Surface Chemistry of Ruthenium Dioxide in Heterogeneous Catalysis and Electrocatalysis: From Fundamental to Applied Research

In recent years, transition metal-catalyzed direct (hetero)arylation reactions of heteroarenes with (hetero)aryl halides and pseudohalides have received significant attention as eco-friendly and economic alternatives to classical methods for the construction of heteroaryl-(hetero)aryl CC bonds by transition metal-catalyzed cross-couplings involving the use of stoichiometric amounts of organometallic reagents. This critical review with 430 references covers the results obtained in the period January 2009 to February 2013 in the area of the transition metal-catalyzed direct inter- and intramolecular (hetero)arylation reactions of heteroarenes containing one heteroatom. Particular attention has been given to illustrate chemo- and site selectivity aspects of these reactions as well applications of these CC bond forming reactions in the synthesis of pharmaceutically relevant compounds, natural products and their analogues and precursors. The most recent advancements into the mechanism(s) of these reactions have also been briefly reported.

Cross‐Coupling of Heteroarenes by CH Functionalization: Recent Progress towards Direct Arylation and Heteroarylation Reactions Involving Heteroarenes Containing One Heteroatom

Single-atom catalysts offer high reactivity and selectivity while maximizing utilization of the expensive active metal component. However, they are susceptible to sintering, where single metal atoms agglomerate into thermodynamically stable clusters. Tuning the binding strength between single metal atoms and oxide supports is essential to prevent sintering. We apply density functional theory, together with a statistical learning approach based on least absolute shrinkage and selection operator regression, to identify property descriptors that predict interaction strengths between single metal atoms and oxide supports. Here, we show that interfacial binding is correlated with readily available physical properties of both the supported metal, such as oxophilicity measured by oxide formation energy, and the support, such as reducibility measured by oxygen vacancy formation energy. These properties can be used to empirically screen interaction strengths between metal–support pairs, thus aiding the design of single-atom catalysts that are robust against sintering.

Interaction trends between single metal atoms and oxide supports identified with density functional theory and statistical learning

CO2 reduction to higher value products is a promising way to produce fuels and key chemical building blocks while reducing CO2 emissions. The reaction at atmospheric pressure mainly yields CH4 via methanation and CO via the reverse water-gas shift (RWGS) reaction. Describing catalyst features that control the selectivity of these two pathways is important to determine the formation of specific products. At the same time, identification of morphological changes occurring to catalysts under reaction conditions can be crucial to tune their catalytic performance. In this contribution we investigate the dependency of selectivity for CO2 reduction on the size of Ru nanoparticles (NPs) and on support. We find that even at rather low temperatures (210 °C), oxidative pretreatment induces redispersion of Ru NPs supported on CeO2 and leads to a complete switch in the performance of this material from a well-known selective methanation catalyst to an active and selective RWGS catalyst. By utilizing in situ X-ray abso...

Low-Temperature Restructuring of CeO2-Supported Ru Nanoparticles Determines Selectivity in CO2 Catalytic Reduction

Steam reforming of ethanol over CuO/CeO2 was studied. Acetaldehyde and hydrogen were mainly produced at 260degreesC. At 380degreesC, acetone was the main product, and 2 mol of hydrogen was produced from 1 mol of ethanol. The formation of hydrogen accompanied by the production of acetone was considered to proceed through the following, consecutive reactions: dehydrogenation of ethanol to acetaldehyde. aldol condensation of the acetaldehyde, and the reaction of the aldol with the lattice oxygen [O(s)] on the catalyst to form a surface intermediate, followed by its dehydrogenation and decarboxylation. The overall reaction was expressed by2C(2)H(5)OH + H2O --> CH3COCH3 + CO2 + 4H(2). Ceria played an important role as an oxygen supplier. The addition of MgO to CuO/CeO2 resulted in the production of hydrogen at lower temperatures by accelerating aldol condensation. (C) 2004 Elsevier B.V. All rights reserved.

Catalytic steam reforming of ethanol to produce hydrogen and acetone

The affinity order among four kinds of major noble metals (Ru, Rh, Pd, and Pt) and CeO2 was investigated by XRD and XAFS techniques. The states of noble metals supported on CeO2 and the sintering behavior of CeO2 on calcination differed depending upon the kinds of noble metals. Rh and Pd retained their Rh O Ce and Pd O Ce bonds even after calcination at 800 °C, and Pt kept its Pt O Ce bond at 500 °C, while Ru on CeO2 formed bulk RuO2 on calcination at temperatures as low as 500 °C. Rh effectively prevented sintering of CeO2. The order of the affinity of the noble metals toward CeO2 was assumed to be Rh > Pd > Pt > Ru.

Affinity order among noble metals and CeO2

Oxidation characteristics of Ru/CeO2 were investigated by N2O decomposition in the presence of C3H6, by temperature programmed desorption (TPD), IR, and XAFS measurements using C3H6 as a probe molecule. Ru/CeO2 (Ru: 5 wt%) had highly dispersed Ru species which formed Ru O Ce and Ru O bonds. This Ru species was in the five-coordination environment and reacted rapidly with allylic hydrogen of C3H6, providing its double bond oxygen. The Ru O site was easily regenerated under oxygen atmosphere. This type of metal-oxo species was not formed for Pt, Pd, and Rh on CeO2. The peculiar oxidation property of Ru/CeO2 catalyst was due to the Ru O species. The optimal preparation procedure for producing Ru O species was to support Ru on hydrated Ce(OH)3 without drying or calcinations. The saturated amount of Ru O species was 2 wt% loading of Ru.

Oxidation characteristics of Ru/CeO2 catalyst

The paper deals with the reactivity of RuO 2 /CeO 2 prepared by a wet reduction method: ruthenium was supported on Ce(OH) 3 , which was precipitated by alkali-hydrolysis of Ce(NO 3 ) 3 , under a reduction condition with formaldehyde at pH 11 and transformed into RuO 2 /CeO 2 catalysts by calcination in air at 773 K. The catalysts were investigated with an ESR technique and were tested for oxidation of propylene. They had no ESR signals by themselves at room temperature, but gave two sets of anisotropic signals upon contact with propylene. These signals were derived from the reaction products between Ru O and propylene. The intensities of the signals were kept unchanged at room temperature for more than 1 h in the absence of excess propylene. The signals decayed in the presence of excess propylene and the upfield signal decayed more rapidly. A prior heating of the catalyst in air at 473 K or above caused the increase in the intensity of the upfield signal. The time course of the signal changes discriminated between ethylene and olefins with allylic hydrogen toward Ru O species.

Reactivity of RuO species in RuO2/CeO2 catalysts prepared by a wet reduction method

Three kinds of Ru/CeO2 catalysts were prepared. The mobility of the oxygen on Ru and their catalytic activity in the wet oxidation of acetic acid was investigated. Ru was present in the form of RuO2, and TPR experiment showed that the reaction, RuO2 + 2H2 → Ru + 2H2O, took place in different temperature ranges depending upon the kind of the catalysts. The catalyst with easily reducible oxygen on Ru had high activity in wet oxidation, and the importance of the release of oxygen from Ru to the reactant was suggested.

Saburo Hosokawa

Papers

Affinity order among noble metals and CeO2

Oxidation characteristics of Ru/CeO2 catalyst

Reactivity of RuO species in RuO2/CeO2 catalysts prepared by a wet reduction method

Reduction behavior of Ru/CeO2 catalysts and their activity for wet oxidation