Item

Abstract

A combination of thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) coupled to mass spectrometry has been applied to study the thermal decomposition of Cu/Zn hydroxycarbonates, which are used as a precursor for the active methanol synthesis catalyst. Original TG and DSC profiles and results of a formal kinetic analysis of the calcination process are compared with transformations occurring in the solid phase, which has been studied by means of in-situ XRD. A series of hydroxycarbonate precursors with different Cu/Zn molar ratios (40/60, 70/30, 80/20) was synthesized under conditions reported as optimum for catalytic performance. The samples contain primarily two crystalline phases, aurichalcite (Cu,Zn)₅(CO₃)₂(OH)₆ and zincian malachite (Cu,Zn)₂CO₃(OH)₂. At least four formal decomposition stages of CO₂ and H₂O evolution cause the major mass loss in the TG experiments. The best-fit quality for the all studied samples was obtained for a four-step competitive reaction model. The experimental TG dependences are adequately described by the n-th order equation and 3D Jander diffusion equation. The effects of the gas flow, sample mass, and water transfer conditions on the reaction pathway were studied. The presence of H₂O vapor in the reaction feed accelerates the decomposition and dramatically changes the reaction TG profile. The decomposition enthalpy of mixed Cu/Zn (80/20) hydroxycarbonate was determined, and the formation enthalpy of the decomposition intermediate, a carbonate-modified oxide, was calculated to be ΔH_f⁰=-633.7±5.6 kJ/mol.