gram of catalyst or support of all the materials used in the present study; for the supports, the corresponding total oxygen storage capacity (OSC) values are also depicted. Taking into account the contribution of support to the total H2 consumption values measure for each catalyst, the theoretical and measured values are in good agreement. Notably, none of the supports shows significant OSC values.

Table 4

Theoretically estimated amounts of H2 need to consume the NiO content of the catalysts; experimentally measured total (NiO + support) hydrogen consumption values; OSC values of the supports

Catalyst or support

Theoretically calculated H2 consumption of NiO particles

(μmol H2 g−1)

Experimentally measured total H2 consumption

(μmol H2 g−1)

OSC

(μmol O2 g−1)

Ni/Al

1217

1150

 

Ni/Zr

1303

1375

 

Ni/Si

1355

1375

 

Al support

 

21

10.5

Zr support

 

67

33.5

Si support

 

64

32.0

3.2.2 XPS (X-ray Photo Electron Spectroscopy)

Figure 5 compares the XPS high-resolution spectra of Ni2p of the catalysts. All catalysts, namely Ni/Al, Ni/Zr and Ni/Si, present a peak corresponding to Ni0 (at 853.5, 853.7, and 853.3 eV). The Ni/Al catalyst present peaks associated with the Ni2+ species belonging to the NiAl2O4 phase at 856.1 and 873.7 eV and to Ni2+ (associated with the presence of NiO phase) at 861.6, 862.0 and 862.0 eV [75, 76, 77]. Based on the area of the deconvoluted peaks it can be stated that there is an effect of the support nature on the reducibility of Ni metal. In particular, the amount of Ni metal is increased from 1.4 to 2.3 and 5.1 for the Ni/Al to Ni/Zr to Ni/Si (Table 5). SiO2 is anticipated to be the less active support and as such interacts the least with the Ni phase. Also, ZrO2 seems to induce the formation of Ni2O3 which is not observed over the other two catalysts, whereas in the case of Al2O3 as support, the formation of the spinel NiAl2O4 phase is being favored. These results are in good agreement with the previously presented TPR profiles (Fig. 4) where the (β-peak and γ-peak) indicate the strong interaction of Ni with Al2O3 support. The discrepancy of the Ni amount as this was calculated based on TPR and XPS can be understood on the basis of the different probing depth of the techniques. It should be noted that the possibility of the samples being reoxidized cannot be discounted, as the XPS experiments were not conducted in situ.

Fig. 5

The Ni2p region of XPS spectra for all catalytic samples

Table 5

XPS data of Ni0, NiO, NiAl2O4 and Ni2O3

Ni/Al

Ni/Zr

Ni/Si

Ni species

Binding energy (eV)

Amount,(%)

Ni species

Binding energy (eV)

Amount, (%)

Ni species

Binding energy (eV)

Amount, (%)

Ni0

853.5

1.4

Ni0

853.7

2.3

Ni0

853.3

5.1

NiAl2O4

856.1

30.4

Ni2O3

856.4

29.6

NiO

856.5

31.5

NiO

861.6

24.8

NiO

862.0

22.5

NiO

862.0

20.7

NiAl2O4

873.7

18.5

NiO

874.0

21.1

NiO

874.0

24.1

NiO

880.0

24.9

NiO

880.1

24.5

NiO

881.0

18.6

3.2.3 Electron Microscopy Analysis

Figure 6 shows the STEM-HAADF images of (a) Ni/Al, (c) Ni/Zr, and (e) Ni/Si reduced catalysts, while on the right hand-side the corresponding EDS spectra from the areas marked with red dashed line are presented (b, d, f). For all samples, a uniform distribution of metallic Ni particles on the respective support could be observed, while for each system the nanoparticles exhibit a spherical morphology. Moreover, the Ni nanoparticles of the Ni/Al catalyst appear to be smaller in comparison with Ni/Zr and Ni/Si. At the same time, the particles of the Ni/Zr are slightly larger and those on the Ni/Si larger still. The size of Ni nanoparticles formed onto the corresponding support as observed by the TEM analysis is in a good agreement and comparable with the calculated average size previously estimated from the XRD patterns. The EDS spectra for each sample reveal the existence of specific elements for the Ni/Al, Ni/Zr, and Ni/Si catalytic systems, while for all samples the Cu that has been detected arises from the TEM grid-support that the catalysts have been deposited prior to TEM investigations.

3.3 Catalytic Performance

3.3.1 Total Conversion and Conversion to Gaseous Products

Figure 7 shows the: (a) total glycerol conversion and glycerol conversion into gaseous products, (b) H2 selectivity and H2 yield, (c) CO2 and CO selectivity, (d) CH4 selectivity, and (e) H2/CO and CO/CO2 molar ratio. According to thermodynamics [6, 7, 8, 9], an increase in temperature from 400 to 750 °C provokes an increase in the gaseous phase products, as at low temperatures the steam reforming reaction is not favored, which causes the vast majority of the organics present in the solution to form intermediate liquid products and carbon deposits. From Fig. 7a it can be seen that below 600 °C the Ni/Si catalyst exhibits lower total glycerol conversion values, however it shows higher conversion of glycerol to gaseous products for the whole temperature range, in comparison with the other two samples. Our catalysts’ performance is in accordance with the literature [51, 78] as higher dispersion of the active phase (Ni0) on the support’s surface (Table 5)is an attribute of a more active catalyst (Ni/Si) in the glycerol steam reforming reaction. Additionally, the surface basic and acid sites are also important factors affecting the catalyst’s gaseous products activity, carbon deposition and deactivation [79, 80, 81, 82]. From the TPD results presented herein (Fig. 3), it can be suggested that the Ni/Si catalyst possess more basic sites than the Ni/Al or the Ni/Zr, but the latter two posses more acidic sites. Based on the results presented herein, it seems more likely that the acidic/basic properties of our catalysts have a more significant influence on gaseous and liquid products selectivity, suggesting different reaction pathways and on carbon deposition affecting the stability, as will be discussed in more detail below.

Fig. 6

Microscopy analysis of reduced catalysts: STEM-HAADF micrograph and EDS analysis of the area marked in red dashed line at STEM-HAADF images. (a, b) Ni/Al, (c, d) Ni/Zr, and (e, f) Ni/Si

Fig. 7

a Total glycerol conversion and glycerol conversion into gaseous products, b H2 selectivity and H2 yield, c CO2 and CO selectivity, d CH4 selectivity, and e H2/CO and CO/CO2 molar ratio [Reaction conditions: C3H8Ο3 (30 v/v%)/H2O (total liquid flow rate = 0.12 ml/min)/He = 71 ml/min, wcatalyst=200 mg, T = 400–750°C]

3.3.2 Gaseous Products’ Selectivity

GC analysis revealed that the main gaseous products were H2, CO2, CO and CH4 (trace amounts of ethylene (C2H4) were also detected). Concerning H2 selectivity and yield (Fig. 7b), the Ni/Al catalyst exhibits the lowest values amongst all samples for the whole temperature range. From Fig. 7c it can be depicted that the CO2 and CO selectivity values range with temperature for all catalysts. Specifically, a decrease in the CO2 production and an increase in the CO can be observed with increasing temperature (T <650 °C) for the Ni/Al sample, while the opposite effect can be seen for the Ni/Zr and Ni/Si catalysts. It should be mentioned that for the latter two catalysts CO2 production increases with temperature (T <550 °C) and then it reveals a sharp drop for 550 °C < T < 650 °C. As for the CH4 selectivity, its values are lower than 10% for the whole temperature range for all catalysts (Fig. 7d).

Thermodynamic analysis of the water-glycerol system indicates that at equilibrium the only additional reaction product in the gas phase is methane, the formation of which is due to the hydrogenation of CO [11, 83, 84, 85, 86]. Additionally, since the decomposition of glycerol to CH4 is highly favored during the reforming process, all of our catalysts seem to have sufficient capacity for reforming the produced CH4 into hydrogen and carbon monoxide [87]. Moreover, there is a strong indication that Ni/Zr and Ni/Si catalysts also facilitate the water gas shift reaction in order to convert CO into CO2. It should be mentioned here that methane selectivity values for T >550 °C are quite lower for the Ni/Zr catalyst compared with the ones for the Ni/Al and Ni/Si.

Thus, it can be said that our experimental results are in accordance with the ones predicted by the thermodynamics as production of H2, CO2 and CO (but not of CH4) was strongly influenced by temperature. Specifically, concerning hydrogen production it can be observed that its selectivity and yield increase as the temperature increases from 400 to 700 °C, reaching for the Ni/Zr catalyst the values of 80.5% and 2.7 moles/mole of glycerol and for the Ni/Si the values of 84.0% and 3.0 moles/mole. It is noted that the corresponding thermodynamically predicted values are as high as SH2 = 95% and YH2 = 6.0 moles per unit mole of glycerol [88, 89].

It can be also seen (Fig. 7e) that the different trend with T of the carbon gaseous substances between catalysts strongly influences the H2/CO and CO/CO2 molar ratios. In fact, for H2/CO, the higher values can be observed for the Ni/Zr and Ni/Si samples at temperature 550 °C (H2/CO equals to 8 and 6, respectively) and for CO/CO2 at 600 °C for the Ni/Al (CO/CO2 equals to 3.5) and at 650 °C for Ni/Si (CO/CO2 equals to 4.5). Interestingly, the CO/CO2 molar ratio value for Ni/Zr sample remains lower than 1 for the whole temperature range, while for the Ni/Si it is almost unity for temperatures between 450 and 600 °C.

A possible explanation is that besides the WGS reaction, the methanation reaction is also favored in backward direction as the reaction temperature increases, which finally leads to increased CO selectivity [88, 89]. During reaction, CO2 is produced as per the overall glycerol reforming reaction which is highly endothermic and WGS reaction, which is mildly exothermic, and a combination of these two leads to form maxima in the thermodynamic CO2 selectivity over the temperature range analyzed, as in the case of the Ni/Zr and Ni/Si catalysts.

Concluding it can be said that Ni/Si catalyst, even if it is less active for T < 600 °C, produces more gaseous products and it reveals higher hydrogen yield for the whole temperature range. Moreover, the Ni/Zr and Ni/Si catalysts facilitate the WGS reaction, producing a gas mixture with a high H2 to CO molar ratio, more suitable for fuel cell applications.

3.3.3 Liquid Products’ Selectivity

In Table 5 the liquid products’ distribution for all catalysts, at various reaction temperatures is presented. Specifically, the presence of acetaldehyde, acrolein, acetone, allyl alcohol, acetic acid, acetol and phenol can be stated for all samples and for the whole range of reaction temperatures (except of acrolein that is detected for T < 500 °C). Other substances, were also detected at trace amounts, such as 2-cyclopenten-1-one (T < 700 °C), propylene glycol (T <600 °C), 2-cyclopenten-1-one, 2-methyl and 2-cyclohexen-1-one for 500 °C < T <600 °C. This can be a ttributed to the fact that the reaction pathway is complex and most probably different for each catalyst, being affected by the specific properties (as the acid and basic sites) of the supporting material. As a result a number of undesirable side reactions occurred thus, producing several carbonaceous by-products especially for lower temperatures [90] (Table 6).

Table 6

Liquid products’ distribution for all samples (Ni/Al, Ni/Zr, Ni/Si) at various reaction temperatures

A/A

Compounds

Reaction temperature (°C)

400

450

500

550

600

650

700

750

1

Acetaldehyde

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

1,3

1,3

2

Acrolein

1,2,3

1,2,3

3

     

3

Acetone

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

4

Allyl Alcohol

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

5

Acetic Acid

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

1,2

6

Acetol

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

2,3

7

2-Cyclopenten-1-one

1,2,3

1,2,3

1,2,3

1,2,3

1,2

1

  

8

2-Cyclopenten-1-one, 2-methyl

  

1,2,3

1,2,3

1

   

9

Phenol

1,2,3

1,2,3

1,2,3

1,2,3

1,2

1

1

 

10

Glycerol

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

1,2,3

11

Propylene glycol

1,2,3

1,2,3

1,2,3

1,3

1

   

12

2-Cyclohexen-1-one

  

1,2,3

1

    

1 Ni/Al, 2 Ni/Zr, 3 Ni/Si

It is well known that acetol formation, through dehydration of glycerol, is observed for catalysts possessing Lewis acidity (as the Ni/Zr), or strong basic sites (as the Ni/Si) [91, 92, 93]. Considering the acrolein production by dehydration of glycerol, catalysts having Brønsted acid sites give high acrolein selectivity [94]. The observed product distributions may state that steam reforming of glycerol is a process that includes dehydration, dehydrogenation and hydrogenolysis reactions [95]. In addition, the SR of these intermediate products may also take place, including steam and dry reforming of methane, particularly at temperatures above 700 °C, which may also result in carbon deposition. Therefore, under SR conditions, the steam gasification and the hydrogenation of the deposited carbon on the catalysts’ surface as well as the Boudouard reaction are also thermodynamically feasible [96].

In order to clarify the distribution of the reaction’s main condensated products, the dependence of their selectivity values, namely acetol (Sacetol), acetone (Sacetone), allyl alcohol (Sallyl alcohol), acetaldehyde (Sacetaldehyde) acetic acid (Sacetic acid) and acrolein (Sacrolein) on temperature, for all samples, is shown in Fig. 8. For the Ni/Al catalyst (Fig. 8a) it can be depicted that acetol was the main liquid product for T < 650 °C with its selectivity value between 30 and 34%. At higher reaction temperatures the main product was acetone (32–40%), while acetaldehyde and acetic acid selectivity values steadily increased with temperature, ranging from 14% (400 °C) to 20% (750 °C) and from 7% (400 °C) to 18% (750 °C), respectively. As for allyl alcohol, its selectivity values were quite constant with T ranging from 18.5 to 23%. Acetol was also the main product for the Ni/Zr catalyst (Fig. 8b), for reaction temperatures lower than 650 °C; selectivity values decreased from 34% (400 °C) to 25% (600 °C). Allyl alcohol and acetic acid were the main products between 650 and 700 °C and 750 °C, respectively. On the other hand, acetone and acetaldehyde selectivity values were quite constant with temperature (T < 700 °C) ranging from 20% (400 °C) to 26% (700 °C) and 12% (400 °C) to 19% (650 °C), respectively. For the Ni/Si catalyst (Fig. 8c), acetol was the main product for reaction temperatures lower than 600 °C (30–40%), allyl alcohol for 600 °C < T <700 °C (27–29%), acetaldehyde for T = 750 °C (31%). Acetone, acetaldehyde and acetic acid selectivities were increasing with reaction temperature (with the only exception T = 750 °C), while acetol and allyl alcohol followed the opposite trend.

Fig. 8

Liquid products selectivity for: a Ni/Al, b Ni/Zr and c Ni/Si [Reaction conditions: C3H8Ο3 (30 v/v%)/H2O (total liquid flow rate = 0.12 ml min−1)/He = 71 ml min−1, wcatalyst=200 mg, T = 400–750°C]

The presented product distribution can be explained according to the scheme proposed by Dumesic and Simonetti [97], where the conversion of glycerol to hydrogen takes place through the formation of a variety of chemical intermediates, such as alcohols and ketones. It has been reported that the dehydration of glycerol can follow two distinct pathways, where the first pathway [I] is through acrolein, which is considered to be much easier than the second pathway [II], which is through acetol [98, 99, 100]. According to the literature, although most of the dehydration sites could be acidic sites, this hypothesis is valid for the formation of acrolein whereas the selectivity to acetol is due to the activity of the basic sites. Also, in the absence of strong Brønsted acidic sites, the acetol or acrolein production reactions lead to non-selective products and facilitate rapid deactivation of the catalyst [99, 100, 101]. Although pure aluminas e.g., η-Al2O3, γ-Al2O3 are poorly active in the reaction, the high selectivity to acetol over the aluminas is related to their largest amount of basic surface AOH groups (as it was indicated from the Ni/Al sample CO2-TPD profile) and acid-base pair sites [22, 23, 63, 102]. It has also been suggested that the NiO phase, if it is partially reduced (as shown from the XPS results) is not active enough in catalyzing glycerol dehydration, but it enhances glycerol hydrogenolysis [102, 103, 104, 105, 106, 107]. As for the Ni/Si catalyst, it can be suggested that due to the neutral SiO2 properties, the glycerol dehydration is not favored, so the reaction pathway [I] is contributing the least. As for the Ni/Zr catalyst, it can be concluded that the high population of the weak and the existence of medium and strong acid sites (NH3-TPD results), together with weak basic ones (CO2-TPD), can mainly lead to acetol production.

3.4 Catalytic Stability

The major challenge in glycerol steam reforming reaction is the minimization of carbon deposition with time on-stream. It has been stated that the thermal decomposition of glycerol [108, 109] and dehydration, dehydrogenation, and condensation of its byproducts [110] are all responsible for coke accumulation. Active phase’s sintering is another factor for catalyst’s deactivation, as the GSR is conducted at relatively high temperatures (usually higher than 400 °C). It is known that sintering is an irreversible, thermal degradation process caused by agglomeration of supported metals and is usually occurring together with coke accumulation. Previous studies on Ru [17], Pt [111], and Ni [30] catalysts have pointed that both sintering and coke formation are taking place during the glycerol steam reforming reaction. One of the proposed strategies to alleviate sintering is to strengthen the interaction between active site and supporting material (strong metal-support interaction, SMSI) by using CeO2 [30], TiO2, or ZrO2 [35] as supports.

Figure 9 shows the stability results for the twenty (20) h time on stream experiments for all catalysts. A drastic drop in the activity of the Ni/Al catalyst can be observed, as values for glycerol total conversion and its conversion into gaseous products (Fig. 8a) decrease from 95 to 70% and from 54 to 22%, respectively. On the contrary, the Ni/Zr and Ni/Si catalysts seem to deactivate at a rather slower rate, as values for glycerol total conversion drop from 80 to 65% and from 89 to 70%, and values for conversion into gaseous products drop from 45 to 39% and 49 to 45%, respectively. From Fig. 9b the dependence of H2 yield and selectivity with time on stream can be observed. As can be seen, H2 selectivity values decrease with time for the Ni/Al and Ni/Zr catalysts, following a sharp decline curve for the Ni/Al (from 90 to 20%). The decline was smoother for the Ni/Zr catalyst starting from the value of 74% and reaching the value of 56% after 20 h, while the Ni/Si catalyst seems to be quite stable. The same trend can be seen for the H2 yield values for all catalysts, with the Ni/Si being the most stable with a final value of 2.5 moles H2/moles glycerol. From Fig. 9c it can be seen that the CO2 and CO selectivity values were quite constant after 20 h in the reaction stream for the Ni/Si catalyst ranging from 75 to 65% and from 21 to 31%, respectively. On the contrary, for the Ni/Al catalyst, the SCO2 and SCO reveal a variation through reaction time as the CO2 selectivity decreases from 76 to 9% and the CO increases from 22 to 71%. As for the Ni/Zr catalyst, a decrease in SCO2 (from 51 to 40%) and an increase to SCO (from 45 to 52% can be observed) with time on stream.

Fig. 9

Time on stream experiments a Total glycerol conversion and glycerol conversion into gaseous products, b H2 selectivity and H2 yield, and c CO2 and CO selectivity [Reaction conditions: C3H8Ο3 (30 v/v%)/H2O (total liquid flow rate = 0.12 ml min−1)/He = 71 ml min−1 wcatalyst=200 mg, T = 600 °C, t = 20 h]

It has already been mentioned that the acid-base properties of the support influences the catalyst reactivity and stability. High acidic supports (e.g., alumina) tend to dehydrate glycerol, yielding undesired coke precursors and clogging the system by subsequent condensation, whereas neutral materials such as silica have substantially greater stability [110]. On the other hand, although acidic supports are not preferred for the GSR reaction, the use of basic materials such as zirconia does not necessarily guarantee better performances [111, 112, 113]. A possible explanation could be that the reduction of the oxide supports could facilitate the synthesis of unsaturated hydrocarbons leading to carbonaceous deposits.

In Table 7 the catalytic performance of the Ni/Al, Ni/Zr and Ni/Si samples described by the reaction metrics at 600 °C, at the beginning (1st measurement) and the end (after 20 h) of the time on stream experiments is shown. It can be observed that the Ni/Al catalyst had the lowest values in terms of glycerol conversion to gaseous products (22.3%), H2 yield (0.75), H2 (19.7%), CO2 (8.78%), allyl alcohol (14.8%) selectivity, and H2/CO molar ratio (0.65) after 20 h time on stream. Moreover, it exhibits the highest selectivity values for CO (70.7%), CH4 (20.5%), acetone (15.8%), acetaldehyde (17.1%) and CO/CO2 molar ratio (8.06) values, amongst all samples. It can be suggested that the Ni/Al catalyst even if is the more active and selective one at the beginning, it turns to be the worst, after 20 h time on stream due to its drastic deactivation.

Table 7

Catalytic performance of the Ni/Al, Ni/Zr and Ni/Si samples at 600 °C, after 20 h of stability test (1st and last measurement)

Reaction metric

Ni/Al

Ni/Zr

Ni/Si

X(C3H8O3), (%)

95.03→70.42

80.26→64.90

89.39→70.29

X(C3H8O3) into gaseous products, (%)

53.84→22.33

44.88→38.61

49.37→44.49

Y(H2), (%)

3.13→0.75

2.33→1.51

3.07→2.51

S(H2), (%)

89.85→19.74

74.26→55.72

88.47→80.55

S(CO2), (%)

76.17→8.78

50.90→39.48

74.63→65.45

S(CO), (%)

22.03→70.70

45.44→52.31

21.23→31.45

S(CH4), (%)

1.79→20.52

3.66→8.21

4.14→3.11

S(acetol), (%)

30.59→39.75

43.37→39.97

22.60→33.03

S(acetone), (%)

16.09→15.80

11.84→11.11

18.84→10.66

S(allyl alcohol), (%)

20.21→14.81

19.65→22.96

23.38→30.46

S(acetaldehyde), (%)

18.73→17.09

14.96→15.77

14.90→13.07

S(acetic acid), (%)

14.38→12.55

10.17→10.19

20.28→12.77

H2/CO

9.51→0.65

3.81→2.49

9.72→5.98

CO/CO2

0.29→8.06

0.89→1.33

0.28→0.48

As mentioned previously, the GSR catalysts’ deactivation may be attributed to various factors as the increased crystallinity of the support, the sintering of the Ni metal particles, the oxidation of the metal sites and the deposition of carbon [114, 115]. For the case of Ni/Al, in accordance with the literature [11, 12, 26, 40] catalyst deactivation is mostly due to the blockage of the active sites by coke precursors formed on the surface acid sites, thereby decreasing the rate of H2 production. The formation of coke deposits is associated with dehydration, cracking and polymerization reactions, which take place on the acid sites of alumina [54]. Besides presenting higher carbon deposition it can be suggested that the Ni active phase sinteringis also responsible for the Ni/Al catalyst rapid deactivation that is associated to a transition of alumina’s crystalline phase during reaction [116, 117]. Concluding, it can be said that the dramatic deactivation that Ni/Al catalyst undergoes during the 20 h time on stream can be mainly attributed to the deposited carbon on its catalytic surface, facilitated by the alumina’s acidic character and possibly to metal particles’ sintering.

Regarding the Ni/Zr catalyst, it exhibited the lowest values at the end of the 20 h time on stream experiments for glycerol total conversion (65%) and acetic acid selectivity (10.2%), and the highest for acetol selectivity (40%); all other reaction metrics values are between the Ni/Al and Ni/Si catalysts’ ones. It can be concluded that the Ni/Zr catalyst seems to be more resistant to deactivation in comparison with the Ni/Al, mainly due to the enhanced basic character of its supporting material (CO2-TPD) and its stronger-metal support interactions (TPR), confirming the high capability of zirconia to stabilize the nickel active phase. Moreover, ZrO2 is known to possess the ability to first adsorb and then dissociate water, thus enhancing the adsorption of steam on its surface and activating the gasification of hydrocarbons in the SR and the water–gas shift reactions [118, 119, 120].

As for the Ni/Si sample, it can be seen that after 20h time on stream it exhibits the highest value for total glycerol conversion (70.3%), glycerol conversion to gaseous products (44.5%), H2 yield (2.5) and H2 (80.5), CO2 (65.5%), allyl alcohol (30.5%), acetic acid (12.8%) selectivities, as well as for the H2/CO molar ratio (6.0). On the other side, it exhibits the lowest values for the CO (31.5%), CH4 (3.11%), acetol (33.0%), acetone (10.7%), acetaldehyde (13.1%) and for the CO/CO2 molar ratio (0.48). These reaction metrics values make the Ni/Si catalyst the one with the best performance of all three catalysts tested herein and this can be attributed to its neutrality (even if it posses both acid and basic surface sites) leading to less condensed byproducts, that are responsible for the accumulation of coke deposits.

3.5 Characterization of Used Catalysts

It has been reported that coke formation can be observed during the pyrolysis and the steam gasification of glycerol in the absence of a catalyst [17, 108, 121, 122, 123] whereas, the GSR also results in coke deposition for most of the catalysts and temperatures that have been investigated [17, 108, 113, 124, 125, 126].

Results of the deposited carbon temperature programmed oxidation (TPO) experiments obtained for the used catalysts after the stability tests (20 h time on stream) are summarized in Fig. 10, where the CO2 concentration at the reactor effluent is plotted as a function of temperature. It is observed that the TPO profile of the Ni/Al catalyst is characterized by the presence of a weak peak at 470 °C and two broad peaks at 550 and 650 °C. For the Ni/Zr catalyst, a very broad peak can be observed at high temperatures, i.e., from 650 to 800 °C, while the profile obtained for the Ni/Si catalyst exhibits a completely different curve, compared to other samples, and is characterized by an intense peak located at 660 °C. The total amount of carbonaceous deposits formed on the studied samples following exposure to reaction conditions was estimated by integration of the respective TPO curves (inset of Fig. 9). It is observed that the lowest amount of carbon has been deposited on the Ni/Al catalyst (≈40%), followed by the other two samples (both ≈ 50%). From the TPO profiles the nature of carbonaceous deposits can be also analyzed. Generally, it can be said that the relatively weak band that was observed at lower temperatures, indicates a minimum presence of amorphous carbon, while the broad band at higher temperatures suggest a greater presence of graphitic carbon [127].

Fig. 10

TPO profiles and total amount of deposited carbon obtained for all samples after exposure to time on stream experiments

The Raman spectra of all used catalytic samples after stability tests are shown in Fig. 11. From the spectra, two characteristic peaks at around 1344 and 1580 cm−1 can be observed, which are attributed to the D-band and the G-band, respectively. The D-band is associated with vibrations of carbon atoms with dangling bonds in an amorphous carbon network, and the G-band corresponds to the stretching vibration of carbon sp2 bonds, which is typically observed in graphitic carbon with high degree of crystallinity, order and symmetry [23, 64]. The relative intensity of D- divided by G-band (I D/I G) can be regarded as a good indicator of the graphitization or the degree of disorder in the carbon structure of the coke formed during the reaction. Thus, smaller I D/I G values indicate higher crystallinity due to higher contribution of the graphitized carbon formed. The I D /I G ratio decreases in the following order: Ni/Si (I D /I G = 1.34) > Ni/Zr (I D /I G = 1.08) > Ni/Al (I D /I G = 0.88) and indicates that the fraction of different carbon types depends mainly on the catalyst’s supporting material nature and specific characteristics.

Fig. 11

Raman spectra of all samples after exposure to time on stream experiments

The presence of carbon over the surface of used catalysts (after exposure to the 20 h time on stream experiments) was also examined by SEM (Fig. 12). The images reveal the appearance of carbon filaments mainly on the Ni/Al catalyst and the absence of such structures on the Ni/Si sample. It is likely that most of the carbon deposited on the latter catalyst is in an amorphous state, as discussed above upon analyzing the Raman results.

Fig. 12

Morphological examination of catalytic samples after exposure to time on stream experiments

4 Conclusions

The aim of this work was to investigate the influence of the support’s nature on the Ni catalysts’ performance for the glycerol steam reforming reaction. It can be said that the support’s specific characteristics determine the active phase’s crystalline size and surface area, as well as the strength of metal-support interactions. Furthermore, the acid-base properties of the supporting material to a great extent define the catalyst’s activity for gaseous products, hydrogen yield, H2/CO molar ratio and liquid products’ distribution with reaction temperature. As a result, its deactivation through carbonaceous intermediates formation on its surface and carbon deposition, affecting its long term stability during time on stream can be also manipulated.

Specifically, comparing all three catalysts tested herein (Ni/Al2O3, Ni/ZrO2, Ni/SiO2) the latter one was proven to be the most stable due to its support’s total neutrality (even if it posses both acid and basic surface sites), leading to less condensed byproducts, considered to be responsible for carbon accumulation and deactivation. Surprisingly, the lowest amount of carbon, as it was revealed by TPO results, has been deposited on the Ni/Al2O3 spent catalyst, even if it’s the most rapidly deactivated one, followed by the other two samples with almost the same amount of deposited carbon. An additional information, though, is that the graphitization degree of the carbon, as it was estimated by the ID/IG ratio (Raman), follows the order Ni/Al2O3 > Ni/ZrO2 > Ni/SiO2 indicating that it is not the quantity, but rather the quality of the carbon deposits that determine catalyst’s deactivation.

It is therefore evident that the nature of the supporting material affects mainly the nickel supported catalysts’ deactivation during the glycerol steam reforming reaction. For a highly stable catalyst to be developed, specific acid-base properties of the support are required that can additionally lead to higher activity for gaseous products and hydrogen selectivity values.

Notes

Acknowledgements

Financial support by the program THALIS implemented within the framework of Education and Lifelong Learning Operational Programme, co-financed by the Hellenic Ministry of Education, Lifelong Learning and Religious Affairs and the European Social Fund, Project Title: ‘Production of Energy Carriers from Biomass by Products. Glycerol Reforming for the Production of Hydrogen, Hydrocarbons and Superior Alcohols’ is gratefully acknowledged. Moreover, the authors also wish to acknowledge financial support provided by the Committee of the Special Account for Research Funds of the Technological Educational Institute of Western Macedonia (ELKE, TEIWM, Grant Number: 80126). L.T. gratefully acknowledges the Bodossaki Foundation for financial support.

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Copyright information

© Springer Science+Business Media New York 2017

Authors and Affiliations

  • N. D. Charisiou
    • 1
  • K. N. Papageridis
    • 1
  • G. Siakavelas
    • 1
  • L. Tzounis
    • 2
  • K. Kousi
    • 3
  • M. A. Baker
    • 4
  • S. J. Hinder
    • 4
  • V. Sebastian
    • 5
  • K. Polychronopoulou
    • 6
  • M. A. Goula
    • 1
    Email author
  1. 1.Laboratory of Alternative Fuels and Environmental Catalysis (LAFEC), Department of Environmental and Pollution Control EngineeringWestern Macedonia University of Applied SciencesKozaniGreece
  2. 2.Composite and Smart Materials Laboratory (CSML), Department of Materials Science & EngineeringUniversity of IoanninaIoanninaGreece
  3. 3.Department of ChemistryUniversity of PatrasPatrasGreece
  4. 4.The Surface Analysis Laboratory, Faculty of Engineering and Physical SciencesUniversity of SurreyGuildfordUK
  5. 5.Chemical and Environmental Engineering Department & Nanoscience Institute of Aragon (INA)University of ZaragozaZaragozaSpain
  6. 6.Department of Mechanical EngineeringKhalifa UniversityAbu DhabiUnited Arab Emirates

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