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.     

Use of potassium conductors in the electrochemical promotion of environmental catalysis

This study summarizes the effect of the electrochemical promotion of catalysis (EPOC) by using potassium conductors in environmental catalytic reactions applied to the removal of several automotive pollutants, such us CO, C₃H₆ and nitrogen oxides (N₂O and NO_x). It has been shown the extraordinarily potential of using Pt/K–βAl₂O₃ in a wide variety of environmental reactions (oxidation, reduction), activating the catalyst at lower reaction temperatures and decreasing the inhibitory effect of poisons such as water in the reaction atmosphere. In addition, a new application of potassium conductors-based electrochemical catalysts has been developed for the NO_x storage/reduction process (NSR). The idea of coupling catalysis and solid-state electrochemistry on this process would allow to improve and simplify the current NSR technology. Finally, the results have been obtained under reaction conditions compatible with the treatment of automotive exhaust emissions. This demonstrates the potential for the practical use of the phenomenon of electrochemical promotion by using potassium conductors on this kind of process.     

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.     

Production of 14-Oxo-<Emphasis Type="Italic">cis</Emphasis>-11-eicosenoic Acid from Lesquerolic Acid by <Emphasis Type="Italic">Sphingobacterium multivorum</Emphasis> NRRL B-23212

The objective of this study was to explore the extent of microbial conversion of lesquerolic acid (14-hydroxy-cis-11-eicosenoic acid; LQA) by whole cell catalysis and to identify the newly converted products. Among compost isolates including NRRL strains B-23212 (Sphingobacterium multivorum), B-23213 (Acinetobacter sp.), B-23257 (Enterobacter cloacae B), B-23259 (Escherichia sp.) and B-23260 (Pseudomonas aeruginosa) the S. multivorum strain was the only microorganism that converted LQA to produce a new product identified as 14-oxo-cis-11-eicosenoic acid by GC-MS and NMR analyses. The conversion yield was 47.4% in 48 h at 200 rpm and 28°C in small shake flask experiments. In comparison, both Acinetobacter and Pseudomonas strains failed to convert LQA to major new products but used LQA apparently as an energy source during fermentation. For structural analysis, 6.88 g of 14-oxo-cis-11-eicosenoic acid was produced from converting 11 g LQA (a 62% yield) in 72 h at 200 rpm and 28 °C in Fernbach flasks using 18-h-old NRRL B-23212 cultures and an improved medium that also contained EDTA and glycerol in lieu of glucose as carbon source. NRRL B-23212 was further identified by 16S rRNA gene sequence analysis as a unique strain of S. multivorum. Therefore, S. multivorum NRRL B-23212 possesses an enzymatic activity presumably a secondary alcohol dehydrogenase for converting LQA to produce 14-oxo-cis-11-eicosenoic acid, a first report that demonstrates the functional modification of LQA by whole cell catalysis.     

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.     

XAFS spectroscopy; fundamental principles and data analysis

The physical principles of XAFS spectroscopy are given at a sufficiently basic level to enable scientists working in the field of catalysis to critically evaluate articles dealing with XAFS studies on catalytic materials. The described data-analysis methods provide the basic tools for studying the electronic and structural properties of supported metal, metal–oxide or metal–sulfide catalysts. These methods include (a) fitting in R-space, (b) application of the difference file technique and (c) control of the fit procedure with k ¹ and k ³ weighting with the help of phase- and amplitude-corrected Fourier transforms. This revised version was published online in June 2006 with corrections to the Cover Date.     

Graphene on ferromagnetic surfaces and its functionalization with water and ammonia

In this article, an angle-resolved photoelectron spectroscopy (ARPES), X-ray absorption spectroscopy (XAS), and density-functional theory (DFT) investigations of water and ammonia adsorption on graphene/Ni(111) are presented. The results of adsorption on graphene/Ni(111) obtained in this study reveal the existence of interface states, originating from the strong hybridization of the graphene π and spin-polarized Ni 3 d valence band states. ARPES and XAS data of the H₂O (NH₃)/graphene/Ni(111) system give an information regarding the kind of interaction between the adsorbed molecules and the graphene on Ni(111). The presented experimental data are compared with the results obtained in the framework of the DFT approach.     

Use of surface response methodology to optimize culture conditions for iturin A production by Bacillus subtilis in solid-state fermentation

Response surface methodology (RSM) was employed to optimize the cultivation conditions of Bacillus subtilis S3 for the enhancement of iturin A, a lipopeptide antibiotic used as biological pesticide, production in solid-state fermentation (SSF). The statistic experimental model predicted a maximum iturin production of 11.435 mg/g-wet solid material. Verification of the calculated maximum was done with experiments that were performed in the culture media representing the optimum combination found, and the iturin A production of 11.447 mg/g-wet solid material (average of three repeats) was obtained when B. subtilis S3 was cultivated at 25 °C for 5 days in solid fermentation containing high gluten flour 10 g and rice bran 50 g in addition to glucose 1.15%, KH₂PO₄ 1.27 mM, MgSO₄ 5.08 mM, peanut oil 1.01%, inoculum 19.49% and water content 44.97%. The iturin A production by B. subtilis S3 was increased significantly by 23%, from 9.26 mg/g-wet solid material to 11.447 mg/g-wet solid material when the strain was cultivated in the optimal medium developed by surface response methodology, as compared to medium conventionally developed by one-factor-at-a-time. The yield of iturin A (11.447 mg/g-wet solid material, with 45% moisture content) produced by B. subtilis S3 reported in this study is the highest reported to date for B. subtilis species in SSF. In addition, the use of rice bran as a substrate in solid-state fermentation for iturin A production by B. subtilis is unique.     

Surface,interface and electronic properties of F8:F8BT polymeric thin films used for organic light‐emitting diode applications

Ultrathin polymeric films consisting of poly(9,9‐di‐n‐octylfluorenyl‐2,7‐diyl) (F8) blended with poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) grown onto PEDOT:PSS/ITO/PET were investigated by X‐ray photoelectron spectroscopy (XPS), depth‐profiling XPS, reflection electron energy loss spectroscopy (REELS) and angle‐dependent X‐ray absorption spectroscopy (XAS) to gain information on the films' electronic, order and interface properties. AFM studies provide valuable information on the films' nanotopographical properties and homogeneity. Spectroscopic ellipsometry and photoluminescence spectroscopy were used also to obtain information on the optoelectronic properties. Well‐ordered films were observed from the XAS analysis, measured at the sulfur K absorption edge. XPS measurements demonstrated that the surface composition of the polymer thin films prepared by a spin‐coating wet‐chemical deposition method matches the expected F8:F8BT blend stoichiometry. The interfacial properties were studied through an argon ion sputtering process coupled to the XPS acquisition, showing an enhancement of oxygen components at the interface. The films' inhomogeneity was verified by AFM images and analysis. We obtained a value of 3.1 eV as the electronic bandgap of the F8:F8BT film from REELS data, whereas analysis of the spectroscopic ellipsometry spectra revealed that the optical bandgap of F8:F8BT has a value of 2.4 eV. A strong green emission was obtained for the produced films, which is in agreement with the expected emission due to the 1:19 ratio of the F8 and F8BT blended polymers. © 2018 Society of Chemical Industry     

Surface properties and catalytic performance of La1−xSrxCrO3 perovskite-type oxides for CO and C3H6 combustion

Catalytic CO oxidation and C₃H₆ combustion have been studied over La_1−xSr_xCrO₃ (x = 0.0–0.3) oxides prepared by solid-state reaction and characterised by X-ray diffraction (XRD), nitrogen adsorption (BET analysis) and X-ray photoelectron spectroscopy (XPS). The expected orthorhombic perovskite structure of the chromite is observed for all levels of substitution. However, surface segregation of strontium along with a chromium oxidation process, leading to formation of Cr⁶⁺-containing phases, is produced upon increasing x and shown to be detrimental to the catalytic activity. Maximum activity is achieved for the catalyst with x = 0.1 in which mixed oxide formation upon substitution of lanthanum by strontium in the chromite becomes maximised.     

Studies on co[poly(ethylene terephthalate-p-oxybenzoate)] thermotropic copolyester (I): Synthesis and thermogravimetric behavior

A series of co[poly(ethylene terephthalate-p-oxybenzoate)] thermotropic copolyester was synthesized from various ratios of two different moieties of either poly(ethylene terephthalate) (PET) or its oligomer as moiety ( II ) with p-acetoxybenzoic acid as moiety ( I ) through the polycondensation process. The polymeric products obtained were then ground and subjected to solid-state polymerization under vacuum for various polymerization times. Thermogravimetric analysis (TGA) and the derivative thermogravimetric analysis (DTG) were performed with these samples obtained. Also, a commercial product, Eastman 10109, was analyzed in comparison with these samples. It was found that the copolyesters made of PET and of its oligomer in a p-oxybenzoate (POB):PET = 80 : 20 composition ratio and subjected to 8 and 4 h solid-state polymerization times, respectively, showed the most similar thermogravimetric behavior with Eastman 10109. © 1993 John Wiley & Sons, Inc.     

9(Z)-Octadecenamide and fatty amides byBacillus megaterium (B-3437) conversion of oleic acid

9(Z)-Octadecenamide, hexadecenamide, tetradecenamide and tetradecanamide were produced by a novel bioconversion of oleic acid withBacillus megaterium NRRL B-3437. Although chemical synthesis is more practical, the bioconversion to fatty amides (5–7% of total recovered lipids) was unique for its requirement of both enzymatic catalysis and equimolar oleic acid/ammonium salt substrates. Purified octadecenamide was obtained by silica gel and high-pressure liquid chromatographic procedures and was characterized by gas chromatography, mass spectrometry, infrared and nuclear magnetic resonance.     

Natural estolides produced by <Emphasis Type="Italic">Pseudomonas</Emphasis> sp. 42A2 grown on oleic acid: Production and characterization

Estolides are a group of FA polyesters resulting from ester bond formation between a hydroxyl or olefinic group of one FA and the terminal carboxyl group of a second FA. These products are commonly found in trace amounts, forming tetraglycerides in several oil seed plants, and have been produced by acid clay and enzymatic catalysis in vitro. In this study, natural estolides produced by a bacterial culture are presented for the first time. Pseudomonas sp. 42A2 produced (E)-10-hydroxy-8-octadecenoic acid and (E)-7,10-dihydroxy-8-octadecenoic acid when grown on oleic acid. It is suggested that these FA were polymerized in culture by a lipase produced by the bacterial strain, resulting in a mixture of estolides. These compounds amounted to 3.8 g/L after 72 h of incubation. LC-MS analysis indicated that the types of estolides formed were dimers (m/z 560–610), trimers (m/z 845–906), tetramers (m/z 1122–1202), pentamers (m/z 1328–1424), and hexamers (m/z 1554–1788), with a relative abundance of 27.5, 19.4, 15, 9.7, and 11%, respectively. This is the first report in which hexamers were detected in a bacterial culture.     

Sex pheromone of European grapevine moth (Lobesia botrana) its chemical transformations in sunlight and heat

Photo- and radical isomerization of (E, Z)-7,9-dodecadien-1-yl acetate (DDA) leads to an equilibrium mixture of all four possible geometric isomers of 7,9-DDA in the ratio ofE,E, 69–76%;Z,E, 11–13%;E, Z, 12–15%; andZ, Z, 1–3%. Iodine catalysis of the isomerization takes place even in dark at room temperature and is probably a radical reaction.     

Site-directed mutagenesis of a highly active Staphylococcus epidermidis lipase fragment identifies residues essential for catalysis

A fragment of Staphylococcus epidermidis lipase gene (Lys-303 to Lys-688) was inserted into plasmid pET-20b(+). The resulting C-terminal His-tagged recombinant protein (43 kDa) was overexpressed in Escherichia coli BL21(DE3) as a highly active lipase and was purified with nickel-coupled resin. Putative catalytic sites were determined by site-directed mutagenesis. Mutant enzymes (S418C and H648K) lost enzyme activities, which strongly suggests that the proposed residues of Ser-418 and His-648 are involved in catalysis. Site-directed mutagenesis showed that in comparison with wild-type enzyme, the M419A and V649l enzymes showed a 2.0- and 4.0-fold increase in the k _cat/K _m′ respectively, but the M419l, M419Q, V649A, and V649L variants lost enzyme activities. The wild-type enzyme and the V649l mutant favored the hydrolysis of p-nitrophenyl esters of butyrate, but the M419A favored decanoate. The results suggested that the amino acid residues (Met-419 and Val-649), following the catalytic triad, could affect the substrate specificity and/or catalytic efficiency. This work was presented at the Biocatalysis Symposium in April 2000, held at the 91st Annual Meeting and Expo of the American Oil Chemists' Society, San Diego, CA.     

Solid-state polycondensation of poly(ethylene terephthalate) recycled from postconsumer soft-drink bottles. II

Poly(ethylene terephthalate) (PET) taken from postconsumer soft-drink bottles was subjected to solid-state polycondensation after cutting into small pieces or after dissolution in trifluoracetic acid, trifluoracetic acid/dichloromethane mixture (50/50%, v/v), or nitrobenzene, and coagulation in methanol. The effect of various reaction parameters such as time and temperature of reaction (180, 200, 220, and 230°C) on intrinsic viscosity [η] and carboxyl and hydroxyl end-group content have been investigated. The highest number average molecular weight, M?_n = 61,400 was obtained from PET (M?_n = 20,300) dissolved in nitrobenzene and solid-state polycondensated by heating under vacuum at 230°C for 8 h. The thermal behavior of solid-state samples was studied by differential scanning calorimetry (DSC); all samples showed a characteristic double endothermal melting peak and no glass-transition temperature. The PET samples taken from the bottles without dissolution were also studied by thermomechanical analysis. The heat distorsion temperatures obtained by this analysis were in very good agreement with the two endothermal melting peaks taken by DSC. This finding indicates that in these samples the crystallites form a coherent matrix and the amorphous phase is dispersed in the voids. © 1995 John Wiley & Sons, Inc.     

Combining XRD and EXAFS with on-Line Catalytic Studies for in situ Characterization of Catalysts

Advances in the study of catalysts by in situ structural characterization, using X-ray absorption spectroscopy (XAFS) and X-ray diffraction (XRD), have recently been achieved and they are illustrated by reference to several examples. Emphasis in the present overview is laid on the study of catalysts under realistic working conditions and therefore in particular on-line gas analysis during activation and/or during catalysis. The examples encompass (a) activation of copper-based methanol catalysts, (b) dynamic changes of copper-based catalysts during methanol synthesis conditions, (c) catalytic partial oxidation on Rh/Al₂O₃ catalysts, and (d) reduction (activation) of copper-promoted high temperature shift catalysts.     

Experimental methods in chemical engineering: X-ray absorption spectroscopy—XAS,XANES, EXAFS

Although X-ray absorption spectroscopy (XAS) was conceived in the early 20th century, it took 60 years after the advent of synchrotrons for researchers to exploit its tremendous potential. Counterintuitively, researchers are now developing bench type polychromatic X-ray sources that are less brilliant to measure catalyst stability and work with toxic substances. XAS measures the absorption spectra of electrons that X-rays eject from the tightly bound core electrons to the continuum. The spectrum from 10 to 150 eV (kinetic energy of the photoelectrons) above the chemical potential—binding energy of core electrons—identifies oxidation state and band occupancy (X-ray absorption near edge structure, XANES), while higher energies in the spectrum relate to local atomic structure like coordination number and distance, Debye-Waller factor, and inner potential correction (extended X-ray absorption fine structure, EXAFS). Combining XAS with complementary spectroscopic techniques like Raman, Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) elucidates the nature of the chemical bonds at the catalyst surface to better understand reaction mechanisms and intermediates. Because synchrotrons continue to be the light source of choice for most researchers, the number of articles Web of Science indexes per year has grown from 1000 in 1991 to 1700 in 2020. Material scientists and physical chemists publish an order of magnitude articles more than chemical engineers. Based on a bibliometric analysis, the research comprises five clusters centred around: electronic and optical properties, oxidation and hydrogenation catalysis, complementary analytical techniques like FTIR, nanoparticles and electrocatalysis, and iron, metals, and complexes.     

Silica-supported tantalum clusters: catalysts for conversion of methane with n-butane to give ethane, propane, and pentanes

Si0₂-supported tantalum clusters were prepared by adsorption of the precursor Ta(CH₂Ph)₅ (Ph is phenyl) on the support followed by treatments in H₂ at 523, 623, and 723 K. The resultant clusters, had approximate average diameters of 0.3, 0.8, and 2 nm, as determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. The samples were tested as catalysts for conversion of methane with n-butane in a once-through flow reactor operated at atmospheric pressure and 523 K, and EXAFS spectroscopy was used to characterize the used catalysts. The results show that (a) the catalysts are active for the conversion of methane with n-butane to give ethane, propane, and pentanes; (b) catalytic activity decreased to nearly zero over a time on stream of 22 h; (c) the catalyst incorporating the smallest clusters exhibited the highest initial activity and that incorporating the largest clusters exhibited the lowest activity; (d) each used catalyst contained clusters of approximately the same nuclearity as the respective fresh catalyst, but with Ta–Ta bond lengths approximately 0.17 ? longer than those found in the fresh catalysts. The data are consistent with catalysis by the supported clusters, and the product distributions are consistent with disproportionation of n-butane accompanied by the reaction of methane with propane to give other alkanes.     

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.