Redox-dependent activation of CO dehydrogenase from Rhodospirillum rubrum

Studies of initial activities of carbon monoxide dehydrogenase (CODH) from Rhodospirillum rubrum show that CODH is mostly inactive at redox potentials higher than −300 mV. Initial activities measured at a wide range of redox potentials (0–500 mV) fit a function corresponding to the Nernst equation with a midpoint potential of −316 mV. Previously, extensive EPR studies of CODH have suggested that CODH has three distinct redox states: (i) a spin-coupled state at −60 to −300 mV that gives rise to an EPR signal termed Cred1; (ii) uncoupled states at <−320 mV in the absence of CO2 referred to as Cunc; and (iii) another spin-coupled state at <−320 mV in the presence of CO2 that gives rise to an EPR signal termed Cred2B. Because there is no initial CODH activity at potentials that give rise to Cred1, the state (Cred1) is not involved in the catalytic mechanism of this enzyme. At potentials more positive than −380 mV, CODH recovers its full activity over time when incubated with CO. This reductant-dependent conversion of CODH from an inactive to an active form is referred to hereafter as “autocatalysis.” Analyses of the autocatalytic activation process of CODH suggest that the autocatalysis is initiated by a small fraction of activated CODH; the small fraction of active CODH catalyzes CO oxidation and consequently lowers the redox potential of the assay system. This process is accelerated with time because of accumulation of the active enzyme.

Understanding the CO Preoxidation and the Intrinsic Catalytic Activity of Step Sites in Stepped Pt Surfaces in Acidic Medium

In order to deepen the knowledge about the origin of the CO preoxidation process and the intrinsic catalytic activity of Pt superficial steps toward CO oxidation, a series of CO stripping experiments were performed on stepped Pt electrodes in acidic medium. For the occurrence of CO preoxidation, it was found that it arises (reproducibly) whenever four interconnected conditions are simultaneously fulfilled: (1) CO adsorption at potentials lower than about 0.2 V; (2) on surfaces saturated with COads; (3) in the presence of traces of CO in solution; (4) in the presence of surface steps. If any of these four conditions is not satisfied, the CO preoxidation pathway does not appear, even though the steps on the electrode surface are completely covered by CO. By controlling the removal of the CO adlayer (voltammetrically), we show that once the CO adlayer has been partially oxidized, the (111) terrace sites of stepped surfaces are released earlier than the (110) step sites. Moreover, if (110) steps are selectively decorated with CO, its oxidation occurs only at potentials ∼150 mV higher than the CO preoxidation peak. Our results systematically demonstrate that step sites are less active to oxidize CO than those ones responsible for the CO preoxidation process. Once the sites responsible for the CO preoxidation are made free, there is no apparent motion of the remaining adsorbed CO layer, suggesting that the activation of the surface controls the whole process, rather than the diffusion of COads toward hypothetically “most active sites”. Voltammetric and chronoamperometric experiments performed on partially covered CO adlayers suggest that adsorbed CO behave as a motionless species during its oxidation, in which the CO adlayer is removed piece by piece. By means of in situ FTIR experiments, the stretching frequency of CO selectively adsorbed on (110) step sites was examined. Band frequency results confirm that those molecules adsorbed on steps are fully coupled with the adsorbed CO on (111) terraces when the surface reaches full coverage.

Ethanol electrooxidation onto stepped surfaces modified by Ru deposition: electrochemical and spectroscopic studies

Oxidation of ethanol on ruthenium-modified Pt(775) and Pt(332) stepped electrodes has been studied using electrochemical and FTIR techniques. It has been found that the oxidation of ethanol on these electrodes takes place preferentially on the step sites yielding CO(2) as the major final product. The cleavage of the C-C bond, which is the required step to yield CO(2), occurs only on this type of site. The presence of low ruthenium coverages on the step sites promotes the complete oxidation of ethanol since it facilitates the oxidation of CO formed on the step from the cleavage of the C-C bond. However, high ruthenium coverages have an important inhibiting effect since the adatoms block the step sites, which are required for the cleavage of the C-C bond. Under these conditions, the oxidation current diminishes and the major product in the oxidation process is acetic acid, which is the product formed preferentially on the (111) terrace sites.

Synthesis, Characterization and Catalytic Activity of Nanocrystalline Ceria Modified With Zirconia

The use of catalysts in chemical and refining processes has increased rapidly since 1945, when oil began to replace coal as the most important industrial raw material. Catalysis has a major impact on the quality of human life as well as economic development. The demand for catalysts is still increasing since catalysis is looked up as a solution to eliminate or replace polluting processes. Metal oxides represent one of the most important and widely employed classes of solid catalysts. Much effort has been spent in the preparation, characterization and application of metal oxides. Recently, great interest has been devoted to the cerium dioxide (CeO2) containing materials due to their broad range of applications in various fields, ranging from catalysis to ceramics, fuel cell technologies, gas sensors, solid state electrolytes, ceramic biomaterials, etc., in addition to the classical application of CeO2 as an additive in the so-called three way catalysts (TWC) for automotive exhaust treatment. Moreover, it can promote water gas shift and steam reforming reactions, favours catalytic activity at the interfacial metal-support sites. The solid solutions of ceria with Group IV transitional-metals deserve particular attention for their applicability in various technologically important catalytic processes. Mesoporous CeO2−ZrO2 solid solutions have been reported to be employed in various reactions which include CO oxidation, soot oxidation, water-gas shift reaction, and so on. Inspired by the unique and promising characteristics of ceria based mixed oxides and solid solutions for various applications, we have selected ceria-zirconia oxides for our studies. The focus of the work is the synthesis and investigation of the structural and catalytic properties of modified and pure ceria-zirconia mixed oxide.

High activity supported gold catalysts by incipient wetness impregnation

It is generally thought that catalysts produced by incipient wetness impregnation (IW) are very poor for low temperature CO oxidation, and that it is necessary to use methods such as deposition-precipitation (DP) to make high activity materials. The former is true, indeed such IW catalysts are poor, and we present reactor data, XPS and TEM analysis which show that this is due to the very negative effect of the chloride anion involved in the preparation, which results in poisoning and excessive sintering of the Au particles. With the DP method, the chloride is largely removed during the preparation and so poisoning and sintering are avoided. However, we show here that, contrary to previous considerations, high activity catalysts can indeed be prepared by the incipient wetness method, if care is taken to remove the chloride ion during the process. This is achieved by using the double impregnation method (DIM). In this a double impregnation of chloroauric acid and a base are made to precipitate out gold hydroxide within the pores of the catalyst, followed by limited washing. This results in a much more active catalyst, which is active for CO oxidation at ambient temperature. The results for DIM and DP are compared, and it is proposed that the DIM method may represent an environmentally and economically more favorable route to high activity gold catalyst production. (C) 2007 Elsevier B.V. All rights reserved.

CeO(2)-catalyzed ozonation of phenol

Three different cerium citrate-based precursors were used for synthesizing CeO(2) through thermal treatment. Three morphological types of CeO(2) were obtained. Characterization of these oxides was carried out by XRD patterns, SEM microscopy, N(2) adsorption isotherms, Raman spectroscopy, zeta potential, and UV/Vis luminescence. Ozonation of phenol catalyzed by CeO(2) was studied as a representative reaction of environmental interest. The differences on the catalytic activity showed by these three oxides could be correlated to amounts of Ce(3+) on CeO(2) surface and, consequently, to the demand for oxygen needed to burn each precursor.

La(2-x)Ce(x)Cu(1-y)Zn(y)O(4) perovskites for high temperature water-gas shift reaction

The performance of La(2-x)Ce(x)Cu(1-y)Zn(y)O(4) perovskites as catalysts for the high temperature water-gas shift reaction (H T-W G S R) was investigated. The catalysts were characterized by EDS, XRD, BET surface area, TPR, and XANES. The results showed that all the perovskites exhibited the La(2)CuO(4) orthorhombic structure, so the Pechini method is suitable for the preparation of pure perovskite. However, the La(1.90)Ce(0.10)CuO(4) perovskite alone, when calcined at 350/700 degrees C, also showed a (La(0.935)Ce(0.065))(2)CuO(4) perovskite with tetragonal structure, which produced a surface area higher than the other perovskites. The perovskites that exhibited the best catalytic performance were those calcined at 350/700 degrees C and, among these, La(1.90)Ce(0.10)CuO(4) was outstanding, probably because of the high surface area associated with the presence of the (La(0.935)Ce(0.065))(2)CuO(4) perovskite with tetragonal structure and orthorhombic La(2)CuO(4) phase.

Effect of adding CaO to ZrO(2) support on nickel catalyst activity in dry reforming of methane

Nickel catalysts with a load of 5 wt% Ni, supported on pure ZrO(2) and ZrO(2) stabilized with 4, 8 and 14 mol% CaO, were prepared by the polymerization method. The samples were characterized by X-ray diffraction (XRD), temperature-programmed reduction with hydrogen (TPR-H(2)), specific surface area (BET) and impedance spectroscopy (IS) and tested in the carbon dioxide reforming of methane. The XRD patterns showed the presence of the oxide precursor (NiO) and the tetragonal phase of CaO-ZrO(2) solid solutions. According to the TPR-H(2) analysis, the reduction of various NiO species was influenced by the support composition. The electrical properties of the support have a proportional effect on the catalytic activities. Catalytic tests were done at 800 degrees C for 6 h and the composition of the gaseous products and the catalytic conversion depended on the CaO-ZrO(2) solid solution composition and its influence on supported NiO species. A direct relation was found between the variation in the electrical conductivity of the support, the nickel species supported on it and the performance in the catalytic tests. (C) 2009 Elsevier B.V. All rights reserved.

Hydrogen purification for fuel cell using CuO/CeO(2)-Al(2)O(3) catalyst

CuO/CeO(2), CuO/Al(2)O(3) and CuO/CeO(2)-Al(2)O(3) catalysts, with CuO loading varying from 1 to 5 wt.%, were prepared by the citrate method and applied to the preferential oxidation of carbon monoxide in a reaction medium containing large amounts of hydrogen (PROX-CO). The compounds were characterized ex situ by X-ray diffraction, specific surface area measurements, temperature-programmed reduction and temperature-programmed reduction of oxidized surfaces; XANES-PROX in situ experiments were also carried out to study the copper oxidation state under PROX-CO conditions. These analyses showed that in the reaction medium the Cu(0) is present as dispersed particles. On the ceria, these metallic particles are smaller and more finely dispersed, resulting in a stronger metal-support interaction than in CuO/Al(2)O(3) or CuO/CeO(2)-Al(2)O(3) catalysts, providing higher PROX-CO activity and better selectivity in the conversion of CO to CO(2) despite the greater BET area presented by samples supported on alumina. It is also shown that the lower CuO content, the higher metal dispersion and consequently the catalytic activity. The redox properties of the ceria support also contributed to catalytic performance. (C) 2010 Elsevier B.V. All rights reserved.

Potential oscillations in a proton exchange membrane fuel cell with a Pd-Pt/C anode

We report in this paper the occurrence of potential oscillations in a proton exchange membrane fuel cell (PEMFC) with a Pd-Pt/C anode, fed with H(2)/100 ppm CO, and operated at 30 degrees C. We demonstrate that the use of Pd-Pt/C anode enables the emergence of dynamic instabilities in a PEMFC. Oscillations are characterized by the presence of very high oscillation amplitude, ca. 0.8 V. which is almost twice that observed in a PEMFC with a Pt-Ru/C anode under similar conditions. The effects of the H(2)/CO flow rate and cell current density on the oscillatory dynamics were investigated and the mechanism rationalized in terms of the CO oxidation and adsorption processes. We also discuss the fundamental aspects concerning the operation of a PEMFC under oscillatory regime in terms of the benefit resulting from the higher average power output. (c) 2010 Elsevier B.V. All rights reserved.

CuO and CuO-ZnO catalysts supported on CeO(2) and CeO(2)-LaO(3) for low temperature water-gas shift reaction

This paper describes an investigation on CuO and CuO-ZnO catalysts supported on CeO(2) and CeO(2)-La(2)O(3) oxides, which were designed for the low temperature water-gas shift reaction (WGSR). Bulk catalysts were prepared by co-precipitation of metal nitrates and characterized by energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), surface area (by the BET method), X-ray photoelectron spectroscopy (XPS), and in situ X-ray absorption near edge structure (XANES). The catalysts` activities were tested in the forward WGSR, and the CuO/CeO(2) catalyst presented the best catalytic performance. The reasons for this are twofold: (1) the presence of Zn inhibits the interaction between Cu and Ce ions, and (2) lanthanum oxide forms a solid solution with cerium oxide, which will cause a decrease in the surface area of the catalysts. Also the CuO/CeO(2) catalyst presented the highest Cu content on the surface, which could influence its catalytic behavior. Additionally, the Cu and Cu(1+) species could influence the catalytic activity via a reduction-oxidation mechanism, corroborating to the best catalytic performance of the Cu/Ce catalyst. (c) 2010 Elsevier B.V. All rights reserved.

Altering the adsorptive and electronic properties of Pt through poly(vinyl alcohol) adsorption

In the present paper we investigated the effect of adsorbed PVA on Pt electrodes on classic electrochemical processes such as hydrogen UPD, oxygen reduction and CO electro-oxidation. Upon adsorption PVA blocks roughly 50% of the hydrogen sites and can not be removed from the Pt surface through cycling in the potential range of 0.05-1.0 V vs. RHE. Potentiodynamic experiments under controlled hydrodynamic conditions provided by rotating disk electrode experiments showed a negative impact of the adsorbed PVA on the oxygen reduction reaction (ORR). Cyclic-voltammetry results revealed that not even CO was able to remove PVA from the Pt surface. Regarding the oxidation of CO, the adsorbed polymer positively shifted the CO oxidation peak potential, therefore higher potentials are required to free the Pt surface from CO poisoning. In situ Fourier transform infrared spectroscopy evidenced that the presence of PVA shifted the linearly bound CO frequency toward higher wavenumbers, a process found to be independent of the Pt surface orientation. In situ electrochemical X-ray absorption spectroscopy results showed that PVA also impacted the electronic properties of platinum by decreasing the occupancy of the Pt conducting 5d band. Our findings clearly support the efforts toward understanding the nature of the interaction between polymers and metallic surfaces as well as the impact on technological applications (e.g. in PEMFCs). © 2013 Elsevier Ltd. All rights reserved.

Novas estratégias no estudo da eletro-oxidação de etanol: nanocatalisadores multimetálisos e análises dos produtos de reação

Pós-graduação em Química - IQ

Catalisadores 'PT' e 'PTSN' modificados com 'SN''O IND.2' para a oxidação eletroquímica de etanol

Pós-graduação em Química - IQ

Estudo das reações de oxidação de metanol e etanol sobre catalisadores bimetálicos suportados preparados por métodos coloidais

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)