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.

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.

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.

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.