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A flame synthesis method has been used to prepare nanosized, high-surface-area Cu-Ce-O, Ni-Ce-O, and Fe-Ce-O catalysts from aqueous solutions of metal acetate precursors. The particles were formed by vaporization of the precursors followed by reaction and then gas to particle conversion. The specific surface areas of the synthesized powders ranged from 127 to 163 m(2)/g. High-resolution transmission electron microscope imaging showed that the particle diameters for the ceria materials are in the range of 3-10 nm, and a thin layer of amorphous material was observed on the surface of the particles. The presence and surface enrichment of the transition-metal oxides (CuO, NiO, and Fe(2)O(3)) on the ceria particles were detected using X-ray photoelectron spectroscopy. Electron energy-loss spectroscopic studies suggest the formation of a core-shell structure in the as-prepared particles. Extended X-ray absorption fine structure studies suggest that the dopants in all M-Ce-O systems are almost isostructural with their oxide counterparts, indicating the doping materials form separate oxide phases (CuO, Fe(2)O(3), NiO) within the host matrix (CeO(2)). Etching results confirm that most of the transition-metal oxides are present on the surface of CeO(2), easily dissolved by nitric acid. The performance of the flame-synthesized catalysts was examined toward water-gas shift (WGS) activity for fuel processing applications. The WGS activity of metal ceria catalysts decreases in the order Cu-Ce-O > Ni-Ce-O > Fe-Ce-O > CeO(2) with a feed mixture having a hydrogen to carbon monoxide (H(2)/CO) ratio of 1. There was no methane formation for these catalysts under the tested conditions.

The kinetics of propane oxidation over Pt/Al 2 O 3 are investigated in this work as a function of O 2 /C 3 H 8 ratio in the 150-300 ˚C temperature range. At high O 2 /C 3 H 8 ratios, the platinum nanoparticles are saturated with oxygen and the reaction rate is zero-order with respect to the oxygen partial pressures in this regime. As the oxygen coverage decreases with decreasing O 2 /C 3 H 8 ratio, the reaction rate increases and the reaction order changes from zero-order to negative-order in the oxygen partial pressure. The reaction rate is controlled to a large extent by the oxygen coverage on the platinum nanoparticles. However, at lower temperatures and higher oxygen pressures there is a slow deactivation of the catalyst that cannot be explained by a slow change in the oxygen coverage. Diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) of adsorbed CO was performed to track the evolution of the nanoparticle structure over the course of the propane oxidation reaction and to determine whether the slow deactivation © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ 2 was caused by reconstruction of the platinum nanoparticles. We found that the platinum nanoparticles are significantly reconstructed during the course of the reaction, including the formation of a platinum oxide (PtO) which has a characteristic CO-DRIFTS band at 2123 cm -1 .The extent of PtO formation decreases with increasing temperature and, as a result, deactivation of the catalyst is less severe at higher temperatures. Unexpectedly, increasing the oxygen partial pressure resulted in less PtO formation. We believe that a different platinum oxide phase (e.g. PtO 2 or Pt 3 O 4 ) is formed at higher oxygen pressures, which is reduced to metallic platinum during CO exposure at 25 ˚C, and therefore is not detectable by CO-DRIFTS. These results are unique because they show how the nanoparticle structure evolves over many hours of propane oxidation, and how the temperature and oxygen pressure influence the reconstruction of the nanoparticles, which has implications for a wide range of reactions not limited to propane oxidation.

Catalytic total oxidation is important in several applications. However, associated models are rather empirical. In this study, a microkinetic model is developed for ethane total oxidation, under fuel-lean conditions on a Pt catalyst using input from density functional theory and Brønsted−Evans−Polanyi linear free energy relations. Reaction orders and the apparent activation energy estimated from the model are in good agreement with experimental values. The inclusion of oxygen coverage effects on the activation of ethane changes the rate-determining step from thermal dehydrogenation to oxidative dehydrogenation of ethane. A significant portion (30%) of the reaction flux proceeds via oxygen insertion reactions to C 2 hydrocarbons (CH 2 C*).

To understand how CO inhibits hydrogen transport across Pd membranes, a 25-μm-thick Pd foil membrane was monitored by infrared-reflection absorption spectroscopy (IRAS) during exposure to H2/CO gas mixtures while the rate of hydrogen permeation across the membrane was measured simultaneously in the 373–533 K temperature range. As the coverage of CO on the membrane surface increases with increasing CO concentration and decreasing temperature, the rate of hydrogen permeation across the membrane decreases. However, in addition to adsorbing on the membrane surface, CO reacts with H2 to form surface-adsorbed methylene (CH2) species and methane in the gas phase. The coverage of methylene increases with decreasing temperature and, therefore, the strong poisoning effect of CO at low temperatures may be due to both CO and methylene species blocking H2 dissociation sites on the membrane surface. The activity and selectivity of the Pd membrane for CO methanation is much higher than expected from previous studies and from the high activation barrier for CO dissociation on Pd. It is possible that the high concentration of defect sites on the polycrystalline Pd foil surface and hydrogen facilitate dissociation of CO at low temperatures. This work demonstrates that spectroscopic observation of membranes under realistic permeation conditions is critical for understanding surface poisoning mechanisms and for rational design of membranes that are resistant to poisoning.

Synthesis of cerium oxide nanocrystallites via precipitation using triethanolamine is reported. The molecular water associated with the cerium nitrate precursor is exploited to generate hydroxyl ions with the help of triethanolamine, facilitating precipitation. The small crystallite diameter (3 nm) in the as prepared powder is believed to result from the limited amount of water present. Solvent type has no effect on the final crystallite size or structure; however, it plays an important role in the dispersion of the nanoparticles with dispersity of the particles increasing with increasing carbon chain length of the solvent alcohol used.

The pillared MWW (PMWW or MCM-36) and pillared MFI (PMFI) zeolites are 2dimensional (2D) catalytically active materials made by pillaring of layered MCM-22(P) and multilamellar MFI precursors, respectively. The single-or near single-unit-cell thick 2D microporous layers in PMWW and PMFI expose comparable number of external surface acid sites (i.e., acid sites enclosed in mesopores between zeolitic layers) to those in micropores, which become important or dominant contributor to the catalytic properties. Although the acidity and catalytic activities of PMWW and PMFI have been studied, modification of their external surfaces and its implications on catalytic reactions are not available. In the present study, we report the tailoring of external surfaces of PMWW and PMFI zeolites by atomic layer deposition (ALD) of silicon (ALD-Si) and aluminum (ALD-Al), respectively. The textural, acidic and catalytic properties of the ALD modified pillared zeolites were investigated using a variety of characterization methods. ALD-Al and ALD-Si modifications kept micropore almost intact, but resulted in significant reduction in mesopore volume and considerable changes in external surface composition and acidity. The catalytic tests showed that intrinsic catalytic behavior of Brønsted acid sites in ALD modified pillared zeolites was similar to their parent counterparts. In diffusion constrained parallel reactions, ALD of Al-or Si-species altered catalyst selectivity. In addition, ALD of Si-species on PMFI zeolite deactivated surface active sites, which resulted in improved catalytic activity in direct methane aromatization reactions under optimal ALD cycles. The study exemplified for the first time that ALD is an effective tool for tuning the surface properties of 2D unit-cell thick zeolites.

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