Co3O4âCeO2 Catalyst for

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The Origin of Active Oxygen in a Ternary CuO/CoO-CeO Catalyst for CO Oxidation x

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Zhigang Liu, Zili Wu, Xihong Peng, Andrew Binder, Songhai Chai, and Sheng Dai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp508487x • Publication Date (Web): 14 Nov 2014 Downloaded from http://pubs.acs.org on November 15, 2014

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The Journal of Physical Chemistry

The Origin of Active Oxygen in a Ternary CuOx/Co3O4-CeO2 Catalyst for CO Oxidation ﹡,†

Zhigang Liu,

﹡,‡

Zili Wu,

Xihong Peng,++ Andrew Binder,§ Songhai Chai,‡ Sheng Dai

﹡,‡,§

†

School of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China

‡

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United

States §

Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600, United States

++

School of Letters and Sciences, Arizona State University, Mesa, Arizona 85212, United States

ABSTRACT: In this paper, we have studied CO oxidation over a ternary CuOx/Co3O4-CeO2 catalyst and employed the techniques of N2 adsorption/desporption, XRD, TPR, TEM, in situ DRIFTS and QMS (Quadrupole mass spectrometer) to explore the origin of active oxygen. DRIFTS-QMS results with labeled 18O2 indicate that the origin of active oxygens in CuOx/Co3O4-CeO2 obeys a model, called as queue mechanism. Namely gas-phase molecular oxygens are dissociated to atomic oxygens and then incorporate in oxygen vacancies located at the interface of Co3O4-CeO2 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 CO2. This process, obeying a queue rule, provides active oxygens to form CO2 from gas-phase O2 via oxygen vacancies and crystalline oxygen at the interface of Co3O4-CeO2.

KEYWORDS: CuOx/Co3O4-CeO2, CO oxidation, Queue mechanism, Interface

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interaction, Oxygen vacancy 1. INTRODUCTION CO oxidation is believed to proceed via 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 Ce3+ and Ce4+ are stable, allowing the oxide to shift between CeO2 and CeO2-x.1, 2 This has been clarified by many studies. For example, Guzman3 reported that CO2, during CO adsorption experiments and without any oxygen in the gas stream, was formed. This result indicates that nanocrystalline CeO2 is able to supply reactive oxygen to the gold active species for the oxidation of CO, which is consistent with the idea of CeO2 acting as an oxygen buffer by releasing-uptaking oxygen through redox processes involving the Ce4+/Ce3+ couple. Liu and Stephanopoulos4 have suggested a reaction model, i.e., Cu+ species were stabilized by the interaction between CuO and CeO2 and the Cu+ species provide surface sites for CO chemisorption while the CeO2 provides the oxygen source through a fast Ce4+/Ce3+ redox cycle. Martìnez-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 CeO2 support. Luo et al.6,7 found that the finely dispersed CuO species in CuO-CeO2 had the highest activity. Moreover, in their further work, they proposed that the reaction may take place at the interface of CuO-CeO2 and the catalyst for CO oxidation is structure-sensitive.

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As for Co3O4-CeO2 binary catalyst, Wang8 investigated the oxygen storage-release capacity. They found that the solubility limit of cobalt oxides in the CeO2 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, Meng9 found that Co3O4 crystallites in Co3O4-CeO2 catalyst with molar ratio of 1:1 are considered to be encapsulated by nano-sized CeO2, with only a small fraction of Co ions exposing on the surface and strongly interacting with CeO2. In addition, they proposed that over Co3O4-CeO2, the CO oxidation should take place preferentially at the interface of Co3O4-CeO2 instead of the surface of Co3O4. However, Chen10 provided the evidence that O2 is supplied by superoxide species on CeO2 in the presence of OH and can diffuse to the interface of noble-metal/CeO2 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

co-shared

oxygen

ions

coupled

with

the

creation

of

high-turnover-frequency active sites which are composed of metastable copper oxide species and oxygen vacancies of two types11. All in all, due to the structural complication of the multi-component oxide catalysts (i.e. CuOx/Co3O4-CeO2), there still exists a lot of controversy about the reaction mechanism, particularly about the reaction pathway and nature of active oxygen. To understand the catalytic origin of these oxidation reactions and to design

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efficient catalysts, it is essential to know the activation route of molecular O2 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, CuOx is supported on Co3O4-CeO2 with Co:Ce atomic ratio of 1:1. The mechanism of CO oxidation over CuOx/Co3O4-CeO2 is investigated by techniques including BET, IR, XRD, TEM, H2-TPR, and in situ DRIFTS-QMS. Here, we provide evidence via oxygen isotopic exchange experiments that gas-phase

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O2 is dissociated into lattice

oxygen, and migrates to react with CO-Cu+ to form CO2 along the interface of Co3O4-CeO2 one by one and this reaction model is called as a queue mechanism. 2. EXPERIMENTAL SECTION 2.1 Catalysts preparation CuOx/Co3O4-CeO2 with Cu:Co:Ce atomic ratios equal to 1:5:5 was synthesized by co-precipitation method and designated as 1Cu5Co5Ce. 0.2416 g Cu(NO3)2·3H2O and appropriate amounts of Ce(NO3)3·6H2O and CoCl2 were dissolved in 100 ml deionized water at room temperature and stirred for 15 min, then 100 ml 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 water and then with 150 ml anhydrous ethanol. The product was dried at 90 oC and heated in air at 500 oC 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. 2.2 Evaluation of Catalytic activity

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Catalytic CO oxidation was tested in a continuous fixed-bed microreactor. 50 mg catalyst without any pretreatment was loaded into a quartz tube (i.d. 4 mm). The feed was 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 48,000 ml/(h·gcat.). A portion of the product stream was extracted periodically with an automatic sampling valve and analyzed using a dual-column gas chromatograph with a thermal conductivity detector (TCD). 2.3 Characterization techniques Surface area, pore volume and pore size distribution were measured by nitrogen adsorption/desorption at -196 oC using a Micromeritics ASAP 2020 surface area and porosity analyzer. The samples were degassed at 200 oC for 2 h prior to the adsorption experiment. The surface area was determined by BET method in 0-0.3 partial pressure range. X-ray diffraction measurement was collected on a Panalytical powder diffractometer operating at 40 mA and 40 kV using CoKα radiation source. The data of 2θ from 20o to 90o were collected with the step size of 10o/min. The average crystallite size of CeO2 was calculated by using Scherrer equation. HR-TEM images were obtained using a JEOL JEM-2110 system operating at 200 kV. Temperature-programmed measurements were performed on a Thermo-Finnigan TPDRO 1100 instrument with a thermal conductivity detector. For H2-TPR test, the quartz tube reactor was loaded with a 50 mg sample in powder form and heated from room temperature to 900 oC in 4% H2/Ar. A heating rate of 10 oC/min and a gas flow rate of 50 ml/min were used.

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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 oC. The exiting stream was analyzed by an online quadrupole mass spectrometer (QMS) (OmniStar GSD-301 O2, Pfeffer Vacuum). Before measurement, the sample powder (30 mg) was treated in situ at 450 oC in 2% O2/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% O2/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% O2/He and 5 ml/min 2%CO/2%Ar/He; O2/CO = 4) at room temperature and ramped up to 250 oC at a rate of 10 oC/min. Switching between 2%

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O2/He (Isotech,

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O2 purity of 99%) and 2%

16

O2/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 CeO2 were yielded via calcination of their corresponding nitrate metal salts at 300 oC for 1 h. And pure Co3O4 was attained by

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co-precipitation as mentioned previously. Pure CuO and Co3O4, shown in Table 1, have very low surface areas, (i.e. 1 m2/g and 16 m2/g, respectively), and CeO2 possesses a relatively high surface area of 82 m2/g. The samples, i.e. 1Cu10Ce, 1Cu10Co and 1Cu5Co5Ce, were prepared by co-precipitation and heated at 500 oC for 1 hour and have surface areas of 187, 20 and 102 m2/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 Co3O4 and CeO2 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 CeO2 are seen at 28.5, 33.1, 47.5, 56.3, 59.1 and 69.5o, respectively. The diffraction peaks are 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 59.5o, 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,

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respectively. This suggests that the addition of Co is beneficial to achieve smaller particle sizes.17 As mentioned in 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 v.s. 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 d spacing 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 nano-sized 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

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achieved, which is favored to achieve a higher catalytic activity. The H2-TPR profiles of 1Cu10Ce, 1Cu10Co and 1Cu5Co5Ce are displayed in Figure 3. The TPR profiles consist of three regions of H2 consumption, spanning the ranges 100-200, 200-500 and 500-900 oC, 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 oC 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.23 We deduce that the existence of Cu+ may affect the reduction of the CuOx at 142 oC, that is to say Cu+ is beneficial to enhance the reduction of CuOx. However, the peak of Cu+ may be negligible and/or overlapped owing to both their small content in the CuOx and the sensitivity of the TPR instrument. For 5Ce5Co, there are many more peaks appearing at 199, 289, 342, 372 and 734 oC, respectively. Vob et al.24 reported three reduction peaks for pure Co3O4: the peak at 272 oC is attributed to the reduction of Co3+ to Co2+, the main signal at 337 oC and the shoulder at 430 oC represent the reduction of CoO to metallic cobalt. Combined with the reduction result of Cu-Ce, the peak at 199 oC is ascribed to the reduction of Co3O4 tightly interacting with CeO2, and the peaks at 289 and 342 oC are attributed to the reduction of Co3O4 to CoO and CoO to metallic cobalt, respectively. The shoulder peak at 372 oC is also ascribed to the reduction of CoO to metallic cobalt. The peak at 734 oC is the reduction of bulk oxygen in CeO212. All the peaks shift to lower temperature in comparison to pure Co3O4. Obviously, Co3O4 has

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modified and improved the reduction properties of CeO2 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/CeO2 system.20,25 In turn, controlling the intimacy of contact between Co3O4 and CeO2, 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 CoOx-CeO2 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 Co3O4 (30 wt%)-CeO2 (70 wt%).24 High dispersed Co3O4 particles in contact with CeO2 with improved thermal stability and enhanced redox properties are found. When Co3+ is incorporated into the CeO2 lattice to substitute Ce4+ 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 Co3O4 and 5Co5Ce, the reduction peaks are shifted towards lower temperatures. Combined with the XRD data, we can infer that the introduction of Cu enhances not only the dispersion of Co3O4 and 5Co5Ce but also the mutual interaction between the two components, thus leading to the enhanced reducibility of Co3O4 and 5Co5Ce. The peaks at 153 and 132

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o

C, for 1Cu10Co and 1Cu5Co5Ce, are ascribed to the reduction of CuCo2O4 and/or

CuOx to Cu, respectively.19 The peaks at 191 and 278 oC for 1Cu5Co5Ce are attributed to the reduction of Co3O4 to CoO and CoO to Co, respectively.19 The corresponding two peaks for 1Cu10Co are at 247 and 368 oC, respectively. 3.2. Reactivity and surface properties of the catalysts Figure 4 demonstrates the catalytic activity of the catalysts for CO oxidation. The pure CuO, CeO2 and Co3O4 have relatively poor catalytic performance. The T50, the temperature for 50% CO conversion, over CuO and Co3O4 are 180 and 138 oC, respectively. As for CeO2, the CO conversion at 250 oC is only 21%. Accordingly, Co3O4 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, T50 of 1Cu10Ce is as low as 73 oC. 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 T50 over 1Cu5Co5Ce is lowered to 64 oC. Hereafter, we can deduce that the three-component catalyst has a noticeable synergistic effect, which plays a crucial role in enhancing the catalytic activity for CO oxidation.

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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. Figure 5A and 5B respectively 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 Ce4+ sites appears around 2175 cm-1 and is 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 CeO2 and/or Co3O4.30,31 The presence of these CO-Cu+ species already upon contact with CO at 25 oC 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/CeO2 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 Figure 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

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on 1Cu5Co5Ce are discovered to be more reactive to O2 even at room temperature than those on 1Cu10Ce as evidenced by their different change in IR band intensity upon 2% O2/He flow. Since the stability of the CO-Cu+ species in inert gas such as He is similar on the two catalysts, the difference in the speed of band intensity change in O2 atmosphere is likely due to production of reactive oxygen species in the presence of well-interacted CeO2 and Co3O4 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 CeO2 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 Figure 6A and 6B, respectively. The CO-Cu+ species at ~2120 cm-1 is observed with the co-existence of IR bands due to gaseous CO2 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 + O2 = 2CO2) at room temperature. For 1Cu10Ce at temperatures up to 150 oC, the IR bands at around 2350 cm-1 are more characteristic of adsorbed CO2 than of gaseous CO2, suggesting lower CO oxidation activity in comparison to 1Cu5Co5Ce in the temperature region below 150 oC.

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 oC. However, in the case of 1Cu5Co5Ce, the

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CO-Cu+ band almost disappears at 100 oC. It is evident that CO-Cu+ species on 1Cu5Co5Ce are noticeably more reactive to oxidation than those on 1Cu10Ce, consistent with the results from Figure 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 mono-dentate carbonates.30 The band at 1218 cm-1, along with those at 1375, 1403, 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. 3.3. The queue mechanism To determine the role of lattice oxygen in 1Cu5Co5Ce and the origin of active oxygen during continuous CO oxidation reaction, to catalyze C16O oxidation by labeled

18

16

O2 treated 1Cu5Co5Ce was used

O2 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 C16O2, appear immediately after introducing C16O and

18

O2.

During the reaction, IR bands at 2340 and 2325 cm-1, ascribing to C16O18O, grow in parallel with the band at 2110 cm-1 (C16O-Cu+). The C16O18O is produced via a

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reaction between C16O and the labeled 18O2. Meanwhile, as the reaction goes on, the intensity of C16O2 bands continues to decline while that of C16O18O bands grows and becomes the dominating one. This indicates that the original

16

O lattice oxygen is

gradually exhausted and labeled 18O 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 C16O2 (m/e = 44) increases simultaneously with the signal of C16O (m/e = 28). The intensity of C16O2 reaches the peak value at about 360 s and then declines with time on stream. By contrast, initially, the intensity of C16O18O (m/e = 46) is lower than that of C16O2 until the reaction has be going on for 720 s. After that, C16O18O 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 16O in the support is removed and leaves vacancies, gas-phase

18

O2 is adsorbed and disociated as

18

O atoms to

incorporate in the oxygen vacancies. Here, it is evident that crystalline oxygen not gas-phase molecular oxygen is the origin of the active oxygen during the CO oxidation. This may be reasonable to explain why C16O18O 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, Co3O4-CeO2 acts as supports and is propose to play a crucial role in providing reactive oxygens.

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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 oC), bulk oxygen migration in CeO2-CoOx can take place. Therefore, in our study, when reaction temperature is as low as or even lower than 250 oC, bulk lattice oxygens can be reasonably excluded from the origin of reactive oxygen in Co3O4-CeO2. In addition, the interaction of Co-O-Ce in Co3O4-CeO2 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 Co3O4-CeO2 is the active oxygen pool. This indicates that the oxygen at the interface of Co3O4-CeO2 actually acts as the active oxygen provider in CO oxidation according to the DFT calculation (illustrated in SI). Furthermore, Zheng et al.33 proposed that molecular O2 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 Co3O4-CeO2 are restored by dissociative adsorption of gaseous O2 present in the reaction environment, these active oxygen may diffuse to the CO-Cu+ to form CO2 due to the magnitude of oxygen vacancy concentration. As a result, it is reasonable to suggest that the origin of active oxygen in Co3O4-CeO2 obeys a reaction model as shown in Figure 8, i.e., a queue mechanism.

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According to this mechanism, adsorbed gas-phase molecular oxygens replenish the lattice vacancies at the interface of Co3O4-CeO2. 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 CO2. CONCLUSIONS To summarize, a ternary catalyst, i.e. 1Cu5Co5Ce, has been synthesized by co-precipitation. 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 Co3O4-CeO2 are proposed to be the origin of the active oxygens. The mechanism of CO oxidation over 1Cu5Co5Ce is called as 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 Co3O4 and CeO2, which may diffuse to CO-Cu+ and react to form CO2. This process, called as a queue mechanism, obeys a queue rule to provide active oxygen among gas-phase molecular oxygen, oxygen vacancy and active crystalline oxygen. ASSOCIATED CONTENT Supporting Information This materials is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding author: *E-mail: [email protected]. Telephone: 86-731-8882-3327. Mailing address: School of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China.

*E-mail: [email protected]. Mail address: Chemical Sciences Division, Oak Ridge National - 17 -



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Laboratory, Oak Ridge, Tennessee 37831, United States;

*E-mail: [email protected]. Mail address: Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States; Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600, United States.

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS 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.21103045, 1210040, 1103312), 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.

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during CO Oxidation Catalyzed by Gold Supported on Nanocrystalline CeO2. J. Am. Chem. Soc. 2005, 127, 3286-3287. (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, 317-332. (5) Martínez-Arias, A.; Hungría, A. B.; Fernández-García, M.; Conesa, J. C.; Munuera, G. Interfacial Redox Processes under CO/O2 in a Nanoceria-Supported Copper Oxide Catalyst. J. Phys. Chem. B, 2004, 108 (46), 17983–17991. (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-CeO2 Catalysts and Their Catalytic Activities for CO Oxidation. J. Phys. Chem. C 2007, 111, 12686-12692. (7) Jia, A. P.; Jiang, S. Y.; Lu, J. Q.; Luo M. F. Study of Catalytic Activity at the CuO-CeO2 Interface for CO Oxidation. J. Phys. Chem. C 2010, 114, 21605–21610. (8) Wang, J.; Shen, M.; Wang, J.; Gao, J.; Ma, J.; Liu, S. CeO2–CoOx Mixed Oxides: Structural Characteristics and Dynamic Storage/release Capacity. Catal. Today 2011, 175, 65-71. (9) Luo, J.Y.; Meng, M.; Li, X.; Li, X.G.; Zha, Y.Q.; Hu, T.D.; Xie, Y.N.; Zhang, J. Mesoporous Co3O4–CeO2 and Pd/Co3O4–CeO2 Catalysts: Synthesis, Characterization and Mechanistic Study of Their Catalytic Properties for Low-Temperature CO Oxidation. J. Catal. 2008, 254, 310-324. (10) Chen, H.L.; Chen, H.T. Role of Hydroxyl Groups for the O2 Adsorption on CeO2 Surface: A DFT + U Study. Chem. Phy. Lett. 2010, 493, 269-272. (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, 370-380.

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(12) Avgouropoulos, G.; Ioannides, T.; Papadopoulou, Ch.; Batista, J.; Hocevar, S.; Matralis, H.K. A Comparative Study of Pt/γ-Al2O3, Au/α-Fe2O3 and CuO-CeO2 Catalysts for the Selective Oxidation of Carbon Monoxide in Excess Hydrogen. Catal. Today 2002, 75 157-167. (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. Topics Catal. 2009, 52 1425-1432. (14) Gamarra, D.; Martinez-Arias, A. Preferential Oxidation of CO in Rich H2 over CuO/CeO2: Operando-DRIFTS Analysis of Deactivating Effect of CO2 and H2O. J. Catal. 2009, 263, 189-195. (15) Tang, X.; Zhang, B.; Li, Y.; Xu, Y.; Xin, Q.; Shen, W.J. Carbon Monoxide Oxidation over CuO/CeO2 Catalysts. Catal. Today 2004, 93-95, 191-198. (16) Wang, H.; Zhu, H.; Qin, Z.; Liang, F.; Wang, G.; Wang, J. Deactivation of a Au/CeO2–Co3O4 Catalyst during CO Preferential Oxidation in H2-rich Stream. J. Catal. 2009, 264, 154-162. (17) Liotta, L.F.; Carlo, G.D.; Pantaleo, G.; Deganello, G. Co3O4/CeO2 and Co3O4/CeO2–ZrO2 Composite Catalysts for Methane Combustion: Correlation between Morphology Reduction Properties and Catalytic Activity. Catal. Comm. 2005, 6, 329-336. (18) Liotta, L.F.; Carlo, G.D.; Pantaleo, G.; Deganello, G. Catalytic performance of Co3O4/CeO2 and Co3O4/CeO2-ZrO2 Composite Oxides for Methane Combustion: Influence of Catalyst Pretreatment Temperature and Oxygen Concentration in the Reaction Mixture. Appl. Catal. B 2007, 70, 314-322.

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(19) Liotta, L.F.; Carlo, G.D.; Pantaleo, G.; Venezia, A.M.; Deganello, G. Co3O4/CeO2 Composite Oxides for Methane Emissions Abatement: Relationship between Co3O4–CeO2 Interaction and Catalytic Activity. Appl. Catal. B 2006, 60, 217-227. (20) Wang, H.; Zhu, H.; Qin, Z.; Wang, G.; Liang, F.; Wang, J. Preferential Oxidation of CO in H2 Rich Stream over Au/CeO2–Co3O4 Catalysts. Catal. Comm. 2008, 9, 1487-1492. (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/CeO2 Catalysts. Appl. Surf. Sci. 2001, 173, 208-220. (22) Xue, L.; Zhang, C.B.; He, H.; Teraoka, Y. Catalytic Decomposition of N2O over CeO2 Promoted Co3O4 Spinel Catalyst. Appl. Catal. B 2007, 75, 167-174. (23) Lee, H.C.; Kim, D.H. Kinetics of CO and H2 Oxidation over CuO-CeO2 Catalyst in H2 Mixtures with CO2 and H2O. Catal. Today 2008, 132, 109-116. (24) Vob, M.; Borgmann, D.; Wedler, G. Characterization of Alumina, Silica, and Titania Supported Cobalt Catalysts. J. Catal. 2002, 212, 10-21. (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 Co3O4–CeO2 Mixed Oxides: Role of Surface Oxygen Vacancies and Bulk Oxygen Mobility in the Catalytic Activity. Appl. Catal. A 2008, 347, 81-88. (26) Liu, Z.G. ; Zhou, R.X.; Zheng, X.M. Comparative Study of Different Methods of Preparing CuO-CeO2 Catalysts for Preferential Oxidation of CO in Excess Hydrogen. J. Mol. Catal. A 2007, 267, 137-142.

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Table 1 Crystallite size, BET surface area and catalytic activity of the samples. Crystallite size (nm)a d(CuO)

d(CeO2)

d(Co3O4)

SA(BET) (m2 g-1)

CuO

27.4

-

-

1

CeO2

-

7.7

-

82

-

Co3O4

-

-

40.0

16

138

5Co-5Ce

-

-

-

89

140

1Cu-10Co

-

-

38.0

20

93

Sample

T50 (oC) 182

1Cu-5Co-5Ce

-

6.9

29.1

102

64

1Cu-10Ce

-

7.0

-

187

73

a

Particle sizes were obtained from CeO2 (111) and Co3O4 (222) reflection, respectively.

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Figure 1. Diffraction patterns of the catalysts.

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Figure 2. High-resolution transmission electron microscopic images of (a) 1Cu5Co5Ce, (b) 1Cu10CO and (c) 1Cu10Ce.

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Figure 3. H2-TPR patterns of the catalysts.

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Figure 4. Light-off curves for the CO oxidation over the catalysts. Reaction condition: Catalyst 50 mg, Flow rate 40 ml/min, 1 vol% CO balance in air.

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Figure 5. IR spectra of CO adsorbed on 1Cu10Ce (A) and 1Cu5Co5Ce (B) at room temperature and subsequent desorption in He and O2/He.

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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.

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Figure 7A. IR spectra collected during C16O oxidation with 18O2 over 1Cu5Co5Ce at room temperature. IR spectra are referenced to the background spectrum collected at rt in helium before introducing the reaction feeds. 7B. Corresponding QMS profiles collected during C16O oxidation with 18O2 over 1Cu5Co5Ce at room temperature.

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Figure 8 A queue mechanism for the origin of active oxygen in a ternary CuOx-Co3O4-CeO2 catalyst for CO oxidation.

o

Co3O4 2(gas)

o2

Co

O

Cu

Ce (abs)

O

o

O

(act)

Co O CeO2

+

CO

CO Cu+

Ce

O

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co2

co2 (ads)

(gas)


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TOC Graphic

o

Co3O4 2(gas)

o2

Co

O

Cu

Ce (abs)

O

o

O

(act)

Co O CeO2

+

CO

CO

(gas)

co2

Cu+ O

Ce

co2

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(ads)

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A queue mechanism for the origination of active oxygen in a ternary CuO-Co3O4-CeO2 catalyst for CO oxidation ﹡,†

ZhiGang Liu,

ZiLi Wu,

﹡,‡

﹡,‡,§

XiHong Peng,++ Andrew Binder,§ SongHai Chai,‡ Sheng Dai,

†

School of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China

‡

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United

States §

Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600, United States

++

School of Letters and Sciences, Arizona State University, Mesa, Arizona 85212, United States

o

Co3O4 2(gas)

o2

Co

O

Cu

Ce

o

O

(abs)

O

(act)

Co O CeO2

+

CO

CO Cu+

O

Ce

1


co2

co2 (ads)

(gas)

Co3O4âCeO2 Catalyst for

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