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International audience; Ring-opening reactions of decalin and MCH were studied over monofunctional acid (WO3/Al2O3) and metal (Ir/Al2O3) catalysts containing, respectively, up to 5.3 at. W/nm(2) and 1.8 wt% Ir. The catalysts were characterized by X-ray diffraction, Raman spectroscopy, low-temperature CO adsorption followed by infrared spectroscopy, and H-2 chemisorption. A reaction network was proposed for both molecules and used to determine the kinetic parameters. Kinetic modeling allowed relating characterization results and catalytic performance. For WO3/Al2O3 catalysts, ring contraction precedes ring opening of both molecules. The evolution of ring contraction activity was consistent with the development of relatively strong Bronsted acid sites. Ring opening occurs according to a classic acid mechanism. For Ir/Al2O3 catalysts, only direct ring opening was observed. Ring opening proceeds mostly via dicarbene mechanism. Analysis of products indicated that monofunctional metal catalysts are better suited than acid solids for upgrading LCO. (C) 2011 Elsevier Inc. All rights reserved.

International audience; The present paper investigates the development of Ir-based catalysts supported on tungstated aluminas for the ring opening reaction of naphthenic molecules using methylcyclohexane (MCH) as a model compound. A series of tungstated aluminas WOx/Al2O3 containing up to 6.0 at.W/nm(2) was prepared. Ir-based catalysts containing 1.2 wt% were obtained by impregnation of these solids. The performance of Ir/WO3/Al2O3 catalysts for methyl cyclohexane ring opening was found to be similar to that observed over the corresponding zirconia-based system indicating that the metal-acid balance (and not the nature of the support) is the key parameter for optimum performance. (C) 2010 Elsevier B.V. All rights reserved.

The relationship between acidity, surface structure and catalytic activity for a series of WOx/Al2O3 catalysts was investigated. The catalysts, containing up to 4.1 atom W/nm2, were prepared by incipient wetness impregnation and studied by N2 physisorption, XRD, in situ Raman and infrared spectroscopy. The W phase was essentially present as surface species. Polymeric W surface species appeared above 1.4 atom W/nm2; their abundance increased with W surface density. Lutidine and CO adsorption measurements, followed by infrared spectroscopy, evidenced the presence of relatively strong Bronsted acid sites. The development of these acid sites was similar to that of the abundance of polymeric W surface species. Similar evolution with W surface density was also observed for the activity for 2-propanol dehydration, an acid-catalyzed reaction, indicating a direct relationship between the abundance of polymeric W surface species, that of relatively strong Bronsted acid sites and the catalytic activity of WOx/Al2O3 catalysts.

The present paper is the second part of a systematic study of the influence of W and Ir loading on the activity of Ir/WO 3 /ZrO 2 –SiO 2 catalysts for the ring-opening reaction of naphthenic molecules using methylcyclohexane (MCH) as a model compound. A series of Si-stabilized tungstated zirconias, WO x /ZrO 2 –SiO 2 , containing up to 3.5 atom W/nm 2 , was prepared. Ir-based catalysts containing up to 1.2 wt% were obtained by impregnation of these solids. Characterization of the metal dispersion and catalyst acidity was described in a previous article. The objective of the present study was to determine the best metal/acid balance for optimal performance of Ir/WO x /ZrO 2 –SiO 2 catalysts in the ring-opening reaction of MCH. Monofunctional (acid WO x /ZrO 2 –SiO 2 or metal Ir/ZrO 2 –SiO 2 ) and bifunctional (Ir/WO 3 /ZrO 2 –SiO 2 ) catalysts were examined. Based on the analysis of the yields and products distributions, a reaction network was proposed, and kinetic data (e.g., activation energies, initial rates) were calculated. Correlations between characterization results obtained earlier (e.g., acidity, dispersion) and catalytic performance are also reported. The monofunctional acid catalysts WO x /ZrO 2 –SiO 2 showed a low selectivity for ring opening. The ring-contraction activity developed for W surface density above a threshold value of 1 atom W/nm 2 . This was attributed to the appearance and the development of a relatively strong Bronsted acidity monitored by infrared measurements. MCH ring contraction and C5 naphthene ring opening occur according to a classic acid mechanism. For low conversions, the monofunctional metal catalysts Ir/ZrO 2 –SiO 2 exhibited significant selectivity for ring opening that decreased with increasing conversion. Because of the lack of ring-contraction products, the observed activity was attributed to the direct ring opening of the MCH. Ring opening and cracking occur according to a dicarbene mechanism. The study of MCH conversion on Ir/WO x /ZrO 2 –SiO 2 catalysts indicated that MCH ring contraction to alkylcyclopentanes occurs before ring opening. The best yields for ring opening were obtained with the 1.2% Ir/WO x /ZrO 2 (1.5 atom of W/nm 2 ). Further increases in W surface density led to a decrease in the indirect ring-opening yield, attributed to a decrease in Ir dispersion. For bifunctional metal/acid catalysts, analysis of the mechanism is less straightforward. The activation energy for C6 ring contraction and indirect C6 ring opening is a function of the metal/acid ratio. For high ratios, indirect ring opening occurs essentially over metallic sites. A decrease in the metal/acid ratio enhances the contribution of acid mechanism.

The oxidation of dibenzothiophene by an oxygen/aldehyde system, using heterogeneous (unsupported and alumina-supported oxides) and homogeneous (acetate salts) catalytic systems based on Mn, Co and Ni, was examined. In all instances, Co- and Mn-based systems were the most active catalysts. In contrast, a markedly lower activity was measured for Ni(II) acetate whereas NiO and the alumina-supported NiO were inactive. The study indicates that involvement of homogeneous catalysis cannot be excluded when using the supported catalysts.

This paper is the first part of a systematic study of the influence of W and Ir loading on the activity of Ir/WO 3 /ZrO 2 –SiO 2 catalysts for the ring-opening reaction of naphthenic molecules using methylcyclohexane (MCH) as a model compound. A series of Si-stabilized tungstated zirconias, WO x /ZrO 2 –SiO 2 , containing up to 3.5 at W/nm 2 was prepared. Ir-based catalysts containing up to 1.2 wt% were obtained by impregnation of these solids. The acidity of Si-stabilized tungstated zirconias, ZSiW x , was monitored by low-temperature CO adsorption followed by infrared (IR) spectroscopy. The results indicated the development of strong and relatively strong Bronsted acidity with increasing W surface density above a threshold of 1 atom W/nm 2 . The study of the Ir-doped solids showed that the texture, structure, and acidity of the ZSiW x solids were not affected by Ir deposition. The characterization of the metallic function was performed by hydrogenation of toluene and by CO adsorption at ambient temperature, followed by IR spectroscopy. Both methods indicated a low and constant Ir dispersion for the y Ir/ZSiW0 ( y = 0.3 , 0.6, 1.2) series. For a given Ir loading (1.2Ir/ZSiW x ), a progressive decrease in the dispersion of Ir metal with increasing W surface density was observed.

International audience; The development of the acidity and the relationship between acidity, catalytic activity, and the surface structure for niobium oxide supported on zirconia were investigated for a series of solids. The catalysts were active for 2-propanol dehydration only above a threshold in Nb loading. The acidity was studied by infrared spectroscopy of adsorbed 2,6-dimethylpyridine as a probe molecule, and the onset of activity was correlated with that of the formation of relatively strong Brønsted acid sites. The variation in the abundance of these sites also correlated with the catalytic activity. Raman, IR, and UV spectroscopy results indicated that the active sites were related to polymeric Nb surface species. These results were compared to those previously reported for the WOx/ZrO2 catalysts

International audience; The relationship between the acidity, catalytic activity, and surface structure for tungsten oxide supported on zirconia was investigated for a series of solids prepared by equilibrium adsorption on monoclinic zirconia. The catalysts were active for propanol dehydration only above a threshold in W loading. The acidity was studied by infrared spectroscopy of adsorbed probe molecules (2,6-dimethylpyridine and CO), and the onset of activity was correlated with that of the formation of relatively strong Bronsted acid sites. The variation in the abundance of these sites correlated with the catalytic activity. Lewis sites were present but could not be directly associated with the activity. Raman, IR, and UV spectroscopy results indicated that the active sites were related to polymeric W surface species.

Two series of molybdena–alumina catalysts were prepared. The first series was prepared by the incipient wetness procedure (1.7–8.8 wt.% Mo) (series IW). A second series was obtained using the equilibrium adsorption (1.3–8.2 wt.% Mo) (series EA) method. X-ray photoelectron spectroscopy (XPS) was used to study the distribution of Mo oxidation states. The average Mo oxidation state estimated from XPS agreed with that obtained by measuring the O 2 consumption on reoxidation, for a given catalyst. The chemisorption of oxygen at −78°C and the isomerization of 1-butene were performed as probes for assaying the number of coordinatively unsaturated sites created by the reduction process. These studies were conducted in order to study possible changes in Mo speciation or distribution, relevant to catalytic reactions, which may occur as a result of the specific features of the preparation method (EA vs. IW). The results showed that for both series of solids, the reducibility of the molybdenum species increases as the metal loading increases, in agreement with the literature. For catalysts with similar loading, the IW series showed a higher degree of reduction with a concomitant increase in the relative abundance of Mo(IV) species attributed to paired double-bonded Mo(IV) entities (as in MoO 2 ). The reducibility data were consistent with the catalytic results and the oxygen chemisorption results. IW preparations (with similar Mo loading) were more active towards the isomerization of 1-butene and chemisorbed larger amounts of oxygen than their EA counterparts. The differences in reducibility can be ascribed to a nonuniform repartition of the molybdenum species between the external and internal surfaces of the alumina, for the IW preparations. Another possible explanation may stem from a decoration effect of the molybdenum species by Al(III) ions. The latter may arise from dissolution of the alumina, which is favored on the EA series due to the long contact time between the solution of the Mo promoter and the support.

Surface and chemical characterization were performed on a manganese-substituted silicoaluminophosphate molecular sieve (MnAPSO-11), and on a manganese-supported silicoaluminophosphate molecular sieve (Mn/SAPO-11). For comparison purposes, the characterization process was also carried out over the parent SAPO-11 molecular sieve. Different characterization techniques were used: XPS, redox cycles, 31 P MAS NMR, and acidity measurements. The transformations of n -butane were carried out over the corresponding platinum promoted solids (Pt/MnAPSO-11, Pt/Mn/SAPO-11 and Pt/SAPO-11). Platinum dispersion was measured by H 2 chemisorption. XPS results indicated that manganese was better dispersed on the MnAPSO-11 solid than on the supported Mn/SAPO-11 catalyst. Redox cycles showed a strong difference between the H 2 (or O 2 ) consumed by each solid. The Mn/SAPO-11 consumed nearly three times as much H 2 (or O 2 ) per Mn atom as the MnAPSO-11 solid. 31 P MAS NMR results showed an increase in the intensity of the side bands, probably due to an anisotropic paramagnetic shift caused by a stronger dipolar interaction between the 31 P and the paramagnetic Mn(II) ions on the MnAPSO-11 sample, when compared with the Mn/SAPO-11 solid. These results suggest a better dispersion of the manganese species on the MnAPSO-11 solid, which would facilitate the above mentioned 31 P –Mn(II) interaction, in agreement with the XPS results. Acidity was measured by pyridine chemisorption at different temperatures. A larger number of (moderate+strong) Bronsted acid sites was found for the MnAPSO-11 solid compared with the SAPO-11 and Mn/SAPO-11 samples. The addition of platinum decreased the acidity. The Pt dispersions were 83%, 68% and 54% for the Pt/SAPO-11, Pt/Mn/SAPO-11 and Pt/MnAPSO-11 solids, respectively. The catalytic results indicate higher yields for the production of isobutane and isobutene over the Pt/MnAPSO-11. A severe decrease in the yield of formation of hydrocarbons with less than four carbon atoms (undesirable side reaction) was also observed for the Pt/MnAPSO-11 system compared with the Pt/SAPO-11 and Pt/Mn/SAPO-11 systems. An ensemble effect is suggested as responsible for the differences observed in the yield of formation of hydrocarbons with less than four carbon atoms. The higher yield and selectivity observed for the formation of isobutane (iso-C4) and isobutene (iso-C4) hydrocarbons over the Pt/MnAPSO-11 solid, was accounted for in terms of the largest number of (moderate+strong) Bronsted acid sites found on this solid. The catalytic and characterization results suggest the incorporation of manganese into the molecular sieve structure for the substituted MnAPSO-11 solid.

The isomerization and metathesis of cyclopropane were carried out over a series of reduced molybdena–alumina catalysts (1.7–13 wt% Mo). The distribution of Mo oxidation states was determined by XPS. The average Mo oxidation state estimated from XPS agreed with that obtained by measuring the O 2 consumption on reoxidation for a given catalyst. A good correlation was found between the catalytic activity for the isomerization reaction and the relative intensity of the XPS peak attributed to paired double-bonded Mo (IV) species (as in MoO 2 ). The results indicate a direct relationship between these Mo (IV) species and the coordinative unsaturated sites involved in the formation of metallocyclobutane. The latter is reportedly the intermediate required for the isomerization reaction to take place.

International audience; Ring-opening reactions of decalin and methylcyclohexane (MCH) over bifunctional catalysts (1.2Ir/WO3/Al2O3) were investigated. A series of catalysts containing up to 5.3 at. W/nm(2) and 1.2 wt.% Ir was prepared. The acidity of the solids was monitored by low-temperature CO adsorption followed by infrared spectroscopy. Characterization of the Ir metal phase was performed by H-2 chemisorption and X-ray diffraction. The activity and product selectivity patterns obtained for the decalin ring-opening reaction were compared with those observed for MCH. For both naphthenes, ring contraction precedes ring opening, suggesting a similar ring-opening mechanism. Kinetic modeling based on the proposed reaction network allowed the determination of the activation energies and initial rates. Based on the yields and products distribution obtained for the decalin reaction, the potential for improvement of the cetane number is discussed. (c) 2012 Elsevier Inc. All rights reserved.

International audience; The development of Brønsted acidity and the relationship between acidity, catalytic performance and structure of niobium oxide were investigated for a series of samples prepared by calcination of niobic acid at temperatures between 423 and 823 K. All solids were active for propan-2-ol dehydration. The Brønsted acidity was monitored by adsorption of lutidine and CO followed by FT-IR. Low temperature CO adsorption measurements showed that all solids exhibit essentially the same medium Brønsted acid strength. A direct relationship between the abundance of Brønsted acid sites monitored by lutidine adsorption followed by desorption at 523 K and propan-2-ol dehydration activity was observed.

Series of rhenium catalysts were prepared by equilibrium adsorption on unmodified alumina and alumina containing F and Mg additives. The uptake of perrhenate on alumina was dependent on the pH and the concentration of nitrate in the impregnation solution. Uptake of perrhenate at pH 6.5 on F modified alumina decreased with increasing F loading; for Mg-modified aluminas, perrhenate uptake at pH 9.3 increased with increasing Mg content. A correlation was observed between perrhenate uptake and the isoelectric point (IEP) of the modified supports. These data are interpreted on the basis of electrostatic attractions which occur between perrhenate ions in solution and alumina surfaces. Competition between nitrate and perrhenate for adsorption sites satisfactorily accounts for the effect of nitrate on perrhenate adsorption.

The oxidative decomposition of dimethyl methyiphosphonate (DMMP) has been studied over Cu-substituted hydroxyapatite catalysts in a flow reactor in the temperature range 373-773 K. By substitution of a portion of the Ca 2+ by Cu 2+ in the hydroxyapatite lattice, the catalytic activity was markedly increased. The composition Cu 2 Ca 8 (PO 4 ) 6 (OH) 2 , herein called Cu2-HA, had the highest activity among these catalysts. All the catalysts showed 100% conversion at the beginning of the reaction, but became deactivated after a period of time. Only CO 2 and H 2 O were produced during the "protection period" of the 100% conversion. With deactivation, CO 2 formation decreased and methanol and dimethyl ether appeared as products. A phosphorus-containing product was not detected in the gas phase. Some condensate of phosphoric acid appeared in the outlet of the reactor after deactivation. Several analytical methods revealed that carbonaceous compounds did not remain on the surface after 773-K oxidative regeneration, but the phosphorus compounds that had accumulated during the reaction were not eliminated. The catalytic activity of Cu-substituted hydroxyapatite was compared to that of conventional supported Pt catalysts. This study suggested that easily made and cheap Cu-substituted hydroxyapatite catalysts could be used as successful alternatives to conventional adsorbents and catalytic systems based on supported noble metals devised for protection against nerve gases in warfare.

Quantitative FTIR studies of alumina hydroxyl groups have been used to measure the surface coverage of V/Al2O3 catalysts. The IR spectra show a preferential consumption of the most basic hydroxyls (3769 cm−1) with the initial V uptake. The integrated area of the hydroxyl region was used to calculate the surface coverage of the catalysts. The surface coverage increased to 0.63 as the V loading increased to 6.7 wt%. The surface coverage results obtained from IR were compared to those measured by ion scattering spectroscopy (ISS) and CO2 chemisorption. The surface coverages measured by ISS were comparable to those obtained by IR. In contrast, CO2 chemisorption appears to overestimate the surface coverage of the catalysts. The overestimation of the surface coverage by the CO2 chemisorption method is attributed to the preferential adsorption of the V phase onto the most basic hydroxyls, the same sites which interact with CO2.

In order to investigate the combined effect of a promoter and a detached support species on a supported metal catalyst, alkali promotion of Rh on the decorative support La[sub 2]O[sub 3] was investigated by X-ray photoelectron spectroscopy, X-ray diffraction, H[sub 2] and CO chemisorption, ethane hydrogenolysis, and cyclopropane hydrogenation. Previously, decorating lanthana moieties have been found to partially suppress ethane hydrogenolysis and cyclopropane hydrogenation on Rh/La[sub 2]O[sub 3]. The major effects of alkali promotion of Rh/La[sub 2]O[sub 3] on these reactions were a decline in apparent activation energy for ethane hydrogenolysis and a moderate additional suppression of cyclopropane hydrogenation activity. Alkali promotion of the more active Rh/SiO[sub 2] reference catalyst, which is not decorated by support entities, resulted in a much more significant suppression of these reactions. For Rh/La[sub 2]O[sub 3], alkali promotion appeared to block sites and/or pathways for hydrogen spillover. In addition, Rb inhibited the Rh-catalyzed decomposition of carbonate species on the lanthana support. No additional effect of alkali promotion was indicated on the [minus]1-eV shift of Rh 3d[sub 5/2] binding energy observed on Rh/La[sub 2]O[sub 3] relative to Rh/SiO[sub 2]. This is consistent with the lack of a Rh 3d[sub 5/2] binding energy shift observed for Rb-promotionmore » of Rh/SiO[sub 2]. The Rb 3p[sub 3/2] binding energy was shifted to the same extent as Rh in Rb/Rh/La[sub 2]O[sub 3], suggesting that the chemical environment for Rb differs between SiO[sub 2]-supported and La[sub 2]O[sub 3]-supported systems or, more likely, that the binding energy shifts observed for this system are just due to differences in the binding energies for the reference carbon on SiO[sub 2] vs on La[sub 2]O[sub 3]. 44 refs., 4 figs., 7 tabs.« less

The distribution of supported species in a series of W/TiO2 catalysts (1.8 – 28 wt% WO3) has been determined by Raman and X-ray photoelectron spectroscopy (XPS, ESCA). The results show that three tungsten species are present on oxidic W/TiO2 catalysts. A tungsten interaction species is formed almost exclusively for catalysts with W loadings lower or equal to 6.7 wt% WO3. WO3 is observed above 6.7 wt% WO3. For W loadings higher than 10 wt%, a disordered W species WDS is also present. The amounts of WDS and WO3 increase with increasing W content above 10 wt% WO3.

Samples containing 8% Mo and l5% W supported on magnesium- and fluoride-modified aluminas were prepared by incipient wetness impregnation and examined in the dried and calcined states using X-ray photoelectron spectroscopy (XPS, ESCA). The presence of fluoride as an additive decreased the dispersion of the Mo and W phases for dried samples. The opposite effect was observed for dried Mg-modified samples. The effects of F and Mg addition largely disappeared when the samples were calcined. The results are interpreted in terms of the interactions between Mo and W species and the basic hydroxyl groups of the alumina.

35-2 FIELD Section Title:Chemistry of Synthetic High Polymers 45, 67 Laboratoire Catalyse et Spectrochimie, ENSICAEN, CNRS, Universite de Caen, Caen, Fr. FIELD URL: written in English; A series of WOx/TiO2 catalysts contg. between 0.6 and 3.4 W at./nm2 was prepd. by pore vol. impregnation using ammonium metatungstate soln. on crystd. TiO2. Characterization of the solids by X-ray diffraction and Raman and IR spectroscopy indicated that the W phase was present essentially as a surface W species. The results also suggested an increased degree of condensation of the W phase with increasing W surface d. The most condensed W species was characterized by a ?(W=O) vibration at 1017 cm-1. The IR or Raman bands attributed to sp. surface species of the solids were correlated with the abundance of relatively strong Bronsted acid sites and isopropanol dehydration activity. A direct correlation was obsd. between propene formation rate and the area of the band at 1017 cm-1, attributed to the most condensed W species. Furthermore, the abundance of this species appeared to be directly related to that of Bronsted acid sites capable of retaining lutidine following desorption at 573 K. The behavior of the WOx/TiO2 system was compared with that reported for the corresponding WOx/ZrO2 solids. Higher intrinsic activity (activity per W atom) was obsd. for WOx/TiO2 system. [on SciFinder(R)]

International audience; Tungstated zirconias consist typically of WOx species deposited on zirconia support. Additives like Si can be added to stabilize the catalyst surface area. The aim of this work is to identify the location of the Si (surface or bulk) as a function of W addition. Zirconia doped with 1.2 wt% Si remained purely tetragonal and exhibited a high specific surface area (150 m(2)/g) following calcination at 1023 K. Infrared quantification of the silanol band at 3745 cm(-1) indicated that Si is essentially present at the surface. The Si-29 NMR chemical shift of the zirconia is characteristic of isolated silicium. This result is corroborated with infrared by the absence of the structural band ascribed to Si-C-Si vibrations. NMR and infrared results indicated that W addition brings about a progressive evolution of the Si environment towards that of silica-like species. These results can be tentatively explained by a ligand enhanced dissolution of the support on W deposition. Si leached from the surface will then aggregate, forming small SiO2-like domains on the zirconia. (c) 2009 Elsevier B.V. All rights reserved.

A series of titania supported cobalt catalysts (0.5-6%) were prepared by incipient wetness impregnation, and were characterized by ESCA, XRD, and hydrogen chemisorption. After calcination at 400 C, a surface CoTiO[sub 3]-like phase was the main species present in the 0.5 and 1% cobalt catalysts. For higher cobalt loadings, discrete Co[sub 3]O[sub 4] particles were formed in addition to surface CoTiO[sub 3]. ESCA indicates that after reduction the cobalt metal particle size (6-13 nm) increases with increasing cobalt loadings, but does not vary with reduction temperature (400-500 C). Hydrogen chemisorption was found to be activated and suppressed. The extent of hydrogen chemisorption suppression increases with increasing reduction temperature and decreasing cobalt particle size. The turnover frequency (based on cobalt dispersion derived from ESCA) for benzene and CO hydrogenation decreases with increasing reduction temperature and decreasing cobalt particle size. The decline in activity correlates with the extent of suppression of H[sub 2] chemisorption. The results were interpreted in terms of a decrease in the fraction of exposed surface cobalt due to site blocking by reduced TiO[sub 3] moieties.

X-ray photoelectron spectroscopy (ESCA or XPS), ion scattering spectroscopy (ISS), X-ray diffraction (XRD), H 2 chemisorption, and gravimetric analysis have been used to characterize a series of Ni/Th/SiO 2 catalysts. The information obtained from surface and bulk techniques has been compared with CO hydrogenation activity and selectivity of the supported nickel catalysts. Comparison of measured ESCA Th/Si intensity ratios with values predicted by a theoretical model shows that the particle size of the Th phase increases from 1.4 to 4.2 nm as the Th/Si atomic ratio increases from 0.0036 to 0.023. ESCA and XRD results also indicate that nickel is present as poorly dispersed NiO on the calcined unpromoted catalyst. Reducibility of the Ni phase in all catalysts exceeds 97%. ESCA data show that Th addition has little effect on Ni metal dispersion up to a Th/Si atomic ratio of 0.015. For higher Th loadings, the Ni dispersion increases from approximately 6 to 9% with increasing Th loading. H 2 chemisorption data suggest that Ni dispersion increases from 4.8 to 9.8% with increasing Th content. The turnover frequency (TOF) for CO hydrogenation calculated using ESCA estimates of Ni dispersion increases by a factor of 9 as the Th/Si atomic ratio increases from 0 to 0.015. For higher Th loadings the TOF decreases, but it remains seven times the value observed for the unpromoted catalyst. These results have been explained in terms of the Th surface coverage indicated by ISS. Th addition also increases olefin selectivity and the yield for higher hydrocarbons.

The adsorption of perrhenate, permanganate, molybdate, tungstate, and chromate on alumina was studied at pH 4.5. The effect of nitrate in the impregnation solution on the adsorption of these ions was examined as well as the effect of washing the freshly prepared catalysts. The uptake of perrhenate and permanganate decreased with increasing nitrate in the impregnation solution; the uptake of molybdate, tungstate, and chromate was unaffected. Washing the catalysts almost completely removed perrhenate, permanganate, and chromate, but allowed a significant amount of molybdate and tungstate to remain on the alumina. It is concluded that tungstate and molybdate adsorb on two types of surface sites, producing loosely and tightly bound surface species. From the amounts of molybdate and tungstate adsorbed, and from EXAFS results from washed and unwashed Mo/Al2O3 samples, both loosely and tightly bound molybdate and tungstate are polymeric in nature. The results are discussed in terms of electrostatic and surface reaction models.

International audience; The surface structure and activity of catalytic systems based on niobium oxide supported on titania were investigated. A series of catalysts with Nb content up to 3.6 atoms/nm2 was prepared by incipient wetness impregnation of the titania support with niobium oxalate solutions. Characterization of the system by X-ray diffraction (XRD) and Raman spectroscopy studies did not show evidence of niobium oxide formation. Surface analysis was performed by X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), Raman and infrared spectroscopy. These results were correlated with Brønsted acidity and catalytic activity determined in our previous study. A direct correlation between the catalytic activity and the abundance of Brønsted acid sites had been observed. Analysis of the surface structure by infrared spectroscopy showed a similar evolution for the intensity of the band attributed to polymerized NbOx.

International audience; Two series of WOx/TiO2 catalysts, containing W surface densities up to 4.4 W atoms/nm2, were prepared by pore volume impregnation of two different supports, titanium oxyhydroxide (amorphous) or titanium oxide (crystallized, 100% anatase). The influence of W surface density and the nature of the support on the surface structure, development of the acidity, and catalytic performances were examined. The texture and structure of the catalysts were investigated by Brunauer−Emmett−Teller measurements, X-ray diffraction (XRD), and Raman and infrared spectroscopy. The catalytic activity was tested for 2-propanol dehydration and n-hexane isomerization. For catalysts obtained by impregnation of titanium oxide, XRD and Raman results showed that W was present as a surface phase. Infrared spectra indicated an increase in the degree of polymerization of W species with increasing W surface density. CO and lutidine adsorption, followed by infrared spectroscopy, showed an increase in the strength and abundance of Brønsted acid sites (measured after lutidine desorption at 573 K) with the W surface density above a threshold of 1.3 W atoms/nm2. The development of Brønsted acidity correlated with the evolution of the infrared bands attributed to polymerized W species. A direct relationship was observed between the concentration of Brønsted acid sites and the catalytic activity for 2-propanol dehydration. Catalytic activity, for n-hexane isomerization, appears to be associated with the presence of highly condensed W species. The catalysts synthesized by impregnation of titanium oxyhydroxide exhibited a comparable behavior. Hence, for a given W surface density, the W surface structure, concentration of Brønsted acid sites, and catalytic performances were similar. Thus, no significant effect of the initial form of the support (titanium oxyhydroxide versus titanium oxide; 100% anatase) was evidenced.

International audience; Tungstated zirconias prepared from W deposition on zirconium oxyhydroxide are reportedly active for alkane isomerization, whereas solids synthesized by impregnation of zirconia are inactive. The origin of the differences between the two preparations is not fully understood. The present paper examines the influence of W surface density and the nature of the support on the surface structure, development of the acidity, and catalytic performance of WOx/ ZrO2 catalysts. Two series of catalysts containing W surface densities up to 5.2 at. W/nm2 were prepared by pore volume impregnation of two different supports: zirconium oxyhydroxide and predominantly tetragonal zirconia (65% tetragonal, 35% monoclinic). The texture and structure of the catalysts were investigated by BET measurements, X-ray diffraction, Raman and infrared spectroscopy. The catalytic activity was tested for 2-propanol dehydration and n-hexane isomerization. For catalysts obtained by impregnation of Zr oxyhydroxide, Raman results showed that W was present as a surface phase. Infrared spectra indicated an increase in the degree of polymerization of W species with increasing W surface density. The development of the acidity was monitored by lutidine adsorption and desorption at 523 K, followed by infrared spectroscopy. The results indicated the presence of a threshold of W surface density at 1.3 at. W/ nm2 for the detection of these acid sites, followed by a progressive increase in their abundance with increasing W surface density. The development of Brønsted acidity correlated with the evolution of the infrared bands attributed to “extensively” polymerized W species. A direct relationship was observed between the abundance of Brønsted acid sites and the catalytic activity for 2-propanol dehydration. For n-hexane isomerization, compared to 2-propanol dehydration, a higher threshold of W surface densities (3.4 at. W/ nm2) for the development of activity was observed. The difference was attributed to stronger Brønsted acid sites required for n-hexane isomerization. The catalysts prepared by impregnation of zirconia exhibited comparable behavior. For a given W surface density, the crystalline composition of the support (tetragonal/monoclinic zirconia), the W surface structure, abundance of Brønsted acid sites, and catalytic performance were similar. Thus, in an apparent variance with some of the results reported in the literature with respect to the influence of preparation methods, no significant effect of the initial form of the support (Zr oxyhydroxide versus predominantly tetragonal zirconia) was evidenced.

The acidity of catalytic systems based on tungsten oxide or niobium oxide supported on titania was compared. Two series with metal contents up to 3.6 atom nm � 2 were prepared by incipient wetness impregnation of the titania support with ammonium metatungstate or niobium oxalate solutions. Characterization of both systems by X-ray diffraction and Raman spectroscopy studies did not show evidence of bulk metal oxide formation. The acidity was monitored by adsorption of 2,6-dimethylpyridine (2,6-lutidine) followed by infrared spectroscopy. The catalytic activity was tested for the reaction of isopropanol dehydration. At a reaction temperature of 403 K, WOx/TiO2 catalysts were inactive for a surface density of W � 1.2 W atom nm � 2 . Above this loading, the activity increased progressively with increasing W content. Similar evolution was observed for the abundance of relatively strong Bronsted acid sites (i.e. able to retain lutidine at 573 K). In contrast, NbOx/TiO2 catalysts were essentially inactive at this reaction temperature and a higher reaction temperature (473 K) was required to reach a comparable catalytic activity. No threshold of Nb loading for the development of catalytic activity was observed. Similar behavior was evidenced for the abundance of medium strength Bronsted acid sites (able to retain lutidine at 523 K). For both systems, a direct correlation between the catalytic activity and the abundance of Bronsted acid sites was observed. # 2005 Elsevier B.V. All rights reserved.

A series of WO(3)/ZrO(2) catalysts with tungsten (W) loadings ranging from 0.5 to 11.4 wt% was prepared by incipient wetness impregnation on a preformed ZrO(2) support. The oxidic catalysts were characterized using XRD, Raman spectroscopy, XPS, ISS, and IR spectroscopy. XRD and Raman results showed that the ZrO(2) support was predominantly present in the monoclinic form. XPS and Raman measurements indicated the formation of increasing amounts of W interaction species for catalysts with W loadings up to 8.8 wt% WO(3). In addition to the W interaction species, bulk WO(3) was also observed for catalysts with W loadingsor = 3.0 wt% WO(3). Comparison of the XPS results with coverage measurements by ISS and CO adsorption suggests that the W surface phase is in the form of two-dimensional polymeric patches for catalysts with W loadings 3.0or = wt% WO(3)or = 4.5. For catalysts with W loadings4.5 wt% WO(3), the results indicated an additional build-up of a bilayer (or multilayer) polymeric W species. Analysis of the hydroxyl region of ZrO(2) by IR spectroscopy showed that initial additions of W occur on the high frequency hydroxyl group. A schematic for the structure of the catalysts has been proposed based on the above observations.

One of the limitations in using CO[sub 2] chemisorption to monitor surface coverage is that the method is based on selective chemisorption of the catalyst support. Thus, only certain support-active phase combinations are suitable for characterization using this technique. The objective of the present study is to examine CO[sub 2] chemisorption on a wider variety of catalysts (Mo, W, V, Cr, and Re supported on Al[sub 2]O[sub 3]). Catalysts are prepared by equilibrium adsorption in order to ensure that the active phase is uniformly distributed over the support.

Catalysts were prepared by subliming Mo(CO)6 onto partially dehydroxylated (PDA) and exhaustively dehydroxylated (DA) alumina made from the same parent preparation (American Cyanamid Aero 100PHF). The chemisorption of NO and CO on these materials was studied using volumetric, Chromatographic, and spectroscopic techniques. ESCA data indicated that metallic Mo crystals formed on Mo(CO)6DA whereas on PDA both Mo4+ and some lower valence state, Mo2+ or Mo0, were present. NO chemisorbed on both preparations at 195 K without releasing either N2O or N2. The chemisorption on the PDA preparations was over tenfold higher than that on the DA-supported catalysts under these conditions, but on raising the temperature to 300 K the difference was reduced to a factor of 2. Moreover, redox chemistry occurred at this higher temperature as evidenced by the release of N2O and N2; this was quantified. The amounts of NO actually chemisorbed under these conditions correlated well with the integrated IR band intensities. These data suggest that lower valence states are oxidized to Mo4+ at 300 K and that the observed IR bands stem from Mo4+(NO)2, irrespective of the initial catalyst. Infrared spectra from residual CO remaining on decomposition of Mo(CO)6 on DA and PDA at increasing temperatures up to 473 K showed bands which could be attributed to residual Mo(CO)6 and/or to subcarbonyl species formed during decomposition. By 573 K, no residual bands could be observed. On adding-back CO at 300 K to the PDA preparation, bands at 1989 and 2170 cmt−1 appeared, suggesting the presence of Mo4+ and residual Mo0. Spectra from similar experiments with the DA preparation demonstrated that chemisorbed Mo(CO)6 was reforming and possibly some subcarbonyl species; there was scant evidence for any higher valence states. The results are discussed in terms of the current literature.

X-ray photoelectron spectroscopy (ESCA or XPS), laser Raman spectroscopy (LRS), and X-ray diffraction (XRD) have been used to characterize the dispersion and chemical state of a series of TiO 2 Al 2 O 3 carriers and a series of 3% Co Al 2 O 3  TiO 2 catalysts. It was found that titania is well dispersed on alumina for Ti loadings less than 14 wt%. The formation of a TiO2: Al2O3 surface phase occurred for carriers in the 1 to 6 wt% Ti range. Formation of anatase occurred, in addition to the TiO2:Al2O3 surface phase, at loadings in excess of 6 wt% Ti, with the amount of anatase increasing with increasing titania loading. ESCA data indicate that the dispersion of cobalt supported on the titania-modified alumina carriers increased with increasing titania loading. XRD and LRS results show that the increase in dispersion can, in part, be ascribed to the formation of a surface cobalt phase at the detriment of the Co3O4 phase.

The influence of boron on the chemical state and dispersion of Co Al 2 O 3 catalysts has been investigated by bulk and surface spectroscopic techniques. Three series of Co Al 2 O 3 catalysts were studied: one containing a constant Co loading of 3 wt% and boron loadings of 0.3 to 3 wt%, and two series containing 0.7 to 10 wt% Co and constant boron loadings of either 0 or 3 wt%. In the absence of boron, Co dispersion decreased with increasing Co loading above 1.5 wt% Co. The presence of boron had little effect on Co dispersion at low Co loadings, while catalysts with Co loadings in excess of 1.5 wt% exhibited an increase in Co dispersion compared to boron-free catalysts. X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy, and gravimetric analysis were used to quantify the Co species present on the catalysts as a function of both Co and boron loading. In the absence of boron, Co exists as tetrahedral and octahedral Co2+ at low Co loadings, while Co3O4 is the primary phase at higher Co loadings. In the presence of boron, Co3O4 formation is suppressed for catalysts with Co loadings less than 8 wt% in favor of octahedral Co2+ and minor amounts of Co borate. The formation of Co3O4 occurs at Co loadings greater than 6 wt% for boron-containing catalysts. A mechanism is described to account for the effect of boron on the dispersion and chemical state of Co.

The isoelectric point (IEP) of γ-alumina was modified by the addition of various amounts of fluoride. The modified aluminas were used as supports for molybdenum catalysts prepared by equilibrium adsorption at pH = 6.5. Also, the adsorption of molybdate on supports containing various amounts of fluoride was studied as a function of pH. The amount of molybdenum adsorbed on the fluoride-modified aluminas was found to decrease as the IEP of the carrier decreased. An electrostatic model is used to interpret the results.

Two molybdenum species (i.e., Mo interaction species and MoO3) are identified and quantitated by Raman spectroscopy for a series of MoTiO22 catalysts. The Mo interaction species is shown to increase with Mo loading up to 6 wt% MoO3 and to level off at higher Mo loadings. MoO3 is detected at 7.5 wt% MoO3TiO2 and shown to increase linearly with further Mo additions. The molybdenum speciation determined from Raman data is shown to correlate with ESCA and X-ray diffraction measurements.

A method is described for the use of X-ray photoelectron spectroscopy (ESCA) for quantitatively determining the distribution of species in supported metal catalysts. The method can determine the distribution of metal species for catalysts which contain both discrete metal oxide and metal surface phases with differing dispersions. The utility of the method is illustrated for a series of CoAl2O3 catalysts containing Co3O4 and both tetrahedrally and octahedrally coordinated Co2+ surface phases. The determination of the distribution of Co species for a series of CoAl2O3 catalysts by the present method is consistent with results from studies using other techniques.

A series of Mo/Al/sub 2/O/sub 3/ (molybdenum/alumina) catalysts was prepared by incipient wetness impregnation at various pH's (4.0, 5.4, 7.1, and 11.0) using a fixed amount of Mo (8% Mo). The effect of pH on the state and dispersion of Mo species in the dried, calcined, and reduced catalysts was investigated by the combined use of two surface sensitive techniques: X-ray photoelectron spectroscopy (XPS, ESCA) and ion scattering spectroscopy (ISS). The surface properties were correlated to thiophene hydrodesulfurization activities for the various catalysts. A tentative interpretation of the influence of pH of the Mo impregnating solution on the surface structure of Mo/Al/sub 2/O/sub 3/ catalysts at various stages of their preparation is proposed. 8 figures, 1 table.