Article pubs.acs.org/JPCC
Catalytic Behavior of Flame-Made Pd/TiO2 Nanoparticles in Methane Oxidation at Low Temperatures Fang Niu, Shuiqing Li,* Yichen Zong, and Qiang Yao †
Key laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: A controlled one-step synthesis of titania-supported palladium (Pd/TiO2) nanoparticles is demonstrated by using a premixed stagnation swirl flame with an ultrafine spray feeding system. The new method produces well-dispersed palladium clusters on the surface of TiO2 particle. The low-temperature methane catalytic oxidation indicates that the flame-made Pd/TiO2 possesses activity apparently higher than that of the impregnated one under the same Pd loading. For the first time, we observe a special pinched hysteresis loop of catalytic activity of Pd/TiO2 nanoparticles from the curves of CH4 conversion versus temperature during heating−cooling cycles. Thermal stability analysis confirms the complex coexistence and transformation of metallic Pd and PdO in Pd/TiO2 particles. Further morphological characterization reveals the oxidation rate is controlled by the metallic Pd content on the particle surface, whereas the deactivation of the catalysts is mainly due to the Pd dispersion decrement. The competition between the Pd dispersion decrement and the PdO reduction at the surface may result in the pinched hysteresis loop of Pd/TiO2 catalysts during low-temperature methane combustion.
1. INTRODUCTION In recent decades, methane oxidation over supported palladium (Pd) catalysts has been extensively studied because of the fact that, among various noble metals, palladium-based catalysts show the highest activity even at reaction temperatures below 600 °C.1,2 The catalytic reactions can shorten the ignition delay time, reduce the ignition temperature, and lead to obtaining complete fuel oxidation at low temperatures with low fuel-to-air ratios. Therefore, the catalysts cover a wide range of applications, such as ignition improvement in supersonic engines, abatement of unburned methane from lean-burn natural gas vehicles, and controlling the formation of CO and NOx emissions.3−5 So far, most studies of nanocatalysts have adopted wet chemistry routes such as sol−gel methods, impregnation, and precipitation processes because of the ability to design and prepare materials with atomic scale accuracy via selfassembly.6−9 However, wet chemistry routes still not only suffer from multistep treatments and long preparation time but also encounter a bottleneck of their scaling-up from lab to industrial applications. More recently, the gaseous aerosol method, e.g., liquid-fed flame synthesis (also known as flame spray pyrolysis), has offered the possibility of precisely controlling the crystal growth and particle size during continuous, high-throughput operation.10,11 We demonstrated the production of ultrafine TiO2 nanocrystals (
