Surface reactions empty site

As NO dissociation produces two atoms from one molecule, the reaction can only proceed when the surface contains empty sites adjacent to the adsorbed NO molecule. In addition, the reactivity of the molecule is affected by lateral interactions with neighboring species on the surface. Figure 4.10 clearly illustrates all of these phenomena [38]. The experiment starts at low temperature (175 K) with a certain amount (expressed in fraction of a monolayer, ML) of NO on the Rh(100) surface. During temperature programming, the SIMS intensities of characteristic ions of adsorbed species are followed, along with the desorption of molecules into the gas phase, as in temperature-programmed desorption (TPD) or temperature-programmed reaction spectroscopy (TPRS) (see Chapter 2). 

FIG. 1 Schematic description of the relevant steps involved in a surface catalyzed reaction within the reactive regime. The cycle starts with the empty sites of the surface (top) and is followed by interactions between reactants and the surface (right branch). Such interaction finally leads to the reaction (bottom) and desorption of the products (left branch), a process which generates new empty sites (top). 

We can think of a heterogeneous catalyst as a collection of active sites (denoted by ) located at a surface. The total number of sites is constant and equal to N (if there is any chance of confusion with N atoms, we will use the symbol N ). The adsorption of the reactant is formally a reaction with an empty site to give an intermediate I (or more conveniently R if we explicitly want to express that it is the reactant R sitting on an adsorption site). All sites are equivalent and each can be occupied by a single species only. We will use the symbol 6r to indicate the fraction of occupied sites occupied by species R, making N6r the number of occupied sites. Hence, the fraction of unoccupied sites available for reaction will be 1 - 0r The following equations represent the catalytic cycle of Fig. 2.7 ... 

In this reaction scheme, hydrogen adsorption (on an empty site ) can occur only after OH removal. Since the adsorption energy of hydrogen is gained only in the second step, this may shift the onset of the reaction to more cathodic potentials. We wUI see later that on a surface covered by Pt monolayer islands that allow an easy formation of Had (see below), a homolytic reaction according to... 

In these expressions is the rate of adsorption of species j, which for A may be written as A + S AS, where A is the gas-phase molecule. S is an empty site on the surface, and AS is the adsorbed molecule. We can consider adsorption as a bimolecular chemical reaction that is proportional to the densities of the two reactants A and S to give... 

Fig. 7.67. Kobosew and Nekrassow model. Recombination is very slow, hence 0H- 1. The desoiption stage is the discharge of H+ on an absorbed H, and this reaction determines the availability of empty sites for fast proton discharge onto the metal. (Reprinted from J. O M. Bockris and S. U. M. Khan, Surface Electrochemistry, Plenum, 1993, p. 316.)...

Fig. 1.18. Distribution of A and B particles on the surface in the annihilation reaction A + B —> 0. For clarity, the distributions of A s and B s have been separated and are shown in the left-hand column and in the right-hand column of the figure, respectively. The results shown correspond to constant and equal fluxes of A and B. The simulation were carried out on a 100 x 100 square lattice, (a) The A and B distribution are complementary. A narrow lane of empty sites separates between them, (b) The long-time (near steady-state) structure of the overlayer developing from the initial condition in (a), (c) The long-time overlayer pattern developing from an initially empty lattice.

At normal temperatures H atoms are very mobile on metal surfaces. We take this into account by the possibility of diffusion steps for the H atoms. A H atom jumps with rate D/z onto the nearest neighbour sites on the lattice. If this site is occupied by N, reaction occurs and an A particle (NH particle) is formed. The same holds if the site is occupied by A or B (NH2) where the products B or NH3 = 0 are formed, respectively. NH3 desorbs immediately from the surface and an empty site is formed. That is we deal with the diffusion-limited reaction system. It is important to note that all the reaction steps discussed above (with the exception of the N2-adsorption) are independent of 7/ and 7m. 

Suppose the activated complex T of the rate-determining step of a reaction, starting from gaseous A and B, is (AaBb) and that it occupies tt sites on the surface. Under the conditions of the reaction this surface may be partially covered by adsorbed A, adsorbed B, or both. Let us suppose that a fraction dA is covered by /lads, each A molecule occupying ta sites, and that a fraction dB is covered by Bada, each B occupying tb sites. A fraction 6f = 1 — dA — 6B of the surface is empty. 

The idea that a metal atom in the zero oxidation state is both a soft acid and soft base can be used to explain surface reactions of metals. Soft bases such as carbon monoxide and olefins are strongly adsorbed on surfaces of the transition metals. Bases containing P, As, Sb, Se, and Te in low oxidation states are strongly adsorbed, blocking the active sites (Pearson, 1966). The clean surfaces are incomplete solids, in that the surface atoms have no nearest neighbors in one of the three-dimensional coordinate system. This means that there are atomic orbitals, both filled and empty, which are not being used to form surface orbitals. 

The steady state reaction rates predicted by Reuter et al. at 600 K in terms of the turnover frequency (TOF) and a summary of the surface structures at this temperature are shown in Figure 1. For most combinations of the CO and 02 partial pressures, the surface is dominated by one adsorbed species and as a result the CO oxidation rate is low. However, if the partial pressures are chosen appropriately, a dynamic equilibrium between adsorbed O, CO, and empty sites exists and the oxidation rate can be large. These regions are shown in white... 

In the steps above, the symbol indicates empty sites or adsorbed intermediates, the symbol indicates a rate-determining step, and the symbol reversible steps. The mechanism involves dissociative adsorption of oxygen, adsorption of ethylene, a surface reaction between adsorbed ethylene and adsorbed oxygen to form the species CH2CH2O, which denotes the oxametallacycle, and finally a reaction/desorption step where the oxametallacycle forms ethylene oxide which is released to the gas phase. 

Takoudis et al. (1981) proposed a model for a bimolecular Langmuir-Hinshelwood surface reaction with two empty sites in its reaction step. The two chemisorbed species were assumed to adsorb competitively on the surface. The two dimensional model with reaction rates as parameters were shown to exhibit oscillations. Bifurcation of this model was also discussed. Takoudis et al. (1982) described a procedure for obtaining necessary and sufficient conditions for the existence of periodic solutions in surface reactions with constant temperature. An analytic method for the analysis of bifurcation to periodic solutions was developed. 

The proposed reaction mechanism includes adsorption, dissociation, surface reaction and desorption steps. The adsorbed species are denoted by H, D and HD, where is an active empty site on the surface. The rates of the elementary steps are given by the following equations for the gas phase species and surface species respectively... 

---