Abstract:

A concise method——isodesorption rate method(IDRM) is presented for the determination of E_d and k_0 from a temperature programmed desorption (decomposition)(TPD) spectrum with no re-adsorption. Based on the Arrhenius equation and the constant heating rate, we have Eqs.(1) and (2)(see the text), where T, b, t, θ, k_0, E_d, and R are temperature, heating rate, time, Surface coverage, pre-exponential factor, activation energy of desorption (decomposition), and standard gas constant, respectively. For the desorption (decomposition) rate of h_i at T_i is equal to that of h_j at T_j (see Fig.1), Eqs.(3)—(5) (1st order) and Eqs.(11)—(13) (2nd order) are deduced. The values of h_i, h_j, θ_i and θ_j may be evaluated by Eqs.(6) and (7), where W_(T_ih_ih_jT_j) is the weight of the paper corresponding to the rectangle T_ih_ih_jT_j, W_0 is that under the spectrum T_Lh_ih_mh_jT_H, and W_i, W_j are that of the peak areas of T≥T_i, T_j respectively. By substituting Eqs.(6) and (7) to Eqs.(3)—(5) and (11)—(13), the resultant Eqs.(8)—(10) (1st order) and (14)—(16) (2nd order) are obtained for determining E_d and k_0. Table 1 shows that the results determinedby different methods coincide with each other quite well. It also proves that the selected kinetic orders of desorption (decomposition) are correct because the relative errors between the E_d′s by IDRM and CTM~[5] are <3% otherwise that will be about 30% when the desorption (decomposition) order is erroneously selected. So the combination of IDRM andCTM is benificial for determining the kinetic order of desorption (decomposition).