Chemisorption temperature coefficient

More recently, Silva et a/.447,448 have found that the temperature coefficients of dEa /dT for a number of stepped Au surfaces do not fit into the above correlation, being much smaller than expected. These authors have used this observation to support their view of the hydrophilicity sequence the low 9 (rs0/97 on stepped surfaces occurs because steps randomize the orientation of water dipoles. Besides being against common concepts of reactivity in surface science and catalysis, this interpretation implies that stepped surfaces are less hydrophilic than flat surfaces. According to the plot in Fig. 25, an opposite explanation can be offered the small BEod0/dT of stepped surfaces is due to the strong chemisorption energy of water molecules on these surfaces. 

Chemisorption. Chemisorption involves heats of adsorption which are large as compared to the heat of van der Waal s adsorption. The term chemisorption implies formation of semi-chemical bonds of the adsorbed gas with the solid surface. Chemisorption may be a process involving measurable activation energy—that is, a measurable rate of adsorption and a measurable temperature coefficient of rate of adsorption. As in the case of hydrogen adsorption on metals, chemisorption may have no measurable rate of adsorption, the adsorption being essentially instantaneous. 

Activated Adsorption. Activated adsorption—that is, adsorption with a measurable rate of adsorption and a measurable temperature coefficient of rate of adsorption—is a type of chemisorption which is, for instance, found in the adsorption of nitrogen on certain metals at elevated temperatures. The difficulties of deciding whether or not true van der Waal s adsorption exists in cases where the heats of adsorption exceed considerably the heats of condensation will become apparent later in the text. 

The term activated adsorption is, as originally designated by H. S. Taylor, a type of adsorption that takes place at a measurably slow rate, associated with a certain temperature coefficient. For the same type of adsorption, the term chemisorption has been used more frequently, in later years. We shall use here the term chemisorption when the heat of adsorption is comparable with the heat evolved in ordinary chemical reactions. 

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. 

Using Equation (13), the external mass transfer coefficient at 723 K was calculated to be 60 cm/s. Since the reactor operating conditions at th s temperature (723 K, slightly above atmospheric pressure, 247 cm /s) were very similar to those of our transient chemisorption experiments, the external mass transfer coefficient calculated above was used for the simulations. 

The experiments discussed above were all carried out with total pressures below 10-4 Torr. However, Hori and Schmidt (187) have also reported non-stationary state experiments for total pressures of approximately 1 Torr in which the temperature of a Pt wire immersed in a CO—02 mixture was suddenly increased to a new value within a second. The rate of C02 production relaxed to a steady-state value characteristic of the higher temperature with three different characteristic relaxation times that are temperature dependent and vary between 3 and 100 seconds between 600 and 1500 K. The extremely long relaxation time compared with the inverse gas phase collision rate rule out an explanation based on changes within the chemisorption layer since this would require unreasonably small sticking coefficients or reaction probabilities of less than 10-6. The authors attribute the relaxation times to characteristic changes of surface multilayers composed of Pt, CO, and O. The effects are due to phases that are only formed at high pressures and, therefore, cannot be compared to the other experiments described here. 

Generally an Arrhenius (exponential) type of relation represents the diffusion coefficient as a fimction of the temperature, with AQa the activation energy of diffusion. Similarly the parameters b and K (9.16) can be expressed with Arrhenius functions with Qa the (isosteric) heat of adsorption. Consequently is also activated with a total apparent activation energy of (Qa AQa). For chemisorption AQa has about the same value as Qa [1]. For physical adsorption the value of AQa is < (0.5-0.66)Qa. Since the surface flux is small at very low temperature as well as very high temperature there must be a maximum. The possibility of observing this maximum depends on the relative magnitudes of Qa and AQa-... 

However, a different definition was used for [426]. This agrees well with the fact that the chemisorption of oxygen on metals usually takes place with only small activation energy [423,427]. The temperature dependence also supports this fact. A decrease of the reaction was observed with increasing temperature of the substrate surface. Ritter [293,298] explains this observation with a reduction of the condensation coefficient a of oxygen. 

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