H chemisorption

Fernandez and Sanchez [18] investigated the kinetics of hydrogen absorption and desorption by activated magnesium powder (several cycles of hydrogen absorp-tion/desorption at 375°C) using a volumetric technique. They pointed out that for-mation/decomposition of metal hydrides comprises a number of steps taking place in series transport to the surface, dissociation, H chemisorption, surface-... 

Fig. 14.9 The variation of H-chemisorption energies at the B3LYP/6-31G(d) level for the chemisorption of one and two hydrogen atoms on the external surface of (3, 3), (4, 4), (5, 5) and (6, 6) armchair single-walled carbon nanotubes (SWNTs) of 9 and 15 carbon layers...

We have shown how hydroxyl groups on the silica surface act as active sites in the modification reaction. The amount of hydroxyls is controlled by the thermal pretreatment of the substrate. APTS molecules are physisorbed to the surface by hydrogen bonding of the amine group to a surface hydroxyl (H). Chemisorption of APTS to the silica surface, in dry conditions, involves the formation of siloxane bonds with release of ethanol (I). Water causes the hydrolysis of the ethoxy groups of the APTS, with formation of silane silanols. These silanols are more reactive than the original alkoxy groups. Siloxane bonds with other silane molecules or with the silica surface are formed with release of water (J). 

Table 4.6. Study of the H chemisorption on a Pt/CcOz catalyst reduced (I h) at 473 K or 773 K evacuated (Ih) ai 773 IC, and further treated with as indicated. Comparison between the quantitative data obtained from TPD-MS and volumetric chemisorption experiments. Data taken from (117).

Also worth of noting is the conclusion drawn in ref. (163) about the influence of the metal precursor on the intensity of the deactivation effects occurring in Rh/Ce02 catalysts. By using H-NMR, the authors investigate the influence of T on the H chemisorption capability of two catalysts prepared respectively from Rh(N03)3 and... 

The physical significance of the above evaluation of as a basis for estimation of ffads.H s the assumption that chemisorption of H involves formation of a quasi-diatomic M—H bond and that the polarity of this bond is characterized by the electronegativity difference, Xm Xh- Directly determined initial heats of adsorption of H, that is, for -> 0, are found (/4) to be related to d> for transition metals, and this effect originates on account of partial charge transfer in H chemisorption determined by the electron affinity of the metal, -O, and characterized inter alia by Xm - Xh-... 

Fig. 18. Pseudocapacitance versus overpotential profiles for H chemisorption at the Ni-Mo composite electrodes. (From Ref. 75.)...

The difference between Pd films and powders is notable. Some of the H retained by the powders may be situated at surface imperfections. However, it is also possible that some of the hydrogen is incorporated within the defect structure of the powders, in much the same way as suggested earlier for Pt blacks. Indeed, Aben and Frieske and Wicke have noted that very high temperatures (> 700 K) are required to desorb all the H from Pd blacks. As with Pt blacks the agreement between surface areas derived from H chemisorption and Kr adsorption may be fortuitous. In any case it is apparent that evacuation at T 300 K causes the desorption of some H from the adsorbed monolayer. Consequently, at these temperatures the excess adsorption may actually correspond to readsorption into the original monolayer. 

H chemisorption measurements were carried out adopting the flow thermal... 

The study was based on temperature programmed techniques, H2 and CO + H chemisorption and FTIR analysis of co-adsorbed CO and H2. The cyclopropane (CP) hydrogenolysis was used as a probe reaction either on clean or CO poisoned catalysts. 

Kesmodel LL, Dubois LH, Somoijai GA (1978) Dynamical DEED study of C H and C H, chemisorption on Pt(lll) evidence for the ethylidyne CH -C group. Chem Phys Lett 56 267... 

This group is currently exploring the use of CO/H chemisorption ratios, H2 D2 exchange rates, and isotopic patterns of ethylene deuterogenation as probes of the nature of supported metals. 

Effect of SMSI on H chemisorption The various M/T102 catalysts were reduced in 250 Torr H2 either at low temperature (200 C LTR samples) or at high temperature (500°C HTR samples), LTR N1/T102 sample was exceptionnally reduced at 300°C to make sure that all nickel was completely reduced. H2 adsorption capacities are listed in Table I. 

Interactions at room temperature When CO is first introduced (Fig.l), a increases irtfantaneously and then remains independent of P 0. The fact that a does not decrease means that CO does not dissociate on titania nor at the interface, otherwise the filling of anionic vacancies by atomic oxygen (Eq,-6) would have decreased substantially a by consuming free electrons. The sharp initial increase, on the contrary, shows that CO chemisorb on Pt with a donor effect probably due to the creation of dipoles as proposed for H. chemisorption which renders ohmic the electrical contact between the metal and its semiconductor support (26, 17, 28)Accordlng to these authors, the creation of a dipole layer decreases the work function of the metal which approaches the electron affinity of the semiconductor, thus suppressing the Schottky barrier. Presently CO adsorbs as a donor molecule on Pt decreasing 0, which allows elec-... 

The electronic metal-support interactions can also account for the lesser extent of SMSI with respect to H. chemisorption and catalysis when the metal loading is increased at constant crystallite size, since for an equivalent quantity of free electrons created by reduction in the support, the higher number of metal atoms present leads to weaker electronic perturbations per atom concerned. 

Compilation of structural parameters for H chemisorption systems. A da is the percentage change in the first interlayer spacing of the substrate computed with respect to the bulk interplanar spacing normal to the surface. The H radius is computed by subtracting the metallic radius of the substrate atom from the derived M-H bond length. 

Rates R at various reaction times (deduced by interpolation) were converted to these turnover numbers (N R/S -iig) in ra°feculGS Per site per second for the reaction by dividing by the number of surface platinum sites ( u. ng) assuming that the number of surface aicnos per m Pt (ng) was 1 25 x lCr, Naturally the Pt surface area S (m per g Pt depends upon the method used (i,e H chemisorption, TPD etc.) as do turnover numbers. 

Little, L. H., Chemisorption and Reactions on Evaporated Metallic Films. (J. R. Anderson, ed.). Academic Press, Now York, 1970. 

Figure 12.10 Steps in the dissociative chemisorption of water at the tip of a silica glass. (a) Tip of crack with approaching water molecule h) chemisorption of water and its alignment (c) the breaking of an Si-O-Si bond and the formation of two Si-OH bonds.

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