A gamma-alumina-supported silver cluster catalyst--Ag/Al(2)O(3)--has been shown to act as an efficient heterogeneous catalyst for oxidant-free alcohol dehydrogenation to carbonyl compounds at 373 K. The catalyst shows higher activity than conventional heterogeneous catalysts based on platinum group metals (PGMs) and can be recycled. A systematic study on the influence of the particle size and oxidation state of silver species, combined with characterization by Ag K-edge XAFS (X-ray absorption fine structure) has established that silver clusters of sizes below 1 nm are responsible for the higher specific rate. The reaction mechanism has been investigated by kinetic studies (Hammett correlation, kinetic isotope effect) and by in situ FTIR (kinetic isotope effect for hydride elimination reaction from surface alkoxide species), and the following mechanism is proposed: 1) reaction between the alcohol and a basic OH group on the alumina to yield alkoxide on alumina and an adsorbed water molecule, 2) C-H activation of the alkoxide species by the silver cluster to form a silver hydride species and a carbonyl compound, and 3) H(2) desorption promoted by an acid site in the alumina. The proposed mechanism provides fundamental reasons for the higher activities of silver clusters on acid-base bifunctional support (Al(2)O(3)) than on basic (MgO and CeO(2)) and acidic to neutral (SiO(2)) ones. This example demonstrates that catalysts analogous to those based on of platinum group metals can be designed with use of a less expensive d(10) element--silver--through optimization of metal particle size and the acid-base natures of inorganic supports.
Dynamic structural changes of Ag in Ag-MFI upon H 2 reduction and subsequent reoxidation treatments were investigated by various characterization methods. Results of in situ time-resolved spectroscopies, UV-vis and quick XAFS, at 573 K show that Ag + ions at a cation-exchange site of preoxidized Ag-MFI are reduced by H 2 to a Ag 4 δ+ cluster. Protons on the zeolite (Brønsted acid sites) are also produced by the H 2 reduction of Ag-MFI as evidenced by IR measurements after D 2 adsorption. At higher reduction temperatures, the aggregation of Ag 4 δ+ clusters results in the formation of larger Ag n (n > 8) clusters, and Ag metal particles appear on the external surface of the zeolite. Upon reoxidation at 573 K, the silver clusters are redispersed to Ag + ions. The rate constant of cluster redispersion at 573 K depends on the oxidants used and increases in the order of NO(0.1%) + O 2 (10%) > O 2 (10%) > NO(0.1%). The activation energy for cluster formation in H 2 was lower than that for cluster redispersion during reoxidation with O 2 . The effect of Ag exchange level on the rate constants and apparent activation energies for Ag cluster formation in H 2 and Ag cluster redispersion in O 2 are also examined.
The mechanism of the H2-assisted selective catalytic reduction of NO with propane (H2−C3H8 SCR) over Ag+-exchanged MFI zeolite (Ag−MFI) was investigated by spectroscopic studies. Transient IR experiments at 573 K indicate that H2 addition increases the rates of C3H8 oxidation to acetate on Ag−MFI and NO oxidation to the weakly adsorbed NO2. IR spectra obtained during steady-state C3H8 SCR indicate that H2 addition increases the coverage of acetate, Ag+−NCO, and Al3+−NCO. The reaction of acetate with NO2 yields, via CH3NO2, NCO species, and their hydrolysis yields NH3 as a final precursor of N2. In situ UV−vis and EXAFS measurements at 573 K shows that, during the H2 + O2 and H2−C3H8 SCR reactions, Ag+ ions and partially reduced Ag4 2+ clusters coexist. ESR spectra obtained after exposing a H2 + O2 gas mixture show the formation of superoxide ions. Combined with the results in our previous studies, the following mechanism is proposed: The Ag4 2+ cluster and protons formed by H2 reduction of Ag+ ions are involved in the reductive activation of molecular oxygen into superoxide ion, which should act as an effective oxidant for C3H8 oxidation to oxygenates such as acetate and NO oxidation to NO2. Thus, H2 addition increases the coverage of surface intermediates (NCO species) and the gas-phase concentration of NO2 and consequently increases the rate of N2 formation.
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