Bi-doped BaBiO3 (x = 0%, 5%, 10%, 15%, and 20%) perovskite oxides by a sol-gel method: comprehensive biological assessment and RhB photodegradation.

The BaBiO3 (BBO) perovskite oxide was prepared via a sol-gel method with different concentrations of Bi nitrate and examined as a photocatalyst for RhB degradation under sunlight, and its antioxidant and antibacterial activities were examined. X-ray diffraction (XRD) indicated the formation of a BaBiO3-BaCO3 (BBO-BCO) binary composite. For the degradation of RhB under solar radiation, high photocatalytic activity (73%) was observed. According to the antibacterial activity study, the addition of Bi enhanced the antibacterial activity of the resulting material against both Gram-positive and Gram-negative microorganisms. The Bi%-BBO (Bi 20%) inhibited 96.23% S. aureus. 10% Bi-BBO as an antioxidant agent had the most efficacious IC50 value of 2.50 mg mL-1. These results seem to suggest that BBO-BCO is a promising catalytic material with potential application in the fields of catalysis and medicine.

Shape-Controlled Pathways in the Hydrogen Production from Ethanol Steam Reforming over Ceria Nanoparticles.

The ethanol surface reaction over CeO2 nanooctahedra (NO) and nanocubes (NC), which mainly expose (111) and (100) surfaces, respectively, was studied by means of infrared spectroscopy (TPSR-IR), mass spectrometry (TPSR-MS), and density functional theory (DFT) calculations. TPSR-MS results show that the production of H2 is 2.4 times higher on CeO2-NC than on CeO2-NO, which is rationalized starting from the different types of adsorbed ethoxy species controlled by the shape of the ceria particles. Over the CeO2(111) surface, monodentate type I and II ethoxy species with the alkyl chain perpendicular or parallel to the surface, respectively, were identified. Meanwhile, on the CeO2(100) surface, bidentate and monodentate type III ethoxy species on the checkerboard O-terminated surface and on a pyramid of the reconstructed (100) surface, respectively, are found. The more labile surface ethoxy species on each ceria nanoshape, which are the monodentate type I or III ethoxy on CeO2-NO and CeO2-NC, respectively, react on the surface to give acetate species that decompose to CO2 and CH4, while H2 is formed via the recombination of hydroxyl species. In addition, the more stable monodentate type II and bidentate ethoxy species on CeO2-NO and CeO2-NC, respectively, give an ethylenedioxy intermediate, the binding of which is facet-dependent. On the (111) facet, the less strongly bound ethylenedioxy desorbs as ethylene, whereas on the (100) facet, the more strongly bound intermediate also produces CO2 and H2 via formate species. Thus, on the (100) facet, an additional pathway toward H2 formation is found. ESR activity measurements show an enhanced H2 production on the nanocubes.

Exceptional Low-Temperature CO Oxidation over Noble-Metal-Free Iron-Doped Hollandites: An In-Depth Analysis of the Influence of the Defect Structure on Catalytic Performance.

A family of iron-doped manganese-related hollandites, K x Mn1-y Fe y O2-δ (0 ≤ y ≤ 0.15), with high performance in CO oxidation have been prepared. Among them, the most active catalyst, K0.11Mn0.876Fe0.123O1.80(OH)0.09, is able to oxidize more than 50% of CO at room temperature. Detailed compositional and structural characterization studies, using a wide battery of thermogravimetric, spectroscopic, and diffractometric techniques, both at macroscopic and microscopic levels, have provided essential information about this never-reported behavior, which relates to the oxidation state of manganese. Neutron diffraction studies evidence that the above compound stabilizes hydroxyl groups at the midpoints of the tunnel edges as in isostructural ß-FeOOH. The presence of oxygen and hydroxyl species at the anion sublattice and Mn3+, confirmed by electron energy loss spectroscopy, appears to play a key role in the catalytic activity of this doped hollandite oxide. The analysis of these detailed structural features has allowed us to point out the key role of both OH groups and Mn3+ content in these materials, which are able to effectively transform CO without involving any critical, noble metal in the catalyst formulation.

Near-ambient XPS characterization of interfacial copper species in ceria-supported copper catalysts.

Catalysts based on combinations of copper and cerium oxides are interesting alternatives to noble metal ones for processes involved in the production/purification of hydrogen produced from hydrocarbons or biomass like the water-gas shift or the preferential oxidation of CO reactions. Active sites for such processes have been proposed to correspond to reduced species formed at the interface between both oxides. The present work provides direct evidence of reduced copper species located at the interface and observed during the course of near-ambient XPS experiments performed over samples of copper oxide supported on ceria nanospheres and nanocubes subjected to interaction with CO at different temperatures. The analysis of XPS results is based on DFT+U calculations employed as a complementary method for the analysis of redox properties of the catalysts and core-level shifts produced upon such redox changes. Differences observed in interfacial redox properties as a function of the ceria support morphology appear to be most useful to explain catalytic properties of this type of system for mentioned processes.

Combinatorial saturation mutagenesis of the Myceliophthora thermophila laccase T2 mutant: the connection between the C-terminal plug and the conserved (509)VSG(511) tripeptide.

A mutant laccase from the Ascomycete Myceliophthora thermophila has been submitted to iterative cycles of combinatorial saturation mutagenesis through in vivo overlap extension in Saccharomyces cerevisiae. Over 180,000 clones were explored, among which the S510G mutant revealed a direct interaction between the conserved (509)VSG(511) tripeptide, located in the neighborhood of the T1 site, and the C-terminal plug. The K(m)(O)(2) value of the mutant increased 1.5-fold, and the electron transfer pathway between the reducing substrate and the T1 copper ion was altered, improving the catalytic efficiency towards non-phenolic and phenolic substrates by about 3- and 8-fold. Although the geometry at the T1 site was perturbed by the mutation, paradoxically the laccase redox potential was not significantly altered. Together, the results obtained in this study suggest that the (509)VSG(511) tripeptide may play a hitherto unrecognized role in regulating the traffic of oxygen through the C-terminal plug, the latter blocking access to the T2/T3 copper cluster in the native enzyme.

Altering the laccase functionality by in vivo assembly of mutant libraries with different mutational spectra.

The generation of diversity for directed protein evolution experiments shows an important bottleneck in the in vitro random mutagenesis protocols. Most of them are biased towards specific changes that eventually confer a predicted and conservative mutational spectrum, limiting the exploration of the vast protein space. The current work describes a simple methodology to in vivo recombine mutant libraries with different nucleotide bias created by in vitro methods. This in vivo assembly was based on the accurate physiology of Saccharomyces cerevisiae, which as host, provided its high homologous recombination frequency to shuffle the libraries in a nonmutagenic way. The fungal thermophilic laccase from Myceliophthora thermophila expressed in S. cerevisiae was submitted to this protocol under the selective pressure of high concentrations of organic solvents. Mutant 2E9 with approximately 3-fold better kinetics than parent type showed two consecutive amino acid changes (G614D -GGC/GAC- and E615K -GAG/AAG-) because of the in vivo shuffling of the mutant libraries. Both mutations are located in the C-terminal tail that is specifically processed at the Golgi during the maturation of the protein by the Kex2 protease. Notoriously, the oxygen consumption at the T2/T3 trinuclear copper cluster was altered and the catalytic copper at the T1 site was perturbed showing differences in its redox potential and geometry. The change in the isoelectric point of C-terminal extension upon mutations seems to affect the folding of the protein at the posttranslational processing steps providing new insights in the significance of the C-terminal tail for the functionality of the ascomycete laccases.

In vitro evolution of a fungal laccase in high concentrations of organic cosolvents.

Fungal laccases are remarkable green catalysts that have a broad substrate specificity and many potential applications in bioremediation, lignocellulose processing, organic synthesis, and more. However, most of these transformations must be carried out at high concentrations of organic cosolvents in which laccases undergo unfolding, thereby losing their activity. We have tailored a thermostable laccase that tolerates high concentrations of cosolvents, the genetic product of five rounds of directed evolution expressed in Saccharomyces cerevisiae. This evolved laccase--R2 variant--was capable of resisting a wide array of cosolvents at concentrations as high as 50% (v/v). Intrinsic laccase features such as the redox potential and the geometry of catalytic copper varied slightly during the course of the molecular evolution. Some mutations at the protein surface stabilized the laccase by allowing additional electrostatic and hydrogen bonding to occur.

Dynamic in situ observation of rapid size and shape change of supported Pd nanoparticles during CO/NO cycling.

Understanding and improving the behaviour of supported precious-metal catalysts for a vast array of environmentally and economically important processes is a central area of research in catalysis. The removal of toxic gases such as CO and NO, without forming others (such as N(2)O), is particularly important. By combining energy-dispersive extended X-ray absorption fine-structure spectroscopy with a vibrational spectroscopy (infrared) and mass spectrometry, at high time resolution, in a single in situ experiment, we dynamically observe and quantify CO-, and subsequent NO-, induced size and shape changes of Pd nanoparticles during CO/NO cycling. In doing so we demonstrate a novel, non-oxidative redispersion (for example, an increase in metal surface area) mechanism, and suggest a model to bridge the structural and reactive functions of supported Pd catalysts.

Impact of alterations near the [NiFe] active site on the function of the H(2) sensor from Ralstonia eutropha.

In proteobacteria capable of H(2) oxidation under (micro)aerobic conditions, hydrogenase gene expression is often controlled in response to the availability of H(2). The H(2)-sensing signal transduction pathway consists of a heterodimeric regulatory [NiFe]-hydrogenase (RH), a histidine protein kinase and a response regulator. To gain insights into the signal transmission from the Ni-Fe active site in the RH to the histidine protein kinase, conserved amino acid residues in the L0 motif near the active site of the RH large subunit of Ralstonia eutropha H16 were exchanged. Replacement of the strictly conserved Glu13 (E13N, E13L) resulted in loss of the regulatory, H(2)-oxidizing and D(2)/H(+) exchange activities of the RH. According to EPR and FTIR analysis, these RH derivatives contained fully assembled [NiFe] active sites, and para-/ortho-H(2) conversion activity showed that these centres were still able to bind H(2). This indicates that H(2) binding at the active site is not sufficient for the regulatory function of H(2) sensors. Replacement of His15, a residue unique in RHs, by Asp restored the consensus of energy-linked [NiFe]-hydrogenases. The respective RH mutant protein showed only traces of H(2)-oxidizing activity, whereas its D(2)/H(+)-exchange activity and H(2)-sensing function were almost unaffected. H(2)-dependent signal transduction in this mutant was less sensitive to oxygen than in the wild-type strain. These results suggest that H(2) turnover is not crucial for H(2) sensing. It may even be detrimental for the function of the H(2) sensor under high O(2) concentrations.

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 degrees C, pH 4.5, yielded values of 26, 0.43, and 0.45 microm 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.

In situ studies of the active sites for the water gas shift reaction over Cu-CeO2 catalysts: complex interaction between metallic copper and oxygen vacancies of ceria.

New information about the active sites for the water gas shift (WGS) reaction over Cu-CeO2 systems was obtained using in-situ, time-resolved X-ray diffraction (TR-XRD), X-ray absorption spectroscopy (TR-XAS, Cu K and Ce L3 edges), and infrared spectroscopy (DRIFTS). Cu-CeO2 nanoparticles prepared by a novel reversed microemulsion method (doped Ce1-xCuxO2 sample) and an impregnation method (impregnated CuOx/CeO2 sample) were studied. The results from all of the samples indicate that both metallic copper and oxygen vacancies in ceria were involved in the generation of active sites for the WGS reaction. Evidence was found for a synergistic Cu-Ovacancy interaction. This interaction enhances the chemical activity of Cu, and the presence of Cu facilitates the formation of O vacancies in ceria under reaction conditions. Water dissociation occurred on the Ovacancy sites or the Cu-Ovacancy interface. No significant amounts of formate were formed on the catalysts during the WGS reaction. The presence of strongly bound carbonates is an important factor for the deactivation of the catalysts at high temperatures. This work identifies for the first time the active sites for the WGS reaction on Cu-CeO2 catalysts and illustrates the importance of in situ structural studies for heterogeneous catalytic reactions.

Unusual physical and chemical properties of Cu in Ce(1-x)Cu(x)O(2) oxides.

The structural and electronic properties of Ce(1-x)Cu(x)O(2) nano systems prepared by a reverse microemulsion method were characterized with synchrotron-based X-ray diffraction, X-ray absorption spectroscopy, Raman spectroscopy, and density functional calculations. The Cu atoms embedded in ceria had an oxidation state higher than those of the cations in Cu(2)O or CuO. The lattice of the Ce(1)(-x)Cu(x)O(2) systems still adopted a fluorite-type structure, but it was highly distorted with multiple cation-oxygen distances with respect to the single cation-oxygen bond distance seen in pure ceria. The doping of CeO(2) with copper introduced a large strain into the oxide lattice and favored the formation of O vacancies, leading to a Ce(1-x)Cu(x)O(2-y) stoichiometry for our materials. Cu approached the planar geometry characteristic of Cu(II) oxides, but with a strongly perturbed local order. The chemical activities of the Ce(1-x)Cu(x)O(2) nanoparticles were tested using the reactions with H(2) and O(2) as probes. During the reduction in hydrogen, an induction time was observed and became shorter after raising the reaction temperature. The fraction of copper that could be reduced in the Ce(1-x)Cu(x)O(2) oxides also depended strongly on the reaction temperature. A comparison with data for the reduction of pure copper oxides indicated that the copper embedded in ceria was much more difficult to reduce. The reduction of the Ce(1-x)Cu(x)O(2) nanoparticles was rather reversible, without the generation of a significant amount of CuO or Cu(2)O phases during reoxidation. This reversible process demonstrates the unusual structural and chemical properties of the Cu-doped ceria materials.