Effects of Tungsten Promoter on Catalytic Performance of Pt/CeO 2 -ZrO 2 -Al 2 O 3 catalyst for the Diesel Exhaust Purification

Transcription

1 Effects of Tungsten Promoter on Catalytic Performance of Pt/CeO 2 -ZrO 2 -Al 2 O 3 catalyst for the Diesel Exhaust Purification Zhengzheng Yang College of Environmental Science and Technology, China West Normal University, Nanchong , China. address: zzyang-catal@foxmail.com Abstract In this work different amount of tungsten promoter was added in the Pt/CeO 2 -ZrO 2 -Al 2 O 3 diesel oxidation catalyst via impregnation method. The catalysts were characterized by X-ray diffraction (XRD), H 2 temperature-programmed reduction (TPR) and NH 3 temperature-programmed desorption (TRD). XRD spectra suggested that tungsten promoter in Pt/CeO 2 -ZrO 2 -Al 2 O 3 catalyst can exist in a highly dispersed state. The H 2 -TPR and NH 3 -TPD results implied that appropriate amount of tungsten in Pt/CeO 2 -ZrO 2 -Al 2 O 3 is advantageous to adjust the surface acidity and promote the low temperature reduction of catalyst. Finally, catalytic measurements indicated that appropriate amount of W in Pt/CeO 2 -ZrO 2 -Al 2 O 3 catalyst can improve the catalytic propylene and CO oxidation activity. The catalyst with 0.5 wt.% amount of W promoter shows the best catalytic performance; the Pt-W/CeO 2 -ZrO 2 -Al 2 O 3 catalyst with 1.0 wt.% amount of W exhibits a good activity and superior durability simultaneously; and much more amount of W promoter will decrease the catalytic performances. Keywords: Diesel oxidation catalyst; Pt-based catalyst; W promoter; Propylene oxidation; CO oxidation 32

2 European International Journal of Science and Technology Vol. 6 No. 2 March Introduction Due to the demand of clear atmospheric environment, exhaust gases regulation of auto has become more strict in the recent decades. [1] Traditional internal purification technology alone aftertreatment systems involved is not able to accomplish regulation limit, so aftertreatment catalysts are necessary to reduce the engine exhaust pollutants. [2] The lean-burn conditions of diesel engines caused unavailability of three-way catalyst that is suitable for theoretical air-fuel ratio conditions. To efficiently purify diesel exhaust, several diesel emission aftertreatment systems were carried out. [3, 4] Diesel oxidation catalyst was one of the simplest of aftertreatment catalysts and have been utilized the longest, first being used to reduce CO and unburned hydrocarbons emissions. [1] Noble metals (Pt, Pd, Rh etc.) supported on CeO 2, ZrO 2 and Al 2 O 3 mixed oxides were applied widely as diesel oxidation catalysts. [5, 6] However, the consuming of considerable amount of noble metals was considered disadvantageous for diesel oxidation catalyst commercial application. The addition of promoter provided an effective means to reduce noble metal consumption and optimize catalytic activity of catalysts. [7, 8] Tungsten was used as a dispersion promoter for Pt based catalyst, due to the tungsten moderator interaction which effects between Pt and supported and modify the metal particle size. [9, 10] Considering all of this, tungsten was added in Pt/CeO 2 -ZrO 2 -Al 2 O 3 diesel oxidation catalyst to enhance the catalytic activity for HC and CO oxidation purification, and the effects of tungsten mixed amount was investigated. 2. Materials and Methods 2.1 Materials Ammonium hydroxide, Ce(NO) 3 6H 2 O, ZrOCO 3 2H 2 O, Al(NO) 3 9H 2 O and (NH 4 ) 6 H 2 W 12 O 40 were purchased from Chengdu Kelong Chemical Reagent Factory (China), Nitrite platinum (Pt(NO 3 ) 2 ) was purchased from Heraeus, all the chemicals were of analytical grade and used without further purification. 2.2 Preparation of Catalysts CeO 2 -ZrO 2 -Al 2 O 3 (CZA) were prepared by co-precipitation. A mixture solutions of Ce(NO) 3 6H 2 O, ZrOCO 3 2H 2 O and Al(NO) 3 9H 2 O in a molar ration of 1:1:20 were mixed with NH 3 H 2 O solutions under stirring. The resulting precipitate was washed, filtered, drying and then calcining at 600 C for 3 hours under air. The obtained CeO 2 -ZrO 2 -Al 2 O 3 mixed oxides powder was used as catalyst support. Pt-W/CZA catalysts with different tungsten loading were prepared by impregnating the CeO 2 -ZrO 2 -Al 2 O 3 powder with Pt(NO 3 ) 2 and (NH 4 ) 6 H 2 W 12 O 40 aqueous solutions. Then the Pt-W/CZA catalyst powders were dried at 120 C overnight and calcined at 500 C for 3 hours. All Pt-W/CZA catalysts contain 1 wt.% Pt and different content of W. The catalysts containing 0 wt.%, 0.5 wt.%, 1.0 wt.%, 1.5 wt.% and 3.0 wt.% of W were referred as Pt/CZA, Pt-0.5W/ CZA, Pt-1.0W/CZA, Pt-1.5W/CZA, Pt-3.0W/CZA, respectively. The simulative km vehicular aged catalysts were obtained by following, the fresh catalysts were placed in the aging gases containing: CO 1500 ppm, C 3 H ppm, NO 200 ppm, 4% CO 2, 5% O 2, 8% vapor, SO 2 50 ppm, N 2 balance at 800 ml/min flow rate, and then aged at 670 C for 15 hours with an additional 15 hours at 250 C. This aging process fits well with the km vehicle aging. [11, 12] The aged samples were referred as Pt/CZA(A), Pt-0.5W/CZA(A), Pt-1.0W/CZA(A), Pt-1.5W/CZA(A), Pt-3.0W/CZA(A), respectively. 33

3 2.3 Catalytic Performance Measurement The catalytic performance measurement was carried out in a fixed-bed flow reactor. About 0.35 g catalyst was placed in a quartz reactor with surrounding electrical heater coil, and then the simulative diesel exhaust gases that regulated by mass-flow controller were flowed through the catalyst. The concentration of simulated diesel exhaust gases were: 1000 ppm CO, 500 ppm C 3 H 6, 500 ppm NO, 6% CO 2, 10% O 2, 10% water vapor, 100 ppm SO 2, and N 2 balance at a gas hourly space velocity of h -1. The CO concentration in the inlet and outlet gases was continually analyzed by an FGA-4100 automotive emission analyzer (Foshan Analytical Instruments, China) and C 3 H 6 was detected by a GC2000 online gas chromatograph (Shanghai Analysis Instruments, China) with flame ionization detector (FID). All catalysts were pre-treated in the reaction gases at 500 C for 3 hours before the testing. 2.4 Instrumentation for Characterization Powder X-ray diffraction (XRD) spectra was detected on a DX-1000 diffractometer (Dandong Instruments, China) with Cu Kα (λ = nm) radiation. The measurement was operated at 25 ma and 40 kv with a range of Hydrogen-temperature programmed reduction (H 2 -TPR) was performed on a chemical adsorption instrument TP-5050 (Xianquan Instrument, China). About 100 mg sample was pretreated under a 25 ml/min N 2 flow at 450 C for 1 h, after cooling down to room temperature, the sample was heated from room temperature to 850 C with a rate of 10 C/min under a 20 ml/min flowing gas mixture of 5 vol%h 2 -N 2. The H 2 consumption was monitored by a thermal conductivity detector (TCD). NH 3 -temperature programmed desorption (NH 3 -TPD) was carried on the chemical adsorption instrument TP-5050 (Xianquan Instrument, China). About 100 mg sample was pretreated under a 30 ml/min Ar flow at 450 C for 1 h, and then adsorbed the mixture gases of 2 vol% NH 3 -Ar at 80 C for 1 h, followed by isothermal desorption under Ar flow at 80 C until no NH 3 was detected. The temperature programmed desorption in Ar from 80 C to 750 C with 10 C/min was finally carried. The NH 3 concentration was monitored by a thermal conductivity detector (TCD). 3. Results and Discussion 3.1 Catalyst Characterization XRD Fig.1 shows the XRD patterns of catalysts. Four peaks were detected in all samples. The peak locating at 29.3 can be assigned to the (101) plane of Zr 0.5 Ce 0.5 O 2. A broad peak at about 34 were ascribed to Zr 0.5 Ce 0.5 O 2 (002) and (110). The superposed characteristic peaks of Zr 0.5 Ce 0.5 O 2 (112) and (200) plane were detected at the 2θ of about 48.7 and The broad peak at can be attributed to (103) and (211) plane of Zr 0.5 Ce 0.5 O 2 (2θ = 57.7, 58 ). 34

4 European International Journal of Science and Technology Vol. 6 No. 2 March 2017 Fig. 1 XRD patterns of the catalysts Moreover, XRD patterns obtained for all samples do not exhibit an evident diffraction peak of Al 2 O 3 phase, indicating that Al 2 O 3 in all catalysts possess an amorphous structure. [13, 14] No diffraction signal of Pt/W were observed in the XRD profiles, which imply that the Pt/W crystal grains were too small to form multiple faces or Pt/W were highly dispersed on the surface of catalysts. [15] TPD Fig. 2 NH 3 -TPD profiles of the catalysts Surface acidity of the catalyst plays a crucial role in the adsorption of hydrocarbons, [16] and hence effect the catalytic activity for hydrocarbon combustion. The surface acidic strength of catalysts were detected by NH 3 -TPD and shown in Fig. 2. All the samples exhibit a NH 3 desorption peak at about 200 C, implying weak acid sites on the catalysts. The peak at about 275 C increased gradually with the increase of W loading, indicating the addition of W enhanced the medium acid sites of catalysts. The peak at about 500 C 35

5 can be attributed to the strong acid specie, and the addition of W distinctly improved the strong acidity of catalysts. [17, 18] Since suitable acidity of catalysts were beneficial to improve the surface adsorption performance of propylene and CO, [16] and hence enhance the catalytic combustion activity. It can be inferred that suitable amount of W addition in the Pt/CZA catalyst can play a favorable role for adjusting the surface acidity of catalyst and hence improving the catalytic propylene and CO oxidation activity TPR Fig. 3 H 2 -TPR profiles of the catalysts H 2 -TPR was carried out to evaluate the reducibility of catalysts. As presented in Fig.3, Pt/CZA catalyst shows three broad peaks at about 100 C, 400 C and 600 C, respectively. The peak around 100 C are [6, 18, 19] attributed to the reduction of noble metal and support surface which shows metal-support interaction. The reduction peak appearing at about 400 C can be associated with surface reduction of CeO 2 -ZrO 2 solid solution, and the peak after 600 C was attributed to the bulk reaction of CeO 2 -ZrO 2 solid solution supports. [6] Because of appropriate amount (0.5 wt.% and 1.0 wt%) of W addition, Pt-0.5W/CZA and Pt-1.0W/CZA catalysts display a larger reduction peak at 100 C than other samples; however, increasing the amount of W cannot lead to increase the area of reduction peak at 100 C, but result in an additional 200 C reduction peak emergence. It can be seen that appropriate amount of W addition can promote the low temperature reduction of Pt/CZA catalyst, which is advantageous to the low temperature catalytic activity. 36

6 European International Journal of Science and Technology Vol. 6 No. 2 March Catalyst performance Fig. 4 C 3 H 6 (a) and CO (b) conversion on the fresh catalysts The relationship between C 3 H 6 conversion and reaction temperature on the fresh catalysts are shown in Fig. 4(a). It can be seen that C 3 H 6 conversion on all catalysts increase with the raising of temperature. The light-off temperature (the temperature that reactant conversion reaches 50%) of Pt/CZA is 268 C. For the W modified catalysts, Pt-0.5W/CZA shows the best catalytic performance, which reaches light-off temperature at 228 C. The light-off temperature of Pt-1.0W/CZA and Pt-1.5W/CZA are 241 C and 252 C, respectively. Further increasing W additive amount, the catalytic activity decreases; light-off temperature of Pt-3.0W/CZA for C 3 H 6 oxidation is 274 C, which is worse than Pt/CZA. Thus it can be seen that moderate amount of W addition in Pt/CZA catalyst can improve the catalytic activity, however excessive amount of W would causes the catalyst activity to decrease. Meanwhile, the catalytic activity of all samples for CO oxidation, as shown in Fig. 4(b), displays the same trend. 37

7 Fig. 5 C 3 H 6 (a) and CO (b) conversion on the aged catalysts The catalytic performance of simulative km vehicular aged catalysts are shown in Fig. 5. Compared with fresh samples, the catalytic activity of all catalysts are decreased after the simulative vehicular ageing. The C 3 H 6 light-off temperature of Pt/CZA(A) is 283 C. Pt-0.5W/CZA(A) and Pt-1.0W/CZA(A) catalysts show better activity than Pt/CZA(A), the light-off temperature for C 3 H 6 conversion are 270 C and 280 C, respectively. The Pt-1.5W/CZA(A) and Pt-3.0W/CZA(A) catalysts display worse performances than the Pt/CZA(A). Moreover, the catalytic performance for CO oxidation exhibits the same trend. Thus it is feasible to indicate that suitable amount of W addition in Pt/CZA catalyst is beneficial to promote the catalytic performance for diesel exhaust gases (C 3 H 6 and CO) oxidation. Taking both fresh and aged activity into account, in this work, W additive amount of 1.0 wt.% is the optimal proportion. 4. Conclusions From the above results, it can conclude that tungsten promoter in Pt/CeO 2 -ZrO 2 -Al 2 O 3 catalyst can exist in a highly dispersed state. Appropriate amount of tungsten in Pt/CeO 2 -ZrO 2 -Al 2 O 3 is advantageous to adjust the surface acidity and promote the low temperature reduction of catalyst, and hence improving the catalytic propylene and CO oxidation activity. The 0.5 wt.% amount of W addition can obviously enhance the catalytic performance of Pt/CeO 2 -ZrO 2 -Al 2 O 3 for propylene and CO oxidation; the Pt-1.0W/CeO 2 -ZrO 2 -Al 2 O 3 catalyst with 1.0 wt.% amount of W promoter exhibits a good activity and superior durability simultaneously; a much more amount of W promoter will decrease the catalytic performances, which probably because of the particle aggregation and phase separation of tungsten. 38

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