Bulk Mixed Oxide Research Articles

Carbon-supported VPO catalysts with fibrous structure: A new family of catalysts for partial oxidation reactions

The present contribution describes the synthesis of a carbon supported VPO catalyst with a fibrous structure. The synthesis method is simple, low-cost, scalable and reproducible that allows the formation of the bulk mixed oxide (VO)2P2O7 (VPP) active phase on the carbon support material. The preparation does not require any special reducing thermal treatments due to the ability of the carbon support to stabilize the VPP active phase since this carbon-based support is stable under oxidizing conditions up to around 400ºC allowing it to be used as for catalytic oxidation reactions. The stability of the carbon-supported VPP catalyst under reaction conditions was examined with in situ Raman under oxidizing/ H2 reducing cycles and n-butane oxidation conditions under both wet and dry conditions. These studies confirm the dynamic character of VPO phases, which interconvert under different conditions, and the stability of carbon-supported VPP phase under different environments. The novel carbon-supported catalyst exhibits an activity behavior like that observed for commercial catalysts that suggests a very promising catalytic material.

Hydrogen-free deoxygenation of industrial vegetable oil waste using Ce, Zr-NiAl catalysts for second-generation biofuels production

High acidic vegetable oil feedstocks and wastes are not suitable for conventional biodiesel production. These wastes could be used to produce cheap hydrocarbon-rich biofuels without affecting food production. In this work, we studied the hydrogen-free fast pyrolysis and deoxygenation of an acidic industrial vegetable oil waste for biofuels and chemicals production, assessing the effect of Ce or Zr on NiAl mixed oxides obtained from layered double hydroxides. Oily waste (OW) was pre-adsorbed on the catalysts using a Catalyst:OW mass ratio of 5:1. Without a catalyst, pyrolysis of OW, containing triglycerides and a high percentage of free fatty acids, produced mainly 1-alkenes, lower carbon chain fatty acids, and fatty acid esters. Catalytic pyrolysis increased hydrocarbons favoring 1-alkenes between propene and 1-heptadecene, as well as alkyl-benzenes. Linear alkylbenzenes are feedstock for the production of biodegradable surfactants. NiAlCe was most selective to polyenes, alkanes and ketones, while NiAl and NiAlZr catalysts were efficient in producing aromatics, alkenes and light hydrocarbons in the GLP and gasoline range (C3-C10). The combination of acidic and metallic properties on high surface area unsupported bulk mixed oxide catalysts can be used to maximize the production of specific-range hydrocarbons for second-generation drop-in biofuels and bio-based surfactant precursors, towards the decarbonization of industry.

Conversion of ethane to ethylene

The synthesis of two novel catalytic materials based on vanadium mixed oxides has been described in present study. The first material was a Vanadium-Phosphorous mixed oxide (VPO) carbon-supported material with fibrous morphology and a very high surface area, and stable under oxidizing conditions up to 350 °C. The other material is a bulk mixed VPO oxide with fibrous structure obtained after optimizing the calcination of the supported carbon fiber catalyst, which presents a very high surface area. Both materials seem to be very attractive as catalysts in a wide range of partial oxidation processes in both liquid and gas phase based on the structural and chemical characterization data.

Activation of Mo and V oxides supported on ZSM-5 zeolite catalysts followed by in situ XAS and XRD and their uses in oxydehydration of glycerol

This paper describes the activation and catalytic performance of a multifunctional catalyst consisting of mixed oxides of vanadium and molybdenum supported on an acidic ZSM-5 zeolite. The zeolite was wet-impregnated with an aqueous solution containing a mixture of NH4VO3 and (NH4)6Mo7O24 precursors at a V/(Mo+V) molar ratio of 0.6. Evolution of the phases during activation (by calcination under 20% O2/He) was followed by synchrotron X-ray diffraction (XRD), but detection of mixed oxides of MoxVyOz was difficult, due to the high degree of dispersion on the zeolite surface. On the other hand, X-ray absorption spectroscopy (XAS) performed simultaneously at the Mo and V K-edges enabled confirmation of formation of the MoV2O8 phase on the acidic ZSM-5 zeolite. The combination of the MoV2O8 phase and the zeolite acid sites produced a highly active catalyst for gas phase glycerol dehydration-oxidation coupled reactions in which acrylic acid was the main product. The multifunctional catalyst presented only 6% of deactivation during a period of 8 h under glycerol stream, while the activities of the bulk mixed oxide and the pure ZSM-5 zeolite decreased by almost 20 and 31%, respectively.

Novel Synthesis Method of porous VPO catalysts with fibrous structure by Electrospinning

The synthesis of two novel catalytic materials based on vanadium mixed oxides has been described in present study. The first material was a Vanadium-Phosphorous mixed oxide (VPO) carbon-supported material with fibrous morphology and a very high surface area, and stable under oxidizing conditions up to 350°C. The other material is a bulk mixed VPO oxide with fibrous structure obtained after optimizing the calcination of the supported carbon fiber catalyst, which presents a very high surface area. Both materials seem to be very attractive as catalysts in a wide range of partial oxidation processes in both liquid and gas phase based on the structural and chemical characterization data.

Development and Optimization of a Catalytic Thruster for Hydrogen Peroxide Decomposition

Hydrogen peroxide is a non toxic, low cost, green monopropellant with a significant potential for applications in satellite microthrusters. It presents a density-specific impulse, similar to hydrazine. A new bulk mixed oxide catalyst for decomposition of hydrogen peroxide has been developed and tested in a 2 N thruster with modular parts, allowing the use of catalytic beds with different diameters and lengths. A method of experiment design with a star configuration was implemented to optimize the catalytic bed geometry, in order to yield complete peroxide decomposition, reduce thruster size and volume and provide maximum thrust.

ChemInform Abstract: Catalysis Science of Bulk Mixed Oxides

AbstractReview: 148 refs.

Catalysis Science of Bulk Mixed Oxides

Bulk mixed oxide catalysts are widely used for various applications (selective oxidation catalysts, electrocatalysts for solid oxide fuel cells, and solid oxide electrolyzers for the production of hydrogen), but fundamental understanding of their structure–performance relationships have lagged in the literature. The absence of suitable surface composition and surface structural characterization techniques and methods to determine the number of catalytic active sites, with the latter needed for determination of specific reaction rates (e.g., turnover frequency (1/s)), have hampered the development of sound fundamental concepts in this area of heterogeneous catalysis. This Perspective reviews the traditional concepts that have been employed to explain catalysis by bulk mixed oxides (molybdates, vanadates, spinels, perovskites, and several other specific mixed oxide systems) and introduces a modern perspective to the fundamental surface structure–activity/selectivity relationships for bulk mixed oxide catalysts. The new insights have recently been made available by advances in surface characterization techniques (low-energy ion scattering, energy-resolved XPS, and CH3OH-IR) that allow for direct analysis of the outermost surface layer of bulk mixed metal oxide catalysts. The new findings sound a note of caution for the accepted hypotheses and concepts, and new catalysis models need to be developed that are based on the actual surface features of bulk mixed oxide catalysts.

Molecular Structural Determination of Molybdena in Different Environments: Aqueous Solutions, Bulk Mixed Oxides, and Supported MoO3 Catalysts

UV−vis diffuse reflectance spectroscopy (DRS) and Raman spectroscopy were used to examine the electronic and molecular structures, respectively, of well-defined Mo(VI) bulk mixed oxide reference compounds ((i) isolated MoO4 or MoO6 monomers, (ii) dimeric O3Mo−O−MoO3, (iii) chains of alternating MoO4 and MoO6 units, (iv) MoO6-coordinated Mo7−Mo12 clusters, and (v) infinite layered sheets of MoO5 units), aqueous molybdate anions as a function of solution pH, and supported MoO3 catalysts (MoO3/SiO2, MoO3/Al2O3, and MoO3/ZrO2). Raman spectroscopy confirmed the identity and phase purity of the different bulk and solution molybdenum oxide structures. UV−vis DRS provided the corresponding electronic edge energy (Eg) of the ligand-to-metal charge transfer (LMCT) transitions of the Mo(VI) cations. A linear inverse correlation was found between Eg and the number of bridging Mo−O−Mo covalent bonds around the central Mo(VI) cation. A relationship between Eg and the domain size (NMo) for finite MoOx clusters, however,...

Structural Determination of Bulk and Surface Tungsten Oxides with UV−vis Diffuse Reflectance Spectroscopy and Raman Spectroscopy

Combined UV−vis diffuse reflectance spectroscopy (DRS) and Raman spectroscopy were applied to examine the electronic and molecular structures, respectively, of well-defined W(VI) bulk mixed oxide reference compounds consisting of (i) isolated WO4 or WO6 monomers, (ii) dimeric O3W−O−WO3, (iii) polymeric chain of alternating WO4/WO6 units, and (iv) WO6-coordinated W9−W18 clusters. Raman spectroscopy was employed to confirm the identity and phase purity of the different tungsten oxide structures. UV−vis DRS provided the corresponding electronic edge energy (Eg) of the ligand-to-metal charge transfer (LMCT) transitions of the W(VI) cations. A correlation between the edge energy and the number of covalent bridging W−O−W bonds around the central W(VI) cation was found with Eg linearly decreasing with increasing number of bridging W−O−W bonds. However, a direct relationship between Eg and the domain size, NW, for finite WOx clusters does not exist. Subsequently, UV−vis and Raman spectroscopy information were applied to determine the local structures of the molecularly dispersed surface W(VI) species present in supported WO3/Al2O3, WO3/ZrO2, and WO3/SiO2 catalysts under ambient and dehydrated conditions.

High Throughput Screening of Low Temperature CO Oxidation Catalysts Using IR Thermography

The catalytic oxidation of carbon monoxide to carbon dioxide is an important process used in several areas such as respiratory protection, industrial air purification, automotive emissions control, CO clean-up of flue gases and fuel cells. Research in this area has mainly focused on the improvement of catalytic activity at low temperatures. Numerous catalyst systems have been proposed, including those based on Pt, Pd, Rh, Ru, Au, Ag, and Cu, supported on refractory or reducible carriers or dispersed in perovskites. Well known commercial catalyst formulations for room temperature CO oxidation are based on CuMn2O4 (hopcalite) and CuCoAgMnOx mixed oxides. We have applied high-throughput and combinatorial methodologies to the discovery of more efficient catalysts for low temperature CO oxidation. The screening approach was based on a hierarchy of qualitative and semi-quantitative primary screens for the discovery of hits, and quantitative secondary screens for hit confirmation, lead optimization and scale-up. Parallel IR thermography was the primary screen, allowing one wafer-formatted library of 256 catalysts to be screened in approximately 1 hour. Multi-channel fixed bed reactors equipped with imaging reflection FTIR spectroscopy or GC were used for secondary screening. Novel RuCoCe compositions were discovered and optimized for CO oxidation and the effect of doping was investigated for supported and bulk mixed oxide catalysts. Another family of active hits that compare favorably with the Pt/Al2O3 benchmark is based on RuSn, where Sn can be used as a dopant (e.g. RuSn/SiO2) and/or as a high surface area carrier (e.g., SnO2 or Sn containing mixed metal oxides). Also, RuCu binary compositions were found to be active after a reduction pretreatment with hydrogen.

High throughput discovery of CO oxidation/VOC combustion and water–gas shift catalysts for industrial multi-component streams

High-throughput and combinatorial approaches have been applied to the discovery of catalysts for selective low temperature CO oxidation/VOC removal using mixed CO/propylene feeds, and for the water–gas shift (WGS) reaction using real post-reformer feeds containing CO, CO2, H2O and H2. The screening approach was based on a hierarchy of qualitative and semi-quantitative primary screens for the discovery of hits, and quantitative secondary screens for hit confirmation, lead optimization and scale-up. For WGS, primary screening was carried out using scanning mass spectrometry. For CO oxidation and VOC removal, parallel IR thermography was the primary screen. Multi-channel fixed bed reactors equipped with imaging reflection FTIR spectroscopy or GC were used for secondary screening. Novel RuCoCe compositions were discovered and optimized for CO oxidation/VOC removal and the effect of doping was investigated for supported and bulk mixed oxide catalysts. For WGS, noble metal-free and Pt-doped CoFeRu mixed oxides as well as Pt on CeO2 and Pt on CeO2/ZrO2 were investigated and a new synergistic PtFeCe ternary composition was discovered. In these cases oxygen vacancies in the ceria lattice are believed to play a key role in the strong and synergistic Pt–Ce interaction. Alkaline metal doping was found to enhance the selectivity towards WGS by suppressing the unselective methanation side reaction and to increase the low temperature catalytic activity.

A Quantum Chemical Study of the Decomposition of Keggin-Structured Heteropolyacids

Heterpolyacids (HPAs) demonstrate catalytic activity for oxidative and acid-catalyzed hydrocarbon conversion processes. Deactivation and thermal instability, however, have prevented their widespread use. Herein, ab initio density functional theory is used to study the thermal decomposition of the Keggin molecular HPA structure through the desorption of constitutional water molecules. The overall reaction energy and activation barrier are computed for the overall reaction HnXM12O40-->Hn-2XM12O39+H2O. and subsequently used to predict the effect of HPA composition on thermal stability. For example, the desorption of a constitutional water molecule is found to be increasingly endothermic in the order silicomolybdic acid (H4SiMo12O40)<phosphomolybdic acid (H3PMo12O40)<silicotungstic acid (H4SiW12O40)<phosphotungstic acid (H3PW12O40), in agreement with the experimental ordering of their thermal stability. The presence of an adjacent Keggin unit may stabilize the structural defect created by the water desorption, thus suggesting that constitutional water loss is an initial step toward the decomposition into a bulk mixed oxide. The equilibrium concentration of defective Keggin units is determined as a function of temperature and water partial pressure. It is concluded that the loss of constitutional water molecules is a plausible deactivation mechanism of the acid catalyst. The intermediate structures along the decomposition path are proposed as possible active sites for oxidation catalysis. The results presented herein provide molecular level insight into the dynamic nature of the heteropolyacid catalyst structure.

Comparison of UV and Visible Raman Spectroscopy of Bulk Metal Molybdate and Metal Vanadate Catalysts

The visible (532 and 442 nm) and UV (325 nm) Raman spectra of bulk mixed metal oxides (metal molybdates and metal vanadates) were compared on the same spectrometer, for the first time, to allow examination of how varying the excitation energy from visible to UV affects the resulting Raman spectra. The quality of the Raman spectra was found to be a strong function of the absorption properties of the bulk mixed oxide. For bulk mixed metal oxides that absorb weakly in the visible and UV regions, both the visible and UV Raman spectra were of high quality and exhibit identical vibrational bands, but with slightly different relative intensities. For bulk mixed metal oxides that absorb strongly in the UV and visible regions and/or strongly in the UV and weakly in the visible regions, the visible Raman spectra are much richer in structural information and of higher resolution than the corresponding UV Raman spectra. This is a consequence of the strong UV absorption that significantly reduces the sampling volume and number of scatterers giving rise to the Raman signal. The shallower escape depth of UV Raman, however, was not sufficient to detect vibrations from the surface metal oxide species that are present on the outermost surface layer of these crystalline mixed metal oxide phases as previously suggested. It was also demonstrated that there is no sample damage by the more energetic UV excitation when very low laser powers and fast detectors are employed, thus avoiding the need of complicated fluidized bed sample arrangements sometimes used for UV Raman investigations. The current comparative Raman investigation carefully documents, for the first time, the advantages and disadvantages of applying different excitation energies in collecting Raman spectra of bulk mixed metal oxide materials.

Influence of preparation technique and iron doping on the structure and reactivity of mixed Fe–Ti–O nanocomposites

Mixed Fe–Ti–O nanocomposites in different compositions were prepared by a wet impregnation technique and compared with the bulk mixed oxide of the same surface composition obtained using the co-precipitation technique. The influence of the preparation method and the iron doping on the structure and reactivity of the mixed oxide were studied by means of various experimental techniques. X-ray photoelectron spectroscopy results indicated a reduction phenomenon at the interface between titania (the support) and iron oxide (the supported) whereas this behaviour was not observed for the bulk mixed oxide. The presence of iron oxide on the surface of titania reduces the hydroxylation. The reactivity with respect to methanol was investigated and the reactivity of titania was found to be increased by the presence of iron oxide on its surface. In particular, the supported sample with a low iron content oxidizes methanol to carbon dioxide at rather low temperatures, which could not be observed for the bulk mixed oxide of the same surface composition. The structures of the supported and bulk mixed oxide samples prepared by the wet impregnation and co-precipitation methods were similar to each other. The structure resembled that of TiO 2 (rutile) and the iron doping does not significantly influence the local environment.

Determination of the Chemical Nature of Active Surface Sites Present on Bulk Mixed Metal Oxide Catalysts

CH3OH temperature programmed surface reaction (TPSR) spectroscopy was employed to determine the chemical nature of active surface sites for bulk mixed metal oxide catalysts. The CH3OH-TPSR spectra peak temperature, Tp, for model supported metal oxides and bulk, pure metal oxides was found to be sensitive to the specific surface metal oxide as well as its oxidation state. The catalytic activity of the surface metal oxide sites was found to decrease upon reduction of these sites and the most active surface sites were the fully oxidized surface cations. The surface V5+ sites were found to be more active than the surface Mo6+ sites, which in turn were significantly more active than the surface Nb5+ and Te4+ sites. Furthermore, the reaction products formed also reflected the chemical nature of surface active sites. Surface redox sites are able to liberate oxygen and yield H2CO, while surface acidic sites are not able to liberate oxygen, contain either H+ or oxygen vacancies, and produce CH3OCH3. Surface V5+, Mo6+, and Te4+ sites behave as redox sites, and surface Nb5+ sites are Lewis acid sites. This experimental information was used to determine the chemical nature of the different surface cations in bulk Mo-V-Te-Nb-Ox mixed oxide catalysts (Mo(0.6)V(1.5)Ox, Mo(1.0)V(0.5)Te(0.16)Ox, Mo(1.0)V(0.3)Te(0.16)Nb(0.12)Ox). The bulk Mo(0.6)V(1.5)Ox and Mo(1.0)V(0.5)Te(0.16)Ox mixed oxide catalytic characteristics were dominated by the catalytic properties of the surface V5+ redox sites. The surface enrichment of these bulk mixed oxide by surface V5+ is related to its high mobility, V5+ possesses the lowest Tammann temperature among the different oxide cations, and the lower surface free energy associated with the surface termination of V=O bonds. The quaternary bulk Mo(1.0)V(0.3)Te(0.16)Nb(0.12)Ox mixed oxide possessed both surface redox and acidic sites. The surface redox sites reflect the characteristics of surface V5+ and the surface acidic sites reflect the properties normally associated with supported Mo6+. The major roles of Nb5+ and Te4+ appear to be that of ligand promoters for the more active surface V and Mo sites. These reactivity trends for CH3OH ODH parallel the reactivity trends of propane ODH because of their similar rate-determining step involving cleavage of a C-H bond. This novel CH3OH-TPSR spectroscopic method is a universal method that has also been successfully applied to other bulk mixed metal oxide systems to determine the chemical nature of the active surface sites.

Oxidation of o-Xylene to Phthalic Anhydride on Sb-V/ZrO2 Catalysts

Zirconia-supported and bulk-mixed vanadiumantimonium oxide catalysts were used for the oxidation of o-xylene to phthalic anhydride. X-ray diffraction, Raman spectroscopy and photoelectron spectroscopy were used for characterization. It was found that vanadium promotes the transition of tetragonal to monoclinic zirconia. The simultaneous presence of Sb and V on zirconia at low coverage led to a preferential interaction of individual V and Sb oxides with the zirconia surface rather than the formation of a binary Sb-V oxide, while at higher Sb-V contents the formation of SbVO4 took place. Sb-V/ZrO2 catalysts showed high activity for o-xylene conversion and better selectivity to phthalic anhydride as compared to V/ZrO2 catalysts. However, their selectivity to phthalic anhydride was poor in comparison to a V/TiO2 commercial catalyst. The improved selectivity of the Sb-containing catalysts is attributed to the blocking of non-effective surface sites of ZrO2, the decrease of the total amount of acid sites and the formation of surface V-O-Sb-O-V structures.

Effect of Preparation Method and Redox Treatment on the Reducibility and Structure of Supported Ceria–Zirconia Mixed Oxide

An Al2O3-supported Ce0.45Zr0.55O2 was prepared with a modified sol–gel procedure designed to maximize the interaction between ceria and zirconia and promote thermal stability of alumina. Its properties, characterized in detail with XRD, TPR, and laser Raman spectroscopy, were compared with those of more conventional preparations. This sample consisted of finely dispersed CexZr1−xO2 and had a high fraction of easily reducible Ce4+. Complete insertion of Zr into the ceria fluorite structure was observed after 900°C oxidation. After reduction at 980°C and reoxidation at 900°C, the mixed oxide particles remained small and furthermore still remained single phase and homogeneous by XRD characterization. Redox treatment enhanced the reducibility of Ce4+ in this sample. One feature of the enhancement, similar to that in bulk mixed oxide of similar composition, was an increase in the ratio of Ce4+ that was reducible at ∼491°C relative to the ones at ∼577°C. A new low-temperature reduction feature appeared at 320°C. This was attributed to the segregation of a small fraction of Ce3+ from the mixed oxide upon reduction and the subsequent formation of a finely dispersed ceria-rich phase on the surface of mixed oxide crystallites upon reoxidation. This phenomenon was particular to the supported samples because Al2O3 was able to stabilize the particle size of the mixed oxide during severe redox and thermal treatments.

Oxygen buffering capacity (OBC) of praseodymium-modified CeO2: influence of the Pr distribution in the ceria host lattice

This work reports on the redox behaviour of different ceria-praseodymia mixed oxides with two different chemical compositions (Ce/Pr molar ratios of 4:1 and 9:1), The samples consist of bulk oxides and ceria-supported samples with varying mixed oxide loading (equivalent to 7 or 12 mixed oxide layers). As revealed by x-ray diffraction and high-resolution electron microscopy, the samples consist of homogeneous fluorite-like mixed oxides. Their redox properties were characterized by means of the oxygen buffering capacity (OBC) technique, which quantitatively evaluates the capacity of the samples for attenuating fast oscillations of the oxygen partial pressure in the He stream flowing through the sample. These oscillations are induced by injecting with a high frequency pulses of oxygen into the inert gas stream. Although the XPS analyses performed on both a bulk and a supported sample showed Ce/Pr molar ratios very close to each other, the bulk mixed oxide exhibits significantly better OBC values. Also remarkable is that an ageing treatment at 1223 K improves the OBC response of the supported mixed oxides. In accordance with the XPS data, the enhancement of their redox properties runs parallel to an increase in the Ce/Pr surface ratio. These results are discussed in terms of the role played by the depth profile distribution of Pr in the different oxide samples.