DEPENDENCE OF THE ELECTROOXIDATION RATES OF CARBON-MONOXIDE AT GOLD ON THE SURFACE CRYSTALLOGRAPHIC ORIENTATION - A COMBINED KINETIC-SURFACE INFRARED SPECTROSCOPIC STUDY

Rate parameters evaluated by linear sweep voltammetry are reported for the electrooxidation of carbon monoxide on six oriented monocrystalline gold surfaces in acidic aqueous perchlorate electrolytes. At a given electrode potential, the rate constants depend markedly upon the crystallographic orientation in the sequence Au(111) < Au(533) approximately Au(100) less-than-or-similar-to Au(221) < Au(210) < Au(110), up to 100-fold differences in rate being observed. Marked inhibition of the notably facile electrode kinetics are observed if 0.1 M H2SO4 is substituted for 0.1 M HClO4 electrolyte. Parallel voltammetric measurements coupled with real-time surface infrared spectroscopy at four surfaces, Au(111), Au(100), Au(110), and Au(210), enable the role of CO reactant adsorption in the surface-dependent catalysis to be assessed. A low-coverage, yet reactive, form of adsorbed CO was detected on Au(110) and (210) from a C-O vibrational (nu-CO) band at 2100-2115 cm-1 which disappears at the onset of the voltammetric wave. The sequence of CO surface concentrations as discerned from infrared spectroscopy, Au(111), Au(100) < Au(110) < Au(210), differs from the above reactivity sequence, signaling the presence of additional factors in the electrocatalysis. Relationships are explored between the surface-dependent rates and the potentials of zero charge or the density of "broken bonds" in the surface lattice (i.e., the average surface coordination number). These correlation suggest that the rate-determining electrooxidation step between coadsorbed CO and H2O (or OH) species is favored either at step sites or on rows of low-coordination metal atoms, such as on Au(110). This is speculated to be due to the ability of such sites to engender CO adsorption and nearby H2O (or OH) coadsorption.