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J. Phys. Chem. C 2008, 112, 8234–8238
Effective CO Oxidation on Endohedral Gold-Cage Nanoclusters Yi Gao, Nan Shao, Satya Bulusu, and X. C. Zeng* Department of Chemistry, UniVersity of Nebraska-Lincoln, Lincoln, Nebraska, 68588 ReceiVed: January 16, 2008; ReVised Manuscript ReceiVed: March 10, 2008
Ab initio calculations are performed to study the CO oxidation on six endohedral gold-cage clusters (W@Au12, Nb@Au13, Zr@Au14, Sc@Au15, Ca@Au16, and Na@Au17). The calculations suggest that three clusters (Nb@Au13, Zr@Au14, and Sc@Au15) can be very effective nanocatalysts. The reaction energy barriers are lower than those associated with either bare gold clusters or gold surfaces and are comparable to those associated with the support Au clusters. The effective CO oxidation can be attributed to the charge transfer from the dopant to the gold cage, the low coordination number for gold atoms on the cages, as well as the fluxionality of the cage. Introduction Gold nanoclusters have attracted considerable attention as new nanocatalysts over the past decade.1–5 Numerious experiments have demonstrated that small-sized gold clusters AuN (N e 20), either in free-standing form or soft-landed on a well-characterized metal oxide support [e.g., MgO(001) surface with oxygen vacancies, i.e., F centers], can oxidize CO into CO2 at low temperatures.6–13 In particular, the gold octamer, when bound to F centers of the magnesia surface, is proven to be very effective for heterogeneous catalysis.12 Detailed studies of the effect of oxide support, quantum-size effect, and coadsorption effect on nanogold catalysis have been reported.8,14–19 Ab initio calculations have also been employed to investigate the CO oxidation on supported or unsupported gold clusters.12,20–24 Sanchez et al. performed density functional theory (DFT) calculations within the local-spin-density approximation to study the CO oxidation on MgO-supported Au8 clusters. It was suggested that both the Eley–Rideal (ER)-type and Langmuir–Hinshelwood (LH)-type reactions entail a low reaction barrier ( 0.25 eV) on the gold cluster with CO preadsorbed are highlighted in bold.
Figure 6. More detailed energy diagram for catalytic reaction CO + O2 f CO2 + O on Sc@Au15.
Figure 7. Energy diagram for catalytic reaction CO + O2 f CO + O-O on Zr@Au14 via the LH reaction pathway.
i.e., between the chemisorbed CO and molecular O2, on all possible sites of the endohedral gold clusters. Results and Discussion As the first step (E1 f E2), strong adsorption of CO is seen on all the six endohedral clusters, with the adsorption energy (∆ECO-ads ) E2 - E1) ranging from -0.82 to -1.39 eV. Among the six clusters, W@Au12 gives rise to the strongest CO adsorption (-1.39 eV). Note that the CO adsorption on the six endohedral gold clusters is stronger than that on a step edge
site on the Au(211) surface (where the adsorption energy ∼ -0.25 eV) and also stronger than that on a bare Au10 cluster (-0.6 eV).34 Interestingly, the adsorption energies are comparable to that on the Au10 cluster supported on a rutile surface (-0.95 eV).21b Note that another recent theoretical study of the CO adsorption on endohedral Na@Au20 cluster showed similar results, although the reported CO adsorption energies (-0.57 to -0.91 eV)20c are slightly weaker than those on the endohedral gold clusters shown in this study. In the second step (E2 f E3), molecular O2 is coadsorbed with CO on the endohedral gold clusters. The adsorption energies (∆EO2-ads ) E3 - E2) vary from –0.09 to –0.26 eV. These values of adsorption energies are very close to those for the O2 adsorption on small-sized bare Aun clusters (n ) 5-10).35 Two forms of O2 adsorption can be observed: (1) O2 binds only to a single Au atom and (2) O2 bridges two Au atoms. In the first form, the O-O bond length is ca. 1.26 Å, slightly longer than that (1.24 Å) of the gas-phase oxygen, which is consistent with previous theoretical studies.21,35 In the second form, the O-O bond length is elongated to 1.32 Å, which represents an O2- superoxo state as seen in an earlier experiment and theoretical studies.20–22,34,36 However, this O2 adsorption is endothermic (0.66 eV for Zr@Au14 as shown in Figure 7), indicating that this form of O2 adsorption is unlikely. Since the O2 adsorption in the first form is much more energetically favorable than that in the second form, hereafter we will only focus on the first form of O2 adsorption. Note that without the CO preadsorption on the gold cluster, the adsorption of a single O2 molecule on the gold cluster is endothermic (e.g., 0.22 eV adsorption energy for Zr@Au14 and 1.24 Å for the O-O bond length), indicating that the CO preadsorption is necessary for the subsequent O2 adsorption. This conclusion is consistent with a recent experiment13 and theoretical calculation.37 The energetically favorable adsorption of O2 (i.e., E4 - E2 > 0.25 eV) on the gold cluster with CO preadsorbed is highlighted in bold in Table 1, and the corresponding schematic energy diagram for catalytic reaction CO + O2 f CO2 + O are shown in Figures 4–6. On the other hand, if the gold cluster is negatively charged, e.g. Ca@Au16-, the adsorption of O2 on the CO preadsorbed gold clusters becomes energetically even more favorable. Our calculation shows that the O2 adsorption energy (with CO preadsorbed) increases from 0.09 to 0.48 eV, and the O-O bond length is elongated to 1.30 Å, much longer than that of O2 adsorbed on the neutral gold clusters. This result suggests that the negatively charged endohedral gold clusters may be more effective as a nanocatalyst than their neutral counterparts. Subsequent to the second step (E3 f E4), the adsorbed CO and O2 form a stable complex CO · O2 (an intermediate). This
CO Oxidation on Endohedral Gold-Cage Nanoclusters TABLE 2: HOMO level (eV) of Preadsorbed and Postabsorbed CO Molecules on W@Au12, Nb@Au13, Zr@Au14, Sc@Au15, Ca@Au16, and Na@Au17, Respectively M@AuN
preadsorbed CO
postadsorbed CO
W@Au12 Nb@Au13 Zr@Au14 Sc@Au15 Ca@Au16 Na@Au17
-5.238 -5.236 -5.558 -5.493 -5.680 -5.736
-5.227 -5.223 -5.356 -5.346 -5.653 -5.601
complex can be characterized by a single C-O bond (∼1.35 Å) and a peroxide O-O bond (∼1.45 Å) with the C and terminal O atoms bound to two neighbor Au atoms. This complex has also been reported in previous theoretical studies of the CO oxidation on gold surface and on small gold clusters.20–22 Our calculations show that the formation of the complex is slightly endothermic on W@Au12 (∆E3 ) E4 - E3: 0.27 eV), but slightly exothermic on other five clusters. Among the five clusters, Sc@Au15 appears to the most favorable to the complex formation because of the largest exothermic energy (-0.30 eV) involved. It is remarkable that the adsorption of the complex on the gold cages (measured by ∆E ) E4 - E1: -0.95 to -1.38 eV) is even stronger than that on the rutile-supported pure Au clusters (-0.45 to -0.73 eV).20 These results suggest that the complex (intermediate) can easily form on most endohedral gold clusters. Once the CO · O2 complex is formed, in the next step (step 4: E4 f E5), the labile O-O peroxide bond can easily break due to the low energy barriers encountered in the step 4 (∆Ebarrier ) E5 - E4: 0.05-0.27 eV). The barrier values are lower than those for the CO oxidation on bare gold clusters (∼0.40 eV)21 and are comparable to those on the supported gold nanoparticles (0.1-0.2 eV).12,20 Last, we found that the final step reaction CO + M@AuN-O-O f M@AuN-O + CO2 is barrierless, consistent with previous theoretical studies.20 Hence, we expect that the catalytic reaction on the cage of endohedral gold clusters is likely to occur well below room temperature. More importantly, since CO and O2 can bind to the endohedral gold clusters more strongly than to the bare gold clusters, we expect that the endohedral gold clusters should lead to more effective CO oxidation, compared with bare gold clusters, on the oxide support.20,21 To gain more insights into the catalytic properties of the endohedral gold clusters, we present a few other calculation results. The charge distribution analysis suggests that Au atoms receive negative charges upon the CO adsorption, consistent with previous theoretical findings that there is charge transfer from CO molecule to the gold cluster where the CO molecule is an electron donor.38 In addition, there exists back-donation from Au atoms to the antibonding π orbitals of CO molecules. This σ-donation/π-back-donation is very similar to the wellknown Blyholder mechanism of catalysis on transition-metal surfaces.39 It should be noted that upon CO adsorption, the HOMO of the endohedral gold-cage clusters, which is contributed mainly by the d orbitals of Au,30 is shifted a bit higher as shown in Table 2. Considering that the antibonding π* orbital of O2 (-6.789 eV) locates below the HOMO of the gold clusters, two conclusions can be drawn: (1) The antibonding π* orbital of O2 can easily receive electrons from the endohedral gold-cage clusters, as pointed out earlier by Ha¨kkinen et al.40 (2) After the CO adsorption, the energy difference between the HOMO level of the gold clusters and the antibonding π* orbital of O2 becomes slightly larger, resulting in stronger O2 adsorption.
J. Phys. Chem. C, Vol. 112, No. 22, 2008 8237 Several previous studies have shown that the presence of undercoordinated Au sites (e.g., defect sites) and a strain in the nanogold clusters are closely associated with the high activity of CO oxidation with supported or unsupported clusters.16,21,22 Here, the coordination number of the Au atom in the endohedral gold-cage clusters ranges from 3 to 6, which is undercoordinated as compared to the bulk. Thus, all the Au atoms on the cages may be viewed as certain defect sites which should lead to higher activity. Finally, our studies show that some geometry changes occur in the endohedral gold clusters in the course of reaction (detailed geometries are shown in the Supporting Information), especially for Ca@Au16 and Na@Au17, which encounter very low reaction barriers (almost barrierless). This behavior is very similar to that for the dissociation of H2 on gold clusters, a reflection of the importance of the fluxionality of the clusters.41 Conclusion In conclusion, the CO oxidation is used as a benchmark to probe the catalytic capability of six endohedral gold-cage clusters. The calculations suggest that three clusters (Nb@Au13, Zr@Au14, and Sc@Au15) can be very effective nanocatalysts. It is found that the catalytic reactions have relatively low energy barriers, even lower than those involved in the known catalytic reactions on the Au(111) and Pt(111) surfaces,42 and comparable with those on the supported Au nanoparticles. Hence, our study suggests that the CO oxidation can proceed below room temperatures on these three endohedral gold-cage clusters. Some insights into the underlying catalytic mechanism for this class of nanocatalysts are obtained. Because of the low coordination number of the Au atoms, the preferential sites of CO adsorption, and the charge transfer from dopant to gold cage as well as the geometry fluxionality, CO oxidation is expected to be more effective on the endohedral gold-cage clusters than on bare gold clusters and possibly more than on oxide supports. Supporting Information Available: Data of Cartesian coordinates, harmonic vibrational frequencies, and full reaction pathways. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgment. We thank Dr. Wei An for helpful discussion. This work was supported by grants from the NSF, the DOE, and the Nebraska Research Initiative and by the Research Computing Facility at the University of Nebraska. References and Notes (1) (a) Haruta, M.; Date, M. Appl. Catal., A 2001, 222, 427. (b) Haura, M. Catal. Today 1997, 36, 153. (c) Haruta, M.; Kobayashi, T.; Samo, H.; Yamada, N. Chem. Lett. 1987, 405. (2) Bond, G. C. Catal. Today 2002, 72, 5. (3) Hammer, B.; Nørskov, J. K. Nature 1995, 376, 238. (4) (a) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1637. (b) Yoon, B.; Ha¨kkinen, H.; Landman, U.; Wo¨rz, A. S.; Antonietti, J.-M.; Abbet, S.; Judai, K.; Heiz, U. Science 2005, 307, 403. (5) Buratto, S. K.; Bowers, M. T.; Metiu, H.; Manard, M.; Tong, X.; Benz, L.; Kemper, P.; Chre´tien, S. Atomic Clusters: 12; Elsevier; 2007, Chaper 4. (6) Kim, T. S.; Gong, J.; Ojifinni, R. A.; White, J. M.; Mullins, C. B. J. Am. Chem. Soc. 2006, 128, 6282. (7) Carrettin, S.; Concepcio´n, P.; Corma, A.; Nieto, J. M. L.; Puntes, V. F. Angew. Chem., Int. Ed. 2004, 43, 2538. (8) Comotti, M.; Li, W.-C.; Spliethoff, B.; Schu¨th, F. J. Am. Chem. Soc. 2006, 128, 917. (9) Okumura, M.; Coronado, J. M.; Soria, J.; Haruta, M.; Conesa, J. C. J. Catal. 2001, 203, 168. (10) Guzman, J.; Carrettin, S.; Fierro-Gonzalez, J. C.; Hao, Y.; Gates, B. C.; Corma, A. Angew. Chem., Int. Ed. 2005, 44, 4778.
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