Collapse in crystalline structure and decline in catalytic activity of Pt nanoparticles on reducing particle size to 1 nm.

Combined experimental and computational studies show that, upon reducing the diameter of Pt nanoparticles down to 1 nm, a collapse in the crystalline structure occurs spontaneously and the thus-induced quantum size effect causes a decline in the catalytic activity toward H2 electrooxidation. Platinum has been playing a vital role in heterogeneous catalysis, especially for low-temperature fuel cells. To maximize Pt utilization, nanoparticles are usually applied, with the particle size expected to be as small as possible. However, the particle-size dependent catalytic activity of Pt, determined by the interplay of surface geometric and electronic factors, is not quite straightforward in the range of a few nanometers.1-4 For COads oxidation,1 methanol oxidation,2 or oxygen reduction,3 the catalytic activity of Pt nanoparticles was found, by most studies, to decrease on reducing the particle size, and the maximum massspecific activity (MSA) was estimated to appear at around 3 nm.2b,3d For hydrogen oxidation reaction (HOR), the situation seems rather different; the catalytic activity of Pt nanoparticles was found to be almost unchanged or even increasing upon reducing the particle size.4 Thus it is still suspected whether the Pt utilization for fuel cell hydrogen anodes can be further increased by reducing the Pt particle size down to 1 nm or even smaller. To explore the possible lower size limit of Pt as a HOR catalyst and to reveal the underlying science is not only a yet unresolved topic of great concern for fuel cell developers but also a general subject of fundamental and technological significance. In the present work, we report a successful preparation of ultrafine Pt/C catalysts with the Pt particle size reduced to about 1 nm, and the resulting changes in the crystalline structure, electronic structure, and catalytic activity toward HOR. The ultrafine Pt nanoparticles were synthesized using the improved impregnation method reported in our previous work5a and by employing mesoporous carbon black CMK-3 as the catalyst support, which was produced according to the well-documented template method.5b,c The BET-specific surface area of CMK-3 is about 1200 m2/g, almost five times of that of XC-72, the most frequently used carbon support for fuel cell catalysts. It turned out that such a high specific surface area greatly enhanced the dispersion of Pt nanoparticles. As shown in Figure 1a, Pt/XC-72, synthesized by the same procedure, shows a clear Pt face-centered cubic (fcc) structure in its X-ray diffraction (XRD) pattern, with a particle size estimated from the line width to be 2.5 nm; whereas Pt/CMK-3 totally loses the crystalline feature in its XRD spectrum, indicating that the Pt particles dispersed on CMK-3 should be extremely small and/or in a noncrystalline state. Figure 1b shows a representative high-resolution transition electron microscopy (HRTEM) image of Pt particles dispersed on CMK-3. In the center of this image, a dark spot, without any crystalline pattern, can be well distinguished from the graphited carbon background and estimated to be around 1 nm in diameter. The lack of crystalline pattern in the particle image is not due to the resolution limitation of the microscope (JEOL JEM-2010FEF, with an ultimate spatial resolution of 0.19 nm) but rather reflects the amorphous nature of the Pt particles. In some locations of the same sample, a few bigger particles (greater than 1.5 nm in diameter) with clear crystalline pattern can be observed (Figure 1c). It can be concluded from the HRTEM observation that most Pt particles in Pt/CMK-3 are around 1 nm in diameter and, more importantly, in an amorphous structure. Such noncrystalline Pt particles are very unusual and, therefore, need further confirmation. An alternative explanation of the noncrystalline spots seen in the HRTEM image might be Pt compounds, such as Pt precursors, instead of metallic Pt. To ascertain the metallic nature of the platinum in the sample, X-ray photoelectron (XPS) examinations have been conducted to compare the oxidation state of Pt in the catalyst before and after the H2 reduction procedure in the preparation. As revealed by the results in Figure S1 and Table 1, it is evident that metallic Pt was formed after H2 reduction. Note that there were also PtO and PtO2 components after H2 reduction, but they should be ascribed to the surface oxidation of Pt nanoparticles on exposure to air. An especially fundamental question is whether the formation of noncrystalline Pt nanoparticles is a natural consequence of reducing the particle size down to 1 nm. To unveil the atomic packing behavior in a Pt nanoparticle as a function of the particle size, molecular dynamic (MD) simulations were carried out using the semiempirical effective medium theory developed by Nørskov and co-workers,7a which has been proven to be accurate in describing structural and thermal properties of fcc metals.7 Figure 1. (a) XRD patterns of Pt/XC-72 and Pt/CMK-3. Pt loadings in both catalysts are 20 wt %. (b) A representative HRTEM image of the Pt particles dispersed on CMK-3. (c) HRTEM images of crystalline Pt particles rarely found in Pt/CMK-3. Published on Web 11/22/2007