Macroscopic measurement desorption

While microscopic techniques like PFG NMR and QENS measure diffusion paths that are no longer than dimensions of individual crystallites, macroscopic measurements like zero length column (ZLC) and Fourrier Transform infrared (FTIR) cover beds of zeolite crystals [18, 23]. In the case of the popular ZLC technique, desorption rate is measured from a small sample (thin layer, placed between two porous sinter discs) of previously equilibrated adsorbent subjected to a step change in the partial pressure of the sorbate. The slope of the semi-log plot of sorbate concentration versus time under an inert carrier stream then gives D/R. Provided micropore resistance dominates all other mass transfer resistances, D becomes equal to intracrystalline diffusivity while R is the crystal radius. It has been reported that the presence of other mass transfer resistances have been the most common cause of the discrepancies among intracrystaUine diffusivities measured by various techniques [18]. 

Flash desorption, although still dependent upon macroscopic wire samples, has made it possible to quantitatively measure rate processes involving the transfer of molecules between the gas phase and the solid. In principle, even the dependence of surface kinetics on atomic structure could be established by studies on macroscopic samples. The specification of surface features below 100A is difficult on such samples. For this purpose measurements in the field emission and ion microscope are more convenient and powerful—they afford a view of the surface on a scale approaching atomic dimensions. However, such work can only be properly carried out against a background of detailed macroscopic information, and it is this sequence from macroscopic measurements to direct observation of atoms that will be followed here. 

The presence of multiple states in flash desorption, which in Section II, C, 1, b, was assigned to a structural effect, has been confirmed by the observation of significant variations in the binding energy of Xe over the surface. More than that, the diminution of the heat of adsorption, deduced from macroscopic measurements, likewise appears to be dependent upon the structure of the surface—in the field emission microscope, the Ilf planes were found to fill in last and with a lower binding energy than typical of rougher planes. 

Surface diffusion can be studied with a wide variety of methods using both macroscopic and microscopic techniques of great diversity.98 Basically three methods can be used. One measures the time dependence of the concentration profile of diffusing atoms, one the time correlation of the concentration fluctuations, or the fluctuations of the number of diffusion atoms within a specified area, and one the mean square displacement, or the second moment, of a diffusing atom. When macroscopic techniques are used to study surface diffusion, diffusion parameters are usually derived from the rate of change of the shape of a sharply structured microscopic object, or from the rate of advancement of a sharply defined boundary of an adsorption layer, produced either by using a shadowed deposition method or by fast pulsed-laser thermal desorption of an area covered with an adsorbed species. The derived diffusion parameters really describe the overall effect of many different atomic steps, such as the formation of adatoms from kink sites, ledge sites... 

Equation 4.3 is formally similar to a complexation reaction between SR(s) and the aqueous solution species on the left side. Indeed, the solid-phase product on the right side can be interpreted on the molecular level as either an outer-sphere or an inner-sphere surface complex. The latter type of adsorbed species was invoked in connection with the generic adsorption-desorption reactions in Eqs. 3.46 and 3.61, which were applied to interpret mineral dissolution processes. In general, adsorbed species can be either diffuse-layer ions or surface complexes,7 and both species are likely to be included in macroscopic composition measurements based on Eq. 4.2. Equation 4.3, being an overall reaction, does not imply any particular adsorbed species product, aside from its stoichiometry and the electroneutrality condition in Eq. 4.4. 

A general experimental result is the difference between measured rates of diffusion in macroscopic experiments and measurements of self-diffusion by spectroscopic techniques such as gradient field NMR [80-84]. The difference between the microscopic measurement and the macroscopic experiment is desorption and reentry of molecules in zeolite microcrystallites. In this respect, it is important to remember the Biot condition, which states the condition when the measured rate of diffusion is independent of the rate of desorption ... 

The majority of sorption kinetic stndies have ntilized either batch or flow-through methods coupled with aqueous measurements for determination of the concentrations of species of interest. More recent work has focused on molecular-scale approaches, including spectroscopic and microscopic techniques that allow for observations at increased spatial and temporal resolution to be made, often in situ and in real time. Complementary to both macroscopic and molecular-scale observations has been the utilization of theoretical techniques, such as molecular mechanics and quantum mechanics, to model surface complexes computationally. It has been through the integration of macroscopic, molecular-scale, and theoretical approaches that some of the most profound observations of sorption-desorption phenomena over the past decades have been made. 

Two TPD measurements of Mg(NH3)6Cl2 can be seen in Fig. 19.3 the two runs are performed on the same sample to demonstrate that the process is reversible. The figure also shows a dense tablet before and after desorption with corresponding masses to show that the material retains its macroscopic shape after desorbing half of the total mass. 

Low field or contact potential measurements on well-defined macroscopic surfaces have an advantage here. The total amount of adsorbed material can be measured separately by flash desorption. Moreover, the contact potential A corresponds to an area average, which is also approached in low field emission measurements. The change in the contact potential in adsorption can therefore be unequivocally related to the dipole moment per adatom through Eq. (32). The difficulty in this approach lies in the preparation of a truly uniform surface of macroscopic size, which has not as yet been accomplished. 

The classical kinetics uses specific models for determining the kinetic parameters, the rate constants, and reaction orders, for reactions in the homogeneous phase. When the reaction is heterogeneous, there are additional adsorptions and desorption parameters, which can be represented by the conventional power low or Langmuir-Hinshelwood Anderson models. The goal is to find out a complete kinetic equation with defined numerical parameters. One resorts to the usual methods, measuring the kinetic constants in a differential reactor, varying temperatures, partial pressures, and space velocities, namely, macroscopic variables [1-3]. 

---