(734b) Activity and Thermal Stability in Well-Defined Platinum/Palladium Bimetallic Catalysts for Hydrocarbon Combustion
Proposed Section: Rational Catalyst Design I
(1)Chemical Engineering, Stanford University, Stanford, CA (2)Department of Chemical Engineering and Material Science, University of California-Irvine (3)SLAC National Accelerator Laboratory, Menlo Park, CA (4)Department of Physics and Astronomy, University of California-Irvine
Activity and Thermal Stability in Well-Defined Platinum/Palladium Bimetallic Catalysts for Hydrocarbon Combustion
Although there is an increasing effort to take advantage of inexpensive base metals in the development of modern materials, rare and expensive metals continue to permeate the fields of thermal and electrocatalysis. Noble metal catalysts, especially when composed of two or more precious metals, often show remarkable activity, making the use of these more expensive materials economically viable. Nevertheless, it remains challenging to establish clear structure-activity-stability relationships using traditional co-impregnation techniques, due to the coexistence of multiple active nanoparticles of different sizes, shapes, and compositions. In this work, we synthesize model bimetallic catalysts with well-defined compositions and properties in order to systematically investigate activity and stability properties in bimetallic catalysts.
We demonstrate a procedure to synthesize small (2.2-3.4 nm), monodisperse bimetallic nanocrystals catalysts, of tunable composition, in order to understand which properties confer advantages in terms of activity for lean methane combustion, with and without the presence of steam. At low temperatures, all bimetallic catalysts show activity similar to Pt/Al2O3. Although such bimetallic catalysts are uniformly much less active than pure Pd/Al2O3catalyst under dry conditions, they are more active at low temperatures in the presence of steam. In terms of stability, the bimetallic nanocrystal catalysts possess unique, composition-dependent stability properties. Despite lower initial combustion rates, hydrothermal aging of the Pd-rich bimetallic catalyst induces both sintering and segregation of a PdO phase in close contact to a Pd/Pt alloy phase, forming more active and highly stable sites for methane combustion.
Approaching this stability problem from the other direction, we investigate fundamental sintering processes in well-defined two-phase catalysts formed by colloidally synthesizing each active phase independently. We carefully distinguish between two fundamental sintering modes in platinum catalysts: low-temperature, surface-mediated diffusion, and high-temperature vapor-phase transport of platinum species. By using highly active palladium catalysts as thermodynamic âplatinum-trapsâ, we can indirectly probe the trajectory of platinum species. Understanding the atomic mechanisms of platinum sintering as a function of temperature and reaction environment can allow us to develop approaches to mitigate these detrimental aging effects. This work correlates activity and stability properties of Pt/Pd bimetallic systems with specific compositions and structures, which provides important guidelines for the preparation of highly active materials with optimal utilization of expensive noble metals.