Chemisorption

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Blue spheres: Red spheres: Red and Green: surface molecules gas molecules molecules react only with hemisorption: Chemical 1.) 2.) 3.) each other to form a new adsorption is an interaction Green spheres: molecular species muchC stronger than physical ad- active sites sorption. In fact, the interaction is an actual chemical bond where electrons are shared between the gas and the solid surface. While physical adsorption takes place on all surfaces if temperature and pressure conditions are favorable, chemisorption only occurs on cer- tain surfaces and only if these sur- faces are clean. Chemisorption, unlike physisorption, ceases when the adsorbate can no longer make direct contact with the surface; it is therefore a single layer process.

Most medicinal, chemical, and petroleum-derived items in Chemisorption Physisorption Many molecules must be activated before they will react ac- common usage are dependent at some stage in their manu- cording to present theories. Activation energy is a measure Near or above dew point Temperature Range: Unlimited facture on a heterogeneous catalytic process. High octane of gas of the energy which must be supplied to them to bring about gasoline, plastics, fertilizers, and herbicides would either be reaction. In some cases the activation energy requirement is prohibitively expensive or unobtainable without them. Like- Enthalpy of Typically Typically such that reaction will proceed only at a measurable rate above (5 - 80 kJ/mol) wise, pollution control would be almost nonexistent were it Adsorption: (80 - 800 kJ/mol) a certain temperature. In others, reaction proceeds rapidly at not for catalysts. Nature of the low temperature, hydrogen on platinum being an example of a Irreversible Reversable adsorption: chemisorption reaction where the activation energy approach- Physical and Chemical Limited to one es zero. Saturation: Multilayer formation layer adsorption differentiated Summary: Adsorption kinetics: Variable Fast Non - Activated process Physisorption Activated process • Chemisorption: (by van der Waals and electrostatic forces) • is a strong (chemical) interaction between the adsorbed This is the most common form of adsorp- Unlike physical adsorption, chemisorption is difficult to re- molecule and the surface molecule of the active site. tion. The molecules are attracted by van verse. In fact, when sufficient energy is applied to remove • is selective and occurs only on certain clean surfaces. der Waals forces, and attach themselves the adsorbed molecules, atoms of the surface material may be • is a single layer process. to the surface of the solid. The molecules carried away with them. For example, when oxygen is chemi- • often requires an activation energy. remain intact, and can be freed easily (the sorbed on charcoal, the application of heat and vacuum results • can take place at elevated temperatures. forces are small, and short-range). in desorption of carbon monoxide. • is always localized. • sites may be homogeneous or heterogeneous according Physical adsorption is the result of a Physical adsorption takes place on all surfaces provided tem- to the energy characteristics of the adsorption site. relatively weak interac- perature and pressure conditions are favorable. Chemisorp- • can be used to measure the area of active metal of a tion between the solid tion, on the other hand, is localized and occurs only on certain catalyst. surface and the gas - a surfaces or surface sites. Meaningful examination of the en- • can be used to determine relative activity among physical attraction. Physi- ergies and sites involved can be achieved only if these sites several catalysts. cally attractive forces in- are cleansed of previously adsorbed molecules. Thus, before • can be used to study degradation of catalytic activity due to time of use and poisoning. volve relatively weak van a chemisorption analysis can proceed, removal from the ac- der Waals forces and low tive sites of any existing atmospheric contamination must be • A catalyst accelerates a chemical reaction without being achieved. consumed in the process. heats of adsorption usually • A homogeneous catalyst is one which is of the same phase not exceeding 80 kJ/mole. Under proper conditions physical adsorption results in ad- as the reactant; for example, a liquid-liquid solution. Physical adsorption does sorbed molecules forming multiple layers. Chemical adsorp- • A heterogeneous catalyst is one which is of different phase not affect the structure or tion occurs only if the adsorptive makes direct contact with as the reactant; for example, a solid-gas interaction. texture of the adsorbent, the active surface; therefore it is a single-layer process. But • A catalyst employs a chemisorption process. and desorption takes place physical and chemical adsorption processes are not exclusive. • The strength and velocity of the chemisorption determines catalytic activity. as conditions are reversed. A layer of molecules may be adsorbed physically on top of an • The total surface of a catalysts may be metallic or metallic Chemical adsorption is a much stronger interaction than underlying chemisorbed layer, or physical adsorption may oc- clusters may be distributed on the surface of a supporting physical adsorption with heats of adsorption up to 800 kJ/ cur on non-active sites of a substrate while chemisorption is material. mole. But heat of adsorption values less than 80 kJ/mole do occurring on the active sites. • Catalysts of practical interest are characterized by high not necessarily rule out chemisorption. During the chemisorp- Physical adsorption diminishes rapidly with temperature el- specific surface area. tion process the adsorbing gas or vapor molecule splits into evation; chemisorption, on the other hand, is enhanced by • TPD and TPR employ chemisorption and investigate de- sorption and reduction as a function of temperature. atoms, radicals, or ions that form a chemical bond with the ad- high temperature. Furthermore, the same surface can display • TPD provides information about the number, strength and sorption site. This interaction involves the sharing of electrons physical adsorption at one temperature and chemisorption at a heterogeneity of chemisorption sites. between the gas and the solid surface and may be regarded as higher temperature. For example, at liquid nitrogen tempera- • TPR is used to gain information on the reducibility of ox- the formation of a surface compound. ture (77 K) nitrogen gas is adsorbed physically on iron but at idic species and provides such information as the number 800 K, an energy level too high for physical adsorption bonds, of reducible species, the activation energy of the site, and nitrogen is adsorbed chemically to form iron nitride. the quantity of hydrogen consumed by the site. Applications Catalysts Fischer-Tropsch Synthesis Fuel Cells Catalytic Reforming Hydrocracking, Hydrodesulfurization, and Hydrodenitrogenation The active surface area and porous Cobalt, iron, etc. based catalysts are used Platinum-based catalysts including Pt/ Catalysts containing platinum, Hydrocracking catalysts typically composed of metal sulfides (nickel, tung- tructure of catalysts have a great in- to convert syngas (carbon monoxide and C, PtRu/C, and PtRuIr/C are often char- rhenium, tin, etc. on silica, alumina, sten, cobalt, and molybdenum) are used for processing feeds containing poly- fluence on production rates. Limiting hydrogen) to hydrocarbons larger than acterized by temperature-programmed or silica-alumina are used for the cyclic aromatics that are not suitable for typical catalytic cracking processes. the pore size allows only molecules methane. These hydrocarbons are rich reduction to determine the number of production of hydrogen, aromatics, and Hydrodesulphurization and hydrodenitrogenation are used for removing sulfur of desired sizes to enter and leave; in hydrogen and do not contain sulfur or oxide phases and pulse chemisorption olefins. These catalysts are commonly and nitrogen respectively from petroleum feeds. The characterization of these creating a selective catalyst that will nitrogen. The characterization of these to calculate: characterized to determine: materials includes: produce primarily the desired prod- materials includes: • Metal surface area • Metal surface area • Temperature-programmed reduction uct. Chemisorption experiments are • Temperature-programmed desorption • Metal dispersion • Metal dispersion • Oxygen pulse chemisorption valuable for the selection of catalysts • Pulse chemisorption • Average crystallite size • Average crystallite size for a particular purpose, qualification of catalyst vendors, and the testing Isomerization Partial Oxidation Catalytic Cracking of catalyst performance over time to Catalysts such as small-pore zeolites Manganese, cobalt, bismuth, iron, Acid catalysts such as zeolites are establish when the catalyst should be (mordenite and ZSM-5) containing noble copper, and silver catalysts used for used to convert large hydrocarbons reactivated or replaced. metals (typically platinum) are used to the gas-phase oxidation of ammonia, to gasoline and diesel fuel. The convert linear paraffins to branched par- methane, ethylene, and propylene are characterization of these materials affins. This increases the octane number characterized using: includes: and value for blending gasoline and im- • Temperature-programmed oxidation • Ammonia chemisorption proves the low temperature flow proper- • Temperature-programmed • Temperature-programmed ties of oil. The characterization of these desorption desorption of ammonia materials includes: • Heat of desorption of oxygen • Temperature-programmed • Temperature-programmed reduction • Heat of dissociation of oxygen decomposition of alkylamines • Pulse chemisorption • Temperature-programmed desorption of aromatic amines

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