Structural and Catalytic Properties of Isolated Pt<sup>2+</sup> Sites in Platinum Phosphide (PtP<sub>2</sub>)
Jiajing Kou, Johnny Zhu Chen, Junxian Gao, Xiaoben Zhang, Jie Zhu, Arnab Ghosh, Wei Liu, A. Jeremy Kropf, Dmitry Zemlyanov, Rui Ma, Xinwen Guo, Abhaya K. Datye, Guanghui Zhang, Liejin Guo, Jeffrey T. Miller
Abstract
This article describes the synthesis and catalytic properties of supported, 2–3 nm platinum phosphide (PtP2) nanoparticles (NPs). Depending on the P loading, two PtP2 structures are formed, that is, a PtP2 surface on a (metallic) Pt core (Pt@PtP2) and single-phase PtP2 NPs. The structures were determined using extended X-ray absorption fine structure , in situ synchrotron X-ray diffraction, and scanning transmission electron microscopy. In PtP2 NPs, Pt2+ ions are geometrically isolated by P22– ions, at a Pt–Pt distance of 4.02 Å, which is much longer than 2.78 Å in (metallic) Pt NPs. The oxidation state of Pt in PtP2 NPs was determined by in situ X-ray absorption near-edge structure and in situ X-ray photoelectron spectroscopy and was found to be consistent with Pt2+ ions even after treatment in H2 at 550 °C. Unlike Pt NPs, which are highly active for propylene hydrogenation at room temperature, PtP2 NPs are not active below about 150 °C, suggesting the absence of metallic surface Pt. In contrast to metallic Pt, which is poorly selective for acetylene hydrogenation, PtP2 NPs display high selectivity toward ethylene. PtP2, also has high olefin selectivity for propane dehydrogenation, although the rate per g Pt is about 7 times lower than that of metallic Pt NPs of the same size. In situ resonant inelastic X-ray scattering spectroscopy shows that the energy of the filled Pt 5d valence orbitals is 1.5 eV lower than that of metallic Pt, which leads to weaker adsorbate binding consistent with its catalytic properties. A H2-stable Pt2+ site suggests different catalytic applications for these catalysts as compared to Pt NPs.