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Mn-promoted bulk iron catalysts with a high specific surface area (82–211 m2·g−1) were synthesized by coprecipitation followed by drying under supercritical conditions. The catalysts were tested in the CO2 hydrogenation to valuable C2-C5 hydrocarbons. The Mn-promoted iron catalysts exhibited better textural properties than the bare Fe2O3 catalyst, allowing better dispersion of the active phase, easier reduction and carburization of iron oxides and, consequently, resulting in higher catalytic activity than the bare Fe2O3 catalyst. The best activity results were obtained by catalyst promotion with a very low amount of Mn (Mn/Fe atomic ratio of 0.05). Upon steady state conditions (T = 340 °C, total pressure of 20 bar and H2/CO2 = 3), this catalyst exhibited high CO2 conversion (44.2%) and selectivity to C2-C4 hydrocarbons (68%, olefin to paraffin ratio of 0.54), while the selectivity to C5+ hydrocarbons, CH4 and CO was about 3.2, 38.5 and 5%, respectively. A close correlation was found between catalyst textural properties and CO2 conversion. The most active MnFe-0.05 catalyst exhibited high stability during 72 h of reaction related to a low amount of soft coke formation and catalyst activation through the formation of the χ-Fe5C2 phase during the on-stream reaction. [ABSTRACT FROM AUTHOR]

The physical characterization of well-defined site-isolated molecular metal complexes and metal clusters supported on metal oxides and zeolites are described. The strengths and limitations of complementary techniques, including transmission electron microscopy, X-ray absorption, infrared, Raman and NMR spectroscopies, are examined and the results have provided a foundation for understanding the complex materials that are typical of industrial catalysts.

Characterization of CO adsorption on a Au/zeolite NaY sample prepared from the precursor Au(super 3+) [CH(sub 3)](sub 2) [C(sub 5) H(sub 7)O(sub 2)] is reported. When a CO molecule is already adsorbed, it stabilizes the Au(super 3+) ion in the new position, and a second CO molecule can be coordinated, thus forming a geminal species.

Mononulcear gold complexes bonded to TiO2 were synthesized from Au(CH3)2(C5H7O2)2. Characterization of the decomposition and conversion into gold nanoclusters on the TiO2 surface were done by time-resolved X-ray absorption and infrared spectroscopies as the temperature of the sample in flowing was ramped up.

Abstract: TiO2-supported rhodium samples synthesized from the Rh6(CO)16 complex were catalytically active and selective for the gas-phase methanol carbonylation at 140°C and atmospheric pressure in the presence of methyl iodide as promoter. Infrared (IR) spectra recorded during catalysis allowed the identification of molecularly adsorbed methanol, together with linear, doubly, and triply-bridged surface methoxy species on Ti4+ sites of the support. IR bands characteristic of rhodium complexes that might be regarded as reaction intermediates were also observed, in addition to bands assigned to surface acetate species attributed to the formation of methyl acetate. Our results reveal that only linear methoxy species on Ti4+ sites react with flowing CO in the presence of CH3I to give methyl acetate, whereas bridged methoxy species and molecularly adsorbed methanol are only spectators in methanol carbonylation. [Copyright &y& Elsevier]

Adsorption of CO on gold supported in zeolite NaY at 85 K led to the formation of (i) various carbonyls and isocarbonyls typical of the zeolite and (ii) carbonyls formed at cationic gold sites (observed in the 2186−2171 cm-1 region). Analysis of the behavior of the bands allows their assignment to carbonyls of Au3+ ions. At temperatures higher than 220 K, CO adsorption led to the formation of a new type of Au3+−CO species (2207 cm-1). Once formed, these complexes could be transformed into the dicarbonyls Au3+(CO)2 when the sample was cooled to 85 K in the presence of CO. The results are explained by migration of Au3+ ions to more accessible positions within the zeolite at increasing temperatures. When a CO molecule is already adsorbed, it stabilizes the Au3+ ion in the new position, and a second CO molecule can be coordinated, thus forming a geminal species. These results are the first evidence of Au3+(CO)2 complexes. [ABSTRACT FROM AUTHOR]

Mononuclear gold complexes in zeolite NaY were synthesized from initially physisorbed Au(CH3)2(C5H7O2), and their reactions with CO in a flow system at 298 K and 760 Torr were investigated by infrared (IR) spectroscopy and mass spectral analysis of the effluent gases. CH4 and CO2 were formed as CO flowed through the sample either steadily or as successive pulses. The results are consistent with the inferences that (a) CO reacted with the supported gold to form gold carbonyls, (b) CH4 formed by reaction of methyl groups on gold with traces of H2O or hydroxyl groups on the zeolite and (c) CO on cationic gold reacted with traces of O2 and/or H2O to form CO2. In samples treated in steadily flowing CO, cationic gold was reduced to zerovalent gold, but the cationic gold in samples exposed to CO pulses was not reduced to zerovalent gold, although CO2 formed. Thus, CO adsorbed on cationic gold reacts to give CO2 in the absence of zerovalent gold, consistent with the inference that gold catalysts for CO oxidation need not contain zerovalent gold. [ABSTRACT FROM AUTHOR]

Gold nanoclusters on TiO2 powder were prepared from adsorbed AuIII(CH3)2(C5H7O2) (dimethyl acetylacetonate gold(III)) and characterized by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies. The samples were tested as catalysts for CO oxidation at 298 K and atmospheric pressure and characterized by EXAFS and XANES with the catalysts in the working state. The XANES results identify Au(III) in the initially prepared sample, and the EXAFS data indicate mononuclear gold complexes as the predominant surface gold species in this sample, consistent with the lack of Au–Au contributions in the EXAFS spectrum. The mononuclear gold complex is bonded to two oxygen atoms of the TiO2 surface at an Au–O distance of 2.16 Å. Treatment of this complex in He or in H2 at increasing temperatures led to formation of metallic gold clusters of increasing size, ultimately those with an average diameter of about 15 Å. The data demonstrate the presence of metallic gold clusters in the working catalysts and also show these clusters alone are not responsible for the catalytic activity. [ABSTRACT FROM AUTHOR]

Highlights: [•] TiO2-supported gold favors the dehydrogenation of ethanol to acetaldehyde. [•] Ethanol is converted mainly to butene and water on the bare TiO2 support. [•] Au atoms tune the reactions of ethanol on TiO2, providing sites for H recombination. [Copyright &y& Elsevier]

Mononuclear gold complexes synthesized from AuIII(CH3)2(acac) in zeolite NaY were characterized by time-resolved X-ray absorption spectroscopy and infrared spectroscopy as they catalyzed CO oxidation at 298 K and 760 Torr in flow systems. Initial contact with a CO + O2 mixture led to the rapid formation of cationic gold complexes in which Au was bonded to approximately two zeolite O atoms, on average. Further contact with CO + O2 led to breaking of an Au-surface oxygen bond, giving a gold carbonyl anchored to approximately one O atom. The process was reversed in the absence of CO and O2.