Tuning of the Copper-Zirconia Phase Boundary for Selectivity Control of Methanol Conversion

Chemical-vapor deposition (CVD) of a Zr(O-tBu)4 precursor on different Cu substrates was used to prepare model systems for ZrOxHy-Cu catalysts and to test their reactivity and selectivity in methanol steam reforming (MSR). A partially hydroxylated and initially fully oxidized submonolayer ZrOxHy surface species results, exhibiting a pronounced catalytic synergism between the ZrOxHy overlayer and Cu only with respect to partial methanol dehydrogenation to formaldehyde. Thus, it differs strongly from in situ grown ZrOxHy layers on Cu formed from an initially bimetallic mixed Zr/ZrOx state under MSR conditions. CVD-grown Zr-OH groups are not stable under MSR conditions; thus reversible in situ hydroxylation and water-activating reaction channels are suppressed. Comparison of the two model systems indicates that only a dedicated Cu-ZrOxHy interface with in situ formed and reversibly hydroxylated sites (accessible only from initially (inter)metallic Cu/Zr species at the surface) leads to water activation, total oxidation of intermediate formaldehyde, and enhanced CO2 selectivity.(VLID)1371551Accepted versio

Selective oxidation of methanol to form dimethoxymethane and methyl formate over a monolayer V2O5 TiO2 catalyst

The oxidation of methanol over highly dispersed vanadia supported on TiO2 (anatase) has been investigated using in situ Fourier transform infrared spectroscopy (FTIR), near ambient pressure X-ray photoelectron spectroscopy (NAP XPS), X-ray absorption near-edge structure (XANES), and a temperature-programmed reaction technique. The data were complemented by kinetic measurements collected in a flow reactor. It was found that dimethoxymethane competes with methyl formate at low temperatures, while the production of formaldehyde is greatly inhibited. Under the reaction conditions, the FTIR spectra show the presence of non-dissociatively adsorbed molecules of methanol, in addition to adsorbed methoxy, dioxymethylene, and formate species. According to the NAP XPS and XANES data, the reaction involves a reversible reduction of V5+ cations, indicating that the vanadia lattice oxygen participates in the oxidation of methanol via the classical Mars-van Krevelen mechanism. A detailed mechanism for the oxidation of methanol on vanadia catalysts is discussed. (C) 2013 Elsevier Inc. All rights reserved

Near Ambient Pressure X ray Photoelectron Spectroscopy Study of Methane Induced Carbon Deposition on Clean and Copper Modified Polycrystalline Nickel Materials

In order to simulate solid-oxide fuel cell (SOFC)-related coking mechanisms of Ni, methane-induced surface carbide and carbon growth was studied under close-to-real conditions by synchrotron-based near-ambient-pressure (NAP) X-ray photoelectron spectroscopy (XPS) in the temperature region between 250 and 600 °C. Two complementary polycrystalline Ni samples were used, namely, Ni foam—serving as a model structure for bulk Ni in cermet materials such as Ni/YSZ—and Ni foil. The growth mechanism of graphene/graphite species was found to be closely related to that previously described for ethylene-induced graphene growth on Ni(111). After a sufficiently long “incubation” period of the Ni foam in methane at 0.2 mbar and temperatures around 400 °C, cooling down to ∼250 °C, and keeping the sample at this temperature for 50–60 min, initial formation of a near-surface carbide phase was observed, which exhibited the same spectroscopic fingerprint as the C₂H₄ induced Ni₂C phase on Ni(111). Only in the presence of this carbidic species, subsequent graphene/graphite nucleation and growth was observed. Vice versa, the absence of this species excluded further graphene/graphite formation. At temperatures above 400 °C, decomposition/bulk dissolution of the graphene/graphite phase was observed on the rather “open” surface of the Ni foam. In contrast, Ni foil showed—under otherwise identical conditions—predominant formation of unreactive amorphous carbon, which can only be removed at ≥500 °C by oxidative clean-off. Moreover, the complete suppression of carbide and subsequent graphene/graphite formation by Cu-alloying of the Ni foam and by addition of water to the methane atmosphere was verified