Catalytic conversion of ethene to butadiene or hydrogenation to ethane on HY zeolite-supported rhodium complexes: Cooperative support/Rh-center route.

Rh(C2H4)2 species grafted on the HY zeolite framework significantly enhance the activation of H2 that reacts with C2H4 ligands to form C2H6. While in this case, the simultaneous activation of C2H4 and H2 and the reaction between these species on zeolite-loaded Rh cations is a legitimate hydrogenation pathway yielding C2H6, the results obtained for Rh(CO)(C2H4)/HY materials exposed to H2 convincingly show that the support-assisted C2H4 hydrogenation pathway also exists. This additional and previously unrecognized hydrogenation pathway couples with the conversion of C2H4 ligands on Rh sites and contributes significantly to the overall hydrogenation activity. This pathway does not require simultaneous activation of reactants on the same metal center and, therefore, is mechanistically different from hydrogenation chemistry exhibited by molecular organometallic complexes. We also demonstrate that the conversion of zeolite-supported Rh(CO)2 complexes into Rh(CO)(C2H4) species under ambient conditions is not a simple CO/C2H4 ligand exchange reaction on Rh sites, as this process also involves the conversion of C2H4 into C4 hydrocarbons, among which 1,3-butadiene is the main product formed with the initial selectivity exceeding 98% and the turnover frequency of 8.9 × 10-3 s-1. Thus, the primary role of zeolite-supported Rh species is not limited to the activation of H2, as these species significantly accelerate the formation of the C4 hydrocarbons from C2H4 even without the presence of H2 in the feed. Using periodic density functional theory calculations, we examined several catalytic pathways that can lead to the conversion of C2H4 into 1,3-butadiene over these materials and identified the reaction route via intermediate formation of rhodacyclopentane.

Dendrimer-mediated synthesis of supported rhodium nanoparticles with controlled size: effect of pH and dialysis.

Rh-dendrimer nanocomposites were synthesized in solution under different conditions and were subsequently used as precursors for the preparation of ZrO2-supported Rh nanoparticles. Elemental analysis, UV-vis, XPS, and STEM measurements were used to estimate the extent of the Rh-dendrimer interactions and to illustrate how the solution pH and dialysis affect the number of Rh atoms complexed with each dendrimer molecule, as well as the final size of the ZrO2-supported Rh particles. When the solution acidity was not controlled and the solution was not purified by dialysis, Rh particles with sizes in the 1-6 nm range were formed on the ZrO2 support. In contrast, the formation of nearly uniform Rh particles was observed when the synthesis was performed under controlled pH and dialysis conditions. Furthermore, the size of these Rh particles can be regulated by controlling the Rh/dendrimer ratio in the original solution.

EXAFS characterization of dendrimer-Pt nanocomposites used for the preparation of Pt/gamma-Al2O3 catalysts.

Pt/gamma-Al2O3 catalysts were prepared using hydroxyl-terminated generation four (G4OH) PAMAM dendrimers as the templating agents and the various steps of the preparation process were monitored by extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS results indicate that, upon hydrolysis, chlorine ligands in the H(2)PtCl(6) and K(2)PtCl(4) precursors were partially replaced by aquo ligands to form [PtCl3(H2O)3]+ and [PtCl2(H2O)2] species, respectively. The results further suggest that, after interaction of such species with the dendrimer molecules, chlorine ligands from the first coordination shell of Pt were replaced by nitrogen atoms from the dendrimer interior, indicating that complexation took place. This process was accompanied by a substantial transfer of electron density from the dendrimer to platinum, indicating that the dendrimer plays the role of a ligand. Following treatment of the H(2)PtCl(6)/G4OH and K(2)PtCl(4)/G4OH complexes with NaBH4, no substantial changes were observed in the electronic or coordination environment of platinum, indicating that metal nanoparticles were not formed during this step under our experimental conditions. However, when the reduction treatment was performed with H2, the formation of extremely small platinum clusters, incorporating no more than four Pt atoms was observed. The nuclearity of these clusters depends on the length of the hydrogen treatment. These Pt species remained strongly bonded to the dendrimer. Formation of larger platinum nanoparticles, with an average diameter of approximately 10 A, was finally observed after the deposition and drying of the H(2)PtCl(6)/G4OH nanocomposites on a gamma-Al(2)O(3) surface, suggesting that the formation of such nanoparticles may be related to the collapse of the dendrimer structure. The platinum nanoparticles formed appear to have high mobility because subsequent thermal treatment in O2/H2, used to remove the dendrimer component, led to further sintering.

Improved CO oxidation activity in the presence and absence of hydrogen over cluster-derived PtFe/SiO2 catalysts.

The catalytic performance of cluster-derived PtFe/SiO(2) bimetallic catalysts for the oxidation of CO has been examined in the absence and presence of H(2) (PROX) and compared to that of Pt/SiO(2). PtFe(2)/SiO(2) and Pt(5)Fe(2)/SiO(2) samples were prepared from PtFe(2)(COD)(CO)(8) and Pt(5)Fe(2)(COD)(2)(CO)(12) organometallic cluster precursors, respectively. FTIR data indicate that both clusters can be deposited intact on the SiO(2) support. The clusters remained weakly bonded to the SiO(2) surface and could be extracted with CH(2)Cl(2) without any significant changes in their structure. Subsequent heating in H(2) led to complete decarbonylation of the supported clusters at approximately 350 degrees C and the formation of Pt-Fe nanoparticles with sizes in the 1-2 nm range, as indicated by HRTEM imaging. A few larger nanoparticles enriched in Pt were also observed, indicating that a small fraction of the deposited clusters were segregated to the individual components following the hydrogen treatment. A higher degree of metal dispersion and more homogeneous mixing of the two metals were observed during HRTEM/XEDS analysis with the cluster-derived samples, as compared to a PtFe/SiO(2) catalyst prepared through a conventional impregnation route. Furthermore, the cluster-derived PtFe(2)/SiO(2) and Pt(5)Fe(2)/SiO(2) samples were more active than Pt/SiO(2) and the conventionally prepared PtFe/SiO(2) sample for the oxidation of CO in air. However, substantial deactivation was also observed, indicating that the properties of the Pt-Fe bimetallic sites in the cluster-derived samples were altered by exposure to the reactants. The Pt(5)Fe(2)/SiO(2) sample was also more active than Pt/SiO(2) for PROX with a selectivity of approximately 92% at 50 degrees C. In this case, the deactivation with time on stream was substantially slower, indicating that the highly reducing environment under the PROX conditions helps maintain the properties of the active Pt-Fe bimetallic sites.

Particle size control in dendrimer-derived supported ruthenium catalysts.

A high-resolution transmission electron microscopy (HRTEM) investigation of a family of supported Ru catalysts prepared from Ru hydroxyl-terminated poly(amidoamine) dendrimer-metal nanocomposite (DMN) precursors has been conducted. Ru particle sizes observed following deposition of DMNs on a HRTEM grid can be controlled within a 0.9-1.4 nm range depending on the metal-to-dendrimer molar ratio. The average particle size in this case correlates well with the theoretically predicted particle size from the molar loading of Ru in the dendrimer. Upon impregnation of Ru-DMNs on Al(2)O(3) and subsequent thermal removal of the dendrimer via reduction at 300 degrees C, significant sintering of the Ru particles was observed. Nevertheless, the resulting supported Ru particles maintained a narrow particle size distribution and average particle size below 2.5 nm. These particle sizes no longer correlate with the metal-to-dendrimer molar ratio but do correlate with the metal-to-dendrimer weight ratio, suggesting that the dendrimer may be acting as a "sintering-control" agent on the catalyst surface. This process is not affected by the surface area of the support, since almost identical particle size distributions were obtained on three different commercial supports.

Low temperature oxidation of CO over cluster-derived platinum-gold catalysts.

The structural and catalytic properties of SiO2- and TiO2 -supported Pt-Au bimetallic catalysts prepared by coimpregnation were compared with those of samples of similar composition synthesized from a Pt2Au4(C{triple bond}CBut)8 cluster precursor. The smallest metal particles were formed when the bimetallic cluster was used as a precursor and TiO2 as the support. FTIR data indicate that highly dispersed Au crystallites in these samples, presumably located in close proximity to Pt, are capable of linearly coordinating CO molecules with a characteristic vibration observed at 2111 cm(-1). The cluster-derived Pt2Au4/TiO2 samples were the only ones exhibiting low-temperature CO oxidation activity, indicating that both the high dispersion of Au and the nature of the support are important factors affecting the catalytic activity for this system.

FTIR studies of CO adsorption on Al2O3- and SiO2-supported Ru catalysts.

The adsorption of CO on Al2O3- and SiO2-supported Ru catalysts has been investigated through FTIR spectroscopy. Deconvolution of the spectra obtained reveals the presence of 11 distinct bands in the case of Ru/Al2O3 and 10 bands in the case of Ru/SiO2, which were assigned to different carbonyl species adsorbed on reduced as well as partially oxidized Ru sites. Although most of these bands on both supports are similar, they exhibit substantial differences in terms of stability. In general, the analogous CO species on Ru/Al2O3 are adsorbed stronger than those on Ru/SiO2, with the most stable species observed being a dicarbonyl adsorbed on metallic Ru (i.e., Ru0(CO)2). Following sintering of the Ru, the ratio of multicarbonyl to monocarbonyl adsorption is reduced substantially because of the lack of isolated sites or small Ru clusters that enable the formation of multicarbonyl species via oxidative disruption. Finally, in the presence of O2, the main features observed correspond to monocarbonyl, dicarbonyl, and tricarbonyl species adsorbed on partially oxidized Run+. The intensities of all bands decrease drastically at temperatures above 210 degrees C because of the onset of CO oxidation, which results in substantially reduced surface coverage.

Effects of reduction temperature and metal-support interactions on the catalytic activity of Pt/gamma-Al2O3 and Pt/TiO2 for the oxidation of CO in the presence and absence of H2.

TiO2- and gamma-Al2O3-supported Pt catalysts were characterized by HRTEM, XPS, EXAFS, and in situ FTIR spectroscopy after activation at various conditions, and their catalytic properties were examined for the oxidation of CO in the absence and presence of H2 (PROX). When gamma-Al2O3 was used as the support, the catalytic, electronic, and structural properties of the Pt particles formed were not affected substantially by the pretreatment conditions. In contrast, the surface properties and catalytic activity of Pt/TiO2 were strongly influenced by the pretreatment conditions. In this case, an increase in the reduction temperature led to higher electron density on Pt, altering its chemisorptive properties, weakening the Pt-CO bonds, and increasing its activity for the oxidation of CO. The in situ FTIR data suggest that both the terminal and bridging CO species adsorbed on fully reduced Pt are active for this reaction. The high activity of Pt/TiO2 for the oxidation of CO can also be attributed to the ability of TiO2 to provide or stabilize highly reactive oxygen species at the metal-support interface. However, such species appear to be more reactive toward H2 than CO. Consequently, Pt/TiO2 shows substantially lower selectivities toward CO oxidation under PROX conditions than Pt/gamma-Al2O3.

Characterization of CuO supported on tetragonal ZrO2 catalysts for N2O decomposition to N2.

A series of tetragonal zirconia-supported CuO oxide catalysts with various CuO loadings were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), ultraviolet and visible diffuse reflectance spectroscopy (UV/vis-DRS), and temperature-programmed reduction (TPR) measurements. The results indicate that the dispersion capacity of copper oxide on this support is approximately 8.6 Cu(2+) ions/nm(2) ZrO(2). The state of the resulting supported copper species depends on the CuO loading. At CuO loadings below the dispersion capacity, only highly dispersed copper ion species are present on the surface of t-ZrO(2). In particular, isolated Cu ions are the predominant species at low loadings. In contrast, pair Cu ions become the most abundant species at loadings near the dispersion capacity. It has been proposed that these dispersed CuO (isolated and paired Cu ions) have a symmetric 5-fold-oxygen-coordination symmetry (C(3)(v) symmetry) and can be described as distorted octahedra with a missing corner or a trigonal bipyramids. Finally, at CuO loadings above the dispersion capacity the formation of crystalline CuO is observed. TPR results reveal that the dispersed Cu ion species have a different reducibility from CuO crystallites, presumably due to strong interactions between these species and the t-ZrO(2) support. The catalytic activity of these CuO/t-ZrO(2) catalysts for the decomposition of N(2)O can also be directly correlated to CuO dispersion, with paired Cu ions being the most active species for this reaction.

Effects of adsorbates on supported platinum and iridium clusters: Characterization in reactive atmospheres and during alkene hydrogenation catalysis by X-ray absorption spectroscopy.

MgO-, SiO2-, and gamma-Al2O3-supported platinum clusters and particles (with average diameters ranging from 11 to 45 A) and zeolite-supported Ir4 clusters (approximately 6 A in diameter) were characterized by extended X-ray absorption fine structure spectroscopy in the presence of H2, O2, ethene, propene, and ethane, as well as under conditions of alkene hydrogenation catalysis. The results indicate that under various atmospheres, the presence of adsorbates affects the smaller platinum clusters (11 A) on gamma-Al2O3 more substantially than the larger platinum particles (i.e., those greater than approximately 21 A in average diameter) on MgO or SiO2. When Pt/gamma-Al2O3 was exposed to H2, the platinum morphology did not change, although the Pt-Pt bond distance increased. In contrast, when the same sample was exposed to O2, complete oxidative fragmentation took place. This processes was reversed following subsequent treatment with H2. Exposure to alkenes changed both the morphology and electron density (as indicated by X-ray absorption near-edge spectra) of the gamma-Al2O3-supported platinum clusters. Under conditions of alkene hydrogenation catalysis at room temperature, the electronic properties and the structure of the platinum clusters were found to depend on the reactant composition and the nature of molecules involved in the reaction process. The effects of the reactant gases on the smaller iridium clusters (Ir4) were substantially less pronounced, apparently as a consequence of the extremely small number of atoms in each iridium cluster.

Quantitative FT-IRRAS spectroscopic studies of the interaction of avidin with biotin on functionalized quartz surfaces.

The interaction of avidin with biotin was studied on functionalized quartz surfaces terminated with 3-aminopropyltrimethoxysilane (3-APTMS), 2,2'-(ethylenedioxy)bis(ethylenediamine) (DADOO), and fourth-generation amine-terminated polyamidoamine (G4-NH2 PAMAM) dendrimers with the use of Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS). In particular, the molecular recognition ability of these surfaces was quantified through FT-IRRAS in combination with the use of an alkyne dicobalt hexacarbonyl probe coupled with avidin. The degree of nonspecific adsorption of avidin was determined by exposure of the amine-terminated and/or biotinylated surfaces to solutions of biotin-saturated avidin. The results indicate that the biotinylated 3-APTMS layer exhibits a very low specific binding capacity for avidin (on the order of 0.15 pmol of avidin/cm2) and substantial nonspecific adsorption. Both the binding capacity and the specificity were greatly improved when the 3-APTMS layer on quartz was modified through serial chemisorption of glutaraldehyde (GA), DADOO, and/or G4-NH2 PAMAM dendrimer layers. Among these layers, the biotinylated G4-NH2 PAMAM dendrimer layer exhibited the highest capacity for avidin binding (2.02 pmol of avidin/cm2) with a specificity of approximately 90%. This effect can be attributed to the efficient packing/ordering of the binding dendrimer layer, leading to a more dense and better organized layer of biotin headgroups on the subsequent biotinylated surface.

FTIR studies of CO adsorption on Rh-Ge/Al2O3 catalysts prepared by surface redox reactions.

A series of bimetallic Al2O3-supported Rh-Ge catalysts was prepared by surface redox reactions under controlled hydrogen atmosphere. The surface properties of these catalysts were probed via in-situ FTIR spectroscopic studies of adsorbed CO and were compared to those of monometallic Rh catalysts that had undergone similar treatments. The results indicate that Ge addition results in the formation and stabilization of smaller rhodium ensembles at the expense of larger Rh0 surfaces. A charge-transfer mechanism from Ge to Rh is also inferred by the IR results for the high Ge loading samples. Air exposure of the catalysts leads to an irreversible segregation of the two metals and formation of large Rh crystallites.

FT-IRRAS spectroscopic studies of the interaction of avidin with biotinylated dendrimer surfaces.

The interaction of avidin with biotin on a functional Au surface containing fourth generation amine-terminated polyamidoamine (G4-NH(2) PAMAM) dendrimers was investigated through the use of Fourier transform infrared reflection-adsorption spectroscopy (FT-IRRAS). The first step in the fabrication of the functional surfaces used was the construction of an aldehyde-terminated self-assembled monolayer (SAM) through the treatment of Au-coated glass slides with ethanol solutions of self-synthesized 2-hydroxypentamethylene sulfide (HPMS). The as-formed aldehyde-terminated monolayer was subsequently immersed in methanol solutions of G4-NH(2) PAMAM dendrimer to obtain well-organized primary amine-terminated surfaces. Biotinylation of the amine-terminated layers thus obtained was accomplished by use of the N-succinimidyl ester of biotin. Each step of the synthetic process, as well as the performance of final surface for protein recognition was monitored by FT-IRRAS. In particular, the molecular recognition ability was examined and quantified by use of an alkyne dicobalt hexacarbonyl probe coupled with avidin. Non-specific adsorption of avidin was determined by exposure of the amine-terminated and/or biotinylated surfaces to solutions of biotin-saturated avidin. The results indicate that the biotinylated G4-NH(2) PAMAM dendrimer layers formed according to this procedure have a high capacity for binding avidin with relatively high specificity. The performance of these layers (i.e. both binding capacity and specificity) improve substantially when 6-mercapto-1-hexanol (MH) is present as a co-adsorbent during the formation of the initial aldehyde-terminated layers. This effect can be attributed to the dilution of the initial aldehyde-terminated SAM, leading to a more favorable spatial arrangement of the subsequent biotinylated surfaces.