Search

We report on theoretical and experimental studies of the reactivity of ethylene with oxygen in two well-known oxygen induced surface reconstructions on silver, the p(2 × 1) reconstruction on the Ag(110) surface and the p(4 × 4) reconstruction on the Ag(111) surface. Density functional theory calculations demonstrate that ethylene can react with oxygen on both surfaces to form an oxametallacycle that can decompose into either ethylene oxide or a CO2 precursor, acetaldehyde. The activation energy associated with acetaldehyde formation is predicted to be 0.4 eV lower than that associated with epoxide formation on both surfaces, though we find lower barriers for all elementary steps on the p(4 × 4) reconstruction due to its unique structural dynamics. Our calculations predict these dynamics make the p(4 × 4) reconstruction active in acetaldehyde formation at room temperature. Experiments performed by exposing the p(4 × 4) reconstruction to ethylene at room temperature support this finding with CO2, the only carbonaceous product formed during temperature-programmed desorption. Our results unambiguously demonstrate that, alone, these oxygen reconstructions are not selective in ethylene epoxidation on silver. [ABSTRACT FROM AUTHOR]

Starting from subsurface Zr0-doped "inverse" Pd and bulk-intermetallic Pd0Zr0 model catalyst precursors, we investigated the dry reforming reaction of methane (DRM) using synchrotron-based near ambient pressure in-situ X-ray photoelectron spectroscopy (NAP-XPS), in-situ X-ray diffraction and catalytic testing in an ultrahigh-vacuum-compatible recirculating batch reactor cell. Both intermetallic precursors develop a Pd0–ZrO2 phase boundary under realistic DRM conditions, whereby the oxidative segregation of ZrO2 from bulk intermetallic PdxZry leads to a highly active composite layer of carbide-modified Pd0 metal nanoparticles in contact with tetragonal ZrO2. This active state exhibits reaction rates exceeding those of a conventional supported Pd–ZrO2 reference catalyst and its high activity is unambiguously linked to the fast conversion of the highly reactive carbidic/dissolved C-species inside Pd0 toward CO at the Pd/ZrO2 phase boundary, which serves the role of providing efficient CO2 activation sites. In contrast, the near-surface intermetallic precursor decomposes toward ZrO2 islands at the surface of a quasi-infinite Pd0 metal bulk. Strongly delayed Pd carbide accumulation and thus carbon resegregation under reaction conditions leads to a much less active interfacial ZrO2–Pd0 state. [ABSTRACT FROM AUTHOR]