Showing papers by "Christopher J. Kiely published in 2006"

Solvent-Free Oxidation of Primary Alcohols to Aldehydes Using Au-Pd/TiO2 Catalysts

Abstract: The oxidation of alcohols to aldehydes with O2 in place of stoichiometric oxygen donors is a crucial process for the synthesis of fine chemicals. However, the catalysts that have been identified so far are relatively inactive with primary alkyl alcohols. We showed that Au/Pd-TiO2 catalysts give very high turnover frequencies (up to 270,000 turnovers per hour) for the oxidation of alcohols, including primary alkyl alcohols. The addition of Au to Pd nanocrystals improved the overall selectivity and, using scanning transmission electron microscopy combined with x-ray photoelectron spectroscopy, we showed that the Au-Pd nanocrystals were made up of a Au-rich core with a Pd-rich shell, indicating that the Au electronically influences the catalytic properties of Pd.

Direct Synthesis of Hydrogen Peroxide from H2 and O2 Using Al2O3 Supported Au−Pd Catalysts

Abstract: The direct synthesis of hydrogen peroxide from H2 and O2 using a range of alumina supported Au, Pd, and Au−Pd metal catalysts is described and discussed in detail for both fresh catalysts and materials that have been aged in storage. The conditions previously identified as being optimal for hydrogen peroxide synthesis (i.e., low temperatures (2 °C) and short reaction residence times) are employed, and the effect of catalyst composition and preparation was investigated in detail for the fresh catalysts. The addition of Pd to the Au catalyst increases the rate of hydrogen peroxide synthesis as well as the concentration of hydrogen peroxide formed. Interestingly, the addition of relatively small amounts of Pd to the Au/Al2O3 catalyst, replacing up to 20% of the Au, significantly increases the formation of hydrogen peroxide. The microstructure of the catalysts was studied using transmission electron microscopy and X-ray photoelectron spectroscopy for both the fresh and the aged catalyst, and the supported bim...

Selective oxidation of CO in the presence of H2, H2O and CO2 utilising Au/α-Fe2O3 catalysts for use in fuel cells

Abstract: Au/Fe2O3 catalysts prepared using co-precipitation are described and discussed for the preferential oxidation of CO in the presence of H2, H2O and CO2. A catalyst prepared using a two-stage calcination procedure (400 °C followed by 550 °C) achieves target conversion and selectivity (>99.5% CO conversion and >50% selectivity, based on O2, for the competing conversion of H2 with O2 at 80–100 °C) for the competitive oxidation of dilute CO in the presence of moist excess H2 and CO2. The effect of the preparation method of the uncalcined precursor is described and the effects of calcination on the catalyst activity in the absence of H2, CO2 and H2O is initially explored. The catalysts are characterised in detail using electron microscopy (TEM), X-ray photoelectron spectroscopy and Mossbauer spectroscopy. For the target conversion to be achieved, it is necessary that the activity for the reverse water gas shift activity (CO2 + H2 → CO + H2O) of the catalyst is suppressed, since under the fuel cell conditions this reaction reforms CO at high CO conversions due to the presence of excess CO2 and H2. It is proposed that the two stage calcination procedure removes active sites for the water gas shift reaction whilst retaining active sites for preferential CO oxidation.

Improvements in the X-ray analytical capabilities of a scanning transmission electron microscope by spherical-aberration correction.

Abstract: A Nion spherical-aberration (Cs) corrector was recently installed on Lehigh University's 300-keV cold field-emission gun (FEG) Vacuum Generators HB 603 dedicated scanning transmission electron microscope (STEM), optimized for X-ray analysis of thin specimens. In this article, the impact of the Cs-corrector on X-ray analysis is theoretically evaluated, in terms of expected improvements in spatial resolution and analytical sensitivity, and the calculations are compared with initial experimental results. Finally, the possibilities of atomic-column X-ray analysis in a Cs-corrected STEM are discussed.

Gallium-doped VPO catalysts for the oxidation of n-butane to maleic anhydride

Abstract: Vanadium phosphate (VPO) catalyst materials have been doped with gallium and subsequently tested for the mild oxidation of n-butane to maleic anhydride. At low Ga concentrations, this impurity is shown to be beneficial for n-butane conversion. For low dopant levels (Ga/V ⩽ 1 at%), the crystallinity of the hemihydrate (VOHPO4·0.5H2O) precursor phase is improved and its specific area is increased relative to the undoped material as a result of decreased platelet thickness. Electron diffraction and energy dispersive X-ray analysis (XEDS) revealed that the Ga is uniformly distributed in a substitutional manner throughout the hemihydrate structure. The presence of Ga also significantly shortens the activation time required to convert the hemihydrate precursor into a well crystallized vanadyl pyrophosphate (VO)2P2O7 phase under an n-butane/air gas flow at 400 °C. The intimate presence of Ga distributed within the VOHPO4·0.5H2O unit cell has also been confirmed by XANES and EXAFS. Such studies also show that the Ga is partially redistributed within the (VO)2P2O7 structure after catalyst activation. A complementary electrical conductivity study on these materials revealed that Ga3+ substitutes for (VO)2+ species in (VO)2P2O7 giving rise to an n-type semiconductivity which partially compensates the natural p-type conductivity character of the (VO)2P2O7 phase. For higher Ga doping levels (Ga/V ≈ 5 at%), the excess of Ga concentrates as a GaPO4 impurity phase, which is shown to have a detrimental effect on the catalytic performance of the Ga-doped VPO catalyst.

Unexpected inversion of enantioselectivity during the hydrogenation of ethyl pyruvate using hydroquinine and hydroquinidine modified Pt/Al2O3

Abstract: In the enantioselective hydrogenation of ethyl pyruvate using hydroquinidine 4-chlorobenzoate modified Pt/γ-Al2O3 catalyst, the sense of the enantioselectivity is a function of the modifier concentration. At low concentration (S)-ethyl lactate is preferred and at higher concentration (R)-ethyl lactate is formed; the opposite trend is observed with hydroquinine 4-chlorobenzoate. This is the first example where enantio-inversion is induced solely as a function of the chiral modifier concentration.

Direct Synthesis of Hydrogen Peroxide from H2 and O2 Using Al2O3 Supported Au—Pd Catalysts.

Abstract: The direct synthesis of hydrogen peroxide from H2 and O2 using a range of alumina supported Au, Pd, and Au−Pd metal catalysts is described and discussed in detail for both fresh catalysts and materials that have been aged in storage. The conditions previously identified as being optimal for hydrogen peroxide synthesis (i.e., low temperatures (2 °C) and short reaction residence times) are employed, and the effect of catalyst composition and preparation was investigated in detail for the fresh catalysts. The addition of Pd to the Au catalyst increases the rate of hydrogen peroxide synthesis as well as the concentration of hydrogen peroxide formed. Interestingly, the addition of relatively small amounts of Pd to the Au/Al2O3 catalyst, replacing up to 20% of the Au, significantly increases the formation of hydrogen peroxide. The microstructure of the catalysts was studied using transmission electron microscopy and X-ray photoelectron spectroscopy for both the fresh and the aged catalyst, and the supported bim...

Cyclic voltammetry as a potential predictive method for supported nanocrystalline gold catalysts for oxidation in aqueous media

Abstract: Nanocrystalline supported Au is a potent catalyst for oxidation of alcohols and polyols in aqueous media using oxygen in which exclusive formation of the mono-acid can be observed. However, relatively non-selective catalysts for the same reaction can also be prepared. Herein we present a detailed characterization study of wholly selective and non-selective Au/graphite catalysts used in the oxidation of glycerol to glyceric acid using a combination of cyclic voltammetry (CV) and transmission electron microscopy (TEM). Analysis using TEM, a technique that is often incisive with supported Au catalysts, in this particular case shows that the non-selective catalysts comprise relatively larger Au crystallites as compared to those of the more selective catalysts. In addition, a set of three Au/C with different selectivities for glyceric acid were characterized using TEM but no discemable differences were observed which could be correlated with the selectivity differences. A detailed study using CV shows distinct differences between the four catalysts. In particular, differences in the activity and selectivity of these Au/graphite catalysts towards glycerol oxidation can be correlated to differences in the relative rates of formation of the selective oxygen species and surface poisons as determined using cyclic voltammetry. It is considered that these cyclic voltammetry correlations can have predictive potential for supported Au catalysts.