Adsorption under steady-state conditions

Figure 53 shows relative rates of C02 formation under steady-state conditions that were recorded with various single-crystal surfaces of Pd as well as with a polycrystalline Pd wire (173). It must be noted that with these experiments no determination of the effective surface areas was performed so that no absolute turnover numbers per cm2 are obtained. Instead, the reaction rates were normalized to their respective maximum values. As can be seen from Fig. 53, all data points are close to a common line which indicates that, in fact, with this reaction the activity is influenced very little by the surface structure. As has been outlined in Section II, the adsorption of CO exhibits essentially quite similar behavior on single-crystal planes with varying orientation. Since the adsorption-desorption equilibrium of CO forms an important step in the overall kinetics of steady-state C02 formation, this effect forms at least a qualitative basis on which the structural insensitivity may be made plausible. 

Under steady-state conditions, the rate of diffusion of A from the bulk solution to the interface is approximately equal to the rate of adsorption of A, and this can be used to eliminate the unmeasured interface concentration CA. Thus,... 

Heterogeneously catalyzed reactions are usually studied under steady-state conditions. There are some disadvantages to this method. Kinetic equations found in steady-state experiments may be inappropriate for a quantitative description of the dynamic reactor behavior with a characteristic time of the order of or lower than the chemical response time (l/kA for a first-order reaction). For rapid transient processes the relationship between the concentrations in the fluid and solid phases is different from those in the steady-state, due to the finite rate of the adsorption-desorption processes. A second disadvantage is that these experiments do not provide information on adsorption-desorption processes and on the formation of intermediates on the surface, which is needed for the validation of kinetic models. For complex reaction systems, where a large number of rival reaction models and potential model candidates exist, this give rise to difficulties in model discrimination. 

Fig. 9.13 where v and AS are plotted against the heat of adsorption q. It must be emphasised that there is only one point for each system since in SCAC the value of V can only be determined under steady-state conditions when the amount of adsorbed molecules equals the amount of desorbed molecules at T = 300 K, since all measurements are made at room temperature. 

The state of the surface of a floating bubble depends on its size. Surfaces of reasonably large bubbles are mobile. As a result, adsorbed surfactants are pulled down to the rear of the bubble, i.e. even under steady-state conditions the value of adsorption on a mobile bubble surface is different from that on an immobile one, T (at the same surfactant concentration in the bulk). 

The model of Nijhuis et al. [39] expHcitly accounts for adsorption/de-sorption at the crystal boundary, assuming Langmuir adsorption kinetics. As the TEX-PEP experiments are conducted under steady-state conditions, this mechanism can be replaced by a simple first-order adsorption/desorption process... 

The consequences for the kinetics of adsorption and desorption are quite obvious. The occupation of adsorption sites by a second species will usually reduce the sticking coefficient. This may lead to the effect that under steady-state conditions, the more strongly held adsorbate is not necessarily most abundant on the surface, as will become obvious in Section 6.3, with the oxidation of CO on Pt. A second species may, however, also increase the sticking coefficient and therefore acts as promoter. This will be outlined in Section 6.1 with the effect of K on the sticking coefficient for nitrogen in catalytic ammonia synthesis. 

In theoretical studies Thullie and Renken [7] showed that operating under periodic variation of the inlet concentration leads to a considerable increase of the reactor performance exceeding the maximal performance obtainable under steady state conditions. The attainable enhancement under periodic operation depends on the sorption equilibrium constants and the sorption rates. Assuming instantaneous equilibrium on S2 and irreversible adsorption on S the model calculations lead to a maximal performance under periodic operation for cycle periods tqtproaching zero (tp- 0)... 

Table 1 summarises the steady state experiments over the zeolite crystals and pellets. The activation energy for the intrinsic rate constant kmtr was 34.4 kJ/mol. The low value indicates that the adsorption enthalpy is of the same order as the reaction enthalpy, i.e. (ERx ,obs = Ekr + EAds). The diffusion coefficient under steady state conditions was found to be an order of magnitude higher than that calculated from Knudsen diffusion (2x10 cmVs) using an average pore size of 3.8x10 cm measured by BET and a tortuosity factor of 4. The estimated... 

Adsorption results shown in Figure 12.6 indicate that a parallel H2 adsorption process takes place simultaneously with H2 chemisorption on Ft when the catalyst has been evacuated at temperatures as low as 423 K. The mechanism of this phenomenon still remains as an open question. To enlighten the nature of this process and the species involved, two t3qjes of experiments have been carried out isotopic exchange under steady-state conditions and temperature programmed desorption tests after D2 adsorption, CO adsorption, and CO-D2 co-adsorption. 

Alternative mechanisms have been recently proposed [78,79] based on a kinetic investigation of NO reduction by n-octane under isothermal (200°C) and steady-state conditions in the presence of H2. The authors built up a mathematical model based on supposed reaction pathways, which account for molecular adsorption of NO and CO and dissociative ones for H2 and 02. The elementary steps, which have been considered for modelling their results are reported in Table 10.3. Interesting kinetic information can be provided by the examination of this mechanism scheme in particular the fast bimolecular... 

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