Catalysts ruthenium dioxide

When referring to Ti02-based photocatalytic systems it is important to note that, in most cases, the semiconducting oxide is associated there with a noble metal or/and a noble metal oxide catalyst. While the role played by these catalysts in (partial) cathodic reactions seems relatively well understood it remains less clear with regard to the photoanodic reactions. In particular, the exact function of the extensively used ruthenium dioxide catalyst has been questioned The role of Ru02 as a hole-transfer catalyst has, for example, been established through laser-photolysis kinetic studies in the case of photo-oxidation of halide (Br and CP) ions in colloidal titanium dioxide dispersions. In fact, the yields of Brf and ClJ radical anions, photogenerated in the course of these reactions. 

On the other hand, comparative reductions of pyridine and 2-methylpyridine in the presence of ruthenium dioxide8 (2% ratio of catalyst to compound) show that the 2-substituted compound is reduced more readily. Indeed, with a lower catalyst ratio the difference is more significant. With a 0.1% ratio of ruthenium dioxide catalyst 2-picoline was completely converted to 2-methylpiperidine in 20 min at 200° and 70 atm pressure. The hydrogenation of pyridine under similar conditions took 90 min. Unpublished work from the same laboratory (37) on the reduction of 2-picoline at room temperature and 2 atm pressure in the presence of a 20% weight ratio of 5% rhodium on alumina shows complete hydrogen uptake in 4 hr as against 6 hr for analytical reagent redistilled pyridine under the same conditions. 

Production of a ruthenium dioxide catalyst for ammonia synthesis by treating ruthenium chloride with aqueous alkali, washing the hydroxide with aqueous magnesium nitrate, and firing the resulting product. V. S. Komarov, M. D. Efros, and G. S. Lemeshono. SU 943204 (1982). 

Ruthenium dioxide or ruthenium-on-carbon are effective catalysts for hydrogenation of mono- and dicarboxylic acids to the alcohol or glycol. High pressures (5,000-10,000 psig) and elevated temperatures (130-225 C) have been used in these hydrogenations 8,12,24). Yields of alcohol tend to be less than perfect because of esterification of the alcohol. Near quantitative yields of alcohol can be obtained by mixing ruthenium and copper chromite catalysts so as to reduce the ester as formed. 

J.-M. Zen, A. S. Kumar and J.-C. Chen, Electrochemical Behavior of Lead-Ruthenium Oxide Pyrochlore Catalyst Redox Characteristics in Comparison with that of Ruthenium Dioxide, J. Mol. Catal. A Chem. 165 (2001) 177-188. 

The ruthenium tetroxide dioxide catalytic system is effective for the oxidation of alkanols, although it will also react with any alkene groups or amine substituents that are present. The catalyst can be used in aqueous acetonitrile containing tetra-butylammonium hydroxide with platinum electrodes in an undivided cell Primary alcohols are oxidised to the aldehyde and secondary alcohols to the ketone [30]. Anodic oxidation of ruthenium dioxide generates the tetroxide, which is the effective oxidising agent. 

Methane is the principal gas found with coal and oil deposits and is a major fuel and chemical used is the petrochemical industry. Slightly less than 20% of the worlds energy needs are supplied by natural gas. The United States get about 30% of its energy needs from natural gas. Methane can be synthesized industrially through several processes such as the Sabatier method, Fischer Tropsch process, and steam reforming. The Sabatier process, named for Frenchman Paul Sabatier (1854—1941), the 1912 Nobel Prize winner in chemistry from France, involves the reaction of carbon dioxide and hydrogen with a nickel or ruthenium metal catalyst C02 + 4H2 —> CH4 + 2H20. 

It was concluded that by combining a hematite nanorod electrode with a suitable water oxidation catalysts, for instance platinum or ruthenium dioxide, the photoelectrochemical activity for direct water splitting applications should increase by a factor of 20. 

Carnahan et al. obtained good yields of alcohols and glycols by hydrogenation of lower mono- and dicarboxylic acids over ruthenium dioxide or Ru-C at 135-225°C and 34-69 MPa H2 (eqs. 10.1 and 10.2).8 In general, the optimum temperature was about 150°C. The chief side reaction was hydrogenolysis of the alcohols, as exemplified in the formation of ethanol from oxalic acid and of butanol and propanol from succinic acid (see eq. 10.2). Platinum and palladium catalysts were ineffective under similar or even more severe conditions. 

However, ruthenium, rhodium, and rhodium-platinum catalysts have been found to be highly effective for the selective hydrogenation of these benzyl-oxygen compounds without loss of the oxygen functions. Thus, benzyl alcohol is hydrogenated to cyclohexanemethanol in high yield over ruthenium dioxide with addition of a small amount of acetic acid (eq. 11.35).114... 

Teuber and Schmitt hydrogenated 5-methoxyindole over ruthenium dioxide in 90% ethanol at elevated temperature and pressure and obtained the corresponding octahy-droindole in a 79% yield (eq. 12.9).11 The hydrogenation was difficult to complete with platinum or rhodium-platinum as catalyst in acetic acid. 

Freifelder and Stone have shown that over ruthenium dioxide, pyridine and its derivatives are readily converted to piperidines under much milder conditions (90-100°C and 7-10 MPa H2) than required for Raney Ni catalyzed hydrogenations.27 Thus, pyridine was hydrogenated to piperidine quantitatively at 95°C and 7-10 MPa H2 in less than 0.5 h over 2 wt% of the ruthenium catalyst (eq. 12.17). At200°C as little as 0.1% of the catalyst was sufficient to carry the hydrogenation to completion in little more than 1 hr. 

According to Freifelder, in most instances ruthenium catalyst is superior to nickel catalyst for the hydrogenation of furans to tetrahydrofurans the hydrogenation can be carried out at 70-100°C and 7 MPa H2.185 He refers to an example in which 2-furfu-rylamine was hydrogenated without solvent over ruthenium dioxide at 100°C and 8 MPa H2 in 10 min, compared to the temperatures of < 150°C and a reaction time of 30 h at 7 MPa H2 that were required for hydrogenation with Raney Ni. Hydrogenation of P-(2-furyl)alkylamines and an A-ethyl-2-furylalkylamine to the corresponding tetrahydro compounds was performed satisfactorily over palladium catalyst in ethanol in the presence of hydrochloric acid at room temperature and 0.62 MPa H2 (eq. 12.102)197 and over 5% Rh-C in neutral solvent at room temperature and 0.15 MPa... 

---