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The paper is focused on evaluation of active centres and impact of adsorbed species on (10%Co+0.5%Pd) / Al2O3 catalyst system performance aiming selectivity optimization. Application of different sets of precursor pretreatment and reduction resulted in catalysts exhibiting high CO conversion or high methane selectivity. A sample of high selectivity was prepared by pretreatment in hydrogen and the performance was determined by lower amount of strongly adsorbed CO, strongly adsorbed carbonate species, and higher amount of reduced metal and bimetallic particles. A more active system was formed by pretreatment in air leading to larger amount of unreduced metal and CO-bridged species on the surface, stable coverage of hydroxyl groups on the support, and medium-strength sites for adsorption of carbonates. Ratios of hydrogen to carbon monoxide adsorption (H/CO) and of strongly to weakly adsorbed CO species appeared as important criteria for catalyst efficiency together with supported metal state, amount of unreduced ions, bimetallic particle formation, and alumina's ability to adsorb CO and CO2. [ABSTRACT FROM AUTHOR]

[Display omitted] • A theoretical database for predicting adsorbate-induced metal–metal bond scission. • Metal adatom formation is generally easier on (100) surfaces than (111) surfaces. • More open facets are required for adatom formation on intrinsically harder metals. • A new perspective for interpreting structure sensitivity in heterogeneous catalysis by metals. Recent surface science discoveries reveal that adsorbates may induce in situ sub-nanometer cluster formation on transition-metal surfaces. To elucidate the structure sensitivity behind this phenomenon, we performed density functional theory calculations to construct an adatom formation energy database for eight fcc metals and 26 adsorbates commonly involved in catalytic reactions. We show that the adatom formation on (100) surfaces is generally easier than that on (111) surfaces. Many adsorbate/metal pairs exist, mostly on Pd, Ni, Rh, Pt, and Ir, for which adsorbates might induce adatom formation under near-ambient conditions on the (100) facet, but not on (111), highlighting the role of more open facets in adatom formation on metals intrinsically harder than Ag, Au, and Cu. Our study offers a new perspective towards understanding structure sensitivity in heterogeneous thermal- and electro-catalytic systems such as methane steam reforming, Fischer-Tropsch synthesis, and ammonia decomposition. [ABSTRACT FROM AUTHOR]

Highlights: [•] IR spectra of CO adsorption are reported for both pressure and temperature as independent influences. [•] We outline the abnormal thermodynamic behavior of the so-called dual sites. [•] We evidence Silanol also interact and describe how SiOH impact CO adsorption. [•] Attributions of surface adsorbed species endorsed in the literature are called into question. [ABSTRACT FROM AUTHOR]

Highlights: [•] Steam reforming of toluene as a model compound of biomass tar, was investigated. [•] Ni/La0.7Sr0.3AlO3−δ catalyst showed higher activity and lower coke formation. [•] Surface adsorbate on steam reforming of toluene were observed using in situ IR. [•] The aromatic ring in toluene strongly adsorbed only on La0.7Sr0.3AlO3−δ . [•] The adsorbate reacted with steam at 673K on Ni/La0.7Sr0.3AlO3−δ . [ABSTRACT FROM AUTHOR]

Abstract: The reaction pathways for the photocatalytic oxidation of ethanol on the TiO2 surface at 30 °C were studied by in situ infrared (IR) spectroscopy. The coverage of ethanol and water was found to play a key role in how the reaction is initiated. The low ethanol coverage on the H2Oad-containing TiO2 surface produced adsorbed formate (HCOO− ad) as a primary intermediate; the high ethanol coverage on the H2Oad-deficient TiO2 surface produced adsorbed acetate (CH3COO− ad) as a major intermediate during the initial period (i.e., 2 min) of the photocatalytic oxidation. The adsorbed species and reaction products observed during in situ IR studies suggest the low-coverage ethanol reaction is initiated by •OH, whereas the high-coverage ethanol reaction is initiated by hole. The hole-initiating ethanol oxidation on the H2Oad-deficient TiO2 surface produced adsorbed acetic acid (CH3COOHad)/CH3COO− ad, and built up photogenerated electrons, giving a parallel increase in the IR intensity of CH3COO− ad and the IR background at 2000 cm−1 (i.e., a measure of photogenerated electrons). As the high-coverage ethanol reaction proceeded toward producing CO2/H2O, adsorbed H2O accumulated and the coverage of CH3CH2OHad/CH3CH2Oad decreased on the TiO2 surface, shifting the reaction from hole-initiating to •OH-initiating. [Copyright &y& Elsevier]

Abstract: Fourier Transform Infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to investigate the species adsorbed on TiO2 related to photocatalytic activity. Dichloroacetyl chloride (DCAC), which is generated during the photocatalytic oxidation of trichloroethylene (TCE), was the only species adsorbed on TiO2. Two types of DCAC were adsorbed – one at low concentration and another at high concentration. While adsorbed phosgene produced from DCAC was adsorbed strongly to TiO2, the adsorption of DCAC was relatively weak. The surface structures of the adsorbed DCAC at low and high concentrations on TiO2 were carboxylate compounds on single Ti–O bonds. The surface structures of carboxylate compounds were resolved to those of adsorbed phosgene, bidentate carbonate compounds, on the two Ti–O bonds during photocatalytic oxidation under UV irradiation. The adsorbed DCAC on TiO2 reduces the photooxidation rate of TCE, and the CHCl2 stretching vibration of adsorbed DCAC was considered to be the inactive species. The adsorbed DCAC and dichloroacetic acid (DCAA) were converted to adsorbed phosgene by photocatalytic oxidation on TiO2. [Copyright &y& Elsevier]

A class of adsorption mechanisms, namely those with a single electron-transfer step whose electronless species graphs are trees, is shown to have equivalent circuits containing only resistors and capacitors. Also for these mechanisms, a “topological” equivalent circuit can be chosen that is closely related to the graph of the reaction mechanism, so that the circuit may be written by inspection of this graph. [Copyright &y& Elsevier]

In situ absorption spectra of methylene blue (MB) adsorbed on an indium-tin-oxide electrode were observed by slab optical waveguide (SOWG) spectroscopy. The absorption spectrum of the MB obtained by the SOWG spectroscopy coincided with that by conventional spectroscopy for the solution phase, and the MB existed as dimer at 0 V vs. Ag/AgCl. Upon the electrode potential of cathodic condition, the absorption spectra gradually changed showing the appearance of the monomeric species as well as the highly aggregated species of MB. [Copyright &y& Elsevier]