The influence of hydrazine reduction on metal dispersion and support morphology in bimetallic Ru---Au/MgO catalysts

The influence of hydrazine reduction on metal dispersion, particle size distribution, and support morphology was investigated for a series of bimetallic Ru-Au/MgO catalysts. The catalysts were prepared by impregnation of MgO with chlorine-containing precursor salt solutions. Hydrazine reduction significantly lowered the residual chlorine content of the catalysts and allowed the formation of high-surface-area MgO. This, in turn, led to much higher Ru metal dispersions as compared to that obtained after H2 reduction alone. X-Ray energy dispersive spectroscopy (EDS) proved that bimetallic Ru-Au aggregates were confined to a particle size range of less than 5 nm. Particles larger than 10 nm contained without exception monometallic Au. The hydrazine-treated catalysts did not show the suppression in catalytic activity for CO hydrogenation that was observed in H2-reduced Ru/MgO catalysts. The increased metal dispersion and restored catalytic activity of Ru in the hydrazine-treated catalysts may be linked to the absence of chlorine contamination of the support.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26244/1/0000324.pd

Metal dispersion of bimetallic catalysts via stepwise chemisorption and surface titration : II. Ru---Au/MgO

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25587/1/0000131.pd

Adsorption-induced conductance changes of thin Pt films and PtPd/TiO2 gas sensors

Chemisorption of H2 and O2 and resulting changes in electrical conductance of a typical gas sensing material, PtPd/TiO2, and thin Pt films on glass are studied and compared. The activation energy of conduction increases as Pt film thickness decreases. Chemisorption of H2 on thin Pt films causes an increase in conductance and activation energy of conduction. O2 chemisorption results in a decrease in conductance and increase in activation energy of conduction. Alteration in the number of charge carriers and reduction in charge carrier mobility are the mechanisms proposed for the observed changes. Compared to thin Pt films, relatively large changes in electrical conductance are observed upon chemisorption of gases on TiO2 supported PtPd. The role of the oxide substrate in the observed chemical interaction and electronic response is discussed. The electronic changes upon adsorption/desorption of gases are reversible for thin Pt films but only partially reversible for TiO2 supported PtPd.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26522/1/0000060.pd

Metal dispersion of bimetallic catalysts via stepwise chemisorption and surface titration : I. Ru---Au/SiO2

A stepwise chemisorption and titration procedure involving H2 and O2 as adsorbates is used to measure the dispersion of the individual metal components in bimetallic Ru---Au/SiO2 catalysts. The experimental approach is based upon differences in O2 adsorption on Ru and Au, respectively, as a function of adsorption temperature. At room temperature, O2 is adsorbed on Ru but not on Au. However, at 473 K both metals chemisorb O2, albeit with a change in adsorption stoichiometry in the case of Ru. Finally, oxygen that is adsorbed on Ru can be titrated with H2 at 373 K, while oxygen that is adsorbed on Au sites does not react with H2 under these conditions. The reliability of the technique is first tested on physical mixtures of Ru/SiO2 and Au/SiO2 of known dispersion and then applied to a series of bimetallic Ru---Au/SiO2 catalysts. The results from this stepwise chemisorption procedure are critically evaluated and compared with previously obtained extensive characterization data on these catalysts. In conjunction with analytical electron microscopy and WAXS results, an estimate of the average surface composition of bimetallic particles is derived.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25586/1/0000130.pd

Selective oxidation of methane over vycor glass, quartz glass and various silica, magnesia and alumina surfaces

The reaction between methane and oxygen was investigated in empty Vycor glass and quartz tubes and over various silica compounds in powder form. In the temperature range of 893-993 K and with methane-to-oxygen ratios close to unity, formaldehyde, ethane and ethene were produced with a combined 53-96% selectivity over Vycor glass reactor surfaces. Quartz reactor surfaces were less active, but gave the same products. Silica compounds in powder form such as Cab-O-Sil, Ludox gels and silicic acid were investigated at temperatures lower than 893 K where the blank activity of the quartz reactor became negligible. These silica compounds followed similar trends of activity as the Vycor or quartz reactor tubes, although at much lower temperatures. High oxygen-to-methane ratios improved the rates of overall methane reaction and C2 formation. Short residence times enhanced the formaldehyde over the carbon monoxide and carbon dioxide selectivity. The reaction rates increased with pressure. However, in contrast to the results on empty reactor surfaces, no ethane was produced over the silica compounds, the ethene yields were lower, and the carbon dioxide production was higher. While the apparent activation energies for the overall methane reaction and for the production of carbon monoxide, carbon dioxide, ethene and ethane were dependent on the nature of the catalytic surface, the apparent activation energy for the formaldehyde formation appeared to be independent of the catalyst and remained constant at about 130 kJ/mol. This might imply that formaldehyde is generated in the gas phase and that carbon monoxide and carbon dioxide can be formed in secondary reactions of formaldehyde involving surface interactions. The effect of the acid/base character of the catalyst was explored by comparing the activities of the silica compounds with those of MgO and [gamma]-Al2O3. MgO and [gamma]-Al2O3 were more active than the silicas, but produced only carbon monoxide and carbon dioxide.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27449/1/0000489.pd

The microstructure of bimetallic Ru---Cu/SiO2 catalysts: A chemisorption and analytical electron microscopy study

Supported bimetallic Ru---Cu/SiO2 catalysts are characterized by transmission electron microscopy, energy dispersive X-ray spectroscopy, electron microdiffraction, and chemisorption. Metal particles up to 4 nm in diameter are bimetallic, while particles larger than 4 nm contain only Cu. Considerable compositional nonuniformity is observed from one individual metal particle to the next. Microdiffraction patterns obtained from individual particles can be attributed to either Ru or Cu suggesting no significant modification in crystallographic structure of either metal component. Addition of Cu to Ru results in a drastic suppression of H2 chemisorption while the extent of O2 chemisorption is not as strongly affected. The suppressed H2 chemisorption capability of Ru in the bimetallic catalysts is an indication of atomic interdispersion of Ru and Cu on the surface of the bimetallic clusters, leading to the break-up of the Ru ensembles which would be necessary for dissociation of molecular hydrogen. The influence of catalyst preparation techniques on the relative interdispersion of Ru and Cu and consequent discrepancies in the Ru---Cu literature are discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26092/1/0000168.pd

Influence of chlorine on the surface area and morphology of TiO2

Changes in BET surface area and morphology of TiO2 (anatase) were studied as a function of temperature and level of chlorine contamination. The objPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25842/1/0000405.pd

Gold-titania interactions: Temperature dependence of surface area and crystallinity of TiO2 and gold dispersion

The influence of temperature on the BET surface area, crystallinity, and anatase/rutile phase transformation of blank TiO2 and Au/TiO2 catalysts is studied. Presence of gold delays the recrystallization of anatase and the phase transformation into rutile. In turn, high gold dispersions are stabilized by TiO2 up to a temperature of 700 [deg]C. Agglomeration of gold into large particles coincides with the phase transformation into rutile at 800 [deg]C. The stability of the gold dispersion does not seem to be due to an SMSI effect. The low metal loading used to impregnate a high-surface-area TiO2 may be responsible for either an incorporation of gold atoms in interstitial positions of the TiO2 lattice, or the trapping of small gold particles in micropores.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/24832/1/0000258.pd

Successive ionic layer deposition: possibilities for gas sensor applications

In this paper we discuss results of research related to design of successive ionic layer deposition (SILD) technology for both preparing porous nano-structured SnO2 films, and surface modification of SnO2 films deposited by spray pyrolysis. This new method of metal oxide deposition has exited high interest, because of this method simplicity, cheapness, and ability to deposit thin nano-structured films on rough surfaces. Method of successive ionic layer deposition (SILD) consists essentially of repeated successive treatments of the conductive or dielectric substrates by solutions of various salts, which form on the substrate surface poorly soluble compounds. It was shown that SILD technology is effective method for above mentioned purposes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/49002/2/jpconf5_15_008.pd