Reassessment of the Resonance Raman Spectra - ACS Publications

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Do Mono-Oxo Sites Exist in Silica-Supported Cr(VI) Materials? Reassessment of the Resonance Raman Spectra Cristina Moisii, David Jeffcoat, Nathan M. Peek, Lambertus van de Burgt, Susannah L Scott, and Albert Edward Stiegman J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03403 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018

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The Journal of Physical Chemistry

Do Mono-oxo Sites Exist in Silica-Supported Cr(VI) Materials? Reassessment of the Resonance Raman Spectra

Cristina Moisii†, David Jeffcoat§, Nathan Peek§, Lambertus van de Burgt§, Susannah L. Scott‡, A. E. Stiegman§*

§ ‡

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

Departments of Chemistry and Chemical Engineering, University of California, Santa Barbara, CA 93106

†

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

* To whom correspondence should be addressed. Email: [email protected]. Tel 850-6446605.

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The monomeric, single-atom oxochromium species present on the surface of silica-supported Cr(VI) catalysts was characterized in detail using resonance Raman (RR) spectroscopy over a range of excitation wavelengths corresponding to the primary electronic transitions of Cr(VI)/SiO2. The findings resolve a long-standing controversy regarding the possible contribution of mono-oxoCr(VI) sites, (SiO)4Cr=O, postulated to co-exist with the wellestablished dioxoCr(VI) sites, (SiO)2Cr(=O)2. Density functional theory (DFT) calculations and a normal coordinate analysis conducted using a chromasiloxane model cluster confirm prior assignments of bands in the non-resonant Raman spectrum at 986 and 1001 cm-1 to the symmetric and anti-symmetric stretching modes, respectively, of the dioxoCr(VI) sites. For all excitation energies, the symmetric stretch shows apparent resonant enhancement. Since all of the electronic transitions are strongly allowed, this finding is consistent with A-term enhancement. UV excitation at 257 nm (into the high energy electronic transition, centered at 271 nm) also results in modest resonant enhancement of the anti-symmetric stretch, due to the low average symmetry of the surface sites. Excitation at 351 nm (into the electronic transition centered at 343 nm) results in a strong increase in the relative intensity of the anti-symmetric stretch, which is likely caused by B-term enhancement. Previously reported evidence for a mono-oxoCr(VI) site consists of a vibrational band observed at ca. 1011 cm-1 and assigned to the Cr=O stretch. However, the band is observed only upon excitation into the lowest energy electronic transition, at 439 nm. We show that excitation into this electronic transition causes photo-induced decomposition. The process depends on the laser power and duration of exposure, and it yields the band previously assigned to a mono-oxo species. The resonance Raman study reported here, in combination with our recent rigorous analysis of the corresponding electronic spectra, lead us to conclude that there is no reliable spectroscopic evidence for the existence of monooxochromate species in highly dispersed Cr/silica materials.


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Introduction Cr(VI) ions dispersed on amorphous silica at monolayer coverages or less are present in a widely-investigated commercial catalyst for light alkane dehydrogenation,1 and to the commercial Phillips polymerization catalyst for the production of high density polyethylene. In both systems, the active sites are formed upon reduction of the isolated, anchored Cr(VI) sites. For the polymerization catalyst, industrial practice involves reducing the sites using either CO or ethylene, depending on the desired polymer molecular weight distribution.2-3 Recent studies have shown that CO reduction proceeds first to a polymerization-inactive Cr(IV) intermediate, before reaching the Cr(II) state in the precatalyst.4-5 Subsequent exposure to ethylene results in quantitative re-oxidation of the Cr(II) sites to the Cr(III) active sites. In ethylene-reduced catalysts, re-oxidation is not temporally distinct from the initial reduction.4 Contemporary research into the Phillips catalyst has focused largely on elucidating the least well-understood aspect of the mechanism, namely, how the initial activation of ethylene by Cr(II) occurs to form the organoCr(III) active sites.4,

6

In contrast, the structure(s) of the

precursor Cr(VI)/SiO2 sites have been studied extensively over the course of several decades. A primary consideration in these early studies was the nuclearity of the Cr(VI) sites, and many papers discussed the distribution of monomeric, dimeric and oligomeric Cr(VI) surface species. Considerable efforts were made to identify and characterize these species, using techniques such as Raman spectroscopy and diffuse reflectance UV-vis spectroscopy.7-12 Early studies of the Phillips catalyst mechanism also considered the possible initiating roles of chromate (CrO42-) and dichromate (Cr2O72-) sites.13-14 Subsequent work involving authentic, isolated Cr(VI) sites prepared by a variety of alternative routes5,

15-17

led to the current consensus that monomeric

chromate sites are the precursors of the polymerization-active sites, although a contemporary review of the field acknowledges that dichromate may still be present in conventional Phillips catalyst formulations, and could play some role in their reactivity.3 The hypothesis that two monomeric Cr(VI) sites with distinctly different coordination geometries co-exist on silica surfaces is relatively longstanding. A 1993 study by Vuurman et al.18 investigated the nature of Cr(VI) dispersed on a variety of supports, including silica. Monomeric Cr(VI) sites on titania, alumina and zirconia surfaces were proposed to have one of two distinct structures: dioxoCr(VI) sites with two terminal Cr=O bonds and two bridging Cr-O-



M bonds to the support (Figure 1a), and mono-oxoCr(VI) sites with a single terminal Cr=O bond and four bridging Cr-O-M bonds (Figure 1b). These structural assignments were based on a comparison of bands observed in the IR and Raman to the spectra of small molecule analogs, specifically, CrOF4 for the mono-oxo site and CrO2Cl2 for the dioxo site. In the case of Cr(VI) supported on silica, the same study proposed the existence of two different structural sites but, since the frequencies of the observed bands were significant lower than on the other supports, the structures were assigned to the di-oxo site and a surface bound “CrO3” species, the latter species was postulated based on the comparison of the vibrational bands to those observed in aqueous CrO42-. In a subsequent study, some of the same authors proposed a slightly different structure on the silica surface: in addition to the aforementioned di-oxoCr(VI) site (Figure 1a), they suggested the presence of a four-coordinate, C3v-symmetric mono-oxoCr(VI) species with one terminal Cr=O bond and three Si–O–Cr bridging bonds. However, since the formulation of (≡SiO)3Cr=O as a neutral species implies a Cr(V) oxidation state, it is unlikely.19

Figure 1. Two postulated structures for the monomeric Cr(VI) sites present on oxide supports (M = Si, Ti, Al and Zr) after high-temperature calcination: (a) the widely-accepted dioxoCr(VI) site, with approx. C2v symmetry; and (b) a proposed mono-oxoCr(VI) site, with approx. C4v symmetry.


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The idea that both monomeric mono-oxo and di-oxoCr(VI) species co-exist on silica surfaces reappeared in later Raman studies by Lee and Wachs (L&W).20-21 In the first of these studies, two high frequency bands were observed at 982 and 1011 cm-1 in resonance Raman spectra collected with 442 nm excitation. The band at 982 cm-1 was assigned to the symmetric stretch of the dioxoCr(VI) species, while the band at 1011 cm-1 was attributed to the terminal Cr=O stretch of the mono-oxoCr(VI) species. These assignments were supported by an observation that reduction with H2 at 500 °C caused the disappearance of the 982 cm-1 band while the 1011 cm-1 remained, leading to the reasonable conclusion that the two bands are associated with different species.20 The assignments were further investigated by

18

O isotopic

18

labeling of the Cr(VI) sites via exposure to H2 O. Raman spectra were collected on-resonance with 325 nm excitation, which excites a different electronic state from that excited in the previous study (422 nm). The reported spectra did not contain a strong band at 1011 cm-1, which led the authors to claim that the dioxoCr(VI) species was observed selectively; the monooxoCr(VI) band was present as a very weak shoulder. Moreover, they argued that only the totally symmetric stretch of the dioxoCr(VI) species is resonantly enhanced. Upon 18O isotopic labeling, the symmetric stretch of the dioxoCr(VI) site at 982 cm-1 shifted to 935 cm-1, while the barely discernable band of the mono-oxoCr(VI) site shifted from 1011 to 967 cm-1.21 Based on simple calculations using the diatomic oscillator approximation, these isotopic shifts were deemed to be consistent with their assignments to a dioxo and mono-oxo species, respectively. More recently, Chakrabarti and Wachs (C&W) have revived and greatly expanded claims for the importance of the mono-oxo species.22-24 Consistent with the original studies by L&W, the newer studies report a band at ca. 1020 cm-1 in the resonance Raman spectrum of Cr/SiO2, observed only using 442 nm excitation, which is assigned to a mono-oxoCr(VI) species. In a study similar to the original work by L&W, the persistence of the 1020 cm-1 band was noted after reduction of the material in H2 at 420 °C. In a temperature programmed reduction study, two overlapping processes were observed, at 420 and 489 °C. They were assigned to reductions of the dioxo and mono-oxo species, respectively. Deconvolution and integration of the data led to the conclusion that the mono-oxo species accounts for 1/3 of the total Cr(VI) present on the surface.23 In order to investigate the mechanistic details of reactions involving supported metal catalysts, it is essential to have a clear understanding of the initial structures of the surface sites.



At low metal loadings, which often give the most active catalysts on a metal-atom basis, monomeric sites usually dominate.3, 25 An important question is whether all of the metal oxide sites share the same basic coordination geometry, or whether multiple sites with significantly different coordination numbers and geometries are present. In the first case, some heterogeneity in the metal coordination environment is expected due to variability in local surface structures, which is manifested in inhomogeneous broadening of spectroscopic bands (both electronic and vibrational). In the second case, different coordination environments are likely to have very different spectroscopic signatures. In both cases, the reactivity may vary considerably from site to site. The preferred ligand environments for a particular transition metal ion depend on its size and oxidation state. Geometries can be reliably predicted using long-established inorganic chemical principles. Such principals allow us to assess the likelihood of proposed structures for dispersed Cr(VI) sites on oxide surfaces, where oxygen atoms derived from the support serve as ligands to the transition metal cation. Thus, the small ionic radius of Cr(VI) (0.26 Å) relative to O2- (1.40 Å) gives a Cr/O radius ratio of 0.19, which strongly favors four-coordinate geometries.26-27 Furthermore, the small size of the metal ion allows it to form strong, highly covalent Cr=O π-bonds, which further disfavors the transformation of dioxoCr(VI) complexes to mono-oxoCr(VI) complexes with higher coordination numbers.28 For example, hydration of CrO2(OH)2 to give the (unknown) CrO(OH)4 was computed to be strongly endothermic.29 Indeed, octahedral Cr(VI) compounds are extremely rare.28 CrO3 itself consists of chains of tetrahedral Cr(VI) ions in the solid state, and all of the known molecular silonates CrO2(OSiR3)2 are four-coordinate.30-32 Among the oxohalides, CrOF4 is the only known mono-oxoCr(VI) compound isostructural with the proposed mono-oxo site in Figure 1b. However, it is unstable and must be synthesized under relatively extreme conditions, either by the fluorination of CrO3 or by reaction of CrO2F2 with KrF2.33-34 Its structure, determined by vibrational spectroscopy in the gas phase and at low temperature in a N2 matrix, is square pyramidal with C4v symmetry.35-36 Consequently, the presence of an abundant species with this very rare and unstable geometry in readily-prepared Cr/SiO2 materials is unexpected, and claims for it require strong evidence. The mineral yedlinite is the only material that is purported to contain Cr(VI) in octahedral coordination, although the oxidation state of the chromium was not fully established in that work.37 While the structure of an oxometal site attached to a surface may differ from its structure


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in a bulk oxide matrix, the former is unlikely to adopt a higher coordination number than the latter. In recent DFT modeling by Handzlik et al., various mono-oxo and dioxoCr(VI) sites were compared on both crystalline and amorphous silicas, and the mono-oxo sites were consistently found to be less energetically favorable.38 Finally, while the [O4Cr=O] structure is highly unlikely on the basis of known Cr(VI) chemistry, mono-oxo complexes with higher coordination numbers higher than four are more common for Cr in oxidation states lower than VI, e.g., the Cr(V) complexes CrOCl4- and CrOCl52-.39 In addition, mono-oxo Cr(IV) sites have been suggested to arise from the reduction of the silica-supported Cr(V) with CO during activation of the oxidized Phillips catalyst, or by oxidation of Cr(II) sites in the reduced Phillips catalyst.4, 40 More generally, the heavier group 6 metals in their maximum oxidation state (VI) are well-known to form both dioxo and mono-oxo complexes, and the radius ratio rule predicts a preference for octahedral coordination for both of these metal ions. Nevertheless, there are many well-characterized compounds with lower coordination numbers, particularly where there is strong metal-ligand multiple bonding.39,

41

Thus the co-existence of both types of sites for oxide-supported Mo(VI) and W(VI) materials is more plausible.42 However, we limit the scope of this study to Cr(VI) surface chemistry. Prior X-ray absorption studies of silica supported Cr(VI) have yielded structural data consistent with four-coordinate Cr(VI) sites. In particular, careful analysis of the EXAFS showed two short Cr=O scattering paths, consistent with dioxoCr(VI) sites.16 If the mono-oxo sites were present at the suggested level of 25% on the surface, it is reasonable to expect that both would be required in the curvefit model. Similarly, when Raman spectra collected under non-resonant conditions were analyzed in some detail, all of the observed modes were assignable to the dioxo structure. In particular, Dines and Inglis (D&I) assigned the two primary bands at 986 and 1001 cm-1 to the symmetric and anti-symmetric stretches, respectively, of the O=Cr=O moiety, on the basis of their Raman study coupled with DFT calculations.43 Their assignment of a band at 919 cm-1 to a mode largely dominated by the Si–O–Cr stretch was also supported by their DFT calculations. Further non-resonant Raman studies by Moisii and Stiegman (M&S) included both polarization measurements and isotopic labeling. The results were generally quite consistent with the D&I assignments, although the polarization anisotropy suggested that the origin of the high frequency band may be more complex (i.e. contain other modes) in addition to the antisymmetric stretch.16 In contrast, claims for the existence of the highly unusual mono-oxoCr(VI)



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sites rest largely on the assignment of a single Raman band at ca. 1010 cm-1, which is observed only under conditions of resonance enhancement at 442 nm.20, 22-23 Thus, a detailed analysis of the resonance Raman (RR) spectra of the Cr(VI)/SiO2 sites was undertaken to evaluate this experimental evidence.

Experimental and Computational Methods Materials. Cr/SiO2 xerogels containing up to 3.0 mol % chromium [(Cr/Cr+Si) x 100 %] were prepared by the sol-gel method, adapting a previous procedure.16,

44

An alcoholic solution of

tetramethyl orthosilicate (TMOS) and the desired quantity of aqueous chromic acid were added to a water/ethanol mixture to form the sol. The stock solution of chromic acid was made using 99.9% CrO3 (Aldrich) and deionized water (10-18 Ω, Barnsted E-Pure system). The solution was sonicated for 5 min in order to obtain a homogeneous mixture. The solution was dispensed into a

1 cm-square polyethylene cuvette, sealed, and allowed to gel, age and

evaporate for about 3-4 months. Each sample was then dried in a programmable furnace whose temperature was ramped to 100 °C at a rate of 0.5 °C/h and held for 72 h. The temperature was then ramped to 500 °C and maintained for 36 h. Finally, the samples were cooled slowly to room temperature over a period of 95 h. A typical B.E.T. surface area of a calcined Cr/SiO2 xerogel is 335 m2/g. Prior to use, each Cr/SiO2 xerogel was fully oxidized to Cr(VI) in flowing O2 (UHP grade) at 500 °C for 36 h. Calcined xerogels were handled under anhydrous and anaerobic conditions at all times. Monoliths were stored under vacuum, in a sample holder Raman Spectroscopy Raman spectra were collected using a micro/Raman spectrograph, JY Horiba LabRam HR800. Non-resonant Raman spectra were excited by a TUI Optics DL 100 grating-stabilized diode laser emitting 80 mW of power at 785 nm. The power at the sample was 6 mW. The spectrograph used a holographic notch filter to couple the laser beam into the microscope (Olympus BX30) by total reflection. The beam was focused on the sample through a microscope objective 5X(Olympus N. A. 0.10). Backscattered radiation was collected by the objective, and laser radiation was filtered out by the notch filter with Raman scattering coupled into the spectrograph through a confocal hole. A 76 mm square 600 line/mm grating dispersed the Raman scatter onto a 1024 X 256 element open electrode CCD detector


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(Wright CCD30-11-0-275) having 26 µm-square pixels thermoelectrically cooled to -70 °C. The detector had a quantum efficiency of 45-22 % in the range 785-900 nm. Resonance Raman Spectroscopy. Resonance Raman spectra were collected on the microRaman system described above. Excitations at 351.1, 363.8, or 457.9 nm were performed with a Coherent I-308 argon ion laser. UV excitation at 257 nm was performed with an Innova Coherent 300C frequency-doubled argon ion laser. The beam was focused on the samples through a microscope objective 15´UV for the 257.2 and 351.1 nm excitations, and 40´ for the 457.9 nm laser line. Gratings with 3600, 2400 and 1800 grooves/mm were used for the 257.2, 351.1 and 457.9 nm excitation laser lines, respectively. The spectral resolution achieved was 6-10 cm-1. The laser power was kept at 50 mW for 257.2 nm and 65 mW for the 351.1 nm excitation. The Raman spectra collection time varied between 5 to 18 h, depending on the strength of the Raman signals for each sample. For the studies at 457.9 nm excitation, the power at the sample, after passage through the 40´ objective, was measured with an Optima 2A power meter. At a laser power 250 mW, the power at the sample was determined to be 15 mW. The power dependent study was carried out by setting the laser to 250 mW and adjusting the power at the sample by means of neutral density filters. Cr/SiO2 xerogels were kept under vacuum for data collection in specially designed spectroscopic cells. Suprasil quartz cells were used for spectra collected at 351.1 and 457.9 nm, while sapphire cells were used for spectra collected at 257.2 nm. The latter were necessary because UV irradiation caused the quartz windows to make an anomalously large contribution to the spectrum. DFT-Calculated Vibrational Spectra. The dioxo chromasiloxane model was built in Avogadro 2 0.8.0, constrained to C2v symmetry and then the bond lengths were optimized. Cartesian coordinates from the Avogadro.cml files were pasted into .com files on the FSU Research Computing Center (RCC) server and formatted along with accompanying .cmd files to use the Gaussian v09 software on the HPC using the SLURM submission format. The B3LYP density functional and 6-311G* basis set were the standard for the calculations in Gaussian v09.70 The Gaussian .log files containing the calculated IR and Raman frequencies were used in Avogadro to observe the animations associated with each frequency mode in order to identify modes involving stretching and bending of the oxo ligands. The Gaussian checkpoint file, reformatted in the .fchk format, was imported into VIBRATZ 2.0 for Linux (Shape Software)53



to convert the Cartesian coordinates from the Gaussian output file into internal coordinates for normal mode analysis of each structure.

Results and Discussion Spectroscopic studies of dispersed metal oxide materials are typically performed with powder samples, via the collection of diffusely scattered light. In the case of electronic spectroscopies, this approach generally precludes the determination of extinction coefficients. In the case of Raman spectroscopies, it also yields poor signal-to-noise ratios and precludes the measurement of reliable polarization ratios. For the specific case of silica-supported metal oxides, we can largely overcome these problems through the use of transparent metal-containing silica monoliths of high optical quality, made through the sol-gel process. These materials, which we pioneered,44-47 allow electronic spectra to be acquired in transmission mode and yield Raman spectra with very good signal-to-noise ratios. Many studies have shown these materials to have spectroscopic and catalytic properties that are identical to catalysts made by conventional wet impregnation. For this study, we dispersed Cr(VI) ions in SiO2 monoliths using a previously published procedure, involving co-condensation of chromic acid with tetramethylorthosilicate.4, 16

Normal mode assessment. The seven-atom dioxoCr(VI) site shown in Figure 1a, with enforced C2v symmetry, has 16 normal modes of which 6 are stretching modes (3 a1, 2 b1 and b2). The 10-atom mono-oxoCr(VI) site shown in Figure 1b, with enforced C4v symmetry, has 24 normal modes of which 9 are stretching modes (3 a1, 2 b1 and 2 e). Since two of these modes are degenerate, only 7 modes are potentially observable, although the lower symmetry in the real material will lift the degeneracy. For supported metal oxides with loadings low enough to ensure the dominance of isolated, monomeric sites, only stretching frequencies of appropriate intensity and high enough energy to avoid overlap with phonon modes of the silica substrate are usually observed in the Raman spectrum. For silica-based materials, silica modes with frequencies above ca. 600 cm-1 are weak in the Raman, facilitating the observation of metal-oxygen stretches (usually in the range 900-1100 cm-1). Raman spectroscopic analysis. In order to draw meaningful conclusions about the degree of resonant enhancement in resonance-Raman spectra, spectra must also be recorded under nonresonant conditions. The Raman spectrum of a Cr(VI)/SiO2 xerogel monolith, acquired using


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784.5 nm excitation, is shown in Figure 2a in the region 700–1200 cm-1. This region of the spectrum contains several bands associated with the Cr species; all bands at lower frequencies (i.e., < 700 cm-1) are readily assigned to intrinsic modes of the silica.48 A rigorous deconvolution of the spectrum in shown in Figure 2b. The bands at 800/844 and 1036/1176 cm-1 are the transverse and longitudinal contributions of the symmetric and anti-symmetric silica modes, respectively.

48-49

The band at 983 cm-1 is the symmetric O3Si-OH stretch of the surface

silanols.50-51

Figure 2. (a) Raman spectra of a Cr(VI) /SiO2 xerogel monolith (0.5 mol % Cr, blue) and a pure silica monolith (magenta), acquired using 785 nm excitation (the spectral intensities are not scaled, but the baselines are corrected), and (b) deconvolution of the Cr(VI)/SiO2 spectrum, with peaks assigned to the Cr site and to the silica support labeled in blue and black, respectively. The experimental spectrum is shown as a dotted line. Three Cr-specific modes appear in the spectral region from ca. 800 to 1000 cm-1 (Figure 2b). As discussed above, the most intense band at 986 cm-1 is assigned to the O=Cr=O



symmetric stretch, while the resolved shoulder at ca. 1002 cm-1 is the corresponding antisymmetric stretch. In C2v symmetry, and using the Mulliken axis convention for this point group, these are the a1 and b1 modes, respectively.52 The weak band at ca. 919 cm-1 is assigned to a mode involving a significant amount of Si-O-Cr stretching.43 Finally, a weak band at 745 cm-1 emerges from the deconvolution. It is clearly visible in the spectrum and is therefore not an artifact of the deconvolution. It is not present in the silica spectrum and may therefore be another, as-yet unassigned, mode associated with Cr. Computational predictions. To assist in interpreting the Raman spectra, and to reevaluate prior assignments, the normal modes associated with the Cr-oxo vibrations were computed using the chromasiloxane cluster model of Dines and Inglis, Figure 3. The Hessian of the DFT output was converted into internal coordinates using the Vibratz53 vibrational analysis program, which generates a generalized valence force field (GVFF) (Table S1). In addition to a normal coordinate analysis,54 this approach also allows the assignment of previously undetected bands that become visible only upon resonance enhancement. We constrained the model compound to C2v symmetry to avail ourselves of group theoretical analysis tools. Calculations in which the structure is allowed to relax result in a lower symmetry C1 structure, the vibrational modes of which were calculated by D&I.43 A comparison of their calculated frequencies and predicted normal modes for the primary stretches of the dioxoCr group shows close agreement with our results.


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Figure 3. Chromasiloxane cluster, constrained to C2v symmetry, used in DFT calculations of the vibrational energies of a dioxoCr(VI) site.

As expected for a small cluster model, some of the normal modes are dominated by atomic motions of the siloxane sub-structure. Modes with minimal contributions from Cr or atoms in the first coordination sphere of Cr were excluded from the analysis below, since they are artifacts of the model. In the spectral region from 800 to 1100 cm-1, four computed vibrational modes have normal coordinate changes dominated by the atomic motion of oxygen atoms bonded to Cr. Scaling of DFT-computed vibrational frequencies is required for agreement with the experimental results, due to the inability of the calculation to fully account for anharmonicity effects.55 The scaled frequencies agree extremely well with the three modes observed spectroscopically, Table 1.56 The modes with computed frequencies of 1003 and 989 cm-1 are anti-symmetric and symmetric stretching (b1 and a1, respectively, in C2v symmetry), and are dominated by motions of the terminal Cr=O atoms (Figure 4a,b). The mode at 916 cm-1 is symmetric (a1) in C2v symmetry. It consists largely of motion of the Si-O-Cr bridging bonds, with some torsional movement of the O=Cr=O group (Figure 4c). Table 1. Comparison of Vibrational Frequencies (cm-1) Involving the Cr Ion, Observed by NonResonant Raman Spectroscopy for Cr(VI)/SiO2 and Computed for a DioxoCr(VI) Model Cluster Normal Mode

18

Unlabeled

Observed

O-Labeled

Calculated

Observed

a,b

Calculated a,b

b1

1002 a

1003

967 d / 954 e

961

a1

986 a

989

935 d / 943 e

941

a1

919

a

916

n.o.

916

b2

n.o.

872

n.o.

709

b2

745 a

717

n.o.

627

a1

396 c

419

383d

386

n.o. = not observed. a This work. b Scaling factor = 0.907. c From ref20 dFrom ref 20. e From ref 16



The relative frequencies of the symmetric and anti-symmetric stretches are governed in large part by the Cr=O/Cr=O interaction force constants (Table S1), which are off-diagonal terms in the Wilson FG matrix.57 Specifically, the coupling between the symmetric and antisymmetric stretches is a function of the cosine of the O=Cr=O angle (although other interactions terms also contribute). The agreement between the measured and calculated frequencies depends on how similar the calculated bond angles are to those present in the material. Conversely, the very good agreement between the calculated and experimental frequencies observed here suggests that the calculated O=Cr=O angle, 109.1° (Table S2), is likely close to the actual value. The calculations also predict a mode at 872 cm-1. It appears to be a stretching mode of b2 symmetry primarily involving the Si–O–Cr internal coordinates, offset by motion of the O=Cr=O unit. This band is likely obscured by the silica mode in the non-resonance-enhanced Raman spectrum, although it may become visible when resonance-enhanced. The calculations also generate a b2 mode at 717 cm-1, which may correspond to a weak band observed at 745 cm1

, although the agreement between calculated and the experimental energies is not as good as

was found for the higher frequency modes. The normal coordinate analysis suggests that this mode involves anti-symmetric motion of all six atoms that comprise chromasiloxane ring (Figure S1a). While it appears to be a reasonable assignment for the 745 cm-1 band, the mode also includes a significant contribution from motions involving the cluster-terminating hydroxyl groups. This is an artifact of the model, and it likely accounts for the underestimation of the vibrational energy. It also makes the assignment somewhat less certain. In general, the limitations of small cluster models are expected to be more severe for low frequency vibrations, which involve coupling of the metal site to the support. Rigorous analyses must take such limitations into account.


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Figure 4. Depiction of atomic motions for the calculated high frequency stretching modes of a dioxoCr(VI) model structure, constrained to C2v symmetry. Finally, we note that L&W reported a low frequency Raman band at 396 cm-1. It was observed with 325 nm excitation after subtraction of the silica background, and assigned to a O=Cr=O bending mode.21 Raman bands obtained solely through background subtraction should be viewed with caution, since Raman intensity can vary from sample to sample. Furthermore, the presence of intense silica modes in this region of the spectrum require a subtraction of two large numbers. Nevertheless, in this case, the calculations combined with normal mode analysis identify a bending mode (a1) at 419 cm-1, in good agreement with the observed band (Figure S1b). In general, isotopic labeling studies are the most definitive way to verify normal mode assignments. Previously, L&W. In the first study,

18 18

O labeling of Cr(VI)/SiO2 was performed by both M&S and by

O was introduced though a sequential process in which Cr(VI) was

reduced in H2 and reoxidized by 18O2, while the second used H218O to incorporate

18

O into the

coordination sphere of Cr(VI) via hydrolysis. In both cases, partial exchange initially yielded intermediate degrees of substitution which added complexity to the spectrum, and made it difficult to observe the predicted isotopic shift of the weak anti-symmetric stretch. However, in both studies the exchange eventually reached a limit in which the spectrum no longer changed significantly, and both gave similar values for the frequencies of the two fundamental modes associated with O=Cr=O stretching upon near-complete

18

O substitution. Specifically, M&S

-1

reported an intense band at 954 cm and weak band at 943 cm-1,16 while L&W reported their frequencies to be 967 and 935 cm-1.21 It is worth pointing out, however, that in order to use isotopic labeling to support spectroscopic assignments, the expected change in frequency must be evaluated properly using the entire normal mode. Often the frequencies of isotopic shifts are determined using a diatomic oscillator approximation instead.21,58 For supported metal oxides, this is rarely a good approximation since the normal modes, and hence their energies, include significant contributions from motions of all of the atoms in the metal coordination sphere.59-60 The frequencies of the symmetric and anti-symmetric stretches for the

18

O-labeled sites were

calculated by DFT, as described above. The frequencies were scaled by the same factor used for 16

O, and the agreement is good (Table 1). The fact that the normal modes of the dioxo structure



primary account for the primary observed Raman bands, combined with the accurate predictions of isotopic shifts using the same structure, support the assignment of the weak high frequency band at 1003 cm-1 to the antisymmetric CrO2 stretch and the intense band at 986 cm-1 to the symmetric CrO2 stretch. They are entirely consistent with the original D&I assignments.43 For the O=Cr=O bending mode observed by L&W at 396 cm-1 after background subtraction and reported by them to shift to 383 cm-1 upon

18

O substitution shifts to 383 cm-1, we calculate an

isotopic shift to 386 cm-1. It is clear from the analysis of the non-resonant Raman spectrum that all of the observed bands are assignable to normal modes associated with the dioxoCr(VI) site, and in particular, the bands at ca. 986 (strong) and 1001 cm-1 (weak) are fully consistent with the symmetric and antisymmetric stretches, respectively, of that site. While it could be argued that the absence of bands in the non-resonant spectra assignable to a mono-oxoCr(VI) site is somehow expected because it is a minor species that can only be observed through resonant enhancement. Such a claim is spectroscopically unlikely, particularly at the surfaces loadings claimed for this species. The totally symmetric metal-oxo stretches for known mono-oxo species such as the vanadyl site are quite intense and observed even at relatively low concentrations (i.e. .5 mol %).46, 60-61 As such, the claim that these species can only be detected under resonant condition and only at one specific wavelength is, in our view, improbable. Resonance Raman enhancement. Resonance Raman scattering differs from non-resonant scattering because the wavelength used to generate the Raman spectrum also excites one or more electronic transitions. Intensities of vibrational modes that are coupled to these electronic transitions can be significantly enhanced. For low-symmetry oxoCr sites, whose electronic spectra consist largely of fully-allowed charge transfer transitions,62 the primary mechanism for resonance enhancement is A-term (Franck-Condon) scattering, in which the electronic transitions couple with and resonantly enhance totally symmetric vibrational normal modes. Although enhancement of non-totally symmetric stretches can also occur through FranckCondon scattering if there is a change in symmetry between the ground and excited state, the most common enhancement mechanism for these modes is B-term (Herzberg-Teller) scattering. In the latter mechanism, non-totally symmetric modes are enhanced by vibronic coupling between the resonant excited state and another excited state of different symmetry.63-64 The mechanism is assigned more readily for small molecules with well-defined point groups,


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compared supported catalysts with a distribution of local site symmetries. For silica-supported chromates, the distribution may include sites with or close to C2v symmetry, although the majority of sites are expected to have much lower symmetries. In principle, for sites with C1 symmetry, all modes can be A-term resonance-enhanced. To explore the resonance Raman spectrum of Cr(VI)/SiO2, four laser wavelengths (257, 351, 364 and 458 nm) were selected to excite electronic transitions of the material (Figure 5). The excited states associated with the four primary UV-vis transitions were assigned in our recent study,62 based on a detailed UV-vis spectroscopic study and time-dependent density functional calculations.

Figure 5. UV-vis absorption spectrum of a Cr(VI) /SiO2 xerogel (0.005 mol % Cr), showing the laser excitation wavelengths used for non-resonant and resonant Raman excitations.

Changes that occur in the Raman spectrum in the spectral region associated with the Croxo stretching modes are shown as a function of excitation wavelength in Figure 6. Resonant excitation (Figure 6b-d) results in a general increase in the relative intensities of the bands at 986 and 1001 cm-1 compared to the non-resonant spectrum (Figure 6a), judged relative to the intensity of the D1 ring mode of silica at 606 cm-1. The intense band at 986 cm-1, associated with the symmetric O=Cr=O stretch, is the most prominent Cr-related vibration for all excitation wavelengths. While we expect the intensity of the symmetric stretch to be A-term-enhanced, the effect appears to be modest.65 However, excitation into the two lowest-energy electronic transitions using the laser lines at 458 and 351 nm clearly provides resonant enhancement of the other bands, including the anti-symmetric O=Cr=O stretch.



Figure 6. Raman spectra of a Cr(VI) /SiO2 xerogel (1.0 mol % Cr), recorded at three different excitation wavelengths: (a) 785 nm (off resonance), (b) 458 nm (on resonance), (c) 364 nm (on resonance), and (d) 351 nm (on resonance), as well as (e) the spectrum of a Cr(VI) /SiO2 xerogel (0.5 mol % Cr) excited at 257 nm (on resonance).

Resonance Raman with 257 nm excitation. Raman spectra of Cr(VI) /SiO2 (0.5 or 1.0 mol % Cr), acquired using 257 nm excitation, are compared with the spectrum of unmodified SiO2 in Figure 7. The spectral quality for the Cr-containing materials is generally poorer than the quality of the spectra for the same materials at other excitation wavelengths (Figure 6). Moreover, the primary Cr-O vibrations are observed more clearly at the lower Cr loading. Both effects arise


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from a common problem inherent in exciting electronic transitions this far into the UV: the strong electronic absorption of the Cr/SiO2 material results in extensive self-absorption of the Raman scattering. In Figure 7b, where the Cr loading is lowered to from 1.0 to 0.5 mol %, the self-absorption effect is reduced and the spectral features are better resolved.

Figure 7. Raman spectra recorded with 257 nm excitation, for two Cr(VI)/SiO2 materials: (a) 1.0 mol % Cr, and (b) 0.5 mol % Cr, as well as (c) unmodified SiO2. * is an artifact from the sapphire cell.

The resonance Raman spectrum of Cr(VI) /SiO2 acquired with 257 nm excitation appears to show enhancement of the symmetric O=Cr=O stretch, although the magnitude of the effect is difficult to judge because the silica bands, whose intensities are typically invariant and are used to assess the relative intensity of the Cr-O modes, are weak and change with Cr loading (due to the self-absorption process described above). In addition, the intensity of the anti-symmetric



O=Cr=O stretch appears to be somewhat enhanced relative to that of the symmetric stretch (although there is considerable overlap of the bands). The greater relative enhancement allows somewhat better resolution of the anti-symmetric stretch than is possible under non-resonant conditions. The Si–O–Cr mode at 919 cm-1 is not visible, although there is evidence for a shoulder at ca. 878 cm-1. This band may be the b2 mode (Figure 4), whose calculated frequency is 872 cm-1 (Table 1). Resonance Raman with 351 and 364 nm excitation. Irradiation at 351 nm excites the second resolved electronic transition of the Cr sites, at close to the band maximum (Figure 5). Our prior study of electronic structure assigned it to a transition to a 1B2 excited state.6 Resonance Raman spectra acquired using 351 nm excitation for a range of CrVI/SiO2 xerogels with varying Cr concentrations are shown in Figure 9. The spectra show very strong resonant enhancement of the anti-symmetric O=Cr=O stretch at 1001 cm-1, when compared to the totally symmetric stretch. The band at 919 cm-1 assigned to the Si–O–Cr stretch, weak though resolved in the non-resonant spectrum, is not obvious in the resonance Raman spectra. However, there is a reasonably well-defined shoulder at 874 cm-1, on the high-energy edge of the silica mode at 820 cm-1. The band was observed with 257 nm excitation (Figure 7), and was assigned to the b2 mode in Figure 4, but it is more strongly enhanced here and is consequently better resolved. Resonant enhancement of an antisymmetric mode typically arises from B-term enhancement, although it is not completely clear how it would occur in this case. In C2v symmetry, the product of the irreducible representations of the b1 vibrational mode and the B2 resonant state generates an A2 representation, which is the required symmetry of the excited state for B-term enhancement.66 Our prior TD-DFT calculations of the Cr(VI)/SiO2 system predict an 1A2 state in reasonable energetic proximity at 274 nm, but the transition is not allowed in C2v symmetry, precluding it from participating in B-term enhancement.62 Presumably, given the heterogeneous nature of the amorphous silica surface, a distribution of slightly different structures exists and some subset of these has the requisite states that can mix to resonantly enhance the antisymmetric stretch.


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Figure 8. Raman spectra recorded with 351 nm excitation, for Cr(VI) /SiO2 xerogels containing (a) 3.0 mol % Cr, (b) 1.0 mol % Cr, and (c) 0.5 mol % Cr, as well as (d) unmodified SiO2. Excitation at 364 nm, which is also in resonance with the 1B2 excited state albeit at lower energy, shows weaker enhancement of the anti-symmetric O=Cr=O stretch than we observe using 351 nm excitation (Figure 9). This arises due to the smaller extinction coefficient on the low energy side of the transition, and from changes in the difference between the excitation frequency and the frequency of the vibronic transition itself. It is worth noting that, in the previous

18

O-labeling study, L&W’s use of a higher excitation energy (325 nm) also did not

result in strong enhancement of the anti-symmetric stretch.21 This excitation falls in a region of the electronic spectrum where the extinction coefficient is much lower, and where considerable overlap of the transitions allows more than one excited state to be populated. Irrespective of the precise origin of resonance enhancement for the antisymmetric stretch, it is interesting that for a distribution of site symmetries, all based on the general dioxoCr(VI) site structure, a specific excitation wavelength can uniquely enhance this mode.



Figure 9. Comparison of Raman spectra (recorded with 364 nm excitation) for two Cr(VI) /SiO2 xerogels containing (a) 1.0 mol % Cr, and (b) 0.5 mol % Cr, as well as for (c) unmodified SiO2. Resonance Raman with 458 nm excitation. Irradiation at this wavelength is in resonance with the lowest energy spectral band of Cr(VI) /SiO2, which was assigned in our previous study to a transition to a 1B1 excited state.62 Here the resonant enhancement is expected to be primarily A-term enhancement of the symmetric O=Cr=O stretch, since there are no electronic transitions of appropriate symmetry to couple to the anti-symmetric O=Cr=O stretch. Thus the resonance Raman spectrum of Cr(VI) /SiO2 (1.0 mol % Cr), collected at an excitation power of 0.2 mW, shows enhancement of the symmetric O=Cr=O stretch at 986 cm-1 with little obvious enhancement of the corresponding anti-symmetric stretch (Figure 10a). Significantly, there is no evidence for a band at ca. 1020 cm-1 that might be attributed to a mono-oxoCr(VI) species.


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Figure 10. Raman spectra recorded with 458 nm excitation, for 1.0 mol % CrVI/SiO2 xerogel, acquired using various laser powers at the sample: (a) 0.2, (b) 2 and (c) 15 mW. The collection time for each spectrum was 1 min.

As the laser power at the sample was increased, the spectrum changed dramatically until, at 15 mW, two sharp bands on the high-energy side of the symmetric stretch became clearly evident at 1012 and 1027 cm-1. In addition, a new band at 786 cm-1 was observed at high power. This band does not appear in the spectra reported by L&W, presumably because it was obscured by subtraction of the silica background.20, 23 The emergence of these new bands as a function of excitation power suggests a photo-induced decomposition and/or rearrangement of the Cr(VI) sites that creates one or more photoproducts that give rise to the new Raman lines. Further confirmation that the new spectral features emerge from a photochemical process was obtained by varying the exposure time at a fixed power. Figure 11 shows the change in intensity of the 986 cm-1 band for the dioxoCr(VI) species relative to the new features as a function of irradiation time.



Figure 11. Raman spectra recorded with 458 nm excitation, for a Cr(VI) /SiO2 xerogel ( 1.0 mol % Cr) irradiated for (a) 1.0 min, (b) 15 min, (c) 30 min, (d) 50 min, and (e) 70 min, with 15 mW laser power at the sample.

A further experiment was carried out to determine whether the photochemical process is reversible. The Raman spectrum of a Cr(VI) /SiO2 xerogel (1.0 mol % Cr) was exposed to the laser (15 mW) for 1 min, to cause the onset of decomposition (Figure 12a), then for an additional 120 min to cause significant formation of the photochemical product (Figure 12b). When irradiation ceased, the sample was allowed to stand in the dark under vacuum overnight. A spectrum recorded the following day shows slight recovery of the 986 cm-1 band of the dioxoCr(VI) site (Figure 12c), indicating that the photoproduct does not readily convert back to the starting structure under these conditions. It worth noting that, at the laser intensity and collection times used, excitation into the higher energy excited states does not lead to emergence of the photo-product, even though the lowest energy excited state is being populated. We


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attribute this result to relatively poor coupling between the higher excited states and the lowenergy emissive state, from which the photochemistry emanates. The decay process will cause a large fraction of the energy to dissipate non-radiatively before the lowest excited state can be populated. Consistent with this explanation, longer exposure to 364 nm excitation ultimately produced changes in the Raman spectra consistent with the photochemical process.

Figure 12. Raman spectra recorded with 458 nm excitation, for a Cr(VI) /SiO2 xerogel (1.0 mol % Cr), irradiated with 15 mW laser power at the sample, for (a) 1.0 min, (b) 120 min, and (c) after the laser was extinguished and the sample was allowed to stand under vacuum in the dark overnight. It seems clear from these results that the Raman peak at ca. 1020 cm-1, assigned by L&W and C&W to a mono-oxoCr(VI) site and observed only upon excitation into the first Cr(VI) absorption band,20, 22-23 is instead associated with a photodegradation process. It might be argued



that the two observed bands at 1012 and 1027 cm-1 are independent of each other, with one arising from the resonance enhanced mono-oxo species and the other from photodecomposition. However the two bands are not independent of each other. This can be seen in Figure 11, which reports the spectral changes as a function of time when the irradiation is at the same power. Under these conditions both bands grow in together maintaining the same relative peak heights. If one of the bands was resonantly enhanced and the other a decomposition product we would expect the resonantly enhanced mode to have a constant intensity while the bands associated with the photodecomposition would grow in over time; this is clearly not the case. An analogous argument can be made from the data in Figure 12, which shows the regrowth of the starting dioxo peaks when the sample is held in the dark overnight. As the bands associated with the dioxo species grow back in, the relative intensities of the two peaks at 1012 and 1027 cm-1 remain constant and are therefore not independent. If they were independent, one being the resonantly enhanced mono-oxo and the other being decomposition product, then the former should remain the same and the latter would become less intense as it was depleted as the dioxo peak grows in. Based on these arguments, we conclude that there is no spectroscopic evidence to support the existence of a mono-oxoCr(VI) species, at the metal loadings used here. Furthermore, the vibrational spectra show only features that are consistent with the normal modes of a dioxoCr(VI) site. This result agrees with our recent re-evaluation of the electronic spectrum of Cr(VI)/SiO2, which found that all of the bands are consistent with the electronic transitions of a dioxoCr(VI) site.62 It also agrees with what is expected by the radius ratio rules and by calculations reported by Handzlik, which show the mono-oxo species to be highly unfavorable.38 In previous studies,22-23 the proposed distribution of mono- and dioxoCr(VI) sites was quantified using temperature programmed reduction (TPR) of Cr(VI)/SiO2 with H2 as the reductant. Two peaks at 420 and 489 °C were assigned to the reduction of dioxo and monooxoCr(VI) sites, respectively, and the ratio of their integrated intensities led to the conclusion that mono-oxoCr(VI) sites represent 1/3 of the total Cr sites. In fact, a number of thorough studies of the H2 reduction of silica supported Cr(VI) oxides have been reported.67-69 The reduction process converts Cr(VI) to Cr(III) with the production of water. In all cases, the higher temperature TPR peak arises from the reduction of polychromates that formed from the water


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catalyzed aggregation of monomers. Given these prior studies, there is little need to assign the two reduction peaks to two distinct monomeric Cr(VI) structures.

Conclusions This study shows the effects of resonant enhancement on the normal modes of vibration associated with the dioxoCr(VI) site supported on silica. In addition to A-term enhancement of the totally symmetric Cr(=O)2 stretch, weak enhancement of the anti-symmetric stretch is also observed with excitation at either 364 and 257 nm into the corresponding absorption bands in the UV-visible spectrum. A strong enhancement of the antisymmetric stretch is observed with 351 nm excitation. We suggest this is likely due to B-term enhancement, although the electronic states responsible are not obvious based on our prior assignments of excited states.62 This finding suggests that a subset of the distribution of surface sites possesses the necessary combination of excited state symmetries to make the enhancement possible. A new band at ca. 874 cm-1, resolved through resonant enhancement, is a previously unobserved mode of the dioxo site. Prior studies suggested that excitation into the lowest allowed electronic transition at 458 nm enhances a band assignable to a mono-oxoCr(VI) species. This study shows that such excitation results in photodecomposition yielding a set of new bands at 786, 1012 and 1027 cm-1. The photodecomposition process is clearly the origin of the band incorrectly assigned to a mono-oxo species. This study also serves to illustrate a general problem in using vibrational spectroscopy to identify molecular structures of surface sites in catalysis. All such species have multiple characteristic vibrational modes that can potentially serve to identify them. While only a subset of the modes may be spectroscopically observable, rigorous spectroscopic assignment requires the analysis of multiple modes, preferably with isotopic labeling interpreted with a correct normal mode analysis to make a credible case for the existence of a particular species. For supported catalysts, a widely-accepted practice is the assignment of a single band in the IR or Raman spectrum to a specific site. As in the case described here, this is usually little more than a guess, and may lead to identification of non-existent species whose roles in catalytic processes are then investigated unproductively at length (e.g., using computational approaches).38 Acknowledgements.



We thank Dr. Eric Dowty of Shapesoft Software for helpful discussion. 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).

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Tables of the generalized valence force field, computed structural parameters, the method of scaling calculated vibrational frequencies.


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Table of Contents Figure


Reassessment of the Resonance Raman Spectra - ACS Publications

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