Bimetallic Model Research Articles

NO + H2 reaction on Pt-Ru/SiO2 catalysts: correlation between nanostructure and catalytic activity and selectivity

Nitric oxide reduction by hydrogen has been studied on Pt-Ru/SiO2 catalysts of various Pt/Ru atomic compositions in the temperature range 298–673 K. Physical characterization showed the presence of bimetallic particles which tend to be Pt-rich. The overall activity of the bimetallic catalysts suggests a dilution of the active component (Pt) in the range 373–523 K. The addition of Ru results in a general improvement of the N2 selectivity and significant modifications in the product distribution are observed as a function of the catalyst composition. A bimetallic particle model is proposed in which various types of surfaces are exposed including those with pure Pt atoms and/or Pt-Ru mixture. This model allows to explain the overall activity and selectivity in the whole series of catalysts.

Novel Bimetallic Model System for Cytochrome P450: Effect of Membrane Environment on the Catalytic Oxidation

Novel Bimetallic Model System for Cytochrome P₄₅₀: Effect of Membrane Environment on the Catalytic Oxidation

Molecular Catalysts for Multielectron Redox Reactions of Small Molecules: The “Cofacial Metallodiporphyrin” Approach

AbstractThe role of metalloenzymes in important biological transformations has attracted increasing attention over the past several decades. Of the many chemical transformations mediated by enzymes, few are as challenging as multielectron redox reactions. Recent studies have revealed a partial structural and mechanistic description of these redox‐active metalloenzymes, but there is much still to be learned regarding the mechanisms of substrate transformation. Due to the complexity of the metalloenzyme systems, simplified model systems are employed to mimic structural or functional features of the enzyme. In multielectron redox enzymes, several metals are probably in‐volved in both substrate binding and the subsequent redox reactions. Thus, functional mimics of multielectron redox enzymes might also need two or more metal centers to be efficacious. The roles of multiple metal centers are to (1) increase the substrate's affinity for the catalyst, (2) increase the rate of electron transfer to the bound substrate, (3) increase the reactivity of the bound substrate, and (4) inhibit deleterious side reactions. Deter‐mining the importance of each factor may help in the development of these catalysts. Cofacial metallodiporphyrins, because of the control they provide over the geometric and electronic properties of the synthetic reaction center, are ideal bimetallic model complexes. The knowledge gained from model studies will help in understanding the mechanisms of metalloenzymes and can be used to design new homogeneous catalysts to effect multielectron transformations.

A Bimetallic Hydroformylation Catalyst: High Regioselectivity and Reactivity Through Homobimetallic Cooperativity

The racemic and meso diastereomers of an electron-rich binucleating tetraphosphine ligand have been used to prepare homobimetallic rhodium norbornadiene complexes. The racemic bimetallic Rh complex is an excellent hydroformylation catalyst for 1-alkenes, giving both a high rate of reaction and high regioselectivity for linear aldehydes, whereas the meso complex is considerably slower and less selective. A mechanism involving bimetallic cooperativity between the two rhodium centers in the form of an intramolecular hydride transfer is proposed. Mono- and bimetallic model complexes in which the possibility for bimetallic cooperativity has been reduced or eliminated are very poor catalysts.

Chemisorption of formate and acetate on cluster models of Rh and bimetallic RhSn clusters

Density functional theory has been applied to the study of the interaction of the formate and acetate species with pure rhodium and bimetallic rhodium-tin model clusters. The calculated equilibrium parameters (optimized geometries, harmonic frequencies and atomic charges) suggest that the internal structure of the formate (and acetate) species is not influenced significantly, however, the metal-formate (and metal-acetate) bond is weakened in the presence of tin atoms. This effect is rationalized in terms of perturbation theory of orbital interaction. In light of our results, a possible explanation is given for the improved selectivity of a RnSn bimetallic catalyst for the hydrogenation of ethyl acetate to ethanol.

Anodic oxidation of methanol at a gold-modified platinum electrocatalyst prepared by RF sputtering on a glassy carbon support

A AuPt bimetallic model catalyst has been prepared on a smooth carbon support by an RF sputtering method, and its electrocatalytic activity for methanol oxidation has been investigated in a sulfuric acid solution. The activity of the sputtered Pt film modified with a small amount of Au deposit was evaluated by the current density in a potential-sweep voltammogram and a steady-state polarization curve. The apparent current density for methanol oxidation based on the geometric area of the electrode was decreased by increasing the Au coverage of the catalyst. In potential-sweep voltammetry, the activity per Pt site exposed to the solution reached a maximum value at 70–80% Au coverage. However, the steady-state activity of the Au-modified Pt catalyst was almost the same as that of the pure Pt film. The XPS measurement showed that the valence band spectrum of the au-modified Pt catalyst prepared by RF sputtering consists of a linear combination of the spectra observed at pure Pt and Au films.

Surface studies with bimetallic model catalyst systems

Some recent experimental results on the interaction of active gases (H 2, D 2, CO, O 2, N 2 and NH 3) with single crystal surfaces of ruthenium and iron covered with various amounts of copper and potassium, respectively, will be reported and reviewed. LEED, AES, TDS, SIMS, UPS, Δϕ and HREELS served to characterize the model surfaces and to identify adsorbed species with respect to binding energies and local coordination. Attention was paid to the interaction of H 2 and CO with Cu/Ru bilayers, and to the reaction of N 2 and H 2 with potassium-promoted Fe surfaces. The uptake of strongly chemisorbed H on Ru(0001) is greatly reduced by Cu atoms (depending on their lateral dispersion) whereas CO adsorption is less affected. Clearly, a dissociative chemisorption process is more easily perturbed than an associative one, as a consequence of the operation of an ensemble effect. Potassium increases the binding energy of the molecular N 2 precursor on Fe and thereby assists the formation of atomic N, which is the important step in NH 3 synthesis.