Search

This work studies the variation in the photocatalytic properties of CdS derived from the insertion of In, Ga, Ag-In and Ag-Ga in the CdS lattice through solvothermal methodology. Solvothermal synthesis of CdS-M photocatalysts has been succesful for the insertion of Ga3+, In3+, Ga3+/Ag+and In3+/Ag+into the hexagonal crystal lattice of one-dimensional CdS. The insertion of In, Ga, Ag-In and Ag-Ga modifies the band gap and the relative position of EVB.CdS modified with In3+or Ga3+shows an increase in the band gap and upshift in the relative position of the valence band energy which leads to a low efficiency hydrogen production. The co-addition of Ag+-In3+or Ag+-Ga3+favours the insertion of Ag+ions into the CdS lattice with narrower band gap. Of all the co-substituted photocatalysts, the CdS-AgGa was the only one that showed a higher photoactivity with respect to the CdS. The increase in the photoactivity of the CdS-AgGa photocatalyst is related to the band gap narrowing and downshift in the relative position of the valence band energy which enhance their visible light absorption and potential for oxidation. The CdS-AgGa photocatalyst shows small segregation of metallic Ag nanoparticles at the surface which also assist in the photoactivity of the sample.

Au/TiO2 and Au/TiO2-CeO2 catalysts were prepared by the sol-gel method and carbon monoxide, hydrogen chemisorption and TEM spectroscopy have been exploited to determine the size of gold particles. The gold nanoparticles (8.1 to 2.1 nm) were deposited by using the deposition-precipitation method. The XRD characterization shows the presence of anatase as the TiO2 crystalline phase; while by XPS spectroscopy, the presence of Au°, Au2O3, Ce3+ and Ce4+ species co-existing in the Au/TiO2-CeO2 catalysts is shown. The characterizations by TPD-CO as well as by TPD-H2 (temperature programmed desorption) showed that on catalysts containing cerium, the gold particle size can be determined with great accuracy by using these chemisorption methods. The gold particle size calculated from either the CO or H2 thermodesorption values is in good agreement with that obtained by High Resolution Transmission Electron Microscopy (HRTEM) and Scanning Transmission Electron Microscopy (STEM) analyses. It was proposed that the TPD-CO and/or TPD-H2 techniques could be helpful for the characterization of the gold particles by TEM; especially when the high contrast in the pictures of the supports containing CeO2 prevents the particle size from being determined.

Two different series of titania-supported iron catalysts prepared by the sol-gel method have been characterised and tested in the combustion of methane at atmospheric pressure. In each series, the effect of metal loading (1, 5 and 10 wt%) and gelation pH (3 and 9) were studied. Characterisation data of calcined catalysts, carried out by specific area measurement, TPR, X-ray diffraction, XPS and EPR revealed no significant changes in metal dispersion. The catalytic activity of the solids were evaluated in the combustion of methane under stoichiometric mixture of CH4/O2from 473 K up to the temperature required for total combustion. The best performance was shown by catalysts having 5 wt.%, which is ascribed to a balance of factors like the presence of a sufficient concentration of iron at the surface, achievement of relatively high dispersion state for iron and the presence of Fe+3ions incorporated to the lattice of titania.

A Cu-Zn-Al precursor was synthesized by coprecipitation of the corresponding cations with sodium carbonate at constant pH and temperature. CuO-ZnO-Al2O3composite oxide support was obtained by calcination (673 K) of the Cu-Zn-Al precursor. Two palladium-modified CuO-ZnO-Al2O3samples were prepared by impregnation of the mixed-oxide support and further calcination (673 K). The presence of remaining CO32-anions in the CuO-ZnO-Al2O3mixed oxide, as a result of incomplete Cu-Zn hydrotalcite phase decomposition, and the hydrothermal-like treatment during the Pd impregnation step, allow the partial reconstruction of the Cu-Zn hydrotalcite-type structure (memory effect). In addition, an enhancement in the CuO crystallinity was obtained for the Pd-modified oxides. A detailed characterization revealed that the hydrotalcite restoration enhances the crystallinity of the copper oxide as a consequence of a crystalline rearrangement of this oxidic phase.

Titania-supported iron (5 wt%) catalysts were prepared by a sol-gel method using different gelation pH and metal precursors (Fe(II) and Fe(III)). Characterization data of calcined catalysts revealed that, irrespective of the nature of the metal precursor, iron is present in all cases as ferric oxide. However, the crystalline phase exhibited by titania does depend on the metal precursor used. The catalytic activity of the catalysts, tested in the combustion of methane at atmospheric pressure, is not related to the dispersion of iron oxide. Thus, Fe³⁺ ions may be obtained in two extreme situations; one highly dispersed in which Fe³⁺ ions are placed in the titania network and another in which large Fe₂O₃ crystals are located on the surface of the catalyst. The former exhibits the best performance in the combustion of methane.© 2002 Society of Chemical Industry

A palladium–copper–zinc catalyst (PdO:CuO:ZnO=2:28:70), prepared by sequential precipitation of the respective cations, was tested in the hydrogenation of CO$\_2$ at high pressure (conditions: 60 bar, CO$\_2$:H$\_2$=1:3 (molar), W/F=0.0675 kg h/m$^3$, 453-513 K). The methanol yield was improved on using this Pd-containing catalyst at all temperatures with respect to the reference copper–zinc catalyst (CuO:ZnO=30:70). This improvement was not due to an additional effect in which palladium was acting as an independent catalytic site but was caused by a synergetic effect of Pd on the active Cu sites. This effect was explained in terms of hydrogen spillover and an increased stability against CO$\_2$ oxidation of the surface copper. Therefore, the present contribution not only supports previous literature findings concerning the hydrogen spillover mechanism but also resulted in a complementary view regarding the role of palladium in Pd-modified CuO-ZnO-based catalysts.

Mixed oxides of the type LaFexNi1−xO3with a perovskite structure were prepared by thermal decomposition of citrate mixtures at temperatures ranging from 773 to 1373 K. Temperature-programmed reduction profiles showed a low temperature step (450–500 K) due to the reduction of Ni3+to Ni2+and a high temperature step (650–1170 K) associated with the reduction of Ni2+to Ni0and Fe3+to Fe0. From the microgravimetric reduction profiles it was established that the samples exhibit oxidative nonstoichiometry LaFexNi1−xO3+δwith δ=0.00–0.08. All the catalysts were tested in the heterogeneous decomposition of hydrogen peroxide (H2O2→H2O+1/2 O2) in alkaline medium. It was found that the active sites for the reaction are the highly oxidized transition metals (NiIIIand FeIV). The concentration of these sites, as determined by iodometric titration, varied with catalyst composition and also with the calcination temperature of the precursors. The compensation effect was observed in all the catalysts investigated for hydrogen peroxide decomposition. The appearance of this effect was interpreted in terms of the catalytic reaction occurring on a heterogeneous catalytic surface, that is, a surface with active sites bearing different activation energies.

The effect of La2O3on the catalytic properties of PtSn/Al2O3reforming catalysts has been investigated. On catalysts with low La2O3content (1 wt%) FTIR CO adsorption spectra show two absorption bands at 2070 and 2122 cm−1. At higher La2O3content (20 wt%) the band at 2070 cm−1diminishes progressively until it completely disappears, while the band at 2122 cm−1increases and shifts to 2130 cm−1. This band is assigned to CO adsorbed on oxidized Pt. This effect is observed on samples on which PtSn–La2O3multiphase coexists. The presence of Pt in an oxidized state is supported by XPS since Pt2+is present on the catalysts containing Pt/Sn and La2O3. On the other hand, XPS revealed the presence of Sn2+and Sn°. For the n-heptane reaction the selectivity pattern of the PtSn/Al2O3–La catalysts shows total inhibition of the formation of benzene, high olefin content, and low n-C7dehydrocyclization products.

The equilibrium parameters for the adsorption of Mo(VI) on γ-Al2O3and of Co(II) and Pt(IV) on MoO3/γ-Al2O3were determined. The adsorption isotherms were performed from aqueous solutions of the corresponding precursors on two different alumina supports. According to the classification given by Giles, L-type-shaped, subgroup 2, adsorption curves were found for the system Mo on γ-Al2O3, L-type, subgroup 1, for the Pt on MoO3/γ-Al2O3, and S-type for Co on the MoO3/γ-Al2O3system. Numerical calculations were carried out for all the isotherms to find the equilibrium parameters. These constants are being used to model the development of Pt, Co, and Mo profiles on MoO3/γ-Al2O3or γ-Al2O3extrudates, respectively, which belong to the new generation of noble-metal–MoO3/γ-Al2O3-supported catalysts to be used in oil-refining processes.

Silica-supported PdZn catalysts have been studied in CO and CO2hydrogenation and in ethylene hydroformylation. The dilution of surface Pd by Zn lowers the hydrogenating capability of the catalysts and favours the production of higher hydrocarbons in CO hydrogenation. The catalyst with a molar ratio Pd : Zn = 3 showed an enhanced ability to insert CO into an M–alkyl bond; this catalyst produced higher oxygenates in the CO hydrogenation and was the most active in all reactions studied.

The selective catalytic reduction of NOxwith CH4in excess oxygen with or without added water was conducted over a series of ion-exchanged mordenites. The fresh catalysts contained both Pt and Co cations in varying proportions. Different treatments of the solid either after or between exchanges were performed in order to learn about the beneficial effect of Pt added to Co-mordenite. Temperature-programmed reduction of the solids together with photoelectron spectroscopy provided information concerning the interaction, migration, and oxidation states of Co and Pt. These studies were performed over the fresh, reduced, and used catalysts. The best formulations have Co/Pt atomic ratios between 15 and 24 and they exhibit better water resistance than Co-mordenites.

Copper–zinc–aluminum (CZA) catalysts prepared by the homogeneous precipitation of chloride salts in the presence of urea to produce hydrogen selectively by partial oxidation and oxyreforming reactions of methanol were used. The crystalline phases of the precursors and of the air-calcined CZA samples were identified by X-ray diffraction. Temperature-programmed reduction revealed that both CuO and CuAl2O4phases were present in the calcined catalysts. Prereduced catalysts were tested in both partial oxidation (O2/CH3OH=0.3–0.4) and oxyreforming reactions (O2/CH3OH/H2O=0.3 : 1 : 1.1) of methanol exhibiting high activity for CH3OH conversion and very high selectivity for H2production. The product distributions obtained in the partial oxidation of methanol (POM) reaction as a function of CH3OH and O2conversion indicate clearly that both conversion and selectivity become strongly dependent on the O2concentration in the gas phase and much less on the catalyst composition investigated. At low methanol conversion, when O2is not completely consumed, combustion and decomposition of methanol are the dominant reactions, however when O2is near or completely consumed, the methanol conversion goes beyond the expected stoichiometric POM values (60 and 80% for O2/CH3OH molar ratios of 0.3 and 0.4, respectively). For the oxyreforming reaction, the simultaneous increase in H2selectivity at the expense of water, with a less marked increase of CO selectivity, points to the contribution of steam reforming of methanol. The Auger parameter of copper of the catalysts used in the partial oxidation (1849.1 eV) and in the oxyreforming (1849.3 eV) reactions indicates that Cu2O species are present on the active catalyst surface. These Cu2O structures, developed on the catalyst surface during on-stream operation, appear to be sufficiently large to be also observe d by X-ray diffraction of the catalysts used in both reactions.

MoO3was added to Rh/SiO2catalysts in order to improve both activity and selectivity towards cinnamyl alcohol during the hydrogenation of cinnamaldehyde in the liquid phase. In the present work, four catalysts were studied. Two Rh–Mo/SiO2catalysts and Rh/SiO2, Mo/SiO2references were prepared by conventional impregnation techniques and characterised by hydrogen chemisorption, X-ray diffraction, transmission electron microscopy, temperature-programmed reduction and X-ray photoelectron spectroscopy. The catalytic behaviour in cinnamaldehyde hydrogenation, after in situ reduction treatments in flowing hydrogen at 773 K shows that Mo oxide clearly promotes the hydrogenation of the carbonyl bond. This is attributed to (i) the presence of both MoO3−xand Rhδ+species which contribute to a polarization of the C=O bond, (ii) the presence of flat surfaces of molybdenum oxides strongly interacting with Rh particles, that provide the appropriate morphology to enhance the selectivity to the unsaturated alcohol, and (iii) the poisoning effect of MoOxon rhodium species by partial coverage of the metallic particles that produce a decrease in the hydrogenation ability and therefore reduce the formation of the saturated alcohol.

The surface of non-stoichiometric nickel and nickel–copper spinel manganites has been investigated by X-ray Photoelectron Spectroscopy (XPS). The oxidation states of the nickel, copper and manganese cations present on the surface of the samples were determined from the analysis of the M 2p3/2core levels (M=Ni, Cu, Mn). In particular, both Cu2+and Cu+were evidenced in the structure whereas only bivalent nickel was observed. The partial substitution of manganese by copper led to a chemical shift towards lower binding energy in the Ni 2p3/2region, which was explained by the displacement of some Ni2+cations from tetrahedral to octahedral sites of the spinel structure. Finally, the surface atomic ratios Ni/Mn for nickel manganites, Ni/(Mn+Cu) and Cu/(Mn+Ni) for nickel–copper manganites, determined from XPS data, were compared to the ratios corresponding to the bulk composition. This study shows in all cases a nickel enrichment at the surface which is not affected by the copper content of the oxide. On the contrary, the ratio Cu/(Mn+Ni) was found to be lower than the corresponding bulk value.

The effect of platinum incorporated into Co-ferrierite catalyst on the selective catalytic reduction of NOxwith CH4was studied by means of in situ DRIFTS technique. NO adsorption on Co- and PtCo-ferrierite catalysts gave dinitrosyl and mononitrosyl species on Co2+ions. The adsorption of NO + O2on Co-ferrierite yielded NO2(NO2δ+) species and also nitrites and nitrates. Similar species were observed on PtCo-ferrierite, although chemisorbed NO2was much more stable since it persisted at reaction temperatures as high as 723 K. The spectra of the pre-reduced bimetallic PtCo-ferrierite catalyst exposed to the CH4+ NO + O2reaction mixture showed bands at 2200–2100 cm-1, which were similar to results for a Pt-free sample but slightly more intense. In addition, strong bands of nitrate, almost unchanged with temperature, were observed. A very stable Co2+–NO2intermediate species was developed upon incorporation of Pt into the base Co-ferrierite catalyst.

Mixed crystalline tin–titanium phosphates with nominal Ti:Sn ratios of 0.5 and 3.0 have been prepared by precipitation of Sn(IV) and Ti(IV) soluble salts with H3PO4. Vanadium was then incorporated by chemical grafting with a VOCl3precursor up to levels of 2% vanadium. Both the tin–titanium phosphates (TS) and their V-loaded counterparts (V/TS3 and V/TS0.5) were characterized by X-ray diffraction, Raman spectroscopy, differential scanning calorimetry and X-ray photoelectron spectroscopy, and their reactivity was tested for the C2H6oxydehydrogenation reaction. It was shown that a part of vanadium segregates as V2O5in the V/TS3 sample, while Sn segregation occurs in V/TS0.5 and this system shown no formation of V2O5crystallites. Both the TS and V/TS systems were found to be active for this reaction, although significant differences were observed depending on the crystallinity and surface enrichment of tin. The V/TS0.5 system displayed the best performance. Selectivity for dehydrogenation decreased only slightly with increasing ethane conversion, whereas the selectivity to CO2increased to the detriment of the selectivity to C2H4because CO selectivity remained close to zero. The high selectivity to C2H4along the C2H6interval explored appears to be related to highly dispersed vanadium species on the catalyst surface.

The oxygen content and the surface composition of pure AlN and Si3N4powders, synthesized by SHS reactions and commercial routes, have been studied using X-ray photoelectron spectroscopy (XPS) and hot gas extraction techniques. Surface stoichiometry is identified and discussed for each powder. The AlN and Si3N4powders synthesized by SHS technology were subjected to a sputtering process with rare gas bombardment (Ar+) in order to progressively remove the uppermost layers. A gradient in the oxygen distribution between the particle surface and the interior was measured. Differences in the oxygen distribution for the AlN and Si3N4powders depending on the route of synthesis have been observed.

Temperature‐programmed in situ Raman spectroscopy is used to understand the effect of surface vanadia coverage on the structure, reducibility and performance for the oxidation of methane on V2O5/SiO2catalysts. The vanadia coverage on silica has no effect on its structure below its dispersion‐limit loading (“monolayer” coverage); however, the interactions among surface vanadia species under reducing conditions become increasingly important. This interaction appears to operate through the sharing of oxygen sites facilitating the reduction, but it does not alter the total reducibility. The probability for this interaction to take place increases with vanadium oxide surface coverage. It is therefore expected that under reaction conditions the catalyst with higher vanadia coverage would have a greater capacity to release oxygen. This would increase the activity per vanadium site.

The state of the metallic phase of PtSn/C catalysts prepared by coimpregnation and successive impregnation on purified and H2O2-functionalized carbons as supports was studied by TPD, TPR, H2chemisorption, XPS, and test reactions. Furthermore, the characteristics of the metallic structures were related with the catalytic behaviors in the selective hydrogenation reaction of carvone in liquid phase. In PtSn/C samples, most of the Sn would be in intimate contact with Pt, and the surface of the reduced bimetallic catalysts would be composed of metallic platinum, Sn(II), and/or Sn(IV) species, and small amounts of zerovalent tin, probably forming alloys with Pt0. The high selectivity of these bimetallic PtSn/C catalysts for the production of unsaturated alcohols in the reaction of carvone hydrogenation would be due to the action not only of the promoter (Sn) but also of the support (carbon).

The study of the activation energies of reduction of stoichiometric and nonstoichiometric potassium-free and potassium-doped NiO was carried out by means of temperature-programmed reduction experiments, using two compared theoretical methods. Nonstoichiometric NiO shows the lower initial activation energy of reduction, whereas stoichiometric NiO shows a higher initial activation energy for starting the autocatalytic nucleation. Studies of the chemical preparation, BET surface areas, X-ray diffraction, X-ray photoelectron spectra, scanning electron microscopy, and measurements of catalytic activities of the nickel samples for the catalytic hydrogenation of 1,6-hexanedinitrile, in a continuous process at 1 atm pressure and 443 K, in the absence of ammonia, were also carried out. Surface areas dcrease when NiO reduction temperature increase. XRD, XPS, TPR, and SEM measurements detect NiO incipiently reduced between 463 and 498 K, a NiO reducibility inhibitor character for potassium, and 99.9% reduction above 573 K for all NiO forms. 100% selectivities with respect to 6-aminohexanenitrile may be obtained at 60% conversions for catalysts with potassium contents of 10.5 × l0−4 g K₂O/g Ni on reduced nonstoichiometric NiO. A mechanism for the continuous process is proposed.Copyright 1993, 1999 Academic Press

The effect of calcination temperature and Cs-promotion on the ternary Cu–Zn–Cr oxide catalyst has been investigated in connection with both structural and catalytic properties in the high pressure CO hydrogenation. As revealed by TPR and XPS, the reduction of copper is inhibited by incorporation of Cs promoter. Under typical reaction conditions, detectable copper phases are reduced to metallic copper. The crystal size of copper particles tends to increase with increasing reduction temperature and simultaneously Cs is segregated to the catalyst surface. Activity results in CO hydrogenation, with and without cofeeding methanol, ethanol, or 1-propanol, showed that methanol or C2+OH yield increased upon Cs-doping of the catalysts. By feeding a given CnOH alcohol, the Cn+OH yield was enhanced, with the most difficult chain growth being for the reaction CH3OH→CH3CH2OH. While a large surface area of the catalysts, with ill-defined crystalline phases, is the key parameter for an enhancement in the productivity and a low increase in the selectivity to higher alcohols, cesium incorporation leads to substantial improvement in C2+OH selectivity.

Fine titanium(IV) oxide amorphous powders containing two different amounts of CuO were prepared by the sol–gel process. The samples were crystallized to anatase by heating. The reduction of CuO–TiO2samples was investigated by temperature-programmed reduction (TPR). Three types of signals were detected: Two of them are strong and were assigned to two different types of copper ions distributed on the surface. The third kind was a weak broad signal and was ascribed to the simultaneous reduction of titanium and a third type of copper ion incorporated into the TiO2network. This was confirmed by electron spin resonance measurements of the samples before and after programmed reductions at various temperatures selected from the TPR profiles. The results showed that the lowest reduction temperature is due to the existence of isolated copper ions distributed on the surface. The second TPR peak was due to a CuO phase with small particle size. These results were consistent with X-ray photoelectron spectroscopy studies, which showed two signals assigned to –Cu–O–Ti–O– and CuO species.

Oxygen chemisorption experiments on prereduced silica-supported vanadia catalysts have been performed in order to determine the optimum conditions for measuring the relative dispersion of vanadium oxide on the silica substrate. By following the extent of vanadia reduction microgravimetrically, it is shown that an appropriate prereduction of the catalyst at 823 K under hydrogen is fundamental in obtaining a reproducible well-defined stoichiometrically reduced vanadium oxide system (V3+). AT 643 K the reduction process is very much slower and requires very long times in order to approach a plateau. Measurement of the binding energies of the V2p3/2core level spectra indicates that high-temperature oxygen chemisorption (643 K) results in a reoxidation of the prereduced vanadium oxide species within crystallites leading to an overestimation of the number of exposed vanadium sites. Chemisorption at low temperatures (298 or 195 K) does not affect the stoichiometrically reduced vanadium oxide species within crystallites, thus allowing the number of exposed surface vanadium sites to be measured.

The catalytic behavior for styrene hydrogenation of two seriesof La-promoted nickel/sepiolite catalysts (xLaNi and NixLa) isreported. Both series were prepared by two-step impregnations,but lanthanum was incorporated first and nickel afterward inNixLa series and the order was reversed inxLaNi samples. Thecatalysts were characterized by temperature-programmed reduction(TPR), kinetics of reduction by hydrogen, photoelectronspectroscopy (XPS), and infrared spectroscopy of adsorbed CO toobtain information on the nature and distribution of nickel andlanthanum in the oxidic and reduced samples. The kinetics ofreduction and XPS data indicate clearly that Ni2+ions interactstrongly with the sepiolite substrate inxLaNi samples and to alesser extent in NixLa series. The IR spectra of CO adsorbed onthe reduced catalysts demonstrate that the promotion ofNi/sepiolite catalyst with lanthana results in a change in thedistribution of CO-adsorbed structures. It appears thatreconstruction of bridge-bonded species occurs via formation ofNi(100) surfaces. Moreover, CO can be adsorbed in a linear formon Ni atoms which are in contact with oxidic phase. The decreaseof the bridged form and relative higher concentration ofunreduced nickel species gives an indication that lanthanum isdeposited on the nickel particles, leading to a decrease in theprobability to have adjacent nickel atoms on the surface. Theobserved stable maximum in activity for the Ni7.1La catalyst iscaused not only by covering by La a larger fraction of thesilicate substrate but also by isolating Ni atomson the surface of Ni crystallites by La2O3patches deposited on it.

The effect of exposure to the UVCON (humidity and UV radiation) accelerated corrosion test of a poly(vinyl chloride) paint applied over cleaned, abraded, and pre-rusted mild steel substrates has been investigated from the interfacial chemistry and adhesion standpoints. The X-ray photoelectron spectroscopy (XPS) data reveal significant differences in the locus and mode of adhesion loss, which depend on the time of exposure and the initial presence of a rust layer on the mild steel. The adhesion values obtained with the pre-rusted mild steel were greater than those obtained with the cleaned and abraded mild steel before exposure. After 15 days of exposure, it was observed that changes in the interfacial surface composition of the PVC/cleaned and abraded mild steel systems might have important positive effects on the adhesion strength values. The XPS data suggest the oxidation of both interfacial surfaces, incorporating polar C-O and C=O species, which would be expected to enhance the adhesion of paint to metal. In contrast, the PVC paint/pre-rusted mild steel system showed no change in adhesion values or interfacial surface composition over the entire period of exposure.

Unsupported copper-aluminum and copper-chromium oxides were prepared by mixing solutions of the nitrates and calcining at 1223 K, followed by treatment with washing solutions of either ammonium carbonate or nitric acid in order to extract uncombined copper oxide. Ammonium carbonate was only effective for removal of dispersed CuO, whereas nitric acid removed both dispersed and crystalline CuO. Catalyst activity for CO oxidation increased with the extraction treatments, particularly with the nitric acid washing solution, indicating that the active sites are copper species derived from CuAl2O4and CuCr2O4spinels rather than from CuO. The CuCr2O4catalyst exhibited higher activity, suggesting that Cu species in tetrahedral coordination lead to higher activity. Also, prereduction with H2produced higher activity than prereduction with CO, which can be attributed to a higher surface concentration of active species or their precursors after the former treatment. Activity results, together with TPR and XPS characterization point out that both Cu0and Cu+species are involved in the mechanism of CO oxidation.

Changes occurring on thermal treatment of the precursor of vanadium/phosphorus mixed oxide, the industrial catalyst for the oxidation ofn-butane, were studied. The precursor was mixed with stearic acid, used as an organic binder for pelletization of the powder. The calcination of the precursor leads to a partially oxidized compound, constituted of an amorphous VIV–P mixed oxide and a crystalline hydrated VV–P–O phase. The calcined compound, when left in a 1% hydrocarbon/air stream for 100 h leads to a “nonequilibrated” catalyst, and after 1000 h to the “equilibrated” catalyst. The catalytic activity of the nonequilibrated and equilibrated catalysts inn-butane andn-pentane oxidation was studied and compared; the chemical–physical features of the two catalysts were studied by means of XRD, FT-IR, chemical analysis, TGA, XPS, and TPD. Only well crystallized (VO)2P2O7was detected in the equilibrated catalyst and a homogeneous distribution of surface centers seems to be present on its surface. In the case of nonequilibrated catalyst, a poorly crystallized (VO)2P2O7is present together with an amorphous VIV–P–O phase and γ-VOPO4; these phases define a heterogeneous distribution of at least two kind of surface centers. This surface heterogeneity gives rise to a catalyst less selective inn-butane oxidation to maleic anhydride and less specific in the conversion ofn-pentane to phthalic anhydride.

This work describes relevant structural features of the ternary Li-Mn-MgO system useful for the reaction of methane oxidative coupling. For a Mn-MgO catalyst, XRD, XPS, and ESR techniques revealed that manganese is incorporated into a Mg₆MnO₈ phase in which the formal oxidation state of manganese is Mn4+. Addition of Li₂CO₃ in small amounts (0.48-1.60 wt.% Li) to the base Mn-MgO catalyst results in partial destruction of the Mg₆MnO₈ phase. TPR and XPS data showed that the reduction of manganese is easier in the bulk Mg₆MnO₈ compound than in the Mn-MgO catalyst and that it is inhibited by lithium. Moreover, the increase in Mn/Mg XPS intensity ratios in the lithium-doped counterparts indicates that lithium incorporation into the base Mn-MgO catalyst enhances manganese dispersion, in agreement with the increase of Mn4+-Mn4+ distance derived from ESR spectra.Copyright 1994, 1999 Academic Press, Inc.

The preparation of a single, well-defined PtSn phase supported on silica was carried out by the definitive interaction of cis-[PtCl₂(PPh₃)₂] and SnCl₂ with the hydroxyl surface groups of silica. After a reduction step, the only metallic phase present on the catalyst was the supported PtSn alloy, which was unambiguously characterized by X-ray diffraction, transmission electron microscopy, energy dispersive X-ray analysis, electron nanodiffraction, and X-ray photoelectron spectroscopy. Its structural properties and improved catalytic behaviour in the CO₂ reaction with C₂H₄ and H₂ or H₂O are contrasted with those of a conventional catalyst prepared from H₂PtCl₆ and SnCl₂.Copyright 1995, 1999 Academic Press, Inc.

The effect of coating the SiO2support of a V2O5oxidation catalyst with different amounts of TiO2has been studied. X-ray photoelectron spectroscopy of the resulting V2O5/TiO2/SiO2showed that the highest dispersion was reached for the low TiO2coverages. The catalysts were prepared by depositing vanadia in a quantity equivalent to one monolayer onto SiO2, TiO2, or TiO2-coated SiO2samples, by impregnation with a vanadyl acetylacetonate solution in ethanol, drying at 348 K, and calcining at 773 K in air. This procedure decreased the surface area by up to 30%. TPR profiles of binary and ternary samples were similar, showing only a single peak, suggesting the presence of just one vanadium species. Electron spin resonance spectra of partially reduced TiO2-coated silica carriers showed that, at very low titania content, titanium ions were present in the silica surface in small clusters. In the high content sample, TiO2covered the silica surface, as in a crystalline TiO2phase. The characteristics of supported vanadium oxide were affected by the TiO2distribution. These V/xTiSi catalysts were found to be active for the selective oxidation of ethanol. The titania coating of the support increased the activity of the supported vanadia as compared to that of the oxide supported on silica, but maintained a similar product distribution. This increase of activity depended on the number of titania monolayers (x), being maximum forx≥1. The selectivity to acetaldehyde also increased withx, being maximum for the same composition. The combination of the two factors allowed the attainment of acetaldehyde yields of 74% per pass at temperatures as low as 473 K. The modifications in the catalytic behavior are discussed in terms of the changes of the support surface and its interaction with the supported vanadia caused by the presence of the titania coating.

Alpha phase of NiMoO4promoted with alkaline (K and Cs) and alkaline earth (Ca, Sr, and Ba) metals has been studied for theoxidative dehydrogenation of butane. The catalysts werecharacterized by BET; X-ray diffraction; atomic absorption;inductively coupled plasma spectroscopy; Fourier transform infrared and Raman, X-ray photoelectron, and ultraviolet–visible diffuse-reflectance spectroscopic techniques; and CO2temperature-programmed desorption. The molybdate structure remains intact after doping. However, while for the Ca-, Sr-, and Ba-doped catalysts, new species were detected as oxide and peroxide compounds, only traces of alkali metals were found in the bulk of the catalysts, for the same loadingsof promoter. For both catalyst series, rate of butane conversiondecreases with increasing promoter content and also with increasingionic radius of the promoter element. A decrease of surface areaaccounts for this trend. Catalyst surface basicity and selectivityto dehydrogenation products appear to be closely related. In fact,increased selectivity to C4products has been observed upon promoter incorporation, specially for K and Cs. An overdoping effectwas detected for barium-doped catalysts which exhibits a maximum ofbasicity at 9% promoter loading. Higher levels of promoterseem to decrease the surface basicity and the C4'sselectivity. At such Ba contents the formation of three-dimensionalbarium compound crystallites occurs, decreasing the amount ofadsorbed CO2. Alkaline earth-promoted NiMoO4appeared to be very promising catalysts for butadiene formation,being almost twice more selective to butadiene than the undopedcounterpart, for the same conversion.

The xerogel–glass transition of SiO2containing various amounts of CuO was studied by thermogravimetry, differential thermal analysis, Fourier transform infrared spectroscopy, diffuse UV–VIS–NIR reflectance, electron spin resonance spectroscopy, and X-ray photoelectron spectroscopy. The samples were prepared by the sol–gel process from tetraethoxysilane, and copper(II) acetate monohydrate. Significant structural changes were observed during the various stages of glass formation. It was concluded that copper(II) ions exist in two different enviroments, i.e., isolated and clustered. The results indicate that the former remains entrapped (doped) in the xerogel and glass, and that the clustered copper(II) ions were transformed into highly dispersed CuO by calcination. The relative amounts of these species depend on the copper(II) concentration and, except for the sample with 0.5% CuO, the amount of isolated copper(II) ions is lower than that of the clustered ions.

Three different Cu–Zn–Al catalysts (Cu : Zn : Al atomic ratio 60 : 38 : 2) were prepared by coprecipitation of the three metals (from their nitrate salts) either by separately precipitating each metal (Cu+Zn+Al), two metals (Cu and Zn coprecipitated+Al) before mixing together, or by precipitating the three of them together. XPS analysis detected a remarkably high Al enrichment at the surface of all calcined or reduced samples, along with a binding energy of the Cu 2p3/2core level peak assigned to Cu2+in CuAl2O4-like environment in the calcined state. ESR spectra of calcined samples showed signals attributed to surface Cu2+species with Al entities in their surroundings for samples in which Cu and Zn are coprecipitated and aged together. The higher Cu dispersion observed after reduction for these catalysts is attributed to the development of such interactions at the surface of Al-containing calcined samples.

The influence of the support on the surface properties and catalytic activity of finely divided ruthenium catalysts is reported. The catalysts were prepared using an organometallic precursor, Ru(acac)2, and three different supports, Al2O3, TiO2and SiO2. In order to study the influence of the particle size on the catalytic performance, the effect of the calcination temperature was also evaluated. XPS suggests that the state of ruthenium is essentially Ru0, and chemisorption measurements indicate a decrease in metal dispersion from catalysts supported on Al2O3> TiO2> SiO2. The turnover number in the o-xylene hydrogenation showed significant differences depending on the support and on the particle size. Additionally, an increase in the selectivity to cis-dimethylcyclohexane with particle size was observed.

Two different series of platinum supported catalysts have been prepared, characterized and tested in the toluene hydrogenation. In each series, the metal loading was constant (1 wt% Pt) and two metal precursors, H2PtCl6 and Pt(acac)2 have been used. Various supports (Al2O3, SiO2, ZrO2 and TiO2) differing in surface areas and chemical properties were used. The catalysts were characterized by means of H2 chemisorption, TPR, surface acidity by TPD of ammonia and XPS. In order to evaluate the catalyst performance the resistance to deactivation by sulphur poisoning was also investigated. Thus, toluene hydrogenation was studied by cofeeding 10 ppm thiophene while n-butane hydrogenolysis was investigated over poisoned catalysts.

This paper reports a study of the effect of the purification and functionalization treatments of a peach pit derived carbon on the properties and performance for nitrobenzene hydrogenation reaction of Pt/C catalysts. Results show that the elimination of inorganic impurities, mainly sulphur, enhances the nitrobenzene hydrogenation rate. Moreover, the functionalization treatments of purified carbon with ozone and hydrogen peroxide have a positive effect both on the Pt dispersion and on the hydrogenation capacity of the catalyst, while the HNO3-treatment has a lower effect. The effect of the different oxidants can be related to the nature of the functional groups developed on the carbon surface. Thus, HNO3-treated carbon displays a high density of both strong and weak acid sites, while H2O2- and O3-treated carbons show an important concentration of weak acid sites but a low concentration of strong acid sites, according to the TPD results. Moreover, the H2PtCl6 isotherms in liquid phase at 298 K show a stronger interaction of the metallic precursor with the carbons of low acidity (like those treated with H2O2 or O3) than with the most acidic carbon (treated with HNO3). Carbons functionalized with weak oxidants, which develop acidic sites with moderate strength and show strong interaction with H2PtCl6 during impregnation, would favour the Pt dispersion on the carbon surface and consequently the catalytic behaviour.

Small amounts of manganese oxide incorporated on MgO enhanced C2+ hydrocarbon selectivity during the oxidative coupling of methane (OCM). Both C2+ selectivity and CH4 conversion were also improved by adding alkaline metal halides. Dosing CH2Cl2 pulses into the CH/4O2 feed stream over alkali promoted MnMgO catalysts led to an improvement in the yield to C2+ hydrocarbons. The extent of this effect and the stability of the alkali phases were found to depend on the type of promoter. Photoelectron spectroscopy (XPS) revealed that all the used catalysts retained important amounts of chlorine in the form of Cl− ions, and simultaneously manganese was oxidized to Mn4+. In the light of quantitative XPS data, a redox mechanism consisting of the decomposition of halocompound at catalyst surface through formation of an oxyhalide of the type MnO2−x2Cl/x has been proposed. The different patterns for CH4 conversion and C2+ selectivity indicate a key role of the alkali promoter and its distribution on the surface, on the formation of this Cl-containing species.

A series of alumina-supported MVSb (MNi, Co, Bi, Sn) oxides and their binary constituents were tested as catalysts for the oxidative dehydrogenation of ethane in the 400–500°C temperature range. The highest selectivity to ethylene of up to 80% was characteristic of the NiVSb oxide composition. The presence of free vanadium and nickel oxides in the catalyst negatively influenced the selectivity to dehydrogenation product. The reaction network was found to be constituted of parallel reactions for ethylene, carbon monoxide and carbon dioxide formation; at longer contact times consecutive reactions of ethylene and carbon monoxide overoxidation also occurred.

Hydrogen production by partial oxidation of methanol (CH3OH+1/2O2↔2H2+CO2) was studied over zinc oxide-supported Pd catalysts. Catalyst performance was investigated as a function of the Pd loading and pretreatment in hydrogen environment under feed ratios O2/CH3OH (molar) of 0.3 and 0.5 at 503–543 K. High yields of hydrogen were obtained under integral operation regime. The hydrogen selectivity showed a strong dependence on the methanol conversion, which suggests that oxidation and reforming steps take place consecutively. Catalyst characterization by temperature-programmed reduction, X-ray diffraction, and X-ray photoelectron spectroscopy revealed that PdZn alloys can be formed upon reduction at moderate temperatures. A shift of +0.7 eV has been observed in the binding energy of Pd 3d5/2core level spectrum of catalyst 1% Pd/ZnO prereduced at temperatures as low as 373 K and catalyst 5% Pd/ZnO reduced at 573 K and above. The relative ease with which PdZn alloy is formed in 1% Pd/ZnO catalyst is explained in terms of a stronger metal–metal oxide interactions of the smaller Pd particles. PdZn alloys were also detected by X-ray diffraction on the 2% Pd/ZnO catalyst after on-stream operation. Pretreatments of the catalysts in hydrogen at high temperatures led to sintering of metallic particles with the subsequent drop in methanol conversion.

Mixed Fe/Cr hydroxyacetate oligomers ranging in composition from 90/10 to 10/90 have been prepared by mixing Fe3+and Cr3+nitrate solutions and then addingn-propylammonium acetate up to an acetate/Cr3+ratio of 2.8 and pH 4. The oligomers were intercalated into colloidal α-zirconium phosphate or precipitated with excessn-propylamine. The precipitates are all amorphous and show differential thermal behavior in comparison with the intercalates. Upon calcination in air, the precipitates become crystalline, showing the characteristic XRD peaks of mixed Fe/Cr oxides. These mixed oxides present unusually high BET surface areas (up to 276 m2· g−1), especially those with low Fe/Cr ratios. The intercalates are poorly crystalline or amorphous at room temperature and upon calcination at 400°C in air or under N2, which indicates that there is no oxide segregation from the interlayers. From these results it is suggested that mixed Fe/Cr oxides are cross-linked to the phosphate layer, forming a pillared structure. Significantly, the mixed oxides segregated from the phosphate surface at 1000°C present XRD patterns identical to the mixed oxides obtained from the precipitates with the same Fe/Cr ratios. The pillared materials show high BET surface areas, from 200 to 306 m2/g, and narrow pore size distributions with pore radius ranging from 8.5 to 13.8 Å. XPS analysis reveals a higher affinity of the phosphate surface for Cr3+at high Fe/Cr ratios from 70/30 to 90/10. In these samples the partial oxidation of Cr(III) to Cr(VI) was observed in the XPS spectra, in spite of the materials being calcined under N2.

The selective oxidation of methane to formaldehyde at atmospheric pressure was studied over a series of silica supported molybdena catalysts. Two silica supports of substantially different surface areas (80 and 200 m2/g) and molybdenum contents ranging from 0 to 3.5 Mo/nm2 were used for this purpose. The catalytic performance of the two catalyst series was compared for a wide range of experimental conditions, including the influence of the oxidant in the feed (O₂ or N₂O). The main products of the oxidation reaction were formaldehyde, CO and CO₂. Small amounts of C₂H₆ and CH₃OH were also identified. For typical reaction conditions, i.e., reaction temperatures 863 K, W/F 4.5 g h/mol and CH₄/O₂ = 10 M both methane conversion and formaldehyde selectivity were higher with O₂ as oxidant. Furthermore, formaldehyde selectivity reached a maximum for molybdenum concentrations ca. 1 Mo/nm2 with both oxidants. The molybdenum exposure on the surface of fresh and spent catalysts was monitored by X-ray photoelectron spectroscopy.Copyright 1993, 1999 Academic Press

Titania-supported 12-phosphomolybdic acid (HPMo/T) and their Ni- and Co-salts (NiPMo/T and CoPMo/T) have been prepared by impregnation and their acid properties tested by means of pyridine adsorption and by the heterogeneous partial oxidation of methanol with oxygen at atmospheric pressure. For sample HPMo/T, calcination at temperatures 523-723 K altered both methanol conversion and product distributions. At 523 K activity was high and reaction appeared to be dominated by acidity as CH3OCH3was the major reaction product, however selectivity to CH3OCH3decreased with the simultaneous increase of HCHO selectivity at 623 K indicating participation of redox properties of the catalyst. These changes have been interpreted as being due to the destruction of the supported heteropoly compound and the parallel change of the acid properties of the resulting catalyst.

The structural and morphological changes of the metal phases which occur during regeneration in hydrogen and air/hydrogen cycles have been studied for Ni-USY catalysts which had been deactivated during the hydrogenation of benzene. Independent of nickel loading in the zeolite, regeneration in hydrogen alone was not sufficient to restore the initial activity. Regeneration in air/hydrogen cycles was more successful for catalysts with higher Ni loadings, but structural transformations of the metal particles occurred during the oxidative regeneration process. Temperature programmed reduction (TPR), photoelectron spectroscopy (XPS) and FTIR of adsorbed CO have been used to identify these modifications.

This review summarizes different catalytic options for the production of syngas and hydrogen starting from simple hydrogen-containing molecules. Particular attention is given to new direct catalytic alternatives in natural gas conversion. Improvements in syngas technology are discussed, including partial oxidation, autothermal reforming, combined reforming and carbon dioxide reforming, and the energy efficiencies of direct and indirect methane conversion are compared. Processes, issues and practical difficulties are discussed with academic and applied efforts presented in parallel. It is emphasized that most of the ongoing research related to the direct processes is at the exploratory stage while technology utilizing indirect approach has advanced to semi-works and initial commercialization plants. The new emerging processes based on partial oxidation features are unique for syngas generation. Further enhancement of such processes plus improvements in other second generation technologies and advances in direct processes are anticipated to provide additional, new attractive paths to the chemical conversion of natural gas. Similarly, on board generation of hydrogen-rich gaseous fuels either for spark ignition engines or for coupled-fuel-cells electric engines is discussed within the scope of both partial oxidation and catalytic decomposition of methanol. A concept based in the thermochemical water splitting, which provides a renewable portable fuel from water in the form of H2, is also presented. Above all, as the chemistry involved in most of these catalytic alternatives takes place under extreme conditions, highly stable catalysts and engineering concepts are being developed.