Atomic regulation of metal catalysts, especially of the active surface, is key to optimizing the catalytic performance. In this work, we tuned surface Pd coordination by varying bismuth loadings in the Pd−Bi alloy system, facilitating different catalytic performances for propane dehydrogenation (PDH) and acetylene semihydrogenation model reactions. In situ X-ray absorption spectroscopy, atom-resolved scanning transmission electron microscopy combined with elemental distribution analysis, infrared spectroscopy, and in situ X-ray photoelectron spectroscopy were employed to characterize the evolution of the surface and bulk structures in Pd−Bi catalysts with changing Bi composition. At low Bi loading, the catalyst nanoparticle (NP) surface was partially transformed into the Pd−Bi intermetallic compound (IMC). The partially alloyed surface has improved catalytic performance compared with Pd NPs. At slightly higher Bi loading, a Pd core−Pd 3 Bi shell structure was formed, which displayed significantly improved selectivity rate and stability. In the Pd 3 Bi IMC surface structure, the adjacent Pd atoms are sufficiently far apart to give catalytically isolated active sites, which significantly enhance the selectivity (>95%) to propylene in PDH and give a higher ethylene selectivity (80%) for acetylene semihydrogenation compared with Pd NPs. At higher Bi loading, a full Pd 3 Bi is formed; however, at even higher loading, an overcoating of excess BiO x leads to a loss in activity. This work demonstrates that in intermetallic alloy catalysts, the surface and bulk structures of the NPs are different with different promoter metal loadings. Importantly, the catalyst performance is not only determined by the alloy structure but also can be significantly affected by the properties of the noncatalytic oxide promoter.

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 P2 2– 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.

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The addition of Pd to Pt-based diesel oxidation catalysts is known to enhance performance and restrict the anomalous growth of Pt nanoparticles when subjected to aging at high temperatures in oxidative environments. To gain a mechanistic understanding, we studied the transport of the mobile Pt and Pd species to the vapor phase, since vapor phase transport is the primary route for sintering in these catalysts. The results are surprising: there is a 30-fold drop in the effective vapor pressure of Pt in the Pt−Pd catalysts compared to monometallic Pt. At the same time, there is a significant enhancement in the vapor pressure of Pd, compared to PdO, which otherwise has a negligible vapor pressure at the aging temperature. Such behavior cannot be explained simply by alloying Pt and Pd in the metallic phase, or a core−shell morphology where a PdO shell covers a Pt core. Transmission electron microscopic examination of catalysts aged up to 50 h in air at 800 °C shows that the particles exhibit a biphasic "Janus"-like structure. The metal and oxide phases are conjoined, exposing a metal and an oxide face to the gas phase. The high mobility of the Pt and Pd allows them to be partitioned into the metal and oxide phases, in apparent thermodynamic equilibrium. The PdO helps to trap mobile PtO 2 and as a result contains high concentrations of Pt oxide, consistent with its role in mitigating the transport of Pt to the vapor phase and preventing the growth of anomalously large particles. In turn, Pt allows Pd to remain metallic, allowing the catalyst to retain both metal and oxide functionality for catalysis. The regeneration of deactivated catalysts typically requires an external input, such as a change in the working environment from reducing to oxidizing or vice-versa. Here, we show that the mobile species, which are primary contributors to catalyst sintering are effectively returned to the active site, hence our use of the term "selfhealing". The detailed insights into the inner workings of the Pt−Pd diesel oxidation catalysts can help provide clues to the design of robust and durable heterogeneous catalysts.