Origin of the High Activity of Mesoporous CeO2 Supported Monomeric

Article pubs.acs.org/JPCC

Origin of the High Activity of Mesoporous CeO2 Supported Monomeric VOx for Low-Temperature Gas-Phase Selective Oxidative Dehydrogenation of Benzyl Alcohol: Role As an Electronic “Hole” Juanjuan Liu,†,⊥ Xin-Ping Wu,‡,⊥ Shihui Zou,† Yihu Dai,† Liping Xiao,† Xue-Qing Gong,*,‡ and Jie Fan*,† †

Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, Hangzhou 310027, People’s Republic of China ‡ Key Laboratory for Advanced Materials, Centre for Computational Chemistry and Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, People’s Republic of China S Supporting Information *

ABSTRACT: Herein, we report a newly developed mesoporous VOx-CeO2 catalyst with dominant monomeric VOx species, which can promote the gas-phase ODH reaction of benzyl alcohol-tobenzaldehyde (BA-to-BAD) by molecular oxygen at a surprisingly low temperature range of 203−243 °C, with a high mass-specific activity of ∼25 mmol·gcat−1·h−1 and TOF of 1367 h−1. To the best of our knowledge, it appears to be the most effective transition metal oxides catalyst for the BA-to-BAD reaction in the gas phase. Experimental measurements and density functional theory (DFT) calculations suggest that the activities of VOx-CeO2 catalysts strongly depend on the polymeric states of surface VOx, with monomeric VOx giving much better performance than bigger VOx clusters. It is important to highlight that a specific monomeric VO3 species, only occurring under the reaction conditions, is identified for the first time to have the key influence on BA oxidation. Moreover, it has been found that the origin for the unique activity of such monomeric VOx-CeO2 can be attributed to its electronic “hole” structure.

1. INTRODUCTION Selective oxidative dehydrogenation (ODH) of alcohols to aldehydes, in particular benzyl alcohol-to-benzaldehyde (BA-toBAD), is of great importance because of its wide applications in cosmetics, perfumery, food, and pharmaceutical industries as the second most important aromatic molecule (after vanillin).1,2 Many oxidizing reagents including permanganate and dichromate are employed in industrial processes, but these stoichiometric oxidants are expensive and produce toxic wastes in the BA-to-BAD transformation process.3 With the increasing environmental and cost-effective concerns, “green” catalysts with high activity and selectivity that can use a clean oxidant such as O2 or H2O2 are required for this reaction. In practice, the oxidation of alcohols with molecular oxygen can be performed in the liquid or gas phase, depending mainly on the thermal stability and volatility of the reagents and products. At present, the liquid phase oxidation of BA-to-BAD with molecular oxygen can be performed over a wide range of Pt-, Pd-, and Au-based catalysts below 100 °C. Hutchings et al. have demonstrated that Au−Pd/TiO2 catalysts are very active for BA oxidation, with high selectivity (>96%) to BAD and high conversion rates (98%. 2.5. DFT Calculations. Total energy density functional theory (DFT) calculations corrected by on-site Coulomb interaction (DFT+U) and London dispersion of the Grimme DFT-D type27 have been performed to study the reactivity of VOx-CeO2(111) (ceria powder catalyst mainly exposes the (111) surface28) for the ODH reaction of BA-to-BAD by using the Vienna ab initio Simulation Package (VASP) package. Detailed information is given as follows: The spin-polarized calculations were carried out with the DFT (PBE)-D2 scheme27 as implemented in VASP. As suggested by Penschke et al.26 the dispersion coefficient (C6) and the vdW radii (R0) for Ce were set to 20.00 J·nm6·mol−1 and 186.0 pm, respectively. Default values of C6 and R0 parameters given by Grimme27 were employed for other elements. The project-augmented wave method (PAW)29 was used at a kinetic energy cutoff of 400 eV to describe the electron−core interaction with Ce (5s, 5p, 6s, 5d, 4f), O (2s, 2p), V (3p, 3d, 4s), C (2s, 2p), and H (1s) shells being treated as valence electrons. In particular, a value of 5.0 eV was used for the Hubbard U parameter to describe the localized 4 f electron. The CeO2(111) surfaces with a lattice parameter of 5.440 Å were modeled by 9-atom-layer slabs which represent p(4 × 4) lateral cells with the bottom three layers being fixed to estimate bulk parameters. To avoid interactions between slabs, all slabs were separated by a vacuum gap greater than 10 Å. The Brillouin-zone integration was performed with use of a 1 × 1 × 1 Monkhorst−Pack grid. All the calculations were converged until the Hellman−Feynman forces on each ion were less than 0.02 eV/Å. Transition states were searched with use of a constrained optimization scheme.30 Adsorption energy of H (Ead[H]) and oxygen vacancy formation energy (Eov) were calculated as follows:

2. EXPERIMENTAL SECTION 2.1. Synthesis of Mesoporous VOx-CeO2. In a typical synthesis, 10 mmol of Ce(NO3)3·6H2O, 40 mmol of HOAc, 24 mmol of HCl (or HNO3), and 1.6 g of F127 (EO96PO70EO96, MW = 12 000 g/mol), VO(acac)2, were dissolved in 30 mL of ethanol. The mixture was stirred vigorously for 2 h and transferred to a Petri dish (diameter 125 mm). The ethanol was evaporated at 40 °C with relative humidity of 30−80%. After the solvent was evaporated, the mixture was transferred into a 65 °C oven and aged for 24 h. The as-synthesized mesostructured hybrids were calcined at 550 °C in air for 6 h (ramp rate 2 deg/min) to obtain the mesoporous VOx-CeO2. 2.2. Synthesis of 1 mol % V-CeO2-Mixed (V2O5) Catalyst. In a conventional treatment process, certain amounts of V2O5 and CeO2 were mechanically mixed. The sample was labeled as 1 mol % V-CeO2-mixed (V2O5) catalyst. 2.3. Characterization. The wide-angle X-ray diffraction (WAXRD) patterns were recorded on a Rigaku Ultimate IV with Cu Kα radiation. Nitrogen adsorption isotherms were measured at −196 °C on a Micromeritics ASAP 2020 adsorption analyzer. Before the adsorption analysis, calcined samples were outgassed under vacuum at 200 °C in the port of the adsorption analyzer. Solid UV/vis adsorption spectra were measured with a Shimadzu UV-2450 spectrophotometer in the diffuse reflectance mode. High-resolution XPS measurements were performed in a VG Scientific ESCALAB Mark II spectrometer equipped with two ultrahigh vacuum (UHV) chambers. All binding energies were referenced to the C 1s peak at 284.6 eV of the surface adventitious carbon. The Raman measurements were performed on a Labor Raman HR-800 with laser excitations at 514.5 nm (before the in-situ Raman

Ead[H ] = −(E[H /VmOn /CeO2 ] − E[VmOn /CeO2 ] 1 − E[H2]) 2

Eov = E[VmOn − 1/CeO2 ] +

1 E[O2 ] − E[VmOn /CeO2 ] 2

where E[H/VmOn/CeO2], E[VmOn/CeO2], E[H2], E[VmOn−1/ CeO2], and E[O2] are the DFT total energies of interaction system of H and vanadium oxide species deposited CeO2(111), vanadium oxide species deposited CeO2(111), H2 molecule, defective vanadium oxide species deposited CeO2(111), and O2 molecule, respectively. 24951

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Before calculating the catalytic reactions, the thermodynamically most stable monomeric, dimeric, and trimeric VOx species on CeO2(111) under experimental conditions were determined (Figure 6) by calculating the surface free energy change per unit area Δγ: Δγ(p , T ) =

1 G[VmOn /CeO2 ] − G[CeO2 ] − mμ[V] A n − μ[O2 ](p , T ) 2

{

}

where G[VmOn/CeO2] and G[CeO2] are the Gibbs free energy of vanadium oxide species deposited CeO2(111) and clean CeO2(111), respectively, μ[V] and μ[O2] are the chemical potentials of the solid vanadium and gas-phase oxygen, respectively, and A is the area of one side of the surface cell. We can rewrite the above equation by considering that (i) Gibbs free energies of the solid components can be equal to the DFT total energies, (ii) the value of μ[V] can be equal to the DFT total energy of the metallic bcc bulk vanadium (Ebulk[V]), and (iii) the surfaces are assumed to be in thermodynamic equilibrium, and μ[O2] can be calculated as follows: 1 1 μ[O2 ](p , T ) = E[O2 ] + Δμ[O](p , T ) 2 2 1 1⎧ = E[O2 ] + ⎨H(p0 , T ) − H(p0 , 0K) 2 2⎩ − TS(p0 , T ) + KBT ln

p[O2 ] ⎫ ⎬ p0 ⎭

Enthalpy (H) and entropy (S) at temperature T were calculated according to the formulas on the Web site of NIST. Accordingly, we can obtain the following equation: Δγ(p , T ) =

1⎧ ⎨E[VmOn /CeO2 ] − E[CeO2 ] − mE bulk [V ] A⎩ ⎪

⎪

−

n n⎧ E[O2 ] − ⎨H(p0 , T ) − H(p0 , 0K) 2 2⎩

− TS(p0 , T ) + KBT ln

Figure 1. The 1.0 mol % VOx-CeO2 catalysts of the (a) HR-SEM and (b) HADDF-STEM images and (c−e) the element mapping of particles.

p[O2 ] ⎫⎫ ⎬⎬ p0 ⎭⎭ ⎪

utilized. As depicted in Figure 2, no characteristic diffraction peaks of V-containing compounds such as V2O5 or CeVO4 were observed, possibly due to their low loading amount, weak

⎪

3. RESULTS AND DISCUSSION Mesoporous VOx-CeO2 metal oxides were prepared via the sol−gel process. Nitrogen sorption data of VOx-CeO2 mixed oxides in Figure S1 (Supporting Information) show that they have narrow pore size distribution (∼4.2 nm), with a typical IV isotherm for mesoporous materials, which is also confirmed by HR-SEM images in Figures 1a and S2a, Supporting Information. Combined with the results discussed above and in Table S1, Supporting Information, we can see that the addition of the vanadium does not influence the morphology and the physical-chemical properties of the CeO2 support. The EDX element mapping measurements of mesoporous VOxCeO2 in Figures 1 and S2 (Supporting Information) show uniform X-ray intensities of V, Ce, and O signals throughout the particles, indicating the homogeneous distribution of the V component in the CeO2 matrix. To better understand the nature of surface vanadium species, WAXRD, XPS, and the UV−vis DRS spectra analyses were

Figure 2. XRD patterns of CeO2 and VOx-CeO2 catalysts before and after reaction. 24952

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crystallization, or good dispersion, which is also confirmed by the EDX element mapping analysis in Figures 1 and S2, Supporting Information. The single XPS peak at 517.1 eV in Figure 3a suggests that all vanadium species are in their fully

Figure 4. Raman spectra of (a) 1.0 mol % VOx-CeO2 with/without BA and (b) 5.0 mol % VOx-CeO2 with/without BA at different operation temperatures. Figure 3. (a) The XPS spectra of the VOx-CeO2 catalysts and (b) UV−vis DRS spectra of CeO2 and VOx-CeO2 catalysts.

ascribed to trimers is observed in 5.0 mol % VOx-CeO2 catalysts,31,33 indicating that the addition of BA can change the lower polymeric states of VOx (dimers/monomers) to trimers. We also conducted the experiment to confirm the 1034 cm−1 band assignment. The results of Raman spectra in Figure S12 (Supporting Information) showed that the assignment of the 1034 cm−1 band to trimers is appropriate. It is generally believed that the surface structures of catalysts play a vital role in the catalysis.34−36 In this study, we also investigated the relationship between VOx polymeric states and their catalytic performances in gas-phase BA oxidation (Figure 5a). All the VOx-CeO2 catalysts exhibit superior activities than the pure CeO2 toward BA-to-BAD conversion below 250 °C with high selectivity (>98.0%). Also, the activities are highly related to the vanadium content. When the V concentration is below 1.0 mol % in VOx-CeO2 mixed oxides, the mass-specific activity vigorously increases with its content increase. It is important to highlight that the TOF of the 0.1 mol % VOxCeO2 sample at 203 °C (bp of BA) is even up to 1367 h−1, 10 times higher than the supported Au catalysts in liquid phase oxidation (TOF < 140 h−1),5 13 times that of the transition metal oxide catalyst of 0.2 mol % Cu-TiO2 (108 h−1) in the gas phase solvent-free BA oxidation.19 In addition, the mass-specific activity (24.7 mmol·gcat−1·h−1) of the 1.0 mol % VOx-CeO2 is 5fold higher than that of 5.0 mol % K-Cu-TiO2,19 close to the 2.5% Au/TiO2 in the liquid-phase solvent-free BA-to-BAD oxidation.3 To the best of our knowledge, it is the highest TOF and reaction rate reported so far for transition metal heterogeneous catalysts for BA-to-BAD oxidation at such low temperatures in the gas phase.37−40 Once the vanadium content

oxidized +5 states. Meanwhile, with the increase of the V loading, the strong absorption in the UV region still occurs, whereas the vis absorption edge of the UV−vis spectra gradually shifted to higher wavelength in Figure 3b, probably due to the formation of surface vanadium species with increasing polymerization degree.31 The polymeric states of vanadium species (VOx) were also detected by the Raman spectra at different operation temperatures (Figure 4), from which one can see that the VO stretching vibration of 1.0 mol % VOx-CeO2 (0.96 V atom·nm−2) is mainly at 1008 cm−1 while that of 5.0 mol % VOx-CeO2 (4.96 V atom·nm−2) is primarily at 1015 cm−1. According to the literature,31,32 the peak at 1008 cm−1 can be assigned to monomeric VOx and that at 1015 cm−1 to dimeric VOx. These results are in good agreement with UV−vis DRS spectra, which suggest that the polymerization degree of 5.0 mol % VOx-CeO2 is larger than that of 1.0 mol % VOx-CeO2, and the V species of 1.0 mol % VOx-CeO2 is dominated by monomeric VOx. To further identify the role of the VOx structure in catalysis, we also collected the Raman spectra with the involvement of BA (Figure 4). It can be seen that the VO vibration intensities of both 1.0 and 5.0 mol % VOx-CeO2 catalysts decrease in the existence of BA. However, the stabilities of vanadium species are quite different. For the 1.0 mol % VOxCeO2 catalyst, no other VO vibration bands appear, suggesting that the monomers are quite stable below 250 °C with BA involvement. By contrast, a new band at 1034 cm−1 24953

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For comparison, mesoporous Cu-CeO2, W-CeO2, Co-CeO2, V-TiO2, V-SiO2, and V-ZrO2 catalysts were also synthesized and applied. The superior activity of VOx-CeO2 is confirmed for all tested M-CeO2 and V-MOx catalysts (Figure S3, Supporting Information), showing there is a synergistic effect between VOx and CeO2 in the gas-phase oxidation as reported in previous work.41 Besides, to confirm the importance of the atomic VOx structures, we also measured the performance of the crystalline V2O5-CeO2 mechanical mixed catalyst. The results in Figure S4 (Supporting Information) showed that the activity of the crystal V2O5 was quite inferior to that of the monomeric VOx with the same vanadium atom amount, implying that the atomic-scale monomeric VOx structure indeed plays a crucial role in the ODH reaction of BA-toBAD in the gas phase. The reuse and stability are two major concerns for a catalyst to be used in practical applications. In this case, we carried out the activity with the time-on-stream test and the life cycle test to assess the stability and durability of the 1.0 mol % VOx-CeO2 on BA-to-BAD oxidation. As shown in Figure S10 (Supporting Information), the initial yield is very high and the sample shows a stable catalytic performance with continuously stable operation (98.0%.

was further increased to 5.0 mol %, the mass-specific activity decreases sharply with the temperature increase, which is opposite to the performances of other VOx-CeO2 catalysts with low vanadium content (Figure 5a), in accordance with the change of the polymeric state of VOx (dimers/monomers) to trimers as monitored by Raman spectroscopy. From the results discussed above, we can conclude that when the vanadium loading is lower than 1.0 mol %, the dominant VOx species are monomers, which are highly active and stable under reaction conditions. The similar apparent activation energies of 0.1, 0.5, and 1.0 mol % VOx-CeO2 (V atom·nm−2: 0.13 vs 0.51 vs 0.96) presented in Figure 5b (kJ mol−1: 29.4 vs 29.1 vs 24.6) also confirm that they have the same or similar type of active sites, and excellent oxidative dehydrogenation capacities due to the significantly lower activation energies. In addition, similar activities of the selected catalysts with different mass shown in Figure S9 (Supporting Information) suggest that the diffusion effect has been eliminated under the reaction conditions in Figure 5. Once the surface vanadium coverage increases to as high as 4.96 V atom·nm−2, dimeric VOx species thus form as a major VOx species. Nevertheless, they are unstable and most of them readily change to inactive trimers with the small amount of existing monomers under the influence of BA involvement and increasing temperature, which results in the activity loss. In other words, the activities of VOxCeO2 catalysts are highly related to the surface VOx polymeric states,25 which are affected by the loading concentration and reaction conditions.

Figure 6. Top and side views of the thermodynamically most stable VOx structures: (a, d) monomeric VO4, (b, e) dimeric V2O5, and (c, f) trimeric V3O6 at CeO2(111) under experimental conditions. Ce atoms are in white, V in gray, lattice O in red, and O atoms of VOx in blue. 24954

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Figure 7. Surface structural evolution within ODH reaction cycles catalyzed by (a) monomeric VOx and (b) clean CeO2(111), dimeric VOx and trimeric VOx. Calculated energy profiles for clean CeO2(111) and monomeric, dimeric, trimeric VOx catalyzed dehydrogenation of BA of the (c) first and (d) second half of the reaction cycle. “TS(O−H)” and “TS(C−H)” are the transition states for O−H and C−H bond breaking; structures X, Y, and Z are the key surface structures. Ce atoms are in white, V in gray, lattice O in red, O atoms of VOx in blue, C in dark gray, H in bright white, and O2 filling the vacancies in green. O-vacancies are marked with “OV”.

controversial with experimental Raman measurements in Figure 4a. However, we cannot exclude the existence of some other VO terminated VOx monomer species like VO3 and VO2, which is just slightly less stable than VO4, and these species could also contribute to the overall Raman signals of VOx monomers. At the same time, the key surface states within the determined reaction cycles, including both the consumption of surface O (Figure S5, Supporting Information) and their (partial) recovery through O2 adsorption, are also illustrated. Surprisingly, we have found that it is not easy to regenerate the thermodynamically most stable VO4 structure, because there is a very high barrier for its formation directly from the new VO4# structure after O2 adsorption (Figure 8). The result suggests that the most favorable VO4 species that can form in the catalyst preparation process may not occur in the steady state reaction process, and the continuous catalytic cycles may be based on the second most stable VO 3 species (see thermodynamic analyses in Figure S13, Supporting Information) displayed in Figure 7a. Actually, it provides the opportunity for the metastable species to show their capabilities in comparison with their most stable states. In addition, it also shows that the corresponding surface monomeric VO3, dimeric V2O5, and trimeric V3O6 species are actually intact within the steady state reaction cycles and it is the CeO2 surface lattice O

Figure 8. Calculated energy profile for regenerating the thermodynamically most stable VO4 structure from VO4#.

nearby that is directly involved in the catalytic reactions through water formation. The complete structural evolution cycles can therefore be described as X → Y → Z → X. Here, all “X” species represent the starting catalytic structure (before any BA adsorption or dehydrogenation) in each system (monomer: VO3; dimer: V2O5; trimer: V3O6), which then evolve into “Y” species after the desorption of BAD and water. All “Z” species 24955

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Table 1. Calculated Most Favorable Adsorption Energies of H (Ead[H]), Oxygen Vacancy Formation Energies (Eov), and the Energy Difference (ΔEH2O) to Generate a Gas-Phase Water from Two Surface Hydroxyls on Key Surface Structuresa Ead[H]/eV Eov/eV ΔEH2O /eV a

VO3b

VO4#, c

clean X

clean Z

dimer X

dimer Z

trimer X

trimer Z

2.57 0.52 2.09

1.93 0.16 1.26

1.21 2.58 2.39

1.19 −0.34 −0.94

1.45 1.86 1.62

1.40 −0.61 −0.72

1.24 2.47 2.45

1.14 −0.43 −0.93

Structures X and Z are the key structures shown in Figure 7b. bMonomer X. cMonomer Z.

are therefore those regenerated by surface oxidation, i.e. X − O → Y; Y + O2 → Z; Z − O → X. Elaborate calculations on various reaction pathways suggested that the oxidation of BA-to-BAD goes through four elementary steps: (i) the adsorption of gas-phase PhCH2OH to give rise to surface PhCH2OH*, (ii) O−H bond breaking (PhCH2OH* to PhCH2O*), (iii) C−H bond breaking (PhCH2O* to PhCHO*), and (iv) the desorption of PhCHO* for the occurrence of gas-phase product PhCHO. The calculated energy profiles for these elementary reaction steps of BA-to-BAD catalyzed by each key surface structure are plotted in Figure 7c,d. From the energy profiles, one can clearly see that the two BA-to-BAD steps (promoted by VO3 and VO4#) within the monomeric VOx supported at CeO2 catalyzed cycle exhibit remarkable activities, such as low H-cleavage barriers. Detailed analyses told us that such low barriers benefit from the close contact of BA with surface monomeric VOx species through van der Waals interactions and H-bonds (see adsorption structures of BA in Figure S7a−h, Supporting Information). Moreover, the H-cleavage at VO3 and VO4# can be facilitated by their oxygen (blue) as they can directly accommodate these H, which is immediately followed by fast H-diffusion (barrier