C–C Coupling on Single-Atom-Based Heterogeneous Catalyst

Compared to homogeneous catalysis, heterogeneous catalysis allows for ready separation of products from the catalyst and thus reuse of the catalyst. C–C coupling is typically performed on a molecular catalyst which is mixed with reactants in liquid phase during catalysis. This homogeneous mixing at a molecular level in the same phase makes separation of the molecular catalyst extremely challenging and costly. Here we demonstrated that a TiO₂-based nanoparticle catalyst anchoring singly dispersed Pd atoms (Pd₁/TiO₂) is selective and highly active for more than 10 Sonogashira C–C coupling reactions (RCH + R′X → RR′; X = Br, I; R′ = aryl or vinyl). The coupling between iodobenzene and phenylacetylene on Pd₁/TiO₂ exhibits a turnover rate of 51.0 diphenylacetylene molecules per anchored Pd atom per minute at 60 °C, with a low apparent activation barrier of 28.9 kJ/mol and no cost of catalyst separation. DFT calculations suggest that the single Pd atom bonded to surface lattice oxygen atoms of TiO₂ acts as a site to dissociatively chemisorb iodobenzene to generate an intermediate phenyl, which then couples with phenylacetylenyl bound to a surface oxygen atom. This coupling of phenyl adsorbed on Pd₁ and phenylacetylenyl bound to O_ad of TiO₂ forms the product molecule, diphenylacetylene

In Situ Surface Chemistries and Catalytic Performances of Ceria Doped with Palladium, Platinum, and Rhodium in Methane Partial Oxidation for the Production of Syngas

Methane partial oxidation (MPO) chemically transforms natural gas into syngas for the production of gasoline. CeO₂ doped with transition-metal ions is one type of catalyst active for MPO. A fundamental understanding of MPO on this type of catalyst is important for the development of catalysts with high activity and selectivity for this process. CeO₂-based catalysts, including Pd-CeO₂-air, Pd-CeO₂-H₂, Pt-CeO₂-air, Pt-CeO₂-H₂, Rh-CeO₂-air, and Rh-CeO₂-H₂, were synthesized through doping noble-metal ions in the synthesis of CeO₂ nanoparticles. The catalytic activity and selectivity in the production of H₂ and CO through MPO on these ceria-based catalysts as well as their surface chemistries during catalysis were investigated. They exhibit quite different catalytic performances in MPO under identical catalytic conditions. In situ studies of their surface chemistries during catalysis, using ambient-pressure X-ray photoelectron spectroscopy (AP–XPS), revealed quite different surface chemistries during catalysis. Correlations between the catalytic performances of these catalysts and their corresponding surface chemistries during catalysis were developed. Differing from the other four catalysts, Rh doped in the surface lattice of a CeO₂ catalyst, including Rh-CeO₂-air and Rh-CeO₂-H₂, is in a complete ionic state during catalysis. Correlations between the in situ surface chemistry and the corresponding catalytic performance show that Rh ions and Pt ions doped in the lattice of CeO₂ as well as metallic Pd nanoparticles supported on CeO₂ are active components for MPO. Among these catalysts, Rh-doped CeO₂ exhibited the highest catalytic activity and selectivity in MPO

Conversion of Methane to Methanol with a Bent Mono(μ-oxo)dinickel Anchored on the Internal Surfaces of Micropores

The oxidation of methane to methanol is a pathway to utilizing this relatively abundant, inexpensive energy resource. Here we report a new catalyst, bent mono­(μ-oxo)­dinickel anchored on an internal surface of micropores,which is active for direct oxidation. It is synthesized from the direct loading of a nickel precursor to the internal surface of micropores of ZSM5 following activation in O₂. Ni 2p_3/2 of this bent mono­(μ-oxo)­dinickel species formed on the internal surface of ZSM5 exhibits a unique photoemission feature, which distinguishes the mono­(μ-oxo)­dinickel from NiO nanoparticles. The formation of the mono­(μ-oxo)­dinickel species was confirmed with X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). This mono­(μ-oxo)­dinickel species is active for the direct oxidation of methane to methanol under the mild condition of a temperature as low as 150 °C in CH₄ at 1 bar. In-situ studies using UV–vis, XANES, and EXAFS suggest that this bent mono­(μ-oxo)­dinickel species is the active site for the direct oxidation of methane to methanol. The energy barrier of this direct oxidation of methane is 83.2 kJ/mol

Restructuring Transition Metal Oxide Nanorods for 100% Selectivity in Reduction of Nitric Oxide with Carbon Monoxide

Transition metal oxide is one of the main categories of heterogeneous catalysts. They exhibit multiple phases and oxidation states. Typically, they are prepared and/or synthesized in solution or by vapor deposition. Here we report that a controlled reaction, in a gaseous environment, after synthesis can restructure the as-synthesized transition metal oxide nanorods into a new catalytic phase. Co₃O₄ nanorods with a preferentially exposed (110) surface can be restructured into nonstoichiometric CoO_1–<i>x</i> nanorods. Structure and surface chemistry during the process were tracked with ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and environmental transmission electron microscopy (E-TEM). The restructured nanorods are highly active in reducing NO with CO, with 100% selectivity for the formation of N₂ in temperatures of 250–520 °C. AP-XPS and E-TEM studies revealed the nonstoichiometric CoO_1–<i>x</i> nanorods with a rock-salt structure as the active phase responsible for the 100% selectivity. This study suggests a route to generate new oxide catalysts

Formation of Second-Generation Nanoclusters on Metal Nanoparticles Driven by Reactant Gases

Heterogeneous catalysis occurs at the interface between a solid catalyst and the reactants. The structure of metal catalyst nanoparticles at the metal–gas interface is a key factor that determines catalytic selectivity and activity. Here we report that second-generation nanoclusters are formed on the initial catalyst nanoparticles as a result of interaction with the reactant molecules when the nanoparticles are in a gas phase at Torr pressure or higher. The formation of the second-generation nanoclusters is manifested by a decrease of the average coordination number of the metal atoms and a shift of their core level energies in the presence of gases. The formation of second-generation nanoclusters increases the number of undercoordinated sites, which are the most active for catalysis in many cases

Reduction of Nitric Oxide with Hydrogen on Catalysts of Singly Dispersed Bimetallic Sites Pt₁Co_<i>m</i> and Pd₁Co_<i>n</i>

The bimetallic catalyst has been one of the main categories of heterogeneous catalysts for chemical production and energy transformation. Isolation of the continuously packed bimetallic sites of a bimetallic catalyst forms singly dispersed bimetallic sites which have distinctly different chemical environment and electronic state and thus exhibit a different catalytic performance. Two types of catalysts consisting of singly dispersed bimetallic sites Pt₁Co_<i>m</i> or Pd₁Co_<i>n</i> (<i>m</i> and <i>n</i> are the average coordination numbers of Co to a Pt or Pd atom) were prepared through a deposition or impregnation with a following controlled calcination and reduction to form Pt₁Co_<i>m</i> or Pd₁Co_<i>n</i> sites. These bimetallic sites are separately anchored on a nonmetallic support. Each site only consists of a few metal atoms. Single dispersions of these isolated bimetallic sites were identified with scanning transmission electron microscopy. Extended X-ray absorption fine structure spectroscopy (EXAFS) revealed the chemical bonding of single atom Pt₁ (or Pd₁) to Co atoms and thus confirmed the formation of bimetallic sites, Pt₁Co_<i>m</i> and Pd₁Co_<i>n</i>. Reduction of NO with H₂ was used as a probing reaction to test the catalytic performance on this type of catalyst. Selectivity in reducing nitric oxide to N₂ on Pt₁Co_<i>m</i> at 150 °C is 98%. Pd₁Co_<i>n</i> is active for reduction of NO with a selectivity of 98% at 250 °C. In situ studies of surface chemistry with ambient-pressure X-ray photoelectron spectroscopy and coordination environment of Pt and Pd atoms with EXAFS showed that chemical state and coordination environment of Pt₁Co_<i>m</i> and Pd₁Co_<i>n</i> remain during catalysis up to 250 and 300 °C, respectively. The correlation of surface chemistries and structures of these catalysts with their corresponding catalytic activities and selectivities suggests a method to develop new bimetallic catalysts and a new type of single site catalysts

Direct Neutron Spectroscopy Observation of Cerium Hydride Species on a Cerium Oxide Catalyst

Ceria has recently shown intriguing hydrogenation reactivity in catalyzing alkyne selectively to alkenes. However, the mechanism of the hydrogenation reaction, especially the activation of H₂, remains experimentally elusive. In this work, we report the first direct spectroscopy evidence for the presence of both surface and bulk Ce–H species upon H₂ dissociation over ceria via <i>in situ</i> inelastic neutron scattering spectroscopy. Combined with <i>in situ</i> ambient-pressure X-ray photoelectron spectroscopy, IR, and Raman spectroscopic studies, the results together point to a heterolytic dissociation mechanism of H₂ over ceria, leading to either homolytic products (surface OHs) on a close-to-stoichiometric ceria surface or heterolytic products (Ce–H and OH) with the presence of induced oxygen vacancies in ceria. The finding of this work has significant implications for understanding catalysis by ceria in both hydrogenation and redox reactions where hydrogen is involved

Methylation state at MBD2 binding sites.

<p>a) Boxplot displaying methylation level at TTE-MBD2 binding sites compared to random (0 = 0% methylation, 1 = 100% methylation). b) Genome wide correlation between TTE-MBD2 enrichment (green) and methylation density, calculated at 1 kb windows ranked by methylation density (dashed line). c) Screenshots from genome browser showing correlation between CpG methylation density (red track) and TTE-MBD2 peaks at KCNN2, ZNF316, and ASCL5.</p

Generation of a tagged MBD2.

<p>a) Schematic presentation of tagging approach: double Ty1 and ER epitopes are inserted at the N-terminal of human full length MBD2. b) Western blot on whole cell lysates from TTE-MBD2 MCF-7 and WT MCF-7. Antibodies against tag (Ty1) and MBD2 are used. GAPDH is shown as loading control. c) Volcano plot showing results from Mass Spectrometric Analysis of immunoprecipitation experiment. The x-axis shows the log of ratios between LFQ intensities in TTE-MBD2 against the control WT. The y-axis display −log10 of the p-value calculated by a permutation-based FDR-corrected <i>t</i> test. The black dots underline Mi2-NuRD complex components within the significantly enriched interactors (grey dots).</p

Methylation levels of MBD2 binding sites in normal and breast cancer.

<p>a) Boxplot displaying methylation levels at TTE-MBD2 binding sites in MCF-7 and HMEC compared to methylation at random regions respectively in MCF-7 and HMEC. Random is corrected for genomic distribution as for <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0099603#pone-0099603-g003" target="_blank">Fig 3A</a>. b) Genome-wide methylation levels in MCF-7 and HMEC calculated in 50 bp sliding windows. c) Dot-plot showing mean methylation for each samples (#patients on X-axis) at all MBD2 binding sites: red dots are indicating mean-methylation at MBD2 sites in tumor samples (MBD2_tumor) and green for healthy samples (MBD2_normal). Same analysis at a random set of sites (as for <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0099603#pone-0099603-g003" target="_blank">Fig 3A</a>) for the two datasets is depicted in grey. On the y-axis methylation levels (0 = 0% methylation, 1 = 100% methylation). d) As for <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0099603#pone-0099603-g006" target="_blank">Fig 6C</a> dot-plot showing mean methylation for each samples (#patients on X-axis) at all MBD2 binding sites and at the subset represented in cluster 4.</p