CO hydrogenation over a rhodium vanadate catalyst: Reduction and regeneration of RhVO4 on SiO2

The hydrogenation of CO over an Rh vanadate (RhVO₄) catalyst supported on SiO₂ (RhVO₄/SiO₂) has been investigated after H₂ reduction at 500°C, and the results are compared with those of vanadia-promoted (V₂O₅–Rh/SiO₂) and unpromoted Rh/SiO₂ catalysts. The mean size of Rh particles, which were dispersed by the decomposition of RhVO₄ after the H₂ reduction, was smaller (41 Å) than those (91–101 Å) of V₂O₅–Rh/SiO₂ and Rh/SiO₂ catalysts. The RhVO₄/SiO₂ catalyst showed higher activity and selectivity to C₂ oxygenates than the unpromoted Rh/SiO₂ catalyst after the H₂ pretreatment. The CO conversion of the RhVO₄/SiO₂ catalyst was much higher than that of V₂O₅–Rh/SiO₂ catalyst, and the yield of C₂ oxygenates increased. We also found that the RhVO₄/SiO₂ catalyst can be regenerated by calcination or O₂ treatment at high temperature after the reaction.     

Hyperfine interactions studies on Pt–In/Nb2O5 catalysts

Pt–In/Nb₂O₅ catalysts were investigated using temperature-programmed reduction (TPR) and time differential perturbed angular correlation (TDPAC) experiments. The results indicated the presence of In–O surface complexes for low loading In/Nb₂O₅ and Pt–In/Nb₂O₅ catalysts after calcination. These complexes did not form a In₂O₃ crystalline phase. After reduction of the Pt–In/Al₂O₃ catalyst, In is present in different states. A fraction of In atoms is bonded to niobia surface, as a surface complex that does not show crystalline structure similar to bulk In₂O₃. Other fraction of In atoms interacts with platinum, in the form of an alloy, in locations that present trigonal symmetry.     

Preparation, characterization and catalytic properties of Co–Nb2O5–SiO2 catalysts

Co–Nb₂O₅–SiO₂ catalysts were prepared using three different sol–gel procedures: (i) the colloidal sol–gel method using NbCl₅ and SiCl₄ as precursors; (ii) the polymeric sol–gel method using niobium ethoxide and tetraethyl-orthosilicate (TEOS); (iii) an intermediate procedure between the colloidal and polymeric sol–gel method in which the precursors were those utilized in the CSG but dissolved in a mixture of anhydrous ethanol and CCl₄. In all procedures, the elimination of the solvent carried out between 80 and 110°C was followed by a reduction in hydrogen flow (30 ml min⁻¹) at 773 K. Following these procedures, samples containing 10 wt.% Co and 15 wt.% niobium oxide (expressed as Nb₂O₅) were obtained. The characterization of the catalysts was performed using various techniques: N₂ adsorption and desorption curves at 77 K, NH₃- and H₂-chemisorption, TPO, XPS, XRD, and solid state ¹H MAS-NMR. Hydrogenolysis of butane was evaluated. The low reaction rates are assigned to the effect of the metal size, whereas the isobutane selectivity as well as the relatively high stability is due to the acidity of the support.     

Nb2O5-supported WO3: a comparative study with WO3/Al2O3

WO₃/Nb₂O₅-supported samples prepared by impregnation are characterised by X-ray diffraction (XRD), Raman spectroscopy and X-ray absorption spectroscopy (XAS) at the W–L₃ absorption edge, as well as temperature programmed reduction (TPR) and FT-IR monitoring of pyridine adsorption. Results are compared with those obtained for WO₃/Al₂O₃ samples prepared in the same conditions, showing that niobia is able to disperse tungsta better than alumina does. Formation of a crystalline WO₃ needs larger tungsten contents on niobia than on alumina, since tungsten solution into niobia is easier than into alumina. Raman and XAS spectra recorded under ambient conditions suggest that similar WO_x species are formed on both supports at tungsten contents 0.5–1 theoretical monolayers; however, TPR results for the low tungsten loaded samples indicate that, when reduction starts (always at temperatures higher than 700 K under H₂/Ar flow) there is a larger concentration of tetrahedral [WO₄] species on alumina, than on niobia. Samples with low tungsten loading have been tested in isopropanol decomposition and ethylene oxidation, following both processes by FT-IR of adsorbed species up to 673 K. Results show that adsorption of ethylene on WO₃/Nb₂O₅ yields acetaldehyde and acetate at 473 K, while this adsorption is non-reactive either on the supports or on WO₃/Al₂O₃. Isopropanol adsorbs dissociatively on both supports, leading to acetone and propene formation on tungsta–niobia, but only propene on tungsta–alumina, probably due to the larger reducibility of the tungsten-containing phases.     

Fischer–Tropsch synthesis over Re-promoted Co supported on Al2O3, SiO2 and TiO2: Effect of water

The activity and selectivity of rhenium promoted cobalt Fischer–Tropsch catalysts supported on Al₂O₃, TiO₂ and SiO₂ have been studied in a fixed-bed reactor at 483 K and 20 bar. Exposure of the catalysts to water added to the feed deactivates the Al₂O₃ supported catalyst, while the activity of the TiO₂ and SiO₂ supported catalysts increased. However, at high concentrations of water both the SiO₂ and TiO₂ supported catalyst deactivated. Common for all catalysts was an increase in C₅₊ selectivity and a decrease in the CH₄ selectivity by increasing the water partial pressure. The catalysts have been characterized by scanning transmission electron microscope (STEM), BET, H₂ chemisorption and X-ray diffraction (XRD).     

Syngas conversion using RhVO4 and Rh2MnO4 catalysts: Regeneration and redispersion of Rh metal by calcination and reduction treatments

The hydrogenation of CO over mixed oxides (RhVO₄, Rh₂MnO₄) supported on SiO₂ has been studied after H₂ reduction at 300°C and at 500°C, and the results compared with those of unpromoted Rh/SiO₂ catalysts. Rh was more highly dispersed (40 Å) after the decomposition of RhVO₄ by the H₂ reduction than those of Rh₂MnO₄/SiO₂ and unpromoted Rh/SiO₂ catalysts. The activity and the selectivity to C₂ oxygenates of the mixed-oxide catalysts after the H₂ reduction were higher than those of the unpromoted Rh/SiO₂ catalysts, but the activity of the RhVO₄/SiO₂ catalyst increased more dramatically after the decomposition by the H₂ reduction at 300°C, and hence the yield of C₂ oxygenates increased. These results suggest that a strong metal–oxide interaction (SMOI) was induced by the decomposition of the mixed oxides after the H₂ reduction. The catalytic activity and selectivity were reproduced repeatedly by the calcination and reduction treatments of the spent (used) catalyst because of the regeneration of RhVO₄ and redispersion of Rh metal.     

SiO2/Nb2O5 sol–gel as a support for HRP immobilization in biosensor preparation for phenol detection

A SiO₂/Nb₂O₅ mixed oxide was prepared by a sol–gel processing method based on TEOS and NbCl₅ as precursors and HCl as catalysts. A material having a specific surface area of 703 m² g⁻¹, average pore diameter of 2.4 nm and 5 wt.% of Nb was obtained. An amperometric peroxidase-based biosensor for phenol was constructed by immobilizing the enzyme onto the SiO₂/Nb₂O₅ sol–gel matrix by adsorption and cross-linking with glutaraldehyde and mixing with graphite powder to make a modified carbon paste. The biosensor performance for phenol detection, investigated in a flow injection system, was based on mediated electron transfer of horseradish peroxidase (HRP), avoiding the direct electron transfer of HRP, which was blocked by the sol–gel matrix. With optimized conditions, a linear response range from 5 to 25 μmol dm⁻³ for phenol was obtained with a sensitivity of 3.2 nA dm³ μmol⁻¹. The detection limit of the biosensor for phenol was 0.5 μmol dm⁻³ and the analytical frequency was 27 samples h⁻¹. The biosensor response was tested for various phenol substrates and the highest response was observed for 2-amino-4-chlorophenol. During 200 determinations, the biosensor kept the same response for phenol. The modified carbon paste retained its activity during 6 months of storage under refrigeration.     

Partial oxidation and ammoxidation of isobutane over Bi-Mo-based composite oxide with Nb2O5

Conversion of isobutane to methacrylonitrile via ammoxidation was studied using mixed catalysts composed of Bi---Mo-based composite oxides and various type of Nb₂O₂. The activity for ammoxidation of isobutane on Nb₂O₅ is fairly poor, and that the products were methacrolein without formation of methacylonitriIe. The activity of Bi---Mo-based composite oxides for the ammoxidation of isobutane was also fairly low and that the selectivity to methacylonitrile and methacrolein was 60–80%. The mixed catalyst composed of Bi---Mo-based composite oxides and amorphous Nb₂O₅ and that supported by γ-Al₂O₃ and SiO₂ which have strong acid properties were improved their catalytic activity with keeping of their selectivity for ammoxidation of isobutane to methacrylonitrile.     

Activity and acidity of Nb2O5-MoO3 and Nb2O5-WO3 in the Friedel-Crafts alkylation

Nb₂O₅ loaded on the supports and mixed with oxides was studied to investigate the activity and acidity for Friedel-Crafts benzylation of anisole. From the study on the loaded catalysts, a preliminary conclusion for the selection of metal oxide was obtained; namely, such an acidic oxide as silica was suitable for the support of Nb₂O₅. Then, MoO₃ and WO₃ were mixed with Nb₂O₅, and prominent high catalytic activity and acidities were observed. Both oxides of Nb₂O₅-MoO₃ and Nb₂O₅-WO₃ showed almost similar behavior with respect to characterization and catalytic activity. Surface area increased, X-ray diffraction (XRD) and Raman bands were lost, acid sites, both Brønsted and Lewis characters generated, and surface acid site density was as high as 2–4 nm⁻². The acid sites were generated on the amorphous metal oxides consisting of Nb and Mo or W oxides, different in nature from those of Nb₂O₅ calcined and un-calcined, and active for Friedel-Crafts benzylation.     

The state of Tin on Pt–Sn/Nb2O5 catalysts

The effect of tin addition on niobia supported catalysts was studied and compared to the properties of alumina supported bimetallic Pt–Sn catalysts. The catalyst surfaces were probed by methylcyclopentane conversion, showing that both the presence of Sn and the reduction of the support caused a decrease in hydrogenolysis activity, favoring the ring enlargement reaction. The thermodynamics of reduction of these systems, evaluated by following the reduction step (temperature programmed reduction — TPR) with a differential scanning calorimeter (DSC), and irreversible H₂ and CO uptakes, allowed to conclude that a Pt–Sn alloy is formed on niobia supported catalysts.     

Selective oxidation of propylene to acrolein over supported V2O5/Nb2O5 catalysts: An in situ Raman, IR, TPSR and kinetic study

The vapor-phase selective oxidation of propylene (H₂CCHCH₃) to acrolein (H₂CCHCHO) was investigated over supported V₂O₅/Nb₂O₅ catalysts. The catalysts were synthesized by incipient wetness impregnation of V-isopropoxide/isopropanol solutions and calcination at 450 °C. The catalytic active vanadia component was shown by in situ Raman spectroscopy to be 100% dispersed as surface VO_x species on the Nb₂O₅ support in the sub-monolayer region (<8.4 V/nm²). Surface allyl species (H₂CCHCH_2*) were observed with in situ FT-IR to be the most abundant reaction intermediates. The acrolein formation kinetics and selectivity were strongly dependent on the surface VO_x coverage. Two surface VO_x sites were found to participate in the selective oxidation of propylene to acrolein. The reaction kinetics followed a Langmuir–Hinshelwood mechanism with first-order in propylene and half-order in O₂ partial pressures. C₃H₆-TPSR spectroscopy studies also revealed that the lattice oxygen from the catalyst was not capable of selectively oxidizing propylene to acrolein and that the presence of gas phase molecular O₂ was critical for maintaining the surface VO_x species in the fully oxidized state. The catalytic active site for this selective oxidation reaction involves the bridging VONb support bond.     

Lean NOx reduction with CO + H2 mixtures over Pt/Al2O3 and Pd/Al2O3 catalysts

The reduction of NO_x by hydrogen under lean burn conditions over Pt/Al₂O₃ is strongly poisoned by carbon monoxide. This is due to the strong adsorption and subsequent high coverage of CO, which significantly increases the temperature required to initiate the reaction. Even relatively small concentrations of CO dramatically reduce the maximum NO_x conversions achievable. In contrast, the presence of CO has a pronounced promoting influence in the case of Pd/Al₂O₃. In this case, although pure H₂ and pure CO are ineffective for NO_x reduction under lean burn conditions, H₂/CO mixtures are very effective. With a realistic (1:3) H₂:CO ratio, typical of actual exhaust gas, Pd/Al₂O₃ is significantly more active than Pt/Al₂O₃, delivering 45% NO_x conversion at 160 °C, compared to >15% for Pt/Al₂O₃ under identical conditions. The nature of the support is also critically important, with Pd/Al₂O₃ being much more active than Pd/SiO₂. Possible mechanisms for the improved performance of Pd/Al₂O₃ in the presence of H₂+CO are discussed.     

MnOx/Pt/Al2O3 catalysts for CO oxidation in H2-rich streams

The catalytic activity of Pt on alumina catalysts, with and without MnO_x incorporated to the catalyst formulation, for CO oxidation in H₂-free as well as in H₂-rich stream (PROX) has been studied in the temperature range of 25–250 °C. The effect of catalyst preparation (by successive impregnation or by co-impregnation of Mn and Pt) and Mn content in the catalyst performance has been studied. A low Mn content (2 wt.%) has been found not to improve the catalyst activity compared to the base catalyst. However, catalysts prepared by successive impregnation with 8 and 15 wt.% Mn have shown a lower operation temperature for maximum CO conversion than the base catalyst with an enhanced catalyst activity at low temperatures with respect to Pt/Al₂O₃. A maximum CO conversion of 89.8%, with selectivity of 44.9% and CO yield of 40.3% could be reached over a catalyst with 15 wt.% Mn operating at 139 °C and λ = 2. The effect of the presence of 5 vol.% CO₂ and 5 vol.% H₂O in the feedstream on catalysts performance has also been studied and discussed. The presence of CO₂ in the feedstream enhances the catalytic performance of all the studied catalysts at high temperature, whereas the presence of steam inhibits catalysts with higher MnO_x content.     

Interactions NO—CO and O2—NO—CO on CuCo2O4/γ-Al2O3 and on γ-Al2O3- and CuCo2O4/γ-Al2O3-supported Pt, Rh and Pt—Rh catalysts, a transient response study

The interactions NO—CO and O₂—NO—CO have been studied onCuCo₂O₄γ-Al₂O₃ and on γ-Al₂O₃- and CuCo₂O₄γ-Al₂O₃-supported Pt, Rh and Pt—Rh catalysts. The deposition of noble metals (Pt, Rh and Pt—Rh) on CuCo₂O₄γ-Al₂O₃ instead of γ-Al₂O₃ is beneficial in: lowering the temperature at which maximum N₂O is formed and decreasing the maximum N₂O concentration attained; lowering the onset temperature of NO to N₂ reduction, and increasing the N₂ selectivity; preserving the activity towards NO to N₂ reduction on a higher level following the concentration step NO + COO₂+ NO + CO and changing the conditions from stoichiometric to oxidizing (50% excess of oxidants). The reason for this behaviour of the CuCo₂O₄γ-Al₂O₃-based noble metal catalysts is the formation (reversible) of a reduced surface layer on the CuCo₂O₄ supported spinel under the conditions of a stoichiometric NO + CO mixture.     

Performance and characterization of Ru/Al2O3 and Ru/SiO2 catalysts modified with Mn for Fischer–Tropsch synthesis

Mn effect and characterization on γ-Al₂O₃-, -Al₂O₃- and SiO₂-supported Ru catalysts were investigated for Fischer–Tropsch synthesis under pressurized conditions. In the slurry phase Fischer–Tropsch reaction, γ-Al₂O₃ catalysts showed higher performance on CO conversion and C₅⁺ selectivity than -Al₂O₃ and SiO₂ catalysts. Moreover, Ru/Mn/γ-Al₂O₃ exhibited high resistance to catalyst deactivation and other catalysts were deactivated during the reaction. From characterization results on XRD, TPR, TEM, XPS and pore distribution, Ru particles were clearly observed over the catalysts, and γ-Al₂O₃ catalysts showed a moderate pore and particle size such as 8 nm, where -Al₂O₃ and SiO₂ showed highly dispersed ruthenium particles. The addition of Mn to γ-Al₂O₃ enhanced the removal of chloride from RuCl₃, which can lead to the formation of metallic Ru with moderate particle size, which would be an active site for Fischer–Tropsch reaction. Concomitantly, manganese chloride is formed. These schemes can be assigned to the stable nature of Ru/Mn/γ-Al₂O₃ catalyst.     

The promoting effect of Nb2O5 addition to Pd/Al2O3 catalysts on propane oxidation

Pd/Nb₂O₅/Al₂O₃ catalysts were investigated on propane oxidation. Diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS) analysis suggested that monolayer coverage was attained between 10 and 20 wt.% of Nb₂O₅. Temperature programmed reduction (TPR) evidenced the partial reduction of niobium oxide. The maximum propane conversion observed on the Pd/10% Nb₂O₅/Al₂O₃ corresponded to the maximum Nb/Al surface ratio. The presence of NbO_x polymeric structures near to the monolayer could favor the ideal Pd⁰/Pd²⁺ surface ratio to the propane oxidation which could explain the promoting effect of niobium oxide.     

The hydrocracking of n-decane over bifunctional Ni-H3PW12O40/SiO2 catalysts

A series of bifunctional Ni-H₃PW₁₂O₄₀/SiO₂ catalysts for the hydrocracking of n-decane were designed and prepared. The evaluation results of the catalysts show that Ni-H₃PW₁₂O₄₀/SiO₂ catalysts possess a high activity for hydrocracking of n-decane and an excellent tolerance to the sulfur and nitrogen compounds in the feedstock. Under the reaction conditions: reaction temperature 300 °C; H₂/n-decane volume ratio of 1500; total pressure of 2 Mpa and the LHSV 2 h⁻¹, the conversion of n-decane over reduced 5%Ni-50%H₃PW₁₂O₄₀/SiO₂ catalysts is as high as 90%, the C₅⁺ selectivity equal to 70%. In order to reveal the structure and nature of the catalysts, a number of characterizations including XRD, Raman, H₂-TPD, NH₃-TPD, XPS and FT-IR of pyridine adsorption were carried out. The characteristic results show that the high activity of the catalysts and high C₅⁺ selectivity can be related to the unique structure of the H₃PW₁₂O₄₀ and its suitable acidity.     

Ammonium complex of niobium as a precursor for the preparation of Nb2O5/Al2O3 catalysts

Ammonium oxalate complex of niobium was investigated as an aqueous precursor for the preparation of x% Nb₂O₅/Al₂O₃ (x=5, 10, 20 and 30 wt.%) samples. Catalysts with the same Nb₂O₅ contents were also prepared from the traditional niobium oxalate/oxalic acid aqueous solution. The catalysts were characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), temperature-programmed reduction (TPR), infrared spectroscopy of chemisorbed pyridine and X-ray photoelectron spectroscopy (XPS). A comparison with the preparation method using the niobium oxalate salt was performed. The results showed that the niobium precursor influence the species growing leading to phases with different reducibility. The XPS revealed the presence of multilayers of niobium oxide on the Nb₂O₅/Al₂O₃ samples prepared by using niobium ammonium oxalate complex, while the ones obtained from niobium oxalate led to Nb₂O₅ particles islands. The addition of niobium oxide decreased the fraction of Lewis acid sites and increased the fraction of Brønsted acid sites, independent of the precursor salt. However, the creation of BAS was more pronounced on the Nb₂O₅/Al₂O₃ samples prepared from niobium oxalate.     

Effect of Rh loading on the performance of Rh/Al2O3 for methane partial oxidation to synthesis gas

Catalytic performance for partial oxidation of methane (POM) to synthesis gas was studied over the Rh/Al₂O₃ catalysts with Rh loadings between 0.1 and 3 wt%. It was found that the ignition temperature of POM reaction increased with the decreasing of the Rh loadings in the catalysts. For the POM reaction over the catalysts with high (≥1 wt%) Rh loadings, steady-state reactivity was observed. For the reaction over the catalysts with low (≤0.25 wt%) Rh loadings, however, oscillations in CH₄ and reaction products (CO, H₂, and CO₂) were observed. Comparative studies using H₂-TPR, O₂-TPD and high temperature in situ Raman spectroscopy techniques were carried out in order to elucidate the relation between the redox property of the Rh species in the Rh/Al₂O₃ with different Rh loadings and the performance of the catalysts for the reaction. Three kinds of oxidized rhodium species, i.e. the rhodium oxide species insignificantly affected by the support (RhO_x), that intimately interacting with the Al₂O₃ surface (RhⁱO_x) and the Rh(AlO₂)_y species formed by diffusion of rhodium oxides in to sublayers of Al₂O₃ [C.P. Hwang, C.T. Yeh, Q.M. Zhu, Catal. Today, 51 (1999) 93.], were identified by H₂-TPR and O₂-TPD experiments. Among them, the first two species can be easily reduced by H₂ at temperature below 350 °C, while the last one can only be reduced by H₂ at temperature above 500 °C. The ignition temperatures of POM reaction over the catalysts are closely related to the temperature at which most of the RhO_x and RhⁱO_x species can be reduced by CH₄ in the reaction mixture. Compared to the Rh/Al₂O₃ with high Rh loadings, the catalysts with low Rh loadings contain more RhⁱO_x species which possess stronger RhO bond strength and are more difficult to be reduced than RhO_x by the reaction mixture. Higher temperature is therefore required to ignite the POM reaction over the catalysts with lower Rh loadings. The oscillation during the POM reaction over the Rh/Al₂O₃ with low Rh loadings can be related to the behaviour of Rh(AlO₂)_y species in the catalyst switching cyclically from the oxidized state to the reduced state during the reaction.     

Syngas production from butane using a flame-made Rh/Ce0.5Zr0.5O2 catalyst

The capability of flame-made Rh/Ce_0.5Zr_0.5O₂ nanoparticles catalyzing the production of H₂- and CO-rich syngas from butane was investigated for different Rh loadings (0–2.0 wt% Rh) and two different ceramic fibers (Al₂O₃/SiO₂ and SiO₂) as plugging material in a packed bed reactor for a temperature range from 225 to 750 °C. The main goal of this study was the efficient processing of butane at temperatures between 500 and 600 °C for a micro-intermediate-temperature SOFC system. Our results showed that Rh/Ce_0.5Zr_0.5O₂ nanoparticles offer a very promising material for butane-to-syngas conversion with complete butane conversion and a hydrogen yield of 77% at 600 °C. The catalytic performance of packed beds strongly depended on the use of either Al₂O₃/SiO₂ or SiO₂ fiber plugs. This astonishing effect could be attributed to the interplay of homogeneous and heterogeneous chemical reactions during the high-temperatures within the reactor.