Ruthenium catalysts carbon monoxide oxidation

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

While many studies have been performed for the oxidation of methanol and carbon monoxide on supported catalyst systems [66,99-103] and Pt-Ru bulk alloys [61,104— 107], relatively few studies have been initiated on single-crystal platinum surfaces modihed with ruthenium. Of those performed these have largely involved the investigation of platinum single crystals modihed by ruthenium dosed electro-chemically [92,93] or spontaneously [80-82,90,91] from aqueous chloride solutions. This approach is discussed in Section 5.4. 

Where the Fischer-Tropsch process has been used on an industrial scale, iron or cobalt are the essential catalyst components. Technical catalysts also contain oxidic promoters, such as alumina and potassium oxide. Ruthenium and nickel are most attractive for academic research since they produce the simplest product packages. Nickel is used for methanation (production of substitute natural gas and removal of carbon monoxide impurities from hydrogen). 

Tranj-dioxoRu(VI) complexes are known to react with olefins according to the classical oxo-transfer mechanism [2] (Fig. 1). The oxoRu(IV) intermediate produced in this process disproportionates readily to give dioxoRu(VI) complex and Ru(II) porphyrin which has strong affinity even towards trace amounts of carbon monoxide. A similar process realized as a side reaction in the rapid oxygenation system would constantly and effectively tie up the catalyst in the catalytically inactive form of Ru (TPFPP)(CO). Indeed, no noticeable changes had been detected in the UV-vis spectrum of the ruthenium porphyrin during the course of Ru (TPFPP)(CO) catalyzed oxidation of cyclohexene. 

The interaction of ruthenium carbonyl, Ru3(C0)j2 with rare earth oxides of high surface area, >50rrrg"l, has been studied. [Ru3(u H)(C0)jq(ii-0M=)] is formed on holmia, but on lanthana only [Ru(C0)o]n species are observed. Reduction of the carbonyl ligands takes place at <573K to give catalysts for the hydrogenation of carbon monoxide with activity and selectivity dependent on the particular rare earth oxide and pretreatment. Over ceria, the product is up to 55 wt% C2-5 olefins. A similar selectivity is obtained over lanthana only after redispersion through a reduction-oxidation-reduction cycle. 

A key question is whether the diatomic molecule in its interaction with metal surfaces remains molecular or dissociates into carbon and oxygen. Broden et al. (3) predicted, by the perturbation of molecular orbitals for CO adsorbed, that only iron could dissociate CO. However, other metals in Group VIII such as nickel (A) ruthenium (5) and rhodium (6) can dissociate CO. Recently Ichikawa et al.(7) observed that disproportionation of CO to CO2 and carbon occurs on small particles of silica-supported palladium. These results show that carbon deposition phenomena may occur via either dissociation of CO on the metals used or disproportionation of CO to CO and carbon on small platinum particles. Cant and Angove (8) studied the apparent deactivation of Pt/Si02 catalyst for the oxidation of carbon monoxide and they suggested that adsorbed CO forms patches and that oxygen atoms are gradually consumed. 

Oxide catalysts, in general, show a smaller degree of activity toward carbon monoxide and hydrogen than the metal catalysts. High pressures and temperatures are required for conversion which is the result of surface reactions. Whereas the high hydrogenating power of cobalt, nickel, and ruthenium orient the hydrogenation of carbon monoxide almost entirely toward hydrocarbons, and the less active iron also produces some alcohols oxide catalysts favor the formation of alcohols. 

J. Assmann, V. Narkhede, L. Khodeir, E. Loffler, O. Hinrichsen, A. Birkner, H. Over, and M. Muhler, On the nature of the active state of supported ruthenium catalysts used for the oxidation of carbon monoxide Steady state and transient kinetics combined with in situ infrared spectroscopy, J. Phys. Chem. B 108, 14634 (2004). 

Carbon monoxide and carbon dioxide are poisons for many hydrogenation catalysts used in ammonia synthesis, refinery processes and petrochemical processes. Therefore, in steam reformers designed to produce hydrogen for hydrogenations, carbon oxides are removed to very low levels, typically a maximum of 5 ppm [7]. The conventional method of achieving this specification is to use a nickel or ruthenium catalyst to convert carbon oxides to methane. The conversion proceeds in accordance with the following methanation reactions ... 

In the above reactions, the oxidation process takes place in the anode electrode where the methanol is oxidized to carbon dioxide, protons, and electrons. In the reduction process, the protons combine with oxygen to form water and the electrons are transferred to produce the power. Figure 9-1 is a reaction scheme describing the probable methanol electrooxidation process (steps i-viii) within a DMFC anode [1]. Only Pt-based electrocatalysts show the necessary reactivity and stability in the acidic environment of the DMFC to be of practical use [2], This is the complete explanation of the anodic reactions at the anode electrode. The electrodes perform well due to the presence of a ruthenium catalyst added to the platinum anode (electrode). Addition of ruthenium catalyst enhances the reactivity of methanol in fuel cell at lower temperatures [3]. The ruthenium catalyst oxidizes carbon monoxide to carbon dioxide, which in return helps methanol reactivity with platinum at lower temperatures [4]. Because of this conversion, carbon dioxide is present in greater quantity around the anode electrode [5]. 

Preliminary results of the reaction between vanadium(iii)-tetrasulpho-phthalocyanine complex with oxygen have been reported these data were compared with those obtained for the corresponding reaction of the hexa-aquo complex ion. The oxidation of methyl ethyl ketone by oxygen in the presence of Mn"-phenanthroline complexes has been studied Mn " complexes were detected as intermediates in the reaction and the enolic form of the ketone hydroperoxide decomposed in a free-radical mechanism. In the oxidation of 1,3,5-trimethylcyclohexane, transition-metal [Cu", Co", Ni", and Fe"] laurates act as catalysts and whereas in the absence of these complexes there is pronounced hydroperoxide formation, this falls to a low stationary concentration in the presence of these species, the assumption being made that a metal-hydroperoxide complex is the initiator in the radical reaction. In the case of nickel, the presence of such hydroperoxides is considered to stabilise the Ni"02 complex. Ruthenium(i) chloride complexes in dimethylacetamide are active hydrogenation catalysts for olefinic substrates but in the presence of oxygen, the metal ion is oxidised to ruthenium(m), the reaction proceeding stoicheiometrically. Rhodium(i) carbonyl halides have also been shown to catalyse the oxidation of carbon monoxide to carbon dioxide under acidic conditions ... 

Ruthenium has a rich chemistry of hydroarylation reactions [22], but it has also been used successfully by Milstein and coworkers [23] as a catalyst for oxidative couplings of the Fujiwara-Moritani type (Figure 4.12). Under an atmosphere of carbon monoxide (6 bar), various ruthenium precursors effectively promoted the reaction of acrylates (e.g., 4g) with benzene (2a) to give a 1 1 ratio of the (E)-cinnamate 5i and methyl propionate 12, rather than the expected hydroarylation product methyl 3-phenylpropionate. Added oxygen (2 bar) could partly take over the role of the reoxidant from the alkene, resulting in an increase in the incorporahon of the alkene into the cinnamate product, giving a ratio of up to 3 1 of the arylated to the reduced acrylate. 

Carbon monoxide (CO), even at trace amounts such as a few ppm levels, can poison Pt catalysts because it can strongly adsorb on the Pt surface, leaving a very small percentage of the Pt surface (e.g., less than 5% at 80°C in the presence of 10 ppm CO) for the HOR. Pt alloys with ruthenium (Ru) and tin (Sn) possess higher CO tolerance, and are thus popular (especially PtRu) as the anode catalysts when H2 is not CO-free. The mechanisms are mainly the accelerated oxidation of CO on PtRu and the reduction of CO adsorption strength on PtSn, respectively. Reaction 1.23 shows how Ru accelerates the... 

Wanat et al. investigated methanol partial oxidation over various rhodium containing catalysts on ceramic monoliths, namely rhodium/alumina, rhodium/ceria, rhodium/ruthenium and rhodium/cobalt catalysts [195]. The rhodium/ceria sample performed best. Full methanol conversion was achieved at reaction temperatures exceeding 550 °C and with O/C ratios of from 0.66 to 1.0. Owing to the high reaction temperature, carbon monoxide selectivity was high, exceeding 70%. No by-products were observed except for methane. 

Figure 4.28 Effect of 21 ppm sulfur dioxide on the activity of a ruthenium/alumina catalyst for the preferential oxidation of carbon monoxide the catalyst contained 1.6 wt.% ruthenium on alumina reaction temperature 150°C [338].

Men et al. investigated selective methanation over nickel and ruthenium catalysts supported by different carriers [344]. A nickel catalyst containing 43wt% nickel, which was doped with 6 wt% calcium oxide and supported by alumina, turned out to be the most active sample. The catalyst produced methane exclusively, from both carbon monoxide and carbon dioxide in the presence of hydrogen. 90% conversion of carbon monoxide could be achieved at a 300 °C reaction temperature with 35% selectivity. The presence of steam reduced the activity of the catalyst but improved its selectivity towards carbon monoxide. When oxygen was added to the feed, the catalyst exclusively converted carbon monoxide into carbon dioxide, and methane formation did not start until all the oxygen had been consumed. 

Dudfield et al. performed investigations of compact aluminium fin heat-exchanger reactors for the preferential oxidation of carbon monoxide [549]. The reactors had the dimensions of 46-mm height, 56-mm width and 170-mm length, which corresponded to a 0.44-L volume and 590-g weight. They contained 2 g of catalyst each [328]. Platinum/ruthenium catalyst formulations of various composition were incorporated into different reactors and tested. Reformate surrogate with a... 

---