Showing papers on "Catalyst support published in 1997"

Design, synthesis, and use of cobalt-based Fischer-Tropsch synthesis catalysts

Abstract: Catalyst productivity and selectivity to C5+ hydrocarbons are critical design criteria in the choice of Fischer-Tropsch synthesis (FTS) catalysts and reactors. Cobalt-based catalysts appear to provide the best compromise between performance and cost for the synthesis of hydrocarbons from CO/H2 mixtures. Optimum catalysts with high cobalt concentration and site density can be prepared by controlled reduction of nitrate precursors introduced via melt or aqueous impregnation methods. FTS turnover rates are independent of Co dispersion and support identity over the accessible dispersion range (0.01–0.12) at typical FTS conditions. At low reactant pressures or conversions, water increases FTS reaction rates and the selectivity to olefins and to C5+ hydrocarbons. These water effects depend on the identity of the support and lead to support effects on turnover rates at low CO conversions. Turnover rates increase when small amounts of Ru (Ru/Co<0.008 at.) are added to Co catalysts. C5+ selectivity increases with increasing Co site density because diffusion-enhanced readsorption of α-olefins reverses, β-hydrogen abstraction steps and inhibits chain termination. Severe diffusional restrictions, however, can also deplete CO within catalyst pellets and decrease chain growth probabilities. Therefore, optimum C5+ selectivities are obtained on catalysts with moderate diffusional restrictions. Diffusional constraints depend on pellet size and porosity and on the density and radial location of Co sites within catalyst pellets. Slurry bubble column reactors and the use of eggshell catalyst pellets in packed-bed reactors introduce design flexibility by decoupling the characteristic diffusion distance in catalyst pellets from pressure drop and other reactor constraints.

Oxidative dyhydrogenation of short chain alkanes on supported vanadium oxide catalysts

Abstract: This paper summarizes the main data appeared in the last years on the oxidative dehydrogenation (ODH) of short chain alkanes on supported vanadium oxide catalysts. The acid-base character of metal oxide support influences the dispersion of vanadium on the surface of the support, as well as the nature of the vanadium species. The reducibility and structure of surface vanadium oxide species and the acid-base character of catalysts, in addition to their catalytic properties in the ODH of C2–C4 alkanes, strongly depend on the metal oxide used as support and the vanadium loading. In this way, it appears that tetrahedral V5+-species are active and selective sites in the ODH of C2–C4 alkanes. The effect of the coordination number and aggregation state of surface vanadium oxide species, and the presence of acid/base sites on the catalytic behavior of supported vanadium oxide catalysts are discussed. It is concluded that these are important factors that must be considered to develop selective catalysts in ODH reactions.

Methanol oxidation as a catalytic surface probe

Abstract: The goal of this review is to present some aspects of the use of a test reaction, i.e., methanol oxidation, as a tool to characterize oxidation catalysts. The selectivity pattern and the formation rates of the reaction products are used to characterize both structural (dispersion) as well as chemical properties (acid-base and redox) on supported oxide catalysts, especially for molybdenum-based systems supported on silica and vanadia on titanium oxide. This highly sensitive technique which gives information on the catalytically active sites at the molecular level characterizes a catalyst at work and is particularly well-adapted to the study of supported catalysts.

Titania−Silica: A Model Binary Oxide Catalyst System

Abstract: Titania−silica mixed oxides are intriguing catalysts and catalyst supports since their surface reactivities depend strongly on composition and homogeneity of mixing. In this review, the synthesis, characterization, and catalytic activity of titania−silica are described, and important structure/property relationships for these materials are emphasized.

Removal of carbon monoxide from hydrogen-rich fuels by selective oxidation over platinum catalyst supported on zeolite

Abstract: Special catalysts — Pt supported zeolites — for the selective oxidation of carbon monoxide in reformed fuels from methanol or natural gas were proposed. They can be applied for the application to polymer electrolyte fuel membrane cells of which anode Pt catalysts suffer serious poisoning by the presence of trace carbon monoxide. The proposed Pt-supported zeolite catalysts can oxidize carbon monoxide much more selectively in a large excess of hydrogen with the addition of a low concentration of oxygen than a conventional Pt-supported alumina catalyst. The selectivity was affected by supports, in the order A type zeolite mordenite X type zeolite alumina. With decreasing oxygen content, an enhanced selectivity was obtained on Pt-zeolite catalysts (approaching 100%). Pt supported on mordenite showed the highest selectivity, with high conversion from carbon monoxide to carbon dioxide among the catalysts examined. It was demonstrated that the oxygen addition can be minimized almost to the stoichiometric amount required for the complete oxidation of carbon monoxide in a large excess of hydrogen.

On the flexibility of the active phase in hydrotreating catalysts

Abstract: This review deals with the structure of the active phase in sulfidic Co Mo and Ni Mo hydrotreating catalysts. Various models describing the catalyst and its interaction with the reaction environment are discussed in the light of the evolution of the active phase during the catalyst life cycle. Special attention is paid to the contribution of Mossbauer Emission Spectroscopy (MES), Extended X-ray Absorption Fine Structure (EXAFS), High Resolution Transmission Electron Microscopy (HR-TEM) and Molecular Modeling to the unraveling of the active phase structure. It is concluded that MoS 2 sintering and Co 9 S 8 (Ni 3 S 2 , NiS) segregation during the catalyst life cycle force the shift of the actual reaction mechanism from that involving a single site or an ensemble of sites to that of a close cooperation between segregated components. The adsorption of reactants can take place in many diferent ways, through the heteroatom or through the ring, on the Mo sites as well as on the Co (Ni) sites. The exact state of each active site depends on the reaction environment. If the catalyst is S deficient, the classical adsorption mechanism involving the S vacancy is dominant. With a fully sulfided catalyst surface the adsorption takes place through S S bonds. The catalyst is a dynamic evolving system adapting itself to its ever changing reaction environment. Each catalyst component fulfills multiple functions in determining the catalyst structure as well as its interaction with the reactant molecules.

Selective NH3 oxidation to N2 in a wet stream

Abstract: Al2O3 and ZSM-5 supported Pd, Rh and Pt catalysts were studied for NH3 oxidation at 200–350°C in comparison with V2O5/TiO2 and Co-ZSM-5 catalysts at 250–400°C. The precious metal catalysts are very active for NH3 oxidation in this temperature range. With the addition of 5% H2O vapor, the NH3 conversion is not affected at high temperatures but decreases at 200–250°C. Generally, the ion exchanged ZSM-5 catalysts are more active than the Al2O3 supported catalysts with an identical metal loading and less affected by water vapor. However, for the supported precious metal catalysts the catalytic selectivity is not significantly affected by catalyst support. The selectivity to N2 is relatively high on Rh and Pd catalysts and low on Pt catalysts, where high levels of N2O were produced on Pt. Some of the catalysts reported here, for example, Pd-ZSM-5, may be quite suitable as a practical catalyst for low temperature NH3 destruction in a wet stream. As a comparison V2O5/TiO2 is quite selective for N2 formation at T

Ziegler-natta catalysts for olefin polymerization

Abstract: A new synthesis of a Ziegler-Natta catalyst uses a multi-step preparation which includes treating a soluble magnesium compound with successively stronger chlorinating-titanating reagents. The catalyst may be used in polymerization of olefins, particularly ethylene, to produce a polymer with low amount of fines, large average fluff particle size and narrow molecular weight distribution. The catalyst has high activity and good hydrogen response.

Gas phase hydrogenation of crotonaldehyde over Pt/Activated carbon catalysts. Influence of the oxygen surface groups on the support

Abstract: Three activated carbons were prepared with different content of oxygen surface complexes and impregnated with aqueous solutions of [Pt(NH34]Cl2. The catalysts were characterized by H2 and CO chemisorption at room temperature, temperature-programmed decomposition (TPD) and X-ray photoelectron spectroscopy, and their catalytic behavior in the vapor phase hydrogenations of benzene and crotonaldehyde (trans-2-butenal) was determined. Metal dispersion is highly dependent on the degree of support oxidation, being lower for the catalyst support containing the highest amount of surface acidic complexes. This is attributed to the decomposition of the surface complexes, which act as anchoring centers for the platinum precursor, upon the reduction treatments at which the catalysts are subjected. The specific catalytic activity in the gas phase hydrogenation of crotonaldehyde is higher as the starting support is more oxidized, and the activity per gram of platinum increases with reduction temperature. The selectivity for the hydrogenation of the carbonilic CO bond instead of the olefinic CC bond is also improved when an oxidized support is used, and the production of the unsaturated alcohol, crotyl alcohol, is enhanced when catalysts are reduced at higher temperature, especially those prepared with oxidized supports.

Mixed transition metal catalyst systems for olefin polymerization

Abstract: The invention encompasses a mixed transition metal olefin polymerization catalyst system suitable for the polymerization of olefin monomers comprising one late transition metal catalyst system and at least one different catalyst system selected from the group consisting of late transition metal catalyst systems, transition metal metallocene catalyst systems or Ziegler-Natta catalyst systems. Preferred embodiments include at least one late transition metal catalyst system comprising a Group 9, 10, or 11 metal complex stabilized by a bidentate ligand structure and at least one transition metal metallocene catalyst system comprising a Group 4 metal complex stabilized by at least one ancillary cyclopentadienyl ligand. The polymerization process for olefin monomers comprises contacting one or more olefins with these catalyst systems under polymerization conditions.

Catalytic pyrogasification of biomass. Evaluation of modified nickel catalysts

Abstract: In a previous work, the pyrolytic gasification of biomass (wood) using a stoichiometric nickel aluminate catalyst in a fluidized-bed reactor gave near-equilibrium yields of products above 650 °C, with 85−90% gas yields and no detectable tar production. Additional tests are reported for a modified nickel−magnesium aluminate stoichiometric catalyst, to give greater physical strength, and for the addition of potassium, as a promoter. The addition of Mg in the catalyst crystal lattice did improve resistance to attrition but resulted in a minor loss in gasification activity and increased coke production. Addition of potassium had little effect. Catalyst deactivation by secondary carbon deposits was demonstrated, and regeneration of the Mg-containing catalysts by burn-off appears to be feasible. The deactivation process was experimentally simulated. A conceptual process for catalytic pyrogasification of biomass was modeled.

Sulphur poisoning of nickel-based hot gas cleaning catalysts in synthetic gasification gas: II. Chemisorption of hydrogen sulphide

Abstract: The effect of different components of gasification gas on sulphur poisoning of nickel catalysts were studied. In addition, the sulphur distribution and content of nickel catalyst beds were analysed to account the poisoning effect of sulphur on the activity of catalysts to decompose tar, ammonia and methane. The desorption behaviour of chemisorbed sulphur from the bed materials was monitored by temperature programmed hydrogenation (TPH). It was established that bulk nickel sulphide was active in decomposing ammonia in high-temperature gasification gas-cleaning conditions. The decomposing activity of methane was not affected by bulk nickel sulphide formation, but that of toluene was decreased. The activity of the catalyst regained rapidly when H2S was removed from the gas. However, the conversion of ammonia was not regained at as high a level as before sulphur addition, most probably due to irreversible sulphur adsorption on the catalyst. The temperature increase could also be used to regenerate the catalyst performance especially in respect to methane and toluene. Sulphur adsorbed on nickel catalysts in different chemical states depends on the process conditions applied. At >900°C the sulphur adsorbed on the catalyst formed an irreversible monolayer on the catalyst surfaces, while at <900°C the adsorbed sulphur, probably composed of polysulphides (multilayer sulphur), was desorbed from the catalyst in sulphur-free hydrogen containing atmosphere. However, a monolayer of sulphur still remained on the catalyst after desorption. The enhanced effect of high total pressure on sulphur-poisoning of nickel catalysts could be accounted for the increased amount of sulphur, probably as a mode of polysulphides, adsorbed on the catalyst.

Propylene epoxidation with hydrogen peroxide and titanium silicalite catalyst: Activity, deactivation and regeneration of the catalyst

Abstract: The epoxidation of propene with hydrogen peroxide and a titanium silicalite catalyst with MFI structure was investigated in detail. The relationship between catalytic activity and propylene oxide selectivity can be explained by the acidification of water coordinated to the active titanium site and its reversible deprotonation to an ate complex, which inhibits the formation of the active species for epoxidation. Pretreatment of the catalyst with neutral or acidic salts improves the propylene oxide selectivity without affecting the catalytic activity. Catalyst deactivation occurs by blocking of the zeolite micropores with propylene oxide oligomers. The deactivated catalyst can be regenerated by refluxing with dilute hydrogen peroxide.

Combustion of carbonaceous materials by Cu-K-V based catalysts. II: Reaction mechanism

Abstract: In Part I the chemical and microstructural nature of Cu K V catalysts for diesel soot combustion was assessed. In this second communication the reaction mechanism of these catalysts is studied by testing their activity through either differential scanning calorimetry (DSC) or temperature-programmed oxidation (TPO), performed on catalyst-carbon mixtures. The following parameters were varied: the particle size of both the catalyst and the carbon powders, the catalyst-to-carbon weight ratio, the catalyst composition (besides the standard Cu K V catalyst defined in Part I, binary catalysts based on a single copper or potassium vanadate + KCl were also considered). On the basis of the obtained results the presence of liquid eutectic phases (whose formation was detected via DSC runs performed in the absence of carbon) is found to be a key factor in determining the catalytic activity of all tested catalysts since it dramatically improves the catalyst-carbon contact. Once liquid phases are formed at suitable temperatures (ranging from 330 to 480°C depending on the catalyst nature), the catalytic combustion likely proceeds according to redox mechanisms.

Catalyst composition containing oxygen storage components

Abstract: The present invention relates to a zirconium, rare earth containing composition comprising zirconium, cerium, neodymium and praseodymium components and the use of this composition in a catalyst composition useful for the treatment of gases to reduce contaminants contained therein and method process to make the catalyst composition. The catalyst has the capability of substantially simultaneously catalyzing the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides.

Two-step synthesis of diphenyl carbonate from dimethyl carbonate and phenol using MoO3SiO2 catalysts

Abstract: The transesterification of dimethyl carbonate with phenol to produce methyl phenyl carbonate was carried out in an autoclave using a variety of solid catalysts. MoO 3 SiO 2 was found to have a very high activity for this transesterification. Thus, a 17.1% yield of methyl phenyl carbonate based on phenol was obtained at 433 K in the presence of MoO 3 SiO 2 . MoO 3 SiO 2 was also an active catalyst for the disproportionation of methyl phenyl carbonate into diphenyl carbonate and dimethyl carbonate. A 48% yield of diphenyl carbonate was attained over MoO 3 SiO 2 at 443 K. These yield values are probably very close to those at the thermodynamic equilibrium.

Spectroelectrochemical Study of the Role Played by Carbon Functionality in Fuel Cell Electrodes

Abstract: X-ray absorption spectroscopy was used to identify specific types of nitrogen and sulfur-based carbon functionality present in the carbon black supports of fuel cell anodes and cathodes. The effects of these functional groups on the electrocatalytic performance of small platinum particles, dispersed on the carbon, during methanol oxidation and oxygen reduction were assessed. Electrodes functionalized with nitrogen had enhanced catalytic activities toward oxygen reduction and methanol oxidation relative to untreated electrodes. Although electrodes with sulfur functionality had higher oxygen reduction activities than untreated carbons, the activity of these electrodes toward methanol oxidation was found to be lower than electrodes manufactured from untreated carbon. It was found that carbon supports functionalized with both nitrogen and sulfur initiated the formation of Pt particles smaller in size than those observed on untreated carbon supports.

Methane interaction with silica and alumina supported metal catalysts

Abstract: Six different transition metals — cobalt, nickel, ruthenium, rhodium, iridium and platinum — have been supported on silica or alumina by means of the incipient wetness method. After drying and calcination at 773 K, they have been characterized by the hydrogen pulse chemisorption technique and by transmission electron microscopy. The interaction of CH4 with the catalyst surface has been studied by a temperature programmed surface reaction of methane and by examining the reactivity with hydrogen of the carbon deposits formed for three hours of reaction at 623 K in a flow of diluted methane. It has been found that the support exerts a great influence in the activity of dehydrogenation of methane and in the kind of carbonaceous species generated. It has also been proved that rhodium catalysts have a special ability to stabilize reactive carbon species (Cβ) hydrogenable between 423 K and 498 K.

Combination Catalysts Consisting of a Homogeneous Catalyst Tethered to a Silica-Supported Palladium Heterogeneous Catalyst: Arene Hydrogenation

Abstract: Transition metal complex catalysts tethered to organic or inorganic supports1 have received much attention in the past few decades because they can, in principle, combine the advantages of homogeneous and heterogeneous catalysts. Such complexes can be easily tethered on silica surfaces through a ligand in the complex which has alkoxyor chlorosilane functional groups that react with surface hydroxyl groups on the SiO2. Silica-supported heterogeneous metal catalysts such as Pd-SiO2, Rh-SiO2, and Pt-SiO2 also have surface hydroxyl groups that could be used to tether transition metal complex homogeneous catalysts. These combination catalysts consisting of a tethered complex on a supported metal (TCSM) catalyst (Figure 1) could function by synergistic action of both catalyst components. For hydrogenation reactions of unsaturated organic substrates, one might imagine that these TCSM catalysts could function in a way that H2 is activated on the supported metal (e.g., Pd, Rh, or Pt) with the resulting hydrogen atoms spilling over onto the silica where they could react with the unsaturated organic substrate that is simultaneously coordinated and activated by the tethered complex. This mechanism for the functioning of a TCSM catalyst depends on the well-known phenomenon of hydrogen spillover on supported metal catalysts.3 In other mechanisms, the tethered complex may interact more directly with molecules that are activated on the supported metal. In this paper, we report an example, the first to our knowledge, of a tethered complex on a supported metal (TCSM) catalyst, whose activity for the hydrogenation of arenes is substantially higher than that of the tethered complex or the supported metal separately. In fact, its activity is higher than that of any reported homogeneous or immobilized metal complex catalyst under the mild conditions of 1 atm of H2 and 40 °C. Two TCSM catalysts were prepared by tethering either of the rhodium isocyanide complexes, RhCl[CN(CH2)3Si(OC2H5)3]3 or RhCl(CO)[CN(CH2)3Si(OC2H5)3]2, to a silica-supported palladium metal catalyst (Pd-SiO2). The rhodium isocyanide complex RhCl(CO)[CN(CH2)3Si(OC2H5)3]2 (Rh-CNR2) was prepared by the reaction of [Rh(CO)2Cl]2 with 4 equiv of CN(CH2)3Si(OC2H5)3 in toluene, in a reaction similar to that described for the synthesis of RhCl(CO)[CNBu]2. The complex RhCl[CN(CH2)3Si(OC2H5)3]3 (Rh-CNR3) was prepared in the reaction of [Rh(COD)Cl]2 (COD ) cyclooctadiene) with 6 equiv of CN(CH2)3Si(OC2H5)3 according to a procedure used for the preparation of RhCl[CN(2,6-xylyl)]3. The toluene solution containing RhCl(CO)[CN(CH2)3Si(OC2H5)3]2 or RhCl[CN(CH2)3Si(OC2H5)3]3 was refluxed with the silica-supported palladium catalyst Pd-SiO2 (Pd, 10 wt %) for 4 h. After filtration, the solid was washed with toluene and then dried in vacuum at room temperature. The resulting tethered catalysts, Rh-CNR2/Pd-SiO2 (Rh content, 1.10 wt %) and Rh-CNR3/PdSiO2 (Rh content, 1.35 wt %), gave IR spectra (DRIFTS) with ν(CN-) and ν(CO) bands (2197 (s) and 2017 (s) cm-1 for RhCNR2/Pd-SiO2; 2176 (s) and 2124 (w) cm-1 for Rh-CNR3/PdSiO2) that are very similar in position and relative intensity to those of the untethered Rh-CNR2 and Rh-CNR3 complexes,4,8 which indicates that the complexes retain their structures after being tethered to the Pd-SiO2 surface. The rates of hydrogenation (Table 1) of toluene to methylcyclohexane at 40 °C while being stirred under 1 atm of H2 in the presence of the TCSM catalysts or the separate homogeneous and heterogeneous catalysts were determined by following the rate of H2 uptake. The catalysts are active from the outset but the TOF (turnover frequency) values increase to a maximum value of 4.8 for Rh-CNR2/Pd-SiO2 after 1 h and to 5.5 for RhCNR3/Pd-SiO2 after 6.5 h. After several hours at the maximum TOF levels, the activities decrease slightly. From the data in Table 1, it can be seen that the Rh-CNR2/Pd-SiO2 catalyst activity (as measured by the maximum TOF, turnover number (TO), or H2 uptake) is at least 7 times greater than that of the simple heterogeneous SiO2-supported Pd (Pd-SiO2), the RhCNR2 complex tethered to just SiO2(Rh-CNR2/SiO2), just the ligand (CN(CH2)3Si(OC2H5)3) tethered to Pd-SiO2(CNR/PdSiO2), or the homogeneous catalyst (Rh-CNR2) even with relatively large amounts of Rh (20 μmol) as compared with 6.3 μmol in Rh-CNR2/Pd-SiO2. Similarly, Rh-CNR3/Pd-SiO2 is at least 9 times more active than Pd-SiO2, homogeneous Rh-CNR3, tethered Rh-CNR3/SiO2, or CNR/Pd-SiO2. The most active TCSM catalyst, Rh-CNR3/Pd-SiO2, has a maximum turnover frequency of 5.5 mol H2/(mol of Rh min) and a turnover number (1) (a) Hartley, F. R. Supported Metal Catalysts; Reidel: Dordrecht, The Netherlands, 1985. (b) Iwasaka, Y. Tailored Metal Catalysts; Reidel: Tokyo, 1986. (c) Cornils, B.; Hermann, W. A. Applied Homogeneous Catalysis with Organometallic Compounds; VCH: Weinheim, 1996; p 351. (2) (a) Blumel, J. Inorg. Chem. 1994, 33, 5050. (b) Capka, M.; Czakova, M.; Wlodzimierz, U.; Schubert, U. J. Mol. Catal. 1992, 74, 335. (c) Allum, K. G.; Hancock, R. D.; Howell, I. V.; McKenzie, S.; Pitkethly, R. C.; Robinson, P. J. J. Organomet. Chem. 1975, 87, 203. (d) Capka, M.; Hetflejs, J. Collect. Czech. Chem. Commun. 1974, 39, 154. (e) Pugin, B. J. Mol. Catal. A: Chem. 1996, 107, 273. (f) Czakova, M.; Capka, M. J. Mol. Catal. 1981, 11, 313. (3) (a) Pajonk, G. M.; Teichner, S. J.; Germain, J. E. SpilloVer of Adsorbed Species; Elsevier: Amsterdam, 1983. (b) Conner, W. C., Jr.; Pajonk, G. M.; Teichner, S. J. AdV. Catal. 1986, 34, 1. (c) Conner, W. C., Jr.; Falconer, J. L. Chem. ReV. 1995, 95, 759. (d) Inui, T.; Fujimoto, K.; Uchijima, T.; Masai, M. New Aspects of SpilloVer Effects in Catalysis; Elsevier: Amsterdam, 1993. (4) Selected data for RhCl(CO)[CN(CH2)3Si(OC2H5)3]2: 1H NMR (CDCl3) δ 3.82 (q, 12H, OCH2CH3), 3.67 (t, 4H, CNCH2), 1.90 (m, 4H, CH2CH2CH2), 1.21 (t, 18H, OCH2CH3), 0.75 (t, 4H, SiCH2); IR (in toluene) ν(CN-) 2192 (s) cm-1, ν(CO) 1996 (s) cm-1. (5) McCleverty, J. A.; Wilkinson, G. Inorg. Synth. 1990, 28, 84. (6) (CH3CH2O)3SiCH2CH2CH2NC was prepared from (CH3CH2O)3SiCH2CH2CH2NHCHO and Cl3COC(dO)Cl following a procedure developed for the synthesis of other alkyl isocyanides (Skorna, G.; Ugi, I. Angew. Chem., Int. Ed. Engl. 1977, 16, 259); IR (in CH2Cl2), ν(CN-) 2150 cm-1; 1H NMR (CDCl3) δ 3.81 (q, 6H, OCH2CH3), 3.38 (m, 2H, CNCH2), 1.78 (m, 2H, CH2CH2CH2), 1.20 (t, 9H, OCH2CH3), 0.72 (t, 2H, SiCH2). (7) Deeming, A. J. J. Organomet. Chem. 1979, 175, 105. (8) Selected data for RhCl[CN(CH2)3Si(OC2H5)3]3: 1H NMR (CDCl3) δ 3.82 (q, 18H, OCH2CH3), 3.58 (t, 4H, CNCH2), 3.46 (t, 2H, CNCH2), 1.85 (m, 6H, CH2CH2CH2), 1.23 (t, 27H, OCH2CH3), 0.73 (t, 6H, SiCH2); IR (in toluene) ν(CN-) 2158 (s), 2119 (m) cm-1. Anal. Calcd for C30H63O9N3Si3ClRh: C, 43.28; H, 7.63; N, 5.05. Found: C, 42.70; H, 7.37; N, 4.57. (9) Giordano, G.; Crabtree, R. H. Inorg. Synth. 1990, 28, 88. (10) Yamamoto, Y.; Yamazaki, H. J. Organomet. Chem. 1977, 140, C33. (11) Pd-SiO2 was prepared by the incipient wetness method by impregnation of SiO2 using an aqueous solution of H2PdCl4, calcining at 500 °C for 4 h and reducing with H2 at 380 °C for 4 h. Figure 1. Conceptual illustration of a TCSM catalyst consisting of a tethered homogeneous complex catalyst on a supported metal heterogeneous catalyst. 6937 J. Am. Chem. Soc. 1997, 119, 6937-6938

Characterization of supported Pd-Cu bimetallic catalysts by SEM, EDXS, AES and catalytic selectivity measurements

Abstract: Pd-Cu/γ-Al2O3 bimetallic catalysts were prepared according to different impregnation sequences of γ-Al2O3 and characterized by XRD, SEM, EDXS and AES. The catalysts were tested for the selective hydrogenation of aqueous nitrate solutions to nitrogen. The reaction selectivity was found to be dependent on the catalyst preparation procedures, which affect the spatial distribution of metallic copper and palladium phases. A catalyst prepared by impregnating γ -Al2O3 with copper followed by palladium gives higher selectivity to nitrogen than a catalyst prepared by impregnating the support with palladium followed by copper. The AES examination shows that in the catalyst exhibiting a higher nitrogen production yield, a reaction zone for the liquid-phase nitrate reduction is located in the interior of particles and covered by a layer of Pd atoms.