Nitrous Oxide Formation over Zeolite-based Catalysts during Ammonia-SCR: The Effect of Framework Structure, Acidity, and Copper Content

Sammanfattning: 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.