Highly selective catalytic reduction of NO via SO₂/H₂O-tolerant spinel catalysts at low temperature

Abstract

Selective catalytic reduction of NO _X by hydrogen (H₂-SCR) in the presence of oxygen has been investigated over the NiCo₂O₄ and Pd-doped NiCo₂O₄ catalysts under varying conditions. The catalysts were prepared by a sol-gel method in the presence of oxygen within 50–350 °C and were characterized using XRD, BET, EDS, XPS, Raman, H₂-TPR, and NH₃-TPD analysis. The results demonstrated that the doped Pd could improve the catalyst reducibility and change the surface acidity and redox properties, resulting in a higher catalytic performance. The performance of NiCo_1.95Pd_0.05O₄ was consistently better than that of NiCo₂O₄ within the 150–350 °C range at a gas hourly space velocity (GHSV) of 4800 mL g⁻¹ h⁻¹, with a feed stream containing 1070 ppm NO, 10,700 ppm H₂, 2 % O₂, and N₂ as balance gas. The effects of GHSV, NO/H₂ ratios, and O₂ feed concentration on the NO conversion over the NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ catalysts were also investigated. The two samples similarly showed that an increase in GHSV from 4800 to 9600 mL h⁻¹ g⁻¹, the NO/H₂ ratio from 1:10 to 1:1, and the O₂ content from 0 to 6 % would result in a decrease in NO conversion. In addition, 2 %, 5 %, and 8 % H₂O into the feed gas had a slightly negative influence on SCR activity over the two catalysts. The effect of SO₂ on the SCR activity indicated that the NiCo_1.95Pd_0.05O₄ possesses better SO₂ tolerance than NiCo₂O₄ catalyst does.

Introduction

Nitrogen oxides (NO _X ), including NO, NO₂, N₂O, N₂O₃, N₂O₄, and N₂O₅, are mainly derived from fossil fuel combustion (Costa and Efstathiou 2007b). NO _X are the major air pollutants that are greatly hazardous to human health and the environment (Xiaoling et al. 2012), causing negative effects such as photochemical smog, acid rain, ozone depletion, ground-level ozone, and greenhouse effects (Qi et al. 2006). A number of techniques have been developed to reduce the emission of NO _X . Among them, the selective catalytic reduction of NO _X by ammonia (NH₃-SCR) is a well-known and widely industrialized NO _X control technology for stationary sources such as power plants and nitric acid plants (Li et al. 2010b). Generally, V₂O₅-WO₃ (MO₃)/TiO₂ is employed as NH₃-SCR catalysts (Grossale et al. 2008). However, many problems are encountered in the use of NH₃-SCR technology, namely catalyst deterioration, NH₃ slip (emissions of unreacted toxic ammonia), ash odor, air heater fouling, and a high running cost (Olympiou and Efstathiou 2011).

H₂-SCR has many advantages; for instance, hydrogen as a reductant does not induce any second pollutants and has high activity to reduce NO _X efficiently at the lowest possible temperature (Kim and Hong 2010; Machida et al. 2001). Especially for industrial sites where H₂ is readily available, H₂-SCR is regarded as a possible alternative for NH₃-SCR. The catalyst is the most central technology in any H₂-SCR process, and its performance directly affects the removal of nitrogen oxides (Koebel et al. 2001; Weirong et al. 2012). Currently, the catalysts of H₂-SCR primarily contain supported noble metal oxides (Jun Yub et al. 2003; Yang and Jung 2009), among which the Pt-based and Pd-based catalysts have been revealed to possess good catalytic activity at relatively low temperatures (Chiarello et al. 2007; Costa and Efstathiou 2007a; Qing et al. 2010; Schott et al. 2009). For example, Costa and Efstathiou (2007a) had reported that the Pt/MgO-CeO₂ catalyst exhibited a maximum of 95 % NO conversion and 78∼92 % N₂ selectivity within the 100–400 °C range. Higher than 90 % NO _X conversion within 170–300 °C was presented in the Pd/SiO₂ catalyst (Qing et al. 2010). Chiarello et al. (2007) investigated the catalytic reduction of NO _X by H₂ over Pd-based catalysts with a support consisting of LaCoO₃. The 0.5 wt% Pd/LaCoO₃ catalyst exhibited a maximum of approximately 100 % NO conversion and over 78 % N₂ selectivity at 150 °C.

Nevertheless, noble metals are rare and expensive and they are sensitive to sulfur poisoning. These factors limit their large-scale applications. Therefore, new highly efficient catalysts need to be searched to replace Pt-based and Pd-based catalysts for H₂-SCR for NO _X . Due to their low price, ready synthesis, and good redox property, transition metal oxides have been widely used as catalysts in various reactions (Auxilia et al. 2014; Chiu et al. 2015). Among them, spinel-type oxides have been widely studied because of their unique structure characteristics, and these oxide catalysts can simultaneously reduce NO _X and soot at comparatively low temperatures (possibly within the range typical of diesel exhaust 150–380 °C) (Fino et al. 2008). Chen et al. (2009) had reported that the CuCoOx/TiO₂ catalyst exhibited a maximum of 98.9 % NO conversion at 200 °C. Du et al. (2014) had reported that Co₃O₄ showed high performance on the removal of low-concentration (10 ppm) NO at room temperature.

Nickel cobaltite (NiCo₂O₄) is a mixed-metal oxide spinel that possesses interesting magnetic properties, rich redox chemistry, good electronic conductivity, and high electrochemical activity (Kim et al. 2000; Rui et al. 2013). In the recent research, NiCo₂O₄ is usually explored as an electrode material (Chi et al. 2006; Xu et al. 2014) and has rare reports about the catalytic reduction performance in the SCR reaction. Wang et al. (2015b) had reported that NiCo₂O₄ possessed the highest catalytic activity within 50–400 °C, with NO _X conversion of more than 70 % at 150 °C and N₂ selectivity of more than 90 % at 100–400 °C. Therefore, we choose nickel cobaltite as a model catalyst.

Generally, doped noble metal can improve the catalytic activity and reduce the operating temperature. Palladium is less expensive and more abundant than platinum (Gaspar and Dieguez 2000), and Pd as an active component shows high activity for H₂-SCR and is highly active for H₂ activation (Li et al. 2012). The exceptional performance is due to the high dispersion of Pd in the catalyst that forms the Pd-NO intermediates, which adsorb more NO on the catalyst surface and are reduce to N₂ by hydrogen with high N₂ selectivity (Li et al. 2008). Rodríguez and Saruhan (2010) reported that the highly active centers could be formed by the interaction sites between Pd and supports and high NO _X conversion and N₂ selectivities could be achieved by the synergistic effects of palladium and perovskites. Moreover, the synergistic effects efficiently enhance H₂ temperature-programmed reduction (H₂-TPR). Besides, Xu et al. (2015) had researched that doped Pd could make NiFe_1.95Pd_0.05O₄ increase acidity, reducibility, and catalytic activity.

NO was selected as a model nitrogen oxide in the simulated gas (Wang et al. 2009), because nitrogen monoxide accounts for 95–99 % of all nitrogen oxide emissions in flue gas (Fritz and Pitchon 1997). And the Pd-doped nickel cobaltite catalyst was successfully prepared through a sol-gel auto-ignition method and exhibited good catalytic performance for the selective catalytic reduction of NO by H₂ in the presence of oxygen at a low temperature. And we have considered the effects of the gas hourly space velocity (GHSV), NO/H₂ ratios, and O₂ feed concentration on the SCR activity. For practical consideration, we also investigated the durability of the catalyst and its tolerance to SO₂ and H₂O, respectively.

Materials and methods

Catalyst preparation

The catalyst used in this research was prepared via a sol-gel method using inorganic salts. All of the chemicals were of analytical grade and used without further purification. The compounds were weighed by analytical balance with the molar ratio of citric acid/Ni(NO₃)₃·6H₂O/Co(NO₃)₃·6H₂O/PdCl₂ = 3:1:1.95:0.05. Then, these compounds were dissolved in 50 mL deionized water with a concentration of 0.1 mol nitrate precursor (Jauhar et al. 2013). The pH of the mixture solution was additionally adjusted to 5–6 by slowly adding the ammonia solution, then the resulting solution was mixed together under stirring at room temperature for 3 h, and the sol was heated at 80 °C to form a wet gel; afterwards, the gel was dried at 130 °C. After drying, the obtained material was ground into fine powder. In order to obtain crystallized NiCo_1.95Pd_0.05O₄, the powder was calcinated in Muffle furnace at 400 °C for 4 h. Spinel NiCo₂O₄ was prepared in similar way with the only difference that merely the molar ratio of citric acid/Ni(NO₃)₃·6H₂O/Co(NO₃)₃·6H₂O/PdCl₂ was 3:1:2.

According to the drying and roasting process, the particles were easy to reunite and affect the catalyst activity, so about 5 mL polyethylene glycol (PEG) 400 was added to ensure high specific surface area and uniform particle size (Fan and Huang 2011).

Catalyst characterization

The as-prepared products were characterized by powder X-ray diffraction (XRD) using a Rigaku D/Max 2550 diffractometer (Japan) with Cu Kα radiation (λ = 1.54056 Å) operating at 40 kV and 100 mA. The elemental composition of the samples was characterized by energy-dispersive spectrometer (EDS) using a Falion 60s spectrometer. The Brunauer–Emmett–Teller (BET) surface area, average particle size, and pore size of the catalysts were measured with a nitrogen adsorption instrument (Micromeritics, TriStar II 3020) using N₂ gas as an adsorbent at the temperature of liquid nitrogen. Prior to BET analysis, the samples were degassed at 300 °C for 3 h.

The X-ray photoelectron spectroscopy (XPS) analysis was conducted in a Quantum 2000 Scanning ESCA Microprobe (Physical Electronics). The instrument uses a focused monochromatic Al Kα X-ray (1486.7 eV) source operated at a 100 W and 100-μm-diameter beam. The binding energy scale was calibrated using the carbon C1s at 284.6 eV for known standards. The de-convolution of XPS peak was performed with the CasaXPS program. The Raman spectroscopy experiments were carried out on an Iuvia microscope instrument (Iuvia Reflerx) with a wavelength of 514.5 nm.

H₂-TPR and temperature-programmed desorption of ammonia (NH₃-TPD) were performed on AutoChem II 2920 equipped with a thermal conductivity detector (TCD) detector. Prior to the H₂-TPR or NH₃-TPD analysis, 50 mg of the oven-dried sample was pretreated in a He stream at 300 °C for 60 min to remove the adsorbed H₂O and other gases followed by cooling to room temperature. After that, the H₂-TPR analysis was performed using a 10 % H₂/Ar mixture at a flow rate of 40 mL/min with a heating rate of 10 °C/min to 800 °C, while the NH₃-TPD analysis was carried out by 10 % NH₃/He mixture with a total flow rate of 40 mL/min at 50 °C for 60 min. After NH₃ adsorption, the sample was purged by He (40 mL/min) for another 60 min. The desorption profile was recorded using a TCD by heating the sample to 600 °C at 10 °C/min under a flow of He (50 mL/min).

Catalyst activity testing

The selective catalytic reduction of NO by hydrogen was carried out in a fixed-bed flow micro-reactor, the catalyst bed temperature was controlled by thermocouple which was interpolated into the fixed-bed reactor, and reactor was heated through an Al-518/518P-type artificial intelligence temperature controller. A sample weighed 1 g, and the reactant gas composites consisted of 1071 ppm NO; 1071–10,710 ppm H₂ (the concentration was based on the ratio of NO to H₂); 0–6 % O₂; 2 %, 5 %, and 8 % H₂O (when used); 100, 300, and 500 ppm SO₂ (when used); and the balance N₂. These gases were fed from compressed cylinders provided by Jia Jie Specialty Gases (Shanghai, China) and adjusted with Brooks thermal mass flow controllers. The catalyst was fixed by silica pellets and quartz wool and placed in the constant temperature zone of the tubular reactor. The total flow of the inlet gas and the gas hourly space velocity were based on the change of gas conditions. The gas effluent stream from the reactor was analyzed by a Chemiluminescent NO–NO₂–NO _X Analyzer (Thermo Scientific, model 42i), and the nitric oxide conversion was indicated using the following equation:

$$ {X}_{\mathrm{NO}}\left(\%\right)=\frac{{\left[\mathrm{NO}\right]}_{\mathrm{inlet}}-{\left[\mathrm{NO}\right]}_{\mathrm{outlet}}}{{\left[\mathrm{NO}\right]}_{\mathrm{inlet}}}\times 100\% $$

(1)

Among them, X _NO represents the NO conversion and [NO]_inlet and [NO]_outlet show the inlet and outlet concentrations of NO in the gas mixture at steady state, respectively.

The Thermo Scientific NO–NO₂–NO _X Analyzer revealed that the NO _X was the sum of NO, NO₂, and less low-state nitrogen oxides, and the ${X}_{{\mathrm{NO}}_X}$ represented the total NO _X conversion rate.

$$ {X}_{{\mathrm{NO}}_X}\left(\%\right)=\frac{{\left[{\mathrm{NO}}_X\right]}_{\mathrm{inlet}}-{\left[{\mathrm{NO}}_X\right]}_{\mathrm{outlet}}}{{\left[{\mathrm{NO}}_X\right]}_{\mathrm{inlet}}}\times 100\% $$

(2)

where the [NO _X ]_inlet and [NO _X ]_outlet show the inlet and outlet concentrations of NO in the gas mixture at steady state, respectively. The N₂ selectivity was calculated as follows:

$$ {S}_{{\mathrm{N}}_2}\left(\%\right)=\frac{X_{{\mathrm{N}\mathrm{O}}_X}\cdot {\left[\mathrm{NO}\right]}_{\mathrm{inlet}}}{X_{\mathrm{N}\mathrm{O}}\cdot {\left[\mathrm{NO}\right]}_{\mathrm{inlet}}}\times 100\% $$

(3)

In this equation, the X _NO·[NO]_inlet expresses the amount of nitric oxide in the transformation, while the ${X}_{{\mathrm{NO}}_X}\cdot {\left[\mathrm{NO}\right]}_{\mathrm{inlet}}$ expresses the amount of nitrogen from the transformation of nitric oxide.

Results and discussion

Catalyst characterization

Structural and textural properties

The XRD patterns of the samples used in this study are shown in Fig. 1a. The major peaks of the samples at ca. 2θ = 18.9°, 31.1°, 36.7°, 38.4°, 44.6°, 55.4°, 59.1°, and 65.1° could be indexed to (111), (220), (311), (222), (400), (422), (511), and (440) crystal planes of spinel NiCo₂O₄, respectively. It could be seen that the samples were a spinel cubic structure which were in good agreement with the standard pattern (JCPDS No. 73-1702) (Xu et al. 2014). While the XRD pattern of NiCo_1.95Pd_0.05O₄ was similar to that of NiCo₂O₄, the diffraction angle theta of (440) crystal planes slightly shifted from 65.1° to 64.9°, which is shown in Fig. 1b. These results indicated that the heavier and larger Pd²⁺ ions substituted the Co²⁺ ions in the structure of the nickel cobaltite spinel, resulting in a small angle deviation of XRD peaks (Kavas et al. 2009). And the Pd doping did not cause a significant change in the crystallinity of the samples, possibly due to the palladium that replaced Co without distorting the spinel structure; therefore, the NiCo_1.95Pd_0.05O₄ sample also maintained the spinel cubic structure.

Fig. 1

a XRD patterns of NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ used in this study. b XRD peak positions of (440) crystal planes

The average crystalline size of NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ was estimated to be 32.2 and 26.7 nm using Scherrer’s formula based on the (422) peak, respectively (Lou et al. 2008). Scherrer’s formula is as follows:

$$ D=\frac{K\lambda }{\beta \cos \theta } $$

(4)

Among which, D is crystalline size, K is constant, λ is X-ray wavelength (0.154056 nm), β is peak half-high width, and θ is the diffraction angle theta. Due to nickel cobaltite’s spinel cubic structure, K should be changed to 0.943 and a half-high width should be converted into a radian system, [(β / 180) × 3.14].

BET and EDS analysis

The BET surface areas, average particle size, and pore size of the studied NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ were summarized in Table 1. The surface areas of the NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ samples were 14.9956 and 12.6566 m² g⁻¹, respectively, while the average particle sizes were 14.0774 and 15.4802 nm, respectively, which had the reverse order compared to the surface area values. Generally, the larger the particle size, the smaller the BET area. The elemental composition of the samples was investigated using an EDS, as shown in Table 1. The Co/Ni atomic ratio (we chose Ni for comparison due to the stoichiometric amount being 1 in all of the studied samples) for NiCo₂O₄ was 2.03, which was consistent with the stoichiometric ratio. Due to the palladium doping, the Co/Ni atomic ratio of NiCo_1.95Pd_0.05O₄ was 1.92, deviating slightly from the theoretical ratio of 1.95. The Pd/Ni atomic ratio was 0.049, which was consistent with the theoretical value of 0.05.

Table 1 Physical and chemical properties of the studied catalysts

H₂-TPR and NH₃-TPD analysis

In order to investigate the reducibility and acidic properties of the NiCo₂O₄ and NiCo_1.95Pd_0.05O₄, the as-prepared samples were characterized by H₂ temperature-programmed reduction (H₂-TPR) and temperature-programmed desorption of ammonia (NH₃-TPD) analysis, respectively. As shown in Fig. 2a, there were three distinct reduction peaks in NiCo₂O₄, and the peaks appeared at 253 and 360 °C, corresponding to the reduction steps of Co³⁺ to Co²⁺ and Co²⁺ to Co⁰, respectively (Gou et al. 2013; Lim et al. 2015), while the peak appeared at 315 °C, which was attributed to the reduction of Ni²⁺ to Ni⁰. However, in comparison to NiCo₂O₄, the H₂-TPR profile of the NiCo_1.95Pd_0.05O₄ catalyst showed one slight peak and one strong broad peak. The slight peak at 129 °C was assigned to the reduction of Pd²⁺ to Pd⁰ (Giraudon et al. 2007; Ling et al. 2015), which was observed in pure Ni-Co spinel. And the strong broad peak within the range of 200–350 °C was formed by three reduction peaks fully overlapped, which corresponded to the reductions of Co³⁺ to Co²⁺ at 223 °C, Ni²⁺ to Ni⁰ at 286 °C, and Co²⁺ to Co⁰ at 316 °C. Compared to NiCo₂O₄, the reduction peaks of NiCo_1.95Pd_0.05O₄ shifted to low temperature, which indicated that the H₂ consumption was raised, enhancing the redox properties of catalysts. In addition, the TPR peak areas were an indicator to determine the reducibility of catalyst; the greater the peak area, the stronger the reducibility. The peak areas of H₂-TPR were provided in Table 2. It could be seen that the Pd-containing nickel cobaltate exhibited relatively higher TPR areas than NiCo₂O₄, indicating that the palladium doping could improve the catalyst reducibility.

Fig. 2

H₂-TPR and NH₃-TPD profiles of NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ used in this study. a H₂-TPR. b NH₃-TPD

Table 2 The peak areas of H₂-TPR and NH₃-TPD

The acidic properties of the as-prepared samples were shown in Fig. 2b. The NH₃ adsorbed on the acid sites and then desorbed at different temperature regions, which was determined by the strength of acid sites and the amount of acid, respectively. The NH₃ desorption below 400 °C corresponded to the weak- and medium-strength acid sites (Imran et al. 2013). Nevertheless, the TPD profile of NiCo₂O₄ showed a broad NH₃ desorption peak from 100 to 350 °C, which was assigned to the NH₃ desorbed by weak- and medium-strength acid sites. Moreover, the acid amount could be determined by the TPD peak areas and the size of peak area corresponded to the amount of acid. Table 2 showed the NH₃-TPD peak areas. Compared to NiCo₂O₄, NiCo_1.95Pd_0.05O₄ slightly increased the peak areas from 100 to 350 °C, indicating that NiCo_1.95Pd_0.05O₄ showed a relatively more acidic amount than NiCo₂O₄.

Raman analysis

NiCo₂O₄ is an inverse spinel with tetragonal (A site) positions occupied by mostly Co³⁺ and octahedral (B site) positions occupied by nearly equal concentrations of Ni²⁺ and Co³⁺ (Iliev et al. 2013). In order to further understand the composition and structural features of the NiCo₂O₄ and NiCo_1.95Pd_0.05O₄, the samples were characterized with Raman spectroscopy. The typical Raman spectrum was shown in Fig. 3; the peaks of NiCo₂O₄ at 183, 465, 509, and 651 cm⁻¹ corresponded to F_2g, E_1g, F_2g, and A_1g models of NiCo₂O₄, respectively, and the Co–O and Ni–O stretching vibrations could be detected in the Raman spectrum. These results were well consistent with previously reported literatures (Babu et al. 2014; Li et al. 2015; Liu et al. 2013). Moreover, the peaks at 181, 418, 501, and 644 cm⁻¹ corresponded to F_2g, E_1g, F_2g, and A_1g models of NiCo_1.95Pd_0.05O₄, respectively. Compared with that of NiCo₂O₄, the Raman spectra of the Pd-substituted nickel cobaltate (NiCo_1.95Pd_0.05O₄) exhibited a shift toward lower wave number, which was the result of the heavier and larger Pd²⁺ ions substituting the Co²⁺ ions in the structure of the nickel cobaltite spinel.

Fig. 3

Raman profiles of NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ used in this study

XPS analysis

In order to investigate the chemical bonding states and compositions of surface elements of as-synthesized NiCo₂O₄ and NiCo_1.95Pd_0.05O₄, the samples were studied by X-ray photoelectron spectroscopy (XPS), and the results were shown in Fig. 4a–d. The Ni, Co, O, and Pd elements were detected for the prepared samples.

Fig. 4

XPS spectra of NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ used in this study. a O1s. b Co2p. c Ni2p. d Pd3d

The high-resolution spectrum for the O1s region in Fig. 4a showed two peaks at binding energies of around 529.1 and 530.8 eV, respectively, which had been denoted as O1 (529.1 eV) and O2 (530.8 eV). The O1 at the low energy of 529.1 eV was typical of the O atoms in the O–Co/Ni bonding. And the O₂ at 530.8 eV was a characteristic of the as-prepared NiCo₂O₄ samples, which corresponded to a number of defect sites with low-oxygen coordination in the material with small particle size (Kim et al. 2000; Liu et al. 2013; Rui et al. 2013). The Co2p binding energies and peak shape were similar for the two preparations (Fig. 4b) and yield binding energies of 779.8 and 794.7 eV for the 2p3/2 and 2p1/2 transitions, respectively (Kim et al. 2000). XPS spectra of Co2p3/2 in the two preparations showed two main peaks of binding energies at 779.4 and 781.4 eV, which were assigned to the surface Co³⁺ and Co²⁺ species, respectively (Wang et al. 2015b). And this indicated that there were only a few Co²⁺ species in the octahedral sites and most of the low-spin Co³⁺ species occupied the octahedral sites (Babu et al. 2014; Kim et al. 2000).

The Ni2p spectra given in Fig. 4c were fitted considering two spin-orbit doublets as a characteristic of Ni²⁺ and Ni³⁺ and two shake-up satellites. The fitting peaks at the binding energy of 854.4 and 871.6 eV were indexed to Ni²⁺, while the fitting peaks at the binding energy of 856.43 and 873.8 eV were ascribed to Ni³⁺, respectively (Li et al. 2015, 2010a; Yu et al. 2016). The Pd3d spectra, as presented in Fig. 4d, showed two peaks at the binding energy of 337.1 eV for 3d5/2 and 342.5 eV for 3d3/2, respectively. The Pd5/2 binding energy was closed to the value of 336.9 which is a characteristic of Pd²⁺, indicating that the Pd doped in the as-prepared NiCo_1.95Pd_0.05O₄ sample (Giraudon et al. 2007; Hu et al. 2011; Ling et al. 2015).

These results exhibited that the surface of the as-synthesized NiCo₂O₄ that contained Ni²⁺, Ni³⁺, Co²⁺, and Co³⁺ (Moni et al. 2014), while the NiCo_1.95Pd_0.05O₄ also contained Pd²⁺ except the common elements.

Catalyst performance

SCR activity of the NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ catalysts

Figure 5 shows the results of the NO conversion and N₂ selectivity over the NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ in the temperature range of 50–350 °C with a GHSV of 4800 mL g⁻¹ h⁻¹ in the presence of 2 % O₂. As shown in Fig. 5a, as the reaction temperature increased, the NO conversion firstly increased then decreased over the NiCo_1.95Pd_0.05O₄ catalyst. While the SCR activity over the NiCo₂O₄ catalyst was always increasing until it reached the maximum NO conversion. The highest NO conversion of the NiCo₂O₄ catalyst was approximately 40.05 % at 350 °C; yet, the N₂ selectivity over the NiCo₂O₄ catalyst was only 60∼80 %, which is illustrated in Fig. 5b. The NO conversion over the NiCo_1.95Pd_0.05O₄ catalyst was higher than 90 % at a suitable temperature of 200–250 °C with the maximum of 94.65 % at 230 °C. And the N₂ selectivity was higher than 90 % in the whole 130–350 °C range, with the maximum closed to 100 % at the 200–270 °C range.

Fig. 5

a The effect of the reaction temperature on NO conversion for H₂-SCR over the NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ catalysts in the presence of 2 % O₂. b N₂ selectivity over the NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ catalysts in the presence of 2 % O₂. c, d The effect of oxygen concentration (0∼6 %) on NO conversion for H₂-SCR over the NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ catalysts. Reaction condition: [NO] = 1070 ppm; [H₂] = 10,700 ppm; [O₂] = 0 %, 2 % (a, b), 4 %, and 6 %; [N₂] = balance; catalyst mass = 1 g; GHSV = 4800 mL g⁻¹ h⁻¹; and T = 50–350 °C. c NiCo₂O₄. d NiCo_1.95Pd_0.05O₄

Moreover, a previous study showed that the content of O₂ was a crucial parameter for the H₂-SCR reaction (Yang et al. 2011; Yuan et al. 2013). The combustion of fuel was not sufficient in the old burners, which resulted in the high oxygen content of 6∼12 %. However, the new type of circulating fluidized bed burns the fuel more efficiently, resulting in the low oxygen content of 4∼6 % (Huilin et al. 2000). Hence, in our study, the highest oxygen content was chosen to be 6 %. In general, oxygen can inhibit NO removal efficiency by oxidizing H₂ to H₂O, but it also oxidizes NO into adsorbed nitrite/nitrate on the surface to enhance the reduction reaction by hydrogen in the H₂-SCR process (Machida et al. 2001; Wei et al. 2012). Hence, the effect of the oxygen concentration ([O₂] = 0 %, 2 %, 4 %, and 6 %) in the feed gas over the NiCo_1.95Pd_0.05O₄ and NiCo₂O₄ catalysts was investigated, which also consisted of 1070 ppm NO/10,700 ppm H₂/x % O₂/N₂ at a temperature range of 50–350 °C with a GHSV of 4800 mL g⁻¹ h⁻¹.

The oxygen content in the flue gas from the circulating fluidized bed boiler is generally 4∼6 % (Huilin et al. 2000). Therefore, experiments were performed to examine the O₂ tolerance of the prepared catalysts. As illustrated in Fig. 5c, under O₂-free reaction conditions, a higher removal efficiency of NO was achieved and an efficiency of 100 % over the NiCo₂O₄ catalysts was stabilized within a temperature range of 250–350 °C. As the O₂ content increased from 0 % to 6 %, the deNO _X performance rapidly decreased and the maximum NO conversion decreased from 100 to 30.77 % at the 350 °C. As shown in Fig. 5d, the high NO conversion of 100 % over NiCo_1.95Pd_0.05O₄ was stabilized within a wider temperature range of 150–350 °C in the absence of oxygen, indicating that the doped Pd improved the catalytic activity and shifted the reaction temperature of the efficiency of 100 % from 250 to 150 °C. When the O₂ increased to 2 %, the maximum NO conversion over NiCo_1.95Pd_0.05O₄ only changed to 94 % at the 230 °C; as the O₂ content increased, the SCR still exhibited good activity for NO reduction, with the maximum NO conversion of 81.16 % with 4 % O₂ at 230 °C and 61 % in the presence of 6 % O₂ at 230 °C, proving that Pd incorporation not only significantly improves the catalytic activity but also increases the O₂ tolerance ability (Wei et al. 2012).

To sum up, the NiCo_1.95Pd_0.05O₄ catalyst showed higher catalytic performance as well as greatly reduced the reaction temperature of the maximum removal efficiency. Hence, the NiCo_1.95Pd_0.05O₄ catalyst was suitable for the selective catalytic reduction of NO by H₂ in the presence of oxygen at a low temperature. Therefore, Pd played a very important role in the nickel cobaltite catalyst in the H₂-SCR reaction. In addition, palladium did not destroy the spinel structure of NiCo₂O₄ when it doped in nickel cobaltite, which was consistent with the XRD results. And from the H₂-TPR and NH₃-TPD activity tests, the NiCo_1.95Pd_0.05O₄ exhibited relatively high TPR areas, reduction level, and slightly larger acidity than NiCo₂O₄. All these factors resulted in the better catalytic performance of NiCo_1.95Pd_0.05O₄.

Effect of GHSV and NO/H₂ ratio

In general, GHSV could significantly affect the NO conversion rate at low temperature, and it was believed that it has less effect on the conversion rate at high temperature (Qi et al. 2006). Consequently, the H₂-SCR activity of the NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ catalysts at different GHSVs (from 4800 to 9300 mL h⁻¹ g⁻¹) and NO and H₂ feed concentration ratios (NO/H₂ = 1:10–1:1) were investigated at a temperature range of 50–350 °C in the presence of 2 % O₂, and the results are shown in Fig. 6.

Fig. 6

Effect of gas hourly space velocity and NO/H₂ ratios on NO conversion for H₂-SCR over the NiCo_1.95Pd_0.05O₄ and NiCo₂O₄ catalysts. Reaction condition: [H₂] = 10,700 ppm (when examined gas hourly space velocity), [NO] = 1070 ppm, [O₂] = 2 %, [N2] = balance, catalyst mass = 1 g, GHSV = 4800 mL g⁻¹ h⁻¹ (when NO/H₂ ratios were examined), and T = 50–350 °C. a NiCo₂O₄. b NiCo_1.95Pd_0.05O₄. c NiCo₂O₄. d NiCo_1.95Pd_0.05O₄

As shown in Fig. 6a, the NO conversion over the NiCo₂O₄ catalyst decreased with the increasing GHSV. When the GHSV was 4800 mL h⁻¹ g⁻¹, the maximum NO conversion was up to 40.5 % at 350 °C, while when the GHSV increased to 6960 and 9300 mL h⁻¹ g⁻¹, the maximum conversion decreased to 35 % and 31 %, respectively. However, in contrast with NiCo₂O₄, the NO conversion faintly decreased over the NiCo_1.95Pd_0.05O₄ catalyst with the increase of the GHSV from 4800 to 9600 mL h⁻¹ g⁻¹ at the reaction temperature of 125–350 °C, which was shown in Fig. 6b. When the GHSV was 4800 mL h⁻¹ g⁻¹, the maximum NO conversion was up to 94 % at 230 °C, and when the GHSV was 9300 mL h⁻¹ g⁻¹, the maximum conversion was approximately 92 % at 230 °C.

These results demonstrated that the GHSV was a crucial parameter for the H₂-SCR reaction and the space velocity determined the residence time of the gas in the catalyst. As GHSV increased, the residence time of feed gas decreased. The NiCo_1.95Pd_0.05O₄ catalyst showed high NO conversion with the increasing GHSV, due to the Pd addition, which enhanced the redox properties of the active catalyst component. And this was consistent with H₂-TPR. Although the increase of GHSV resulted in the decrease of the reaction gas residence time in the catalyst, the activity of the active component was enhanced in the Pd-doped catalyst, resulting in more reduction reaction sites for NO and only a slight decrease of NO conversion. Therefore, the Pd-doped NiCo_1.95Pd_0.05O₄ catalyst showed better catalytic performance than the NiCo₂O₄ catalyst at different GHSVs (from 4800 to 9300 mL h⁻¹ g⁻¹).

The variation trend of NO conversion over the sample catalysts was roughly similar as the increase of reaction temperature at different NO/H₂ ratios, the NO conversion firstly increased then decreased over the NiCo_1.95Pd_0.05O₄ catalyst with the increasing reaction temperature, but the removal of NO over the NiCo₂O₄ catalyst increased as the temperature increased. As shown in Fig. 6c, the NO conversion over the NiCo₂O₄ catalyst decreased as the NO and H₂ feed concentration ratio increased within a temperature range of 170–350 °C in the presence of 2 % O₂. When the NO/H₂ ratio was 1:10, the maximum NO conversion was 41 %. When the NO/H₂ ratio was changed to 1:5 and 1:1, the NO conversion rate was 34 % and 31 %, respectively, exhibiting that higher H₂ concentration that resulted in higher NO conversion. Nevertheless, compared with the NiCo₂O₄ catalyst, the NiCo_1.95Pd_0.05O₄ catalyst showed higher catalytic activity at different NO and H₂ ratios, which is shown in Fig. 6d. When the NO/H₂ ratio was 1:10, the maximum NO conversion was more than 95 % at a reaction temperature of 230 °C. Actually, the economic factors must be considered. Industrially, the lower NO/H₂ ratio was used to obtain a relatively high NO conversion. When the NO/H₂ ratio was changed to 1:5 and 1:1, the NO conversion rate was 76 % and 37 %, respectively. Therefore, the doped Pd largely improved the catalytic activity and NiCo_1.95Pd_0.05O₄ exhibited higher catalytic performance than NiCo₂O₄ in the common reaction condition.

Effect of the presence of H₂O and SO₂

H₂O and SO₂ were usually contained in the industrial flue gases, which could cause a deactivation on SCR catalysts, and the general content of water was 2∼10 %. At the same time, the SCR catalysts are sensitive to sulfur poisoning since sulfur compounds could deposit on the active sites of catalysts and deactivate them irreversibly (Chang et al. 2013; Lee et al. 2013; Yin et al. 2015). Therefore, it was important to investigate the effect of H₂O and SO₂ on NO conversion over selected catalysts.

The effect of H₂O and SO₂ on the selective catalytic reduction of nitric oxide with hydrogen over the NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ catalyst was demonstrated in Fig. 7. As shown in Fig. 7a, when 2 %, 5 %, and 8 % H₂O was introduced to feed gas, the NO conversion of NiCo₂O₄ decreased rapidly from 36.8 %, 36.5 %, and 36.3 % to 33.2 %, 32.5 %, and 31.8 %, respectively, and then the NO conversion recovered slowly to 34.2 %, 33.4 %, and 32.6 % after the removal of H₂O, respectively. Compared with the initial value, the decrement of NO conversion was 7.07 %, 8.49 %, and 12.95 %, respectively. It could be seen that the conversion decreased more as the content of H₂O increased, which was due to the water-irreversible dissociative adsorption on the active sites of the catalyst (Burch and Coleman 1999; Leicht et al. 2012). And the NO conversion recovered a little, which was due to the reversible competitive adsorption by water. Hence, the influence of water on the catalyst was both reversible and irreversible.

Fig. 7

Effect of H₂O ([H₂O] = 2 %, 5 %, and 8 %) and SO₂ ([SO₂] = 100, 300, and 500 ppm) on NO conversion for H₂-SCR over the NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ catalysts. Reaction condition: [NO] = 930 ppm (950 ppm, when SO₂ was added), [H₂] = 9300 ppm (9500 ppm, when SO₂ was added), [O₂] = 2 %, [N2] = balance, catalyst mass = 1 g, GHSV = 9300 mL g⁻¹ h⁻¹, and flow rate = 155 mL/min. a NiCo₂O₄, T = 350 °C, when H₂O was added. b NiCo_1.95Pd_0.05O₄, T = 230 °C, when H₂O was added. c NiCo₂O₄, T = 350 °C, when SO₂ was added. d NiCo_1.95Pd_0.05O₄, T = 230 °C, when SO₂ was added

However, the behavior of H₂O poisoning of the NiCo_1.95Pd_0.05O₄ catalyst was quite different. As shown in Fig. 7b, after 2 %, 5 %, and 8 % H₂O was introduced into the inlet gas, the NO conversion showed a slight decrease approximately from 87.2 % to 86.5 %, 84.5 %, and 80.5 %, respectively, which probably was due to the H₂O competitive adsorption with NO as well as with H₂. The water on the catalyst could form the additional surface hydroxyls because of the dissociative adsorption of water (Kijlstra et al. 1996), and the surface hydroxyls would neutralize the acid sites, resulting in the reduction of SCR activity. For the Pd-doped catalyst, the surface acidity was enhanced. The multi-acid neutralized the hydroxyls and decreased the water poisoning on active sites of the catalyst. Therefore, the NiCo_1.95Pd_0.05O₄ catalyst has a better H₂O tolerance than NiCo₂O₄. Furthermore, the influence of surface hydroxyls neutralizing the acid sites was irreversible; when the H₂O was off, the acidity of acid sites could not recover, and hence, the NO conversion over the NiCo_1.95Pd_0.05O₄ catalyst could not restore to the original level, which was consistent with Fig. 7b.

Moreover, the SO₂ tolerance and regenerability of NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ catalysts were also examined. As shown in Fig. 7c, when the 100 ppm SO₂ was added to the reactant gas, the NO conversion of NiCo₂O₄ at 350 °C decreased rapidly from 35.8 % to 31.6 % in 150 min and only recovered to 32.6 % after the removal of SO₂, manifesting the inhibition effect of SO₂ on the SCR activity over NiCo₂O₄ (Chang et al. 2013; Yin et al. 2015). When the SO₂ increased to 300 and 500 ppm, the NO conversion decreased rapidly to lower than 27.5 % and 14.5 % and only recovered to 30.6 % and 21.6 % after the removal of SO₂, respectively. The decrease of NO conversion was mainly due to the blocking of the active sites by the formation of metal sulfates and/or sulfites (Li et al. 2011). The sulfated species formed on the catalytic center inhibited the SCR activity, which resulted in the decrease of NO conversion. After the supply of SO₂ was cut off, the NO conversion recovered slowly in a certain amount, which was mainly due to the regeneration of the part of sulfated catalysts by the hydrogen (Wang et al. 2015a).

The SO₂ poisoning behavior to the NiCo_1.95Pd_0.05O₄ catalyst at 230 °C was quite similar. As shown in Fig. 7d, after 100, 300, and 500 ppm SO₂ was introduced to the inlet gas, the NO conversion of NiCo_1.95Pd_0.05O₄ decreased steadily from 88.1 % to 83.8 %, 80.5 %, and 51.6 % in 130 min, respectively. The decrement of NO conversion was 4.88 %, 8.63 %, and 41.43 % compared to the initial value; however, the decrement value of NiCo₂O₄ was 11.73 %, 23.18 %, and 59.50 %, respectively. This was attributed to the improvement of catalytic activity by enhanced surface acidities and redox properties, which was consistent with NH₃-TPD and H₂-TPR. After 100, 300, and 500 ppm SO₂ was off, the NO conversion of NiCo_1.95Pd_0.05O₄ could efficiently recover to 84.4 %, 82.8 %, and 65.5 %, respectively, indicating that the deactivation was partially reversible.

These results demonstrated that the NiCo_1.95Pd_0.05O₄ catalyst possesses better SO₂ tolerance than NiCo₂O₄ catalyst does.

Conclusions

In this study, the NiCo₂O₄ and NiCo_1.95Pd_0.05O₄ catalysts were investigated for NO selective reduction by hydrogen in the presence of O₂. The results showed that the NiCo₂O₄ catalyst had a certain effect for NO removal, while the Pd-containing NiCo_1.95Pd_0.05O₄ catalyst exhibited better SCR activity with the maximum NO conversion of 94.65 % at 230 °C and the N₂ selectivity of 100 %. Moreover, the prepared NiCo_1.95Pd_0.05O₄ exhibited higher H₂O and SO₂ tolerance than NiCo₂O₄.