Search

Bifunctional catalysts are widely applied to chemical and petrochemical industry. Herein nickel was introduced by ionic exchange in HZSM5 zeolite resulted in a Ni/HZSM5 catalyst that contains both H+ and Ni2+ as compensation cations of the zeolite framework. This catalyst has been characterized by using different in-situ techniques as Dispersive X-ray Absorption (DXAS), X-ray Absorption Fine Structure (XAFS), Diffuse Reflectance Infrared Fourier-Transform (DRIFTS) spectroscopies, Temperature Programmed Reduction experiments (TPR) under both H2 and isobutane atmospheres, and by X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM) after exposure to H2 or isobutane. The reduction of Ni2+ species under isobutane started at about 120 °C, meanwhile under H2 stream the reduction started at 350 °C. Under isobutane, nickel undergoes a sequential reduction from Ni2+ to Ni+ and then to Ni0 and the number of Bronsted acid sites of the catalyst increased. By increasing the temperature (500 °C), sintering occurs and nickel particles are formed.

Cu-ZnGaMO (M = Al, Zr) catalysts prepared by a surfactant-free sol-gel method were characterized and tested in the reverse water-gas shift (RWGS) reaction. Their catalytic behavior was analyzed under 0.1–3 MPa at 250–325 °C and using a reactant mixture of CO2/H2 = 1/3. The catalytic results are analyzed in the light of the characteristics of the catalysts. The Cu-ZnGaZrO catalysts were highly selective for the RWGS reaction; a major role of surface copper and interface oxygen vacancies is proposed. CO2 conversion and CO selectivity increased on increasing the reaction temperature. CO2 conversion reaches 16.8% with 99.7% selectivity for CO under 0.1 MPa at 325 °C over the Cu-ZnGaZrO1 catalyst. It is suggested that CO and methanol production proceed via independent reaction pathways.

In this paper, the effect of Ga in Pt/Ga-TiO2 catalysts for photocatalytic H2 production from aqueous solutions of ethanol is studied. The fresh and post-reaction catalysts were characterized using different techniques including BET, XRD, TPR, SEM-EDX, XPS, CO chemisorption and in situ DRIFTS. The presence of Ga in Pt/TiO2-based catalysts increased the Pt dispersion and decreased the relative amount of surface oxidized Pt species. Pt/Ga-TiO2 catalysts showed more stable behaviour during the photocatalytic reaction than the reference Pt/TiO2. The Pt/0.2Ga-TiO2 catalyst showed a higher initial rate of production of H2 and produced a higher total amount of H2 than the monometallic Pt/TiO2. The promoter effect of Ga is analysed in the light of the surface characteristics of the catalysts.

Experimental and theoretical evidences show that carbon vacancies determine the catalytic behavior of vanadium carbides in the CO2 conversion to CO via the Reverse Water Gas Shift (RWGS) reaction. Two VCx samples, one mostly containing stoichiometric VC and the other being C deficient, mainly V8C7, were synthesized, characterized, and studied. The samples show different CO2 adsorption heats, which correlate with those calculated using Density Functional Theory (DFT) on suitable models. The sample containing more V8C7 shows a higher CO2 conversion and CO selectivity and a lower apparent activation energy, being a stable catalyst over long-time tests. DFT calculations confirm that C vacancies in V8C7 are responsible for the observed catalytic behavior, allowing reactants to adsorb more strongly and lowering the energy barrier for both H2 and CO2 dissociation steps. The present work highlights the importance of such native point defects in the transition metal carbides surface chemistry and catalytic properties.

This paper reports the study of substoichiometric ethanol steam reforming (ESR) (H2O/ethanol = 1.6 mol ratio) for hydrogen production over new Ni/Y2O3 La2O3 ZrO2 catalysts. ZrO2-based supports containing Y2O3 and/or La2O3 were prepared by precipitation using active carbon as the templating agent and the Ni-catalysts were then prepared by impregnation. CO2 was used as regenerating agent and its effect was analysed as a function of the characteristics of the catalysts. The CO2 treatment proved to be a powerful tool for regenerating deactivated Ni catalysts through mild oxidation of carbon deposits; this phenomenon is related to the basic characteristics of the catalysts. For Ni/12Zr29Y13La, a high H2 yield and the smallest amount of poorly ordered carbon deposits were determined; full ethanol conversion was achieved during the last 50 h of a long-term (280 h) experiment with periodic CO2 treatments.

Differences in the gas-phase photocatalytic degradation of NH3 and ethanol in the presence of water over two commercial TiO2 materials with different structural characteristics (Evonik's P25 (P25) and Sigma Aldrich's anatase (SA)) were established here using FTIR. The materials were characterized using different techniques including BET, XRD, UV–vis DRS and XPS. The interaction of CO probe molecule with the oxides was evaluated via calorimetry and by in-situ DRIFTS coupled to on-line mass spectrometry. This allowed us to determine differences in the surface properties of the two TiO2 materials. NOx− surface species became evident during the photodegradation of adsorbed NH3 over both SA and P25. However, while surface bidentate nitrate species remained strongly bound to P25, the surface species were fully eliminated from SA. Differences in the photocatalytic degradation of ethanol over P25 and SA were also determined. Ethanol adsorption and its evolution were largely independent of the presence of oxygen for SA, but oxygen affected these processes for P25.

A series of CuZnGa catalysts was synthesized by microwave-assisted method and their performance for CO 2 hydrogenation to methanol was measured under a range of reaction conditions (temperatures between 250 °C and 270 °C and pressure up to 4.5 MPa). The influence of the preparation method on the physicochemical properties of catalysts was studied by N 2 adsorption, XRD, TPR, XPS, N 2 O titration and CO 2 adsorption followed by temperature programmed desorption and calorimetry. The results indicate that the one-pot microwave-assisted precipitation method used in the preparation of the (CuZnGa) MW catalyst, promoted copper and Ga 2 O 3 dispersion. Moreover, the mean basic site strength of (CuZnGa) MW was higher than that of the other catalysts prepared. All of these factors were found to affect its catalytic performance greatly in terms of CO 2 conversion and methanol selectivity. (CuZnGa) MW was highly stable and produced 4.87 mol methanol kg cat −1 h −1 at 250 °C with a selectivity of approximately 50% ( P = 4.5 MPa, GHSV = 3000 h −1 , CO 2 /H 2 = 1/3). The strong interaction between copper and support prevented metallic copper sintering, affording high stability of catalyst. The promoting effect was related to an intimate association between highly dispersed copper species and strong basic ZnO species in the presence of small particles of Ga 2 O 3 .

This paper reports hydrogen production from oxidative steam reforming of 1-propanol and propylene glycol over Ni-M/Y2O3-ZrO 2 (10% wt/wt Y2O3; M = Ir, Pt, Ru) bimetallic catalysts promoted with K. The results are compared with those obtained over the corresponding monometallic catalyst. The catalytic performance of the calcined catalysts was analyzed in the temperature range 723-773 K, adjusting the total composition of the reactants to O/C = 4 and S/C = 3.2-3.1 (molar ratios). The bimetallic catalysts showed higher hydrogen selectivity and lower selectivity of byproducts than the monometallic catalyst, especially at 723 K. Ni-Ir performed best in the oxidative steam reforming of both 1-propanol and propylene glycol. The presence of the noble metal favours the reduction of the NiO and the partial reduction of the support. The NiO crystalline phase present in the calcined catalysts was transformed to Ni during oxidative steam reforming. The adsorption and subsequent reactivity of both 1-propanol and propylene glycol over Ni-Ir and Ni catalysts were followed by FTIR; C-C bond cleavage was found to occur at a lower temperature in propylene glycol than in 1-propanol.

The influence of the addition of a noble metal (Ru, Rh, Ir or Pd) on the catalytic performance of Co/ZnO was studied with respect to oxidative steam reforming of the bio-butanol raw mixture (butanol/acetone/ethanol = 6/3/1, mass ratio). Bimetallic catalysts, CoM/ZnO (M = Ru, Rh, Ir, Pd) had higher conversion values, decreased deactivation and higher H2 concentration in the outlet gas than monometallic Co/ZnO. However, significant differences were found in both raw mixture conversion and H2 concentrations for the different CoM/ZnO, depending on M. CoRh/ZnO presented the best catalytic behavior (65.1 mol% hydrogen in the outlet after 100 h of reaction at 500 °C with 84% conversion). Characterization of catalysts post-reaction by Raman spectroscopy and TPO showed that less ordered and less abundant carbonaceous deposits were formed over bimetallic catalysts than over the monometallic one; CoRh/ZnO had the lowest amount of such coke.

The interaction of ethanol and its evolution to n-butanol over different MgAl mixed oxides derived from hydrotalcite precursors is studied by FTIR and in situ DRIFT-MS experiments. The evolution of different surface species is monitored as a function of thermal treatments. Acetaldehyde appears as a primary intermediate which converts over the mixed oxides to C4 products through the aldol condensation reaction. However, acetaldehyde also produces surface carboxylate species which compete for the active sites and make the condensation process less efficient.

The interaction of VO2+ species with Mg6Al2(OH)16(CO3)·n(H2O) hydrotalcite (HTC) and the corresponding HTC-derived mixed oxide (MO, obtained by calcination of HTC at 800 °C) is reported. Vanadium was loaded (2–20 wt.-%) onto HTC and onto MO using an aqueous solution of VOSO4. After the spontaneous reaction, solids were filtered, treated at 120 °C and then characterised. The characterisation techniques used include spectroscopic, structural, textural and thermal analyses, such as FTIR, Raman, UV/Vis DRS, XRD, BET, EPR, TG/DTA and TPO/TPR. In this work, the nature of the vanadium species on HTC and MO as a function of vanadium loading was studied. The presence of VIII species in the layers of HTC and anionic VV species in the interlayers have been proposed, whereas on MO VIV species are easily oxidised by air producing oligomerised VV supported species.

The residual accelerations that are typically present in microgravity environments (g-jitters) contain a broad spectrum of frequencies and may be modeled as stochastic processes. Their effects on the quality of the semiconductor crystals are analyzed here quantitatively with direct numerical simulation. In particular we focus on the dopant segregation effects due to thermosolutal convection as a function of the parameters characterizing the statistics of the stochastic force. The numerical simulation is specified for material parameters of two doped semiconductors (Ge:Ga and GaAs:Se) in realistic conditions of actual microgravity environments. As a general result, we show that the segregation response is strongly dominated by the low-frequency part of the g-jitter spectrum. In addition, we develop a simplified model of the problem based on linear response theory that projects the dynamics into very few effective modes. The model captures remarkably well the segregation effects for an arbitrary time-dependent acceleration of small amplitude, while it implies an enormous reduction of computer demands. This model could be helpful to analyze results from real accelerometric signals and also as a predictive tool for experimental design.

Oxidative steam reforming (OSR) of n-propanol was studied over new Ni catalysts (ca. 7% Ni wt/wt) supported on Y 2O 3-ZrO 2 oxides with different yttrium content (2-41 % Y 2O 3 wt/wt). Materials were characterized by X-ray diffraction, temperature-programmed reduction, X-ray photoelectron and Raman spectroscopy, scanning electron microscopy with energy dispersive X-ray analysis and high resolution transmission electron microscopy. Samples were used in calcined form and tested in the temperature range 673-773 K using a reactant feed of n-propanol/water/O 2 at a molar ratio 1/9/0.5. Hydrogen production is related with the support composition and Ni dispersion. Copyright

This paper reports the hydrogen production through the steam reforming of a bioresource-derived butanol mixture (butanol:acetone:ethanol=6:3:1 mass ratio) over supported cobalt-based catalysts. The support plays an important role for the catalytic behavior and Co/ZnO exhibits the best catalytic performance compared to Co/TiO(2) and Co/CeO(2). Moreover, a higher hydrogen yield is obtained over bimetallic Co-Ir/ZnO, which shows an increase in H(2) selectivity and a decrease in CH(4) selectivity under steam reforming conditions, compared to Co/ZnO. Raman results of the used catalysts indicate that the addition of Ir could prevent the coke formation to prolong the catalyst stability.

The use of lanthanum as a vanadium trap in USY-based catalysts was studied. Lanthanum was introduced in a HUSY zeolite by different methods: impregnation, precipitation and ionic exchange. Catalysts were characterized by means of XRD, XPS, FTIR, TEM and textural properties (surface-area and microporous volume). The catalysts were also tested in the catalytic reactions of 2-propanol dehydration and n-hexane cracking. The prevention of zeolite damage caused by vanadium depended on the lanthanum localization in the zeolite, which in turn was a function of the method of lanthanum introduction. Samples prepared by precipitation predominantly showed lanthanum species on the zeolite surface. On the other hand, when ionic exchange was used for the lanthanum introduction, it was located in the zeolite channels and cavities. The impregnation method produced a catalyst in which lanthanum was distributed on the zeolite surface but also in the channels and cavities and this catalyst showed a high tolerance to the vanadium presence, lower zeolite damage in steaming and less amount of acid sites poisoned by vanadium.

The partial oxidation of methane is studied at 673–873 K over new Ru-based catalysts supported on TiO2–ZrO2 with different TiO2 content. Supports were prepared by a sol–gel method, and RuCl3 and RuNO(NO3)3 were used as ruthenium precursors to prepare the catalysts (1–2 wt% Ru). The effect of the reaction temperature on the catalytic behavior is analyzed, along with the support composition and the Ru precursor used.

Ru-based catalysts supported on Ta2O5–ZrO2 and Nb2O5–ZrO2 are studied in the partial oxidation of methane at 673–873 K. Supports with different Ta2O5 or Nb2O5 content were prepared by a sol–gel method, and RuCl3 and RuNO(NO3)3 were used as precursors to prepare the catalysts (ca. 2 wt.% Ru). At 673 K high selectivity to CO2 was found. An increase of temperature up to 773 K produced an increase in the selectivity to syngas (H2/CO = 2.2–3.1), and this is related with the transformation of RuO2 to metallic Ru as was determined from XRD and XPS results. At 873 K and with co-fed CO2 an increase of the catalytic activity and CO selectivity was found. A TOF value of 5.7 s−1 and H2/CO ratio ca. 1 was achieved over Ru(Cl)/6TaZr. Catalytic results are discussed as a function of the support composition and characteristics of Ru-based phases.

Cerium was introduced into a USY-based catalyst using three methodologies: precipitation, impregnation and ionic-exchange. The CeUSY catalysts were poisoned with vanadium and deactivated with steam at 1073 K. The catalysts were characterized by HRTEM-EDX, FTIR, BET, EPR, XPS and by the test of catalytic dehydration of 2-propanol. The results indicate that the presence of a V–Ce–EFAL multi-component interaction strongly affects the vanadium reduction and its migration, also inhibiting the cerium oxide sintering. HRTEM measurements with analysis of energy dispersive X-rays on the impregnated catalyst revealed a preferential V–Ce interaction in the extra framework aluminum (EFAL) domains. The FTIR results strongly support that this V–Ce–EFAL interaction occurs via hydroxyl groups. The impregnated catalysts showed greater vanadium tolerance than catalysts prepared by other methodologies of cerium introduction.

This paper reports a study on the effect of the support composition in Pt/Ta 2 O 5 –ZrO 2 catalysts in regard to their catalytic behaviour in the vapour phase selective hydrogenation of crotonaldehyde (2-butenol). Two Ta 2 O 5 –ZrO 2 supports with different Ta/Zr ratios and a pure Ta 2 O 5 were prepared by a sol–gel route. The platinum catalysts were characterized by CO chemisorption, infrared spectroscopy of adsorbed CO and X-ray photoelectron spectroscopy (XPS). The best catalyst in terms of activity and selectivity to crotyl alcohol (2-butenol) was Pt/Ta 2 O 5 , and this behaviour is tentatively related to the metal–support interaction. On the other hand, under the experimental conditions used, the acidic properties of the supports could not be correlated to the catalytic behaviour of these materials.

The effect of iron promoter on cobalt-based catalysts, active in the ethanol steam-reforming, was studied. Fe x Co 3 - x O 4 ( 0 ⩽ x ⩽ 0.60 ) oxides prepared by co-precipitation and an Fe-doped Co 3 O 4 prepared by wetness impregnation are analysed. The activation process of the oxides under reaction conditions was studied by in situ X-ray diffraction (XRD); the activation depended on the iron content of the oxides. The systems were characterized by means of temperature programmed reduction (TPR), XRD and tested in the ethanol steam-reforming reaction at 623–673 K. An optimal iron loading that gives rise to a high H2selectivity and catalyst stability was determined.

New unsupported Co catalysts were studied in the ethanol steam-reforming reaction by in situ diffuse reflectance infrared spectroscopy and on-line mass spectrometry (MS) techniques. The initial ethoxy surface species evolved to surface-acyl species and to acetaldehyde intermediate at temperatures ca. 473 K. The subsequent formation of surface-acetate species occurs at ca. 573 K. Further, the acetate species mainly evolve above 573 K to H2 and CO2 products. The route to ketone formation was inhibited on these materials. The presence of surface-hydroxyl groups, which are probably related to the formation of a CoO phase under the reforming conditions, was determined. Alkaline addition shows a beneficial effect on the ethanol steam-reforming over bulk Co catalysts.

The evolution of Co3O4 during ethanol steam-reforming was investigated under operando conditions by using XRD measurements. The study was carried out at atmospheric pressure as a function of reaction temperature, flowing a 1/6 ethanol/water mixture. XRD measurements and catalytic evaluation were simultaneously accomplished using a reactor-cell specially designed for these purposes. Up to 548 K, only the presence of Co3O4 crystalline phase was determined, and under these conditions ethanol was dehydrogenated to acetaldehyde. At higher temperatures, Co3O4 reduced to crystalline CoO and small metallic cobalt particles. Simultaneously, the material became active for the ethanol steam-reforming reaction. At 623 K, the material which showed by XRD the presence of both CoO and Co phases, was very selective in the steam-reforming of ethanol.

International audience; The preparation of binary materials based on Ta2O5-ZrO2 by a sol-gel method and with a wide range of compositions is reported (Ta2O5 nominal content from 4 to 74% w/w). Materials were characterized by X-ray diffraction, and infrared and Raman spectroscopy. Their acidic characteristics were evaluated from the model reaction of 2-propanol dehydration and the adsorption of CO at 77 K and 2,6-dimethylpyridine at 373 K, both followed by infrared spectroscopy. Increasing tantalum content brings about a development of a surface TaOx phase followed by a progressive formation of aggregates of amorphous Ta2O5. Increased stabilization of tetragonal zirconia was also observed with increasing Ta loading. The surface area, number and strength of Bro¨nsted acid sites, and the catalytic activity in the 2-propanol dehydration increase with the increase in tantalum content up to ca. 50% w/w Ta2O5. A surface area of 103 m2 g-1 was obtained for a solid with a Ta2O5 content of 53% w/w after calcination at 873 K. The abundance of Bro¨nsted acid sites, determined by lutidine adsorption, and their strength, determined by CO adsorption at 77 K, correlated with the catalytic activity in the 2-propanol dehydration.

ZnO-supported Ni and Cu as well as bimetallic Co-Ni and Co-Cu catalysts containing ca. 0.7 wt% sodium promoter and prepared by the co-precipitation method were tested in the ethanol steam-reforming reaction at low temperature (523–723 K), using a bioethanol-like mixture diluted in Ar. Monometallic ZnO-supported Cu or Ni samples do not exhibit good catalytic performance in the steam-reforming of ethanol for hydrogen production. Copper catalyst mainly dehydrogenates ethanol to acetaldehyde, whereas nickel catalyst favours ethanol decomposition. However, the addition of Ni to ZnO-supported cobalt has a positive effect both on the production of hydrogen at low temperature (

We examined the effect of the activation process on the structural and morphological characteristics of a cobalt-based catalyst for Fischer–Tropsch synthesis. A 10 wt.% Co/SiO 2 catalyst prepared by wet impregnation was separately activated under H 2 , CO or a H 2 /CO mixture. The structural changes during activation from 298 to 773 K were studied by in situ X-ray diffraction. Catalysts were examined by SEM, TEM, XPS and in situ DRIFT-MS. The H 2 /CO activation produced redispersion of cobalt particles and simultaneous carbon nanostructures formation. The catalyst showed the highest performance in the Fischer–Tropsch synthesis after the H 2 /CO activation.

The direct transformation of ethanol to ethyl acetate was studied over supported palladium catalysts with low palladium content (1%, w/w Pd) at 1 MPa pressure. SiO2, Al2O3, ZnO, SnO2 and WO3-ZrO2 (29%, w/w WO3) were used as supports. The catalytic behaviour of samples was related to the characteristics of the support and the presence of mono- or bimetallic entities. The best catalytic results were obtained with ZnO- and SnO2-supported catalysts, which can easily lead to alloyed palladium phases.

Microcalorimetric and infrared studies of ethanol and acetaldehyde adsorption were carried out on fresh and deactivated ZnO-supported cobalt catalysts (Co/ZnO and Co/ZnO(d), respectively) as well as on ZnO support alone. The results were used to analyze the catalytic behavior of these materials for ethanol and acetaldehyde steam-reforming reactions. The Co/ZnO(d) sample contained extensive carbon deposition as shown by Raman spectroscopy and transmission electron microscopy. On fresh Co/ZnO, the adsorption energetics of ethanol and acetaldehyde (an intermediate in the ethanol reforming reaction) were similar. Under steam-reforming conditions at low conversion values of ethanol, acetaldehyde was selectively yielded. The presence of surface acetate species was shown from IR spectra following acetaldehyde adsorption. Besides that, the Co/ZnO catalyst was active and showed a high selectivity toward the reforming products, H2 and CO2, when the steam reforming of acetaldehyde was carried out at low conversion values. In contrast, on the deactivated sample, the strongest adsorption sites of ethanol have disappeared, and acetaldehyde was adsorbed with higher energy with respect to ethanol, resulting in the blockage of the active sites; a poorer catalytic performance in both ethanol and acetaldehyde steam-reforming reactions is observed. The presence of acetate species after adsorption of acetaldehyde on Co/ZnO(d) was not shown. The polymerization of acetaldehyde over Co/ZnO(d) was related to the decomposition of acetaldehyde under reforming conditions to give CO and CH4.

The ethanol steam-reforming reaction was studied by in situ diffuse reflectance infrared Fourier transform spectroscopy and on-line mass spectrometry over ZnO and a Co/ZnO catalyst prepared from impregnation of Co2(CO)8. Acetaldehyde is the primary product. The presence of cobalt favors the reforming of acetaldehyde, via the CC scission of bidentate acetate species. Dimethyl ketone is produced as a by-product on ZnO. The roles of ZnO support and cobalt are discussed on the basis of surface species and their subsequent evolution.

The transformation of Co3O4 during steam-re-forming of ethanol was investigated at atmospheric pressure in the 523−673 K temperature range using a bioethanol-like mixture. The initial and after reaction materials were characterized by the Brunauer−Emmett−Teller method, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and in situ magnetic measurements. The performance in the ethanol steam-re-forming of the initial material and of those materials obtained under the reaction conditions was determined. During the activation process, in situ magnetic measurements and catalytic evaluation were carried out simultaneously. At 573 K, ethanol is dehydrogenated to acetaldehyde over Co3O4. At higher temperatures, Co3O4 is reduced to CoO and small metallic cobalt particles. Simultaneously, the material becomes active for the ethanol steam-re-forming reaction. Up to 5.2 mol of H2/mol of ethanol is obtained at temperatures as low as 623 K, and only methane (

The effect of sodium promoter on cobalt–zinc oxide catalysts was studied. Catalysts with different sodium contents (up to 1% (w/w)) were prepared by a coprecipitation method. They were studied in the steam reforming of ethanol between 523 and 723 K and characterized by transmission electron microscopy, X-ray diffraction, Raman and infrared spectroscopies, X-ray photoelectron spectroscopy, and in situ diffuse reflectance infrared spectroscopy. A high segregation of sodium on the surface of catalysts took place. The sodium addition had a positive effect on the steam-reforming reaction of ethanol. Under total conversion and in the 623–723 K temperature range, the production of hydrogen from an ethanol–water (C 2 H 5 OH:H 2 O=1:13 molar) mixture (bioethanol) increased (5–8%) with sodium content. In addition, the incorporation of sodium resulted in the stabilization of the catalysts toward deactivation. In particular, the sodium-promoter effect was related to a decrease in carbon deposition, as evidenced by transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy.

The ethanol steam-reforming reaction was studied over ZnO-supported cobalt catalysts (10 wt.% Co). Catalysts were prepared by impregnation of nitrate and carbonyl cobalt precursors. Characterization was accomplished by transmission electron microscopy (TEM), Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), and in situ techniques: magnetic measurements, and diffuse reflectance infrared spectroscopy (DRIFT) coupled to mass spectrometry. The use of Co 2 (CO) 8 as precursor produced a catalyst that was highly stable and selective for the production of CO-free hydrogen at reaction temperature as low as 623 K. The only by-product was methane and selectivity of 73% to H 2 and 25% to CO 2 was obtained. Under reaction conditions, the catalyst showed 92% of reduced cobalt, mainly as small particles.

Silica-supported trimetallic catalysts containing Pt, Sn and a group 13 metal (PtSnM, M=Ga, In, Tl) were prepared by consecutive impregnation steps from cis-[PtCl2(PPh3)2] and chloride precursors. X-ray diffraction (XRD), transmission electron microscopy (TEM), selected-area electron diffraction (ED) and energy dispersive X-ray analysis (EDX) showed large platelet-like particles of PtSn1−xMx phases. PtSnGa catalyst with a Pt/(Sn+Ga) molar ratio of 1.72 showed a bimodal particle distribution and a Pt phase was identified. Differences in surface structures were also revealed by the performance of catalysts in the dehydrogenation of n-hexane. For PtSnIn and PtSnTl (Pt/(Sn+M) molar ratio of about 1) the dehydrogenation was favoured. In contrast, PtSnGa catalyst yielded hydrogenolysis products. Photoelectron spectra showed the Pt 4f7/2 level at a binding energy of 70.0–71.8 eV in all cases. Moreover, the FT-IR spectra of chemisorbed CO on the PtSnGa showed a slight shift in the ν(CO) toward higher values with respect to the monometallic catalyst, pointing to an electronic effect in accordance with photoelectron spectroscopy.

Various supported cobalt catalysts (Co/MgO, Co/Al2O3, Co/SiO2, Co/TiO2, Co/V2O5, Co/ZnO, Co/La2O3, Co/CeO2 and Co/Sm2O3), were characterised by magnetic measurements under ethanol steam-reforming conditions, in situ diffuse reflectance infrared spectroscopy and UV-Vis-NIR diffuse reflectance spectroscopy. Their magnetic behaviour was analysed as a function of reaction temperature and hydrogen treatment and was related to the cobalt species in the catalysts and to their catalytic behaviour in the steam-reforming of ethanol. The catalysts were prepared from Co2(CO)8 and used without previous calcination or reduction. All samples showed a paramagnetic or diamagnetic behaviour before reaction. The catalysts that performed well in the steam-reforming of ethanol presented under reaction conditions, metallic (ferromagnetic) cobalt particles and oxidised cobalt species. An easy exchange between small metallic cobalt particles (produced under H2 or under ethanol steam-reforming conditions) and oxidised cobalt species (produced under ethanol steam-reforming conditions) was found in these catalysts.

Niobia-modified silica was used as support for nickel-based catalysts. Catalysts with different nickel loading were prepared by successive incipient-wetness impregnation of toluenic nickel octanoate solutions. The samples were characterised after both calcination and reduction by TEM-EDX techniques and tested in the ethane hydrogenolysis. Their catalytic behaviour was compared to that of related silica-supported nickel catalysts. A high suppression of hydrogenolysis activity (93–99%) was determined for the niobia-modified silica-supported nickel catalysts which do not show nickel particles on silica.

The reaction between ethanol and water was studied in the temperature range 573–723 K at atmospheric pressure over supported cobalt catalysts. Cobalt-loaded catalysts (1%) were prepared by impregnation of Co2(CO)8 on MgO, γ-Al2O3, SiO2, TiO2, V2O5, ZnO, La2O3, CeO2, and Sm2O3. Ethanol steam reforming occurred to a large degree over ZnO-, La2O3-, Sm2O3-, and CeO2-supported catalysts; CO-free hydrogen was produced. Samples were characterized after catalytic tests by high-resolution transmission electron microscopy, electron diffraction, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed hydrogen reaction. Depending on the support, different cobalt-based phases were identified: metallic cobalt particles, Co2C, CoO, and La2CoO4. The extent and nature of carbon deposition depended on the sample and on the reaction temperature. ZnO-supported samples showed the best catalytic performances. Under 100% ethanol conversion, selectivity up to 73.8% to H2 and 24.2% to CO2 was obtained.

This study reports new gallium-promoted copper-based catalysts prepared by co-impregnation of methoxide–acetylacetonate (acac) precursors from methanolic solutions onto silica and zinc oxide supports. Catalyst performance in the CO2 hydrogenation to methanol was investigated at 2 MPa and temperatures between 523 and 543 K. A high activity and selectivity for ZnO-supported catalysts was found, which also showed a high stability in terms of both activity and selectivity. The maximum value for the activity was 378 g MeOH/kg cat h at 543 K, with a selectivity of 88% towards methanol production. The high performance of these materials in the CO2 hydrogenation is related to the presence of Ga2O3 promoter and highly dispersed Cu+ species on the surface, determined by XPS and Auger on used catalysts.

Gallium-promoted copper-based catalysts supported on ZnO and SiO2, respectively, Cu-Ga/ZnO and Cu-Zn-Ga/SiO2, were tested in the hydrogenation of CO2 to methanol under a range of experimental conditions. Long-term methanol synthesis operation and catalytic tests under high pressure (up to 5 MPa) at 523 K indicated that the catalysts were highly effective. The methanol synthesis was sensitive to the presence of small amounts of methane and ethane in the CO2/H2 source, producing significant deactivation of the catalysts, especially of Cu-Zn-Ga/SiO2, and promoting the formation of by-products. After reaction with hydrocarbon-containing CO2/H2 feed, TEM experiments showed copper-based particles covered by an amorphous layer that is probably carbonaceous in nature. XPS allowed determination of differences in surface composition for catalysts tested with clean or hydrocarbon-containing CO2/H2. These differences allowed the performance of Cu-Zn-Ga/SiO2 to be related to the presence of Cu+ and the quantity of gallium in the near-surface regions, which was lower for the catalyst tested under hydrocarbon-containing CO2/H2 feed.

The hydrogenation of carbon monoxide over Co/SiO 2 catalysts obtained by reduction of precursors prepared by the reaction of dicobaltoctacarbonyl with silica gives rise to a mixture of alcohols and hydrocarbons. The chain growth probability α for hydrocarbon synthesis is similar to that observed over conventional catalysts prepared by the ammonia method ( α =0.74) with a comparable metal dispersion (metal particle size 4 nm). Alcohol formation yields a smaller chain growth probability ( α =0.42). Magnetic measurements have shown that in the latter catalyst, small cobalt clusters (∼1 nm) are likely to be present together with the cobalt particles (4 nm), this observation reconciles apparently conflicting results from the literature. It has been speculated that alcohol formation might be related to the presence of these clusters according to a concerted mechanism. Furthermore, the observation of large amounts of acetates by temperature-programmed hydrogenation, not detected in hydrocarbon selective catalysts, suggests that they may play a role in ethanol synthesis.

In this study, gallium-promoted copper-based catalysts prepared by impregnation methods on silica and ZnO supports, were examined for the hydrogenation of CO 2 to methanol. The surface characteristics of catalysts depended on the support and were related to their catalytic performance. Silica-supported catalysts tested at reaction temperatures between 523 and 543 K were highly selective and stable. The selectivity to methanol was around 99%, the conversion to CO was very low and negligible amounts of hydrocarbons were formed. The use of hydrophobic silica enhanced the performance of the catalyst in terms of activity, selectivity and stability. The modification of properties of copper particles is related to the presence of very small particles of Ga 2 O 3 on the surface.

Magnesia-supported bimetallic Pt–Sn catalysts with various metal contents and Pt/Sn atomic ratios were prepared using the bimetallic complex cis-[PtCl(SnCl3)(PPh3)2] or the monometallic complex cis-[PtCl2(PPh3)2] and SnCl2 as metal precursors. Catalysts were characterised by X-ray diffraction, transmission electron microscopy techniques and X-ray photoelectron spectroscopy. The bulk and surface composition of the catalysts was related to the catalytic behaviour in the vapour phase hydrogenation of crotonaldehyde (but-2-enal). It is shown that the various preparation methods produce catalysts with distinct properties and catalytic behaviour. For catalysts prepared with the bimetallic complex, the highest selectivity for the unsaturated alcohol (but-2-en-1-ol) was achieved for the catalyst containing larger bimetallic particles, which are associated with a higher surface concentration of oxidised tin species. The catalyst prepared with separate metal precursors contains a higher amount of residual chlorides, which significantly affect the catalytic activity but only slightly alter the selectivity for the hydrogenation of the carbonyl bond.

The reaction between CO2 and C2H4 in the presence of H2 over Pt and PtSn-based (SiO2, Al2O3, MgO) supported catalysts has been studied at 34 bar of total pressure and 423 K by in situ diffuse reflectance infrared Fourier transform spectroscopy and on-line mass spectrometry experiments. Over silica-supported PtSn alloy, the reaction yields 2-hydroxypropanoic acid through a C2-carbonylic species and CO2 nondissociatively coordinated. C2H4 chemisorption on PtSn/SiO2 produces a C2-carbonylic species which interacts with the hydroxyl groups of SiO2, and this C2-carbonylic species evolves to acetaldehyde at 423 K. 2-Hydroxypropanoic acid is also produced over PtSn/SiO2 from acetaldehyde, CO2, and H2, but not from ethanol and CO2.

Silica-supported PdZn catalysts have been studied in CO and CO2 hydrogenation 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.

Alumina- and silica-supported bimetallic Pt–Sn catalysts with different Pt/Sn atomic ratios have been prepared by using the bimetallic complex cis-[PtCl(SnCl3)(PPh3)2], the monometallic complex cis-[PtCl2(PPh3)2] and SnCl2 as metal precursors. Catalysts have been characterised by X-ray diffraction and transmission electron microscopy techniques, including energy-dispersive X-ray microanalysis, electron diffraction experiments and lattice-fringe microanalysis. The phase composition of the different catalysts has been related to the catalytic behaviour in the vapour phase hydrogenation of crotonaldehyde (but-2-enal). Results show that catalysts containing platinum–tin alloy phases with large particles are more selective towards the hydrogenation of the carbonyl bond to yield crotyl alcohol. The presence of oxidised tin species on the surface of the large alloy particles promotes the hydrogenation of the carbonyl bond. In situ diffuse reflectance infrared spectroscopy studies of crotonaldehyde adsorption under He or H2 reveal differences between bimetallic catalysts and monometallic, and these differences have been related to the different catalytic behaviour of the catalysts.

A silica-supported CuCo catalyst has been prepared by anchoring a Cu–Co bimetallic complex. Evolution of the species under vacuum and CO treatments has been followed by in situ FTIR. A breakdown of the initial complex with CO at 373 K gives [Co(CO) 4 ] − . After total decarbonylation, a subsequent CO treatment followed by FTIR indicates the presence of metallic Cu and very small particles of cobalt. The catalyst obtained after an H 2 -treatment at 473 K has been characterized by XRD, TEM and XPS techniques. The presence of large Cu particles and highly-dispersed subnanometric cobalt particles is inferred. Coordination of CO on these very small particles of metallic cobalt could give Co 4 (CO) 12 . Catalytic behaviour in the hydroformylation of ethylene and in the hydrogenation of CO is related to the highly dispersed cobalt, which probably interacts with Cu. An easy CO insertion into a metal–alkyl bond is favoured over this CuCo catalyst.

Magnesia-supported platinum–tin catalysts with a Pt content 0.25–3% and atomic ratio Pt/Sn = 1 were prepared from a Pt–Sn bimetallic complex. Characterization of catalysts by XRD, TEM, EDX, XPS and CO singleton frequency indicated the formation of well-dispersed bimetallic particles, the size of which decreased with the metal content. Both dilution and electronic effects on Pt by Sn were evidenced by IR experiments of CO adsorption. The behavior of catalysts in isobutane dehydrogenation at 753 K and atmospheric pressure was related to their structural characteristics and compared to silica- and alumina-supported catalysts prepared by a similar method, and to the respective monometallic catalysts. Magnesia-supported bimetallic catalysts were the most selective to isobutene production. Selectivities higher than 99% were achieved for conversion values of 11%.

An anisotropic heat di⁄usion coeƒcient is introduced in order to study some interfacial growth phenomena. This anisotropy has been incorporated in a phase field model which has been studied numerically to reproduce some fundamental solidification situations (needle crystal growth) as well as the dynamics of a nematic—smectic-B interface. As a general result, we find that dendrites grow faster in the lower heat di⁄usion direction. Simulation results are compared with experiments with remarkable qualitative agreement. ( 1998 Elsevier Science B.V. All rights reserved.

The interaction between CO 2 and the silica-supported PtSn alloy gave rise to an infrared band at 2339 cm −1 which was assigned to the coordination of this molecule to the bimetallic phase. The coadsorption of CO and CO 2 indicates that both compete for similar coordination sites on the supported PtSn alloy. A CO singleton vibration frequency at ca. 2054 cm −1 was determined for both Pt/SiO 2 and PtSn/SiO 2 catalysts. A dilution effect caused by tin in the silica-supported PtSn alloy is proposed. The presence of isolated Pt sites in the particles of this alloy determine their unique capability to coordinate the CO 2 molecule in the active sites.

Silica-supported Pt‐Sn catalysts were prepared by two-step impregnation from [PtCl2(PPh3)2] and SnCl2 solutions of appropriate concentrations to yield Pt/Sn atomic ratios ranging from 0.2 to 5.0. In these systems, the presence of true Pt‐Sn alloys was confirmed by X-ray diffraction, transmission electron microscopy, energy dispersive X-ray analysis and electron nanodiffraction. Pt and PtSn alloy phases were found on catalysts with Pt/Sn> 1, PtSn alloy alone on the catalyst with Pt/SnD 1 and PtSn and PtSn2 alloys, together with Sn in the catalysts with Pt/Sn< 1. All these catalysts were tested in the skeletal reactions of n-hexane at 753 K and atmospheric pressure. The selectivity of Pt changed significantly when alloyed with tin. For Sn-rich compositions a segregation of tin toward the catalyst surface was shown by photoelectron spectroscopy, and high hydrogenolysis selectivity and fast deactivation were observed. In contrast, Pt-rich catalysts, in which a well defined PtSn alloy was observed, were much more stable and exhibited high selectivity to dehydrogenation reaction while maintaining low conversions to benzene and hydrogenolysis products. This selectivity pattern can be interpreted in terms of a change in adsorption properties due to differences in the number of adjacent Pt atoms required for the various reaction pathways. c ∞ 1997 Academic Press

Hydroformylation of ethylene and CO hydrogenation were studied over cobalt-based catalysts derived from reaction of Co2(CO)8 with ZnO, MgO and La2O3 supports. At 433 K a similar activity sequence was reached for both reactions: Co/ ZnO > Co/La2O3 > Co/MgO. This confirms the deep analogy between hydroformylation and CO hydrogenation into alcohols. In the CO hydrogenation the selectivity towards alcohol mixture (C1-C3) was found to be near 100% at 433 K for a conversion of 6% over the Co/ZnO catalyst; this catalyst showed oxo selectivity higher than 98% in the hydroformylation of ethylene. Magnetic experiments showed that no metallic cobalt particles were formed at 433 K. It is suggested that the active site for the step that is common to both reactions is related to the surface homonuclear Co2+/[Co(CO)4]− ion-pairing species.

Alumina-supported tungsten and molybdenum oxide catalysts with a wide range of MO3 loadings were studied in the metathesis of propene. The catalysts were activated by several procedures in order to correlate the oxidation state of Mo or W, as characterized by XPS, and the acidic character, as determined by IR of adsorbed pyridine, with the main metathesis reaction and side olefin reactions. The catalytic activity of the samples under standard conditions was found to be invariant with the pretreatment (air or He). This leads to the conclusion that the activation of catalysts takes place under the propene atmosphere during the first minutes of reaction. XPS studies of fresh and spent catalysts showed that the active sites for metathesis seem to be W(VI) or Mo(VI) species. Both MoO 3 Al 2 O 3 and WO 3 Al 2 O 3 catalysts are active in parallel reactions such as polymerization, isomerization and cracking. The product distribution for the metathesis reaction differed greatly from that expected. Thus, for all catalysts the ethene/butene ratio is lower than 1 and isobutene was the main component of the C4 fraction, its proportion increasing with the metal oxide content. Studies by IR of adsorbed pyridine showed the Bronsted acid sites generated on MoO3 or WO3 in samples with high metal oxide content to be responsible for the lateral acid-catalyzed reactions.