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The local structures of rhodium complexes derived from the immobilization of Wilkinson’s complex, RhCl(PPh3)3, on SBA-15 silica functionalized with primary–amine, secondary–amine, or diphenylphosphine groups within the mesoporous channels were characterized by a series of techniques including XRD, HR-TEM, multinuclear (13C/29Si/31P) solid-state NMR, 2D 31P{1H} HETCOR NMR, XPS, and Rh K-edge EXAFS. Immobilization of RhCl(PPh3)3 through covalent bond formation with different functional groups grafted to the silica surface lead to variations in the local structure of the Rh center that has important implications for catalysis. The immobilized Rh complexes demonstrated high activity for the addition of alkynes with thiols (hydrothiolation) or sulfonic acids (hydrosulfonation) with excellent regio- and stereoselectivity under mild reaction conditions. This work demonstrates the elucidation of the local structure of the immobilized Rh complexes requires a complimentary multi-technique characterization approach that probes both the metal center itself and surrounding ligands.

Silica-supported Pd and PdZn nanoparticles of a similar size were evaluated for neopentane hydrogenolysis/isomerization and propane hydrogenolysis/dehydrogenation. Monometallic Pd showed high neopentane hydrogenolysis selectivity. Addition of small amounts of Zn to Pd lead Pd–Zn scatters in the EXAFS spectrum and an increase in the linear bonded CO by IR. In addition, the neopentane turnover rate decreased by nearly 10 times with little change in the selectivity. Increasing amounts of Zn lead to greater Pd–Zn interactions, higher linear-to-bridging CO ratios by IR and complete loss of neopentane conversion. Pd NPs also had high selectivity for propane hydrogenolysis and thus were poorly selective for propylene. The PdZn bimetallic catalysts, however, were able to preferentially catalyze dehydrogenation, were not active for propane hydrogenolysis, and thus were highly selective for propylene formation. The decrease in hydrogenolysis selectivity was attributed to the isolation of active Pd atoms by inactive metallic Zn, demonstrating that hydrogenolysis requires a particular reactive ensemble whereas propane dehydrogenation does not.

Isotopic labeling and density functional theory (DFT) were used to determine the mechanism for acetylene hydrogenation and oligomerization on well-defined intermetallic nickel–zinc catalysts. The primary benefit of adding zinc to nickel is a reduction in oligomeric species formation which leads to higher ethylene selectivity. The production of ethane is not highly dependent on zinc content; therefore, ethane production is not a good descriptor of ethylene selectivity since acetylene may also be converted to higher molecular weight products. Analysis using DFT and Langmuir–Hinshelwood kinetics shows that the large decrease in the adsorption energy of acetylene on intermetallic NiZn compared to pure Ni is responsible for the observed increase in ethylene selectivity. The adsorption energy of acetylene appears to be a descriptor for carbon–carbon bond formation since a high adsorption energy leads to an increased coverage of C2 species and an increased rate of carbon–carbon bond formation.

We present a thermodynamic study of the chelate effect for a series of P–P, N–N, and P–N bidentate ligands binding to PdCl2(MeCN)2 in acetonitrile (MeCN), using isothermal titration calorimetry (ITC) to measure the solution-phase binding thermodynamics. The chelate effect is considered to be an entropic effect, that is, the enthalpic contributions attributed to binding for comparable monodentate and bidentate ligands are nearly identical, meaning that changes in ΔG are due to favorable changes in ΔS upon displacement of bound solvent molecules. However, our results demonstrate that enhanced enthalpic contributions (i.e. large and exothermic) control the stability of the chelated complexes and generally are accompanied by large entropic penalties. We discuss the contribution of solvent reorganization and its role in enthalpy–entropy compensation for the binding equilibria studied.

The influence of particle size on the poisoning of ethylene hydrogenation by CO was studied over a series of catalysts composed of nearly monodisperse Pt nanoparticles (1.7–7.1 nm) encapsulated in mesoporous silica (SBA-15). The turnover frequency at 403 K in the presence of 0.5 Torr CO was ∼2 × 10−2 s−1 (compared with ∼102 s−1 in the absence of CO). The apparent activation energy in the absence and presence of 0.2 Torr CO was ∼10 and 20 kcal mol−1, respectively. The pressure dependency changes significantly in the presence of CO; reaction orders in hydrogen were 1/2 in the presence of CO at 403 K and noncompetitive with regard to co-adsorption with C2H4. In the absence of CO at similar temperatures, H2 adsorption was primarily irreversible (first-order dependence), and H2 and C2H4 compete for the same sites. Ethylene orders at 403 K were first order in the presence of 0.2 Torr CO and remained unity with increasing CO pressure. At similar reaction conditions in the absence of CO, ethylene had an inhibitory effect (negative reaction order) on the overall hydrogenation reaction. The change in C2H4 and H2 kinetics suggests strong competitive adsorption between C2H4 and CO for the same type of site, whereas H2 apparently adsorbs on distinct surface sites due either to steric hindrance or H2-induced CO desorption. Incorporation of a quasi-equilibrated CO adsorption step into a noncompetitive Langmuir–Hinshelwood mechanism predicts the experimentally observed pressure dependencies and a doubling of the apparent activation energy. Hydrogenation of ethylene in the presence of 1 Torr CO was examined under reaction conditions at 403 K by infrared spectroscopy; the only surface species identified under reaction conditions was linear-bound CO. The hydrogenation of ethylene on clean Pt catalysts was structure-insensitive and remains insensitive in the presence of CO; rates decreased only by a factor of two with increasing particle size.

The influence of substituent effects and CO poisoning were examined during the hydrogenation/dehydrogenation of cycloalkenes (cyclohexene and 1- and 4-methylcyclohexene) on a Pt(111) single crystal. Reaction rates for both hydrogenation and dehydrogenation decreased when a methyl group was added to the cycloalkene ring. The location of a methyl group relative to the C C double bond was influential in the overall kinetics for both reaction pathways. All cycloalkenes demonstrated “bend-over” Arrhenius behavior, after which rates for hydrogenation and dehydrogenation decreased with increasing temperature (inverse Arrhenius behavior). This is explained in terms of a change in surface coverage of the reactive cycloalkene. The potential importance of hydrogen effects is discussed. Introduction of CO in the Torr pressure range (0.015 Torr) led to a decrease in turnover frequency and increase in apparent activation energy for both the hydrogenation and dehydrogenation of all cycloalkenes. Sum frequency generation (SFG) surface vibrational spectroscopy revealed that upon adsorption, the three cycloalkenes form a surface species with similar molecular structure. SFG results under reaction conditions in the presence of CO demonstrated that the cycloalkene coverage is low on a CO-saturated surface. Substituted cyclohexenes were more sensitive than cyclohexene to the presence of adsorbed CO, with larger increases in the apparent activation energy, especially in the case of dehydrogenation. A qualitative explanation for the changes in activity with temperature and the increase in apparent activation energy for cycloalkene hydrogenation/dehydrogenation in the presence of CO is presented from a thermodynamic and kinetic perspective.

The kinetics of isopropyl alcohol (IPA) dehydrogenation over carbon-supported Pt and three bimetallic Cu Pt catalysts were examined and compared with previous results for carbon-supported Cu. Adsorption of H 2 and CO, dissociative chemisorption of N 2 O, and the titration of adsorbed O atoms by H 2 and CO were used to determine surface compositions and metal dispersion. Monometallic platinum catalysts had a higher specific activity than monometallic Cu catalysts and the bimetallic catalysts; for example, at 448 K and 14 Torr IPA, for an activated carbon heat treated at 1223 K as the support, the turnover frequency (TOF) on the Pt catalyst was 0.11 s −1 , the TOF on the Cu catalyst was 0.020 s −1 , and the TOFs on the three bimetallic catalysts were between 0.006 and 0.043 s −1 . Reaction orders were typically between 0 and 1 2 for IPA and slightly negative for acetone and H 2 . Increasing the Cu loading reduced the number of surface Pt atoms, and the kinetic behavior became similar to that of monometallic copper catalysts. The apparent activation energy for acetone production from IPA on Pt/C was 6.8 kcal mol −1 , compared with 21 kcal mol −1 for the monometallic Cu/C catalysts. Apparent activation energies for the Cu Pt/C catalysts (7–9 kcal mol −1 ) were comparable to those of Pt/C at lower temperatures and decreased further to ∼ 3 kcal mol −1 above 443 K. This decrease is not due to diffusional limitations and can be explained by a decrease in the surface coverage of IPA. A Langmuir–Hinshelwood model invoking cleavage of the alcohol O H bond to form a surface isopropoxide species as the rate-determining step fit the data well and accounted for the shift in the apparent activation energy. Physically meaningful values of enthalpies and entropies of adsorption were obtained from the fitting parameters contained in the L-H rate expression. Additional evidence for adsorbed IPA and acetone was provided by infrared spectra of the catalysts obtained under reaction conditions.

For accurate catalyst comparisons and kinetic modeling, data free of mass and heat transfer effects must be acquired over a range of temperature and reactant concentrations using well-characterized heterogeneous catalysts. For semibatch reactors operated at constant pressure, selectivity vs conversion profiles at several temperatures, preferably obtained with catalysts reduced in situ, are required to allow complicated reaction networks to be defined. Vapor-phase isopropyl alcohol (IPA) dehydrogenation over carbon-supported Cu catalysts in a differential fixed-bed reactor was studied and compared to UHP Cu powder and a Cu chromite catalyst. Low dispersions of Cu (ca. 0.02–0.17) were obtained in all these catalysts. Cu dispersed on an activated carbon heat-treated at 1223 K had the highest turnover frequency (0.052 s−1 at 448 K) of all the Cu/C catalysts, more than double that on an industrial Cu chromite catalyst; thus the rate with this 5.0% Cu/C catalyst (6.0 μmol s−1 g−1) was close to that of the chromite catalyst containing 41% Cu (10.6 μmol s−1 g−1). The steady-state selectivity to acetone was 100% for all catalysts except those containing the nitric-acid-treated carbon. In the absence of Cu, the nitric-acid-treated carbon produced propylene under steady-state reaction conditions, and reduction at 573 K decreased activity compared to reduction at 423 K. This dehydration reaction is associated with the presence of oxygen-containing acidic groups on the C surface. The apparent activation energy for acetone formation was typically near 20 kcal mol−1 for all the Cu/C catalysts and the Cu powder, but it was near 12 kcal mol−1 for the Cu chromite catalyst. A DRIFT spectrum under reaction conditions indicated the presence of an isopropoxide species and molecularly adsorbed IPA on the surface. A Langmuir–Hinshelwood mechanism, which assumed removal of the first hydrogen atom as the rate-determining step and incorporated adsorbed IPA, hydrogen, acetone, and a surface isopropoxide species into the site balance, fit the kinetic data well and gave physically meaningful values for the enthalpies and entropies of adsorption.

Reaction steps for the reaction of hydrodechlorination on Pd were investigated by means of isotope-exchange experiments. The reversibility of bond scission for the two C–Cl bonds in CF3CFCl2 was investigated by following 37Cl enrichment in the reactant and products. No enrichment of 37Cl was observed in the reactant CF3CFCl2 but the product CF3CHFCl was enriched. Thus, the scission of the first C–Cl bond is irreversible; once the reactant loses the first chlorine it desorbs only as a product. However, after the first C–Cl bond is broken, the second chlorine may be exchanged with the pool of surface chlorine. Isotope exchange experiments between D2 and HCl during hydrodechlorination of CF3CFCl2 showed that the forward rate and reverse rate for the overall reaction H2+2Cl*=2HCl+2* were nearly 400 times faster than the overall hydrodechlorination reaction rate. Thus, this experiment confirms that this step is in quasi-equilibrium.

Catalyst T RED (K) CO uptakea (μmol g−1) COT/CuT COirr/CuT ‘O’ uptakeb (μmol g−1) Cu0 disp. dc (nm) Total Irreversible 4.96% Cu/AC–HNO3 423 170.0 62.0 0.218 0.079 12.1 0.031 35.5 473 35.4 6.1 0.045 0.008 18.3 0.047 23.5 573 19.6 0.0 0.025 0.0 16.5 0.042 26.0 4.89% Cu/AC–ASIS 423 54.0 0.0 0.070 0.0 0.0 0.0 0.0 473 54.6 0.0 0.071 0.0 8.8 0.023 48.1 573 51.6 0.0 0.067 0.0 16.6 0.043 25.5 5.01% Cu/AC–HTT–H2 423 79.4 5.7 0.101 0.007 54.0 0.137 8.0 473 83.5 5.6 0.106 0.007 58.9 0.149 7.4 573 59.1 0.0 0.075 0.0 67.3 0.171 6.4 0.98% Cu/AC–HTT–H2 473 52.5 1.1 0.340 0.007 11.1 0.144 7.6 573 53.2 0.0 0.345 0.0 8.9 0.115 9.5 0.63% Cu/GF-IE 573 0.0 0.0 0.0 0.0 0.9d 0.018 60.6 Cu powder 573 – – – – 1.8d 2.3 × 10−4 ∼4800 Cu chromite 573 55.0 18.3 0.009 0.003 181.5 0.056 19.6 a Volumetric uptakes at 75 Torr corrected for irreversible uptake on support. b Uptakes at 363 K and 75 Torr N2O determined gravimetrically. c Cu crystallite size based on d = 1.1/(2Oad/CuT). d Determined volumetrically.