A. Martínez-Arias

Inverse CeO2/CuO Catalyst As an Alternative to Classical Direct Configurations for Preferential Oxidation of CO in Hydrogen-Rich Stream

A novel inverse CeO(2)/CuO catalyst for preferential oxidation of CO in H(2)-rich stream (CO-PROX) has been developed on the basis of a hypothesis extracted from previous work of the group (JACS 2007, 129, 12064). Possible separation of the two competing oxidation reactions involved in the process (of CO and H(2), respectively) is the key to modulation of overall CO-PROX activity and is based on involvement of different sites as most active ones for each of the two reactions. Achievement of large size CuO particles and adequate CeO(2)-CuO interfacial configurations in the inverse catalyst apparently allows appreciable enhancement of the catalytic properties of this kind of system for CO-PROX, constituting an interesting alternative to classic direct configurations so far explored for this process. Reasons for such behavior are analyzed on the basis of operando-XRD, -XAFS, and -DRIFTS studies.

NO reaction at surface oxygen vacancies generated in cerium oxide

NO adsorption on CeO2 pre-outgassed at different temperatures (Tv) has been studied by EPR and FTIR. The results are compared with those obtained by measuring the products of the reaction, performed at 323 K, between NO and CeO2 samples activated by previous heating under N2 flow. EPR results indicate that, for Tv 573 K, NO molecules are adsorbed in radical form at 77 K and react at 298 K with surface oxygen vacancies generating diamagnetic products. FTIR spectra show correspondingly the formation of hyponitrites, nitrites and nitrates upon NO adsorption at 298 K on CeO2 outgassed at different temperatures. The hyponitrites can decompose at ambient temperatures to produce N2O; the process leads to modification (probably reoxidation) of the surface centres initially present. This can be followed by O2 adsorption and EPR spectroscopy, which shows that O2– adsorbed on the modified vacancies is less stable than on CeO2 without NO adsorbed, its reaction leading in some cases to the formation of peroxynitrate-type radicals. The reaction at 323 K of NO on the pretreated CeO2 surface produced N2O for pretreatment temperatures TF 573 K, in amounts increasing with TF. The results indicate that surface centres containing associated oxygen vacancies are active sites for this process.

Novel Polyphenol Oxidase Mined from a Metagenome Expression Library of Bovine Rumen BIOCHEMICAL PROPERTIES, STRUCTURAL ANALYSIS, AND PHYLOGENETIC RELATIONSHIPS

RL5, a gene coding for a novel polyphenol oxidase, was identified through activity screening of a metagenome expression library from bovine rumen microflora. Characterization of the recombinant protein produced in Escherichia coli revealed a multipotent capacity to oxidize a wide range of substrates (syringaldazine > 2,6-dimethoxyphenol > veratryl alcohol > guaiacol > tetramethylbenzidine > 4-methoxybenzyl alcohol > 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) >> phenol red) over an unusually broad range of pH from 3.5 to 9.0. Apparent Km and kcat values for ABTS, syringaldazine, and 2,6-dimetoxyphenol obtained from steady-state kinetic measurements performed at 40 °C, pH 4.5, yielded values of 26, 0.43, and 0.45 μm and 18, 660, and 1175 s-1, respectively. The Km values for syringaldazine and 2,6-dimetoxyphenol are up to 5 times lower, and the kcat values up to 40 times higher, than values previously reported for this class of enzyme. RL5 is a 4-copper oxidase with oxidation potential values of 745, 400, and 500 mV versus normal hydrogen electrode for the T1, T2, and T3 copper sites. A three-dimensional model of RL5 and site-directed mutants were generated to identify the copper ligands. Bioinformatic analysis of the gene sequence and the sequences and contexts of neighboring genes suggested a tentative phylogenetic assignment to the genus Bacteroides. Kinetic, electrochemical, and EPR analyses provide unequivocal evidence that the hypothetical proteins from Bacteroides thetaiotaomicron and from E. coli, which are closely related to the deduced protein encoded by the RL5 gene, are also multicopper proteins with polyphenol oxidase activity. The present study shows that these three newly characterized enzymes form a new family of functional multicopper oxidases with laccase activity related to conserved hypothetical proteins harboring the domain of unknown function DUF152 and suggests that some other of these proteins may also be laccases.

New Pd/CexZr1−xO2/Al2O3 three-way catalysts prepared by microemulsion: Part 1. Characterization and catalytic behavior for CO oxidation

The state of a series of palladium catalysts supported on ceria–zirconia and ceria–zirconia/alumina in the calcined state and under a reactive CO+O2 atmosphere has been characterized by a combination of electron transmission microscopy, infrared and electron paramagnetic resonance spectroscopies, and the results obtained used to interpret the catalytic behavior displayed by the samples in the CO oxidation reaction. The Ce/Zr promoter enhances the catalytic properties of the noble metal by favoring the formation of the metallic state and through creation of anionic vacancies in the Ce–Zr component even from room temperature. The optimum promoting effect in the CO oxidation is observed for palladium particles in contact with three-dimensional-like Ce–Zr particles due to the appropriate nature and high number of the anion vacancies present at their surface. Compared with classical ceria-based systems, CO oxidation is here maximized for Pd-supported on the bulk material and not on mixed ceria-containing/alumina catalysts. These differences are discussed on the basis of the effect of the ceria and ceria–zirconia promoter particle size on the nature and number of active sites.

Spectroscopic study of oxygen adsorption as a method to study surface defects on CeO2

Adsorption of O2, using either normal oxygen or 17O-enriched mixtures, on CeO2 outgassed at different temperatures has been studied by EPR and FTIR. Different signals, assigned to O2– species bonded to surface cerium ions, were observed depending on the vacuum treatment temperature Tv, a parameter that determines the type of defects generated at the CeO2 surface. Depending on their linewidths and lowest g values, the O2– signals observed in EPR spectra can be grouped into two types, related to species absorbed at isolated and aggregated oxygen vacancies, respectively. The EPR parameters indicate that the bonds of these species to the surface have different degrees of covalency, which might also influence their IR absorption coefficient. While the two oxygen atoms in the O2– species formed on isolated surface oxygen vacancies are EPR equivalent, they are non-equivalent in those formed on aggregated vacancies produced at the higher temperatures. The different EPR signal parameters and the variations in the intensities of the latter, observed with different Tv and thermal treatments of the samples after adsorption, indicate that EPR of adsorbed O2– and related species can be used as a probe to study the generation and properties of defects on CeO2 surfaces.

New Pd/CexZr1−xO2/Al2O3 three-way catalysts prepared by microemulsion: Part 2. In situ analysis of CO oxidation and NO reduction under stoichiometric CO+NO+O2

Abstract The light-off behavior for CO oxidation and NO reduction under stoichiometric CO+NO+O2 gas mixtures of a series of palladium catalysts supported on ceria–zirconia, ceria–zirconia/alumina and alumina has been examined by means of catalytic activity tests, and by DRIFTS, XANES and EPR spectroscopies. The results show the promoting effect of the Zr–Ce mixed oxide on both reactions. The extent of promotion depends on the characteristics of the promoter entities present for each catalyst and on the nature of the reaction. As observed in the absence of NO, CO oxidation was mainly enhanced by contact between Pd and 3D aggregated promoter entities, although an inhibiting effect of NO on CO oxidation, attributed to passivation of the interface sites due to formation of oxidized states of Pd, was observed at low temperatures. In contrast, the higher activity for NO reduction over catalysts supported on ceria–zirconia/alumina suggests that most active sites for that reaction involve Pd interacting with dispersed promoter entities.

EPR study of the radicals formed upon UV irradiation of ceria-based photocatalysts

EPR measurements reveal remarkable differences on the type of radicals produced after UV illumination of TiO2, CeO2 and 0.8% CeO2/TiO2 photocatalysts. Photoactivation of the TiO2 sample in vacuum results in the formation of Ti4+–O− species and a small amount of Ti3+ centers. In the presence of adsorbed oxygen, irradiation of this material also generates Ti4+–O3− radicals. In the case of the CeO2/TiO2 catalyst, the ceria component is present in a highly dispersed state, as indicated by XRD and UV–Vis diffuse reflectance spectra (DRS) results. Accordingly, the only type of Ce4+–O2− adducts generated on the CeO2/TiO2 sample are indicative of the presence of two-dimensional patches of ceria on the anatase surface. On the other hand, photoactivation of the CeO2/TiO2 sample in the presence of oxygen also leads to the formation of some Ti4+–O− and Ti3+ centers. In the case of the CeO2 sample, superoxide radicals are observed upon irradiation in vacuum and subsequent oxygen adsorption. Further irradiation of this material in the presence of oxygen increases the amount of Ce4+–O2− radicals and simultaneously generates new species, which are tentatively assigned to Ce4+–O2H radicals. Photocatalytic activity was tested for toluene oxidation, and the results obtained show that the photodegradation rate is slightly lower for CeO2/TiO2 than for the TiO2 sample. However, the selectivity towards benzaldehyde (6–13%) is comparable for both materials. In the case of CeO2, the photo-oxidation rate is an order of magnitude lower than for TiO2, although mineralization of toluene is almost complete. Photoactivity results are discussed in connection with the characteristics of the radicals observed.

Selective Oxidation of Propane to Acrylic Acid on MoVNbTe Mixed Oxides Catalysts Prepared by Hydrothermal Synthesis

MoVNbTe mixed oxides have been prepared by both hydrothermal synthesis and slurry methods and have been tested in the selective oxidation of propane to acrylic acid. For comparative purpose, ternary metal oxides have also been prepared and tested. Characterisation results (X-ray diffraction and EPR) show important differences between the catalysts prepared hydrothermally and one prepared by a slurry method. The catalysts prepared hydrothermally show a higher activity and selectivity to acrylic acid than those prepared by slurry method. A reaction network for the partial oxidation reaction is tentatively proposed from the catalytic results obtained during the oxidation of propane and propylene on these catalysts.

Nature and catalytic role of active silver species in the lean NOx reduction with C3H6 in the presence of water

A study of the lean NOx reduction activity with propene in the presence of water over Ag/Al2O3 catalysts with different silver loadings (1.5–6 wt%) has been done using X-ray diffraction, ultraviolet–visible spectroscopy, transmission electron microscopy, and in situ diffuse reflectance infrared and X-ray absorption spectroscopies under reaction conditions. The catalysts were prepared by an impregnation method employing EDTA complexes that allow highly dispersed silver phases to be obtained, which are stabilized under reaction conditions by strong interactions with the support. It is shown that the active species corresponds to silver aluminate-like phases with tetrahedral local symmetry. The role of silver in the reaction mechanism is shown to be mainly in the activation of NOx and propene species. In particular, the silver entities have been found to offer a new reaction path for propene activation which involves generation of acrylate species as a partially oxidized active intermediate. Differences between two active catalysts containing 1.5 and 4.5 wt% of Ag suggest that optimization of the SCR activity can be related to the oxygen lability of the tetrahedral silver aluminate-like phase present in the catalyst. As postulated previously, the high nonselective propene oxidation activity of the highest loaded sample (with 6 wt% Ag) appears to be related to formation of metallic silver surface states at low reaction temperatures which are active for NO dissociation.  2003 Elsevier Science (USA). All rights reserved.

Redox interplay at copper oxide-(Ce, Zr)Ox interfaces: influence of the presence of NO on the catalytic activity for CO oxidation over CUO/CeZrO4

A catalyst composed of copper oxide supported on cerium–zirconium mixed oxide (CuOx/CeZrO4) has been studied with respect to its activity for CO oxidation under stoichiometric conditions employing either oxygen or oxygen with a small amount of NO as oxidant. The nature of copper oxide entities, as well as the redox properties of the catalyst following interactions with CO and O2–NO, has been studied by XPS, EPR, and static NO adsorption infrared spectroscopy while in situ DRIFTS has been employed to follow processes occurring at the catalyst surface under reaction conditions. Characterization of the copper oxide species in both fully oxidized and partially reduced states reveals that they are significantly affected by interactions with the underlying support. On the basis of catalytic activity results and in combination with analysis of the evolution of particular Cu+ carbonyls, comparing the CO oxidation reaction in the presence and absence of NO, it is proposed that the two basic factors affecting the catalytic performance of this type of system are the facility for achieving a partially reduced state for the copper oxide phase at the interfacial zone and the redox properties of the CuOx/CeZrO4 interface.