Optimalizace obsahu cesia ve směsných kobaltových oxidech pro katalytický rozklad N2O

Abstract

This dissertation deals with N2O catalytic decomposition over mixed oxides containing active species – cobalt and manganese, which were modified with cesium promoter. A series of Co4MnAlOx mixed oxides modified by cesium in concentration range of 0.6 – 4.5 wt. % was prepared by calcination of hydrotalcite-like precursor with molar ratio Co:Mn:Al = 4:1:1. Subsequently formed mixed oxide was impregnated with cesium by pore filling method. Two different salts (Cs2CO3 a CsNO3) were used for impregnation. Samples was characterized by several analytical methods – AAS, XRD, SBET, TPR-H2, TPD-CO2, TPD-NH3, XPS and TAD. Experimental measurements of N2O catalytic decomposition was carried out in an integral fixed bed stainless steel reactor in temperature range of 210 – 450 °C. The specific surface area of Co4MnAlOx mixed oxide was 92 m2/g and no significant changes were observed after cesium modification. Spinel phase was identified by XRD analysis in catalyst samples. The sample basicity increased with increasing amount of cesium. Samples modified by cesium using CsNO3 exhibited relatively worse reducibility than ones modified by cesium using Cs2CO3. Thermal alkali desorption showed that cesium on the catalyst surface is stable until 550 °C. The catalyst activity increased with increasing cesium amount. The highest catalytic activity was achieved over catalysts with cesium amount in concentration range of 3 – 4 wt. % at conditions simulating the waste gas from nitric acid plant. Used impregnation salt had no influence on catalytic activity. Based on these laboratory results, optimal cesium amount of 3.4 wt. %. was chosen for preparing of pilot plant catalyst designated as AST3. AST3 catalyst was tested for N2O emission reduction in the waste gas in pilot plant reactor connected to tail gas from nitric acid production in BorsodChem MCHZ Ostrava. The high N2O conversion value of 64 – 94 % was achieved at space velocity 11 130 l.lbed-1.h-1 and 450 °C. These values were dependent on variable reaction mixture composition. No deactivation was observed after 142 days of operation. It was found that cesium acts as electron promoter. Small amount of cesium promotes easier scission of N2O molecule on Cs–Co interface. The shift of temperature maxima of TPR-H2 peaks to lower temperatures with increasing cesium amount confirmed easier Co3+ reduction which is related to weakening Co–O bond which leads to lowering of oxygen residence time on the catalyst surface. TPD-O2 tests confirmed that optimal amount of cesium promotes faster oxygen desorption from the catalyst surface, which is a crucial step for N2O catalytic decomposition.