Oxidation of Co

One of the best understood reactions is the catalytic oxidation of carbon monoxide on Pt or Pd 

Particularly, the formation of C02 by the oxidation of CO over platinum metal catalysts is the reaction that has been studied most extensively in the past years and whose elementary steps appear to be best understood. This reaction is an important process in automobile catalytic converters [92] and its mechanism proceeds as follows [129]  

We consider next perhaps the bet understood catalyzed reaction the oxidation of CO over group VIII metal catalysts. The reaction is an important environmental one since it involves the conversion of CO to CO2 in automobile catalytic converters. The mechanism is straightforward  

It is Langmuir-Hinshelwood in type, and the usually observed rate law is 

As with any system, there are complications in the details. The CO sticking probability is high and constant until a 0 of about 0.5, but then drops rapidly [306a]. Practical catalysts often consist of nanometer size particles supported on an oxide such as alumina or silica. Different crystal facets behave differently and RAIRS spectroscopy reveals that CO may adsorb with various kinds of bonding and on various kinds of sites (three-fold hollow, bridging, linear) [307]. See Ref 309 for a discussion of some debates on the matter. In the case of Pd crystallites on a-Al203, it is proposed that CO impinging on the support 

Perhaps the most fascinating detail is the surface reconstruction that occurs with CO adsorption (see Refs. 311 and 312 for more general discussions of chemisorption-induced reconstructions of metal surfaces). As shown in Fig. XVI-8, for example, the Pt(lOO) bare surface reconstructs itself to a hexagonal pattern, but on CO adsorption this reconstruction is lifted [306] CO adsorption on Pd( 110) reconstructs the surface to a missing-row pattern [309]. These reconstructions are reversible and as a result, oscillatory behavior can be observed. Returning to the Pt(lOO) case, as CO is adsorbed patches of the simple 1 x 1 structure (the structure of an undistorted (100) face) form. Oxygen adsorbs on any bare 1 x 1 spots, reacts with adjacent CO to remove it as CO2, and at a certain point, the surface reverts to toe hexagonal stmcture. The presumed sequence of events is shown in Fig. XVIII-28. 

With certain critical Pco/Poi ratios, structural oscillations can be observed [306]. Patterns of stationary and/or traveling waves can actually be seen by means of photoemission electron microscopy (see Ref. 313, and note Section XVIII-7B. Such behavior can be modeled mathematically (e.g.. Refs. 214, 314). 

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Some fascinating effects occur in the case of CO on Pt(lOO). As illustrated in Fig. XVI-8, the clean surface is reconstructed naturally into a quasi-hexag-onal pattern, but on adsorption of CO, this reconstruction is lifted to give the bulk termination structure of (110) planes [56]. As discussed in Section XVIII-9E very complicated changes in surface structure occur on the oxidation of CO... 

A catalyst may play an active role in a different sense. There are interesting temporal oscillations in the rate of the Pt-catalyzed oxidation of CO. Ertl and coworkers have related the effect to back-and-forth transitions between Pt surface structures [220] (note Fig. XVI-8). See also Ref. 221 and citations therein. More recently Ertl and co-workers have produced spiral as well as plane waves of surface reconstruction in this system [222] as well as reconstruction waves on the Pt tip of a field emission microscope as the reaction of H2 with O2 to form water occurred [223]. Theoretical simulations of these types of effects have been reviewed [224]. 

Bonn M, Funk S, Hess C, Denzier D N, Stampfl C, Soheffler M, Wolf M and ErtI G 1999 Phonon versus eleotron-mediated desorption and oxidation of CO on Ru(OOI) Science 285 1042... 

The oxidation of CO to CO2, whieh is essential to eontrolling automobile emissions, has been extensively studied beeause of the relative simplieity of this reaetion. CO oxidation was the first reaetion to be studied using the surfaee seienee approaeh and is perhaps the most well understood heterogeneous eatalytie reaetion [58]. The simplieity of CO oxidation by O2 endears itself to surfaee seienee studies. Both reaetants are diatomie moleeules whose adsorption... 

Figure A3.14.il. Spiral waves imaged by photoelectron electron microscopy for the oxidation of CO by O2 on a Pt(l 10) single crystal under UHV conditions. (Reprinted with pennission from [35], The American Institute of Physics.)...

The vapor cloud of evaporated droplets bums like a diffusion flame in the turbulent state rather than as individual droplets. In the core of the spray, where droplets are evaporating, a rich mixture exists and soot formation occurs. Surrounding this core is a rich mixture zone where CO production is high and a flame front exists. Air entrainment completes the combustion, oxidizing CO to CO2 and burning the soot. Soot bumup releases radiant energy and controls flame emissivity. The relatively slow rate of soot burning compared with the rate of oxidation of CO and unbumed hydrocarbons leads to smoke formation. This model of a diffusion-controlled primary flame zone makes it possible to relate fuel chemistry to the behavior of fuels in combustors (7). 

The radicals and other reaction components are related by various equiUbria, and hence their decay by recombination reactions occurs in essence as one process on which the complete conversion of CO to CO2 depends. Therefore, the hot products of combustion of any lean hydrocarbon flame typically have a higher CO content than the equiUbrium value, slowly decreasing toward the equiUbrium concentration (CO afterburning) along with the radicals, so that the oxidation of CO is actually a radical recombination process. 

Precious Meta.1 Ca.ta.lysts, Precious metals are deposited throughout the TWC-activated coating layer. Rhodium plays an important role ia the reduction of NO, and is combiaed with platinum and/or palladium for the oxidation of HC and CO. Only a small amount of these expensive materials is used (31) (see Platinum-GROUP metals). The metals are dispersed on the high surface area particles as precious metal solutions, and then reduced to small metal crystals by various techniques. Catalytic reactions occur on the precious metal surfaces. Whereas metal within the crystal caimot directly participate ia the catalytic process, it can play a role when surface metal oxides are influenced through strong metal to support reactions (SMSI) (32,33). Some exhaust gas reactions, for instance the oxidation of alkanes, require larger Pt crystals than other reactions, such as the oxidation of CO (34). 

A TWC catalyst must be able to partition enough CO present in the exhaust for each of these reactions and provide a surface that has preference for NO adsorption. Rhodium has a slight preference for NO adsorption rather than O2 adsorption Pt prefers O2. Rh also does not cataly2e the unwanted NH reaction as does Pt, and Rh is more sinter-resistant than Pt (6). However, the concentrations of O2 and NO have to be balanced for the preferred maximum reduction of NO and oxidation of CO. This occurs at approximately the stoichiometric point with just enough oxidants (O2 and NO ) and reductants (CO, HC, and H2). If the mixture is too rich there is not enough O2 and no matter how active the catalyst, some CO and HC is not converted. If the mixture is too lean, there is too much O2 and the NO caimot effectively compete for the catalyst sites (53—58). 

Some studies of potential commercial significance have been made. For instance, deposition of catalyst some distance away from the pore mouth extends the catalyst s hfe when pore mouth deactivation occui s. Oxidation of CO in automobile exhausts is sensitive to the catalyst profile. For oxidation of propane the activity is eggshell > uniform > egg white. Nonuniform distributions have been found superior for hydrodemetaUation of petroleum and hydrodesulfuriza-tion with molybdenum and cobalt sulfides. Whether any commercial processes with programmed pore distribution of catalysts are actually in use is not mentioned in the recent extensive review of GavriUidis et al. (in Becker and Pereira, eds., Computer-Aided Design of Catalysts, Dekker, 1993, pp. 137-198), with the exception of monohthic automobile exhaust cleanup where the catalyst may be deposited some distance from the mouth of the pore and where perhaps a 25-percent longer life thereby may be attained. 

The kinetics of the oxidation of CO on a platinum surface indicate that CO and oxygen are adsorbed to about the same extent. The rate of oxidation depends on the oxygen partial pressure when CO is in excess, and on tire CO partial pressure when oxygen is in excess. 

Further oxidation of the pollutants outside the combustion chamber. This oxidation may be either by normal combustion or by catalytic oxidation. These systems require the addition of air into the exhaust manifold at a point downstream from the exhaust valve. An air pump is employed to provide this air. Figure 31-2 illustrates an engine with an air pump and distribution manifold for the oxidation of CO and hydrocarbons (HC) outside the engine. 

The catalytic properties of the shock-modified rutile whose defect properties have been reported in previous sections of this chapter have been studied in a flow reactor used to measure the oxidation of CO by Williams and coworkers [82G01, 86L01]. As shown in Fig. 7.7 the effect of shock activation is substantial. Whereas the unshocked material displays such low activity that an effect could only be observed at the elevated temperature of 400 °C, the shock-modified powder shows substantially enhanced catalytic activity with the extent of the effect depending on the shock pressure. After a short-time transient is annealed out, the activity is persistent for about 8 h. Although the source of the surface defects that cause the activity is not identified, the known annealing behavior of the point defects indicates that they are not responsible for the effect. 

Fig. 7.7. Studies of the catalytic activity of shock-modified rutile in the oxidation of CO shows greatly enhanced catalytic activity, which is strongly influenced by the shock conditions [86G01].

The ZGB lattiee gas model is an oversimplified approaeh to the aetual proeesses involved in the eatalytie oxidation of CO. Consequently several attempts have been made in order to give a more realistie deseription. Some of them are the following (i) The inelusion of A desorption [19,38-40] eauses the first order IPT to beeome reversible and slightly rounded, in qualitative agreement with experiments (Fig. 3). (ii) The influenee of lateral interaetions between reaetants adsorbed on the eatalyst surfaee have been eonsidered by various authors, e.g., [38,41,42]. (iii) Studies on the influenee of the fraetal nature of the eatalyst surfaee were motivated by the faet that most eatalysts are eonstituted by small fraetal (metallie) elusters dispersed on a fraetal support. The fraetal surfaees have been modeled by means of... 

So far, none of the simulations of the ZGB model diseussed above attempt to deseribe one of the most faseinating and well doeumented [2,16,16] behaviors of the eatalytie oxidation of CO on some erystallographie planes of Pt, namely the oeeurrenee of oseillations. After the early simulation work of Moller et al. [62], very reeently numerous lattiee-gas models have been studied. There are various meehanisms that might be invoked in order to explain the oseillations observed experimentally [15,62,67-74]. A simple approaeh is to introduee an inert speeies eapable of bloeking surfaee sites. Also, oseillations ean be due to adsorbate-indueed surfaee reeonstruetions of the surfaee. So, let us diseuss both approaehes in detail. 

Recently, Vigil and Willmore [67] have reported mean field and lattice gas studies of the oscillatory dynamics of a variant of the ZGB model. In this example oscillations are also introduced, allowing the reversible adsorption of inert species. Furthermore, Sander and Ghaisas [69] have very recently reported simulations for the oxidation of CO on Pt in the presence of two forms of oxygen, namely chemisorbed atomic O and oxidized metal surface. These species, which are expected to be present for reaction under atmospheric pressure, are relevant for the onset of oscillatory behavior [69]. 

It is well known that the catalytic oxidation of CO on certain Pt surfaces exhibits oscillatory behavior, within a restricted range of pressures and temperatures, which are coupled with adsorbate-induced surface phase transitions [16,17]. In fact, in their clean states the reconstructed surfaces of some crystallographic planes, e.g. Pt(lOO) and Pt(llO), are... 

M. Ehsasi et al. Steady and nonsteady rates of reaction in a heterogeneously catalyzed reaction Oxidation of CO on platinum, experiments and simulations. J Chem Phys 97 4949-4956, 1989. 

J. Mai, A. Cashes, W. von Niessen. A Monte Carlo simulation of the catalytic oxidation of CO on DLA clusters. Chem Phys Lett 277 197-202, 1993. 

CO Dehydrogenase, also incorporating Fe and catalysing the oxidation of CO to CO2. The attachment of CO to a nickel centre coordinated to perhaps four 5-donors is postulated. 

Other variations of the dual-bed scheme exist as a combination of thermal oxidizing reactors and catalytic reducing reactors. The Questor company has developed a reactor with three zones the first zone is a thermal reactor with limited air to raise the temperature of the exhaust gas, the second zone is a catalytic bed of metallic screens to reduce NO, and the third zone is another thermal reactor where secondary air is injected to complete the oxidation of CO and hydrocarbons (45). 

The relatively high cost and lack of domestic supply of noble metals has spurred considerable efforts toward the development of nonnoble metal catalysts for automobile exhaust control. A very large number of base metal oxides and mixtures of oxides have been considered, especially the transition metals, such as copper, chromium, nickel, manganese, cobalt vanadium, and iron. Particularly prominent are the copper chromites, which are mixtures of the oxides of copper and chromium, with various promoters added. These materials are active in the oxidation of CO and hydrocarbons, as well as in the reduction of NO in the presence of CO (55-59). Rare earth oxides, such as lanthanum cobaltate and lanthanum lead manganite with Perovskite structure, have been investigated for CO oxidation, but have not been tested and shown to be sufficiently active under realistic and demanding conditions (60-63). Hopcalities are out-... 

There are few studies in the literature on the kinetics and mechanism of oxidation over base metal oxides. Blumenthal and Nobe studied the oxidation of CO over copper oxide on alumina between 122 and 164°C. They reported that the kinetics is first order with respect to CO concentration, and the activation energy is 20 kcal/mole (77). Gravelle and Teichner studied CO oxidation on nickel oxide, and found that the kinetics is also first order with respect to CO concentration (78). They suggested that the mechanism of reaction is by the Eley-Rideal mechanism... 

The most complete study on the oxidation of CO and hydrocarbons was reported by Kuo et al. (91). Their study was done on a copper chromite catalyst under conditions that simulate exhaust gases. They found that CO oxidation kinetics is very accurately represented as first order in CO... 

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