Pt L3-edge XANES studies about the hydrogen adsorption on small Pt particles

Influence of adsorbates on the electronic structure, bond strain, and thermal properties of an alumina-supported Pt catalyst

We describe the results of an X-ray absorption spectroscopy (XAS) study of adsorbate and temperature-dependent alterations of the atomic level structure of a prototypical, noble metal hydrogenation and reforming catalyst: ∼1.0 nm Pt clusters supported on gamma alumina (Pt/γ-Al(2)O(3)). This work demonstrates that the metal-metal (M-M) bonding in these small clusters is responsive to the presence of adsorbates, exhibiting pronounced coverage-dependent strains in the clusters' M-M bonding, with concomitant modifications of their electronic structures. Hydrogen and CO adsorbates demonstrate coverage-dependent bonding that leads to relaxation of the M-M bond strains within the clusters. These influences are partially compensated, and variably mediated, by the temperature-dependent electronic perturbations that arise from cluster-support and adsorbate-support interactions. Taken together, the data reveal a strikingly fluxional system with implications for understanding the energetics of catalysis. We estimate that a 9.1 ± 1.1 kJ/mol strain exists for these clusters under H(2) and that this strain increases to 12.8 ± 1.7 kJ/mol under CO. This change in the energy of the particle is in addition to the different heats of adsorption for each gas (64 ± 3 and 126 ± 2 kJ/mol for H(2) and CO, respectively).

Determination of CO, H2O and H2 coverage by XANES and EXAFS on Pt and Au during water gas shift reaction

The turn-over-rate (TOR) for the water gas shift (WGS) reaction at 200 degrees C, 7% CO, 9% CO(2), 22% H(2)O, 37% H(2) and balance Ar, of 1.4 nm Au/Al(2)O(3) is approximately 20 times higher than that of 1.6 nm Pt/Al(2)O(3). Operando EXAFS experiments at both the Au and Pt L(3) edges reveal that under reaction conditions, the catalysts are fully metallic. In the absence of adsorbates, the metal-metal bond distances of Pt and Au catalysts are 0.07 A and 0.13 A smaller than those of bulk Pt and Au foils, respectively. Adsorption of H(2) or CO on the Pt catalysts leads to significantly longer Pt-Pt bond distances; while there is little change in Au-Au bond distance with adsorbates. Adsorption of CO, H(2) and H(2)O leads to changes in the XANES spectra that can be used to determine the surface coverage of each adsorbate under reaction conditions. During WGS, the coverage of CO, H(2)O, and H(2) are obtained by the linear combination fitting of the difference XANES, or DeltaXANES, spectra. Pt catalysts adsorb CO, H(2), and H(2)O more strongly than the Au, in agreement with the lower CO reaction order and higher reaction temperatures.

Hydrogen chemisorption on Al2O3-supported gold catalysts

Hydrogen is dissociatively adsorbed on the gold particles in Au/Al(2)O(3) catalysts, as demonstrated by a combination of in-situ X-ray absorption spectroscopy, chemisorption, and H/D exchange experiments. This chemisorption of hydrogen induces changes in the Au L(3) and L(2) X-ray absorption near-edge structures. The gold atoms on corner and edge positions dissociate the hydrogen, which does not spill over to the face sites. Therefore, the average number of adsorbed hydrogen atoms per surface gold atom increases with decreasing particle size. With temperature, the hydrogen uptake by supported gold increases or remains constant, whereas it decreases for platinum. Furthermore, in H/D exchange experiments, the activity of Au/Al(2)O(3) increases strongly with temperature. Thus, the dissociation and adsorption of hydrogen on gold is activated.