The influence of annealing temperature on NH3-SCR performance of CexZr1-x/UiO-66 bimetal MOFs

MOFs possess large surface area and high porosity, but they will convert into derivatives when subjecting to high temperature due to its poor thermal stability. Herein, a series of Ce-Zr bimetal MOFs derivatives were prepared by directly pre-annealing of CexZr1−x/UiO-66 bimetal MOFs at different temperatures under air atmosphere. The as-prepared samples were used as catalysts for selective catalytic reduction of NOx with NH3. The catalytic activity test results indicated that pre-annealing temperature had an obvious promoting effect on NH3-SCR performance. C0.5Z0.5/U-400 catalyst achieved a NOx conversion of 92% at 350 ºC and a complete N2 selectivity in a wide temperature range of 150–500 ºC. Various characterization tools were employed to reveal the effects of pre-annealing temperature on the microstructure and chemical properties of C0.5Z0.5/U bimetal MOFs and its derivatives. The pre-annealing treatment could lead to a distinctive decrease in specific surface area, demonstrating that C0.5Z0.5/U bimetal MOFs gradually converted into derivatives and there was an obvious agglomeration phenomenon between derived metal oxides. But a relatively large specific surface area and fluffy porous microstructure could be obtained for pre-annealing C0.5Z0.5/U at 400 ºC for 2 h. It was conducive to exposure of more active sites, thus enhancing catalytic performance. Moreover, C0.5Z0.5/U-400 catalyst not only possessed a higher Ce3+/(Ce3+ + Ce4+) ratio and a better reduction ability, but also had more abundant oxygen vacancies to adsorb and activate reactant gases. In-situ DRIFTS results indicated that NH3-SCR reactions over C0.5Z0.5/U-400 catalyst followed both E-R and L-H mechanisms.