Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

188 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSquantity adsorbed at any instant can be calculated from the residual pressure.The adsorption rate is then found from the rate of pressure change with time.Apparatus and vacuum conditions must be arranged to correspond with the surfacearea available for adsorption and the rate.(a) Conventional techniqueThe essential parts of a volumetric apparatus are a closed system of knownvolume, a source of adsorbate and a pressure measuring device. This type ofapparatus is more commonly used for the determination of adsorption isotherms,but kinetics can be measured provided that the adsorbent surface is large and theresponse of the pressure measuring device is much faster than the rate of adsorption.These criteria are usually only satisfied by powders or films which show'activated' adsorptions. Bond3 has discussed the phenomenon of activated adsorption.Although the results may often be ascribed to contamination or incorporationinto the bulk of the material, in other instances a genuine activated chemisorptionis found.McLeod gauges were used as the pressure measuring device in the olderbut thermal conductivity gauges or other instruments with an electrical output anda fast response have obvious advantages". A recent publication by Marcus andSyversod" describes an apparatus equipped for high temperature studies and highspeed pressure measurement (Fig. 2). Liquid adsorbates can be drawn into acapillary (V,) before vaporisation into the measuring volume (Vl). The adsorptioncell is surrounded by a vapour jacket and is also attached to a circulatingpump which gives a circulation rate of about 1 1. min-'. A Bourdon tube with thefree end attached to the centre plate of a differential condenser forms the pressuresensing element. Changes in capacitance are converted into an output voltagewhich can be recorded. The response characteristics allow a pressure change fromYVaporreservoirFig. 2. Constant volume apparatus for the study of adsorption kinetics, equipped for hightemperature studies and high speed pressure measurements. From ref. 82.