Direct chemisorption

Park S, Tong YY, Wieckowski A, Weaver MJ. 2002a. Infrared spectral comparison of electrochemical carhon monoxide adlayers formed by direct chemisorption and methanol dissociation on carbon-supported platinum nanoparticles. Langmuir 18 3233-3240. 

Data obtained by Rettner [12] also show that the sticking probability had negligible dependence on the incidence angle for the precursor-mediated chemisorption regime and a very weak dependence on incidence angle in the direct chemisorption region. Rettner et al. postulate that this could possibly be due to the effective corrugation of the surface potential or multiple interactions of the molecule with the surface. 

Several investigations have been undertaken on a number of metal surfaces in order to elucidate the dynamics and mechanisms behind the direct chemisorption of CH4 at higher kinetic energies. 

Similar to the trends seen earlier by Refiner et al. [40] of CH4 on the W(1 1 0) surface, Lee et al. also observed the enhancing effect of vibrational excitation on the direct chemisorption of CH4. From this data, Lee et al. also propose, like Refiner et al., that vibrational energy may be more efficient... 

Good agreement between the calculated and experimental values offered justification for extrapolation of these models to higher gas temperatures. Figure 17, displays the calculated contributions from the trapping mediated and direct chemisorption, along with the total, combined contribution to the... 

The rapid decrease in So(E ) observed below 0.15 eV on Pt(5 3 3) (Fig. 18) has also been observed on the Pt(l 1 1) surface [134] and is consistent with a trapping mechanism where the need to dissipate energy limits the probability of adsorption, and subsequent dissociation, via the physisorbed precursor. In order to assess the contribution of the physisorption mediated channel, the contribution to sticking directly via the chemisorbed channel must be subtracted from the measured So. The proportion of So derived from the direct chemisorption channel on Pt(5 3 3) at Ex = 0.05 eV is significantly higher than on Pt(l 1 1) (ca. 10%) [137]. Once this direct contribution is subtracted, the dependence S0(Ts)can be used to obtain kinetic parameters relating to the partition of the physisorbed precursor. This is achieved... 

Figure 9 Adsorption process of NO on Pd particles supported on MgO(l 00). (a) Global adsorption probability as a function of surface temperature and for various particle sizes (from Ref. [89]). (b) Schematic representation of die elementary processes in die molecular adsorption of NO on supported Pd particles (1) quasi-elastic redection on die bare support, (2) physisorption-diffusion-desorption from the bare support, (3) direct chemisorption on die Pd particles, (4) NO chemisorption on the Pd particles via a precursor physisorbed state on die bare support. Xs is die mean diffusion length of die NO molecules on the support and p is die width of die collection zone around die Pd particles.

Fig. 3.11. Schematic representation of the reverse-spillover for gas molecules impinging on a Pd/MgO model catalyst. (1) Reflection from the MgO, (2) adsorption-desorption from the MgO, (3) direct chemisorption on the Pd cluster, (4) capture by a Pd cluster of a molecule physisorbed on the MgO...

In cases not readily amenable to direct chemisorption, titration of specific sites is possible with probe reactions. The reaction must be an irreversible interaction between a gas and surface site, such that only one event occurs per site. The first example of this technique was the titration of Pt-O sites. The surface of supported platinum is first oxidized to form no more than a monolayer. Then follows reaction with hydrogen... 

As TEDA is present on all modem types of whetlerite, military filters exhibit a rather good retention of radioactive gases by direct chemisorption. However, one has to bear in mind this is only a retention, not a destruction. This means that the filter itself will become a radioactive source, much like when retaining radioactive aerosol particles (see section 2.3.8), and has to bo replaced, and disposed of, as soon as possible. 

Now we need to talk about physical adsorption, which, though it is not generally considered to be a crucial factor in surface reactions, is of importance in relation to the topic as a whole. One might say that physical adsorption does things a bit differently from chemisorption, as is shown in Table 3.1. The analysis of physical adsorption can be complicated, even more so than for chemisorption, because attractive-repulsive interactions are involved that may be more complex than direct chemisorptive interactions. 

Park, S., Y.T. Tong, A. Wieckowski, and M.J. Weaver, Infrared spectral comparison of electrochemical carbon monoxide adlayers formed by direct chemisorption and methanol dissociation on carbon-supported platinum nanoparticles. Langmuir, 2002.18(8) pp. 3233-3240 Park, S., Y. Tong, A. Wieckowski, and M.J. Weaver, Infrared reflection-absorption properties of platinum nanoparticle films on metal electrode substrates control of anomalous opticalejfects. Electrochemistry Communications, 2001. 3(9) pp. 509-513 Park, S., P.K. Babu, A. Wieckowski, and M.J. Weaver, Electrochemical infrared characterization of CO domains on ruthenium decorated platinum nanoparticles. Abstracts of Papers of the American Chemical Society, 2003. 225 pp. U619-U619... 

An additional and very important dimension is provided by the fact that many of the species shown in Figure 8.6 can exist in two different conformations designated syn and anti. This applies to butadiene whether as the di-7r forms I and III or the TTsa forms lA and lllB (IIIA can only adopt the syn form), and to the half-hydrogenated butenyls formed therefrom. Now in the gas phase the anti conformation of butadiene is preferred to the syn by a factor of about 20 at room temperature,so direct chemisorption into any of these structures (not going via II) automatically favours the former, and this geometry is maintained after the first hydrogen atom is added. The 713-butenyls cannot interconvert, except via IV, V and VII a temporary detachment of the r-bond from the surface, followed by... 

According to the adsorption theory, passivity of chromium and the stainless steels, because of their pronounced affinity for oxygen, can occur by direct chemisorption of oxygen from the air or from aqueous solutions, and the equivalents of oxygen so adsorbed were found [22] to be of the same order of magnitude as the equivalents of passive film formed on iron when passivated either anodically, by concentrated nitric acid, or by exposure to chromates. Similarly, oxygen in air can adsorb directly on iron and passivate it in aerated alkaline solutions, or also in near-neutral solutions if the partial pressure of oxygen is increased sufficiently. 

Lithographic gold electrodes were modified in two ways (1) formation of a SAM of the bipodal alkanethiol DT2 followed by covalent linking of whole anti F. tularensis antibody and (2) direct chemisorption of F. tularensis antibody fragments Francisella tularensis Step and sweep amperometry 4.5 ng/mL for the lipopolysac-charide antigen isolated from F. tularensis and 31 bacteria/ mL for the F. tularensis bacteria/- Dulay et al. (2014)... 

The particular reactivity of bare Si02 for the production of HCHO is a matter of debate and has not yet been completely rationalized. Parmaliana et al. [113] pointed out that the performance of the silica surface in CH4 partial oxidation is controlled by the preparation method. For several commercial Si02 samples, the following reactivity trend has been established, based on the preparation method precipitation > sol-gel > pyrolysis. The activity of such silicas has been correlated with the density of surface sites stabilized under steady-state conditions acting as O2 activation centers [114], and the reaction rate was the same for all the silicas when expressed as TOF (turnover frequency). Klier and coworkers [115] reported the activity data for the partial oxidation of CH4 by O2 to form HCHO and C2 hydrocarbons over fumed Cabosil and silica gel at temperatures ranging from 903 to 1953 K under ambient pressure. They observed that short residence times enhanced HCHO (and C2 hydrocarbon) selectivity, suggesting that HCHO did not originate from methyl radicals, but rather from methoxy complexes formed upon direct chemisorption. 

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