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Jun 12, 2017 - Department of Mechanical Engineering, Indian Institute of Technology Jodhpur, Old Residency Road, Ratanada, Jodhpur, Rajasthan. 342011 ...
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Absolute Molecular Orientation of Isopropanol at Ceria (100) Surfaces: Insight into Catalytic Selectivity from the Interfacial Structure Benjamin Doughty,*,†,# Sriram Goverapet Srinivasan,†,§,# Vyacheslav S. Bryantsev,*,† Dongkyu Lee,‡ Ho Nyung Lee,‡ Ying-Zhong Ma,† and Daniel A. Lutterman*,† †

Chemical Sciences Division and ‡Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Department of Mechanical Engineering, Indian Institute of Technology Jodhpur, Old Residency Road, Ratanada, Jodhpur, Rajasthan 342011, India S Supporting Information *

ABSTRACT: The initial mechanistic steps underlying heterogeneous chemical catalysis can be described in a framework where the composition, structure, and orientation of molecules adsorbed to reactive interfaces are known. However, extracting this vital information is the limiting step in most cases due in part to challenges in probing the interfacial monolayer with enough chemical specificity to characterize the surface molecular constituents. These challenges are exacerbated at complex or spatially heterogeneous interfaces where competing processes and a distribution of local environments can uniquely drive chemistry. To address these limitations, this work presents a distinctive combination of materials synthesis, surface-specific optical experiments, and theory to probe and understand molecular structure at catalytic interfaces. Specifically, isopropanol was adsorbed to surfaces of the model CeO2 catalyst that were synthesized with only the (100) facet exposed. Vibrational sum-frequency generation was used to probe the molecular monolayer and, with the guidance of density functional theory calculations, was used to extract the structure and absolute molecular orientation of isopropanol at the CeO2(100) surface. Our results show that isopropanol is readily deprotonated at the surface, and through the measured absolute molecular orientation of isopropanol, we obtain new insight into the selectivity of the (100) surface to form propylene. Our findings reveal key insight into the chemical and physical phenomena taking place at pristine interfaces, thereby pointing to intuitive structural arguments to describe catalytic selectivity in more complex systems.

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INTRODUCTION To describe chemical reactivity and selectivity at surfaces, a detailed physical picture of the molecular structure and orientation is the first essential piece of information needed. The adsorption of molecules to the surface and overall chemical makeup ultimately represent the first in a series of mechanistic steps that underlie the catalytic conversion of one species to another.1 While a series of recent review articles have described how the characteristics of metal oxide surfaces can be tied to product observation, including (1) the coordination environment of the surface cations, (2) the oxidation state of the cations, and (3) the redox properties of the surface, there is little information gathered from the reactive species themselves on the surfaces of interest.2−4 Extracting this essential information is difficult with conventional spectroscopic methods due to the presence of relatively few chemical species at the interface as compared to those in the bulk.5−12 The disproportionate number of bulk species thus buries the weak signals from the surface species, and with that, detailed chemical information on the © XXXX American Chemical Society

mechanisms governing reactivity and selectivity at the interfaces is lost. These problems are worsened at complex or spatially heterogeneous interfaces where competing processes and different exposed surface facets can uniquely drive chemistry. Thus, to advance the understanding of reactivity and selectivity of catalysts, the initial mechanistic steps must be well understood and framed in an intuitive physical model. Alcohol adsorption and reactivity is a classic probe for characterizing the catalytic properties of metal oxide materials.13 In this sense, ethyl and propyl alcohols are often used to characterize the acidity or basicity of an oxide surface.13 Reactions of alcohols are thought in general to proceed via dehydration pathways to produce alkenes and water on acidic oxides or dehydrogenation pathways on basic oxides producing aldehydes and hydrogen.14 This generalization is however an Received: April 6, 2017 Revised: June 7, 2017 Published: June 12, 2017 A

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Figure 1. Optimized configurations of an isopropanol molecule on a (2 × 2 × 1) supercell of the CeO2(100) surface. Ce atoms are depicted as yellow; O atoms are red; C atoms are brown; and H atoms are white.

to conventional materials, whereas notable advances in catalyst design are pushing toward novel materials with enhanced selectivity and efficiency, which are not generally commercially available. In this regard, the present work focuses on studies at well-defined CeO2(100) thin films synthesized in house to serve as a model platform for SFG studies and density functional theory (DFT) calculations from which molecular-scale details regarding chemical structure and absolute molecular orientation can be readily extracted. This distinctive combination of designer film synthesis, nonlinear optical spectroscopy, and first-principles theory, as will be shown below, allows for a complete characterization of the molecular orientation of isopropanol at CeO2(100) surfaces and provides new insight underlying the selectivity of CeO2 catalysis.

extreme oversimplification, and alcohols can; in fact do, produce a mixture of both dehydration and dehydrogenation products.15−17 Furthermore, reactions on heterogeneous powders sample the ensemble-averaged reactivity and selectivity of a multitude of exposed facets and chemical environments, thus obscuring pertinent molecular-scale phenomena of interest.18 This leads to the question: what governs the specificity of a catalyst, and what are the associated mechanistic steps that drive this chemistry? To address this question, this work presents a distinctive combination of surface-specific optical experiments and theory to probe and understand the molecular environment at pristine catalytic interfaces. Specifically, well-defined interfaces of the model CeO2 catalyst are synthesized with only one exposed facet via pulsed laser deposition. A layer of isopropanol is adsorbed and probed using surface-specific vibrational sum-frequency generation (SFG) spectroscopy to extract vibrational spectra and molecular orientation. In SFG experiments, two incident laser pulses are focused in space and overlapped in time at a sample interface. The intense fields induce polarizations in the interfacial species that can oscillate the driving frequencies, harmonics of those frequencies (i.e., second harmonic generation), as well as at the sum and difference frequencies and thereby generate new frequencies of light that describe the interfacial species. If a molecule has resonances that overlap with the driving laser bandwidth, these signals are enhanced and serve to map out the spectral response. Thus, vibrational SFG is the nonlinear analogue of conventional vibrational spectroscopies, such as FT-IR and Raman, but selectively probes interfacial species where the symmetry is broken.5−12 SFG and related nonlinear spectroscopies have been used to extract detailed information on molecular orientation,19−26 structure,27−31 ordering,32−36 surface potential,37−39 and energetics of interfacial processes,40−43 at flat and nanomaterial interfaces to name a few examples. In parallel, theoretical efforts have been used to make sense of the often complicated spectra. These include pioneering efforts to understand model aqueous44−46 and chemically reactive47,48 interfaces using a combination of MD and ab initio methods. Our approach builds on these studies but instead focuses on tailormade interfaces and the chemical selectivity underlying these reactive surfaces. Of particular relevance here are pioneering studies probing chemical reactions at metallic49−57 and semiconducting58−60 interfaces. Only somewhat more recently have reports appeared probing well-defined catalytic semiconductor oxide interfaces with a single exposed facet. These include recent studies of methanol59,61 and acetonitrile62,63 at TiO2(110) surfaces using SFG methods to provide new insight into the chemistry at reactive oxide interfaces. Despite these advances, the study of catalytic interfaces using SFG remains somewhat limited

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RESULTS AND DISCUSSION Density Functional Theory Calculations. DFT calculations of isopropanol at CeO2(100) interfaces were performed to assist in the assignment and interpretation of SFG measurements. Details of these calculations can be found in the Supporting Information (SI) section. Briefly, our calculations begin by considering six different initial configurations of isopropanol on the dipolar CeO2(100) surface, modeled as an oxygen-terminated (2 × 2 × 1) supercell with half of the oxygen ions moved from one surface to the other. From these configurations, four different local minima were identified, as shown in Figure 1. In all cases, the oxygen atom of the isopropanol molecule moved to or remained at a bridging site, indicating that the bridge site is the preferred binding site for an isopropanol molecule on the CeO2(100) surface. This is similar to earlier calculations of ethanol on the CeO2(100) surface, which showed the bridge site to be the preferred binding site.64 The most stable configuration of the isopropanol molecule was found to be a deprotonated state with the dissociated proton bound to a nearest-neighbor (NN) oxygen site. This proton could either lie flat on the surface, forming a hydrogen bond with the next nearest neighbor (NNN) oxygen atom (Figure 1a, at 0.25 monolayer (ML) coverage, denoted as 0.25 ML-NN-1 hereafter), or point away from the surface (Figure 1b, denoted 0.25 ML-NN-2 hereafter). The inclusion of potential oxygen vacancies on the surface did not cause any change in the preferred adsorption site of the isopropanol molecule (Figure S1 in the SI section), although at higher vacancy concentrations, the dissociated proton pointed away from the surface. On the pristine CeO2(100) surface, a local minimum for the isopropanol adsorbed in its molecular state was also found (Figure 1c) but was energetically higher by 0.58 eV with respect to the most stable 0.25 ML-NN-1 configuration and thus unlikely to be present in experimental conditions at room B

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isopropanol molecules in the gas phase. It should be noted that the precise coverage of isopropanol is not known in this work due to limitations in both measurements and computation. To explore finer ranges of coverage increasingly large systems are required for simulations that quickly become computationally prohibitive. As such, we treat the measured SFG data as being representative of the coverages on the order of 0.25 and 0.5 ML and assign bands to both coverages. Continued work with control over the isopropanol coverage is currently underway in our laboratories. Dissociation of isopropanol molecules on the surface did not induce a partial reduction of the surface Ce4+ ions in any of the configurations that we have studied, indicating that the deprotonation step does not involve a redox reaction. Normal mode analysis was performed on the 0.25 ML-NN-1, 0.25 ML-NN-2, and 0.5 ML configurations shown in Figure 1 and Figure S2a and summarized in Table 1. Notably, the −OH stretch frequency falls in the −CHn stretching region for the 0.25 ML-NN-1 and 0.5 ML configurations due to the formation of a hydrogen bond with an NNN oxygen atom. A scaling factor (0.9623) was applied to the calculated transition frequencies to bring into line the calculated and experimentally measured −CH3 asymmetric stretching frequencies. It is very important to note that while calculated structures are energetically distinct the structures of the adsorbed isopropyl groups are nearly identical. The difference in adsorption energies arises from the orientation of the −OH group on the catalyst surface. Note that there is no in-plane ordering for the randomly distributed isopropanol, and as such the orientational angles extracted from SFG measurements serve to describe the average out-of-plane orientations. To quantify this, we performed ab initio Born−Oppenheimer molecular dynamics (MD) simulations to determine the orientational angles of −(CH3)2 and −CH group vibrations relative to the surface normal at both 0.25 and 0.5 ML coverages. To account for potential interactions from inadvertently adsorbed water from the atmosphere, we also simulated a system where one isopropanol molecule was coadsorbed with three water molecules (denoted 0.25 ML + 0.75 ML-H2O hereafter). Since van der Waal’s interactions may play an important role in determining the orientation of the isopropanol molecules at the surface, these calculations used the DFT-D3 method65 to describe the two-body dispersion interactions between the adsorbed isopropanol molecules and the surface. Details of the simulation setup are provided in the SI. The results summarized in Figure 3 show that the orientation of the C−H bond with respect to the surface normal (Figure 3a) is relatively insensitive to the specific surface coverage. For instance, the mean C−H angle is approximately the same for 0.25 ML (80° ± 8.3°) and 0.5 ML coverages (79° ± 6.5°), while this angle becomes slightly

temperature. Similarly, configurations beginning with the proton located at subsurface sites (Figure 1d) were significantly more unfavorable (by ca. 1.13 eV compared to the most stable configuration). Given the energetic difference between the associated and dissociated isopropanol−CeO2(100) complexes, we conclude that only dissociated species are present at the interface. Given the small energetic gap between the two dissociated conformations and the energy of the IR light used in measurements, we expect both nearest-neighbor conformations (0.25 ML-NN-1 and 0.25 ML-NN-2) to be present in experiments. The adsorption free energy of isopropanol molecules on the CeO2(100) surface was computed as a function of surface coverage to identify the most likely coverage at room temperature. Details of the simulation setup, procedure to compute the adsorption free energies and the optimized structures of the isopropanol molecules on the surface at various coverages are provided in the SI. Figure 2 shows the variation in

Figure 2. Variation in the adsorption free energy of isopropanol molecules at various coverages as a function of temperature at 1 atm pressure.

the adsorption free energy at various coverages as a function of temperature at 1 atm pressure. It was found that 1 ML coverage of isopropanol is highly unfavorable at all temperatures due to the steric repulsion between the methyl groups of the adjacent molecules on the surface, as shown in Figure S2c. 0.5 ML is expected to be the most likely coverage until ca. 380 K, beyond which the surface is likely to be covered 25% with isopropanol molecules. Beyond ca. 790 K, the surface is expected to be devoid of any isopropanol adsorbates. At those temperatures, the most favorable configuration consists of a bare CeO2(100) surface and

Table 1. Summary of Observed Bands and Assignments for Isopropanol at CeO2(100) Surfaces band

assignment

A (arb. units)

ω (cm−1)

0.25 ML-NN-1

0.25 ML-NN-2

0.5 ML

f a g b c h d e j i

−CH −CH −CH3-ss + FR −CH3-ss + FR FR FR −OH + FR −CH3-as −CH3-as −CH3-as

3.7 ± 0.3 30.2 ± 5.2 27.1 ± 1.1 35.3 ± 5.5 11.9 ± 20.3 91.4 ± 1.8 101.2 ± 53.3 −34.8 ± 25.2 50.0 ± 4.3 114.5 ± 1.0

2836.8 ± 0.2 2842.8 ± 0.9 2859.8 ± 0.2 2860.2 ± 0.5 2895.2 ± 3.6 2902.4 ± 0.2 2916.1 ± 2.0 2938.3 ± 2.1 2948.4 ± 1.2 2951.4 ± 0.1

2783.5 2783.5 2869.0 2869.0 2916.6 2949.1 2949.1 2949.1

2829.7 2829.7 2859.7 2859.7 3520.8 2937.3 2937.3 2937.3

2819.4 2819.4 2864.3 2864.3 2947.8 2943.7 2943.7 2943.7

C

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Figure 3. Variation in the angle made by the (a) C−H bond vector and (b) the −CH3 angle bisector with the surface normal at 300 K temperature.

larger upon the inclusion of water molecules (83° ± 6.6° at 0.25 ML + 0.75 ML H2O coverage). A more significant effect of surface coverage was seen in the orientation of the −CH3 bisector with the surface normal (Figure 3b). While an increase in the coverage of isopropanol molecules from 0.25 to 0.5 ML resulted only in a 2° change in mean orientation (45° ± 7.6° for 0.25 ML coverage vs 47° ± 5.4° for 0.5 ML coverage), the addition of water molecules caused a decrease of this angle by ∼8°, resulting in a mean orientation angle of 37° ± 4.6° for 0.25 ML + 0.75 ML H2O coverage. A more significant effect of surface coverage was seen in the orientation of the −CH3 bisector with the surface normal (Figure 3b). These results show that in the presence of water molecules on the surface steric repulsion causes the −CH3 groups of the isopropanol molecule to be located further away from the surface and the orientation of the −CH3 bisector to be closer to the surface normal, as compared to the case with no water molecules on the surface. As will be shown below, the orientation of isopropanol in the presence of water molecules is not consistent with SFG measurements, and therefore we do not believe water is playing a substantial role in the present experiments. These results are compared with the molecular orientation of isopropanol determined using SFG orientational methods below.19,30 Vibrational Sum-Frequency Generation. The as-grown CeO2(100) surfaces were placed in the SFG spectrometer described previously;36,66 its essential operating conditions are provided in the Supporting Information. Details on the thin-film synthesis and characterization of the CeO2 thin film are supplied in the Supporting Information. Isopropanol was drop cast on the CeO2(100) surface and allowed to dry for several minutes before laser experiments began. It was observed that the SFG signal was stable over the course of several hours, indicating the strong adsorption of isopropanol to the surface even in ambient conditions and minimal reactivity at room temperature, as expected.13 Figure 4 shows SFG spectra taken in different polarization combinations denoted as SSP, PPP, and SPS that describe the polarization of radiated SFG (S-), incident nearinfrared (S-), and mid-IR pulses (P-), respectively. Transition frequencies for the observed bands are extracted from the SFG data by fitting the measured SFG intensity to the equation8,12,24,30,31,34,53,55 n

ISFG = ANR +

∑ q

Aq ωIR − ωq + i Γq

Figure 4. SFG spectra of isopropanol at CeO2(100) interfaces collected in the SSP (a), PPP (b), and SPS (c) polarization combinations. Fits are solid red lines, and extracted band positions are indicated with dashed gray lines, labeled alphabetically and summarized in Table 1. A representative error bar is indicated for each data set.

2

made over n-resonances; Aq is the amplitude of the qth-resonance; ωIR and ωq are the frequencies of the excitation light and the molecular resonance, respectively; and Γq is the line width. In this work, the semiconducting CeO2 interface results in substantial

(1)

where ISFG is the observed SFG signal intensity; ANR is the nonresonant contribution to the SFG signal; the summation is D

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and 0.5 ML configurations have nearly the same transition frequency at 2869, 2860, and 2864 cm−1, respectively, which are not resolved individually in the present SFG measurements. This leaves bands c and h, which are not predicted by normal mode calculations and as such are assigned to Fermi resonances. The uncertainties quoted in Table 1 are obtained from fitting the SFG spectra and thus represent the uncertainties of the extracted parameters, which somewhat underestimate the true experimental error. It should be noted that the work of Liu et al. probed both gauche and cis conformers of isopropanol in their high-resolution Raman measurements. Since isopropanol is deprotonated under the conditions studied here, no such conformational difference would be expected. Having reliably assigned the bands in the SFG spectrum, the extracted amplitudes can be used to calculate the orientation of various functional groups at the well-defined interface. As will be shown below, the orientation of isopropanol at CeO2(100) surfaces reveals insight into the selectivity of this catalyst, which is the focus of this report. Following standard procedures, Fresnel factors were calculated for the incoming and outgoing light fields5,8,24 assuming an interfacial index of refraction of 1.18, which is a commonly used value for organic species far away from electronic resonance.8 Changing the value of the interfacial index of refraction to match a range of generally accepted values (i.e., between 1.18−1.20) does not strongly affect the extracted orientational angles (shifts by only ∼4° over the range for the −CH bond angle) or the chemical insight gained from them. The choice of interfacial index to be 1.18 is supported by excellent agreement with the orientational angle for the −CH group with theory, as will be shown below. The bulk indices of refraction for CeO2 were estimated from published work69 to be 2.375, 2.250, and 2.125 for the SFG, NIR, and IR light, respectively. Using the intensity ratio of −CH strengths obtained in the SSP and PPP spectra and a hyperpolarizability tensor ratio of 0.3245,68 we find that the −CH bond angle lies at θCH = 73 ± 1° with respect to the surface normal. This is in excellent agreement with our ab initio molecular dynamics simulations, which show an average −CH angle of θCH = 79° relative to the surface normal at 0.5 ML coverage (similar angles are also observed for 0.25 ML as mentioned above). The methyl groups are treated in a unified atom model detailed by Cremer and co-workers.30 This reduces the tetrahedral structure surrounding the central carbon to an effective trigonal planar molecule with three distinct functional groups (i.e., −CH, −CO, and −C(CH3)2). Unfortunately, using the measured amplitude ratios for the CH3-as modes in all three polarization combinations does not yield a unique solution for the set of equations but rather a select family of possible twist− tilt angle combinations, given by ψ and θ, respectively, that the methyl groups could possibly take. The family of solutions is shown in Figure 5 as solid blue lines (relevant equations and data given in SI). To identify the correct pair of angles describing the methyl group orientations, we first note that the angle between the −CH and −C(CH3)2 groups should be ∼120° based on the effective trigonal planar structure discussed above. Thus, the dot product between the two vectors pointing along the respective bond axes (or bisector) can be used to constrain the allowed solutions such that the −CH bond axis is oriented at the independently determined θCH angle. Specifically, we find that

nonresonant contributions that are observed in the SFG spectra shown in Figure 4 as a nonzero baseline. On the basis of SFG polarization selection rules,67 recent highresolution Raman results,68 and the above-described DFT calculations, we can confidently assign the resolved bands for isopropanol adsorbed to CeO2(100) surfaces. The most obvious feature contained in all three spectra is the band centered near 2946 cm−1, which is readily attributed to the CH3-asymmetric stretching (CH3-as) modes of isopropanol at the CeO2(100) interface. This assignment follows the selection rules for asymmetric stretching modes, where the SFG intensity is largest in PPP and SPS combinations.8 Notably, this band position is red-shifted by ∼30 cm−1 relative to the same transition observed at the neat isopropanol−air interface.24,30 This is readily explained by our DFT calculations that show by removing the proton from the −OH group of the isopropanol molecule an overall red shift of vibrational frequencies is expected (see Table 1). Thus, given the substantial shift directly observed in our measurements corroborated by DFT calculations, we can confidently assert that isopropanol is deprotonated at the CeO2(100) interface, even at room temperature. It should be noted that a possible contribution from an unresolved and previously undetected Fermi resonance might be invoked to explain the difference in the SSP CH3-as mode band position relative to the other polarization combinations. Global fitting of the data did not improve the quality of the fits, particularly for the SSP combination. Since the orientation of the methyl groups is largely dictated by the ratio of PPP/SPS CH3-as signals as will be discussed below (see SI, Figure S5), the difference in the band position for this particular band does not affect our results or the conclusions drawn from them. Similarly, bands a and f are assigned to the −CH stretch of isopropanol; this assignment differs from assignments made in other reports8,24,30,31 but is in excellent agreement with both our newly presented calculations as well as very recent high-resolution Raman measurements and calculations.68 This assignment also follows SFG polarization selection rules8 for −CH vibrations lying nearly in the surface plane as predicted from our ab initio calculations (see Figure 3a). Our calculations further show that the transition frequency of the −OH group in 0.25 ML-NN-1 and 0.5 ML structures is expected to be near 2932 cm−1 (i.e., the average of 2916.6 and 2947.8 cm−1). This considerable shift from typical gas-phase −OH stretch frequencies (∼3650 cm−1) is due to the formation of a hydrogen bond with a surface oxygen atom. Indeed, some features, labeled as bands c, d, and h, are resolved in the SFG spectrum in Figure 4 near 2916 cm−1 that could represent this vibrational mode. Band d found at 2916 cm−1 agrees most favorably with the predicted value from theory, and as such this band is tentatively assigned to the −OH stretch from the 0.25 ML-NN1 structure. This observation highlights the utility of SFG to selectively study the vibrational structure of molecules adsorbed to surfaces where the chemical composition differs from the bulk. It should be noted that this transition is likely overlapped with Fermi resonances (FR) resulting from the mixing of bending overtones with fundamental stretching frequencies.24,30,68 Thus, while band d is readily observed and could be assigned to the −OH stretch, it should be considered an unresolved mixture of two different vibrations. Similarly, based on recent isotopic Raman measurements and theory, bands b and g are assigned to a combination of CH3-symmetric stretch (CH3ss) resonances and contributions from Fermi resonances.68 This assignment is supported by our calculations that indicate the symmetric stretching modes of 0.25 ML-NN-1, 0.25 ML-NN-2,

sin ψ =

−1 csc θCH csc θ − cot θCH cot θ 2

(2)

Red dashed lines in Figure 5 illustrate the possible sets of orientational values that the methyl groups can take on given the E

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functional groups the approach detailed by Rao and Eisenthal is used.19 Initially, we set the molecular plane to coincide with the laboratory X−Z plane such that the coordinates of the −C(CH3)2 group can be represented as the direction cosine (l0CH3,x, l0CH3,y, l0CH3,z) = (0, 0, 1), and the −CH group is positioned at (l0CH,x, l0CH,y, l0CH,z) = (cos(−30°), 0, cos(120°)). To rotate the molecule to the laboratory frame we apply two consecutive rotations: first a ψ = 49° twist about the laboratory and molecular Z-axis is applied followed by a tilt, θ = 62°, about the laboratory Y-axis as shown in Figure 6a and 6b, respectively. For convenience, we chose to rotate about ψ before tilting about the laboratory Y-axis by θ. This permits one to avoid rotation about the −C(CH3)2 axis (molecular z-axis) after tilting about the laboratory Y-axis, which involves multiple rotation operations. These two rotations place the −C(CH3)2 group at (cos(152°), cos(90°), cos(62°)) and the −CH group at (cos(44°), cos(130°), cos(74°)). As expected, the angle of the −CH group relative to the laboratory Z-axis agrees with the measured value within uncertainty. To determine the absolute molecular orientation, which is the angle of the molecular normal (green arrow in Figure 6c) with respect to the surface normal, we take the cross product of the rotated C(CH3)2 and −CH direction cosines to obtain a new vector normal to the molecular plane given by (cos(72°), cos(55°), cos(55°)). The projection of this vector onto the laboratory Z-axis then provides the absolute orientational angle, measured here to be φ = 48°, of isopropanol at the CeO2(100) interface. In comparison, ab initio MD simulations predict the average angle of the vector cross product of the −CH3 angle bisector and −CH bond vector with the surface normal to be 76°(±6.9°) at the most stable 0.5 ML coverage below 380 K. This angle increased to 81°(±6.4°) at 0.25 ML coverage and further to 85°(±3.9°) in the presence of water molecules on the surface. Figure 7 shows the probability distribution for this angle at various surface coverages and a temperature of 300 K. One can see that at lower coverages the most probable angle of the vector cross product with the surface normal is close to 90°, indicating that the two −CH3 groups of isopropanol are nearly equidistant from the surface. Inclusion of one and two oxygen vacancies on the surface at 0.25 ML coverage, corresponding to a partial reduction of 50% and 100% of the surface Ce4+ ions to Ce3+, did not cause an appreciable change in the orientation of the molecule. In the optimized configurations shown in Figure S1 the absolute orientational angle varied between 77° and 85° for one and two surface oxygen vacancies, respectively. Differences in measured and calculated absolute orientations are somewhat

Figure 5. Allowed angles from SFG intensity ratios are bound by the solid blue lines, whereas allowed orientational angles that satisfy the constraints described in the text are bound by red dashed lines. The intersection of these regions represents the only allowed orientations of the methyl groups that satisfy all of the observed data.

measured −CH orientational angle and associated uncertainty. Since these requirements must satisfy simultaneously the measured set of possible −C(CH3)2 orientations (solid blue lines in Figure 5), the regions where the curves overlap represent the only physical solution. This region allows us to extract the orientation twist and tilt angles for the −C(CH3)2 group, which are found to be ψ = 49 ± 1° and θ = 62 ± 1°, respectively. Our ab initio MD calculations predict that the tilt angle of the −CH3 groups will be ∼47° at 0.5 ML coverage, which is in reasonable agreement with the SFG measurements. The uncertainties in the extracted amplitudes obtained from fitting are included in the evaluation of the orientational angles. Details of this procedure can be found in the Supporting Information. The measured orientational information provides the necessary information to uniquely define the absolute molecular orientation of the isopropyl group at CeO2(100) surfaces at room temperature. The absolute molecular orientation describes how the molecules are arranged on the surface, and through this, we aim to gain insight into chemical selectivity through the obtained structure. To obtain the absolute molecular orientation, we first position the isopropanol molecule such that the −C(CH3)2 group is pointed in the +Z direction as shown in Figure 6a. Blue arrows in Figure 6 represent the unit vectors pointing along the various bond axes/bisector that serve to define the effective molecular plane. To track the orientations of the individual

Figure 6. Rotation of isopropanol to the laboratory frame through successive rotations, noted along the respective bond axes with appropriate angles, as discussed in the text. F

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conformation wherein one of the −CH3 groups lies closer to the surface, with the methyl H−Osurf contact distance of only 2.402 Å. In fact, the departure of theory and experiment points to the challenges present in modeling polar surfaces such as CeO2(100) at experimentally realized coverages. Performing calculations in the presence of coadsorbed water or via inclusion of van der Waals forces (see SI for details) cannot simultaneously replicate the spectral signatures or the orientations obtained from experiment. This might imply some degree of heterogeneity in the surfaces being studied and/or a more complicated polarcompensated reconstruction71 not included in the calculations. Alternatively, contributions from Fermi resonances to the −CH stretching mode intensity would systematically change the extracted orientational angles, possibly leading to a different molecular orientation. Similarly, controlling the surface coverages could help to determine if adsorbate interactions impact the ordering and orientation at the surface. Continued work aimed at probing potential changes in orientation and eventual onsets of reactivity at elevated temperatures at these and other interfaces is in progress in our laboratories. While these efforts are ongoing, the relatively minor differences between experimentally and theoretically obtained orientations serve to illustrate the synergy between experiment and theory to provide a more complete picture of the chemistry at surfaces. This unique combination of nonlinear spectroscopy, theory, and materials synthesis promises to provide new avenues to the understanding of chemical selectivity at these and other reactive interfaces.

Figure 7. Probability distribution for the angle made by the vector cross product of the −CH3 angle bisector and the −CH bond vector with surface normal at 300 K.

expected since small differences in angles of functional groups (i.e., a few degrees difference for the measured and predicted angles) are compounded when calculating the absolute orientation. As such, this angle is an extremely sensitive metric for evaluating theory based on experimental observables, or vice versa. New Insight into Catalytic Selectivity. This result provides novel insight into the initial mechanistic steps involved in the selective reaction of isopropanol at CeO2 surfaces. Initially, we find experimentally and theoretically that the proton on the −OH group is removed by the surface oxygen species, even at room temperature. Our calculations further show that the deprotonated isopropanol is bound to two Ce atoms at the surface on a bridge site. This deprotonation step follows expectations based on the basicity and acidity of the surface and molecules and agrees with separate experimental efforts indicating the presence of deprotonated isopropanol at ceria interfaces.18,29,70 To produce propylene, which is the dominant product at elevated temperatures, one must further abstract a proton from a methyl group. This is in contrast to the minor product, acetone, which would require the removal of the proton from the −CH group. It is interesting that the absolute orientation sketched in Figure 6c shows the molecule assuming the necessary configuration prereaction (i.e., at room temperature) to allow for the eventual loss of a methyl proton. Given this intuitive interfacial chemical structure and knowledge of CeO2 selectivity, an extrapolated mechanism for the elevated temperature reaction to proceed can be formed: upon adsorption, the −OH group is immediately deprotonated, and the isopropyl moiety assumes a “prereacted” configuration with a methyl group directed toward the surface. This is directly observed in our measurements as well as predicted by theory. It is then expected that on raising the temperature the energetic barrier for proton removal from the methyl and oxygen loss is overcome, and the molecule rearranges to form propylene after which it can desorb. This is based on previous work showing that the dominant product in the reaction of isopropanol with CeO2 nanocubes with exposed (100) facets is propylene.18,29,65 The present results reveal an apparent prereaction configuration that points to the selectivity of the CeO2(100) surfaces to form propylene from isopropanol, namely, that for the reaction to occur the methyl must be in close proximity to the surface, which is directly observed in our SFG measurements, even at room temperature. It should be noted that while our DFT calculations do not quantitatively match the experiments they do qualitatively predict that the isopropanol molecules assume a prereacted

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CONCLUSION This works presents a new paradigm for studies of chemical selectivity at catalyst interfaces that takes advantage of precision materials synthesis, surface-specific optical experiments, and firstprinciples theory. The selectivity of isopropanol reacting at elevated temperatures with CeO2(100) interfaces to form propylene was postulated through direct orientational analysis of SFG spectra. It was experimentally found that the twist and tilt angles for the −C(CH3)2 group are ψ = 49 ± 1° and θ = 62 ± 1°, respectively, whereas the −CH bond is oriented at a θCH = 73 ± 1° angle relative to the surface normal. These angles allowed us to define the absolute molecular orientation, which is the angle the molecular normal takes with respect to the surface normal, and was found here to be φ = 48°. Our results unambiguously show that isopropanol is deprotonated at the surface and, through the orientational analysis, appears to arrange to a “prereaction” geometry to allow for H-abstraction from a methyl group at elevated temperatures. This prereaction configuration hints at the origin of the selectivity of the CeO2 catalyst to form propylene vs acetone as the dominant reaction product. This key insight into the chemical and structural phenomena taking place at well-defined facets underlies the same chemistry at complex and spatially heterogeneous interfaces.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b03272. Materials synthesis and characterization including XRD and AFM images of the CeO2 surfaces used in experiments, equations and figures describing the orientation of methyl groups at CeO2, and details of the DFT and MD calculations (PDF) G

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AUTHOR INFORMATION

Corresponding Authors

*Benjamin Doughty. E-mail: [email protected]. *Vyacheslav S. Bryantsev. E-mail: [email protected]. *Daniel A. Lutterman. E-mail: [email protected]. ORCID

Sriram Goverapet Srinivasan: 0000-0003-3984-1547 Vyacheslav S. Bryantsev: 0000-0002-6501-6594 Dongkyu Lee: 0000-0003-4700-5047 Ho Nyung Lee: 0000-0002-2180-3975 Ying-Zhong Ma: 0000-0002-8154-1006 Daniel A. Lutterman: 0000-0002-4875-6056 Author Contributions #

B.D. and S.G.S. contributed equally to this work. B.D., Y.Z.M., and D.A.L. carried out the SFG measurements, data analysis, and interpretation. S.G.S. and V.S.B. performed DFT calculations, molecular dynamics simulations, and their analysis. The CeO2 epitaxial thin film was prepared and characterized by D.L. and H.N.L. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division (B.D., Y.-Z.M., and D.A.L.), Materials Sciences and Engineering Division (D.L. and H. N. L.), and the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office (S.G.S. and V.S.B.). This research used computational resources at the Orcinus computing facility of WestGrid/Compute Canada and the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, supported by the Office of Science of the U.S. Department of Energy under contract No. DE-AC05-00OR22725.

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