H2-Dissociation

Flammer B, Scheffler M, Jacobsen K W and Norskov J K 1994 Multidimensional potential energy surface for H2 dissociation over Cu(111) Phys.Rev. Lett. 73 1400... 

Gundersen K, Jacobsen K W, Norskov J K and Hammer B 1994 The energetics and dynamics of H2 dissociation on... 

The above problem with H2 dissociation is a matter of wave function construction. The functional form of a restricted single-determinant wave function will not allow a pair of electrons in an orbital to separate into two different orbitals. Wave function construction issues were addressed in greater detail in Chapters 3 through 6. 

Role of Atomic Hydrogen. The stable hydrogen diatomic molecule (H2) dissociates at high temperature (>2000°C) or in a high current-density arc to form atomic hydrogen. The dissociation reaction is highly endothermic (AH = 434.1 KJmofi). 

These two determinants produce equivalent, but asymmetric densities. In addition, the energies obtained from these densities are the same, i. e. E[Pl] = E[p2]. If we now insert these two densities in equation (5-24) it is clear that the energy will be invariant to the choice of Wj and w2. If we choose w, = w2 = 1/2 we will also arrive at the physically correct, i. e. symmetric density. A very similar reasoning can be used for the H2 dissociation. We again have two equivalent Kohn-Sham spin densities corresponding to... 

Lee et al.97 investigated the HF, HC1, and H2S dissociation reactions in water clusters. A good agreement between the DFT(BLYP) and the MP2 results was reported for binding energies (within 2 kcal/mol) and geometries. The studies showed also that at least four water molecules are needed for ionic products of dissociation to coexist. 

This study permits to discuss the efficiency of new probe molecules for the characterization of basic sites in zeolites. For MBOH and for methylacetylene, the observed frequencies shift account for the variations in the basic strength of the zeolitic framework oxygen atoms. Interestingly, methylacetylene also informs on the environment of the basic sites and H2S dissociation brings information on the amount of strong basic sites. These results show the high potential and the complementarity of these protic probes to describe the strength, concentration and environment of basic sites. 

An elementary rate law or an equilibrium constant was assigned to each step involved in the detailed reaction mechanism of FTS. Both the H2 dissociative... 

Figure 4.61 displays the potential-energy curves for insertion in the three spin states. All three spin multiplicities lead to local minima at small R, but only the triplet is strongly bound with respect to asymptotic 3Ti + H2 dissociation products, and we focus primarily on this species. 

The dissociation of H2 (or D2) is heterolytic from H+ and H (or D+ and D ) on these metal oxides. This is different from conventional metal-based hydrogenation catalysts where H2 dissociates homolytically and 2H molecules are formed. 

O2 and H2 dissociation kinetics are better at higher temperatures (>400 °C), low-cost electrode structures of high surface area Ni and oxides such as spinels or perovskites to replace the very effective, but costly, Pt catalysts have been sought. 

---