Rates of Chemisorption

The rate at which molecules of a gas strike a surface, in molecules/(sec) (cm surface), is plilTimk T). If 5 is the fraction of the collisions which result in chemisorption, that is, the sticking probability, the rate of adsorption 

CHAPTER 9 KINETICS OF FLUID-SOLID CATALYTIC REACTIONS 

This ideal expression for the rate of adsorption does not usually agree with experimental data. Observed rates decrease so rapidly with increasing coverage 6 that they can be explained only if the activation energy increases with 9. Also, the condensation coefficient may vary with d. These variations may be caused by surface heterogeneity that is, the activity of the sites varies, so that different sites possess different values of a and E. The most active sites would have the lowest activation energy and would be occupied first. Alternately, interaction forces between occupied and unoccupied sites could explain the deviations. In any event, it is necessary to rewrite Eq. (9-3) as 

It is instructive to compare the Langmuir equation, Eq. (8-1), with Eq. (9-4). If the latter is correct, then k in Eq. (8-1) is a function of surface coverage. However, the reason a and E are functions of 9 is that the first two postulates of the Langmuir treatment (see Sec. 8-4) are not satisfied experimentally that is, in real surfaces all sites do not have the same activity, and interactions do exist. 

For many cases of chemisorption the variation in rate with surface coverage can be accounted for entirely in the exponential term of Eq. (9-4), because a 6) and 1 — 6 are so much weaker functions. This leads to the result (for constant temperature) 

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The presence of alkali promoters on the substrate surface can affect both the rate of chemisorption, (e.g. on K/Rh(100))55 and the adsorptive capacity... 

C/min to 140°C, (3) hold for 2 hours, and (4) heat to 500°C at 3°/min. Oxygen was introduced at the time the temperature reached 140°C. The increase in temperature after the isothermal (140°C) region led to an increase in the rate of chemisorption, up to the temperature at which combustion (burn-off) becomes the dominant process resulting in rapid weight loss (ca. 270°C). 

It is important to note that the principal features of hydrogen chemisorption, which are summarized above, apply equally well to other adsorbents than zinc oxide, for instance to chromium oxide. A satisfactory theory therefore must not depend on specific properties of zinc oxide. In this connection let us recall the important experiments of Pace and Taylor (14) and Kohlschiitter (15), who found that the slow rates of chemisorption of hydrogen and deuterium on chromium oxide, zinc oxide-chromium oxide, and nickel on kieselguhr, were identical within experimental error. It would be interesting to perform such an experiment on zinc oxide because it permits one to make a decision on the nature of the slow activated step (16). 

Here, again, the exponential term is of primary importance in determining the rate of chemisorption and the amount of the surface concentration of the chemisorbed atoms. Considering the other terms to be constant, therefore, integration gives... 

Fig. 7a. Rate of chemisorption of oxygen on nickel oxide at 25°C and at various oxygen partial pressures, according to Engell and Hauffe. The chemisorbed volume in cubic centimeters is plotted against log (< + Zo), where (is the time of chemisorption in minutes, and to a constant.

Fig. 8, Rate of chemisorption of hydrogen on CrjOj at a pressure of 0,5 atm. and at 184°C, as measured by Burwell and Taylor and evaluated by Taylor and Thon.

According to the experimental result that the rate of reaction is proportional to Pn,o, the electron consuming step (37a) is the slowest, and is therefore rate determining for the chemisorption. From (37a) we obtain for the rate of chemisorption, in the case of a p-type catalyst... 

Weisz (22) and Morrison (31) have pointed out that if there is a barrier at the surface of a semiconductor, and if electrons from the bulk of the material must cross this barrier for chemisorption to occur, then the rate of chemisorption may be limited by the rate at which electrons can cross the barrier. The rate at which electrons can cross a barrier is proportional to the number of electrons with energy greater than the barrier height, or... 

If the activation energy for chemisorption is appreciable, the rate of chemisorption at low temperature may be so slow that, in practice, only physical adsorption is observed. 

Figure 5.3 shows how the extent of gas adsorption on to a solid surface might vary with temperature at a given pressure. Curve (a) represents physical adsorption equilibrium and curve (b) represents chemisorption equilibrium. The extent of adsorption at temperatures at which the rate of chemisorption is slow, but not negligible, is represented by a non-equilibrium curve, such as (c), the location of which depends on the time allowed for equilibrium. 

Molecular hydrogen does not dissolve easily into iron with a smooth surface at temperatures below 200°C. Atomic hydrogen, however, enters the iron easily even at room temperature (218). In the case of molecular hydrogen it is the activation energy at the surface which governs the process. On a smooth or contaminated iron surface, it is the rate of chemisorption which governs the total rate. We shall return to this special case in Sec. X,4. 

The rate of chemisorption at a pressure of 20 cm. mercury could be represented by... 

Studies of corrosion processes, detailed in Section 4.4.3, have demonstrated the capability of SAW devices to monitor relatively low rates of chemisorption, including the conversion of a thin copper film to CU2S at an initial rate of 4% of one molecular monolayer/day. The use of SAW devices to monitor the real-time desorption of species from a metal film in response to a temperature ramp has been shown to yield information about both the energy and extent of chemisorption [114]. TSM studies of chemisorption of O2 and CO on very thin Ti films were used to determine that the oxide being formed is Ti203 and that the oxidation depth is approximately one nm [137]. For further discussion and additional examples of chemisorption, the reader is referred to Section 5.4.4.3, where these... 

The rate of chemisorption may be estimated from the kinetic theory formula for wall collisions [Eq. (VII.6.6)] ... 

The technically desirable conditions of anode potentials smaller than 1(X) mV vs. RHE, imply very small rates of process (5d) at either platinum or platinum-alloy PEFC anode catalysts, as can be seen, for example, from the RDE results reported in [18d,e]. The PEFC anode catalyst is thus required to electro-oxidize hydrogen in the presence of significant coverage by CO. The rate of sequence (5b) -I- (5c) can be enhanced by anodic overpotential as long as process (5c) significantly limits the rate of this sequence. Since reaction (5c) is a fast and potential-driven process, at relatively low anodic overpotentials the rate of sequence (5b) -I- (5c) could become fully controlled by the rate of chemisorption of H atoms (Eq. (5b)) on a catalyst surface with few CO-free sites. 

In studying the chemisorption of hydrogen on carefully reduced nickel the author has actually observed that a minute quantity of the vapor of stop-cock grease or of mercury vapor from a pressure gage appreciably affect the rate of chemisorption in so far as these contaminants reduce considerably the rate of adsorption and produce the effects typical for the so-called activated adsorption. Incomplete reduction of nickel oxide to the metal leads to a similar result. This can be avoided by repeated reduction and subsequent evacuations of the metal sample at 400°C. for a week. A typical result obtained with an exhaustively reduced nickel specimen is shown in Fig. 1. In view of these findings, the activated adsorption of hydrogen on other reduced metal catalysts frequently reported in the earlier literature might have been caused by contamination effects. 

Attempts have been made by Eyring and Sherman (29) and by Okamoto, Horiuti, and Hirota (30) to evaluate the activation energy for the chemisorption of hydrogen on carbon or nickel on the assumption that the surface atoms behave as isolated atoms. The calculated values, although too high, vary markedly with the spacing of surface atoms. Quantitatively, however, we must at present rely upon experimental data. The absolute rates of chemisorption will be calculated here using the observed temperature coefficient. 

The absolute rate of chemisorption of gaseous molecules 5, V, on the bare and plane surface of a catalyst can be given, assuming that the transmission coefficient equals unity (31), by... 

Davis obtained a value of Ae of 12.68 kcal./mole from the observed chemisorption rate of nitrogen on tungsten and compared it with a value of Ae of 10 kcal./mole derived according to equation (3). A calculation from the relation that Ae is 10 kcal. - - %RT of the rate by means of equation (6) leads to the result that T is 7.1 X 10 . as shown in Table I. The initial rate of chemisorption calculated on the basis of an activation energy deduced from the observed rate is not reliable because the rates... 

Calculaled and Observed Rates of Chemisorption on Bare Surfaces of Metallic Catalysts... 

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