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A platinum single crystal electrode with ordered step and terrace structures was used to probe site-dependent aspects of methanol surface chemistry. Infrared spectral features of the adsorbed CO reaction intermediate were measured in parallel experiments with Pt(335) (Pt(s)-[4( 11 1) x (loo)]) and Pt( 11 1) electrodes.Experiments confirm that methanolic CO formation is inhibited at Pt( 11 1) at potentials in the hydrogen adsorption region, and they reveal that methanol dissociative chemisorption proceeds to a limited extent in this potential region at the corrugated Pt(335) surface plane, where independent vibrational spectral features for CO at step and terrace sites were detected. IntroductionThe catalytic oxidation of methanol at noble metal electrodes is a technologically important process. Complete oxidation to COZ and water yields six electrons per molecule of methanol, making it attractive for use in fuel A primary complication is the formation of adsorbed intermediates, such as carbon monoxide (CO), which block catalytic surface sites and thereby inhibit energy-producing pathways. An important strategy for optimizing methanol oxidation rates has involved the study of reactions at well-characterized electrode surface^.^-^ These include basic investigations of methanol surface chemistry at well-ordered surface planes of single crystal materials (Refs 2, 3 and 4b-9 and references therein).A fruitful approach involving the use of single crystal electrodes has been to examine the effect of surface corrugation on the rate and specificity of methanol oxidation pathway^.^.^^^.^ Recent work has compared methanol oxidation processes at flat (1 11) and (100) surface planes and stepped (1 10) and high index surface plane^.^ Structurally well-defined surfaces allow systematic studies of surface stereochemical influences, and the stepped surfaces also serve as models for practical catalyst^,'^-'^ which typically contain a high defect density. In the case of methanol oxidation, voltammetric and chronoamperometric studies at single crystal platinum electrodes have shown that the fastest reaction rates occur at clean, highly corrugated surfaces but that these surfaces rapidly deactivate through poisoning mechanism^.^ At least for the low index surface planes of platinum, methanolic surface deactivation processes have been examined in greater detail by using spectroscopic methods. In particular, infrared spectroscopy has been an important tool for investigating methanol surface electrochemistry under reaction conditions (cf. refs 2, 7, 8, and 15). These experiments detect the C-0 stretching vibrational features of adsorbed CO intermediates. Spectra indicate the onset potential for methanolic CO poison formation, the coordination environment of the poison (e.g., terminal or bridging), and the extent of CO aggregation (island formation). At Pt( 11 l), Pt( loo), and Pt( 110) electrodes, infrared spectroscopy has been applied to probe methanol surface electrochemistry in aqueous electrolyte solutions.2~7~s~16-20 The methanolic...