Activation of Lattice Oxygen of TiO₂ by Pd²⁺ Ion: Correlation of Low-Temperature CO and Hydrocarbon Oxidation with Structure of Ti_1–xPd_xO_2–x (x = 0.01–0.03)

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.

Mn Ion substituted CeO2Nano spheres for Low Temperature CO Oxidation: The Promoting Effect of Mn Ions

“…It may be concluded that oxygen deficiency is more in Ce 0.80 Mn 0.20 O 2‐δ due the presence of Mn 2+ ions . The vacancy between Ce +4 and Mn +2 is one oxygen atom, which is most helpful for oxygen storage and loosing whereas Mn +3 , Mn +4 ions are not contribute the vacancy of oxygen atom . Based on above results Mn +2 content is increased from 10 % Mn ion substitution to 20 %, beyond that content of Mn +2 is decreasing.…”

“…Ce 0.90 Mn 0.10 O 2‐δ and Ce 0.80 Mn 0.20 O 2‐δ catalysts have Mn 2+ ion about 4.5 % and 15 % respectively, whereas, Ce 0.50 Mn 0.50 O 2‐δ does not have +2 oxidation state. It may be concluded that oxygen deficiency is more in Ce 0.80 Mn 0.20 O 2‐δ due the presence of Mn 2+ ions . The vacancy between Ce +4 and Mn +2 is one oxygen atom, which is most helpful for oxygen storage and loosing whereas Mn +3 , Mn +4 ions are not contribute the vacancy of oxygen atom .…”

High rates of catalytic hydrogen combustion with air over $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ Ti 0.97 Pd 0.03 O 2 - δ coated cordierite monolith

“…Binding energies of Ti(2p) core level are exactly the same that of in Ti 0.97 Pd 0.03 O 2−δ powder. 23 In Figure 5( 23 The oxidation state of Pd ion will be in +2 state, but not +4 state in Ti 1−x Pd x O 2−δ , as was found by the extensive studies on Pd earlier. 30 3.4 H 2 + O 2 recombination reaction 0.5 mL of pure H 2 was injected into the 2 L reservoir and circulated over a single monolith at a flow rate of 2 L/min.…”

“…In an earlier study, Pd was dispersed in TiO 2 in ionic state and high catalytic activity for CO oxidation and hydrocarbon oxidation have been observed. 23 In present study, we expect high catalytic activity of Ti…”

Noble metal ions in CeO2and TiO2: synthesis, structure and catalytic properties

“…Reprinted from[48] and[55] with permission of American Chemical Society. Reprinted from[48] and[55] with permission of American Chemical Society.…”