Heats, calorimetric, of adsorption

Most microporous adsorbents have a range of micropore size, as evidenced, for example, by a variation in or in calorimetric heats of adsorption with amount adsorbed [227]. As may be expected, a considerable amount of effort has been spent in seeing how to extract a size distribution from adsorption data. 

It is for this reason that stepped curves are convenient to represent accurately the evolution of the calorimetric heats of adsorption (gd)cai expressed... 

Bosworth and Rideal (98) also measured the rate of change of the C.P.D. of an oxygenated W filament which was maintained at temperatures varying from 1270° to 1930° K. Again the experimental conditions permitted the heat of evaporation to be calculated by the Clapeyron equation and the value recorded at low coverage—about 150 kcal./mole—is in satisfactory agreement with the measured calorimetric heat of adsorption of O2 on W (24). 

The data shown in Figure 9.10 indicate both the kind of data that may be obtained by direct calorimetric study of gas adsorption and some evidence of the effect of preheating on the properties of surfaces. The figure shows the calorimetric heat of adsorption of argon on carbon black. The broken line indicates the behavior of the untreated black, and the solid line is the same adsorbent after heating at 2000°C in an inert atmosphere, a process known as graphitization. The horizontal line indicates the heat of vaporization of argon. 

FIG. 9.10 Calorimetric heats of adsorption as a function of coverage for argon on carbon bis at 78K. The dashed line represents untreated black the solid line is after graphitization at 2000° The horizontal line is the heat of vaporization of argon. (Redrawn with permission from R. Beebe and D. M. Young, J. Phys. Chem., 58, 93 (1954).)... 

Similarly, our forcefield works equally well for unsaturated halocarbons. For example, calorimetric heats of adsorption for trichloroethylene in the same three faujasite zeolites are in excellent agreement with our (N.V.T) Monte Carlo simulations [16]. Our results at "zero" loading suggest, unlike hydrocarbons, an analogy between the adsorption processes of saturated and unsaturated halocarbons. 

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. 

Adsorption of oxygen, at room temperature, on samples of NiO(200°) or NiO(260°) containing preadsorbed carbon monoxide changes the color of the oxides to black and increases the electrical conductivity from 10-1 to 1.6 x lO- ohm-i cm-i (2S, 41). Calorimetric heats of adsorption of oxygen are much higher (Table V) (Fig. 16) than in the... 

There has been considerable experimental uncertainty for some time about the connections between calorimetric and isosteric heats. Hill (18) showed that in the reversible isothermal process, Eq. (56) must be replaced by Eq. (57). Kington and Aston (87) derived the analogous Eq. 58 for the reversible adiabatic process and then proceeded to show experimentally that their adiabatic calorimeter actually behaves reversibly. Thus for six values of 6 from 1.16 to 1.33, qa is larger than qBt by successive values of 84, 107, 143, 128, 154, and 111 cal./mole. But qa is larger than the right-hand side of Eq. (58), i.e., qst properly corrected, only by the successive values —48, —25, +10, —9, +14 and —34 cal./mole, which is excellent agreement in view of the estimated maximum error of 15 cal./mole in qa and +15 cal./mole in q,t. Hence the work of Kington and Aston completely clarifies for the first time the relations between isosteric and calorimetric heats of adsorption. 

Integral calorimetric heats of adsorption or heats of immersion, together with a calculation of at one temperature may be used, as shown by Jura and Hill (92) and Drain and Morrison (9). 

Fig. 20. The variation of the calorimetric heat of adsorption of carbon monoxide with surface coverage on evaporated metal films of a number of transition metals. [Redrawn from Brennan and Hayes (80). Reproduced by permission of the Royal Society.]...

The close similarity between adsorption of CO on tungsten and molybdenum is shown by the work of Jackson and Hooker (82) in a LEED study on the Mo(llO) face when they observed a complex ordered structure formed by heating the crystal to 1300°K in the presence of CO. This surface structure shows a marked resemblance to that obtained by May and Germer on W(llO) (71) after annealing the crystal to between 600-900°K. On molybdenum the removal of the additional structure was not observed until about 1800°K, whereas on W(llO) CO was observed to desorb at 1100°K. This result is puzzling in the light of the flash filament studies and calorimetric heats of adsorption on polycrystalline samples which indicate a weaker adsorption process for CO on poly crystalline molybdenum compared with tungsten. 

It can be seen (Fig. 20) from the results of Brennan and Hayes 80) on evaporated iron films that the initial CO calorimetric heat of adsorption is low (about 40 kcal/mole) in comparison with the 100 kcal/mole measured on tungsten and its nearest neighbors. This value obtained for the initial heat of adsorption of CO on iron films is very close to the values measured by these authors on all the other group VIII elements studied (Ni, Co, Rh, Pd, and Pt). 

Manganese has not been studied extensively in its adsorption properties. However, on evaporated films of the metal, Trapnell 179) reports a rapid adsorption at 0°C and Brennan and Hayes 80) measured a calorimetric heat of adsorption of about 80 kcal/mole on their films. 

Fig. 2. Calorimetric heat of adsorption Qd for CO, adsorbed at 300 K on clean Pd(100) (0 ML C) as well as Pd(100) precovered with various amounts of carbon (0.05-0.45 ML C), as a function of CO coverage 0 [97Yeo].

In other words, the medium-specific contribution is not easily measured or quantified for probe molecules in interaction with solid surfaces. Moreover, in the case of microporous solids, the short-distance interactions known as "confinement effects" are even more difficult to evaluate. In all comparisons of experimental data one should be aware that the reactivity of probe base molecules is largely influenced by the size of adsorbates and micropore dimensions. As a result, the acidity scales based on the free energy of proton transfer to a specific base are expected to depend on the choice of reference base. This fact has been confirmed experimentally, as calorimetric heats of adsorption of various bases on, e.g., zeolites, depend on the base chosen. For example, a ZH zeolite may be a stronger acid... 

Ramesh, R., et al, Isosteric and calorimetric heats of adsorption of methanol on coal. Energy Fuels, 6(3), 239-241 (1992). 

Finally, the calorimetric heats of adsorption of quadrupolar (CO2) and non-polar (CH4, C2H6, SFe) gases on silicalite were determined by Dunne et al. [200], who studied the effect of adsorbate size and polarity on the energetics of adsorption. 

The method is here applied to a very simple but instructive case CO adsorbed via electrostatic polarization, [23] on Na-MFl and K-MFl pre-outgassed at T = 673 K. The adsorption represents in both cases an ideal process, which is characterized (within the experimental error) by a Langmuir-like behavior, as evidenced by the adsorption isotherms (vide supra Fig. 1.7 in Sect. 1.3.1). The calorimetric heat of adsorption was ca. 35 and 28 kJ mol for Na-MFI and K-MFl, respectively (vide supra Sect. 1.4.2.3, Fig. 1.14a, b). The half-coverage equilibrium pressure (obtained by the adsorption isotherms) were pi/2 = 200Torr for Na-MFI and 850 Torr for K-MFI. In both cases the reference state for the gas phase was p° = I Torr, as done in previous work (see Ref. [18, 97]). The obtained standard adsorption entropy was AaS° = -151J mol for Na-MFI and — 140J mol forK—MH. 

S. Cerny, M. Smutek, F. Buzek, The calorimetric heats of adsorption of hydrogen on platinum films. J. Catal. 38, 245-256 (1975)... 

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