The use of in situ X-ray absorption spectroscopy in applied fuel cell research

For a detailed understanding and systematic optimization of fuel cell systems, in situ studies are an indispensable tool, as they provide information on the catalyst structure in different operation conditions. X-ray absorption spectroscopy (XAS) is in particular suitable for operando investigations, since it does not require ultra high vacuum conditions or long-range order in the sample. Furthermore, it provides in situ information on oxidation state, adsorbed species and catalyst structure, and thus complements ex situ information, e.g. from X-ray diffraction (structure), X-ray photoelectron spectroscopy (oxidation state) and FTIR (adsorbates) nicely. In a spectroelectrochemistry experiment, XAS can be combined with different electrochemical techniques in order to satisfy different needs and scientific aims. Spectra of both a Pt–Ru anode catalyst and a Pt–Co cathode catalyst were recorded at different potentials, while measuring the current-potential characteristics of a single cell. So-called half-cell measurements, where the former fuel cell cathode was used with hydrogen as the reference electrode, were performed in water and ethanol to obtain a more detailed mechanistic insight into the ethanol electrooxidation. From a more industrial point of view, different catalysts were tested with a fast potential cycling protocol simulating rapid load changes in a vehicle.     

Solid-state kinetics and catalytic behavior of selective oxidation catalysts from time-resolved XAFS investigations

Time-resolved measurements are required to elucidate time-dependencies of the electronic and geometric structure of a catalyst under changing reaction conditions. Monitoring the evolution of the bulk structure of a catalyst under changing conditions reveals the solid-state kinetics of the corresponding reaction. X-ray absorption spectroscopy (XAS) permits to reveal quantitative phase composition and average valence together with the evolution of the local structure. Hence, combining time-resolved XAS with simultaneous catalysis measurements may elucidate correlations between catalytic performance, the catalyst state under reaction conditions, and its solid-state kinetics. Here, results from time-resolved in situ XAS investigations of various molybdenum-based selective oxidation catalysts are compared and discussed. Model systems (i.e. α-MoO₃, hexagonal MoO₃ supported on SBA-15, and H₄[PVMo₁₁O₄₀]) suitable to distinguish structural effects and promotion by additional metal centers have been studied under changing reaction conditions. Correlations between reduction and oxidation solid-state kinetics and catalytic performance reveal the dependence of the selectivity of the catalyst on its electronic structure. In particular the re-oxidation kinetics and the average valence under reaction conditions appear to be determined by the defect structure of the underlying catalyst bulk.     

X-ray voltabsorptometry on redox proteins

Biological X-ray absorption spectroscopy (BioXAS) is able to describe the metal environment in a metalloprotein and is sensitive to metal oxidation state. Coupling of BioXAS and electrochemistry permits the characterization of different oxidation states and avoids uncontrolled protein redox state changes due to X-ray beam irradiation. XAS spectroelectrochemistry requires electrochemical cells specifically designed to meet the requirements of both XAS measurements and electrochemical effectiveness in potential control. In this context, this paper describes a new cell tested with different types of working electrodes developed for BioXAS, in particular for in situ studies of redox proteins. The XAS electrochemical measurements of a relatively high-molecular-weight protein (Cu,Zn superoxide dismutase) for which it is difficult to observe direct electrochemistry have been achieved.New working electrodes, capable of fast and unmediated electron transfer, are described. The cell permits to isolate protein redox states and to measure X-ray absorption intensity during a potential scan (X-ray voltabsorptometry).     

Catalysts at work: From integral to spatially resolved X-ray absorption spectroscopy

Spectroscopic studies on heterogeneous catalysts have mostly been done in an integral mode. However, in many cases spatial variations in catalyst structure can occur, e.g. during impregnation of pre-shaped particles, during reaction in a catalytic reactor, or in microstructured reactors as the present overview shows. Therefore, spatially resolved molecular information on a microscale is required for a comprehensive understanding of theses systems, partly in ex situ studies, partly under stationary reaction conditions and in some cases even under dynamic reaction conditions.Among the different available techniques, X-ray absorption spectroscopy (XAS) is a well-suited tool for this purpose as the different selected examples highlight. Two different techniques, scanning and full-field X-ray microscopy/tomography, are described and compared. At first, the tomographic structure of impregnated alumina pellets is presented using full-field transmission microtomography and compared to the results obtained with a scanning X-ray microbeam technique to analyse the catalyst bed inside a catalytic quartz glass reactor. On the other hand, by using XAS in scanning microtomography, the structure and the distribution of Cu(0), Cu(I), Cu(II) species in a Cu/ZnO catalyst loaded in a quartz capillary microreactor could be reconstructed quantitatively on a virtual section through the reactor. An illustrating example for spatially resolved XAS under reaction conditions is the partial oxidation of methane over noble metal-based catalysts. In order to obtain spectroscopic information on the spatial variation of the oxidation state of the catalyst inside the reactor XAS spectra were recorded by scanning with a micro-focussed beam along the catalyst bed. Alternatively, full-field transmission imaging was used to efficiently determine the distribution of the oxidation state of a catalyst inside a reactor under reaction conditions. The new technical approaches together with quantitative data analysis and an appropriate in situ catalytic experiment allowed drawing important conclusions on the reaction mechanism, and the analytical strategy might be similarly applied in other case studies. The corresponding temperature profiles and the catalytic performance were measured by means of an IR-camera and mass spectrometric analysis. In a more advanced experiment the ignition process of the partial oxidation of methane was followed in a spatiotemporal manner which demonstrates that spatially resolved spectroscopic information can even be obtained in the subsecond scale.     

Experimental aspects of X-Ray absorption spectroscopy

X-ray absorption spectroscopy (XAS), including X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine-structure spectroscopy (EXAFS), provides physical and chemical information on almost any element regardless of matrix or conditions. Experimental factors that are critical to successful measurements, including general beamline configuration and energy resolution, in situ cell design, detector gases and common artifacts are discussed. This review is intended to assist during experimental setup and data collection.     

Oscillatory CO Oxidation Over Pt/Al2O3 Catalysts Studied by In situ XAS and DRIFTS

Fresh and mildly aged Pt/Al₂O₃ model diesel oxidation catalysts with small and large noble metal particle size have been studied during CO oxidation under lean burn reaction conditions to gain more insight into the structure and oscillatory reaction behaviour. The catalytic performance, CO adsorption characteristics using in situ DRIFTS and oxidation state using in situ XAS were correlated. Stable and pronounced oscillations only occurred over the catalyst with smaller particle sizes. Characteristic for this catalyst are low-coordinated surface Pt sites (more corner and edge atoms) which seem to become oxidized at elevated temperature as evidenced by in situ DRIFTS and in situ XAS. In situ XAS further uncovered that the oxidation of the Pt surface starts from the end of the catalyst bed and the oxidation state oscillates like the catalytic activity.     

Fischer-Tropsch synthesis: characterization and catalytic properties of rhenium promoted cobalt alumina catalysts☆

The unpromoted and promoted Fischer-Tropsch synthesis (FTS) catalysts were characterized using techniques such as X-ray diffraction (XRD), temperature programmed reduction (TPR), X-ray absorption spectroscopy (XAS), Brunauer-Emmett-Teller surface area (BET SA), hydrogen chemisorption and catalytic activity using a continuously stirred tank reactor (CSTR). The addition of small amounts of rhenium to a 15% Co/Al₂O₃ catalyst decreased the reduction temperature of cobalt oxide but the percent dispersion and cluster size, based on the amount of reduced cobalt, did not change significantly. Samples of the catalyst were withdrawn at increasing time-on-stream from the reactor along with the wax and cooled to become embedded in the solid wax for XAS investigation. Extended X-ray absorption fine structure (EXAFS) data indicate significant cluster growth with time-on-stream suggesting a sintering process as a major source of the deactivation. Addition of rhenium increased the synthesis gas conversion, based on catalyst weight, but turnover frequencies calculated using sites from hydrogen adsorption and initial activity were similar. A wide range of synthesis gas conversion has been obtained by varying the space velocities over the catalysts.     

Ethanol electro-oxidation on carbon-supported Pt-Ru, Pt-Rh and Pt-Ru-Rh nanoparticles

This work investigates the effects of carbon-supported Pt, Pt-Ru, Pt-Rh and Pt-Ru-Rh alloy electrocatalysts on the yields of CO₂ and acetic acid as electro-oxidation products of ethanol. Electronic and structural features of these metal alloys were studied by in situ X-ray absorption spectroscopy (XAS). The electrochemical activity was investigated by polarization experiments and the reaction intermediates and products were analyzed by in situ Fourier Transform Infra-Red Spectroscopy (FTIR). Electrochemical stripping of CO, which is one of the adsorbed intermediates, presented a faster oxidation kinetics on the Pt-Ru electrocatalyst, and similar rates of reaction on Pt-Rh and Pt. The electrochemical current of ethanol oxidation showed a higher value and the onset potential was less positive on Pt-Ru. However, in situ FTIR spectra evidenced that the CO₂/acetic acid ratio is higher for the materials with Rh, mainly at lower potentials. These results indicate that the Ru atoms act mainly by providing oxygenated species for the oxidation of ethanol intermediates, and point out an important role of Rh on the CC bond dissociation.     

On the contribution of X-ray absorption spectroscopy to explore structure and activity relations of Pt/ZrO2 catalysts for CO2/CH4 reforming

X-ray absorption spectroscopy (XAS) has proven to be a very useful technique in characterizing metal-based catalysts exposed to extreme operating conditions. The technique allows in situ evaluation of structural parameters (XAFS) and electronic properties (XANES). The elucidation of the nature and state of Pt-based catalysts in dry reforming of methane with carbon dioxide is presented as case study to show the contribution and potential of XAS to explore property/performance relationships for heterogeneous catalysts. Pt/ZrO₂ is an active and stable catalyst for the reaction between CH₄ and CO₂ to synthesis gas (H₂/CO). The activity and stability of the catalyst is strongly influenced by the catalyst pretreatment (calcination/reduction). The combination of hydrogen chemisorption, IR spectroscopy, XPS and XAS is shown to be suitable to track the changes of the state of the catalyst. In particular, it will be demonstrated, how XAFS helped to correctly attribute variations in the chemisorptive properties of Pt/ZrO₂ after severe temperature treatment to partial and reversible decoration of the small Pt particles with fragments of the oxide support. In situ tracking of the reduction of the catalysts by XANES additionally helped to semiquantitatively assess the partial reduction of the ZrO₂. Finally, XANES helped to demonstrate that CO₂ exposure under these severe conditions did not lead to detectable levels of surface oxidation of Pt. Based on XANES, IR spectroscopy and kinetic measurements it is concluded that in dry reforming activation of methane occurs on Pt, while CO₂ is activated on the support and the two entities react at the metal–support interface. This revised version was published online in June 2006 with corrections to the Cover Date.     

In situ XAS with high-energy resolution: The changing structure of platinum during the oxidation of carbon monoxide

The catalytically active species during the oxidation of carbon monoxide over a real alumina-supported platinum catalyst under atmospheric pressure are determined by combining in situ high-energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD XAS) at the Pt L₃ edge and kinetic measurements. Catalysts were prepared by incipient wetness impregnation. The oxidation of carbon monoxide occurred in two distinctive regimes, a high-activity regime and a low-activity regime, which have high and low rates of reaction respectively. In the low-activity region, the catalyst is poisoned by carbon monoxide, limiting the dissociative adsorption of oxygen. In the high-activity regime, all CO is desorbed from the surface, increasing the rate of reaction. The low carbon monoxide concentration enables oxygen to react to the surface in this reaction regime generating a more reactive surface. HERFD showed that different phases are active depending on the reaction conditions in nano-sized catalyst particles.     

Effect of nitridation on the electronic environment of vanadium in VAlO(N) powder catalysts, used for the ammoxidation of propane

VAlO(N) oxynitrides are promising novel catalysts for the ammoxidation of propane to acrylonitrile. These catalysts are obtained after nitridation of a VAlO oxide precursor in NH₃ or during the reaction in a NH₃/C₃H₈/O₂ mixture. The local structure around the vanadium atoms in the oxide precursor and in several catalysts that have been used in catalytic tests is studied by X-ray absorption spectroscopy (XAS). The nitridation in NH₃ was followed with in situ XAS. The oxide precursors have tetrahedral co-ordinated vanadium, while after full nitridation octahedrons are found and a small decrease in vanadium oxidation state occurs. The catalysts used in the ammoxidation reaction consist of a mixture of these tetrahedrons and octahedrons.     

Synthesis and characterisation by X-ray absorption spectroscopy of a suite of seven mesoporous catalysts containing metal ions in framework sites

MCM-41 type mesoporous silicas have been prepared in which one or more of the following elements are accommodated in framework sites: titanium, iron, chromium, vanadium, manganese, boron and aluminium. XRD and FTIR are used as aids to characterisation, which is achieved chiefly — and to a degree that arrives at valence states, bond lengths and coordination numbers of the metal ion — by X-ray absorption spectroscopy (XAS). Ti-containing MCM-41, as well as the Fe-, V- and Cr-containing variants, yield self-consistent, XAS-based, structural data of the respective metal-ion sites. Some of these (especially those containing Ti) are exceptionally good catalysts for the selective oxidation of large organic molecules such as limonene and norbornene.     

An in-situ Reduction/Oxidation XAS Study on the EL10V8 VO x /TiO2(Anatase) Powder Catalyst

The structural changes of the supported vanadium oxide in the V₂O₅/TiO₂(anatase) EUROCAT EL10V8 powder catalyst during reduction and oxidation at 420 and 490 °C were studied with in-situ X-ray absorption spectroscopy (XAS). The Vanadium K-edge XAS results are compared with pure bulk V₂O₅. For the reduction–oxidation cycle at 420 °C, similar structural changes as for bulk V₂O₅ were observed for the supported vanadium oxide: a reduction to the VO₂ structure and re-oxidation back to V₂O₅. After reduction at 490 °C however, a different structure was obtained: very regular “VO₆” octahedra with a V^2.8+ valence. This may point to a structural support effect.     

Aqueous Phase Glycerol Reforming by PtMo Bimetallic Nano-Particle Catalyst: Product Selectivity and Structural Characterization

A carbon supported PtMo aqueous phase reforming catalyst for producing hydrogen from glycerol was characterized by analysis of the reaction products and pathway, TEM, XPS and XAS spectroscopy. Operando X-ray absorption spectroscopy (XAS) indicates the catalyst consists of bimetallic nano-particles with a Pt rich core and a Mo rich surface. XAS of adsorbed CO indicates that approximately 25% of the surface atoms are Pt. X-ray photoelectron spectroscopy indicates that there is unreduced and partially reduced Mo oxide (MoO₃ and MoO₂), and Pt-rich PtMo bimetallic nano-particles. The average size measured by transmission electron microscopy of the fresh PtMo nano-particles is about 2?nm, which increases in size to 5?nm after 30?days of glycerol reforming at 31?bar and 503?K. The catalyst structure differs from the most energetically stable structure predicted by density functional theory (DFT) calculations for metallic Pt and Mo atoms. However, DFT indicates that for nano-particles composed of metallic Pt and Mo oxide, the Mo oxide is at the particle surface. Subsequent reduction would lead to the experimentally observed structure. The aqueous phase reforming reaction products and intermediates are consistent with both C?CC and C?COH bond cleavage to generate H₂/CO₂ or the side product CH₄. While the H₂ selectivity at low conversion is about 75%, cleavage of C?COH bonds leads to liquid products with saturated carbon atoms. At high conversions (to gas), these will produced additional CH₄ reducing the H₂ yield and selectivity.     

A comparison study of catalytic oxidation and acid oxidation to prepare carbon nanotubes for filling with Ru nanoparticles

Ru catalyst confined within the channels of multiwall carbon nanotubes (MWCNTs) is prepared by opening tube ends via mixed concentrated acid oxidation or catalytic oxidation, followed by filling via wet impregnation. The catalyst filling ratio is characterized by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The defects and functional groups are detected and quantified by Raman, fourier transformer infrared spectroscopy (FTIR) and XPS. The results show that high catalyst filling efficiency (∼80%) can be achieved with catalytic oxidation pre-treatment. The acid oxidation process can introduce more oxygen-containing functional groups such carboxyl and phenol groups. Both FTIR and XPS provide evidence for functional groups removal during thermal treatment. The effects of the nanotube length distributions, defects and functional groups on filling efficiency have been discussed intensively.     

Use of a New Polymer Anchored Cu(II) Azo Complex Catalyst for the Efficient Liquid Phase Oxidation Reactions

A new polymer anchored copper(II) azo complex has been synthesized and characterized by using scanning electron microscope (SEM), thermogravimetric analysis (TGA), elemental analysis, atomic absorption spectroscopy (AAS) and spectrometric methods like diffuse reflectance spectra of solid (DRS) and Fourier transform infrared spectroscopy (FTIR). The immobilized Cu(II) catalyst shows excellent catalytic activity in oxidation of cyclohexene, styrene, benzyl alcohol and ethylbenzene in presence of tert-butylhydroperoxide (TBHP) as an oxidant. The effects of different solvents, oxidants, temperature, substrate oxidant ratio and amount of catalyst were also studied. The catalytic results reveal that the polymer-anchored Cu(II) azo complex catalyst can be recycled more than five times without appreciable loss in the catalytic activity.     

Modulation Excitation X-Ray Absorption Spectroscopy to Probe Surface Species on Heterogeneous Catalysts

The advantages and open questions of the combination of modulation excitation spectroscopy and phase sensitive detection (PSD) with X-ray absorption spectroscopy (XAS) for the analysis of heterogeneous catalysts at work are reviewed. The characteristic spectral signatures of two different edges (Pd K and Pt L₃) are described in relation to the red-ox chemistry of Pd/Al₂O₃ and Pt/Al₂O₃ with respect to NO reduction by CO and CO oxidation, respectively. Both examples demonstrate that PSD makes XAS sensitive to potentially active species for the catalytic reaction.     

Selective growth of single-walled carbon nanotubes over Co–MgO catalyst by chemical vapor deposition of methane

The selective synthesis of SWCNTs with narrow chirality and diameter distribution by methane decomposition over a Co–MgO catalyst is reported. Raman spectroscopy, temperature programmed oxidation (TPO), UV–Vis–NIR absorption spectroscopy, and nitrogen physisorption were used to probe SWCNTs morphology, reaction selectivity, SWCNTs chirality and diameter distribution, and carbon yield. The catalyst was examined by nitrogen physisorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), and UV–Vis-diffuse reflectance spectroscopy to elucidate the structure and chemical state of the species responsible for SWCNT growth. The results established a clear link between the degree of dispersion of Co species inside the MgO lattice and the catalyst activity and selectivity for SWCNT growth. High dispersion and stabilization of Co species influenced catalytic activity for methane decomposition and the high SWCNT selectivity. The yield of carbon and SWCNT selectivity increased with an increase in temperature, however, SWCNTs diameter distribution shifts to larger diameter tubes as synthesis temperature was increased.     

磺酸化无定形碳催化剂的制备及其应用

采用加热回流法制备了磺酸化无定形碳固体酸催化剂。通过电感耦合等离子体发射光谱(ICP-AES)、酸碱电位滴定、傅里叶变换红外光谱(FTIR)、X射线粉末衍射(XRD)、扫描电镜(SEM)和热重分析(TGA)等方法对催化剂进行了表征。结果表明,催化剂结构中—SO3H负载量为2.03 mmol/g,与其他固体酸类催化剂相比,磺酸化无定形碳在催化环酮类化合物的Baeyer-Villiger氧化反应以及苯甲醚和苄醇的傅-克烷基化反应中均显示了良好的催化活性。所得催化剂热稳定性好,制备过程简单。     

Lab Scale Fixed-Bed Reactor for Operando X-Ray Absorption Spectroscopy for Structure Activity Studies of Supported Metal Oxide Catalysts

Lab scale fixed-bed reactor is applied for operando transmission X-ray absorption spectroscopy (XAS) for structure–activity studies of supported metal oxide catalysts under real reaction conditions. This setup includes many properties of an optimal fixed-bed reactor for operando transmission XAS studies. For instance, it is usable in a wide range of temperature (up to 1,000 °C), pressure and space velocity. Besides, this operando setup can be used for transmission XAS measurements in a wide edge energy range. The potential of this reactor for operando transmission XAS is demonstrated by, as examples, the three-way catalytic performance of Pd/Al₂O₃/CeZrO₂ and Rh/Al₂O₃.