Thermokinetic investigation of binary Cu/Zn hydroxycarbonates as precursors for Cu/ZnO catalysts

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, 60/40, 80/20) was synthesized under conditions reported as optimum for catalytic performance. The samples contain primarily two crystalline phases, aurichalcite (Cu,Zn) 5 (CO 3 ) 2 (OH) 6 and zincian malachite (Cu,Zn) 2 CO 3 (OH) 2 . At least four formal decomposition stages of CO 2 and H 2 O 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 H 2 O 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 f 0 =-633.7±5.6 kJ/mol.

Cu,Zn-based catalysts for methanol synthesis: On the effect of calcination conditions and the part of residual carbonates

“…It was demonstrated that calcination at mild heating rates results in better dispersed CuO and ZnO, thus preventing aggregation of oxides and leading to higher dispersion of metal particles in the reduced catalyst. A detailed kinetic analysis of the decomposition process of ZM and AU was published by our group recently [12]. We showed that the decomposition of the HT-CO 3 is significantly influenced by the transport conditions of the evolved water.…”

“…Evolution of the gas phase during reaction was monitored with a quadrupole mass spectrometer (Pfeiffer, QMS200 Omnistar). The kinetic data obtained in the TG experiments were processed using Netzsch software and are presented elsewhere [12]. The Thermokinetics program package was used for processing kinetic dependences and for component kinetic analysis.…”

“…XRD confirmed that both precursor materials were phase pure. The original XRD pattern and the results of Rietveld refinement are presented elsewhere [12].…”

“…50 mg (100-200 μm particle diameter, diluted with 0.7 g of SiO 2 ) were loaded into a 6 mm inner diameter stainless steel reactor tube using a bed volume of 1.7 mL, which resulted in a GHSV of 3500 h Based on the determined formal decomposition mechanism and thermokinetic parameters [12] several temperature programs were calculated enabling calcination of AU with the constant mass loss rate ( Figure 1). Table 1 demonstrated for other systems [21].…”

“…In water containing atmosphere the carbonate decomposes more rapidly and already at lower temperatures [9,12]. The evolved water during decomposition promotes the crystallization of CuO.…”