Reduction of Nitric Oxide by Acetylene on Ir Surfaces with Different Morphologies: Comparison with Reduction of NO by CO

Reduction of nitric oxide (NO) by acetylene (C₂H₂) has been investigated by temperature-programmed desorption (TPD) on planar Ir(210) and faceted Ir(210) with tunable sizes of three-sided nanopyramids exposing (311), (311̅), and (110) faces. Upon adsorption, C₂H₂ dissociates to form acetylide (CCH) and H species on the Ir surfaces at low C₂H₂ precoverage. For adsorption of NO on C₂H₂-covered Ir, both planar and faceted Ir(210) exhibit high reactivity for reduction of NO with high selectivity to N₂ at low C₂H₂ precoverage, although the reaction is completely inhibited at high C₂H₂ precoverage. Coadsorbed C₂H₂ significantly influences dissociation of NO. The N-, H-, and C-containing TPD products are dominated by N₂, H₂, CO, and CO₂ together with small amounts of H₂O. For adsorption of NO on C-covered Ir(210) at fractional C precoverage, formation of CO₂ is promoted while production of CO is reduced. Reduction of NO by C₂H₂ is structure sensitive on faceted Ir(210) versus planar Ir(210), but no evidence is found for size effects in the reduction of NO by C₂H₂ on faceted Ir(210) for average facet sizes of 5 nm and 14 nm. The results are compared with reduction of NO by CO on the same Ir surfaces. As for NO+C₂H₂, the Ir surfaces are very active for reduction of NO by CO with high selectivity to N₂ and the reaction is structure sensitive, but clear evidence is found for size effects in the reduction of NO by CO on the nanometer scale. Furthermore, coadsorbed CO does not affect dissociation of NO at low CO precoverage whereas coadsorbed CO considerably influences dissociation of NO at high CO precoverage. The adsorption sites of CCH+H on Ir are characterized by density functional theory

Surface Stability of Pt₃Ni Nanoparticulate Alloy Electrocatalysts in Hydrogen Adsorption

Nanoparticles of Pt/Ni alloys represent state of the art electrocatalysts for fuel cell reactions. Density functional theory (DFT) based calculations along with in situ X-ray absorption spectroscopy (XAS) data show that the surface structure of Pt₃Ni nanoparticulate alloys is potential-dependent during electrocatalytic reactions. Pt₃Ni based electrocatalysts demonstrate preferential confinement of Ni to the subsurface when the electrode is polarized in the double layer region where the surface is free of specifically adsorbed species. Hydrogen adsorption triggers nickel segregation to the surface. This process is facilitated by a high local surface coverage of adsorbed hydrogen in the vicinity of the surface confined Ni due to an uneven distribution of the adsorbate(s) on the catalyst’s surface. The adsorption triggered surface segregation shows a non-monotonous dependence on the electrode potential and can be identified as a breathing of the catalyst as was proposed previously. The observed breathing behavior is relatively fast and proceeds on a time scale of 100–1000 s

Ultrasmall CoO(OH)_<i>x</i> Nanoparticles As a Highly Efficient “True” Cocatalyst in Porous Photoanodes for Water Splitting

The coupling of light absorbers to cocatalysts with well-designed optical and catalytic properties is of fundamental importance for the development of efficient photoelectrocatalytic devices for solar-driven water splitting. We achieved an effective loading of visible-light-active porous hybrid photoanodes for water photooxidation with ultrasmall (∼1–2 nm), highly disordered CoO­(OH)_<i>x</i> nanoparticles using a two-step impregnation method. Under visible light (λ > 420 nm) irradiation, the resulting photoanodes significantly outperformed photoanodes loaded with conventional cobalt-based cocatalyst (Co-Pi) comprising larger nanoparticles (∼5 nm) in terms of both Faradaic efficiency of oxygen evolution (by the factor of 2) and performance stability under long-term irradiation. A combination of STEM, XAS, cyclic voltammetry, and photoelectrochemical techniques was used to elucidate the advantages of using ultrasmall CoO­(OH)_<i>x</i> nanoparticles as cocatalysts. Specifically, due to the high transparency of ultrasmall CoO­(OH)_<i>x</i> nanoparticles in the visible range, higher loading of porous photoanodes with cobalt catalytic sites can be achieved, while the photocurrent losses due to parasitic light absorption by the cocatalyst are minimized. Notably, a significant enhancement in stability of ultrasmall CoO­(OH)_<i>x</i> nanoparticles in borate electrolytes as compared to phosphate electrolytes has been observed. EXAFS data recorded before and after photoelectrocatalysis indicated that the effect of the electrolyte on the stability can be explained by the difference in structural ordering dictated by different interaction of the electrolyte anions with cobalt ions, as corroborated by DFT calculations. This study highlights the strong impact of structural and optical properties of cocatalysts as well as the strong influence of the electrolyte composition on the activity and stability of photoelectrocatalytic systems comprising transition metal oxide electrocatalysts

Direct Formation of C–C Double-Bonded Structural Motifs by On-Surface Dehalogenative Homocoupling of <i>gem</i>-Dibromomethyl Molecules

Conductive polymers are of great importance in a variety of chemistry-related disciplines and applications. The recently developed bottom-up on-surface synthesis strategy provides us with opportunities for the fabrication of various nanostructures in a flexible and facile manner, which could be investigated by high-resolution microscopic techniques in real space. Herein, we designed and synthesized molecular precursors functionalized with benzal <i>gem</i>-dibromomethyl groups. A combination of scanning tunneling microscopy, noncontact atomic force microscopy, high-resolution synchrotron radiation photoemission spectroscopy, and density functional theory calculations demonstrated that it is feasible to achieve the direct formation of C–C double-bonded structural motifs <i>via</i> on-surface dehalogenative homocoupling reactions on the Au(111) surface. Correspondingly, we convert the sp³-hybridized state to an sp²-hybridized state of carbon atoms, <i>i</i>.<i>e</i>., from an alkyl group to an alkenyl one. Moreover, by such a bottom-up strategy, we have successfully fabricated poly­(phenylenevinylene) chains on the surface, which is anticipated to inspire further studies toward understanding the nature of conductive polymers at the atomic scale