Article pubs.acs.org/JPCC
Ethanol Conversion on Cyclic (MO3)3 (M = Mo, W) Clusters Zhenjun Li,† Zongtang Fang,‡ Matthew S. Kelley,‡ Bruce D. Kay,† Roger Rousseau,*,† Zdenek Dohnalek,*,† and David A. Dixon*,‡ †
Fundamental and Computational Sciences Directorate and Institute for Integrated Catalysis, Pacific Northwest National laboratory, PO Box 999, Richland, Washington 99352, United States ‡ Department of Chemistry, The University of Alabama, Shelby Hall, Box 870336, Tuscaloosa, Alabama 35487, United States S Supporting Information *
ABSTRACT: The reactions of ethanol (CH3CH2OD) over cyclic (MO3)3 (M = Mo, W) clusters were studied experimentally and computationally. The cyclic clusters were prepared by sublimation of MoO3 and WO3 powders in a vacuum. To evaluate the cluster activity in dehydration, dehydrogenation, and condensation reactions, they were suspended in an ethanol matrix on an inert substrate, graphene monolayer on Pt(111). The reaction products formed upon heating were followed and quantified using temperature-programmed desorption. The experimental results were corroborated using coupled cluster CCSD(T) calculations at DFT optimized geometries that provide quantitative molecular-scale information on the reaction mechanisms. The dehydration and dehydrogenation of ethanol probe both the Lewis/Brønsted acid/ base and redox properties of the metal centers. The overall conversion of the alcohol is governed by the Lewis acidity of the metal center, and product selectivities, as determined by the relative weights of dehydrogenation and dehydration, are governed by the reducibility of the metal center.
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INTRODUCTION Oxides of molybdenum and tungsten are an important class of catalytic materials with applications including isomerization of alkanes and alkenes, partial oxidation of alcohols, selective reduction of nitric oxide, and metathesis of alkenes.1−10 While many studies have focused on their structure−function relationships, the nature of the catalytic activity is still being extensively investigated. There is a general agreement that the activity of supported MOx (M = W, Mo) catalysts is correlated with the presence of acidic sites, type of oxide support, delocalization of electron density, structure of oxide domains, and presence of protons.10−12 In our prior studies, we focused on the preparation and characterization of model WO3 catalysts with well-defined structural motifs that allow for unambiguous determination of their catalytic activity.1,13−20 Sublimation of WO3 powders in vacuum led to cyclic (WO3)3 clusters in the gas phase, which were subsequently used to prepare a variety of model systems ranging from unsupported (WO3)3 clusters deposited into reactive alcohol matrices,1,14 to isolated (WO3)3 clusters supported on oxide substrates,13,19,20 to epitaxial as well as high surface area films composed of (WO3)3.16,18 The conversion of alcohols via dehydration, dehydrogenation, and condensation was used as model catalytic reactions, allowing us to explore factors that control the activity and branching into various pathways.1,14,18,19 For (WO3)3 suspended in matrices of small (C1−C4) aliphatic alcohols, we found that dehydration is a dominant conversion pathway for all alcohols with the © 2014 American Chemical Society
exception of methanol and ethanol where both aldehyde and ether products were also observed in significant amounts.1 Here, we prepare, for the first time, cyclic (MoO3)3 clusters and compare their reactivity with that of (WO3)3. We selected the dehydration and dehydrogenation of ethanol as model reactions to probe both the Lewis/Brønsted acid/base and redox properties of the metal centers. These studies provide us with a unique opportunity to quantitatively compare and contrast the activity of identical structural building blocks with different chemical identities. The experimental results are corroborated using coupled cluster with single and double excitations with an approximate triples correction (CCSD(T)) calculations21 at optimized density functional theory (DFT) geometries that provide quantitative molecular-scale information on the reaction mechanisms allowing us to unambiguously identify the leading chemical descriptors, which determine catalytic activity and product selectivity. Moreover, the high level of quantitative accuracy of CCSD(T) is mandatory for comparing the relative reactivity of two different metal centers with multiple energetically similar reaction channels. Received: January 9, 2014 Revised: February 5, 2014 Published: February 7, 2014 4869
dx.doi.org/10.1021/jp500255f | J. Phys. Chem. C 2014, 118, 4869−4877
The Journal of Physical Chemistry C
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Article
METHODS Experiment. The experiments were carried out in an ultrahigh vacuum (UHV) molecular beam scattering chamber (∼1 × 10−10 Torr) described previously.1 The polished Pt(111) single crystal (disk 10 mm in diameter, 1 mm thick, Princeton Scientific) was mounted on a manipulator cooled by a closed cycle helium cryostat and resistively heated via spot-welded Ta leads. The temperature was measured using a K-type thermocouple spot-welded to the rear side of the sample. Pt(111) was cleaned using standard procedures including a sequence of neon-ion bombardment at 300 K, O2 annealing at 1000 K (5 min, 2 × 10−7 Torr), and annealing in UHV at 1200 K. A graphene monolayer was prepared by exposing Pt(111) to decane at high temperatures and its quality confirmed by lowenergy electron diffraction (LEED) and Kr temperatureprogrammed desorption (TPD).22 A high-temperature effusion cell (CreaTec) was used to sublimate WO3 (99.95%, Aldrich) and MoO3 and prepare (WO3)3 and (MoO3)3 gas phase clusters. The flux of generated gas phase clusters was stabilized at ∼1 × 1013 MO3/(cm2 s) (evaporation temperature of ∼1390 and ∼745 K for WO3 and MoO3, respectively) and monitored using a quartz crystal microbalance (Inficon). Deposited MO3 was removed from Pt(111) by Ne+ sputtering. Ethanol (SigmaAldrich, 99.5%) was purified with repeated freeze−pump−thaw cycles and dosed using an effusive molecular beam. TPD experiments (ramp rate 2 K/s) were performed using a quadrupole mass spectrometer (UTI). TPD Spectra Deconvolution. Figure 1 shows raw TPD spectra for deuterated ethanol, CH3CH2OD, at m/z = 46 amu, H2O at 18 amu, D2O at 20 amu, ethylene (CH2CH2) at 27 amu, ethanol (CH3 CH2 OH) at 45 amu, acetaldehyde (CH3CHO) at 43 amu, and diethyl ether ((CH3CH2)2O) at 74 amu. To obtain the data presented in Figure 3, the contribution of CH3CH2OD reagent to the corresponding
masses of the products has been subtracted using the cracking pattern determined from the CH3CH2OD multilayer desorption region at low temperatures (130−170 K). Mass spectra and absolute desorption rates of ethylene, acetaldehyde, ethanol, and D2O were determined by dosing known amounts of these molecules with a flux calibrated molecular beam and measuring their TPD spectra.23 The resulting measured reference mass spectra (cracking patterns) of the dosed molecules were further utilized to subtract the contribution of D2O to H2O signal at 18 amu (30%) and the contribution of CH3CH2OH to CH3CHO at 43 amu (∼5%) and to CH2CH2 at 27 amu (∼20%). Theory. On the basis of extensive studies of the properties and reactions of group 4 and 6 transition metal oxide clusters,24−32 we used density functional theory (DFT) with the B3LYP33,34 exchange-correlation functional for the geometry optimizations and frequency calculations. For the transition state optimizations, the synchoronous transit-guided quasi-Newton (STQN) method was employed.35,36 The augcc-pVDZ basis set was used for H, C, and O,37 and the aug-ccpVDZ-PP basis set with a relativistic effective core potential38,39 was used for Mo and W. The DFT calculations were performed with the Gaussian 09 program package.40 Final single point energies at the optimized B3LYP geometries were calculated at the coupled cluster CCSD(T)21 level and at the DFT level with the M06 exchange-correlation functional41 using the basis sets given above at the B3LYP-optimized geometries. Selected geometry optimizations with the M06 functional showed energy differences to those using the B3LYP-optimized geometries of
