Design of atomistic models of the little-known palladium oxide PdO 2 - PowerPoint PPT Presentation

Design of atomistic models of the little-known palladium oxide PdO 2 Diana Fabušová 1 , K. Tokár 1,2 , M. Derzsi 1,3 1 Advanced Technologies Research Institute, Faculty of Materials Science and Technology in Trnava, Slovak University of Technology in Bratislava, 917 24 Trnava, Slovakia 2 Institute of Physics, Slovak Academy of Sciences, 845 11 Bratislava, Slovakia 3 Center of New Technologies, University of Warsaw, Żwirki i Wigury 93, 02089 Warsaw, Poland diana.fabusova@gmail.com

INTRODUCTION • Palladium and its oxides are important catalyst in many catalytic reactions with diversity of technological applications. • The only well studied and technologically exploited phase of palladium with oxygen is palladium monoxide PdO. • One polymorph of palladium dioxide PdO 2 was also reported. It was obtained in high-p,T synthesis [ Shaplygin, 1978 ], but it is poorly characterized . • Existence of only one PdO 2 polymorph is surprising since at least 5 polymorphs are known for Pt dioxide PtO 2 . • Our aim is to predict other stable PdO 2 polymorphs using Density Functional Theory modelling. The 2nd International Online Conference on Crystals 10-20 Diana Fabušová: PdO2 from ab initio 2 NOVEMBER 2020 ONLINE

PdO 2 modelled in known MO 2 structures • 19 unique structural types were taken from ICSD Modelled types # 1 VO 2 crystallographic database 2 CaCl 2 • DFT optimization was performed for the models after 3 CoO 2 4 Cdl 2 substitution of the metal atom (M) for Pd: M → Pd 5 BaSi 2 6 Ni 0,5 Mn 1.5 O 4 • Enthalpies of formation were calculated: 7 MnO 2 • in respect to elements: 8 α -PbO 2 9 NbO 2 Pd+O 2 -> PdO 2 ( Δ H 1 ) 10 Rutil 11 VO 2 (oF96) • in respect to PdO: 12 PdF 2 PdO + ½ O 2 -> PdO 2 ( Δ H 2 ) 13 Brookite 14 Anatase 15 VO 2 (aP12) • Relative stability at high pressures. 16 VO 2 (HT) 17 Fluorit • Dynamical Stability calculations. 18 Pyrit 19 HgO 2 The 2nd International Online Conference on Crystals 10-20 Diana Fabušová: PdO2 from ab initio 3 NOVEMBER 2020 ONLINE

PdO 2 modelled in known MO 2 structures Modelled types # The modelled structures belong to 6 structural families. 1 VO 2 2 CaCl 2 • Basic building block in 3 CoO 2 Structural families 4 Cdl 2 all modelled strucures is 5 BaSi 2 6 Ni 0,5 Mn 1.5 O 4 octahedron [MO 6 ] Porous structures 7 MnO 2 8 α -PbO 2 Layered structures 9 NbO 2 10 Rutil Rutile group 11 VO 2 (oF96) O 12 PdF 2 Post-rutiles M 13 Brookite 14 Anatase Fluorite group 15 VO 2 (aP12) 16 VO 2 (HT) Complex structures 17 Fluorit 18 Pyrit 19 HgO 2 The 2nd International Online Conference on Crystals 10-20 Diana Fabušová: PdO2 from ab initio 4 NOVEMBER 2020 ONLINE

CRYSTAL STRUCTURES Rutile group • Formed by infinite chains of edge-shared octahedra. • The chains are interconnected through corners. linear chains Zig-zag chains Types: rutile, CaCl 2 , α -PbO 2 , NbO 2 , Types: brookite The 2nd International Online Conference on Crystals 10-20 Diana Fabušová: PdO2 from ab initio 5 NOVEMBER 2020 ONLINE

CRYSTAL STRUCTURES Post-rutile group • In the post-rutile structures, the rutile-like chains of octahedra share apart from corners, also edges. • They are known also as compressed rutiles. Types: VO 2 (HT) Types: anatase The 2nd International Online Conference on Crystals 10-20 Diana Fabušová: PdO2 from ab initio 6 NOVEMBER 2020 ONLINE

CRYSTAL STRUCTURES Layered structures • Formed by layers of octahedra sharing edges. • Various stackings of layers are possible (AAA or ABAB stacking). • The layers can be interconnected by peroxo-bridges. Peroxo-bridge Type: HgO 2 Types: CoO 2 , CdI 2 , BaSi 2 The 2nd International Online Conference on Crystals 10-20 Diana Fabušová: PdO2 from ab initio 7 NOVEMBER 2020 ONLINE

CRYSTAL STRUCTURES Porous structures • Characteristic feaure of porous structures is presence of empty channels (1D voids). • The stuctures are formed by single or double rutile-like chains sharing corners. Type: V O 2 Type: MnO 2 Type: Ni 0.5 Mn 1.5 O The 2nd International Online Conference on Crystals 10-20 Diana Fabušová: PdO2 from ab initio 8 NOVEMBER 2020 ONLINE

COMPUTER DETAILS • All structure models were optimized with DFT functional PBE modified for solids (PBEsol). • All models were optimized in the pressure range 0-100 kb. • DFT calculation were performed in program VASP. • Lattice dynamics was calculated in the program Phonopy. • Visualization of crystal structures was done in VESTA. The 2nd International Online Conference on Crystals 10-20 Diana Fabušová: PdO2 from ab initio 9 NOVEMBER 2020 ONLINE

PdO 2 : ENTHALPY OF FORMATION Δ H 1 : Pd+O 2 → PdO 2 0,0 • All models stable in respect to Pd and O 2 19 • 13 models stable in respect to PdO and O 2 -0,3 18 Δ H 1 [eV/FU] • All structures observed for PtO 2 are stable -0,6 also for PdO 2 17 • Rutile structure (#10) is not the ground 16 15 -0,9 state 14 • Lowest-E structure is VO 2 type -1,2 13 9 10 11 12 -1,5 8 7 6 5 3 4 2 1 The 2nd International Online Conference on Crystals 10-20 Diana Fabušová: PdO2 from ab initio 10 NOVEMBER 2020 ONLINE

PdO 2 : ENTHALPY OF FORMATION Δ H 1 : Pd+O 2 → PdO 2 1,1 0,0 • All models stable in respect to Pd and O 2 19 Δ H 2 [eV/FU] • 13 models stable in respect to PdO and O 2 -0,3 0,8 18 Δ H 1 [eV/FU] • All structures observed for PtO 2 are stable -0,6 0,5 also for PdO 2 17 • Rutile structure (#10) is not the ground 16 15 -0,9 Δ H 2 : PdO+ ½ O 2 → PdO 2 0,2 state 14 • Lowest-E structure is VO 2 type -1,2 -0,1 13 9 10 11 12 -1,5 -0,4 8 7 6 5 3 4 2 1 The 2nd International Online Conference on Crystals 10-20 Diana Fabušová: PdO2 from ab initio 11 NOVEMBER 2020 ONLINE

PdO 2 : ENTHALPY OF FORMATION Δ H 1 : Pd+O 2 → PdO 2 1,1 0,0 • All models stable in respect to Pd and O 2 Palladium peroxides (O 2 2- ) 19 Δ H 2 [eV/FU] are unstable • 13 models stable in respect to PdO and O 2 -0,3 0,8 18 Δ H 1 [eV/FU] • All structures observed for PtO 2 are stable -0,6 0,5 also for PdO 2 17 • Rutile structure (#10) is not the ground 16 15 -0,9 Δ H 2 : PdO+ ½ O 2 → PdO 2 0,2 state 14 • Lowest-E structure is VO 2 type -1,2 -0,1 13 9 10 11 12 -1,5 -0,4 8 7 6 5 3 4 2 1 The 2nd International Online Conference on Crystals 10-20 Diana Fabušová: PdO2 from ab initio 12 NOVEMBER 2020 ONLINE

PdO 2 : ENTHALPY OF FORMATION Δ H 1 : Pd+O 2 → PdO 2 1,1 0,0 • All models stable in respect to Pd and O 2 19 Δ H 2 [eV/FU] • 13 models stable in respect to PdO and O 2 -0,3 0,8 18 Δ H 1 [eV/FU] • All structures observed for PtO 2 are stable -0,6 0,5 also for PdO 2 17 • Rutile structure (#10) is not the ground 16 15 -0,9 Δ H 2 : PdO+ ½ O 2 → PdO 2 0,2 state 14 • Lowest-E structure is VO 2 type -1,2 -0,1 13 9 10 11 12 -1,5 -0,4 8 7 6 5 3 4 2 1 Polymorphs observed in PtO 2 The 2nd International Online Conference on Crystals 10-20 Diana Fabušová: PdO2 from ab initio 13 NOVEMBER 2020 ONLINE

PdO 2 : ENTHALPY OF FORMATION Δ H 1 : Pd+O 2 → PdO 2 1,1 0,0 • All models stable in respect to Pd and O 2 19 Δ H 2 [eV/FU] • 13 models stable in respect to PdO and O 2 -0,3 0,8 18 Δ H 1 [eV/FU] • All structures observed for PtO 2 are stable -0,6 0,5 also for PdO 2 17 • Experimentally observed rutile structure 16 15 -0,9 Δ H 2 : PdO+ ½ O 2 → PdO 2 0,2 (#10) is not the ground state 14 • Lowest-E structure is VO 2 type -1,2 -0,1 13 9 10 11 12 -1,5 -0,4 8 7 6 5 3 4 2 1 Rutile type (observed in experiment) The 2nd International Online Conference on Crystals 10-20 Diana Fabušová: PdO2 from ab initio 14 NOVEMBER 2020 ONLINE

PdO 2 : ENTHALPY OF FORMATION Δ H 1 : Pd+O 2 → PdO 2 1,1 0,0 • All models stable in respect to Pd and O 2 19 Δ H 2 [eV/FU] • 13 models stable in respect to PdO and O 2 -0,3 0,8 18 Δ H 1 [eV/FU] • All structures observed for PtO 2 are stable -0,6 0,5 also for PdO 2 17 • Rutile structure (#10) is not the ground 16 15 -0,9 Δ H 2 : PdO+ ½ O 2 → PdO 2 0,2 state 14 • Lowest-E structure is VO 2 type -1,2 -0,1 13 9 10 11 12 -1,5 -0,4 8 7 6 5 3 4 2 1 VO 2 type (ground state) Rutile type The 2nd International Online Conference on Crystals 10-20 Diana Fabušová: PdO2 from ab initio 15 NOVEMBER 2020 ONLINE

PdO 2 : DYNAMICAL STABILITY • We have calculated impact of atomic Phonon dispersion curves of PdO 2 in VO 2 type vibrations on crystal stability by calculation (dynamically stable) of phonon dispersion curves. • Positive (real) values of energies indicate dynamical stability. • Negative (imaginary) values of energies indicate dynamical instability. • All PdO 2 models that are stable in respect to PdO are also dynamically stable . • Only rutile structure is dynamically unstable . • Possible reasons: • Failure of DFT method, • different structure observed in experiment. The 2nd International Online Conference on Crystals 10-20 Diana Fabušová: PdO2 from ab initio 16 NOVEMBER 2020 ONLINE