Kinetics desorption

There are two commonly employed techniques for studying thermal desorption and several variations on each of them. The interpretation of kinetic data is fraught with difficulties and to obtain kinetic parameters such as kinetic order and activation energy from desorption requires some knowledge of the desorption mechanism. We therefore leave discussion of data analysis until a later section. 

Additional peaks caused by adsorbate-adsorbate repulsion are seen for H2 desorbing from single crystal surfaces at high initial coverages [378, 386]. 

Temperature programmed desorption of H2 from the reduced catalyst has been reported to proceed at 90-240 °C [405] or to show 4 peaks [406]. The kinetics of thermal desorption of H2 from the catalyst is second order [407]. Evidence that chemisorbed hydrogen remains on the surface after evacuation at 550 °C for 24 h has been reported [408]. 

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TPD Temperature-programmed desorption [171, 172] The surface is heated and chemisorbed species desorb at characteristic temperatures Characterization of surface sites and desorption kinetics... 

Reider G A, Hdfer U and Heinz T F 1991 Desorption-kinetics of hydrogen from the Si(111)7 7 surface J. Chem. Phys. 94 4080-3... 

In this review we put less emphasis on the physics and chemistry of surface processes, for which we refer the reader to recent reviews of adsorption-desorption kinetics which are contained in two books [2,3] with chapters by the present authors where further references to earher work can be found. These articles also discuss relevant experimental techniques employed in the study of surface kinetics and appropriate methods of data analysis. Here we give details of how to set up models under basically two different kinetic conditions, namely (/) when the adsorbate remains in quasi-equihbrium during the relevant processes, in which case nonequilibrium thermodynamics provides the needed framework, and (n) when surface nonequilibrium effects become important and nonequilibrium statistical mechanics becomes the appropriate vehicle. For both approaches we will restrict ourselves to systems for which appropriate lattice gas models can be set up. Further associated theoretical reviews are by Lombardo and Bell [4] with emphasis on Monte Carlo simulations, by Brivio and Grimley [5] on dynamics, and by Persson [6] on the lattice gas model. 

Our last example of the utility of the transfer matrix approach concerns the thermodynamics and desorption kinetics of H/Rh(311) obtained from... 

Additional applications of the transfer matrix method to adsorption and desorption kinetics deal with other molecules on low index metal surfaces [40-46], multilayers [47-49], multi-site stepped surfaces [50], and co-adsorbates [51-55]. A similar approach has been used to study electrochemical systems. 

In one dimension the truncation of the equations of motion has been worked out in detail [59]. This has allowed an accurate examination of the role of diffusion in desorption, and implications for the Arrhenius analysis in nonequilibrium situations. The largest deviations from the desorption kinetics of a mobile adsorbate obviously occur for an immobile adsorbate... 

Along similar hnes, one can study the desorption kinetics in the presence of precursors. 

Adsorption-Desorption Kinetics and Chromatographic Band Broadening... 

For further development of the quantitative treatment of the desorption kinetics, the work of Redhead (31) and of Carter (32) is of great impor-... 

If a generalization to an arbitrary order x of the desorption kinetics is also performed, the expressions (28) transform into... 

Thus, for the second-order desorption kinetics and the hyperbolic heating schedule, the peaks are symmetric about Tm in the scale (1/T). The first-order peaks are asymmetric in this scale, exhibiting a steeper descent than ascent. These considerations suggest that the hyperbolic heating schedule is especially favorable for an analysis of the peak shapes and for the detection... 

Its main features are given by the use of a stream of inert carrier gas which percolates through a bed of an adsorbent covered with adsorbate and heated in a defined way. The desorbed gas is carried off to a detector under conditions of no appreciable back-diffusion. This means that the actual concentration of the desorbed species in the bed is reproduced in the detector after a time lag which depends on the flow velocity and the distance. The theory of this method has been developed for a linear heating schedule, first-order desorption kinetics, no adsorbable component in the entering carrier gas (Pa = 0), and the Langmuir concept, and has already been reviewed (48, 49) so that it will not be dealt with here. An analysis of how closely the actual experimental conditions meet the idealized model is not available. 

Even if the peak behavior fits well for a given apparent desorption order, the real kinetic situation may be a different one. As a rate controlling step in a second-order desorption, random recombination of two particles is assumed most frequently. However, should the desorption proceed via a nonrandom recombination of neighboring particle pairs into an ordered structure, the resulting apparent first-order desorption kinetics is claimed to be possible (36). The term pseudo-first-order kinetics is used in this instance. Vice versa, second-order kinetics of desorption can appear for a nondissociative adsorption, if the existence of a dimer complex is necessary before the actual desorption step can take place (99). A possibility of switching between the apparent second-order and first-order kinetics by changing the surface coverage has also been claimed (60, 99, 100). 

The first-order and second-order kinetics of desorption are by far the most common and practically considered cases. Less than first-order desorption kinetics indicates multilayer adsorption or transport limited desorption (101). An actual significance of the third-order kinetics in desorption has been found recently by Goymour and King (102, 103). 

Recently, a quantitative lateral interaction model for desorption kinetics has been suggested (103). It is based on a statistical derivation of a kinetic equation for the associative desorption of a heteronuclear diatomic molecule, taking into account lateral interactions between nearest-neighbor adatoms in the adsorbed layer. Thereby a link between structural and kinetic studies of chemisorption has been suggested. 

Subsequently one plots InNo vs tHe and extrapolates to tHe=0. This plot provides the 02 desorption kinetics at the chosen temperature T. The intersect with the N0 axis gives the desired catalyst surface area NG (Fig. 4.8) from which AG can also be computed. More precisely NG is the maximum reactive oxygen uptake of the catalyst-electrode but this value is sufficient for catalyst-electrode characterization. 

The right-hand part of Fig. 7.7 corresponds to the second-order desorption of nitrogen atoms from a rhodium surface. As the desorption reaction corresponds to N -I- N —> N2 -I- 2 the rate is indeed expected to vary with A characteristic feature of second-order desorption kinetics is that the peaks shift to lower temperature with increasing coverage, because of the strong dependence of the rate on coverage. 

Equation (12) also contains a pre-exponential factor. In Section 3.8.4 we treated desorption kinetics in terms of transition state theory (Figure 3.14 summarizes the situations we may encounter). If the transition state of a desorbing molecule resembles the chemisorbed state, we expect pre-exponential factors on the order of ek T/h = 10 s . However, if the molecule is adsorbed in an immobilized state but desorbs via a mobile precursor, the pre-exponential factors may be two to three orders of magnitude higher than the standard value of 10 s . ... 

A similar though more complicated expression exists for second-order desorption kinetics ... 

Desorption Rates. Using the above model for the temperature jump associated with pulsed laser heating, the rate of desorption versus time and the total number of molecules desorbed from a finite surface area heated by the laser can be calculated. For the particular case of first-order desorption kinetics, the desorption rate is ... 

In Ae hydrogen TPD spectrum, Ae lower-temperature peak occurs at 355 K, characteristic of desorption of surface hyAogen Ni(lOO). Hence it is defined by Ae desorption kinetics... 

McEwen JS, Eichler A. 2007. Phase diagram and adsorption-desorption kinetics of CO on Ru(OOOl) from first principles. J Chem Phys 126 094701. 

Pfniir H, Feulner P, Menzel D. 1983. The influence of adsorpate interactions on kinetics and equilibrium for CO on Ru(OOl)—II. Desorption kinetics and equilibrium. J Chem Phys 79 4613. 

Schwarz JA. 1979. Adsorption-desorption kinetics of H2 from clean and sulfur covered Ru(OOOl). Surf Sci 87 525. 

Quigley MS, Honeyman BD, Santschi PH (1996) Thorium sorption in the marine enviromnent equilibrium partitioning at the Hematite/water interface, sorption/desorption kinetics and particle tracing. Aquat Geochem 1 277-301... 

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