Potential energy surface dissociative adsorption

Before a detailed presentation of the ab initio dynamics simulations, first the fundamental difference between atomic and molecular adsorption on the one hand and dissociative adsorption on the other hand has to be addressed. Then I will briefly discuss the question whether quantum or classical methods are appropriate for the simulation of the adsorption dynamics. This section will be followed by a short introduction into the determination of potential energy surfaces from first principles and their continuous representation by some analytical or numerical interpolation schemes. Then the dissociative adsorption and associative desorption of hydrogen at metal and semiconductor surfaces and the molecular trapping of oxygen on platinum will be discussed in some detail. 

Figure 4 Illustration of the steering effect on a potential energy surface with a coexistence of purely attractive and repulsive paths towards dissociative adsorption. Three typical trajectories corresponding to the low, medium and high kinetic energy regime are included.

N2 dissociation on Fe crystal planes seems to be an example also of the presence of mixed adsorption channels, which has led to some confusion over the detailed nature of the potential energy surface for this system. Ertl et al. (1982) have claimed that there is a zero net barrier on Fe(lll), with adsorption dominated by precursor kinetics, whereas highly activated adsorption is measured in supersonic beam experiments (Rettner and Stein, 1987). This probably relates to the different regimes of measurement, Ertl using low gas temperatures, while Rettner and Stein varied the gas energy. 

The surface electronic structure of Mg (0001) was analysed by angle resolved photoemission (Karlsson et al, 1982), Two sharp peaks due to surface states were identified. No experimental studies on H adsorption on single crystalline Mg are available. Self consistent calculations of the potential energy surface for a molecule on Mg (0001) were performed by N rskov et al, (1981), They reveal (Fig, 11) an activation barrier for adsorption into a mobile precursor state and an activation barrier for dissociation which depends strongly on the adsorption site geometry. 

Quantum dynamics on dissociative adsorption of the prototype activated system H2(D2)/Cu(111) have been essential, together with an accurate potential energy surface [21], to reproduce experimental observables with chemical accuracy [19]. 

Arboleda, N.B. Kasai, H. Potential energy surfaces for H2 dissociative adsorption on Pt(lll) surface-effects of vacancies. Surf. Interface Anal. 40 (2008), pp. 1103-1107. 

Figure 12.5 Schematic potential energy surface for dissociative adsorption-desorption [adapted from ErtI (1982)]. In the drawing, the barrier to dissociation-recombination occurs when the molecule is already at the surface and is along the bond distance. This will lead to vibrationally excited desorbed molecules. Note also the precursor well along the approach to the (physical) surface. This well will slow down the approaching reactant but may not be deep enough to insure that it fully accommodates to the surface.

Figure 6.35. Potential energy diagrams for adsorption and dissociation of N2on a Ru(0001) surface and on the same surface with a monoatomic step, as calculated with a density functional theory procedure. [Adapted from S. Dahl, A. Logadottir, R. Egberg, J. Larsen, I. Chorkendorff,...

Figure 6.1 Schematic potential energy diagram for atomic and molecular nitrogen adsorption on a clean and K-covered Fe(100) surface. Curve (a) is for N2 + Fe(100) curve (b) is for N2 + Fe(100)-K. Note the lowering of the activation energy for dissociation from 3 kcalmol-1 to zero. (Reproduced from Ref. 3).

Figure 1.1 Schematic representation of a well known catalytic reaction, the oxidation of carbon monoxide on noble metal catalysts CO + Vi 02 —> C02. The catalytic cycle begins with the associative adsorption of CO and the dissociative adsorption of 02 on the surface. As adsorption is always exothermic, the potential energy decreases. Next CO and O combine to form an adsorbed C02 molecule, which represents the rate-determining step in the catalytic sequence. The adsorbed C02 molecule desorbs almost instantaneously, thereby liberating adsorption sites that are available for the following reaction cycle. This regeneration of sites distinguishes catalytic from stoichiometric reactions.

Fig. 6-3S. Potential energy curves for water adsorption on metal surface in the states of molecules and hydrozjd radicals c = energy r = reaction coordinate solid curve = adsorption as water molecules and as partially dissociated hydroxj4 and hydrogen radicals broken curve = adsorption of completely dissociated oxygen and hydrogen radicals.

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