Henning Lichtenberg

Fe and Mn‐Based Catalysts Supported on γ‐Al2O3 for CO Oxidation under O2‐Rich Conditions

MnOx and FeOx‐based catalysts supported on γ‐Al2O3 (0.1–20 wt %) were prepared by using two methods: incipient wetness impregnation and single‐step flame spray pyrolysis. The effect of the structural properties and composition on the CO oxidation activity was systematically evaluated and correlated with the preparation methods. The characterization of the samples by XRD, X‐ray absorption spectroscopy, TEM, and temperature‐programmed reduction by hydrogen revealed that, in contrast to the use of incipient wetness impregnation, flame spray pyrolysis leads to the formation of highly dispersed homogeneously distributed FeOx and MnOx species. A partial incorporation of Fe and Mn ions into the γ‐Al2O3 lattice for low metal oxide loadings and for samples prepared by flame spray pyrolysis was observed. In general, the CO oxidation activity increased with the transition metal oxide loading. Below 200 °C, Mn‐based catalysts demonstrated the highest catalytic performance. However, the addition of water decreased the performance, especially at lower temperatures, which demonstrates the competitive adsorption on the active sites. The presence of NO had no effect on the CO conversion. A significant effect of the preparation method on the catalytic performance was observed during hydrothermal aging. The superior distribution of the active species obtained by flame spray pyrolysis leads to thermally more stable catalysts.

Identification of the iron oxidation state and coordination geometry in iron oxide- and zeolite-based catalysts using pre-edge XAS analysis

Analysis of the oxidation state and coordination geometry using pre-edge analysis is attractive for heterogeneous catalysis and materials science, especially for in situ and time-resolved studies or highly diluted systems. In the present study, focus is laid on iron-based catalysts. First a systematic investigation of the pre-edge region of the Fe K-edge using staurolite, FePO4, FeO and α-Fe2O3 as reference compounds for tetrahedral Fe(2+), tetrahedral Fe(3+), octahedral Fe(2+) and octahedral Fe(3+), respectively, is reported. In particular, high-resolution and conventional X-ray absorption spectra are compared, considering that in heterogeneous catalysis and material science a compromise between high-quality spectroscopic data acquisition and simultaneous analysis of functional properties is required. Results, which were obtained from reference spectra acquired with different resolution and quality, demonstrate that this analysis is also applicable to conventionally recorded pre-edge data. For this purpose, subtraction of the edge onset is preferentially carried out using an arctangent and a first-degree polynomial, independent of the resolution and quality of the data. For both standard and high-resolution data, multiplet analysis of pre-edge features has limitations due to weak transitions that cannot be identified. On the other hand, an arbitrary empirical peak fitting assists the analysis in that non-local transitions can be isolated. The analysis of the oxidation state and coordination geometry of the Fe sites using a variogram-based method is shown to be effective for standard-resolution data and leads to the same results as for high-resolution spectra. This method, validated by analysing spectra of reference compounds and their well defined mixtures, is finally applied to track structural changes in a 1% Fe/Al2O3 and a 0.5% Fe/BEA zeolite catalyst during reduction in 5% H2/He. The results, hardly accessible by other techniques, show that Fe(3+) is transformed into Fe(2+), while the local Fe-O coordination number of 4-5 is maintained, suggesting that the reduction involves a rearrangement of the oxygen neighbours rather than their removal. In conclusion, the variogram-based analysis of Fe K-edge spectra proves to be very useful in catalysis research.

Continuous microfluidic synthesis of colloidal ultrasmall gold nanoparticles: in situ study of the early reaction stages and application for catalysis

A continuous microfluidic setup was developed to study colloidal synthesis of gold nanoparticles using tetrachloroauric acid as precursor, sodium borohydride as reducing agent and PVP as stabilizer. The setup consists of pressurized vessels that allow pulsation-free flow of reactants and a microfluidic chip with integrated micromixers essential for efficient mixing with small mixing time (2 ms) followed by a meandering microchannel. The microfluidic chip enables recording X-ray absorption spectra (XAS) in situ at different positions along the microchannel at high flow rates approaching turbulent mixing conditions and thus to correlate reaction time with changes in the nanoparticle structure. Significant contributions of oxidized gold could be observed after the first 6 ms of the reaction, whereas after 10 ms principally all gold appeared to be in a metallic state. The nanoparticles obtained were characterized ex situ by various complementary techniques. The resulting nanoparticles had average diameter of 1.0 nm and narrow size distributions compared with those produced in a batch reactor. Depositing the nanoparticles on TiO2 resulted in catalysts with two different Au loadings (0.7 and 1.7 wt% Au/TiO2) which exhibited good CO oxidation activity.

Structure and reducibility of a Fe/Al2O3 catalyst for selective catalytic reduction studied by Fe K-edge XAFS spectroscopy

EXAFS and pre-edge information from the Fe K-edge absorption spectra is used in this study to characterise the local environment and geometry of Fe-centres in a 1% Fe/Al2O3 model catalyst. The EXAFS results reveal clusters of 2-3 Fe oxo-moieties dispersed on the Al2O3-support. The Fe3+ centres are coordinated by 6 O-atoms in a strongly distorted octahedral geometry. This is supported by the pre-edge peak, which is far more intense than in α-Fe2O3 absorption data acquired for comparison. For preliminary investigations, catalytic tests for selective catalytic reduction (SCR) of NOx by ammonia and in situ XANES studies of Fe/Al2O3 are compared with previously published data for Fe/zeolite systems. The low SCR activity of Fe/Al2O3 (compared to Fe/zeolite catalysts) in which the Fe3+ species do not change their electronic state under SCR-relevant conditions (reference temperature 250°C in our case) correlates well to the significantly higher temperatures required to reduce these species to Fe2+. The difference in reducibility (and consequently in the SCR-activity) between the two systems probably results from differences in the structure and the electronic interaction of the Fe oxo-moieties and the particular catalyst support.

In situ characterization of catalysts and membranes in a microchannel under high-temperature water gas shift reaction conditions

Microreactor technology with high heat transfer in combination with stable catalysts is a very attractive approach for reactions involving major heat effects such as methane steam reforming and to some extent, also the high temperature water gas shift (WGS) reaction. For this study Rh/ceria catalysts and an ultrathin hydrogen selective membrane were characterized in situ in a microreactor specially designed for x-ray absorption spectroscopic measurements under WGS conditions. The results of these experiments can serve as a basis for further development of the catalysts and membranes.

Cell Designs for In Situ and Operando Studies

The design of appropriate spectroscopic cells for in situ and operando XAFS studies of heterogeneous catalysts has been a very active field during the past decades as the investigation of catalysts at work has become a powerful approach to improve the activity and selectivity of catalysts in a rational manner. This chapter reviews criteria for choosing the appropriate cell design and underlines its significance using several examples of in situ and operando cells for studying heterogeneous catalysts, sensors, and electrocatalysts and for deriving structure-performance relationships. This strongly contributes to a better understanding of the dynamics of functional materials and their knowledge-based improvement.

Operando XAS studies on catalysts for energy-related processes

Synchrotron based in situ and operando X-ray absorption spectroscopy (XAS) has evolved into a powerful tool to analyze structural changes of catalysts under realistic reaction conditions as a basis for structure activity relationships. The design of appropriate sample cells is crucial to achieve the best compromise between high quality data acquisition and sample environments similar to those in industrial reactors. The following three selected examples illustrate the high potential of this technique for various relevant energy related applications. Data were acquired at DESY, ESRF and ANKA.

Flame made ceria supported noble metal catalysts for efficient H2 production via the water gas shift reaction

Rh/ceria catalysts were synthesized by flame spray pyrolysis for high temperature water gas shift (WGS) reactions. These catalysts show a high specific surface area due to a high degree of nanocrystallinity. X-ray absorption spectroscopy (XAS) unraveled the formation of small Rh particles under WGS reaction conditions. The catalytic activity was examined at atmospheric pressure by measuring CO conversion as a function of temperature. Some methane formation was observed above 310°C.