Recent Altamira Notes have discussed the use of several different chemisorption techniques to determine crystallite sizes for supported metal catalysts.These techniques, temperature-programmed desorption, static chemisorption, and pulse chemisorption, can all be described as "chemical" methods because they rely on some way of monitoring the chemisorption or desorption of molecules on metal surfaces. A second general approach to the determination of metal crystallite sizes involves the use of techniques which may be described as "physical" methods. The objective of this Note is to compare the chemical methods described in earlier Notes to several physical methods discussed below.

There are 3 fundamental ideal types of reactors. Laboratory reactors are almost exclusively related to these ideal forms.  Larger reactors,  pilot-plant or commercial scale,  can be mathematically described usually by deviations from these ideal reactors. Additional complications to the descriptive or "design" equations are introduced by the presence of multiple phases.

In the study of reactions on heterogeneous catalysts over the past 40 years, much use has been made of transient kinetic techniques in order to provide insight into surface reaction processes and mechanisms. These techniques have typically employed at reaction conditions the use of stopping/ starting the flow of one of the reactants or of pulsing the reactants. With the exception of experiments studying the exchange reaction between molecules at steady flow (1), it is difficult to extrapolate the results from these transient, non-steady-state studies to interpret the nature of surface reaction under steady-state conditions.

Designing and using laboratory-scale reactors for studying catalysts can be a very confusing process. Laboratory catalytic reactors are used for a variety of objectives including catalyst screening and evaluation, determination of reaction data, simulation of commercial process conditions, studies of catalyst pretreatments or aging, etc. Each of these objectives lends itself to a specific design of the reactor in order to achieve results which are meaningful. This Altamira Note discusses three common applications of laboratory-scale reactors and the suitability of various reactor types for these different research and development applications.
 

Many industrial processes involve multi-phase systems, and perhaps the most complex is the gas-liquid-solid or slurry reactor. The complexity of this system arises from the simultaneous presence of mass and heat transport involving all three phases in addition to reaction. Although the transport phenomena can often be theoretically evaluated, by assuming limiting cases, the addition of the reactor operating characteristics and slurry physical properties introduces further complexities. Empirical correlation based on a particular reactor at actual operating conditions must then be utilized.

The specific surface area plays an important role in the purification, processing, mixing, tableting, and packaging capabilities of pharmaceuticals in tablet form.  It also has an influence on the expiry date, dissolution rate, and efficacy.

All solid powders, and in particular those with apparent surface areas > 400 m2/g, are bound to contain micropores. The importance of micropores lies in that materials containing micropores generally have most of their apparent surface area residing in them. Hence, all applications of materials (adsorbents, catalysts) that depend on their accessible surface area must deal with the possible effects arising from the presence of micropores in the materials.

Temperature-programmed desorption (TPD) of basic molecules from the surface of zeolites has been extensively used to measure their acid properties. The TPD experiment consists of sorbing a base molecule on the material of interest and, while flushing the surface with an inert gas, linearly ramping the temperature and measuring the desorption of the base. By quantitatively measuring the amount of base desorbed and noting the temperature(s) of desorption, information can be obtained on both the intrinsic and extrinsic acid properties in a single experiment (See Figure 1).

One of the most important properties which characterizes a supported metal catalyst is its specific metal surface area. This information may be gained by a number of different experimental techniques (1, 2) and is usually reported as the average metal crystallite diameter of the catalyst. This parameter provides a means of comparing catalysts prepared by different methods or in different laboratories. A previous Altamira Note (September 1989) discussed how surface area measurements may be obtained from temperature-programmed desorption data for supported metal catalysts. This Note will focus on how the same information is obtained from a more traditional chemisorption method known as volumetric or static chemisorption

Previous issues of Altamira Notes have discussed different selective chemisorption techniques and how they may be used to determine the specific metal surface area of supported metal catalysts.   One additional technique commonly used for the same purpose is pulse chemisorption. This method is one of the simplest, most straightforward ways to measure adsorbate uptake by a metal surface; however, as with most other measurements in catalysis, interpretation of the results can be problematic if the nature of the catalyst system and the experiment itself are not well-understood. This Altamira Note discusses the principles of pulse chemisorption experiments as well as some common experimental observations