Promotion of surface reactions

Clearly the activity of the surface relative to promotion of surface reactions differed depending on whether ethane or propane was used as a feedstock. This finding Is not surprising since Tsai and Albright (5) report that dynamic equilibria occur during pyrolysis relative to the following oxidation of surface with steam vs reduction of surface with gaseous components and carbon (or coke) deposition vs removal of coke with steam. The present results are, however, the first to show conclusively that surface activity depends on the feedstock surface activity must be related In some way to the levels of both surface oxides and surface carbon. 

The basis of the rate acceleration by this host is an increased effective molarity within the assembly cavity. This principle has been demonstrated with other supramolecular compounds that possess a defined inner space [27, 28]. This is a powerful but narrow capability of these assemblies, employing size- and shape-complementarity to bring molecules together in the promotion of bimolecular reactions. Importantly, this phenomenon does not depend on perturbation of the potential energy surface to effect the rate accelerations. 

A variety of surface reactions have been observed with other metals on a-Si H (Nemanich 1984). For example, a similar silicide is formed with platinum and nickel at 200 °C and with chromium at 400 °C. Aluminum and gold form intermixed phases at low temperature, but do not form silicides. Instead both metals promote low temperature crystallization of the a-Si H film. Dendritic crystallization occurs at 200 °C at a gold contact, giving a very non-uniform interface, and aluminum causes crystallization at 250 C. The resulting Schottky contact for gold is surprisingly ideal, but is very poor for aluminum. 

MVD of ruthenium on Pt(llO) has been shown to provide an ideal system for the study of the promotion of electrocatalytic reactions on a well-characterized Pt-Ru alloy surface [85,86]. In transfer studies, XPS, LEISS, and LEED have been used to characterize the Pt(110)-Ru alloy system, and TPD and stripping voltammetry used to investigate the chemisorption behavior of CO, and the promotion of CO electro-oxidation as a function of incorporated ruthenium. The facile incorporation of ruthenium in the relatively open-packed Pt(110)-(1 x 2) surface provided an ideal model for the alloy system. It is also interesting to note also that the clean Pt(llO) surface exhibits the highest hydrogen oxidation currents of the three basal planes of platinum [108]. 

In contrast, the effect of the adsorbed and alloyed ruthenium on the electrooxidation of CO has shown that promotion of the reaction is only evident if ruthenium is present in the top surface layer. Hence the mediation of the oxidizing species by top-layer ruthenium in the provision of Pt-Ru ensembles, rather than the modification of CO adsorption by ruthenium, promotes the electro-oxidation reaction [85,86]. 

Also notable in the direction of classical catalyst improvement is the research conducted by Rhodes et al.22 A series of coprecipitated promoters were evaluated on ferrochrome catalyst activity at temperatures between 350 and 440 °C. It was found that activity decreases in the following order Hg > Ag > Ba > Cu > Pb > unpromoted > B. A noticeable compensation effect observed in the correlation between preexponential factors and apparent activation energies led the authors to conclude that these promoters might only influence the CO adsorption on catalyst rather than the course of surface reactions. 

Nano/microporous cellulose (NMC) prepared after removal of lignin from wood cellulose was found suitable for the development of cold pasteurization" processes acting as a biofilter for cell removal. It was also used successfully as biocatalyst in food fermentations acting as both cell immobilization carrier and as promoter of biochemical reactions, even at extremely low temperatures. The cumulative surface area of the NMC pores was found to be 0.8 to 0.89 m g" as indicated by porosimetry analysis. This surface is relatively small compared with other porous materials such as y-alumina however, using a natural organic material is attractive from the point of view that it is safer for bioprocess applications and is better accepted by consumers. The NMC/immobilized yeast biocatalyst increased the fermentation rate and was more effective at lower temperatures compared with free cells. Furthermore, the activation energy E, of fermentation was found to be 28% lower than that of free cells, indicating that it is an excellent material to promote the catalytic action of cells for alcoholic fermentation. 

Janjam, S., Peethala, B.C., Zheng, J.P., et al., 2010b. Electrochemical investigation of surface reactions for chemically promoted chemical mechanical pohshing of TaN in tartaric add solutions. Mater. Chem. Phys. 123, 521—528. 

The pyrolysis results obtained using the Vycor reactor were used to make various comparisons. Increased temperatures and decreased partial pressures of entering propane both result in increased ethylene yields. Such findings are consistent with the general trends reported by many previous investigators for pyrolyses of other hydrocarbons and specifically by those (3,4) who have investigated propane pyrolyses in metal reactors. The ore-treatments of metal surfaces also affected product composition. Less surface reactions occur on H S-treated metal surface as compared to untreated surfaces. Oxy n-treated surfaces that have metal oxides on the surface tend to promote undesired surface reactions and to produce considerably more carbon oxides. 

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