Adsorption temperature coefficients

The Hg/dimethyl formamide (DMF) interface has been studied by capacitance measurements10,120,294,301,310 in the presence of various tetraalkylammonium and alkali metal perchlorates in the range of temperatures -15 to 40°C. The specific adsorption of (C2H5)4NC104 was found to be negligible.108,109 The properties of the inner layer were analyzed on the basis of a three-state model. The temperature coefficient of the inner-layer potential drop has been found to be negative at Easo, with a minimum at -5.5 fiC cm-2. Thus the entropy of formation of the interface has a maximum at this charge. These data cannot be described... 

This backdonation of electron density from the metal surface also results in an unusually low N-N streching frequency in the a-N2 state compared to the one in the y-N2 state, i.e. 1415 cm 1 and 2100 cm"1, respectively, for Fe(l 11)68. Thus the propensity for dissociation of the a-N2 state is comparatively higher and this state is considered as a precursor for dissociation. Because of the weak adsorption of the y-state both the corresponding adsorption rate and saturation coverage for molecular nitrogen are strongly dependent on the adsorption temperature. At room temperature on most transition metals the initial sticking coefficient does not exceed 10 3. 

This model was fitted to the data of all three temperature levels, 375, 400, and 425°C, simultaneously using nonlinear least squares. The parameters were required to be exponentially dependent upon temperature. Part of the results of this analysis (K6) are reported in Fig. 6. Note the positive temperature coefficient of this nitric oxide adsorption constant, indicating an endothermic adsorption. Such behavior appears physically unrealistic if NO is not dissociated and if the confidence interval on this slope is relatively small. Ayen and Peters rejected this model also. 

Catalysts are porous and highly adsorptive, and their performance is affected markedly by the method of preparation. Two catalysts that are chemically identical but have pores of different size and distribution may have different activity, selectivity, temperature coefficient of reaction rate, and response to poisons. The intrinsic chemistry and catalytic action of a surface may be independent of pore size, but small pores appear to produce different effects because of the manner and time in which hydrocarbon vapors are transported into and out of the interstices. 

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. 

Hence, the temperature coefficient of k1 having been measured, for an absolute calculation of k only kt) and bo must be known, and not the heat of adsorption, X. At the moment we are concerned with b0. A simple statistical estimate can be based on the assumption that in the absence of adsorption energy the adsorption space is filled at a proportion given by the ratio of the molecular adsorption volume (liquid volume Fm) to the molecular gas volume... 

We shall now go a step farther and consider the intramolecular mechanism by asking what changes are effected within the molecules during the reaction as a consequence of their adsorption by the catalyst. The study of temperature coefficients is useful for this purpose. This yields the energy of activation, q, according to... 

We should thus anticipate that the energy of activation of the adsorption compounds in heterogeneous catalysis would vary with the nature of the catalyst. It is however, as we have seen, a difficult matter to determine the value of E from the temperature coefficient alone, for the surface of the catalyst may not be equally covered with the reactant over the temperature range of investigation and in addition, as we have had occasion to note, the surface of the catalyst is by no means homogeneous. 

A C.P.D. method was adopted by Bosworth and Rideal (95, 119) to investigate the evaporation of Na from a W filament. Desorption was accompanied by a negative drift in the S.P. when the coated filament was held at a temperature in the range 610° to 795° K., and the resulting S.P.-time curves were converted into coverage-time curves by the use of calibration data previously obtained. The results represent the mutual effect of adsorption and desorption processes on the W filament. Hence, the heat of evaporation E wav iiaay be calculated from the temperature coefficient of... 

For some models adsorption or storage is important. For example, oxygen storage is important in a 3-way catalysis, a catalyst may contain a hydrocarbon storage component for improved low-temperature performance, and ammonia storage is important for ammonia SCR (selective catalytic reduction). Clearly, this sort of behaviour needs to be included in the final model. The nature of the measurements depends on the exact system being studied and will be discussed in more detail later. Suffice to say, from measurements at steady state, the heats of adsorption and coefficients of... 

An interesting feature of this slow adsorption was pointed out by Taylor (6) in a discussion of early data obtained on zinc oxide between 132° and 184°C. (7). From the temperature coefficient of the slow adsorp-... 

The acidic sites on iron oxides are believed to be FeOH sites (32), much like the well-known SiOH sites on silica. Heats of adsorption on iron oxide of bases of known Cg and Eg, having appreciably different ratios of Cg to Eg ("hardness" or "softness"), allow estimation of the and for the acidic sites of iron oxide. Our initial studies were done by measuring adsorption isotherms at two or more temperatures (Figure 7) and from the temperature coefficient of the equilibrium constant K the enthalpy of adsorption was calculated. In Figure 7 the adsorption data is plotted as a Langmuir isotherm ... 

There are shortcomings in this work, however, and we expect to solve these soon. Adsorption is a slower process than most of us realize (25), and at 25°C the adsorption of pyridine onto iron oxide takes about three days to reach equilibrium. The results of Figure 7 with pyridine and those with triethylamine were obtained in about one hour. However Fm was the same for the two temperatures, for the slopes are exactly equal for the two lines. We are now using a flow microcalorimeter to measure the evolution of heat upon adsorption and we are adding a UV sensor to detect concentration changes this combination should give accurate heats of adsorption and desorption. We will then be able to compare these direct measurements of heats of adsorption with those obtained from the temperature coefficients of adsorption isotherms. 

The temperature coefficient for both the pyridine and triethylamine adsorption was small, and will need checking by calorimetric measurements. 

The surface was actually a film of native aluminum oxide it did not adsorb pyridine but did adsorb chloroform showing the oxide to have no acid sites, but basic sites. Treatment of the aluminum oxide with aqueous carbonate solutions clearly enhanced the basicity, as evidenced by stronger adsorption of chloroform. By observing the temperature coefficient of adsorption isotherms with ellipsometry one can actually determine heats of adsorption on a square centimeter of flat surface. 

The other factor that can show the influence of kinetic, catalytic, and adsorption effects on a diffusion-controlled process is the temperature coefficient.10 The effect of temperature on a diffusion current can be described by differentiating the Ilkovic equation [Eq. (3.11)] with respect to temperature. The resulting coefficient is described as [In (id,2/id,iV(T2 — T,)], which has a value of. +0.013 deg-1. Thus, the diffusion current increases about 1.3% for a one-degree rise in temperature. Values that range from 1.1 to 1.6% °C 1, have been observed experimentally. If the current is controlled by a chemical reaction the values of the temperature coefficient can be much higher (the Arrhenius equation predicts a two- to threefold increase in the reaction rate for a 10-degree rise in temperature). If the temperature coefficient is much larger than 2% °C-1, the current is probably limited by kinetic or catalytic processes. 

The temperature coefficients for currents that are limited by adsorption vary and may have negative values. 

Clarke, Kassel, and Storeh5 interpret the slow adsorption on chromic oxide gel as in part a diffusion of an already adsorbed layer to secondary centres of adsorption but Burwell and Taylor criticize the whole diffusion hypothesis, partly on the grounds that the temperature coefficient of the rate of adsorption is not that to be expected for diffusion.6 It may not, however, be easy to distinguish between the gross characteristics of an activated diffusion along the surface, and of an activated adsorption indeed it seems just admissible to consider the former as a localized form of the latter. 

The adsorption of each of the reactants and products on cobalt molybdate was studied under conditions as close as possible to reaction conditions. Butene and thiophene both showed strong temperature dependence, adsorption of the latter in particular being slow at the lower reaction temperatures. The temperature coefficients were 8.5 and 9.5 kcal. per mole, respectively. H2S adsorbed quickly and desorbed at a rate proportional to coverage, and hydrogen apparently behaved in the same way. Only relatively weakly bound or free hydrogen appeared to be reactive, but adsorbed hydrogen probably modified the adsorption of thiophene and of butene. 

The integral heat of adsorption is the difference between the heat of immersion of the clean adsorbent and the heat of immersion of the adsorbent, with n2 moles of X2 adsorbed upon it. This calorimetric heat of adsorption is to be compared with the heat of adsorption calculated from the temperature coefficient of the integral free energy change by Equation 6. 

Entropy Change. The entropy change in the adsorption of water on barium sulfate is shown in Figure 8. The integral entropy, AS, calculated from AG and A ffl decreases monotonically with increasing amount adsorbed. AS can also be calculated from the temperature coefficient of v by... 

Where W is the equilibrium adsorption capacity. Wo is the total volume of the micropores aecessible to the given adsorbate, k is a characteristic constant related to the pore structure of the adsorbent, P is an affinity coefficient, Csai is the saturation concentration in the gas phase of liquid adsorbate at the adsorption temperature T, and C is the eoncentration of adsorbate vapor in equilibrium. Plotting ln(W) versus [RTln(Csai/C)] the parameters K/p and Wo in the DR equation were determined by the slope and the intercept of the linear lines respeetively. The obtained results and correlation coefficients are eompiled in Table 2. This Table shows the DR equation parameters and the... 

Chromatographic separations of the isotopic isomers of molecules are of interest first because they afford a convenient technique for analysis of mixtures, secondly because scale up in the future might afford economically feasible separations of macroscopic amounts of material, and finally because the values of the separation factors and their temperature coefficients are of intrinsic theoretical interest. This last follows because such data are straightforwardly related to the understanding of isotope effects on solution and adsorption processes and of the intermolecular forces which give rise to these effects. We have approached the general problem... 

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. 

We now turn to the numerous observations made on the activated adsorption of hydrogen on reduced copper. In this case a definite, large temperature coefficient or activation energy Ae has been found. It was calculated by the conventional formula... 

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