Novel embedded Pd@CeO2 catalysts: a way to active and stable catalysts

1-wt% Pd-CeO2 catalysts were prepared by co-precipitation of Pd nanoparticles with ceria (Pd@CeO2-CP), by a microemulsion procedure (Pd@CeO2-ME), and by normal impregnation of Pd salts (Pd/CeO2-IMP) in order to test the concept that Pd-CeO2 catalysts could be more stable for the water-gas-shift (WGS) reaction when the Pd is embedded in CeO2. Initial WGS rates measured at 250 degrees C were similar for the Pd@CeO2-CP and Pd/CeO2-IMP, indicating that Pd was accessible for gas-phase reactions on both catalysts. Pd@CeO2-CP exhibited better stability for WGS than did Pd/CeO2-IMP but exposure to the WGS environment at 400 degrees C still caused a decrease in activity. Physical characterization of the Pd@CeO2-ME implied that the core-shell nanoparticles underwent condensation that resulted in a low surface area and poor Pd accessibility. However, the Pd@CeO2-ME sample exhibited good stability for WGS, suggesting that more effective encapsulation of Pd can limit the sintering of the metal phase, thus resulting in stable catalysts under high temperature reaction conditions

Electronic structure of bis(imino)pyridyl ruthenium complexes and reactivity toward alkyne coupling and insertion

This dissertation addresses three principal topics: (1) the insertion of acetylene into the Ru-Si bonds of chloro(organosilyl)ruthenium complexes, (2) the electronic structure, reactivity, and solution behavior of dimeric, formally ruthenium(I) hydride and ruthenium(0) complexes, and (3) the reactivity of alkynes with low-valent ruthenium complexes bearing bis(imino)pyridyl, [N3] ligands. Acetylene inserts into the Ru-Si bond of ruthenium silyl chloride complexes [N3]RuCl(SiMe2Cl) and [N3]RuCl(SiMeCl 2) to give β-silylvinyl complexes. A kinetic distribution of ( Z)- and (E)-β-silylvinyl products is formed initially upon reaction of acetylene with [N3]RuCl(SiMe2Cl). The thermodynamic (E)-silylvinyl product is formed by both a fast process involving either direct anti insertion or rapid Z/E isomerization from an (unobserved) intermediate form of a (Z)-β-silylvinyl complex and a slow isomerization of the observed kinetic isomer. Experimental and computational evidence indicates that C=C bond rotation via a zwitterionic transition state with Ru=C character is an accessible pathway for Z to E isomerization of the β-silylvinyl complexes. Dimeric, formally Ru(0) complexes {[N3]Ru}2(μ-η 1:η1-N2) react with H2 to generate the formally Ru(I) hydride complexes {[N3]Ru(H)} 2(μ-η1:η1-N2). Calculated and experimental ligand metrical parameters for both the Ru(0) and Ru(I) complexes, as well as DFT calculations, are consistent with one-electron reduction of the bis(imino)pyridine ligand. The resultant metalloradical for the formally Ru(0) dimers may in part explain the propensity these complexes to react with H2 to add only a single H-atom to each metal center. Crossover experiments demonstrate that dimeric Ru(I) and Ru(0) split into monomers and recombine, and confirm that the Ru(0) complexes remain dimeric in solution and that facile hydride transfer occurs between Ru(0) and Ru(I) hydride monomers. The reaction of the low-valent complex [κ2-N 3]Ru(η6-MeC6H5) with acetylene generates the formally zerovalent ruthenium acetylene complex [N3]Ru(C 2H2), which catalyzes cyclotrimerization of acetylene to benzene. Spectroscopic and structural data for [N3]Ru(C2H 2) are consistent with significant participation of the acetylenic orbitals orthogonal to the RuC2 plane (π⊥) in metal-ligand bonding. Reaction of [κ2-N3]Ru(η 6-MeC6H5) with diphenylacetylene generates the metallacyclopentadiene complex [N3]Ru(1,2,3,4-tetraphenylruthenacyclopenta-2,4-diene), an analogue for cyclotrimerization and linear coupling intermediates

Electronic structure of bis(imino)pyridyl ruthenium complexes and reactivity toward alkyne coupling and insertion

This dissertation addresses three principal topics: (1) the insertion of acetylene into the Ru-Si bonds of chloro(organosilyl)ruthenium complexes, (2) the electronic structure, reactivity, and solution behavior of dimeric, formally ruthenium(I) hydride and ruthenium(0) complexes, and (3) the reactivity of alkynes with low-valent ruthenium complexes bearing bis(imino)pyridyl, [N3] ligands. Acetylene inserts into the Ru-Si bond of ruthenium silyl chloride complexes [N3]RuCl(SiMe2Cl) and [N3]RuCl(SiMeCl 2) to give β-silylvinyl complexes. A kinetic distribution of ( Z)- and (E)-β-silylvinyl products is formed initially upon reaction of acetylene with [N3]RuCl(SiMe2Cl). The thermodynamic (E)-silylvinyl product is formed by both a fast process involving either direct anti insertion or rapid Z/E isomerization from an (unobserved) intermediate form of a (Z)-β-silylvinyl complex and a slow isomerization of the observed kinetic isomer. Experimental and computational evidence indicates that C=C bond rotation via a zwitterionic transition state with Ru=C character is an accessible pathway for Z to E isomerization of the β-silylvinyl complexes. Dimeric, formally Ru(0) complexes {[N3]Ru}2(μ-η 1:η1-N2) react with H2 to generate the formally Ru(I) hydride complexes {[N3]Ru(H)} 2(μ-η1:η1-N2). Calculated and experimental ligand metrical parameters for both the Ru(0) and Ru(I) complexes, as well as DFT calculations, are consistent with one-electron reduction of the bis(imino)pyridine ligand. The resultant metalloradical for the formally Ru(0) dimers may in part explain the propensity these complexes to react with H2 to add only a single H-atom to each metal center. Crossover experiments demonstrate that dimeric Ru(I) and Ru(0) split into monomers and recombine, and confirm that the Ru(0) complexes remain dimeric in solution and that facile hydride transfer occurs between Ru(0) and Ru(I) hydride monomers. The reaction of the low-valent complex [κ2-N 3]Ru(η6-MeC6H5) with acetylene generates the formally zerovalent ruthenium acetylene complex [N3]Ru(C 2H2), which catalyzes cyclotrimerization of acetylene to benzene. Spectroscopic and structural data for [N3]Ru(C2H 2) are consistent with significant participation of the acetylenic orbitals orthogonal to the RuC2 plane (π⊥) in metal-ligand bonding. Reaction of [κ2-N3]Ru(η 6-MeC6H5) with diphenylacetylene generates the metallacyclopentadiene complex [N3]Ru(1,2,3,4-tetraphenylruthenacyclopenta-2,4-diene), an analogue for cyclotrimerization and linear coupling intermediates

Magnetism and EPR Studies of Binuclear Ruthenium Hydride Binuclear Species Bearing Redox-Active Ligands

The binuclear complex {[N₃]­Ru­(H)}₂­(μ-η¹:η¹-N₂) ([N₃] = 2,6-(ArylNCMe)₂­C₅H₃N and Aryl = mesityl or xylyl) contains two formally Ru­(I), d⁷ centers linked by a bridging dinitrogen ligand, although the odd electrons are substantially delocalized onto the redox non-innocent pincer ligands. The complex exhibits paramagnetic behavior in solution, but is diamagnetic in the solid state. This difference is attributed to intermolecular “π-stacking” observed in the solid state, which essentially couples unpaired electrons on each half of the complex to form delocalized 22-center-2-electron covalent bonds. Introduction of a bulky <i>t-</i>butyl group on the ligand pyridine ring prevents this intermolecular association and allows further investigation of the magnetic behavior and electronic structure of the binuclear species. The interaction of the unpaired electrons in the two halves of the complex has been probed with magnetic susceptibility and perpendicular and parallel mode EPR measurements, revealing a weakly antiferromagnetically coupled system with a thermally accessible triplet excited state. In addition, the mixed valent, <i>S</i> = ¹/₂, {[N₃]­Ru­(H)}­(μ-η¹:η¹-N₂)­{[N₃]­Ru} system has also been observed via perpendicular mode EPR and was used to quantify the growth of the thermally accessible triplet state of the dihydride complex using parallel mode EPR

Heterogeneous Catalysts Need Not Be so “Heterogeneous”: Monodisperse Pt Nanocrystals by Combining Shape-Controlled Synthesis and Purification by Colloidal Recrystallization

Well-defined surfaces of Pt have been extensively studied for various catalytic processes. However, industrial catalysts are mostly composed of fine particles (e.g., nanocrystals), due to the desire for a high surface to volume ratio. Therefore, it is very important to explore and understand the catalytic processes both at nanoscale and on extended surfaces. In this report, a general synthetic method is described to prepare Pt nanocrystals with various morphologies. The synthesized Pt nanocrystals are further purified by exploiting the “self-cleaning” effect which results from the “colloidal recrystallization” of Pt supercrystals. The resulting high-purity nanocrystals enable the direct comparison of the reactivity of the {111} and {100} facets for important catalytic reactions. With these high-purity Pt nanocrystals, we have made several observations: Pt octahedra show higher poisoning tolerance in the electrooxidation of formic acid than Pt cubes; the oxidation of CO on Pt nanocrystals is structure insensitive when the partial pressure ratio <i>p</i>_O2/<i>p</i>_CO is close to or less than 0.5, while it is structure sensitive in the O₂-rich environment; Pt octahedra have a lower activation energy than Pt cubes when catalyzing the electron transfer reaction between hexacyanoferrate (III) and thiosulfate ions. Through electrocatalysis, gas-phase-catalysis of CO oxidation, and a liquid-phase-catalysis of electron transfer reaction, we demonstrate that high quality Pt nanocrystals which have {111} and {100} facets selectively expose are ideal model materials to study catalysis at nanoscale

Quantifying global soil carbon losses in response to warming

The majority of the Earth's terrestrial carbon is stored in the soil. If anthropogenic warming stimulates the loss of this carbon to the atmosphere, it could drive further planetary warming. Despite evidence that warming enhances carbon fluxes to and from the soil, the net global balance between these responses remains uncertain. Here we present a comprehensive analysis of warming-induced changes in soil carbon stocks by assembling data from 49 field experiments located across North America, Europe and Asia. We find that the effects of warming are contingent on the size of the initial soil carbon stock, with considerable losses occurring in high-latitude areas. By extrapolating this empirical relationship to the global scale, we provide estimates of soil carbon sensitivity to warming that may help to constrain Earth system model projections. Our empirical relationship suggests that global soil carbon stocks in the upper soil horizons will fall by 30 ± 30 petagrams of carbon to 203 ± 161 petagrams of carbon under one degree of warming, depending on the rate at which the effects of warming are realized. Under the conservative assumption that the response of soil carbon to warming occurs within a year, a business-as-usual climate scenario would drive the loss of 55 ± 50 petagrams of carbon from the upper soil horizons by 2050. This value is around 12-17 per cent of the expected anthropogenic emissions over this period. Despite the considerable uncertainty in our estimates, the direction of the global soil carbon response is consistent across all scenarios. This provides strong empirical support for the idea that rising temperatures will stimulate the net loss of soil carbon to the atmosphere, driving a positive land carbon-climate feedback that could accelerate climate change