Mechanisms metal surface decomposition

Analysis of thermal decomposition of molecules on hot surfaces of solids is of considerable interest not only for investigation of mechanisms of heterogeneous decomposition of molecules into fragments which interact actively with solid surfaces. It is of importance also for clarifying the role of the chemical nature of a solid in this process. Furthermore, pyrolysis of molecules on hot filaments made of noble metals, tungsten, tantalum, etc., is a convenient experimental method for producing active particles. Note that it allows continuous adjustment of the intensity of the molecular flux by varying the temperature of the filament [8]. 

Interest in the adsorption of sulfur-containing molecules at metal surfaces been stimulated by a desire to elucidate the decomposition mechanisms of thiols during the catalytic removal of sulfur from feedstocks and the position of thiols as the favoured head groups for adsorbates used to construct self-assembled monolayers. We shall not survey the extensive self-assembled film literature but restrict our discussion to the simpler thiols. 

The uncertainty in the mechanism of antiwear tribofilm formation derives in part from observations that exposing metal surfaces to heated ZDDP/oil solution forms films similar to those generated in a tribochemical way. From the utility standpoint, both thermal and tribochemical films seem to provide protection from wear. Thus, the current model involves both a tribochemical and thermooxidative component for the decomposition of ZDDP and tribofilm formation (Aktary et al., 2001 Bancroft et al., 1997 Fuller et al., 1997 and 1998 Martin, 1999 Willermet et al., 1995b Yin et al., 1997a). 

Decomposition of formic acid on transition and posttransition metal surfaces has drawn a great deal of interest in recent years (121-137). The process shows distinct periodic regularities and, therefore, is well suited for the BOC-MP analysis. We want to understand the mechanism of HCOOH decomposition, in particular (1) why formate HCOO is the prevailing intermediate, (2) what is the preferred coordination mode, V or t)2, for HCOO, and (3) how HCOO decomposes further into C02 and CO. 

The differences in the temperature dependence and the shape of the kinetic curves for metal- and AC-catalyzed reactions point to the apparent dissimilarities in the mechanism of methane decomposition in the presence of metal and carbon catalysts. The nature of active sites responsible for the efficient decomposition of methane over the fresh surface of carbon catalysts is yet to be understood. 

This chapter is concerned with the work reported in the literature on the steam reforming df hydrocarbons which has been done since 1974 when the earlier review by Ross was written. For continuity some reference has had to be made to research covered in that review and some work before 1974 not described there is included here. Hydrocarbon steam reforming is still a process of major importance for the manufacture of hydrogen, synthesis gases, and town gas and, in the last five years, for the production of substitute natural gas. The study of reactions between hydrocarbon and steam on catalytic surfaces has continued to be an area of interest, throwing light on the mechanism of hydrocarbon decomposition and on the properties, of metal surfaces. 

A mechanism has been postulated to account for the growth of filamentous carbon produced from metal catalyzed decomposition of acetylene ( 5, 6). This mechanism, which is outlined in the schematic diagram, Figure 2, involves the diffusion of carbon through the catalyst particle from the hotter leading face, on which exothermic decomposition of the hydrocarbon occurs, to the cooler trailing faces, at which carbon is deposited from solution. Excess carbon buildup occurs at the exposed particle face and is transported by surface diffusion around the peripheral surfaces of the particle to form an outer skin on the filament. 

The actual mechanism of decomposition on a metal surface can be described by the following reactions (S and S+ stand for the uncharged and charged parts of the metal surface respectively, for a given surface area (S) + (S+) = const. = a. 

The chemistry of acetate on transition metal surfaces is important for a variety of selective oxidation processes. Methanol and vinyl acetate syntheses are two such important oxidation chemistries where acetate intermediates have been postulated. In VAM synthesis, acetate is a critical intermediate in both VAM formation, as well as in its decomposition to CO2. The latter unselective decarboxylation path becomes important at higher operating temperatures. Understanding the mechanism for decarboxylation and VAM synthesis may ultimately aid in the design of new catalyst formulations on new operating conditions. 

On the basis of an infrared study of the adsorption and reaction of methanol and dimethyl ether over alkali metal cation exchanged zeolites, we propose a reaction mechanism for the decomposition of methanol over alkali cation exchanged zeolites. Additionally, formaldehyde adsorption is performed on these molecular sieves and attempts will be made to correlate its adsorption structure with the surface reactivity. 

The intermediate formation and decomposition of PH upon sorption of PH3 on metal surfaces is discussed in Section 1.3.1.5.11, p. 287. The reaction of the spectroscopically observed intermediate PH (from the reaction of red phosphorus with atomic H) with atomic oxygen yielded PC and PO2 [1,2] the same mechanism was also formulated as a step in the reactions of PH3 with O atoms [3]. Cocondensation of PH (from UV photolysis of PH3) with CO in an Ar matrix at 12 K gives HPCO [4]. 

The discussion in this paper appties to wide classes of phase transformations that occur by a nucleation and growth mechanism. Examples of such transformations indude many common transformations, such as boiling and freezing of a liquid and condensation of a vapor, as well as some much less well known transformations, such as oxidation of a metal surface and formation of voids in nudear reactor materials. There are some types of phase transformations, such as spinoidal decomposition, that occur as soon as they are thermodynamically allowed and do not require nudeation. 

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