Selective catalytic reduction of nitrogen oxides under oxidizing conditions over sol-gel derived alumina catalysts.
Cavataio, John Vito
2002
Abstract
Silver and platinum alumina catalysts prepared by a single-step sol-gel procedure, sol-gel alumina samples modified by an acid or base, and H<sub> 3</sub>PO<sub>4</sub> modified sol-gel alumina impregnated with platinum were studied on a laboratory flow reactor system for the lean NOx reduction in the presence of excess oxygen. An optimal loading of 5 wt% for silver alumina was determined to achieve the best lean NOx reduction by propylene. Different hydrocarbon species significantly altered the NOx activity. Among the reductant types tested, n-octane achieved the best performance followed by propylene, ethylene, propane, and finally toluene. Using n-octane as the reductant, 100% NOx conversion was achieved from 350°C to 460°C. At equivalent moles of hydrogen, the 5% platinum alumina achieved 95% NOx conversion at 100°C with hydrogen as compared to 48% NOx conversion at 250°C for propylene. For the NOx/O<sub>2</sub>/H<sub>2</sub> reaction, the N<sub>2</sub>O selectivity has a strong inverse relationship to temperature yielding the highest N<sub>2</sub>O selectivity at the lowest temperature. Increasing the individual reactant concentrations (O<sub>2</sub>, H<sub>2 </sub>, or NOx) on the reaction showed a negative order relationship with respect to O<sub>2</sub>, a positive first order with H<sub>2</sub>, and a zero order with NO. The thermal stability of acid or base modified alumina was characterized in terms of pore morphology and surface area during sintering experiments. The addition of H<sub>3</sub>PO<sub>4</sub> to the boehmite sol during the pepitzation stage of alumina preparation significantly increased the thermal stability. In fact, after calcination at 1100°C for 16 hours, the H<sub> 3</sub>PO<sub>4</sub>-alumina sample retained a surface area of 125M<super> 2</super>/g with no indication of alpha-alumina. The best NOx activity was observed with NH<sub>4</sub>OH and H<sub>2</sub>O modified alumina samples exhibiting large pore volumes and pore-size distributions. By exposure to 1000°C for 16 hours, severely sintered platinum impregnated on HNO<sub>3</sub> and H<sub>3</sub>PO<sub>4</sub> modified sol-gel alumina modestly affected the NOx reduction by hydrogen whereas the use of propylene significantly degraded the NOx performance. Furthermore, compared to HNO<sub> 3</sub>-alumina, platinum supported on H<sub>3</sub>PO<sub>4</sub>-alumina exhibited lower N<sub>2</sub>O selectivity and higher net NOx conversion regardless of the type of reductant, calcination temperature, platinum dispersion, and platinum loading.Subjects
Alumina Catalysts Catalytic Reduction Conditions Derived Nitrogen Oxides Over Oxidizing Selective Sol-gel Solgel Under
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