WO3 Based Composite Prepared by Ammonium Metatungstate for Selective Catalytic Reduction of NOx

The nitrogen oxides (NOx) produced by mobile and stationary sources are the main sources of photochemistry and smog acid rain. Ammonia selective catalytic reduction (SCR) is one of the most effective methods for removing NOx. It is widely used in power plants and other industrial environments.Due to its high oxygen storage capacity and excellent redox characteristics of conversion between Ce4+ and Ce3+, the application of cerium oxide-based oxides in the NH3-SCR reaction has attracted more and more attention. However, compared with vanadium catalysts, the SO2 tolerance is a great challenge for CeO2 SCR catalysts applied in stationary sources especially at low and mediate temperatures.

Introducing tungsten trioxide (WO3) is a method to enhance the physical properties of catalysts. Thus, WO3 based composite has been prepared by ammonium metatungstate for selective catalytic reduction of NOx. The synthesis process of TiO2, CeO2/TiO2 and CeO2-WO3/TiO2 catalysts catalyst are prepared by a one-step sol-gel method:

Firstly, mix butyl titanate (0.1 mol), absolute ethanol (2.9 mol), deionized water (1.9 mol), nitric acid (0.1 mol), appropriate amount of cerium nitrate and ammonium metatungstate under continuous vigorous stirring at room temperature to obtain a transparent Yellow sol. After drying at 80°C for 24 hours, the sol turns into a dry gel. The dry gel is crushed and sieved to 60-100 mesh, and then calcined in air at 500°C for 5 hours. CeO2 is prepared by direct decomposition of cerium nitrate, dried at 80°C for 24 hours, and then calcined in air at 500°C for 5 hours. The catalyst is expressed as CexWyTi, where x and y represent the mass percentages of CeO2 and WO3 and TiO2, respectively.

The activity measurement was carried out in a fixed-bed quartz reactor (inner diameter = 8 mm), using 0.35 g of catalyst, 100-250 μm. The simulated gas used for all catalytic activity measurements contains 1000 ppm NO, 1000 ppm NH3, 3 vol.% O2, 10 vol.% H2O (when in use), 500 ppm SO2 (when in use), and N2 as the balance gas. The concentrations of NO, NO2, SO2 and O2 are continuously monitored by a gas analyzer (350 Pro, Testo). The activity data was collected when the SCR reaction reached a steady state for half an hour at each temperature. The formation of N2O was found to be negligible (<10 ppm), so this product will not be discussed further.

In summary, WO3 based composite has been prepared by ammonium metatungstate for selective catalytic reduction of NOx. The addition of WO3 can increase the SCR activity and significantly improve the tolerance of the Ce-Ti oxide catalyst to H2O and SO2. In a wide temperature range of 275-500 °C, a 90.0-98.5% NO conversion rate was obtained on the Ce20W10Ti catalyst with a GHSV of 90,000 h-1.

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