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Studies in Surface Science and Catalysis, volume 147 X. Bao and Y. Xu (Editors) 9 Elsevier B.V. All rights reserved.

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Effect of support structure on methane combustion over PdO/Zr02 L.-F. Yang*, Y.-X. Hu, D. Jin, X. -E He, C.-K. Shi, J.-X. Cai Department of Chemistry, State Key Laboratory of Physical Chemistry for Solid Surface, Xiamen University, Xiamen 361005, China 1. INTRODUCTION Recently the catalytic combustion technology has been widely applied for various industrial processes [1-3]. For higher energy transfer efficiency and lower emission of air pollutants, it has long been necessary in practice to develop a catalyst with better ignition activity. Since palladium has been known to be the most active component for methane combustion, consequently investigations on catalyst support become rather attractive [4-6]. In previous studies, palladium loaded on conventional oxide support materials, such as ZrO2, A1203 and SnO2 demonstrated fairly good catalytic activity for methane combustion. The exclusion of SiO2 as a candidate for the support reveals that the surface structure of oxide support influences on the PdO dispersion and its reactivity [7]. An appropriate interaction between PdO and surface lattice of oxide gives rise to more active sites for methane combustion. As elucidated by Juszczyk's in-situ infrared spectroscopic experiments, the active PdO phase is stabilized by insertion of the Pd 2+ into A106 octahedron. The extent of this interaction between support and active metal component (SMI) is apt to be affected by the kind of support, preparation parameter, thermal history, and even the palladium source. In the end, the effect of surface lattice on PdO distribution is the determinate [8]. In order to ascertain the relationship between PdO phase and support structure, Pd catalysts with Pd loaded on alumina supports with various crystal structure have been widely investigated in which the template effect of oxide surface lattice is evident. In this study, we have investigated Pd supported on zirconia support with different crystal structure for low temperature combustion of methane, and emphasis was given to the relationship between active phase and support structure. Corresponding author. Tel: +86-592-2185944; Fax: +86-592-2183047; E-mail address: [email protected]

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2. EXPERIMENTAL 2.1. Sample preparation Zirconia supports with monoclinic or monoclinic-tetragonal mixed lattice were synthesized by co-precipitation method from ZrO(NO3)2 aqueous solution. After calcination at an assigned temperature, these support powders were dipped into aqueous solution of palladium nitrate. The wet precursor was dried and calcined at 800~ for 5 hours in air. The loading of palladium in the resultant catalysts was from 0.5wt.%, l wt.% to 3wt.%. The samples are designated as [Pd wt.% ]Pd/[support type (m/t+m)]-ZrO2. 2.2. Characterization of catalysts The phase component of calcined catalyst was analyzed by XRD method, XRD patterns were recorded with a Rigaku Rotaflex D/Max-C Diffractometer with scanning rate of 4deg./min using monochromatic CuKoradiation (X =0.15406 nm). Specific surface area was measured by nitrogen adsorption technique at liquid nitrogen temperature on a Carlo Elba 1900 Sorptomatic instrument. The dispersion of palladium component of the catalyst was measured by pulse-adsorption of CO in a flow of carrier gas. Prior to measuring, the catalyst was reduced by hydrogen at 400 ~ The dispersion of palladium was calculated from the CO uptake by assuming a stoichiometry of [CO]/Pd surface=1. 2.3. Measurement of activity Catalytic activity tests were performed using a quartz tubular fixed bed microreactor equipped with a sheathed thermocouple monitoring the reaction temperature. Combustion reaction was conducted at atmospheric pressure, and a gaseous mixture of CH4 (2 vo1.%), 02 (8 vo1.%) and N2 (90 vol.%) was supplied at a space velocity of 48,000h -~. Methane conversion in the effluent gas was analyzed by an on-line gas chromatograph (Shimadzu GC-14B) equipped with a thermal conductivity detector (TCD) 2.4. Temperature-programmed reaction The temperature-programmed desorption (TPD) of oxygen was carried out in a flow system to observe oxygen desorption from the catalysts. About 50mg catalyst particles (dp 0.25~0.45mm) were fixed in the middle of the quartz tubular reactor by packing quartz wool at both ends. Prior to the experiment, the sample was oxidized in an air flow (20ml/min) at 800~ for 30min.After cooling to room temperature, the purging gas was switched to helium (impurity< lppm). The sample bed was heated at a rate of 20~ after gaseous oxygen was purged out. During this heating stage, the concentration of oxygen in the effluent gas was monitored by a quadrupole mass spectrometer (Balzers QMS

471 200 Omnistar). The acidity sites on catalysts and supports were characterized by ammonia temperature-programmed desorption (NH3-TPD) experiments, which were identical to O2-TPD except by dilute ammonia was used in the pretreatment. The signal of fragment with m/e=15 reflected the NH3 concentration in effluent so that it was not interfered by the water signals. 3. RESULTS AND DISCUSSION

3.1. Catalytic activity for methane combustion XRD patterns (Fig. 1) show that the thermodynamically metastable t-ZrO2 remains about 30 at.% when the final catalysts are acquired. On both monoclinic and monoclinic-tetragonal mixed supports, PdO presents no characteristic reflection even when the Pd loading attains 3 wt.%. This implies that PdO fails to aggregate into large crystallite on zirconia support. Further investigation by CO pulse reveals that the dispersion of Pd component on these two zirconia supports is quite different. It is shown in Table 1 that the monoclinic zirconia evidently promotes the dispersion of palladium component thereon. This is more pronounced on the lower Pd-loading samples. In the case of 0.5wt.% Pd supported on monoclinic zirconia, the dispersion is 23.8%, nearly quadruple the value for the sample supported on t+m type zirconia, of which the specific surface area is even three fold larger than that for the m-type support. The high dispersion of palladium on the less available surface implies a favorable interaction between PdO and m-ZrO:.

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472 Table 1 Characteristics of Pd/ZrO2 catalysts SSA m2/g

Samples 3Pd/m-ZrO2 1Pd/m-ZrO2 0.5Pd/m-ZrO2 3Pd/t+m-ZrO2 1Pd/t+m-ZrO2 0.5Pd/t+m-ZrO2

Pd dispersion % 5.0 14.2 23.8 2.8 3.1 6.3

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Co-feeding of methane and air was adopted to simulate the operating condition of methane combustion. An obvious improvement in the ignition performance was acquired and a growth of activity increased rapidly by reaction temperature for the case of m-ZrO2 supported catalysts, especially, for the lower Pd-loading samples. These results were shown in Fig.2. The promotion of support structure on metal dispersion undoubtedly contributes to the catalytic activity. Considering that the nature of active site for methane combustion is still obscure at present, Pd dispersion is unable to reflect directly the number of active sites; therefore, it seems unreasonable to evaluate the catalytic activity by turnover frequency.

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Fig. 3. O2-TPD patterns of supported Pd catalysts in sequence from top to bottom: 3Pd/m-ZrO2, 1Pd/m-ZrO2, 0.5Pd/m-ZrO2, 3Pd/t+m-ZrO2, 1Pd/t+m-ZrO2, 0.5Pd/t+m-ZrO2 Fig. 4. NH3-TPD patterns of supported Pd catalysts in sequence from top to bottom: 3Pd/t+m-ZrO2, 1Pd/t+m-ZrO2, 0.5Pd/t+m-ZrO2, t+m-ZrO2

3.2. PdO species on the ZrO2 By means of O2-TPD technique (Fig. 3), several PdO species that could be separated by thermal stability were revealed on the surface of zirconia support. It is evident that there is a rather stable oxygen species on the surface of PdO/t+m-ZrO2. Comparative blank experiment with t+m-ZrO2 excludes that oxygen desorbs from the support. So the oxygen desorption at high temperature originates from the PdO species intensively interacted with zirconia. This PdO species is unseen on the m-ZrO2 supported catalyst. It is thought to be related to the decrease of Pd dispersion and catalytic activity of t+m-ZrO2 supported catalysts. In the catalytic redox cycle, the reducibility of active PdO phase is predominant for the overall activity. The excessively strong Pd-O bond in the intensively interacted PdO species leads to its inertia to catalytic reaction. Combined the catalytic activity and O2-TPD patterns, a consistent relationship between ignition activity and the oxygen desorption at moderate temperature emerges. This could be explained by the viewpoint of our previous study [9,10],

474 that the well-crystallized PdO phase by epitaxy meet the requirement of structural sensitivity for methane activation and become the main active phase for methane combustion. 3.3. Acid effect on Pd distribution

A comparative study about these two supports would be rather informative. For tetragonal zirconia it is meta-stable under low temperature (<1100), plenty of acidity sites caused by charge inhomogeneity have been detected on its surface by NH3-TPD technique, uniformity for PdO distribution was disrupted thereby. As shown by NH3-TPD (Fig. 4), the Pd component preferentially situates on the acidic site. This inhomogeneity leads to the decrease of Pd dispersion; moreover, catalytic activity may be depressed by reducing the ratio of sensitive crystal plane, which meets the requirement on the activation of methane. 4. CONCLUSION Palladium component acquires better dispersion on the monoclinic zirconia support than on the support containing tetragonal phase, since the m-ZrO2 offers uniform potential field on the surface. And as the result of stacking the PdO layer epitaxially along the m-ZrO2, a family of crystal planes, which was sensitive on activation of methane, is formed; therefore, an obvious improvement on catalytic activity for methane combustion is achieved. ACKNOWLEDGEMENTS The financial support and funding given to the G1999022400 and 20203014 projects by MoST (National Ministry of Science and Technology) and NSFC (National Natural Science Foundation) of China are gratefully acknowledged. REFERENCES

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