Effects of Adsorbate Coverage and Bond‐Length Disorder on the d‐Band Center of Carbon‐Supported Pt Catalysts

Determination of the factors that affect the d‐band center of catalysts is required to explain their catalytic properties. Resonant inelastic X‐ray scattering (RIXS) enables direct imaging of electronic transitions in the d‐band of Pt catalysts in real time and in realistic environmental conditions. Through a combination of in situ, temperature‐resolved RIXS measurements and theoretical simulations we isolated and quantified the effects of bond‐length disorder and adsorbate coverage (CO and H₂) on the d‐band center of 1.25 nm size Pt catalysts supported on carbon. We found that the decrease in adsorbate coverage at elevated temperatures is responsible for the d band shifts towards higher energies relative to the Fermi level, whereas the effect of the increase in bond‐length disorder on the d‐band center is negligible. Although these results were obtained for a specific case of non‐interacting support and weak temperature dependence of the metal–metal bond length in a model catalyst, this work can be extended to a broad range of real catalysts.     

Identifying Different Types of Catalysts for CO2 Reduction by Ethane through Dry Reforming and Oxidative Dehydrogenation

The recent shale gas boom combined with the requirement to reduce atmospheric CO₂ have created an opportunity for using both raw materials (shale gas and CO₂) in a single process. Shale gas is primarily made up of methane, but ethane comprises about 10 % and reserves are underutilized. Two routes have been investigated by combining ethane decomposition with CO₂ reduction to produce products of higher value. The first reaction is ethane dry reforming which produces synthesis gas (CO+H₂). The second route is oxidative dehydrogenation which produces ethylene using CO₂ as a soft oxidant. The results of this study indicate that the Pt/CeO₂ catalyst shows promise for the production of synthesis gas, while Mo₂C‐based materials preserve the C? C bond of ethane to produce ethylene. These findings are supported by density functional theory (DFT) calculations and X‐ray absorption near‐edge spectroscopy (XANES) characterization of the catalysts under in situ reaction conditions.     

Surface science approach to the solid-liquid interface: surface-dependent precipitation of Ni(OH)2 on α-Al2O3 surfaces

Surface-dependent precipitation: The adsorption of Ni(II) complexes in aqueous solution on (0001) and (1102) α-Al(2)O(3) single-crystal surfaces has been studied (see the X-ray absorption spectra obtained for parallel and perpendicular polarization directions). The use of planar model systems emphasizes the crucial role of the Al(2)O(3) orientation for Ni dispersion with practical implications in catalyst preparation procedures.     

The state of the iron promoter in tungstated zirconia catalysts.

The activity and selectivity of tungstated zirconia (WZ) for the conversion of n- into isopentane are dramatically enhanced when the catalyst is modified with Pt and Fe. The state of iron in these catalysts was hitherto only poorly characterized. Therefore, in the present work we investigated the structural and electronic properties of iron in WZ catalysts containing 1 wt% Pt and 1 wt% Fe2O3, by a combination of spectroscopic techniques, namely X-ray absorption spectroscopy (XAS), in situ electron paramagnetic resonance (EPR), and M?ssbauer spectroscopy. In the oxidized catalyst, iron is present as Fe(III) and predominantly forms a surface solid solution in which the isolated Fe(III) ions are located in a distorted octahedral environment. A small amount of the total iron (around 10%) is present in the form of small iron oxide particles. Both iron species can be reduced in H2 and then easily reoxidized on exposure to air at room temperature. We infer that the promoter action of iron in these catalysts is intimately related to its redox properties and specifically affects the dehydrogenation activity of the materials.     

Effect of Non‐Specifically Adsorbed Ions on the Surface Oxidation of Pt(111)

The oxidation processes of a Pt(111) electrode in alkaline electrolytes depend on non‐specifically adsorbed ions according to in situ X‐ray diffraction and infrared spectroscopic measurements. In an aqueous solution of LiOH, an OH_ad adlayer is formed in the first oxidation step of the Pt(111) electrode as a result of the strong interaction between Li⁺ and OH_ad, whereas Pt oxidation proceeds without OH_ad formation in CsOH solution. Structural analysis by X‐ray diffraction indicates that Li⁺ is strongly protective against surface roughening caused by subsurface oxidation. Although Cs⁺ is situated near the Pt surface, the weak protective effect of Cs⁺ results in irreversible surface roughening due to subsurface oxidation.     

Cu‐Based Catalyst Resulting from a Cu,Zn,Al Hydrotalcite‐Like Compound: A Microstructural,Thermoanalytical, and In Situ XAS Study

A Cu‐based methanol synthesis catalyst was obtained from a phase pure Cu,Zn,Al hydrotalcite‐like precursor, which was prepared by co‐precipitation. This sample was intrinsically more active than a conventionally prepared Cu/ZnO/Al₂O₃ catalyst. Upon thermal decomposition in air, the [(Cu_0.5Zn_0.17Al_0.33)(OH)₂(CO₃)_0.17] ? mH₂O precursor is transferred into a carbonate‐modified, amorphous mixed oxide. The calcined catalyst can be described as well‐dispersed “CuO” within ZnAl₂O₄ still containing stabilizing carbonate with a strong interaction of Cu²⁺ ions with the Zn–Al matrix. The reduction of this material was carefully analyzed by complementary temperature‐programmed reduction (TPR) and near‐edge X‐ray absorption fine structure (NEXAFS) measurements. The results fully describe the reduction mechanism with a kinetic model that can be used to predict the oxidation state of Cu at given reduction conditions. The reaction proceeds in two steps through a kinetically stabilized Cu^I intermediate. With reduction, a nanostructured catalyst evolves with metallic Cu particles dispersed in a ZnAl₂O₄ spinel‐like matrix. Due to the strong interaction of Cu and the oxide matrix, the small Cu particles (7 nm) of this catalyst are partially embedded leading to lower absolute activity in comparison with a catalyst comprised of less‐embedded particles. Interestingly, the exposed Cu surface area exhibits a superior intrinsic activity, which is related to a positive effect of the interface contact of Cu and its surroundings.     

Controlled assembly of a heterogeneous single-site ethylene trimerization catalyst as probed by X-ray absorption spectroscopy

X-ray absorption spectroscopy at the Cr K- and L(2,3)-edges was used to study the assembling process of a heterogeneous Cr-based single-site catalyst. The starting point was a Phillips-type system with monochromate species anchored on a silica surface, which was first reduced to a variety of different surface Cr(II) species. The reduced sample was modified with a 1,3,5-tribenzylhexahydro 1,3,5-triazine (TAC) ligand in the presence of CH(2)Cl(2) as solvent to yield a heterogeneous single-site Cr-based catalyst active in the trimerization of ethylene. The molecular structure of the resultant catalytic material consists of distorted octahedral Cr(III) species. The extended X-ray absorption fine-structure (EXAFS) spectroscopy fitting procedure in R space up to 2.5 A showed that the synthesis leads to coordination with a TAC ligand. The fit also shows that it was possible to complete the six-fold environment around Cr(III) with two oxygen atoms and one chloride ligand. This chloride ligand is formed in a redox process from the solvent and is responsible for the oxidation of surface Cr(II) to Cr(III). The obtained geometry and the local environment of the surface complex are discussed in light of its homogeneous counterpart and confirm the single-site characteristics of the prepared catalytic material.