[Show abstract][Hide abstract] ABSTRACT: H/D exchange experiments on a Cu/ZnO/Al2O3 catalyst have shown that methanol synthesis and RWGS display a strong thermodynamic isotope effect, which is attributed to differences in the zero-point energy of hydrogenated vs. deuterated species. The effect is larger for methanol synthesis and substantially increases the equilibrium yield in deuterated syngas. In the kinetic regime of CO2 hydrogenation, an inverse kinetic isotope effect of H/D substitution was observed, which is stronger for methanol synthesis than for CO formation suggesting that the two reactions do not share a common intermediate. Similar observations were also made on other catalysts such as Cu/MgO, Cu/SiO2, and Pd/SiO2. In contrast to CO2 hydrogenation, the CO hydrogenation on Cu/ZnO/Al2O3 did not show such a strong kinetic isotope effect indicating that methanol formation from CO2 does not proceed via consecutive reverse water gas shift and CO hydrogenation steps. The inverse KIE is consistent with formate hydrogenation being the rate-determining step of methanol synthesis from CO2. Differences in the extent of product inhibition by water, observed for methanol synthesis and reverse water gas shift indicate that the two reactions proceed on different surface sites in a parallel manner. The consequences for catalyst design for effective methanol synthesis from CO2 are discussed.
Journal of Catalysis 08/2015; 328. DOI:10.1016/j.jcat.2014.12.016 · 6.07 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Deactivation behavior is an important topic in catalyst development. In case of methanol synthesis the conventional Cu/ZnO/Al2O3 system is commonly known to be prone to sintering, however, information about the structural development during deactivation or the sintering mechanism(s) are scarce. We present a systematic deactivation study on three different Cu/ZnO/Al2O3 catalysts which are aged under constant conditions and periodically analyzed using kinetic measurements and N2O chemisorption. A power law model for the catalyst activity with time on stream is derived. Furthermore it is found, that the presence of water provokes a steep loss in active surface area and specific activity. Also, the TEM particle size distributions generated during the aging treatment are evaluated and discussed.
[Show abstract][Hide abstract] ABSTRACT: There is a great interest in electrochemical water splitting for the efficient utilization of sustainable energy. As an alternative to high-priced materials, carbon offers a considerable potential. However, carbon is limited as an electrode material for oxygen evolution reaction (OER) due to its thermodynamic instability against electrochemical oxidation. In this study, we investigated the electrochemical degradation of multiwall carbon nanotubes (MWCNTs) under the acidic OER environment. Electrochemical oxidation of MWCNTs induces structural changes and the formation of oxygen-containing functional groups on the carbon surface. We carried out electrochemical, microstructural and spectroscopic analysis in order to investigate the degradation of MWCNTs. By changing of the electrochemical properties of MWCNTs during the oxidation process, carbon is initially activated and then can be kinetically stabilized in the OER condition.
[Show abstract][Hide abstract] ABSTRACT: X-ray photoelectron spectroscopy (XPS) is a widely used technique for characterizing the chemical and electronic properties of highly ordered carbon nanostructures, such as carbon nanotubes and graphene. However, the analysis of XPS data—in particular the C 1s region—can be complex, impeding a straightforward evaluation of the data. In this work, an overview of extrinsic and intrinsic effects that influence the C 1s XPS spectra—for example, photon broadening or carbon–catalyst interaction—of various graphitic samples is presented. Controlled manipulation of such samples is performed by annealing, sputtering, and oxygen functionalization to identify different CC bonding states and assess the impact of the manipulations on spectral line shapes and their binding energy positions. With high-resolution XPS and XPS depth profiling, the spectral components arising from disordered carbon and surface-defect states can be distinguished from aromatic sp2 carbon. These findings illustrate that both spectral line shapes and binding energy components must be considered in the analysis of potentially defective surfaces of carbon materials. The sp2 peak, characteristic of aromatic carbon, features a strong asymmetry that changes with the curvature of the sample surface and, thus, cannot be neglected in spectral analysis. The applied deconvolution strategy may provide a simple guideline to obtaining high-quality fits to experimental data on the basis of a careful evaluation of experimental conditions, sample properties, and the limits of the fit procedure.
[Show abstract][Hide abstract] ABSTRACT: The development of efficient energy conversion systems requires precise engineering of electrochemical interfaces and thus asks for in situ techniques to probe the structure and the composition of the dynamic electrode/electrolyte interfacial region. This work demonstrates the potential of the Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAPXPS) for in situ studies of processes occurring at the interface between a metal electrode and a liquid electrolyte. By using a model membrane-electrode assembly of a high temperature phosphoric acid-imbibed proton exchange membrane fuel cell, and combining NAPXPS measurements with the density functional theory, it was possible to monitor such fundamental proceccess as dissociation and migration of the phosphoric acid within a nanostructured Pt electrode under polarization.
Chemical Science 07/2015; DOI:10.1039/C5SC01421B · 9.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Abstract The role of surface structure and defects in the oxidative coupling of methane (OCM) was studied over magnesium oxide as a model catalyst. Pure, nano-structured MgO catalysts with varying primary particle size, shape and specific surface area were prepared by sol–gel synthesis, oxidation of metallic magnesium, and hydrothermal post treatments. The initial activity of MgO in the OCM reaction is clearly structure-sensitive. Kinetic studies reveal the occurrence of two parallel reaction mechanisms and a change in the contribution of these pathways to the overall performance of the catalysts with time on stream. The initial performance of freshly calcined MgO is governed by a surface-mediated coupling mechanism involving direct electron transfer between methane and oxygen. The two molecules are weakly adsorbed at structural defects (steps) on the surface of MgO. The proposed mechanism is consistent with high methane conversion, a correlation between methane and oxygen consumption rates, and high C2H4 selectivity after short times on stream. The water formed in the OCM reaction causes sintering of the MgO particles and loss of active sites by degradation of structural defects, which is reflected in decreasing activity of MgO with time on stream. At the same time, gas-phase chemistry becomes more important, which includes the formation of ethane by coupling of methyl radicals formed at the surface and the partial oxidation of C2H6. The mechanistic concepts proposed in this work (Part I) will be substantiated in Part II by spectroscopic characterization of the catalysts (Schwach et al. ).
Journal of Catalysis 06/2015; DOI:10.1016/j.jcat.2015.05.007 · 6.07 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A series of pure, nanostructured magnesium oxides prepared by different synthesis techniques that show different initial, but similar steady-state activity in the oxidative coupling of methane (OCM) (Schwach et al., submitted for publication) has been studied by infrared and photoluminescence spectroscopy in the dehydroxylated state before the reaction and after catalysis. The abundance of structural defects, in particular mono-atomic steps, on the dehydroxylated MgO surface characterized by a band in the FTIR spectrum of adsorbed CO at 2146 cm−1 and Lewis acid/base pairs probed by co-adsorption of CO and CH4 correlate with the initial rates of both methane consumption and C2+ hydrocarbon formation. Infrared spectroscopy evidences strong polarization of C-H bonds due to adsorption of methane on dehydroxylated MgO surfaces that contain a high number of mono-atomic steps. It is postulated that these sites effectively promote intermolecular charge transfer between adsorbed methane and weakly adsorbed oxygen that leads to the dissociation of one C-H bond in the methane molecule and simultaneous formation of a superoxide species. Heterolytic splitting of C-H bonds in the presence of oxygen at the surface of dehydroxylated MgO already at room temperature has been proven by the appearance of an EPR signal associated with superoxide species that are located in close vicinity to a proton. With time on stream, MgO sinters and loses activity. The deactivation process involves the depletion of mono-atomic steps and the reconstruction of the MgO termination under formation of polar and faceted surfaces.
Journal of Catalysis 06/2015; DOI:10.1016/j.jcat.2015.05.008 · 6.07 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The stepwise substitution of Al by Cr and Ga leads to quaternary LDH precursors for Cu/ZnM2O4 (M = Al, Ga, Cr) catalysts. With the substitution of Al by Cr the interaction of the Cu phase with the oxide matrix is gradually weakened, which is caused by the participation of the chromium oxide phase in the redox processes during catalyst preparation. Such reactive Cr oxide matrix is less efficient than the inert Al oxide matrix in stabilizing the special microstructure of Cu/ZnM2O4 catalysts. These weakened interactions led to a lowering of the Cu particle embedment, coinciding with a pronounced Cu crystallite growth during reduction. Both effects partially compensate each other and a maximum in Cu surface area is observed for intermediate Cr contents. In the Ga-substituted catalysts, two distinct Cu species were found for high Ga contents. This is attributed to the presence of partially crystalline spinel and the resulting different strength of interface interaction of the CuO phase with the crystalline and the amorphous oxide. After reduction Cu catalysts with similar average Cu particle sizes as well as Cu surface areas were obtained. In both sample series, the catalytic activity in methanol synthesis does not scale with the Cu surface area and the experiments show that a strong interaction to the oxide is necessary to gain stability and activity of the Cu phase. Al substitution thus confirms that interface interactions between Cu and the oxide seem to beneficially affect the activity of the Cu particles and the optimal catalyst requires a compromise of exposed surface and interface.
[Show abstract][Hide abstract] ABSTRACT: A series of differently loaded palladium-iron catalysts was prepared by a controlled co-precipitation method of the nitrate precursors, in order to ensure homogeneous Pd particle size-distribution. After characterization of the pre-catalysts by various techniques, different controlled reduction conditions were applied to investigate the interactions within the Pd-iron system, containing reversible and irreversible processes like phase transformations, SMSI, sintering and alloying. Strong indications for the reversible surface decoration of the Pd nanoparticles with iron oxide species via strong metal-support interaction were found by the combined results of DRIFTS, CO-chemisorption, TEM and XPS measurements. This SMSI state was found to be unstable. It was observed independent of bulk phase or palladium particle size. Catalytic CO-oxidation was found to be a suitable test reaction for the study of the phenomenon: higher activity as well as oxidative deactivation of the SMSI state was observed by investigating the light-off behavior in repeated, temperature-programmed cycles as well as by isothermal measurements. The instability was found to be higher in case of higher Pd dispersion. In addition, bulk properties of the Pd-Fe system, like alloying, were investigated by detailed XRD measurements.
Applied Catalysis A General 05/2015; 502. DOI:10.1016/j.apcata.2015.04.010 · 3.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this work, the geometric and electronic structure of N species in N-doped carbon nanotubes (NCNTs) is derived by X-ray photoemission (XPS) and absorption spectroscopy (NEXAFS) of the N1s core excitation. Substitutional N species in pyridine-like configuration and another form of N with higher thermal stability are found in NCNTs. The structural configuration of the high thermally stable N species, in literature often referred to as graphitic N, is assessed in this work by a combined theoretical and experimental study as a three-fold substitutional N species in an NCNT basic structural unit (BSU). Furthermore, the nature of the interaction of those N species with a Pd metal center immobilized onto NCNTs is of σ-type donation from filled orbital of the N atom to empty orbital of the Pd atom and a π back-donation from filled Pd atomic orbital to anti-bonding orbital of the N atom. We have found that the interaction of pyridine N with Pd is characterized by a charge transfer typical of a covalent chemical bond with partial ionic character, consistent with the chemical shift observed in the Pd3d core level of divalent Pd. Graphitic N sites interact with Pd by a covalent bond without any charge redistribution. In this case the electronic state of the Pd corresponds to metallic Pd nanoparticles electronically modified by the interaction with the support. The catalytic reactivity of these samples in hydrogenation, CO oxidation, and oxygen reduction reaction (ORR) allowed clarifying some aspects of the metal carbon support interaction in catalysis.
[Show abstract][Hide abstract] ABSTRACT: Carbon is emerging as an important metal-free catalyst for multiple types of heterogeneous catalysis, including thermocatalysis, photocatalysis, and electrocatalysis. However, the study of mechanisms for carbon catalysis has been impeded at an early stage due to the lack of quantitative research, especially the intrinsic kinetics (e.g., intrinsic TOF). In many carbon-catalyzed reactions, the surface oxygenated groups were found to be the active sites. Recently, we have shown that these oxygenated groups could be identified and quantified via poisoning by small organic molecules; however, these small molecules were toxic. As most of the oxygenated groups are acidic groups, they could also be identified and quantified with respect to the acid properties. More importantly, the method based on acid properties is very green and environmentally benign, because only inorganic bases are added. In this work, the acid properties of carbon nanotubes (CNTs) treated by concentrated HNO3 were thoroughly studied by mass titration and Boehm titration. The two titration methods were also compared to the conventional methods for acidity analysis including NH3 pulse adsorption, NH3-TPD, and FT-IR. Boehm titration was very effective to quantify the carboxylic acid, lactone, phenol, and carbonyl groups, and the findings were consistent with the results from XPS and NH3 pulse adsorption. These CNTs were applied in the oxidative dehydrogenation (ODH) of ethylbenzene, and the activity of these catalysts exhibited a good linear dependence on the number of carbonyl groups. The value of TOF for the carbonyl group obtained from Boehm titration was 3.2 × 10–4 s–1 (245 °C, atmosphere pressure, 2.8 kPa ethylbenzene, 5.3 kPa O2). For better understanding the acidity of nanocarbon, these CNTs were also applied in two acid-catalyzed reactions (Beckmann rearrangement and ring opening), and a good linear relationship between the conversion and the number of acidic sites was found.
[Show abstract][Hide abstract] ABSTRACT: The promoting effect of Al, Ga and Mg on the support in Cu/ZnO catalyst for methanol synthesis has been investigated. Different unpromoted and promoted ZnO supports have been synthesized and impregnated with Cu metal in a subsequent step. All materials, supports, calcined and activated catalysts have been characterized by various methods including contactless (microwave) conductivity measurements in different gas atmospheres. Small amounts of promoters were found to have significant influence on the properties of the oxide support, concerning textural as well as electronic properties. We found correlations between the conductivity of the ZnO support and the activity of the catalyst in the reverse water gas shift reaction (rWGS) as well as in methanol synthesis. In rWGS the activation energy and reaction order in H2 are decreased upon promotion of the ZnO support with the trivalent promoters Al3+ and Ga3+ indicating an electronic promotion. In methanol synthesis, results point to a structural promotion by Al3+ and Ga3+. A detrimental effect of Mg2+ doping was observed in both reactions. This effect is discussed in the context of the reducibility of ZnO under reaction conditions, which can be tuned by the promoter in different ways. The reducibility is seen as a critical property for the dynamic metal support interaction of the Cu/ZnO system.
[Show abstract][Hide abstract] ABSTRACT: The cover picture shows four different pathways for methanol synthesis over Cu-based catalysts depending on the feed composition and the presence of the Zn-promoter. In their Full Paper, F. Studt et al. present a DFT-based microkinetic model and consistent experimental data that explain the Janus-faced character of Cu in COx hydrogenation allowing only one path, CO or CO2 hydrogenation, to be effective at a time. The industrial Zn-promoter accelerates CO2 conversion over Cu/ZnO, but can simultaneously act as a poison for CO hydrogenation. On the other hand, CO2 in the feed was found to poison unpromoted catalysts such as Cu/MgO, which was highly efficient for CO hydrogenation. The article highlighted by this cover can be found on p. 1105 ff. of Issue 7, 2015.
[Show abstract][Hide abstract] ABSTRACT: In industrially relevant Cu/ZnO/Al2O3 catalysts for methanol synthesis, the strong metal support interaction between Cu and ZnO is known to play a key role. Here we report a detailed chemical transmission electron microscopy study on the nanostructural consequences of the strong metal support interaction in an activated high-performance catalyst. For the first time, clear evidence for the formation of metastable “graphite-like” ZnO layers during reductive activation is provided. The description of this metastable layer might contribute to the understanding of synergistic effects between the components of the Cu/ZnO/Al2O3 catalysts.
[Show abstract][Hide abstract] ABSTRACT: The influence of redox dynamics of a Ni/MgAl oxide catalyst for dry reforming of methane (DRM) at high temperature was studied to correlate structural stability with catalytic activity and coking propensity. Structural aging of the catalyst was simulated by repeated temperature-programmed reduction/oxidation (TPR/TPO) cycles. Despite a very high Ni loading of 55.4 wt.%, small Ni nanoparticles of 11 nm were obtained from a hydrotalcite-like precursor with a homogeneous distribution. Redox cycling gradually changed the interaction of the active Ni phase with the oxide support resulting in a crystalline Ni/MgAl2O4-type catalyst. After cycling the average particle size increased from 11 to 21 nm – while still a large fraction of small particles was present – bringing about a decrease in Ni surface area of 72%. Interestingly, the redox dynamics and its strong structural and chemical consequences were found to have only a moderate influence on the activity in DRM at 900 °C, but lead to a stable attenuation of carbon formation due to a lower fraction of graphitic carbon after DRM in a fixed-bed reactor. Supplementary DRM experiments in a thermobalance revealed that coke formation as a continuous process until a carbon limit is reached and confirmed a higher coking rate for the cycled catalyst.
[Show abstract][Hide abstract] ABSTRACT: In recent years, an enhanced interest in chemical processes utilizing carbon dioxide as chemical feedstock can be observed. It is driven by the raising global concerns with respect to the anthropogenic greenhouse gas emission. Politics and society are aiming for a reduction or limitation of the CO 2 release. Catalytic dry reforming of methane (DRM) represents one process, which converts carbon dioxide and methane – both greenhouse gases – into synthesis gas. Synthesis gas has a well-established chemistry and can be used for example as feedstock in methanol synthesis, oxo synthesis or in Fisher-Tropsch synthesis. DRM: CH 4 + CO 2 ⇌ 2 CO + 2 H 2 ΔH r = 247 kJ/mol Nickel-based catalysts are the accepted commercial catalysts for DRM because of their high activity, reasonable stability and moderate cost and availability. However, coke formation and therefore catalyst deactivation is a major obstacle of the DRM process . In this work a stable nickel catalyst using a Ni, Mg, Al hydrotalcite-like precursor has been supported on a spherical α-alumina support. The catalyst has been tested in a tubular fixed-bed reactor appyling a capillary sampling technique developed earlier by one of the authors . The method provides profiles of gas species concentrations, gas temperature and surface temperature, which can be analyzed in terms of reaction pathways, mass and heat transport limitation, and validate CFD simulations.
48. Jahrestreffen Deutscher Katalytiker, Weimar, Germany; 03/2015
[Show abstract][Hide abstract] ABSTRACT: We report on a combined density functional theory and the experimental study of the O1s binding energies and X-ray Absorption Near Edge Structure (XANES) of a variety of oxygen species on Ag(111) and Ag(110) surfaces. Our theoretical spectra agree with our measured results for known structures, including the p(N × 1) reconstruction of the Ag(110) surface and the p(4 × 4) reconstruction of the Ag(111) surface. Combining the O1s binding energy and XANES spectra yields unique spectroscopic fingerprints, allowing us to show that unreconstructed atomic oxygen is likely not present on either surface under equilibrium conditions at oxygen chemical potentials typical for ethylene epoxidation. Furthermore, we find no adsorbed or dissolved atomic species whose calculated spectroscopic features agree with those measured for the oxygen species believed to catalyze the partial oxidation of ethylene.
[Show abstract][Hide abstract] ABSTRACT: The selective propane oxidation catalyst MoVTeNb oxide M1 was investigated by microwave conductivity, synchrotron X-ray photoelectron, soft X-ray absorption and resonant photoelectron spectroscopy under reaction conditions to identify the influence of steam on the electronic bulk and surface properties. Steam increases significantly both the conversion of propane and the selectivity to the target product acrylic acid. The increased catalytic performance comes along with a decreased conductivity, a modification of the surface chemical and electronic structure with an enrichment of covalently bonded V5+ species at the extent of Mo6+, a decreased work function and hence polarity of the surface and a modified valence band structure. The higher degree of covalency in metal oxide bonds affects the mobility of the free charge carriers, and hence explains the decrease of the conductivity with steam. We could furthermore prove that a subsurface space charge region depleted in electrons and thus an upward bending of the electronic band structure is induced by the reaction mixture, which is however not dependent on the steam content.