No abstract

Lattice oxygen of TiO 2 is activated by the substitution of Pd ion in its lattice. Ti 1−x Pd x O 2−x (x = 0.01−0.03) have been synthesized by solution combustion method crystallizing in anatase TiO 2 structure. Pd is in +2 oxidation state and Ti is in +4 oxidation state in the catalyst. Pd is more ionic in TiO 2 lattice compared to Pd in PdO. Oxygen storage capacity defined by "amount of oxygen that is used reversibly to oxidize CO" is as high as 5100 μmol/g of Ti 0.97 Pd 0.03 O 1.97 . Oxygen is extracted by CO to CO 2 in absence of feed oxygen even at room temperature which is more than 20 times compared to pure TiO 2 . Rate of CO oxidation is 2.75 μmol g −1 s −1 at 60 °C over Ti 0.97 Pd 0.03 O 1.97 and C 2 H 2 gets oxidized to CO 2 and H 2 O at room temperature. Catalyst is not poisoned on long time operation of the reactor. Such high catalytic activity is due to activated lattice oxygen created by the substitution of Pd ion as seen from first-principles density functional theory (DFT) calculations with 96 atom supercells of Ti 32 O 64 , Ti 31 Pd 1 O 63 , Ti 30 Pd 2 O 62 , and Ti 29 Pd 3 O 61 . The compounds crystallize in anatase TiO 2 structure with Pd 2+ ion in nearly square planar geometry and TiO 6 octahedra are distorted by the creation of weakly bound oxygens. Structural analysis of Ti 31 Pd 1 O 63 which is close to 3% Pd ion substituted TiO 2 shows that oxygens associated with both Ti and Pd ions in the lattice show bond valence sum of 1.87, a low value characteristic of weak oxygen in the lattice compared to oxygens with valence 2 and above in the same lattice. Exact positions of activated oxygens have been identified in the lattice from DFT calculations.

A series of Ce1‐xMnxO2‐δ (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5) nanocatalysts were prepared by solution combustion synthesis. XRD results confirmed the absence of bulk manganese oxide in Ce1‐xMnxO2‐δ indicating the substitution of Mn ion in ceria matrix. The percentage C and N doping in Ce1‐xMnxO2‐δ was quantified with a CHN analyzer, whereas, the mesoporous nature of the catalysts was confirmed by N2 physisorption analysis. Raman, H2‐TPR results confirmed the increasing oxygen vacancies on Mn substitution, whereas, UV‐VIS DRS and XPS confirmed the multiple oxidation states of Mn ion. SEM and TEM analysis highlighted the near spherical morphology and nanoparticle size, respectively. The rate of CO oxidation was found to be the highest for Ce1‐xMnxO2‐δ (x=0.2), which was assigned due to the combined effect of highest amount of Mn+2 and/or due to the highest oxygen vacancies.

Palladium substituted in cerium dioxide in the form of a solid solution, Ce 0·98 Pd 0·02 O 1·98 is a new heterogeneous catalyst which exhibits high activity and 100% trans-selectivity for the Heck reactions of aryl bromides including heteroaryls with olefins. The catalytic reactions work without any ligand. Nanocrystalline Ce 0·98 Pd 0·02 O 1·98 is prepared by solution combustion method and Pd is in +2 state. The catalyst can be separated, recovered and reused without significant loss in activity.

Ti0.97Pt2+ 0.03O1.97 and Ti0.97Pt4+ 0.03O2 have been synthesized by a solution combustion method using alanine and glycine as the fuels, respectively. Both crystallize in anatase TiO2 structure with 15 nm average crystallite size. X-ray photoelectron spectroscopy (XPS) confirmed Pt ions are in the 2+ state in Ti0.97Pt0.03O1.97 (alanine) and 4+ state in Ti0.97Pt0.03O2 (glycine). The rate of CO oxidation occurring over Ti0.97Pt2+ 0.03O1.97 (0.76 μmol·g–1·s–1) is ∼10 times more than that over Ti0.97Pt4+ 0.03O2 at 60 °C (0.08 μmol·g–1·s–1). A large shift in 100% hydrocarbons conversion to lower temperature was observed for Pt2+ ion-substituted TiO2 relative to that for Pt4+ ion-substituted TiO2. After reoxidation of the reduced compound by H2 as well as CO, Pt ions are stabilized in mixed valences, 2+ and 4+ states. The role of oxide ion vacancy has been demonstrated by CO oxidation and H2 + O2 recombination reactions in the presence and absence of O2. We analyze the activated lattice oxygens upon substitution of Pt2+ and Pt4+ ions in TiO2, using first-principles density functional theory (DFT) calculations with supercells of Ti31Pt1O63, Ti30Pt2O62, and Ti29Pt3O61 for Pt2+ ion substitution and Ti31Pt1O64, Ti30Pt2O62, and Ti29Pt3O61 for Pt4+ ion substitution in TiO2. We find that the local structure of Pt2+ ion has a distorted square planar geometry and that of Pt4+ ion has an octahedral geometry similar to that of Ti4+ ion in pure TiO2. The change in coordination of Pt2+ ion gives rise to weakly bonded oxygens, and these oxygens are involved in high rates of catalytic reaction. Thus, the high catalytic activity results from synergistic roles of Pt2+ ion and oxide ion vacancy and weakly bonded lattice oxygen.