Bimetallic Ag–Ir/Al2O3 catalysts prepared by electroless deposition: Characterization and kinetic evaluation

Abstract

A series of alumina-supported Ag–Ir bimetallic catalysts having controlled and incremental coverages of Ag have been successfully prepared in an optimized electroless deposition bath. The starting monometallic Ir/γ-Al2O3 catalyst was highly dispersed (volume-surface mean diameter of 1.1nm) with a narrow particle size distribution, as demonstrated by H2 chemisorption and scanning transmission electron microscopy (STEM). While both catalytic (Ag on Ir) and autocatalytic (Ag on Ag) deposition were observed, coverages of Ag on Ir (measured by H2 chemisorption) were successfully varied up to θAg=0.85 by varying Ag weight loadings up to 0.81wt.%. In situ transmission Fourier transform infrared spectroscopy (FTIR) of CO adsorption demonstrated that the Ag is indiscriminately deposited on all types of Ir surface sites during the ED process. Kinetic studies of CO oxidation revealed that higher coverages of Ag resulted in higher turnover frequencies. Comparison of reaction orders in CO and O2 on monometallic Ir and the most active bimetallic catalyst (θAg=0.37) suggests a bifunctional effect, where Ag provides a non-competitive source of adsorbed oxygen for reaction with CO adsorbed on Ir. In addition, further increases in TOF at higher Ag coverages argue for the presence of a geometric effect on smaller Ir ensembles.