Carbon monoxide over supported metals

Dynamic reactor studies are not new, but they have not been widely used in spite of the fact that they can provide a wealth of information regarding reaction mechanisms. In this research, oxidation of carbon monoxide over supported cobalt oxide (C03O4) was studied by both dynamic and conventional steady state methods. Among metal oxides, cobalt oxide is known to be one of the most active catalysts for CO and hydrocarbon oxidation, its activity being comparable to that of noble metals such as palladium or platinum. 

Adsorptive and Catalytic Properties of Carbon Monoxide and Carbon Dioxide Over Supported Metal Oxides... 

In hydrogenation of carbon monoxide over supported catalysts of technetium metal between 270 and 500 °C in a helium flow, predominantly methane was formed. The conversion of CO into methane increased with increasing temperature. Conversion yields up to 100 vol% of CH4 were obtained over a TC/7-AI2O3 catalyst [23]. 

Cant, N., Hicks, P. and Lennon, B. (1978). Steady state oxidation of carbon monoxide over supported noble metal catalysts with particular reference to platinum, J. Catal., 54, pp. 372-383. Taylor, K. (1984). Catalysis, Science and Technology, J. Anderson and M. Boudart (eds), Vol. 5. Springer Verlag, Berlin, p. 119. 

Ceint NW, Hicks PC, Lennon BS (1978) Steady-state oxidation of carbon monoxide over supported noble metals with particular reference to platinum. J Catal 54 372-383... 

This reaction serves for removal of carbon monoxide from gas mixtures and is usually carried out over supported metal catalysts. In reforming techniques, carbon monoxide, poisonous for the catalyst in fuel cells, is removed in such a way. It is also applied in automobiles for reducing the exhaust gas carbon monoxide to an environmentally acceptable level. 

Al-Ammar AS, Webb G (1978) Hydrogenation of acetylene over supported metal catalysts Part 1 - Adsorption of [ C] Acetylene and [ C] ethylene on silica supported rhodium, iridium and palladium and alumina supported palladium. J Chem Soc Earaday Trans 74 195 Al-Ammar AS, Webb G (1979) Hydrogenation of acetylene over supported metal catalysts Part 3 - [ C] tracer studies of the effect of added ethylene and carbon monoxide on the reaction catalyzed by silica-supported palladium, rhodium and iridium. J Chem Soc Faraday Trans 75 1900... 

Takenaka, S., Shimizu, T. and Otsuka, K. (2004) Complete removal of carbon monoxide in hydrogen-rich gas stream through methanation over supported metal catalysts. International Journal of Hydrogen Energy, 29, 1065-1073. 

Chemisorption of carbon monoxide on transition metal surfaces (either single crystals or supported clusters) is a tool of general use to study the active sites present over this type of solid surfaces [42]. CO adsorption on noble metals has been the subject of a large number of papers. For instance, the adsorption behaviour of CO on surfaces of Pt single crystals, polycrystalline Pt films, and supported Pt catalysts has been discussed in terms of adsorbed species or adsorption structures formed during interaction of CO with the metal surface. 

As wc know, carbon nanofibers have been widely used in various areas, such as in electronic components, as polymer additives, for gas storage, and as catalyst support [94]. Via the decomposition of hydrocarbons or carbon monoxide over Ni, Co or Fe catalysts, a large quantity of carbon fibers can be produced. Moreover, these carbon fibers with different sizes and shapes, such as straight, bent, thin and helical, etc., could be synthesized by controlling the reaction conditions [95-99]. With carbon nanofibers as template, Ueda and co-workers prepared a number of single and binary transition metal oxide nanofibers and/or nanotubes, including LaMnOs and LaFeOs nanofibers and nanotubes [99-102]. 

The success of the correlation of catalytic behavior with bulk Mossbauer parameters by Skalkina et al. is also reflected in the work of Tops0e and Boudart (96). As discussed earlier, these authors found a decrease in the isomer shift of the octahedral iron ions in a lead-promoted Cr-Fe304 carbon monoxide shift catalyst, indicative of an increased covalency of these ions. Schwab et al. (203) have proposed a correlation of the activity for CO oxidation by ferrites with the octahedral ions in these materials, and the electron transfer required for this catalytic process may be facilitated by an increased covalency of the metal ions (204). In view of these suggestions, the lead-promoted catalyst is expected to possess a higher catalytic activity for the CO shift reaction than an unpromoted catalyst, as evidenced by the Mossbauer parameters of these two samples. This has in fact been shown experimentally to be the case (96). For the reverse CO shift reaction over supported europium (176), the success of the correlation between catalytic activity and the Mossbauer parameters (in this case the reducibility) has already been noted in Section III, A, 4. 

Over most reported Au catalysts, CO oxidation takes place at the junction perimeter between Au NPs and the metal oxide supports. Carbon monoxide is adsorbed on the edges, corners and steps of Au NPs. Molecular oxygen is adsorbed on the support surfaces and may be activated at the oxygen defect sites at the perimeter interfaces, where the two adsorbates react to form C02 in the gas phase. At the perimeter interfaces Au is assumed to exist as Au3 +, which might be stabilized through bonding with OH-. 

A representative comparison of the effect of the catalyst bed geometry on methane conversion and product selectivity over a range of methane/air ratios is shown in Fig 4 Unlike typical supported catalysts, where the catalyst is well-dispersed and submicrometer-sized, the noble-metal catalysts in these methane oxidation reactions were basically films with micrometer-sized surface features (Other tests on both extruded cordiente and alumina foam monoliths with lower catalyst loading resulted in similar carbon monoxide production but lower hydrogen yields than those illustrated in the figure, which provided evidence that the reaction is catalyst-dependent and not initiated by the monoliths or gas... 

A Mdssbauer investigation of the reduction of iron oxide (0.05 wt % Fe) and iron-oxide-with-palladium (0.05 wt % Fe, 2.2 wt % Pd), carried upon 7 -Al203, reveals that supported ferric ion alone, under hydrogen, yields ferrous ion only at 500—700 °C this reduction takes place at room temperature with the bimetallic catalyst and proceeds to form a PdFe alloy at 500 °C. Similar effects are found in reduction by carbon monoxide, which yields iron-palladium metal clusters at 400 °C. The view is taken that migration over T7-A1203 is not involved but that activated hydrogen transfers only at bridgeheads on the contact line between the metal and iron oxide. 

In the late 1960s, new antipollution initiatives were enacted to reduce nitrogen oxides, carbon monoxide, and lead pollutants from automotive exhaust. Nitrogen oxides were responsible for the brown haze that hung over cities that can still be seen today. The advent of the catalytic converter, a small canister that contained heavy metal catalysts embedded on a ceramic support, helped oxidize carbon monoxide and reverse the reaction that produced nitrogen oxides. However, lead in the exhaust stream deactivated the catalysts in the catalytic converter. The only solution was to remove tetraethyllead from the gasoline. 

Deactivation during carbon monoxide oxidation carried over alumina supported copper and copper-chromite catalysts has been examined. Prereduction of the oxidic precursor with CO increased catalyst activity, but prereduction with hydrogen led to less activity increase due to copper metal crystallization. 

Methane reforming with carbon dioxide over Pt/ZrOj proceeds in a complex sequence of reaction steps involving decomposition of methane on the Pt to CH, (average value of x=2) and H2. In the second set of reactions COj reduction occurs via initial formation of a carbonate type species at the metal-support boundary. The reaction between the surface bound carbon (from methane dissociation) and the carbonate (from CO2 activation) yields carbon monoxide via a formate intermediate. A stable catalyst can only be achieved if the two sets of reactions are balanced. 

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