H2 adsorption

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. 

H2 adsorption is weak on the anatase surfaces [8], No dissociative adsorption of H2 takes place over the smooth surfaces of Au at temperatures below 473 K [9,10]. On small Au particles, adsorption is possible at low temperature. Dissociative adsorption of H2 can be accelerated by the negatively charged molecular oxygen species at steps, edges, comers of Au particles [5]. 

Figure 5 presents data for the non-lnteractlng Rh/S102 catalyst at similar pressures and at 48, 158, and 333 °C. Even with the scatter In the 333°C data, there Is an obvious transition In the spectra as the temperature Is Increased. The predominant peak around 10 rad/sec diminishes and the one around. 5 rad/sec Increases to dominate the spectrum, a trend similar to that observed In the Rh/T102 spectra. Presumably, these trends are the result of differences In apparent activation energies for H2 adsorption and desorption on the various types of sites. 

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) ... 

H2S adsorption on the (2x2)-S covered Pt(lll) surface at IlOK contrasts with adsorption on the clean surface. On the (2x2)-S surface no complete dissociation Is observed at low temperature Instead, H2S partially dissociates to form an adsorbed SH Intermediate with a characteristic bend vibration at 585 cm . Heating adsorbed SH on the (2x2)-S covered surface leads to a SH+H recombination reaction not observed on clean Ft. The recombination process removes the excess SH so that the stable, high coverage (/3 X /3)R30 -S lattice can be formed. 

The long-term stability of the Ru/Ti02 catalyst was studied under various reaction conditions and the spent catalysts were characterized for assessing the reasons of deactivation. It was observed that the rate exhibits a rapid reduction at the initial several hours of reaction, followed by a slow and continuous deactivation. Analysis of the spent catalyst, by H2 adsorption after removing surface carbon, showed that the initial rapid reduction of activity is mainly due to metal sintering, while the continuous and slow deactivation is related to the occurrence of the SMSl phenomenon at the later part of the catalyst bed, where reducing conditions prevail. In order to avoid these processes which lead to catalyst deactivation, Ti02... 

Figure 3.50. Reversible (weak chemisorption) and irreversible (strong chemisorption) H2 adsorption on AI2O3 supported Ni catalyst at T= 323 K (Xu Xiaoding, 1998).

The most essential question is why the CO-free sites are secured for H2 adsorption and oxidation. Watanabe and Motoo proposed a so-called bifunctional mechanism originally found at Pt electrodes with various oxygen-adsorbing adatoms (e.g., Ru, Sn, and As), which facilitate the oxidation of adsorbed COad at Pt sites [Watanabe and Motoo, 1975a Watanabe et al., 1985]. This mechanism has been adopted for the explanation of CO-tolerant HOR on Pt-Ru, Pt-Sn, and Pt-Mo alloys [Gasteiger et al., 1994, 1995], and recently confirmed by in sim FTIR spectroscopy [Yajima et al., 2004]. To investigate the role of such surface sites, we examined the details of the alloy surface states by various methods. 

The 5% Pd/C catalysts, with an increasing amount of tin (from 0 to 1%), were activated in the same way described above and titrated with both carbon monoxide and hydrogen sulfide, up to 800 Torr, at 30°C. Figure 15.4 indicates a constant volume of carbon monoxide adsorbed, as expected from the above relationship for a fixed amoimt of %Pd on the catalyst. However, there does appear to be a relationship between H2S adsorption and the Sn/Pd ratio at constant 5 wt% Pd concentration on the catalyst. When there is little or no tin associated with the Pd-catalyst, the H2S is irreversibly adsorbed, resulting in high voliunetric uptake of... 

Oudar and co-workers studied the dissociative chemisorption of hydrogen sulfide at Cu(110) surfaces between 1968 and 1971.3,14 As in the case of Ni(110) described below, a series of structures were identified, which in order of increasing sulfur coverage were described as c(2 x 2), p(5 x 2) and p(3 x 2). In contrast to nickel, the formation of the latter phase is kinetically very slow from the decomposition of H2S and could only be produced at high temperatures and pressures. The c(2 x 2) and p(5 x 2) structures were confirmed by LEED,15 17 but the p(3 x 2) phase has not been observed by H2S adsorption since Oudar and colleagues work. 

Figure 10.6 STM images of the Ni(l 11) (5 /3 x 2)S phase and a model for the structure proposed to explain the decreased density of nickel within the islands, (a) 15.0 x 16.5 nm image showing the three possible domains of the (5 /3 x 2)S structure the brighter part of the image corresponds to an adlayer that has developed on top of a nickel island formed during H2S adsorption, (b) 1.8 x 2.9 nm atomically resolved image of the (5 /3 x 2)S structure, (c) Proposed clock structure for the (5 /3 x 2)S phase that accounts for the reduced nickel density in the sulfur adlayer. (Reproduced from Refs. 23 and 25).

The results of a similar experiment with adsorbed hydrogen is shown in Fig. 2.3b. Only one desorption peak was observed in the temperature range studied [50], The desorption peak temperature lies at 420 K for the experiment with 0.8 L and is shifted to lower temperatures as the H2 concentration increases indicating second order desorption kinetics. Surface states with desorption temperatures at 165 K, 220 K, 280 K and 350 K were reported for the adsorption of H2 and D2 at 120 K [51]. Thermal desorption experiments after H2 adsorption at 350 K show only one desorption state at ca. 450 K [52],... 

Based on the assumption that hydrogen atoms are adsorded in pairs, instead of the expression of dissociative hydrogen atom adsorption, it s adequate to use H2 adsorption as the fractional surface coverage for the rate equation. 

Dillon, A.C., J.L. Blackburn, P.A. Parilla, Y. Zhao, Y.-H. Kim, S.B. Zhang, A.H. Mahan, J.L. Alleman, K.M. Jones, K.E.H. Gilbert, M.J. Heben. Discovering the mechanism of H2 adsorption on aromatic carbon nanostructures to develop adsorbents for vehicular applications. Materials Research Society Symposium Proceedings, 837 (Materials for Hydrogen Storage, 2004), 2005, pp. 117-123. 

Morales F., de Smit E., de Groot F.M.F., Visser T., and Weckhuysen B.M. 2007. Effects of manganese oxide promoter on the CO and H2 adsorption properties of titania-supported cobalt Fischer-Tropsch catalysts. J. Catal. 246 91-99. 

The first of these changes reflects the difference between the rates of H2 adsorption and consumption in the reduction of NO. 

The decrease in 0 jq occurs for two reasons a) inhibition of the readsorption of desorbing NO as a consequence of H2 adsorption and b) the consumption of adsorbed NO by reduction. It should be noted that the dissociation of NO, and hence the rate of NO reduction is accelerated by the creation of vacant sites. The increase in 0V, seen in Fig. 13, can be ascribed to a consumption... 

Comparison of H2 Adsorption Capacities of Activated Carbons and Metal-Organic Frameworks... 

For the interaction of H2 with energies greater the 0.18eV, Rice etal. reported that all cases of H2 adsorption were accompanied by dissociation of the molecule. This is in agreement with experimental observation, where the chemisorption of intact H2 molecules has not been reported. Furthermore, for all cases they report that both H atoms chemisorb to the surface rather than reflect back into the gas phase. The dissociative adsorption of H2 was accompanied by an energy release of between 2.5 and 4.3 eV. The energy released was shown to enhance the initial mobility of the H atoms on the surface, and was reported to be somewhat independent of the surface temperature. Once the energy was dissipated to the surface, however, the mobility of the H atoms decreased sharply. 

Ligand effect CO adsorption is lowered by alloying, thus decreasing CO coverage and increasing sites available for H2 adsorption/ dissociation and oxidation. 

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