The Effect of Flame Temperature on Structure of Ceria-Supported Pt Nanoparticles in Flame-Assisted Spray Pyrolysis

Supported metal catalysts are used in many applications such as combustion gas purification catalysts, gas reforming systems, and chemical product synthesis processes. Supported metal catalysts generally have a structure with high specific surface area and high dispersion of precious metal to improve catalytic activity and economic efficiency. One of the big challenges in these catalysts is to prevent deterioration of catalytic activity caused by thermal aging and following aggregation of precious metal nanoparticles in high-temperature environment. To achieve both high thermal stability and high catalytic activity, it is necessary to properly control the oxide and metal particle sizes and metal/oxide interface structure. However, in the case of wet processes such as the impregnation method, precise structural control requires complex multi-step processes, which can be disadvantage toward practical application. The aim of this study is to synthesize supported metal catalysts with high catalytic activity and thermal stability in a simple process. For this purpose, we synthesized Pt/CeO₂ nanoparticles by flame-assisted spray pyrolysis using a burner diffusion flame, and the effects of material precursor concentration and flame conditions on the structure of the synthesized particles were investigated. In addition, the CO oxidation properties and thermal stability of the particles synthesized by flame-assisted spray pyrolysis were evaluated and compared with those of particles prepared by the impregnation method.<br/>A coaxial flow diffusion flame burner with a double-tube structure was used for the particle synthesis experiments. A mixture of oxygen and nitrogen is supplied to the outer tube, and a mixture of nitrogen and methane containing a precursor solution atomized by an ultrasonic transducer (2.4 MHz) is supplied to the inner tube. The synthesized Pt/CeO₂ particles consisted of submicron-scale CeO₂ with 10 nm-scale Pt and aggregation of fine CeO₂ less than 10 nm with highly dispersed Pt (1 nm or less) [1]. When the flame temperature was varied by changing the ratio of oxygen and nitrogen supplied to the burner outer tube, more fine particles were produced at higher flame temperatures. Submicron-level particles may have formed by the droplet-to-particle route, in which the precursor droplets do not evaporate completely and nucleation occurs within the droplet. On the other hand, fine nanoparticles may have formed by the gas-to-particle route, in which the droplets completely evaporate and nucleation occurs in the gas phase. The evaporation of precursor droplets is enhanced at higher flame temperature, resulting in the sift of particle formation route from droplet-to-particle to gas-to-particle. The fine Pt/CeO₂ nanoparticles synthesized by the high-temperature flame showed CO catalytic activity comparable to that of particles prepared by the impregnation method. Moreover, the thermal stability of the Pt particles on CeO₂ by flame synthesis was better than that of the impregnation method.<br/><br/>References:<br/>[1] T. Nagasawa, K. Matsumoto, N. Minegishi, H. Kosaka, <i>Energy Fuels</i> 35, 12380-12391 (2021)