Surface modification combustion

We are concerned with both homogeneous gas-phase chemistry and heterogeneous surface chemistry. Certainly in combustion, gas-phase chemistry is usually dominant. However, there may be good reason to be concerned with heterogeneous chemistry, for example, on the relatively cool walls of a combustion chamber. Moreover there are emerging materials-synthesis and surface-modification techniques that depend on flame-surface interactions. 

M.C. Branch, N. Sullivan, M. Ulsh, and M. Strobel. Surface Modification of Polypropylene Films by Exposure to Laminar, Premixed Methane-Air Flames. Proc. Combust. Inst., 27 2807-2813,1998. 

Y. Shibuya. Surface Modification of Metals by Using the Combustion Flame of 02-C2H2. High Temp. Mater. Processes, 13 173-180,1993. 

Summarizes the known hydrogen effects on internal combustion engine and fuel cell components, suggesting surface modifications that improve stability and performance... 

In conclusion future efforts in structural bulk and surface modifications are needed urgently unless ceria will stay an academic subject for research. This is illustrated by the following figure (Fig. 13.6) showing percentage of conversion (hydrocarbon combustion) versus temperature using a catalyst containing ceria as support or ceria-other oxide solid solution. Two dashed temperature zones are clearly evidenced ... 

It has been established that both the change in activity and the oscillation in environmentally important processes, such as total combustion of hydrocarbons and NO abatement under lean fuel conditions, is the result of transient states during the reaction. The transient state is always a result of fast surface modification by and can be induced by the change in the surface condition of the catalysts e.g. restructuring of the surface, addition of promoters, etc or by the strong interaction between the surface and the reactants in the reacting system. 

The complexity of the reaction rate transients, which consist of one fast and one slow stage, is in agreement with the cyclic voltammetric evidence abont the existence of differently accessible regions for surface charging. The first rapid step (a) is believed to be dne to accumulation of promoting species over the gas-exposed catalyst surface by the mechanism of backspillover, while the second step (b) is due to current-assisted chemical surface modification. Since no correlation between potential transients and reaction rate transients was manifested, a dynamic approach is justified and the applied current —rather than the catalyst overpotential— may be an appropriate parameter to describe the transient behavior of ethylene combustion rate at electrochemically promoted Ir02AfSZ film catalysts. For the interpretation of the fast transient steps (a) and (c), a dynamic model of electrochemical promotion has been developed, as presented in detail in Section 11.3. 

Machida, M., Sato, A., Kijima, T., Inoue, H., Eguchi, K., and Arai, H. Catalytic properties and surface modification of hexaalnminate microcrystals for combustion catalyst. Catal Today 1995, 26, 239-245. 

B.J. Smith and A.R. Marder, Characterization of Chromium Diffusion (Chromize) Coatings in a High Temperature Coal Combustion Atmosphere, Surface Modification Technologies IV, T.S. Sudarshan, D.G. Bhat, and M. Jeandin, Ed., TMS, 1991, p 471... 

The combustion of a hydrocarbon fuel, under controlled conditions generates a flame plasma which modifies the surface of a polyolefin. The depth of modification is 5 -10 nanometers (10 meter). Since it is a surface modification, the bulk properties of the polymer are not affected. 

The basic approach taken in the analytical studies of composite-propellant combustion represents a modification of the studies of double-base propellants. For composite propellants, it has been assumed that the solid fuel and solid oxidizer decompose at the solid surface to yield gaseous fuel and oxidizing species. These gaseous species then intermix and react in the gas phase to yield the final products of combustion and to establish the flame temperature. Part of the gas-phase heat release is then transferred back to the solid phase to sustain the decomposition processes. The temperature profile is assumed to be similar to the situation associated with double-base combustion, and, in this sense, combustion is identical in the two different types of propellants. 

H2 production from ethanol (as well as methanol) employs these methodologies either as such or after slight modifications, especially in the ATR process, wherein a separate combustion zone is usually not present (Scheme 3). A mixture of ethanol, steam and 02 with an appropriate ethanol steam 02 ratio directly enters on the catalyst bed to produce syngas at higher temperature, around 700 °C.18,22 The authors of this review believe that under the experimental conditions employed, both steam reforming and partial oxidation could occur on the same catalyst surface exchanging heats between them to produce H2 and carbon oxides. The amount of 02 may be different from what is required to achieve the thermally neutral operation. Consequently the reaction has been referred to as an oxidative steam reforming... 

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