In Situ Calorimetric Study: Structural Effects on Adsorption and

ARTICLE pubs.acs.org/JPCC

In Situ Calorimetric Study: Structural Effects on Adsorption and Catalytic Performances for CO Oxidation over Ir-in-CeO2 and Ir-on-CeO2 Catalysts Jian Lin,†,‡ Lin Li,† Yanqiang Huang,† Wansheng Zhang,† Xiaodong Wang,*,† Aiqin Wang,† and Tao Zhang*,† † ‡

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China ABSTRACT: In this study, the formation processes of CO2 and carbonates during CO oxidation over Ir-in-CeO2 and Ir-onCeO2 catalysts at 40 °C were compared. An in situ pulse calorimetry, in combination with H2-TPR, Raman, DRIFTS, and adsorption microcalorimetry techniques, was used to reveal the effect of structural differences of these two catalysts on the adsorption and catalytic performances for CO oxidation. It was found that encapsulating Ir in CeO2 greatly weakened the Ce
O bond strength on Ir-in-CeO2. This led to the formation of more oxygen vacancy sites, promoting the accumulation of carbonates. Comparatively, supporting Ir on CeO2 showed a higher Ce
O bond strength and less oxygen vacancy sites on Ir-onCeO2 but exposed more surface accessible Ir sites, which favored the CO adsorption. During CO oxidation, more oxygen vacancy sites on Ir-in-CeO2 resulted in the formation of carbonates with 0.06 monolayers coverage initially before the production of CO2, whereas surface Ir sites on Ir-on-CeO2 facilitated the adsorbed CO reacting with the oxygen species to directly produce CO2.

1. INTRODUCTION Ceria (CeO2) is widely used in catalysis as an oxygen buffer and an active support for noble metals.1 The CeO2 supported noble metal (NM-on-CeO2) catalysts have been used in many reactions including CO oxidation, preferential oxidation of CO (PROX), and water-gas shift (WGS).2
5 In this type of catalyst, the CeO2 was proposed to enhance the dispersion and stability of metal components, which behaved as active sites.6
8 Comparatively, the CeO2 sites could operate in a catalytic cycle and the catalysts with noble metal (NM) encapsulated by CeO2 (NM-inCeO2) can enhance the thermal stability of nanosized NM and also exhibit high reactivity even without the direct participation of the NM sites.9
13 It has been suggested that different reactive sites existed on these two types of NM-on-CeO2 and NM-inCeO2 catalysts, which affected their catalytic performances. There is increasing interest and importance to pursue the role of surface NM sites and activated CeO2 sites on the catalytic performances. Hardacre et al. found that CO oxidation was strongly dependent on the CeO2 coverage on Pt (111) sites. The catalyst of Pt metal fully encapsulated by CeO2 behaved much higher activity than the bare Pt itself.14 Martínez-Arias et al. thought that mutual interactions between Pt and CeO2 led to an enhancement of the reducibility of both components, providing the sites for activation of both CO and O2.15 Venezia et al. suggested that the presence of small Au particles was not the main requisite for a higher activity of CO oxidation but the strong interaction between ionic Au and CeO2 to enhance the CeO2 reducibility was more important.16 Furthermore, Deng et al. directly proved that NM sites just acted as a spectator on the Au-CeO2 or Pt-CeO2 catalyst for PROX or WGS. By leaching the r 2011 American Chemical Society

surface metallic Au or Pt, these catalysts were still effective.17 Comparatively, Wieder et al. found that alumina supported Pd@CeO2 core
shell catalyst easily suffered from deactivation during WGS reaction, probably due to the coverage of Pd by CeO2 to block the access of reactants.18 Recently, Yeung et al. found that these two types of reactive sites resulted in different products. The exposed Pt sites led to easy CO methanation on Pt-on-CeO2 while the activated CeO2 sites on Pt-in-CeO2 were favorable for WGS.19,20 All of these cases indicated a typical relationship between structure and catalytic performance on NM-CeO2 catalysts; however, little detailed information has been yet provided about the structural effects on the adsorption strengths of reactants, further correlating them with the reaction processes. Previously, we developed an Ir-on-CeO2 catalyst, which exhibited better performances than Ir supported on SiO2 or Al2O3 ones for PROX.21 Nevertheless, in situ DRIFTS results under PROX reaction displayed a stronger band attributed to hydrogenbonded water on this catalyst than on Ir-in-CeO2, suggesting a poor selectivity for CO oxidation due to the extensively surface Ir sites. With Ir encapsulated in CeO2, the CeO2 activated by Ir formed the active sites for CO oxidation, which kept a high selectivity to CO2 on Ir-in-CeO2. The Ir-in-CeO2 and Ir-onCeO2 catalysts have a strong structure
activity relationship for CO oxidation during PROX process.22 To discriminate the role of surface Ir and CeO2 sites of these Ir-CeO2 catalysts on the Received: May 9, 2011 Revised: July 7, 2011 Published: July 19, 2011 16509

dx.doi.org/10.1021/jp204288h | J. Phys. Chem. C 2011, 115, 16509–16517

The Journal of Physical Chemistry C

ARTICLE

adsorption or even catalytic processes for CO oxidation, calorimetric method, a powerful tool to provide direct information on the bonding strength between adsorbed species and catalyst surface,23
26 was here exploited. The accurate values of adsorption heats of CO, O2, and CO2 were measured by a microcalorimetry technique. These were then correlated to the evolved heats during CO oxidation evaluated by an in situ pulse calorimetry to explore the formation processes of CO2 and carbonates.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The Ir-in-CeO2 catalyst was prepared by coprecipitation method, where an aqueous solution of H2IrCl6 and Ce(NO3)3 was added to a heated NaOH solution (80 °C) to form a black precipitate with pH of around 9. After aging for 2 h, the suspension was filtered and washed with deionized water several times. Then the sample was dried at 60 °C overnight and finally calcined at 400 °C for 2 h. The CeO2 support was prepared by precipitation from Ce(NO3)3 under similar conditions. The Ir-on-CeO2 catalyst was prepared by deposition precipitation procedure, where the CeO2 support was suspended in an aqueous solution of H2IrCl6 and the pH was adjusted to around 9 using an aqueous solution of 0.2 M NaOH at 80 °C. After the same drying and calcining treatment as the Ir-in-CeO2 catalyst, the final sample was obtained. As a comparison, Ir-on-SiO2 catalyst was prepared by incipient impregnation of SiO2 with aqueous solution of H2IrCl6. The Ir loadings of all catalysts were fixed at around 1.6 wt % (the actual Ir loadings determined by ICP were 1.50 wt % for Ir-in-CeO2, 1.47 wt % for Ir-on-CeO2, and 1.80 wt % for Ir-on-SiO2). There were no chlorine detected on either Ir-in-CeO2 or Ir-on-CeO2 by XRF characterization ( Ir-on-CeO2 > CeO2; on the other hand, the Ir-in-CeO2 presents a lower lattice parameter than the Ir-on-CeO2 analyzed from XRD results. These results indicate that the Ir-in-CeO2 has the most intrinsic defect. It has been proved that the presence of surface defect, such as large size oxygen vacancy clusters, would promote the transformation of Ce4+ to Ce3+ for CeO2-based materials.39,40 Therefore, the formation of more oxygen vacancy is facile on Irin-CeO2 than on Ir-on-CeO2 during reduction treatment. 3.4. DRIFTS of CO Adsorption. The CO adsorption behaviors on Ir-in-CeO2 and Ir-on-CeO2 were investigated by DRIFTS technique. For comparison, the same experiments on CeO2 and Ir-on-SiO2 were also performed. As shown in Figure 5, CO adsorbs in a form of carbonates (1465, 1504 cm
1 for monodentate carbonates; 1215 and 1396 cm
1 for bridging carbonates; and 1033, 1294, 1537, 1580 cm
1 for bidentate carbonates) on the pure CeO2 and linearly (around 2070 cm
1) on Ir sites of Iron-SiO2.41
43 When CO adsorbs on Ir-in-CeO2 and Ir-on-CeO2 catalysts, the band of CO adsorption on Ir sites is observed in addition to the carbonates. From the intensities of the band for CO adsorption on Ir sites, we can clearly see that the Ir-in-CeO2 has much less accessible Ir sites than the Ir-on-CeO2. On the other hand, the band intensities of carbonates, especially bridging and bidentate carbonates on Ir-in-CeO2 are much higher than that on Ir-on-CeO2, possibly due to the creation of more oxygen vacancy sites by encapsulating Ir in CeO2 for CO adsorption. 3.5. O2, CO, and CO2 Adsorption Microcalorimetry. Adsorption microcalorimetry is a special technique to detect the interaction between the probe gas and the catalyst surface. Table 2 and Figures 6
8 show a comparison of the initial heats and saturation uptakes of O2, CO, and CO2, as well as microcalorimetric profiles for the adsorption heat versus the coverage θ (θ: the ratio of the uptake for each pulse to their saturation uptake). All of the CeO2, Ir-in-CeO2, Ir-on-CeO2, and Ir-onSiO2 samples were prereduced by H2 at 400 °C before the measurements. As shown in Table 2 and Figure 6, a heat plateau around 469 kJ/mol is evolved within the coverage from 0 to 0.9 for O2 adsorption on reduced CeO2, much higher than that of 347 kJ/mol on reduced Ir-in-CeO2, indicating that encapsulating Ir in CeO2 can lower the Ce
O bond strength. This was expected because the presence of metal facilitated the redox process of CeO2.32,44 16512

dx.doi.org/10.1021/jp204288h |J. Phys. Chem. C 2011, 115, 16509–16517

The Journal of Physical Chemistry C

ARTICLE

Table 2. Initial Heats and Saturation Uptakes for O2, CO, and CO2 Adsorption on CeO2, Ir-in-CeO2, Ir-on-CeO2, and Ir-onSiO2 at 40 °C O2 heat sample

CO

uptake

heat

CO2

uptake

heat

uptake

(kJ/mol) (μmol/g) (kJ/mol) (μmol/g) (kJ/mol) (μmol/g)

CeO2

469

220

141

41

141

185

Ir-in-CeO2

347

257

142

108

142

422

Ir-on-CeO2

400

195

156

104

139

202

Ir-on-SiO2

346

32

153

68

Figure 8. Differential heats of CO2 adsorption at 40 °C as a function of surface coverage on CeO2 (2), Ir-in-CeO2 (b), and Ir-on-CeO2 (9).

Figure 6. Differential heats of O2 adsorption at 40 °C as a function of surface coverage on CeO2(2), Ir-in-CeO2 (b), Ir-on-CeO2 (9), and Iron-SiO2 (O).

Figure 7. Differential heats of CO adsorption at 40 °C as a function of surface coverage on CeO2 (2), Ir-in-CeO2 (b), Ir-on-CeO2 (9), and Iron-SiO2 (O).

The initial adsorption heat on Ir-on-SiO2 is only 346 kJ/mol, similar to the reported value of O2 adsorption on Ir sites.45 The simultaneous adsorption of O2 on both Ir and CeO2 sites contributs to the produced initial heat of 400 kJ/mol on reduced Ir-on-CeO2; thus, the adsorption heat on reduced CeO2 sites of Ir-on-CeO2 should be higher than 400 kJ/mol. Therefore, the Ce
O bond strength on Ir-on-CeO2 is higher than that on Ir-inCeO2. On the other hand, the saturation uptake of O2 adsorption on reduced Ir-in-CeO2 (257 μmol/g) is higher than that on reduced Ir-on-CeO2 (195 μmol/g), despite some of the O2

adsorbed on the Ir sites (32 μmol/g on Ir-on-SiO2). According to the density of surface oxygen atoms of CeO2 (1.3  1015 atoms/cm2),46 the amount of oxygen vacancy was calculated around 20% on Ir-inCeO2, higher than 14% on Ir-on-CeO2, indicating that encapsulating Ir in CeO2 leads to more oxygen vacancy on Ir-in-CeO2. As shown in Table 2 and Figure 7, the initial heat of CO adsorption on CeO2 is 141 kJ/mol. As coverage increased, this heat decreases quickly to physisorption field ( 0.6 on CeO2, Ir-in-CeO2, and Ir-on-CeO2. This demonstrates that the presence of Ir hardly imposes effect on the adsorption strength of CO2. Nevertheless, it should be noted that the saturation uptake of CO2 on Ir-in-CeO2 (422 μmol/g) is much higher than that on Ir-on-CeO2 (202 μmol/g), whereas CO2 does not adsorb on Ir-on-SiO2 as shown in Table 2, suggesting that CO2 was mainly adsorbed on the CeO2 sites and more carbonates were accumulated on Ir-in-CeO2 than on Ir-on-CeO2 (0.16 MLs (monolayers) vs 0.09 MLs) since CO2 was easily adsorbed on CeO2 sites as carbonates.49,50 3.6. In Situ Pulse Calorimetry. 3.6.1. Dosing Pulses of CO on the Oxygen Preadsorbed Samples. The adsorption strength of oxygen species on the ceria-based catalysts played an important role in CO oxidation.51,52 CO reacting with the adsorbed oxygen species on reduced CeO2, Ir-in-CeO2, and Ir-on-CeO2 samples was investigated. These samples were first submitted to pulses of O2 until the saturation adsorption. Then pulses of CO were 16513

dx.doi.org/10.1021/jp204288h |J. Phys. Chem. C 2011, 115, 16509–16517

The Journal of Physical Chemistry C

ARTICLE

Figure 9. Percentages of CO adsorbed/reacted (b,O) and the evolved heat (2,Δ) on the oxygen-preadsorbed Ir-in-CeO2 (solid symbol) and Ir-on-CeO2 (hollow symbol) samples during exposure to pulses of CO at 40 °C.

injected onto these oxygen-preadsorbed samples until nearly no consumption of CO. The percentages of adsorbed/reacted CO and the correspondingly evolved heat with the CO pulse number at 40 °C are shown in Figure 9. CO hardly adsorbs on CeO2, which is ascribed to the high Ce
O bond strength to inhibit the subsequent adsorption/ reaction of CO. Comparatively, about 22% to 6% of CO can adsorb/react from first to seventh pulse of CO on Ir-in-CeO2 with the evolved heat of 250
280 kJ/mol, indicating that the lower Ce
O bond strength on Ir-in-CeO2 facilitates the adsorbed oxygen species reacting with CO, either to form CO2 (reaction 1) or carbonates (reaction 2). On the basis of the standard reaction enthalpy of CO oxidation (reaction 3) and the adsorption heat of O2 (reaction 4), the heat of reaction 1 was calculated as
110 kJ/mol. According to reactions 1 and 5, the heat of reaction 2 was calculated as
252 kJ/mol. ΔH ¼
110kJ=mol½c

COðgÞ þ OðadÞ f CO2 ðgÞ

ð1Þ COðgÞ þ OðadÞ f carbonatesðadÞ

ΔH ¼
252kJ=mol½c

ð2Þ COðgÞ þ 1=2O2 ðgÞ f CO2 ðgÞ

ΔH ¼
284kJ=mol½a

ð3Þ 1=2O2 ðgÞ f OðadÞ

ΔH ¼
174kJ=mol½b

CO2 ðgÞ f carbonatesðadÞ

ð4Þ

ΔH ¼
142kJ=mol½b ð5Þ

[a] Calculated according to the standard value from ref 53. [b] Measured in the present study. [c] Calculated from [a] and [b]. The measured heat (250
280 kJ/mol) close to the calculated heat of carbonates formation (reaction 2) and almost no production of CO2 in this process demonstrate that CO could react with preadsorbed oxygen to produce carbonates on Ir-inCeO2. A buildup of around 0.06 MLs of carbonates is estimated.

Figure 10. Percentages of O2 (0) and CO (O) adsorbed/reacted and conversion to CO2 (g), and corresponding heat (Δ) evolved with the dose number of CO+O2 (2:1) pulse on CeO2 at 40 °C.

Figure 11. Percentages of O2 (9) and CO (b) adsorbed/reacted and conversion to CO2 (f), and corresponding heat (2) evolved with the dose number of CO + O2 (2:1) pulse on Ir-in-CeO2 at 40 °C.

It should be noted that the amount of CO adsorbed/reacted is much lower on Ir-on-CeO2. The evolved heat is 260
270 kJ/mol and nearly no release of CO2 is detected, implying the similar formation of carbonates on Ir-on-CeO2. However, the higher Ce
O bond strength leads to the difficulty of adsorbed oxygen reacting with CO, thus the produced coverage of carbonates is only 0.03 MLs after injecting pulses of CO on oxygen-preadsorbed Ir-on-CeO2 sample. 3.6.2. Dosing Pulses of CO+O2. To gain insight into the effects of adsorption behaviors of CO, O2, and CO2 on the catalytic performances, we injected pulses of CO+O2 (2:1) successively onto CeO2, Ir-in-CeO2, and Ir-on-CeO2 catalysts and analyzed the processes of CO oxidation. Figures 10
12 show the percentages of O2 and CO adsorbed/reacted, CO2 yield, and the corresponding heat evolved with pulses of CO+O2 (2:1) at 40 °C. For CeO2, with the pulse from first to fifth, the percentages of O2 and CO adsorbed/reacted decrease quickly from 100% and 18% to 15% and 2%, whereas hardly any CO2 is produced with the evolved heat increasing from 325 to 375 kJ/mol (Figure 10). The ratio of consumed CO/O2 decreases with the pulse of reactants, indicative of O2 preferentially adsorbed on reduced CeO2. No production of CO2 is observed in the first 4 pulses despite almost complete adsorption/reaction of O2 and high consumption 16514

dx.doi.org/10.1021/jp204288h |J. Phys. Chem. C 2011, 115, 16509–16517

The Journal of Physical Chemistry C

Figure 12. Percentages of O2 (9) and CO (b) adsorbed/reacted and conversion to CO2 (f), and corresponding heat (2) evolved with the dose number of CO + O2 (2:1) pulse on Ir-on-CeO2 at 40 °C.

of CO from 55% to 25% on Ir-in-CeO2 (Figure 11). In addition, the evolved heat of 250
280 kJ/mol is close to the heat of CO adsorption/reaction on the oxygen-preadsorbed sample. These suggest the simultaneous adsorption/reaction of O2 and CO, as well as the subsequent carbonates formation (reactions 4 and 2) first. The amount of accumulated carbonates is around 0.06 MLs, close to that of CO pulses on oxyge-preadsorbed Ir-in-CeO2 sample. It can be calculated from the equilibrium of oxygen element that some of O2 was adsorbed on the oxygen vacancy sites, the amount of which is around 7.4%. According to O2 adsorption microcalorimetry results, there remains 12.6% oxygen vacancy sites after the fourth pulse. With further pulses of CO +O2, CO2 can gradually release and the conversion reaches the highest (around 6%) at the eighth pulse while the corresponding evolved heat decreases from 270 to 210 kJ/mol and remains constant from then on. As compared with Ir-in-CeO2, the percentages of O2 adsorbed/reacted are lower, while the percentages of CO adsorbed/reacted and CO2 produced are higher on Ir-on-CeO2 (Figure 12). A little CO2 (around 1%) is produced with higher percentage of CO adsorbed/reacted (69% on Ir-on-CeO2 compared with 55% on Ir-in-CeO2) even for the first pulse. With increasing the pulses, the production of CO2 increases and reaches the maximum of around 17% at the eighth pulse. It has been suggested that CO adsorbed on NM sites could directly react with the oxygen on CeO2 to produce CO2.8,54,55 The higher CO consumption with more CO2 production implies the importance of adsorbed CO on Ir sites for CO oxidation to CO2. The evolved heat is a little lower than that on Ir-in-CeO2 and keeps constant at 180 kJ/mol from the eighth pulse, which is consistent with a higher conversion of CO to CO2 on Ir-onCeO2 since the evolved heat of CO2 formation is lower than that of carbonates formation.

4. DISCUSSION It is well accepted that there is an interaction between noble metal and ceria for NM-CeO2 catalysts. The combination of NM with CeO2 can lower the CeO2 reduction temperature.29 Although the precise role of NM on facilitating the CeO2 reduction is still obscure, two alternative explanations have been proposed: hydrogen spillover and electron transfer effect. As for Ir-on-CeO2, the more exposed Ir sites, the easier the reduction of

ARTICLE

surface oxygen species on CeO2.56 The hydrogen activated on Ir sites can spillover to the CeO2 surface and promoted the CeO2 reduction. As most of Ir is encapsulated by CeO2 for Ir-in-CeO2, it shows a higher reduction temperature than Ir-on-CeO2 (Figure 3). However, more intrinsic defect sites exist (Figure 4b), and the Ce
O bond strength is much weakened on Ir-in-CeO2 (Figure 6), which demonstrates a direct electron interaction and a higher degree of contact between Ir and CeO2. Analogously, when CeO2 was deposited over PGM (Pt group metals), its reducibility was thought to be dependent on the work function of the underlying metals. The junction effect between CeO2 and a metal with higher work function provided the driving force.57,58 This effect led to higher consumption of H2 to extract the surface oxygen of CeO2 and then more O2 adsorption could adsorb on the oxygen vacancy sites for Ir-in-CeO2 (Tables 1 and 2). Thus more surface Ir sites exist on Ir-on-CeO2 while more oxygen vacancy sites on Ir-in-CeO2. The existence of different reactive sites (surface Ir and oxygen vacancy sites) may affect the adsorption property and further reaction mechanism for CO oxidation. There are two widely accepted reaction routes: the Mars van Krevelen (redox cycle) and the Langmuir
Hinshelwood (L-H) mechanism.52,59,60 The steps associated with the former are as follows: the CeO2 is oxidized and the CO reacts with surface oxygen on CeO2; then the produced CO2 desorbs from the surface and the resulting oxygen vacancy is refilled by oxygen. Alternatively, the steps with the latter are as follows: reaction between adsorbed molecules on surface sites (NM and CeO2 sites independently) happens; then the product CO2 desorbs to gas phases. As for CeO2, too high Ce
O bond strength led to the difficulty of adsorbed oxygen species to react with CO and then a lower production of CO2 or carbonates as shown in Figure 10. With Ir encapsulated in CeO2, an obviously lower Ce
O bond strength and further presence of more oxygen vacancy sites exist on Ir-in-CeO2 (Table 2). During CO oxidation, the adsorbed oxygen species on Ir-in-CeO2 can react with CO to produce carbonates (reaction 2) initially, ascribed to the preferential formation of carbonates on oxygen vacancy sites.49,52,59 However, only 7.4% oxygen vacancy sites are favorable for the formation of carbonates. After formation of 0.06 MLs carbonates, the CO2 is produced gradually on the remaining 12.6% oxygen vacancy sites. Generally, the temperature (K) for desorption of adsorbed species nearly equates four times of adsorption heat value (kJ mol
1).61,62 According to the CO2 adsorption heat, the temperature for carbonates decomposition is around 560 K, higher than the reaction temperature of 313 K (40 °C). Thus the production of CO2 is not originated from the decomposition of carbonates, but through a redox cycle (reactions 4 and 1), that is, CO directly reacts with the adsorbed oxygen species to produce CO2. Comparatively, the adsorbed CO on the exposed Ir sites could directly react with the oxygen species on CeO2 to produce CO2 via the L-H mechanism on Ir-on-CeO2. The produced CO2 can be easily transformed to carbonates (reaction 5), as the presence of oxygen vacancy is favorable for the accumulation of carbonates. However, the relatively fewer amount of oxygen vacancy sites (14%) leads to accumulation of less carbonates during CO oxidation on Ir-on-CeO2. Therefore, the formation of CO2 and carbonates is affected by the surface states of these two Ir-CeO2 catalysts during CO oxidation. The existence of surface Ir sites is favorable for the production of CO2, while the existence of oxygen vacancy sites results in the formation of carbonates. Regarding the effects of different reactive sites on the production of CO2 and carbonates, we can imagine that the presence of 16515

dx.doi.org/10.1021/jp204288h |J. Phys. Chem. C 2011, 115, 16509–16517

The Journal of Physical Chemistry C more exposed Ir sites would be helpful for CO oxidation, and the accumulated carbonates on oxygen vacancy sites may decompose to CO2 with the rise of reaction temperature. In another study, we constructed a dual bed reactor system of upper bed Ir-inCeO2 and lower bed Ir-on-CeO2 to provide a wide operating temperature window (80
200 °C) for CO preferential oxidation in H2 rich stream,56 which exerted their respective structural advantages for CO oxidation.

5. CONCLUSIONS The structural effects of Ir-in-CeO2 and Ir-on-CeO2 on the adsorption and catalytic performances for CO oxidation were reported. Encapsulating Ir in CeO2 weakened Ce
O bond strength significantly, which promoted the formation of more oxygen vacancy sites on Ir-in-CeO2. Comparatively, supporting Ir on CeO2 showed a higher Ce
O bond strength but exposed more accessible Ir sites, which was in favor of the CO adsorption on Ir-on-CeO2. With CO dosing on oxygen-preadsorbed samples, the evolved heats were 250
280 kJ/mol, consistent with the enthalpy of CO reacting with adsorbed oxygen to form carbonates. The adsorption strength of preadsorbed oxygen species affected the formation of carbonates; the formed carbonates were calculated as 0.06 MLs on Ir-in-CeO2, whereas the higher Ce
O bond strength led to formation of only 0.03 MLs coverage of carbonates on Ir-on-CeO2. With coadsorption/ reaction of CO + O2, CO could react with the oxygen species to preferentially form carbonates on the oxygen vacancy sites on Ir-in-CeO2, while the existence of Ir sites facilitated the adsorbed CO reacting with the oxygen species to directly produce CO2 on Ir-on-CeO2. ’ AUTHOR INFORMATION Corresponding Author

*(X.D.W.): Tel.: 86-411-84379680. Fax: 86-411-84685940. E-mail: [email protected]. (T.Z.): Tel.: 86-411-84379015. Fax: 86-411-84691570. E-mail: [email protected].

’ ACKNOWLEDGMENT Financial supports provided by the National Natural Science Foundation of China (Nos. 20803079, 21003119, and 21076211) are gratefully acknowledged. ’ REFERENCES (1) Trovarelli, A. Catal. Rev. Sci. Eng. 1996, 38, 439–520. (2) Hilaire, S.; Wang, X.; Luo, T.; Gorte, R. J.; Wagner, J. Appl. Catal., A 2001, 215, 271–278. (3) Deluga, G. A.; Salge, J. R.; Schmidt, L. D.; Verykios, X. E. Science 2004, 303, 993–997. (4) Mari~no, F.; Descorme, C.; Duprez, D. Appl. Catal., B 2004, 54, 59–66. (5) Wang, X. Q.; Rodriguez, J. A.; Hanson, J. C.; Gamarra, D.; Martinez-Arias, A.; Fernandez-Garcia, M. J. Phys. Chem. B 2006, 110, 428–434. (6) Bera, P.; Gayen, A.; Hegde, M. S.; Lalla, N. P.; Spadaro, L.; Frusteri, F.; Arena, F. J. Phys. Chem. B 2003, 107, 6122–6130. (7) Nagai, Y.; Hirabayashi, T.; Dohmae, K.; Takagi, N.; Minami, T.; Shinjoh, H.; Matsumoto, S. J. Catal. 2006, 242, 103–109. (8) Oran, U.; Uner, D. Appl. Catal., B 2004, 54, 183–191. (9) Golunski, S.; Rajaram, R.; Hodge, N.; Hutechings, G. J.; Kiely, C. J. Catal. Today 2002, 72, 107–113.

ARTICLE

(10) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301, 935–938. (11) Cargnello, M.; Gentilini, C.; Montini, T.; Fonda, E.; Mehraeen, S.; Chi, M.; Herrera-Collado, M.; Browning, N. D.; Polizzi, S.; Pasquato, L.; Fornasiero, P. Chem. Mater. 2010, 22, 4335–4345. (12) Rogatis, L.; Cargnello, M.; Gombac, V. L.; Montini, T.; Fornasiero, P. ChemsusChem 2010, 3, 24–42. (13) Cargnello, M.; Wieder, N.; Montini, T.; Gorte, R.; Fornasiero, P. J. Am. Chem. Soc. 2010, 132, 1402–1409. (14) Hardacre, C.; Ormerod, R. M.; Lambert, R. M. J. Phys. Chem. 1994, 98, 10901–10905. (15) Martínez-Arias, A.; Coronado, J. M.; Catalu na, R.; Conesa, J. C.; Soria, J. J. Phys. Chem. B 1998, 102, 4357–4365. (16) Venezia, A. M.; Pantaleo, G.; Longo, A.; Carlo, G. D.; Casaletto, M. P.; Liotta, F. L.; Deganello, G. J. Phy. Chem. B 2005, 109, 2821–2827. (17) Deng, W.; Jesus, J. D.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Appl. Catal., A 2005, 291, 126–135. (18) Wieder, N.; Cargnello, M.; Bakhmutsky, K.; Montini, T.; Fornasiero, P.; Gorte, R. J. Phys. Chem. C 2011, 115, 915–919. (19) Yeung, C. M. Y.; Yu, K. M. K.; Fu, Q. J.; Thompsett, D.; Petch, M. I.; Tsang, S. C. J. Am. Chem. Soc. 2005, 127, 18010–18011. (20) Yeung, C. M. Y.; Tsang, S. C. J. Phys. Chem. C 2009, 113, 6074–6089. (21) Huang, Y. Q.; Wang, A. Q.; Wang, X. D.; Zhang, T. Inter. J. Hydro. Energy 2007, 32, 3880–3886. (22) Huang, Y. Q.; Wang, A. Q.; Li, L.; Wang, X. D.; Su, D. S.; Zhang, T. J. Catal. 2008, 255, 144–152. (23) Bars, J. L.; Auroux, A.; Forissier, M.; Vedrine, J. C. J. Catal. 1996, 162, 250–259. (24) Tripathi, A. K.; Kamble, V. S.; Gupta, N. M. J. Catal. 1999, 187, 332–342. (25) Maroto-Valiente, A.; Guerrero-Ruiz, A.; Rodriguez-Ramos, I. Thermochimi. Acta 2001, 379, 195–199. (26) Xia, X. Y.; Strunk, J.; Busser, W.; Comotti, M.; Sch€uth, F.; Muhler, M. J. Phys. Chem. C 2009, 113, 9328–9335. (27) Li, L.; Wang, X. D.; Shen, J. Y.; Zhou, L. X.; Zhang, T. Chin. J. Catal. 2003, 24, 872–876. (28) Lin, J.; Li, L.; Wang, X. D.; Huang, Y. Q.; Wang, A. Q.; Zhang, T. Sci. China Ser. B 2010, 40, 1409–1414. (29) Harrison, B.; Diwell, A. F.; Hallett, C. Platinum Met. Rev. 1988, 32 (2), 73–83. (30) Das, T. K.; Kugler, E. L.; Dadyburjor, D. B. Ind. Eng. Chem. Res. 2009, 48, 10796–10802.  ao, J. J. M.; Pereira, (31) Carabineiro, S. A. C.; Bastos, S. S. T.; Orf~ M. F. R.; Delgado, J. J.; Figueiredo, J. L. Appl. Catal., A 2010, 381, 150–160. (32) Zhang, B. C.; Tang, X. L.; Li, Y.; Cai, W. J.; Xu, Y. D.; Shen, W. J. Catal. Commun. 2006, 7, 367–372. (33) Hickey, N.; Fornasiero, P.; Kaspar, J.; Gatica, J. M.; Bernal, S. J. Catal. 2001, 200, 181–193. (34) Guzman, J.; Carrettin, S.; Corma, A. J. Am. Chem. Soc. 2005, 127, 3286–3287. (35) Wang, X. Q.; Hanson, J. C.; Liu, G.; Rodriguez, J. A.; IglesiasJuez, A.; Fernandez-García, M. J. Chem. Phys. 2004, 121, 5434–5444. (36) Gamarra, D.; Munuera, G.; Hungría, A. B.; Fernandez-García, M.; Wang, X. Q.; Hanson, J. C.; Rodriguez, J. A.; Martínez-Arias, A. J. Phys. Chem. C 2007, 111, 11026–11038. (37) Taniguchi, T.; Watanabe, T.; Sugiyama, N.; Subramani, A. K.; Wagata, H.; Matsushita, N.; Yoshimura, M. J. Phys. Chem. C 2009, 113 (46), 19789–19793. (38) Wu, Z. L.; Li, M. J.; Howe, J.; Meyer, H. M.; Overbury, S. H. Langmuir 2010, 26 (21), 16595–16606. (39) Dutta, P.; Seehra, M. S.; Shi, Y.; Eyring, E. M.; Ernst, R. D. Chem. Mater. 2006, 18, 5144–5146. (40) Liu, X. W.; Zhou, K. B.; Wang, Lei.; Wang, B. Y.; Li, Y. D. J. Am. Chem. Soc. 2009, 131, 3140–3141. (41) Holmgren, A.; Andersson, B.; Duprez, D. Appl. Catal., B 1999, 22, 215–230. 16516

dx.doi.org/10.1021/jp204288h |J. Phys. Chem. C 2011, 115, 16509–16517

The Journal of Physical Chemistry C

ARTICLE

(42) Bera, P.; Camara, A.; Hornes, A.; Martínez-Arias, A. J. Phys. Chem. C 2009, 113, 10689–10695. (43) Solymosi, F.; Rasko, J. J. Catal. 1980, 62, 253–263. (44) Martínez-Arias, A.; Gamarra, D.; Fernandez-García, M.; Wang, X. Q.; Hanson, J. C.; Rodriguez, J. A. J. Catal. 2006, 240, 1–7. (45) Guil, J. M.; Masia, A. P.; Paniego, A. R.; Menayo, J. M. Thermochimi. Acta 1998, 312, 115–124. (46) Madier, Y.; Descorme, C.; Le Govic, A. M.; Duprez, D. J. Phys. Chem. B 1999, 103, 10999–11006. (47) Yang, Z.; Woo, T.; Hermansson, K. Chem. Phys. Lett. 2004, 396, 382–392. (48) Huang, M.; Fabris, S. J. Phys. Chem. C 2008, 112, 8643–8648. (49) Song, Z.; Liu, W.; Nishiguchi, H. Catal. Commun. 2007, 8, 725–730. (50) Li, C.; Sakata, Y.; Arai, T.; Domen, K.; Maruya, K.; Onishi, T. J. Chem. Soc. Faraday Trans 1989, 85, 929–943. (51) Pushkarev, V. V.; Kovalchuk, V. I.; L. d’Itri, J. J. Phys. Chem. B 2004, 108, 5341–5348. (52) Shapovalov, V.; Metiu, H. J. Catal. 2007, 245, 205–214. (53) CRC Handbook of Chemistry and Physics, 84th ed.; CRC Press: Boca Raton, FL, 2003
2004. (54) Zhu, H. Q.; Qin, Z. F.; Shan, W. J.; Shen, W. J.; Wang, J. G. J. Catal. 2005, 233, 41–50. (55) Xu, J.; Mullins, D. R.; Overbury, S. H. J. Catal. 2006, 243, 158–164. (56) Lin, J.; Huang, Y. Q.; Li, L.; Qiao, B. T.; Wang, X. D.; Wang, A. Q.; Zhang, T. Chem. Eng. J. 2011, 168, 822–826. (57) Frost, J. C. Nature 1988, 334, 577–580. (58) Acerbi, N.; Tsang, S. C.; Golunski, S.; Coller, P. Chem. Commun. 2008, 1578–1580. (59) Boaro, M.; Giordano, F.; Reccia, S.; Santo, V. D.; Giona, M.; Trovarelli, A. Appl. Catal., B 2004, 52, 225–237. (60) Shekhtman, S. O.; Goguet, A.; Burch, R.; Hardacre, C.; Maguire, N. J. Catal. 2008, 253, 303–311. (61) Li, M. S.; Shen, J. Y. J. Catal. 2002, 205, 248–258. (62) Li, L.; Wang, X. D.; Wang, A. Q.; Shen, J. Y.; Zhang, T. Thermochim. Acta 2009, 494, 99–103.

16517

dx.doi.org/10.1021/jp204288h |J. Phys. Chem. C 2011, 115, 16509–16517