Carbonaceous species effects

In the case of NO + C3H6 treatment, N2 formation started immediately after switching feed stream to NO + O2 in spite of small amount of deposited carbonaceous material. The carbonaceous material deposited upon treatment with C3H6 + NO is extremely reactive, showing that carbonaceous species effective for NO reduction is probably produced by the coexistence of C3H6 and NO. 

Heterogeneous catalysts for hydrocarbon conversion may require metal sites for hydrogenation-dehydrogenation and acidic sites for isomerisation-cyclisation and these reactions may be more or less susceptible to the effect of carbonaceous overlayers depending on the size of ensembles of surface atoms necessary for the reaction. In reality we must expect species to be transferred and spilled-over between the various types of sites and if this transfer is sufficiently fast then it may affect the overall rate and selectivity observed. If there is spillover of a carbonaceous species [4] then there may be a common coke precursor for the carbonaceous overlayer on the two types of site. Nevertheless, the rate of deactivation of a metal site or an acidic site in isolation may be very different from the situation in which both types of site are present at a microscopic level on the same catalyst surface. The rate at which metal and acid sites deactivate with carbonaceous material may of course not be identical. Indeed metal sites may promote the re-oxidation of a carbonaceous species in TFO at a lower temperature than the acid sites would allow on their own and this may allow differentiation of the carbonaceous species held on the two types of site. 

The goal of maximum energy generation by oxidation of carbonaceous species often thwarted detailed examination of occasional selective oxidations, such as ethylene oxidation to acetaldehyde on Pd or Au (28, 29, 370) or to ethylene oxide on Ag (330) or methanol and benzyl alcohol oxidation to formates and benzaldehyde, respectively (6-32, 54, 250, 333). Product yields were usually determined at one potential only or even galvanostatically (330), and the combined effects of potential, catalyst, reactant concentration, and cell design or mixing on reaction selectivity are unknown at present. Thus, reaction mechanisms on selective electrocatalysis are not well understood with few exceptions. For instance, ethylene oxidation on solid pal-... 

The effect of temperature, contact time and reactant concentration on the kinetics of NO reduction by CsHs and by CsHg over Pt/Al203 under lean-bum conditions have been investigated and kinetic models which satisfactorily fit the data have been developed. The results suggest that with CsHs the Pt surface is dominated by carbonaceous species, while with CsHs adsorbed atomic oxygen is the main species on the Pt surface. This difference in the state of the Pt surface results in different mechanisms for NOx reduction. Thus, with CsHe, NOx reduction seems to occur via the dissociation of adsorbed NO on the Pt surface, while with CsHg, NOx reduction appears to occur via spill-over of NO2 from the Pt metal onto the AI2O3 support where it reacts with CjHs-derived species to form N2 and N2O. 

It was found that the amount of N2 (and also N2O) formation decreases with increasing offset time At from 0 to 1 second for the lean mixture B. The amoxmt and peak shape of the N2 formed during the NO pulse at At = 1 second are identical to those observed when NO is pulsed over a preoxidised surface. This indicates that one second after the propene/Oa pulse no residual carbonaceous species are present on the surface which can reduce NO. When the next propene/02 pulse enters the catalyst, two seconds after the NO pulse, still some N containing adspecies are on the surface as some N2 (and also N2O) formation is visible. At At = 0 and 0.01 seconds more oxygen leaves the catalyst after the propene/02 pulse then at larger offset times. Also more N2 is formed during the NO pulse at At = 0 and 0.01 seconds. This effect can be caused by a competition between O2 and NO for a direct reaction with propene or reaction products of propene. Another possibility is that O2 and NO compete for reduced adsorption sites. Rottlander et al. [14] recently reported similar results with TAP experiments on a Pt/ZSM-5 catalyst. They proposed that carbon containing surface species, formed from propene, are mainly responsible for the NOx reduction at T < 600 K. 

Nevertheless, it is necessary to return to the main point of interest here, which is the effect of deactivat ion and the accumulation of carbonaceous species on the Pt surface on the structure sensitivity or insensitivity exhibited (as inferred from the dependence of K on the platinum surface area)- Turnover numbers so calculated after 10 min reaction time decrease rapidly with increasing platinum surface area in a hyperbolic manner (and increase almost linearly with the mean platinum particle size) for reactions followed at both 3L3K and 295K, in contrast to the results obtained by Boudart over a narrower range of Pt surface area, Such behaviour very unusual but has been reported for structure sensitivity in cyclopentadiene hydrogenation on supported copper. However, the turnover numhers were almost independent of platinum surface area and... 

It is important to understand the detrimental effects caused by gross fuel starvation as the anodic current that would have been supplied by the oxidation of hydrogen or the fuel is now being supplied by the oxidation of the carbon support to form carbonaceous species such as carbon monoxide and carbon dioxide. Gross fuel starvation results in irreversible damage to the carbon support and the loss of active site loss as the carbon corrodes. 

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