Starch bio-template synthesis of W-doped CeO2 catalyst for selective catalytic reduction of NOx with NH3: influence of ignition temperature.

A novel tungsten-doped CeO2 catalyst was fabricated via the sweet potato starch bio-template spread self-combustion (SSC) method to secure a high NH3-SCR activity. The study focuses on the influence of ignition temperature on the physical structure and redox properties of the synthesized catalyst and the catalytic performance of NOx reduction with NH3. These were quantitatively examined by conducting TG-DSC measurements of the starch gel, XRD analysis for the crystallites, SEM and TEM assessments for the morphology of the catalyst, XPS and H2-TPR measurements for the distribution of cerium and tungsten, and NH3-TPD assessments for the acidity of the catalyst. It is found that the ignition temperature shows an important role in the interaction of cerium and tungsten species, and the optimal ignition temperature is 500 °C. The increase of ignition temperature from 150 °C is beneficial to the interactions of species in the catalyst, depresses the formation of WO3, and refines the cubic CeO2 crystallite. The sample ignited at 500 °C shows the biggest BET surface area, the highest surface concentration of Ce species and molar ratio of Ce3+/(Ce3++Ce4+), and the most abundant surface Brønsted acid sites, which are the possible reasons for the superiority of the NH3-SCR activity. With a high GHSV of 200,000 mL (g h)-1 and the optimal ignition temperature, Ce4W2Oz-500 can achieve a steadily high NOx reduction of 80% or more at a lowered reduction temperature in the range of 250~500 °C.

Environment-friendly magnetic Fe-Ce-W catalyst for the selective catalytic reduction of NO x with NH3: influence of citric acid content on its activity-structure relationship.

The influence of the citric acid content on the structural and redox properties of a magnetic iron-cerium-tungsten mixed oxide catalyst prepared through a microwave-assisted citric acid sol-gel method is investigated via TG-DTG-DSC, XRD, N2 adsorption-desorption, XPS, H2-TPR and NH3-TPD. Additionally, the NH3-SCR activity of the magnetic FeCeW-m (m = 0.25, 0.5 and 1.0) catalysts are also studied. The results indicate that an increase in citric acid content strengthens the sol-gel reaction between citric acid and metal ions and promotes the formation of the γ-Fe2O3 crystallite not α-Fe2O3. Meanwhile, it decreases the BET surface area and pore volume of the catalyst. Furthermore, the surface concentration of iron species on the catalyst is enhanced when the molar ratio of citric acid/(Fe + Ce + W) increases from 0.25 to 1.0, but its surface absorbed oxygen and total oxygen concentration decrease. The magnetic FeCeW-0.5 catalyst shows the best reducibility at temperatures below 790 °C. The increase in the citric acid content inhibits the formation of acid sites in the catalyst, thus the magnetic FeCeW-0.25 catalyst possesses the most Lewis acid sites and Brønsted acid sites among the catalysts. The enhancement in citric acid content is beneficial to improve the SCR reaction rates normalized by the surface area of the catalyst. This catalyst exhibits high anti-SO2 and H2O poisoning, and the molar ratio of citric acid/(Fe + Ce + W) affects the adsorption of NO x species on its surface.