A CeO 2 / ZrO 2-TiO 2 catalyst for the selective catalytic reduction of NO x with NH 3

A CeO2/ZrO2-TiO2 catalyst was prepared by a stepwise precipitation approach for the selective catalytic reduction of NOx with NH3. Through the control of pH value and precipitation time during preparation, the function of the CeO2/ZrO2-TiO2 catalyst could be controlled and the structure with highly dispersed CeO2 (with redox functions) on the surface of the ZrO2-TiO2 (with acidic functions) could be obtained. Characterizations revealed that the obtained CeO2/ZrO2-TiO2 catalyst could exhibit superior redox functions, outstanding adsorption and activation properties for NOx and NH3, and enhanced charge imbalance, which is the main reason for the excellent catalytic performance of the catalyst.


Introduction
Selective catalytic reduction of NOx with NH3 (NH3-SCR) has been widely applied for the removal of NOx generated from stationary sources for many years, and it has also become the dominant technology for the control of NOx emission from diesel vehicles. 13][4] In the NH3-SCR reaction, both redox functions and acidic functions of the catalyst are needed.Therefore, highly disperse of active sites and close coupling of redox with acid sites are the key factors for the design of a high efficiency NH3-SCR catalyst.

Experimental
The CeO2/ZrO2-TiO2, with a Ce/Zr/Ti molar ratio of 1:0.5:2, was prepared by a precipitation method.The reaction was performed on a fixed-bed quartz flow reactor.The effluent gas, including NO, NH3, NO2 and N2O, was continuously analyzed by an online FTIR gas analyzer (Nicolet Antaris IGS analyzer).Characterization methods, including XRD, XPS, H2-TPR, NOx-TPD, and NH3-TPD, were used to investigate the catalyst.

Results and discussion
Due to the strong acidity of the added Ti(SO4)2 (as the precursor of the Ti), the initial pH of the mixed solution during the preparation of CeO2/ZrO2-TiO2 dropped to be 1.1.After heating, the pH of the solution increased gradually due to the hydrolysis of urea, and some white suspended particles were produced in the first hour.After 4 hours of reaction, the pH of the solution increased to be 6.1 and the suspended particles turned to be light yellow.After 12 hours of heating, the pH reached ca.7.0 and the color of the suspended particles was yellow.The particles with different precipitation time (1 h, 4 h, 6 h, and 12 h) were collected and then prepared to be catalyst samples.Interestingly, the activity test showed a remarkably enhancement of the NOx conversions over the samples with the increase of precipitation time (Figure 1).
The surface metal atomic concentrations of the samples with different precipitation times were analyzed by XPS, and the variations of Ce, Zr, and Ti concentrations with precipitation time were shown in Figure 2. On the sample of 1 h precipitation, just Ti and Zr, without Ce, were detected.With the increase of precipitation time, the samples showed a gradually increase of surface Ce concentration.At the same time, both of the surface Ti and Zr concentrations gradually decreased with the increase of precipitation time.Considering the variation of the solution pH during the preparation, the formation process of the CeO2/ZrO2-TiO2 can be proposed as follows: the Ti and Zr species were firstly co-precipitated when the pH of the mixed solution increased, and then the Ce species was precipitated uniformly onto the precipitated Zr-Ti species due to the further increase of pH.Finally, a CeO2/ZrO2-TiO2 catalyst with higher surface Ce concentration than Ti and Zr was obtained.Through the control of the hydrolysis of urea, the variation of the solution pH can be controlled, and then we can control the precipitation process, which is very important for the formation of highly dispersed CeO2 on ZrO2-TiO2.Thus, the obtained catalyst can present excellent NH3-SCR performance.
Characterizations, including XRD, XPS, H2-TPR, NOx-TPD, and NH3-TPD, were carried out for the CeO2/ZrO2-TiO2, and this catalyst showed superior redox functions, outstanding adsorption and activation properties for NOx and NH3, and enhanced charge imbalance.

Conclusions
In this study, a CeO2/ZrO2-TiO2 catalyst, with highly dispersed CeO2 (with redox functions) on the surface of the ZrO2-TiO2 (with acidic functions), was developed for the NH3-SCR.The catalyst showed superior redox functions, outstanding adsorption and activation properties for NOx and NH3, and enhanced charge imbalance.Thus, it exhibited excellent NH3-SCR performance.

Figure 1 .
Figure 1.The pH variation of the mixed solution during the preparation of the CeO2/ZrO2-TiO2 catalysts.NOx conversions of the obtained samples with different precipitation time were inserted.

Figure 2 .
Figure 2. Surface metal atomic concentrations of the CeO2/ZrO2-TiO2 samples with different precipitation times.