Oxidation of carbon monoxide

The success of Haruta s early work lay in his choice of preparation method and support. Gold particles of the necessary small size were first obtained by coprecipitation (COPPT) and later by deposition-precipitation (DP) (see Sections 4.2.2 and 4.2.3) classical impregnation with HAuCLj does not work. The choice of support is also critical transition metal oxides such as ferric oxide and titania work well, whereas the more commonly used supports, such as silica and alumina, do not work well or only less efficiently. This strongly suggests that the support is in some manner involved in the reaction. 

The goal of this chapter is to consider the difficulties of comparing results from different laboratories, to point out agreement and disagreement in results and their interpretation, to discuss mechanism(s), to raise questions, and to summarise the present position. Because of the great extent of the literature and the speed with which it grows, we may have inadvertently missed some important papers if we have, we apologise. 

Sensitivity of Gold Catalysts towards Reaction Conditions 

Large variations in activity have been reported for catalysts of apparently similar composition.4,11 Activity depends not only on the obvious variables 

Oxidation of Carbon Monoxide. Few other branched-chain explosions have been studied in the context of direct temperature measurements. Carbon monoxide is 

A full kinetic scheme has been proposed by Yang. - Its important feature is the identification of glow, oscillation, and explosion with different values of oxygen atom concentration. In its simplest form it reproduces the kinetic equations analysed earlier by Gray. This is a two-variable isothermal scheme in which branching is inhibited by the production of a metastable intermediate  

Gray associates the branching agent X and intermediate Y with the spedes O and CiO respectively. He concludes that this system will show three distinct types of behaviour no reaction if the branching factor (=kb — ktt) is negative, damped oscillations when is greater than 0 but less than a critical value, mid explosion when is greater than a critical value. The scheme cannot exhibit limit-Qrcle behaviour characteristic of successive undamped oscillations. 

The oxidation of carbon monoxide takes place in the car exhaust cleaning by means of catalysts based on the platinum metals and represents the simplest heterogeneously catalyzed reaction [54,55]. It involves chemisorption of CO and dissociative chemisorption of oxygen, and COad -f Oad react with each other to CO2 via the Langmuir-Hinshelwood mechanism. As an example. Fig. 6.15 shows the ordered structures formed by these adsorbates on a Rh(l 11) surface [56]. The CO molecules are in this case bonded to the surface on top and always exhibit the tendency to form densely packed adlayers if the coverage becomes high enough (Fig 6.15a). The O atoms, on the other hand, occupy 

In the following, we will concentrate on the behavior of the Pt(l 11) surface. In contrast to Pt(l 0 0) and Pt(l 10) (which will be discussed in Chapter 7), this most densely packed plane does not reconstruct. 

The progress of the reactionis sketched in Fig. 6.16. In this case, adsorbed CO forms a densely packed c2 x 4 layer occupying both the on top and bridge sites [57], while adsorbed O gives rise to the same 2x2 structure as with Rh(l 11). Consequently, the same mixed structure, as depicted in Fig. 6.15c, will be formed if both adsorbates are present on the surface. The operation of the Langmuir-Hinshelwood mechanism was estabhshed through modulated molecular beam experiments that enabled derivation of the kinetic parameters in the framework of the Langmuir approximation [58]. (The CO2 formed is so weakly held at the surface that it is immediately released into the gas phase [59].) 

There are, however, strong indications that the actual mechanism involves more complex aspects. The activation energy of lOOkJ/mol indicated in Fig. 6.16 holds only for small coverages, while it is only half as high at higher coverages [58]. From 

FIGU RE6.16. Mechanism and potential diagram for catalytic CO oxidation at a Pt(l 11) surface at low coverages (energies in kj/mol). (See color insert.) 

Catalytic Oxidation of Carbon Monoxide. - This reaction has been used by several authors as a simple test reaction in the field of catalytic oxidation. Hirota et al.115 conclude from tracer experiments that this follows an oxidation-reduction mechanism in which lattice oxygen is used. In the mechanism proposed, two neighbouring (V=0) groups are successively reduced by CO and are then simultaneously reoxidized. 

Goldwasser and Trimm116 propose a Langmuir-Hinshelwood mechanism 

Davydov et al.44 report on the basis of e.s.r. and i.r. studies that CO adsorption takes place on V3+and V4+-ions. Mori etal.61,119 suggest that the oxidation of CO takes place on active sites such as steps, kinks, or vacancies and that (V=0) groups are much less active. This also explains the observation that Ti02-supported catalysts are less active for this reaction than are unsupported ones, in contrast to the promoting effect observed in hydrocarbon oxidation. Their results do not agree, however, with those of Roozeboom etal.,113 who find a promoting effect also for CO oxidation. 

In 2002, the group of Rolinson [21] reported the use of gold-titania composite aerogels for room temperature oxidation of carbon monoxide. Alkanethiolate monolayer-protected gold clusters with a well controlled size distribution were added to the Ti(IV) precursor during the sol-gel chemistry. After calcination the Au particles aggregate to average diameters of 5-10 nm. 

Partial oxidation of methanol has many commerdal appHcahons for the production of formaldehyde, methyl formate and dimethyl ether (dehydration)  

It was already pointed out in section 4 of chapter IV that the definition of one rate-determining step is meaningful for a sequence of reactions leading to one final product under steady-state conditions. The above classification is restricted to net reactions of this type. The oxidation of carbon monoxide to carbon dioxide is an example. 

The investigations of carbon monoxide adsorption on platinized platinum [29,30] were recently supplemented [31] to allow the establishment of similar relations as on smooth platinum. Sokolsky and coworkers [29, 30] concluded that physical adsorption of carbon monoxide occurs at about 10°C in 0.5 M H2SO4 on platinized platinum. Conversion to CO2 and H2 was postulated for temperatures between 20 C to 30°C. Initial conversion and subsequent chemisorption were suggested between 50 °C and 70 °C. Binder, Kohling, and Sandstede [32] concluded from measurements on Raney platinum electrodes in phosphoric acid and sulfuric acid solutions that chemical conversion is the first step in the anodic oxidation of CO at temperatures between 90°C and 150°C. 

It had been noticed by the author [33] that a certain portion of the adsorbed carbon monoxide is oxidized at less positive potentials than the rest (see the short arrest between about 0.3 V and 0.55 V of curve c in Fig. 15) The transition from the lower to the upper arrest is sometimes accompanied [31] by an overshoot (see curves a, b, c, d in Fig. 39). Adsorbed carbon monoxide that is oxidized in the upper arrest was designated [31] as type I, and the corresponding charge as Qj. The adsorbed species oxidized in the short arrest were denoted [31] as type II, and the charge as Q2. It was demonstrated [31] by gas chromatography that the net reaction in acid solutions 

The rate of oxidation was found [31] to depend upon U and the coverage 62/362 of type II species in the following way  

9 with an 0L2 ri2 value smaller than 1 is consistant with the assumption of reaction 10 

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There has been a general updating of the material in all the chapters the treatment of films at the liquid-air and liquid-solid interfaces has been expanded, particularly in the area of contemporary techniques and that of macromolecular films. The scanning microscopies (tunneling and atomic force) now contribute more prominently. The topic of heterogeneous catalysis has been expanded to include the well-studied case of oxidation of carbon monoxide on metals, and there is now more emphasis on the flexible surface, that is, the restructuring of surfaces when adsorption occurs. New calculational methods are discussed. 

Engei T and Erti G 1978 Eiementary steps in the cataiytic oxidation of carbon monoxide on piatinum metaisTIdv. Catal. 28 1... 

Other important uses of stannic oxide are as a putty powder for polishing marble, granite, glass, and plastic lenses and as a catalyst. The most widely used heterogeneous tin catalysts are those based on binary oxide systems with stannic oxide for use in organic oxidation reactions. The tin—antimony oxide system is particularly selective in the oxidation and ammoxidation of propylene to acrolein, acryHc acid, and acrylonitrile. Research has been conducted for many years on the catalytic properties of stannic oxide and its effectiveness in catalyzing the oxidation of carbon monoxide at below 150°C has been described (25). 

A variety of instmments are available to analyze carbon monoxide in gas streams from 1 ppm to 90%. One group of analyzers determines the concentration of carbon monoxide by measuring the intensity of its infrared stretching frequency at 2143 cm . Another group measures the oxidation of carbon monoxide to carbon dioxide electrochemically. Such instmments are generally lightweight and weU suited to appHcations requiring portable analyzers. Many analyzers are equipped with alarms and serve as work area monitors. 

Carbon dioxide can cause product contamination through ammonium carbonate formation. Ammonium carbonate may also form by oxidation of carbon monoxide by cupric ion (eq. 27) ... 

CO Oxidation Catalyzed by Palladium. One of the best understood catalytic reactions occurring on a metal surface is the oxidation of carbon monoxide on palladium ... 

Very recently, considerable effort has been devoted to the simulation of the oscillatory behavior which has been observed experimentally in various surface reactions. So far, the most studied reaction is the catalytic oxidation of carbon monoxide, where it is well known that oscillations are coupled to reversible reconstructions of the surface via structure-sensitive sticking coefficients of the reactants. A careful evaluation of the simulation results is necessary in order to ensure that oscillations remain in the thermodynamic limit. The roles of surface diffusion of the reactants versus direct adsorption from the gas phase, at the onset of selforganization and synchronized behavior, is a topic which merits further investigation. 

H. P. Kaukonen, R. M. Nieminen. Computer simulations studies of the catalytic oxidation of carbon monoxide on platinum metals. J Chem Phys 97 4380- 386, 1989. 

E. V. Albano. Monte Carlo simulation of the oxidation of carbon monoxide on fractal surfaces. Surf Sci 255 351-359, 1990. 

Perhaps the most familiar example of heterogeneous catalysis is the series of reactions that occur in the catalytic converter of an automobile (Figure 11.12). Typically this device contains 1 to 3 g of platinum metal mixed with rhodium. The platinum catalyzes the oxidation of carbon monoxide and unburned hydrocarbons such as benzene, C6H6 ... 

Catalysts in an oxidized state showed high activity in the oxidation of carbon monoxide [nickel catalysts (146) ] and hydrogen [molybdenum catalysts (146a)]. 

Surface Studies with the Vacuum Microbalance High-Temperature Reactions Earl A. Gulbransen The Heterogeneous Oxidation of Carbon Monoxide Morris Katz... 

Kolbel et al. (K16) examined the conversion of carbon monoxide and hydrogen to methane catalyzed by a nickel-magnesium oxide catalyst suspended in a paraffinic hydrocarbon, as well as the oxidation of carbon monoxide catalyzed by a manganese-cupric oxide catalyst suspended in a silicone oil. The results are interpreted in terms of the theoretical model referred to in Section IV,B, in which gas-liquid mass transfer and chemical reaction are assumed to be rate-determining process steps. Conversion data for technical and pilot-scale reactors are also presented. 

Oxidation of carbon monoxide by metal ions and homogeneous catalysis of the water gas shift reaction and related processes. J. Halpern, Comments Inorg. Chem., 1981,1, 3-15 (42). 

The second step is the oxidation of carbon monoxide to carbon dioxide ... 

Drivers for Performing the Oxidation of Carbon Monoxide to Carbon Dioxide... 

Haruta, M., Kobayashi, T, Sano, H. and Yamada, N. (1987) Novel Gold Catalysts for the Oxidation of Carbon Monoxide at a Temperature far Below 0°. Chemistry Letters, 16, 405-408. 

Mechanisms of the Oxidation of Carbon Monoxide and Small Organic Molecules at Metal Electrodes... 

In this chapter, we have summarized (recent) progress in the mechanistic understanding of the oxidation of carbon monoxide, formic acid, methanol, and ethanol on transition metal (primarily Pt) electrodes. We have emphasized the surface science approach employing well-defined electrode surfaces, i.e., single crystals, in combination with surface-sensitive techniques (FTIR and online OEMS), kinetic modeling and first-principles DFT calculations. 

Gilman S. 1964. The mechanism of electrochemical oxidation of carbon monoxide and methanol on platinum. II. The reactant-pair mechanism for electrochemical oxidation of carbon monoxide and methanol. J Phys Chem 68 70-80. 

Love B, Lipkowski J. 1988. Effect of surface crystallography on electrocatalytic oxidation of carbon monoxide on Pt electrodes. ACS Symp Ser 378 484. 

McCallum C, Pletcher D, 1978. An investigation of the mechanism of the oxidation of carbon monoxide adsorbed onto a smooth Pt electrode in aqueous acid. J Electroanal Chem 70 277. 

Hayden BE, Murray AJ, Parsons R, Pegg DJ. 1996. UHV and electrochemical transfer studies on Pt(110)-(1 X 2) The influence of bismuth on hydrogen and oxygen adsorption, and the electro-oxidation of carbon monoxide. J Electroanal Chem 409 51-63. 

Watanabe M, Motoo S. 1975a. Electrocatalysis by ad-atoms. Part in. Enhancement of the oxidation of carbon monoxide on platinum by mthenium ad-atoms. J Electroanal Chem 60 275-283. 

Kabbabi A, Faure R, Durand R, Beden B, Hahn F, Leger JM, Lamy C. 1998. In situ FTIRS study of the electrocatalytic oxidation of carbon monoxide and methanol at platinum-ruthenium bulk alloy electrodes. J Electroanal Chem 444 41-53. 

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