Impact of gas treatment of CuAl-LDH on NO reduction by CO under oxidative conditions

Abstract

Removal of NO from cement-type industrial flue gases can be achieved by its catalytic reduction with CO (NO-Selective Catalytic Reduction) over a suitable noble metal free catalyst. Cu-Al mixed metal oxides derived from parent Layer-Double Hydroxide (LDH) after thermal decomposition under different pre-treatment gas conditions: inert (He), oxidising (air), reducing gas (1 %CO in He) at 500 °C have been used to achieve it. Additionally, a mixed oxide was prepared by the treatment of the LDH under 100 % CO2 at 500 °C, a novel method for synthesis of mixed oxides is reported in this study. Different characterization techniques (N2-Physisorption, XRD, H2-TPR, TGA and FTIR) were used to investigate physico-chemical properties of CuAl mixed oxide materials to determine their role on the catalytic performances. It was observed that gas treatments did not affect the crystal size (∼20 nm) of the corresponding Cu-Al mixed oxide except for CO/He treatment, where highly crystalline phases of Cu(0) were formed predominantly instead of CuO as in other Cu-Al mixed oxide samples. Further, the catalytic tests were performed using a simulated industrial gas composition from a cement industry consisting of CO (0.13 vol%), CO2 (20 vol%), NO (475 ppm), O2 (8.8 vol%) and without or with H2O (8.2 vol%), the rest being balanced by He. Both NO reduction and oxidation reactions occurred in the presence of water over CuAl-CO/He, CuAl-He, CuAl-CO2 and CuAl-air samples. However, only CuAl-CO2 was the only sample active for the NO reduction (N2 selectivity was 83 %) in the absence of water in the reaction feed. Moreover, under wet reaction conditions, CuAl-CO2 sample showed similar catalytic activity like its CuAl-air and CuAl-He counterparts (NO reduction yield of 13–16 % at peak temperatures of 420–400 °C). The plausible formation of monodentate and/or bidentate carbonates onto the material during the CO2-treatment at 500 °C (inferred from TGA and FTIR measurements) was proposed to promote NO reduction activity in the absence of water while under wet conditions, these carbonate species supposedly reacted with the water species and formed in active formiate species. This resulted in a mixed oxide with material properties similar to that of the CuAl-air and CuAl-He samples and therefore, led to similar catalytic performance. Finally, our study showcased the effect of novel CO2 treatment for CuAl-LDH precursor in improving the performances for NO reduction.