Search

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.

The redox behavior of platinum(Pt) promoted tungstated zirconia catalysts, which are active in the isomerization of light alkanes is investigated by applying FTIR and EPR techniques. EPR shows evidence of formation of W(super 5+) at reduction temperatures above 473 K for the unpromoted catalyst and room temperature when platinium is present.

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]

Adsorption of NO and its coadsorption with oxygen on WO3–ZrO2 as well as the reactivity of the surface NOx species toward propene have been studied by FTIR spectroscopy and temperature-programmed desorption. NO adsorption on WO3–ZrO2 results in the formation of N2O (2282, 2243, and 1224 cm−1), NO+ (2140 and 2119 cm−1), surface nitrates (1614, 1570, and 1230 cm−1), and nitrosyls (1936 cm−1) created on cationic sites, which are affected by nitrates. Except for N2O, the other species are developed with time. Addition of oxygen to the NO–WO3–ZrO2 system results mainly in an increase in the concentration of the surface nitrates. The latter are stable up to ca. 673 K and are thermally decomposed, yielding NO and NO2, at higher temperatures. The nitrates interact with propene at ca. 473 K, giving an organic deposit which is decomposed to isocyanates (2296, 2278, and 2250 cm−1). These species easily react with a NO + O2 mixture, producing dinitrogen. The assignments of the IR bands are supported by 15N-labeling experiments and analysis of the spectral region where the overtones and combination modes of surface nitrites and nitrates appear. Finally, some new aspects of the mechanism of selective catalytic reduction of NOx by hydrocarbons are discussed. [Copyright &y& Elsevier]

Raman spectroscopy and XRD are used to study the preparation of alumina-supported Mo catalysts, with emphasis on the molecular level understanding of the influence of the ageing step (i.e. leaving the catalyst in contact with water after impregnation and before drying) on Mo speciation. For incipient wetness impregnation with heptamolybdate ((NH4)6Mo7O24), long oxide/water contact times (ageing) favor alumina dissolution and consequently increase the formation and crystallization of the resulting compound, (NH4)3[Al(OH)6Mo6O18] on Al2O3. Precipitation of this compound leads to the formation of MoO3 after calcination. On the contrary, minimizing the oxide/water contact time (and therefore alumina dissolution) by freeze-drying the system after impregnation inhibits the formation and crystallization of (NH4)3[Al(OH)6Mo6O18] and therefore drastically limits MoO3 formation after calcination. [Copyright &y& Elsevier]