Activation of Lattice Oxygen of TiO2 by Pd2+ Ion: Correlation of Low

Nov 12, 2012 - Peng Zhang , Yanlong Yu , Enjun Wang , Jingsheng Wang , Jianghong Yao , and Yaan Cao. ACS Applied Materials & Interfaces 2014 6 (7), ...
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Activation of Lattice Oxygen of TiO2 by Pd2+ Ion: Correlation of Low-Temperature CO and Hydrocarbon Oxidation with Structure of Ti1−xPdxO2−x (x = 0.01−0.03) Bhaskar Devu Mukri,† Gargi Dutta,† Umesh V. Waghmare,‡ and M. S. Hegde*,† †

Solid State and Structural Chemistry Unit, Indian Institute of Science Bangalore 560012 Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India

‡

ABSTRACT: Lattice oxygen of TiO2 is activated by the substitution of Pd ion in its lattice. Ti1−xPdxO2−x (x = 0.01−0.03) have been synthesized by solution combustion method crystallizing in anatase TiO2 structure. Pd is in +2 oxidation state and Ti is in +4 oxidation state in the catalyst. Pd is more ionic in TiO2 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 Ti0.97Pd0.03O1.97. Oxygen is extracted by CO to CO2 in absence of feed oxygen even at room temperature which is more than 20 times compared to pure TiO2. Rate of CO oxidation is 2.75 μmol g−1 s−1 at 60 °C over Ti0.97Pd0.03O1.97 and C2H2 gets oxidized to CO2 and H2O 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 Ti32O64, Ti31Pd1O63, Ti30Pd2O62, and Ti29Pd3O61. The compounds crystallize in anatase TiO2 structure with Pd2+ ion in nearly square planar geometry and TiO6 octahedra are distorted by the creation of weakly bound oxygens. Structural analysis of Ti31Pd1O63 which is close to 3% Pd ion substituted TiO2 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. KEYWORDS: solution combustion method, TiO2, CO and HC oxidation, XPS, DFT

1. INTRODUCTION Among the noble metals or platinum group metals (PGM) for catalysis, palladium is the cheapest and most commonly used metal. There exists an extensive literature on Pd-catalyzed chemical reactions. For example, 5%Pd/carbon is well-known for hydrogenation catalysis.1 Direct synthesis of hydrogen peroxide from H2 and O2 is catalyzed by PdO on fluorinated Al2O3 and ZrO2,2 Au−Pd alloy supported on zeolites, TiO2 or Fe2O3 or acid treated carbon.3−6 High turnover frequency for solvent free oxidation of alcohols to aldehydes is achieved on Au−Pd/TiO2.7 Pd/TiO2 catalyst modified by adsorption of triphenyl phospine and phenyl sulfide enhanced selectivity in acetylene hydrogenation.8 Surfaces of the support were thus made acidic such that Pd can be in partial ionic state. Palladium-catalyzed reactions of organic halides with olefins pioneered by Richard Heck is well-known where palladium acetate is the material catalyzing C−C coupling reactions.9 Recently, we have shown that Pd2+-ion-substituted CeO2 is highly active for Heck coupling reactions.10 To understand the catalytic action and mechanism, it is desirable to know the nature of Pd in the catalyst whether in ionic or partially ionic or zerovalent state. We have been pursuing the idea of noble metal ions as the active sites for catalytic oxidation−reduction reactions.11 Noble metal ions are stabilized by the substitution of noble metals in reducible oxides such as CeO2 forming single phase Ce1−xMxO2−δ (M = Ru, Rh, Pd, Pt).11 Pd-substituted CeO2 has been studied for activation of C−H bonds in CH4.12 TiO2 is a nontoxic reducible © 2012 American Chemical Society

oxide support and noble metal ions can be substituted in TiO2. Indeed, Ti1−xPdxO2−x (x = 0.01) where Pd is in +2 oxidation state showed high rates of NOx reduction by CO,13 high rates of hydrogen−oxygen recombination at 40−50 °C,14 and good photo catalytic activity for CO oxidation.15 In a multicomponent catalyst, it is difficult to identify active sites and also elucidate mechanism of catalytic reactions. Instead, if we can design a uniform solid catalyst16,17 where the catalyst is a single phase solid having definite structure, it is possible to identify active sites for catalysis. For example, oxygen associated with square planar (Cu(I)) in YBa2Cu3O7 is identified as the weak lattice oxygen with oxygen bond valence sum equal to 1.85 is selectively utilized for CO oxidation.17,18 Such an approach can lead to rational synthesis of new catalysts. Activation of lattice oxygen in TiO2 with substitution19 of Au and noble metals in ZnO has been shown by DFT calculations.20 Earlier works have shown that noble metal ion in CeO2 activates the lattice oxygen.21,22 With this background, we have synthesized single phase Ti1−xPdxO2‑δ (x = 0.01, 0.02, 0.03). Three atomic percent Pd-ion-substituted TiO2, namely, Ti0.97Pd0.03O1.97, showed extremely high rates of CO and hydrocarbon oxidation at 40−50 °C. Here we show that high rates of CO oxidation activity of Ti1−xPdxO2−δ is due to activation of the lattice oxygen by the Received: June 2, 2012 Revised: November 6, 2012 Published: November 12, 2012 4491

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Table 1. Rietveld Refined Lattice Parameters of TiO2 and Ti1−xPdxO2−δ (x = 0, 0.01, 0.02, and 0.03) catalysts

a

c

cell volume

Rf

RB

TiO2 Ti0.99Pd0.01O2‑δ Ti0.98Pd0.02O2‑δ Ti0.97Pd0.03O2‑δ

3.779(1) 3.779(0) 3.779(0) 3.778(1)

9.497(1) 9.491(1) 9.487(0) 9.473(0)

135.62 135.54 135.48 135.21

2.8 3.8 3.9 2.7

2.6 2.8 3.1 2.5

Figure 2. Powder XRD patterns of (a) 3% PdO impregnation on TiO2, (b) 3% Pd metal impregnation on TiO2, and (c) 3% Pd ion substituted TiO2. (sigma aldrich), and 1.11 mmol of glycine (SD Fine Chem. Limited) as fuel in a 300 mL crystallizing dish. The components were fully dissolved in 15 mL of water. The solution was kept in preheated furnace at 350 °C. The combustion takes place after the dehydration and the solid product is left behind. Similarly, pure TiO2 and 1 at % and 2 at % Pd in TiO2 were prepared by taking corresponding stoichiometric amounts of precursors. To compare catalytic properties of Pd-ion-substituted TiO2, we prepared 3 at % PdO as well as 3 at % Pd metal impregnated on TiO2 as follows: pure TiO2 was first prepared by combustion method. Pd(NO3)2 solution to the extent of 3 at % Ti taken as TiO2 mixed with stoichiometric amount of glycine was added to pure TiO2; it was stirred well and combustion reaction was carried out at 350 °C. The oxidized Pd on TiO2 was obtained by this method. PdO on TiO2 was then reduced to Pd metal by hydrazine hydrate solution at room temperature. The solid was separated and dried at 110 °C. XRD patterns were recorded on a Philips X’Pert Diffractometer at a scan rate of 0.20°/min with a 0.017° step size in the 2θ range between 10 and 90°. X-ray photoelectron spectra were recorded on Thermo Fisher Scientific Multilab 2000 instrument with Al Kα (1486.6 eV) X-ray. Binding energy (BE) of the core levels reported here are with reference to C(1s) peak at 284.5 eV. For transmission electron microscopy, compound was dispersed in acetone and dropped on the holey carboncoated Cu-grid and recorded the images by FEI Tecnai 20 instrument at 200 kV. For catalytic study, catalyst was loaded in a quartz micro reactor of 30 cm length and 0.4 cm diameter plugged with ceramic wool at both ends. CO and hydrocarbon (HC) oxidation were carried out over 150 mg of the catalyst at a gas flow rate of 100 cc/min. CO oxidation reaction was carried out with stoichiometric ratio of CO: O2 (1:1/2 vol %) at a space velocity of 47 770 h−1. A mixture of

Figure 1. Rietveld refined XRD profile of (a) Ti0.99Pd0.01O2−δ, (b) Ti0.98Pd0.02O2−δ ,and (c) Ti0.97Pd0.03O2−δ.

substitution of Pd2+ ion in TiO2 from a combined study of catalysis, crystal structure, electronic structure, and first-principles DFT calculations.

2. EXPERIMENTAL SECTION Three compounds Ti1−xPdxO2−δ (x = 0.01, 0.02 and 0.03) were prepared by solution combustion method.23 In a typical synthesis for Ti0.97Pd0.03O2−δ, we have taken 9.7 mmol of TiO(NO3)2 solution, which is prepared by Ti(OC3H7)4 (sigma Aldrich), 0.3 mmol of Pd(NO3)2 4492

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Figure 3. (a) Bright-field image of Ti0.97Pd0.03O2−δ and electron diffraction (inset); (b) HRTEM image of Ti0.97Pd0.03O2−δ and (c) energy-dispersive X-ray analysis (EDX) from the image. ions following the bond-valence method. Bond valence S is defined as (a) S = (R/R0)−N (for Ti−O bonds) and (b) S = exp[−(R − R0)/B] (for Pd−O bonds), where R is the bond length, R0 is the length of a bond of unit valence, and N and B are fitted parameters.27 The atomic valence V is obtained by summing the bond valences associated with a particular ion, given by V = ∑iSi. The bond-valence method has been earlier used to probe the weakly bound oxygens in Ce1−xZrxO228 and Ce1−xTixO229 to correlate oxygen storage capacity (OSC). For the relaxed structures, we used finer mesh of k-points (6 × 6 × 3 mesh) to determine the electronic density of states (DOS) and partial density of states (PDOS). These calculations are performed using postprocessing modules of the Quantum-ESPRESSO package.30 As the substitution of Pd at Ti sites accompanied by introduction of oxygen vacancies can result in deviation from the ideal oxidation states and changes in the occupation of d-orbitals of the transition metals, we assessed sensitivity of our results with calculations carried out including the onsite correlations with Hubbard U parameter of 5 eV.

ethylene (875 ppm), acetylene (875 ppm), propylene (470 ppm), propane (470 ppm), CO2 (12.4%), and rest helium calibrated gas mixture were obtained from Chemix Specialty Gases and Equipment, India. 5% O2 in helium was also obtained. Gaseous products were analyzed by an online gas chromatograph (Mayura analytical PVT LTD, India) with TCD (thermal conductivity) and FID (flame ionization) detectors. DFT Calculations. Our total energy calculations are based on DFT with a local density approximation (LDA) to the exchange correlation energy of electrons. Interaction between valence electrons and ionic cores is treated using ultrasoft pseudopotentials.24 Single-particle Kohn−Sham wave functions (density) are represented with a plane wave basis truncated with an energy cutoff of 30 Ry (180 Ry). We use PWSCF25 implementation of the DFT with periodic boundary conditions. Pd substitution in TiO2 is treated using periodic tetragonal supercells of 32 formula units (96 atoms) of anatase TiO2 (a = 7.57 Å, c =19.03 Å). Structure of four cells Ti31Pd1O63, Ti31Pd1O60, Ti30Pd2O62, and Ti29Pd3O61 are optimized until the forces on atoms are less than 1 mRy/bohr. One oxygen vacancy has been created per Pd2+ ion substitution. We have tried two different configurations of positions of the vacancy and Pd for the substitution in Ti29Pd3O61 supercell. Integrals over the Brillouin zone were sampled on 4 × 4 × 2 k-point Monkhorst−Pack26 mesh. The bond distances determined from the optimized structures are used to calculate valencies of oxide

3. RESULTS AND DISCUSSION XRD, TEM and XPS studies. The Rietveld refined XRD profiles of Pd2+ ion substituted TiO2, namely, Ti1−xPdxO2−x (x = 0.01, 0.02, and 0.03) are shown in Figure1. The patterns are 4493

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Figure 5. % CO Conversion over (a) Ti1−xPdxO2−δ (x = 0.01, 0.02, and 0.03) and (b) 3% PdO on TiO2 and 3% Pd metal on TiO2.

square planar as seen from the DFT calculations presented later in this paper. Pd metal in contact with TiO2 induces strong metal support interaction.32 Pd metal particle loaded on TiO 2 upon CO adsorption reduces Ti 4+ ion gradually. In the combustion synthesis of Ti 1−xPdxO 2−x, Pd metal particles are not found and Pd ions are substituted for Ti4+ sites. Isolated reduction of TiO2 to Ti4O7 phases are not reached in these compounds. We have examined XRD patterns of 3 at % PdO and 3 at % Pd metal impregnated on TiO2. Powder XRD patterns of PdO on TiO2 and Pd metal on TiO2 are given in panels a and b in Figure 2, respectively. Three atomic percent Pd-ionsubstituted TiO2 is shown in Figure 2c (same as Figure 1c) for ready comparison. Diffraction lines of PdO are present when PdO was loaded in TiO2 (Figure 2a). Pd metal lines are observed in the Pd metal impregnated TiO2 (Figure 2b). But in the combustion synthesized sample (Figure 2c), Pd metal or PdO diffraction lines are absent. Thus, the presence of PdO and Pd metal diffraction lines in panels a and b in Figure 2, respectively, in the impregnated samples and their absences in the combustion synthesized sample (Figure 2c) demonstrates the substitution of Pd in TiO2 and the formula can be written as Ti1−xPdxO2−δ (x = 0.01, 0.02, 0.03). The crystallite sizes were determined by the Scherrer’s formula: size (t) = 0.9λ/βcosθ,33 where, λ is the wavelength of X-ray (Cu Kα), β is the full width at half maxima (fwhm) in radians and θ is the diffraction angle. fwhm was calculated by using the equation β = (Utan2 θ + Vtan θ + W)1/2. U, V, and W values were obtained from the Rietveld refinement. The average crystallite size of these compounds is found to be in the range of 13 ± 2 nm. Crystallite sizes agree with the high-resolution transmission electron microscopy (HRTEM) images of Ti0.97Pd0.03O2−δ shown in images a and b in Figure 3.

Figure 4. Pd (3d) core level XPS of (a) Ti1−xPdxO2−δ (x = 0.01, 0.02, and 0.03), (b) 3% PdO impregnation on TiO2, and (c) 3% Pd metal impregnation on TiO2.

indexed to Anatase TiO2 (Tetragonal, I41/amd, JCPDS no. 21−1272). One to two atomic percent Pd metal or PdO can be detected by slow scan in the XRD. Further, in Rietveld refined profile, difference plot would give distinct peaks due to Pd metal or PdO if present. Indeed, 1−3 at % Pd metal or PdO impregnate on TiO2 show XRD lines due to Pd metal or PdO. Absence of Pd metal or PdO diffraction lines in Figure 1a−c indeed confirm Pd ion is substituted in TiO2. Cell parameters obtained by Rietveld refinement are given in Table 1. There is very little change in the lattice parameters compared to pure anatase TiO2. Ionic radius of 6 coordinated Ti4+ ion is 0.605 Å and that of Pd2+ ion is 0.86 Å.31 If Pd ion was substituted for Ti4+ ion in six coordination, there should have been an increase in lattice parameters or cell volume. On the contrary, there is little variation in the lattice parameters and if at all, a slight decrease in cell volume from 135.6 to 135.5 Å3 is observed. Therefore, Pd ion in these oxides is not in octahedral coordination. Ionic radius of Pd2+ ion in square planar coordination is 0.64 Å,31 which is close to Ti4+ ion in octahedral coordination. Becuse for every Pd2+ ion substitution one oxide ion vacancy needs to be created for charge balance, Pd is likely to be in square planar coordination. Indeed, Pd is found to be nearly 4494

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Figure 6. (a) Fractional CO conversion VS W/F plot for Ti0.97Pd0.03O2−δ; (b) rate vs temperature plot derived from slopes of % CO conversions over Ti0.97Pd0.03O2−δ; inset: ln(r) vs 1000/T plot of CO oxidation over Ti0.97Pd0.03O2−δ..

Selected area electron diffraction pattern given in the inset of Figure 3a can be indexed to TiO2 and the rings due to either PdO or Pd are absent. Lattice fringes in Figure 3b correspond to (101) planes of TiO2. Lattice fringes of PdO or Pd metals could not be detected in the high resolution lattice images from Ti0.97Pd0.03O2−δ sample. Energy-dispersive X-ray analyses (EDAX) on this image were recorded and indeed Pd metal X-rays were detected as shown in Figure 3c. Analysis shows that Pd is present to the extent of 3.5% which is close to the composition taken in the preparation. Therefore, absence of Pd or PdO in the HRTEM images from Ti0.97Pd0.03O2−δ and the presence of Pd from EDAX study confirm substitution of Pd ion in the lattice. X-ray photoelectron spectroscopy (XPS) of combustion synthesized compounds indeed show that Pd is in +2 state. In Figure 4a, Pd(3d) core level spectra of Ti0.99Pd0.01O2−δ, Ti0.98Pd0.02O2−δ and Ti0.97Pd0.03O2−δ are shown. The binding energy of Pd(3d5/2) is 337.8 eV. In panels b and 4cin Figure 4, Pd(3d) spectra of PdO and Pd metal impregnated on TiO2 are presented. Pd(3d5/2) peak of Pd metal and PdO are at 335.1 and 336.4 eV, respectively, and the binding energies agree with the literature.34,35 Pd is in +2 state in PdO. Binding energy of Pd(3d5/2) in PdCl2 is at 338 eV and at 338.7 eV in PdSO4 where formal oxidation state of Pd is +2.35 Thus, if the Pd is more ionic as in PdCl2 or PdSO4, Pd(3d) core level binding energy is higher compared to less ionic Pd in PdO. Pd ion is substituted in highly ionic TiO2 lattice and therefore higher binding energy of Pd(3d) is expected. Ti(2p3/2) core level spectra in all the three compounds are observed at 458.8 eV indicating Ti is present in +4 oxidation state. Therefore, the formula of Pd-ion-substituted catalysts can be written as Ti4+1−xPd2+xO2−2−x (x = 0.01, 0.02, 0.03). Catalytic CO and Hydrocarbon Oxidation. CO conversion vs temperature−light off curves with Ti1−xPd xO2−x (x = 0.01, 0.02, and 0.03) are shown in Figure 5a. Temperature at 50% conversion (T50) decreases with increasing Pd concentration in the catalyst. With Ti 0.98 Pd 0.02 O 2−x and Ti0.97Pd 0.03O2−x, even at room temperature, about 10% CO conversion occurs. 100% CO conversion occurs around

Figure 7. % CO conversion over Ti0.97Pd0.03O2−δ at different temperatures (a) 40 and (b) 60 °C.

60 °C with Ti 0.97Pd0.03O2−x. CO conversion light of curves with 3 at % PdO and 3 at % Pd metal impregnated on TiO2 are shown in Figure 5b. T50 for 3% Pd metal on TiO 2 is 100 °C and that with 3% PdO is 90 °C. T50 decreases to 40 °C when the same amount of Pd is substituted in TiO2. Thus, Pd-ion-substituted catalyst is far superior to Pd metal or PdO impregnated on TiO2. To determine actual rates of CO oxidation, we carried out catalytic reaction under differential reactor condition following the equation: Rate (r) = (F/W)x, where F = flow rate of CO molecules (mol/s), W = weight of the catalyst and x is the fractional conversion. In this experiment, Ti0.97Pd0.03O2−x catalyst was studied. We varied the weight of the catalyst, keeping the flow rate of 100 cc/min with 1% CO and 0.5% O2 the same for all the experiments. In Figure 6a, we show fractional CO conversion (χ) vs W/F plots. Rates were 4495

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Figure 8. % Hydrocarbon oxidation over (a) Ti0.99Pd0.01O2−δ, (b) Ti0.98Pd0.02O2−δ, and (c) Ti0.97Pd0.03O2−δ.

Figure 10. Pd (3d) core level XPS of Ti1−xPdxO2−δ (x = 0.01, 0.02, and 0.03) (a, a′, a′′) after CO oxidation by lattice oxygen and (b, b′, b′′) after oxidation with stream oxygen.

We can see extremely high rate of 2.75 μmol g−1s−1 at 60 °C. The activation energy from ln(rate) vs 1000/T plot (inset of Figure 6b) is 17 kcal/mol. This value compares well with Ce0.98Pd0.02O2−δ catalyst of 16 kcal/mol.21 Thus, Pd ion substituted TiO2 is highly active for CO oxidation at temperatures much below 100 °C. Activation energies calculated from the conversion below 20% from Figure 5b with 3% PdO and 3% Pd on TiO2 are 20.1 and 19.8 kcal/mol, respectively. The stability of the catalyst under catalytic reaction condition was examined with CO oxidation reaction carried out for 24 h continuously at 40 and 60 °C as shown in Figure 7. At 40 °C, 30% CO conversion and at 60 °C, 100% CO conversion continued for 24 h. Catalyst is not poisoned on long time reaction. At such low temperature, most common poison is carbonate ion covering the surface because of adsorption of CO2 back on the catalyst surface. On this catalyst, CO 2 adsorption is inhibited because, Ti4+ ion is small and highly acidic. Acidity of metal ions can be correlated with Fajan’s ratio, defined by ionic charge by ionic radius in pm.36 Fajan’s ratio for Ti4+ ion is 66 pm (4/0.605 = 66 pm) which is extremely high compared to a carbonate stabilizing ion such as barium ion with Fajan’s ratio of 13 pm.37 Au/TiO2 system does show deactivation and a mechanism of bicarbonate formation is shown to be OH adsorption on Au sites.38 In our study, reaction is carried out in absence of H2O and there is no deterioration due to bicarbonate. Further study

Figure 9. % CO oxidation by lattice oxygen over (a) TiO2, (b) Ti0.99Pd0.01O2‑δ, (c) Ti0.98Pd0.02O2−δ, and (d) Ti0.97Pd0.03O2‑δ for two cycles.

calculated from the slope of χ vs W/F plots at different temperature and in Figure 6b, rate as a function of temperature is given. 4496

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Figure 11. Ti (2p) core level XPS of Ti1−xPdxO2−δ (x = 0.00, 0.01, 0.02, and 0.03) (a) after CO oxidation by lattice oxygen as in Figure 8 and (b) after oxidation with stream oxygen at 350 °C.

XPS studies were carried out on the CO reduced and reoxidized catalysts and in Figure 10, Pd(3d) spectra are given. Clearly, Pd2+ in the catalyst is reduced to Pd0 on reduction by CO as seen from the binding energy at 335.1 eV. The catalyst when oxidized by stream oxygen, Pd is oxidized to +2 state. In Figure 11, Ti(2p) spectra are shown after CO oxidation and after oxidation of reduced catalysts. Ti4+ ion is partially reduced even up to +2 state with binding energy at 455.5 eV. On oxidation, reduced Ti ion is oxidized to +4 state. Thus, the catalyst undergoes reversible reduction−oxidation. Estimation of extent of oxygen that can be reversibly utilized for catalytic oxidation of CO, defined as oxygen storage capacity (OSC) is 250, 1240, 2220, and 5100 μmol/g for TiO2, Ti1−xPdxO2−x (x = 0.01, 0.02, and 0.3), respectively. These results show huge enhancement of OSC upon Pd ion substitution in TiO2 and OSC is very high value compare with Ce0.90Zr0.10O239 and Pt/Ce0.75Zr0.25O2.40 If only the Pd ion in the substituted TiO2 were to be reduced to Pd0, the values should have been 125, 250, and 375 μmol/g. Thus, more than 10−20 times increase in the amount of CO oxidation demonstrated lattice oxygen activation upon Pd ion substitution in TiO2. Having found activation of lattice oxygen of Ti1−xPdxO2−x (x = 0.01, 0.02, and 0.3) by CO/TPR and CO oxidation studies, search for activated oxygen was attempted by DFT calculations. Simulated Structures by DFT Calculations. We first determined the structure of pure TiO2 in anatase phase and find that the Ti−O bond lengths are within typical errors of their experimental values (for planar Ti−O bonds of 1.93 Å and two epical Ti−O bonds of 1.97 Å). For the Pd-substituted TiO2, we used a 2 × 2 × 2 supercell containing 32 TiO2 formula units (96 atoms), and substituted up to three Pd

with moisture along with CO + O2 needs to be done to rule out the formation of bicarbonate. Hydrocarbons: propane, propylene, ethylene, and acetylene mixture along with oxygen was passed over the catalyst and light off curves are given in Figure 8 with the three catalysts. Here also, reaction temperature decreased with increase in Pd content. It should be noted that 100% oxidation of acetylene occur at room temperature. With Ti0.97Pd0.03O2−x catalyst, activation energy determined from conversions below 20% are 37.5, 17.7, and 5.7 kcal/mol for ethylene, propylene and propane, respectively. Temperature-Programmed Reduction (TPR) with CO and OSC. One % CO in helium was passed at a rate of 100 cc/ min over 150 mg of TiO2 and Ti1−xPdxO2−x (x = 0.01, 0.02 and 0.03). In Figure 9, % CO to CO2 conversions are shown. TPR was carried out up to 550 °C. Here lattice oxygen is utilized for CO oxidation. The reduced catalyst was oxidized by a stream of pure oxygen at 350 °C for 10 min. TPR with only CO/He was repeated. As can be seen from the figure, TPR profiles repeat indicating restoration of lattice oxygen by stream oxygen. The reaction can be written as follows Ti1 − xPdxO2 − x + δCO → Ti1 − xPdxO2 − x − δ + δCO2 Ti1 − xPdxO2 − x − δ + δ /2O2 → Ti1 − xPdxO2 − x

Extent of reduction by CO was estimated from the reduction of known wt. of CuO by passing same amount of CO. Accordingly, composition of the reduced compounds are: TiO1.98, Ti0.99Pd0.01O1.89, Ti0.98Pd0.02O1.70, and Ti0.97Pd0.03O1.55. Reduction of only Pd2+ ion in the catalyst to Pd0 would not be sufficient to account for the decrease of oxygen in the catalyst. Ti4+ ion is also reduced in the process. 4497

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Figure 12. Structures of Pd-substituted anatase TiO2:(a) Ti31Pd1O63, (b) Ti30Pd2O62; (c, d) two configurations of Ti29Pd3O61.

Ti ions at (0.5, 0.5, 0.0) and (0.5, 0.5, 0.5) sites are substituted with Pd and creating two oxygen vacancies for Ti 30Pd2O62, as shown in Figure 12b. Three Ti ions are replaced by Pd2+ ion at (0.5, 0.5, 0.5); (0.5, 0.5, 0.0), and (0.5, 0.0, 0.5) sites in one configuration, and (0.5, 0.5, 0.5); (0.25, 0.25, 0.25) and (0.75, 0.75, 0.75) in another configuration of Ti 29Pd3O61, to simulate 9% substitution of Pd. At the same time, to maintain oxidation states, three oxygen vacancies were created in each case (Figure 12c, d). Bond lengths of Ti−O and Pd−O and their variation were obtained from the relaxed structures, and used in bond valence analysis to

atoms at Ti sites. Since the Pd substitution is between 3 and 9%, which is quite low, we do not expect notable changes in the lattice parameters, as supported by weak variation in lattice constants with Pd concentration observed here experimentally (see Table 1). Hence, we have carried out only the internal optimization of the supercells to determine the minimum energy structures of Pd substituted TiO2. For 3% substitution of Pd, one Ti ion in the coordinate at (0.5, 0.5, 0.5) is replaced by Pd2+ ion while introducing an oxygen vacancy in the planar geometry with the formula of Ti31Pd1O63, as shown in Figure 12 a). At 6% substitution, two 4498

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understand the reactivity of the catalyst, identifying the activated oxygen sites. Figure 13a shows Ti−O bond distribution in Ti32O64 cell with two types of bonds: each Ti is coordinated to four oxygens

Figure 14. Valence of oxygens in Pd-ion-substituted TiO2.

oxygen ions with valence of 1.87 and 1.91. In Ti30Pd2O62, strongly bonded oxygen ions have valence of 2.43 and weakly bonded ones have valence of 1.88 and 1.89. Finally, in Ti29Pd3O61, strongly bonded oxygen ions have valence of 2.33, 2.43 and weakly bonded oxygen ions have valence of 1.89, 1.90. Oxygen ions with valence of about 1.85 to 1.87 are indeed weakly bonded as seen in YBa2Cu3O7.41 In pure PdO, the Pd is coordinated to four oxygens in square-planar geometry with Pd−O distances at 2.01 Å as shown in Figure 15a. In the TiO2 anatase structure, Ti is coordinated with four square planar oxygens with Ti−O distances of 1.93 Å and two epical oxygens at 1.98 Å (Figure 15b). Coordination around Ti and Pd, in the Pd-ion-substituted TiO2, namely, Ti31Pd1O63, are given in Figure 15c. Four oxygen ions around Pd ion are in a distorted square planar geometry. The four Pd−O bonds are at 1.99 and 2.01 Å, which are close to those in pure PdO. But, the angle O1−Pd−O2 is ∼150° and O3−Pd−O4 is ∼210°. Angles of O1−Pd−O3 and O1−Pd−O4 as well as O2−Pd−O3 and O2−Pd−O4 are ∼93°. Thus, Pd is in a distorted square planar geometry. O5 is the epical oxygen connected to Pd at a longer distance of 2.36 Å, but it is the bridging oxygen for the two TiO6 octahedra with Ti−O5 at 1.85 Å. O6, O7, O8, and O9 are other four oxygen ions in one TiO6 octahedron and O10, O11, O12, O13 are the four other oxygens of another TiO6 octahedron of two Ti ions adjacent to the Pd ion. Among these, the planar oxygens O6, O7, O8 and O10, O11 and O12 are at 1.95−1.98 Å and the epical oxygens O9 and O13 are at 2.01 Å. These distances need to be compared with Ti−O bond lengths in TiO6 octahedra of pure TiO2 (see Figure 15b), where the planar oxygens are at 1.93 Å and epical oxygens are at 1.98 Å. Oxygen ions are coordinated by metal ions and bond valence sums of each of the oxygen have been determined. Oxygen ions O1 and O2 bonded to Pd are epical to Ti, and have a total valence of 1.87. Similarly, O3 and O4 have the valence of 2.00, and the bridging oxygen O5 between two Ti and epical to Pd has a valence of 1.91. Oxygens O6 to O13 have higher

Figure 13. Metal−oxygen bond-length distribution in Pd-ionsubstituted TiO2.

at 1.93 Å and two other oxygens at 1.97 Å. Upon substitution of Pd2+, there is distortion in the structure as shown by spread in the metal−oxygen bonds to long and short distances compared to pure TiO2. In Ti31Pd1O63, Pd is coordinated to four oxygens: two bonds at 1.99 Å, another two bonds at 2.00 Å (Figure 13b). In Ti30Pd2O62, both the Pd’s have similar distribution of bonds: two Pd−O bonds are at 1.99 Å, two others are at 2.00 Å (Figure 13c). In Ti29Pd3O61, the first Pd has bonds of the following type: two Pd−O bonds at 1.99 Å, two bonds at 2.00 Å (Figure 13d); the second Pd has bonds of the following type: two Pd−O bonds at 1.97 Å, two bonds at 1.98 Å and the third Pd has bonds of the following type: two Pd−O bonds at 1.97 Å and two bonds at 1.98 Å . Therefore, four Pd−O bonds in the three compounds are in nearly square planar geometry with Pd−O short bonds between1.97 to 2.0 Å. Bond distributions for both Ti−O and Pd−O bonds in all the simulated compounds are given in figure 13. Ti−O bonds are distorted with reference to pure TiO2 by the creation of longer and shorter Ti−O bonds. Both the structures with three Pd ions considered at higher concentration of substitution (the formula Ti29Pd3O61) bear a similar trend in distribution of bond-lengths. Further insight into activation of oxygen can be gleaned from bond valence analysis, see Figure 14 for the valence of oxygen ions in pure TiO2 and Ti1−xPdxO2−x. The valence of oxygen ions in Ti1‑xPdxO2‑x show large deviation from 2.0 (which is the valence of oxygen ions in pure TiO2). In Ti31Pd1O63, we find two distinct types of oxygen ions: strongly bonded oxygen ions with valence of 2.43 and weakly bonded 4499

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Figure 15. (a) Square planar coordination of Pd2+ in PdO, (b) octahedral coordination of Ti4+ in anatase TiO2, and (c) distorted square planar of Pd2+ and octahedral of Ti4+ near to the Pd2+ ion in Ti31Pd1O63, showing the valence of oxygens that are near to the Pd2+ ion.

Figure 16. Valence of (a) titanium and (b) palladium in Ti31Pd1O60 after removal of three oxygens, O1, O2, and O5.

valence sums between 1.95 and 2.04. Therefore, the two oxygen ions O1, O2 bonded to Pd are weak and hence amenable to

extraction at ease. One of the oxygens ion associated with two Ti ions, O5, is also weakened because of the Pd ion in the lattice. 4500

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Figure 17. Electronic density of states of (a) Ti32O64 and (b) Ti31Pd1O63; partial density of states of (c) Ti32O64 and (d) Ti31Pd1O63.

Extraction of one of the oxygen ions O1 or O2 would render Pd2+ to Pd0, and the second oxygen extraction associated with Pd would render Ti4+ to Ti3+ state. Removal of the third oxygen, O5 would render Ti3+ state to Ti2+ state. The distribution of titanium valence and palladium valence of the reduced structure, Ti31Pd1O60, by removing these three oxygens, O1, O2, and O5, is shown in Figure 16. Decrease in valence of Titanium and Palladium from ideal values to lower values indeed the confirmation of our expectation. The concentration of Pd for Ti31Pd1O63 as shown in Figure 15c corresponds to 3% Pd substitution, which correlates with experiment. This is what we observe in the XPS of Ti0.97Pd0.03O1.97 (Figures 10 and 11) when oxygens are extracted from the catalyst. The electronic density of states and partial density of states of Ti32O64 and Ti31Pd1O63 are shown in Figure 17. As shown in Figure 17, the electronic density of states of Pd-substituted TiO2 and its projection on to atomic orbitals clearly show that the d-states of Pd appear at the top of the valence band, and will indeed play an important role as frontier orbitals in catalytic activity. Finally, similar calculations with inclusion of the Hubbard U correction for on-site correlations do result in slightly narrower bands and wider band gap, though the overall electronic structure remains unchanged qualitatively (Figure 18). Thus, our interpretation of the LDA-based calculations and corresponding identification of the activated oxygen sites are expected to be robust. Experimentally we found the d-states

Figure 18. Comparison of dos of Ti31Pd1O63 obtained by LDA and LDA + Hubbard U Calculations.

of Pd contribution in valence band by XPS and it is shown in Figure 19. In Figure 19a, we have shown the valence band spectra of pure TiO2 and Ti0.97Pd0.03O2−δ and d-states of Pd ion obtained by subtracting of both valence band spectra which is shown in Figure 19b. The structure obtained from the DFT calculations indeed reveal the specific oxygen ions that are activated and d-states of Pd role for the catalytic reaction. 4501

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Figure 19. (a) Valence band XPS of (i) pure TiO2 and (ii) Ti0.97Pd0.03O2‑δ. (b) 4d- states of Pd is obtained by subtracting of TiO2 and Ti0.97Pd0.03O2−‑δ valence band spectra.

4. CONCLUSIONS Ti1−xPdxO2−x (x = 0.01, 0.02 and 0.03) crystallizes in anatase structure with Pd in 2+ and Ti in 4+ states. Pd2+ ion substituted TiO2 shows much higher catalytic activity compared to PdO or Pd metal impregnated on TiO2. High rates of CO and HC oxidation even at room temperature are correlated with activation of lattice oxygen from DFT calculation, where we have shown that oxygen ions bonded to both Pd and Ti in the vicinity of Pd ion are activated.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.S.H. and G.D. thank the Council of Scientific and Industrial Research, India, for fellowships. We also thank Anumol, Materials Research Centre, Indian Institute of Science, Bangalore, for recording TEM images.

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REFERENCES

(1) Edwards, J. K.; Solsona, B. E.; Ntaininjua, N. E.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Science 2009, 323, 1037. (2) Lunsford, J. H. J. Catal. 2003, 216, 455. (3) Li, G.; Edwards, J. K.; Carley, A. F.; Hutchings, G. J. Catal. Today 2007, 122, 361. (4) Edwards, J. K.; Solsona, B. E.; Landon, P.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutching, G. J. J. Catal. 2005, 236, 69. (5) Edwards, J. K.; Solsona, B. E.; Landon, P.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Hutching, G. J. J. Mater. Chem. 2005, 15, 4595. 4502

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