Catalytic Activity and Surface Characterization Study of Pd Supported on Nanocrystalline and Polycrystalline CeO 2
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"**Departmentof Chemical Engineering, Tianjin University, Tianjin 300072, PRC ***AFRL/MLQ, 139 Barnes Drive, Tyndall AFB, FL 32403-5323, USA "****Departmentof Chemical Engineering, Box 870203, The University of Alabama, Tuscaloosa, AL 35487, USA
*****Darmstadt University of Technology, FB 21-Materials Science Department, Thin Films Division, Petersenstrasse 23, 64287 Darmstadt, GERMANY ABSTRACT The catalytic activity of polycrystalline and nanocrystalline CeO2-supported Pd (Pd/pCeO 2 and Pd/nCeO2) has been determined as a function of temperature and Pd loading. While the untreated nCeO2 support gives 50% methane conversion at 420'C, the untreated pCeO2 support exhibits little activity under the conditions examined due to its low surface area. A Pd loading of 5 wt% increases the activity of pCeO2 to 50% conversion at 2600C, while a 40 wt% Pd loading on nCeO2 exhibits a relatively smaller activity increase, yielding 50% conversion at 240°C. On a mass basis the 40 wt% Pd/nCeO2 catalyst is the most active tested in this study, but it is less active than the 5 wt% Pd/pCeO2 catalyst on a surface-area basis. Furthermore, the activity of the 40 wt% Pd/nCeO 2 catalyst does not decrease during 100 hrs of exposure to CH4 and 02 at 2500C. X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) have been used to characterize the surfaces of both bare supports and Pd-containing catalysts before and after
exposure to reactor conditions. The XPS results reveal that the Pd surface concentration is more than an order of magnitude higher for 5 wt% Pd/pCe02 than for 5 wt% Pd/nCeO2 due to the larger surface area of nCeO2 and that the 40 wt% Pd/nCeO2 catalyst has a lower Pd loading on a surfacearea basis than the 5 wt% Pd/pCeO2 catalyst. Most of the supported Pd is in the form of PdO, but
higher PdO2/PdO ratios are observed for both CeO2 supports compared to Pd supported on ZrO 2 or C0304. Furthermore, a significant amount of metallic Pd forms on Pd/nCeO2 but not on
Pd/pCeO2 during reaction. The nanocrystalline and polycrystalline CeO2 behave differently chemically which is consistent with the fact that the nanocrystalline catalysts are less active on a surface-area basis. Accumulation of H20 on the Pd/pCeO 2 surface during reaction is significant but not on the Pd/nCe02 surface. This suggests that the rate limiting step may be H20 desorption on Pd/pCeO2 while for Pd on nCe02 adsorption of methane appears to be the slow step. The ISS data indicate that the outermost atomic layer of Pd/nCeO 2 consists mostly of 0 and C, which is not the case for Pd/pCeO2. Site blockage by these species may also contribute to the lower activity on a surface-area basis of Pd/nCe02 compared to Pd/pCe02. INTRODUCTION The use of natural gas as an alternative automotive fuel could potentially lead to reduced levels of C02, NO,, and SOx emissions and decreased reliance on imported oil (1). However, an obstacle to widespread use of natural gas as an automotive fuel is the need to eliminate unburned methane from engine exhaust streams bec
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