Ceria has recently shown intriguing hydrogenation reactivity in catalyzing alkyne selectively to alkenes. However, the mechanism of the hydrogenation reaction, especially the activation of H, 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 H dissociation over ceria via in situ inelastic neutron scattering spectroscopy. Combined with in situ ambient-pressure X-ray photoelectron spectroscopy, IR, and Raman spectroscopic studies, the results together point to a heterolytic dissociation mechanism of H 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.

Amorphous alloys structurally deviate from crystalline materials in that they possess unique short-range ordered and long-range disordered atomic arrangement. They are important catalytic materials due to their unique chemical and structural properties including broadly adjustable composition, structural homogeneity, and high concentration of coordinatively unsaturated sites. As chemically reduced metal-metalloid amorphous alloys exhibit excellent catalytic performance in applications such as efficient chemical production, energy conversion, and environmental remediation, there is an intense surge in interest in using them as catalytic materials. This critical review summarizes the progress in the study of the metal-metalloid amorphous alloy catalysts, mainly in recent decades, with special focus on their synthetic strategies and catalytic applications in petrochemical, fine chemical, energy, and environmental relevant reactions. The review is intended to be a valuable resource to researchers interested in these exciting catalytic materials. We concluded the review with some perspectives on the challenges and opportunities about the future developments of metal-metalloid amorphous alloy catalysts.

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 TiO-based nanoparticle catalyst anchoring singly dispersed Pd atoms (Pd/TiO) 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 Pd/TiO 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 TiO 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 Pd and phenylacetylenyl bound to O of TiO forms the product molecule, diphenylacetylene.

Direct conversion of methane to chemical feedstocks such as methanol under mild conditions is a challenging but ideal solution for utilization of methane. Pd O single-sites anchored on the internal surface of micropores of a microporous silicate exhibit high selectivity and activity in transforming CH to CH OH at 50-95 °C in aqueous phase through partial oxidation of CH with H O . The selectivity for methanol production remains at 86.4 %, while the activity for methanol production at 95 °C is about 2.78 molecules per Pd O site per second when 2.0 wt % CuO is used as a co-catalyst with the Pd O @ZSM-5. Thermodynamic calculations suggest that the reaction toward methanol production is highly favorable compared to formation of a byproduct, methyl peroxide.

Bimetallic catalysts are one of the main categories of metal catalysts due to the tunability of electronic and geometric structures through alloying a second metal. The integration of a second metal creates a vast number of possibilities for varying the surface structure and composition of metal catalysts toward designing new catalysts. It is well acknowledged that the surface composition, atomic arrangement, and electronic state of bimetallic catalysts could be different from those before a chemical reaction or catalysis based on ex situ studies. Thanks to advances in electron-based surface analytical techniques, the surface chemistry and structure of bimetallic nanoparticles can be characterized under reaction conditions and during catalysis using ambient pressure analytical techniques including ambient pressure XPS, ambient pressure STM, X-ray absorption spectroscopy and others. These ambient pressure studies revealed various restructurings in the composition and arrangement of atoms in the surface region of catalysts under reaction conditions or during catalysis compared to that before reaction. These restructurings are driven by thermodynamic and kinetic factors. The surface energy of the constituent metals and adsorption energy of reactant molecules or dissociated species on a metal component are two main factors from the point of view of thermodynamics. Correlations between the authentic surface structure and chemistry of catalysts during catalysis and simultaneous catalytic performance were built for understanding catalytic mechanisms of bimetallic catalysts toward designing new catalysts with high activity, selectivity, and durability.

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 m or Pd1Co n (m and n 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 m or Pd1Co n 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 m and Pd1Co n . 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 m at 150 °C is 98%. Pd1Co n 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 m and Pd1Co n 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.

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 CoO1-x 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 CoO1-x 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.

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.