Preparation and characterization of inexpensive heterogeneous catalysts for air pollution control: Two case studies
Relatively inexpensive heterogeneous catalysts for two reactions of great importance in air pollution control, NO reduction and VOC combustion, were prepared and characterized. Apart from their common practical goal and the frequent need for simultaneous removal of air pollutants, these reactions share a similar redox mechanism, in which the formulation of more effective catalysts requires an enhancement of oxygen transfer. For NO reduction, supported catalysts were prepared by adding a metal (Cu, Co, K) using ion exchange (IE) and incipient wetness impregnation (IWI) to chars obtained from pyrolysis of a subbituminous coal. The effects of pyrolysis temperature, between 550 and 1000 Â°C, on selected catalyst characteristics (e.g., BET surface area, XRD spectrum, support reactivity in O 2) are reported. For IE catalysts, the surface area increased in the presence of the metals while the opposite occurred for IWI catalysts. For the Co-IE catalysts, the highest surface area was obtained at 700 Â°C. The XRD results showed that, except for Cu (which exhibited sharp Cu 0 peaks), the catalysts may be highly dispersed (or amorphous) on the carbon surface. For the C-O 2 reaction the order of (re)activity was K â‰« Co > Cu for IE catalysts and K > Cu > Co for IWI catalysts. For NO reduction the orders were K > Co > Cu (IE catalysts) and Cu > Co > K (IWI catalysts). In all cases the catalytic (re)activity for NO reduction was lower than that exhibited for the C-O 2 reaction. The K-IE and Cu-IWI catalysts appeared to be the most promising ones, although further improvements in catalytic activity will be desirable. Some surprising results regarding CO and CO 2 selectivity are also reported, especially for Co catalysts. In VOC combustion, the effect of the nature of ion B (Fe and Ni) on the partial substitution of ion A (Ca for La) in ABO 3 perovskites (e.g., LaFeO 3 and LaNiO 3) and on their catalytic activity was studied. The perovskite-type oxides were characterized by means of surface area measurements, XRD, temperature-programmed desorption (TPD) and temperature-programmed reduction (TPR). The effect of partial substitution of La 3+ by Ca 2+ was more significant for the La 1-xCa xFeO 3 perovskites. In this case, the electronic perturbation is compensated by an oxidation state increase of part of Fe 3+ to Fe 4+. The TPD results revealed that, at higher substitution levels, oxygen vacancies are also formed to preserve electroneutrality. For the La 1-xCa xNiO 3 perovskites, the characterization results showed no evidence of large differences in electronic properties as calcium substitution increases. The La 1-xCa xNiO 3 perovskites exhibited lower activity than the simple LaNiO 3 perovskite, whereas for the La 1-xCa xFeO 3 substituted perovskites the most active catalyst (exhibiting the lowest ignition temperature) was obtained at the highest substitution level, La 0.6Ca 0.4FeO 3. The performance of both groups of catalysts is briefly discussed in terms of redox processes, in which the interplay between oxygen transfer and electron transfer requires further elucidation for the improvement of catalytic activity. Â© 2007 Elsevier B.V. All rights reserved.
|Título según WOS:||Preparation and characterization of inexpensive heterogeneous catalysts for air pollution control: Two case studies|
|Título según SCOPUS:||Preparation and characterization of inexpensive heterogeneous catalysts for air pollution control: Two case studies|
|Título de la Revista:||CATALYSIS TODAY|
|Fecha de publicación:||2007|
|Página de inicio:||208|