The aim of the present work is to provide a deeper understanding of gold catalysis for CO electrooxidation in alkaline media, through a combined electrochemical, spectroscopic, and DFT study. Voltammetric and spectroscopic measurements evidence that the amount of CO irreversibly adsorbed on gold increases as the adsorption potential becomes more negative (vs SHE). This explains why higher CO coverages can be achieved in more alkaline solutions, since the value of adsorption potential vs RHE becomes more negative vs SHE with increasing pH. On the other hand, the combination of FTIRRAS experiments and DFT calculations shows that the adsorption site of irreversibly adsorbed CO on Au(111) depends on the value of the adsorption potential. It is concluded that CO adsorption on top sites takes place at all studied potentials, and hollow and bridge sites also become occupied for adsorption potentials lower and higher than 0 V vs RHE, respectively. However, it should be noted that our DFT calculations give values of the CO binding energies that are not strong enough to explain CO irreversible adsorption. This may be partly attributed to the fact that OH coadsorption is not included in the calculations. Indeed, this work presents two experimental facts that suggest that CO adsorption on gold promotes the coadsorption of OH species: (i) CO irreversibly adsorbed on Au(111) and Au(100) leads to an unusual voltammetric feature, whose charge indicates the stabilization of one OH species per adsorbed CO species; (ii) the apparent transfer coefficient of this unusual state is close to unity, suggesting that it is due to a presumed structural transformation coupled to OH adsorption. Finally, the effect of the adsorption potential on the bulk CO electrooxidation is also studied. It is found that, on Au(111), an increased occupation of CO on multifold (hollow) sites seems to result in a less efficient catalysis. However, on Au(110), an increased coverage of CO on top sites does not produce any significant change in catalysis.

The catalytic activity of gold towards the oxidation of carbon monoxide in the gas phase, the liquid phase, and electrochemically, has attracted substantial interest. [1][2][3][4][5][6][7][8][9][10][11][12] The mechanism for the remarkable activity of gold in CO oxidation, especially when dispersed as nanoparticles on an oxidic support, is still a subject of much debate. The high activity of gold for the oxidation of CO in the aqueous phase, and particularly in alkaline media, [4] was originally reported in the electrochemistry literature. [5,6] Gold is the most active electrode material for oxidizing CO dissolved in an aqueous solution, and is superior to, for example, platinum. [6] We have previously reported that CO may be adsorbed irreversibly on single-crystalline gold electrodes in alkaline media; on Au(111) and Au (100)- (5 20) surfaces, the CO adsorption was accompanied by an unusual reversible peak in the cyclic voltammogram at circa 0.4 V versus RHE (RHE = reversible hydrogen electrode).[12] We ascribed this peak to the enhanced co-adsorption of OH onto the CO-covered gold surface. Such an enhancement of OH adsorption onto the gold surface by adsorbed CO would still be advantageous for the oxidation of CO in solution, even if the irreversibly adsorbed CO may only be oxidized off the surface at relatively high potentials (circa 0.8 V). Indeed, the COpromoted adsorption of OH may lead to the oxidation of CO in solution. For such a model, a simplified rate equation for the CO oxidation current on a gold surface would be:where two electrons are transferred when CO is oxidized to CO 2 , F is the Faraday constant, k is the oxidation rate constant, q CO* is the coverage of oxidizable CO* molecules adsorbed on the surface when CO is in solution (as opposed to, and in addition to, the more strongly irreversibly adsorbed CO molecules that are oxidized at higher potential when there is no CO in solution), and q OH is the OH coverage on the surface, all at a fixed applied potential. If the OH adsorption energy is enhanced by the presence of CO on the surface by a linear dependence on the CO coverage, q OH could be expressed as:where q CO is now the total CO coverage, although we do not really know how to distinguish in this model between strongly (irreversibly) and weakly adsorbed CO molecules (only when CO is in solution). If both q CO* and q CO depend on the concentration of CO in solution (c CO ), then it is straightforward to show that the reaction order r in c CO is given by:Therefore, this model predicts a reaction order in c CO of r > 1 if g > 0; g > 0 expresses the idea that CO promotes the adsorption of its own oxidant (OH), and thereby possibly enhancing its own oxidation. Herein, we provide support for this self-promotion mechanism using DFT calculations and the experimental determination of the reaction order in CO. The binding energies of OH and CO on a clean Au(111) surface in a 3 3 unit cell (0.11 monolayer) are given in Table 1. The binding energy for OH "atop" (À2.32 eV) is presented for comparison...

In this work, a systematic study on the adsorption of atomic and molecular hydrogen and carbon oxides on cubic (001) and hexagonal (0001) WC surfaces by periodical density functional theory is reported. Calculations have been performed by employing the Perdew–Burke–Ernzerhof exchange correlation functional with van der Waals corrections to account for the dispersive force term. In addition, dipole corrections were applied for W- and C-terminated hexagonal WC(0001) surfaces. Good agreement is found between calculated and reported data for representative bulk properties. Regarding surface properties, our results indicate that atomic hydrogen adsorbs quite strongly while H2 does, in general, dissociatively on the studied surfaces, with very small energy barriers (<0.35 eV) for the cleavage of the H–H bonds. The C sites of the carbide play an essential role in the binding of H atoms and the cleavage of H–H bonds. Studies examining the interaction of tungsten carbide with CO and CO2 also evidence the importance of C sites. The reactivity of C- and W-terminated (0001) hexagonal WC surfaces significantly differs. Atomic hydrogen, carbon monoxide, and CO2 are more stable on a C- than on a W-terminated surface, and only this latter termination is able to cleave spontaneously a C–O bond of the CO2 molecule. This difference in reactivity may open a number of possibilities for fine-tuning the selectivity of the resulting material or designing compounds catalytically active for specific reactions by carefully adjusting the proportion of C, W, and mixed terminations during the synthesis procedure.

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The catalytic activity of gold towards the oxidation of carbon monoxide in the gas phase, the liquid phase, and electrochemically, has attracted substantial interest. [1][2][3][4][5][6][7][8][9][10][11][12] The mechanism for the remarkable activity of gold in CO oxidation, especially when dispersed as nanoparticles on an oxidic support, is still a subject of much debate. The high activity of gold for the oxidation of CO in the aqueous phase, and particularly in alkaline media, [4] was originally reported in the electrochemistry literature. [5,6] Gold is the most active electrode material for oxidizing CO dissolved in an aqueous solution, and is superior to, for example, platinum. [6] We have previously reported that CO may be adsorbed irreversibly on single-crystalline gold electrodes in alkaline media; on Au(111) and Au (100)- (5 20) surfaces, the CO adsorption was accompanied by an unusual reversible peak in the cyclic voltammogram at circa 0.4 V versus RHE (RHE = reversible hydrogen electrode).[12] We ascribed this peak to the enhanced co-adsorption of OH onto the CO-covered gold surface. Such an enhancement of OH adsorption onto the gold surface by adsorbed CO would still be advantageous for the oxidation of CO in solution, even if the irreversibly adsorbed CO may only be oxidized off the surface at relatively high potentials (circa 0.8 V). Indeed, the COpromoted adsorption of OH may lead to the oxidation of CO in solution. For such a model, a simplified rate equation for the CO oxidation current on a gold surface would be:where two electrons are transferred when CO is oxidized to CO 2 , F is the Faraday constant, k is the oxidation rate constant, q CO* is the coverage of oxidizable CO* molecules adsorbed on the surface when CO is in solution (as opposed to, and in addition to, the more strongly irreversibly adsorbed CO molecules that are oxidized at higher potential when there is no CO in solution), and q OH is the OH coverage on the surface, all at a fixed applied potential. If the OH adsorption energy is enhanced by the presence of CO on the surface by a linear dependence on the CO coverage, q OH could be expressed as:where q CO is now the total CO coverage, although we do not really know how to distinguish in this model between strongly (irreversibly) and weakly adsorbed CO molecules (only when CO is in solution). If both q CO* and q CO depend on the concentration of CO in solution (c CO ), then it is straightforward to show that the reaction order r in c CO is given by:Therefore, this model predicts a reaction order in c CO of r > 1 if g > 0; g > 0 expresses the idea that CO promotes the adsorption of its own oxidant (OH), and thereby possibly enhancing its own oxidation. Herein, we provide support for this self-promotion mechanism using DFT calculations and the experimental determination of the reaction order in CO. The binding energies of OH and CO on a clean Au(111) surface in a 3 3 unit cell (0.11 monolayer) are given in Table 1. The binding energy for OH "atop" (À2.32 eV) is presented for comparison...