Oxidation of CO at low temperatures

The oxidation of CO at low temperatures was the first reaction discovered as an example of the highly active catalysis by gold [1]. Carbon monoxide is a very toxic gas and its concentration in indoor air is regulated to 10-50 ppm depending on the conditions [61]. An important point is that CO is the only gas that cannot be removed from indoor air by gas adsorption with activated carbon. On the other hand, metal oxides or noble metal catalysts can oxidize CO at room temperature. 

Kandoi S, Gokhale AA, Grabow LC, Dumesic JA, Mavrikakis M (2004) Why Au and Cu are more selective than Pt for preferential oxidation of Co at low temperature Catal Lett 93 93-100... 

A viable electrocatalyst operating with minimal polarization for the direct electrochemical oxidation of methanol at low temperature would strongly enhance the competitive position of fuel ceU systems for transportation appHcations. Fuel ceUs that directiy oxidize CH OH would eliminate the need for an external reformer in fuel ceU systems resulting in a less complex, more lightweight system occupying less volume and having lower cost. Improvement in the performance of PFFCs for transportation appHcations, which operate close to ambient temperatures and utilize steam-reformed CH OH, would be a more CO-tolerant anode electrocatalyst. Such an electrocatalyst would reduce the need to pretreat the steam-reformed CH OH to lower the CO content in the anode fuel gas. Platinum—mthenium alloys show encouraging performance for the direct oxidation of methanol. 

The presence of both poisoning species and intermediate reaction products requires the development of new electrocatalysts able to break the C-C bond and to oxidize adsorbed CO at low temperatures [94]. Numerous studies have been carried out to develop more active electrocatalysts for ethanol eleetrooxidation. Besides Pt, other metals such as gold, rhodium, and palladium have been investigated as anode catalysts for ethanol eleetrooxidation and show certain activities [105]. On gold in an acid medium, the oxidation reaction leads mainly to the formation of acetaldehyde [94]. The oxidation of ethanol on rhodium proceeds mainly through the formation of acetic acid and CO. 

NiOl-3), and Be(0H)2 ). These gold catalysts are active in the oxidation of CO at a temperature as low as -70 C. However, coprecipitation is valid only for a selected group of metal oxides as mentioned above, because the precipitation rates of support metal hydroxide and gold hydroxides and their affinity might determine in the dispersion of gold. 

Also in 1980s Haruta recognized that supported gold nanocrystals can be highly effective catalysts for the oxidation of CO at very low temperatures (Fig. 4.6) and in particular at temperatures below 0°C. This is a surprisingly high activity and is not replicated by other metals. Haruta recognized that the method of preparation... 

Combining Figs. 17.4 and 17.5 into Fig. 17.6, we see that, although MO and C are favored over M and CO at low temperatures, there is a temperature Tr above which the reverse becomes true, for the diagram as drawn. In other words, above Tr, carbon (usually supplied as coke, i.e., metallurgical-grade coal from which the volatiles have been distilled) is thermodynamically capable of reducing the metal oxide to the metal. 

This oxide is one of the most active catalytic agents for the oxidation of CO near room temperature, if it is not contaminated with adsorbed material in its preparation. Its activity was discovered by Whitesell and Frazer (12). This oxide represents probably the first successful low temperature simple catalyst of industrial importance. It was prepared by treatment of potassium permanganate with sulfuric acid, followed by treatment of the product with concentrated nitric acid. The precipitated hydrated oxide was carefully washed, oxidized, and dried. This yielded a material which was catalytically active for CO oxidation at a temperature as low as — 20°C. It was rapidly poisoned by adsorbed water, but the activity could be regenerated if the water was removed by heating below sintering temperature. 

The FeO(lll) film is, indeed, extremely stable and chemically inert under conditions typically used in UHV-based experiments. However, the situation changes dramatically in the mbar range of pressures. At low temperatures studied here (400 450 K), Pt(lll) is inactive in CO oxidation due to the well-known blocking effect of CO on O2 dissociation. The nanometer-thick FesO lll) films shows some activity, but it is negligible as compared to ultrathin FeO(lll) films, which showed an order of magnitude higher reaction rate under the same conditions (Figure 4.4.7A). Therefore, it is the thin FeO overlayer on Pt that is responsible for the enhanced reactivity of encapsulated Pt particles in CO oxidation. 

The activation energy is lower over Pt than over Ir, Rh, Pd and Ru catalysts [87]. CO adsorption inhibits dissociation of O2 at low temperatures. Because of CO desorption, inhibition becomes less with increased temperatures. The overall rate of oxidation decreases at higher temperatures because of the decrease in surface coverage. 

The practical application of CO oxidation is growing, especially in relation to the development of polymer electrolyte fuel cells. An ongoing attempt is focused on the selective CO oxidation in H2 stream. The key challenging question related to the development of direct methanol fuel cells is whether CO oxidation can proceed at low temperatures, even under a strongly acidic environment. 

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