Nitrous Oxide Formation over Zeolite-based Catalysts during Ammonia-SCR: The Effect of Framework Structure, Acidity, and Copper Content
The emission control of anthropogenic nitrous oxide (N2O), a by-product formed through fossil- and renewable fuel combustion, agricultural activities, and industrial chemical processes, has attracted large considerations due to its substantial contribution in global warming and ozone layer depletion. Selective catalytic reduction with ammonia (NH3-SCR) is the most prevailing technology for the abatement of nitrogen oxides (NOx) in the exhaust gases from lean-burn processes, with the possible formation of N2O. Therefore, the development of catalysts for efficient NOx reduction with no or minor N2O formation is of major importance. Hence, zeolite-based catalysts exchanged with copper have shown to be efficient catalysts for NOx reduction owing to their high catalytic performance under practical reaction conditions. This work aims to increase the understanding of the N2O formation during NH3-SCR, in particular studying the effect of different parameters, from zeolite framework structure, ammonia storage capacity of the zeolites, to Si/Al and Cu/Al molar ratios. Three different zeolites with varying pore sizes, from small to medium and large pore zeolites (SSZ-13, ZSM-5 and beta), were chosen to investigate their performance as SCR catalysts. A range of SSZ-13 samples with Si/Al molar ratios of 6, 12, and 24 were prepared by hydrothermal crystallization, and exchanged with copper, Cu/Al= 0-0.4 molar ratios, to investigate the effect of the Si/Al and Cu/Al molar ratios on the ammonia storage capacity and the SCR performance of the samples. Furthermore, the role of the sample pretreatment on the SCR performance was investigated for the SSZ-13 sample with Si/Al= 12 molar ratio. The prepared samples were studied by flow reactor experiments and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to evaluate the catalytic activity and selectivity, and to monitor the evolution of surface species during reaction. Copper ions as active sites in the zeolite catalyzing the SCR reaction by NO activation and formation of NO+ and/or surface nitrate species. During SCR, the nitrate species can subsequently react with NH3 and form ammonium nitrate (AN) as an intermediate, which partially contributes to N2O formation upon decomposition. In order to understand the system in more detail, we have investigated important factors such as Si/Al and Cu/Al molar ratio and temperature on the NH3 storage capacity of the samples based on SSZ-13. Temperature programmed desorption by NH3 (NH3-TPD) carried out to characterize the nature of the different acid sites in the zeolite. It is revealed that the samples with low Si/Al molar ratio provide higher NH3 storage capacity, which increases with increasing Cu loading. After NH3-TPD, SCR experiments were subsequently performed resulting in higher NOx conversion and N2O formation by increasing the Cu content for all samples. Results from DRIFTS showing vibrational peaks associated with N2O in accordance with the flow reactor findings. Moreover, the role of pretreatment for the NH3-SCR performance was evaluated for the sample with Si/Al= 12, and the results show that the pretreatment in NH3 and NO, in the absence of O2, reveals higher low-temperature activity for standard SCR compared to the pretreatment including O2.
In situ IR spectroscopy
Si/Al molar ratio