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DOI: 10.1039/C1RA00431J (Communication) RSC Adv., 2011, 1, 1200-1203

Reduction of Cu2O nanocrystals: reactant-dependent influence of capping ligands and coupling between adjacent crystal planes†

Qing Hua abc, Kai Chen b, Sujie Chang abc, Huizhi Bao abc, Yunsheng Ma c, Zhiquan Jiang a and Weixin Huang *abc
aHefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China
bCAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Hefei 230026, China
cDepartment of Chemical Physics, University of Science and Technology of China, Hefei 230026, China. E-mail: huangwx@ustc.edu.cn.; Fax: +86-551-3600437; Tel: +86-551-3600435

Received 8th July 2011 , Accepted 11th September 2011

First published on 27th September 2011

Abstract

A systematic study of the reduction behavior of various types of uniform Cu2O nanocrystals reveals that the surface blocking effect exerted by the capping ligand on the reduction behavior of the Cu2O nanocrystals depends not only on the type of capping ligand but also on the reactant and that coupling between adjacent crystal planes occurs for Cu2O nanocrystals exposing two types of crystal planes during the reduction reactions and exerts great influence on the reduction kinetics of the involved crystal planes.

The fundamental understanding of heterogeneous catalysis is of great importance but also remains a great challenge. Surface science studies of single crystal/single crystal thin film model catalysts have provided deep insights into elementary surface reactions involved in heterogeneous catalysis at the atomic level,1 however, a so-called “materials gap” exists between single crystal/single crystal thin film model catalysts and powder catalysts consisting of polycrystalline nanoparticles in which a single nanoparticle exposes various types of crystal planes determined by its shape. Therefore, caution should be taken when the fundamental understanding acquired from single crystal and single crystal thin film model catalysts is extended to practical catalytic reactions employing powder catalysts. Ertl et al.2 directly observed that the reaction rate of hydrogen oxidation on a 40 nm region of Pt(100) plane on a platinum tip displayed sustained temporal oscillations under the same conditions where the reaction rate was steady on an extending Pt(100) single crystal and concluded that such a kinetic difference was due to the coupling between the Pt(100) plane and other adjacent platinum crystal planes exposed on the platinum tip via the migration of reactive surface species. Similar coupling between adjacent crystal planes has also been reported in other systems of metal model catalysts.3–7 These results point to an important concept that the reaction kinetics of a polycrystalline nanoparticle cannot be regarded as a simple superposition of the contributions from each individual crystal plane exposed on the polycrystalline nanoparticle surface. Recently, the great progress in the controlled synthesis of inorganic materials makes it possible to prepare nanocrystals with a uniform and well-defined shape that can serve as model catalysts for the fundamental study of heterogeneous catalysis.8,9 In this communication, we for the first time validate the concept of the coupling between adjacent crystal planes on an oxide nanocrystal in reduction reactions employing various types of Cu2O nanocrystals with a uniform and well-defined shape as model catalysts.

An important issue encountered when employing nanocrystals with a uniform and well-defined shape as catalysts is the influence of capping ligands remaining on the nanocrystal surface inherited from the nanocrystal preparation process. A surface blocking effect in which the chemisorbed capping ligand decreases the number of accessible surface active sites for reactants has been generally observed;10–12 however, a promotion effect has also been reported via the modification of the electronic structure and local chemical environment of the nanocrystals by the chemisorbed capping ligands.13–16 In this communication, we have also comparatively studied the reduction of Cu2O nanooctahedra capped with polyvinylpyrrolidone (PVP) and capped with oleic acid (OA) by H2 and CO. The influence of PVP and OA capping ligands on the reduction of Cu2O nanooctahedra by H2 is observed to be different from that on the reduction of Cu2O nanooctahedra by CO. These results reveal a novel vision of reactant-dependent influence of capping ligands on the surface reactivity of nanocrystals.

Fig. 1 and Fig. S1† show SEM images of various types of uniform Cu2O nanocrystals with a well-defined shape that were prepared following Zhang et al.'s17 and Liang et al.'s18 recipes, including cubic Cu2O nanocrystals with sizes of 400–700 nm without capping ligands (denoted as Cu2O–6f), octahedral Cu2O nanocrystals with sizes of about 300 nm capped with PVP (denoted as Cu2O–8f–PVP), octahedral Cu2O nanocrystals with sizes of 200–800 nm capped with OA (denoted as Cu2O–8f–OA), rhombic dodecahedral Cu2O nanocrystals with sizes of 600–900 nm capped with OA (denoted as Cu2O–12f–OA), {100} truncated octahedral Cu2O nanocrystals with sizes of about 300 nm capped with PVP (denoted as Cu2O–14f–PVP), and {110} truncated octahedral Cu2O nanocrystals with sizes of about 600 nm capped with OA (denoted as Cu2O–20f–OA). Cu2O–6f, Cu2O–8f–PVP, Cu2O–8f–OA, Cu2O–12f–OA, Cu2O–14f–PVP, and Cu2O–20f–OA expose {100}, {111}, {111}, {110}, {100} & {111}, and {110} & {111} crystal planes, respectively.7,8 The XRD and Cu 2p XPS results (Fig. S2†) clearly demonstrate that their crystal phases have cubic fcc Cu2O structure and their surfaces also remain as Cu2O. PVP chemisorbed on Cu2O–8f–PVP and Cu2O–14f–PVP can be directly seen from their N 1s XPS spectra (Fig. S3†), however, OA chemisorbed on Cu2O–8f–OA, Cu2O–12f–OA and Cu2O–20f–OA cannot because its C 1s and O 1s XPS signals cannot be distinguished from those arising from adventitious carbon and Cu2O in the C 1s and O 1s XPS spectra (Fig. S3†), but it was observed that the C 1s XPS peak intensity of Cu2O–8f–OA, Cu2O–12f–OA and Cu2O–20f–OA is stronger than that of other Cu2O nanocrystals.


SEM images of uniform Cu2O nanocrystals with different shapes: (a) cubes; (b) octahedra capped with PVP; (c) octahedra capped with OA; (d) rhombic dodecahedra capped with OA; (e) {100} truncated octahedra capped with PVP; (f) {110} truncated octahedra capped with OA.
Fig. 1 SEM images of uniform Cu2O nanocrystals with different shapes: (a) cubes; (b) octahedra capped with PVP; (c) octahedra capped with OA; (d) rhombic dodecahedra capped with OA; (e) {100} truncated octahedra capped with PVP; (f) {110} truncated octahedra capped with OA.

The reducibility of oxide catalysts strongly correlates with their catalytic performance. We have thus studied the reduction behaviours of various types of Cu2O nanocrystals in both H2 and CO. Fig. 2 shows the H2-TPR and CO-TPR profiles of Cu2O–8f–PVP, Cu2O–8f–OA and Cu2O–6f. Their reducibility in H2 follows the order of Cu2O–8f–PVP > Cu2O–8f–OA > Cu2O–6f but those in CO follows the order of Cu2O–8f–PVP > Cu2O–6f > Cu2O–8f–OA. Cu2O–8f–PVP and Cu2O–8f–OA expose their {111} crystal planes, but when reduced by either H2 or CO, the reduction temperature of Cu2O–8f–PVP is much lower than that of Cu2O–8f–OA, which clearly manifests the influence of the chemisorbed capping ligand on the reducibility of the Cu2O nanocrystals. The chemisorbed capping ligand is also likely to reduce Cu2O nanooctahedra at elevated temperatures. We thus studied the reaction between Cu2O nanooctahedra and the chemisorbed capping ligand by heating the nanocrystals in an Ar stream and monitoring the production of CO2. The results (Fig. S4†) demonstrate that the reaction between Cu2O nanooctahedra and the chemisorbed capping ligand occurs at higher temperatures than that between Cu2O nanooctahedra and CO and produces CO2 whose intensity only amounts to several hundredths of that of CO2 produced by the reaction between Cu2O nanooctahedra and CO. Therefore, the contribution from the reduction of Cu2O nanooctahedra by the chemisorbed capping ligand to H2-TPR and CO-TPR reactions can be neglected. The observed different reduction behaviors between Cu2O–8f–PVP and Cu2O–8f–OA reasonably arise from the different surface blocking effects of chemisorbed PVP and OA on the surface reactivity of the Cu2O nanooctahedra. The much lower reduction temperatures of Cu2O–8f–PVP in either H2-TPR or CO-TPR than those of Cu2O–8f–OA clearly demonstrate that chemisorbed PVP exerts a much weaker surface blocking effect on the access and chemisorption of H2 and CO to the surface of Cu2O nanooctahedra than chemisorbed OA.


H2-TPR and CO-TPR profiles of Cu2O octahedra capped with PVP (Cu2O–8f–PVP) and capped with OA (Cu2O–8f–OA), and Cu2O cubes (Cu2O–6f).
Fig. 2 H2-TPR and CO-TPR profiles of Cu2O octahedra capped with PVP (Cu2O–8f–PVP) and capped with OA (Cu2O–8f–OA), and Cu2O cubes (Cu2O–6f).

We19 have recently reported that the Cu2O{111} crystal plane exposed by Cu2O nanooctahedra has coordination-unsaturated Cu(I) ions but that the Cu2O{100} crystal plane exposed by Cu2O nanocubes has not (Fig. S5†), therefore the Cu2O{111} crystal plane exhibits a much stronger activity towards the chemisorption and activation of H2 and CO than the Cu2O{100}crystal plane. Due to the weak surface blocking effect exerted by chemisorbed PVP, Cu2O–8f–PVP is reduced at lower temperatures either by CO or by H2 than Cu2O–6f.19 Chemisorbed OA exerts a much stronger surface blocking effect than chemisorbed PVP and thus Cu2O–8f–OA is more difficult to reduce either by CO or by H2 than Cu2O–8f–PVP; more interestingly, when reduced by H2, the reduction temperature of Cu2O–8f–OA is lower than that of Cu2O–6f, however, when reduced by CO, the reduction temperature of Cu2O–8f–OA is higher than that of Cu2O–6f. This result demonstrates that the surface blocking effect exerted by chemisorbed OA on Cu2O nanooctahedra towards the access and chemisorption of CO is more profound than that towards the access and chemisorption of H2. This can be explained by the fact that the dynamical size of H2 is much smaller than that of CO and thus H2 can penetrate the ligand shell more easily than CO. Therefore, the surface blocking effect exerted by the chemisorbed capping ligand on the surface reactivity of the nanocrystals is not only dependent on the type of capping ligand but also dependent on the type of reactant. This is reasonable since both the capping ligand and the reactant are involved in the surface blocking effect.

Fig. 3 presents the H2-TPR and CO-TPR profiles of Cu2O–6f, Cu2O–8f–PVP and Cu2O–14f–PVP. Cu2O–6f and Cu2O–8f–PVP expose their {100} and {111} crystal planes, respectively, and Cu2O–14f–PVP exposes both {100} and {111} crystal planes. In both H2-TPR and CO-TPR profiles, the reduction of Cu2O–14f–PVP initiates at similar temperatures to those of Cu2O–8f–PVP, which can be attributed to the reduction of the {111} crystal plane on Cu2O–14f–PVP; however, the reduction feature of the {100} crystal plane on Cu2O–6f can be hardly observed in the reduction of Cu2O–14f–PVP, and the reduction of Cu2O–14f–PVP completes at a much lower temperature than that of Cu2O–6f. Therefore, the reduction kinetics of the {100} crystal plane on Cu2O–14f–PVP differ much from those on Cu2O–6f, which can be attributed to the occurrence of coupling between adjacent {111} and {100} crystal planes on Cu2O–14f–PVP during the course of the reduction reaction. The {111} crystal plane on a Cu2O–14f–PVP nanocrystal can chemisorb and activate H2 or CO and thus be reduced at relatively low temperatures; meanwhile, the activated species on the {111} crystal plane can migrate to the adjacent {100} crystal plane on the Cu2O–14f–PVP nanocrystal and results in its reduction at temperatures where the {100} crystal plane on Cu2O–6f cannot be reduced. Therefore, due to the coupling between adjacent {111} and {100} crystal planes in the reduction reactions, the reduction kinetics of Cu2O–14f–PVP exposing the {111} and {100} crystal planes are obviously not a simple superposition of the contributions from the individual {111} and {100} crystal planes.


H2-TPR and CO-TPR profiles of Cu2O cubes (Cu2O–6f), Cu2O octahedra capped with PVP (Cu2O–8f–PVP), and {100} truncated octahedra capped with PVP (Cu2O–14f–PVP).
Fig. 3 H2-TPR and CO-TPR profiles of Cu2O cubes (Cu2O–6f), Cu2O octahedra capped with PVP (Cu2O–8f–PVP), and {100} truncated octahedra capped with PVP (Cu2O–14f–PVP).

Coupling was also observed to occur between adjacent {111} and {110} crystal planes on Cu2O–20f–OA during the reduction reactions. Fig. 4 shows the H2-TPR and CO-TPR profiles of Cu2O–8f–OA, Cu2O–12f–OA and Cu2O–20f–OA that respectively expose the {111}, {110}, and {111} & {110} crystal planes. The Cu2O{110} crystal plane has no coordination-unsaturated Cu(I) ions but the Cu2O{111} crystal plane has (Fig. S5†),20 therefore, Cu2O–8f–OA is more facilely reduced either by H2 or by CO than Cu2O–12f–OA. However, due to the strong surface blocking effect exerted by chemisorbed OA, the reduction of Cu2O–8f–OA occurs at relatively high temperatures, and thus the difference between the reducibility of Cu2O–8f–OA and Cu2O–12f–OA resulting from their different surface structures seems not to be as much as that between Cu2O–8f–PVP and Cu2O–6f. In both H2-TPR and CO-TPR profiles, the reduction of Cu2O–20f–OA initiates at similar temperatures to those of Cu2O–8f–OA, which can be attributed to the reduction of the {111} crystal plane on Cu2O–20f–OA; however, the reduction kinetics of Cu2O–20f–OA exposing the {111} and {110} crystal planes are obviously not a simple superposition of the contributions from the individual {111} and {110} crystal planes. This also demonstrates the occurrence of coupling between adjacent {111} and {110} crystal planes of Cu2O–20f–OA during the course of the reduction reaction, in which the activated H2 or CO species formed on the more reactive {111} crystal plane can migrate to the adjacent {110} crystal plane on the Cu2O–20f–OA nanocrystal and promote its reduction at temperatures where the {110} crystal plane on Cu2O–12f–OA cannot be reduced.


H2-TPR and CO-TPR profiles of Cu2O octahedra (Cu2O–8f–OA), Cu2O rhombic dodecahedra (Cu2O–12f–OA), and {110} truncated Cu2O octahedra (Cu2O–20f–OA) that all are capped with OA.
Fig. 4 H2-TPR and CO-TPR profiles of Cu2O octahedra (Cu2O–8f–OA), Cu2O rhombic dodecahedra (Cu2O–12f–OA), and {110} truncated Cu2O octahedra (Cu2O–20f–OA) that all are capped with OA.

Therefore, our above results clearly demonstrate that coupling occurs between adjacent crystal planes on a Cu2O nanocrystal exposing two types of crystal planes during the reduction reaction via the migration of reactive surface species due to the different reducibility of crystal planes exposed on such a Cu2O nanocrystal. In this case, the reduction kinetics of a Cu2O nanocrystal exposing two types of crystal planes are not a simple superposition of the contributions from each individual crystal plane. The concept of coupling between adjacent crystal planes on a polycrystalline nanoparticle during the reaction is very important when considering the so-called “materials gap” between single crystal/single crystal thin film model catalysts and powder catalysts. Such a concept has been previously validated by surface science studies of model systems of metallic catalysts.2–7 Our results for the first time confirm this concept in powder oxide catalysts under realistic reaction conditions. Since the crystal plane exposed on the oxide surface determines not only its surface structure but also its surface composition, the catalytic activity and surface reactivity of the oxide surface are sensitively dependent on the exposed crystal plane.9,19,21–23 During heterogeneous catalytic reactions, a concentration gradient of surface species will form between adjacent crystal planes on an oxide nanoparticle which exhibit different catalytic activity, resulting in the coupling between adjacent crystal planes via the migration of surface species. Therefore, coupling between adjacent crystal planes on an oxide nanoparticle might be a general phenomenon during heterogeneous catalytic reactions catalyzed by oxide catalysts.

In summary, we have systematically investigated the reduction of various types of Cu2O nanocrystals with a uniform and well-defined shape by H2 and CO. The surface blocking effect exerted by the capping ligand on the reduction behaviour of the Cu2O nanocrystals depends not only on the type of capping ligand but also on the reactant, providing a novel vision of the influence of capping ligand on the surface reactivity and catalytic activity of nanocrystals. Coupling between adjacent crystal planes occurs for Cu2O nanocrystals exposing two types of crystal planes during the reduction reactions and exerts great influence on the reduction kinetics of the involved crystal planes, greatly deepening the fundamental understanding of heterogeneous catalytic reactions catalyzed by oxide nanoparticles.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (grant 21173204), the solar energy project of Chinese Academy of Sciences, National Basic Research Program of China (2010CB923302), MOE program for PCSIRT (IRT0756), the Fundamental Research Funds for the Central Universities (WK2060030005), and the MPG-CAS partner group program.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, Fig. S1 (SEM images), Fig. S2 (XRD patterns), Fig. S3 (XPS spectra), Fig. S4 (CO2-TPRS spectra in Ar) and Fig. S5 (optimized structures of Cu2O nanocrystals). See DOI: 10.1039/c1ra00431j
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