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.     

In situ TPR/TPO-Raman studies of dispersed and nano-scaled mixed V-Nb oxides on alumina

Various catalysts containing niobium and vanadium oxides supported on alumina were prepared by wet impregnation via aqueous solution using several precursors. The total loading of V and Nb oxides were below their dispersion limit on alumina. Vanadyl sulfate, ammonium metavanadate and ammonium niobate(V) oxalate were the precursors for supported vanadia and niobia. The reduction/oxidation properties were studied by conventional TPR/TPO and TPR/TPO-Raman. Surface vanadium oxide species tend to increase their polymerization degree upon TPR/TPO cycles. A broad weak feature near 900 cm⁻¹ appears associated to V³⁺–O–Al³⁺ bond vibration in the reduced vanadia-alumina catalysts. Niobia appears to retard vanadia reduction. Regarding supported niobia, a fraction of surface niobia is significantly more reducible than surface vanadia and another fraction is significantly less reducible. The more reducible niobia appears associated to an incipient Nb–Al–O phase that may account for a fluorescence background observed in the Raman spectra. The less reducible niobia phases appears associated to dispersed niobium oxide species on alumina. Niobium has an effect on vanadia reduction profiles in VNb/Al₂O₃ system.     

Surface organometallic chemistry on metals in water: Chemical modification of platinum catalyst surface by reaction with hydrosoluble organotin complexes: application to the selective dehydrogenation of isobutane to isobutene

Surface organo-metallic chemistry on metals can be a new route to generate supported bimetallic catalysts. According to previous works on Pt–Sn catalysts, the reaction of tetra n-butyl-tin on the reduced platinum surface leads to well-defined bimetallic catalysts which are very active and selective in the dehydrogenation of isobutane into isobutene. The presence of tin not only isolates the surface platinum atoms from each other (EXAFS) and thus prevents a fast deactivation by decreasing the processes of C–C bond cleavage but also favors the regeneration processes under air. So far the catalyst preparations were carried out either in the gas phase or in organic solution (e.g. heptane). However, in order to meet the industrial criteria of process simplicity, there is a need to carry out such catalyst preparation in water.

In this work, Pt–Sn/Al₂O₃ and Pt–Sn/SiO₂ catalysts was prepared by reacting tris n-butyl-tin hydroxide on the platinum surface, in water solution under atmospheric pressure of hydrogen. The kinetics of the reaction was followed by measuring the amount of butane evolved as a function of time. The solids obtained were characterized by CO, O₂ or H₂ chemisorption and electron microscopy (CTEM and EDAX). Clearly, the (n-Bu)₃Sn(OH) reacts selectively on the platinum surface and not with the support, with evolution of butane, leading to a bimetallic catalyst where the platinum atoms are isolated from each other by the tin atoms. Very high selectivities (>95%) and activities were obtained for the reaction of isobutane dehydrogenation into isobutene and it was concluded that surface organo-metallic chemistry on metal in water can be an alternative route to prepare well-defined supported bimetallic Pt–Sn catalysts.     

Hydrogenation of carvone on Pt–Sn/Al2O3 catalysts

The effect of Sn concentration in Pt–Sn/Al₂O₃ catalysts prepared by different procedures on the catalytic behavior in the carvone hydrogenation in liquid phase was studied. Results indicated that the increase of the Sn amount added to Pt modified the catalytic behavior, favoring the formation of unsaturated ketones and the simultaneous production of small quantities of unsaturated alcohols as reaction intermediaries. On the other hand, Pt/Al₂O₃ catalyst only produced carvomenthone as the main reaction intermediary.     

Pt–Sn/C electrocatalysts for methanol oxidation synthesized by reduction with formic acid

Carbon supported Pt–Sn alloy catalysts were prepared by reduction of Pt and Sn precursors with formic acid, and their electrocatalytic activity for methanol oxidation was compared with commercial Pt/C and Pt₇₅Sn₂₅/C electrocatalysts. By X-ray diffraction analysis it was found that the Pt lattice parameter increases with the addition of Sn, indicative of alloy formation. It was confirmed that Sn exhibits cocatalytic activity for CO oxidation. The onset potential for the methanol oxidation reaction of the Pt–Sn electrode was approximately 0.1 V smaller than that on Pt both at room temperature and at 90 °C. The best performance in a direct methanol fuel cell was obtained using the Pt₇₅Sn₂₅/C alloy catalyst prepared by the formic acid method as the result of an optimal balance of Sn content, degree of alloying and metal particle size.     

EXAFS study on Pd–Pt catalyst supported on USY zeolite

Local structure around Pd and Pt in the bimetallic Pd–Pt catalysts supported on ultra stable Y (USY) zeolite (SiO₂/Al₂O₃=680) was investigated by an extended X-ray absorption fine structure (EXAFS) method during oxidation, reduction, and sulfidation. The Pt L III-edge EXAFS spectra showed that a new bond that was significantly different from Pt–Pt to Pt–Pd metallic bonds was formed in the bimetallic Pd–Pt (4:1) reduced catalysts supported on USY zeolite. This new bond may reflect the ionic properties of Pt through the Pt–Pd interaction. Furthermore this new bond survived sulfidation indicating that the bond has a cationic property and sulfur-tolerance property. The Pt–Pd ionic interaction in these catalysts allows some of the Pd metal to survive as metallic phase. The existence of this metallic phase under sulfidation condition may result in high activity of Pd–Pt (4:1) catalyst supported on USY zeolite in the aromatics hydrogenation.     

Low temperature oxidation of CO over tin-modified Pt/SiO2 catalysts

Low temperature oxidation of CO over alloy type Sn–Pt/SiO₂ catalysts with different Sn/Pt ratios has been investigated at different CO partial pressure using thermal programmed oxidation (TPO) technique and time on stream (TOS) experiments. The introduction of tin into platinum strongly increased the activity of the catalyst. The activity had a maximum, which depended on both the Sn/Pt (at./at.) ratio and the CO partial pressure. TOS experiments revealed the aging of the Sn–Pt/SiO₂ catalysts. FTIR and Mössbauer spectroscopy has been used to follow compositional and structural changes of Sn–Pt/SiO₂ catalysts during the catalytic run. The results show that the in situ formed, highly mobile “Snⁿ⁺–Pt” ensemble sites are responsible for high activity, while formation of relatively stable SnO_x type surface species are involved in the catalyst deactivation.     

Preparation, characterization and reactivity of Pd/Nb2O5 catalysts in hexa-1,5-diene hydrogenation

Palladium/niobia catalysts are prepared by various methods involving either gas or liquid phase reduction. Although giving rise to average or low dispersion, the reduction of palladium precursors in a liquid medium (hydrazine or ethylene–glycol) appears to be a promising method since a low dispersion favors the activity in the hydrogenation of hexa-1,5-diene in liquid phase. The substitution of alumina by niobia improves the fractional selectivity and the yield of hex-1-ene in all cases. A very good global selectivity is also observed.     

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.     

Synthesis of platinum–tin/alumina reforming catalysts from a well-defined platinum–tin precursor complex

The complex [(CH₃)₄N]₃[Pt(SnCl₃)₅] was selected as a molecular precursor to prepare PtSn/γ_c-Al₂O₃ reforming catalysts. The spectroscopic fingerprints of the starting complex were obtained by ¹⁹⁵Pt and ¹¹⁹Sn NMR and diffuse reflectance UV–visible spectroscopy. A series of supported catalysts were synthesized by wet impregnation of alumina with a solution of the precursor in acetone.

Well-dispersed species are obtained for Pt loadings below 1 wt.%; at higher loadings, a second species is formed that has spectroscopic features reminiscent of the initial complex and precipitates as a separate phase. Apparently, the Pt–Sn bonds are hydrolyzed in the low-loading species and preserved in the high-loading species.

The thermal transformations of PtSn/γ_c-Al₂O₃ catalysts are also studied and compared with those of the bulk precursor. In particular, it is shown that the nature of the atmosphere of thermal treatment (neutral or oxidizing) can orient the final catalyst towards preservation of an intimate Pt–Sn interaction, or towards demixtion.     

Influence of the preparation method on the catalytic behaviour of PtSn/TiO2 catalysts

PtSn/TiO₂ catalysts containing 2 wt% Pt and a Pt:Sn atomic ratio of 2:1 and 1:1 were prepared by coimpregnation or successive impregnation method with aqueous solutions of SnCl₂·2H₂O and H₂PtCl₆·6H₂O of a commercial TiO₂ (P25, from Degussa). Both catalyst series, independent of the preparation method, were reduced at 473 and 773 K. XPS results show that tin was in an oxidized state after reduction at 473 K, and that a fraction was in the metallic state after reduction at 773 K. By use of in situ FTIR spectroscopy of adsorbed CO, the presence of bimetallic Pt–Sn phases was assessed after reduction at 773 K. Microcalorimetric analysis of CO adsorption enthalpy indicates that reduction at 773 K causes the appearance of a more heterogeneous distribution of active sites, as well as a loss in the amount of sites. The catalytic activity for the gas phase hydrogenation of crotonaldehyde was greatly improved when the catalysts were prepared by coimpregnation, at both reduction temperatures. The selectivity toward crotyl alcohol was higher after reduction at 773 K and independent of the preparation method, although it increased with the amount of tin, suggesting a promoting effect of tin on this reaction.     

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.     

Cumene hydrocracking and thiophene HDS on niobia-supported Ni, Mo and Ni–Mo catalysts

Results are reported on the XPS characterization and catalytic activity in cumene hydrocracking (2.8 MPa, 623 K) and thiophene HDS (2.8 MPa, 523–573 K) of sulfided Ni, Mo and Ni–Mo catalysts supported on alumina and on pure and phosphated niobia. From the XPS results, evidence was obtained for the formation of a surface niobium sulfide with stoichiometry close to NbS₂ during catalyst sulfidation. Sintering of supported nickel during sulfidation occurred to a much smaller extent with the niobia-supported catalysts than with the alumina-supported ones. The dispersion of alumina-supported molybdenum was little influenced by sulfidation, whereas, with the niobia supports, the molybdenum surface concentration increased with sulfidation. With the alumina support, the Ni–Mo combination caused the dispersion of the sulfided nickel to be improved, possibly due to formation of a NiMoS phase. This was not observed with the niobia-supported catalysts.

Reasonable linear correlations were also found between the intrinsic activity for cumene hydrocracking and the amount of sulfided niobium in the catalysts, but the catalysts supported on phosphated niobia had a higher intrinsic activity than the ones supported on pure niobia. In thiophene HDS, the activity of the niobia-supported nickel catalysts was much larger than the activity of the alumina-supported ones. The activity of the niobia-supported molybdenum catalysts was smaller than that of the alumina-supported catalyst. With the bimetallic catalysts, little or no synergy was observed with the niobia-supported catalysts, in sharp contrast with the alumina case.     

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.     

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.     

NOx storage-reduction catalysts based on hydrotalcite: Effect of Cu in promoting resistance to deactivation

Novel NO_x storage-reduction (NO_xSR) catalysts prepared by Pt and/or Cu impregnation of Mg–Al (60:40) hydrotalcite (HT)-type compounds show better performances in NO_x storage than Pt–Ba/Al₂O₃ Toyota-type NO_xSR catalysts at reaction temperatures lower than 250 °C. The presence of Pt or Cu considerably enhances the activity, with the former more active. The nature of the HT source, however, also influences performance. The co-presence of Pt and Cu slightly worsens the low temperature activity, but considerably promotes the resistance to deactivation after severe hydrothermal treatment and in the presence of SO₂. This effect is attributed to both the possibility of formation of a Pt–Cu alloy after reduction, and the modification of the HT induced during the deposition of Cu. The overall Pt–Cu/HT performances are thus superior to those of the Pt–Ba/Al₂O₃ Toyota-type NO_xSR catalysts.     

The effect of ceria content on the performance of Pt/CeO2/Al2O3 catalysts in the partial oxidation of methane

The effect of CeO₂ loading (1–20 wt.%) on the properties and catalytic behaviors of CeO₂–Al₂O₃-supported Pt catalysts on the partial oxidation of methane was studied. The catalysts were characterized by S_BET, X-ray diffraction (XRD), temperature-programmed reduction (TPR) and oxygen storage capacity (OSC). XRD and TPR results showed that the pretreatment temperature of the support influences on the amount of CeO₂ with fluorite structure. The pretreatment temperature of the support and CeO₂ loading influenced the morphology of Pt. OSC analysis showed a significant increase in the oxygen storage capacity per weight of CeO₂ for samples with high CeO₂ loading (12 and 20 wt.%). TPR analyses showed that the addition of Pt promotes the reduction of CeO₂. This effect was more significant for the catalysts with high CeO₂ loading (≥12 wt.%). The dispersion of Pt, measured by the rate of cyclohexane dehydrogenation, increases with increasing of the pretreatment temperature of the support. It was shown that the kind of the support is very important for obtaining of catalysts resistant to carbon formation. The catalysts with high CeO₂ loading (≥12 wt.%) showed the highest catalytic activity and stability in the reaction of partial oxidation of methane due to a higher Pt–CeO₂ interface.     

A study of the behaviour of Pt supported on CeO2–ZrO2/Al2O3–BaO as NOx storage–reduction catalyst for the treatment of lean burn engine emissions

The behaviour of a Pt(1 wt.%) supported on CeO₂–ZrO₂(20 wt.%)/Al₂O₃(64 wt.%)–BaO(16 wt.%) as a novel NO_x storage–reduction catalyst is studied by reactivity tests and DRIFT experiments and compared with that of Pt(1%)–BaO(15 wt.%) on alumina. The former catalyst, designed as a hydrothermally stable sample, is composed of an alumina modified with Ba ions and an overlayer of ceria-zirconia. The results pointed out that during the calcination barium ions migrates over the surface of the catalyst which thus show a good NO_x storage–reduction behaviour comparable with that of Pt–BaO on alumina, although Ba ions result much better dispersed.     

Support effects in Pt/TiO2–ZrO2 catalysts for NO reduction with CH4

ZrO₂–TiO₂ mixed oxides, prepared using the sol–gel method, were used as supports for platinum catalysts. The effects of catalyst pre-reduction and surface acidity on the performance of Pt/ZT catalysts for the reduction of NO with CH₄ were studied. The diffuse reflectance infrared Fourier transformed (DRIFT) spectra of CO adsorbed on the Pt/ZT catalysts, and also on the Pt/T and Pt/Z references, pre-reduced at 773 K in hydrogen, revealed that an SMSI state is developed in the Ti-rich oxide-supported platinum catalysts. However, no shift in the binding energy of Pt 4f_7/2 level for Pt/T and Pt deposited on Ti-rich support counterparts pre-reduced at 773 K was found by photoelectron spectroscopy. The DRIFT spectra of the catalysts under the NO+O₂ co-adsorption revealed the appearance of nitrite/nitrate species on the surface of the Zr-containing catalysts, which displayed acidic properties, but were almost absent in the Pt/T catalyst. The intensity of these bands reached a maximum for the Pt/ZT(1:1) catalyst, which in turn exhibited a larger specific area. In the absence of oxygen in the feed stream, the NO+CH₄ reaction showed DRIFT spectra assigned to surface isocyano species. Since the intensity of this band is higher for the Pt/ZT (9:1) catalyst, it seems that such species are developed at the Pt–support interface.     

Kinetic studies of CH4 oxidation over Pt–NiO/δ-Al2O3 in a fluidised bed reactor

The oxidation of CH₄ over Pt–NiO/δ-Al₂O₃ has been studied in a fluidised bed reactor as part of a major project on an autothermal (combined oxidation–steam reforming) system for CH₄ conversion. The kinetic data were collected between 773 and 893 K and 101 kPa total pressure using CH₄ and O₂ compositions of 10–35% and 8–30%, respectively. Rate–temperature data were also obtained over alumina-supported monometallic catalysts, Pt and NiO. The bimetallic Pt–NiO system has a lower activation energy (80.8 kJ mol⁻¹) than either Pt (86.45 kJ mol⁻¹) and NiO (103.73 kJ mol⁻¹). The superior performance of the bimetallic catalyst was attributed to chemical synergy. The reaction rate over the Pt–NiO catalyst increased monotonically with CH₄ partial pressure but was inhibited by O₂. At low partial pressures (<30 kPa), H₂O has a detrimental effect on CH₄ conversion, whilst above 30 kPa, the rate increased dramatically with water content.