In-situ polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) is used to study CO adsorption at elevated pressures and temperatures on Pd nanoclusters deposited on crystalline SiO 2 thin films. The PM-IRAS data indicate that CO dissociates on the Pd nanoclusters at 185 mbar and at temperature >600 K. Combined STM and PM-IRAS data show that the Pd nanoclusters are three-dimensional and consist predominantly of 〈111〉 facets with an average diameter and height of 3.5 and 0.7 nm, respectively. STM data indicate no change in the morphology or sintering of the Pd nanoclusters between 300 and 650 K. For comparison, CO adsorption was investigated on a Pd(111) single-crystal surface using PM-IRAS under similar conditions (P CO ) 133 mbar, T ) 175-750 K). Comparison of the PM-IRAS data for CO adsorption on Pd(111) and Pd nanoclusters indicate that in contrast to the Pd nanoclusters, CO adsorbs molecularly on the Pd(111) surface in a reversible manner, i.e., no dissociation.
The selective oxidation of styrene on clean and modified Ag(100) surfaces has been studied by synchrotron fast XPS and temperature-programmed reaction spectroscopy. By following the time dependence of surface species, it is unequivocally demonstrated that the necessary and sufficient conditions for epoxide formation are oxygen adatoms and pi-adsorbed alkene molecules. Increased oxygen coverage and coadsorbed Cs have pronounced and opposite effects on epoxidation selectivity, consistent with the view that the valence charge density on O(a) is pivotal in determining this property. Submonolayer quantities of Cs nitrate generated in situ open a new, low-temperature ultraselective, epoxidation pathway thought to involve direct oxygen transfer from the oxyanion to the alkene.
Scanning tunneling microscopy (STM), in conjunction with the nucleation and growth of Au clusters, has been used to identify and quantify various types of defects on ordered, SiO 2 thin films grown on Mo(112). On a low-defect surface, Au clusters nucleate and grow at line defects with metal deposition at room temperature, whereas deposition at 850 K leads to cluster decoration primarily at step edges. On a highly defective surface, clusters nucleate and grow at point defects (oxygen vacancies and/or oxygen vacancy complexes) on the terraces, with some clusters grown on oxygen vacancy complexes remaining even after an 850 K anneal. The average cluster density for low Au coverages deposited at room temperature is identical to that obtained for the same Au coverage deposited at 850 K, consistent with complete titration of point defects by the nucleating clusters.
STM, NEXAFS, XPS, and TPR have been used to characterize the adsorption and reactivity of methyl pyruvate on Pt{111}. In the absence of coadsorbed hydrogen, methyl pyruvate polymerizes at room temperature yielding polymer chains, partly dendritic. The STM and XPS data provide independent estimates of the average length, found to be ∼9 monomer units. NEXAFS shows that this polymer contains CdO bonds and no CdC bonds; the CdO bonds are inclined at 64°( 5°with respect to the metal surface. It is proposed that polymerization occurs by hydrogen elimination from the monomer, followed by an aldol condensation that involves elimination of methanol. This mechanism is in excellent accord with the intramolecular bonding, shape, and reactivity of the polymer deduced from the NEXAFS, STM, and TPR results. Coadsorbed hydrogen completely suppresses polymerization. These findings suggest that irreversible deactivation during start-up or steady-state operation of Pt catalysts during enantioselective hydrogenation of alkyl pyruvates can be due to hydrogen starvation which results in polymerization of the prochiral reactant.
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