The influence of metal-support interactions on the catalytic properties of Pd/La2O3

Abstract Evidence is presented for the existence of metal-support interactions between Pd and La2O3. XPS spectra indicate that the Pd 3d 5 2 binding energy is shifted below the value of bulk Pd by as much as 0.7 eV. The negative binding energy shift increases with decreasing Pd dispersion. The chemisorption of CO at 298K is suppressed on the Pd microcrystallites, and the amount of suppression also increases with decreasing Pd dispersion. This latter effect exerts a strong influence on the morphology of the Pd surface during methanol synthesis. The turnover frequency for CH3OH synthesis from CO and H2 correlates with the Pd morphology: Pd(100) planes are 2.8 times more active than Pd(111) planes. The division of the Pd surface into a mixture of these two planes is inferred from the infrared spectrum of adsorbed CO. For a fixed Pd morphology, the CH3OH turnover frequently is independent of Pd dispersion. By contrast, the CH4 turnover frequency decreases with increasing Pd dispersion, and it is insensitive to the distribution of Pd(100) and Pd(111) planes.

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