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PtO2 crystallizes in the hexagonal P6_3mc space group. The structure is two-dimensional and consists of two PtO2 sheets oriented in the (0, 0, 1) direction. Pt4+ is bonded to six equivalent O2- atoms to form edge-sharing PtO6 octahedra. All Pt–O bond lengths are 2.06 Å. O2- is bonded in a distorted trigonal non-coplanar geometry to three equivalent Pt4+ atoms.
PtO2 is Hydrophilite structured and crystallizes in the orthorhombic Pnnm space group. The structure is three-dimensional. Pt4+ is bonded to six equivalent O2- atoms to form a mixture of corner and edge-sharing PtO6 octahedra. The corner-sharing octahedral tilt angles are 55°. All Pt–O bond lengths are 2.04 Å. O2- is bonded in a distorted trigonal planar geometry to three equivalent Pt4+ atoms.
PtO2 is Cuprite structured and crystallizes in the cubic Pn-3m space group. The structure is three-dimensional. Pt4+ is bonded to four equivalent O2- atoms to form corner-sharing PtO4 tetrahedra. All Pt–O bond lengths are 1.94 Å. O2- is bonded in a linear geometry to two equivalent Pt4+ atoms.
K3(PtO2)2 crystallizes in the orthorhombic Cmcm space group. The structure is three-dimensional. there are two inequivalent K sites. In the first K site, K is bonded in a 6-coordinate geometry to six equivalent O atoms. There are a spread of K–O bond distances ranging from 2.86–2.91 Å. In the second K site, K is bonded in a 4-coordinate geometry to four equivalent O atoms. All K–O bond lengths are 2.69 Å. There are two inequivalent Pt sites. In the first Pt site, Pt is bonded in a distorted square co-planar geometry to four equivalent O atoms. All Pt–O bond lengths are 2.02 Å. In the second Pt site, Pt is bonded in a distorted square co-planar geometry to four equivalent O atoms. All Pt–O bond lengths are 2.04 Å. O is bonded to four K and two Pt atoms to form a mixture of distorted face, edge, and corner-sharing OK4Pt2 octahedra. The corner-sharing octahedra tilt angles range from 0–70°.
Zn(PtO2)3 crystallizes in the orthorhombic Cmmm space group. The structure is three-dimensional. there are two inequivalent Pt+3.33+ sites. In the first Pt+3.33+ site, Pt+3.33+ is bonded to six O2- atoms to form a mixture of edge and corner-sharing PtO6 octahedra. The corner-sharing octahedral tilt angles are 59°. There are four shorter (2.05 Å) and two longer (2.07 Å) Pt–O bond lengths. In the second Pt+3.33+ site, Pt+3.33+ is bonded in a square co-planar geometry to four equivalent O2- atoms. All Pt–O bond lengths are 1.99 Å. Zn2+ is bonded in a distorted body-centered cubic geometry to eight O2- atoms. There are four shorter (2.20 Å) and four longer (2.45 Å) Zn–O bond lengths. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded to three Pt+3.33+ and one Zn2+ atom to form a mixture of distorted edge and corner-sharing OZnPt3 tetrahedra. In the second O2- site, O2- is bonded to two equivalent Pt+3.33+ and two equivalent Zn2+ atoms to form a mixture of edge and corner-sharing OZn2Pt2 tetrahedra.
PtO2 is Hydrophilite structured and crystallizes in the tetragonal P4_2/mnm space group. The structure is three-dimensional. Pt4+ is bonded to six equivalent O2- atoms to form a mixture of edge and corner-sharing PtO6 octahedra. The corner-sharing octahedral tilt angles are 53°. There are two shorter (2.01 Å) and four longer (2.04 Å) Pt–O bond lengths. O2- is bonded in a trigonal planar geometry to three equivalent Pt4+ atoms.
CoPt3O6 crystallizes in the orthorhombic Cmmm space group. The structure is three-dimensional. Co2+ is bonded in a body-centered cubic geometry to eight O2- atoms. There are four shorter (2.19 Å) and four longer (2.41 Å) Co–O bond lengths. There are two inequivalent Pt+3.33+ sites. In the first Pt+3.33+ site, Pt+3.33+ is bonded in a square co-planar geometry to four equivalent O2- atoms. All Pt–O bond lengths are 1.99 Å. In the second Pt+3.33+ site, Pt+3.33+ is bonded to six O2- atoms to form a mixture of distorted edge and corner-sharing PtO6 octahedra. The corner-sharing octahedral tilt angles are 60°. There are four shorter (2.05 Å) and two longer (2.06 Å) Pt–O bond lengths. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded to one Co2+ and three Pt+3.33+ atoms to form a mixture of distorted edge and corner-sharing OCoPt3 tetrahedra. In the second O2- site, O2- is bonded to two equivalent Co2+ and two equivalent Pt+3.33+ atoms to form a mixture of edge and corner-sharing OCo2Pt2 tetrahedra.
CaPt2O4 crystallizes in the tetragonal P4_2/mmc space group. The structure is three-dimensional. Ca2+ is bonded in a body-centered cubic geometry to eight equivalent O2- atoms. All Ca–O bond lengths are 2.50 Å. Pt3+ is bonded in a square co-planar geometry to four equivalent O2- atoms. All Pt–O bond lengths are 2.01 Å. O2- is bonded in a see-saw-like geometry to two equivalent Ca2+ and two equivalent Pt3+ atoms.
CdPt3O6 crystallizes in the orthorhombic Cmmm space group. The structure is three-dimensional. there are two inequivalent Pt+3.33+ sites. In the first Pt+3.33+ site, Pt+3.33+ is bonded to six O2- atoms to form PtO6 octahedra that share corners with two equivalent PtO6 octahedra, edges with four equivalent CdO8 hexagonal bipyramids, and edges with two equivalent PtO6 octahedra. The corner-sharing octahedral tilt angles are 53°. There are two shorter (2.04 Å) and four longer (2.07 Å) Pt–O bond lengths. In the second Pt+3.33+ site, Pt+3.33+ is bonded in a square co-planar geometry to four equivalent O2- atoms. All Pt–O bond lengths are 2.01 Å. Cd2+ is bonded to eight O2- atoms to form distorted CdO8 hexagonal bipyramids that share edges with two equivalent CdO8 hexagonal bipyramids and edges with eight equivalent PtO6 octahedra. There are four shorter (2.33 Å) and four longer (2.53 Å) Cd–O bond lengths. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded to three Pt+3.33+ and one Cd2+ atom to form a mixture of distorted edge and corner-sharing OCdPt3 tetrahedra. In the second O2- site, O2- is bonded to two equivalent Pt+3.33+ and two equivalent Cd2+ atoms to form a mixture of distorted edge and corner-sharing OCd2Pt2 tetrahedra.
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 PtO2 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.