Heterogeneous Catalysts DOI: 10.1002/ange.201200903 Electronic Modulation of a Copper/Zinc Oxide Catalyst by a Heterojunction for Selective Hydrogenation of Carbon Dioxide to Methanol** Fenglin Liao, Ziyan Zeng, Clive Eley, Qin Lu, Xinlin Hong,* and Shik Chi Edman Tsang* In the past decades, global warming caused by an increasing concentration of atmospheric carbon dioxide (CO 2 ) and the depletion of fossil fuels have received much attention by scientists and governmental agencies. [1, 2] Catalytic conversion of CO 2 to liquid fuels or other valuable chemicals has a positive impact on these important environmental and energy issues. [3, 4] In particular, the hydrogenation of CO 2 to methanol is very attractive because of its position as a high energy density liquid fuel and a key platfrom chemical (for manufacture of formaldehyde, methyl-tert-butyl ether, and acetic acid). The convenient production of hydrogen at large scale from renewable energy sources (solar energy, hydro- power, biomass, or excess chemical heat) also supports this new green process. [5, 6] The majority of research on catalytic studies of CO 2 hydrogenation has been using modified industrial methanol catalysts for the hydrogenation of synthesis gas (CO/H 2 ), which contained Cu and ZnO as the main components together with an alumina support and different modifiers. [7–9] Ni was recently claimed to display a higher turnover frequency than Cu. [10] Up to now, the exploitation of the catalyst is slow because of the lack of knowledge on both hydrogenation reactions and the fundamental understanding of the important material interactions in the catalyst formu- lation. Regarding the hydrogenation of synthesis gas many models have been proposed to define the active sites and the synergetic interactions of the copper and zinc oxide. [11] For example, ZnO is regarded to provide active sites for spillover hydrogen [12] or as a structure-directing support controlling the dispersion, morphology, and specific activity of the copper particles. [13] We have previously reported a significant shape effect of ZnO on the interaction with copper for the synthesis of methanol from hydrogenation of CO 2 . The electron-richer polar (002) face containing surface-terminated oxygen ions in the platelike ZnO nanocrystal showed a much stronger material synergy with copper than other crystal facets, which gave a higher selectivity (around 70 %) towards methanol obtained from the hydrogenation of CO 2 . On the other hand, the charge-neutral ZnO rod crystal surfaces (010, 110, and 101 faces) showed only a selectivity of around 40 % towards methanol when they were physically mixed with copper particles of the same size. [14] Here, we report, for the first time, a dramatic enhance- ment in the electron density of ZnO by encapsulating CdSe (electron rich quntum dot) into the ZnO rod as core–shell structure. Using ZnO rod/CdSe of controlled morphology as the model support for copper, a selectivity of 75 % towards methanol production from hydrogenation of CO 2 was real- ized. The formation of heterojunctions between CdSe and ZnO rods promotes the electron transfer thermally at the Schottky–Mott junction between modified ZnO and Cu, which increases the methanol selectivity of the reaction. The procedure for preparing CdSe core–ZnO shell heterojunc- tions at different compositions (samples 1–5) can be found in the Supporting Information. The use of preformed CdSe nanoparticles followed by the growth of a ZnO rod material was adopted and verified by inductive coupled plasma (ICP) method. The X-ray powder diffraction (XRD) data in Figure 1a confirm that all the samples show characteristic peaks of a rod-shaped crystalline wurtzite structure of ZnO without phase alternation (see also the Supporting Information). The diffraction peaks clearly shifted to a lower angle when the content of encapsulated CdSe was increased. However, no crystalline phase of CdSe (samples 1, 2, and 3) was observed, suggesting a good dispersion of CdSe within the ZnO matrix. CdSe also adopts the Wurtzite structure (27–29% lattice mismatch with respect to ZnO depending on the particular phases at the interface). The successful synthesis of a core– shell CdSe/ZnO structure has been reported. [15] However, we identified small CdSe phases at higher loadings (samples 4 and 5, see the Supporting Information). The lattice expansion (left shift) in the CdSe-modified ZnO samples can be attributed to the replacement of O 2À by Se 2À anions (Se 2À is larger than O 2À ) at the extensive interface between the two phases (see the Supporting Information). High-resolution transmission electron microscopy (HRTEM) studies on the samples have been conducted. [*] F. Liao, Z. Zeng, Dr. X. Hong, Prof. S. C. E. Tsang College of Chemistry and Molecular Sciences Wuhan University, Wuhan, 430072 (PR China) E-mail: hongxl@whu.edu.cn F. Liao, C. Eley, Dr. Q. Lu, Prof. S. C. E. Tsang Wolfson Catalysis Centre, Department of Chemistry University of Oxford, Oxford, OX1 3QR (UK) E-mail: edman.tsang@chem.ox.ac.uk Dr. Q. Lu Naval Research Laboratory 4555 Overlook Ave., S.W, Chemistry Division, Code 6112 Washington, DC 20375-5342 (USA) [**] This research was supported by the NSFC (grant number 20903074) and the Fundamental Research Funds for Central Universities of China. TEM characterization was performed by the University of St Andrews (UK) through the EPSRC access scheme. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201200903. A ngewandte Chemi e 1 Angew. Chem. 2012, 124,1–5  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! Ü Ü