1 Introduction

The major sources of global energy production are combustibles like coal, natural gas, oil and fuel. However, a critical issue of these fossil fuels is the output of CO2 causing global warming. Therefore, sustainable strategies of energy supply are of marked relevance, especially for the important transport sector. Promising concepts of transportation imply biogenic fuels, synthetic fuels originated from biomass, fuel cells, batteries and hybrid engines. Furthermore, H2 combustion engines are also developed with particular focus on fleets of trucks and busses fuelled by regional filling stations. Sustainable production of H2 is associated with the electrolysis of water powered by photovoltaic and wind generators. However, a serious issue of H2 combustion engines is the output of NOx, which is associated with the formation of thermal NO, thus demanding the after-treatment of the exhaust to meet emission standards. For this purpose, the catalytic NOx reduction by H2 (2 NO + 2 H2 → N2 + 2 H2O) is a favourable tool using the engine slip of H2, whereas injection of H2 to the exhaust is also possible. As a consequence, the supply of an additional reductant such as urea is not necessary. Aqueous solution of urea (”AdBlue”) is employed for the on-board production of NH3 required for the selective catalytic reduction of NOx (SCR), which is well-established for diesel passenger cars and trucks. In SCR, NO is reduced by NH3 to yield N2 and H2O (4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O). Traditional SCR catalysts are comprised of V2O5/WO3/TiO2, while for diesel vehicles Fe and Cu based zeolites are presently taken [1].

Furthermore, in the lean exhaust of passenger cars NOx storage reduction catalysts (NSR) are also applied consisting of Pt, Pd and Rh as well as NOx adsorbents like Al2O3, CeO2 and BaCO3. The operation of NSR includes periodic storage and reduction of NOx [2]. The reduction is performed under rich conditions temporarily induced by engine management and is substantially enhanced by the presence of H2 [3].

Moreover, the reduction of NOx by H2 also occurs effectively in the excess of O2 when using Pt catalysts. Their operation range lies between 65 and 200 °C and is therefore relevant for low exhaust temperatures of lean-burn engines. However, a serious drawback is the marked selectivity of N2O of up to 50%, whereas Mo and Co promotors are shown to decrease N2O [4, 5]. Additionally, Ru, Ir, Rh, Pd and Ag [6,7,8,9] as well as mixed oxides like perovskites [10,11,12,13] and spinels [8] reveal lean H2-deNOx activity, but also include substantial production of N2O. A recent review also features several TiO2-based precious metal catalysts with significant activity under lean conditions [14, 15]. Nevertheless, clearly diminished N2O formation combined with high performance below 200 °C is only reported for a few catalysts, particularly Pt/La0.7Sr0.2Ce0.1FeO3 [10, 16], Pt/MgO-CeO2 [17,18,19] and Pt/WO3/ZrO2 [20]. Furthermore, Pd/WO3/ZrO2 catalysts exhibit selective formation of N2 upon lean H2-deNOx, but their activity is limited to a rather narrow operation range [21]. High activity and selectivity of N2 is also evidenced for LaCoO3-supported Pd [22]. Moreover, Ag/Al2O3 catalysts show an enhancing effect of H2 for both NOx reduction by hydrocarbons [23] as well as SCR [24].

The present paper aims to study Al2O3/SiO2-based Pt catalysts with and without WOx promotor for the NOx reduction by H2 under O2-rich conditions. WOx was shown to reveal high efficiency for lean H2-deNOx on Pt and Pd catalysts [15, 20, 21, 25]. The catalysts were prepared, thoroughly characterised and tested in synthetic model exhaust of H2 combustion engines towards low-temperature deNOx. Additionally, the H2-deNOx performance was correlated with the physical–chemical properties to unravel features, which drive the activity as well as N2 selectivity of the catalysts. Also, possible strong metal support interaction (SMSI) was evaluated to enlighten the effect of the WOx promotor.

2 Experimental Section

2.1 Catalyst Preparation

A series of commercially available Al2O3/SiO2 carriers with SiO2 proportions of 20, 30 and 40 wt% was used (Siralox 20 HPV, Siralox 30 HPV and Siralox 40 HPV, Sasol); respective codes are AlSi-20, AlSi-30 and AlSi-40. The carriers were modified with the active component Pt, whereas AlSi-20 was additionally coated with tungsten oxide promotor before introducing Pt. The codes of the tungsten-modified catalysts reflect the mass fraction of tungsten present in the sample, e.g. 11W/AlSi-20. The introduction of WOx and Pt was carried out by incipient wetness impregnation implying complete absorption of a defined volume of an aqueous solution of Pt(NO3)2 (Chempur) and (NH4)6H2W12O40∙H2O (Honeywell), respectively. In accordance with previous work [20] the load of Pt was adjusted to be 0.25 wt% referring to bare substrate, and for AlSi-20 the content of W was varied between 0 and 85 wt% also referring to bare substrate. The highest loading of tungsten is related to a theoretical monolayer of planar WO3 [26]. For special investigations, samples with a Pt load of 2.0 wt% were also prepared. After each impregnation, samples were dried in air for 1 h at 80 °C. Subsequently, the catalysts were activated by dosing a mixture of 10 vol% H2 and 90 vol% N2; temperature was ramped from 20 to 300 °C with 1.7 K/min and end temperature was held for 30 min. Finally, the samples were conditioned in static air at 500 °C for 5 h.

2.2 Catalyst Characterization

N2 physisorption was made on a TriStar II (Micromeritics). From the equilibrated N2 adsorption isotherms taken at -196 °C, BET surface area (SBET) and total specific pore volume (Vp) were determined. Prior to N2 exposure, sample was outgassed in vacuum (10–1 mbar) at 350 °C for 16 h to remove adsorbed components, particularly H2O. The BET surface area was derived from the adsorption data recorded at p/p0 ratios from 0.05 to 0.20.

Acidity of the samples was characterized by temperature-programmed desorption of ammonia (NH3-TPD). Before the analysis, respective sample was pelletised, granulated and sieved to a size of 125—250 µm to avoid discharge upon TPD. A sample mass of 500 mg was introduced into the quartz glass tube reactor (i.d. 8 mm) and pre-treated in N2 flow (500 ml/min, STP) at 450 °C for 1 h. After cooling to 50 °C in N2, the sample was exposed to a gas mixture (500 ml/min, STP) consisting of 1000 vppm NH3 and N2 (balance) until saturation was reached (min. 30 min). Weakly adsorbed NH3 was then removed by flushing with N2 until the NH3 outlet fraction was below 5 vppm. Finally, TPD was started by heating at a rate of 10 K/min in flowing N2 (500 ml/min, STP). Temperature was recorded by two K-type thermocouples fitted directly in front and behind the sample. NH3 was continuously monitored by non-dispersive infrared spectroscopy (NDIR, X-Stream X2GP, Emerson).

Temperature-programmed desorption of CO (CO-TPD) was made to quantify the number of active Pt site and to check possible SMSI effect. A sample mass of 2.00 g, which was prepared like for NH3-TPD, was introduced into the quartz glass tube reactor (i.d. 8 mm) and pre-treated in a mixture of 5 vol% O2 and 95 vol% Ar at 500 °C for 15 min followed by flushing with pure Ar for another 20 min (500 ml/min, STP). At the same temperature, the sample was reduced by 3000 vppm CO in Ar (500 ml/min, STP) for 15 min and cooled down to 50 or 350 °C in Ar flow. After saturation at 50 or 350 °C with 3000 vppm CO in Ar (100 ml/min, STP), it was flushed with Ar and CO-TPD was started (150 ml/min, STP) by heating to 700 °C at the rate of 20 K/min. Temperature was recorded by a K-type thermocouple fitted directly in front of the sample. Desorbing CO and CO2 was monitored by NDIR (X-Stream Enhanced XEGK, Emerson). For quantification of the active Pt sites, the CO and CO2 TPD profiles were integrated. The evolution of CO2 is ascribed to the oxidation of desorbed CO by the catalyst surface. A blank CO-TPD of the bare supports provided only minor amounts of CO and CO2, which were subtracted from the respective TPD traces of the catalysts. The amount of active Pt sites (na(Pt)) was calculated based upon a CO/Pt adsorption stoichiometry of 1 [27]. Furthermore, from this number of active centres and the total abundance of Pt present in the catalyst (nPt) the dispersion of Pt (DPt = na(Pt) / nPt) was derived, while the mean size of Pt particles (dPt = 6 VPt / (aPt DPt)) was calculated by assuming spheres; VPt amounts to 15.10 Å3 and is the volume of a Pt atom present in the bulk metal, aPt is equal to 8.07 Å2 and represents the surface area of a Pt atom located on a polycrystalline surface [27].

The size of Pt particles was also evaluated by a FEI Quanta FEG 250 scanning electron microscope (SEM) in BSE mode using an acceleration voltage of 2 kV. SEM was coupled with energy-dispersive X-ray spectroscopy (EDX).

X-ray diffraction (XRD) of the powder samples was performed at room temperature on a D8 Discover (Bruker-AXS) with Bragg–Brentano configuration, Fe-filtered Co-Kα radiation and VANTEC-1 detector. Diffractograms were taken from 5° to 100° in 2θ mode at a step width of 0.5° and an integration time of 250 s. The patterns were evaluated by using the database Powder Diffraction Files (PDF).

Laser raman spectroscopic (LRS) analyses were conducted by inVia Raman microscope (Renishaw) equipped with Nd:YAG laser (532 nm, 100 mW), grating with 1800 lines per mm and CCD array detector. The spectra were collected under ambient conditions from 10 to 1800 cm−1 at a resolution of 1.6 cm−1, an exposure time of 5–20 s and a laser power of 1–10 mW, while accumulating 3–12 scans to a spectrum.

Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) using CO as probe molecule was made to check interaction of support and Pt. Investigations were made on a Tensor 27 FTIR spectrometer (Bruker) equipped with Praying Mantis reflectance optics (Harrick Scientific) and MCT detector. The sample compartment was purged with N2 to avoid diffusion of air. The heatable stainless steel IR cell (Harrick Scientific) contained ZnSe windows and was connected to a gas-handling system. For the DRIFTS investigation, respective catalyst was introduced into the sample cup of the cell and was heated in N2 flow at 500 °C for 15 min. Then, it was cooled to 200 °C under flowing N2 and a mixture of 4 vol% H2 and 96 vol% N2 was adjusted for 10 min to reduce the Pt component. After this, it was cooled to 50 °C in N2 flow and a background spectrum was collected; whereas in some tests other temperatures (65–150 °C) were also established. Subsequently, the sample was exposed to a flowing mixture of 1 vol% CO and 99 vol% N2. After saturation, a spectrum was taken under CO/N2. Then, it was flushed with N2 for 15 min to remove CO from the gas phase and another spectrum was recorded. The total gas flow was always kept at 200 ml/min (STP). Spectra were collected from 600 to 4000 cm−1 with a resolution of 0.9 cm−1, while 200 scans were accumulated to a spectrum resulting in an acquisition time of approx. 5 min.

X-ray photoelectron spectroscopy was performed with a High Pressure VG ESCALAB photoelectron spectrometer equipped with a special gas cell, which allows acquiring XPS spectra at elevated pressures, i.e. up to 0.02 mbar [28, 29]. The non-monochromatic Mg Kα line was used as the primary excitation. For calibration, the Au4f7/2 (binding energy BE = 84.0 eV), Ag3d5/2 (BE = 368.3 eV) and Cu2p3/2 lines (BE = 932.7 eV) from metallic gold, silver and copper foils were used [30]. Residual gas pressure was better than 2·10−9 mbar. Spectral analysis and data processing were made with XPSPeak 4.1 software. The BE values were determined after subtraction of Shirley background and analysis of line shapes. Curves were fitted by Gaussian–Lorentzian functions for each XPS region. The Si2p line (BE = 102.5 eV [31]) of substrate was chosen as internal standard for spectrum calibrating. For in-situ XPS, samples were placed into a special basket made of tantalum foil with steel grid blown round with gases and heated by resistivity. K-type thermocouple spot-welded to the bottom of basket was used for measurements at temperatures up to 180 °C to evaluated changes on the catalyst surface in the relevant H2-deNOx activity window. In addition to analyses in vacuum the following gas mixtures were used: 1) 0.0005 mbar of NO; 2) blend of 0.0005 mbar NO, 0.0020 mbar H2 and 0.0100 mbar O2 (p(NO): p(H2): p(O2) = 1: 5: 20, total pressure: 0.0125 mbar). Gas phase was controlled by a mass spectrometer with secondary electron multiplier detector (Pfeiffer Vacuum QME 220).

2.3 H2-DeNOx Studies

The catalytic H2-deNOx investigations were performed on a laboratory bench using a synthetic model exhaust typical for lean H2 combustion engines. The powder samples (200 mg) were charged into the quartz glass tube reactor (i.d. 8 mm), fixed with quartz wool and heated to 350 °C. Subsequently, the model exhaust was added and temperature was decreased to 80 °C with a rate (β) of 2.0 K/min. The feed consisted of 500 vppm NO, 2000 vppm H2, 6.0 vol% O2, 10 vol% H2O and N2 as balance; total flow was kept at 400 ml/min (STP) corresponding to a space velocity (S.V.) of 160,000 h−1. Two dosing lines were employed, whereas in the first line H2, NO and a part of N2 (200 ml/min, STP) was added. In the second one, H2, O2 and remaining N2 was supplied. For production of H2O, the later gas mixture was passed over a reactor unit with Pt/Al2O3 balls heated to 220 °C. Both lines were united in front of the reactor section. All gases were controlled by independent mass flow controllers (Bronkhorst).

Temperature was measured by two K-type thermocouples each located directly in front of and behind the catalyst bed. Maximum difference of inlet and outlet temperature was 8 K. The simultaneous analyses of NO, NO2, N2O, NH3, and H2O was carried out by a hot measuring FTIR spectrometer (MULTI-GAS Analyzer 2030, MKS Instruments). O2 was monitored by a lambda probe (LSU 4.9, Bosch).

For assessment of the catalysts, the conversion of NOx (X(NOx) = 1—y(NOx)out/y(NOx)in) and selectivity of N2O (S(N2O) = 2 y(N2O) / (y(NOx)in—y(NOx)out)) were used. The latter is exclusively presented for X(NOx) > 20% to minimize error propagation. As a consequence, S(N2O) data refer to a reduced temperature range as compared to the X(NOx) features.

3 Results and Discussion

The most important physical–chemical properties of the catalysts are summarized in Table 1 demonstrating rather similar BET surface area for the WOx-free samples (455–500 m2/g). Contrary, the BET surface area of the WOx-loaded catalysts is decreased with inclining content of tungsten, i.e. from 386 m2/g to 169 m2/g. This effect is in line with the negligible surface area of bare tungsten oxide (approx. 5 m2/g [32]). Furthermore, the WOx-free catalysts reveal a similar number of available Pt sites (2.7–4.4 µmol/g), which is attributed to the resembling BET surface area of the supports. However, the available Pt sites of the WOx-loaded Pt/AlSi-20 samples are strongly diminished from 3.3 to 0.2 µmol/g in accordance with the drastic decline in BET surface area (Fig. 1). As a consequence, the Pt dispersion of these catalysts is decreased from 26 to 1%.

Table 1 BET surface area (SBET), specific pore volume (Vp), number of available Pt sites (na(Pt)), Pt dispersion (D(Pt)) and estimated Pt particle size (d(Pt)) of the catalysts used
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Fig. 1
figure 1

Correlation of BET surface area and number of available Pt sites of the Pt/AlSi-20 catalysts with and without load of tungsten (the line is a guide for the eyes)

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The XRD patterns of the bare supports (not shown) display one major SiO2 reflex (PDF 00–050-0511), which increases with inclining proportion of Si, while the expected Al2O3 reflexes appear predominantly attributed to γ-Al2O3 (PDF 01–075-0921). Very similar reflexes are observed for the tungsten-loaded Pt catalysts (Fig. 2). For the WOx-promoted samples with a W load of 45 and 85 wt% clear reflexes ascribed to orthorhombic WO3 (PDF 00-020-1324) are observed, whereas they are more pronounced for the higher W content. Contrary, only very weak reflexes of WO3 exist in the sample with the lowest W content (11 wt%) indicating clear presence of amorphous tungsten oxide entities (Fig. 2). This interpretation is substantiated by the LRS trace of this sample evidencing signals of amorphous WOx species at approx. 980 cm−1 (νs(W=O)) and 888 cm−1 (νas(W=O)) related to polymeric WO6 units (Fig. 3) [33]. Furthermore, the catalyst with a W fraction of 45 wt% reveals relatively broad features at about 700 and 800 cm−1, which are referred to slightly distorted (W=O) stretching vibrations of bridging (W–O–W) groups. Additionally, a peak is observed at approx. 270 cm−1 ascribed to the bending vibration of bridging (W–O–W) moieties [34]. These LRS features are associated with crystalline WO3 species, which is in line with XRD [34]. However, a signal at approx. 980 cm−1 (νs(W=O)) also appears indicating additional existence of amorphous WOx entities as found for the catalyst with a W content of 11 wt%. Moreover, the sample with 85 wt% W exhibits well defined peaks of crystalline WO3 located at 270 (δ(WOW)), 715 and 805 cm−1 (ν(WOW)), which is in good agreement with XRD. Additionally, LRS implies a minor peak at 1000 cm−1 (νs(W=O)) showing some presence of amorphous WOx species as well [33]. It should also be mentioned that the 11W/AlSi-20-supported reference sample with a Pt load of 2.0 wt% provides a very similar LR spectrum.

Fig. 2
figure 2

X-ray diffraction patterns of the Pt/AlSi-20 catalysts with different loads of tungsten (SiO2 (x), WO3 ( +), γ-Al2O3 (°))

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Fig. 3
figure 3

Laser raman spectra of the Pt/AlSi-20 catalysts with and without load of tungsten

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Furthermore, all the X-ray diffraction patterns of the samples with a Pt loading of 0.25 wt% exhibit no Pt-related reflexes, as expected from the little Pt proportion. Contrary, the reference catalysts with 2 wt% Pt (Supplement Fig. 1) indicate clear reflexes ascribed to elemental Pt (PDF 00-004-0802). Calculation of the Pt crystallite size for 2Pt/AlSi-20 and 2Pt/11W/AlSi-20 using the Scherrer equation results in 16 nm for the former and 17 nm for the later catalyst. These particle sizes (Supplement Fig. 2) are confirmed by analysis of TEM images providing a d50 of 17 nm (2Pt/AlSi-20) and 23 nm (2Pt/11W/AlSi-20), respectively.

The catalytic H2-deNOx studies show that the three Pt/AlSi catalysts reveal prominent conversion of NOx in a narrow temperature window from 120 to 160 °C with significant selectivity of N2O (Fig. 4); no NH3 was detected for any of the catalysts. Moreover, the Pt/AlSi samples indicate rather similar performances, which closely resemble Pt/Al2O3 reported in the literature [20, 35]. Pt/AlSi-20 tentatively exhibits the highest maximum conversion of NOx (57%) including a N2O selectivity of about 50%. It is also worth to notice that the temperature of maximum conversion of NOx coincides with the temperature of minimum selectivity of N2O (140 °C). This is associated with fast NO dissociation on the Pt sites enabling the recombination of adjacent N atoms to yield N2. Contrary, at lower temperatures NO dissociation is not as fast thus leading to combination of NO and N adsorbates and pronounced formation of N2O, respectively [36]. Furthermore, preliminary investigations demonstrated that the temperature ramp adjusted provokes steady state. Additionally, blank experiments made with the empty reactor provide no reduction of NOx clearly evidencing the catalytic effect of the samples.

Fig. 4
figure 4

NOx conversion (left) and N2O selectivity (right) in H2-deNOx on Pt/AlSi-20, Pt/AlSi-30 and Pt/AlSi-40. Conditions: m = 200 mg, y(NO) = 500 vppm, y(H2) = 2000 vppm, y(O2) = 6 vol%, y(H2O) = 10 vol%, N2 balance, F = 400 ml/min (STP), S.V. = 160,000 h−1, β = 2 K/min

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The introduction of 11 wt% W to Pt/AlSi-20 clearly enhances H2-deNOx as reflected by the maximum NOx conversion, which increases from 58 to 72% (Fig. 5). Additionally, the minimum selectivity of N2O is declined from ca. 50 to 20%. Moreover, the operation window of Pt/11W/AlSi-20 is significantly broadened ranging from 100 to 300 °C. It is also noteworthy that the temperature of maximum conversion of NOx (110 °C) is shifted to lower temperatures, whereas the temperature of minimum selectivity of N2O has changed to higher temperatures (160 °C) as compared to the unpromoted catalyst. Additionally, the increase in the W content from 11 to 45 wt% results in decreasing NOx reduction (maximum deNOx: 57%) while leaving the N2O selectivity mostly unaffected. When inclining the tungsten proportion even further to the theoretical monolayer of planar WO3 (85 wt%), the maximum NOx conversion is further decreased to 50%. However, the selectivity of N2O is not significantly influenced below 160 °C, but above it is diminished to 20%. Furthermore, the 11W/AlSi-20-supported reference catalyst with 2 wt% Pt shows slightly higher NOx conversion (up to 90%) with very similar N2O selectivity (Supplement Fig. 3).

Fig. 5
figure 5

NOx conversion (left) and N2O selectivity (right) in H2-deNOx on Pt/AlSi-20 with a W load of 11, 45 and 85 wt%. Conditions: m = 200 mg, y(NO) = 500 vppm, y(H2) = 2000 vppm, y(O2) = 6 vol%, y(H2O) = 10 vol%, N2 balance, F = 400 ml/min (STP), S.V. = 160,000 h−1, β = 2 K/min

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The combination of the H2-deNOx results with the physical–chemical features of the samples shows that in each series of promoted and unpromoted catalysts the number of available Pt sites correlates well with the maximum NOx conversion (X(NOx)max) taken as a measure for the activity (Fig. 6). However, as higher NOx conversions are achieved for the WOx containing catalysts at a lower number of active Pt sites, the promoting effect of tungsten oxide is evident.

Fig. 6
figure 6

Correlation of maximum NOx conversion and number of available Pt atoms of all the catalysts (the lines are guides for the eyes); H2-deNOx conditions are demonstrated in Figs. 4 and 5

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Furthermore, from the abundance of active Pt present in Pt/AlSi-20 (W load: 85 wt%) a mean particle size (dPt) of 96 nm is derived (Table 1). Indeed, SEM coupled with EDX clearly confirms the substantial existence of Pt particles with sizes of approx. 100 nm (Fig. 7). Contrary, no Pt particles were found for the remaining WOx-loaded samples due to the limited resolution of the SEM in accordance with the small particle sizes deduced (4 and 11 nm, Table 1). The SEM studies clearly confirm that the increase in WOx load, associated with decreasing BET surface area, results in growing Pt particles affecting the catalytic performance. Thus, a strong metal support interaction (SMSI), which potentially implies decoration of Pt particles by WOx species partially reduced during H2-deNOx, appears unlikely. A possible SMSI effect was also checked by CO-TPD using Pt/AlSi-20 with and without W promotor (11 wt%). As a result of the TPD studies each performed after CO expsoure at 500 °C and subsequent saturation at 50 °C or 350 °C, the atomic ratio of available Pt sites of both samples remained almost the same (0.75 at 50 °C and 0.72 at 350 °C) thus excluding a substantial SMSI effect.

Fig. 7
figure 7

SEM image of Pt/AlSi-20 with a tungsten load of 85 wt%. Some Pt particles are exemplarily marked by white arrows

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Additionally, the W4f7/2 XP spectra of 11W/AlSi-20 and reference catalyst 2Pt/11W/AlSi-20 demonstrate a BE of 36.0 eV clearly ascribed to WO3 species (Fig. 8) [37]. Position, shape and width of this W4f peak of both samples do not indicate any existence of extra states of tungsten, which could support SMIS effect. Moreover, tungsten spectra do not change under in-situ XPS conditions using the gas mixtures (Sect. 2.2) up to 180 °C.

Fig. 8
figure 8

XPS W4f spectra of 2Pt/11W/AlSi-20 and 11W/AlSi-20 recorded in vacuum at room temperature

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According to XPS data Pt4f7/2 BE is 71.3 eV for 2Pt/AlSi-20 sample and 71.2 eV for 2Pt/11W/AlSi-20 sample corresponding to metallic Pt (Fig. 9) [30]. During in-situ XPS experiments, performed with the different gas phase compositions between room temperature and 180 °C (Supplement Fig. 5), Pt4f7/2 line shifts in the range from 71.3 to 71.8 eV for 2Pt/AlSi-20 and 71.2–71.4 eV for 2Pt/11W/AlSi-20, which is still metallic platinum BE. No changes of chemical state of platinum under the established reaction conditions were found thus also excluding significant SMSI effects. Note that the Pt particle sizes of both samples demonstrated above (d50(2Pt/AlSi-20) = 17 nm and d50(2Pt/11W/AlSi-20) = 23 nm) are out of range of XPS size effects [29]. Hence, size effects are not relevant for the spectrum interpretation. Small shifts of Pt4f line (well seen with respect to Al2p line), found under in situ XPS experiments, correlating with gas phase composition and sample temperature (Supplement Fig. 5) in the range from room temperature to 180 °C, can be explained by the fine electronic effects, particularly due to formation of surface and subsurface oxygen at platinum particles. It should be mentioned, that ex situ XPS study does not show the difference in platinum spectra for W-modified and non-modified samples (Fig. 9). Divergence between samples 2Pt/11W/AlSi-20 and 2Pt/AlSi-20 becomes apparent during in situ XPS experiments (Supplement Fig. 5). This observation can be explained by the heterogeneity of the Pt/AlSi-20 catalyst discussed below in the CO-DRIFTS section, particularly, surface structure and surface defects resulting in the difference in accumulation of the oxygen associated with Pt.

Fig. 9
figure 9

XPS Pt4f (solid line) and Al2p (dashed line) spectra of 2Pt/11W/AlSi-20 and 2Pt/AlSi-20 recorded in vacuum at room temperature

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The interaction of the WOx-modified AlSi-20 supports with the Pt component was also investigated by DRIFTS using CO as probe molecule (Fig. 10). The DRIFT spectrum of Pt/AlSi-20 with 11 wt% W collected after exposure to CO at 50 °C shows a band at 2065 cm−1, which is referred to stretching vibrations of CO linearly coordinated to Pt sites (ν(CO)) [38]. Bridging CO species expected at ca. 1850 cm−1 do not appear. The intensity of the ν(CO) band clearly declines with increasing load of tungsten in line with the number of active Pt sites. Consequently, the sample with 85 wt% W indicates only negligible intensity [39]. Since no significant shift of the ν(CO) band occurs for the three tungsten-loaded catalysts and the band is very close to that of the WOx-free sample (2070 cm−1), specific electronic interaction of the crystalline and amorphous WOx species with Pt is rather excluded. Additionally, the location of the ν(CO) band is obviously not affected by the mean Pt particle size of the tungsten-loaded samples, which ranges from 4 to 96 nm (Table 1). Indeed, the effect of Pt particle size on the CO stretching vibration is known from the literature for Pt/SiO2 and Pt/Al2O3, but it only appears for much smaller particles below approx. 2 nm [38], as similarly discussed for XPS. From the DRIFTS studies it is deduced that the H2-deNOx performance of the WOx-loaded Pt/AlSi-20 catalysts is primarily driven by the number of active Pt sites as demonstrated in Fig. 6.

Fig. 10
figure 10

DRIFT spectra of bare and WOx-loaded Pt/AlSi-20 after exposure to CO at 50 °C followed by N2 flushing

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Furthermore, it should be stated that the DRIFT spectrum of the W-free Pt/AlSi-20 sample shows a band at 2070 cm−1 with a marked shoulder at 2095 cm−1 both ascribed to ν(CO) vibration mode of linearly coordinated CO species [38]. The splitting of the DRIFTS feature evidences considerable heterogeneity of the Pt sites, which implies CO adsorbates with weaker binding strength to the active component. As a result, some blue shift of the ν(CO) vibration occurs relative to the 2070 cm−1 band. Note that the feature at 2095 cm−1 completely vanishes when exposing Pt/AlSi-20 to CO at 65 °C, while the band at 2070 cm−1 clearly remains (not shown). This disappearance of the 2095 cm−1 band already at slightly increased temperature substantiates the low stability of the corresponding adsorbates. It may be supposed that this heterogeneity of the Pt/AlSi-20 catalyst, which includes less stable adsorbates and likely less active Pt sites, results in lower H2-deNOx efficiency as compared to the WOx-promoted catalysts.

Moreover, additional insights into the role of the WOx promotor were gained from H2-deNOx investigations made at an extremely high space velocity (480,000 h−1). These studies were carried out with lesser catalyst mass using Pt/AlSi-20 with and without 11 wt% W, which both reveal very similar numbers of available Pt sites (Table 1). As expected from the increased S.V., the NOx conversion of both catalysts is diminished as referred to the lower space velocity (160,000 h−1, Fig. 12), whereas the W-promoted sample again shows higher H2-deNOx performance (Fig. 11, left). In contrast to the studies at the lower S.V., the catalysts exhibit significant NH3 formation below 160 °C with a broader and stronger appearance for the unpromoted catalyst. The NH3 yield is attributed to the reaction of N adsorbates, originated from NO dissociation, with hydrogen species on the Pt sites [40]. Obviously, at the high space velocity NH3 is not completely converted. In line with literature [41], the NH3 conversion mainly occurs by reaction with O2 to form N2 and N2O [41], which starts as low as approx. 90 °C (Figs. 12 and 13). Like at lower S.V. the N2O formation is higher for the unpromoted catalyst (Fig. 11, right). As the decreased NH3 formation on the 0.25Pt/11W/AlSi-20 catalyst correlates with higher NOx conversions, it may be deduced that ammonia species are also involved in the NOx reduction, in addition to the H2-deNOx reaction on the Pt sites [36]. For this reason, NH3 (100 vppm) was added to the gas flow in presence and absence of H2 to check a possible contribution of the NH3-SCR reaction. These studies performed with both catalysts were conducted at selected temperatures providing stationary conditions to exclude ammonia storage effects.

Fig. 11
figure 11

NOx conversion (left) as well as N2O and NH3 formation (right) in H2-deNOx on Pt/AlSi-20 with and without a W load of 11 wt%. Conditions: m = 50 mg, y(NO) = 500 vppm, y(H2) = 2000 vppm, y(O2) = 6 vol%, y(H2O) = 10 vol%, N2 balance, F = 400 ml/min (STP), S.V. = 480,000 h−1, β = 2 K/min

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Figure 12 demonstrates that the NH3-SCR reaction in the absence of H2 results in only little NOx conversion on both catalysts (maximum 16% at approx. 160 °C). Therefore, it is unlikley that SCR plays a major role in H2-deNOx. Note that due to the NH3 proportion of 100 vppm maximum NOx conversion of only 20% is possible according to the stoichiometry of SCR reaction (5 NO + 5 NH3 + O2 → 5 N2 + 6 H2O). Additionally, the NH3-SCR tests substantiate high NH3 conversion on both catalysts (approx. 90% at 160 °C) due to the oxidation of ammonia (4 NH3 + 3 O2 → 2 N2 + 6 H2O), which clearly exceeds the NOx reduction. Contrary, the NH3 oxidation to NO can be excluded at temperatures below 200 °C [41]. Marked NH3 oxidation even at low temperatures is typical for Pt catalysts and implies strong formation of N2O (2 NH3 + 2 O2 → N2O + 3 H2O) as evidenced for both catalysts investigated (Fig. 12) [42]. Additionally, some N2O formation is also ascribed to the SCR reaction on Pt in line with literature [43]. It should also be stated that the W-loaded catalyst provides slightly higher NH3 conversion between 100 and 140 °C with lower yield of N2O. In the H2-deNOx reaction (Fig. 11), this superior N2 selectivity may contribute to the lower N2O production of Pt/11W/AlSi-20, associated with the oxidation of NH3 formed as side-product. Indeed, tungsten promotion was previously reported as beneficial for the oxidation of NH3 to N2 on Pt/W/ZrO2 catalysts [42].

Fig. 12
figure 12

NH3 and NOx conversion (left) as well as N2O formation (right) in NH3-SCR on Pt/AlSi-20 with and without a W load of 11 wt% (the lines are guides for the eyes). Conditions: m = 50 mg, y(NO) = 500 vppm, y(NH3) = 100 vppm, y(O2) = 6 vol%, y(H2O) = 10 vol%, N2 balance, F = 400 ml/min (STP), S.V. = 480,000 h−1

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Furthermore, when simultaneously dosing 2000 vppm H2 and 100 vppm NH3 (Fig. 13, left), the NOx conversion on Pt/AlSi-20 is clearly higher as compared to the experiment without H2 (Fig. 12, left). For instance at 140 °C, the NOx conversion amounts to 36%, whereas in NH3-SCR it is 8% only; also, slightly higher NOx reduction is obtained compared to bare H2-deNOx (31%, Fig. 11). Interestingly, at around 140 °C the NH3 conversion seems to be smaller in the presence of H2. This effect is explained by the formation of NH3 by the H2-deNOx reaction, as indicated in Fig. 11, formally decreasing the conversion of NH3. Additionally, the significant production of N2O (Fig. 12, right) is referred to H2-deNOx as well as the follow-up reactions of NH3, namely NH3 oxidation and SCR.

Fig. 13
figure 13

NH3 and NOx conversion (left) as well as N2O formation (right) upon NH3-SCR in the presence of H2 on Pt/AlSi-20 with and without a W load of 11 wt% (the lines are guides for the eyes). Conditions: m = 50 mg, y(NO) = 500 vppm, y(H2) = 2000 vppm, y(NH3) = 100 vppm, y(O2) = 6 vol%, y(H2O) = 10 vol%, N2 balance, F = 400 ml/min (STP), S. V. = 480,000 h−1

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In the investigation of Pt/11W/AlSi-20 performed in simultaneous presence of H2 and NH3 (Fig. 13), the NH3 conversion is increased above 110 °C referred to the study without H2 (Fig. 12). Also, the NOx conversion is markedly above that observed in H2-deNOx, i.e. without additional NH3 supply; at 140 °C deNOx amounts to 56% with H2/NH3, whereas with H2 only it is 44% and with NH3 it is 6%. These results clearly demonstrate a synergetic effect of H2 and NH3 in the NOx reduction on the W-promoted catalyst. Moreover, it is apparent that the lower N2O formation in the H2-deNOx reaction on Pt/11W/AlSi-20 (Fig. 11) coincides with the diminished N2O production during the NH3 oxidation (Fig. 12). Therefore, it is inferred that ammonia formed in H2-deNOx enhances the NOx reduction on the one hand side, while on the other hand side its oxidation results in lesser proportion of N2O.

It is known from literature that NH3 originated from H2-deNOx on W-promoted Pt catalysts can adsorb on Bronsted acid W-OH sites [15, 25] leading to the formation of NH4+ species. The NH3-TPD analysis of W-promoted and bare AlSi-20 (Supplement Fig. 4) shows that the promotor slightly increases the NH3 uptake capacity from 1.4 µmol/m2 to 1.7 µmol/m2. Additionally, it is assumed that in H2-deNOx some hydrogen spillover from Pt to adjacent W = O species, evidenced by LSR (Fig. 3), can occur increasing the number of W-OH Bronsted acid sites in proximity of the Pt particles [44, 45]. From the discussion of above deNOx and TPD studies as well as the cited literature it is drawn the conclusion that amonium species, formed at the interface of Pt and the WOx/Al2O3/SiO2 support, may react with NO likely adsorbed on Pt to yield N2 and H2O. This participation of ammonia surface species resaonably explains the increased H2-deNOx performance and lower N2O selctivity of the W-promoted Pt/AlSi-20 catalyst compared to the unpromoted sample.

4 Conclusion

The Al2O3/SiO2-based Pt catalysts with and without WOx promotor revealed a strong dependency of their lean H2-deNOx activity by the number of active Pt sites. The presence of the tungsten oxide promotor led to a clear increase in NOx conversion as compared to the WOx-free catalysts, whereby the presence of amorphous WOx entities, very probably polymeric WO6 units, seems to be most advantageous. Interestingly, tungsten oxide was also reported to enhance the H2-deNOx conversion on related ZrO2-based Pt and Pd catalysts [20, 21]. For the W-promoted catalysts, a high BET surface area was beneficial to increase the number of available Pt sites.

Additionally, NH3 appeared as side-product of the H2-deNOx reaction, which was assumed to contribute to the NOx conversion. The superior activity of the W-promoted catalysts compared to the W-free samples was ascribed to ammonia species at the Pt-WOx/Al2O3/SiO2 interface participating in the NOx reduction, in addition to the H2-NOx reaction on the Pt sites. Moreover, the more homogeneous nature of the Pt sites of the W-promoted catalysts might also assist the H2-deNOx efficiency. Contrary, a SMSI effect of the tungsten oxide promotor was clearly excluded.

The best catalyst (Pt/11W/AlS-20) provided maximum NOx conversion of approx. 70% implying a significantly broadened temperature window from 100 to 300 °C, even in presence of H2O. Simultaneously, the selectivity towards N2O was lowered to approx. 20%. The wide temperature window and the pronounced activity in the low temperature region (< 200 °C) makes this catalyst a promising basis for the efficient NOx reduction in H2 combustion engines keeping NOx emissions within current and future emission limits.