Nonlinear desorption activation energy from TPD curves : analysis of the influence of initial values for the regression procedure

Thermal Programmed Desorption (TPD) is a powerful technique for materials and catalysts characterization. By analysing TPD curves it is possible to calculate important parameters as the desorption activation energy E_d, that depends on the surface coverage (θ) by a non linear polynomial function, i.e.E_d (θ)=∑_(k=0)^N▒〖α_k (1-θ)^k 〗. The Polanyi-Wigner equation -dθ/dT=(A(θ)/β) θ^n exp((-E_d (θ))/RT) can be used as theoretical basis to calculate this parameter, by a fitting regression procedure starting from experimental TPD data. Different degrees (k) for this polynomial equation and different initial values of the frequency factor A(θ) were considered and discussed to obtain univocal value of desorption energy. Three different Pt and Co based catalysts, suitable for hydrogenation reactions, have been considered as case studies for the application and validation of the proposed calculation procedure.