Density Functional Theory Study of Methanol Steam Reforming on Co(0001) and Co(111) Surfaces

We report a periodic density functional theory (DFT) study of the methanol steam reforming (MSR) reaction on the Co(0001) and Co(111) surfaces. Thermochemistry and activation barriers for all elementary steps of two commonly accepted mechanisms, CH₂O decomposition and H₂COOH formation, were calculated along with the water gas shift (WGS) reaction. The adsorption energies on Co(0001) and Co(111) are within 0.05 eV for all the MSR intermediates examined, which suggests the same catalytic activity for both surfaces. On the basis of both the thermochemistry and barriers, CH₂O decomposition into CHO and CO is favored over H₂COOH formation on the Co(0001) surface. The strong CO binding on Co(0001) limits its WGS activity to convert CO into CO₂. Our results of the MSR and WGS pathways suggest that Co will not show high selectivity toward CO₂ for MSR, which matches the limited experimental data available. A simple Langmuir equilibrium model was applied to study the surface coverages on Co. The results show that O* and OH* coverages on Co are higher than on other transition metals such as Pt, Pd, and Cu due to the facile H₂O activation on the surface, and reaction steps involving O–H bond breaking and forming may be facilitated by O* and OH*. The results also suggest that Co is more susceptible than other transition metals to oxide formation under steam reforming conditions, especially under high water to alcohol ratios

Thermal Fluctuations in the Structure of Naturally Chiral Pt Surfaces

The intrinsic chirality of metal surfaces with kinked steps (e.g. Pt(643)) endows them with enantiospecific adsorption properties (D. S. Shell, Langmuir, 14, 1998, 862). To understand these properties quantitatively the impact of thermally-driven step wandering must be assessed. The authors derive a lattice-gas model of step motion on Pt(111) surfaces using diffusion barriers from Density Functional Theory. This model is used to examine thermal fluctuations of straight and kinked steps

Facet Dependence of CO₂ Reduction Paths on Cu Electrodes

Experimental results have shown that CO₂ electroreduction is sensitive to the surface morphology of Cu electrodes. We used density functional theory (DFT) to evaluate the thermodynamics and kinetics of CO₂ reduction pathways on Cu(100) and Cu(111) with the aim of understanding the experimentally reported differences in CO₂ reduction products. Results suggest that the hydrogenation of CO* to hydroxymethylidyne (COH*) or formyl (CHO*) is a key selective step. Cu(111) favors COH* formation, through which methane and ethylene are produced via a common CH₂ species under high overpotential (<−0.8 V vs RHE). On Cu(100), formation of CHO* is preferred and ethylene formation goes through C–C coupling of two CHO* species followed by a series of reduction steps of the C₂ intermediates, under relatively lower overpotential (−0.4 to −0.6 V vs RHE). Further reduction of these C₂ intermediates, however, require larger potentials (∼−1.0 V vs RHE) and conflicts with the experimentally observed low potential pathway to C₂ products on Cu(100). Calculations show that the presence of (111) step sites on the flat (100) terrace can reduce the overpotential for C₂ production on the Cu electrode, which may be present on Cu(100) due to reconstruction. On Cu(100), a change in CO* coverage from low to high with increasing negative applied potential can trigger a switch from ethylene/ethanol to methane/ethylene as the reduction products by affecting the relative stability of CHO* and COH*

CO Poisoning Effects on FeNC and CN_<i>x</i> ORR Catalysts: A Combined Experimental–Computational Study

CO poisoning as a probe for oxygen reduction reaction (ORR) active sites was examined on both iron–nitrogen coordinated catalysts supported on carbon (FeNC) and nitrogen-doped carbon nanostructures (CN_<i>x</i>). Rotating disk electrode (RDE) experiments show a partial decrease in the ORR activity of FeNC catalysts after CO exposure and no decrease for CN_<i>x</i> catalysts. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy experiments after exposure to CO show a peak at 2033 cm^–1 (associated with linearly bound CO) on the FeNC catalysts and no peaks associated with adsorbed CO on the CN_<i>x</i> catalyst surface. Density functional theory (DFT) calculations incorporating dispersion interactions of the adsorption energy of O₂ and CO were performed on a total of 16 proposed active sites for the FeNC and CN_<i>x</i> catalysts. On CN_<i>x</i>, all the sites show weak CO adsorption and only O₂ molecules are expected to adsorb, which matches the experimental observation of no poisoning. Several FeNC sites show CO binding energies similar in strength to that seen for Pt(111), but only two sites, namely FeN₄/C pyridinic and FeN₄/C pyrrolic, are found to bind CO stronger than O₂. DFT results suggest that the partial poisoning (instead of complete poisoning as found on Pt catalysts) observed in experiments is due to only some fraction of the active sites being blocked. DFT-derived CO stretch frequencies on FeNC show a similar redshift as observed in the DRIFTS experiments, which further confirms that the FeNC catalysts adsorb CO while the CN_<i>x</i> catalysts only show weak physisorption