CO, oxidation

The following sections deals with the reactions studied within this work. For UHV and ambient experiments CO oxidation, for UHV ethene hydrogenation and for ambient conditions photocatalytic water splitting is introduced. As these model reactions are extensively studied in surface science only a brief overview is given. Further, the survey is limited to findings on Pt surfaces and Pt nanoparticles, in the case of photocatalysis, to semiconductor based systems (i.e. CdS). 

The conversion of CO and O2 into CO2 in the gas phase has a free enthalpy of -283kJ/mol and is therefore thermodynamically favored [1, 2], However, in order to initiate this reaction, the activation energy for the dissociation of O2 has to be overcome, lowered e.g. by a heterogeneous catalyst. The reaction occurs on Pt (and other group VIII metals) surfaces via a Langmuir-Hinshelwood (LH) mechanism [3-8] with the following steps (Eqs.2.1-2.4, represent surface adsorption sites). 

Schweinberger, Catalysis with Supported Size-selected Pt Clusters, Springer Theses, DOl 10.1007/978-3-319-01499-9 2, 

Recently the CO oxidation reaction on supported Pt particles (of different sizes) was studied under applied conditions (elevated pressures and temperatures, as well as steady-state conditions) and by means of different techniques. Monitoring changes in plasmon frequency (INPS, Sect. 5.2.2) the reaction as a function of the mole fraction (at ambient pressures) was measured on Pt catalysts (2-20 nm size) on Si02 and proved to be able to detect CO poisoning [28], comparable to UHV results. Similar sized catalysts were investigated under near atmospheric pressures (in a 

In conclusion, the CO oxidation mechanism on Ft, and other d-metals is well understood and serves as a benchmark reaction to characterize reactivity. However, with respect to behavior for supported metal particles and small clusters under ambient conditions, there is still the need for studies in order to fully understand the role of the size, particularly with respect to the electronic structure. 

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Fig. XVIII-27. Specific rates of CO oxidation on single crystal and supported catalysts as a function of temperature. (From Ref 308. Reprinted with permission from American Chemical Society, copyright 1988.)...

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... 

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. 

Figure A3.10.25 Arrhenius plots of CO oxidation by O2 over Rli single crystals and supported Rli/Al203 at PCO = PO2 = 0.01 atm [43]. The dashed line in the figure is the predicted behaviour based on the rate constants for CO and O2 adsorption and desorption on Rli under UHV conditions.

Szanyi J, Kuhn W K and Goodman D W 1994 CO oxidation on palladium 2. A combined kinetic-infrared reflection absorption spectroscopic study of Pd(IOO) J. Phys. Chem. 98 2978... 

Nettesheim S, von Oertzen A, Rotermund FI FI and ErtI G 1993 Reaction diffusion patterns in the catalytic CO-oxidation on Pt(110) front propagation and spiral waves J. Chem. Rhys. 98 9977-85... 

Macroscopic properties often influence tlie perfoniiance of solid catalysts, which are used in reactors tliat may simply be tubes packed witli catalyst in tlie fonii of particles—chosen because gases or liquids flow tlirough a bed of tliem (usually continuously) witli little resistance (little pressure drop). Catalysts in tlie fonii of honeycombs (monolitlis) are used in automobile exliaust systems so tliat a stream of reactant gases flows witli little resistance tlirough tlie channels and heat from tlie exotlieniiic reactions (e.g., CO oxidation to CO,) is rapidly removed. 

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]. 

Because Pd(II) salts, like Hgtll) salts, can effect electrophilic metallation of the indole ring at C3, it is also possible to carry out vinylation on indoles without 3-substituents. These reactions usually require the use of an equiv. of the Pd(ll) salt and also a Cu(If) or Ag(I) salt to effect reoxidation of the Pd. As in the standard Heck conditions, an EW substitution on the indole nitrogen is usually necessary. Entry 8 of Table 11.3 is an interesting example. The oxidative vinylation was achieved in 87% yield by using one equiv. of PdfOAcfj and one equiv. of chloranil as a co-oxidant. This example is also noteworthy in that the 4-broino substituent was unreactive under these conditions. Part B of Table 11.3 lists some other representative procedures. 

L = P(CH3)3 or CO, oxidatively add arene and alkane carbon—hydrogen bonds (181,182). Catalytic dehydrogenation of alkanes (183) and carbonylation of bensene (184) has also been observed. Iridium compounds have also been shown to catalyse hydrogenation (185) and isomerisation of unsaturated alkanes (186), hydrogen-transfer reactions, and enantioselective hydrogenation of ketones (187) and imines (188). 

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 ... 

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. 

Fig. 12. Schematic potential energy diagram illustrating the changes associated with the individual reaction steps in CO oxidation on Pd (71).

A. L. Boehman, S. Niksa, and R. J. Moffatt,H Comparison of Rate Earnfor CO Oxidation OverPt on Mlumina, SAE 930252, Society of Automotive Engineers, Warrendale, Pa., 1993. 

Catalyst contamination from sources such as turbine lubricant and boiler feed water additives is usuaUy much more severe than deactivation by sulfur compounds in the turbine exhaust. Catalyst formulation can be adjusted to improve poison tolerance, but no catalyst is immune to a contaminant that coats its surface and prevents access of CO to the active sites. Between 1986 and 1990 over 25 commercial CO oxidation catalyst systems operated on gas turbine cogeneration systems, meeting both CO conversion (40 to 90%) and pressure drop requirements. 

In summary, external recycle reactors are expensive and their usefulness is limited. They can be practical for simple chemical systems where no condensation can occur and neither high pressure nor high temperature is needed. For example Carberry et al (1980) preferred an external recycle reactor over a spinning basket reactor for the study of CO oxidation in dry air at atmospheric pressure. 

Substrate Compounds Aerobic Anaerobic Fermentation Oxidation Co-oxidation... 

FIG. 8 Plot of the fraction of vacant sites as a function of the coverage of inert species (X) during CO oxidation. The squares are determined using Monte Carlo simulations with a fixed X coverage using a grid of 256 x 256 sites. The arrows depict how the system evolves. The production of CO2 is proportional to the number of vacant sites. (From Ref. 68.)... 

A. Cashes, J. Mai, W. von Niessen. A Monte Carlo study of the CO oxidation on probabilistic fractals. J Chem Phys 99 3082-3091, 1993. 

P. Moller, K. Wetzl, M. Eiswirth, G. Ertl. Kinetic oscillations in the catalytic CO oxidation Computer simulations. J Chem Phys 55 5328-5334, 1986. 

B. J. Brosilow, E. Gulari, R. Ziff. Boundary effects in a surface reaction model for CO oxidation. J Chem Phys 99 1-5, 1993. 

A. P. J. Jansen, R. M. Nieminen. A Monte Carlo study of CO oxidation with oscillations induced by site blocking. J Chem Phys 706 2038-2044, 1997. 

V. N. Kusovkov, O. Kortluke, W. von Niessen. Kinetic oscillations in the catalytic CO oxidation on Pt single crystal surfaces Theory and simulation. J Chem Phys 705 5571-5580, 1998. 

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