Design, Identification, and Evolution of a Surface Ruthenium(II/III) Single Site for CO Activation.

RuII compounds are widely used in catalysis, photocatalysis, and medical applications. They are usually obtained in a reductive environment as molecular O2 can oxidize RuII to RuIII and RuIV . Here we report the design, identification and evolution of an air-stable surface [bipy-RuII (CO)2 Cl2 ] site that is covalently mounted onto a polyphenylene framework. Such a RuII site was obtained by reduction of [bipy-RuIII Cl4 ]- with simultaneous ligand exchange from Cl- to CO. This structural evolution was witnessed by a combination of in situ X-ray and infrared spectroscopy studies. The [bipy-RuII (CO)2 Cl2 ] site enables oxidation of CO with a turnover frequency of 0.73×10-2  s-1 at 462 K, while the RuIII site is completely inert. This work contributes to the study of structure-activity relationship by demonstrating a practical control over both geometric and electronic structures of single-site catalysts at molecular level.

Adsorption of U(VI) on Stoichiometric and Oxidised Mackinawite: a DFT Study.

Naturally occurring minerals, such as the iron sulfide mackinawite, play a key role in the remediation of uranium from groundwater systems. Here, density functional theory (DFT) is used to investigate the interaction of uranium with the most stable surface of stoichiometric mackinawite, {001}-S. The high reactivity of the mineral toward oxygen may affect its ability to sequester uranium; therefore, two models of oxidized mackinawite are also used to study the effect of surface oxidation on adsorption. Weak adsorption of mononuclear uranyl(VI) complexes is found on stoichiometric mackinawite; however, equivalent adsorption modes on the oxidized mackinawite models generally exhibit stronger adsorption. Some of the most energetically stable DFT structures are found to match well with experimental extended X-ray absorption fine structure (EXAFS) data. The implications for the proposed use of mackinawite as a scavenger material for uranium in groundwater systems are discussed.

A DFT and KMC based study on the mechanism of the water gas shift reaction on the Pd(100) surface.

We present a combined density functional theory (DFT) and Kinetic Monte Carlo (KMC) study of the water gas shift (WGS) reaction on the Pd(100) surface. We propose a mechanism comprising both the redox and the associative pathways for the WGS within a single framework, which consists of seven core elementary steps, which in turn involve splitting of a water molecule followed by the production of an H-atom and an OH-species on the Pd(100) surface. In the following steps, these intermediates then recombine with each other and with CO leading to the evolution of CO2, and H2. Seven other elementary steps, involving the diffusion and adsorption of the surface intermediate species are also considered for a complete description of the mechanism. The geometrical and electronic properties of each of the reactants, products, and the transition states of the core elementary steps are presented. We also discuss the analysis of Bader charges and spin densities for the reactants, transition states and the products of these elementary steps. Our study indicates that the WGS reaction progresses simultaneously via the direct oxidation and the carboxyl paths on the Pd(100) surface.

Catalytic and mechanistic insights of the low-temperature selective oxidation of methane over Cu-promoted Fe-ZSM-5.

The partial oxidation of methane to methanol presents one of the most challenging targets in catalysis. Although this is the focus of much research, until recently, approaches had proceeded at low catalytic rates (<10 h(-1)), not resulted in a closed catalytic cycle, or were unable to produce methanol with a reasonable selectivity. Recent research has demonstrated, however, that a system composed of an iron- and copper-containing zeolite is able to catalytically convert methane to methanol with turnover frequencies (TOFs) of over 14,000 h(-1) by using H(2)O(2) as terminal oxidant. However, the precise roles of the catalyst and the full mechanistic cycle remain unclear. We hereby report a systematic study of the kinetic parameters and mechanistic features of the process, and present a reaction network consisting of the activation of methane, the formation of an activated hydroperoxy species, and the by-production of hydroxyl radicals. The catalytic system in question results in a low-energy methane activation route, and allows selective C(1)-oxidation to proceed under intrinsically mild reaction conditions.

Detection of key transient Cu intermediates in SSZ-13 during NH3-SCR deNO x by modulation excitation IR spectroscopy.

The small pore zeolite Cu-SSZ-13 is an efficient material for the standard selective catalytic reduction of nitrogen oxides (NO x ) by ammonia (NH3). In this work, Cu-SSZ-13 has been studied at 250 °C under high conversion using a modulation excitation approach and analysed with phase sensitive detection (PSD). While the complementary X-ray absorption near edge structure (XANES) spectroscopy measurements showed that the experiments were performed under cyclic Cu+/Cu2+ redox, Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) experiments provide spectroscopic evidence for previously postulated intermediates Cu-N([double bond, length as m-dash]O)-NH2 and Cu-NO3 in the NH3-SCR deNO x mechanism and for the role of [Cu2+(OH-)]+. These results therefore help in building towards a more comprehensive understanding of the reaction mechanism which to date has only been postulated in silico.

Room temperature methoxylation in zeolite H-ZSM-5: an operando DRIFTS/mass spectrometric study.

At high loading, methanol reacts under ambient conditions with acidic hydroxyls of H-ZSM-5 to methoxylate framework oxygen; while a significant proportion remains hydrogen bonded to the framework with a protonated geometry. The findings not only explain the data which have been published so far, but also pave a way forward for potential unravelling of the initial reaction steps in the relevant chemical processes.

Identification of single-site gold catalysis in acetylene hydrochlorination.

There remains considerable debate over the active form of gold under operating conditions of a recently validated gold catalyst for acetylene hydrochlorination. We have performed an in situ x-ray absorption fine structure study of gold/carbon (Au/C) catalysts under acetylene hydrochlorination reaction conditions and show that highly active catalysts comprise single-site cationic Au entities whose activity correlates with the ratio of Au(I):Au(III) present. We demonstrate that these Au/C catalysts are supported analogs of single-site homogeneous Au catalysts and propose a mechanism, supported by computational modeling, based on a redox couple of Au(I)-Au(III) species.