Insight into the low-temperature DeNOx performance of copper-based oxides: Perspective from the valence state distribution
Understanding the effect of CuO x valence state specifically is significant for constructing high-performance Cu based oxide catalyst for low-temperature selective catalytic reduction of NO with NH 3 (NH 3 -SCR). In this work, CuO x with different valence state including Cu 2 O, Cu 2 O-CuO and CuO particles are synthetized as model catalysts, which present same variation laws in catalytic performance at 180 ℃, 210 ℃ and 240 ℃. NH 3 -SCR catalytic performance of Cu 2 O-CuO heterojunction is the best, CuO exhibits good catalytic activity, while Cu 2 O possesses excellent N 2 selectivity. Experimental results combined with density functional theory (DFT) calculations reveal that Cu 2+ favor to form monodentate nitrate and possess strong ability of activation and dehydrogenation of NH 3 . The energy barrier for the form of NH 2 * is 79.5 kJ/mol, which is much lower than that for Cu + . While Cu + is beneficial to NH 3 and NO x adsorption and the form of bidentate nitrate, which have a positive impact on improving the selectivity of N 2 . Moreover, the collaboration of Cu 2+ and Cu + facilitates the redox capability and the form of surface adsorbed oxygen species, thus the reaction progress is further promoted. The SCR reaction catalyzed by three samples are dominated mainly by Langmuir-Hinshelwood (L-H) mechanism at 180 ℃. While at 240 ℃, the reaction over Cu 2 O follows Eley-Rideal (E-R) mechanism, during which the rate-limiting step is the internal hydrogen transfer procedure (NH 2 NO*→NHNOH*). Over CuO, the reaction follows L-H mechanism, the process of atomic isomerism (NHNOH*+ *→N 2 *+H 2 O*) is the rate-determining step. The energy barriers are 136 and 146 kJ/mol, respectively. This work may supply important theoretical support for the preparation of high-performance Cu-based oxide catalysts.
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