G. Parravano

Chemical reactivity of supported gold : IV. Reduction of NO by H2

was 150 to 300°C 10 6 pn

Cyclopropane hydrogenation on Ru and RuAu catalysts

6 100 Torr, and p!+/pHa = 1. The Au preparations were characterized by wide-angle X-ray scattering and transmission electron microscopy. No characteristic effect on the rate of reaction was observed by varying the Au precursor salt or the preparative method. There was an effect of the support on the reaction selectivity to Nt,

Bimetallic ruthenium-gold-on-magnesia catalysts chemicophysical properties and catalytic activity

02~supported preparations showing the lowest and AllOo-supported preparations the highest value of the selectivity. The rate of the IIs-1~2 equilibration was fastest on Au supported on SiO,. It is argued that the different tendency of the supports employed to act as electron donor-acceptor with supported Au influences t,he electron density at the surface Au site. The interpretation is consistent with the relative acid-base ranking of the supports and of the precursor Au salt. An increased electron density at the Au site increases electron back donation from metal to adsorbed NO, weakening the N-O bond and facilitating the formation of Nz over that of NHS. This interpretation of the role of the support in modifying the electronic conditions at the Au site is empIoyed to suggest that the higher HZ-D, equilibration rate on Au-SiOe results from a more ionic hydrogen chemisarption bond. Suggestions on the morphoIo~ca1 and chemical nature of the Au which interacts with the support are advanced in the context of the highly heterogeneous morphology of the supported Au, as revealed by the characterization methods employed. The meaning of these findings in the framework of present ideas for high Nz selectivity in the NO reduction as well as the impact of the conclusions from the present study on other important reactions (CO + HZ, NZ + HZ) are pointed out.

Oxygen chemisorption on supported gold

The rate of reaction between cyclopropane and hydrogen was measured in a flow system on unsupported Ru, Ru/SiO,, and Ru/MgO. The effect of the addition of Au to Ru/MgO was also investigated. The temperature was in the range 30-170°C; partial pressures were 0.01 5 P,, 5 0.1 atm, 0.1 5 P,,, 5 0.5 atm; and total pressure was 1 atm. The reaction proceeded via three different routes, namely:

Surface composition and oxygen chemisorption on Ag-Pd alloys

Abstract Bimetallic Ru-Au catalysts supported on MgO were prepared, their atomic composition varying from 100% Ru to 100% Au. Samples were characterized after impregnation and drying at 110 °C and after further reduction by hydrogen at 400 °C. X-Ray line broadening analysis, small-angle X-ray scattering, transmission electron microscopy, gas chemisorption, diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS) analysis were used. Catalytic activity was measured for the oxygen transfer between CO and CO 2 . The Ru and Au precursor compounds appear to be modified, and have probably interacted in the bimetallic samples, already after impregnation and drying. Relevant amounts of metal oxides are found in some “reduced” catalysts; a metal-support interaction is proposed for Au/MgO. The amounts of the Ru and Au oxidized species found by EXAFS in the examined bimetallic sample are significantly different from those found in Ru/MgO and Au/MgO, respectively. The DR spectra of the bimetallic samples are not a simple combination of those of the monometallic catalysts. The Ru/Au atomic ratios measured by XPS are higher than the analytical ones. The fraction of surface Ru atoms, measured by hydrogen chemisorption, increases on decreasing the bulk Ru/Au atomic ratio. The Au/MgO sample has an activity which is two orders of magnitude higher than that of the remaining samples. All of this suggests the existence of a Ru surface enrichment and also an Ru-Au chemical interaction. These conclusions are also supported by an ir study of CO chemisorbed on the same samples.

Study of the equilibrium surface composition of Co1−yFe2+yO4—I: Cation composition with stoichiometric oxygen content

Oxygen chemisorption on supported gold corresponded to an Au/O ratio of 2:1 at 200/sup 0/C and 3-6 mm Hg oxygen initial pressure. The chemisorptions were conducted at 170/sup 0/-450/sup 0/C on catalysts that comprised 0.54-4.25Vertical Bar3< by wt of gold on alumina, silica, and magnesium oxide, that were pretreated in hydrogen or evacuated at up to 450/sup 0/C, and that were characterized by wide-angle X-ray scattering and transmission electron microscopy.

Isotopic mixing in carbon monoxide catalyzed by zinc oxide

Ag-Pd alloys prepared by room temperature coprecipitation from solutions of the corresponding nitrates have been subjected to surface analysis by Auger electron spectroscopy. Previous work on the adsorption of O2 on the same alloys showed that except for very high Pd concentration the adsorption heat was independent of alloy composition. On untreated and Ar ion sputtered alloys Pd enrichment in the surface layers was found, while on thermally annealled alloys (<0.1 Ton or 1 atm of H2 or O2), surface segregation of Ag took place. This is the expected result from surface tension and/or atomic size considerations. The relation of these findings with the earlier data on the chemisorption heat of oxygen is discussed. The implications of these conclusions upon present models of metallic alloys is pointed out.

An AES study of the surface composition of cobalt ferrites

Abstract The equilibrium composition at the surface of the mixed spinel Co 1− y Fe 2+ y O 4 is calculated as a function of the composition parameter y and of the degree of inversion. The ionic standard chemical potentials include the Born-Mayer repulsive energy and the vibrational contribution. The calculated results turn out to be weakly dependent on the bulk inversion, and indicate a pronounced tendency of iron to segregate at the surface. These results are compared and found to be in agreement with some experimental data obtained by AES. In a subsequent paper we will take into account the gas-phase and the charge-transfer effects.

Study of the equilibrium surface composition of Co1−yFe2+yO4—II: The role of vacant and chemisorbed oxygen and charge transfer effect

The rate of the isotopic mixing in CO has been studied at 3OO”C, for CO partial pressures from 6 to 100 Torr and a total pressure of 250 Torr on ZnO catalysts. Significant deviations from a first-order rate in pco were found. The rate of oxygen exchange between ZnO and gasphase CO was also measured and the results were employed to calculate the fraction of surface sites active for the CO isotopic mixing. Values on the order of 0.001 were found. The turnover rate and surface collision efficiency varied between 0.7 and 107 min-1 and 0.13 and 2.24 X lo-*, respectively. Hz additions to CO increased the rate of isotopic mixing, whereas the rate of Hz + Dz was decreased by the presence of CO. The Hz + DP rate was faster than that of isotopic mixing in CO, but as the ratio p~,/p~o decreased the rates became about equal. It is argued that on ZnO samples, in which the rate of CO isotopic mixing and the rate of ZnO-CO oxygen exchange were influenced in a similar manner by the CO pressure, the isotopic mixing in CO took place via the ZnO oxygen, while oxide oxygen participation was not kinetically significant for ZnO samples in which the two reactions had different kinetics. The crucial factor controlling the path followed by the isotopic mixing in CO seems to be the surface Zn/O ratio, since a close correlation was found between the former and the reaction kinetics of the CO isotopic mixing reaction. Solid-state conditions which may vary the Zn/O surface ratio (foreign additions) are indicated. The implications of these findings to the problem of product selectivity from CO-H, mixtures reacting on metal oxide surfaces are discussed.

Abstract of articles to be published in the journal of physics and chemistry of solidsStudy of the equilibrium surface composition of Co1−yFe2+yO4 II. The role of vacant and chemisorbed oxygen and charge transfer effect

A series of cobalt ferrite samples, Co1−yFe2 + yO4, where −0.1 ⩽ y ⩽ 0.1, was examined by AES. A remarkable Fe surface enrichment was observed near the stoichiometric composition for y both positive and negative, and found to be in quite good agreement with theoretical calculations. The submission of samples to various treatments, at different temperatures and gas atmospheres, produced changes in the surface composition, in a fashion associated with the oxygen concentration both in gas and solid phase.