Planar versus Nonplanar Pd Clusters: Stability and CO Oxidation

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Planar versus Non-planar Pd Clusters: Stability and CO Oxidation Activity of Pd Clusters with and without TiO2(110) Substrate Yanle Li, Chunyan Liu, Zhichen Pu, Linquan Bao, Yan Zhu, and Jing Ma J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 22, 2019

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The Journal of Physical Chemistry

Planar versus Non-planar Pd Clusters: Stability and CO Oxidation Activity of Pd Clusters with and without TiO2(110) Substrate Yanle Li†,§, Chunyan Liu‡,§, Zhichen Pu†, Linquan Bao†, Yan Zhu*,† and Jing Ma*,† †Key

Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical

Engineering, Nanjing University, Nanjing 210023, P. R. China ‡College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, P. R. China

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Abstract: Metal clusters with different size and shape may have very different optical, electronic, magnetic, and catalytic properties. The possible geometrical structures of Pdm (m=3 – 12, the number of Pd atoms) have been explored by the combination of a modified Morse-potential driven genetic algorithm search with density functional theory (DFT) calculations. By using the clustering analysis of DFT data, the studied Pdm clusters were grouped into two families of m>8 (with longer average interatomic distance and larger atomization energy) and m8 with longer bond length but higher NAE values, the other m 2.14 > 1.96 > 1.66, in e-), simultaneously giving the same order for the CO oxidation activity, 2D Pd8/TiO2(110) > 3D 6CO@Pd8/TiO2(110) > 2D 2CO@Pd8/TiO2(110) > 3D Pd8/TiO2(110) (Figure 10a). The charge transfer played an important role in the CO oxidation on the Pd8/TiO2(110) systems. To investigate the electronic properties and metal valence states of the reaction systems, we performed X-ray photoelectron spectroscopy (XPS) on the Pd3/TiO2(110) and Pd NPs/TiO2(110). The Pd 3d XPS spectra of Pd3/TiO2(110) are shown in Figure 10b. The peaks at binding energies of 335.7 and 341.0 eV were assigned to Pd0, and those at binding energies of 336.9 and 342.1 eV were associated with Pd2+, respectively.38 Especially, the shift toward higher binding energy was found for the Pd3/TiO2(110) and Pd NPs/TiO2(110) catalysts (Figure 10c), indicating that the charge transfers indeed from the Pd to the TiO2 substrate, which was well in accordance with our theoretical results. Although Pd7 and Pd8 differ by only one atom, they are quite different from each other in terms of some behaviors. Firstly, the most stable 2D Pd8/TiO2(110) is lower in energy than the 3D one, which is in contrast with the Pd7 cluster. Secondly, when located on TiO2(110), with the addition of CO, the supported 3D Pd7 cluster transforms into the 2D one, while the 3D Pd8 cluster does not. Thirdly, our calculations show that the CO oxidation performance of the supported Pd8 clusters is more complex than that of the reported Pd7 one. The difference between Pd7 and Pd8 reflects the 22


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mystery nature of nano-clusters that clusters with sizes of a few nanometres or less often have properties that change evidently with the addition of a single atom.

Figure 10. (a) The TS barrier correlated well with the charge transfer for four studied systems. XPS profiles of (b) Pd3/TiO2(110) and (c) Pd NPs/TiO2(110).The green line and magenta line represent the Pd2+ and Pd0 spectra, respectively.

The conclusions drawn in this work are also consistent with some experimental and 23



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computational results of other metal clusters. The experimental data of Pt8 cluster supported on TiO2(110) indicated that the geometrical structure changes from a planar structure to a 3D structure at Pt8, and the corresponding CO oxidation activity and activation energy varied at the same where 2D-to-3D transition happened.14 This result may also provide a rationale for our research of the stability and CO oxidation activity of 3D versus 2D Pd cluster. Moreover, the inhomogeneous distribution of charge on the metal cluster was also discussed by Hong et al.39 for the supported Au13 cluster, which was correlated with its higher activity in methanol decomposition and the trend was similar to our results. From the pathway of CO oxidation on Pd1, we found that the reactivity of the supported Pd1 was not superior to 2D Pd8 (Figure S8), although one would have naively expected the single site to be more reactive. Recent development in the precise control of isolated single-atom catalysts by chemical methods will promote the advance of single-atom catalysts research.40 The catalysis on the low-dimensional materials is expected to bring new picture and products in both theoretical simulations and experimental synthesis.

Conclusions We systematically investigated the structures and stability of free-standing 2D/3D Pd8, as well as the CO oxidation activity of the supported low dimensional Pd1, Pd3 and 2D/3D Pd8 clusters. The 2D unsupported Pd8 cluster was less stable than 3D Pd8 one. It was hard for the 3D Pd8 cluster to transform to the 2D one only with adsorbing more CO molecules. However, if the 2D Pd8 cluster was located on TiO2(110), the 2D Pd8 cluster became the preferred structures. This was due to the 24


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2D Pd8 cluster contacting more electronegative O atoms through the more significant charge transfer between the 2D Pd8 and TiO2(110). The reaction pathway contained the TS1, intermediate, and TS2 were computed when adsorbed CO and O2 reacted on Pdm/TiO2(110) (m = 1, 3, 8) surfaces. The simultaneous adsorption of CO and O2 could somewhat alter the shapes of surpported Pd8 clusters. The barriers of the rate-determining steps of CO oxidation on the TiO2 supported low dimensional Pd1, Pd3 and 2D Pd8 clusters were close to each other, but lower than that on the supported 3D Pd8 clusters. This was supported by our CO oxidation experiments on Pd3 cluster and Pd NPs supported on TiO2(110)surfaces. The 2D cluster was more favorable for the CO oxidation than the 3D one, which was rationalized from more significant interfacial charge transfer. The XPS spectra showed that the electron donation from Pd to TiO2, which was in agreement with our theoretical results.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: The TiO2(110) surface model and its energy for k points test; A serial of Pd4~Pd12 isomers and their energy properties; The structures of unsupported and supported Pd8 clusters; The charge distribution; The experimental characterizations of Pd NPs and the Pd3 cluster; The dynamics behavior of CO on the free standing Pd8 cluster; The O2 molecule located on the TiO2(110) surface; The catalytic cycle of Pd1 single atom catalysis. (PDF) 25



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AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected]. Phone: + 86-25-89681772

*E-mail:

[email protected]. Phone: +86-25-89681696

ORCID Jing Ma: 0000-0001-5848-9775 Yan Zhu: 0000-0002-5993-4110 Author Contributions §These

authors contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program of China (2017YFB0702600, 2017YFB0702601) and the National Natural Science Foundation of China (No. 21673111 and 21773109). We are grateful to the High Performance Computing Centre of Nanjing University for providing the IBM Blade cluster system.

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Planar versus Nonplanar Pd Clusters: Stability and CO Oxidation

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