Infrared Spectroscopic Detection of CO Formed at Step and Terrace Sites on a Corrugated Electrode Surface Plane during Methanol Oxidation

Abstract

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...

The role of the surface structure in the oxidation mechanism of methanol

“…From the previous studies of methanol oxidation on various Pt stepped surfaces, the results seem to be quite controversial when the role of the presence of different steps on (1 1 1) terraces and its effect is discussed [15][16][17]. So the point of our work is to make clear the influence of different steps and the step density on Pt(1 1 1) and (1 0 0) surfaces for the methanol oxidation reaction.…”

Electrocatalytic Oxidation and Adsorption Rate of Methanol at Pt Stepped Single‐Crystal Electrodes and Effect of Ru Step Decoration: A DEMS Study

“…Wang et al20 compared the potentiodynamic oxidation of methanol at smooth polycrystalline platinum, Pt(111), and Pt(332) electrodes and found that methanol oxidation is enhanced with increasing step density without an appreciable increase of the CO 2 current efficiency; hence, they concluded that the defect sites enhance both parallel pathways of methanol oxidation. Shin and Korzeniewski21 suggested that an increase of the step density catalyzes methanol decomposition and the methanolic CO formation is inhibited on Pt(111) at the potential of the hydrogen adsorption. Tripkovic and Popovic22 reported that the initial surface activity for methanol oxidation on stepped electrodes decreases with increasing step density whereas Housmans et al7 found that the overall oxidation rate of methanol increases with increasing step density.…”

Electroanalytical Measurements at Electrodes Modified with Metal Nanoparticles

“…Because of a higher density of step and kink sites, a surface plane with a high Miller index generally has a greater catalytic activity than one with a low Miller index [81,82] . A challenge in optimizing the geometry of NPs for catalysis is that particles with high -index facets are diffi cult to synthesize.…”