J. Phys. Chem. C 2009, 113, 4161–4167
4161
LnxPdyTi1-x-yO6 Catalysts: Formation of Oxygen Vacancy and Identification of the Active Site for CO Oxidation Fang Wang†,‡ and Gongxuan Lu*,† State Key Laboratory for Oxo Synthesis and SelectiVe Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China, and Graduate School of Chinese Academy of Sciences, Beijing 100039, P. R. China ReceiVed: NoVember 6, 2008; ReVised Manuscript ReceiVed: January 3, 2009
The catalytic activities for CO oxidation at low temperature on various LnxPd1-x-yTiO6 catalysts have been investigated. The results showed that the catalytic activities of PdyTi1-yO6 catalysts could be promoted significantly by the addition of rare earth elements. CexPdyTi1-x-yO6 showed the highest activity among these catalysts. Evidence from characterization by XRD, XPS, TPR, and BET indicated that the synergistic interaction between Pd and Ce in the CexPdyTi1-x-yO6 catalyst was stronger than that of any other rare earth element dopant. The doping resulted in the formation of oxygen vacancies and the higher ionization of Pd species. Three kinds of structure have been proposed according to the distribution of Pd and oxygen vacancies in TiO2. Results of computer simulations show that the structure with an oxygen vacancy adjacent to each Pd atom is predicted to be favored, which might be due to the high stability of Pd2+ in a square-planar environment. A possible reaction mechanism for CO oxidation over these Ln-doped LnxPdyTi1-x-yO6 catalysts is discussed. 1. Introduction During the past several decades, low-temperature catalytic oxidation of CO has attracted considerable attention because of its significance in gas purification, CO gas sensors, and environmental pollution control, among other areas.1-4 Catalysts comprising Pd supported on metal oxides such as Al2O3,5 SiO2,6 CoOx,7 CeO2,8,9 and especially TiO210 have been extensively used in CO oxidation and surface characterization studies. Furthermore, doping rare earth ions into TiO2 has become a hot research area11-16 by virtue of their efficient promotion effect on photocatalytic activity. Therefore, some research groups have started to use the rare-earth-ion-doped TiO2 as a support to investigate their catalytic activity for CO oxidation. Recently, Zhu et al.17,18 reported that Pd/CeO2-TiO2 catalysts prepared by sol-gel precipitation exhibit a high activity for CO oxidation at low temperature. However, ceria cannot represent the other rare earth oxides, because of its special properties, such as a higher reducibility and oxygen storage capacity. Although Dai et al.19 investigated the catalytic performances of rare-earthelement-doped Au/TiO2 catalysts in detail, they simply related the promotional performances to the higher dispersion of active species. Moreover, in our previous work,20 we considered that the catalytic performance of the Pt/SiO2 catalysts can be promoted significantly by doping with rare earth elements through adjustments of the basicity of the support surface. However, for low-acidity supports, such as TiO2, modification of the surface basicity might not play a dominant role in the process of CO oxidation. To the best of our knowledge, although the structural and electronic properties of the Pd/TiO2(110) system have been extensively studied using a wide variety of experimental techniques,21-25 studies focused on investigating Pd adsorption * Corresponding author. Tel.: +86 931 4968178. Fax: +86 931 4968178. E-mail:
[email protected]. † Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences.
on Ln-doped TiO2 are relative scarce. These results make it clear that the effects of rare earth elements are far from being wellunderstood. Therefore, understanding the nature of such interfaces becomes one of the most appealing current challenges for materials scientists, and more theoretical work on this system is needed. The objective of the present investigation is to provide insight into the structure of Ln-doped LnxPdyTi1-x-yO6 and to theoretically analyze the distribution of Pd atoms and clusters. Therefore, a series of LnxPdyTi1-x-yO6 catalysts were prepared using the incipient impregnation method, and a comprehensive study of their catalytic performances for low-temperature oxidation of CO was conducted. Through light-off tests and various characterizations including N2 adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction (TPR), it was found that the catalytic activity was correlated with the conditions of the catalyst pretreatment, the state of Pd species, and the interaction between Pd and the support. Based on these examinations and computer simulations, we propose a possible reaction mechanism for CO oxidation over these catalysts and use it to explain these observations. 2. Experimental Section 2.1. Catalyst Preparation. Degussa P25 TiO2 powder (70-30% anatase) was used. PdyTi1-yO6 was prepared by incipient wetness impregnation. LnxPdyTi1-x-yO6 catalysts were prepared by coimpregnation and sequential impregnation. The Pd species were deposited by impregnation from an aqueous solution of H2PdCl4, whereas the rare earth elements were deposited by impregnation from an aqueous solution of Ln(NO3)x (Ln ) La, Ce, Sm, and Dy). In the process of sequential impregnation, P25 was first impregnated with the additives, and then the supports prepared were impregnated with the Pd species. After each impregnation, all the samples were dried at 120 °C overnight and then calcined in air at 300 °C for 3 h. After calcination, the prepared catalysts were washed to
10.1021/jp809821u CCC: $40.75 2009 American Chemical Society Published on Web 02/13/2009
4162 J. Phys. Chem. C, Vol. 113, No. 10, 2009 eliminate chlorine contamination. Washing was continued until no Cl- ions could be detected by AgNO3 solution in the filtered solution. The loading of Pd was 2%, and the loading of Ln2O3 was 10% for all of the catalysts. 2.2. Characterization of Catalysts. Powder X-ray diffraction (XRD) analysis was performed to verify the species in the catalysts. XRD patterns of the samples were recorded on a Rigaku D/MAX-RB X-ray diffractometer with a Cu KR target operated at 50 kV and 40 mA with a scanning speed of 0.5°/ min and a scanning angle (2θ) range of 10-90°. The average grain sizes, D, were determined from the XRD pattern according to the Scherrer equation
D)
kλ β cos θ
where k is a constant (shape factor, about 0.9), λ is the X-ray wavelength (0.15418 nm), β is the full width at half-maximum (fwhm) of the diffraction line, and θ is the diffraction angle. The values of β and θ for anatase were taken from the anatase (101) diffraction line. The amounts of rutile in the samples were calculated using the equation26
(
XR ) 1 + 0.8
IA IR
)
-1
where XR is the mass fraction of rutile in the sample and IA and IR are the X-ray integrated intensities of the (101) reflection of the anatase and the (110) reflection of rutile, respectively. The chemical states of the atoms in the catalyst surface were investigated by X-ray photoelectron spectroscopy (XPS) on a VG ESCALAB 210 Electron Spectrometer (Mg KR radiation; hν ) 1253.6 eV). XPS data were calibrated using the binding energy of C 1s (284.6 eV) as the standard. H2 temperatureprogrammed reduction (H2 TPR) of the catalyst was performed in a conventional flow system built in our laboratory at atmospheric pressure and at a linearly programmed rate of 10 °C /min from 20 to 800 °C (5% H2 in an Ar stream with a flow rate of 40 mL/min). A sample of 0.05 g was used for each run. The amount of H2 consumed was determined by a thermal conductivity detector (TCD). Before each measurement, the samples were purged with dry air at 300 °C for 1 h. The specific surface area of the catalyst was measured by the BrunauerEmmett-Teller (BET) method on a Micromeritics ASAP-2010 apparatus at liquid nitrogen temperature with N2 as the absorbent at 77 K. 2.3. Activity Measurement. Catalytic performance tests were carried out in a fixed-bed continuous-flow quartz reactor (i.d. ) 5 mm) under standard pressure from 80 to 250 °C. Typically, 0.1 g of the catalyst was used in each run. Partial catalysts were reduced in situ by 50% H2 in a N2 stream at 200 °C for 1 h prior to use. The total flow rate of the feed gas was 60 mL/min [gas hourly space velocity (GHSV) ) 36000 h-1]. The feed gas consisted of 2.5% CO and 20% O2 in N2 balance. Argon was used as the carrier gas, and nitrogen was used as the internal standard for gas analysis. The gas-phase effluents were analyzed by online chromatographs equipped with TCDs. At the end of the catalytic tests, the catalyst was cooled under a N2 stream and stored for characterization. 3. Results and Discussion 3.1. Characterization of Catalysts. 3.1.1. XRD. XRD was used to investigate the phase composition of TiO2 and the Pd particle size in various supported Pd catalysts. XRD patterns of LnxPdyTi1-x-yO6 catalysts are shown in Figure 1. The peaks at 25.34°, 36.99°, 48.08°, and
Wang and Lu
Figure 1. XRD patterns of various Pd catalysts prepared by the coimpregnation method: (a) PdyTiO6, (b) LaxPd1-x-yTiO6, (c) CexPd1-x-yTiO6, (d) SmxPd1-x-yTiO6, (e) DyxPd1-x-yTiO6.
TABLE 1: Lattice Parameters, Average Crystallite Sizes of the Anatase Phase, and Pd Particle Sizes of Various LnxPd1-x-yTiO6 Catalysts sample
a ) b (Å)
c (Å)
crystal size (nm)
Pd particle size (nm)
PdyTiO6 LaxPd1-x-yTiO6 CexPd1-x-yTiO6 SmxPd1-x-yTiO6 DyxPd1-x-yTiO6
3.782 3.784 3.786 3.787 3.785
9.456 9.579 9.574 9.538 9.480
53.5 37.0 36.2 30.1 30.2
4.8 2.5 3.2 3.8
62.78° correspond to the reflections from the (101), (004), (200), and (204) crystal planes, respectively, of anatase. Those at 27.47°, 36.17°, 37.92°, and 53.94° can be assigned to the characteristic peaks of the (110), (101), (200), and (211) crystal planes, respectively, of rutile. Therefore, the anatase and rutile phases can be detected in all of the samples. In the PdyTi1-yO6 sample, anatase is the dominant phase, and the mass fraction is 72%. However, the relative ratios of anatase to rutile were not reduced in the Ln-doped LnxPdyTi1-x-yO6 catalysts as reported previously.15,27 This observation suggests that the effects of rare earth elements on the transformation from anatase to rutile are inconspicuous at the low temperature of 300 °C. In addition, the average crystallite size of the anatase phases of TiO2 in the different samples can be calculated by applying the Scherrer equation on the anatase (101) diffraction peaks. The calculated average crystallite size of anatase phase is 53.5 nm in PdyTi1-yO6, while those in the La2O3-, CeO2-, Sm2O3-, and Dy2O3-doped LnxPdyTi1-x-yO6 catalysts are 37.0, 36.2, 30.1, and 30.2 nm, respectively (Table 1). These results reveal that the crystal lattice of TiO2 can be changed by doping with Ln3+ ions, owing to the different atomic sizes between Ln3+ (0.106-0.0908 Å) and Ti4+(7.45 Å). Thus, the crystal lattice deformation results in a decrease of the crystal size of TiO2. It is worth noting that no crystalline phase attributed to rare earth oxides can be found in the XRD patterns. One possible reason is that the contents of rare earth oxides in the samples are below the detection limit of this technique. Another is that rare earth ions might partially substitute Ti4+ ions and insert into the crystal lattice of TiO2, because the radii of Ln3+ are much smaller than that of Ti4+. Therefore, to clarify the situation, the lattice parameters of the anatase phase were calculated according to the results shown in Figure 1, and the results are included in Table 1. We can find that the lattice parameters of the anatase phase can be changed by the addition of Ln2O3. This result is consistent with the second reason for the undetectable rare earth oxides, confirming that Ln3+ ions are indeed partially substituted for Ti4+ ions and insert into the crystal lattice of TiO2. In addition, no obvious crystallite formation of the Pd species can be found in PdyTi1-yO6 catalyst, which indicates that the
LnxPdyTi1-x-yO6 Catalysts
Figure 2. XPS spectra of various palladium catalysts: (a) PdyTiO6, (b) LaxPd1-x-yTiO6, (c) CexPd1-x-yTiO6, (d) SmxPd1-x-yTiO6, (e) DyxPd1-x-yTiO6.
low metal content might lead to a high dispersion of Pd and, therefore, that the Pd particles are too small to be detected by XRD analysis. However, there is only a broader diffraction peak of PdO at 33.2° for LaxPd1-x-yTiO6, CexPd1-x-yTiO6, SmxPd1-x-yTiO6, and DyxPd1-x-yTiO6 catalysts, and the grain sizes calculated by the Scherrer formula are 4.8, 3.5, 3.8, and 4.2 nm, respectively. These results suggest that the dispersion of Pd species cannot be improved by the addition of rare earth oxides, which is slightly different from the former reports of Ln-doped Pt/SiO2 catalysts.20 Thus, the effect of Ln dopants on TiO2 is distinguished from that on the acidic support SiO2. 3.1.2. XPS. Supported Pd catalysts were further characterized by XPS to investigate the main elements and their chemical states. Pd (3d5/2, 3d3/2) core-level spectra obtained from LnxPd1-x-yTiO6 catalysts are presented in Figure 2. The Pd (3d5/2, 3d3/2) spectrum obtained from Pd1-yTiO6 catalyst (Figure 2a) is quite broad, suggesting that several Pd species might coexist. The Pd (3d5/2) peak at low binding energy of 335.4 eV can be related to metallic Pd species, whereas that at higher binding energy of 337.4 eV can be assigned to Pd4+,28 which might be due to partial substitution of Pd2+ ions in TiO2.10,29 In contrast, the spectra obtained from the Ln-doped LnxPd1-x-yTiO6 catalysts are narrow, indicating that only one Pd species exists. The Pd (3d5/2) binding energies of LaxPd1-x-yTiO6 (Figure 2b) and CexPd1-x-yTiO6 (Figure 2c) catalysts are both higher than that of Pd2+ in PdO, suggesting that the Pd species substituted in TiO2 mostly appear as PdO2 in virtue of their higher ionization. However, PdO is the main Pd species in SmxPd1-x-yTiO6 (Figure 2d) and DyxPd1-x-yTiO6 (Figure 2e). It should be noted that the Pd (3d5/2) peaks shift toward lower binding energies with decreasing ionic radius from La to Dy, namely, lanthanide constriction. In addition, the surface concentrations of Pd and the rare earth elements are listed in Table 2. It can be found that the Pd species mostly distribute on the surface of the support in PdyTiO6. However, the surface concentrations of Pd species in the LnxPd1-x-yTiO6 catalysts are much lower than those in Pd1-yTiO6, because partial transfer of Pd species into the subsurface of the support occurs. This result indicates that the interactions between active Pd species and supports can be enhanced by the addition of rare earth oxides. Furthermore, the Ln (3d) binding energies indicate that La, Sm, and Dy species appear as La2O3, Sm2O3, and Dy2O3 in the LnxPd1-x-yTiO6 catalysts, whereas Ti (2p3/2) corresponds to TiO2 with Ti in the 4+ oxidation state. As shown in Figure 3, the line shape and width of Ce (3d5/2) are different from those of CeO2, suggesting that partial Ce species still appear as Ce2O3. Furthermore, the relative atomic contents of CeO2 and Ce2O3 are 60% and 40%, respectively. 3.1.3. TPR. The catalyst can be reduced and oxidized in our fixed-bed system for CO oxidation. Thus, temperature-programmed reduction (TPR) measurements were used to evaluate the reducibility of various supported Pd catalysts. H2 TPR
J. Phys. Chem. C, Vol. 113, No. 10, 2009 4163 profiles of the original Pd1-yTiO6 catalyst and Ln-doped LnxPd1-x-yTiO6 catalysts are all shown in Figure 4. It can be found that the H2 TPR profile of Pd1-yTiO6 (4e) exhibits only one reduction peak centered at about 70 °C; according to the previous reports, this reduction peak can be related to the reduction of PdO.30-32 However, there is no H2 desorption peak for the decomposition of palladium hydride indicating that no metallic Pd species appears in Pd1-yTiO6, which is in conflict with the XPS result. Herein, we consider that the metallic Pd might disperse on the surface of support and can be oxidized easily at room temperature. In addition, the consumption of hydrogen shows that an almost complete reduction of palladium oxides to metallic Pd occurs, which might be due to the poor interaction between Pd species and the support in the Pd1-yTiO6 catalyst. In contrast, the H2 consumptions for Pd reduction in the Ln-doped LnxPd1-x-yTiO6 catalysts are much lower than that in Pd1-yTiO6, suggesting that partial palladium oxide reduction can occur. This fact indicates that the interaction between Pd species and the support can be promoted by the addition of rare earth elements, which is consistent with the XRD results. Moreover, comparing the TPR profiles of the Ln-doped LnxPd1-x-yTiO6 catalysts, it can be found that the reduction temperatures of the palladium oxides shift toward higher temperature with decreasing Ln3+ ionic radius. These results suggest that palladium oxides are likely to exist in different structural environments, owing to the different interactions between Pd and the support in various Ln-doped LnxPd1-x-yTiO6 catalysts. Therefore, the structures of the LnxPd1-x-yTiO6 catalysts are analyzed in detail below. 3.1.4. BET. The pore structure parameters of various Pd catalysts from the N2 adsorption-desorption isotherms are given in Table 2. The surface area, pore volume, and average pore size of the Pd1-yTiO6 catalyst are 40.2 m2/g, 0.20 cm3/g, and 18.0 nm, respectively. It is interesting to note that the surface areas and pore volumes of the Ln-doped LnxPd1-x-yTiO6 catalysts are much larger than those of the Pd1-yTiO6 catalyst, whereas their pore sizes are slightly smaller than that of the Pd1-yTiO6 catalyst. These facts show that the Ln dopants cause increases in the surface areas and pore volumes and a decrease in the pore sizes, which might be due to the appearance of the lattice distortion and structure defects in the TiO2 lattice. The adsorption-desorption isotherms (not shown) for the Ln-doped LnxPd1-x-yTiO6 catalysts are similar in shape to that for the Pd1-yTiO6 catalyst, suggesting that the framework of mesoporous TiO2 has a better thermal stability and the distributions of rare earth oxides are regular. However, it should be noted that the dispersions of the pore sizes on the doped catalysts are much wider; therefore, the coexistence of micropores and mesopores might be one reason for the larger surface areas of the Ln-doped catalysts compared to Pd1-yTiO6. 3.2. Activity Tests. To examine the effects of reaction temperature on the catalytic activity behaviors of different supported Pd catalysts, tests of CO oxidation activity were carried out under the conditions described in the Experimental Section. Comparisons of CO oxidation activities over various Ln-doped LnxPd1-x-yTiO6 catalysts prepared by coimpregnation and sequential impregnation methods are shown in Figure 5. It can be found that the minimum temperatures for CO elimination (T100) over the Pd1-yTiO6 catalysts doped by La, Ce, Sm, and Dy that were prepared by the coimpregnation method are 180, 120, 210, and 260 °C, respectively, whereas for the corresponding catalysts prepared by the sequential-impregnation method, the T100 values increase to 230, 130, 240, and 280 °C, respectively. It is known that the minimum temperature for CO
4164 J. Phys. Chem. C, Vol. 113, No. 10, 2009
Wang and Lu
TABLE 2: XPS Data and Textural Properties of Different Catalysts Obtained by N2 Adsorption binding energies (eV) catalyst PdyTiO6 LaxPd1-x-yTiO6 CexPd1-x-yTiO6 SmxPd1-x-yTiO6 DyxPd1-x-yTiO6
Ti 2p3/2 Ln 3d5/2 458.5 458.6 458.3 458.4 458.4
surface concentration (at. %)
Pd 3d5/2
335.4, 337.4 839.0 337.7 882.5 337.4 1083.2 336.4 336.4
Ln
Pd
0 34.5 10.6 9.6 1.0
2.0 0.9 1.1 0.8 1.0
elimination is higher and the catalytic activity for CO oxidation is lower. Thus, the results confirm that the activities are very sensitive to the preparation procedures, and the performances of coimpregnated catalysts are superior to those of the sequentially impregnated catalysts. Therefore, the light-off curves of CO conversion on the original Pd1-yTiO6 and Ln-doped LnxPd1-x-yTiO6 catalysts prepared by the coimpregnation method are presented in Figure 6. It can be found that CO cannot be eliminated over the Pd1-yTiO6 catalyst until the reaction temperature increases to 240 °C, indicating the low catalytic performance of the Pd1-yTiO6 catalyst. Furthermore, it can be found that the catalytic activity for CO oxidation over Dy-doped DyxPd1-x-yTiO6 is slightly lower than that on Pd1-yTiO6, indicating that the additive effect of Dy2O3 is very poor. Fortunately, the effects of the other rare earth oxides are substantial, especially at higher reaction temperatures. Thus, a stepwise change can also be observed in the 50% conversion temperatures (T50), which are 145, 85, and 175 °C on the Pd1-yTiO6 catalysts doped by La2O3, CeO2, and Sm2O3,
Figure 3. XPS spectra of Ce 3d5 over CexPd1-x-yTiO6.
Figure 4. H2 TPR profiles of (a) PdyTiO6, (b) LaxPd1-x-yTiO6, (c) CexPd1-x-yTiO6, (d) SmxPd1-x-yTiO6, (e) DyxPd1-x-yTiO6.
Figure 5. Activity comparison over various supported Pd catalysts prepared by coimpregnation and sequential-impregnation methods.
surface area (m2/g) pore volume (cm3/g) average pore size (nm) 40.2 42.4 46.5 42.6 42.3
0.20 0.22 0.22 0.21 0.21
21.0 20.6 19.0 19.5 19.6
TABLE 3: Physical Chemistry Parameter of Rare Earth Elements and Ti element
metallic radius (Å)
electronegativity (eV)
ionization potential (eV)
La Ce Sm Dy Ti
1.88 1.83 1.80 1.77 2.00
1.10 1.12 1.17 1.22 1.54
19.18 20.20 23.4 22.8 43.27
respectively. Moreover, the corresponding 100% conversion temperatures (T100) are 170, 120, and 210 °C, respectively. Herein, we can conclude that the catalytic activities for CO oxidation over various Pd catalysts prepared by the coimpregnation method decrease in the order of CexPd1-x-yTiO6 > LaxPd1-x-yTiO6 > SmxPd1-x-yTiO6 > Pd1-yTiO6 > DyxPd1-x-yTiO6. Metallic radii, electronegativities, and third ionization potentials of the rare earth elements and Ti are reported in Table 3. One can see that, except for the heavy rare earth element of Dy, the electronegativities and ionization potentials are both increased with decreasing metallic radii. In addition, the effects of the third ionization potential on the minimum temperature for CO elimination were investigated, and the results are shown in Figure 7. It can be seen that, except for the Ce-doped CexPd1-x-yTiO6 catalyst, the temperatures for CO elimination (T100) over LnxPd1-x-yTiO6 catalysts increase with increasing ionization potential. It is known that the ability of losing electron is inversely proportional to the ionization potential, so the lower ionization potential is beneficial to electron transfer from the rare earth element to the oxygen that is bonded to Pd.
Figure 6. Effect of reaction temperature on the activity of palladium catalysts prepared by the coimpregnation method.
Figure 7. Effects of the third ionization potential of rare earth elements and the fourth ionization potential of Ti on the minimum temperature for CO elimination.
LnxPdyTi1-x-yO6 Catalysts
J. Phys. Chem. C, Vol. 113, No. 10, 2009 4165
Figure 8. Activity comparison over various supported Pd catalysts in the calcined (coimpregnation) and reduced states.
Furthermore, the presence of the extra electrons favors the existence of Pd species with higher-value states. Although both the nature of the active palladium phase and the elucidation of the mechanism for CO oxidation are still debated, herein, we consider that the palladium oxides are major active sites. Therefore, the ionization potential of rare earth element is higher, and the catalytic activity for CO oxidation over LnxPd1-x-yTiO6 catalyst is lower. Finally, because the oxygen mobility is significantly important for the successful progress of the CO oxidation reaction,33,34 the increased oxygen mobility resulting from the synergism of Ce and Pd species is one possible reason for the best catalytic performance being observed for the CexPd1-x-yTiO6 catalysts. Comparisons of the catalytic activity for CO oxidation over the LnxPd1-x-yTiO6 catalysts with different pretreatments (as calcined, after being prereduced with H2 at 200 °C for 1 h) are shown in Figure 8. It can be seen that the effects of prereduction on the catalytic performance of various supported Pd catalysts are obviously different. The catalytic activities on Pd1-yTiO6 and LaxPd1-x-yTiO6 samples can be inhibited by the reduction pretreatment, and the minimum temperatures of CO elimination (T100) increase from 240 and 180 °C to 260 and 220 °C, respectively. However, no obvious change in the catalytic activity can be detected on the reduced CexPd1-x-yTiO6 catalyst, which is different from the previous result,35 owing to the different preparation method for CeO2-TiO2. Finally, the activities for CO oxidation over SmxPd1-x-yTiO6 and DyxPd1-x-yTiO6 catalysts can be promoted significantly, as the minimum temperatures of CO elimination decrease by 40 and 20 °C, respectively. These facts suggest that the reaction mechanisms for CO oxidation at low temperature might be different over the Ln-doped LnxPd1-x-yTiO6 catalysts. 3.3. Possible Structure of Ln-Doped LnxPd1-x-yTiO6. The possible adsorption sites of Pd on the perfect TiO2(110) surface are shown in Figure 9. Sanz et al.36 found that the main contribution to the energy for Pd adsorption on the perfect TiO2 surface comes from Pd polarization. Therefore, Pd single atoms and dimers prefer to adsorb on the surface, which results in the high dispersion of Pd on the Pd1-x-yTiO6 catalyst surface. Moreover, Pd deposition is accompanied by a strong polarization of the adsorbed atoms, which eventually transfers some electron density toward the surface; thus, upon deposition almost no Pd oxidation occurs. This is consistent with the XPS result obtained from Pd1-x-yTiO6. In addition, Asaduzzaman et al.37 reported that subsurface sites involving Ti substitution are more stable than any surface adsorption sites. Moreover, S1 is the preferred subsurface site for V, where V has substituted the Ti atom and the latter occupies the I1 interstitial site. Therefore, V preferentially occupies subsurface sites. Compared to V, Pd has a similar ionic radius and a larger electronegativity, which indicate that Pd has a higher bond enthalpy with oxygen and that the Pd adsorption site is more stable. Thus, the appearance of Pd species partially substituting for Ti4+ in Pd1-x-yTiO6 is reasonable.
Figure 9. (a) Top view of the supercell. Different possible adsorption sites are labeled 1-5. (b) Side view of the TiO2(110) surface. Black and gray balls represent O and Ti atoms, respectively. Different substitutional (S) and interstitial (I) sites are shown.
According to the law of energy conservation before and after the reaction, the change in energy of the overall system is the energy that is required for the products. Therefore, the Ln-doped TiO2 formation energy (Ef) is defined as38
Ef ) ETiO2:Ln - ETiO2 - ELn + ETi where ETiO2:Ln and ETiO2 are the total energies of Ln-doped TiO2 and pure TiO2, respectively, in the same size supercell and ELn and ETi are the energies of Ln atoms and Ti atoms, respectively, in the elementary substance. Zhao et al.39 considered Ef to be closely correlated with the ionization potentials and electron configurations of the dopants. Furthermore, the formation energies of La-, Ce-, Sm-, and Dy-doped are 6.25, 1.58, 5.26, and 7.22 eV, respectively, which indicates that, except for Ladoped, the Ef values increase with decreasing ionic radius. The exceptional phenomenon for the La-doped material might be due to the absence of 4f states in the electron configuration of La. Certain surface relaxations in Ln-doped TiO2 mainly involve distortion of the geometry around the protruded and in-plane oxygen atoms. In addition, this perturbation also affects the neighboring atoms, and the extent of the relaxations grows with the cell dimensions. Hence, on one hand, Pd cluster growth can be hindered by surface defects as Pd atoms preferentially occupy oxygen vacancies, which indeed can act as efficient nucleation sites. On the other hand, the adsorbed Pd can more easily diffuse into subsurface sites owing to the surface relaxation. The structure of anatase has tetragonal symmetry and belongs to the same point group, but it contains an inversion center perpendicular to the [001] axis that gives it body-centered symmetry with a space group I41/amd. Note that there is a diamond glide along the c axis in the ab plane (see Figure 10a). The structures of the Ln-doped LnxPdyTi1-x-yO6 catalysts were studied, and calculations were performed in ab plane allowing for the three-dimensional octahedral tilt. Structure 1 (Figure 10b) contains no O vacancy, structure 2 (Figure 10c) contains an O vacancy adjacent to Pd, and structure 3 (Figure 10d) contains an O vacancy between two Ti atoms but not adjacent to Pd. In structure 1, the substitution of one Pd for one Ti with no accompanying vacancy results in a dramatic decrease in symmetry, which can be exhibited by a change in band lengths. The three nearest Ti neighbors all have two distinct sets of bond lengths, that is, four short bonds of about 1.94 Å and two long bonds of about 1.98 Å. The distorted Ti octahedron is the result of the formation of strong bonds (∼2.03 Å) to the small Pd4+ cation. The bond length of 2.03 Å is typical of Pd4+ in an O6 environment (rO2- ) 1.42 Å and rPd4+ ) 0.615 Å). Comparing
4166 J. Phys. Chem. C, Vol. 113, No. 10, 2009
Figure 10. Computer simulations of (a) anatase and (b-d) LnxPd1-x-yTiO6: (b) structure 1, where Pd is not accompanied by an O vacancy; (c) structure 2, where Pd is adjacent to an O vacancy; (d) structure 3, which contains an O vacancy between two Ti atoms, not adjacent to Pd (lower plane, not shown). Pd is yellow, Ln is light blue, Ti is green, and O is pink.
the two structures with an oxygen vacancy, we consider that the higher-energy structure is structure 3, because the reduction of Pd4+ to Pd2+ is more favorable than the reduction of Ti4+. As shown in Figure 10c, the Pd in structure 2 is not fivecoordinate square-pyramidal but four-coordinate square-planar. The first four shorter bonds are, on average, 2.04 Å, and the fifth one is 2.66 Å. This longer bond occurs as a result of the apical O in the TiO5 square pyramid moving away from the Pd. Therefore, the O vacancy in structure 2 is further stabilized by the formation of a square-planar environment around Pd2+, and rPd2+ ) 0.64 Å. However, this does not occur in structure 3, where the O vacancy is between two Ti atoms (Figure 10d). All six PdsO bond lengths in structure 3 are around 2.02 Å, whereas the two TiO5 units surrounding the vacancy seem to contract inward. These bond lengths are similar to those seen in structure 1, where there is no O vacancy. The Pd species in structure 2 is more different than those in structures 1 and 3, as both structures 1 and 3 contain Pd4+ in an octahedral environment and have the same d states, although they are shifted in energy. In contrast, in structure 2, Pd is stabilized in a square-planar environment as Pd2+; thus, the new d states closer to the Fermi level are populated, and previously unfilled bands above the Fermi level decrease in intensity. Therefore, the Pd species in structure 2 is less ionized than that in structures 1 and 3. In addition, comparison of structures 1 and 3 shows that the Pd species in structure 3 is slightly more ionized than that in structure 1, because the O vacancy between two Ti atoms causes the octahedron around Pd to contract slightly, consequently forming stronger and more ionic Pd-O bonds. 3.4. Mechanism. In line with the XPS results and the analysis of the possible structure of the Ln-doped LnxPdyTi1-x-yO6 catalysts, we consider that LaxPdyTi1-x-yO6 and CexPdyTi1-x-yO6 exist as structure 1 or structure 3, which contains Pd4+ in an octahedral environment. However, SmxPdyTi1-x-yO6 and DyxPdyTi1-x-yO6 exist as structure 2, where Pd is stabilized in a square-planar environment as Pd2+. These results might be
Wang and Lu due to the different crystal structures of Ln2O3; that is, the crystal structure of La2O3 and Ce2O3 is hexagonal, and the crystal structure of Sm2O3 is rhombohedral, whereas the crystal structure of Dy2O3 is body-centered cubic. Therefore, three structures might exist together in PdyTi1-yO6, owing to the weak interaction between Pd and the support. Moreover, partial metallic Pd species still adsorb on the surface of TiO2. It can be found that the catalytic activity is closely related to the ionization of Pd species: the more ionization, the higher the catalytic activity. In other words, the active sites for CO oxidation are different on various LnxPd1-x-yTiO6 catalysts. According to the change in catalytic activity over prereduced LnxPd1-x-yTiO6, we deem that the Pd species substituted in TiO2 can be moved out of the TiO2 matrix and reduced to metallic Pd by the reduction pretreatment in Pd1-yTiO6 and LaxPd1-x-yTiO6. This is due to the poor interaction between Pd and the supports, which is consistent with the previous report.40 In contrast, for CexPd1-x-yTiO6, no Pd species that substituted in TiO2 can be moved out of matrix because of the strong interaction between Pd and the support. Therefore, the activity for CO oxidation over prereduced CexPd1-x-yTiO6 exhibits no detectable change from that of the calcined material. On the contrary, the catalytic activities over SmxPd1-x-yTiO6 and DyxPd1-x-yTiO6 can be promoted significantly by the reduction pretreatment. The abnormal phenomenon shows that the effects of prereduction on structure 2 are different from those on structures 1 and 3. According to the analysis of the LnxPd1-x-yTiO6 structures, we find that Pd is stabilized in a square-planar environment as Pd2+; therefore, the d states are closer to the Fermi level than those in structures 1and 3. Thus, the formation of oxygen vacancies is easier than the reduction of Pd2+ to metallic Pd, which can be supported by the higher reduction temperature of PdO. It is known that the presence of oxygen vacancies is beneficial to the adsorption of CO and active oxygen species. Herein, we consider that the presence of oxygen vacancies deduced by the reduction pretreatment might be responsible for the promotional catalytic performance over SmxPd1-x-yTiO6 and DyxPd1-x-yTiO6. Furthermore, we detect that, except for LaxPd1-x-yTiO6, the catalytic activities for CO oxidation over LnxPd1-x-yTiO6 catalysts do match well with the Ln-doped formation energies. These results indicate that the interaction between the rare earth elements and TiO2 plays an important role in the high catalytic activity. 4. Conclusions The catalytic activities for CO oxidation at low temperature on various LnxPd1-x-yTiO6 catalysts have been investigated. The results show that the activities of the catalysts are very sensitive to the preparation procedures and pretreatment. The performances of coimpregnated catalysts are superior to those of sequentially impregnated catalysts and the activities decrease in the following order: CexPd1-x-yTiO6 > LaxPd1-x-yTiO6 > Pd1-yTiO6 > SmxPd1-x-yTiO6 > DyxPd1-x-yTiO6. Based on the experimental data and computer simulation analysis, we believe that LnxPd1-x-yTiO6 catalysts exist in distinct structures by virtue of the different Ln-doped formation energies. Therefore, the effects of prereduction on the catalytic activities for CO oxidation over various LnxPd1-x-yTiO6 catalysts are extremely different. In summary, on one hand, the catalytic activity is related to the ionization of Pd species: the more ionization, and the higher the catalytic activity. On the other hand, the catalytic activity does match well with the Ln-doped formation energy, indicating that the interaction between the rare earth element and TiO2 plays an important role in CO oxidation catalytic activity.
LnxPdyTi1-x-yO6 Catalysts Acknowledgment. The 973 programs (2007CB210204, 2007CB613305, and 2009CB220003) are acknowledged for financial support of this work. References and Notes (1) Kang, Y. M.; Wan, B. Z. Appl. Catal. A: Gen. 1995, 128, 53. (2) Inui, T.; Ono, Y.; Takagi, Y.; Kim, J. B. Appl. Catal. A: Gen. 2000, 202, 215. (3) Yuan, Y.; Kozlova, A. P.; Asakura, K.; Wan, H.; Tsai, K.; Iwasawa, Y. J. Catal. 1997, 170, 191. (4) Wang, G. Y.; Li, X. M.; Bai, N.; Zhang, W. X.; Jiang, D. Z.; Wu, T. H. Chem. J. Chin. UniV. 2001, 22, 431. (5) Skotak, M.; Karpi’nski, Z.; Juszczyk, W.; Pielaszek, J.; epi’nski, L. K.; Kazachkin, D. V.; Kovalchuk, V. I.; d’Itri, J. L. J. Catal. 2004, 227, 11. (6) Lambert, S.; Cellier, C.; Grange, P.; Pirard, J. P.; Heinrichs, B. J. Catal. 2004, 221, 335. (7) Liotta, L. F.; Carlo, G. D.; Pantaleo, G.; Venezia, A. M.; Deganello, G.; Merlone Borla, E.; Pidria, M. Appl. Catal. B: EnViron. 2007, 75, 182. (8) Pozdnyakova, O.; Teschner, D.; Wootsch, A.; Kro¨hnert, J.; Steinhauer, B.; Sauer, H.; Toth, L.; Jentoft, F. C.; Knop-Gericke, A.; Paa´l, Z.; Schlo¨gl, R. J. Catal. 2006, 237, 17. (9) Oh, S. H.; Hoflund, G. B. J. Phys. Chem. A 2006, 110, 7609. (10) Roy, S.; Hegde, M. S.; Ravishankar, N.; Madras, G. J. Phys. Chem. C 2007, 111, 8153. (11) Butler, E. C.; Davis, A. P. J. Photochem. Photobiol. A: Chem. 1993, 70, 273. (12) Yu, J. C.; Lin, J.; Kwok, R. W. M. J. Phys. Chem. 1998, 102, 5094. (13) Serpone, N.; Lawless, D. Langmuir 1994, 10, 643. (14) Karakitsou, K. E.; Verykios, X. E. J. Phys. Chem. 1993, 97, 1184. (15) Lin, J.; Yu, J. C.; Lo, D.; Lam, S. K. J. Catal. 1999, 183, 368. (16) Boschloo, G.; Hagfeldt, A. Chem. Phys. Lett. 2003, 370, 381. (17) Zhu, H. Q.; Qin, Z. F.; Shan, W. J.; Shen, W. J.; Wang, J. G. J. Catal. 2004, 225, 267. (18) Dong, G. L.; Wang, J. G.; Gao, Y. B.; Chen, S. Y. Catal. Lett. 1999, 58, 37.
J. Phys. Chem. C, Vol. 113, No. 10, 2009 4167 (19) Ma, Zh.; Overbury, S. H.; Dai, Sh. J. Mol. Catal. A: Chem. 2007, 273, 186. (20) W, F.; Lu, G. X. J. Power Sources 2008, 181, 120. (21) Xu, C.; Lai, X.; Zajac, G. W.; Goodman, D. W. Phys. ReV. B. 1997, 56, 13464. (22) Jak, M. J. J.; Konstapel, C.; van Kreuningen, A.; Verhoeven, J.; Frenken, J. W. M. Surf. Sci. 2000, 457, 295. (23) Jak, M. J. J.; Konstapel, C.; van Kreuningen, A.; Verhoeven, J.; Frenken, J. W. M. Surf. Sci. 2001, 474, 28. (24) Bowker, M.; Stone, P.; Bennett, R.; Perkins, N. Surf. Sci. 2002, 497, 155. (25) Howard, A.; Mitchell, C. E. J.; Egdell, R. G. Surf. Sci. 2002, 515, L504. (26) Spurr, R. A.; Myers, H. Anal. Chem. 1957, 29, 760. (27) Lin, J.; Yu, J. C. J. Photochem. Photobiol. A: Chem. 1998, 116, 63. (28) Bi, Y.Sh.; Lu, G. X. Appl. Catal. B: EnViron. 2003, 41, 279. (29) Roy, S.; Hegde, M. S.; Sharma, S.; Lalla, N. P.; Marimuthu, A.; Madras, G. Appl. Catal. B: EnViron. 2008, 84, 341. (30) Figoli, N. S.; Argentiere, P. C. L.; Arcoya, A.; Seoane, X. L. J. Catal. 1995, 155, 95. (31) Seoane, X. L.; Figoli, N. S.; Argentiere, P. C. L.; Gonzalez, J. A.; Arcoya, A. Catal. Lett. 1997, 47, 213. (32) Yang, C.; Ren, J.; Sun, Y. Catal. Lett. 2002, 84, 123. (33) Ramaker, D. E.; de Graaf, J.; van Veen, J. A. R.; Koningsberger, D. C. J. Catal. 2001, 203, 7. (34) Damyanova, S.; Perez, C. A.; Schmal, M.; Bueno, J. M. C. Appl. Catal. A: Gen. 2002, 234, 271. (35) Zhu, H. Q.; Qin, Zh.F.; Shan, W. J.; Shen, W. J.; Wang, J. G. J. Catal. 2005, 233, 41. (36) Sanz, J. F.; Ma’rquezA., J. Phys. Chem. C 2008, 111, 3949. (37) Asaduzzaman, A. M.; Kru¨ger, P. J. Phys. Chem. C 2008, 112, 4622. (38) Cui, X. Y.; Medvedeva, J. E.; Delley, B.; Freeman, A. J.; Newman, N.; Stampfl, C. Phys. ReV. Lett. 2005, 95, 256404. (39) Zhao, Z. Y.; Liu, Q. J. J. Phys. D: Appl. Phys. 2008, 41, 085417. (40) Nishihata, Y.; Mizuki, J.; Akao, T.; Tanaka, H.; Uenishi, M.; Kimura, M.; Okamoto, T.; Hamadak, N. Nature 2002, 418, 164.
JP809821U
