Preferential CO Oxidation in Hydrogen: Reactivity of Core−Shell Nanoparticles

Abstract

We report on the first-principles-guided design, synthesis, and characterization of core-shell nanoparticle (NP) catalysts made of a transition metal core (M = Ru, Rh, Ir, Pd, or Au) covered with a approximately 1-2 monolayer thick shell of Pt atoms (i.e., a M@Pt core-shell NP). An array of experimental techniques, including X-ray diffraction, Fourier transform infrared spectroscopy, high resolution transmission electron microscopy, and temperature-programmed reaction, are employed to establish the composition of the synthesized NPs. Subsequent studies of these NPs' catalytic properties for preferential CO oxidation in hydrogen-rich environments (PROX), combined with Density Functional Theory (DFT)-based mechanistic studies, elucidate important trends and provide fundamental understanding of the reactivity of Pt shells as a function of the core metal. Both the PROX activity and selectivity of several of these M@Pt core-shell NPs are significantly improved compared to monometallic and bulk nonsegregated bimetallic nanoalloys. Among the systems studied, Ru@Pt core-shell NPs exhibit the highest PROX activity, where the CO oxidation is complete by 30 degrees C (1000 ppm CO in H(2)). Therefore, despite their reduced Pt content, M@Pt core-shell NPs afford the design of more active PROX catalysts. DFT studies suggest that the relative differences in the catalytic activities for the various core-shell NPs originate from a combination of (i) the relative availability of CO-free Pt surface sites on the M@Pt NPs, which are necessary for O(2) activation, and (ii) a hydrogen-mediated low-temperature CO oxidation process that is clearly distinct from the traditional bifunctional CO oxidation mechanism.

CO Oxidation Mechanism on CeO2-Supported Au Nanoparticles

“…57 We found that this is the case for small Au NPs as well. As presented by Nilekar et al in transition metal-Pt shell NPs, 57 modifying Au NPs with alloying elements would be a promising way for the lower CO adsorption energy and Figure 7. CO oxidation by Au 13 of CeO 2 -supported Au 13 NPs is shown with Au 13 @CeO 2 -STO in (a) and Au 13 @CeO 2 -3VAC in (b).…”

Shape control of bimetallic nanocatalysts through well-designed colloidal chemistry approaches

“…A higher coordination number will lead to a deeper d-band center. 54 In addition, lattice strain caused by twinned defects 12 or the core-shell interface 5,55 has a strong influence on the interaction between two adjacent surface atoms and thus affects the electronic structure of surface atoms. If the lattice is compressed, the d-band center will be deeper.…”

Bifunctional anode catalysts for direct methanol fuel cells

“…16 Further, theoretical methods and the available computational power have reached a level where quantum mechanical simulations can be used for understanding and directing catalyst design. [17][18][19][20] Guided by first principle predictions of the CO tolerance of ternary Pt-Ru-M alloys, Strasser et al discovered an active class of Co-rich Pt-Ru-Co methanol oxidation catalysts. 21,22 The superiority of this system was later confirmed in an independent study by Cooper and McGinn.…”