Desorption surface intermediate

Cavitation induced turbulence also enhances the rates of the desorption of intermediate products from the catalyst active sites and helps in continuous cleaning of the catalyst surface. 

The resulting radicals are not usually observed, but thermal desorption products indicate the nature of the surface intermediates. Molybdenum(V) dispersed on silica also gives rise to 0 and O2 ions when exposed to N2O and O29 respectively. The 0 ion on this surface may be used to activate methane and ethane in a catalytic cycle which leads to their partial oxidation. 

The components of the starting mixture are in rapid adsorption-desorption interaction with the surface. For example, a part of adsorbed -hexane desorbs as -hexane another part reacts to give benzene. If benzene formation involves an n-hexene surface intermediate, this hexene—the concentration of which may be eventually so small that it does not appear in the gas phase—interacts with the inactive hexene in the starting material and increases its specific radioactivity. 

A heterogeneously catalysed reaction involves several steps (Mady et al, 1976) (i) mass transport of fluid reactants to the surface, (ii) chemisorption of reactants on the surface, (iii) diffusion and chemical reaction at the surface and (iv) desorption and diffusion of products from the surface. Step (iii), involving the formation of surface intermediates, is the key step. Formation of surface intermediates, which ultimately give rise to products, was first proposed by Sabatier (see Burwell, 1973) and strikingly demonstrated by Sachtler Fahrenfort (1960) in the decomposition of formic acid... 

The most significant observation is the large differences in reactivities of the three forms of oxygen ions, with O" Oj OJ. This is well illustrated by the reaction with ethylene where O- ions react readily at — 60°C, Oj ions react at 25°C with a half-life of ca. 5 min, whereas only one-third of the Oj ions react after 2 hr at 175°C. The authors propose a number of surface intermediates (Table XIV) in the oxidation reactions based on analysis of the desorption products and IR studies. 

The scheme involves a number of unknown surface intermediates, while the appearance of product molecules in the gas phase depends on the relative rates of adsorption, desorption, formation and conversion. The calcu-... 

For the cases where gas phase cyclohexenes do not appear to be intermediates, the question arises as to the nature of the surface reaction. Thus, does cyclohexane simultaneously lose six hydrogen atoms via the sextet mechanism (T3) originally proposed by Balandin in 1929, or does the reaction take place in a stepwise fashion without desorption of intermediate products According to the sextet theory, the active catalyst unit is an aggregate of metal atoms which must be spaced within certain definite limits consistent with the geometry of the cyclohexane ring. While there... 

The production of butadiene from butene involves at least three surface intermediates adsorbed butene, 7t-allyl, and butadiene. One or more of these may be particularly vulnerable to attack by gas-phase oxygen on a-Fe203. From the temperature programmed desorption experiments, it was found that the products of isomerization, selective oxidation, and combustion... 

Fig. 6. Schematic representation of the competitive processes in butene oxidation for (a) a-Fe203, (b) y-Fe203. Deg degradation of surface intermediates BD production and desorption of butadiene C02 production and desorption of C02.

At present we have evidence for the complexity of higher temperature adsorption/desorption phenomena while, in general, the kinetic characteristics observed for many catalytic reactions are perhaps deceptively simple. The estimations of the concentrations of the participating surface intermediates are, in contrast, experimentally very difficult. Mechanistic investigations of many heterogeneous catalytic processes yield insufficient information to allow clear distinctions to be drawn between alternative reaction modelsf 125). 

Finally, new methods of analysis have recently been developed that may allow characterization of single atoms on surfaces such as atomic force microscopy.9 In certain cases, in situ experiments can be done such as the study of electrodes, enzymes, minerals and biomolecules. It has even been shown that one atom from a tip can be selectively placed on a desired surface.10 Such processes may one day be used to prepare catalysts that may enhance selectivity. Other methods that show promise as regards detection of surface catalytic intermediates are temperature programmed desorption techniques.11 Selective poisoning of some surface intermediates with monitoring via temperature programming methods may also allow the preparation of more selective catalysts. 

In contrast to the acetaldehyde decarbonylation, reactions with ethanol over Rh (111) did not lead to formation of methane but rather to an oxametallocycle via methyl hydrogen abstraction. These data suggest that ethanol formed over supported rhodium catalysts may not be due to hydrogenation of acetaldehyde. This study shows how surface science studies of model catalysts and surfaces can be used to extract information about reaction mechanisms since the nature of surface intermediates can often be identified by methods such as temperature programmed desorption and high resolution electron energy loss spectroscopy. 

Consider an example from nucleation and growth of thin films. At least three length scales can be identified, namely, (a) the fluid phase where the continuum approximation is often valid (that may not be the case in micro- and nanodevices), (b) the intermediate scale of the fluid/film interface where a discrete, particle model may be needed, and (c) the atomistic/QM scale of relevance to surface processes. Surface processes may include adsorption, desorption, surface reaction, and surface diffusion. Aside from the disparity of length scales, the time scales of various processes differ dramatically, ranging from picosecond chemistry to seconds or hours for slow growth processes (Raimondeau and Vlachos, 2002a, b). 

Many similar studies have been done at surface science conditions using step-function concentration signals. For example, Takagi et al. (52) studied the adsorption and desorption of CO on Ni(lOO) via infrared reflection absorption spectroscopy (IRAS). In one case, CO dosing was turned on for 214 s and then turned off. The response of the spectrum of the surface intermediate was followed by IRAS. Of course, under these high-vacuum... 

Methanol reactions have also been studied on polycrystalline wafers of UO2 [76]. Two parent desorption states existed for methanol adsorbed at 90 K. Molecularly adsorbed methanol desorbed at 110 K, and methanol generated by surface recombination of methoxides and protons desorbed at 180 K. Carbon (Is) XPS demonstrated that methanol dissociatively adsorbed on the urania surface and that methoxide was the only surface intermediate present above 150 K. Primary reaction products were methane and carbon monoxide at 480 K. Oxygen atoms not removed from the surface as CO were incorporated into the oxide surface isotopically labeled U 02 surfaces did not exchange oxygen with methoxide to produce C 0 [76]. 

Kinetics and Mechanisms of Hds Reactions. - Earlier work and outstanding problems are summarized in the earlier Report. Much of the work reviewed in the present Report concerns thiophen and benzo- and dibenzothiophens that have been of particular interest as model compounds in residual oil and coal hds. Recent reviews include discussions of mechanisms. An outstanding question is whether S elimination from a heterocyclic ring is preceded by ring hydrogenation. The obvious, but important, point has been made that failure to detect a compound in the gas phase, specifically tetrahydrothiophen during thiophen hds, does not exclude the possible importance of a reaction path in which a particular compound is a surface intermediate, since its surface reaction may be faster than its desorption. 

Mechanism and Kinetics. The most detailed study of the reaction mechanism has been made by Wachs and Madix. They used isotopic tracers and flash desorption to study the species produced when methanol is adsorbed on an oxygen-doped copper (110) single-crystal surface. While the results of such a study are of considerable interest, they are not necessarily representative of a copper catalyst continuously exposed to reaction conditions. From the desorption spectra, methanol shows exchange only of the hydroxy-hydrogen surface methoxide was identified as the most populous surface intermediate. As formaldehyde and hydrogen also appeared to be produced from the same intermediate, the mechanism (21)—(24) was proposed for the selective reaction ... 

The literature pertaining to the catalytic properties of magnetite focuses primarily on the water-gas shift reaction. A number of reaction kinetics studies have been reported in which WGS reaction pathways have been proposed (1,2,7-18), In short, two types of mechanisms have been put forward, these being the adsorptive and regenerative mechanisms. In the adsorptive pathway, reactants adsorb on the surface where they react to form surface intermediates, followed by decomposition to products and desorption from the surface (12-18), Support for this adsorptive mechanism has been provided by tracer studies and apparent stoichiometric number analyses. Two such adsorptive mechanisms consistent with experimental observations are shown below. 

The experiments of acrolein oxidation in absence of molecular oxygen confirm a dense adsorption layer of oxidized Cj-compounds, acrylic anion + acrylic acid, figure 11. It is remarkable that despite of the increasing adsorption of the oxidized Cj-compounds the rate of acrolein consumption is hardly reduced. IR analysis of surface intermediates have lead to the assumption that deprotonation of chemisorbed acrolein and oxidation of the intermediate to acrylic anion are fast reactions. Therefore, the protonation of the acrylic anion and the following desorption of acrylic acid are regarded as rate determining steps leading to accumulation of the acrylic anion on the surface of the catalyst [12]. 

In our discussion of the influence of structure on the turnover rate our understanding is frequently hampered by lack of information on the ratedetermining step and the most abundant surface intermediate. It would be logical to consider the structure sensitivity of the rate of an elementary step, such as the desorption of a chemisorbed gas. Results on temperature programmed desorption as a function of particle size might be simpler to interpret than those of global reactions consisting of a sequence of steps. However, few such data are available. 

Since rank a = 8, the surface intermediates are linearly independent and, hence, a direct RR involves no more that 9+l = 8 + l= 9 elementary reactions. It is further observed that 5i, sz, 53, and S5 (adsorption and desorption steps) should be involved in all full RRs. That is, it is not possible to obtain the OR by omitting these 4 elementary reactions. The remaining 9-4 = 5 elementary reactions involved in a RR need to be selected from among S4, sy, ss, s% s lo, S12, sw, Sis and sn. Thus, the total number of RRs does not exceed the number of ways 5... 

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