Surface reaction data interpretation

In this article we describe novel approaches aimed at making surface analytical data easier to obtain and interpret. These include 1) a small, low cost reaction facility designed to work... 

Cycled Feed. The qualitative interpretation of responses to steps and pulses is often possible, but the quantitative exploitation of the data requires the numerical integration of nonlinear differential equations incorporated into a program for the search for the best parameters. A sinusoidal variation of a feed component concentration around a steady state value can be analyzed by the well developed methods of linear analysis if the relative amplitudes of the responses are under about 0.1. The application of these ideas to a modulated molecular beam was developed by Jones et al. ( 7) in 1972. A number of simple sequences of linear steps produces frequency responses shown in Fig. 7 (7). Here e is the ratio of product to reactant amplitude, n is the sticking probability, w is the forcing frequency, and k is the desorption rate constant for the product. For the series process k- is the rate constant of the surface reaction, and for the branched process P is the fraction reacting through path 1 and desorbing with a rate constant k. This method has recently been applied to the decomposition of hydrazine on Ir(lll) by Merrill and Sawin (35). 

However, due to the difficulties in calculating ion yields in SIMS, quantitation of the data is not very reliable, and their work was not conclusive. We have determined here that the reaction of chemisorbed ethylene to form ethylidyne is first order in ethylene coverage. A noticeable isotope effect was observed, with activation energies of 15.0 and 16.7 Kcal/mole for C H and 02 respectively. These values are smaller than those calculated from TDS, but the differences can be reconciled by including the recombination of hydrogen atoms on the surface in the interpretation of the thermal desorption experiments. 

For a problem involving surface chemistry, the next step is to execute the Surface Chemkin Interpreter, which reads the user s symbolic description of the surface-reaction mechanism. Required thermodynamic data can come from the same Thermodynamic Database used by Chemkin or from a separate Thermodynamic Database compiled for surface species. Both Interpreters provide the capability to add to or override the data in the database by user input in the reaction description. The Surface Chemkin Interpreter extracts all needed information about gas-phase species from the Chemkin Linking File. (Thus the Chemkin Interpreter must be executed before the Surface Chemkin Interpreter.) Like the Chemkin Interpreter, the Surface Chemkin Interpreter also provides a printed output and a Linking File. Again, the Surface Linking File is read by an initialization subroutine in the Surface Subroutine Library that makes the surface-reaction mechanism information available to all other subroutines in the Library. 

Surface reactions are very important in both theoretical and applied research. Experimental information on the individual reaction steps is difficult to obtain and the interpretation of the data is not easy. However, investigation of individual steps of a surface reaction can be obtained by using theoretical models, where discrete lattices are used to represent the surface. Depending on the number of different particles and on the adsorption and reaction steps, the models are classified as A + A 0, A + B 0, A + 2 —> 0,... 

We would, therefore, agree with Bond s conclusion (3) that application of the transition state theory to heterogeneous reactions has not so far provided insight into the mechanisms of surface reactions and that the failures of the theory are generally more significant than the successes. We do not accept that the use of the theory of absolute reaction rates in the interpretation of kinetic data provides a general and reliable method for the estimation of the concentration of surface active sites but conclude that results should always be considered with reference to appropriate quantitative supporting evidence (133). 

Thus, due to the shortcomings of currently available statistical procedures and the restricted data included in many reports of kinetic studies, it is at present impracticable to calculate a parameter that provides a realistic measure of the accuracy of obedience of (log A, E) values to the compensation equation. While this objective may become realizable in the future, we are at present restricted to the use of the linear regression formula as a semiquanti-tative approximation. Results obtained using this approach, in a comparative analysis of the kinetic data available in the literature for a wide variety of surface reactions, are tabulated in Section III and some judgments concerning the relative accuracy of fit of data for different systems to Eq. (2) can be made. Interpretation of the significance of the observed trends must include consideration of the possibilities that the observed relationships... 

Interpretation of the mechanisms of the hydrocarbon desorption reactions mentioned above was considered (31,291) with due regard for the possible role of clay dehydration. While this water evolution process is not regarded as a heterogeneous catalytic reaction, it is at least possible that water loss occurs at an interface (293) so that estimations of preexponential factors per unit area can be made. On this assumption, Arrhenius parameters (in the units used throughout the present review) were calculated from the available observations in the literature and it was found (Fig. 9, Table V, S) that compensation trends were present in the kinetic data for the dehydration reactions of illite (+) (294), kaolinite ( ) (293,295 298), montmorillonite (x) (294) and muscovite (O) (299). If these surface reactions are at least partially reversible,... 

By use of the proper experimental conditions and Ltting the four models described above, it may be possible to arrive at a reasonable mechanistic interpretation of the experimental data. As an example, the crystal growth kinetics of theophylline monohydrate was studied by Rodriguez-Hornedo and Wu (1991). Their conclusion was that the crystal growth of theophylline monohydrate is controlled by a surface reaction mechanism rather than by solute diffusion in the bulk. Further, they found that the data was described by the screw-dislocation model and by the parabolic law, and they concluded that a defect-mediated growth mechanism occurred rather than a surface nucleation mechanism. 

The variation in the peak-separation values (AEp) for the cyclic voltammetric data of Table 9.9 may be interpreted in terms of heterogeneous electron-transfer kinetics, but the most reasonable explanation is uncompensated resistance (especially for py and MeCN) and surface reactions (especially for the metal electrodes). 

Before applying the vacuum microbalance or any similar method to the study of the rate of a particular surface reaction it is essential to understand as much as possible concerning the chemistry of the main reaction and the possible side reactions which may occur in a given system. This requires detailed thermochemical calculations to be made for all conceivable reactions to determine the specifications for the vacuum system and furnace tubes, the preparation of the specimens, the experimental procedures, and the interpretation of the data obtained. Kinetic theory calculations should be applied to aid in interpretation of the rates of certain vacuum and low-pressure reactions. 

Cluster modeling of possible chemisorption states and of possible intermediate states in surface reactions can to a first approximation be useful in guiding experiments or interpretations of experimental data for surface reactions (23-25). One important and enlightening result (6, 26, 27) in metal carbide cluster chemistry will be used here to illustrate this particular point because it bears directly on the importance of multicenter C-H-M bonding for hydrocarbon fragments in metal chemistry. 

For interpretation of CLPC data two relationships are used (1) the dependence of the apparent differences of potentials of two sequential reactions Atp, at a distance y along a profile, Atpa = f(y) and (2) polarisation curves. Using these it is possible to determine the size of an ore body projection to the surface measuring profile, the mineral composition of an ore body and reserves of minerals. Ryss (1983) describes an example of CLPC data interpretation on a section at Rudny Altay (Fig. 2-48). The end faces of the horizontal ore bodies correspond to extremes of Acpa(y) (Fig. 2-48B). The group of ore bodies is considered as a horizontal cylinder with radius ro, depth of axis h. Also we... 

Measurement of the adsorption enthalpy AHa revealed a linear decrease with carbon number. By applying Eqn. (3.53) the activation energy for the surface reaction, Ea2, was estimated. The data in Fig. 3.9 clearly shows that 32 has reasonable (and positive) values, while it remains fairly constant for n > 8, supporting the kinetic interpretation. 

A difficulty in applying a simple one-dimensional vertical explanation for the sequence of redox reactions, and for changes in the depth of the oxic-suboxic interface, is that horizontal transport appears to play a dominant role. Diffusion and mixing are much more rapid along, rather than across, equal density or isopycnal surfaces. Reactions occurring at the side boundaries of the Black Sea may have a strong influence on the distribution of properties observed in the interior (e.g., 40). In addition, the chemocline of the Black Sea appears to be subject to rapid lateral ventilation (15, 42) with waters of different histories and pathways. Both dissolved and particulate components are affected in this way (40). Unfortunately, data are not yet available for developing one- or two-dimensional horizontal and vertical interpretations. 

The direct method for enthalpy determination is calorimetry [55-67]. This technique enables the heat of surface reactions to be measured in more complicated situations, e.g. in the pH regions outside the pzc region, and also in the case of (specific) adsorption of species other than p.d. ions. However, the measured heat is a sum of the contributions of all reactions taking place at the interface, so that in interpretation of data one meets the problem of distinguishing between the different contributions. An additional problem is to account for electrostatic effects. On the other haxid, one can always express the results as the enthalpy per amount of adsorbed species or per surface charge. When doing so the... 

Consider a case where the true order of the surface reaction is 2 [according to Eq. (10-2)] but the rate is diffusion controlled, so that Eq. (10-3) is applicable. Experimental data plotted as rate vs would yield a straight line. If diffusion were not considered, and Eq. (10-2) were used to interpret the data, the order would be identified as unity—a false conclusion. This simple example illustrates how erroneous conclusions can be reached about kinetics of a catalytic reaction if external mass transfer is neglected, a... 

Perhaps the main thing to realize about potential energy surfaces for chemical reactions, is that we know very little about them. The large amount of experimental data which is being accumulated can, as yet, be used to furnish only rather indirect and imprecise information on the surface involved. Ab initio calculations in this field are, therefore, in the rare position of playing a vital complementary role to experimental results in aiding the choice of potential energy surface used to interpret specific reactions. 

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