Origin of Active Oxygen in a Ternary CuO x /Co 3 O 4 CeO 2 Catalyst for CO Oxidation

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1 pubs.acs.org/jpcc Origin of Active Oxygen in a Ternary CuO x /Co 3 O 4 CeO 2 Catalyst for CO Oxidation Zhigang Liu,*, Zili Wu,*, Xihong Peng, Andrew Binder, Songhai Chai, and Sheng Dai*,, School of Chemistry and Chemical Engineering, Hunan University, Changsha , China Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Department of Chemistry, University of Tennessee, Knoxville, Tennessee , United States School of Letters and Sciences, Arizona State University, Mesa, Arizona 85212, United States Downloaded via ARIZONA STATE UNIV on September 4, 2018 at 22:03:55 (UTC). See for options on how to legitimately share published articles. *S Supporting Information ABSTRACT: We have studied CO oxidation over a ternary CuO x /Co 3 O 4 CeO 2 catalyst and employed the techniques of N 2 adsorption/desporption, XRD, TPR, TEM, in situ DRIFTS, and QMS (quadrupole mass spectrometry) to explore the origin of active oxygen. DRIFTS-QMS results with labeled 18 O 2 indicate that the origin of active oxygens in CuO x /Co 3 O 4 CeO 2 obeys a model, called a queue mechanism. Namely gas-phase molecular oxygens are dissociated to atomic oxygens and then incorporated in oxygen vacancies located at the interface of Co 3 O 4 CeO 2 to form active crystalline oxygens, and these active oxygens diffuse to the CO Cu + sites thanks to the oxygen vacancy concentration magnitude and react with the activated CO to form CO 2. This process, obeying a queue rule, provides active oxygens to form CO 2 from gas-phase O 2 via oxygen vacancies and crystalline oxygen at the interface of Co 3 O 4 CeO INTRODUCTION CO oxidation is believed to proceed via the Mar van Krevelen mechanism over ceria-based catalysts; namely, it involves the removal of surface lattice oxygen by CO and consequent annihilation of vacancies by gas phase oxygen, which results from the fact that Ce 3+ and Ce 4+ are stable, allowing the oxide to shift between CeO 2 and CeO 2 x. 1,2 This has been clarified by many studies. For example, Guzman 3 reported that CO 2, during CO adsorption experiments and without any oxygen in the gas stream, was formed. This result indicates that nanocrystalline CeO 2 is able to supply reactive oxygen to the gold active species for the oxidation of CO, which is consistent with the idea of CeO 2 acting as an oxygen buffer by releasing uptaking oxygen through redox processes involving the Ce 4+ /Ce 3+ couple. Liu and Stephanopoulos 4 have suggested a reaction model; i.e., Cu + species were stabilized by the interaction between CuO and CeO 2, and the Cu + species provide surface sites for CO chemisorption while the CeO 2 provides the oxygen source through a fast Ce 4+ /Ce 3+ redox cycle. Martiǹez-Arias et al. 5 concluded that the CuO species in both fully oxidized and partially reduced states were significantly affected by the interaction with underlying CeO 2 support. Luo et al. 6,7 found that the finely dispersed CuO species in CuO CeO 2 had the highest activity. Moreover, in their further work, they proposed that the reaction may take place at the interface of CuO CeO 2 and the catalyst for CO oxidation is structure-sensitive. As for Co 3 O 4 CeO 2 binary catalyst, Wang 8 investigated the oxygen storage-release capacity. They found that the solubility limit of cobalt oxides in the CeO 2 was 5 mol % based on Co/ (Co + Ce), and cobalt oxides with multiple valences contributed to the majority of oxygen storage-release capacity. In addition, Meng 9 found that Co 3 O 4 crystallites in Co 3 O 4 CeO 2 catalyst with molar ratio of 1:1 are considered to be encapsulated by nanosized CeO 2, with only a small fraction of Co ions exposing on the surface and strongly interacting with CeO 2. In addition, they proposed that over Co 3 O 4 CeO 2 the CO oxidation should take place preferentially at the interface of Co 3 O 4 CeO 2 instead of the surface of Co 3 O 4. However, Chen 10 provided the evidence that O 2 is supplied by superoxide species on CeO 2 in the presence of OH and can diffuse to the interface of noble-metal/ceo 2 for CO oxidation. A preliminary mechanism involving surface lattice oxygen of CuO and oxygen vacancy participation is proposed to fully explicate the synergistic process leading to light-off, and the cause of light-off is attributed to the formation of coshared oxygen ions coupled with the creation of high-turnoverfrequency active sites which are composed of metastable copper oxide species and oxygen vacancies of two types. 11 All in all, due to the structural complication of the multicomponent oxide catalysts (i.e., CuO x /Co 3 O 4 CeO 2 ), there still exists a lot of controversy about the reaction mechanism, particularly about the reaction pathway and nature of active oxygen. Received: August 22, 2014 Revised: November 14, 2014 Published: November 14, American Chemical Society 27870

2 To understand the catalytic origin of these oxidation reactions and to design efficient catalysts, it is essential to know the activation route of molecular O 2 on the catalyst surface. Our work sheds some light on how CO oxidation reaction takes place and where the active oxygen originates in 1Cu5Co5Ce. In the present study, CuO x is supported on Co 3 O 4 CeO 2 with Co:Ce atomic ratio of 1:1. The mechanism of CO oxidation over CuO x /Co 3 O 4 CeO 2 is investigated by techniques including BET, IR, XRD, TEM, H 2 -TPR, and in situ DRIFTS-QMS. Here, we provide evidence via oxygen isotopic exchange experiments that gas-phase 18 O 2 is dissociated into lattice oxygen and migrates to react with CO Cu + to form CO 2 along the interface of Co 3 O 4 CeO 2 one by one, and this reaction model is called a queue mechanism. 2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation. CuO x /Co 3 O 4 CeO 2 with Cu:Co:Ce atomic ratios equal to 1:5:5 was synthesized by coprecipitation method and designated as 1Cu5Co5Ce g of Cu(NO 3 ) 2 3H 2 O and appropriate amounts of Ce(NO 3 ) 3 6H 2 O and CoCl 2 were dissolved in 100 ml of deionized water at room temperature and stirred for 15 min; then 100 ml of NaOH solution (0.375 M) was added dropwise to the above solution under vigorous stirring. After stirring for 30 min, the obtained precipitate was centrifuged and washed, first with 150 ml of water and then with 150 ml of anhydrous ethanol. The product was dried at 90 C and heated in air at 500 C for 1 h. 1Cu10Co (Cu:Co = 1:10), 1Cu10Ce (Cu:Ce = 1:10), and 5Co5Ce (Co:Ce = 5:5) were synthesized via a similar process as mentioned above Evaluation of Catalytic Activity. Catalytic CO oxidation was tested in a continuous fixed-bed microreactor. 50 mg of catalyst without any pretreatment was loaded into a quartz tube (i.d. 4 mm). The feed consisted of 1 vol % CO (balanced in air), and the flow rate of the reactant stream was 40 ml/min, equivalent to a space velocity of ml/(h g cat. ). A portion of the product stream was extracted periodically with an automatic sampling valve and analyzed using a dualcolumn gas chromatograph with a thermal conductivity detector (TCD) Characterization Techniques. Surface area, pore volume, and pore size distribution were measured by nitrogen adsorption/desorption at 196 C using a Micromeritics ASAP 2020 surface area and porosity analyzer. The samples were degassed at 200 C for 2 h prior to the adsorption experiment. The surface area was determined by the BET method in the partial pressure range. X-ray diffraction measurement was collected on a Panalytical powder diffractometer operating at 40 ma and 40 kv using Co Kα radiation source. The data of 2θ from 20 to 90 were collected with the step size of 10 /min. The average crystallite size of CeO 2 was calculated by using the Scherrer equation. HR-TEM images were obtained using a JEOL JEM-2110 system operating at 200 kv. Temperatureprogrammed measurements were performed on a Thermo- Finnigan TPDRO 1100 instrument with a thermal conductivity detector. For the H 2 -TPR test, the quartz tube reactor was loaded with a 50 mg sample in powder form and heated from room temperature to 900 C in4%h 2 /Ar. A heating rate of 10 C/min and a gas flow rate of 50 ml/min were used. In situ diffuse reflectance infrared spectroscopy (DRIFTS) measurement was performed on a Nicolet Nexus 670 spectrometer equipped with a MCT detector cooled by liquid nitrogen and an in situ chamber (HC-900, Pike Technologies) which allows the sample heated up to 900 C. The exiting stream was analyzed by an online quadrupole mass spectrometer (QMS) (OmniStar GSD-301 O 2, Pfeffer Vacuum). Before measurement, the sample powder (30 mg) was treated in situ at 450 Cin2%O 2 /He with a flow rate of 25 ml/min to eliminate water traces. After cooling to room temperature in a He flow (25 ml/min), the background spectrum was collected for spectral correction. Then, 2% CO/ 2% Ar/He (25 ml/min) was introduced to the in situ chamber for adsorption. After holding at room temperature for 30 min, the sample was purged with He (25 ml/min) for 10 min and then switched to 2% O 2 /He for another 10 min. Difference IR spectra were collected during the CO adsorption and desorption process. In CO oxidation experiment, the pretreated sample was exposed to the reaction mixture (20 ml/min 2% O 2 /He and 5 ml/min 2% CO/2% Ar/He; O 2 /CO = 4) at room temperature and ramped up to 250 C at a rate of 10 C/ min. Switching between 2% 18 O 2 /He (Isotech, 18 O 2 purity of 99%) and 2% 16 O 2 /He was also done during CO oxidation reaction. IR spectra were recorded continuously to follow the surface changes during the reaction. 3. RESULTS AND DISCUSSION 3.1. Characterization of the Catalysts. In this study, pure CuO and CeO 2 were yielded via calcination of their corresponding nitrate metal salts at 300 C for 1 h, and pure Co 3 O 4 was attained by coprecipitation as mentioned previously. Pure CuO and Co 3 O 4, shown in Table 1, have very low surface Table 1. Crystallite Size, BET Surface Area, and Catalytic Activity of the Samples crystallite size a (nm) sample d(cuo) d(ceo 2 ) d(co 3 O 4 ) S A (BET) (m 2 g 1 ) T 50 ( C) CuO CeO Co 3 O Co-5Ce Cu-10Co Cu-5Co-5Ce Cu-10Ce a Particle sizes were obtained from CeO 2 (111) and Co 3 O 4 (222) reflection. areas (i.e., 1 and 16 m 2 /g, respectively), and CeO 2 possesses a relatively high surface area of 82 m 2 /g. The samples, i.e. 1Cu10Ce, 1Cu10Co, and 1Cu5Co5Ce, were prepared by coprecipitation and heated at 500 C for 1 h and have surface areas of 187, 20, and 102 m 2 /g, respectively. Apparently, when Cu and/or Co were introduced, the surface areas of the samples were remarkably enlarged. This is attributed to the addition of transition metals. As reported, the doping of transition metals to ceria would greatly increase their surface areas. 12,13 Typically, the larger the surface area of the catalyst is, the more the active centers are exposed. And in turn, this results in a higher catalytic performance. 14,15 Figure 1 reveals the XRD patterns of the catalysts and the crystallite sizes of Co 3 O 4 and CeO 2 in these catalysts are calculated from the line broadening of the most intense XRD reflections, using the Sherrer equation. The distinct fluorite oxide diffraction peaks of CeO 2 are seen at 28.5, 33.1, 47.5, 56.3, 59.1, and 69.5, respectively. The diffraction peaks are 27871

3 The Journal of Physical Chemistry C Figure 1. Diffraction patterns of the catalysts. indexed to (111), (200), (220), (311), (222), and (400) planes, matching well those of the face-centered cubic fluorite structure of CeO2.16 The characteristic peaks of Co3O4 with spinel structure are also exhibited at 31.3, 36.8, 44.8, and 59.5, respectively.16 However, XRD peaks of CuO cannot be detected under the detection sensitivity in all samples. It indicates that CuO species are likely present in an amorphous state and/or with a relatively high dispersion on the carriers.6 The particle sizes of ceria in 1Cu10Ce and 1Cu5Co5Ce are 7.0 and 6.0 nm, respectively. This suggests that the addition of Co is beneficial to achieve smaller particle sizes.17 As mentioned in the literature,18,19 the mutual interaction between CeO2 and Co3O4 has a determining effect on the catalytic performance of CeO2 Co3O4 based catalysts. Thereafter, smaller particle sizes of CeO2 and Co3O4 are beneficial to yield more interfacial contact and propagate more active centers and higher activity.18,19 Additionally, 1Cu5Co5Ce, in comparison with 5Co5Ce, has smaller particle sizes (i.e., 6.0 vs 7.5 nm for CeO2). This means Cu can be helpful to attain smaller particles for CeO2 Co3O4 based catalysts. In light of the absence of CuOx patterns in XRD, we can deduce that introduction of Cu and Co gives rise to a smaller particle size and enhances the interaction among Cu, Co, and Ce. In particular, the mutual interaction between CeO2 and Co3O4 has been strengthened. The HRTEM images of 1Cu5Co5Ce, 1Cu10Co, and 1Cu10Ce are illustrated in Figure 2. The reflection with dspacing values of 0.31 and 0.27 nm are observed and attributed to the CeO2 (111) plane and CeO2 (200) plane, respectively.20 In the catalysts, besides the lattice plane (111) and (200) of fluorite CeO2, the reflection of Co3O4 (311) with a spacing value of 0.25 nm is also detected. However, no reflections related to CuOx species are detected in all samples, implying that CuOx is likely amorphous and highly dispersed. As for 1Cu5Co5Ce, CeO2 and Co3O4 are both observed in the same zone. This suggests that CeO2 and Co3O4 are well mixed in nanosized scale. Typically, smaller particle sizes of catalysts would be helpful for improving mixing. Consequently, the mutual interaction between the interface of CeO2 and Co3O4 would be strongly achieved, which is favored to achieve a higher catalytic activity. The H 2 -TPR profiles of 1Cu10Ce, 1Cu10Co, and 1Cu5Co5Ce are displayed in Figure 3. The TPR profiles consist of three regions of H2 consumption, spanning the Figure 2. High-resolution transmission electron microscopic images of (a) 1Cu5Co5Ce, (b) 1Cu10CO, and (c) 1Cu10Ce. ranges , , and C, associated with a reduction of CuOx, Co3O4, and CeO2, respectively.21,22 In the case of 1Cu10Ce, there are three peaks at 142, 161, and 768 C, and they represent the reduction of amorphous CuOx strongly interacting with CeO2 and less associated with CeO2 and bulk oxygen in CeO2, respectively.12 As for the CuOx species, a small amount of Cu+ should exist in the Cu2+ pool

4 the two components, thus leading to the enhanced reducibility of Co 3 O 4 and 5Co5Ce. The peaks at 153 and 132 C, for 1Cu10Co and 1Cu5Co5Ce, are ascribed to the reduction of CuCo 2 O 4 and/or CuO x to Cu, respectively. 19 The peaks at 191 and 278 C for 1Cu5Co5Ce are attributed to the reduction of Co 3 O 4 to CoO and CoO to Co, respectively. 19 The corresponding two peaks for 1Cu10Co are at 247 and 368 C, respectively Reactivity and Surface Properties of the Catalysts. Figure 4 demonstrates the catalytic activity of the Figure 3. H 2 -TPR patterns of the catalysts. We deduce that the existence of Cu + may affect the reduction of the CuO x at 142 C; that is to say, Cu + is beneficial to enhance the reduction of CuO x. However, the peak of Cu + may be negligible and/or overlapped owing to both their small content in the CuO x and the sensitivity of the TPR instrument. For 5Ce5Co, there are many more peaks appearing at 199, 289, 342, 372, and 734 C, respectively. Vob et al. 24 reported three reduction peaks for pure Co 3 O 4 : the peak at 272 C is attributed to the reduction of Co 3+ to Co 2+ ; the main signal at 337 C and the shoulder at 430 C represent the reduction of CoO to metallic cobalt. Combined with the reduction result of Cu Ce, the peak at 199 C is ascribed to the reduction of Co 3 O 4 tightly interacting with CeO 2, and the peaks at 289 and 342 C are attributed to the reduction of Co 3 O 4 to CoO and CoO to metallic cobalt, respectively. The shoulder peak at 372 C is also ascribed to the reduction of CoO to metallic cobalt. The peak at 734 C is the reduction of bulk oxygen in CeO All the peaks shift to lower temperature in comparison to pure Co 3 O 4. Obviously, Co 3 O 4 has modified and improved the reduction properties of CeO 2 due to interfacial interactions which shape the redox and electronic properties of the active phase. 25 In particular, an enhanced reducibility and surface affinity of ceria for molecular CO might well contribute to what is affecting the reactivity of the Co/CeO 2 system. 20,25 In turn, controlling the intimacy of contact between Co 3 O 4 and CeO 2, the synthesis route could exert an essential influence on the physicochemical properties and reactivity of the catalyst. 20,25 Doping ceria with transition metals is a well-known way to modify the redox properties, enhance the oxygen mobility, and improve the catalytic activity. 13,26,27 Physical and catalytic properties of CoO x CeO 2 binary systems can be modulated depending on the Co/Ce ratio and preparation method. 25 The best results have been attained for Co/Ce atomic ratio close to 1:1, corresponding to the composition of Co 3 O 4 (30 wt %) CeO 2 (70 wt %). 24 Highly dispersed Co 3 O 4 particles in contact with CeO 2 with improved thermal stability and enhanced redox properties are found. When Co 3+ is incorporated into the CeO 2 lattice to substitute Ce 4+ cations, a solid solution is formed. The unbalanced charges and the lattice distortion which takes place within can bond less stable oxygen species. 28 As for 1Cu10Co and 1Cu5Co5Ce, in comparison with Co 3 O 4 and 5Co5Ce, the reduction peaks are shifted toward lower temperatures. Combined with the XRD data, we can infer that the introduction of Cu enhances not only the dispersion of Co 3 O 4 and 5Co5Ce but also the mutual interaction between Figure 4. Light-off curves for the CO oxidation over the catalysts. Reaction conditions: catalyst 50 mg, flow rate 40 ml/min, 1 vol % CO balance in air. catalysts for CO oxidation. The pure CuO, CeO 2, and Co 3 O 4 have relatively poor catalytic performance. The T 50, the temperature for 50% CO conversion, over CuO and Co 3 O 4 are 180 and 138 C, respectively. As for CeO 2, the CO conversion at 250 C is only 21%. Accordingly, Co 3 O 4 achieved the highest catalytic activity. Bulk cobalt oxide has also been compared to other transition metals during preferential CO oxidation in the presence of hydrogen and has shown the highest activity. 29 However, though cobalt oxide is quite active for CO oxidation, bulk cobalt oxide can be reduced to lower valences including metallic cobalt in the excess hydrogen present under PROX reaction and deactivates. 29 Alternatively, 1Cu10Ce, 1Cu10Co, and 5Co5Ce have a noticeable higher activity. Particularly, T 50 of 1Cu10Ce is as low as 73 C. This implies the activity of binary catalysts has been tremendously improved, similar to reports elsewhere. 26,27 However, when Cu, Ce, and Co were mixed together to give 1Cu5Co5Ce, the catalytic activity is further increased and T 50 over 1Cu5Co5Ce is lowered to 64 C. Hereafter, we can deduce that the threecomponent catalyst has a noticeable synergistic effect, which plays a crucial role in enhancing the catalytic activity for CO oxidation. The nature of surface sites on the catalysts is probed by CO adsorption followed by IR spectroscopy. Here we compare 1Cu10Ce and 1Cu5Co5Ce samples in the IR study in order to gain insights into the effect of Co on the surface sites. Figures 5A and 5B show the IR spectra from adsorbed CO on the two samples at room temperature. On both surfaces, an intense band is observed at 2112/2119 cm 1 with a shoulder at 2065 cm 1. Both IR bands are not due to adsorbed CO species on Co sites since Co is not present in the 1Cu10Ce sample. The IR bands are also not due to CO adsorbed on Ce sites since adsorbed CO on Ce 4+ sites appears around 2175 cm 1 and is 27873

5 well-interacted CeO 2 and Co 3 O 4 in close proximity to Cu species in 1Cu5Co5Ce so that CO species adsorbed on Cu + sites can be more easily oxidized than just being adjacent to CeO 2 in the case of 1Cu10Ce. In order to further investigate the reactivity of surface carbonyl species, CO oxidation was followed in the in situ IR cell, and IR spectra collected during the light-off process over 1Cu10Ce and 1Cu5Co5Ce are shown in Figures 6A and 6B, Figure 5. IR spectra of CO adsorbed on 1Cu10Ce (A) and 1Cu5Co5Ce (B) at room temperature and subsequent desorption in He and O 2 /He. not stable upon removal of gas phase CO. 30 Both bands can be ascribed to adsorbed CO on Cu + sites in dispersed copper oxide with a possibly different interaction with CeO 2 and/or Co 3 O 4. 30,31 The presence of these CO Cu + species already upon contact with CO at 25 C is consistent with the easy reduction of copper in the catalyst, taking into account that the fully oxidized state of copper is present in the initial heated catalysts. 31 Room temperature reduction of copper in this catalyst upon interaction with CO has been previously demonstrated by EPR and XPS. 32 A similar observation is also made on a CuO/CeO 2 system where two carbonyl bands were observed at 2109 and 2094 cm 1 upon CO interaction and ascribed to two different CO Cu + species with the band at lower wavenumber to adsorbed CO on smaller sized copper oxide clusters dispersed on ceria. 32 Comparison of Figures 5A and 5B shows that the addition of cobalt leads to decreased portion of the 2065 cm 1 band relative to 2112 cm 1 one and a shift of 2112 cm 1 band to 2119 cm 1, indicating that the interaction between copper oxide and ceria is modified by the presence of cobalt. Moreover, the CO Cu + species on 1Cu5Co5Ce are discovered to be more reactive to O 2 even at room temperature than those on 1Cu10Ce as evidenced by their different change in IR band intensity upon 2% O 2 /He flow. Since the stabilities of the CO Cu + species in inert gas such as He are similar on the two catalysts, the difference in the speed of band intensity change in O 2 atmosphere is likely due to production of reactive oxygen species in the presence of Figure 6. IR spectra of 1Cu10Ce (A) and 1Cu5Co5Ce (B) during CO light-off process. The reaction temperature indicated in the spectra increases from the bottom to the top. respectively. The CO Cu + species at 2120 cm 1 is observed with the coexistence of IR bands due to gaseous CO 2 at 2361 and 2333 cm 1 starting at room temperature on 1Cu5Co5Ce, suggesting CO adsorbed at Cu + sites has been converted through the reaction (i.e., 2CO + O 2 = 2CO 2 ) at room temperature. For 1Cu10Ce at temperatures up to 150 C, the IR bands at around 2350 cm 1 are more characteristic of adsorbed CO 2 than of gaseous CO 2, suggesting lower CO oxidation activity in comparison to 1Cu5Co5Ce in the temperature region below 150 C. For both samples, the intensity of the CO Cu + band decreases gradually when the reaction temperature increases due to increased oxidation activity of the adsorbed CO species and possibly simultaneous thermal desorption. The CO Cu + bond on 1Cu5Ce is weak but remains evident when the reaction temperature increases to 150 C. However, in the case of 1Cu5Co5Ce, the CO Cu

6 band almost disappears at 100 C. It is evident that CO Cu + species on 1Cu5Co5Ce are noticeably more reactive to oxidation than those on 1Cu10Ce, consistent with the results from Figures 5A and 5B. In addition to the formation of CO Cu + species, strong IR bands due to carbonate or related species are observed in the spectral region below 1800 cm 1. As for 1Cu10Ce in Figure 6A, the peaks at 1551 and 1264 cm 1 are ascribed to bidentate carbonates. The band at 1350 cm 1 is attributed to the symmetric stretching of the terminal CO bonds in poly- or monodentate carbonates. 30 The band at 1218 cm 1, along with those at 1375, 1403, and 1060 cm 1 and a shoulder at 1625 cm 1, are attributed to bicarbonate species. 31,32 In the case of 1Cu5Co5Ce (Figure 6B), the bands can be assigned primarily to bicarbonate (1600, 1395, 1299, and 1214 cm 1 ), bidentate carbonate (1561 and 1343 cm 1 ), and unindentate carbonate (1461 cm 1 ). 32 The formation of these carbonate species clearly indicates that CO can readily reduce the surface of these catalysts even at room temperature Queue Mechanism. To determine the role of lattice oxygen in 1Cu5Co5Ce and the origin of active oxygen during continuous CO oxidation reaction, 16 O 2 -treated 1Cu5Co5Ce was used to catalyze C 16 O oxidation by labeled 18 O 2 at room temperature and followed by in situ IR spectroscopy. As shown in Figure 7A, two bands at 2360 and 2340 cm 1, assigned to gaseous C 16 O 2, appear immediately after introducing C 16 O and 18 O 2. During the reaction, IR bands at 2340 and 2325 cm 1, ascribing to C 16 O 18 O, grow in parallel with the band at 2110 cm 1 (C 16 O Cu + ). The C 16 O 18 O is produced via a reaction between C 16 O and the labeled 18 O 2. Meanwhile, as the reaction goes on, the intensity of C 16 O 2 bands continues to decline while that of C 16 O 18 O bands grows and becomes the dominating one. This indicates that the original 16 O lattice oxygen is gradually exhausted and labeled 18 O atoms load these oxygen vacancies. This trend is in agreement with the QMS data as shown in Figure 7B. At the beginning, the QMS signal of C 16 O 2 (m/e = 44) increases simultaneously with the signal of C 16 O(m/e = 28). The intensity of C 16 O 2 reaches the peak value at about 360 s and then declines with time on stream. By contrast, initially, the intensity of C 16 O 18 O(m/e = 46) is lower than that of C 16 O 2 until the reaction has be going on for 720 s. After that, C 16 O 18 O becames the main product. Evidently, at the initial step of CO oxidation, the origin of active oxygen stems from the support, in which the oxygens are labeled as 16 O. Moreover, crystalline 16 O in the support is preferetial to react with actived CO and plays a main role in CO oxidation. Only when 16 O in the support is removed and leaves vacancies, gas-phase 18 O 2 is adsorbed and dissociated as 18 O atoms to incorporate in the oxygen vacancies. Here, it is evident that crystalline oxygen and not gas-phase molecular oxygen is the origin of the active oxygen during the CO oxidation. This may be reasonable to explain why C 16 O 18 O becames the main product at prolonged reaction time as illustrated in Figure 7. Normally, the activation of gas-phase molecular oxygen mainly occurs on the surface of the support in catalysts and is probably related to the surface oxygen vacancy concentration and distribution. 33 And here, as for 1Cu5Co5Ce, Co 3 O 4 CeO 2 acts as supports and is propose to play a crucial role in providing reactive oxygens. Typically, the active oxygen may come from the bulk lattice oxygen. But the migration of bulk lattice oxygen can happen only at high reaction temperatures. Wang et al. 28 reported that only at higher temperatures (above 300 C) bulk oxygen Figure 7. (A) IR spectra collected during C 16 O oxidation with 18 O 2 over 1Cu5Co5Ce at room temperature. IR spectra are referenced to the background spectrum collected at rt in helium before introducing the reaction feeds. (B) Corresponding QMS profiles collected during C 16 O oxidation with 18 O 2 over 1Cu5Co5Ce at room temperature. migration in CeO 2 CoO x can take place. Therefore, in our study, when reaction temperature is as low as or even lower than 250 C, bulk lattice oxygens can be reasonably excluded from the origin of reactive oxygen in Co 3 O 4 CeO 2. In addition, the interaction of Co O Ce in Co 3 O 4 CeO 2 may weaken Co O bonds, making it easier to provide active oxygen than their bulk counterparts. 28 Meng et al. 9 also proposed that the interfacial oxygen of Co 3 O 4 CeO 2 is the active oxygen pool. This indicates that the oxygen at the interface of Co 3 O 4 CeO 2 actually acts as the active oxygen provider in CO oxidation according to the DFT calculation (illustrated in Supporting Information). Furthermore, Zheng et al. 33 proposed that molecular O 2 is adsorbed on the defects of the (110) surface of a rutile single crystal and that the oxygen diffusion rate is dependent on the surface oxygen vacancy density. Luo et al. 27 reports that there exists the magnitude of oxygen vacancy concentration surrounding the active centers, which may enhance the diffusion of active oxygen to the reactive sites. Herein, when the oxygen vacancies at the interface of Co 3 O 4 CeO 2 are restored by dissociative adsorption of gaseous O 2 present in the reaction environment, these active oxygen may diffuse to the CO Cu + to form CO 2 due to the magnitude of oxygen vacancy concentration

7 As a result, it is reasonable to suggest that the origin of active oxygen in Co 3 O 4 CeO 2 obeys a reaction model as shown in Figure 8, i.e., a queue mechanism. According to this Figure 8. A queue mechanism for the origin of active oxygen in a ternary CuO x Co 3 O 4 CeO 2 catalyst for CO oxidation. mechanism, adsorbed gas-phase molecular oxygens replenish the lattice vacancies at the interface of Co 3 O 4 CeO 2. Thanks to the magnitude of oxygen vacancy concentration surrounding the reactive centers of CO Cu +, active oxygens migrates to CO Cu + and reacts to form CO CONCLUSIONS To summarize, a ternary catalyst, i.e. 1Cu5Co5Ce, has been synthesized by coprecipitation. With respect to the mechanism of CO oxidation over 1Cu5Co5Ce, Cu + ions are proposed to be the active sites and form Cu + CO species, and the lattice oxygens located at the interface of Co 3 O 4 CeO 2 are proposed to be the origin of the active oxygens. The mechanism of CO oxidation over 1Cu5Co5Ce is called a queue mechanism and described as that gas-phase molecular oxygen dissociated to atom oxygen and incorporated into an oxygen vacancy located at the interface of Co 3 O 4 and CeO 2, which may diffuse to CO Cu + and react to form CO 2. This process, called a queue mechanism, obeys a queue rule to provide active oxygen among gas-phase molecular oxygen, oxygen vacancy, and active crystalline oxygen. ASSOCIATED CONTENT *S Supporting Information Computational details and simulation results. This material is available free of charge via the Internet at AUTHOR INFORMATION Corresponding Authors * liuzhigang@hnu.edu.cn; Tel (Z.L.). * wuz1@ornl.gov (Z.W.). * dais@ornl.gov (S.D.).. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This research is sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy. Part of the work including DRIFTS was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Science, U.S. Department of Energy. The research (Z. G. Liu) is also supported partly by Natural Science Foundation of China (No , , and ), the Fundamental Research Funds for the Central Universities and the Heavy Oil State Key Laboratory in China. Andrew Copple is acknowledged for the critical review of the manuscript. REFERENCES (1) Wu, Z. L.; Li, M. J.; Overbury, S. H. On the Structure Dependence of CO Oxidation over CeO 2 Nanocrystals with Welldefined Surface Planes. J. Catal. 2012, 285, (2) Royer, S.; Dupez, D. Catalytic Oxidation of Carbon Monoxide over Transition Metal Oxides. ChemCatChem 2011, 3, (3) Guzman, J.; Carrettin, S.; Corma, A. Spectroscopic Evidence for the Supply of Reactive Oxygen during CO Oxidation Catalyzed by Gold Supported on Nanocrystalline CeO 2. J. Am. Chem. Soc. 2005, 127, (4) Liu, W.; Flytzani-stephanopoulos, M. Total Oxidation of Carbon- Monoxide and Methane over Transition Metal Fluorite Oxide Composite Catalysts: II. Catalyst Characterization and Reaction- Kinetics. J. Catal. 1995, 153, (5) Martínez-Arias, A.; Hungría, A. B.; Fernańdez-García, M.; Conesa, J. C.; Munuera, G. Interfacial Redox Processes under CO/ O 2 in a Nanoceria-Supported Copper Oxide Catalyst. J. Phys. Chem. B 2004, 108 (46), (6) Luo, M. F.; Song, Y. P.; Lu, J. Q.; Wang, X. Y.; Pu, Z. Y. Identification of CuO Species in High Surface Area CuO-CeO 2 Catalysts and Their Catalytic Activities for CO Oxidation. J. Phys. Chem. C 2007, 111, (7) Jia, A. P.; Jiang, S. Y.; Lu, J. Q.; Luo, M. F. Study of Catalytic Activity at the CuO-CeO 2 Interface for CO Oxidation. J. Phys. Chem. C 2010, 114, (8) Wang, J.; Shen, M.; Wang, J.; Gao, J.; Ma, J.; Liu, S. CeO 2 CoO x Mixed Oxides: Structural Characteristics and Dynamic Storage/ Release Capacity. Catal. Today 2011, 175, (9) Luo, J. Y.; Meng, M.; Li, X.; Li, X. G.; Zha, Y. Q.; Hu, T. D.; Xie, Y. N.; Zhang, J. Mesoporous Co 3 O 4 CeO 2 and Pd/Co 3 O 4 CeO 2 Catalysts: Synthesis, Characterization and Mechanistic Study of Their Catalytic Properties for Low-Temperature CO Oxidation. J. Catal. 2008, 254, (10) Chen, H. L.; Chen, H. T. Role of Hydroxyl Groups for the O 2 Adsorption on CeO 2 Surface: A DFT + U Study. Chem. Phys. Lett. 2010, 493, (11) Wang, J. B.; Tsai, D. H.; Huang, T. J. Synergistic Catalysis of Carbon Monoxide Oxidation over Copper Oxide Supported on Samaria-Doped Ceria. J. Catal. 2002, 208, (12) Avgouropoulos, G.; Ioannides, T.; Papadopoulou, Ch.; Batista, J.; Hocevar, S.; Matralis, H. K. A Comparative Study of Pt/γ-Al 2 O 3, Au/α-Fe 2 O 3 and CuO-CeO 2 Catalysts for the Selective Oxidation of Carbon Monoxide in Excess Hydrogen. Catal. Today 2002, 75, (13) Martinez-Arias, A.; Gamarra, D.; Fernandez-Garcia, M.; Hornes, A.; Belver, C. Spectroscopic Study on the Nature of Active Entities in Copper Ceria CO-PROX Catalysts. Top. Catal. 2009, 52, (14) Gamarra, D.; Martinez-Arias, A. Preferential Oxidation of CO in Rich H 2 over CuO/CeO 2 : Operando-DRIFTS Analysis of Deactivating Effect of CO 2 and H 2 O. J. Catal. 2009, 263, (15) Tang, X.; Zhang, B.; Li, Y.; Xu, Y.; Xin, Q.; Shen, W. J. Carbon Monoxide Oxidation over CuO/CeO 2 Catalysts. Catal. Today 2004, 93 95, (16) Wang, H.; Zhu, H.; Qin, Z.; Liang, F.; Wang, G.; Wang, J. Deactivation of a Au/CeO 2 Co 3 O 4 Catalyst during CO Preferential Oxidation in H 2 -Rich Stream. J. Catal. 2009, 264, (17) Liotta, L. F.; Carlo, G. D.; Pantaleo, G.; Deganello, G. Co 3 O 4 / CeO 2 and Co 3 O 4 /CeO 2 ZrO 2 Composite Catalysts for Methane Combustion: Correlation between Morphology Reduction Properties and Catalytic Activity. Catal. Commun. 2005, 6, (18) Liotta, L. F.; Carlo, G. D.; Pantaleo, G.; Deganello, G. Catalytic performance of Co 3 O 4 /CeO 2 and Co 3 O 4 /CeO 2 -ZrO 2 Composite Oxides for Methane Combustion: Influence of Catalyst Pretreatment Temperature and Oxygen Concentration in the Reaction Mixture. Appl. Catal., B 2007, 70,

8 (19) Liotta, L. F.; Carlo, G. D.; Pantaleo, G.; Venezia, A. M.; Deganello, G. Co 3 O 4 /CeO 2 Composite Oxides for Methane Emissions Abatement: Relationship between Co 3 O 4 CeO 2 Interaction and Catalytic Activity. Appl. Catal., B 2006, 60, (20) Wang, H.; Zhu, H.; Qin, Z.; Wang, G.; Liang, F.; Wang, J. Preferential Oxidation of CO in H 2 Rich Stream over Au/CeO 2 Co 3 O 4 Catalysts. Catal. Commun. 2008, 9, (21) Jiang, X. Y.; Lu, G. L.; Zhou, R. X.; Mao, J. X.; Chen, Y.; Zheng, X. M. Studies of Pore Structure, Temperature-Programmed Reduction Performance, and Micro-Structure of CuO/CeO 2 Catalysts. Appl. Surf. Sci. 2001, 173, (22) Xue, L.; Zhang, C. B.; He, H.; Teraoka, Y. Catalytic Decomposition of N 2 O over CeO 2 Promoted Co 3 O 4 Spinel Catalyst. Appl. Catal., B 2007, 75, (23) Lee, H. C.; Kim, D. H. Kinetics of CO and H 2 Oxidation over CuO-CeO 2 Catalyst in H 2 Mixtures with CO 2 and H 2 O. Catal. Today 2008, 132, (24) Vob, M.; Borgmann, D.; Wedler, G. Characterization of Alumina, Silica, and Titania Supported Cobalt Catalysts. J. Catal. 2002, 212, (25) Liotta, L. F.; Ousmane, M.; Carlo, G. D.; Pantaleo, G.; Deganello, G.; Marci, G.; Retailleau, L.; Giroir-Fendler, A. Total Oxidation of Propene at Low Temperature over Co 3 O 4 CeO 2 Mixed Oxides: Role of Surface Oxygen Vacancies and Bulk Oxygen Mobility in the Catalytic Activity. Appl. Catal., A 2008, 347, (26) Liu, Z. G.; Zhou, R. X.; Zheng, X. M. Comparative Study of Different Methods of Preparing CuO-CeO 2 Catalysts for Preferential Oxidation of CO in Excess Hydrogen. J. Mol. Catal. A: Chem. 2007, 267, (27) Luo, M. F.; Ma, J. M.; Lu, J. Q.; Song, Y. P.; Wang, Y. J. High- Surface Area CuO CeO 2 Catalysts Prepared by a Surfactant- Templated Method for Low-Temperature CO Oxidation. J. Catal. 2007, 246, (28) Wang, X.; Rodriguez, J. A.; Hanson, J. C.; Gamarra, D.; Martinez-Arias, A.; Fernandez-Garcia, M. In Situ Studies of the Active Sites for the Water Gas Shift Reaction over Cu CeO 2 Catalysts: Complex Interaction between Metallic Copper and Oxygen Vacancies of Ceria. J. Phys. Chem. B 2006, 110, (29) Woods, M. P.; Gawade, P.; Tan, B.; Ozkan, U. S. Preferential Oxidation of Carbon Monoxide on Co/CeO 2 Nanoparticles. Appl. Catal., B 2010, 97, (30) Hadjiivanov, K. I.; Vayssilov, G. N. Characterization of Oxide Surfaces and Zeolites by Carbon Monoxide as An IR Probe Molecule. Adv. Catal. 2002, 47, (31) Wang, H. Q.; Wang, Z.; Zhu, J.; Li, X. W.; Liu, B.; Gao, F.; Dong, L.; Chen, Y. Influence of CO Pretreatment on the Activities of CuO/γ-Al 2 O 3 Catalysts in CO + O 2 Reaction. Appl. Catal., B 2008, 79, (32) Binet, C.; Daturi, M.; Lavalley, J. C. IR Study of Polycrystalline Ceria Properties in Oxidized and Reduced States. Catal. Today 1999, 50, (33) Zheng, Z. F.; Teo, J.; Chen, X.; Liu, H. W.; Yuan, Y.; Waclawik, E. R.; Zhong, Z.; Zhu, H. Y. Correlation of the Catalytic Activity for Oxidation Taking Place on Various TiO 2 Surfaces with Surface OH Groups and Surface Oxygen Vacancies Aluminium Oxide Adsorbent Special Sol-Gel Technique Acid Sites Catalytic Propene Oxidation. Chem. Eur. J. 2010, 16,