A METHOD OF CALCULATING ADSORPTION ENTHALPY DISTRIBUTION USING AMMONIA TEMPERATURE-PROGRAMMED DESORPTION SPECTRUM UNDER ADSORPTION EQUILIBRIUM CONDITIONS

A method, called the ac-TPD method, was presented for calculating the density distribution function of the adsorption enthalpy of ammonia by utilizing the temperature-programmed desorption (TPD) spectrum of amm onia from solid catalysts measured under complete adsorption equilibri um conditions. TPD spectra were measured using a gas stream containing ammonia, at a relatively large concentration compared to the amount o f ammonia desorbed from the catalyst. Under these conditions, the conc entrations of ammonia both inside the catalyst particles and in the ga s phase are kept almost constant during the experiment. This situation meant that the amount of ammonia remaining on the catalyst was at an equilibrium state and dependent on the temperature and concentration o f ammonia. Therefore, it could be assumed that the adsorption equilibr ium of ammonia on acid sites can be expressed by a Langmuir-Hinshelwoo d type equation, which has an adsorption enthalpy and a pre-exponentia l adsorption equilibrium constant corresponding to the strength of an acid site. The overall TPD spectrum is represented by the sum of the s pectra from acid sites with different acid strengths. Under these assu mptions, the desorption temperature was related to the adsorption enth alpy of ammonia, and a relationship was derived between the overall TP D spectrum and the density distribution function of the adsorption ent halpy. The distribution function obtained by this method can be used f or evaluating acidic properties of solid catalysts. The heat flux requ ired for the desorption of ammonia from acid sites during the TPD expe riment was calculated using the obtained distribution. The value was f ound to agree well with that directly measured by a differential scann ing calorimetric apparatus. The density distribution of the adsorption enthalpy (Q) was compared to the distribution of activation energy fo r desorption of ammonia (E) by a method, called dc-TPD, which was repo rted previously by us. This comparison gave the relationship between Q and E, and it was found that the Q value was smaller than the E value by about 15 kJ mol(-1). (C) 1997 Elsevier Science B.V.