Non-dissociative adsorption

Non-dissociative, dissociative. If a molecule is adsorbed without fragmentation, the adsorption process is non-dissociative. Adsorption of carbon monoxide is frequently of this type. If a molecule is adsorbed with dissociation into two or more fragments both or all of which are bound to the surface of the adsorbent, the process is dissociative. Chemisorption of hydrogen is commonly of this type. 

Reaction (I) only involves a non-dissociative adsorption. Therefore, it is expected that K2 Ki and the denominator in equation (7) is dominated by the term K2PD2 In that case, the observed order in deuterium will be near -1 and the order in CP will be close to +1. This is indeed observed in the experiment, with an order of -0.82 in D2 and +0.87 in CP. 

On the other hand, the absorption band due to a carbonyl group (at about 1725 cm ) displays, at high potentials, a lower intensity on PtSn (the absorption band located close to 1725 cm remains as a shoulder even at high potentials) than on Pt (Fig. 34b), which indicates that the formation of C2 species (presumably acetaldehyde) resulting from a non-dissociative adsorption of ethanol is lower on a PtSn catalyst. 

In the case of non-dissociative adsorption of a diatomic molecule, two items of geometrical information can be extracted from the ARUPS data, the first being the bond angle between the molecular axis and the surface. The important assumption here is that certain major features of the spectrum should be determined by the adsorbate alone and be independent of the substrate, particularly if the initial states are not involved in the bond to the surface. This is the case for the 4a and Irt states of CO on Ni(lOO), for example. 

It has been reported that methanol is formed from C0-H2 reaction over silica-supported Pd, Pt, Ir catalysts (5). The behaviour of the metal was found to be influenced by the carrier (6,7,8, 9). The selectivity in methanol was discussed in terms of acid-base properties of the support which influenced the non dissociative adsorption of CO on the metal required for oxygenated hydrocarbon formation, in terms of electronic interaction between the metal and the support or in terms of stabilization by the carrier of oxidized metal cations which would adsorb CO non dissociatively. We have studied the C0-H2 reaction at 553 K, 30 atmospheres over Pt supported on a variety of oxides. The characteristics of the catalysts are given in table 2... 

Here, ka, kd and km are the rate coefficients for adsorption, desorption, and migration from the intrinsic precursor state, k m and are the rate coefficients for migration and desorption from the extrinsic precursor state, kv is the rate coefficient for transfer from the chemisorbed state to the intrinsic precursor state, and a and a are the trapping probabilities for molecules incident at intrinsic and extrinsic precursor sites, respectively. Direct transfer from gas phase to chemisorbed state or vice versa is included through the probability, sc, for adsorption and the rate coefficient, ftc, for desorption [427]. In order to generalise the rate expressions, we now introduce a group of terms F(0) which are only functions of the surface coverage 0. For a particular case, such as non-dissociative adsorption, these terms may be evaluated and inserted into the appropriate rate expression. 

Fd is the occupation probability that a site (or sites) exists in a configuration which can lead to desorption (= 0 for non-dissociative adsorption). 

Fm is the probability that an intrinsic precursor, in hopping, moves to a site configuration where an extrinsic precursor state can exist (=0 for non-dissociative adsorption). 

F is the probability that a collision takes place at an intrinsic precursor site [= (1 — 0) for non-dissociative adsorption]. 

In the simplest cases, where there is no interaction between adsorbed species, eqn. (40) reduces to the Langmuir isotherm form, for dissociative or non-dissociative adsorption with pairwise interactions between adsorbed species, eqn. (40) is equivalent to the Fowler—Guggenheim [320] isotherm. 

Such relationships indicate two types of behaviour. First, considering the non-dissociative adsorption ... 

In general, hydrogen dissociates easily over metallic surface, but reports on non-dissociative adsorption of hydrogen on Ag systems are available in the literature. Hydrogen is... 

Obviously, the assumption of non-dissociative adsorption as well as that of the reaction being first order in both reactants are gross simplifications, since both methane and oxygen will dissociate prior to the reaction, and the further surface reaction mechanism will involve several consecutive and parallel reaction steps. Nevertheless, the model should allow for a qualitative test of the assumption that site competition between methane and oxygen account for the observed trend in ignition temperatures. 

While the assumption of only one type of surface sites for all adsorbants turned out to be an important factor in our results, assuming dissociative or non-dissociative adsorption did not change the results much. Obviously, under high temperature conditions, free surface sites do not play an important role in the surface reaction mechanism, while rather the competition for adsorbed oxygen appears to be the limiting factor for the different parallel surface reaction paths. 

When the CNDO method was applied to studies of water-metal systems, the results obtained seemed very different. In studies by Kuznetsov et on the adsorption of the H2O monomer by several metals, Cu, Ag, Au, Zn, Cd and Hg, in which the different crystallographic orientations of the surface for each metal were considered, surprisingly the hollow site was reported to be preferred. The stabilization energy in this work was reported to be extremely strong, from -66 kJmol for mercury up to -296 kJmol for gold. These values seem to be severe overestimates when compared with experimental estimates of the energy of approximately -40 to —60kJmol for the non-dissociative adsorption of the water molecule on metals.f ... 

Figure 3.26. Dissociative and non dissociative adsorption of CO on elements of the periodic table at temperature and CO hydrogenation reaction temperature.

M8 Surface reaction 02 non-dissociative adsorption ro2i+fcH4]=>p]... 

According to the above hypotheses, in case of non dissociative adsorption it is possible to derive the number of occupied sites No at a given time t by ... 

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