Tailoring the Electronic Structure of Mesoporous Spinel Î³-Al2O3 at

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Tailoring the Electronic Structure of Mesoporous Spinel γ‑Al2O3 at Atomic Level: Cu-Doped Case Liangjie Fu and Huaming Yang* Department of Inorganic Materials, School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China S Supporting Information *

ABSTRACT: The well crystallized mesoporous spinel gamma alumina (γAl2O3) has been widely studied as an important catalyst support. However, the tailoring of alumina catalyst is still an unresolved challenge when considered at the atomic level. On the basis of experimental characterization, it is found that the electronic structure of γ-Al2O3 could be continuously tailored by metal doping via one-pot route. Combined with density functional theory (DFT) calculations, the nature of geometric and electronic structure evolution upon Cu doping in γ-Al2O3 is investigated. The structure of γ-Al2O3 and the influence of intrinsic defects are studied at first. Considering the nano effect and the charge compensation effects of these intrinsic defects, the doping mechanism of Cu inside γ-Al2O3 lattice are in detail explored at the atomic level. At low doping level the excess electrons from VO or Ali would over compensate Cu dopants, leading to the formation of Cu+ species. As the doping level increases, the preferred doping sites change from Oh sites to Td sites, while electrons from Ali correlate the Cu species, and an electronic phase transition is observed.

1. INTRODUCTION As an important support in functional materials, alumina, especially ordered mesoporous alumina are recently reviewed for catalytic applications.1−3 The mesoporous gamma alumina, inherently inert as an unreducible oxide, are widely used in many fields due to their crystallinity, high surface area and surface basicity. The well-defined mesoporous aluminum oxides4−8 with high surface area and pore volume are widely employed in environmental remediation, water−gas shift reactions,9 aldol reaction,10 CO2 capture,11 and hydrogen production.12 Generally, the impregnation method gives disperse metal catalysts on the alumina surface, with excellent catalytic properties. The recent successful preparation of ordered mesoporous alumina13 stimulated the one-pot synthesis of alumina-supported metal oxides with well-developed mesoporosity, relatively high surface area, and crystalline pore walls.14 In amorphous alumina, metal dopants coexist with intrinsic defects and might form species with varied structures, influenced by local structure, doping concentration, and chemical environment.15,16 The one-pot method gives disperse metal dopants in the alumina lattice, with the interaction between each other influenced by the distance. As expected, metal species homogeneously distributed in the support structure might also alter the thermally stablility, isoelectric point, and electronic properties. Cu is always an essential part in high-performance materials and has been intensively studied due to its redox properties.17 The catalytic properties of CuO nanoparticles supported on other oxides have higher catalytic activity than pure CuO © 2014 American Chemical Society

because the supported oxides have large surface area and Cu species are highly dispersed over the support.18−20 CuO/Al2O3 or copper aluminate (CuAl2O4) was used as catalyst for defluoridation of water21 or hydrogenation of the carbonyl group.22 CuO was known active during the degradation of some organic compounds but suffered photocorrosion due to its redox properties.23 The Cu2+ ions in sol−gel prepared Cu/ Al2O3 are dispersed in the bulk, which will give steady catalytic activity, less affected by surface environment.24 In contrast to the formation of bulk CuO at high copper loadings on γ-Al2O3 as recognized long ago, the agglomeration of Cu species can be suppressed by the one-pot synthesis method. As for the typical Cu-doped γ-Al2O3 systems, according to UV−vis spectra,25 it was believed that the ratio of Cu doping at Td/Oh site increased with the Cu content in γ-Al2O3 lattice. The absorption band around 700 and 1500 nm are assigned to d−d transition of Cu2+(Oh) or Cu2+(Td) species, while the absorptions at 300−600 nm are still not clarified.26−36 However, the band at 400−500 nm was assigned to threedimensional Cu+ clusters, bulk-like CuAl2O4,37 or ligand to metal charge transfer (LMCT) between oxygen and copper,29,38 but the above discussion is far from conclusive due to the lack of quantitative relation of absorption intensity with Cu content in the Oh/Td crystal field symmetry, which still needs to be examined. Furthermore, there has been still a Received: November 17, 2013 Revised: June 12, 2014 Published: June 16, 2014 14299

dx.doi.org/10.1021/jp502204h | J. Phys. Chem. C 2014, 118, 14299−14315

The Journal of Physical Chemistry C

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analysis, the doping mechanism and electronic structure evolution of Cu-doped Al2O3 catalysts are studied.

debate upon the valence state, coordination of Cu within Cudoped γ-Al2O3, and whether the two Cu 2p peaks should be assigned to Cu+/Cu2+,34,39 disperse CuO/bulk CuO,40 CuO/ CuAl2O4 structures,41 or Cu(Oh)/Cu(Td).31 Correspondingly, it was found that the octahedral (Al3+octa)/tetrahedral (Al3+tetra) coordination ratio for γ-Al2O3 could vary upon metal doping,14 but there was no detailed mechanism. The correlation between the dopant states increases with the doping level, but how far can metal doping tailor the electronic structure of the inert support is still an open question. For example, one of the key concepts when doping metal oxide to increase the reducibility is the size of the dopants. Doping with a cation that is smaller than the host cation can create both short and long metal−oxygen bonds, with the oxygen atoms bonded by long bonds to the metal being weaker and therefore easier to remove.42 Decades ago, the distribution of transitionmetal ions at Oh/Td sites in spinel structure was discussed by crystal field theory, and the orbital split was determined from optical and magnetic data.43 The classical calculations have indicated that the structure of the spinels is determined under the balance of many factors, e.g., the Madelung energy, the Born repulsive energy, ordering energy of cations, site preference energy, and polarization energy.44 Since the correlation between metal dopants and the intrinsic defects might change as the doping level increases, in order to achieve reasonable transport properties, the concentration of the defects inducing the impurity band must exceed a certain percolation limit, which is material dependent.45 However, the theoretical studies to date are mainly focused on “surface spinel” structures,46−49 which do not apply in the bulk doping case, and research works on detailed doping mechanism of metal into the Al2O3 structure and studies referring electronic structures are rare.50,51 While the role of In doping on the formation of a PtSn alloy phase on alumina surface are explored by DFT calculations,52 the intrinsic reason for the electronic structure evolution of those metal-doped samples is still ambiguous.14,53 In this article, tailoring of the mesoporous alumina support is performed by copper doping in gamma alumina (MA-xCu, where x is a Cu/Al molar ratio in the precursor) by the one-pot synthesis. The morphology, geometric structure, electronic structure and optical properties of the samples were in detail characterized by X-ray diffraction (XRD), N2 porosimetry, transmission electron microscopy (TEM), solid-state nuclear magnetic resonance (NMR), H2-temperature-programmed reduction (H 2-TPR), X-ray photoelectron spectroscopy (XPS), and ultraviolet−visible spectroscopy (UV−vis). Furthermore, first-principle calculations were performed in detail to elucidate the nature of the obscured experimental phenomena, based on which a better understanding of the geometric and electronic structure of these metal-doped Al2O3 catalysts can be achieved, and the cooperative effect of between Cu dopants and intrinsic defects in γ-Al2O3 can then be understood. Considering charge compensation effects, a systematic searching for the preferred Cu doping sites inside γ-Al2O3 lattice is performed to elucidate the detailed doping mechanism. The energetically favorable structures at different doping levels are listed, in which some proper structures are selected for further analysis of experimentally characterized Cudoped Al2O3 structures. The influence of Cu-doping upon tuning of the electronic structures and optical properties of Al2O3 is carefully examined. Together with the experimental

2. EXPERIMENTAL METHODS 2.1. Materials Synthesis. Cu-doped mesoporous γ-Al2O3 catalyst was synthesized using a similar procedure to our previous work:15 For pure alumina (MA) sample, appoximately 2.0 g of Pluronic P123 (Sigma-Aldrich) was dissolved in 20.0 mL of ethanol and allowed to stir for about 3 h. Then, appoximately 0.84 g of citric acid (99.5%, Sinopharm Chemical Reagent Co., Ltd.) and 7.5 g of Al(NO3)3·9H2O (99.5%, Sinopharm Chemical Reagent Co., Ltd.) were added into 20 mL of ethanol with vigorous stirring. Once dissolved, the two solutions were combined, and 10.0 mL of ethanol were used to thoroughly transfer the aluminum nitrate solution. The combined solution was covered with PE film, and further stirred at room temperature for about 5 h. The final solution was slowly evaporated at 60 °C in drying oven. After 3 days of aging, calcination was then carried out by slowly increasing temperature from room temperature to 800 °C (1 °C/min) and held at 800 °C for 4 h in air. MA-xCu samples were prepared by adding (CH3COO)2Cu·H2O (99%, Tianjin Kemiou Chemical Reagent Co., Ltd.) to the aluminum nitrate solution, with the remaining synthesis kept the same as for MA. The ratio of copper to aluminum was adjusted accordingly (2%, 5%, 7%, 10%, and 15%). 2.2. Characterization. Powder XRD patterns were collected on a D/MAX2550VB+ X-ray diffractometer (Cu Kα radiation, steps of 2°/min). N2 adsorption−desorption isotherms were obtained at −196 °C on a Micromeritics ASAP 2010 Sorptometer using a static adsorption procedure. Brunauer−Emmett−Teller (BET) surface areas were determined from the linear region of the BET plot. Pore size distributions were calculated from the adsorption isotherms by the Barrett−Joyner−Halenda (BJH) model, and the total pore volumes were estimated from the N2 uptake at P/P0 = 0.995. Transmission electron microscopy (TEM) images were recorded on a JEM-2100F field emission source transmission electron microscope (JEOL) operating at 200 kV. X-ray photoelectron spectroscopy (XPS) spectra were recorded on K-Alpha 1063 spectrometer (Thermo Fisher Scientific) using monochromatic Al−Kα as the excitation source. The pressure in the analysis chamber was 7, at which doping level the phase transition is possible. The increasing orbital coupling between Cu(Oh) and Cu(Td) species at higher doping level is influenced by surrounding Cu−O local structure, leading to the formation

of CuAl2O4-like structures. Combined with DFT calculations, the assignment of Cu 2p peaks can be clarified (Figure 6b). Among the various species that might exist, the most favored Cu species (highlighted in bold) at each doping level in γ-Al2O3 are proposed based on their corresponding formation energy. To sum up, a Cu doping mechanism is proposed. While formation of intrinsic defects in nanoparticles can be also strongly favored due to the nanosize effect, the charge compensation effect and the orbital coupling under various local structutres (Oh or Td) are the key factors for the doping mechanism. We suggest that by using the one-pot synthesis method the Cu dopants (brown) are well dispersed in the γAl2O3 lattice but correlated by the excess electrons (solid circle) introduced by Ali (gray) under Coulomb screening effect (dash circle). Hence, theoretically, a series of ordered mesoporous spinel alumina, with the electronic structure continuously tailored by metal doping, might be synthesized accordingly. The coordination environment of the metal in the γ-Al2O3 structure and the electronic structure change upon metal doping is still an open question to date. Diffuse reflectance spectroscopy (DRS) and IR have been applied extensively in the study of the coordination of Cu2+ in γ-Al2O3 since decades ago.25,33,35,98 In the following, we combined ab initio results with the available experimental spectroscopic information to explore the mysteries remaining. 3.8. Geometric Structure of Cu-Doped γ-Al2O3. A detailed interpretation of absorption spectra of Cu2+ species in compounds with tetragonally deformed octahedral environment or near planar is complicated,99 and the local structures need to be first examined. For Cu doping in γ-Al2O3, the Cu−O polyhedra (Figure S11a,b, Supporting Information) distorted a little with the relaxed Al−O bond (approximately 1.7 to 2.2 Å) in the rather loosely bonded system of this transition phase Al2O3. The Cu−O bond length was (0.1−0.2 Å) longer than Al−O, attributed to the larger atomic radius of Cu over Al. The slight elongation of three Cu−O bonds at one side of the Oh structure (Figure S11a, Supporting Information) or one Cu−O bond in the Td structure (Figure S11b, Supporting Information) makes Cu atoms paramagnetic and lowers the total energy at several hundred meV. Interestingly, the paramagnetic state only slightly above the diamagnetic state is suggested to be accessible at room temperature. We then 14306



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Figure 7. Normalized image intensities in two directions from HRTEM regions and the corresponding atomic structure and local structure around Cu.

Figure 8. Total density of states (DOS) and atom-projected density of states (PDOS) for PDOS of Cu-doped γ-Al2O3. Both paramagnetic and diamagnetic configurations for (a,b) Cu(Oh) and (c,d) Cu(Td) are given. Partial charge density corresponding to the 3d orbitals, as pointed by arrows, are given. The isosurface levels are 0.02 e/Å3. In order to keep consistency, the allowed transition was always from green to brown.

checked that the local structure of Cu+ species coexisted with intrinsic defects (Figure S11c, Supporting Information). At low

doping level, for the Cu−VO species, the excess electrons from VO would stimulate the transition of Cu(Oh)/Cu(Td) species 14307



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Figure 9. Total density of states (DOS) and atom-projected density of states (PDOS) for the far configurations at low doping level, such as (a) Cu(Oh,Oh) + Ali far, (b) Cu(Td,Oh) + Ali far, (c) Cu(Td,Oh,Td) + Ali far, Cu(Oh,Oh,Oh) + Ali far, (d) Cu(Td,Oh) + Ali, Cu(Td,Td) + Ali, and (e)Cu(Td,Td,Td) + Ali and Cu(Oh,Oh,Oh) + Ali.

to Cu(IV)/Cu(III) species. For the Cu++Ali species, however, the excess electrons from Ali would only lengthen the Cu−O bond, with the bond lengths around 0−0.2 Å longer than that of Cu2+ species. In real cases, the alumina system is occasionally positively charged (Figure S11d, Supporting Information), i.e., the excess electrons might be compensated by the acceptor type defects at distance or surface adsorbate, then the Cu−O bond lengths of Cu+ species will be gradually decreased and finally resemble that of Cu2+ species. It should be mentioned that reduction of the 2+ oxidation state of Cu to 1+ oxidation state occurs only after two excess electrons are gained. At higher doping level, because of the charge compensation effect, the electrons from Ali will be averaged over the surrounding Cu species. Thus, the total Cu−O bond length distribution is statistically a combination of the length of the above species, of about 0.05 Å longer than Al−O on average. However, it should be noted that it is the increase of Ali that leads to the slight downward shift of six main reflections in XRD simulations (Figure S12, Supporting Information), in line with that in experimental XRD patterns (Figure S1, Supporting Information). For the MA-xCu sample at medium doping level (x = 7), as can be seen from the HRTEM images (Figure 7), the framework of the mesopore structure is highly crystallized, and the measured interplanar distances are close to the (222) and (100) surface of spinel

alumina from cross sections of the images, in agreement with DFT results. 3.9. Electronic Structure, Valence Band, and Reduction Ability of Cu-Doped γ-Al2O3. For the substitution of Cu to Al, the whole band shifts downward (Figure S13, Supporting Information), while the dopant state lies above the valence band maximum compared to the pure γ-Al2O3. The Cu 3d states overlap and mix with the O 2p states and increase the width of the valence band, indicating that the CBM and VBM of Cu-doped γ-Al2O3 are lowered from those of pure γ-Al2O3 by 1 and 0.7 eV, respectively. With a large part of the Cu 3d states extended into the band gap with a maximum about 1.5 eV from the valence band maximum of γ-Al2O3, the excitation from these occupied Cu 3d states to conduction band also lead to a decrease of the photon excitation energy and induce more significant extrinsic visible light absorption, in agree with the UV−vis absorption at 600−800 nm observed in Cu(Oh)-doped γ-Al2O3.25,26,32,33,36,67 According to the Jahn−Teller theorem, any degenerate electronic system will spontaneously distort in such a way as to remove the degeneracy. The absorption spectra of transitionmetal atoms vary with the surrounding environment, from water solution to crystal oxide.43 It is well-known that for Cu2+ species the five d-shell orbitals split into a triplet t2g state and a doublet eg state. However, DFT calculations for the Cu2+(d9) 14308



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Figure 10. Total density of states (DOS) and atom-projected density of states (PDOS) for near configurations (a) Cu(Td,Oh) + Ali near and Cu(Td,Td) + Ali near; (b) Cu(Td,Oh,Oh) + Ali near and Cu(Td,Oh,Td) + Ali near.

Ali seems to make Cu species over compensated and t2g orbitals and eg orbitals degenerated, showing some metallic Cu0 features. Considering the charge compensation effect, by removing one electron from the Cu(Oh) + Ali system, we then have the pseudo-Cu+ species with full 3d orbitals. From the energy point of view (Figure 4), while Ali coexisted with Cu dopants, both far and near configurations (with Cu atoms locate far and near from each other) are stable. In Figure 9 we listed the electronic structures of several representative species with far configurations for Cu doping concentration from approximately 1.5% to 5%, such as (a) Cu(Oh,Oh) + Ali far, (b) Cu(Td,Oh) + Ali far, (c) Cu(Td,Oh,Td) + Ali far, Cu(Oh,Oh,Oh) + Ali far, (d) Cu(Td,Oh) + Ali far and Cu(Td,Td)+Ali far, and (e) Cu(Td,Td,Td) + Ali far and Cu(Oh,Oh,Oh) + Ali. Since there is 50% chance of having Cu dopants at the next-nearest neighbors at higher doping level above 4.8% (the averaged distance between Cu dopants will be less than 8 Å), some near configurations are given in Figure 10. The electronic structures of several representative species are given for Cu(Td,Oh) + Ali near, Cu(Td,Td) + Ali near, Cu(Td,Oh,Td) + Ali near, and Cu(Td,Oh,Oh) + Ali near species. At low doping level, take the Cu(Oh,Oh) + Ali far species, for example, it is found that the electronic structure varies with the location of Ali. While in the case where Ali locates in the middle region between the two Cu atom (structure shown in Figure S9, Supporting Information), the charge compensation of excess electrons from Ali would result in two Cu1+δ(Oh) species with the highest unoccupied d orbitals partially filled (Figure 9a). However, while in the case where Ali locates near one Cu atom, which is about 1 eV more stable than former case, the charge compensation would result in one Cu+(Td) species near Ali and one partially reduced Cu2+(Oh) species (denoted as Cu1+δ(Oh)) at a distance, as can be interpreted

in Oh environment (Figure S14, Supporting Information) show that the tetragonally deformed Cu(Oh) species have split energy levels of dx2−y2, dz2 orbitals, and the nonequivalent occupation of dx2−y2 and dz2 orbitals is influenced by local distortion. The slight structure distortions (Figure S11a,b, Supporting Information) lead to the nonequivalent occupation and orbital hybridization for Cu2+(Oh) (Figure 8a,b) and Cu2+(Td) (Figure 8c,d) species. The spin polarization, increasing with the orbital splitting, results in the transition of magnetic state from diamagnetic to paramagnetic property. Generally, the d−d transition of Cu2+ species is always from the occupied t2g orbitals to the highest unoccupied eg orbital, i.e., from dxy, dyz, or dzx orbitals (green) to the hybridized orbital (brown, mainly dx2−y2 or hybridized with dz2) for Cu2+(Oh) species (Figure 8a,b), and from eg orbitals to t2g orbitals, i.e., dx2−y2 or dz2 orbitals (green) to the hybrid orbitals (brown, dxy, dyz, and dzx orbitals) for Cu2+(Td) species (Figure 8c,d). It should be noted that the electronic structures are strongly influenced by the local bonding structures of Cu−O, whether compressed or lengthened in each directions (Figure S11a,b, Supporting Information). Take Cu2+(Oh) species, for example, the randomly varied Cu−O bond lengths in γ-Al2O3 lattice will alter the highest unoccupied eg orbitals to be mainly dz2 orbital (Figure 8a) or dx2−y2 (Figure 8b). This kind of Jahn−Teller distortion of Cu2+ complex was also observed at hydrated Cuzeolites.100,101 Similar separation of Cu 3d states is found by Gallino et al. using a hybrid DFT methods.102 In the real case, because of the excess electrons from intrinsic defects, the electronic structures are more complicated. The gaining of electrons from intrinsic defects, such as Ali, will lift up the Fermi level. It is suggested that the electron states of the Cu+ species are lifted higher compared with Cu2+ species, and the excess electron will occupy the highest eg orbital (Figure S14, Supporting Information). Gaining all three electrons from 14309



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Figure 11. Total density of states (DOS) and atom-projected density of states (PDOS) for Cu+(Oh) + VO, Cu+(Td) + VO, Cu(Oh,Oh) + VO far, and Cu(Oh,Oh) + VO near.

3d orbitals) 3 eV above VBM. Interestingly, after checking the electronic structure of other combinations such as Cu(Oh,Td) + VO, Cu(Td,Oh) + VO, and Cu(Td,Td) + VO, it is observed that the two electrons introduced by VO are located at the adjacent Cu atom, leading to the formation of reduced species such as, Cu+−VO, Cu1+δ−VO−Cu1+δ, and Cu2+···VO−Cu+, etc. Overall, it is found that due to the charge compensation effect and the nano effect, the electrons from Ali have also correlated the Cu species and an electronic phase transition has occurred. We also examined the electronic properties of Cudoped cases by nonspinel and spinel-like models. The calculated Mulliken charge, PDOS results, and charge density distributions are similar for the case without intrinsic defects (Figure S14, Supporting Information). Unlike in defective spinel structure, since there is no vacancy site for Ali to occupy in nonspinel structure, the introduction of interstitial type of Ali in Cu-doped nonspinel model will induce local lattice structure distortion around Cu, leading to the transformation from Cu(Oh) to Cu(III) or Cu(II) (Figure 12). Alternatively, the interstitial type of Cui will lead to similar local distortion (Cu(Oh) + Cui far) or aggregation (Cu(Oh) + Cui near), depending on the distance between each other (Figure S15, Supporting Information). Interestingly, the calculated electronic properties seem to behave similarly with spinel case, such as the PDOS results, charge state, and spin state of Cu dopants, even for species at higher doping level (Figure S15, Supporting Information, Cu(Oh,Oh,Oh) + Ali). Furthermore, it is observed that further distortion around Cu dopant resulted in the increase of Al(Td) amount, with the total energy several hundred meV lower (Figure S16, Supporting Information). Combined with the DFT calculated electronic structures, the XPS valence band spectra of MA-xCu (Figure 13) can be

from PDOS results (Figure 9b). Hence, it is understandable that the electronic structure of the far configurations at low doping level, such as Cu(Td,Oh,Td) + Ali far, Cu(Oh,Oh,Oh) + Ali far, Cu(Td,Oh) + Ali, and Cu(Td,Td) + Ali (Figure 9c,d), in which Cu atoms are far from each other, can be interpreted as a simple combination of Cu+−Cu1+δ, bridged by the excess electrons from Ali. Occasionally, the Cu1+δ species might sometimes be partially oxidized to Cu2+ (Figure 9d). This observation also holds for other species formed at higher doping levels such as Cu(Td,Td,Td) + Ali and Cu(Oh,Oh,Oh) + Ali (Figure 9e). While the PDOS of Cu(Td,Oh) + Ali far species (Figure 9b) can be interpreted as a combination of Cu+(Td)−Cu1+δ(Oh), the PDOS of Cu(Td,Oh) + Ali near species shows some hybridization between the 3d orbitals of Cu(Oh) and Cu(Td) atoms (Figure 10a). The outmost d orbitals of the Cu atoms are partially correlated by Ali, and as the Cu doping content increases, the orbital coupling between Cu atoms become so strong that the highest occupied d orbitals of adjacent Cu atoms hybridize with each other (Figure 10b). It should be mentioned that the electronic structure of Cu species also changes with the distance between Cu dopants, as can be seen from the higher oxidation state of Cu2+(Oh) (Figure 9d) than Cu1+δ(Oh) (Figure 9b) with the same Cu(Td,Oh) + Ali combination. Furthermore, the electronic structures for Cu species coexisting with VO (Figure 11), such as Cu+(Oh) + VO, Cu+(Td) + VO, and Cu(Oh,Oh) + VO (the most favored structure with VO at VO1 site in near and far configurations, Figure S10, Supporting Information) species, are also given. The presence of VO seems to reduce the adjacent Cu and introduce localized defect states (partially hybridized with Cu 14310



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Figure 12. (top) Structure relaxation and (down) PDOS of Cu(Oh,Oh) + Ali, Cu(Oh,Td) + Ali, and Cu(Td,Td) + Ali doped γ-Al2O3 for nonspinel model. The charge and spin state of Cu are given. The unoccupied (brown) and highest occupied (yellow) orbitals are shown.

interpreted to elucidate the influence of Cu doping upon the Fermi level shift. The calculated results agreed well with the experimental; both have two main peaks near 5 and 10 eV. The electron states at the upper part (0−2 eV) of valence band are mainly contributed by Cu2+ species (blue arrowed), while the lower part (2−4 eV) is contributed by Cu2+ species (light blue arrowed). It is worth noting that for Cu-doped alumina, the band tail near VBM is mainly contributed by the occupied 3d orbitals of Cu species, with an orbital splitting of approximately 2 eV. It is proposed that a hard electrophilic attack will occur close to the catalytic active Cu−O region. The TPR experiment (Figure S17, Supporting Information) shows that the Cu2+ species are greatly increased upon higher doping level, and reduction of MA-xCu catalysts is easier than the aMA-xCu catalysts,15 due to the improved charge transfer ability of crystal phase.

Figure 13. Experimental XPS valence band spectra of MA-xCu samples.

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Figure 14. (a) Calculated optical absorption of γ-Al2O3 and Cu-doped γ-Al2O3. Some representative Cu species for (red dot) Cu(Oh), (blue dot) Cu(Td), (red dash) Cu+(Oh) + VO, (blue dash) Cu+(Td) + VO, (yellow line) Cu(Oh,Oh) + Ali far, (red line) Cu(Oh,Oh,Oh) + Ali far, (green line) Cu(Td,Oh,Oh) + Ali far, (blue line) Cu(Td,Td,Td) + Ali far, (violet line) Cu(Td,Oh+) + Ali, (violet dash) Cu(Td+,Oh) + Ali, (violet dot) Cu(Td,Td) + Ali, (black dash) Cu(Oh,Oh,Oh) + Ali, (black dot) Cu(Td,Td,Td) +Ali, and (gray dot) Cu(Td,Td,Td) + Ali near are given. (b) Absorption band assignment of the UV−vis spectra of MA-xCu.

3.10. Calculated Optical Properties of Cu-Doped γAl2O3. From the electronic structure, optical properties of Cudoped γ-Al2O3 have been calculated (Figure 14a, the 30% overestimate of PBE functional for the d−d splitting of Cu in alumina are included) in order to interpret the experimental UV−vis spectra (Figure 14b). According to the discussion in the section above, the red shift of absorption edge from 350 to 450 nm can be attributed to the excitation from the highest occupied d orbital to the conduction band of the various Cu+ species at each doping level, and the absorption band around 400 nm can be attributed to the well dispersed Cu+−Cu1+δ species (yellow). The red-shift of the later band is attributed to the Cu1+δ−Cu1+δ species that are correlated by Ali. The absorption band around 600 nm can be attributed to the characteristic Cu(Oh) species, which are formed at medium doping level (Figure 14a, dot), or reduced Cu1+δ(Td)/ Cu(Td)+VO species. The absorption band around 600−800 nm is assigned to Cu(Oh) + VO species or the well dispersed Cu1+δ−Cu1+δ species that emerged at doping concentration higher than 2.4%, in contrast with previous DRS studies.25,33,67,68 According to the trends of favored doping site from Cu(Oh) to Cu(Td) as suggested in section 3.4, the shift of absorption band from 400 to 500 nm at higher doping concentration (Figure 14b, brown) is originated from the electronic phase transition from Cu(Oh,Oh,Oh) to Cu(Td,Td,Td) (Figure 14a), in agreement with electronic structure evolution. It is also found that the aggregated species generally introduced a background absorption like the Cu(Td,Td,Td) near species (Figure 14a, gray). In contrast to the amorphous case, where the d−d transition occurs locally,15,16 the d−d transition intensity here is shifted by the charge transfer in gamma alumina lattice with long-range order.

to exist as Cu+ species or partially reduced Cu+ species due to the coulomb screening. As a result, the Cu+ + Ali species, possessing diamagnetic properties, are popular at the low doping level. Then, as the distance between Cu species decreases with the increased doping concentration, Cu2+ species are formed at higher doping level. Though the formation of aggregated Cu species such as Cu(Oh,Td) + Ali, Cu(Td,Td) + Ali, Cu(Td,Oh,Td) + Ali, etc. are energetically favored, the dispersed Cu2+ species correlated by Ali are most popular at the medium doping level of x = 7. The increasing orbital coupling between Cu(Oh) and Cu(Td) species at higher doping level is influenced by the distance between the dopants, which is highly related to the dopant distribution, depending upon the detailed preparation procedure. In this regard, the one-pot method is crucial to obtain an even distribution of metal dopants in alumina. Correspondingly, the increased optical absorption upon the gradual increase of copper content in γ-Al2O3 is elucidated by DFT modeling. While optical absorption bands in the visible light range are introduced by Cu doping, the DFT calculations suggest that the visible light absorption is not significantly improved by Cu at low doping level (MA-2Cu) due to the preferred formation of Cu+ species. While the absorption band due to the d−d transition of the dispersed Cu2+(Oh) species (around 600 nm, lower than the classic d−d transition of 800 nm in solution) are enhanced at higher doping level (MA7Cu), the absorption bands between 300 and 600 nm due to these d orbital correlated Cu species, such as Cu(Oh,Td) + Ali Cu(Td,Oh,Td) + Ali, Cu(Td,Td,Td) + Ali, etc. (formed at high doping level, MA-15Cu) depending on the degree of dispersion of Cu dopants, can be improved by one-pot method. While Cu doping in γ-alumina and amorphous alumina15 both give Cu+ species in the structure at low doping level, they introduce CuAl2O4-like structure and CuO nanoparticles at high doping level, respectively, due to the higher long-range order of γalumina over amorphous alumina. The analysis of electronic structure evolution indicates that, due to the charge compensation effect, the electrons from the widely distributed Ali have correlated the Cu species, and there is a phase transition from Mott-insulator to charge transfer-type at high doping level. The charge compensation effect, nano

4. CONCLUSIONS Tailoring of the mesoporous gamma alumina support is demonstrated by Cu doping. The MA-xCu supports, possessing large surface area with a narrow pore-size distribution concentrated at 7 nm was synthesized and characterized in detail. For Cu doping in γ-Al2O3 systems, at low doping level, because of the feedback of charge compensation effect by (major) excess Al atom or (minor) oxygen vacancy, Cu prefers 14312



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effect, and the orbital coupling under various local structures (Oh or Td) are the key factors for the doping mechanism. Although the results gained in this study are focused upon understanding the nature of Cu doping in γ-Al2O3 systems, they may shed some light for the study of other metal doping in γAl2O3 at atomic level, which is crucial for future development of γ-Al2O3 support. Considering the tailored geometric and electronic properties, we suggest that Cu-doped mesoporous γ-Al2O3 with better stability and extended Fermi level might have potential applications in environmental remediation, water−gas shift reactions, aldol reaction, CO2 capture, and hydrogen production.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S18, and the details on formation energy calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(H.Y.) Phone: +86-731-8883 0549. Fax: +86-731-8871 0804. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Fund for Distinguished Young Scholars (51225403), the Hunan Provincial Natural Science Fund for Innovative Research Groups and the Specialized Research Fund for the Doctoral Program of Higher Education (20120162110079). All computations were performed at the High Performance Computing Center of Central South University.

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REFERENCES

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Tailoring the Electronic Structure of Mesoporous Spinel Î³-Al2O3 at

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