Adsorbed CO oxidation

Figure 1. Solid lines - cyclic voltammetric curves for hydrogen and oxygen adsorption at the three Pt single crystal planes investigated determined from the pure (CO free) 0.1 M HC10, solution. Dotted lines - cyclic voltammetric curves of adsorbed CO oxidation (note the sensitivity for the CO oxidation curves has been ten fold attenuated. Sweep rate 50 mv/s.

The mechanism for CO oxidation over platinum group metals has been established from a wealth of data, the analysis of which is beyond the scope of this chapter. It is quite evident that surface science provided the foundation for this mechanism by directly showing that CO adsorbs molecularly and O2 adsorbs... 

The first step consists of the molecular adsorption of CO. The second step is the dissociation of O2 to yield two adsorbed oxygen atoms. The third step is the reaction of an adsorbed CO molecule with an adsorbed oxygen atom to fonn a CO2 molecule that, at room temperature and higher, desorbs upon fomiation. To simplify matters, this desorption step is not included. This sequence of steps depicts a Langmuir-Hinshelwood mechanism, whereby reaction occurs between two adsorbed species (as opposed to an Eley-Rideal mechanism, whereby reaction occurs between one adsorbed species and one gas phase species). The role of surface science studies in fomuilating the CO oxidation mechanism was prominent. 

The CO oxidation occurring in automobile exhaust converters is one of the best understood catalytic reactions, taking place on Pt surfaces by dissociative chemisoriDtion of to give O atoms and chemisoriDtion of CO, which reacts with chemisorbed O to give CO, which is immediately released into the gas phase. Details are evident from STM observations focused on the reaction between adsorbed O and adsorbed CO [12]. 

CO oxidation catalysis is understood in depth because potential surface contaminants such as carbon or sulfur are burned off under reaction conditions and because the rate of CO oxidation is almost independent of pressure over a wide range. Thus ultrahigh vacuum surface science experiments could be done in conjunction with measurements of reaction kinetics (71). The results show that at very low surface coverages, both reactants are adsorbed randomly on the surface CO is adsorbed intact and O2 is dissociated and adsorbed atomically. When the coverage by CO is more than 1/3 of a monolayer, chemisorption of oxygen is blocked. When CO is adsorbed at somewhat less than a monolayer, oxygen is adsorbed, and the two are present in separate domains. The reaction that forms CO2 on the surface then takes place at the domain boundaries. 

In this study, Pt/AliOj having high activity for CO oxidation and different affinities for fee adsorption of CO and Hi was selected as a catalyst/adsorbent In a conventional packed bed reactor (PBR), fee surface of fee catalyst is dominantly covered by COads with small amotmt of Oads fee CO conversion is therefore low. Several investigations on periodic operation have illustrated feat fee reaction front wife comparable amount of fee two adsorbed species leads to enhancement of fee CO conversion. Conceptually, this type of the reaction front should be generated by application of a CMBR, as well. Figure 1 illustrates an image of... 

CO oxidation, an important step in automotive exhaust catalysis, is relatively simple and has been the subject of numerous fundamental studies. The reaction is catalyzed by noble metals such as platinum, palladium, rhodium, iridium, and even by gold, provided the gold particles are very small. We will assume that the oxidation on such catalysts proceeds through a mechanism in which adsorbed CO, O and CO2 are equilibrated with the gas phase, i.e. that we can use the quasi-equilibrium approximation. 

Catalytic oxidations on the surface of oxidic materials usually proceed according to the Mars-Van Krevelen mechanism [P. Mars and D.W. van Krevelen, Chem. Eng. Sci. 3 (1954) 41], as illustrated in Fig. 9.17 for the case of CO oxidation. Instead of a surface reaction between CO and an adsorbed O atom, CO2 is formed by reaction between adsorbed CO and an O atom from the metal oxide lattice. The vacancy formed is filled in a separate reaction step, involving O2 activation, often on defect sites. 

Figure 10.7 shows the temperature dependence of CO oxidation rate on a rhodium surface, as reported by Bowker et al. It shows that the rate of reaction maximizes when both reactants, adsorbed CO and O, are present in comparable quantities at a temperature where the activation barrier of the reaction can be overcome. 

Figure 10.8. Temperature-programmed reaction of NO and CO on two surfaces of rhodium. The initially molecularly adsorbed NO dissociates entirely at relatively low temperatures, but NO does not desorb. Note the difference in selectivity and reactivity between the surfaces on Rh(lOO) most of the CO oxidizes to CO2 and the reaction already...

On the other hand, adsorbed CO can be oxidized through the reac-... 

The crucial aspect is thus to determine the fate of the ( CHO), species. Possible mechanisms for its oxidative removal are schematically shown in Fig. 9. From this scheme, it appears that the desorption of the formyl species can follow different pathways through competitive reactions. This schematic illustrates the main problems and challenges in improving the kinetics of the electrooxidation of methanol. On a pure platinum surface, step (21) is spontaneously favored, since the formation of adsorbed CO is a fast process, even at low potentials. Thus, the coverage... 

The rate-determining step (rds) of the reaction on platinum is the oxidation of adsorbed CO with adsorbed hydroxyl species [step (26)]. The current density of the methanol electrooxidation can be obtained from the following equatiorf ... 

Another very interesting result obtained from these FURS measurements is the difference between adsorbed CO obtained from dissolved CO and that from the dissociation of adsorbed methanol. The shift in wave number is more important with dissolved CO. These shifts may also be correlated with the superficial composition of the alloys, and it was observed that the optimized composition for the oxidation of CO (about 50 at.% Ru) is different from that for the oxidation of methanol (about 15 at.% Ru). FTIR spectra also revealed that the amount of adsorbed CO formed from methanol dissociation is considerably higher on R than on Pt-Ru. For a Ptog-Ru-o i alloy, the amount of linearly adsorbed CO is very small (Fig. 8), suggesting a low coverage in the poisoning species. Moreover, by observing the potentials at which the COj IR absorption band appears, it is possible to conclude that the oxidation of both (CHO)ads and (CO)acis species occurs at much lower potentials on a R-Ru alloy electrode than on pure Pt. 

Sinee no oxidation of methanol was observed on pure Ru, it is probable that there is no -( CHO)ads on Ru. However, adsorption of CO was observed on an Ru eleetrode from solution eontaining dissolved CO. The adsorbed CO observed on a R-Ru eleetrode during the adsorption of methanol may arise mainly from the dissoeiation of methanol at Pt sites, followed by the migration of sueh CO speeies toward the neighboring Ru sites. The removal of adsorbed CO by oxidation with adsorbed OH may oeeur by one of two ways (1) at R sites aeeording to reaetions (26) or (27) and (25), and (2) at Ru sites aeeording to reaetion (26 ) ... 

Room temperature CO oxidation has been investigated on a series of Au/metal oxide catalysts at conditions typical of spacecraft atmospheres CO = 50 ppm, COj = 7,000 ppm, H2O = 40% (RH) at 25 C, balance = air, and gas hourly space velocities of 7,000- 60,000 hr . The addition of Au increases the room temperature CO oxidation activity of the metal oxides dramatically. All the Au/metal oxides deactivate during the CO oxidation reaction, especially in the presence of CO in the feed. The stability of the Au/metal oxide catalysts decreases in the following order TiOj > FejO, > NiO > CO3O4. The stability appears to decrease with an increase in the basicity of the metal oxides. In situ FTIR of CO adsorption on Au/Ti02 at 25 C indicates the formation of adsorbed CO, carboxylate, and carbonate species on the catalyst surface. 

Adsorbed CO on a metal surface is one of the simplest adsorbates and has attracted significant interest within the areas of fundamental surface science, catalysis, and electrochemistry. An understanding of the oxidation mechanism of adsorbed CO is important to design and develop electrocatalysts for fuel cells [69-73] and the surface dynamics of adsorbed CO on electrode surfaces in electrolyte solutions is, therefore, very important. 

In-situ ATR-FTIR spectroscopic study of electro-oxidation of methanol and adsorbed CO at Pt-Ru alloy. J. Phys. Chem. B, 108, 2654-2659. 

---