Pseudo-single crystal electrochemistry on polycrystalline electrodes : visualizing activity at grains and grain boundaries on platinum for the Fe2+/Fe3+ redox reaction

The influence of electrode surface structure on electrochemical reaction rates and mechanisms is a major theme in electrochemical research, especially as electrodes with inherent structural heterogeneities are used ubiquitously. Yet, probing local electrochemistry and surface structure at complex surfaces is challenging. In this paper, high spatial resolution scanning electrochemical cell microscopy (SECCM) complemented with electron backscatter diffraction (EBSD) is demonstrated as a means of performing ‘pseudo-single-crystal’ electrochemical measurements at individual grains of a polycrystalline platinum electrode, while also allowing grain boundaries to be probed. Using the Fe2+/3+ couple as an illustrative case, a strong correlation is found between local surface structure and electrochemical activity. Variations in electrochemical activity for individual high index grains, visualized in a weakly adsorbing perchlorate medium, show that there is higher activity on grains with a significant (101) orientation contribution, compared to those with (001) and (111) contribution, consistent with findings on single-crystal electrodes. Interestingly, for Fe2+ oxidation in a sulfate medium a different pattern of activity emerges. Here, SECCM reveals only minor variations in activity between individual grains, again consistent with single-crystal studies, with a greatly enhanced activity at grain boundaries. This suggests that these sites may contribute significantly to the overall electrochemical behavior measured on the macroscale

In situ Investigations of Structure-Activity Relationships of a Cu/ZrO2Cu/ZrO_2 Catalyst for the Steam Reforming of Methanol

Structure–activity relationships of a nanostructured Cu/ZrO2 catalyst for the steam reforming of methanol (MSR) were investigated under reaction conditions by in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) combined with on-line mass spectrometry (MS). Temperature-programmed activation by reduction in hydrogen or by reduction in a mixture of methanol and water (feed) was studied by time-resolved Cu K edge XANES and TG/DSC/MS measurements. Small and disordered CuO particles were identified as the main copper phase present in the precursors. After extended time on stream and treatment at 673 K in hydrogen, no significant sintering of the copper particles or deactivation of the reduced Cu/ZrO2 catalysts was detected, indicating a superior stability of the material. The initially low steam-reforming activity of the Cu/ZrO2 catalyst after reduction in hydrogen could be significantly increased by a temporary addition of oxygen to the feed. This increased activity after oxidative treatment is correlated with an increasing amount of oxygen in the copper particles. 63Cu NMR studies detected only a minor degree of microstrain in the active copper phase of the Cu/ZrO2 catalyst. The decreased reducibility of CuO/ZrO2, the low degree of microstrain, and the correlation between the amount of oxygen remaining in the copper particles and the catalytic activity indicate a different metal support interaction compared with Cu/ZnO catalysts

Catalyzed SnO₂ Thin Films: Theoretical and Experimental Insights into Fabrication and Electrocatalytic Properties

SnO₂ thin films are studied experimentally and from first-principles as model supports for Pt nanoparticle catalysts in an acidic environment. SnO₂ thin film supports are attractive model systems because composition, microstructure, and surface termination can be tailored by varying the deposition parameters. SnO₂ films are synthesized by reactive dc magnetron sputtering, and the effects of the deposition conditions on the physicochemical and electrochemical properties are investigated experimentally and theoretically. Variation of the deposition conditions results in limited long-range order SnO or SnO₂ films. Annealing in either case leads to well-crystallized SnO₂ films, but with different growth directions. Films deposited as SnO₂ show only growth along the [110] direction, while SnO₂ films formed from deposited SnO show no preferred orientations. Hybrid density functional theory (DFT) suggests that growth along the [110] direction is driven by (110) being the lowest energy surface, while the loss of orientation in the SnO derived films originates from an almost degenerate set of surface energies at the SnO|SnO₂ equilibrium. The oxygen reduction reaction activity of Pt nanoparticles depends on the SnO₂ film orientation. A 2-fold higher catalytic activity is observed for Pt nanoparticles on the SnO₂ film without preferential orientation compared to Pt on SnO₂ grown along the [110] direction, pointing to the presence of strong surface-dependent metal–support interaction