Letter pubs.acs.org/JPCL
Alcohol Dehydration on Monooxo WO and Dioxo OWO Species Zhenjun Li,† Břetislav Šmíd,‡ Yu Kwon Kim,§ Vladimír Matolín,‡ Bruce D. Kay,*,† Roger Rousseau,*,† and Zdenek Dohnálek*,† †
Chemical and Materials Sciences Division, Fundamental and Computational Sciences Directorate and Institute for Integrated Catalysis, Pacific Northwest National Laboratory, PO Box 999, Mail Stop K8-88, Richland, Washington 99352, United States ‡ Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holešovičkách 2, 18000 Prague 8, Czech Republic § Department of Energy Systems Research and Department of Chemistry, Ajou University, Suwon 443-749, South Korea S Supporting Information *
ABSTRACT: The dehydration of 1-propanol on nanoporous WO3 films prepared via ballistic deposition at ∼20 K has been investigated using temperature-programmed desorption, infrared reflection absorption spectroscopy, and density functional theory. The as-deposited films are extremely efficient in 1propanol dehydration to propene. This activity is correlated with the presence of dioxo OWO groups, whereas monooxo WO species are shown to be inactive. Annealing of the films induces densification that results in the loss of catalytic activity due to the annihilation of OWO species. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis
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W(VI) and Brønsted basicity of oxygen in the mono- and dioxo species.13 Examples of SEM and TEM images from a nanoporous WO3 film revealing the filament-like morphology and complex pore structure are shown in Figure 1. The mechanism of nanoporous film growth via BD is based on limited surface diffusion of the deposited species at low substrate temperatures (hit and stick), which leads to preferential growth of asperities. At oblique angles of incidence, such asperities develop during the growth into the filamentous nanoporous structures displayed in Figure
ungsten oxide is an important early transition-metal oxide with applications in heterogeneous catalysis, photocatalysis, electronic devices, and corrosion protection.1−3 In catalysis, tungsten oxide clusters supported on other oxides are primarily active for isomerization of alkanes and alkenes, partial oxidation of alcohols, selective reduction of nitric oxide, and metathesis of alkenes.4−9 Whereas many studies have focused on structure−function relationships, the nature of high catalytic activity is still being extensively investigated. There is general agreement that the activity of supported tungsten oxide catalysts is correlated with the presence of acidic sites, where the catalytic activity is strongly affected by the type of oxide support, delocalization of electron density, structures of tungsten oxide domains, and the presence of protons.4,7,10 In this study, we focus on comparing the reactivity of monooxo WO and dioxo OWO species prepared on nanoporous WO3 films. The films are deposited via ballistic deposition (BD) of gas-phase (WO3)3 clusters. 1-Propanol is used as a probe molecule to study the dehydration reaction using temperature-programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). The results demonstrate that under ultrahigh vacuum (UHV) conditions the reaction proceeds efficiently only on dioxo OWO species, whereas isolated monooxo WO species are inactive. These results are consistent with our prior studies of alcohol dehydration on unsupported (WO3)3 clusters in alcohol matrices and (WO3)3 clusters supported on TiO2(110) and FeO(111)8,11,12 and are qualitatively correlated with the theoretically determined differences in Lewis acidity of © 2012 American Chemical Society
Figure 1. SEM and TEM images of a nanoporous WO3 film deposited at an angle of 85° and a substrate temperature of 20 K. The amount of deposited WO3 corresponds to 100 ML of dense WO3. Received: July 5, 2012 Accepted: July 30, 2012 Published: July 30, 2012 2168
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Figure 2. (A) Kr TPD spectra obtained after a saturation dose of Kr at 39 K. (B) IRAS spectra from the nanoporous WO3 films. (C) TPD spectra of propene (monitored at 41 amu) obtained after a saturation dose of 1-propanol at 20 K. The results presented in panels A−C were obtained as a function of annealing temperature, Tan, on a 10 ML thick nanoporous WO3 films deposited at a 65° angle of incidence and a Pt(111) substrate at 20 K. (D) Areas of Kr TPD peak, WO IRAS feature, and propene TPD peak from panels A−C, respectively, shown as a function of Tan. Krypton and propene TPD peak areas are normalized relative to their monolayer peak areas on Pt(111) (not shown).
1. A detailed discussion of porous film growth and characterization can be found elsewhere.14−18 Evolution of the surface area of nanoporous films as a function of deposition angle and substrate temperature can be measured using adsorption of weakly bound gases (e.g., Ar, Kr, N2).14−16,19 Such experiments are analogous to Brunauer− Emmett−Teller (BET) isotherm measurements but offer vastly enhanced sensitivity that allows for the determination of absolute surface areas as small as ∼1 mm2. Previously, we found that films deposited with the incident (WO3)3 flux at 65° from the surface normal and a substrate temperature of 20 K possess specific surface area of 560 m2/g,14 which is at least an order of magnitude higher than any previously reported specific surface area for WO3.20 A set of Kr TPD spectra from the film grown at 65° and 20 K is shown in Figure 2A as a function of substrate annealing temperature, Tan. A broad range of desorption temperatures are observed in all spectra, indicating a distribution of adsorption sites with a wide range of binding energies. The integral of the surface area versus Tan is plotted in Figure 2D (magenta circles). To characterize further the chemical nature of the surface sites, we employed IRAS (Figure 2B). Our prior studies of matrix isolated (WO3)3 clusters showed that the symmetric and asymmetric OWO stretching modes are at 975 and 1014 cm−1.12 As the amount of Kr in the matrix decreases, the asymmetric mode shifts to lower frequencies and decreases in intensity. At the same time a broad phonon band develops below ∼990 cm−1. Finally, for WO3 deposited at 30 K without Kr, only a single, broad feature at 1022 cm−1 is observed, and the asymmetric mode becomes completely screened by the intense phonon band.12 The stretching frequency of isolated WO species has been also shown to lie in the same spectral region.21 Therefore, the normalized integrated area of the observed band as a function of Tan is used as a measure of the concentration of all WO species (Figure 2D, green triangles). Interestingly the area of the WO feature decreases with increasing Tan in an identical manner as the film surface area. To explore alcohol dehydration on nanoporous WO3, we adsorbed 1-propanol as a probe molecule and followed propene formation in TPD as a function of Tan. The resulting propene TPDs are shown in Figure 2C. For the as-deposited WO3 film, an intense propene desorption peak is observed at 441 K. This peak reflects reaction-limited desorption (physisorbed propene desorbs at ∼135 K).22 The asymmetric line shape and constant temperature of the propene desorption peak are indicative of first-order reaction kinetics. A complete set of TPD spectra for
all reaction products including water and recombinatively desorbed 1-propanol is shown in Figure S1 (Supporting Information (SI)). The amount of propene decreases with increasing Tan, and practically no propene is observed on the film annealed to 800 K. This extremely low activity is comparable to low alcohol conversion yields observed on epitaxial (WO3)3-(3 × 3) films grown on Pt(111) at 700 K (Figure S2, SI)23 and (WO 3) 3 clusters supported on FeO(111).11 The integrated propene yield as a function Tan is shown in Figure 2D (black squares). Surprisingly, the propene yield decreases much more rapidly with increasing Tan than the integrated surface area and the integrated WO intensity measured by Kr TPD (Figure 2A) and IRAS (Figure 2B), respectively. We hypothesize that the more rapid decrease in the reactivity as compared with the surface area or the intensity of the WO stretch results from the thermal conversion of dioxo OW O to monooxo WO species, as schematically shown in eq 1.
This hypothesis is consistent with the previously observed high reactivity of dioxo OWO species of unsupported (WO3)3 clusters8,12 and low reactivity of isolated monooxo WO species on FeO(111). To elucidate the differences in the reactivity of WO and OWO groups, we have carried out theoretical studies of ethanol dehydration on a linear (WO3)3 cluster (schematically shown below). This cluster possesses two dioxo OWO species, one on each terminal four-coordinated tungsten (blue) and one monooxo WO species on the central fivecoordinated tungsten (red), and therefore is expected to serve well as a model for nanoporous WO3, where both O WO and WO species are present.
Three possible reaction schemes are considered in the density functional theory (DFT) calculations: the dehydration of one and two ethanol molecules on OWO and the dehydration of one ethanol molecule on WO. We start by summarizing the dehydration of two ethanol molecules on O WO sites, which is analogous to the dehydration on O WO species on cyclic (WO3)3 clusters.8,12 The complete 2169
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monooxo group with the molecularly bound ethanol. This scheme clearly shows that on the monooxo WO site ethanol desorption will be strongly favored over dissociation. We further follow the path of the unfavorably created ethoxy (and hydroxy) species in Figure 3 and find that β-hydrogen elimination is endothermic by ΔE = 21 kcal/mol (comparable to ΔE = 18 kcal/mol on the OWO site) but is hindered by a very high energy barrier of ∼50 kcal/mol, primarily due to poor geometric alignment of the alkyl chain with the monooxo WO group. Final water desorption requires 17 kcal/mol. On the basis of the results of our DFT calculations, we conclude that alcohol dehydration is favorable on dioxo O WO species, regardless of whether one or two alcohols are adsorbed. In contrast, no favorable dehydration path is found on monooxo WO species; therefore, desorption of intact alcohol molecules dominates. These conclusions are in qualitative agreement with the properties of WO3 calculated by Li and Dixon.13 In particular, the lower alcohol and water binding energies on a WO group relative to a OWO group are qualitatively consistent with the lower Lewis acidity of these sites. Additionally, the deprotonation that is favored on OWO but unfavorable on WO is consistent with the higher proton affinity (Brønsted basicity) of dioxo oxygens as compared with monooxo and bridging oxygens.13 On the basis of the theoretical results, we conclude that the propene observed in Figure 2C has to result from 1-propanol dehydration on dioxo OWO species. A simple kinetic analysis of the experimental data (Figure 2) further supports this conclusion. In the simplest case, the annealing driven annihilation of WO bonds for both dioxo OWO and monooxo WO species will follow the same kinetics (eq 1, k1 = 2k2). To show that this is indeed the case, in Figure 4 we plot
reaction scheme including all intermediates and reaction barriers is summarized in Figure S3 (SI). The adsorption of the first ethanol molecule is exothermic by ΔE = −30 kcal/mol and is followed by slightly exothermic deprotonation (ΔE = −6 kcal/mol) over a relatively large barrier of 18 kcal/mol that will be favored over alcohol desorption (ΔE = 30 kcal/mol). Adsorption of the second ethanol molecule is also exothermic by ΔE = −10 kcal/mol. The exothermic deprotonation of the second ethanol leads to the formation of molecularly bound water (ΔE = −15 kcal/mol), which readily desorbs (only 13 kcal/mol required). Water desorption will dominate over alcohol recombination (ΔE = 15 kcal/mol) due to a high recombination barrier of 19 kcal/mol. This H2O desorption step is critical because it prevents the back reaction to molecular ethanol. Further β-hydrogen eliminations of the first and second ethoxy species are endothermic by 28 and 18 kcal/mol with barriers of 27 and 30 kcal/mol, respectively. The final formation of the water molecule is also endothermic (ΔE = 10 kcal/mol) with a barrier of 26 kcal/mol. The reaction scheme for the dehydration of one ethanol molecule on OWO is shown in Figure 3 (blue) and in
Figure 3. Reaction mechanisms for ethanol dehydration on monooxo WO (red) and dioxo OWO (blue) species on the linear (WO3)3 cluster determined via DFT calculations.
more detail in Figure S4 (SI). The adsorption and deprotonation of the first ethanol molecule is identical with the already discussed scheme for two ethanol molecules (Figure S3, SI). The β-hydrogen elimination in the absence of the second ethanol molecule is endothermic by 18 kcal/mol and proceeds over a barrier of 30 kcal/mol. The energy required to overcome this barrier is identical to the desorption energy of ethanol from its molecularly bound state, providing for highly competing processes. The lower energy of the deprotonated species as compared with molecularly bound ethanol is expected to set up kinetic conditions that will favor β-hydrogen elimination over the desorption. Therefore, dehydration is expected to dominate, even for the case of a single alcohol molecule on OWO. The energetics of the reaction scheme change for monooxo WO species (Figure 3, red trace) and in more detail in Figure S5 (SI). By comparison, molecularly bound ethanol is significantly less stable on this site than on the dioxo OW O site (21 vs 30 kcal/mol). The differences are further exacerbated in the deprotonation step that requires hydrogen transfer to monooxo or to bridging oxygen. The latter is highly endothermic (ΔE = 22 kcal/mol, not shown in Figure 3) and unfavorable in comparison with ethanol desorption (ΔE = 21 kcal/mol). The former path is more favorable but still endothermic with ΔE = 4 kcal/mol with a large barrier of ∼30 kcal/mol due to poor geometric alignment of the
Figure 4. Data: The normalized dioxo OWO concentration determined from the propene yield as a function of normalized total concentration of monooxo WO and dioxo OWO species determined from WO stretching mode area. Model: Normalized OWO concentration as a function of normalized total WO + OWO concentration obtained for a set of sequential reactions shown in eq 1.
the normalized dioxo OWO concentration determined from the propene yield (Figure 2C,D) as a function of the normalized total concentration of monooxo WO and dioxo OWO species determined from WO stretching mode area (Figure 2B,D). The analytic solution of the sequential reaction scheme (see eq 1) is described in detail in the SI, and the particular case where k1 = 2k2 is shown by the solid blue 2170
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curve in Figure 4. As one can see, this simple model describes the observed population quite well. Only a slight improvement is obtained when k1/k2 is varied, and the best fit is obtained for k1/k2 = 4 (dashed blue curve, Figure 4). The observation of similar annihilation rates of WO bonds in both dioxo O WO and monooxo WO species indicates that the kinetics is controlled by annealing induced densification and morphological changes, implying that the relative thermal stabilities of the mono and dioxo species are not radically different. In summary, we employed novel nanoporous WO3 films as model catalysts to compare the relative activity of dioxo, O WO, and monooxo, WO, species toward alcohol dehydration. The combined experimental and theoretical evidence clearly demonstrates that only OWO species are active. The high activity of the OWO moiety is shown to be a result of both the kinetically and thermodynamically favorable deprotonation step. In contrast, the deprotonation on the WO moiety is shown to be unfavorable both kinetically and thermodynamically. The differences in the thermodynamics of the resulting alkoxy/hydroxy pairs can be reconciled based on the differences in Lewis acidity of the tungsten and Brønsted basicity of the oxygen of the dioxo OWO and monooxo WO species.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected];
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS A part of this work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences and performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle. B.S. and V.M. were supported by the Ministry of Education of the Czech Republic under grant ME08056.
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METHODS The experiments were carried out in a UHV molecular beam scattering chamber (7 × 10−11 Torr) previously described.23 The Pt(111) single crystal mounted on a closed-cycle Hecooled manipulator was used for the WO3 growth. A clean Pt(111)-(1 × 1) surface was prepared using a standard cleaning procedure.23 Porous tungsten oxide thin films were prepared via sublimation of (WO3)3 at an oblique angle of incidence on Pt(111) at 20 K using a high-temperature effusion cell (CreaTec) operating at 1160 °C (flux ∼5.2 × 1013 WO3 /(s·cm2)). IRAS spectra were collected with a Bruker Equinox 55 spectrometer (typically 1000 scans at a resolution of 4 cm−1). Prior to use, 1-propanol (99.9%, Sigma-Aldrich) was purified using freeze−pump−thaw cycles and introduced using a neat, 300 K, quasi-effusive molecular beam directed normal to the surface. All TPD spectra were obtained using a line-of-sight quadrupole mass spectrometer (UTI) with a linear heating rate of 1.0 K/s. Calculations were performed employing DFT with a gradient corrected functional for exchange and correlation,24 as implemented in the CP2K package.25,26 Core electrons were modeled as norm-conserving pseudopotentials, and the wave functions expanded in a double-zeta Gaussian basis set with a plane-wave auxiliary basis of 300 Ry energy cutoff. Calculations of all reaction coordinates were performed using the climbing image nudged-elastic-band method (CI-NEB)27,28 employing 12 replicas. The transition states for each reaction in our mechanism were examined by a normal-mode analysis to verify that it represented a saddle point on the potential energy surface.
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ASSOCIATED CONTENT
S Supporting Information *
TPD spectra following 1-propanol adsorption on nanoporous and ordered WO3 films. Reaction schemes for dehydration of one and two alcohol molecules on mono WO and dioxo OWO species. Solution for kinetics of sequential reactions. This material is available free of charge via the Internet at http://pubs.acs.org 2171
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