Adsorption and Decomposition of Acetylene on Planar and Faceted Ir(210)

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 (C(2)H(2)) 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), (311[overline]), and (110) faces. Upon adsorption, C(2)H(2) dissociates to form acetylide (CCH) and H species on the Ir surfaces at low C(2)H(2) precoverage. For adsorption of NO on C(2)H(2)-covered Ir, both planar and faceted Ir(210) exhibit high reactivity for reduction of NO with high selectivity to N(2) at low C(2)H(2) precoverage, although the reaction is completely inhibited at high C(2)H(2) precoverage. Coadsorbed C(2)H(2) significantly influences dissociation of NO. The N-, H-, and C-containing TPD products are dominated by N(2), H(2), CO, and CO(2) together with small amounts of H(2)O. For adsorption of NO on C-covered Ir(210) at fractional C precoverage, formation of CO(2) is promoted while production of CO is reduced. Reduction of NO by C(2)H(2) 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 C(2)H(2) 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+C(2)H(2), the Ir surfaces are very active for reduction of NO by CO with high selectivity to N(2) 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.

Reduction of NO by CO on unsupported Ir: bridging the materials gap

Temperature programmed desorption (TPD) and density functional theory (DFT) are used to investigate adsorption sites and reaction of coadsorbed NO and CO on planar Ir(210) and faceted Ir(210) with tailored sizes of three-sided nanopyramids exposing (311), (31 1) and (110) faces. Both planar and faceted Ir(210) are highly active for reduction of NO by CO with high selectivity to N(2), which is accompanied by simultaneous oxidation of CO. Evidence is found for structure sensitivity in adsorption sites and reaction of coadsorbed NO and CO on faceted Ir(210) versus planar Ir(210). Strong interaction between NO and CO at high NO exposure and one-monolayer CO pre-coverage results in "explosive" evolution of N(2) and CO(2) on planar Ir(210) and size effects in reduction of NO by CO on faceted Ir(210) for average facet size ranging from 5 to 14 nm without change in facet structure.

First-principles studies on oxygen-induced faceting of Ir(210)

Density functional theory calculations were performed to obtain an atomistic understanding of facet formation on Ir(210). We determined geometries and energetics of clean and oxygen-covered surfaces of planar Ir(210) as well as Ir(311) and two types of Ir(110) surfaces, which are involved in faceting by forming three-sided nanopyramids. Using the energies together with the ab initio atomistic thermodynamics approach, we studied the stability of substrate and facets in the presence of an oxygen environment. Our results show that facets are stable over the entire temperature range at which oxygen is adsorbed on the surface at coverages >/=0.45 physical ML, supporting the picture of a thermodynamic driving force. We also investigated the dependence of the phase diagram on the choice of the exchange-correlation functional and obtained qualitatively the same behavior. Finally, this work helps to better understand reactivity and selectivity of O-covered planar and faceted Ir surfaces in catalysis.

Facet stability in oxygen-induced nanofaceting of Re(1231)

The stability of the various facets in oxygen-induced faceting of Re(1231) has been studied by low-energy electron diffraction, scanning tunneling microcopy, and synchrotron-based high-resolution X-ray photoemission spectroscopy. When Re(1231) is annealed at 800-1200 K in oxygen (10(-7) Torr), the surface becomes completely covered with nanometer-scale facets, and its morphology depends on the substrate temperature and oxygen exposure. Especially, the (1121) facet competes with the (1011) facet in determining the surface morphology, and the stability of each facet relies on oxygen coverage. Using density functional theory, the O-Re binding energies on the facets for various oxygen concentrations are calculated to explain how the oxygen coverage affects the anisotropy of surface free energy, which in turn determines the morphology of the faceted surface.

Decomposition of Ammonia and Hydrogen on Ir Surfaces: Structure Sensitivity and Nanometer-Scale Size Effects

The adsorption and decomposition of ammonia and hydrogen have been studied on surfaces of clean planar Ir(210) and clean nanoscale-faceted Ir(210), which are prepared from the same crystal in situ. We find evidence for structure sensitivity in recombination and desorption of H2 and in thermal decomposition of NH3 on clean planar Ir(210) versus clean faceted Ir(210). Moreover, the decomposition kinetics of NH3 on faceted Ir(210) exhibit size effects on the nanometer scale, which is the first observation of size effects in surface chemistry on an unsupported monometallic catalyst with controlled and well-defined structure and size.