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Chemisorption

An Introduction to Chemical Adsorption Analytical Techniques and Methods

Paul A. Webb

This article is a condensed version of a more comprehensive article titled “Chemical Adsorption,” as an Analytical Technique, available on Micromeritics’ web site, www.micromeritics.com.

Introduction Catalysts are used in a variety of surface energy distribution, and the molecules forming multiple layers. applications from the production of surface area of the solid. A plot of Chemisorption, on the other hand, consumer goods to the protection the quantity of molecules adsorbed only proceeds as long as the of the environment. Optimum versus pressure at constant tem- adsorptive can make direct contact design and efficient utilization of perature is called the adsorption with the surface; it is usually catalysts requires a thorough isotherm. considered to be a single-layer understanding of the surface process. structure and surface chemistry of Physical adsorption (“physis- the active material. sorpton”) is the result of relatively A characteristic of physical adsorp- weak Van der Waal’s interaction tion is that almost all the adsorbed Chemical adsorption (“chemisorp- forces between the solid surface molecules can be removed by tion”) analysis techniques provide and the adsorbate - a physical evacuation at the same temperature much of the information necessary attraction. Physical adsorption is at which adsorption occurred. to evaluate catalyst materials in the easily reversed. Heating accelerates desorption design and production phases, as because it makes readily available well as after a period of use. Depending on the gas and solid, to the adsorbed molecules the the adsorption phenomenon also energy necessary to escape the Although a catalyst and the reac- can result in the sharing of elec- adsorption site. tants and products can be of many trons between the adsorbate and forms, this article addresses the solid surface - a chemical A chemically adsorbed molecule is commonly used heterogenous bond. This is chemical adsorption strongly bound to the surface and catalysts. and unlike physisorption, chemi- cannot escape without the influx of sorption is difficult to reverse. A a relatively large quantity of energy Differentiating physical and significant quantity of energy compared to that necessary to chemical adsorption usually is required to remove liberate a physically bound mol- chemically adsorbed molecules. ecule. This energy is provided by A distinctive characteristic of a heat and often very high tempera- solid material is a distribution of Physical adsorption takes place on tures are required to clean a surface weak surface energy sites. Gas or all surfaces provided that tempera- of chemically adsorbed molecules. vapor molecules can become bound ture and pressure conditions are to these sites. This generally favorable. Chemisorption, how- Physisorption tends to occur only at describes the adsorption phenom- ever, occurs only between certain temperatures near or below the enon. adsorbents and adsorptive species boiling point of the adsorptive at and only if the surface is cleaned the prevailing pressure. This is not The quantity of molecules taken up of previously adsorbed molecules. the case with chemisorption. by the surface depends on several Chemisorption usually can take conditions and surface features Under proper conditions, physical place at temperatures well above including temperature, pressure, adsorption can result in adsorbed the boiling point of the adsorptive. www.micromeritics.com

Chemisorption

The relationship of chemi- 5) diffusion of products away quantity of reactant molecules sorption to catalysis from the surface of the taken up by active sites upon each A catalyst is a material that affects catalyst to allow recycling to injection. Initial injections may be the rate of a chemical reaction. A step 1 chemisorbed totally; upon satura- catalyst cannot cause a reaction that tion none of the later injections otherwise would not occur; it only Predicting the efficiency of steps 1 will be chemisorbed, indicating can increase the rate at which the and 5 is aided by analytical saturation. The number of mol- reaction approaches equilibrium. techniques such as physical ecules of gas chemisorbed is adsorption and mercury directly related to the active The surface of an ‘active’ metal is porosimetry, which characterize surface area of active material. composed of chemisorption sites. the porosity of the catalyst bed, Supported catalysts are those on catalyst monolith, or the individual The quantity of gas chemisorbed which finely divided grains of the grains of catalyst material. per gram of sample combined with active metal are deported on a Characterizing steps 2, 3, and 4 is the knowledge of the stoichiometry support material. Those grains the domain of chemisorption of the reaction and the quantity of located on the surface of the analyses. active metal mixed with support support are available to react with material during formulation of the the adsorptive. Chemisorption techniques & catalyst allows the percent metal methods for the evaluation dispersion to be calculated. This If the accelerated rate of reaction of catalysts can be an important indicator of the performance of the catalyst and an was simply due to an increased Chemisorption analyses may be important economic measure of concentration of molecules at the applied to determine a catalyst’s how efficiently the expensive surface, catalysis could result from relative efficiency in promoting a active metal is being employed in a physical adsorption of the reac- particular reaction, or used to catalyst product. tants. This is not the case; chemi- study catalyst poisoning and in sorption is an essential step, monitoring the degradation of Temperature-Programmed Desorp- apparently altering the reactant (the catalytic activity over time of use. adsorbed molecule) to make it more tion (TPD), Temperature- Programmed Reduction (TPR) and receptive to chemical reaction. The Isothermal chemisorption analyses Temperature-Programmed Oxida- dependence of catalysis on forming are performed by two chemisorp- tion (TPO) are three non-isother- active surface bond intermediates is tion techniques: a) static volumet- mal methods for characterizing one reason why chemisorption as ric chemisorption, and b) dynamic catalysts. Temperature-pro- an analytical technique is so (flowing gas) chemisorption. The grammed desorption typically does fundamental in the study of cataly- volumetric technique is convenient not employ a vacuum, better sis. for obtaining a high-resolution simulating conditions found in measurement of the chemisorption actual industrial applications. In Stages of a heterogeneous catalytic isotherm from very low pressure to the TPD analysis, materials are reaction cycle are: atmospheric pressure at essentially placed in a sample cell and pre- any temperature from near ambient treated to clean the active surfaces. 1) diffusion (transport) of reac- to 1000 oC or greater. tants to the surface of the catalyst Next, a selected gas or vapor is chemisorbed onto the active sites Pulse chemisorption, a flowing gas until saturation is achieved, after 2) chemisorption of reactant(s) technique, typically is performed which the remaining molecules are at ambient pressure. After the flushed out with an inert gas. 3) surface reactions among sample has been cleaned in a flow chemisorbed species of inert gas, small quantities of a Temperature (energy) is increased reactant are injected until the at a controlled rate while a constant 4) liberation of products from sample is saturated. A calibrated flow of inert gas is maintained over catalysts thermal conductivity detector the sample. The inert gas and any (TCD) is used to determine the desorbed molecules are monitored www.micromeritics.com

Chemisorption

by a thermal conductivity detector. calculated using the Clausius- 1st order kinetics commonly The TCD signal is proportional to Clapeyron equation. This expres- expressed as -kq, where k is the the quantity of molecules desorbed sion describes the isosteric heat of rate constant, the negative sign as thermal energy overcomes the adsorption in terms of pressure, indicating a reduction in coverage binding energy. Quantities desorbed temperature, and the gas constant with time, and q represents the at specific temperatures provide and is particularly applicable to current degree of surface coverage. information about the number, data obtained by volumetric strength, and heterogeneity of the adsorption techniques. The rate constant k can be ex- chemisorption sites. pressed in Arrhenius form, The isosteric heats of adsorption Temperature-programmed reduc- over a range of coverage can be A exp(-EA/RT), where EA is the tion is mainly used to study the obtained from adsorption isosteres, activation energy for desorption, T reducibility of species such as which are plots of pressure vs. is absolute temperature, and R the metal oxides dispersed on a temperature at a constant volume gas constant. A is known as the pre- support. This involves flowing a adsorbed. The isosteres are ex- exponential factor. stream of diluted hydrogen (or tracted from a family of isotherms another reducing agent) over the obtained for the same material at Combining the relationships and sample as the sample temperature is different temperatures. The slope of equations presented above ulti- increased. The quantity of hydro- an isostere plotted on a logarithmic mately yields an expression for gen consumed and the temperature scale (lnP vs 1/T)n provides one activation energy in terms of profile under which the reduction data point (qst, n), where n repre- variables that can be determined by took place are measured. A plot of sents the degree of coverage TPD analyses. quantity of hydrogen consumed associated with the isostere. A plot versus temperature can produce one of similar points for different Summary or more peaks and the data ob- degrees of coverage describes the Chemisorption is a fundamental tained reveal the number of reduc- surface energy distribution as a process in heterogeneous catalysis. ible species in the sample, as well function of coverage. This informa- Understanding the chemisorption as their activation energies. tion aids in predicting the activity process associated with a catalyst of a catalyst toward a specific and reactant is key to controlling Temperature-programmed oxida- chemical reaction at a specific the composition and manufacture of tion is performed to examine the temperature. extent to which a catalyst can be re- catalysts and for catalyst evalua- tion. Therefore, analytical instru- oxidized. Usually the sample is Activation energy also can be ments capable of measuring pretreated and the metal oxides are deduced from data obtained by the chemical and physical adsorption reduced to the base metal. The dynamic chemisorption technique, and desorption isotherms and those sample is heated at a uniform rate particularly TPD. The process by capable of analyzing temperature- as the reactant gas, typically 2% this method is in the opposite programmed reactions are power- oxygen, is applied to the sample in direction as that described for static ful tools in the study of catalysis. pulses or, alternatively, as a steady volumetric technique. In the present stream. The oxidation reaction case, heat (energy) is applied and, occurs at a specific temperature and as temperature increases, molecules the resulting uptake of oxygen is are liberated in order of weakest quantified. bonding. The desorbed molecules are swept away and no re- Surface energies adsorption is allowed to occur. The rate of change of surface coverage, When a solid surface is exposed to or loading, is related to the rate of an adsorptive, the most energetic change in temperature. sites are occupied first. The heat of adsorption at a specific degree of The rate of simple molecular surface coverage (loading) can be desorption may be modelled using