Surface residence time

The surface residence time, tsurf, is related to the heat of adsorption, AH, and temperature, T, through a Frenkel-type relationship ... 

If we assume that t0= 10 13 s (vibrational frequency)-1, then for a heat of adsorption AH of 40 kJ mol-1 and a surface temperature of 295 K the residence time zsurf is 3 x 10-6 s and for 80 kJ mol-1 it is 102 s as T decreases the value of the surface residence time increases rapidly for a given value of AH. Decreasing the temperature is one possible approach to simulating a high-pressure study in that surface coverage increases in both cases the reaction, however, must not be kinetically controlled. 

Wilson MH, Limbird LE (2000) Mechanisms regulating the cell surface residence time of the a2A-adrenergic receptor, Biochem 39 693-700... 

In this paper selectivity in partial oxidation reactions is related to the manner in which hydrocarbon intermediates (R) are bound to surface metal centers on oxides. When the bonding is through oxygen atoms (M-O-R) selective oxidation products are favored, and when the bonding is directly between metal and hydrocarbon (M-R), total oxidation is preferred. Results are presented for two redox systems ethane oxidation on supported vanadium oxide and propylene oxidation on supported molybdenum oxide. The catalysts and adsorbates are stuped by laser Raman spectroscopy, reaction kinetics, and temperature-programmed reaction. Thermochemical calculations confirm that the M-R intermediates are more stable than the M-O-R intermediates. The longer surface residence time of the M-R complexes, coupled to their lack of ready decomposition pathways, is responsible for their total oxidation. 

Many studies address the effect of promoters such as K and Mn on Fe-based catalysts. Dry et al (23) suggest that the alkali promoter weakens the C-O bond and enhances its rate of dissociation it also strengthens the metal-C bond, the surface residence time of adsorbed chains, and the probability of chain growth. In the presence of Mn, termination to olefins predominates (24-26). Our results suggest that we must also consider the effect of promoters and of catalyst treatment on a-olefin readsorption. Perhaps the presence of alkali also enhances a-olefin readsorption reactions leading to heavier products whereas Mn does not. 

Figure 8 Angular-resolved flux distribution of CO molecules scattered from clean and fully CO saturated Ru(0 0 01) at 60° incidence angle. In the case of the clean surface, two different surface temperatures have been applied. The first has a CO-surface residence time longer than the measuring time, the second a CO-residence time in the micro-second regime. The fluxes of both distributions have been multiplied by a factor of 10 to compensate for the large loss of signal due to sticking. From Riedmuller et al. [55],...

Excess diamine (ODA) component on the other hand leads to undesirable chemical reactions. For instance, it was proposed that under conditions of excess ODA, following imidization, the free amino groups react with imide carbonyl groups resulting in the formation of imine links. These imine links are unstable beyond 300°C, consequently, the films also exhibit poor thermal stability. The optimum relative fluxes need to be determined for each polymer separately taking into consideration the differences in the surface residence time of the species. 

Snibson A, Greaves JL, Soper NDW, et al. Ocular surface residence times of artificial tear solutions. Cornea 1992 11 288-293. 

Figure 30.9. LPCAT treatment under stronger plasma conditions for a longer time yields the more wettable surface. However, the sessile droplet contact angle of a paint on Parylene C surface (resident time 0) is low and minimal change occurred with LTCAT treatment. Thus, the adhesion problem is not due to the wetting difficulty.

With Diminished Signaling and Cell Surface Residency Time... 

If no readsorption occurs, the 1. order rate constant is equal to the inverse of the average surface residence time (t) ... 

In Figure 4, the normalized transient responses for the masses 15, 17 and 40 ( CH, CH and Ar, respectively) are shown for an experiment using the Co/Re/AljOj catalyst. The relatively smooth curves make the calculation of the areas under the curves, and thus the surface residence times, straightforward. The two CO transients are omitted from the figure for simplification. 

Table 3 shows the experimentally obtained values for the CO conversion, the methane selectivity, the number of active surface species reacting to methane and the corresponding surface residence time for all of the SSITKA experiments. 

A comparison of the surface residence times, x, reveals no significant difference for the experiments. From this observation, it can be concluded that the variation in methane 5deld is due to a difference in the amount of active sites. The intrinsic site activity for the methanation reaction remains unchanged after the water treatment, and it is also unaffected by the presence of the rhenium promotor. It is proposed that rhenium supports the reduction of cobalt, but the water pretreatment leads to a reoxidation of the active metal, particularly the fraction of the cobalt metal that is reduced only when rhenium is present. 

Influence of water treatment on CO conversion, X(CO), the selectivity to methane, S(CH ), the number of active surface species reacting to methane, N, and their surface residence time, x(CH ) for Co/Al O, and Co/Re/AljOj. 

IN SSITKA, Np and the mean surface residence time of these most active reaction intermediates (xp) are determined. After a step-change between two reactant streams containing different isotopes of a reactant without disturbing other reaction conditions or reaction (as long as an H2/D2 switch is not used), the distributions of isotopically labeled products are monitored using a mass spectrometer. Tp is first determined by integration of the normalized isotopic transient of a product relative to an inert tracer (usually Ar) that delineates gas phase hold-up (see Figure 1 for the case of methanation). Np is then calculated from... 

Figure 1 Typical SSITKA transients with area between CH4 and Ar determining t, surface residence time of methane intermediates...

We define the formation of a surface chemical bond to be adsorption accompanied by charge transfer and charge redistribution between the adsorbate and the substrate, producing strong bonds of covalent or ionic character. Heats of adsorption on the order of 63 kJ/mole (15 kcal /mole) or larger would certainly indicate the formation of a chemical bond, leading to long surface residence times r [r = tq exp (A//ajs/ 7 )], even at elevated temperatures, compared to to (to == 10 sec) related to vibrational times for surface atoms. 

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