NO_2对Cu/SAPO-34分子筛催化剂上NH_3选择性催化还原NO性能的影响

主要考察了NO2对Cu/SAPO-34分子筛催化剂在整个温度范围内(100-500°C)NH3选择性催化还原(SCR)NO性能的影响.研究所使用样品为新鲜Cu/SAPO-34催化剂在750°C下水热处理4 h的稳定期样品.通过X射线衍射(XRD)和扫描电子显微镜(SEM)对样品的结构以及形貌进行表征,采用SCR活性评价、动力学实验以及原位漫反射傅里叶变换红外光谱(in situ-DRIFTS)表征催化剂的性能以及催化剂表面物种的变化.活性评价实验结果表明,NO2会抑制催化剂的低温(100-280°C)活性,但其存在会提高催化剂的高温(280°C以上)活性.与此同时,随着反应物中NO/NO2的摩尔比例减少,由于NH4NO3物种的分解,副产物(N2O)的浓度增大.动力学结果表明,Cu/SAPO-34催化剂上快速SCR反应的表观活化能(Ea=64.02 kJ?mol-1)比标准SCR反应的表观活化能(Ea=48.00 kJ?mol-1)更大.In situ-DRIFTS实验结果表明NO比NO2更容易在催化剂表面形成硝酸盐,并且NO2更容易与吸附在Br?nsted酸性位上的NH3物种反应生成NH4NO3.低温下,催化剂表面的NH4NO3物种会覆盖SCR反应的活性位,造成活性降低,但在高温时,形成的NH4NO3物种一部分会被NO还原为N2,而另一部分会直接热分解为N2O,造成催化剂的选择性降低.

Effect of NO2 on the Selective Catalytic Reduction of NO with NH3 over Cu/SAPO-34 Molecular Sieve Catalyst

This study investigated the effects of NO₂ on the selective catalytic reduction (SCR) of NO by NH₃ over Cu/SAPO-34 catalyst at temperatures ranging from 100 to 500 ℃. The Cu/SAPO- 34 sample was hydrothermally treated at 750 ℃ for 4 h to obtain a de-greened sample and X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the structure of the catalyst. SCR activity test, kinetic analysis, and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ-DRIFTS) were all applied to evaluate the changes in catalytic activity in the presence of various NO/NO₂ ratios. The SCR results for different NO/NO₂ molar ratios demonstrated that NO₂ inhibited the NO_x removal efficiency over the Cu/SAPO- 34 catalyst at low temperatures (100-280 ℃), but enhanced the efficiency at high temperatures (above 280 ℃). The amount of N₂O was observed to increase with decreasing NO/NO₂ ratios, owing to the decomposition of NH₄NO₃. The kinetic results showed that the fast SCR reaction exhibited a higher apparent activation energy (E_a=64.02 kJ·mol^-1) than that of the standard SCR reaction (E_a=48.00 kJ·mol^-1) over Cu/SAPO-34 catalyst. The results of in situ-DRIFTS showed that NO₂ did not efficiently generate nitrate species on Cu2+ sites compared with NO, and that some nitrate species combined with NH₄₊ on Brønsted acid sites to generate NH₄NO₃. The inhibitory effect of NO₂ at low temperatures is evidently caused by deposited NH₄NO₃ covering the active sites of Cu/SAPO-34 catalyst, while these NH₄NO₃ species can be reduced by NO or thermally decomposed as the temperature increases.