Mechanisms of Carbon Monoxide and Methanol Oxidation at Single-crystal Electrodes

Abstract

The electrochemical oxidation of carbon monoxide and methanol on single-crystal noble metal electrodes has been studied using cyclic voltammetry, chronoamperometry, in situ FTIR spectroscopy, online electrochemical mass spectrometry, and theoretical methods. The oxidation of CO was found to be enhanced by steps and defects. Furthermore, the surface diffusion rate was found to have a significant influence on the kinetics of the oxidation process: for high diffusion rates, such as the oxidation of CO on platinum, the kinetics can be described by a mean field model, while for low diffusion rates, such as CO oxidation on rhodium in sulfuric acid, a nucleationand-growth model was found to be more suitable. Voltammetric and mass spectrometric measurements on the oxidation of methanol on platinum indicate that steps enhance the overall reaction rate. In general, the selectivity towards the direct oxidation pathway through soluble intermediates was found to be higher in the absence of strongly adsorbing anions. In both perchloric and sulfuric acid, this selectivity was also found to increase with increasing step density. In sulfuric acid, Pt(111) shows the highest relative contribution for the direct pathway of all surfaces studied in that electrolyte. Based on these results, a detailed reaction scheme for the electrochemical oxidation of methanol is presented.

Bifunctional anode catalysts for direct methanol fuel cells

“…Next, the single crystal was transferred to the de-aerated CuSO 4 and H 2 SO 4 solution. The Cu UPD profile of the crystal was first measured by cycling the crystal between potentials of 0.2 V and 0.45 V at 5 mV s À1 .…”

“…Steps and defects on Pt are known to decrease the potential for CO oxidation and thereby also help the MOR. 4 Surfaces with relatively weak binding of CO (such as the group 11 metals), however, are not generally able to activate methanol sufficiently. Alloy catalysts have been proposed to overcome these technical issues.…”

Structural Effects on Methanol Oxidation on Single Crystal Electrodes of Palladium

“…Therefore, the MOR has been also studied in HClO 4 solution in which perchloric anion is not adsorbed on Pt electrodes as strongly as (bi)sulfate anions. Some papers report the order of the activity for the MOR: Pt(111) < Pt(110) < Pt(100), 3,5,6 whereas Wieckowski et al report the activity series: Pt(111) < Pt(100) ¹ Pt(110).…”

ATR-SEIRAS study of CO adsorption and oxidation on Rh modified Au(111-25 nm) film electrodes in 0.1 M H2SO4

“…Gómez et al [16] studied the role of surface heterogeneities (ordered steps and defects) on the oxidation of CO adsorbed at Rh (111) electrodes and found that the more ordered is the Rh(111) surface, the slower the reaction proceeds. Housmanns et al investigated CO electro-oxidation on "ordered" and "disordered" Rh[n(111)-(111)] single crystal electrodes [17][18][19][20][21] and proposed that the CO electro-oxidation follows a Langmuir-Hinshelwood type nucleation and growth mechanism controlled by the slow surface mobility of adsorbed CO. Anions were also shown to play an important role in the CO oxidation mechanism on Rh electrodes [19]. In our electrochemical study of the three low-index Rh single crystal surfaces [22] we demonstrated that the reaction pathway of CO oxidation on all three low-index Rh surfaces proceeds according to the Langmuir-Hinshelwood mechanism and is controlled by the formation of OH ads at steps and defect sites, followed by a complex growth process on terrace sites.…”