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Introduction to Chemical Adsorption Analytical Techniques and Their Applications to Catalysis Paul A

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Introduction to Chemical Adsorption Analytical Techniques and Their Applications to Catalysis Paul A MIC Technical Publications January 2003 Introduction to Chemical Adsorption Analytical Techniques and their Applications to Catalysis Paul A. Webb Micromeritics Instrument Corp., Norcross, Georgia 30093 Abstract The chemical adsorption isotherm reveals information about the active surface of a material and has been em- ployed for many years as a standard analytical tool for the evaluation of catalysts. Temperature-programmed reac- tion techniques have emerged from the 1950’s as an indispensable companion to chemisorption isotherm analyses in many areas of industry and research. This paper provides an introduction to these analytical techniques. Keywords: chemical adsorption, temperature programmed reactions, TPD, TPR, TPO, catalyst 1. Introduction and 800 kJ/mole for chemical bonds. A chemical Optimum design and efficient utilization of cata- bond involves sharing of electrons between the ad- lysts require a thorough understanding of the surface sorbate and the adsorbent and may be regarded as the structure and surface chemistry of the catalytic mate- formation of a surface compound. Due to the bond rial. Chemical adsorption (chemisorption) analyses strength, chemical adsorption is difficult to reverse. can provide much of the information needed to Physical adsorption takes place on all surfaces evaluate catalyst materials in the design and produc- provided that temperature and pressure conditions are tion phases, as well as after a period of use. The re- favorable. Chemisorption, however, is highly selec- quired analytical equipment can be relatively inex- tive and occurs only between certain adsorptive and pensive, simple to operate, and fast compared to al- adsorbent species and only if the chemically active ternate equipment capable of obtaining the same in- surface is cleaned of previously adsorbed molecules. formation. Under proper conditions, physical adsorption can 2. Differentiating physical and chemical adsorp- result in adsorbed molecules forming multiple layers. tion Chemisorption, in the typical case, only proceeds as A solid material usually exhibits a heterogeneous long as the adsorptive can make direct contact with distribution of surface energy. Gas, vapor, or liquid the surface; it is therefore a single-layer process. molecules may become bound to the surface if they Exceptions can exist if the adsorptive is highly polar approach sufficiently close to interact. The discus- such, NH3 being an example. Both physical and sions in this paper are confined to the adsorption (and chemical adsorption may occur on the surface at the desorption) of gases or vapors on (or from) solid sur- same time; a layer of molecules may be physically faces. The solid is called the adsorbent; the gas or adsorbed on top of an underlying chemisorbed layer. vapor molecule prior to being adsorbed is called the The same surface can display physisorption at one adsorptive and while bound to the solid surface, the temperature and chemisorption at a higher tempera- adsorbate. ture. For example, at liquid nitrogen temperature (77 K) nitrogen gas is adsorbed physically on iron Physical adsorption is the result of a relatively but at 800 K, an energy level too high for physical weak solid-gas interaction. It is a physical attraction adsorption bonds, nitrogen is adsorbed chemically to resulting from nonspecific, relatively weak Van der form iron nitride (Moore). Waal's forces and adsorption energy usually not ex- ceeding 80 kJ/mole, with typical energies being con- 3. General applications of gas sorption as analyti- siderably less. Physically adsorbed molecules may cal tools diffuse along the surface of the adsorbent and typi- A variety of surface features can be determined cally are not bound to a specific location on the sur- from observations of the adsorption process. The face. Being only weakly bound, physical adsorption quantity of molecules taken up by a surface depends is easily reversed. on several variables including temperature, pressure, Adsorption also can result in a surface complex, a surface energy distribution, and the surface area and union much stronger than a physical bond with heats porosity of the solid. The relationship between the of adsorption up to about 600 kJ/mole for C-N bonds quantity of molecules adsorbed and the pressure at INTRODUCTION TO CHEMICAL ADSORPTION ANALYTICAL TECHNIQUES AND THEIR APPLICATIONS TO CATALYSIS constant temperature is called the adsorption iso- aluminum component. The end product is a highly therm. porous active metal ‘sponge’, all other scaffolding materials having been removed. Another unsup- Physical adsorption and desorption isotherms are ported metal catalyst is produced by fusing or sinter- important in characterizing the overall surface. The ing metal oxides with promoters, thus forming a net- slightest change in the shape of the plotted isotherm work of pores throughout the metal mass (van der is indicative of a particular surface feature. Analyses Laan). An example of this is a fused iron ammonia of physical adsorption isotherm data reveal the total synthesis catalyst. surface area, mesopore and micropore volume and area, total pore volume, the distribution of pore vol- Zeolitic catalysts form another group. Zeolites are ume and area by pore size, and surface energy distri- hydrated aluminosilicates and are used widely in the bution. Thus, physical adsorption is an important tool chemical industry and refining operations. Their ac- in the study of catalysts, particularly in evaluating the tivity is influenced by the silica to alumina ratio. support structure. Amorphous silica-alumina catalysts have a lower activity than zeolitic catalysts and are applied in mild The chemical adsorption isotherm also evaluates hydrocracking operations. An acidic surface is re- the surface, but is selective in that it only probes the quired for cracking and the acidity is associated with active areas—those capable of forming a chemical the oxygen atoms attached to aluminum atoms. bond with the adsorptive gas or vapor. This selectiv- ity involves both the probe molecule and the molecu- 5. Chemisorption and catalysis lar or atomic composition of the active material, so The catalytic process can be generalized by a the choice of probe molecules is much more impor- short sequence of steps as follows. Consider reactant tant in chemisorption tests. Although isothermal molecules A and B in the gaseous bulk above a solid chemisorption tests are important in characterizing surface that supports an ensemble of active metal active surfaces, temperature-programmed tests are sites S. If molecule A is chemically adsorbed on one even more so. The basic form of data produced by of the active sites, a surface complex A is formed. these tests resembles a chromatogram of temperature Next, A reacts with B forming molecule A+B, which versus quantity desorbed. escapes the site, thus regenerating site S. 4. Catalyst Basics Sorption of at least one of the reactant molecules A catalyst affects the rate of a chemical reaction. is required for catalysis to occur. If the accelerated ‘Rate' is the most important word in this description rate of reaction simply was due to the concentration because a catalyst cannot induce a reaction that is not of molecules at the surface, catalysis would result permissible under the laws of thermodynamics. A from physical adsorption of the reactants. Chemi- catalyst only can increase the rate at which the reac- sorption is a essential step, the adsorbed molecule tion approaches equilibrium. forming an intermediate surface complex that is more receptive to chemical reaction. The dependence of Heterogeneous catalysts include metals, metal catalysis on chemisorption is one reason why chemi- oxides and solid acids. Pure metals may be employed sorption is such an informative analytical technique as solid catalysts, or may be dispersed as small grains in the study of catalysis—the chemistry occurring in on the surface of a supporting material such as TiO2, the application of the catalyst is being observed di- ZrO2, Al2O3, or SiO2. The preparation of a sup- rectly in the laboratory. ported catalyst involves selecting precursors of the active components and any necessary promoters and The performance of a catalyst depends on several mixing them in a solvent. The mixture forms a pre- variables. First, adsorption sites must be both nu- cipitant, or is used to coat an inert carrier, or is used merous and available to the reactant molecules. In to impregnate a carrier. Ultimately, the active metal some cases, grains of active metal are at the surface, or precursor is dispersed on the carrier. The product but there also are grains located below the surface is dried, mixed with a binder or forming agent, then and unavailable to the reactants. Adsorption sites ground, pelletized, extruded, or otherwise shaped. simply being located on the surface is insufficient to Finally, the material is calcined and activated by oxi- assure optimum performance. For example, some dation, reduction, or other means. potential adsorption sites may be located deep within a micropore that is too narrow for the reactant mole- One type of unsupported catalyst is composed of pure cule to enter or for the reaction product to exit; in this metal. Raney metal catalysts, for example, are pre- case the surface site cannot be an active participant in pared by dissolving an aluminum-nickel alloy in a chemisorption. A site could be located along a tortu- solution of sodium hydroxide, which dissolves the -2- INTRODUCTION TO CHEMICAL ADSORPTION
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