Transient CO oxidation

Figure 2a The rate of formation of CO2, in arbitrary units (a.u.), as a function of time for transient CO oxidation O2 was dosed to a coverage of 0.5 ML at 310 K and CO was dosed at various sample temperatures.

Two commercial catalysts (PtRh and PdRh) were treated in N2+H2O (or Air+H20) at 900, 1000 or llOO C (Lab. ageing) or aged on an engine bench for 25, 50 or 200h. OSC (transient CO oxidation) and catalytic activity for CO, HC and NO abatment Oight-ofif temperatures and conversion at 450°C) were measured over these aged catalysts. Correlations between OSC values and catalytic activity are discussed for a possible On-Board Diagnostic application. 

Many elements of a mathematical model of the catalytic converter are available in the classical chemical reactor engineering literature. There are also many novel features in the automotive catalytic converter that need further analysis or even new formulations the transient analysis of catalytic beds, the shallow pellet bed, the monolith and the stacked and rolled screens, the negative order kinetics of CO oxidation over platinum,... 

Figure 8.27. Transient effect of current on the rate of CO oxidation on Pt (solid lines) and on catalyst potential (broken lines) inlet compositions and temperatures (a) pco=0.47 kPa, po2-10 kPa, T=412°C (b) pco=2.9 kPa, po2=0.40 kPa, T=555°C.33 Reprinted with permission from Academic Press.

In order to assess the role of the platinum surface structure and of CO surface mobility on the oxidation kinetics of adsorbed CO, we carried out chronoamperometry experiments on a series of stepped platinum electrodes of [n(l 11) x (110)] orientation [Lebedeva et al., 2002c]. If the (110) steps act as active sites for CO oxidation because they adsorb OH at a lower potential than the (111) terrace sites, one would expect that for sufficiently wide terraces and sufficiently slow CO diffusion, the chronoamperometric transient would display a CottreU-hke tailing for longer times owing to slow diffusion of CO from the terrace to the active step site. The mathematical treatment supporting this conclusion was given in Koper et al. [2002]. 

Figure 6.2a shows chronoamperometric transients for CO oxidation recorded on three different stepped electrodes for the same final potential. Clearly, the electrode with the higher step density is more active, as it oxidizes the CO adlayer in a shorter period of time. Figure 6.2b shows a fit of a transient obtained on a Pt(15, 15, 14) electrode (terrace 30 atoms wide) by both the mean field model [(6.5), solid line] and the N G model [(6.6), dashed line]. The mean field model gives a slightly better fit. More importantly, the mean field model gives a good fit of all transients on all electrodes. 

Without the direct pathway contribution, this equation may either yield an increasing or decreasing current transient, depending on the value of A ox/ dec- If this ratio is larger than 4, i.e., if methanol decomposition is slow compared with CO oxidation, then the current is predicted to increase with time. Experimentally, this simation has been observed for a low methanol concentration and an almost perfect Pt(l 11) electrode [Housmans and Koper, 2003], which both lead to a low methanol decomposition rate. Typically, however, current transients decrease with time, suggesting that the rate... 

Figure 2. Three-dimensional plots of the CO oxidation transients determined in the single potential step experiments on the Pt(100) electrode in 0.10 M HC10., solution. The potential step was applied from E = +0.40 V to Ep displayed on the third axis of the figure.

The oxidation of CO by Oj over group VIII metal catalysts has been the subject of a large body of ultrahigh vacuum surface science and high pressure catalysis work due to its importance in pollution control. Currently, the removal of CO as CO2 from automobile exhaust is accomplished by catalytic converters which employ a supported Pt, Pd, and Rh catalyst. The importance of CO oxidation has led to numerous recent studies of the kinetics of this reaction on supported metal catalysts and transient kinetic studies on polycrystalline foils , which have sought to identify and quantify the parameters of the elementary mechanistic steps in CO oxidation. 

The presence of various types of Au sites and carbonate/carboxylate species as well as variation in the OH intensity of Ti02 (not shown here) during the reaction suggests that CO oxidation over AuCb catalyst could follow the carboxylate mechanism which involves the reaction of adsorbed CO with OH to produce a carbonate/carboxylate species on Au cations and the decomposition of carboxylate to COz-iS, 10) Transient infrared study needs to be employed to further verily the role of carbonate/carboxylate species in the reaction pathway. (13)... 

Lewandowski and Ollis have proposed a simple kinetic model describing the transient photocatalytic oxidation of aromatic contaminants [50]. The model considered three chemical species an aromatic contaminant, preadsorbed onto the catalyst in the dark and refreshed continuously from the gas phase a strongly bound, recalcitrant reaction intermediate and final reaction products (CO or CO2), assumed for simplicity to be strictly gas-phase species. The model also assumed that two types of catalyst site were present on the photocatalyst surface, with the first suitable for the adsorption of aromatic contaminants, as well as reaction intermediates, and the second type considered to be more polar in nature, suitable only for adsorption of partially oxidized reaction intermediates. 

Zhou, X., Barshad, Y. Gulari, E. 1986 CO oxidation on Pd/Al203. Transient response and rate enhancement through forced concentration cycling. Chem. Engng Sci. 41(5), 1277-1284. 

It is very convenient and helpful in the selection of a sound kinetic model to have a simple experimental technique which can seriously distinguish between the rival kinetic models. To meet this necessity, our recent works have proposed the transient reponse method [7 ]. In the present study, the transient response method is typically applied to distinguish between the rival kinetic models in CO oxidation over a silver catalyst derived from the Hougen-Watson procedure. It is also shown how the best kinetic parameter-set can be determined among the rival parameter-sets by using the transient response method. 

However, if some intrinsic size effects are visible in the CO oxidation at steady state they remain small we will now see the case of transient reaction regimes. 

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