A Reassessment of the Electronic Structure of Cr(VI) Sites Supported

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A Reassessment of the Electronic Structure of Cr(VI) Sites Supported on Amorphous Silica and Implications for Cr Coordination Number Nathan M. Peek, David B. Jeffcoat, Cristina Moisii, Lambertus van de Burgt, Salvatore Profeta, Susannah L Scott, and Albert Edward Stiegman J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12079 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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The Journal of Physical Chemistry

A Reassessment of the Electronic Structure of Cr(VI) Sites Supported on Amorphous Silica and Implications for Cr Coordination Number Nathan M. Peek§, David B. Jeffcoat §, Cristina Moisii†, Lambertus van de Burgt§, Salvatore Profeta Jr.§, Susannah L. Scott‡, A. E. Stiegman§*

§

Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306

†

Department of Science, Engineering & PE, Eastern Florida State College, Cocoa, FL 32922

‡

Department of Chemical Engineering, University of California, Santa Barbara, CA 93106

* To whom correspondence should be addressed. Email: [email protected]

ACS Paragon Plus Environment

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Abstract The electronic structure of isolated Cr(VI) sites supported on silica was re-investigated using multiple, complementary electronic spectroscopies applied to transparent xerogel monoliths. The absorption spectrum exhibits three previously reported peaks, at 22,800, 29,100 and 41,500 cm-1, as well as a previously unresolved band at ca. 36,900 cm-1. The emission is a long-lived red luminescence with λmax = 13,600 cm-1, emanating from the lowest excited state. Assignment of the excited states was facilitated using time-dependent density functional theory (TD-DFT) calculations performed on cluster models. All of the observed electronic transitions and their energies are accounted for by dioxoCr(VI) sites. The lowest energy observed excitation at 22,800 cm-1 populates a singlet excited state, while the emitting state is the corresponding triplet state, accessed by intersystem crossing from the singlet state. Spectroscopic bands observed at 29,100, 36,900 and 41,500 cm-1 were assigned, based on the TD-DFT calculation, to spin-allowed transitions that are consistent with emission polarization anisotropy measurements. Small variations in site symmetry at Cr result principally in inhomogeneous broadening of the spectral bands, as well as a red-edge effect in the photoemission spectrum. There is no evidence for a significant contribution from five-coordinate mono-oxoCr(VI) sites.


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Introduction

Cr(VI) ions dispersed on oxide (typically, silica or alumina) supports have been the subject of considerable study due, in large part, to their roles as catalyst precursors in commercial Phillips ethylene polymerization and several commercial processes for light alkane dehydrogenation.1-2 In addition, the materials are of interest as photo-oxidation catalysts because of their long excited state lifetimes and absorption in the visible.3-6 Cr(VI)/SiO2 has been characterized using a variety of spectroscopic techniques, including X-ray absorption spectroscopy (EXAFS and XANES), vibrational spectroscopies (Raman, resonance-Raman and IR) and diffuse reflectance electronic spectroscopy.7-10 Many studies have attempted to use UV-vis spectroscopy to distinguish different types of Cr sites as a function of loading. In particular, as the Cr loading increases, specific bands emerge that have been assigned to oligomeric species such as dichromates, Figure 1, as well as Cr2O3 crystallites.11 At low Cr loadings, most studies have concluded that the majority of the monomeric Cr(VI) sites have a dioxochromate structure, with two terminal oxygen ligands and two bridging oxygen ligands linking Cr to the silica surface (Figure 1a). However, suggestions of monooxochromate sites have been made for many years. In earlier studies, the case for the coexistence of dioxo and mono-oxo sites were based largely on the assignment of one of two vibrational bands observed in the Raman spectrum to each of these two species, as well as arguments based on tentative assignment of bands observed in the UV-vis spectrum.12-16 Subsequently, it was shown that the two Raman bands are the symmetric and antisymmetric vibrational modes of the dioxo site, and that, in fact, there are no vibrational bands in materials with low Cr loadings that can be assigned to a mono-oxo structure.7, 17 Recently, the proposal for the existence of a mono-oxo site has been revived, primarily, though not exclusively, due to the appearance of a new band in the resonance Raman spectrum, obtained by laser excitation into the first electronic excited state.18-19


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Figure 1. Various structures proposed for Cr(VI) sites supported on amorphous silica: (a) dioxochromate site, (b) dichromate site, and (c) mono-oxochromate site.

The most widely proposed structure, shown in Figure 1a, has nominal C2v symmetry at Cr, although the actual symmetry is lower due to bonding to the silica. Furthermore, since the silica is amorphous, a distribution of site geometries exists. Nevertheless, an analysis of the Raman spectra of Cr(VI)/SiO2 based on strict C2v symmetry (or small deviations from it) showed that all of the observed bands can be assigned based on that simple structure.17 In addition, this structure is consistent with the Cr K-edge EXAFS. It is generally taken to be the starting structure in the generation of the active sites of the Phillips catalyst.1, 7, 10, 20 Electronic spectra of Cr(VI) dispersed and supported on silica or silica-alumina, acquired using diffuse reflectance spectroscopic methods, have been interpreted in terms of majority dioxochromate sites.9, 21-25 All of these studies report three broad absorption bands at approx. 21,500, 38,600 and 41,700 cm-1 (in order of increasing relative intensity) for samples with low Cr loadings (≤ 0.5 wt %) calcined at ≥ 500 °C. Since Cr(VI) is a d0 metal ion, all bands are necessarily ligand-to-metal charge transfer transitions. Notably, the spectrum of Cr(VI)/SiO2 is qualitatively consistent with those of other dioxoCr(VI) compounds with C2v symmetry such as CrO2F2 and CrO2Cl2, and is nearly identical to that of CrO2(OH)2.25-26 In this work, we undertake a re-investigation of the electronic structure of the isolated Cr(VI) site supported on silica, employing both electronic absorption and emission spectroscopic measurements. In order to obtain high-quality spectra, we exploit transparent sol-gel monoliths of silica-supported Cr(VI). We developed and pioneered the use of these materials for the spectroscopic characterization of silica-supported metal oxides.7,

20, 27-30

Since the spectra are

collected in transmission mode, key parameters such as extinction coefficients and emission


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polarization anisotropies can be determined. Using this information, we can make rigorous assignments of the electronic transitions, facilitated by density functional theoretical computations. This study provides a comprehensive understanding of the important ground and excited state electronic properties of Cr(VI)/SiO2, which can be used to describe both its thermal and photochemical behaviors. This study lead to a markedly deeper understanding of the Cr(VI) silica surface, which will yield new insights into the nature of the catalytically active sites on supported Cr(VI) catalysts and on the possible precursor species that, upon reduction, yield the active sites in the Phillip’s catalyst.

Experimental and Computational Methods Preparation of Cr/SiO2 Xerogels. Transparent, porous silica xerogels containing between 0.5 and 3.0 mol% Cr (defined as mol Cr/(mol Cr + mol Si) × 100 %) were made by cocondensing tetramethylorthosilicate (TMOS) with CrO3 in an aqueous 2-propanol solution, following published procedures.7, 29 The sols were allowed to gel, then aged over a period of 2–3 months, after which they were slowly dried and calcined in a furnace at 500 °C. This procedure results in transparent yellow-orange monoliths (the color intensity depending on the Cr concentration) in which the CrVI sites are well-dispersed throughout the silica matrix. UV-vis Absorption Spectroscopy. UV-visible spectra were collected on a Perkin-Elmer Lambda 900 spectrophotometer in transmission mode, with the beam passing through the Cr/SiO2 sol-gel monolith. The sample was placed in a sealed, high-temperature/high-pressure (HTHP) spectroscopic cell (International Crystal Laboratories) equipped with sapphire windows. All samples were handled and measurements carried out under strict anaerobic and anhydrous conditions. Fluorescence Spectroscopy. Fluorescence spectra were collected on a Spex Fluorolog II equipped with 0.22 m double monochromators (Spex 1680) and a 450 W Hg/Xe lamp. All emission and emission excitation experiments were performed using 0.005% Cr/SiO2 sol-gel monoliths. Right-angle collection was used for both emission and emission excitation spectra, and cutoff filters were used to suppress second-order excitation lines. For low temperature studies, samples were mounted on an APD Model DE-202 cryo-stat, shrouded, and evacuated. Spectra were corrected for the lamp profile and the detector response. Emission spectra reported


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in wavenumber units were corrected in the standard way by the multiplications of the intensities by the wavelength squared, (λ(nm))2, for the bandpass variability in spectra collected at fixed wavelength resolution.31 Density Functional Theory Calculations. TD-DFT calculations were carried out at the Florida State University Research Computing Center using Gaussian v09 software. Calculations were carried out using the B3LYP density functional and 6-311G* basis set. Electronic transitions of the model complexes described in the text were obtained from the Gaussian output. Full experimental and computational details are provided in the Supporting Information. Results and Discussion

Electronic Spectroscopy of Cr(VI)/SiO2 The electronic spectrum of a dilute Cr(VI) silica xerogel monolith (0.005 mol % Cr), collected in transmission mode, is shown in

Figure

2a.

The absorption spectrum is

characterized by two relatively intense low-energy bands at [λmax(ε(L mol-1 cm-1)] = 22,300 (624) and 29,300 (4020) cm-1, consistent with prior reports.7, 32 At higher energy, others have reported a single band, based on diffuse reflectance spectroscopy. However, the higher quality of the transmission spectrum obtained here with the transparent monolith reveals the presence of two overlapping but resolved bands: a peak maximum at 42,900 (12,450) cm-1 and a clear shoulder at ca. 37,600 (8,270) cm-1. The failure to resolve two transitions in diffuse reflectance mode is likely caused by complications due to the background absorption (the commonly-used MgO reference performs poorly), which distorts the spectrum in the UV region by artificially decreasing the reflectance.33


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Figure 2. Electronic spectra of a 0.005 mol % Cr(VI)/SiO2 xerogel: (a) electronic absorption spectrum (blue) and photoemission spectrum (red), as well as (b) emission excitation spectra recorded at room temperature (cyan) and 10 K (green). All four electronic absorption bands are broad (e.g., fwhm ≈ 4500 cm-1 for the lowest energy absorption band), due largely to inhomogeneous broadening caused by the distribution of site symmetries on the amorphous silica surface (vide infra). Thus we must consider that inhomogeneous broadening may obscure other, weaker transitions. To evaluate this possibility, we performed a deconvolution of the electronic spectrum. A minimum of six Gaussian bands are required to reproduce the full spectral envelope of Cr(VI)/SiO2, Figure 3. We note that


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considerable caution should be exercised in deconvoluting complex spectra, since spurious bands are easy to generate, but lack physical meaning. The four lowest energy bands correspond very well to the four resolved electronic transitions in Figure 2; notably, no additional bands are required to generate the observed spectrum. At higher energies (>42,500 cm-1), the broad, unresolved spectrum may consist of transitions related to Cr(VI) as well as contributions from impurities in and scattering from the silica. In this region, our deconvolution includes a broad band that essentially corrects for the rising background, as well as two Gaussian bands at 47,420 and 49,850 cm-1 that are not resolved in the experimental spectrum. We judge them to be likely artifacts of deconvolution, and we view them simply as parameters whose sum reproduces the congested, high-energy region of the spectrum. Nevertheless, we are confident that the deconvolution of the four observed low energy transitions is correct, and that they do not mask other allowed transitions (although weak or non-allowed transitions may still be present). The peak maxima for the Gaussian deconvolutions peaks for the first four transitions as determined by the fitting routine are at 22,800, 29,100, 36,900 and 41,500 cm-1. We will henceforth use these values to identify the electronic transitions.


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Figure 3. Electronic absorption spectrum (dots) recorded for a 0.005 mol % Cr(VI)/SiO2 xerogel, and its deconvolution obtained using several Gaussian band shapes, as well as the simulated spectrum (red line).

The photoemission spectrum is displayed relative to the absorption spectrum in Figure 2a. The Cr(VI) sites exhibit red luminescence emanating from the lowest excited state, with λmax at 13,620 cm-1. It is important to note that there is no overlap between the photoemission spectrum and the lowest energy absorption band. If such overlap existed, it would occur at approximately the zero-point energy transition (E00). Furthermore, the apparent Stokes shift between the maxima of the first absorption band and the emission band is much larger (> 9000 cm-1) than is usually observed when the emitting state is populated by direct excitation (for which the Stokes shift reflects normal coordinate changes between the ground and excited states and is typically < 1000 cm-1).34 These observations strongly suggest that the electronic transition associated with the lowest energy absorption band does not populate the emitting state directly. Instead, the lowest energy allowed excited state must be coupled to a weak, forbidden transition that is not directly observed spectroscopically. Furthermore, since the magnitude of the extinction coefficient for the lowest energy absorption band (ε = 624 L mol-1 cm-1) suggests that it is a fully allowed transition, the emitting state must be a triplet excited state populated by intersystem crossing from the lowest singlet excited state. These spectroscopic properties are fully consistent with prior studies of the electronic structure of well-defined dioxoCr(VI) sites in ZSM-11 silicalite.32, 35 In that system, the Cr(VI) sites are located at crystallographically-defined lattice positions, and their spectra exhibit very little inhomogeneous broadening. The emitting site is a low-lying triplet, for which no direct transition from the singlet ground state to the triplet state is observed. As in the amorphous silica system, there is no overlap between the emission and lowest energy band in the excitation spectrum that might define a zero-point energy transition; the apparent Stokes shift is a large 4,600 cm-1. However, the uniformity of the Cr(VI) sites in ZSM-11 gives rise to extremely welldefined vibrational structuring of the emission. Using the position of the lowest energy vibronic band, E00 was estimated to be 16,500 cm-1. We also reported similar photophysical properties for a silica-supported mono-oxovanadium(V) site, (≡SiO)3V=O, whose emission is a long-lived phosphorescence from a triplet excited state.30, 36


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Emission excitation spectra, collected by monitoring the emission at 16,390 cm-1 at either room temperature or 10 K, are shown in Figure 1b. Significantly, the excitation spectra consist of three well-resolved bands at 20,900, 26,800 and 35,100 cm-1. They replicate qualitatively the three principal transitions observed in the absorption spectrum. A small, barely resolved band at 41,000 cm-1 in the low-temperature excitation spectrum appears to correspond to the fourth and highest-energy absorption band at 41,500 cm-1, although its proximity to the cutoff for the excitation lamp in the emission spectrometer makes this assignment tentative. Nevertheless, the general agreement between the absorption and excitation spectra indicates that all four bands observed in the absorption spectrum are coupled to the same low-energy emitting state and are, therefore, transitions of a single structural type of Cr(VI) species. Consistent with this conclusion, photoemission spectra collected using different excitation wavelengths of 22,200, 35,100, 37,000, or 38,600 cm-1 are identical (Figure S1). On this basis, we can definitively rule out the possibility that alternative Cr(VI) structures such as dichromates or a mono-oxochromate (Figure 1b,c), or impurities such as the traces of Cr(III) or Cr(V) sites commonly found in Phillips catalysts, give rise to any of the four absorption bands observed in Figure 3. As is typical of excitation spectra, the relative intensities of the bands in Figure 2b do not necessarily replicate those of the absorption spectrum in Figure 2a, since the intensities reflect the efficiency of intersystem crossing and internal conversion processes that couple the higher excited states and the emitting state. In addition, while the transitions in the excitation and absorption spectra of a discrete molecule are expected to have precisely the same energies, this is clearly not the case for Cr(VI)/SiO2: the energy maxima of the excitation bands are consistently lower than those of the corresponding absorption bands, by approx. 1,800-2,100 cm1

. The excitation bands are also narrower. Both are expected consequences of the distribution of

sites present in the heterogeneous catalyst. Variations in the site environment result in slight differences in electronic structure, and not all sites are necessarily emissive. (Obviously, the excitation spectrum reflects only the subset of sites that are emissive.) The observed energy shifts indicate that emission occurs principally from sites whose coordination environments correspond to lower energy excited states. This phenomenon has been observed for luminescent molecules distributed in glassy solvents and polymer matrices, where it gives rise to the well-known “red-edge effect”.31,


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Thus excitation on the extreme red edge of the lowest absorption band yields an emission that is red-shifted relative to the emission observed with higher energy excitation. Consistent with this interpretation, emission spectra of Cr(VI)/SiO2 acquired with progressively lower frequency excitations over the first absorption band show a clear red-edge effect (Figure 4). This result provides the first direct spectroscopic evidence of which we are aware for the site distribution in the dioxoCr(VI) sites of the Phillips catalyst (although reactivity evidence, in the form of low active site fractions and high polymer dispersities, as well as the adsorption of probe molecules such as CO to reduced Cr sites, have long suggested such a distribution).1 More significantly, it shows how the site distribution varies continuously in energy, presumably reflecting the range of minor structural variations in the coordination sphere of Cr(VI).38 While we cannot relate the distribution of emitting sites directly to their subsequent reactivity, the presence of a reactive subset of sites with specific electronic properties is consistent with the finding that the catalytically active sites are a subset of the total sites available.

Figure 4. Comparison of emission spectra for a 0.005 mol % Cr(VI)/SiO2 xerogel, collected at excitation frequencies of (a) 22,220 cm-1, (b) 20,000 cm-1, (c) 18,870 cm-1, and (d) 18,180 cm-1, showing a red-edge effect in the shift of the emission maximum to lower energy as the exciting wavelength decreases.

The nature of the electronic transitions, in relation to the emitting state as well as to each other, was further explored via measurement of the excitation polarization anisotropy (r). Its value depends on the angular displacement between the transition moment vectors for the


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absorption and emission processes.31 For a rigid, isotropically-dispersed molecule whose absorption and emission transition moments lie along the same axis, this angle is zero and the anisotropy will have a value of 0.40. Conversely, when the absorption and emission transition moment vectors are perpendicular to each other, the value of r will be -0.02. Since heterogeneous catalysts have a distribution of local site geometries which cause the intensity and energy of a given electronic transition to vary, the value of the polarization anisotropy associated with a specific transition is necessarily an average value. For the first (i.e., lowest energy) transition of Cr(VI)/SiO2, the anisotropy is high, at ca. 0.23 (Figure 5). Thus, it is reasonable to suggest that the state populated by the lowest energy electronic transition and the emitting state have the same symmetry. For the second and third (next two lowest energy) transitions, the anisotropy values at the band maxima are approximately the same, and negative (-0.07). In a low-symmetry point group such as C2v, the transition moment vectors for the dipole-allowed transitions to the 1A1, 1B1 and 1B2 excited states are mutually orthogonal, i.e., all are either 0 or 90° relative to the emitting state. Since the transition moment vectors for transitions to the second and third electronic states must also be perpendicular to that of the emitting state, the two excited states they populate must necessarily have symmetries different from that of the first electronic transition. This finding is important for assigning each of the transitions unambiguously using the results of TD-DFT calculations (vide infra).


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Figure 5. Polarization anisotropy (r, red) of the bands in the emission excitation spectrum (blue) for 0.005 mol % Cr(VI)/SiO2. The emission intensity was monitored at 16,400 cm-1 (610 nm).

Photophysical Characterization of the Emitting State To support the assignment of the observed emission to phosphorescence from a triplet excited state, its luminescence lifetime was measured. Emission decay data were collected by exciting at 450 nm and monitoring at 626 nm. The decays shown in Figure 6 are not simple exponentials, but are consistent with a distribution of surface sites that yields a distribution of lifetimes. The emission decays were analyzed using the method of Albery, which assumes a Gaussian distribution of ln(k) values, where k is the first-order rate constant for the emission decay, and a dimensionless parameter γ reflecting the width of the Gaussian distribution.39 Consistent with the lowest excited state being a triplet, the average lifetimes are relatively long, with values of 0.56 µs at room temperature and 5.00 µs at 77 K. Interestingly, the γ values from the Albery fit differ at room temperature (1.9) and 77 K (3.8). Thus the distribution of emitting sites is larger at the lower temperature. Since the emission quantum yield generally increases at lower temperatures, it is reasonable to expect that a higher number of emitting species is observed. Finally, a standard test to explore the spin multiplicity of the emitting state involves measurement of the emission in the presence of O2. Strong quenching by the triplet ground state of O2 is observed (Figure S2), confirming the triplet assignment of the emitting state.


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Figure 6. Emission decays (blue), monitored at 16000 cm-1 (626 nm) and two different temperatures, for 0.05 mol % Cr(VI)/SiO2 excited at 22,2000 cm-1 (450 nm). The red lines are the non-linear least-squares fits of the Albery function to the observed decays. Computational Models for (≡ ≡SiO)2CrO2 Sites Many computational studies of Cr ions supported on silica and other oxides have been conducted, largely with the aim of developing a molecular-level understanding of the Phillips polymerization process.40-45 These studies have mostly, although not exclusively, focused on the energetics of possible mechanistic pathways for the activation and polymerization of ethylene at reduced Cr sites. In contrast, Dines and Inglis performed a careful density functional theory investigation of a dioxoCr(VI) site using small chromasiloxane ring structures as models, with the goal of interpreting that site’s vibrational modes. In particular, they assigned the vibrational spectrum of the dioxoCr(VI) site rigorously, and identified the Raman band associated with its weak antisymmetric stretching mode.17 More recently, Handzlik et al. probed the effect of the local silica environment on the energies of model dioxoCr(VI) sites supported on extended, crystalline silica surfaces and silica-alumina supports.46-47 That work also evaluated the possibility of alternative mono-oxoCr(VI) structures, but found all such sites to be much higher in energy. In this study, we performed time-dependent density functional theory (TD-DFT) calculations to assign the ground and excited state electronic structures of dioxoCr(VI) sites. We chose to use primarily simple clusters containing various chromasiloxane ring structures (Figure 6).17, 40, 43 Models containing smaller, strained chromasiloxane rings (I and II) were proposed to be more catalytically relevant than structures with larger, less strained chromasiloxane rings.48


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Nevertheless, a model with a larger ring (III) derived from another structure used by Dines and Inglis17 was also included for comparison. Finally, we included a mono-oxochromate model structure (IV), because of persistent suggestions that it is a significant contributor to the population of monomeric Cr(VI) species on silica.12-13, 18-19

Figure 7. Structures of cluster models used in the calculations: two dioxoCr(VI) models with strained chromasiloxane rings, based on a model used by Dines and Inglis,17 (I) restricted to C2v symmetry, and (II) the corresponding geometry-optimized model, (III) a larger, unstrained chromasiloxane ring (modified based on a model used by Dines and Inglis, with geometry optimization), and (IV) mono-oxoCr(VI) model. Color scheme: magenta Cr, red O, blue Si, gray H.

Excited State Electronic Structures In Td symmetry, each of the promotions t1 e and t2 e gives rise to two, triply-degenerate excited states.25 In the studies of the electronic structure of the chromate ion, these transitions have previously been observed and assigned.25, 49-51 Reduction of the chromate symmetry to C2v generates A1, A2, B1 and B2 excited states. The A2 state arises from a dipole-forbidden transition from the ground state; therefore, there are just five fully-allowed excited states: 1A1, two 1B1 and two 1B2, along with their corresponding triplet states. Historically, the interpretation of electronic transitions has relied heavily on measurements of spectroscopic parameters such as band intensity, emission polarization anisotropy and, where possible from single-crystal data, polarization direction of the absorption band. Assignments were based on states inferred from likely one-electron promotions and their expected properties (i.e., polarization) obtained using group theoretical considerations. We used this approach to assign the electronic spectra of a silica-supported oxovanadium site.30, 36


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In recent years, the use of TD-DFT to compute the excited states of metal complexes has provided, for the first time, a generally reliable method for assigning the bands observed in electronic spectra.52-53 To assign the transitions of Cr(VI)/SiO2, we initially performed TD-DFT calculations using model I, fixed at C2v symmetry with axes were chosen to be consistent with Mulliken convention with Z as the highest symmetry axis and the terminal Cr(=O)2 group lying in the YZ plane.54 We chose this structure as a starting point because it allows us to use group theoretical methods in our analysis, and because our calculations show that its one-electron molecular orbitals do not differ significantly from those of the relaxed, lower symmetry model II. Obviously, in the real system, the site symmetry is lower than C2v; we consider the spectroscopic consequences of lower symmetry below. The transition energies and symmetries of the calculated excited states, as well as the principal one-electron promotions that contribute to them, are listed in Table 1. Full details of these calculations are given in the Supporting Information. The reported transition energies are the weighted average of these one-electron promotion energies. There is no precise correspondence between these calculated energies and the experimentally measured energies of spectroscopic transitions. TD-DFT calculations predict the energies of adiabatic transitions between the bottoms of the potential energy wells for both the ground and excited states. In contrast, λmax values in the experimental spectrum represent Franck-Condon maxima, i.e., the energies of the most probable vibronic transitions in direct excitation from the ground state to the excited state, consistent with the Born-Oppenheimer approximation. Such energies are always higher than the zero-point energies transitions (E00), i.e., the energy differences between the lowest vibrational states (v = 0) of the ground and excited electronic states. Energies calculated by TD-DFT are smaller than Franck-Condon maxima and are closer, but not identical, to E00. For most bands, the E00 transition energy is difficult to determine in the absence of resolved vibronic structure; however, it can be estimated using the red edge of the absorption transition where the band intensity approaches zero. Thus, for the lowest energy, fully-allowed electronic transition of Cr(VI)/SiO2, the spectrum returns to baseline at approximately 18,000 cm-1, which we assume to be close to E00. TD-DFT was also used to compute the energy of the transition to the lowlying triplet state, 3B1, at 19,600 cm-1.


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Table 1. Comparison of Observed Energies for Allowed Electronic Transitions with Calculated Transition Energies to Singlet Excited States for Model Structure Ia Constrained to C2v Symmetry Transition Observed b Calculated Contributing OnePartial Intensity Excited Transition Energy f State Electron Transitions Energy λmax -1 c -1 Coefficients (cm ) (cm ) 0.90 (81%) HOMO LUMO 1 HOMO-2 LUMO 0.39 (15%) 0.0015 22,800 24,342 B1 1 HOMO-3 LUMO+1 -0.14 (2%) 29,100

2

30,537

36,433 3

36,900 36,681

42,380

See Figure 7.

Figure 3.

HOMO-1 LUMO+1 0.62 (39%) HOMO-6 LUMO -0.62 (39%) HOMO-3 LUMO+2 -0.42 (18%) HOMO-12 LUMO -0.16 (2%) HOMO-8 LUMO HOMO-12 LUMO HOMO-5 LUMO+1

0.66 (44%) 0.58 (34%) -0.21 (4%)

0.0125

1

B2

0.0058

1

B2

0.0317

1

A1

0.0080

1

A1

41,500

4

a

0.92 (84%) HOMO-3 LUMO HOMO LUMO+1 0.23 (5%) HOMO-2 LUMO+1 0.17 (3%) HOMO-2 LUMO+1 0.83 (69%) -0.40 (16%) HOMO LUMO+1 HOMO-9 LUMO+1 0.20 (4%) HOMO-4 LUMO+1 -0.18 (3%)

c

b

HOMO-13 LUMO 0.76 (58%) 0.37 (14%) HOMO-11 LUMO 1 42,798 0.0051 B2 HOMO-4 LUMO+1 0.31 (9%) HOMO-6 LUMO+2 0.22 (5%) Based on Gaussian deconvolution of the experimental spectrum, as shown in

Values in parentheses represent the percent contribution of each one-electron

promotion to the intensity of the specified transition, calculated from the square of the transition coefficients times 100.

Spectroscopic Assignments Based on C2v Symmetry In comparisons between the energies and relative intensities of observed and calculated electronic transitions, precise agreement is unlikely, due to inherent limitations of the theory, the chosen model system, and the nature of the material which often contains a distribution of sites. As such, the calculations yield transition energies somewhat higher than the observed absorption


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maxima. Nevertheless, the calculated transition energies, relative intensities and symmetries are consistent with the spectroscopic data and, therefore, serve as a reliable guide in assigning the observed spectroscopic features. The lowest energy, fully-allowed transition in the calculated absorption spectrum is the 1A1 1B1 transition, whose energy is 24,300 cm-1. It is observed in the UV-vis spectrum at λmax = 22,800 cm-1. Consistent with the spectroscopic considerations discussed above, the emitting state is the 3

B1 excited state, populated by intersystem crossing from 1B1. The 3B1 state is predicted to be the

lowest energy triplet state. The energy required for direct promotion to it (not observed spectroscopically) is calculated to be 19,600 cm-1, corresponding to a singlet-triplet splitting of approx. 4,700 cm-1. However, since the observed emission maximum occurs at 13,600 cm-1, direct excitation to the triplet state must occur at a much lower energy than 19,600 cm-1. As described above, the estimated value of E00 for the low-lying triplet state of Cr(VI)/ZSM-11 silicalite is 16,500 cm-1. Similarly, for the tetrahedral chromate ion, the transition to the lowlying 3T1 state was observed in single crystal studies at ca. 16,000 cm-1.55 Thus, the calculated transition energy to the 3B1 triplet state seems anomalously high and, based on these experimental precedents, should be closer to 16,000 cm-1. In the case of the higher-energy allowed transitions, the agreement between the energies of calculated and observed spectroscopic bands is closer. The second allowed absorption band, observed at 29,100 cm-1, is assigned to the transition to a 1B2 state, with a calculated transition energy of 30,500 cm-1. Consistent with its polarization anisotropy, its transition moment is perpendicular to that of the first singlet excited state (1B1). The higher energy region of the absorption spectrum is characterized by two resolved bands. The spectral envelope is broad, suggesting several overlapping transitions, although deconvolution indicates that the envelope can be adequately described by just two Gaussian bands at 36,900 and 41,500 cm-1 (Figure 3). At even higher energies, contributions from impurities in the silica gel matrix start to interfere. The TD-DFT calculations predict allowed transitions to a 1B2 state at 36,400 cm-1 and a 1A1 state at 36,700 cm-1, both of which may contribute intensity in this region. However, the calculated oscillator strength for the 1A1 → 1B2 transition is small relative to the 1A1 → 1A1 transition, suggesting that the contribution of the former to the spectrum is minor. We also note that the transition to the 1A2 state, which is non-allowed in C2v symmetry, also occurs in this energy region (35,300 cm-1). In the lower symmetry environment which likely reflects the real system,


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this transition is allowed and may make a contribution to the spectrum. Finally, based on the TDDFT calculations, the highest-energy peak observed at 41,500 cm-1 has contributions from excitation to a 1A1 state at 42,400 cm-1 with the possibility of some contribution from the weaker transition to a 1B2 state at 42,800 cm-1.

Spectroscopic Consequences of Small Variations in the DioxoCr(VI) Site Geometry The heterogeneous Phillips catalyst (Cr(VI)/SiO2) has a distribution of dioxoCr(VI) sites (vide supra) whose symmetries are, for the most part, lower than C2v. DFT optimization of the geometry of the C2v symmetry structure I yielded structure II (Figure 7). Since it is possible for such cluster models to relax in ways not possible for a real site attached to a solid surface, structure optimizations of small chromasiloxane models may not represent silica structures accurately. The resulting geometry change is significant. In C2v symmetry, the Cr(VI) ion lies in the –O–Si–O–Si–O– mirror plane. Upon relaxation, the mirror plane is lost as the Cr(=O)2 group bends out of the plane. A minor degree of twisting removes the two-fold rotation axis, thus reducing the site symmetry to C1. Nevertheless, the relaxed geometry at Cr(VI) in II resembles structures reported in calculations of silica-supported Cr(VI) on extended silica surfaces.46 The TD-DFT calculations were repeated with the fully relaxed, C1-symmetry model II, in order to understand how its electronic spectrum differs from that of model I constrained to the C2v symmetry. The energies of the allowed transitions are given in Table S6. As expected, there are more allowed transitions due to the symmetry reduction, however, the oscillator strengths and energies of the most intense transitions are close in energy to those of I. For example, the lowest energy singlet-singlet transition is calculated to occur at 24,600 cm-1, compared to 24,300 cm-1 for the C2v structure. The energy of the lowest-lying triplet state of II also lies at a similar energy above the singlet ground state relative to the energy difference of I (20,200 vs 19,600 cm1

, respectively) and remains significantly higher than the experimental value (13,600 cm-1). This

finding further suggests that the anomalously high computed energy of the lowest triplet state is not an artifact of enforced C2v symmetry. The measured UV-vis spectrum (Figure 8a) is compared to spectra simulated from the TD-DFT calculations in Figure 8. Both the calculated spectra for models I and II reproduce the first three low-energy transitions observed in the experimental spectrum (Figure 8b,c). The spectra also reproduce well the position of the highest energy band at 41,500 cm-1, although the


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computed intensity relative to that of the other transitions is lower. This likely reflects additional contributions to the intensity of the high-energy region of the experimental spectrum from impurities in the silica. The spectrum of a dioxoCr(VI) site embedded in a larger, 8-membered chromasiloxane ring III is shown in Figure 8d. The spectrum also reproduces the energies of the first three transitions reasonably well, albeit with relative intensities that differ from both the experimental spectrum and from the simulated spectra of the models with smaller, six-membered chromasiloxane rings (models I and II). However, the highest energy band at ca. 41,500 cm-1 is not generated in the calculated spectrum for the larger ring structure. This finding suggests that larger, unstrained chromasiloxane ring structures may be less well-represented on the silica surface than smaller, strained, and presumably more reactive, chromasiloxane rings, however, it is difficult to make this claim rigorously. Nevertheless, the finding supports our previous XANES investigation, which showed that strained chromasiloxane rings are more abundant in more active Phillips catalysts.48


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Figure 8. Comparison of electronic absorption spectra: (a) experimental spectrum for Cr(VI)/SiO2, and simulated spectra based on TD-DFT calculations for (b) dioxoCr(VI) site with enforced C2v symmetry (model I), (c) dioxoCr(VI) site with C1 symmetry (model II), and (d) dioxoCr(VI) site with larger chromasiloxane ring (model III), and (e) a mono-oxoCr(VI) site (model IV). Vertical lines show the energies of the one-electron promotions that comprise the transitions. The half-width of each bands at half height was set to 4,000 cm-1 in the simulations.

Re-assessment of Putative Mono-oxoCr(VI) Site Contributions As discussed in the introduction, there have long been suggestions that mono-oxoCr(VI) sites represent a significant fraction of the monomeric Cr(VI) sites present on silica surfaces.12-13, 15-16

In the most recent proposed structure for these sites, the Cr(VI) ion has nominal C4v

symmetry, with a single terminal Cr=O bond and four bridging Cr–O–Si linkages to the surface


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(model IV in Figure 7).18-19 Its presence was deduced principally from the appearance of a single Raman band at 1020 cm-1, which was observed only under resonance conditions by exciting into the lowest energy transition 1A1 → 1B1. On the basis of a temperature-programmed reduction study, it was further claimed that these mono-oxoCr(VI) sites represent one-third of all Cr(VI) sites, with the remainder being the well-established dioxoCr(VI) sites. The electronic spectra described above provide no evidence consistent with the presence of a significant amount of a structurally distinct species such as a mono-oxoCr(VI) site. Such a species should have charge transfer bands with extinction coefficients similar to those of the dioxoCr(VI) site, readily observed in the absorption spectrum, as well as clearly separable from bands associated with the dioxoCr(VI) site due to their lack of coupling to the emissive state of the dioxoCr(VI) site. It is clear from the emission excitation spectrum in Figure 2b that all of the resolved bands in the absorption spectrum are electronically coupled to the emissive state. TD-DFT calculations were employed to obtain a more detailed assessment of the expected spectroscopic properties of a hypothetical mono-oxoCr(VI) site, represented by model IV (Figure 7). The calculated energies of its electronic transitions are given in Table S8, and a simulated spectrum for this species is shown in Figure 8e. It differs significantly from the experimental spectrum. For example, the lowest energy transition at λmax = 20,700 cm-1 is predicted to be by far the most intense band. Given the very large difference between the calculated and observed spectra, it is hard to rationalize the presence of such a species. While it might be difficult to detect at low concentrations, these results suggest that the proposed high abundance of mono-oxoCr(VI) sites is unlikely. This finding is consistent with DFT calculations by Handzlik et al., who reported that a variety of dioxoCr(VI) sites are significantly more stable than mono-oxoCr(VI) sites.46

Possible Contributions of Oligomeric Cr(VI) Sites Finally, electronic spectra of Cr(VI)/SiO2 have often been used to justify the presence, and even estimate the amounts, of dichromates (Figure 1b) or larger CrxOy oligomers.23,

56

In

particular, resolved high energy bands were assigned to specific oligomers and their intensities were used to quantify the amounts of each species. There are a number of problems with this approach. Firstly, band assignments were often made via comparison with discrete molecular species such as dichromate salts. Since electronic transitions of silica-supported Cr(VI) sites


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involve occupied orbitals associated with the support, the electronic transitions of the molecular species that have no such interactions with the silica support are necessarily inadequate to describe them. Secondly, and more importantly, many of the bands assigned to oligomeric chromates are very similar to those of the monomeric dioxoCr(VI) site characterized here. For example, dichromate salts, which are often used as a small molecule reference,19 show a series of broad UV-vis bands at ca. 27,000, 37,000 and 40,000 cm-1.57 Weckhuysen et al. monitored changes in the electronic spectra as a function of Cr loading on silica.58 A low Cr loading, typically at or 1 wt. % the monomeric species dominates and accounts for the observed bands (we note that the materials used here are at .008 wt %) . As Cr loading increases the observed bands change due to the formation of oligomeric species, however, even at 8 wt % there are no bands that are unambiguously characteristic of a specific species with many of the resolved bands being close in energy and relative intensity to those observed at low concentration. As such, separating their overlapping contributions in any quantitative way from those of the monomeric dioxoCr(VI) sites is highly problematic.

Conclusions The electronic spectrum of Cr(VI)/SiO2 was measured, and compared to the spectrum simulated based on TD-DFT calculations for a dioxoCr(VI) model with enforced C2v symmetry. The observed and computed energies agree well, and the assignments are rigorously consistent with key spectroscopic observations such as the excitation spectrum and the polarization anisotropy of the emission. The long-lived emission from the Cr(VI) sites is assigned to phosphorescence from a low-lying 3B1 state, populated by intersystem crossing from a 1B1 excited state. The observed red-edge effect in the electronic spectrum of the silica-supported material is direct evidence for a distribution of closely-related dioxoCr(VI) site structures that contribute to inhomogeneous broadening of the spectral bands. Consistent with this finding, TDDFT calculations using three similar but different models for the dioxoCr(VI) site show that the spectroscopic transitions do not vary dramatically in terms of their number and relative intensities, although they do differ slightly in energy. Several other findings are also significant. As the excitation spectrum indicates, all four resolved bands are coupled to the same emitting state, indicating that they all arise from a single species: no bands are unaccounted for, and there are no large deviations between excitation and


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absorption spectra. This allows us to effectively rule out the presence of species with vastly different structures, such as mono-oxoCr(VI) sites, at least in any significant amounts. This conclusion is further supported by the simulated spectrum of a mono-oxoCr(VI) site, which differs dramatically from the experimental spectrum. We conclude that all monomeric Cr(VI) sites have essentially the same primary coordination environment, i.e., (≡SiO)2CrO2. Small variations in the basic geometry based on this coordination environment contribute to the heterogeneity of the catalyst. The elucidation of such small but kinetically important differences for catalytically active sites in amorphous materials remains very challenging, for example, because of the difficulty in separating and assigning the broad overlapping UV-visible bands of similar components.

However, rigorous studies that make use of all of the available

spectroscopic and photophysical information can provide at least some reliable, definitive information about the structures of catalytic sites.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental methods for the synthesis of the Cr(VI)/SiO2 monoliths. Full experimental details of all spectroscopic procedures and computational methods. Computational results from the TDDFT calculations.

Acknowledgements We thank Prof. Eugene DePrince for helpful discussions. This work was carried out with funding provided by the Catalysis Science Initiative of the U.S. Department of Energy, Basic Energy Sciences (DE-FG02-03ER15467).


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A Reassessment of the Electronic Structure of Cr(VI) Sites Supported

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