Molecular hydrogen dissociative chemisorption

To exhibit such an active and selective catalytic effect, the catalyst must be a fairly good hydrogenation catalyst that is able to activate molecular hydrogen. It must also activate carbon monoxide without dissociating it. A nondissociative chemisorption permits the hydrogenation of carbon monoxide to occur on both oxygen and carbon. Considering the formation of surface methoxide in the second mechanism [Eq. (3.43)], a further requirement is that the catalyst not form a too stable metal methoxide. 

When a contamination is present which produces a dipole layer, as sulfur does, the dissociative chemisorption of molecular hydrogen is given by (Fig. 40) ABC D ) there is an activation energy (difference between levels C and A) the heat of adsorption is severely reduced it is even negative (endothermic chemisorption). The dissolution of molecular hydrogen proceeds less easily than in the case of a pure-iron surface. The kinetic energy has to be sufficient to overcome the difference between C ... 

Chemisorption of hydrogen — Process leading to the formation of strongly bound (chemisorbed) hydrogen atoms on an adsorbent (mostly on metal) either via the dissociative adsorption of molecular hydrogen (H2) or, in the case of electrified interfaces, by charge transfer process occurring, for instance, with H+(H30+) or H20 species... 

So far the 3D flat-surface model has been quite successful in providing qualitative and even some quantitative dynamics information for hydrogen dissociation on metals such as the role of hydrogen vibration and rotation in dissociative adsorption on Cu(lll) (104,114,117-119). However, the inherent limitation of the flat-surface model dictates that it cannot provide information on surface corrugation and its effect on molecular adsorption. One would like to investigate the effect of rotational orientation of diatomic molecules on chemisorption in the presence of surface corrugation. In order to obtain... 

We show in this subsection some calculational results for the H2/Cu(lll) system which has become the prototype for studying dissociative chemisorption on surfaces. The results presented in this subsection are obtained from calculations at the 3D flat-surface level using a LEPS PES (117). We are interested in rovibrational effect of hydrogen on dissociation. In particular, we will present some interesting results showing the effect of molecular orientation and homonuclear symmetry on dissociative chemisorption of hydrogen on Cu(lll). 

Although lower-level calculations produced varying results, recent advanced ab initio and density functional theory (DFT)-based studies provide more reliable data. All the theoretical investigations noted above were, however, focused on chemisorption on the basal (0001) plane of graphite, and the results can be summarized as follows. Hydrogen atoms prefer adsorption sites directly above carbon atoms of graphite, and a stable phase is available based on the more advanced ab initio or DET methods. The dissociative chemisorption of molecular hydrogen... 

Volumetric steady-state gas permeation tests at elevated temperatures are typically used to characterize the performance of paUadium membranes. Electrochemical methods are also effective, even at high temperatures [90, 166]. Permeation through palladium depends on a solution-diffusion mechanism, including the steps of chemisorption and dissociation into atoms, absorption into the metal, diffusion through the metal lattice, transfer from the bulk metal to the opposite side, and recombination into molecules for desorption [167, 168]. Difiusion of molecular hydrogen through boundary layers adjacent to the surface is also necessary. 

Hydrogen interacts with metals in three principal ways (i) by dissociative chemisorption at the surface (ii) by physical adsorption as molecules at very low temperatures and (iii) by dissolution or occlusion. As we shall see, to these three extreme forms have been added numerous intermediate states of various lifetimes and stabilities, some of which may have importance in catalysis. There is for example clear evidence for a molecular state formed at about 100 K on stepped surfaces saturated with atomic hydrogen (on Ni(510), Pd(510) and (210) ) this is distinct from a molecular precursor stzlt such as that seen with deuterium on Ni( 111) at 100 K. The role of such states will be discussed further below (Sections 3.2.2 and 3.3.3). The small size and electronic simplicity of the hydrogen atom formed by dissociation enable it to bond to metal surfaces in different ways, and simple-minded notions about its forming only a single covalent bond to another atom have to be abandoned. 

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