Hydrogen adsorption

The reversible adsorption of H2 and of D2 on Cr203 at liquid-air temperatures, with initial heats of adsorption of 5.1 and 5.4 kcal./mole respectively (105), are active in the exchange of H2 and D2 and we must, consequently, assume that both gases are adsorbed in the atomic form. We may think of a sharing of electrons between Cr3+ ions and H atoms, while also an electron transfer from the H atom to a Cr3+ ion, forming a Cr2+ ion, may be considered. Both possibilities may contribute to the real situation, the bond being a resonance between them  

At room temperature H2 is chemisorbed by Cr203 in a different way (106) the initial heat of adsorption is far higher, viz. 72 kcal./mole, and on desorption H20 is given off. We must assume that on adsorption OH ions are formed from 02- ions and metal ions are reduced to a lower valency 

After adsorption of H2 in this way an enhancement of the tendency to chemisorb 02 should be expected which, indeed, is found to be the case. After desorption of water, one may expect chemisorbed oxygen to fill up 

A general conclusion regarding H2 adsorption on alkali modified metal surfaces is that alkali addition results in a pronounced decrease of the dissociation adsorption rate of hydrogen as well as of the saturation coverage. 

On K modified Ni(100) and Ni(lll)62,63 and Pt(lll)64 the dissociative adsorption of hydrogen is almost completely inhibited for potassium coverages above 0.1. This would imply that H behaves as an electron donor. On the other hand the peaks of the hydrogen TPD spectra shift to higher temperatures with increasing alkali coverage, as shown in Fig. 2.22a for K/Ni(lll), which would imply an electron acceptor behaviour for the chemisorbed H. Furthermore, as deduced from analysis of the TPD spectra, both the pre-exponential factor and the activation energy for desorption 

1 Conductivity changes show up as background shifts in the IR spectra. 

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Hayden B E and Lament C L A 1989 Coupled translational-vibrational aetivation in dissoeiative hydrogen adsorption on Cu(110) Phys. Rev. Lett. 63 1823... 

Butler D A, Hayden B E and Jones J D 1994 Precursor dynamics in dissociative hydrogen adsorption on W(100) Chem. Phys. Leff. 217 423... 

The major role of TOF-SARS and SARIS is as surface structure analysis teclmiques which are capable of probing the positions of all elements with an accuracy of <0.1 A. They are sensitive to short-range order, i.e. individual interatomic spacings that are <10 A. They provide a direct measure of the interatomic distances in the first and subsurface layers and a measure of surface periodicity in real space. One of its most important applications is the direct determination of hydrogen adsorption sites by recoiling spectrometry [12, 4T ]. Most other surface structure teclmiques do not detect hydrogen, with the possible exception of He atom scattering and vibrational spectroscopy. 

Shoji F, Kashihara K, Sumitomo K and Oura K 1991 Low-energy recoil-ion spectroscopy studies of hydrogen adsorption on Si(100)-2 x i surfaces Surf. Sc/. 242 422-7... 

Roux C D, Bu H and Rabalais J W 1992 Hydrogen adsorption site on the Ni 110 -p(1 time-of-flight scattering and recoiling spectrometry (TOF-SARS) Surf. Sc/. 271 68-80... 

Fukunishi Y and Nakatsu] H 1992 Modifications for ab initio calculations of the moderately large-embedded-cluster model. Hydrogen adsorption on a lithium surface J. Chem. Phys. 97 6535-43... 

Nishihara C and Nozoye H 1995 influence of underpotentiai deposition of copper with submonolayer coverage on hydrogen adsorption at the stepped surfaces Pt(955), Pt(322) and Pt(544) in sulfuric acid solution J. Electroanal. Chem. 396 139-42... 

S g of ethyl glycinate hydrochloride were dissolved in 400 cc of ethanol and 33.5 g of salicylic aldehyde were added. It is refluxed for half an hour and cooled. 38 cc of triethylamlne and 25 g of Raney nickel are then added whereafter hydrogenation is carried out at room temperature and under atmospheric pressure. After hydrogen adsorption was complete, the mixture was filtered and the alcohol evaporated off. The residue was taken up with acidified water, extracted with ether to eliminate part of the by-products, consisting mainly of o-cresol, then made alkaline with ammonia and extracted with ethyl acetate. The solvent was removed in vacuo and the residue crystallized from ether/petroleum ether. 36.7 g of o-hydroxybenzyl-aminoacetlc acid ethyl ester melting at 47°C are obtained. 

Fig. 8.4 Mechanisms involving embrittlement of the metal, (a) Crack-tip adsorption, (b) hydrogen adsorption, (c) decohesion by hydrogen influx to dilated lattice and (d) crack extension due to brittle hydride particle forming at crack tip...

A number of metals have the ability to absorb hydrogen, which may be taken into solid solution or form a metallic hydride, and this absorption can provide an alternative reaction path to the desorption of H,. as gas. In the case of iron and iron alloys, both hydrogen adsorption and absorption occur simultaneously, and the latter thus gives rise to another equilibrium involving the transfer of H,<,s across the interface to form interstitial H atoms just beneath the surface ... 

In order for the reaction to proceed, hydrogen adsorption must be followed by its diffusion over the surface amongst other mobile adsorbed species... 

Hydrogen adsorption from solution Oxygen adsorption from solution Underpotential deposition of metals Adsorption of probe molecules from solution ... 

We present expressions for reaction rates and steady-state concentrations using the simplified assumption that Cads hydrogenation to CH4 occurs in one reaction step. We also assume that Oads removal is fast and that hydrogen adsorption is not influenced by the other adsorbates. 

Figure 2. Hydrogen adsorption and desorption Isotherms for rhodium catalysts. Solid lines denote total adsorption and dashed lines denote reversible adsorption. The meaning of symbols Is as follows ...

HREELS and TFD have played a unique role In characterizing the surface chemistry of systems which contain hydrogen since many surface techniques are not sensitive to hydrogen. We have used these techniques to characterize H2S adsorption and decomposition on the clean and (2x2)-S covered Ft(111) surface (5). Complete dissociation of H,S was observed on the clean Ft(lll) surface even at IlOK to yield a mixed overlayer of H, S, SH and H2S. Decomposition Is primarily limited by the availability of hydrogen adsorption sites on the surface. However on the (2x2)-S modified Ft(lll) surface no complete dissociation occurs at IlOK, Instead a monolayer of adsorbed SH Intermediate Is formed (5) ... 

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