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.     

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.     

Effects of small MoO3 additions on the properties of nickel catalysts for the steam reforming of hydrocarbons: II. Ni---Mo/Al2O3 catalysts in reforming, hydrogenolysis and cracking of n-butane

Addition of molybdenum to nickel catalysts has a favourable effect on their properties in the steam reforming of hydrocarbons. Some attempts were made to explain the mechanism of promoter action by determining the properties of such catalysts in hydrogenolysis and cracking reactions. With small amounts of promoter (≤0.5 wt.%) advantageneous changes in selectivity of steam reforming and disadvantageneous changes in n-butane hydrogenolysis were observed. The promoter does not affect practically the catalyst properties in n-butane cracking. The effect of molybdenum was compared with that of potassium promoter applied in the industry.     

Preparation of rhodium niobate compound by chemical mixing methods: Effect of preparation method on SMSI behavior of RhNbO4/SiO2 catalysts

Rhodium niobate was prepared as a complex oxide supported on silica via tetra-alkyl ammonium and citrate complexation. After calcination at 750 °C, the complex oxide was formed essentially in the monophasic columbite structure as determined by means of XRD. The two silica-supported RhNbO₄ catalysts were tested for their activity toward ethane hydrogenolysis, and a large difference in SMSI behavior was observed for the two preparations.     

Conversion of CCl2F2 (CFC-12) in the presence and absence of H2 on sol–gel derived Pd/Al2O3 catalysts

Conversion of CCl₂F₂ in the presence (hydrogenolysis) and absence of hydrogen was investigated on Al₂O₃, AlF₃ and Pd/Al₂O₃ xerogel and aerogel catalysts. CCl₂F₂ was found to form CClF₃ and CCl₃F on Al₂O₃ and AlF₃ in the presence and absence of hydrogen as well as on the Pd/Al₂O₃ catalysts in the absence of hydrogen. Overall activity increased during the hydrogenolysis reactions at 230°C as a function of time which was paralleled by a significant increase in the yield of CClF₃ formed through a Cl/F-exchange reaction. X-ray diffraction patterns of the spent catalyst recovered after 3 h of hydrogenolysis confirmed the presence of Pd(C) (Pd–carbon solid solution) and AlF₃ phases on Pd/Al₂O₃ catalysts indicated that the carbon incorporation into the Pd lattice and the transformation of Al₂O₃ to AlF₃ starts at the initial stage of the reaction. It was concluded that AlF₃ is responsible for the Cl/F-exchange reactions. CH₄, a complete hydrogenation product, is formed during hydrogenolysis. Another route for its formation is the reaction between hydrogen in the gas phase and the interstitial carbon.     

Characterization by Mössbauer spectroscopy of trimetallic Pd–Sn–Au/Al2O3 and Pd–Sn–Au/SiO2 catalysts for denitration of drinking water

The reduction of nitrate using a catalytic process is one of the most interesting ways to solve the problem of drinking water pollution by this compound. The key parameter of this technique is the selectivity toward nitrogen formation. Palladium/tin-based bimetallic catalysts are well suited for this purpose, but the selectivity of these catalysts is not high enough for a direct application of this process. In the present study, alumina- and silica-supported catalysts were prepared by successive deposition of tin and gold onto palladium particles by using controlled surface reaction. The characterization of trimetallic Pd–Sn–Au catalyst evidenced that trimetallic catalysts supported on silica present a palladium/tin/gold phase. The catalytic test showed that this type of catalyst is very active and selective in nitrate and nitrite reduction. Moreover, the addition of gold improves the stability and the selectivity toward nitrogen formation of the catalyst compared to the parent Pd–Sn catalyst.     

Characterization of Ni-Mo/Al2O3 catalysts prepared by simltaneous impregnation of the active components

Three Ni-Mo/Al₂O₃, catalysts were prepared, containing similar amounts of NiO (2.8%) and MoO₃ (13.5%), by simultaneous impregnation of the active components, varying the nature of the solubilizing agent (ammonia, citric acid and phosphoric acid). The catalysts were characterized with respect to texture (surface area and pore volume), presence of crystalline phases (XRD), distribution of active components (EPMA) and nature of the active phases (DRS). Catalytic activity was measured using thiophene hydrogenolysis as a model reaction. A commercial Ni-Mo catalyst of similar composition was characterized by the same methods.

The catalyst prepared in the phosphoric acid medium (catalyst P) had a significantly lower surface area. In phosphorus-containing catalysts there was a marked tendency of this element to be deposited on the external region of the catalyst extrudates. The DRS spectra showed that Mo was present predominantly as polymolybdate species in all cases. In the industrial catalyst (catalyst I) the presence of nickel spinel was clearly observed. Catalyst P had an absorption minimum in the visible region displaced to higher wavelength compared to the catalysts prepared in ammonia (catalyst A) and citric acid (catalyst C) media. The observed order of activity was P > C > A > I.     

Activity and selectivity of Pt-Sn/Nb2O5 in n-heptane conversion

The catalytic properties of niobia and alumina supported platinum and platinum-tin catalysts were evaluated at the conversion of n-heptane at 773K. Over a niobia supported platinum catalyst the reaction proceeded via a monofunctional path, with a high selectiviy for oleflns. This was explained by the formation of a SMSI state, leading to the coverage of platinum particles by NbO_x and the formation of metal-support interfacial sites. The presence of tin inhibited the formation of a SMSI state, causing a decrease in the olefins/toluene ratio.     

V2O5 catalysts supported on Al2O3–SiO2 mixed oxide: V, H MAS solid-state NMR, DRIFTS and methanol oxidation studies

Alumina–silica mixed oxide, synthesized by the sol–gel technique, was used as a support for dispersing and stabilizing the active vanadia phase. The catalysts were characterized employing ⁵¹V and ¹H solid-state MAS NMR, diffuse reflectance FT-IR, BET surface area measurements. The partial oxidation activities of the catalysts were tested using methanol oxidation as a model reaction. ⁵¹V solid-state NMR studies on the calcined catalysts showed the peaks corresponding to the presence of both tetrahedral and distorted octahedral vanadia species at low vanadia loadings and with an increase in V₂O₅ content, the ⁵¹V chemical shifts corresponding to amorphous V₂O₅ like phases were observed. DRIFTS studies of the catalysts indicated the vibrations corresponding tetrahedral vanadia species at low and medium loadings and at high V₂O₅ contents the vibrations corresponding V=O bonds of V₂O₅ agglomerates were observed. The V/Al–Si catalysts exhibited high selectivity for the dehydration product dimethyl ether in the methanol partial oxidation studies showing the predominance of the acidic nature of the alumina–silica support over the redox properties of the active vanadia phase.     

Ethylene dimerization over NiSO4 supported on Fe2O3-promoted ZrO2 catalyst

The NiSO₄ supported on Fe₂O₃-promoted ZrO₂ catalysts were prepared by the impregnation method. Fe₂O₃-promoted ZrO₂ was prepared by the coprecipitation method using a mixed aqueous solution of zirconium oxychloride and iron nitrate solution followed by adding an aqueous ammonia solution. No diffraction line of nickel sulfate was observed up to 20 wt.%, indicating good dispersion of nickel sulfate on the surface of Fe₂O₃–ZrO₂. The addition of nickel sulfate (or Fe₂O₃) to ZrO₂ shifted the phase transition of ZrO₂ (from amorphous to tetragonal) to higher temperatures because of the interaction between nickel sulfate (or Fe₂O₃) and ZrO₂. 15-NiSO₄/5-Fe₂O₃–ZrO₂ containing 15 wt.% NiSO₄ and 5 mol% Fe₂O₃, and calcined at 500 °C exhibited a maximum catalytic activity for ethylene dimerization. NiSO₄/Fe₂O₃–ZrO₂ catalysts was very effective for ethylene dimerization even at room temperature, but Fe₂O₃–ZrO₂ without NiSO₄ did not exhibit any catalytic activity at all. The catalytic activities were correlated with the acidity of catalysts measured by the ammonia chemisorption method. The addition of Fe₂O₃ up to 5 mol% enhanced the acidity, surface area, thermal property, and catalytic activities of catalysts gradually, due to the interaction between Fe₂O₃ and ZrO₂ and due to consequent formation of Fe–O–Zr bond.     

K-, CeO2-, and Mn-promoted Ni/Al2O3 catalysts for stable CO2 reforming of methane

Reforming of methane with carbon dioxide into syngas over Ni/γ-Al₂O₃ catalysts modified by potassium, MnO and CeO₂ was studied. The catalysts were prepared by impregnation technique and were characterized by N₂ adsorption/desorption isotherm, BET surface area, pore volume, and BJH pore size distribution measurements, and by X-ray diffraction and scanning electron microscopy. The performance of these catalysts was evaluated by conducting the reforming reaction in a fixed bed reactor. The coke content of the catalysts was determined by oxidation conducted in a thermo-gravimetric analyzer. Incorporation of potassium and CeO₂ (or MnO) onto the catalyst significantly reduced the coke formation without significantly affecting the methane conversion and hydrogen yield. The stability and the lower amount of coking on promoted catalysts were attributed to partial coverage of the surface of nickel by patches of promoters and to their increased CO₂ adsorption, forming a surface reactive carbonate species. Addition of CeO₂ or MnO reduced the particle size of nickel, thus increasing Ni dispersion. For Ni–K/CeO₂–Al₂O₃ catalysts, the improved stability was further attributed to the oxidative properties of CeO₂. Results of the investigation suggest that stable Ni/Al₂O₃ catalysts for the carbon dioxide reforming of methane can be prepared by addition of both potassium and CeO₂ (or MnO) as promoters.     

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.     

Role of ZnO in Cu/ZnO/Al2O3 catalyst for hydrogenolysis of butyl butyrate

For hydrogenolysis of butyl butyrate (BB), a series of Cu/ZnO/Al₂O₃ catalysts with different metal compositions were prepared, and characterized by N₂O chemisorption for measuring Cu surface area and by chromatographic experiment for determining the heat of BB adsorption. As a result, the presence of ZnO in Cu-based catalysts was found to enhance the catalytic activity of Cu due to dual function of ZnO. The Cu surface area was linearly correlated with the butanol productivity, demonstrating that ZnO exerts the structural function in Cu/ZnO/Al₂O₃ catalysts. Additionally, the role of ZnO as a chemical contributor was revealed such that its presence leads to lower activation energy of the surface reaction, thus resulting in higher Cu catalytic activity obtained at a low temperature such as 200 °C. Consequently, optimizing the Cu/Zn ratio in Cu/ZnO/Al₂O₃ catalyst is required to tune its structural and chemical characteristics of Cu metals, and thus to obtain a higher activity on the hydrogenolysis reaction.     

Strong rhodium–niobia interaction in Rh/Nb2O5, Nb2O5–Rh/SiO2 and RhNbO4/SiO2 catalysts: Application to selective CO oxidation and CO hydrogenation

The extent of Rh–niobia interaction in niobia-supported Rh (Rh/Nb₂O₅), niobia-promoted Rh/SiO₂ (Nb₂O₅–Rh/SiO₂) and RhNbO₄/SiO₂ catalyst after H₂ reduction has been investigated by H₂ and CO chemisorption measurements. These catalysts have been applied to selective CO oxidation in H₂ (CO+H₂+O₂) and CO hydrogenation (CO+H₂), and the results are compared with those of unpromoted Rh/SiO₂ catalysts. It has been found that niobia (NbO_x) increases the activity and selectivity for both the reactions.     

Carbon dioxide reforming of ethanol over Ni/Y2O3–ZrO2 catalysts

Supported nickel catalysts of composition Ni/Y₂O₃–ZrO₂ were synthesized in one step by the polymerization method and compared with a nickel catalyst prepared by wet impregnation. Stronger interactions were observed in the formed catalysts between NiO species and the oxygen vacancies of the Y₂O₃–ZrO₂ in the catalysts made by polymerization, and these were attributed to less agglomeration of the NiO during the synthesis of the catalysts in one step. The dry reforming of ethanol was catalyzed with a maximum CO₂ conversion of 61% on the 5NiYZ catalyst at 800 °C, representing a better response than for the catalyst of the same composition prepared by wet impregnation.     

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.     

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.     

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.     

Aromatization of hydrocarbons over Pt- and Pt-Re-containing zeolite-Al2O3 catalysts

The typical reaction of ethane in a hydrogen flow over Pt-Re/ZSM-Al₂O₃ catalysts in hydrogenolysis of the C₂H₆ molecule. Hydrogenolysis activity is thereby enhanced by a high dispersion of the metal phase. However, in a helium atmosphere, the formation of aromatics can be observed which reaches a maximum for ZSM contents between 50 and 70 wt % and surpasses the hydrogenolysis activity of the pure Pt-Re/ZSM catalyst.

The utilization of this ability to aromize during the conversion of higher hydrocarbons is hindered by the high hydrocracking activity of the zeolites. However, by adding small amounts of both a ZSM and a Y type zeolite to Al₂O₃, the aromatization activity and selectivity in the conversion of n-hexane and n-heptane in the presence of about 300 ppm water vapour can be improved.     

Role of CeO2 in Ni/CeO2–Al2O3 catalysts for carbon dioxide reforming of methane

Ni catalysts supported on γ-Al₂O₃, CeO₂ and CeO₂–Al₂O₃ systems were tested for catalytic CO₂ reforming of methane into synthesis gas. Ni/CeO₂–Al₂O₃ catalysts showed much better catalytic performance than either CeO₂- or γ-Al₂O₃-supported Ni catalysts. CeO₂ as a support for Ni catalysts produced a strong metal–support interaction (SMSI), which reduced the catalytic activity and carbon deposition. However, CeO₂ had positive effect on catalytic activity, stability, and carbon suppression when used as a promoter in Ni/γ-Al₂O₃ catalysts for this reaction. A weight loading of 1–5 wt% CeO₂ was found to be the optimum. Ni catalysts with CeO₂ promoters reduced the chemical interaction between nickel and support, resulting in an increase in reducibility and stronger dispersion of nickel. The stability and less coking on CeO₂-promoted catalysts are attributed to the oxidative properties of CeO₂.