Creation of Redox Adsorption Sites by Pd - American Chemical

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India, Chemical. Engineering Department, Indian Institute of...
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J. Phys. Chem. C 2007, 111, 8153-8160

8153

Creation of Redox Adsorption Sites by Pd2+ Ion Substitution in nanoTiO2 for High Photocatalytic Activity of CO Oxidation, NO Reduction, and NO Decomposition Sounak Roy,† M. S. Hegde,† N. Ravishankar,‡ and Giridhar Madras*,§ Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India, Chemical Engineering Department, Indian Institute of Science, Bangalore 560012, India, and Material Research Center, Indian Institute of Science, Bangalore 560012, India ReceiVed: September 19, 2006; In Final Form: March 24, 2007

The present study aims at developing a new photocatalyst for NO reduction and CO oxidation by creating redox adsorption sites and utilizing oxide ion vacancy in titania. For this purpose, Pd ion-substituted TiO2, Ti1-xPdxO2-δ, was synthesized for the first time by solution combustion method. The photocatalytic activity was investigated with increasing Pd substitution and the optimum concentration was found to be 1 atom % Pd ion in TiO2. The reduction of NO was carried out both in the presence and in the absence of CO. Despite competitive adsorption of NO and CO on the Pd2+ sites, the reduction of NO was 2 orders of magnitude higher than unsubstituted TiO2. High rates of photooxidation of CO with O2 over Ti1-xPdxO2-δ are observed at room temperature. It was shown that enhanced CO oxidation at Pd2+ ion site and O2 or NO photo dissociation at oxide ion vacancy is responsible for the enhanced catalytic activity.

Introduction Nitrogen oxides (NOx), produced primarily by automobile sources and stationary sources, contribute significantly to environmental problems like producing photochemical smog, acid rain, and depletion of stratospheric ozone and lead to health hazards.1,2 Extensive research work has been carried out to reduce NOx to N2 by various carbonaceous reductants such as CO, hydrocarbon, or non-carbonaceous reductants like H2 and NH3 in selective catalytic reduction.3-8 Though NO reduction by H2 over Pd-supported catalysts occurs around 100 °C,7 other reactions require higher temperature, so alternative techniques need to be developed. An alternative method of reducing NOx is by the use of photocatalysis. Direct photocatalytic decomposition of NO would yield N2 and O2. CO oxidation by NO can induce decomposition of NO. The latter is more economical because of lower-energy consumption and operating costs. However, the decomposition of NOx into N2 and N2O has become a major challenge.9-12 The photocatalytic decomposition of NO has been investigated in presence of semiconductors (e.g., ZnO, WO3, TiO2). Among these, TiO2 has the highest photocatalytic activity, and it is a material of choice because of its nontoxicity, high stability, and low cost. The basic principle for such photocatalytic reactions is generation of e- and h+ that can serve as redox reactants. The band gap of anatase phase of TiO2 is 3.2 eV, and the oxidation and reduction potentials of the valance and the conduction bands are +2.95 and -0.25 V, respectively.13 The reduction potential of NO to N2 is +3.36 V and CO oxidation potential to CO2 is -0.106 V,14-16 indicating that the reduction of NO by CO on TiO2 surface is possible. As CO is produced from the combustion process and highly poisonous, it is used as a reductant of NOx. It has been reported that * To whom correspondence should be addressed. E-mail: giridhar@ chemeng.iisc.ernet.in. Phone: +91-80-2293-2321. Fax: +91-80-2360-0683. † Solid State and Structural Chemistry Unit. ‡ Chemical Engineering Department. § Material Research Center.

photocatalytic NO reduction by CO produces N2, N2O, and CO2. Anpo and his co-workers9 have studied photocatalytic decomposition of NO on TiO2 and found that anatase TiO2, with a large surface area and numerous OH groups, exhibits a high efficiency for the decomposition of NO in a flow system. Lim et al.l0 have investigated the photocatalytic decomposition of NO on Degussa P-25 TiO2 in an annular flow type reactor and showed the conversion of NO to NO2, N2O and N2 by photocatalysis increases with light intensity, residence time and decreasing initial NO concentration. Bowering et al.14 have showed that the photocatalytic activity of Degussa P-25 TiO2 decreases with the increasing pretreatment temperature. The photooxidation of CO in presence of TiO2 has been investigated and the yield of CO2 is less in TiO2 compared to Au/TiO2.17 The in situ photocatalytic NO reaction on the catalysts M/TiO2 (where M ) Cu, V, Cr)18 has been investigated with FTIR. It was shown that many NOx species, such as NO+, NO2, NO2+, NO3, N2O, and NOH, were formed during reaction. It is wellknown19 that metal substitution in TiO2 influences the degradation of dyes/organics in the liquid phase. Therefore, the investigation of metal impregnation and metal substitution in TiO2 is of great interest. The objective of the study is to synthesize a photocatalyst for NOx removal based on utilizing the oxide ion vacancy and redox adsorption site. In this context, nanosized Pd metal substituted and Pd metal impregnated TiO2 were synthesized by solution combustion method. The catalyst was characterized by a wide variety of techniques. The photocatalytic activity of Ti1-xPdxO2-δ was investigated for NOx reduction and compared with the other catalysts reported in the literature. Experimental Preparation of Catalysts. TiO(NO3)2 was prepared from titanium isopropoxide TiO(i-pr)4. 10 mL of TiO(i-pr)4 was added to ice cold (4 °C) water slowly with continuous stirring. Hydrolysis of TiO(i-pr)4 produces a white precipitate of TiO(OH)2. TiO(OH)2 was dissolved in minimum amount of

10.1021/jp066145v CCC: $37.00 © 2007 American Chemical Society Published on Web 05/18/2007

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concentrated nitric acid. Ti ion concentration in the solution was estimated by colorimetric method. In the solution combustion method to synthesize 1 atom % Pd/TiO2 [1%Pd/TiO2(comb)], TiO(NO3)2, PdCl2, and glycine (NH2CH2COOH) was taken in the molar ratios 0.99 : 0.01 : 1.1. In a typical preparation, 1 g of TiO(NO3)2 (in solution), 0.0095 g PdCl2 and 0.444 g glycine were taken in 300 mL borosilicate dish. The solution was introduced in to a muffle furnace maintained at 350 °C. The solution boiled with frothing and foaming with concomitant dehydration. At the point of its complete dehydration, the redox mixture ignites yielding a voluminous finely dispersed solid product. The reaction can be written as follows:

9(1 - x)TiO(NO3)2 + 10(1 - x)C2H5NO2 + 9xPdCl2 f 9Ti(1-x)PdxO2-x + 20(1 - x)CO2 +14(1 - x)N2 + (25 - 34x)H2O + 18 HCl Similarly, 0.5 atom % Pd/TiO2, 0.75% atom Pd/TiO2, and 2% and 3% atom Pd/TiO2 were synthesized. 1% Pd/Al2O3(imp) and 1% Pd/TiO2(imp) were made by impregnation method. In this technique, solution combustion synthesized solid powder TiO2 was dispersed in water with hydrazine hydrate. Calculated quantity of PdCl2 was added dropwise with vigorous stirring. The color of the solution became black due reduction of Pd2+ to Pd° conversion. The solid product was centrifuged and dried. Solution combustion synthesized Al2O3 was taken in water with hydrazine hydrate for the preparation of 1% Pd/Al2O3(imp) and the preparation was carried out in the same way as 1% Pd/ TiO2(imp). Pd nano metal powder was prepared from PdCl2, by reducing PdCl2 solution by hydrazine hydrate. Characterization of Catalysts. Powder X-ray diffraction (XRD) patterns of oxide samples were recorded on a Phillips X’Pert diffractometer using Cu KR radiation at a scan rate of 2θ ) 0.5°/min. No thermal pretreatment was carried out before XRD analysis. Transmission electron microscopic studies of powders were carried out using a JEOL JEM-200CX transmission electron microscope operated at 160 kV. X-ray photoelectron spectra (XPS) of the as prepared catalysts and used catalysts were recorded on an ESCA-3 Mark II VG scientific spectrometer using Al KR radiation (1486.6 eV). Binding energies reported are with respect to C (1s) at 285 eV and were measured with a precision of (0.2 eV. Infrared data were taken from FTIR spectrometer spectrum 1000 (Perkin-Elmer) in the range of 400-4000 cm-1. Photoluminescence spectra were taken in Luminescence spectrometer (LS 55, Perkin-Elmer) after exciting the samples at 285 nm. BET surface area for 1% Pd/TiO2 catalyst was determined by nitrogen adsorptiondesorption method at liquid nitrogen temperature using a Quantachrome NOVA 1000 surface area analyzer and the value was 50 m2/g. Photocatalytic Studies. The catalytic reactions were carried out in a continuous flow photo reactor system equipped with a quadruple mass spectrometer SX200 (VG Scientific Ltd, England) and a Gas Chromatograph (Mayura Analytical, India) equipped with Chromosorb 101 column (J. V. Scientific, USA) using a thermal conductivity detector for online gas analysis. The reaction proceeds without any prior activation thermal treatment of the catalyst. The catalyst samples were placed in a cell of 35 mm diameter with a quartz window where the radiation from the 125 W medium-pressure mercury lamp was illuminated. The average energy of the light emitted was 3.5 eV with corresponding photon flux of 6 × 10-6 mol of photon cm-2 s-1. Catalysts were spread over the area corresponding to 20 mm diameter. Distance between the cell and

Figure 1. (a) XRD patterns of Ti1-xPdxO2-δ (x ) 0.005, 0.0075, 0.01, 0.02, 0.03) and 1% Pd/TiO2 (imp), 1% Pd/Al2O3. * ) rutile phase. (b) Enlarged XRD patterns of Ti0.99Pd0.01O1.99 and 1%Pd/TiO2 (imp) in the 2θ range of 30-45°.

the lamp was ∼4 cm. A water circulation system was used around the lamp to remove the infrared radiation as well maintain the temperature at 35 °C. The rate of water circulation was adjusted so as to maintain the temperature. To compare the data obtained from UV radiation with solar light experiments were also done with a 400 W Xe lamp with UVcut filter of below 350 nm. The reaction temperature was monitored by chromel-alumel thermocouple dipped in the catalyst bed. The gaseous products were sampled through a fine leak valve via differentially pumped sampling chamber to an ultrahigh-vacuum (UHV) system housing the quadrupole mass spectrometer. N2O and CO2 were analyzed by GC. Flow rate was 20 sccm with 0.5 vol % NO and 0.5 vol % CO (5000 ppm each). Gases were from M/S Bhuruka Gases Ltd. (India) with 5.14 vol % CO in He, 4.74 vol % NO in He and 99.99% pure O2 in this study. Results and Discussion A. Characterization. XRD. X-ray diffraction patterns of 0.5%, 0.75%, 1%, 2%, and 3% Pd/TiO2(comb), 1% Pd/TiO2(imp), and 1% Pd/Al2O3(imp) are shown in Figure 1a. The diffraction lines of TiO2 are indexed to anatase phase up to 1% Pd substitution. Pd metal peaks (111) were not found in the XRD patterns of 0.5%, 0.75%, and 1% Pd/TiO2(comb). With 2% Pd/TiO2(comb), the major phase is still anatase, and rutile TiO2 lines appear. With 3% Pd/TiO2(comb), rutile phase goes

Redox Adsorption Sites by Pd2+ Ion Substitution

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Figure 2. Rietveld refinement of Ti0.99Pd0.01O1.99. Observed (O), calculated (-) and difference XRD patterns. The vertical bars represent Bragg position.

up to 30%. Further, 2% and 3% Pd/TiO2(comb) gives Pd (111) peaks. The catalyst shows broad XRD lines and the crystallite sizes of 1% Pd/TiO2(comb) calculated using Scherrer method are in the range of 8-9 nm. Pure TiO2 synthesized by combustion method crystallizes in anatase phase with 5-7 nm in size. The diffraction lines due to PdO were also not present in any of the catalysts. However, 1% Pd/TiO2(imp) and 1% Pd/ Al2O3(imp) indeed show Pd (111) peaks. To see if 1% Pd/TiO2(comb) gives any Pd metal, 2θ range from 30-45° is expanded and in Figure 1b, 1% Pd/TiO2(comb) and 1% Pd/TiO2(imp) is given. Absence of Pd (111) peak in the combustion-synthesized sample shows the formation of Ti0.99Pd0.01O1.99. Figure 2 shows the Rietveld refinement of 1% Pd/TiO2(comb). The cell parameter of anatase TiO2 are a/b ) 3.7865 Å and c ) 9.5102 Å. RBragg and Rf are 2.65 and 2.09, respectively, taking Pd2+ ion in Ti4+ (0.00, 0.250, 0.375) site with a good profile fitting. The difference plot does not show detectable peaks due to Pd or PdO. The lattice parameters of pure anatase TiO2 in Rietveld refinement are a ) b ) 3.7904 Å and c ) 9.5067 Å. The present study is mainly with Pd concentration up to 1% and the TiO2 is only in anatase phase. TEM. The TEM image of 1% Pd/TiO2 is shown in Figure 3a. The particle sizes measured from the image are in the range of 8-10 nm, which agrees well with the XRD. The ring type diffraction pattern of 1% Pd/TiO2(comb) and Pd/TiO2(imp) is presented in Figure 3b. In the TEM picture of Pd/TiO2(imp), the Pd metal diffraction lines which is absent in 1%Pd/TiO2(comb). XPS. In Figure 4a, Pd(3d5/2,3/2) core level spectra of Pd metal powder, 1% Pd/TiO2 (imp), 1% Pd/Al2O3, PdO and 1% Pd/ TiO2 (comb) are presented. Pd(3d5/2,3/2) peaks at 335 eV and 340.5 indicate Pd metal in zerovalent state. In the Pd metal impregnated on anatase TiO2 also, binding energy of Pd(3d5/2,3/2) peaks is at 335 and 340.5 eV, respectively. Pd(3d5/2,3/2) peaks at 336.8 and 342.2 eV in the PdO sample shows Pd in 2+ state. In the combustion synthesized Pd/TiO2, Pd(3d5/2) core level

Figure 3. (a) TEM images of Ti0.99Pd0.01O1.99. (b) The ring-type diffraction pattern of Ti0.99Pd0.01O1.99 and Pd/TiO2(imp).

binding energy is at 337.2 eV. Binding energy of Pd ions in TiO2 is higher than that of Pd2+ in PdO. XPS of Pd(3d5/2) in 2% Pd/TiO2 (comb) is also at 337.2 eV (not shown) indicating Pd2+ ion substitution in TiO2. In Ce1-xPdxO2-δ also, Pd(3d5/2,3/2) peaks were at higher binding energy at 338.0 and 342.8 eV.20 Therefore, Pd2+ ions in TiO2 is more ionic than Pd2+ in PdO. Thus, Pd in the combustion synthesized TiO2 is in 2+ state. It may be noted that in the XRD, PdO diffraction lines were not observed. This suggests that Pd2+ ions are substituted in TiO2 matrix in the combustion method. Based on XRD, TEM and XPS, 1% Pd/TiO2(comb) can be represented as Ti0.99Pd0.01O1.99. Because Pd is in 2+ state in the Ti site, there has to be oxide

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XPd IPd σTiλTiDE(Ti) ) XTi ITi σPdλPdDE(Pd)

Figure 4. (a) Pd (4d) core-level XPS of Pd metal, 1%Pd/Al2O3 (imp), 1%Pd/TiO2(imp), PdO, Ti0.99Pd0.01O1.99. (b) Ti (2p) core-level spectra of unsubstituted TiO2 and Ti0.99Pd0.01O1.99. (c) VB region of TiO2 and Ti1-xPdxO2-δ (x ) 0.005-0.02).

ion vacancy for charge balance. For each Pd2+ ion, there need to be at least one oxide ion vacancy. In Figure 4b Ti(2p3/2,1/2) core level spectra are shown for pure TiO2 and Ti0.99Pd0.01O1.99. Clearly Ti (2p3/2) peak was at 459 eV. Similar line shape and same line width show that Ti is indeed in 4+ state in Ti0.99Pd0.01O1.99 catalyst. Surface concentration of Pd ion Ti0.99Pd0.01O1.99 and Pd metal dispersed over TiO2 is estimated from the relative intensities of Pd (3d5/2,3/2) and Ti (2p3/2,1/2) core level spectra:21

where X, I,σ, λ, and DE are the surface concentration, intensity, photoionization cross section, mean escape depth, and geometric factors respectively. Mean free path values were taken from Penn21 and photoionization cross sections were obtained from Scofield.22 Accordingly, surface concentration of Pd2+ ion in Ti0.99Pd0.01O1.99 is 2.3% as against 9.4% in the Pd metal impregnated on TiO2. Thus, Pd ion is indeed substituted in TiO2 crystallites. In Figure 4c, XPS valance band region of TiO2 and Ti1-xPdxO2-δ (x ) 0.005 - 0.02) are shown. As can be seen from the figure, with increase in Pd2+ ion substitution, Pd (4d) band occupies in the band gap region (0-3 eV) of TiO2. Photoluminescence Studies. Figure 5 shows the PL spectra of all the catalysts under study. TiO2 shows the maximum luminescence and followed by 1%Pd/TiO2(imp) and Ti1-xPdxO2-δ. The intensity of the peak decreases with further increase (above 1%) in Pd2+ ion concentration in Ti1-xPdxO2-δ. B. Catalytic Studies. NO Reduction by CO. The NO reduction by CO was carried out over the catalysts with equimolar mixture of 0.5:0.5 vol % NO and CO at a flow rate of 20 cm3/min. 150 mg of Ti0.99Pd0.01O1.99 was taken in the cell and UV light was illuminated and the output gas was analyzed. The only products on reaction of NO and CO were N2 and N2O and CO2. Similarly, Ti1-xPdxO2-δ (x ) 0.05, 0.075, 0.02, 0.03), 1% Pd/TiO2(imp), 1% Pd/Al2O3(imp), and Pd metal were also studied for NO+CO photocatalytic reaction. The steady-state NO conversion under flow condition is shown in Figure 6a. The conversion of NO was 20% and 80%, respectively, over 1% Pd/TiO2 (imp) and Ti0.99Pd0.01O1.99. The dissociation of NO was not observed over either Pd metal powder or Pd metal impregnated over Al2O3. This indicates that TiO2 is essential for photocatalysis. In Figure 6b, the dependence of % NO conversion as a function of Pd concentration is shown. Pure TiO2 did not show measurable NO conversion under the 0.5 vol % NO and CO, 20 cm3/min flow condition. With an increase in Pd2+ ion content (x) in Ti1-xPdxO2-δ, NO conversion increased with x from 0.005 to 0.01. On increasing x (Pd2+ ion) to 0.02, % NO conversion decreased and with x ) 0.03, NO conversion was not observed. The highest NO conversion of 80% was observed with x ) 0.01 (Ti0.99Pd0.01O1.99). This shows that there is an optimum Pd2+ ion concentration for NO conversion. With an increase in Pd2+ ion concentration; the Pd2+(4d) band becomes broader and the photoluminescence decreases significantly at 2 and 3% leading to lesser conversion. Decrease in NO conversion over x ) 0.02 and 0.03 catalyst is also correlated with increase of electron density due to Pd2+(4d) band in the band gap region (0-3 eV) of pure TiO2. With an increase in Pd2+ ion concentration, increase in h+-e- pair recombination (shorting effect) should occur lowering the rate of photocatalytic reaction. To investigate whether NO conversions occur under visible light, experiments with 400 W Xe lamp on 150 mg of Ti0.99Pd0.01O1.99 was carried out keeping the gas flow rate same. There was no observable NO conversion. The moment Xe lamp was replaced by UV lamp, NO conversion started, and again 80% conversion was observed as shown in Figure 7. This indicates that the conversion occurs only in the presence of ultraviolet radiation. NO conversion experiments with 0.5 vol % NO, 0.5 vol % CO over different amount of Ti0.99Pd0.01O1.99 catalyst was carried out. Percent NO conversion is linear up to 150 mg of the

Redox Adsorption Sites by Pd2+ Ion Substitution

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Figure 5. Photoluminescence spectra of TiO2, Ti1-xPdxO2-δ (x ) 0.005, 0.0075, 0.01, 0.02, 0.03).

catalyst. Reactions occur at room temperature, and there was no increase in the temperature of the catalyst. Under steadystate condition, NO conversion rate was 5.3 × 10-7 mol g-1 s-1 over Ti0.99Pd0.01O1.99 with 125 W Hg vapor lamp. Measurable NO conversion over pure TiO2 was not observed at 0.5 vol % NO and 0.5 vol % CO with 20 cm3/min. Therefore, the conversion was studied under batch condition, where 0.5 vol % NO and 0.5 vol % CO was filled in the photo reactor and light was put on for 100 s. NO conversion could be detected to a small extent and the reaction rate was estimated to be 1 × 10-9 mol g-1 s-1. Thus, NO conversion over Ti0.99Pd0.01O1.99 is at least 100 times more than unsubstituted TiO2 with UV light. Conversion profile of NO versus temperature has been plotted in Figure 6(c). NO reduction by CO over TiO2 is negligible even up to 750 °C. However, NO reduction by CO over Ti0.99Pd0.01O1.99 takes place around 250-300 °C. NO-TPD. To determine the adsorption of NO on the catalysts, 1 cm3 (4.74% NO, in He) was pulsed over 0.3 g of TiO2 and Ti0.99Pd0.01O1.99 at room temperature. Accordingly, NO adsorption is 1.4 × 10-5 mols/g over unsubstituted TiO2 and 8.5 × 10-6 mol/g over Ti0.99Pd0.01O1.99. Temperature program desorption of NO was carried out over Ti0.99Pd0.01O1.99 and TiO2. NO was passed over both the catalysts for 30 min at room temperature. TPD of NO was carried in He. Figure 8 shows the TPD curve for TiO2 and for Ti0.99Pd0.01O1.99. Complete desorption occurs as seen from repeated adsorptiondesorption up to 400 °C. TPD of NO over Ti0.99Pd0.01O1.99 shows two distinct NO desorption peaks one at 70 °C and at 250 °C. The low-temperature peak is absent over unsubstituted TiO2, and NO desorption occurs only at higher temperature ∼325 °C. The high-temperature desorption is accompanied by N2O, N2, and O2, indicating dissociative chemisorption on TiO2 and on Ti0.99Pd0.01O1.99. Low-temperature desorption of NO over Ti0.99Pd0.01O1.99 is not accompanied by N2O, O2, and N2 indicating molecular adsorption. This study demonstrates molecular as well as dissociative chemisorption of NO over Ti0.99Pd0.01O2-δ and only dissociative chemisorption over TiO2. NO Dissociation. Conversion of NO by CO to N2, N2O, and CO2 should involve photo dissociation of NO. In order to investigate the dissociation of NO, only NO was passed over the unsubstituted TiO2 and Ti0.99Pd0.01O1.99 in presence of UV light. 45% of NO conversion was observed for Ti0.99Pd0.01O1.99 (Figure 9), whereas there was no NO conversion over unsubstituted TiO2. The photo dissociation in light on/off mode was

Figure 6. (a) NO reduction by CO in presence of 125 high-pressure mercury vapor lamp over Ti0.99Pd0.01O1.99, 1% Pd/TiO2(imp), 1% Pd/ Al2O3(imp), and Pd metal. (b) NO photo reduction by CO over Ti1-xPdxO2-δ (x ) 0.005, 0.0075, 0.01, 0.02, 0.03). (c) Thermal reduction of NO by CO over TiO2 and Ti0.99Pd0.01O1.99.

carried out. In the first run, % NO dissociation was 45%. With subsequent on/off condition, % NO dissociation decreased as shown in Figure 10a. Product analysis showed that the ratio N2 to N2O is 2:1. For such product ratio, the photodissociation should follow the reaction

12NO f 4N2 + 2N2O + 5O2

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Figure 7. NO + CO reaction over Ti0.99Pd0.01O1.99 in presence of 400 W Xe lamp and 125 W mercury vapor lamp. Figure 9. NO dissociation over Ti0.99Pd0.01O1.99 in presence of UV light.

Figure 8. TPD of NO over Ti0.99Pd0.01O1.99 and TiO2.

This reaction indicates that oxygen should be evolved, which was not observed experimentally. Further, each time, when light is put on, % NO conversion decreased (Figure 10a). This suggests that O2 is adsorbed by Ti0.99Pd0.01O1.99 and further dissociation of NO is hindered. However, when NO + CO was passed over the same catalyst, 80% NO conversion was continuously observed as shown in Figure 10b. There was no deterioration of NO conversion in presence of CO. Further, after exposing with UV light for at least 2500 s, when NO dissociation stops, NO + CO mixture was passed and 80% of NO conversion is restored. Thus even though NO dissociation was limited, the % NO conversion remained the same indicating that NO dissociated to N and O, with the oxygen being utilized to form CO2. To investigate what happened to the dissociated oxygen, IR of photo irradiated and un-irradiated Ti0.99Pd0.01O1.99 and TiO2 in presence of NO was analyzed. Clearly, NO2 species formation on both TiO2 and Ti0.99Pd0.01O1.99 surfaces (ν ) 1385 cm-1, symmetric NO2 stretching18,23) is observed (Figure 11).

Figure 10. Repeated cycles of photocatalytic reduction of (a) NO by CO and NO dissociation (b).

It may be noted that in absence of UV irradiation also, 1385 cm-1 adsorption peak appears over both the catalysts. Therefore, NO2 like species is formed on TiO2 as well as on Ti0.99-

Redox Adsorption Sites by Pd2+ Ion Substitution

Figure 11. FT-IR spectra Ti0.99Pd0.01O1.99 and TiO2 after NO adsorption in presence and in absence of UV light.

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Figure 13. Variation of the conversion of NO with W/F.

Figure 14. Potential energy curves of NO and O2 scaled with TiO2 conduction band.

Figure 12. Photocatalytic CO oxidation over Ti0.99Pd0.01O1.99.

Pd0.01O1.99. Though adsorption of NO occurs on unsubstituted TiO2, photodissociation of NO occurs only on Ti0.99Pd0.01O1.99 and not on unsubstituted TiO2. Therefore, primary requirement of photodissociation of NO seems to be creation of oxide ion vacancy; oxide ion vacancy is present in Ti0.99Pd0.01O1.99 and is absent in unsubstituted TiO2. Therefore, NO adsorbed on specific sites in Ti0.99Pd0.01O2-δ is dissociative. Therefore, in the absence of CO, there is partial conversion of NO to NO2 via NOPd + “O” f NO2 and NO2 seems to block the oxide ion vacancy sites. CO Oxidation by O2. The oxidation of CO was investigated with Ti0.99Pd0.01O1.99 in presence of UV light. CO and O2 was taken in 1:1 vol % at a flow rate of 20 cm3/min. Figure 12 shows 20% CO conversion at steady state after 200 s and the rate of CO conversion was estimated to be 2 × 10-7 mol g-1 s-1 confirming photodissociation of O2 over the catalyst. In a separate experiment, photooxidation of CO by N2O was studied and photooxidation of CO was not observed with N2O because no photo dissociation of N2O occurs. Some studies24 have shown that the photo activity of titania is increased in presence of water due to increase of surface hydroxyl groups. However, experiments conducted with moist Ti0.99Pd0.01O1.99 showed no significant increase in photo activity. This was further

confirmed by heating Ti0.99Pd0.01O1.99 at 500 °C to remove hydroxyl groups and this heat-treated sample also showed 80% conversion of NO. The Ti0.99Pd0.01O1.99 was reduced in H2 at 500 °C for 8 h and the reduced sample was black in color indicating Ti in 3+ state. This reduced sample did not show any photocatalytic activity indicating the ionic state of Ti4+ in the material plays an important role in photocatalytic activity. To see the effect of temperature on the photocatalytic activity, photocatalytic reaction of CO+NO was carried out at 100 °C by heating the cell. The conversion was not higher than 80%. Figure 13 shows the variation of the conversion of NO with W/F. It could be interesting to compare the photocatalytic decomposition rate of NO obtained by previous studies. The comparison is shown in Table 1. From the table, it is apparent that the Pd substituted TiO2 shows very high rates compared to other catalysts reported in literature. Photocatalytic activity over Pd2+ ion substituted TiO2 in the form of Ti0.99Pd0.01O1.99 is at least 2 orders of magnitude higher than unsubstituted TiO2. The rates of Ti0.99Pd0.01O1.99 are higher than 1% Pd/TiO2(imp) as seen from Figure 6a. Figure 14 shows the potential energy curves of NO and oxygen scaled with the conduction band of titania. Valance band of Ti0.99Pd0.01O1.99 consists of Pd2+(4d) band at or near CB separated by O (2p) band. With increase in Pd2+ ion concentration, Pd2+(4d) band becomes broader and PL should subsequently decrease and indeed, at 3%, PL is very low where no photo reduction is seen.

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TABLE 1: Comparison of Rate and Experimental Condition of Different Catalyst for Photocatalytic Reduction of NO by CO and CO Oxidation by O2

catalysts

reactor configuration

lamp

TiO2 (synthesized by sol-gel process) Ti0.99Pd0.01O2-d

NO + CO 400 W MPML flow reactor flow reactor bank of 12 black lights (λ ) 300-400 nm) 100 W mercury arc lamp flow reactor 125 W HPML flow reactor

TiO2 Au/TiO2

200 W mercury

0.5/1/2 wt%Pt/TiO2 Ti0.99Pd0.01O1.99

two 4 W black light 125 W HPML

TiO2 (Degussa P25) (Preheated at 200°C) TiO2 (Degussa P25)

CO + O2 flow reactor

Thus, Pd2+ ion up to 1 atom % can be considered as isolated ion in TiO2 matrix. Such a compound would give an e--h+ pair with UV light, and electron lifetime can be as high as that without Pd2+ ion. Isolated Pd2+ ions forms adsorption site for CO. O2 molecule with a size of ∼2.4 Å (O-O bond distance 1.2 Å) can get adsorbed over an oxide ion vacancy site of ∼2.8 Å size (rO2- ) 1.4 Å). Valance band of Ti0.99Pd0.01O1.99 is combined with potential energy level diagram25 of O2 and O2-. EF of Ti0.99Pd0.01O1.99 is scaled with zero of the O2 potential energy curve. Electrons excited to the CB can jump to O2 molecule forming O2- and its potential energy curve crosses that of O2 leading to dissociation to O- (2P) and O (3P). COPd + O (3P) can form CO2 once “O”(3P) is formed. Thus, the role of Pd2+ ion is to create site for CO adsorption and photoexcitation is to transfer e- to the adsorbed oxygen molecule. Probability of O2 adsorption seems to be enhanced due to the creation of oxide ion vacancy. Adsorption probability of CO is very high over Pd2+ ion, and in the absence of Pd2+ ion (i.e., pure TiO2), it is very low. Schaub et al.26,27 and Campbell28 have shown O2 adsorption over the oxide ion vacancies on the TiO2 surfaces supporting proposed mechanism. Thus, enhanced photooxidation rate of CO with O2 over Ti0.99Pd0.01O1.99 compared to unsubstituted TiO2 is due to high CO adsorption over Pd2+ ion and high oxygen dissociation over oxide ion vacancies. Potential energy curve of NO is combined with VB of Ti0.99Pd0.01O1.99 in Figure 14. Potential energy of NO- is not available. But NO + e- forming NO- can similarly dissociate into N (4S) and O- (2P), and we predict it to cross the potential energy curve of NO as shown in the figure. Adsorption of NO occurs on both on Pd2+ and on anionic vacancy (Figure 8). Under UV exposure, the adsorbed NO in the anionic vacancy alone dissociates and this dissociation does not occur in the absence of oxide ion vacancy on unsubstituted TiO2. The photo dissociation of NO over Ti0.99Pd0.01O1.99 is similar to that of O2. Conclusions In order to develop a photocatalyst with high activity, Pd, substituted nanotitania (8-10 nm) was synthesized for the first time. The catalyst Ti1-xPdxO2-δ crystallizes in anatase phase, with Pd in the 2+ state. The surface of the compound thus contains Ti4+, Pd2+, O2-, and the oxide ion vacancy sites. The favored surface is (110) as seen from TEM. Several interesting results are observed in this study. The presence of Pd2+ ion for adsorption and UV light are both essential for the photo dissociation of NO. NO is photodissociated on the oxide ion vacancy site on the surface. While the oxidation of CO to CO2

plug flow reactor flow reactor

inlet concentration of gas (ppm)

rate (µmol g-1 s-1)

1818 (NO) 1.6 (NO)

0.172 0.003

14 29

1 (NO) 5000 (NO)

0.001 0.53

30 present work

30 (CO)

0.093 0.139

17

200 (CO) 10,000 (CO)

0.198

refs

31 present work

is always high, the photo dissociation of NO is high only in presence of CO. A significantly lesser conversion was observed for NO dissociation in presence of 1% Pd/TiO2 (imp) compared to that of Ti0.99Pd0.01O1.99. Further, an optimum concentration of Pd was observed for reaction and was explained based on photoluminescence. References and Notes (1) Fritz A.; Pitchon V. Appl. Catal., B 1997, 13, 1. (2) Bosch H.; Janssen F. Catal. Today 1988, 2, 369. (3) Granger, P.; Delannoy, L.; Lecomte, J. J.; Dathy, C.; Praliaud, H.; Leclercq, L.; Leclercq, G. J. Catal. 2002, 207, 202. (4) Cho, B. K. J. Catal. 1994, 148, 697. (5) Holles, J. H.; Switzer, M. A.; Davis, R. J. J. Catal. 2000, 190, 247. (6) Macleod, N.; Corpley, R.; Keel, J. M.; Lambert, R. M. J. Catal. 2004, 221, 20. (7) Wolf, C. A.; Nieuwenhuys, B. E. Surf. Sci. 2000, 469, 196. (8) Cant, Noel, W.; Chambers, Dean, C.; Liu, Irene, O. Y. J. Catal. 2005, 231, 201. (9) Zhang, J.; Ayusawa, T.; Minagawa, M.; Kinugawa, K.; Yamashita, H.; Matsuoka, M.; Anpo, M. J. Catal. 2001, 198, 1. (10) Lim, T. H.; Jeong, S. M.; Kim, S. D.; Gyenis, J. J. Photohem. Photobiol., A 2000, 234, 209. (11) Hashimoto, K.; Wasada, K.; Osaki, M.; Shono, E.; Adachi, K.; Tokai, N.; Kominami, H.; Kera, Y. Appl. Catal. B 2001, 30, 429. (12) Ichiura, H.; Kitaoka, K.; Tanaka, H. Chemosphere 2003, 51, 855. (13) Mills, A.; LeHunte, S. J. Photohem. Photobiol., A 1997, 108, 1. (14) Bowering, N.; Walker, G. S.; Harrison, P. G. Appl. Catal. B 2006, 62, 208. (15) Serpone, N.; Pelizzetti, E. Photcatalysis. Fundamentals and Applications, 1st ed.; Wiley: Chichester, U.K., 1989. (16) Milazzo, G.; Caroli, Standard Tables of Standard Electrode Potentials; John Wiley & Sons: Chichester, U.K., 1978. (17) Hwang, S.; Lee, M. C.; Choi, W. Appl. Catal. B 2003, 46, 49. (18) Wu, J. C. S.; Cheng, Y. T. J. Catal. 2006, 237, 393. (19) Subramanian, V.; Wolf, E.; Kamat, P. V. J. Phys. Chem. B 2001, 105, 11439. (20) Priolkar, K. R.; Bera, P.; Sarode, P. R.; Hegde, M. S.; Emura, S.; Kumashiro, R.; Lalla, N. P. Chem. Mater 2002, 14, 2120. (21) Penn, D. R. J. Electron Spectrosc. Relat. Phenom. 1976, 9, 29. (22) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129. (23) Valyon, J.; Hall, W. K. J. Phys. Chem. 1993, 97, 1204. (24) Premkumar, J. Chem. Mater 2005, 17, 944. (25) Illenberger, E.; Momigny, J. Gaseous Molecular Ions: An Introduction to Elementary Process Induced by Ionization; Steinkopff Verlag: Darmstadt, Germany, 1992; Vol 2, p 280. (26) Schaub, R.; Wahlstrom, E.; Ronnau, A.; Laegsgaard, E.; Stengaard, I.; Besenbacher, F. Science 2003, 299, 377. (27) Wahlstrom, E.; Vestergaard, E. B.; Schaub, R.; Ronnau, A.; Vestergaard., M.; Laegsgaard, E.; Stengaard, I.; Besenbacher, F. Science 2004, 303, 511. (28) Campbell, C. T. Science 2003, 299, 357. (29) Ibusuki, T.; Takeuchi, K. J. Mol. Catal. 1994, 88, 93. (30) Minglin, Y.; Suantseng, Y.; Hunghuang, J.; Chao, C.; Chihchen, C.; Wang, A. EnViron. Sci. Technol. 2006, 40, 1616. (31) Zhang, M.; Jin, Z.; Zhang, J.; Zhang, Z.; Dang, H. J. Mol. Catal., A 2005, 225, 59.