## 3 Results and Discussions  
### Catalytic Performance Test  
Figure 1 shows the influence of different Ce/W molar ratios of the Ce-W-Ti catalysts on the conversion of NO in the temperature range of 150-450 \({}^{\circ}\)C. Almost no catalytic activity was found for W\({}_{0.4}\)Ti at a temperature below 250 \({}^{\circ}\)C, while the conversion of NO increased quasi-linearly from 4.6% to 64.7% when increasing the temperature from 250 to 450 \({}^{\circ}\)C. As for the Ce\({}_{0.4}\)Ti catalyst, the NO conversion increased from 5.8% to 85.0% in the tested temperature range (150-450 \({}^{\circ}\)C). The addition of Ce into the W-Ti catalyst dramatically improved NO conversion and shifted the operation temperature of the catalysts to a lower temperature. A maximum NO conversion of 90.1% to 100% was obtained between 250 \({}^{\circ}\)C and 450 \({}^{\circ}\)C over the Ce\({}_{0.2}\)W\({}_{0.2}\)Ti catalysts. However, further increasing the doping amount of Ce or W decreased the NO conversion as the value was in the range of 74.8% to 98.8% for Ce\({}_{0.3}\)W\({}_{0.1}\)Ti and 57.5% to 93.2% for Ce\({}_{0.1}\)W\({}_{0.3}\)Ti, respectively. The selectivity of N\({}_{2}\) showed a similar trend. As for W\({}_{0.4}\)Ti, the N\({}_{2}\) selectivity decreased from 98.0% at 300 \({}^{\circ}\)C to 83.8% at 450 \({}^{\circ}\)C, while this value decreased from 99.7% to 90.4% for Ce\({}_{0.4}\)Ti. The N\({}_{2}\) selectivity was significantly enhanced when using the Ce-W-Ti catalysts, 100% N\({}_{2}\) selectivity was achieved over Ce\({}_{0.2}\)W\({}_{0.2}\)Ti catalyst between 150 \({}^{\circ}\)C and 350 \({}^{\circ}\)C and further increasing the temperature to 450 \({}^{\circ}\)C slightly decreased the N\({}_{2}\) selectivity to 98.5%. The results of NO conversion and N\({}_{2}\) selectivity indicate that strong interactions exist between Ce and W species in the Ce-W-Ti catalysts, which could contribute to the reaction performance of NH\({}_{3}\)-SCR process. A series of characterizations were carried out to analyze the physico-chemical properties of the Ce-W-Ti catalysts and the relationships between the catalyst structure and the reaction performance.  
Figure 2 shows the effect of SO\({}_{2}\) on the NH\({}_{3}\)-SCR activity over the best-performed Ce\({}_{0.2}\)W\({}_{0.2}\)Ti catalyst at 300 \({}^{\circ}\)C with and without H\({}_{2}\)O over 10 h. Before introducing SO\({}_{2}\) and H\({}_{2}\)O into the catalyst layer, the NO conversion rate is 100% at a steady state. When 500 ppm SO\({}_{2}\) was added, the catalytic activity of Ce\({}_{0.2}\)W\({}_{0.2}\)Ti dramatically decreases to around 60% in 8 h [22]. Previous work confirmed that the SO\({}_{2}\) could react with NH\({}_{3}\) and form the NH\({}_{4}\)HSO\({}_{4}\), which may block the active sites on the surface of Ce\({}_{0.2}\)W\({}_{0.2}\)Ti catalyst [26]. On the other hand, the formation of highly thermal stable sulfate species of Ce(SO\({}_{4}\))\({}_{2}\) and Ce\({}_{2}\)(SO\({}_{4}\))\({}_{3}\) was also expected on the catalyst surfaces, which would reduce the relative concentration of Ce\({}^{3+}\)/Ce\({}^{4+}\) redox pair on the catalyst surface, and consequently impose a negative effect on the catalytic activity [27; 28]. The co-introduction of 500 ppm SO\({}_{2}\) and 5% H\({}_{2}\)O significantly affects the catalytic activity of Ce\({}_{0.2}\)W\({}_{0.2}\)Ti. The catalytic activity is reduced more quickly compared to the case of introducing SO\({}_{2}\) alone with only 65% after 8 h. After cutting off SO\({}_{2}\) and H\({}_{2}\)O from the feeding gas, the activity slightly recovers to around 70%. It is obvious that the addition of W species into the original Ce-Ti catalyst remarkably enhances the resistance of catalysts towards SO\({}_{2}\) and H\({}_{2}\)O. Similar observations were also reported in previous work of cerium-based catalysts [27; 29].