Mechanistic studies on the conversion of NO gas on urea-iron and copper metal organic frameworks at low temperature conditions: in situ infrared spectroscopy and Monte Carlo investigations

Abstract

Nitrogen oxides (NOx) emissions from high temperature combustion processes under fuel-lean conditions continue to be a challenge for the energy industry. Selective catalytic reduction (SCR) has been possible with metal oxides and zeolites. There is still the need to identify catalytic materials that are efficient in reducing NOx to environmentally benign nitrogen gas at temperatures lower than 200°C. Metal-organic frameworks (MOFs) emerged as a class of highly porous materials with unique physical and chemical properties. This study is motivated by the lack of systematic investigations on SCR using MOFs under industrially-relevant conditions. Here, we investigate the extent of NO conversion with two commercially-available MOFs; Basolite F300 (Fe-BTC) and HKUST-1 (Cu-BTC), mixed with solid urea as a source for the reductant, ammonia gas. For comparison, experiments were also conducted using cobalt ferrite (CoFe2O4) as a non-porous counterpart to relate its reactivity to those obtained from MOFs. Fourier-transform infrared spectroscopy (FTIR) was utilized to identify gas and surface species the temperature range 115 -180°C. Computational analysis was performed using Monte Carlo (MC) simulations to quantify adsorption energies of different surface species. The results show that the rate of ammonia production from the in situ solid urea decomposition was higher using CoFe2O4 than Fe-BTC and Cu-BTC, and that there is very limited conversion of NO on the mixed solid urea-MOF systems due to site blocking. The main conclusions from this study is that MOFs have limited abilities in converting NO under low temperature conditions, and that surface regeneration requires additional experimental steps.