Identification of the active species generated from supported Pd catalysts in Heck reactions: an in situ quick scanning EXAFS investigation

Quick scanning extended X-ray absorption fine structure (QEXAFS) studies in the subsecond time scale have been performed to gain insight into the reaction mechanism of Heck-type C-C coupling reactions in the presence of supported Pd-based catalysts. Using a specially designed in situ EXAFS cell, both the solid catalyst and the liquid reaction mixture during the reaction of phenyl bromide (PhBr) with styrene were monitored. Soluble Pd species were only, but rapidly, detected in the liquid reaction phase once the reaction temperature of 150 °C was reached. At the same time, the conversion of PhBr started, and during the following "active phase" of the catalyst hardly any changes in the corresponding EXAFS and XANES spectra were observed. The present species could be identified as colloidal Pd(0) clusters with a size of ～2 nm estimated from the corresponding EXAFS spectra. The QEXAFS mode not only allowed monitoring rapid changes in the second time scale but also permitted minimization of effects caused by the heterogeneity of the systems. When the reaction rate started to decrease, pronounced changes in the EXAFS spectra were observed, which were attributed to an increased formation of bromo-palladates ([PdBr(4)](2-), [Pd(2)Br(6)](2-)). In addition to the liquid-phase species, significant changes were observed for the solid catalyst that was also probed in situ during the reaction. The originally oxidized Pd catalyst was efficiently reduced upon heating. Additionally, growth of the supported Pd particles was observed by both EXAFS and STEM. The above results confirm the role of the soluble molecular Pd species as the catalytically active species and clarify their conjunction with the in situ formed Pd colloids. Furthermore, the investigation demonstrates the potential of the QEXAFS not only for monitoring rapid changes during catalysis but also for gaining deeper insight into the mechanism of such complex industrially important systems under relevant reaction conditions.     

In situ investigation of the oxidative addition in homogeneous Pd catalysts by synchronised time resolved UV-Vis/EXAFS

Simultaneous structuro-kinetic information obtained via time resolved stopped-flow/UV-Vis spectroscopy/dispersive EXAFS (EDE) experiments elucidated a two-step process for the addition of iodobenzene to [(Ph3P)2Pd(dba)].     

Copper-supported catalysts in methanol synthesis and water gas shift reaction

The comparative studies of physicochemical properties of Cu-support catalysts (support = ZnO, Al₂O₃, Cr₂O₃, ZnAl₂O₄, FeAlO₃, or CrAl₃O₆) and their catalytic activity in methanol synthesis and water gas shift reaction were the main goal of this work. The promotion effect of copper addition in both reaction was proved. The formation of spinel type structure CuCr₂O₄, ZnAl₂O₄ and binary oxide CrAl₃O₆, FeAlO₃ during calcination process was confirmed by XRD technique. Results showed that 20% Cu/FeAlO₃ had the best performance in water gas shift reaction. The best selective and active catalyst in methanol synthesis was 20% Cu/ZnAl₂O₄.     

On the mechanism of low-temperature water gas shift reaction on copper

Periodic, self-consistent density functional theory (DFT-GGA) calculations are used to investigate the water gas shift reaction (WGSR) mechanism on Cu(111). The thermochemistry and activation energy barriers for all the elementary steps of the commonly accepted redox mechanism, involving complete water activation to atomic oxygen, are presented. Through our calculations, we identify carboxyl, a new reactive intermediate, which plays a central role in WGSR on Cu(111). The thermochemistry and activation energy barriers of the elementary steps of a new reaction path, involving carboxyl, are studied. A detailed DFT-based microkinetic model of experimental reaction rates, accounting for both the previous and the new WGSR mechanism show that, under relevant experimental conditions, (1) the carboxyl-mediated route is the dominant path, and (2) the initial hydrogen abstraction from water is the rate-limiting step. Formate is a stable "spectator" species, formed predominantly through CO2 hydrogenation. In addition, the microkinetic model allows for predictions of (i) surface coverage of intermediates, (ii) WGSR apparent activation energy, and (iii) reaction orders with respect to CO, H2O, CO2, and H2.     

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.     

Identification of relevant active sites and a mechanism study for reverse water gas shift reaction over Pt/CeO_2 catalysts

Reverse water gas shift(RWGS) reaction can serve as a pivotal stage in the CO_2 conversion processes,which is vital for the utilization of CO_2.In this study,RWGS reaction was performed over Pt/CeO_2 catalysts at the temperature range of 200-500℃ under ambient pressure.Compared with pure CeO_2,Pt/CeO_2catalysts exhibited superior RWGS activity at lower reaction temperature.Meanwhile,the calculated TOF and E_a values are approximately the same over these Pt/CeO_2 catalysts pretreated under various calcination conditions,indicating that the RWGS reaction is not affected by the morphologies of anchored Pt nanoparticles or the primary crystallinity of CeO_2.TPR and XPS results indicated that the incorporation of Pt promoted the reducibility of CeO_2 support and remarkably increased the content of Ce~(3+) sites on the catalyst surface.Furthermore,the CO TPSR-MS signal under the condition of pure CO_2 flow over Pt/CeO_2catalyst is far lower than that under the condition of adsorbed CO_2 with H_2-assisted flow,revealing that CO_2 molecules adsorbed on Ce~(3+) active sites have difficult in generating CO directly.Meanwhile,the adsorbed CO_2 with the assistance of H_2 can form formate species easily over Ce~(3+) active sites and then decompose into Ce~(3+)-CO species for CO production,which was identified by in-situ FTIR.     

Nanocrystalline gold and gold-palladium alloy oxidation catalysts: a personal reflection on the nature of the active sites

The discoveries that supported gold nanoparticles are exceptionally efficient catalysts for a whole range of reactions is one of the most surprising revelations in the recent history of chemistry. For many years gold was considered too noble to be chemically interesting. However, when divided to the nano-scale, small gold nanoparticles comprising relatively few atoms, are very efficient in catalysing a range of redox and chemical synthesis reactions. In this perspectives article some of the recent research in this rapidly evolving field is highlighted, and the nature of the active site for oxidation reactions is commented on.     

A HYSCORE investigation of bimetallic titanium-vanadium microporous catalysts: elucidating the nature of the active sites

Vanadium and titanium bimetallic AlPO-5 molecular sieves have been synthesized and characterized by means of Electron Spin Echo detected EPR and Hyperfine Sublevel Correlation (HYSCORE) spectroscopy. Direct evidence for framework substitution of redox-active Ti ions and VO(2+) units at Al sites is provided through the detection of large (31)P hyperfine couplings.     

DFT investigation of the interaction between gold(I) complexes and the active site of thioredoxin reductase

The binding of the [Au(PMe₃)]⁺ fragment to cysteine, selenocysteine and the tetrapeptides H₂NGlyCysAGlyCOOH (A = Cys, Sec) has been investigated by DFT methods as a model for the binding of gold(I) to the selenium-containing active site of thioredoxin reductase. The calculations demonstrate both a higher acidity of Se-H compared to S-H and a stronger binding of gold at the selenium site compared to sulphur. Se-H dissociation at the selenium site increases the reducing power of the tetrapeptide H₂NGlyCysSecGlyCOOH whilst gold coordination at selenium has the opposite effect.     

Analysis of in situ EXAFS data of supported metal catalysts using the third and fourth cumulant

X-Ray absorption spectra of supported Pt catalysts with various Pt cluster sizes were collected between 77 and 673 K, in inert and hydrogen atmospheres. When analyzing these spectra with the standard EXAFS equation, a Pt-Pt bond contraction and a large increase in the inner potential correction were observed with increasing temperature. These errors are up to 0.08 A and 10 eV for clusters of 1 nm diameter. They were corrected by including the third and fourth cumulants as fit parameters. Fit guidelines were developed to analyze EXAFS data of supported metal catalysts collected at elevated temperatures, allowing for asymmetry and broadening or sharpening of the pair distribution function. These comprise fixing fit parameters, using different k-weightings and identifying trends in a series of experiments. By fitting the EXAFS spectra using these guidelines, it was determined that in small Pt clusters the Pt-Pt bond is 0.10 A shorter than in bulk Pt. These contracted bonds relax to distances near that of bulk Pt upon hydrogen chemisorption.     

New approaches to the preparation of highly efficient chromium-containing oxide catalysts for the water gas shift reaction

It was found experimentally that the solutions of Cr³⁺ nitrate and the nitrates of other metals that are the constituents of Cr-containing catalysts can be prepared by dissolving a corresponding metal (for example, cast iron and electrolytic copper) in a solution of chromic anhydride and nitric acid to reach the quantitative reduction of Cr⁶⁺ without the formation of nitrogen oxides. Analogously, the oxidation of Fe²⁺ cations to Fe³⁺ coupled with the reduction of hexavalent chromium can be performed. The precipitation of Fe³⁺, Cr³⁺, and Cu²⁺ ions at a ratio of Fe: Cr = 9 and a concentration of Cu²⁺ to 20 at % can result in the formation of a partially hydrated oxide with the hydrohematite structure—a dispersed and highly defective oxide structure with a high specific surface area more than 300 m²/g and a higher thermal stability, as compared with the goethite phase (α-FeOOH). The dehydration of hydrohematite occurred at a noticeable rate at temperatures higher than 400°C. Hydrohematite promoted with copper cations exhibited high activity below 400°C; this can decrease the starting temperature of the adiabatic high-temperature WGSR to 300°C or below.     

WOx/ZrO2 catalysts prepared by anionic exchange: in situ Raman investigation from the precursor solutions to the calcined catalysts

W/ZrO(2) catalysts were prepared using anionic exchange of peroxotungstate species with hydroxyl groups of zirconium hydroxide at low pH. The solids were dried and calcined under air at 700 degrees C. Each step of this novel method of preparation was investigated by Raman spectroscopy. A reference sample was also prepared by incipient wetness impregnation of ZrO(2).n(H2O) with an ammonium tungstate solution and characterized throughout its preparation process. Complementary data were collected from X-ray diffraction, chemical analysis, surface area measurements, and thermal analysis. The Raman spectra of the H2WO4-H2O2 precursor solutions evidenced the presence of (W2O3(O2)4(H2O)2)(2-) dimers. These low-nuclearity species were exchanged with zirconium hydroxide at low pH. The Raman spectra of the dried solids did not reveal peroxotungstate species but were typical of tetrahedral (WO4)(2-) species. A slight agglomeration of W species was observed with an increase in the W content. However, for an equivalent W loading, a higher W dispersion was obtained by anionic exchange, compared to the impregnation method. Furthermore, a remarkable homogeneity of the exchanged samples was evidenced by the micro-Raman spectra. The in situ Raman spectra recorded during calcination characterized both crystalline phases and supported tungsten species. Significant modifications were observed during the calcination process. The exchanged and the impregnated samples, with the same W loading, evidenced a similar type of tungsten species with one W=O bond. However, their behavior during calcination up to 700 degrees C was different. This was attributed to different strengths of interaction with the support. Moreover, the spectra recorded after calcination on various points of the exchanged sample with a high W content revealed a better spatial homogeneity than the impregnated one.     

沉淀剂对铜锰复合氧化物变换催化剂构效的影响

以CuSO4.5H2O和MnSO4.H2O为原料,KOH和NaOH为沉淀剂制备了铜锰复合氧化物,考察了其变换反应催化性能,利用XRD、低温氮气吸附法、TG、H2-TPR等对所合成样品进行了表征。以KOH和NaOH为沉淀剂所得沉淀终产物的物相组成和织构明显不同,分别为层状结构碱式硫酸铜Cu4SO4(OH)6.H2O及无定形锰氧化物和Cu2+1O和Mn3O4混合物。两种物相组成和织构完全不同的沉淀终产物焙烧后都生成Cu1.5Mn1.5O4固熔体,在变换反应条件下均转化为Cu和MnO,但其催化性能却有明显差异。以NaOH为沉淀剂,得到以Cu2+1O和Mn3O4复合体为主的沉淀终产物,焙烧及还原后保持了较高的织构稳定性,提高了样品的活性和热稳定性。而以KOH为沉淀剂得到以层状结构碱式硫酸铜Cu4SO4(OH)6.H2O和无定形锰氧化物为主的沉淀终产物,在焙烧过程发生的演变极其复杂,削弱了铜锰组分协同效应,造成其活性和热稳定性极差。研究结果表明,NaOH作沉淀剂所制备样品的织构稳定性、催化活性显著高于以KOH作沉淀剂所制备样品,且热稳定性良好。     

Inverse gas chromatographic investigation of the active sites related to CO adsorption over Rh/SiO2 catalysts in excess of hydrogen

In the present work, the novel methodology of reversed-flow gas chromatography (RF-GC) is extended in a topic of contemporary scientific and technological interest, such as the effect of hydrogen in the "topography" of the active sites related to CO adsorption. This study concerns CO adsorption on a silica-supported Rh catalyst, at 90 degrees C. The topography of the catalyst in the absence of hydrogen consists of both randomly and islands of CO bound over chemisorbed CO molecules. In contrast, under H2-rich conditions, the observed topography is almost entirely patchwise ascribed to long-range lateral attractions between adsorbate molecules. In excess of hydrogen, CO adsorption is shifted at higher lateral attractions values which correspond to weaker adsorbate-adsorbent interactions and lower surface coverage. This provides an indication of a H2-induced desorption, which can be attributed to the formation of an H-CO complex desorbing from the catalyst surface below the temperature required for CO desorption, in the absence of H2, and it may explain the well-known enhancement of the rate of selective CO oxidation in excess of hydrogen by H2.     

In situ time-resolved characterization of Au-CeO2 and AuOx-CeO2 catalysts during the water-gas shift reaction: presence of Au and O vacancies in the active phase

Synchrotron-based in situ time-resolved x-ray diffraction and x-ray absorption spectroscopies were used to study the behavior of nanostructured {Au+AuO(x)}-CeO(2) catalysts under the water-gas shift (WGS) reaction. At temperatures above 250 degrees C, a complete AuO(x)-->Au transformation was observed with high catalytic activity. Photoemission results for the oxidation and reduction of Au nanoparticles supported on rough ceria films or a CeO(2)(111) single crystal corroborate that cationic Au(delta+) species cannot be the key sites responsible for the WGS activity at high temperatures. The rate determining steps for the WGS seem to occur at the gold-ceria interface, with the active sites involving small gold clusters (<2 nm) and O vacancies.     

CeO_2-Al_2O_3负载金催化剂用于水煤气变换反应的催化活性

采用浸渍法和沉积-沉淀法制备了CeO2-Al2O3复合氧化物,比较了复合氧化物负载纳米金催化剂对水煤气变换反应的催化活性。通过N2物理吸附、XRD、TEM、H2-TPR等表征手段对复合氧化物及其负载金催化剂的物相和结构进行分析,发现复合氧化物的制备方法及其焙烧温度对其比表面积、孔结构及水煤气变换反应活性有明显的影响。与沉积-沉淀法相比,浸渍法制备的CeO2-Al2O3复合氧化物具有较大的CeO2晶粒尺寸,经500℃焙烧后再负载金,所得催化剂具有更高的活性,250℃时CO转化率可达78.1%。     

New insights in the electrocatalytic proton reduction and hydrogen oxidation by bioinspired catalysts: a DFT investigation

In this paper, we present a DFT study of the proton reduction mechanism catalyzed by the complex [Ni(P?(H)N?(H))?](2+), bioinspired from the hydrogenases. A detailed analysis of the reactive isomers is discussed together with the localizations of the transitions states and energy minima. The reactive catalytic species is a biprotonated Ni(0) complex that can show different conformations and that can be protonated on different sites. The energies of the different conformations and biprotonated species have been calculated and discussed. Energy barriers for two different reaction mechanisms have been identified in solvent and in gas phase. Frequency calculations have been performed to check the nature of the energy minima and for the calculations of entropic energetic terms and zero point energies. We show that only one conformation is mostly reactive. All the others species are nonreactive in their original form, and they have to pass through conformational barriers in order to transform in the reactive species.     

Comparative studies of Pd, Ru, Ni, Cu/ZnAl2O4 catalysts for the water gas shift reaction

The activity of monometallic Pd, Ru, Ni and Cu catalysts supported on spinel ZnAl₂O₄ for water gas shift reaction (WGS) was investigated. The physicochemical properties of each catalyst was studied by XRD, TPR, BET and chemisorption methods. The highest activity was obtained for Cu/ZnAl₂O₄ among the catalysts tested. The activation process carried out in a reducing atmosphere 5%H₂-95%Ar in the case of Cu/ZnAl₂O₄ system lead to the catalytic activity improvement. In the case of copper catalysts, the water gas shift reaction proceeded by the redox surface mechanism between Cu⁰/Cu⁺. The PdZn alloy formation after reduction at 350°C was shown.