Effect of Thin Film Coating and Precoverage on H Dissociation and H Adsorption and Absorption

The ability to deposit monolayers of metal atoms on single crystal metallic substrates and the formation of multilayer structures initiated a surge of activity in surface and interface science, which has also had an impact on H sorption and permeation studies. The modification of structure, electronic structure and magnetism at the interface and in the overlayer are widely studied properties [138-142]. Strongly enhanced magnetic moments were found for some transition metal overlayers, as were induced magnetic moments at the interface of otherwise nonmagnetic elements. Epitaxial constraints limit lattice expansion due to H absorption in Nb-Ta superlattices and depress superconductivity [143]. A survey of magnetic effects in hydrogenated superlattices is given in Chapter 7.1.

It was noticed [144] that the deposition of more than two monolayers of Pd or Pt dramatically enhances the H uptake rate of Nb or Ta substrates. It is the variation in the surface atomic and electronic structure and the weakening of subsurface bonding, not just protection from surface oxidation, that are at the root of the enhanced uptake [145]. The use of "catalytic overlayers" was also successfully implemented in switchable mirrors by depositing a thin Pd capping layer on top of the optically active metal hydride. The switching kinetics of such a switchable mirror - and consequently its hydrogen uptake rate - is mainly determined by the thickness of the catalytic Pd cap layer [146,147]. In particular, it was observed that a minimum Pd thickness is required for a sufficient hydrogen uptake. The effect was explained by an encapsulation of the clusters by a reduced yttrium oxide layer after exposure to hydrogen [148], the so-called Strong Metal Support Interaction (SMSI) [149]. There is evidence that this explanation holds also for similar switchable mirror systems [150], and its impact (slowing down of the kinetics) might be reduced by blocking the interdiffusion of the layers [151, 152].

It is well known from catalysis that electropositive (e.g. Na, Cs, K) and electronegative (e.g. S, O, C, Cl) adatoms decrease or increase the reaction rate and thus poison or promote the reaction, respectively [153-155]. Alkali-metal influenced adsorption on transition metals was reviewed by Bonzel [154]. Coadsorption of alkali metals and H, or D, on Al(100) revealed that the sticking coefficient and dissociation rate are extremely weak (~10-4 at all alkali coverages [156]). Upon exposing alkali-covered metal substrates to a beam of atomic H or D, alkali hydride formation was observed.

The effect of oxygen precoverage of transition metals on the H2 chemisorption increases with the degree of oxide formation and was investigated in relation to the inhibition of H sorption and embrittlement and SMSI in catalysis. Monolayer amounts of oxygen reduce the H2 adsorption and desorption on single and polycrystalline transition metals by orders of magnitude [42, 157, 158]. On poly-crystalline metals oxygen precoverage seems to deteriorate somewhat less than on monocrystals: oxidation of polycrystalline Ti caused a decreased D2 absorption rate; complete inhibition of the reaction occurred at oxide thicknesses exceeding 2 nm

[159]. Ko and Gorte observed a complete suppression of H2 adsorption even for TiO [160]. At elevated temperatures some substrate metals dissolve surface oxygen, a mechanism which is particularly important in the activation of getter alloys [85, 161]. From detailed volumetric studies Fromm and coworkers [162] conclude that thin suboxide layers, formed during the initial stages ofoxidation or by partial oxide reduction in H2, do not impede the H2 reaction drastically. This was found in particular for Ni overlayers, where a strong inhibition of hydrogen absorption occurs only for fully oxidized, that is NiO layers [163]. A decrease in the H2 reaction probability of typically one order of magnitude was observed for a Ti substrate film covered with 3 nm thick metal overlayers after precoverage of the overlayers with 10 monolayers of oxygen. The results indicate that H2 dissociation is strongly impeded if the oxygen precoverage exceeds a critical value which is given by the maximum amount of oxygen that can be adsorbed with sticking probability of one in the initial stage of oxidation. H2 on TiO2 dissociates in neither the rutile nor anatase structure modifications [119]. TiO2 is used, as mentioned earlier, as an effective coating to prevent embrittlement [126]. Various authors have reported that transition metal overlayers on the oxide layer restore the original uptake rate of the substrate [159, 162, 164]. This clearly indicates that oxidation reduces the dissociation rate and is in agreement with the observed rather fast diffusion of atomic H across TiO channels [124, 125] mentioned earlier.

The adsorption of sulfur on clean transition metal surfaces completely inhibits H2 dissociation and H adsorption (from gas phase and cathodic in aqueous acid medium) [165-168]. A major effect seems to be the blocking of H adsorption sites, possibly even the blocking of several H sites by one S atom. Marcus and Protopopoff [167] conclude from S and H adsorption experiments on Pt(110) and Pt(111) that one S atom blocks approximately 8 and 12 H adsorption sites, respectively. Self-consistent linearized APW calculations of the electronic structure perturbations induced by S on the Rh(001) surface reveal a substantial reduction in the density of states at the Fermi level, even at nonadjacent sites [169]. Thus blocking is explained by local electronic structure arguments. Using density functional theory calculations of sulfur-poisoned Pd surfaces, this argument could be made precise. Wilke and Scheffler showed that adsorbed sulfur builds up energy barriers which hinder the dissociation and that, because these energy barriers are located in the entrance channel of the dissociation-reaction pathway of hydrogen, this hindrance is particularly effective [170]. Surface poisoning by adsorption of sulfur species is known to inhibit H desorption from Pd hydride [171].

In an effective medium approach to poisoning and promotion by Norskov et al. [172] evidence is given for the importance of empty (antibonding) states near the Fermi level. Competition between the closed-shell kinetic energy repulsion and the attraction due to the gradual filling of the antibonding level determines the height of the molecular adsorption barrier, the depth of the molecular wall and the height of the dissociation barrier in the reaction path. Possible ways of altering the balance between the two competing terms are the addition of a partially empty d-band around the Fermi level (transition metal atoms) or changes in the work function of the surface.

The close relation between inhibitors of H embrittlement and H2 dissociation poisons was noticed early by Berkowitz et al. [173]. The source of hydrogen determines whether, for example surface S, which slows down the H2 ^ 2H-reaction in both directions, inhibits or promotes embrittlement. For dissolved H in the host metal, for example during electrochemical treatment, S prevents recombinative desorption and thus promotes embrittlement. In contrast, S prevents dissociation and solution of gaseous H2, for example in a H2-pipeline tube, and thus inhibits H embrittlement under these circumstances.

Continue reading here: Relevance of Surface Reactions for Bulk Absorption

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