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. 2024 Apr 29;14(10):7463–7477. doi: 10.1021/acscatal.3c05847

Structural Changes of Ni and Ni–Pt Methane Steam Reforming Catalysts During Activation, Reaction, and Deactivation Under Dynamic Reaction Conditions

Enrico Tusini †, Maria Casapu †, Anna Zimina †,‡, Dmitry E Doronkin †,‡, Heike Störmer §, Laurent Barthe ∥, Stephanie Belin ∥, Jan-Dierk Grunwaldt †,‡,*
PMCID: PMC11110164  PMID: 38779186

Abstract

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Ni-based catalysts are the most widely used materials to produce H2 in large-scale methane steam reformers under stationary conditions. For domestic applications such as fuel cells, H2 production involves the exposure of the catalysts to more dynamic conditions due to the daily startup and shutdown operation mode, making Ni-based catalysts susceptible to oxidation and deactivation. In this context, we report a systematic investigation of the structural changes occurring for monometallic Ni/MgAlOx and bimetallic NiPt/MgAlOx catalysts during methane steam reforming under transient conditions, comprising catalyst activation, operation, and deactivation processes. Besides extensive catalytic tests, the samples prepared by incipient wetness impregnation were characterized by complementary methods, including N2-physisorption, X-ray diffraction, H2-temperature-programmed reduction, and electron microscopy. Next, the structure of the Ni and Pt species was monitored under reaction conditions using time and spatially resolved in situ/operando X-ray absorption spectroscopy. The results obtained show that before catalyst activation by H2-reduction, nickel diffuses into the support lattice and forms mixed oxides with magnesium. In the activated catalysts, Ni is present in the metallic state or alloyed with Pt. A clear beneficial effect of the noble metal addition was identified on both the activity and stability of the bimetallic NiPt/MgAlOx catalyst. In contrast, the pronounced oxidation and reincorporation of Ni into the support lattice were observed for the monometallic sample, and these catalyst deactivation effects are hindered in the bimetallic Ni–Pt catalyst. Overall, the outcome of our study not only helps in understanding the catalyst activation/deactivation processes at an atomic level but also provides the basis for the rational development of improved methane steam reforming catalysts.

Keywords: operando characterization, X-ray absorption spectroscopy, methane steam reforming, bimetallic catalysts, hydrogen, transient conditions

1. Introduction

The increase in population and the standard of living in the last decades caused an expansion in energy demand and consumption. Consequently, the exploitation of natural resources such as coal, oil, and gas increased, raising the emission of greenhouse gases (GHG) into the atmosphere to alarming levels. Hydrogen, which is an important raw material widely used in the chemical industry,1,2 is now regarded as a possible solution to replace nonrenewable fossil fuels for a sustainable future. Based on the production methods, hydrogen sources can be divided into three main categories:3 (i) gray hydrogen: fossil fuels derivative, involving CO2 released into the atmosphere; (ii) blue hydrogen: fossil fuel-based production, with carbon capture, utilization, and storage (CCUS)-implemented systems, and (iii) green hydrogen: based on renewable resources. In spite of the great efforts and technical improvements done for a green hydrogen transition in the last decades, methane steam reforming (MSR) still holds for the vast majority of hydrogen production4 to these days. Among numerous applications, hydrogen usage gained additional attention recently in polymer electrolyte fuel cells (PEFC), which could be applied for stationary-distributed power stations in buildings, residences in urban areas, or remote regions. These distributed smart grids have several advantages, including the ability to provide both heat and electricity in a flexible way, thus leveling off the peaks in the power demand. Nonetheless, the lack of a robust hydrogen delivery infrastructure makes the technology not appealing to the final customer. A possible solution would be to use a fuel reformer, which is able to convert hydrogen-rich fuels into molecular hydrogen when required, by using fuels such as liquefied petroleum gas (LPG) and methane. Preferentially, the reforming reaction used is methane steam reforming since among the other possibilities—partial oxidation, autothermal reforming, and dry reforming— it is the most mature technology and yields the highest hydrogen concentration.5,6 However, in contrast to the industrial-scale reformer, which operates under stationary conditions, systems for domestic purposes such as small-scale fuel reformers must withstand the transient conditions of numerous activity cycles. The bottleneck is, thus, to find a catalyst that can tolerate such rapid and rough changes in temperature and reaction mixture.

Ni-based catalysts supported on metal oxides (Al2O3, MgO, and their mixed oxides) have been intensively studied and industrially used for MSR.7−9 The high catalytic activity and relatively low cost made them the perfect compromise for the industrial application of MSR. Nevertheless, Ni-based catalysts suffer from severe deactivation mechanisms, such as coke deposition, Ni oxidation, and sintering.8,10,11 According to Matsumara and Nakamori’s12 X-ray diffraction (XRD) measurements of the catalysts supported on SiO2 and Al2O3, prereduced Ni particles are progressively oxidized by steam under steady-state reaction conditions at 500°C. In contrast, on TiO2 at the same reaction temperature Ni particles were reported to remain in the metallic state over 100 h testing time.13 With respect to coke formation, low temperatures are known to promote carbon deposition if the H2O/CH4 ratio is close to stoichiometric.14 To overcome these challenges, various synthesis methods have been applied for preparing Ni-catalysts on different supports and additional promoters were added for tuning the properties of the carrier or Ni particles.9 The catalyst support is known to play a major role in affecting not only the state of Ni but directly contributing to the reaction mechanism, e.g., adsorption of reactants or as an oxygen storage component during coke removal.15,16 The interactions of Ni with the Al- or Mg-based supports have been intensively studied in the literature. Generally, a weak interaction results in better reducibility of the active metal but poor stabilization against sintering; a strong interaction hinders the reduction of the active metal phase but allows the stabilization of small nanoparticles. Crucial factors for the modulation of metal–support interactions are the synthesis procedure and the calcination temperature.17−19 Several studies20−22 reported the formation of NiAl2O4 spinel phase after the thermal treatment of Ni supported on Al2O3, which then requires temperatures above 750 °C to be reactivated. Due to their similar atomic radius, the intercalation of Ni into Mg-based supports is also possible at any concentration.23,24 In both cases, small nickel nanoparticles are formed upon reduction, which further prevents deactivation like carbon deposition and sintering.25 With respect to the catalyst promoters, the doping of Ni particles with a secondary metal seems to be an effective solution against deactivation. Particularly, platinum-group metals (PGM) decorate or form alloys with Ni, often leading to improved activity and stability.26−28

These aspects have been extensively investigated during the past years but mostly by pre- and post-mortem analysis of catalysts. Despite monitoring the evolution of the catalyst during methane steam reforming (in situ/operando) would be highly valuable for a more comprehensive understanding of these systems, only a small number of such studies have been reported.29,30 This is probably due to the harsh reaction conditions of the MSR process. By operando Fourier-transform infrared spectroscopy measurements on a series of Ni/SiO2 catalysts, important reaction intermediates and rate-determining steps were identified for steam reforming and dry reforming of methane.31 In a study by Duarte et al.,32 the cerium oxidation state in CeO2-doped Rh/Al2O3 catalysts was determined by in situ X-ray absorption spectroscopy (XAS) measurements during methane steam reforming at 500 °C, which allowed to relate its contribution to water activation and alumina stabilization by CeAlO3 formation. By using the same approach, the effect of CeO2 and Sm2O3 dopants on the Rh state and sintering behavior was also demonstrated in a subsequent study.33 Particularly, hard X-ray-based techniques appear very promising due to the possibility of simultaneously obtaining information about the electronic state and the local structure surrounding the adsorbing atom under realistic reaction conditions. For such in situ/operando studies, different cells have been constructed over the past years to mimic realistic reaction conditions.34,35 Concerning the steam reforming of methane, this reaction requires high temperatures (up to 900 °C) even during catalyst activation, and only a few in situ/operando setups exist, which are capable of fulfilling these conditions.36−38 For instance, La Fontaine et al.39 recently reported a reaction environment able to reach 1000 °C with various heating/cooling ramp rates, which is suitable also for methane steam reforming studies. Analogous, Eggart et al.38 conducted in situ/operando XAS investigations up to 1000 °C with a newly developed cell.

In this context, the present work focuses on elucidating the structure of the active species in 15%Ni/MgAlOx and 15%Ni-1%Pt/MgAlOx catalysts under transient reaction conditions between room temperature and 900 °C, which are characteristic for small-scale fuel reformers with daily startup and shutdown (DSS) operation. Complementary in situ/operando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) measurements are used to reveal structural changes of Pt and Ni species together with support phase variations during catalyst activation and light-off/light-out cycles over a broad temperature range. Time and spatially resolved XAS measurements were conducted at Ni K and Pt L3 edges under transient methane steam reforming reaction conditions to understand the activation/deactivation behavior of such monometallic and bimetallic catalysts.

2. Experimental Methods

2.1. Catalyst Preparation

Commercially available hydrotalcite from SASOL GmbH (PURAL MG70) with Al2O3/MgO = 30:70 was used as support for both mono- and bimetallic Ni–Pt catalysts after calcination at 900° for 12 h in static air. The resulting material is referred to as MgAlOx in the following discussion. In the next step, the incipient wetness impregnation method (IWI) was used to add aqueous solutions of tetraamine platinum nitrate (Pt(NH3)4(NO3)2) and nickel nitrate hexahydrate (Ni(NO3)2·6H2O) precursors. The bimetallic Ni–Pt catalyst was prepared via coimpregnation. The metal concentrations selected for the mono- and bimetallic catalysts are, respectively, 15 wt % Ni and 1 wt % Pt. The resulting samples were dried at 80° for 12 h and subsequently calcined at 500° for 6 h in static air. In the following discussion, the mono- and bimetallic samples are denoted as Ni/MgAlOx and NiPt/MgAlOx, respectively.

2.2. Catalyst Characterization

The Brunauer–Emmett–Teller (BET) surface area40 of the samples was determined by N2 physisorption at −196 °C using a BELSOPRP-mini instrument (MicrotracBEL, Osaka, Japan). Prior to the analysis, the samples were degassed at 200 °C for 2 h.

The crystallinity of the catalyst components was assessed via XRD using an X′Pert PRO instrument (PANalytical, Malvern, United Kingdom). The patterns were recorded between 5 and 120° using Cu Kα radiation (wavelength = 1.54 Å) with a step size of 0.017°.

Temperature-programmed reduction with H2 (H2-TPR) was performed on an AutoChem II instrument (Micromeritics, Norcross, USA) by heating the sample at 10 °C/min from room temperature to 900 °C while dosing 50 mL/min of 10%H2/Ar. Before the reduction step, adsorbed species were removed by heating the sample in 50 mL/min of He from room temperature to 200 °C.

For all catalyst samples, Ni and Pt metal loadings were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) using an OPTIMA 4300 DV spectrometer (PerkinElmer, Waltham, USA).

2.3. Operando X-ray Absorption Spectroscopy Study

The operando XAS measurements were performed at the ROCK beamline39,41 of SOLEIL Synchrotron (Saint-Aubin, France). For these experiments, the quick-EXAFS42,43 (QEXAFS) acquisition mode was used with a time resolution of 2 Hz. Energies were selected via two independent crystal channel-cut monochromators (Si(111), Si(220)). The “edge jump” operation mode of the ROCK beamline39,41 was used to monitor simultaneously the Ni K (8333 eV) and Pt L3 (11564 eV) edges (the edge switch required ca. 1 min). During transient reaction conditions, the middle of the catalyst bed was monitored with a beam size of about 450 μm × 350 μm (H × V). For selected temperatures, three positions along the catalyst bed were screened (beginning, middle, and end of a catalyst bed of 6 mm, with about 2.5 mm in between the measured positions), in order to identify gradients in the oxidation state and structure for Ni and Pt during the reaction. For energy calibration, reference metal foils of Ni and Pt were recorded together with each experimental spectrum. For initial data collection and merging, a graphical user interface (GUI) developed at SOLEIL was used.39,41 Further data evaluation was performed using Athena and Artemis data analysis software using IFEFFIT.44 The extended X-ray absorption fine structure (EXAFS) data χ(k) collected at room temperature were extracted, background subtracted, and subsequently analyzed in the k-space range of 3–12 Å–1 for both edges. For the Ni K edge, the Fourier-filtered R-space selected for the fitting is 1.0–3.1 Å, while for the Pt L3 edge, the R-space selected is 1.0–2.5 Å. The passive electron reduction factor (S02) estimated from the foil fitting is 0.78 for Pt and 0.83 for Ni. Complex systems such as catalytic materials require a mixture of various scattering paths to obtain a meaningful structure refinement. In this study, the Ni metal structure was used to model the Ni–Ni shell (ICSD collection code 260169), and NiMgOx was used for the Ni–Mg second scattering shell (ICSD collection code 13774). The Ni–O and Ni–Ni (second scattering shell) scattering paths are modeled starting from NiO (ICSD collection code 184918), while the Ni–Pt interaction (obtained via Pt L3 edge data fitting) is modeled using a NiPt alloy structure (ICSD collection code 105318). Coordination numbers (CN), mean square deviation of interatomic distances (σ2), interatomic distances (R), and energy shift (δE0) are refined during the fitting. The overall misfit from data and fitting is expressed via the R factor.

For the operando experiments, 5 mg of catalyst (1:1 dilution with MgAlOx support, sieve fraction 100–200 μm) was loaded into a quartz capillary of 1.5 mm diameter (wall thickness of 0.02 mm). An operando XAS cell recently developed at the ROCK beamline was used for all experiments.39 The cell can be used in both fluorescence and transmission data acquisition modes. Additionally, linear and uniform heating and cooling are provided between room temperature and 1000 °C, which was highly important for studying the catalysts during the methane steam reforming reaction. In the first step, the catalysts underwent a prereduction treatment (5% H2/He, 50 mL/min total flow) while continuously monitoring Ni and Pt structure via continuously collecting QEXAFS spectra. Before and after each change in the reaction environment, a series of EXAFS spectra were recorded as well. Afterward, the reaction mixture (5000 ppm of CH4, 1.5% H2O in He; 50 mL/min total flow) was supplied to the catalyst using mass flow controllers (Bronkhorst) and a water vapors gas saturator (at RT) at a WHSV of 106 mL gcatalyst–1 h–1 (comparable to the lab scale experiments). This gas mixture with a steam-to-carbon (S/C) ratio of 3 was selected to prevent the capillary reactor obstruction by carbon deposit formation, which is known to occur for S/C < 2.45,46 Reaction products were monitored with a quadrupole mass spectrometer (Pfeiffer Vacuum, Omnistar GSD 320) and an online FTIR gas analyzer (MKS, Multigas 2030). Changes in the chemical state of the catalysts were monitored in the middle of the catalyst bed under transient reaction conditions (light-off/light-out cycles) by QEXAFS measurements at Ni K and Pt L3 edges. Additionally, 3 different positions in the catalyst bed (beginning, middle, and end) were investigated at various temperatures during cooling down from 900 °C to room temperature after 3 consecutive activity light-offs.

2.4. Operando X-ray Powder Diffraction Study

Operando X-ray diffraction experiments were conducted at the DiffAbs beamline of SOLEIL Synchrotron (Saint-Aubin, France) during transient reaction conditions similar to those applied for the operando XAS measurements: (i) prereduction in 5%H2/He; (ii) light-off/light-out cycles in the reaction mixture (5000 ppm of CH4, 1.5% H2O in He; 50 mL/min total flow). The energy selected was 8.3 keV, which was tuned using a double Si(111) crystal (3–23 keV) and two Rh-coated mirrors. A Circular Hybrid Pixel Array Detector47 (CirPad) was used, which consists of 20 XPAD modules assembled together in a circular arch geometry, capable of covering angles up to 135° and an angular opening of 0.0115°. Due to the geometry of the CirPad, a gas blower (FMB Oxford, UK) was used to heat the capillary reactor. Light-off (LOff) and light-out (LOut) experiments were conducted in a similar manner as during the operando XAS experiments.

2.5. Catalytic Tests

Systematic catalytic tests were performed for all mono- and bimetallic catalysts in a plug flow reactor, similar to the one used for the operando XAS and XRD experiments. Because of the high temperature required for the reaction, a specially designed high-temperature oven was used.38 Briefly, about 5 mg of sieved catalyst powder was pretreated in a 5%H2/N2 gas flow for 1 h at 900 °C. Afterward, the reaction was conducted in 5000 ppm of CH4 and 1.5% H2O, balanced to 50 mL/min in N2 (roughly 106 mL/h/gcat).

3. Results and Discussion

3.1. Catalyst Characterization

Despite the high specific surface area of the as-received MG70 support (233 m2/g), a pronounced drop was measured after calcination at 700 °C for 5 h (88 m2/g). After Ni and Pt deposition, the BET surface area did not change significantly, amounting to 72 m2/g for 15 wt % Ni/MgAlOx, 86 m2/g for 1 wt % Pt/MgAlOx, and 72 m2/g for 15% Ni-1% Pt/MgAlOx catalysts. Figure 1 shows the corresponding X-ray diffractograms of the MgAlOx support as well as of the monometallic and bimetallic Pt–Ni catalyst samples. In the monometallic Ni/MgAlOx sample, diffraction peaks corresponding to MgAl2O4, MgO, and NiO phases can be observed. The simultaneous presence of MgO and MgAl2O4 is due to the nonstoichiometry of the starting hydrotalcite support. The identification of the individual NiO and MgO contributions is difficult due to the similarities of the unit cells. The same applies to MgAl2O4 and the possibly formed NiAl2O4 phases. However, a shift toward higher 2θ is noticed for the Ni/MgAlOx catalyst compared to the pure support,48,49 which is probably caused by the intercalation of Ni2+ into the MgO lattice. Due to the relatively low loading, Pt addition does not seem to affect the catalyst’s crystalline structure. Nonetheless, the absence of characteristic patterns for metallic Pt, as observed for the monometallic Pt/MgAlOx sample, indicates a possible interaction with Ni species, leading to a higher noble metal dispersion in the bimetallic catalyst.

Figure 1.

Figure 1

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XRD patterns of the as-prepared mono- and bimetallic 15% Ni-1% Pt/MgAlOx catalysts.

The reduction behavior of Ni species in the Ni/MgAlOx and NiPt/MgAlOx catalysts was first investigated by means of H2-TPR. The profiles of the thermal conductivity detector signals (Figure 2) recorded for the different samples show clear differences depending on the catalyst composition. For Ni-containing samples, two separate reduction regions are recognized: (i) a low-temperature region from 200 to 500 °C and (ii) a high-temperature region from 500 to 900 °C. The low-temperature region presents peaks around 409 and 339 °C, respectively, for Ni/MgAlOx and NiPt/MgAlOx. At this temperature, the hydrogen consumption is mainly attributed to the presence of NiO particles on the catalyst surface, not in close contact with the support.50 Between 600 and 750 °C, the reacting species are reported to be Ni2+ ions in the outermost layer and subsurface of mixed oxide phases.48,51

Figure 2.

Figure 2

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H2-TPR profiles measured by a thermal conductivity detector (TCD) for the mono- and bimetallic samples using 10% H2/Ar (50 mL/min, 100 mg sample, 10 °C/min heating rate).

At even higher temperatures, reduction peaks are mostly ascribed to the formation of the hardly reducible NiO–MgO solid solution,50,3,52 sometimes also referred to as NixMgyOz or NiMgOx, with a variable composition depending on the sample calcination temperature and the quantity of Ni present in the material. However, even though MgO represents the vast majority of the carrier material, the high reduction temperature was attributed in the literature to the possibly formed NiAl2O4.53,54 The reduction of both phases, NixMgyO and NiAl2O4, as well as of other Ni species cannot be clearly elucidated by an H2-TPR experiment but require a more detailed analysis as shown in the following discussion. Interestingly, Pt addition promotes the reduction of all Ni species, which is most probably caused by the hydrogen spillover effect after its dissociation on Pt.55 Furthermore, the presence of platinum has been linked to an increase in the dispersion of nickel sites,56 affecting as well the reduction temperature.

In the next step, all catalysts were studied by in situ X-ray absorption spectroscopy and in situ X-ray diffraction to further understand the structure of Ni and Pt species in the as-prepared samples and during reduction with H2 (catalyst activation). In Figure 3, the XANES and the Fourier-transform (FT) k2-weighted EXAFS spectra of the mono- and bimetallic catalysts are shown. The Ni K edge XAS spectra collected at room temperature for the monometallic Ni/MgAlOx sample (Figure 3(a)) in an inert atmosphere are depicted alongside those obtained for the metallic Ni foil, NiO, and NiMgOx references. The pre-edge feature appearing at 8333 eV for NiO NiMgOx and Ni/MgAlOx is a fingerprint of the octahedral coordination of Ni with 6 oxygen atoms. This peak derives from the 1s → 3d orbital transition, and its intensity directly correlates with the degree of geometrical distortion.57 Noteworthy is the similarity of the XANES spectra obtained for the Ni/MgAlOx sample with that of the NiMgOx reference in terms of profile and edge step magnitude. This implies that the coordination environment and the electronic structure of Ni in the monometallic sample resemble those present in the NiMgOx reference. The differences between the white line of Ni foil, NiO, NiMgOx, and the catalyst are illustrated in the inset of Figure 3(b). For Ni/MgAlOx, the strong Ni–Mg interaction leading to the formation of a mixed oxide phase is additionally supported by the linear combination analysis (LCA) of the XANES data: the Ni–Mg mixed oxide represents 67% among nickel phases, while the NiO fraction is only 32%. According to previous studies,58,23 Ni ions substitute Mg into the MgO lattice, and the percentage of Ni in the lattice depends on the preparation method and calcination temperature. Analogous to the monometallic sample, the same features are observed in the Ni K edge XANES spectra of the bimetallic NiPt/MgAlOx catalyst (Figure 3b). The lower intensity of the edge could be explained by the slightly larger amount of NiO phase present in this sample, which, according to the LCA, is about 35%.

Figure 3.

Figure 3

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XANES (a–c) and FT-EXAFS (d–f) spectra recorded at Ni K and Pt L3 edges for the as-prepared catalysts (not phase-shift corrected, k2-weighted EXAFS). The spectra of the NiO, NiMgOx, PtO2, and Ni and Pt foil reference samples are included for comparison.

The local structure around the Ni atoms was additionally investigated by the analysis of the FT-EXAFS data reported in Figure 3(d,e). The results of the EXAFS data fitting (more details in Tables S1–S6 and Figures S10−S18) show that Ni is partially incorporated into the MgO lattice for both catalysts. In the as-prepared monometallic Ni/MgAlOx sample, Ni–O is the first scattering path at 2.09 Å with a coordination number of 6.2 ± 0.5, which corresponds to Ni in octahedral coordination to 6 O atoms. The second shell at 2.90 Å was found to be represented the best by a Ni–O–Mg bond in a Ni–Mg mixed oxide, in line with previous literature.58,23 The coordination number obtained is 6.3 ± 0.4, which is lower than the value expected for the second scattering shell of a mixed oxide phase. Although the LCA analysis of the XANES region suggests the presence of NiO, no clear indication of a Ni–Ni scattering path is found at 2.95 Å in the EXAFS fitting, which could be due to the similar bond distances to those present in the Ni–Mg mixed oxide. FT-EXAFS analysis of the bimetallic NiPt/MgAlOx sample (Figure 3(e)) resulted in a similar outcome for the Ni surrounding atoms. In this case, a coordination number of 6.5 ± 0.4 was obtained for the first Ni–O scattering shell at 2.09 Å, while the Ni–O–Mg coordination sphere was found at 2.89 Å. Similarly as obtained for the monometallic Ni catalyst, the coordination number for the second coordination shell is 6.5 ± 0.6. Hence, the analysis of the XANES and EXAFS data indicates that the composition of the initial state is not so different for the mono- and bimetallic samples.

Figure 3(c) shows the XANES spectra recorded at the Pt L3 edge for the bimetallic NiPt/MgAlOx catalyst together with those of PtO2 and Pt foil. In the as-prepared sample, Pt was found in the oxidized state, with a comparable white line intensity as obtained for the PtO2 reference. This suggests that Pt is well-dispersed in Ni/MgAlOx. Previous studies reported the formation of Pt–O–Mg bonds, leading to an anchoring effect that prevents Pt from sintering.59,60 However, in our study, we observed the formation of PtO2 particles in the monometallic Pt/MgAlOx sample, which is an indicator of a rather moderate interaction with the support if temperatures up to 500 °C are applied for the catalyst preparation. With the aim of better understanding the nature of Pt–Ni and Pt–support interactions, the analysis of the FT-EXAFS spectra was also carried out for the data obtained at the Pt L3 edge. The backscattering peak centered at 2.0 Å with a coordination number of 5.3 ± 0.5 corresponds to the first coordination shell with oxygen. The absence of further features between 2 and 3 Å suggests that there is hardly any interaction of Pt with Ni after the calcination procedure. Interestingly, there is the presence of a backscattering path at 3.0 Å, which previously was assigned to a Pt–O–Mg interaction.60

3.2. Activation of Ni- and Ni–Pt-based Catalysts by High-Temperature Reduction

For methane steam reforming, previous investigations showed that Ni in a reduced state is the most active species,26 which is typically achieved by reductive treatments at high temperatures. To identify all structural changes occurring during this procedure, in situ XAS and in situ XRD measurements were conducted during catalyst reduction in 5%H2/He. Figure 4(a) shows the LCA and the in situ 2D-XANES maps collected at the Ni K edge during H2-TPR of the monometallic Ni/MgAlOx sample between room temperature and 900 °C. In the reductive atmosphere, the white line (WL) intensity of the Ni K edge, which originates from the 1s → 4p electronic transition, slowly starts to decrease above 650 °C and reaches the level of the metallic reference around 850 °C. Furthermore, the energy shift of the edge position observed at high temperatures indicates the formation of metallic Ni. The reduction temperature is well above the one expected for bulk NiO,50 which is in accordance with our previous findings in Figure 2, and is due to NiMgOx presence in the catalyst. Based on the LCA of the XANES region, about 93% Ni0 is present at the end of the reductive treatment (catalyst activation procedure).

Figure 4.

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LCA of the in situ XAS spectra and the corresponding 2D-XANES map at the Ni K edge for (a) Ni/MgAlOx and (b) NiPt/MgAlOx and at the Pt L3 edge for (c) NiPt/MgAlOx catalysts during H2-TPR retreatment. Complementary in situ XRD patterns of (d) Ni/MgAlOx and (e) NiPt/MgAlOx catalysts during heating to 900 °C in 5%H2/He at a ramp rate of 5 °C/min.

For the bimetallic NiPt/MgAlOx catalyst, a similar reduction profile was observed for nickel (Figure 4(b)). The presence of Pt appears to increase the reduction rate, slightly shifting the onset reduction temperature to 300 °C. For the same catalyst, the reduction of Pt is completed at about 500 °C (Figure 4(c), starting above 150 °C). Interestingly, a shoulder appears at around 11575 eV in the Pt L3 XANES above 400 °C. This feature (cf. reports in literature61,62) has been assigned to the hybridization of Ni and Pt empty orbitals in Ni–Pt alloys.62 Overall, the impact of Pt on the reduction of the Ni species is moderate. This is probably due to the rather low concentration of the noble metal combined with the high stability of Ni-containing mixed oxides. However, at the end of the H2-TPR procedure, i.e., 900 °C, all Ni is present in the metallic state in comparison to only 93%, as found for the monometallic sample.

Complementary in situ XRD patterns collected during catalyst reduction are reported in Figure 4(d,e) for the monometallic and bimetallic samples, respectively. Upon exposure to 5%H2/He and increasing the temperature up to 850 °C, the main features appearing for both catalysts are the metallic Ni reflections at 2θ = 50° (200) and 73° (220). For both samples, they are only observed around 800 °C. After 1 h reduction, the Ni reflections at 44, 50, and 73° of the monometallic sample are sharper compared to the bimetallic counterpart. In this regard, the differences in crystallite size could be due to the strong sintering of Ni in the absence of a Pt promoter.

The detailed structure of Ni and Pt species after catalyst activation was identified by evaluating the FT-EXAFS spectra collected for the mono- and bimetallic catalysts after H2-reduction. Visual comparison of the FT-EXAFS spectrum obtained for the reduced NiPt/MgAlOx sample with those of Ni references indicates a complete resemblance with that of metallic Ni. This assumption is further validated by the Ni–Ni coordination shell at 2.48 Å, which is identical to that of Ni in its metallic form. The fit of the first coordination shell yields a coordination number of 9.8 ± 0.3, which is low compared to that present in bulk Ni (i.e., 12). The difference could be caused not only by the size63 but also by the flat shape of the formed metallic Ni particles,64 which are still interacting to a certain extent with the MgAlOx support. In Figure 5(b), the FT-EXAFS spectrum recorded at the Pt L3 edge for the as-prepared and activated NiPt catalysts is shown together with those of PtO2 and Pt foil references. After catalyst reduction, the expected Pt–Pt backscattering of metallic Pt does not appear in the bimetallic sample. The main scattering shell located at 2.55 Å has been previously attributed to Pt–Ni in alloyed particles.65 Hence, during the hydrogen treatment at high temperatures, Pt is not only reduced but also forming an alloy with Ni. As the alloying process is favored at moderate temperatures,66−68 Pt will probably interact with the metallic Ni that is already available in this temperature range. The changes in the XANES profile (Figure 4(b)) indicate that the alloying process starts at 400 °C. As the temperature increases, Ni species are further reduced, which favors the increase of the Ni share in the alloy. The fitting of the FT-EXAFS data (Table S5) obtained at the end of the activation procedure indicates that Pt is in close contact with Ni, with a coordination number of 9.0 ± 0.2. The relatively high coordination number suggests that Pt is mostly inside the Ni lattice, possibly forming intermetallic Ni1–xPtx alloys or single-atom alloys.65,69

Figure 5.

Figure 5

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FT-EXAFS data recorded at (a) Ni K edge and (b) Pt L3 edge (k2-weighted, k = 3–12 Å–1, not phase-shift corrected) of the mono- and bimetallic catalysts before and after H2-reduction.

HAADF-STEM EDX images acquired for the activated catalysts show a broad particle size distribution for both the Ni/MgAlOx and Ni–Pt/MgAlOx samples (Figure 6). For the monometallic Ni/MgAlOx catalyst, the particle sizes range between 4 and 10 nm. Additionally, agglomerates are present on the mixed oxide support. In the bimetallic catalyst, very small Pt-rich particles (1–2 nm) are distinguished besides Ni-rich particles of sizes mostly between 2 and 10 nm (Figure 6(a–d)). This indicates a rather heterogeneous distribution of the noble metal in the bimetallic catalyst. On the one side, this could be caused by the special interactions of both Pt and Ni with the MgAlOx support. On the other side, the different reduction temperatures of Ni and Pt species seem to favor the formation of Ni–Pt alloys with different compositions. Additionally, the rather high reduction temperature of 900 °C used for catalyst activation could lead to the sintering of Ni particles and partial segregation of Pt on the alloyed particle surface.67 Due to the high sensitivity of the catalysts in this study to the ambient atmosphere exposure and also to the relatively high temperature required for the alloying/realloying process, the observed formation and evolution of the Pt–Ni alloyed particles could not be supplemented by other surface sensitive techniques, e.g., XPS or diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS).

Figure 6.

Figure 6

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HAADF-STEM images with EDX mapping of Ni/MgAlOx (a–d) and NiPt/MgAlOx (e–h) samples after the H2-TPR activation. The corresponding particle size distribution for Ni in the mono- (i) and bimetallic (l) catalysts.

3.3. Catalyst Structural Changes During Methane Steam Reforming

Methane conversion curves acquired for the monometallic Ni and bimetallic NiPt catalysts are shown in Figure 7(a,b), respectively. The catalytic performance was evaluated between 200 and 900 °C for a specific steam-to-carbon ratio (S/C) of 3, after catalyst activation in a 5%H2/N2 gas flow for 1 h at 900 °C. The activity and stability of the catalysts were assessed by 3 subsequent heating (LOff) and cooling (LOut) cycles. A about 50 °C lower onset temperature was measured for the SR reaction over the bimetallic catalyst in comparison to that recorded for the monometallic Ni/MgAlOx sample. Additionally, a progressive deactivation was noticed for the monometallic Ni catalyst, which is indicated by the higher T50 (temperature at 50% CH4 conversion) measured during the second and third LOff/LOut cycles. In contrast, Pt addition leads to an increase in stability, in terms of the CH4 reforming rate, throughout the three testing cycles.

Figure 7.

Figure 7

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Catalytic activity and selectivity obtained during the first MSR light-off for (a) Ni/MgAlOx and (b) NiPt/MgAlOx catalysts using a quartz capillary reactor (top); overview on the T50 temperatures (of 50% CH4 conversion) measured during the heating step of the three light-off/light-out MSR cycles between 50 and 900 °C (bottom).

In order to reveal the structural changes that cause the activity decrease for Ni/MgAlOx or the higher stability of the bimetallic sample, both Ni and Pt species were studied during operando XAS experiments at the Ni K and Pt L3 edges. Figure 8 depicts the variations in the oxidation state recorded at the middle position of the catalyst bed during the first activity cycle to 900 °C for the monometallic and bimetallic catalysts (the corresponding activity is reported in Figure S1(a)). As shown in Figure 5(a,b), upon catalyst activation in a reductive atmosphere, Ni and Pt are both in a metallic state with Pt entirely alloyed with Ni. A slight oxidation of Ni species occurs immediately after exposure of the catalyst to the reaction mixture, which is more pronounced for the bimetallic catalyst (20% vs only 10% Ni2+ species in Ni/MgAlOx) probably due to the smaller Ni particle size in this sample. Ni oxidation at low temperatures has been previously assigned to H2O, CO2, or traces of oxygen in specific cases.70,71 For the monometallic catalyst, the oxidation state of Ni increases during heating in the reaction mixture from RT to 450 °C, where the fractions of metallic Ni and NiO are 48 and 42%, respectively, with the rest being present as NiMgOx. Further increase in temperature results in an inversed trend in the speciation of nickel: Ni0 becomes predominant at 900 °C and reaches 90% among all other species. The trend in the oxidation state clearly correlates with the reaction course and H2/CO formation, which are able to reduce nickel back to the metallic state at high temperatures.

Figure 8.

Figure 8

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2D-XANES maps and corresponding LCA of the Ni K and Pt L3 edge XANES spectra collected during the 1st Light Off of MSR for (a) Ni/MgAlOx, (b) NiPt/MgAlOx (Ni K edge), and (c) NiPt/MgAlOx (Pt L3 edge).

Ni species in the NiPt/MgAlOx catalyst show some mild oxidation during heating that reaches its maximum at 300 °C, just before the methane steam reforming onset. At this temperature, the extent of Ni oxidation is completely different compared to the monometallic counterpart (Ni0 is 70%, s. above). With a further temperature increase, Ni2+ concentration decreases likewise as for the Ni/MgAlOx sample. At 750 °C it amounts to ∼10%, and apparently increases slightly above this temperature to 13%. The relatively broad temperature window for Ni reduction under reaction conditions despite the initial catalyst activation could indicate a possible reintercalation of Ni into the support crystal structure at lower temperatures. However, the presence of Pt in the bimetallic sample seems to attenuate the oxidation process. The shielding role of Pt toward Ni oxidation in bimetallic catalysts could be caused by the recurring diffusion of Pt toward the surface of bimetallic particles during high-temperature treatments (prereduction or under reaction conditions), as reported by Ahmadi et al. in a study on the behavior of PtNi nanoparticles during different annealing treatments.72

In contrast to the changes observed for Ni species, the profile of the Pt L3 edge XANES spectra does not show any new features during heating to 900 °C in the reaction mixture. This indicates that Pt remains mostly in an alloyed and reduced state throughout the steam reforming process, in line with the noble character of this element (less prone to oxidation). Furthermore, this implies that Ni atoms in the close vicinity of Pt do not change their local structure and alloyed state, i.e., are not oxidized.

The sensitivity of Ni toward steam oxidation at low temperatures is visible also during cooling down from 900 to 200 °C. In the electronic Supporting Information, the linear combination analysis (LCA) for the operando XANES data and the corresponding catalytic activity of the mono- and bimetallic samples are provided (Figure S2). For the Ni/MgAlOx catalyst, the activity is maintained down to 500 °C when the methane concentration starts to increase. However, the oxidation state of Ni at the middle of the catalyst bed remains unchanged down to 350 °C. According to Figure S2(c), below 350 °C, the concentration of the reductive gaseous products H2 and CO decreases, whereas the large excess of H2O (S/C 3:1) is predominant. Consequently, Ni0 share drops from 90% at 350 °C to 67% at 250 °C. In the same temperature range, NiO concentration increases from 9 to 21%. The difference of up to 100% is due to the appearance of NiMgOx, most probably at the interface of Ni particles with the MgAlOx support.

For the bimetallic catalyst, the results shown in Figure S2(d) reveal a similar trend in Ni reoxidation at low temperatures. The final concentrations of Ni0 and Ni2+ species are different: Ni0 concentration drops from 85% at 300 °C to 72% at 200 °C, while NiO share increases from 11 to 17%. Hence, the noble metal addition contributes to catalyst stabilization by increasing the Ni reoxidation resistance. Nonetheless, owing to the low noble metal concentration, the effect of Pt on Ni is limited. Additionally, at temperatures below 500 °C and especially under oxidation conditions, the oxidation and segregation of Ni on the surface of PtNi alloys is as well expected.73 In the next step, NiMgOx species are formed. In the bimetallic catalyst investigated here, these species amount to about 11%, which is comparable to the 12% found in the monometallic Ni/MgAlOx sample.

In line with laboratory tests (Figure 7), a decrease in activity was observed during the following activity cycles for both samples, which is clearly caused by the oxidation of Ni species. For the monometallic catalyst, a further increase of NiO and NiMgOx concentrations was noticed during the second and third LOffs (Figures S3–S5). During the second LOff NiO amounted to 46% and NiMgOx 23% before the methane steam reforming ignition around 600 °C. Analogous to the behavior observed during the first reaction cycle, Ni0 concentration gradually decreases in the bimetallic NiPt/MgAlOx catalyst from 69 to 60% until 300 °C (Figure S3(d)), while Ni2+ increases accordingly from 20 to 30% as NiO and 10% as NiMgOx. Once the reaction ignites, Ni species are reduced again to the metallic state. However, this process is slow until 900 °C for the monometallic sample, whereas for the bimetallic catalyst, the reduction achieves apparently a steady state around 600 °C during the second LOff. Similar to that observed during the first cooling, Ni is partially reoxidized in both catalysts once the activity drops, amounting to a total of 44% NiO for the monometallic and 27% for the bimetallic sample. The progressive oxidation of Ni species is more pronounced for the monometallic catalyst also during the third methane steam reforming LOff (Figure S5), whereas the presence of Pt maintains 50% CH4 conversion at 450 °C, and 68% of Ni is in the metallic state for the bimetallic sample.

3.4. Catalyst Bed Structural Profiles

Overall, it could be confirmed that metallic nickel is the most active species obtained by catalyst reduction at high temperatures. However, the exposure of the catalyst to water vapors at temperatures below the methane steam reforming onset leads to partial oxidation under transient reaction conditions, a process retarded by the presence of Pt presence. Additionally, due to the strong interaction with the support, a fraction of Ni2+ is reincorporated into the MgO lattice. This outcome is in line with that obtained by similar studies on the catalyst state at different stages of the steam reforming process for daily startup and shutdown operation modes.74,28,75 However, for a more precise evaluation of the extent of catalyst deactivation, which occurs for the monometallic and bimetallic NiPt catalysts, spatially resolved operando XAS measurements were conducted at the beginning (inlet), middle (mid), and end (outlet) positions of the catalyst bed. This approach allows us to accurately assign the drop in activity over time to the catalyst structural changes occurring in a certain zone of the reactor or in the entire catalyst bed, which, for an industrial application, corresponds to the end of the catalyst lifetime.

Figure 9 depicts the Ni K edge XANES spectra measured at three positions of the catalyst bed for the monometallic and bimetallic catalysts at selected temperatures during the extinction (LOut of the third activity cycle) of methane SR reaction between 900 and 150 °C. While Figure 9 shows the changes in the spectral features due to the temperature and gas mixture variations along the catalyst bed, the general trends in the evolution of Ni phases obtained by LCA of the XANES region are reported in Figure 10 for the inlet, middle, and outlet positions of the catalyst bed. In line with the results reported in previous sections, pronounced changes are noticed around the white line and edge position of the Ni K edge XANES spectra, as outlined in the figure insets. The increase of the white line and the shift of the edge position toward higher energy correlate with the oxidation of Ni species, resulting in NiO and NiMgOx formation. The comparison of the Ni state at the three different positions is shown in Figure 10 (results of LCA for the Ni K edge XANES region) for different reaction temperatures. For the monometallic Ni/MgAlOx catalyst (Figure 9(a,b)), more pronounced changes are noticed at the start (inlet) of the catalyst bed, with the oxidation of Ni starting around 500 °C at a CH4 conversion of 75% (Figure S9). At 400 °C and 20% CH4 conversion, about 90% of nickel is present as Ni2+ in NiO and NiMgOx, an amount that remains almost unchanged during further temperature decreases. In contrast, for the bimetallic catalyst, the oxidation of Ni at the beginning of the catalyst bed is observed only around 400 °C at an overall methane conversion of 25%. In both cases, the oxidation of Ni species at this position in the catalyst bed is in line with the methane conversion profile along the reactor, progressing from the inlet toward the outlet zones. Besides the overall decrease in methane conversion during light-out (Figure S9), the inlet zone is exposed to the highest water concentration and the lowest amount of reducing gases, i.e., H2 and CO, which generate the most favorable conditions for nickel oxidation.

Figure 9.

Figure 9

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Spatially resolved operando XAS measurements at the Ni K edge at selected temperatures during the 3rd LOut (cooling from 900 to 150 °C) of CH4 steam reforming, respectively, for Ni/MgAlOx (a) and for NiPt/MgAlOx (b); inlet denotes the beginning of the catalyst bed, while outlet represents the end of the catalyst bed.

Figure 10.

Figure 10

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Linear combination analysis of Ni K edge XANES spectra acquired for (a) Ni/MgAlOx and for (b) NiPt/MgAlOx catalysts during MSR at temperatures between 900 and 50 °C (LOut) for different positions of the catalys bed.

Moving downstream in the catalyst bed, the oxidation of Ni species in the monometallic sample occurs only at further lower temperatures: between 450 and 300 °C and 400–250 °C at the middle and the end positions, respectively. This is due to the lower steam concentration and especially due to the more reducing gas atmosphere generated by the reaction products, i.e., H2 (Figure S9), at these positions in the reactor. For the bimetallic NiPt/MgAlOx catalyst, a large portion of nickel remains in a reduced state even at very low methane conversion. While at the beginning of the catalyst bed, 40% of Ni is oxidized below 400 °C, and only about 20% of Ni species are in the 2+ state for the middle and outlet positions. Furthermore, at these catalyst bed positions, Ni oxidation seems to occur only below 250 °C, a process outside of the activity window for methane steam reforming. At this temperature, only about 10% of CH4 is converted to H2 and CO2 (Figure S9). Despite the low amount of Pt in the bimetallic catalyst and its uneven distribution, a high stabilization effect of the Ni metallic state is gained. The observed decrease in activity for the NiPt/MgAlOx catalyst seems rather to follow the thermodynamic equilibrium conversion, also considering the higher dissociation energy for CH4 over Ni–Pt bimetallic surfaces versus pure Ni.76,77 However, we cannot exclude that the decrease in activity is caused by the surface oxidation of PtNi particles at these low temperatures,72,73 which may lead to a catalyst passivation effect.

With respect to long-term effects on catalyst structure, an overview of the evolution of Ni species in the catalyst bed from the as-prepared state upon activation and after each of the three consecutive heating/cooling cycles between room temperature and 900 °C is given in Figure 11. Ni speciation was obtained by LCA of the XANES spectra acquired at the beginning, middle, and end positions, as shown for the reduced catalyst after exposure to the reaction mixture at room temperature. As described above, by applying a calcination step to an impregnated Ni or NiPt/MgAlOx catalysts, the solid-state reaction between NiO and MgAlOx support is promoted with the formation of NiMgOx species. After reduction at 900 °C, Ni is present in the metallic state, partially alloyed with Pt in the bimetallic catalyst.

Figure 11.

Figure 11

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Linear combination analysis of the Ni K edge XANES spectra collected at room temperature at the beginning (inlet), middle, and end (outlet) positions of the catalyst bed for Ni/MgAlOx (a–c) and NiPt/MgAlOx (d–f) at different stages of the experimental procedure.

The sensitivity of Ni species toward oxidation is detected already during the initial exposure to the reaction mixture. According to the operando XAS results, this process is induced by the water vapors present in the gas mixture below the steam reforming onset temperature (Figure 7). Probably due to the smaller Ni-containing particles present in the NiPt sample, especially the bimetallic catalyst seems to suffer from steam oxidation under these conditions, which leads to the formation of about 18% NiO in the NiPt/MgAlOx sample. However, a progressive and more pronounced Ni oxidation is observed for the monometallic sample during the following activity cycles.

Furthermore, low reaction temperatures promote the formation of Ni–Mg mixed oxide in addition to NiO. This is confirmed by the analysis of the FT-EXAFS spectra obtained for the Ni/MgAlOx and NiPt/MgAlOx catalysts at room temperature after consecutive LOff/LOut cycles (Figure 12). During the stepwise cooling to room temperature, both processes start at the beginning of the reactor and rapidly evolve at downstream positions. Less than 20% metallic Ni is present at all positions of the catalyst bed at the end of the third catalytic cycle in the monometallic sample. The analysis of the corresponding FT-EXAFS data uncovers not only the formation of NiO but especially the gradual conversion of this species into the mixed oxide phase due to the solid-state reaction with MgO (Figures 12(d), S6, and S7), starting from the beginning of the catalyst bed and moving toward the outlet of the reactor. In contrast, despite its low concentration, Pt is able to drastically stabilize Ni species. Also in this case, the inlet of the catalyst bed is foremost affected as the conversion of methane is minor at this position in the reactor. However, the concentration of Ni2+ species shows no major variations at the three different positions during the second and third catalytic cycles for the bimetallic catalyst. This behavior is most probably related to Pt promotion of the catalyst stability.

Figure 12.

Figure 12

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XANES (a–c) and FT-EXAFS (not phase-shift corrected) (d–f) spectra recorded at Ni K and Pt L3 edges at the beginning (inlet), middle, and end (outlet) positions of the catalyst bed; the XAS spectra of NiO, NiMgOx, PtO2, Ni, and Pt foil reference samples are shown as well.

4. Conclusions

In this study, the evolution of Ni-based mono- and bimetallic catalysts for hydrogen production by methane steam reforming was systematically analyzed under dynamic reaction conditions that are characteristic of daily startup/shutdown operation cycles. Structural changes of the oxidation state and the local structure of Ni and Pt in 15%Ni/MgAlOx and 15%Ni-1%Pt/MgAlOx catalysts were evaluated between room temperature and 900 °C by in situ/operando XAS, whereas the overall phase composition was determined by in situ XRD. During the reductive activation treatment, metallic Ni particles are formed in the monometallic catalyst, while alloyed NiPt particles are generated in Ni–Pt/MgAlOx. Despite most Ni particles containing Pt, a heterogeneous distribution of the noble metal was identified by HAADF-STEM investigations. In situ XAS measurements uncovered that this outcome is caused by the different reduction temperatures of the platinum and nickel oxides. Under reaction conditions, the oxidation of Ni species was observed. This is a pronounced and progressive process that occurs over consecutive activity cycles, in particular for the monometallic Ni/MgAlOx catalyst. Spatially resolved XAS measurements performed along the catalyst bed uncovered the start of Ni oxidation at the beginning of the catalyst bed below 500 °C, whereas the middle and end positions are only affected at lower temperatures while methane conversion decreases during cooling down of the reactor. Less than 20% of Ni is in the metallic state after 3 light-off/light-out cycles at the end of the catalyst bed, most probably present at the core of larger Ni-containing particles. Additionally, the analysis of the FT-EXAFS data uncovered the reintercalation of Ni upon oxidation into MgO lattice, leading to the formation of mixed oxide phases. Consequently, a gradual decrease in activity was observed for the monometallic catalyst during successive catalyst heating and cooling cycles in the reaction gas mixture.

Despite the relatively low loading of Pt, the noble metal presence in the bimetallic Ni–Pt/MgAlOx catalyst as alloyed Pt–Ni particles leads to a lower extent of oxidation for Ni species at low temperatures and thus too strong catalyst stabilization. According to the operando XAS results, the formation of Ni2+ as NiO and NiMgOx phases was found also in the bimetallic sample but amounting to only 20–35% of the nickel species. With only a small variation along the catalyst bed, this Ni-phase distribution was identified already after the first light-off/light-out cycle in the reaction mixture, and no further progress was recognized during the following activity tests. This behavior could be directly correlated with the activity profile obtained for this sample. In contrast to Ni evolution, Pt remains in the alloyed state with no noticeable structural changes during transient reaction conditions.

Hence, the use of in situ/operando characterization methods allowed for the first time to gain a comprehensive insight into the evolution of Ni and Ni–Pt/MgAlOx catalysts under the demanding reaction conditions of methane steam reforming. In this way, clear correlations between the Ni and Pt states and the catalyst activity could be derived over the broad temperature range of the MSR, which could serve as a fundamental basis for the rational improvement of this class of catalysts.

Acknowledgments

This work was funded by the European Union′s Horizon 2020 Research and Innovation program under the Marie Sklodowska-Curie Actions—Innovative Training Network (MSCA-ITN) grant agreement 813748 (BIKE Project) and the MTET program of the HGF. The work at ROCK beamline was supported by a public grant overseen by the French National Research Agency (ANR) as part of the “Investissements d′Avenir” program (reference: ANR-10-EQPX-45). Angela Deutsch is acknowledged for the help during the N2-physisorption measurements.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscatal.3c05847.

  • Additional information on the catalytic activity, 2D-XANES map, and EXAFS data evaluation are reported (PDF)

Author Contributions

E.T. conducted the study, performed the catalyst preparation, XAS characterization, and data analysis, and wrote the manuscript. M.C. was involved in the XAS experiments, study design, data discussion, and paper writing. D.D. and A.Z. helped with the XAS data evaluation; H.S. performed the TEM measurements; S.B. and L.B. supported the measurements at the ROCK beamline and the sample environment; J.-D.G. helped during the design of the study, discussion on the results, and paper writing. All the authors commented and gave input to the manuscript.

The authors declare no competing financial interest.

Supplementary Material

cs3c05847_si_001.pdf (3.2MB, pdf)

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