Atomic Structure of Pd-, Pt-, and PdPt-Based Catalysts of Total Oxidation of Methane: In Situ EXAFS Study

In this study, 3%Pd/Al2O3, 3%Pt/Al2O3 and bimetallic (1%Pd + 2%Pt)/Al2O3 catalysts were examined in the total oxidation of methane in a temperature range of 150–400 °C. The evolution of the active component under the reaction conditions was studied by transmission electron microscopy and in situ extended X-ray absorption fine structure (EXAFS) spectroscopy. It was found that the platinum and bimetallic palladium-platinum catalysts are more stable against sintering than the palladium catalysts. For all the catalysts, the active component forms a “core-shell” structure in which the metallic core is covered by an oxide shell. The “core-shell” structure for the platinum and bimetallic palladium-platinum catalysts is stable in the temperature range of 150–400 °C. However, in the case of the palladium catalysts the metallic core undergoes the reversible oxidation at temperatures above 300 °C and reduced to the metallic state with the decrease in the reaction temperature. The scheme of the active component evolution during the oxidation of methane is proposed and discussed.

Lignin-to-chemicals: Application of catalytic hydrogenolysis of lignin to produce phenols and terephthalic acid via metal-based catalysts

Lignin is the only renewable aromatic material in nature and contains a large number of oxygen-containing functional groups. High-value and green utilization of "lignin-to-chemicals" can be realized via using lignin to produce fine chemicals such as phenols and carboxylic acids, which can not only reduce the waste of lignin in the process of lignocellulosic biomass treatment, but gradually make the substitution of traditional fossil fuels come true. The hydrogenolysis process under catalysis of metal catalyst has high product selectivity and less impurity, which is suitable for the production of same type or single fine chemicals. Hydrogenolysis of lignin via metal catalysts to produce lignin oil, and further modification of functional groups (e.g. methoxyl, alkyl and hydroxyl group) of depolymerized monomers in the bio-oil to yeild phenols and terephthalic acid are reviewed, and catalytic mechanisms are briefly summarized in this paper. Finally, the problems of lignin catalytic conversion existing currently are investigated, and the future development of this field is also prospected.

Spiers Memorial Lecture: Understanding reaction mechanisms in heterogeneously catalysed reactions

Heterogeneous catalysis lies at the heart of the chemical and fuel manufacturing industries and hence is a cornerstone of many economies. Many of the commercially operated heterogeneous catalysts have remained basically unchanged for decades, undergoing small but important optimisation of their formulations. Yet we all acknowledge that there is a continuous drive towards improved catalysts or designing new ones. At the heart of these studies has been the need to gain an improved understanding of the reaction mechanism for these important reactions since this can unlock new ways to improve catalyst design and, of course, the ultimate aim is to design catalysts based on the detailed understanding of the reaction mechanism. These advanced studies have been aided in the last decade by two key factors, namely: (a) access to advanced characterisation techniques based on synchrotron methods and aberration-corrected microscopy that can probe the nature of the active site, and (b) the application of high-level computational methods to understand how the reactants and products interact at the active site. In this paper this theme will be explored using two examples to bring out the complexity in gaining an understanding of a reaction mechanism. Using the zeolite H-ZSM-5 as an example of a single site catalyst, the mechanism of the conversion of methanol to the first hydrocarbon carbon-carbon bond will be discussed. In this section the use of model reactants and reaction probes will be used to try to differentiate between different mechanistic proposals. The second example explores the use of gold catalysts for CO oxidation and acetylene hydrochlorination. In both these examples the importance of advanced characterisation and theory will be highlighted.

Fabrication of Pd/CeO2 nanocubes as highly efficient catalysts for degradation of formaldehyde at room temperature

The highly active Pd/CeO2 nanocube interface guarantees a high percentage of metallic Pd and the surface active O species is responsible for the complete decomposition of formaldehyde.

Rational Synthesis of Porous Graphitic-like Carbon Nitride Nanotubes Codoped with Au and Pd as an Efficient Catalyst for Carbon Monoxide Oxidation

The precise fabrication of efficient catalysts for CO oxidation is of particular interest in a wide range of industrial and environmental applications. Herein, a scalable method is presented for the controlled synthesis of graphitic-like porous carbon nitride nanotubes (gC₃N₄NTs) codoped with Au and Pd (Au/Pd/gC₃N₄NTs) as efficient catalysts for carbon monoxide (CO) conversion. This includes the activation of melamine with nitric acid in the presence of ethylene glycol and metal precursors followed by consecutive polymerization and carbonization. This drives the formation of porous one-dimensional gC₃N₄NT with an outstanding surface area of (320.6 m² g^-1) and an atomic-level distribution of Au and Pd. Intriguingly, the CO conversion efficiency of Au/Pd/gC₃N₄NTs was substantially greater than that for gC₃N₄NTs. The approach thus presented may provide new avenues for the utilization of gC₃N₄ doped with multiple metal-based catalysts for CO conversion reactions which had been rarely reported before.

Engineering surface defects and metal–support interactions on Pt/TiO2(B) nanobelts to boost the catalytic oxidation of CO

The thermally reduced Pt/TiO₂(B) catalysts show high catalytic activity and good water resistance for the catalytic oxidation of CO.

Pt–Au/CeO2 catalysts for the simultaneous removal of carbon monoxide and formaldehyde

A series of Pt–Au/CeO₂ catalysts were prepared via the impregnation deposition–precipitation (IDP) and reduction–deposition precipitation (RDP) methods.

A ligand coordination approach for high reaction stability of an Au–Cu bimetallic carbon-based catalyst in the acetylene hydrochlorination process

A novel high stability gold catalyst for acetylene hydrochlorination was designed and prepared by adding trichloroisocyanuric acid (TCCA) as a ligand and Cu as a synergistic metal, which can promote the development of a non-mercury catalyst for China's PVC industry.

Rational control of nano-scale metal-catalysts for biomass conversion

This feature article discusses the rational control of nano-scale metal catalysts for catalytic biomass transformation.

Catalytic activity of bimetallic catalysts highly sensitive to the atomic composition and phase structure at the nanoscale

The ability to determine the atomic arrangement in nanoalloy catalysts and reveal the detailed structural features responsible for the catalytically active sites is essential for understanding the correlation between the atomic structure and catalytic properties, enabling the preparation of efficient nanoalloy catalysts by design. Herein we describe a study of CO oxidation over PdCu nanoalloy catalysts focusing on gaining insights into the correlation between the atomic structures and catalytic activity of nanoalloys. PdCu nanoalloys of different bimetallic compositions are synthesized as a model system and are activated by a controlled thermochemical treatment for assessing their catalytic activity. The results show that the catalytic synergy of Pd and Cu species evolves with both the bimetallic nanoalloy composition and temperature of the thermochemical treatment reaching a maximum at a Pd : Cu ratio close to 50 : 50. The nanoalloys are characterized structurally by ex situ and in situ synchrotron X-ray diffraction, including atomic pair distribution function analysis. The structural data show that, depending on the bimetallic composition and treatment temperature, PdCu nanoalloys adopt two different structure types. One features a chemically ordered, body centered cubic (B2) type alloy consisting of two interpenetrating simple cubic lattices, each occupied with Pd or Cu species alone, and the other structure type features a chemically disordered, face-centered cubic (fcc) type of alloy wherein Pd and Cu species are intermixed at random. The catalytic activity for CO oxidation is strongly influenced by the structural features. In particular, it is revealed that the prevalence of chemical disorder in nanoalloys with a Pd : Cu ratio close to 50 : 50 makes them superior catalysts for CO oxidation in comparison with the same nanoalloys of other bimetallic compositions. However, the catalytic synergy can be diminished if the Pd50Cu50 nanoalloys undergo phase segregation into distinct chemically-ordered (B2-type) and disordered (fcc-type) domains. This finding is significant since it provides a rational basis for streamlining the design and preparation of Pd-based nanoalloy catalysts in terms of atomic structure and phase state.

Support Morphology-Dependent Catalytic Activity of Pd/CeO₂ for Formaldehyde Oxidation

To eliminate indoor formaldehyde (HCHO) pollution, Pd/CeO2 catalysts with different morphologies of ceria support were employed. The palladium nanoparticles loaded on {100}-faceted CeO2 nanocubes exhibited much higher activity than those loaded on {111}-faceted ceria nanooctahedrons and nanorods (enclosed by {100} and {111} facets). The HCHO could be fully converted into CO2 over the Pd/CeO2 nanocubes at a GHSV of 10,000 h(-1) and a HCHO inlet concentration of 600 ppm at ambient temperature. The prepared catalysts were characterized by a series of techniques. The HRTEM, ICP-MS and XRD results confirmed the exposed facets of the ceria and the sizes (1-2 nm) of the palladium nanoparticles with loading amounts close to 1%. According to the Pd 3d XPS and H2-TPR results, the status of the Pd-species was dependent on the morphologies of the supports. The {100} facets of ceria could maintain the metallic Pd species rather than the {111} facets, which promoted HCHO catalytic combustion. The Raman and O 1s XPS results revealed that the nanorods with more defect sites and oxygen vacancies were responsible for the easy oxidation of the Pd-species and low catalytic activity.

Facile SILAR approach to air-stable naked silver and gold nanoparticles supported by alumina

A synthetically convenient and scalable SILAR (successive ion layer adsorption and reaction) method is used to make air-stable films of silver and gold nanoparticles supported on alumina scaffolds. This solution-based deposition technique yields particles devoid of insulating capping agents or ligands. The optical properties of the nanoparticle films were investigated using femtosecond transient absorption spectroscopy. A linear absorption arising from intraband excitation (775 nm laser pulse) is seen only for Au nanoparticles at low intensity. However, both Au and Ag particles exhibit plasmon resonance responses at high excitation intensity via two photon absorption of the 775 nm pump pulse. The difference in optical response to near-IR laser excitation is rationalized based on the known density of states for each metal. To demonstrate the potential applications of these films, alumina-supported Ag nanoparticles were utilized as substrates for surface enhanced Raman spectroscopy, resulting in a 65-fold enhancement in the Raman signal of the probe molecule rhodamine 6G. The exceptional stability and scalability of these SILAR films opens the door for further optical and photocatalytic studies and applications, particularly with ligand-free Ag nanoparticles that typically oxidize under ambient conditions. Additionally, isolating plasmonic and interband electronic excitations in stable AgNP under visible light irradiation could enable elucidation of the mechanisms that drive noble metal-assisted photocatalytic processes.

Monodispersed bimetallic PdAg nanoparticles with twinned structures: formation and enhancement for the methanol oxidation

Monodispersed bimetallic PdAg nanoparticles can be fabricated through the emulsion-assisted ethylene glycol (EG) ternary system. Different compositions of bimetallic PdAg nanoparticles, Pd₈₀Ag₂₀, Pd₆₅Ag₃₅ and Pd₄₆Ag₅₄ can be obtained via adjusting the reaction parameters. For the formation process of the bimetallic PdAg nanoparticles, there have two-stage growth processes: firstly, nucleation and growth of the primary nanoclusters; secondly, formation of the secondary nanoparticles with the size-selection and relax process via the coalescence or aggregation of the primary nanoclusters. The as-prepared PdAg can be supported on the carbon black without any post-treatment, which exhibited high electro-oxidation activity towards methanol oxidation under alkaline media. More importantly, carbon-supported Pd₈₀Ag₂₀ nanoparticles reveal distinctly superior activities for the methanol oxidation, even if compared with commercial Pt/C electro-catalyst. It is concluded that the enhanced activity is dependant on the unique twinning structure with heterogeneous phase due to the dominating coalescence growth in EG ternary system.

Synthesis of fluorescent metal nanoparticles in aqueous solution by photochemical reduction

A facile green chemistry approach for the synthesis of sub-5 nm silver and gold nanoparticles is reported. The synthesis was achieved by a photochemical method using tyrosine as the photoreducing agent. The size of the gold and silver nanoparticles was about 3 and 4 nm, respectively. The nanoparticles were characterized using x-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy and photoluminescence spectroscopy. Both silver and gold nanoparticles synthesized by this method exhibited fluorescence properties and their use for cell imaging applications has been demonstrated.

CO oxidation on colloidal Au(0.80)Pd(0.20)-Fe(x)O(y) dumbbell nanocrystals

We report a colloidal synthesis of Au(0.80)Pd(0.20)-Fe(x)O(y) dumbbell nanocrystals (NCs) derived from Au(0.75)Pd(0.25) NCs by metal oxide overgrowth. We compared the catalytic activity of the two types of NCs in the CO oxidation reaction (CO + 1/2O(2) → CO(2)), after they had been dispersed on an alumina nanopowder support. In both cases, the surface active sites were identified by means of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The enhanced catalytic performance of the dumbbell NCs (Au(0.80)Pd(0.20)-Fe(x)O(y)) catalyst over that of the initial Au(0.75)Pd(0.25) NCs could be correlated to the presence of the epitaxial connection between the Fe(x)O(y) and the Au(0.80)Pd(0.20) domains (as the main factor). Such connection should result in an electron flow from the metal oxide (Fe(x)O(y)) domain to the noble metal (Au(0.80)Pd(0.20)) domain and appears to influence favorably the nature and composition of the catalytically active surface sites of the dumbbells. Our experiments indicate indeed that, when the metal alloy domain is attached to the metal oxide domain (that is, in the dumbbell), surface Pd species are more active than in the case of the initial Au(0.75)Pd(0.25) NCs and also Au(δ-) sites are formed that were not present on the initial Au(0.75)Pd(0.25) NCs.

Relativistic effects in chemistry: more common than you thought

Relativistic effects can strongly influence the chemical and physical properties of heavy elements and their compounds. This influence has been noted in inorganic chemistry textbooks for a couple of decades. This review provides both traditional and new examples of these effects, including the special properties of gold, lead-acid and mercury batteries, the shapes of gold and thallium clusters, heavy-atom shifts in NMR, topological insulators, and certain specific heats.