Environmental catalysis: a solution for the removal of atmospheric pollutants

Abstract

Atmospheric pollutants such as nitrogen oxides (NOx), carbon monoxide, and volatile organic compounds (VOCs), chiefly emanating from industrial activities and transportation vehicles, are harmful to human health. Catalysis is one of the most effective and economic technologies to control serious air pollution problems; however, the key issue is the availability of high-performance catalysts. Great efforts have been made to develop catalytic materials that can be used for eliminating atmospheric pollutants. Compared to bulk materials, nanosized or porous materials possess larger surface areas or more abundant pores, which are beneficial for the diffusion, adsorption, and activation of the reactants. Therefore, nanosized or porous materials exhibit higher catalytic performance than bulk materials. In the past decades, the related research has focused on the synthesis and environmental applications of nanosized or porous catalysts. NOx is one of the chemicals responsible for acid rain. NOx emissions mainly come from coal-fired power plants, chemical industries, and automotive vehicles. At present, the major NOx removal technique is its selective catalytic reduction (SCR) using reducing regents such as ammonia, urea, hydrogen, carbon monoxide, and light hydrocarbons. The most widely used catalyst is V2O5–WO3/TiO2, and its catalytic performance is associated with the properties of support, V2O5 and WO3 loading, catalyst surface area, and porous structure. Recently, several research groups have investigated the SCR of NOx with ammonia or hydrogen over supported transition metal or precious metal catalysts. For example, Ma’s group [1] examined the poisoning and regeneration effect of alkali metal deposition over commercial V2O5–WO3/TiO2 catalysts for the SCR of NO with NH3, and found that the doping of alkali metals (e.g., Na2O and K2O) could poison the V2O5–WO3/TiO2 catalyst. Moreover, the poisoning effect of Na2O was more significant than that of K2O, but the Na2O-doped catalyst more readily regenerated than the K2O-doped one. The authors also observed that the doping of alkali metals leads to a decrease in the desorbed ammonia amount and the destabilization of acid sites, and such deactivation is related to a decrease in the concentration of chemisorbed oxygen. Chen’s group [2] studied the influence of the Ce/Zr molar ratio on the SCR of NOx with NH3 over the Fe2O3–WO3/ CexZr1-xO2 monolith catalyst, and claimed that there was a close relationship between the NH3-SCR catalytic activity and the Ce/Zr molar ratio. Among all the catalysts, the Fe2O3–WO3/Ce0.68Zr0.32O2 catalyst showed the best performance (over 90 % NOx, with a N2 selectivity of more than 99 %, could be removed at 200–500 C and 30,000 h), and the catalyst was even resistant to moisture and sulfur dioxide. The authors assigned the excellent catalytic performance of Fe2O3–WO3/Ce0.68Zr0.32O2 to its good textural and structural properties and the larger amounts of surface Fe, Ce, and reactive oxygen species (ROS). Liu’s group [3] worked on the SCR of NOx by hydrogen over a modified Pd/TiO2–Al2O3 catalyst under lean-burn conditions. They observed that the NOx conversion and N2 selectivity above 200 C could be enhanced by doping 2 wt% Sn or Ni to the Pd/TiO2–Al2O3 catalyst, and that N2 selectivity and high-temperature activity were much higher over Pd–Sn/TiO2–Al2O3 than over Pd/TiO2– H. Dai (&) Beijing Key Laboratory for Green Catalysis and Separation, Key Laboratory of Beijing on Regional Air Pollution Control, Key Laboratory of Advanced Functional Materials, Ministry of Education, and Laboratory of Catalysis Chemistry and Nanoscience, Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China e-mail: hxdai@bjut.edu.cn