Higher Catalytic Activity of Nano-Ce - American Chemical Society

Tinku Baidya,† A. Marimuthu,‡ M. S. Hegde,† N. Ravishankar,§ and Giridhar Madras*,‡. Solid State and Structural Chemistry Unit, Department of...
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J. Phys. Chem. C 2007, 111, 830-839

Higher Catalytic Activity of Nano-Ce1-x-yTixPdyO2-δ Compared to Nano-Ce1-xPdxO2-δ for CO Oxidation and N2O and NO Reduction by CO: Role of Oxide Ion Vacancy Tinku Baidya,† A. Marimuthu,‡ M. S. Hegde,† N. Ravishankar,§ and Giridhar Madras*,‡ Solid State and Structural Chemistry Unit, Department of Chemical Engineering, Material Research Centre, Indian Institute of Science, Bangalore 560012, India ReceiVed: July 19, 2006; In Final Form: October 2, 2006

Nano-Ce0.73Ti0.25Pd0.02O2-δ solid solution, prepared by solution combustion method, has been synthesized. The catalyst has been characterized by XRD, HRTEM and XPS. Synergistic interaction of Pd with Ti/Ce is very strong because of ionic substitution. The reducibility of Ce0.73Ti0.25Pd0.02O2-δ is about 5 times higher compared to that of Ce0.98Pd0.02O2-δ below 140 °C. The easy removal of oxygen from the more reducible Ti containing support plays a major role in showing higher catalytic activity of this material for CO oxidation and N2O and NO reduction by CO. The catalyst shows 100% N2 selectivity above 240 °C and high reaction rates compared to other catalysts reported in the literature. It has been shown that oxide ion vacancy creation in the support promotes the adsorption and dissociation of NO or N2O at a lower temperature. Kinetic models based on a bifunctional mechanism were used to determine the reaction rate coefficients.

Introduction Strict automotive regulations all over the world have necessitated the development of new catalysts to reduce automotive exhaust gases such as CO, “HC”, and NOx. Finding an efficient catalyst working at low temperature around 200 °C remains a challenge. Because of this, several studies have examined the use of oxide support such as alumina and ceria-modified alumina. The substitution of Zr ion in CeO2 forming Ce1-xZrxO2 showed enhanced redox properties compared to CeO2. Ce1-xZrxO2 can be reduced to a greater extent at a lower temperature than CeO2 in H2, and ZrO2 cannot be reduced by hydrogen.1-8 Thus, Ce1-xZrxO2 (x ∼ 0.3-0.5) is a favored support for Pt, Pd, and Rh for exhaust catalysts because this oxide acts as a material showing high oxygen storage capacity (OSC). An alternative oxide support Ce1-xTixO2 (0 e x e 0.4), where both Ce as well as Ti are reducible, can be synthesized in the parent fluorite structure.9 Ce1-xTixO2 can be reduced to a larger extent at a lower temperature than either CeO2 or TiO2, and we have recently shown that Pt2+ ions can be substituted in the lattice that showed high rates of CO to CO2 oxidation.10 The effect of ceria addition on NO reaction has been reported in several investigations. A number of catalysts based on Pt, Pd, and Rh dispersed on CeO2/Al2O3 and CeO2-ZrO2 have been studied for NOx reduction.11-13 In a recent study on NO + CO as well as N2O + CO over Pd and Rh/CeOx/Al2O3, Holles et al.14 concluded that the presence of oxide vacancy is crucial for NO dissociation. Rao et al.15,16 also showed the role of Ce3+ h Ce4+ + e couple for N2O or N2 formation over reduced Rh/ CeO2-ZrO2. All of these studies reflect that oxide ion vacancy on the support takes part in NO or N2O to N2 conversion, but further N2O to N2 conversion over these catalysts occurs at ∼300 °C or above. This occurs because active metal sites are predominantly occupied by CO and NO molecules, thus N2O * Corresponding author. E-mail: [email protected]. Phone/ Fax: +91-80-2293-2321. † Solid State and Structural Chemistry Unit. ‡ Department of Chemical Engineering. § Material Research Centre.

adsorption and further dissociation is difficult at low temperature. Therefore, there is a need for a catalyst for NOx reduction with high N2 specificity at lower temperatures. The drawback of the noble metal impregnated CeO2 or Ce1-xZrxO2 is that a strong metal-support interaction cannot be completely achieved. The metal particles generally act with the support at the interface.12,14 Thus, a high dispersion of noble metal, either atomically or ionically, as well as a greater reducibility of the support is the primary focus for the selection of a good catalyst.17,18 However, among the metals investigated for NOx reduction, Pd is found to show higher catalytic activity for NOx reduction.19,20 In search of new catalysts to convert NOx showing 100% N2 selectivity and capable of working at low temperatures, we have been trying to develop new materials by modifying the support and/or the active material. Here, we report the synthesis, structure, and catalytic activity of nano-Ce0.75-xTi0.25PdxO2-δ prepared by the solution combustion method. The enhancement of catalytic activity of the CeO2 matrix by modification with Ti substitution has been primarily discussed. The higher extent of reduction at a low-temperature correlates to the higher N2 selectivity. CO oxidation and N2O or NO reduction by CO occurs at a much lower temperature over Ce0.75-xTi0.25PdxO2-δ compared to Ce1-xPdxO2-δ with 100% N2 selectivity in the temperature range of 250 °C. It has been shown that lattice oxygen can be more easily exchanged from Ce0.75-xTi0.25PdxO2-δ compared to Ce1-xPdxO2-δ in all of the reactions. In addition, we have proposed kinetic models based on a bifunctional mechanism to determine the reaction rate coefficients. Experimental Section Ce0.73Ti0.25Pd0.02O2-δ was prepared by the solution combustion method taking (NH4)2Ce(NO3)6, PdCl2, TiO(NO3)2 (in solution) and glycine in the mole ratio 0.73:0.02:0.0.25:2.42. TiO(NO3)2 was prepared by dissolving TiO(OH)2 in a minimum amount of HNO3. TiO(OH)2 was prepared by hydrolyzing TiCl4 in H2O at 4 °C. Chloride ion was removed by repeated washing of TiO(OH)2.

10.1021/jp064565e CCC: $37.00 © 2007 American Chemical Society Published on Web 11/30/2006

Nano-Ce0.73Ti0.25Pd0.02O2-δ

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Pd (2 atom%) impregnated over combustion synthesized Ce0.75Ti0.25O2 was prepared by reducing PdCl2 by hydrazine hydrate. X-ray diffraction (XRD) data of the oxides and the Pdsubstituted samples were recorded on a Philips X’Pert diffractometer at a scan rate of 0.5° min-1 with 0.02° step size in the 2θ range between 20 and 100°. The refinement was done using the FullProf-fp2k program varying 17 parameters simultaneously. X-ray photoelectron spectra (XPS) of PdO, Ce0.75Ti0.25O2, Ce0.75-xTi0.25PdxO2-δ 2 atom% Pd/Ce0.75Ti0.25O2(impreg), and a few hydrogen reduced oxides were recorded in an ESCA-3 Mark II spectrometer (VG Scientific Ltd., England) using Al KR radiation (1486.6 eV). Binding energies were calibrated with respect to C(1s) at 285 eV with a precision of (0.2 eV. For XPS analysis, the powder samples were made into 0.5 mm thick, 8 mm diameter pallets, and placed into an ultrahigh vacuum (UHV) chamber at 10-9 Torr housing the analyzer. Hydrogen uptake experiments as a function of temperature were carried out by passing 5% H2 in Ar at 30 mL/min flow rate over 50 mg of oxide, and the amount of H2 uptake is detected by a TCD detector, which is calibrated against the uptake of H2 with a known amount of CuO. The catalytic experiments were carried out in a temperature programmed reaction (TPR) system equipped with a Quadrupole mass spectrometer SX200 (VG Scientific Ltd., England) for product analysis in a packed bed tubular quartz reactor (dimension 25 cm × 0.4 cm) at atmospheric pressure. N2O and CO2 have the same mass of 44 just as N2 and CO. N2O and CO2 were analyzed by gas chromatograph and NO from the mass spectrometer. From the total conversion of NO and N2O + CO2 peak at 44 together with N2O/CO2 ratio from GC. N2, CO2, and N2O are obtained by taking the total conversion of NO. The volumetric ratio of NO:CO was confirmed from GC. Different weights of the catalyst (40/80 mesh size) diluted with SiO2 (30/60 mesh size) were loaded in the reactor of column length of 1.1 cm, and the ends were plugged with ceramic wool. All of the reactions were carried out as a function of temperature with a linear heating rate of 5° min-1 with a total flow of 100 sscm (GHSV of 43 000 h-1). The reactions were carried out in differential mode (rate ) F.x/W; F is the flow rate in mol/s, W is the weight of the catalyst, and x is the fractional conversion). The rates are estimated from the slope of the x versus W/F plot. Before the catalytic experiments, the as-prepared catalyst was heated in O2 flow at 300 °C for 1 h followed by degassing in He flow to the experimental temperature to remove the residual moisture. The nonlinear least-square technique in Polymath software is used to test the model. In this technique, the optimized values are obtained by minimizing the sum of squared differences of the experimental rate and calculated rate. N

σ ) 2

∑ i)1

(rexp - rmodel)2 N-K

where N is the number of runs and K is the number of parameter to be optimized. Results XRD Studies. Ce1-xTixO2 (x ) 0.0-0.4) crystallized in the fluorite structure. A total of 25% Ti substituted CeO2 was chosen for Pd ion substitution. To see if a Pd ion is substituted for Ce/Ti ions, the XRD patterns were refined by Rietveld method.

Figure 1. Rietveld refined observed (O), calculated (-), and difference XRD patterns of Ce0.73Ti0.25Pd0.02O2-δ.

In Figure 1a,b, observed, calculated, and difference plots for Ce0.75Ti0.25O2 and Ce0.73Ti0.25Pd0.02O2-δ are shown, respectively. Rp and RBragg values are 1.92 and 2.93 for Ce0.75Ti0.25O2 and 1.94 and 2.90 for the Pd substituted sample. Lattice parameter of Ce0.75Ti0.25O2 is 5.3912 Å, whereas the lattice parameter for the palladium substituted sample is 5.3992 Å. The ionic radii of Ce4+ and Ti4+ ions in VIII coordination are 0.97 and 0.74 Å, respectively, and that of Pd2+ in VI coordination is 0.86 Å. In the Pd substituted compound, Ti is actually substituted by Pd. Thus, there is an increase in the lattice parameter which supports Pd ion substitution in the Ce0.75Ti0.25O2 support. Since Pd is expected to be in the +2 state, there has to be oxide vacancy for charge balancing purposes and so a lower coordination of Pd ion than that of Ce4+ is expected. In the XRD of Ce0.73Ti0.25Pd0.02O2-δ, Pd metal or PdO diffraction lines could not be detected. To confirm this further, Pd0 nanoparticles were dispersed over Ce0.75Ti0.25O2 to the same (2%) extent, and in Figure 2, XRD patterns of Ce0.75Ti0.25O2, Ce0.73Ti0.25Pd0.02O2-δ and 2 atom%Pd + Ce0.75Ti0.25O2 (impreg.) are shown with 10 times expansion in the Y scale. The Pd(111) peak is clearly visible in the Pd metal impregnated sample, and it is absent in the combustion synthesized Pd containing sample. Thus, the XRD study showed Pd ion substitution in the Ce1-xTixO2 (x ) 0.25) lattice. HRTEM Studies. A high-resolution TEM image and an electron diffraction pattern of combustion synthesized 2 atom%/ Ce0.75Ti0.25O2 is shown in Figure 3, panels A and B, respectively. The fringes spacing at ∼3.12 Å corresponds to (111) layers of Ce0.75Ti0.25O2. The electron diffraction pattern clearly shows the

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Figure 2. XRD patterns of (a) Ce0.75Ti0.25O2, (b) 2 atom% Pd/Ce0.75Ti0.25O2 (impreg.), and (c) Ce0.73Ti0.25Pd0.02O2-δ.

Baidya et al.

Figure 4. XPS of Pd(3d) core level in (a) 2 atom% Pd/Ce0.75Ti0.25O2 (impreg.), (b) PdO, (c) Ce0.73Ti0.25Pd0.02O2-δ, and (d) Ce0.73Ti0.25Pd0.02O2-δ (reduced at 100 °C in H2).

Figure 5. XPS of Ti(2p) core level from (a) TiO2 (anatase), (b) Ce0.75Ti0.25O2, (c) Ce0.73Ti0.25Pd0.02O2-δ, and (d) Ce0.73Ti0.25Pd0.02O2-δ (reduced at 100 °C in H2).

Figure 3. HRTEM image of Ce0.73Ti0.25Pd0.02O2-δ (A) and ED pattern of Ce0.73Ti0.25Pd0.02O2-δ (B) and 2 atom% Pd/Ce0.75Ti0.25O2 (impreg.) (C).

fluorite structure of the compound, and it is completely crystalline. There is no lattice fringe or ring due to Pd metal, and fringes due to Pd metal particles separated at 2.25 Å are absent. Figure 3C shows the ED pattern of 2 atom% PdCe0.75Ti0.25O2 (impreg.), and clearly a Pd(111) ring is visible as indicated by arrow. The absence of the diffraction ring due to Pd metal in the combustion synthesized material shows substitution of Pd in the lattice forming Ce0.73Ti0.25Pd0.02O2-δ solid solution. The average particle size obtained from the low magnification image is ∼200 Å. XPS Studies. Figure 4a-d shows the Pd(3d) spectra of 2 atom% Pd/Ce0.75Ti0.25O2 (impreg.) (a), PdO (b), Ce0.73Ti0.25Pd0.02O2-δ (c), and Ce0.73Ti0.25Pd0.02O2-δ (d) reduced at 100 °C. The binding energy of Pd impregnated on Ce0.75Ti0.25O2 at 335.2

eV confirms Pd in the zero valence state (Figure 4a). The Pd(3d5/2) peak in PdO (Figure 4b) is at 336.4 eV corresponding to Pd in the +2 state. The binding energies of Pd0 and Pd2+ in Pd metal and PdO agree well with the reported literature values.18,21 The binding energy of the Pd ion in the Ce0.73Ti0.25Pd0.02O2-δ catalyst is at 337.7 eV (Figure 4c) which is higher than that of Pd2+ in PdO. The binding energy of the Pd2+ ion in Pd(NO3)2 is 338.2 eV.22 Therefore, Pd is in the +2 state, but it is more ionic than that in PdO and less ionic than in Pd(NO3)2. On reduction in H2 at 100 °C, Pd(3d5/2) peaks due to Pd0 and Pd2+ states are clearly observed (Figure 4d). Thus, Pd in the as prepared Ce0.73Ti0.25Pd0.02O2-δ is in the +2 state and gets reduced partially to Pd0 with H2 at 100 °C. Ti(2p) spectra of the Ti ion in TiO2, Ce0.75Ti0.25O2, and Ce0.73Ti0.25Pd0.02O2-δ are shown in Figure 5a-c, respectively. Ti(2p3/2) binding energies in these compounds are at 458.9 eV corresponding to a Ti ion in the +4 state. To check the reducibility of Ti and Ce under reduction conditions, Ti(2p) as well as Ce(3d) spectra of the sample reduced in H2 at 100 °C

Nano-Ce0.73Ti0.25Pd0.02O2-δ

Figure 6. XPS of Ce(3d) core level from (a) Ce0.75Ti0.25O2, (b) Ce0.73Ti0.25Pd0.02O2-δ, and (d) Ce0.73Ti0.25Pd0.02O2-δ (reduced at 100 °C in H2). (+) and (*) sign corresponds to satellites due to 3+ and 4+ state of Ce.

Figure 7. Valence band spectra of (a) pure CeO2, (b) TiO2 (anatase), (c) Ce0.73Ti0.25Pd0.02O2-δ, and (d) Ce0.73Ti0.25Pd0.02O2-δ (reduced at 100 °C).

were also recorded. In Ce0.73Ti0.25Pd0.02O2-δ, Ti(2p3/2) binding energy is 458.9 eV, which matches with that of pure TiO2. In the reduced samples, there is a shift of 0.7 eV toward the lower binding energy at 458.2 eV (Figure 5d) which means Ti4+ ions can be reduced to the Ti3+ state even at 100 °C in the presence of substituted Pd2+ ions. The Ti3+ ion in Ti2O3 is observed at 458.2 eV.23 Ti4+ in TiO2 does not get reduced to Ti3+ at 100 °C by hydrogen. Figure 6a-c shows the normalized Ce(3d) spectra of Ce0.75Ti0.25O2 and Ce0.73Ti0.25Pd0.02O2-δ. Ce is essentially in +4 oxidation state with 3d5/2 binding energy at 901 eV along with characteristic satellites. When reduced at 100 °C, Ce0.73Ti0.25Pd0.02O2-δ gets partially reduced to Ce3+ as can be seen from the satellites due to the Ce3+ state. Pure Ce0.75Ti0.25O2 does not show Ti3+ or Ce3+ states when reduced with H2 at 100 °C. Thus, the Pd2+ ion in Ce1-xTixO2 induces reduction of Ti4+ to Ti3+ and part of the Ce4+ to Ce3+ state. Figure 7a-d shows the valence band spectra of CeO2, TiO2, Ce0.73Ti0.25Pd0.02O2-δ, and Ce0.73Ti0.25Pd0.02O2-δ (reduced at 100 °C). As can be seen from the figure, the electron density near the fermi level is increased in the reduced sample. The valence

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Figure 8. H2-TPR profiles of (a) pure CeO2, (b) Ce0.75Ti0.25O2, (c) pure PdO (d) Ce0.98Pd0.02O2-δ, and (e) Ce0.73Ti0.25Pd0.02O2-δ.

band of CeO2 or TiO2 consists of a O2-(2p) band below 3 eV from the EF. A significant increase in the electron density just below EF is observed upon reduction of the sample which can be assigned to Ce(4f1) + Ti(3d1) + Pd(4d0) states. This is consistent with the core level shifts of Pd(3d), Ti(2p), and Ce(3d) states toward lower binding energies. Thus, core level spectra of Pd(3d), Ti(2p), and Ce(3d) and the VB spectra shows that, even at 100 °C, Pd2+ and Ti4+ ions get reduced to Pd0 and Ti3+, and the Ce4+ ion is partially reduced to the +3 state. It may be mentioned here that a distinct change in color from pale yellow to gray black is observed on H2 exposure at 100 °C, and on exposure to air or O2, the color changes from gray black to pale yellow indicating reoxidation. Hydrogen Uptake Studies. H2 uptake is used extensively to characterize the reducibility of oxygen species in CeO2 and related compounds as well as to understand the metal-support interaction.2,5,24 H2-TPR profiles of Pd compounds along with pure PdO and the corresponding supports are shown in Figure 8. Notice the Y scales are different in the plots for different compounds. Pure CeO2 shows H2 uptake from 350 °C with a low-temperature peak at ∼500 °C, which has been attributed to surface cerium reduction followed by bulk cerium reduction beyond 550 °C. The area under the peak up to 700 °C corresponds to 5.2 cm3 of H2 per gram of CeO2. Reduction of Ti substituted compounds starts at 300 °C, and Ce0.75Ti0.25O2 is reduced to Ce0.75Ti0.25O1.82. This composition corresponds to complete reduction of Ti4+ to Ti3+ and 8% of Ce4+ to Ce3+. The result is consistent with XPS studies. PdO is reduced at 80 °C, and the area under the peak corresponds to PdO + H2 f Pd + H2O. A small negative peak after PdO reduction is attributed to desorption of H2 from the dissociation of PdHx. A similar kind of negative peak in H2-TPR of oxidized Pd on Al2O3 has been reported earlier.25 H2 uptake starts at ∼30 °C in Ce0.98Pd0.02O2-δ with a hydrogen adsorption peak at ∼65 °C, and a small peak at 400 °C due to CeO2 reduction is seen. Taking the low-temperature H2 peak at 65 °C, the ratio of H/Pd is 4 in Ce0.98Pd0.02O2-δ. In the case of Ce0.73Ti0.25Pd0.02O2-δ, the H/Pd ratio is 17. A ratio of H/Pd greater than 2 indicates reduction of support because Pd2+ + 2H can give Pd0. Thus, Ce0.75Ti0.25O2 upon substitution with Pd2+ ion is more easily reducible at a temperature as low as 90 °C. The composition of the reduced oxide corresponds to Ce0.73Ti0.25Pd0.02O1.82, and it

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Figure 10. (a) Partial pressure (O2) vs reaction rate over Ce0.73Ti0.25Pd0.02O2-δ at 70 °C. (b) Experimental data and model fitting for CO oxidation over Ce0.73Ti0.25Pd0.02O2-δ and Ce0.98Pd0.02O2-δ catalysts.

Figure 9. (a) Fractional conversion vs W/F plot for CO + O2 (2:2) reaction over Ce0.73Ti0.25Pd0.02O2-δ. (b) Arrhenius plot of CO oxidation for Ce0.73Ti0.25Pd0.02O2-δ, Ce0.98Pd0.02O2-δ and 2 atom% Pd/Ce0.75Ti0.25O2 (impreg.) catalysts. (c) TPR plot of CO oxidation over Ce0.73Ti0.25Pd0.02O2-δ and Ce0.98Pd0.02O2-δ (without oxygen).

becomes Ce0.73Ti0.25Pd0.02O1.74 when reduced up to 700 °C. Thus, there is a huge decrease in the reduction temperature of Ce0.75Ti0.25O2 in the presence of 2% Pd ion in the lattice. Thus, Ti4+ to Ti3+ and Ce4+ to Ce3+ reduction temperature decreased in the presence of Pd2+ ion. The results are consistent with XPS studies. From XRD, XPS, and H2 uptake studies, as-prepared 2 atom% Pd/Ce0.75Ti0.25O2 can thus be represented as Ce0.73Ti0.25Pd0.02O2-δ where Pd is in the +2 state and to that extent there is oxide ion vacancy. Catalytic Reactions I. CO + O2 Reaction. CO oxidation was carried out with a CO:O2 ratio of 2:2 vol % with different weight of the catalysts. Fractional conversion with W/F was plotted at different temperatures and is shown in Figure 9a. The weight of the catalyst, W, was varied from 25 to 150 mg, whereas F, the flow rate

(mol/s), was kept constant. The plot is linear up to nearly 60% conversion, and the reaction rates at different temperatures were determined from the slopes of the linear region. The rates are much higher over Pd substituted Ce1-xTixO2 compared to Pd substituted CeO2. Figure 9b shows the Arrhenius plot for the temperature dependence of the reaction rate in presence of Ce0.73Ti0.25Pd0.02O2-δ, Ce0.98Pd0.02O2-δ, and 2 atom% Pd/Ce0.75Ti0.25O2 (impreg.). Activation energies of these catalysts respectively are 13.0, 16.0, and 31.5 kcal mol-1. Clearly, Ti and Pd ion substituted ceria shows highest catalytic activity with lowest activation energy. The possible reason for the higher rate of CO oxidation with Ce0.73Ti0.25Pd0.02O2-δ compared to that of Ce0.98Pd0.02O2-δ could be the higher oxygen storage capacity of Ce1-xTixO2 compared to CeO2 at lower temperature. To test this, the TPR of CO to CO2 was carried out by passing CO only at 0.5 cm3/min at a total flow rate of 133 cm3/min. In Figure 9c, %CO conversion to CO2 over 250 mg of the catalysts is shown. The conversion of CO to CO2 reaction is nearly 20% at a much lower temperature in the presence of Ce0.73Ti0.25Pd0.02O2-δ, whereas only less than 5% conversion is obtained in Ce0.98Pd0.02O2-δ. This experiment suggests that exchange of stream oxygen with lattice oxygen is much higher in Ce0.73Ti0.25Pd0.02O2-δ compared to Ce0.98Pd0.02O2-δ. The dependence of the rate of CO oxidation on the partial pressure of oxygen was determined by varying the O2 partial pressure from 0.005 to 0.06 atm as shown in Figure 10a. The reaction order with respect to O2 is 0.78 when the partial pressure is below stoichiometric ratio and slowly decreases to zero at higher partial pressures of oxygen. The reaction rate increases with partial pressure up to the stoichiometric ratio of CO to O2 and becomes independent of O2 with excess O2. In

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the case of this reaction with a noble metal, catalyst supported on alumina/silica show a strong CO inhibition effect, so that, over reducible materials like ceria/titania, the negative values are lower and may be even possible. The results obtained in this study are similar to those reported for CO oxidation in the presence of single crystals as well as on Pd/CeO2-ZrO2.26-28 Oxygen is likely to be adsorbed on an oxide vacancy site without hindering the CO adsorption on the metal site. Thus, the reaction rate increases when O2 is in stoichiometric ratio. At higher oxygen partial pressures, the availability of more dissociated oxygen makes the CO oxidation independent of oxygen. A model of CO oxidation is proposed where CO is adsorbed on the Pd2+ site. Oxygen from the lattice as well as the stream oxygen molecules dissociating over the oxide ion vacancy is considered. Competitive adsorption of CO and O2 on Pd2+ ion is not considered because adsorption of oxygen on Pd2+ is improbable. The reaction mechanism can be written as follows:

CO(g) + SPd h COads

(1)

COads + OL f CO2(g) + V ¨ o2- + SPd

(2)

¨ o2- f OL + OX O2(g) + 2V

(3)

Here OL is the lattice oxygen, Ox is the dissociated oxygen on intrinsic oxide ion vacancy site due to Pd2+ ion substitution, V ¨ o2- is the oxide vacancy. It is assumed that OL and Ox are the same kinetically. It should be noted that due to substitution of the Pd2+ ion in the lattice, there is one oxide ion vacancy per Pd2+ ion for charge balance. From EXAFS studies done earlier, we had shown that oxide ion vacancy is present around Pd2+ ions.18 The kinetic parameters could be predicted using a model which can be derived from the above two sites mechanism: CO adsorption on Pd2+ ion and O2 on oxide ion vacancy. By assuming the surface coverage by CO adsorption is described by a Langmuir isotherm, ΘCO ) K1CCOΘV1 where K1 ) k1/k-1, ΘV1 refers to fractional vacant site on Pd site and ΘV1 refers to fractional vacant oxide ion vacancy. The site balance on the metal surface is given by ΘV1 + ΘCO ) 1. Substituting ΘV1, ΘCO ) K1CCO/1 + K1CCO. O2 adsorption is proportional to ΘV2 as observed experimentally.29 Therefore, the rate of adsorption of O2 on the support can be written as rO2 ) 2k3CO2ΘV2. The steady-state mass balance for the species O(ad) is ΘO ) 2k3CO2Θv2/k2ΘCO. The rate of formation of CO2 can be written as rCO2 ) k2ΘCOΘO, where ΘO is the fractional coverage of the oxide ion.29 Thus

rCO2 )

2K1k3k2CCOCO2 K1k2CCO + 2k3CO2(1 + K1CCO)

The experimental data is fitted to the above model for both Ce0.73Ti0.25Pd0.02O2-δ as well as Ce0.98Pd0.02O2-δ. Figure 10b shows the experimental and model fitting for the reaction. The values of the optimized rate parameters are given in Table 1a,b. The adsorption rate constant (k3) is estimated experimentally by defining k3 as [RT/2πM]1/2σS where σ is the surface area occupied by 1 g of catalyst and is estimated to be 39.2 m2/gcat for Ce0.73Ti0.25Pd0.02O2-δ and 35 m2/gcat for Ce0.98Pd0.02O2-δ by the BET method, S is assumed to be 0.5, and M is molecular weight of oxygen.30 As can be seen, the K1 ()k1/k-1) value for Ce0.73Ti0.25Pd0.02O2-δ is 6.07 xT exp(18010/T) which is higher than 108.25xT exp(13550/T) reported by Cho for Rh/Al2O3.30

TABLE 1: Rate Parameters for the CO + O2 Reaction over (a) Ce0.73Ti0.25Pd0.02O2-δ and (b) Ce0.98Pd0.02O2-δ parameter (a) K1 (cm3 mol-1) k2 (mol gm-1 s-1) k3 (cm3 gm-1 s-1) (b) K1 (cm3 mol-1) k2 (mol gm-1 s-1) k3 (cm3 gm-1 s-1)

value 6.07 xT exp(18010/T) 77.89 exp(-5700/T) 3.98 × 106 xT 3000 xT exp(1912/T) 1.316 × 1018 exp(-20980/T) 3.55 × 106 xT

Clearly, k2 is very high over Ce0.73Ti0.25Pd0.02O2-δ compared to Ce0.98Pd0.02O2-δ. This proves that CO oxidation utilizing the lattice oxygen is higher over Ce0.73Ti0.25Pd0.02O2-δ compared to that of Ce0.98Pd0.02O2-δ (Figure 9c). The value of E2/R of 5700 K for Ce0.73Ti0.25Pd0.02O2-δ is smaller compared to 7150 K for Rh/Al2O3.29 This confirms that less activation energy is required for CO2 formation over Ce0.73Ti0.25Pd0.02O2-δ compared to Ce0.98Pd0.02O2-δ. The ratio of k2 values for Ce0.73Ti0.25Pd0.02O2-δ and Ce0.98Pd0.02O2-δ catalyst is 5.9 × 10 - 17 exp(15280/T). This indicates that CO oxidation rate is faster over Ce0.73Ti0.25Pd0.02O2-δ below 150 °C. It is clearly noted that CO oxidation starts at much lower temperature over Ce0.73Ti0.25Pd0.02O2-δ. The oxide ion vacancies are nucleophilic in nature and thus facilitate the dissociation of molecular oxygen, which enhance the CO oxidation rate at low temperature over Ce0.73Ti0.25Pd0.02O2-δ compared to Ce0.98Pd0.02O2-δ. The CO oxidation over Ce0.98Pd0.02O2-δ is faster above 150 °C which can be explained as follows. Removal of oxygen from the distorted lattice is easier; thus, at high temperature, the CO oxidation rate over Ce0.73Ti0.25Pd0.02O2-δ is so fast that the oxide vacancy cannot be refilled from the stream oxygen at the same as its removal. Thus, at high temperatures, a slow increase in rate may be due to a lower availability of dissociated oxygen. Nibbelke et al.31 showed the CO + O2 reaction follows both a monofunctional and bifunctional mechanism over Pt-Rh/γAl2O3-CeO2. In this catalyst, the monofunctional mechanism occurs on the metal particle surface and the bifunctional one takes place at the noble metal-support interface. In contrast, in the present catalysts, the noble metal is ionically substituted in the reducible support. Pd is present in the +2 state and oxide ion vacancies around the Pd ion are created for charge balancing.18 Adsorption of the O2 molecule on the Pd2+ site is not favorable because further oxidation of the Pd ion is difficult. Therefore, the Pd2+ ion is the only site available for CO adsorption. Thus, the advantage of ionic dispersion is that both CO as well as O2 have independent adsorption sites at atomic distances. Hence, the bifunctional mechanism is more appropriate than combined mechanisms in the present catalyst at any temperature. Since Ce0.73Ti0.25Pd0.02O2-δ shows much higher catalytic activity than that of Ce0.98Pd0.02O2-δ, other reactions were investigated only in the presence of Ce0.73Ti0.25Pd0.02O2-δ. II. N2O + CO Reaction. N2O reduction by CO was carried out with a ratio of 0.5:0.5 vol % varying the catalyst loading from 25 to 150 mg. Rates were calculated from the slope of the linear region of W/F vs fractional conversion plot shown in Figure 11a. Figure 11b shows the Arrhenius plot, and the activation energies for CO2 formation (CO conversion) and N2O conversion are 6.3 and 13.4 kcal mol-1, respectively. Clearly, the CO to CO2 reaction occurs at a much lower temperature utilizing lattice oxygen, and N2O dissociation occurs at a higher temperature.

836 J. Phys. Chem. C, Vol. 111, No. 2, 2007

Baidya et al. k-4. By assuming pseudo steady for the species O(ad), ΘO ) k6CN2OΘv2/k5ΘCO. Based on the oxygen and vacancy balance on the support, Θv2 + ΘO ) 1 and

Θ V2 ) 1+

ΘO )

1 k6CN2O k5ΘCO

k6CN2O k6CN2O +

k5K4CCO 1 + K4CCO

the rate of formation of CO2 is rCO2 ) k5ΘCOΘO and thus

rCO2 )

Figure 11. (a) Fractional conversion vs W/FN2O plot for N2O + CO (0.5:0.5) reaction over Ce0.73Ti0.25Pd0.02O2-δ. (b) Arrhenius plot of N2O reduction by CO in N2O+CO reaction over Ce0.73Ti0.25Pd0.02O2-δ. (c) Experimental data and model fitting for N2O reduction by CO over Ce0.73Ti0.25Pd0.02O2-δ.

A model for N2O reduction by CO was developed. CO is adsorbed on a metal site and reacts with lattice oxygen to produce CO2, and thus, an oxide ion vacancy is created. N2O adsorption and dissociation has been assumed to be occurring through single step; that is, N2O is adsorbed on an oxide vacancy site and simultaneously gets dissociated filling the oxide ion vacancy. The mechanism can be written as follows:

K4k5k6CN2OCCO k6CN2O(1 + K1CCO) + k5K4CCO

In the above expression, the rate coefficients can be expressed in terms of the pre-exponential factor and activation energy (Table 2). Figure 11c shows the experimental and model fitting for the above reaction. The ratio of k6/k5, 3.02 exp(-1830/T), is the ratio for the rate of CO oxidation to N2O conversion. This indicates that CO oxidation is faster at low temperature and N2O conversion is faster at higher temperature as shown in Figure 11b. This could happen because at higher temperature, the availability of lattice oxygen is low due to faster removal by CO. Since more oxide ion vacancy is present at this condition, N2O dissociation is expected to be faster than CO oxidation. III. NO + CO Reaction. NO reduction by CO was carried out over Ce0.73Ti0.25Pd0.02O2-δ with an equimolar mixture of 0.5:0.5 vol % NO and CO gases. The rates were calculated from the slope of the linear region of fractional conversion (x) vs W/FNO plot as shown in Figure 12a. The outlet gas concentrations containing N2, N2O, and CO2 were analyzed, and the variations of the fractional conversion with the weight of the catalyst for all outlet gases were plotted. Figure 12b shows the Arrhenius plot for NO conversion, CO oxidation, and N2O dissociation. The activation energies are ENO ) 12.63 kcal mol-1, ECO/CO2 ) 12.6 kcal mol-1, and EN2O ) 15.73 kcal mol-1. N2O/CO2 were analyzed and in Figure 12c, the temperature profiles of NO, CO2, N2O, and N2 are shown. Clearly, at 240 °C, N2 selectivity is 100%. In the inset of the figure, N2O reduction by CO for the same 0.5:0.5 vol % N2O/CO flow is shown, and it can be seen that, over this catalyst, the N2O reduction temperature coincides with that of 100% N2 specificity for the NO + CO reaction. The reaction mechanism based on the previous mechanism can be written as

CO(g) + SPd h COads

(7)

NO(g) + SPd h NOads

(8)

CO(g) + SPd h COads

(4)

¨ o2 - + SPd COads + OL f CO2(g) + V

(9)

COads + OL f CO2(g) + V¨ o2 - + SPd

(5)

NO(g) + V ¨ o2 - h “O” - N

(10)

N2O(g) + V¨ o2 - f N2(g) + OL

NOads + “O”-N f N2O(g) + OL + SPd

(11)

(6)

¨ o2 - f N2(g) + OL N2O(g) + V

(12)

A rate equation can be derived using the above bifunctional mechanism. The fraction of metal surface occupied by CO atom can be given as ΘCO ) K4CCO/(1 + K4CCO), where K4 ) k4/

Because Ce0.73Ti0.25Pd0.02O2-δ has a higher catalytic activity at lower temperature, the mechanism has been modified to that

Nano-Ce0.73Ti0.25Pd0.02O2-δ

J. Phys. Chem. C, Vol. 111, No. 2, 2007 837

Figure 13. Experimental data and model fitting for NO reduction by CO over Ce0.73Ti0.25Pd0.02O2-δ.

TABLE 3: Rate Parameters for NO-CO Reaction parameter 3

-1

K7 (cm mol ) K8 (cm3 mol-1) K10 (cm3 mol-1) k9 (mol gm-1 s-1) k11 (mol gm-1 s-1) k12 (cm3 gm-1 s-1)

Figure 12. (a) Fractional conversion vs W/FNO plot for NO+CO (0.5: 5) reaction Ce0.73Ti0.25Pd0.02O2-δ. (b)Arrhenius plot of CO oxidation, NO conversion, N2O formation in the NO + CO reaction over Ce0.73Ti0.25Pd0.02O2-δ. (c) TPR plot for NO, N2O, N2, and CO2 analysis in the NO + CO (0.5:0.5) reaction over Ce0.73Ti0.25Pd0.02O2-δ.

value 6.07 xT exp(18010/T) 2018 xT exp(11460/T) 97.86 xT exp(11430/T) 8.481 × 10-6exp(-428/T) 4.922 × 109 exp(-11650/T) 4.82 × 1014exp(-3913/T)

by NO adsorbed on the palladium site and ΘNO2 is the fractional coverage by NO adsorbed on oxide vacancy site. By assuming a Langmuir adsorption isotherm for both NO and CO on the metal surface, the fraction of sites occupied by the CO and NO species on the metal site is ΘCO ) (K7CCO)/(1 + K7CCO + K8CNO) and ΘNO1 ) (K7CNO)/(1 + K7CCO + K8CNO). By assuming equilibrium adsorption of NO on the support for step 10, ΘNO1 ) K10CNOΘV2. The steady-state mass balance for the species OS2 is ΘOS2 ) (k11ΘNO1K10CNO + k12CN2O)/ (k9ΘCO1)ΘV2, where ΘOS2 is the fractional coverage of oxide ion on the surface. The total species balance on the support is Θv2 + ΘO + ΘNO2 ) 1. Based on the total species balance on the support, the vacancy on the support, ΘV2, is obtained and substituting

rNO )

2[k12CN2O + K8K10k11CNO2/(1 + K7CCO + K8CNO)] K8K10k11CNO2 k12CN2O 1 + K10CNO + + (1 + K7CCO) k9K7CCO k9K7CCO

TABLE 2: Rate Parameters for N2O-CO Reaction parameter (cm3

mol-1)

K4 k5 (mol gm-1 s-1) k6 (cm3 gm-1 s-1)

value 6.07 xT exp(18010/T) 365 exp(-1410/T) 1100 exp(-3240/T)

proposed by Granger et al.19 from steps 10-12. The formation of N2O (step 11) occurs because of the close proximity of the nitrogen atom of NOads and the nitrogen atom of NO adsorbed on the vacancy site through O. The 100% N2O decomposition temperature is the same in both NO + CO as well as in N2O + CO reactions. Thus, N2O adsorption and the simultaneous decomposition mechanism is expected to be the same in both reactions, and hence, step 12 follows the same as in the N2O + CO reaction. From the overall reaction stoichiometry, the rate of disappearance of NO can be written as rNO ) 2(rN2 + rN2O). The rate of formation of N2 and N2O can be written as rN2 ) k12CN2OΘV2 and rN2O ) k11ΘNO1ΘNO2 where ΘNO1 is the fractional coverage

For the above expression, the optimized values of the rate coefficients are given in Table 3. Figure 13 shows the experimental and model fitting for the reaction. As can be seen that adsorption equilibrium constant K8 is higher than K10. This is expected because NO adsorbed on Pd through “N” form strong bond compared to NO adsorption through “O” on the oxide ion vacancy site. Since k9 is much higher than k11, as soon as OL is formed, it is consumed by CO(ads) that leads to formation of CO2 at higher rate. Similarly, k12 is much higher than k11, and therefore, N2O dissociation is much faster than its formation and it leads to higher N2 selectivity. The NO + CO reaction follows the bifunctional mechanism in the whole temperature range in the present investigation. In contrast, Granger et al.12 showed the change of the combined mechanism (monofunctional and bifunctional) at low temperature to the monofunctional mechanism occurring at high temperature over Pt-Rh/γ-Al2O3-CeO2 because of the unavailability of sufficient lattice oxygen at the interface resulting in

838 J. Phys. Chem. C, Vol. 111, No. 2, 2007 the loss of synergism. The support will have a negligible effect behaving like an irreducible support. Since the ionically substituted catalyst is different from the impregnated one and each ion is separated from the other, the monofunctional mechanism is simply not possible in the whole range of 100% conversion. Metal sites are predominantly occupied by CO or NO, and necessary oxygen must be supplied from the lattice which is refilled by NO or N2O dissociation. At hightemperature NO or N2O dissociation is also fast enough to replenish the oxide vacancy and the synergism process remains intact. Discussion The emphasis of this work was to develop a new catalyst that is capable of high catalytic activity and 100% N2 selectivity for NO + CO reactions at low temperature. For this purpose, we have been trying to develop material based on new active material or modifying the support. To this end, we show that the oxide ion vacancy in the support plays a pivotal role in enhancing the reaction rates. CeO2 is a reducible support, but Ce0.75Ti0.25O2 is reducible to a greater extent. Substitution of the Pd2+ ion in Ce0.75Ti0.25O2 enhances reducibility further. This is shown by H2-TPR as well as CO-TPR experiments. From EXAFS and ab initio calculations, a higher OSC of Ce1-xTixO2 is shown to be due to distortion of the oxide sub lattice. Smaller Ti4+ ion in Ce4+ site in CeO2 is shown to have 4 + 4 coordination with 4 long and 4 short TisO bonds; oxygen associated with the long TisO bond is weak, and they are reduced more easily.32 It is this factor which is responsible for easy oxide ion vacancy creation in the lattice. Pd2+ ion substitution in Ce1-xTixO2 has been achieved by a unique solution combustion method. That the Pd2+ ion is the active site has been demonstrated earlier.17,18 Every Pd2+ ion is associated with an oxide ion vacancy, and thus, in Ce1-x-yTixPdyO2-δ, Pd2+ and oxide ion vacant sites are created. Ce0.75Ti0.25O2 could be reduced to Ce0.75Ti0.25O1.79 when heated in H2 up to 700 °C, whereas Ce0.73Ti0.25Pd0.02O2-δ is reduced to Ce0.73Ti0.25Pd0.02O1.82 in H2 up to 100 °C only. This is attributed to the electronic interaction of the Pd ion in Ce1-xTixO2. The Pd2+ ion is reduced at 50 °C, and electron transfer from Pd0 to Ti4+(3d) is facile due to the overlap of Pd(4d) and Ti(3d) orbitals. In addition Ce(4f0) can get populated to Ce(4f1). The oxide ion vacancy is nucleophilic in nature. Accordingly, reductant molecules get adsorbed at the Pd2+ site and oxidant molecules at oxide ion vacancy sites. In the case of CO oxidation by O2, CO is adsorbed on the Pd2+ site, but NO is adsorbed both on oxidant and a reductant sites. NO adsorbed on oxide ion vacancy site can get dissociated which NO on Pd2+ site remain molecular. Since the Pd2+ and oxide ion vacant sites are close to each other, “N”-O (Pd2+ site) + N-“O” (oxide ion vacancy site) f N2O is to be expected. An increase in the rate dissociation of NO is achieved by increasing the oxide ion vacant site due to CO oxidation by lattice oxygen as shown in CO/TPR. Thus, an increase in N2 specificity is achieved mainly due to an increase in the NO adsorption in the oxide ion vacant site. High rates of CO oxidation and NO conversion at relatively low-temperature could be explained by the bifunctional character of the catalyst. An increase of NO dissociation rate due to adsorption probability of NO on the oxide ion vacancy site seems to be responsible for higher N2 specificity. The high rates of CO oxidation as well as NO and N2O conversions could be fitted for the model presented only when adsorption of NO on

Baidya et al. the Pd2+ as well as oxide ion vacant sites is taken into account. Thus, the structure of the catalyst with Pd2+ ion substituted in Ce1-xTixO2 is primarily responsible for oxide ion vacancy in addition to easy removal of oxide ions (easy reducibility) of the lattice oxygen by Ti4+ ionic substitution. The catalyst is therefore distinctly different than just Pd metal particles dispersed on an oxide support. Conclusions 1. Ce0.75-xTi0.25PdxO2-δ solid solution was prepared by single step solution combustion method. 2. A similar extent of reduction occurs below 140 °C in Ce0.75-xTi0.25PdxO2-δ as against Ce0.75Ti0.25O2 up to 700 °C. The H/Pd ratio is 17 over Ce0.73Ti0.25Pd0.02O2-δ, whereas the same is 4 for Ce0.98Pd0.02O2-δ. 3. Lattice oxygen plays the role in decreasing temperature of CO oxidation, N2O or NO reduction by CO, significantly. 4. 100% N2 selectivity is obtained above 240 °C. Acknowledgment. The authors gratefully acknowledge financial support from the Department of Science and Technology, Government of India. References and Notes (1) Mamontov, E.; Egami, T.; Brezny, R.; Koranne, M.; Tyagi, S. J. Phys. Chem. B 2000, 104, 11110. (2) Fornasiero, P.; Balducci, G.; Monte, R. D.; Kasper, J.; Sergo, V.; Gubitosa, G.; Ferrero, A.; Graziani, M. J. Catal. 1996, 164, 173. (3) Trovarelli, A. Catalysis by Ceria and Related Materials; Imperial College Press: London, 2002. (4) Mausi, T.; Ozaki, T.; Machida, Ken-ichi.; Adachi, Gin-Ya.; J. Alloys. Compd. 2000, 303, 49. (5) Fornasiero, P.; Fonda, E.; Monte, R. D.; Vlaic, G.; Kasˇper, J.; Graziani, M. J. Catal. 1999, 177, 187. (6) Monte, R. D.; Kasˇper, J. J. Mater. Chem. 2005, 63, 15. (7) Bekyarova, E.; Fernasiero, P.; Kasˇper, J.; Graziani, M. Catal. Today 1998, 45, 179. (8) He, H.; Dai, H. X.; Ng, L. H.; Wong, K. W.; Au, C. T. J. Catal. 2002, 206, 1. (9) Luo, M.; Chen, J.; Chen, L.; Lu, J.; Feng, Z.; Li, C. Chem. Mater. 2001, 13, 197. (10) Baidya, T.; Gayen, A.; Hegde, M. S.; Ravishankar, N.; Dupont, L. J. Phys. Chem. B 2006, 110, 5262. (11) Kolli, T.; Rahkamma-Tolonen, K.; Lassi, U.; Sivima¨ki, A.; Keiski, R. L. Catal. Today 2005, 100, 297. (12) Granger, P.; Delannoy, L.; Lecomte, J. J.; Dathy, C.; Praliaud, H.; Leclercq, L.; Leclercq, G. J. Catal. 2002, 207, 202. (13) Lecomte, J.; Granger, P.; Leclercq, L.; Lamonier, J.-F.; Aboukais, A. Colloid Surf. A 1999, 158, 241. (14) Holles, J. H.; Switzer, M. A.; Davis, R. J. J. Catal. 2000, 190, 247. (15) Fornasiero, P.; Rao, G. R.; Kasper, J.; Elario, F. L.; Graziani, M. J. Catal. 1998, 175, 269. (16) Rao, G. R.; Fernasiero, P.; Di, Monte, R.; Kasper, J.; Vlaic, G.; Balducci, G.; Meriani, S.; Gubitosa, G.; Cremona, A.; Graziani, M. J. Catal. 1996, 162, 1. (17) Bera, P.; Patil, K. C.; Jayaram, V.; Subbanna, G. N.; Hegde, M. S. J. Catal. 2000, 196, 293. (18) Priolkar, K. R.; Bera, P.; Sarode, P. R.; Hegde, M. S.; Emura, S.; Kumashiro, R.; Lalla, N. P. Chem. Mater. 2002, 14, 2120. (19) Granger, P.; Lamonier, J.-F.; Sergent, N.; Aboukais, A.; Leclercq, L.; Leclercq, G. Top. Catal. 2001, 16/17, 89. (20) Roy, S.; Marimuthu, A.; Madras, G.; Hegde, M. S. Appl. Catal. B (in print). (21) Brun, M.; Bertlet, A.; Bertolini, J. C. J. Electron Spectrosc. Relat. Phenom. 1999, 104, 55. (22) Briggs, D.; Seah, M. P. Practical Surface Analysis; John Wiley & Sons Ltd.: New York, 2004. (23) Rao, C. N. R.; Sarma, D. D.; Vasudevan, S.; Hegde, M. S. Proc. R. Soc. London A 1979, 367, 239. (24) Marecot, P.; Pirault, L.; Mabilon, G.; Prigent, M.; Barbier, J. Appl. Catal. A, EnViron. 1994, 5, 57. (25) Lieske, H.; Vo¨lter, J. J. Phys. Chem. 1985, 89 (10), 1842. (26) Nieuwenhuys, B. AdV. Catal. 2000, 44, 259.

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