Catalytic Activity of 0.4% Rh Supported ZrP 2 O 7 Catalyst for the NO-CO-C 2 H 6 -O 2 Reactions Under Modulated Air-to-Fuel Condition

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1 SUST Journal of Science and Technology, Vol. 21, No. 1, 214; P: Catalytic Activity of.4% Rh Supported ZrP 2 O 7 Catalyst for the NO-CO-C 2 H 6 -O 2 Reactions Under Modulated Air-to-Fuel Condition (Submitted: November 13, 213; Accepted for Publication: March 18, 214) Ahmed Jalal Samed Department of Chemistry, Shahjalal University of Science & Technology, Bangladesh. ajf-che@sust.edu Abstract Catalytic activity of the.4% Rh/ ZrP 2 O 7 catalyst for the NO-CO-C 2 H 6 -O 2 reactions under modulated air-to-fuel ratio (A/F) conditions were studied with a view to evaluate its catalytic performance under both oxidizing and reducing environment. In situ EXAFS experiments were carried out further to evaluate its catalytic performance. This newly developed catalyst exhibited outstanding 85% NO removal efficiency in oxidizing environment where conventional.4% Rh/ZrO 2 catalyst showed 45% NO removal efficiency. The strong Rh-O-P linkage has been confirmed by EXAFS investigation both in oxidizing and reducing environment as well, which is believed to be the crucial reason for exhibiting excellent catalytic performance of this catalyst. This newly developed catalyst having strong anchoring effect of Rh nano particles on ZrP 2 O 7 support along with high thermal stability has the potential to serve as a lean-burn denox catalyst. Keywords: ZrP 2 O 7, Modulated air-to-fuel, Oxidizing, Reducing, Lean-burn. 1. Introduction After more than two decades of stringent automotive pollution control, the abatement of NOx emission still continues to be difficult problem to tackle. Air-to-fuel (A/F) ratio during combustion is an important parameter of the internal combustion engines of automobiles and hence imparts substantial impacts on autocatalysis technology. The normal 14.6 part air to 1 part fuel ratio is considered stoichiometric (A/F=14.6) value which produces exhaust gas that contains the right balance of CO, H 2, and hydrocarbon (HC) and traditional three-way catalyst (TWC) works under this stoichiometric condition [1, 2]. But to keep pace with high fuel price, lean burn technology has been installed in the spark ignited gasoline engine to improve fuel economy and reduce CO 2 emission, which operate higher than stoichiometric A/F ratio having excess oxygen for better combustion of gasoline. The diesel engine also operates in excess oxygen and is famous for its high efficiency which attracts auto consumers in Europe. The conventional three-way catalyst (TWC) fed with precious noble metal becomes totally ineffective during leanmode due to presence of excess O 2 [3,4]. To improve the efficiency of TWC, researchers in academia and industry around the Globe are keep continuing their efforts, unfortunately feasible solution for lean burn engines still has to be found. Precious Rh becomes essential ingredients of current TWC to control NOx emission for its intrinsic activity to reduce NO selectively to N 2 [5-8]. The scarcity and high price of Rh make it desirable to find ways to minimize the Rh content in the catalytic converter. Only reducing the contributive amount of Rh can help preserving this scarcest metal as no suitable substitute has yet been developed. But in real practice the scenario is opposite, higher amount of Rh loading becomes usual. Lessening of precious metals in autocatalysis is urgently requested. One way to address this critical issue is to develop suitable support material for this precious Rh catalyst for satisfactory catalytic activity with threshold amount of precious metal. Ikeue et al. studied temperature programmed catalytic activity of different metal phosphates and focused on the importance of Rh supported metal phosphates [9]. Previously we reported a noble method to synthesize ZrP 2 O 7 by hydrothermal method followed by post-calcination [1], then studied thermal stability of this ZrP 2 O 7 as support for the.4% Rh metal [11]. Here I report the catalytic performance of the.4% Rh/ ZrP 2 O 7 catalyst using EXAFS experiment in oxidizing as well as reducing environment and also focus on catalytic activity of this newly developed catalyst under modulated A/F condition.

2 Catalytic Activity of.4% Rh Supported ZrP 2 O 7 Catalyst for the NO-CO-C 2 H 6 -O 2 33 Reactions Under Modulated Air-to-Fuel Condition 2. Experimental 2.1 Preparation of Catalyst First zirconium oxyhydroxide, {ZrO(OH) 2 } was prepared from zirconyl nitrate to use this as a precursor of Zr as follows. A solution of.2 mol zirconyl nitrate in 3 ml deionized water was prepared and stirred well for 1 min to get clear solution. Then, aqueous ammonia solution (1 M) was added dropwise slowly to this solution with vigorous stirring until ph of the solution was 1. This white gel thus obtained was filtered by centrifugal separation and washed with alkaline water several times and dried in air at 11 C for 12 h. ZrP 2 O 7 was synthesized hydrothermally. A 3.53 g of Zirconium oxyhydroxide was dispersed in 9. ml distilled water and stirred at room temperature (RT) for an half an hour. Then 5.76 g of H 3 PO 4 (85 %) acid was added drop wise slowly to this slurry and stirred 6 h at RT. Finally this composite was transferred to the teflon tube within the stainless steel autoclave and was heated in a oven by autogenious pressure at 2 C for 18 h without stirring. The white composite gel thus obtained centrifuged, washed with deionized water, dried over night in vacuum to get the final product. This product was further calcined in air at 9 C for 5h to get pure single phase of ZrP 2 O 7. Rh-loaded on ZrP 2 O 7 (.4 wt % as Rh metal) was prepared by impregnation of an aqueous solution of Rh(NO 3 ) 3, followed by drying in air (1 C, 1 h), then it was calcined in air at 6 C, for 3h. Details of the synthetic procedure was mentioned in our previous paper [9]. All chemicals used in this study were high purity (99 %) reagent grade and purchased from Wako Chemicals Ind. Ltd. Japan. 2.2 Characterization Powder X-ray diffraction (XRD) measurement was performed using monochromated Cu Kα radiation (3 kv, 2 ma, Rigaku Multiflex). Specific surface area was calculated by BET method (S BET ) from N 2 adsorption isotherms measured at 77 K (Belsorp, Bel Japan, Inc.). Microstructure observation was performed on a transmission electron microscope (TEM, FEI TECNAI F2, 2 kv). X-ray absorption fine structure, EXAFS of Rh K-edge was recorded on the BL14B2 station at SPring-8 of Japan Synchrotron Radiation Research Institute. A Si (311) double-crystal monochromator was used. The spectra were recorded at room temperature in a transmission mode. For in-situ EXAFS measurement, powder catalysts for EXAFS measurement were pressed into a disk about 1 g to give edge jump of.2. The incident and transmitted X- rays were monitored in 17- and 31-cm-long ionization chambers filled with Ar and Kr 3%+Ar 7% gasses, respectively. EXAFS data were processed using a REX 2 program (Rigaku). 2.3 Catalytic Measurement Catalytic tests were carried out under in a conventional flow reactor equipped with a water cooled infrared image furnace at atmospheric pressure under modulating A/F condition (13.7 A/F 15.1). During the experiment, a simulated exhaust gas mixture containing NO (.5%), CO (.51%), C 3 H 6 (.39%), O 2 (.1.8%), H 2 O (1%), and He (balance) supplied at 1 cm 3 min -1 (W/F = 5. X 1-4 g min cm -3 ) were used. In this mode, O 2 concentration in the gas feed was varied with time on stream, but concentrations of other gases (NO, CO, C 3 H 6, and H 2 O) and the rate of total gas feed were fixed at constant. The average rate of A/F scan was set at 2.2 h -1. The effluent gas was analyzed using a Pfeiffer GSD311 mass spectrometer and a Horiba VA3 NDIR gas analyzer. 3. Result and Discussion ZrP 2 O 7 Zr 3 (PO 4 ) 4 (c) Intensity (b) (a) θ /deg Figure 1: XRD Patterns of (a) as Synthesized Zr 3 (PO 4 ) 4 (b) ZrP 2 O 7 Calcined at 9 C 5h Air, and (c).4% Rh / ZrP 2 O 7

3 34 Ahmed Jalal Samed Figure 1 shows the XRD patterns of ZrP 2 O 7. Crystalline ZrP 2 O 7 single phase without any impurity appeared after calcinations at 9 C. As discussed in our previous paper, it belongs to cubic system of space group Pa 3 [1]. For our current catalytic application we utilized this pure crystalline ZrP 2 O 7 single phase as an active support of.4% Rh metal. After loading of.4% Rh by wet impregnation method, the ZrP 2 O 7 phase remains intact {Fig 1(c)}. This catalyst was found thermo stable up to 12 C [1] and is quite enough for high temperature application. ZrP 2 O 7 Rh R h 1nm Figure 2. TEM Images of.4% Rh/ZrP 2 O 7 after Ageing at 9 C for 25 h in 1% H 2 O/ Air. Figure 2 shows TEM micrograph. From this micrograph it is evident that Rh nano particles are well dispersed in nano size over ZrP 2 O 7 support. Conversion efficiencies of NO, CO, C 3 H 6 in a simulated exhaust gas composition with variable A/F ratio at fixed temperature were studied over.4% Rh/ZrP 2 O 7 and Rh/ZrO 2 catalyst in a flow reactor by scanning from fuel rich to fuel lean and reversing this scanning with a view to evaluate TWC performance. As ZrO 2 is a well known commercial support in auto-catalyst, I compared my result with this support material. Results are summarized in Fig wt% Rh/ZrO 2.4 wt% Rh/ZrP 2 O 7 1 reducing rich oxidizing lean 1 reducing rich oxidizing lean 8 8 NO Conversion / % CO C 3 H 6 NO Conversion / % CO C 3 H A/F A/F Figure 3. Catalytic Conversion of CO, NO, and C 3 H 6 at 39 C under Modulated A/F Condition in the Range 13.7 A/F 15.1.

4 Catalytic Activity of.4% Rh Supported ZrP 2 O 7 Catalyst for the NO-CO-C 2 H 6 -O 2 35 Reactions Under Modulated Air-to-Fuel Condition Figure 3 exhibits catalytic conversion of CO, NO, and C 3 H 6 at 39 C over.4% Rh/ZrP 2 O 7 and Rh/ZrO 2 catalyst when A/F was modulated in the range 13.7 A/F For each gas conversion, hysteresis behaviors were observed because of the different extent of surface oxidation depending on the direction of A/F scan. For both catalysts conversions of CO and C 3 H 6 were high at around stoichiometric and lean conditions (A/F 14.6), their conversions in a rich environment (reducing environment) deceased with decreasing A/F from The important difference is apparent in the lean environment (oxidizing environment). In the case of.4% Rh/ZrP 2 O 7 catalyst NO conversion was found 85%, whereas for the.4% Rh/ZrO 2 catalyst NO conversion decreased drastically to 45%. To justify this interesting and valuable findings next in-situ EXAFS were taken reducing the.4% Rh/ZrP 2 O 7 catalyst in a stream of 5% H 2 /He starting from room temperature to 8 C, then cooling at room temperature and subsequent oxidation in a stream of 5% O 2 /He from room temperature to 8 C. Results of this investigation are shown in figure 4. FT magnitude / a.u reduction 1 TPR in 5% H 2 /He Distance / Å 8 C 75 C 7 C 65 C 6 C 55 C 5 C 45 C 4 C 35 C 3 C oxidation 25 C 2 C 15 C 1 C RT TPRO in 5% O 2 /He Rh-O Rh-Rh Rh-O Rh-Rh Rh 3+ PO 4 Rh PO Distance / Å Figure 4: In-situ EXAFS of Rh/ZrP 2 O 7. Left Hand Side Represents Reduction Treatment and Right Hand Side Represents Subsequent Oxidation Treatment. During reduction treatment, peak intensity of Rh-O bond gradually decreased and Rh-Rh metallic bond distance increased followed by shifting of Rh-O bond distance to higher value. Interestingly at higher temperature during reduction treatment Rh-O peak did not disappear completely. After cooling down the system at room temperature when the catalyst was re-oxidized, peak intensity of Rh-Rh metallic bond distance decreased and Rh-O bond distance increased followed by shifting towards lower value. Again interestingly in oxidizing environment at higher temperature, Rh-Rh metallic bond did not disappear completely and found co-existed along with Rh-O bond. Based on the above observation it can be concluded that Rh-O-P linkage has been preserved by Rh/ZrP 2 O 7 catalyst under oxidizing and reducing environment. This anchoring effect of Rh nano particle on the support ZrP 2 O 7 is the key reason for exhibiting high NO removal efficiency in oxidizing environment (almost double) compared to traditional ZrO Conclusion Excellent catalytic activity of the.4% Rh/ ZrP 2 O 7 catalyst under both oxidizing as well as reducing environment has been reported here. Catalytic activity of traditional catalyst.4% Rh/ZrO 2 dropped remarkably for NO removal in oxidizing zone whereas this newly developed.4% Rh/ZrP 2 O 7 catalyst exhibited superb performance both in oxidizing as well as reducing environment keeping strong Rh-O-P linkage. This is due to the strong anchoring effect of Rh nano sized particles on ZrP 2 O 7 support material. In Addition Rh nano particles are well dispersed on the support ZrP 2 O 7, which enhances catalytic activity of this catalyst for reducing CO, C 3 H 6 also. This new findings will shed light on future generation de-nox catalyst. Local structure analysis and details of metalsupport interaction related experimental studies are currently under investigation.

5 36 Ahmed Jalal Samed Acknowledgement Author is grateful to Prof. Masato Machida, Kumamoto University for his cordial help and Prof K Ikeue for his valuable suggestions especially in EXAFS spectral analysis. References [1] Goula, G. Katzourakis, P. Vakakis, N. Papadam, T. Konsolakis, M. Tikhov, M. and Yentekakis, I. V. Catal. Today, 127, 199 (27). [2] Kaspar, J. Fornasiero, P. and Hickey, N. Catal. Today, 77, 419 (23). [3] Matsumoto, S. Yokota, K. Doi, H. Kimura, M. Sekizawa, K. and Kasahara, S. Catal. Today, 22, 127 (1994). [4] Yentekakisa, I. V. Telloub, V. Botzolakib, G. and Rapakousios, I. A. Appl. Catal. B 56, 229 (24). [5] Heck, R. M. Farrauto, R. J. and Gulati, S.T. Catalytic Air Pollution Control, page 522, 3rd Edition, John Willey & Sons, Hoboken, New Jersey, (29). [6] Shelef, M. Catal. Rev. Sci. Eng., 11, 1 (1975). [7] Taylor, K. C. Catal. Rev. Sci. Eng., 35, 457 (1993). [8] Gandhi, H. S. Graham, G. W. and McCabe, R. W. J. Catal., (23). [9] Ikeue, K. Muakami, K. Hinokuma, S. Uemura, K. Zhang, D. and Machida, M. Bull. Chem. Soc. Jpn., 83 (3) 291, (21). [1] Samed, A. J. Zhang, D. Hinokuma, S. and Machida, M. Journal of the Ceramic Society of Japan, 119, (211). [11] Samed, A. J. Uemura, K. Ikeue, K. and Machida, M. International Journal of Environmental Studies, 7(4), (213).