Letter pubs.acs.org/JPCL
Support Shape Effect in Metal Oxide Catalysis: Ceria-NanoshapeSupported Vanadia Catalysts for Oxidative Dehydrogenation of Isobutane Zili Wu,*,†,‡ Viviane Schwartz,*,† Meijun Li,‡ Adam J. Rondinone,† and Steven H. Overbury†,‡ †
Center for Nanophase Materials Sciences and ‡Chemical Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *
ABSTRACT: The support effect has long been an intriguing topic in catalysis research. With the advancement of nanomaterial synthesis, the availability of faceted oxide nanocrystals provides the opportunity to gain unprecedented insights into the support effect by employing these well-structured nanocrystals. In this Letter, we show by utilizing ceria nanoshapes as supports for vanadium oxide that the shape of the support poses a profound effect on the catalytic performance of metal oxide catalysts. Specifically, the activation energy of VOx/CeO2 catalysts in oxidative dehydrogenation of isobutane was found to be dependent on the shape of ceria support, rods < octahedra, closely related to the surface oxygen vacancy formation energy and the numbe of defects of the two ceria supports with different crystallographic surface planes. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis catalytic CO oxidation performance on Cu oxide thin films formed upon surface reconstruction of cubic Cu2O and octahedral Cu2O shows drastic dependence on the surface structure of the underlying Cu2O nanocrystals.17 In this work, we report the utilization of ceria rods (r-CeO2) and octahedra (o-CeO2) as supports for vanadia catalysts and show that the shape of the ceria nanocrystals influences the catalytic performance of vanadia in isobutane ODH. The two ceria nanoshapes were synthesized via a hydrothermal method and have been well-characterized by XRD, electron microscopy, and Raman spectroscopy in our recent work.16,18 It was shown that the ceria rods and octahedra were terminated by mainly {110} + {100} and {111} facets, respectively. Vanadia was loaded onto these ceria nanoshapes via the incipient wetness impregnation method.19 Due to the difference in surface area of these ceria nanoshapes (BET surface areas of rods and octahedra are 93 and 12 m2/g, respectively), the loading of vanadia is expressed in surface vanadium density, V/nm2. At similar V loading of 2 V/nm2, 2 V/r-CeO2 and 2 V/o-CeO2 samples were tested for isobutane ODH reaction from 523 to 673 K with an O2/isobutane ratio of 2:1. Figure 1A presents the Arrhenius plot of isobutane ODH catalyzed by the two samples, from which the apparent activation energy can be calculated to be 33 and 47 kJ/mol for 2 V/r-CeO2 and 2 V/o-CeO2, respectively. The selectivity to isobutene over the two catalysts is plotted versus conversion in Figure 1B. The selectivity ranges from 20 to 50% on both
S
upported vanadium oxide catalysts are widely used in a variety of industrial applications and show great potential in a number of redox reactions.1−4 The support materials have a profound effect on the performance of vanadia catalysts in various oxidation reactions and thus have been investigated extensively. Current research has been exclusively concentrated on the effect of support compositions that are generally classified as reducible and nonreducible. It was shown that variation of support materials can result in several orders of magnitude difference in reactivity of surface VOx species in alcohol oxidation and alkane oxidative dehydrogenation (ODH) reactions. This was usually attributed to a difference in the Sanderson electronegativity of the metal cation in the support oxides.5,6 More recently, the reactivity of supported VOx was correlated with the oxygen vacancy formation energy of the supports,7,8 that is, the less the energy needed to form an O-vacancy on the support, the lower the activation energy for the rate-determining step, C−H bond cleavage, in alkane ODH .7,8 In this respect, it is possible to tune the activity of vanadia species by changing the surface crystal planes of the support material because O-vacancy formation energy is dependent not only on the composition but also on the crystallographic surfaces of the support.9 Therefore, it is expected that the shape in addition to the composition of a support can have a profound effect on the reactivity of supported vanadia catalysts. Such a shape effect was predicted by DFT study of vanadia supported on tetragonal ZrO2 with different crystallographic surfaces.10 To our knowledge, this kind of support shape effect in metal oxide catalysis has yet to be demonstrated, though it was already shown for supported metal catalysts.11−13 The recent advances in synthesis of nanoshaped metal oxides14−17 provide opportunities to study such an effect. For example, the © 2012 American Chemical Society
Received: April 25, 2012 Accepted: May 17, 2012 Published: May 17, 2012 1517
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Figure 1. (A) The Arrhenius plot for isobutane ODH over 2 V/r-CeO2 and 2 V/o-CeO2. (B) Selectivity to isobutene versus conversion over the two catalysts.
Figure 2. (A) Visible (λ = 632.8 nm) and (B) UV (λ = 325 nm) Raman spectra of dehydrated 2 V/r-CeO2 and 2 V/o-CeO2. Peaks marked with ∗ denote the plasma line from the laser, and peaks with + represent trace phosphate species on o-CeO2; see the Experimental Methods section for more information.
features, (1) a broad band centered at around 860 cm−1 due to the V−O−Ce mode and (2) much sharper bands between 1000 and 1050 cm−1 due to the VO stretch mode.20 According to our recent Raman study of VOx supported on ceria nanoparticles19 and a combined STM and IR study of V/ CeO2(111),21 the bands at 1020 and 1036 cm−1 on the 2 V/rCeO2 can be assigned to the VO modes of dimeric and trimeric VOx, while the rather broad one at 1027 cm−1 for 2 V/ o-CeO2 is attributable to a mixture of dimeric and trimeric species. The UV Raman spectra (Figure 2B) are less informative on the structure of surface VOx but more informative on the defect sites of the ceria support due to resonance Raman effect.16,19,22 Comparing the relative intensity of the band at 592 cm−1 (defect sites related band) to that of 464 cm−1 (F2g mode), the number of defect sites on 2 V/rCeO2 is obviously more abundant than that on 2 V/o-CeO2. Considering the similar surface density of VOx, this difference is
catalysts as the isobutane conversion varies from 4 to 12%. The rod-supported VOx has marginally higher selectivity than the octahedra-supported one at similar conversion levels. Apparently, the shape of the ceria support poses an effect mainly on the reaction barrier and slightly on the reaction pathways (selectivity). Before attributing these differences solely to the support shape effect, we need to determine if the reactivity differences are also related to the surface VOx species on the differently shaped ceria. To approach this, the first step is to investigate the structure of surface VOx because it has been documented that the structure of surface VOx can play an important role in the activity and selectivity in alkane ODH reactions.1−4 Raman spectroscopy was employed here to characterize the structure of surface VOx species on both ceria supports, and the spectra are shown in Figure 2. The visible Raman spectra (Figure 2A) of the two 2 V samples are generally characterized by two 1518
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Figure 3. (A) Visible (λ = 632.8 nm) Raman spectra of dehydrated 0.5, 2, and 10 V/r-CeO2; (B) Arrhenius plots for the three catalysts in the isobutane ODH reaction.
about three consequences that can impact the reaction activation energy. First, the oxygen vacancy formation energy on ceria is surface-structure-dependent, as indicated by DFT calculations,24 following the sequence {110} < {100} < {111}. Therefore, it is expected that in the rate-determining step of isobutane ODH, the surface oxygen on the {110}- and {100}terminated rods would be more reactive to abstract H from the adsorbed reactant than that on the {111}-terminated octahedra, thus leading to a lowered apparent activation energy, as exhibited in Figure 1. Our results support the attribution of the difference in the reactivity of supported VOx in ODH reactions to the variation of the oxygen vacancy formation energy of the supports.7 The reactive surface oxygen in VOx/CeO2 is mostly related to the V−O−Ce moiety because it was demonstrated that vanadia prefers to titrate the most labile oxygen sites on the ceria surface.19 Second, the number of defect sites is dependent on the shape of the ceria nanocrystals.16 The UV Raman spectra in Figure 2B show that there are more defect sites available on 2 V/r-CeO2 than on 2 V/o-CeO2. It has been suggested by DFT calculation that the presence of a defect site (O-vacancy) around VOx can effectively lower the reaction barrier in the C−H breaking step in the methanol ODH reaction over the V/TiO2 system.7 Because it was shown that VOx species tends to interact closely with the defect sites on the ceria surface,19 we can analogously propose that this also occurs for isobutane ODH on VOx/ CeO2, that is, the reactive oxygen on 2 V/r-CeO2 with defect sites nearby has more flexibility to interact with the adsorbed reactant and abstract the H atom. This results in a lower activation energy for the more defective 2 V/r-CeO2 than that for the 2 V/o-CeO2. The much higher activation energy for the 10 V/r-CeO2 sample than that for the 0.5 V and 2 V/r-CeO2 samples is likely a result of a combination of the larger oxygen vacancy formation energy and the smaller number of defect sites because it was shown from H2-TPR and Raman studies that the surface lattice oxygen reducibility is retarded and the number of defect sites is decreased by increasing loading of VOx on ceria.19 The presence of more defect sites on 2 V/rCeO2 than on 2 V/o-CeO2 is also likely responsible for the
mainly due to the support shape where {110}- and {100}terminated rods are more defective than {111}-terminated octahedra.16 Therefore, the Raman result suggests that the VOx on both ceria supports are two-dimensionally dispersed species with different degrees of clustering while the two supports show different numbers of defect sites. To further explore the structure effect of VOx on the isobutane ODH reaction, we vary the surface density of VOx (0.5, 2, and 10 V/nm2) but keep the support shape fixed by using ceria rods. The variation in surface density of VOx results in different structures of surface VOx species, as depicted in Figure 3A. Namely, VOx monomer is seen on 0.5 V with a single VO stretch at 1008 cm−1; the dimer (1020 cm−1) and trimer (1028 cm−1) are seen for 2 V; and a mixture of dimer, trimer, polymer (1041 cm−1), crystalline V2O5 (VO mode at 995 cm−1), and CeVO4 (VO4 asymmetric stretching mode at 864 cm−1) is seen for 10 V.19 The catalytic performance of these three V/r-CeO2 samples in isobutane ODH is displayed in Figure 3B, where the Arrhenius plot is shown in the temperature range between 543 and 673 K. Despite the different mix of the monomer, dimer, and trimer VOx species for the 0.5 V and 2 V samples, the activation energy is about the same on the two samples (∼33 kJ/mol), suggesting that the catalysts share a similar active center, most likely the V−O−Ce site.20,23 At the highest VOx loading, the 10 V sample gives a higher activation energy at 48 kJ/mol. Therefore, it becomes clear that the reaction barrier for isobutane ODH is rather independent of the VOx structure as long as the VOx species are 2D-structured on the ceria surface. Going back to 2 V/r-CeO2 and 2 V/o-CeO2, the VOx species are well-dispersed dimers and trimers on both supports, and their structures do not change even after the ODH reaction (see Raman spectra in Figure S1 in the Supporting Information (SI)). Furthermore, the shapes of the two ceria supports are well kept after the ODH reaction at 673 K, as seen by electron microscopy images (see Figure S2 SI). We can now confidently attribute the difference in the catalytic performance in isobutane ODH over 2 V/r-CeO2 and 2 V/o-CeO2 to the support shape effect. Specifically, the support shape brings 1519
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catalyst, previous study indicated that the effect is relatively limited at an XPS P/Ce ratio of
