Activation of CO2 on Ni/La2O3: non-isothermal kinetic study on the basis of thermogravimetric studies

The thermo-catalytic CO2 activation was studied over supported nickel catalyst (Ni/La2O3) obtained by reduction of the perovskite LaNiO3. At temperatures lower than 700 A degrees C, a superficial CO2 activation over the catalyst was observed and a partial structural change of the support to lanthanum oxycarbonate was determined by in situ XRD. The oxycarbonate species decomposed at around 750 A degrees C, regenerating the original support La2O3 and producing CO2 (determined by TGA-MS). Above 750 A degrees C, CO2 consumption accompanied by CO production and a weight increase were observed by TGA-MS. These observations suggest the oxidation of metallic nickel to NiO. The oxidation process finished at around 1180 A degrees C, the phases obtained were NiO/La2NiO4. The mechanism of oxidized species formation was proposed according to the kinetic parameters obtained by TGA-MS under CO2 atmosphere. Activation energies and pre-exponential factors were calculated for the different steps of the mechanism proposed. Additionally, a kinetic compensation effect was observed for the activation of CO2 on NiO/La2O3 catalyst. This compensation effect exhibited a slope change at around 850 A degrees C, which suggested a modification of the mechanism at this temperature.