Caracterizacion de catalizadores PtGe utilizados para PROX

JOSÉ M. RAMALLO-LÓPEZ; HERNÁN BIDEBERRIPE; GERARDO F. SANTORI; SANTIAGO J. FIGUEROA; FÉLIX G. REQUEJO; MÓNICA L. CASELLA; GUILLERMO J. SIRI

Madrid, España

Simposio; International Symposium on "Catalysis for Clean Energy and Sustainable Chemistry"; 2008

Instituto de Catalisis y Petroleoquímica

In the present contribution, we study Pt and PtGe catalysts supported on SiO2 by EXAFS and XANES experiments with the aim to obtain a detailed knowledge of the location and state of the species of the metallic phases, in order to understand their performance in the preferential oxidation reaction of CO in the presence of high concentrations of hydrogen. The Fourier transform of the EXAFS spectrum at the Pt L3 edge of the Pt/SiO2 catalyst shows the same peaks as that of a foil of Pt, only differing in their intensities. This indicates that Pt is completely reduced forming small metallic clusters. From a fit of the experimental data, we obtained an average coordination number for the first Pt-Pt shell of 8.3 } 1, which would correspond to spherical metallic particles of 1.5 nm in diameter. XANES spectrum of the monometallic catalyst is very similar to that of the foil of Pt, except that a shoulder in the white line is observed. Similar results have been previously reported for different supported Pt systems and have always been attributed to the chemisorption of H2 on the metallic surface of the particles. Contrary to what has been observed for other bimetallic systems as PtSn, there is no formation of Pt-Ge bimetallic phase. In effect, the Fourier transform of the EXAFS spectrum for the PtGe0 catalyst presents the same peaks as that of the foil of Pt, the same that was observed for the monometallic catalyst. The fit of the first Pt-Pt shell gives and average coordination number of 8.0 } 1, indicating that the Pt does not suffer any structural changes because of the adding of Ge to the system and it maintains its structure forming metallic nanoparticles. XANES results show an important increase of the white line intensity, indicating an increase of the 5d hole density of Pt. This result is showing that, although the addition of Ge does not result in the direct interaction of the metals forming an inter-metallic compound, it results in modifications of the electronic behavior of Pt, leading to an important charge transfer to the Ge. Figure 2 Conversion of CO to CO2 as a function of temperature. Pt/SiO2 (œ); PtGe/SiO2 fresh ( ); 2 cycles (∆); 4 cycles (›)  No changes are observed in the EXAFS spectra at the Pt-L3 and Ge-K edges of bimetallic samples after successive oxidation-reduction treatments, what shows the great stability of the Pt and Ge structures. The conversion of CO is an important effect when Ge is added, evidenced for an increase in the CO conversion for a given temperature. The existence of ionic Ge confers the active site the capacity to promote the O2 adsorption. No variation in the catalytic behavior for the PROX reaction for samples subjected to several oxidation-reduction cycles were observed what confirms these last EXAFS results. is an important effect when Ge is added, evidenced for an increase in the CO conversion for a given temperature. The existence of ionic Ge confers the active site the capacity to promote the O2 adsorption. No variation in the catalytic behavior for the PROX reaction for samples subjected to several oxidation-reduction cycles were observed what confirms these last EXAFS results.