An In Situ Investigation Using Sum Frequency Spectroscopy

J. Phys. Chem. C 2008, 112, 219-226

219

Acetonitrile Adsorption on Polycrystalline Platinum: An In Situ Investigation Using Sum Frequency Spectroscopy S. Beau Waldrup and Christopher T. Williams* Department of Chemical Engineering, UniVersity of South Carolina, Columbia, South Carolina 29208 ReceiVed: August 1, 2007; In Final Form: September 25, 2007

Sum frequency spectroscopy (SFS) has been used to probe the interface between Pt films and liquid consisting of neat acetonitrile or acetonitrile/solvent mixtures. In the case of the neat acetonitrile, vibrational features associated with CtN (2239 cm-1) and C-H stretching (2939 cm-1) from adsorbed acetonitrile were observed. The peaks on Pt were more significantly downshifted from bulk values compared to Al2O3, indicating different adsorption configurations on the surfaces. Significant downshifting of the primary νCtN stretch (2239-2229 cm-1) was observed with changing acetonitrile concentration in solution, with alcohols (EtOH and MeOH) causing larger effects than water. In addition, a secondary, downshifted νCtN band (2229-2211 cm-1) is observed whose intensity is invariant with liquid-phase concentration, indicating a more strongly bound species. Similar shifts were observed for the CH3 stretching mode of acetonitrile in ethanol, methanol, and water. These frequency shifts in both the CtN and C-H stretching regions are attributed to a combination of decreased dipole-dipole coupling and charge-transfer effects.

Introduction Heterogeneous liquid-phase nitrile hydrogenation is an important industrial reaction for the formation of amines, which are used to produce certain textiles, surfactants, nylons, and fungicides.1 Throughout the extensive literature involving heterogeneous nitrile hydrogenation, it is obvious that little is known about the exact reaction intermediate species that reside on the surface.2-9 Due to the interest in identification of surface adsorbates and intermediates at solid-liquid interfaces, there has been a recent drive to apply spectroscopic methods to such systems. Spectroscopic techniques such as infrared reflectionabsorption spectroscopy (IRAS),10,11 attenuated total reflectioninfrared (ATR-IR)12-16 spectroscopy, and surface enhanced Raman spectroscopy (SERS)17-19 have been successful in identification of surface species. However, such linear optical techniques are often limited in their ability to observe surface species due to bulk-phase spectral interference. This is especially the case when these bulk-phase signals have identical or nearly identical vibrational frequencies, making it very difficult to isolate surface vibrational resonances. Nonlinear optical spectroscopy can overcome these difficulties and has therefore proven useful in studying such “buried” interfaces.20 The nature of nonlinear optical spectroscopies is such that they allow for perfect interface selectivity, thereby providing direct spectroscopic information about surface species. One such nonlinear optical technique that has proven useful in the study of catalytic interfaces is vibrational sum frequency spectroscopy (SFS).21,22 We have been attempting to apply several of these approaches to the study of nitrile adsorption and hydrogenation on catalytic supports and metals. Our previous ATR-IR studies of Al2O3 and Pt/Al2O3 in nitrile/hexane mixtures have revealed that acetonitrile adsorbs in two configurations on the catalysts * To whom correspondence should be addressed. Address: Department of Chemical Engineering, Swearingen Engineering Center, University of South Carolina, Columbia, SC 29208. Fax: (803)777-8265. E-mail: [email protected].

surface: a weakly bound end-on species and an imine-type species.13 The latter was found to be reactive toward hydrogen, suggesting its importance in the nitrile hydrogenation mechanism. We have also recently examined the adsorption of acetonitrile on Al2O3 with SFS, confirming the presence of endon species on the surface.23,24 In these studies, total internal reflection (TIR) SFS was used, and the orientation of the adsorbed acetonitrile was determined. In the present study, we have examined a sputter-deposited thin film of Pt on Al2O3 using TIR-SFS. Due to the structure of this model catalyst surface, it provides more representative results of an actual catalyst surface than could a flat (i.e., single crystal) surface. Adsorption has been studied in neat acetonitrile and over a range of acetonitrile/ethanol, acetonitrile/methanol, and acetonitrile/water mixture compositions. The results reveal at least two different adsorbed species with varying degrees of interaction with the surface. Observed concentration-dependent vibrational trends are attributed to a combination of dipoledipole coupling losses and charge-transfer effects. Experimental Section Materials and Prism Apparatus. The chemicals used in all of the experiments were as follows: neat liquid anhydrous acetonitrile (Alpha Aesar, 99.9+%), ethanol (Sigma-Aldrich, 99.5+%), methanol (VWR, 99.9+%), and 17+ MΩ deionized water made by an in house purification unit. A 0.5-mL-volume Teflon flow cell is used for in situ SFS experiments and has been described previously.23,24 The prism that was used as the TIR element was an Al2O3 coated CaF2 (ISP Optics) equilateral prism (60°, 0.5 × 0.5 “ face). The prism was polished on three faces to 60/40 (scratch/dig) and 1/4λ at 632 nm. A 100 nm (rms ) 4.5 nm) Al2O3 film was coated on one face of the prism by magnetron sputtering as described previously.24 During the SFS experiments, the prism is mounted on its edge in the flow cell, which itself is mounted on a three-way micrometer-driven stage used to make fine adjustments for proper optical alignment.

10.1021/jp076154j CCC: $40.75 © 2008 American Chemical Society Published on Web 12/07/2007

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Sum Frequency Spectroscopy. The laser system has been described in detail elsewhere,23,24 but the essentials of the system are as follows. A 1064 nm Nd:YAG laser (Continuum) operating at 20 Hz with 20 ps pulses is used to pump an OPG/OPA (LaserVision) which generates a 532 nm visible beam and a tunable infrared beam (0.8-5 µm). This system is capable of delivering visible powers of 20-30 µJ/pulse in all spectral regions, with infrared powers of 50-80 µJ/pulse in the cyano spectral region (2200-2300 cm-1) and 200-250 µJ/pulse in the methyl spectral region (2800-3000 cm-1). SFG is detected using a photomultiplier tube (Electron Tubes Limited) for both the sample and reference channel. The sample line SF light is filtered using a monochromator (Acton Inc.), a polarization selecting cube (CVI Inc.), and a band-pass filter (Semrock). The reference SFG is collected using the transmitted signal from barium titanate (BaTiO3) fused into a glass slide and is filtered using a band-pass filter (Semrock). There are several polarization combinations that can be utilized in SF spectra. However, when metals are present in the system, spectra arising from s-polarized visible and (especially) infrared combinations are vastly reduced in intensity. It is for this reason that all of the spectra in this work are ppp polarized, referring to p polarized SF, p polarized visible, and p polarized infrared. The peak positions reported are subject to an error of (1 cm-1. Platinum Film Deposition. The platinum thin films with thicknesses of 7 ( 0.1 nm were deposited on the surface of the prism using plasma sputtering deposition. The thickness was monitored using a quartz crystalline microbalance. This thickness corresponded to an optimal value that was thick enough to avoid damage to the film by the focused lasers while being thin enough as to allow the reflected SFG signal to be detected.25 Prior to each application of Pt to the prism, the Pt was removed from the surface by rubbing the surface in ethanol. The prism surfaces were imaged using atomic force microscopy (AFM). All AFM measurements were carried out using a PicoSPM AFM (Molecular Imaging) operated in the acoustically driven, intermittent contact (“tapping”) mode. Standard silicon AFM probes (Mikromasch Ultrasharp NSC12/3) having cantilever spring constants of 2.5-8.5 N/m and resonance frequencies from 120 to 190 kHz were used. The manufacturer estimates that the probe tip radius of curvature is no smaller than about 5-10 nm. All AFM measurements were performed with the samples in air at ambient temperature. Spectral Modeling. Modeling of SF spectra was performed using an “in house”-written MATLAB program. Typically modeling of SFG spectra is accomplished by using eq 1.20,24

ISFG )

i |χ(2) NRe

+

∑q ω

IR

e-(ωq-ωq ) /2σq Aq 0 2

i ISFG ) |χ(2) NRe +

∑q ∫ω

ωh l

x2πσ

2 q

2

dωq|2

(ωIR - ωq + Γqi)

(2)

where σ is the standard deviation of the introduced Gaussian distribution. Note here that the full integral should technically be from zero to infinity; however, a good approximation can be achieved at limits above and below the resonance (ωh, high, and ωl, low) where the spectral values are no longer perturbed by the resonance. In addition, in some cases where two different molecules with vibrations in the same spectral region were present, it is more rigorous to split the phase difference apart into absolute phases. The resulting equation is shown in eq 3.

e-(ωq-ωq ) /2σq Aqeiδq 0 2

ISFG )

iφ |χ(2) NRe

+

∑q ∫ω

ωh

∑r ∫ω

l

ωh l

x2πσ

2

dωq +

q (ωIR - ωq + Γqi) -(ωr-ωr0)2/2σr2 e Areiδr

x2πσ

2

dωr|2 (3)

2 r

(ωIR - ωr + Γri)

where the parameters φ, δq, and δr are the absolute phase of the nonresonant, species q resonant, and species r resonant contributions, respectively. When solving these equations, the exact value of the absolute phase of the nonresonant term has been taken to be 2π. Thus, the other two phases are determined through the regression. With this model, data that contained molecular resonances from two different species were more accurately fit than with the original model. Finally, the effect of changes in the magnitude of the Fresnel factor over the spectral range was included in the model. In many SFS models, the Fresnel factors are assumed to be invariant over the frequency range of interest. However, this assumption is not always appropriate in the case of neat (or high concentration) liquids in contact with surfaces, since the refractive index of the liquid can vary extensively near molecular resonances.27,28 Especially for TIR-SF measurements, the Fresnel factors can exhibit large variations with refractive index. To include the Fresnel factors in the model, the intensity relationship for the ppp polarized spectra was first considered

Ippp ∝ |-Lx,SFKx,VISKz,IRχxxz - Lx,SFKz,VISKx,IRχxzx + Lz,SFKx,VISKx,IRχzxx +Lz,SFKz,VISKz,IRχzzz|2 (4)

Aq

|

- ωq + Γqi

2

(1)

The parameters given in this equation, χNR, Aq, ωq, Γq, and , refer to the nonresonant susceptibility, resonant signal strength, resonant frequency, resonant line width, and the phase difference between the nonresonant and resonant term, respectively. This model incorporates homogeneous broadening due to vibrational dephasing and relaxation, by using a Lorentzian-type function as shown in the second term. We and others have found this to be an effective model in the absence of significant inhomogeneous broadening. However, in the case of metal surfaces (as in the present study), inhomogeneous broadening can become significant. To account for this effect, the model was modified to include a Gaussian-type function, as described by Bain et al.26 This change results in the following equation:

Here, L is the Fresnel factor for the sum frequency beam, Ki is the Fresnel factor of the ith beam, either visible or infrared, and χabc is the susceptibility tensor for the abc contribution. The mathematical equations for these Fresnel factors have been reported previously.24 If the Fresnel factors are examined over the regions of interest, it can be determined that the Lz,SFKz,VISKz,IR combination is the dominant contribution assuming that the magnitude of the χ terms are roughly similar. For example, if the magnitude of the Lz,SFKz,VISKz,IR term is compared with the magnitude of the Lx,SFKz,VISKx,IR term, it is over 40 times larger, and on average the pure z term is over 30 times larger than the other terms. With this assumption, the intensity becomes proportional to the Fresnel factors times the susceptibility tensor component. Incorporating the Fresnel factors, the final modeling equations are shown in eqs 5 and 6 for one and two molecular species respectively.

Acetonitrile Adsorption on Polycrystalline Platinum

[

J. Phys. Chem. C, Vol. 112, No. 1, 2008 221

i ISFG ) |(Lz,SFKz,VISKz,IR) χ(2) NRe +

∑q ∫ω

e-(ωq-ωq ) /2σq Aq 0 2

ωh

x2πσ

l

[

]

2

dωq |2 (5)

(ωIR - ωq + Γqi)

2 q

iφ ISFG ) |(Lz,SFKz,VISKz,IR) χ(2) NRe +

∑q ∫ω

ωh l

e-(ωq-ωq ) /2σq Aqeiδq

x2πσ

∑r ∫ω

ωh l

0 2

2 q

2

dωq +

(ωIR - ωq + Γqi) e-(ωr-ωr ) /2σr Areiδr 0 2

x2πσ

2 r

2

(ωIR - ωr + Γri)

]

dωr |2 (6)

These two equations were used to model all of the spectra in this work. Results and Discussion Surface Characterization of Pt/Al2O3/CaF2. Characterization of the Pt/Al2O3/CaF2 surface morphology was done using AFM imaging as shown in Figure 1. Figure 1a shows an image of the Al2O3/CaF2 surface. It can be seen that the surface is very rough and has many island type structures with lateral dimensions of 50-100 nm. As reported previously, the rootmean-square (rms) height value for this surface is 4.5 nm, which is approximately seven times the roughness observed for a bare CaF2 surface.24 Figure 1b shows an image of a 7 nm film of Pt on the Al2O3/CaF2 surface. The rms height is essentially unchanged by the addition of Pt. However, visual inspection of the surface indicated that the Pt covers the entire surface. Conductivity measurements reveal an average conductivity of ∼15 kΩ per square (25 mm × 25 mm) lending to the fact that the film is continuous over the surface. Neat Acetonitrile Adsorption. Throughout the literature, acetonitrile (AN) adsorption on a variety of metals has been studied in different liquid and gas-phase environments. Perhaps not surprisingly, the peak positions observed for adsorbed AN are found to vary widely. As a baseline for studying solvent effects, neat AN adsorption on polycrystalline Pt films was first examined. Figure 2 shows typical SF spectra of neat acetonitrile adsorbed on Pt in the cyano stretching region. The spectrum shows one predominant peak centered at 2239 cm-1. This peak is attributed to the νCtN stretch of “end-on” adsorbed acetonitrile, bound through the nitrogen to the Pt surface. This species has been observed previously on both oxide23,24,29 and Pt30-32 surfaces under different gas and liquid-phase environments with varying peak positions. The observed red-shift of approximately 14 cm-1 from the bulk liquid peak33-36 can be attributed to a significant interaction of Pt with the cyano nitrogen. The SF spectrum of acetonitrile adsorbed on Al2O3 is also shown in Figure 2 for comparison. The predominant peak is observed at 2242 cm-1 and has also been attributed to the νCt 24 N peak of the “end-on” adsorbed acetonitrile. In comparison, the peak observed on Pt is more significantly downshifted from the bulk value. This difference is due to altered modes of adsorption. In the case of Pt, electrons are backbonded to the

Figure 1. (a) AFM image of the Al2O3/CaF2. The height scale bar is in angstroms. (b) AFM image of a 7 nm Pt film on Al2O3/CaF2. The height scale bar is in angstroms.

antibonding 2pπ* orbital of the acetonitrile.37 In the case of Al2O3, hydrogen bonding of the adsorbed species with native reduced oxide species occurs.24 Accordingly, the increased bandwidth of acetonitrile adsorbed on Pt suggests a different interaction than on Al2O3. The increased red-shift of this resonance on Pt as compared to Al2O3 further suggests the stronger adsorption on the former. The SF spectrum of neat acetonitrile adsorbed on Pt in the methyl stretching region is shown in Figure 3. In this region, there is again one predominant peak of interest at 2939 cm-1 that is attributed to adsorbed acetonitrile. This peak is assigned to the ν(A)C-H (symmetric) vibration of acetonitrile bound to the surface. Similar peaks have been observed previously on both oxide24,29 and on Pt30-32 surfaces, but like the CtN stretch, the peak position varies widely. In the present case, the peak is slightly red-shifted from the bulk value of 2944 cm-1.34-36 The SF spectrum during acetonitrile adsorption on Al2O3/CaF2 is also shown in Figure 3 for comparison. The ν(A)C-H (symmetric) stretch in this case is observed at 2942 cm-1. As with the cyano stretch, the increase in the red-shift away from the bulk values suggests stronger adsorption on the Pt versus the oxide surface. Last, there are some other broad peaks in Figure 3 between 2850 and 2915 cm-1. These peaks cannot be attributed to acetonitrile; however, as reported previously,24 they

222 J. Phys. Chem. C, Vol. 112, No. 1, 2008

Figure 2. SF spectra of the cyano stretching region for acetonitrile adsorbed on a 7 nm film of Pt on Al2O3/CaF2 (circles) and on bare Al2O3/CaF2 (triangles).

likely arise from residual carbonaceous residue on the surface of the prism from exposure to air. Acetonitrile Adsorption in Various Solvents. Nitrile hydrogenations and other liquid-phase catalytic reactions are often carried out in the presence of solvents. Unlike typical carrier gas (e.g., He, Ar, and N2) that is used in gas-phase catalytic studies, such solvents are expected to interact significantly with the catalyst surface. In addition, solvent molecules interact directly with the reactants themselves, effecting their adsorption configuration. Before exploration of the solvent effects the adsorption of acetonitrile from the gas phase was also examined. Such a measurement yields a spectrum for acetonitrile species that is not perturbed by liquid solvent or bulk liquid acetonitrile. Gas phase acetonitrile adsorption was carried out by replacing the liquid AN in the cell with a flow of AN-saturated N2 gas. A scan of the cyano stretching region after reaching steady state (Supporting Information, Figure S1) revealed a similar spectrum to that of neat acetonitrile, with the primary νCtN peak at 2233 cm-1. This downshifted result suggests that the surface structure of acetonitrile adsorbed from the gas and liquid phase are similar. However, the lower frequency would suggest a decreased degree of dipole coupling, perhaps due to lower coverage. To explore solvent effects in acetonitrile adsorption on Pt, three polar solvents were examined. Acetonitrile adsorption from ethanol was first studied, with Figure 4 showing a series of SF spectra in the cyano stretching region for varying nitrile concentrations. Spectra were obtained in 10 vol % increments over a range from pure acetonitrile to 20%. In this series, the predominant νCtN peak of end-on adsorbed acetonitrile is clearly present at all concentrations, with two major trends apparent. First, as ethanol is added to the liquid, the intensity of the peak decreases. This suggests the displacement of acetonitrile molecules on the surface by ethanol solvent. Second, the primary

Waldrup and Williams

Figure 3. SF spectra of the methyl stretching region for acetonitrile adsorbed on a 7 nm film of Pt on Al2O3/CaF2 (circles) and on bare Al2O3/CaF2 (triangles).

peak position red-shifts from 2239 cm-1 at 100 vol % to 2229 cm-1 at 20 vol %. These changes are accompanied by the presence of a secondary feature between 2229 and 2211 cm-1, which is first resolved at 60% AN and remains even to the lowest concentrations. Adsorption experiments were also conducted using both methanol (Figure 5) and water (Figure 6). A similar trend in the peak positions of the νCtN stretch is observed for both solvents. For MeOH, the shift (2239-2230 cm-1) is slightly smaller than EtOH, and likewise for H2O (2239-2234 cm-1), it is smaller as well. The additional lower frequency peak, between 2229 and 2211, is also observed in MeOH and H2O with similar concentration dependence. These peaks and their frequency ranges are summarized in Table 1. The nature of these surfaces is clarified by considering their concentration-dependence more quantitatively. By tracking the relative intensity of each peak as obtained from curve fitting and plotting versus liquid-phase concentration, a rough idea of the coverage variations is obtained. This type of analysis was completed in a previous study involving AN adsorption on Al2O3, wherein an isotherm relationship was developed through the resonant susceptibly.24 Assuming that the average orientation remains constant with coverage (or only changes slightly), then the peak strength (A) will be proportional to the surface coverage. The relevant plots are shown alongside the corresponding spectra in Figures 4-6 for ethanol, methanol, and water, respectively. In all three cases, the largest peak (i.e., 2229-2239 cm-1) is observed to monotonically increase with acetonitrile concentration. This suggests that this moiety has comparatively similar adsorption strengths to these three solvents. In contrast, the coverage of the satellite peak (22292211 cm-1) is relatively invariant with concentration. This trend suggests that the second species is more strongly adsorbed on Pt than its higher frequency counterpart.

Acetonitrile Adsorption on Polycrystalline Platinum

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Figure 4. (A) Concentration-dependent SF spectra in the cyano stretching region for acetonitrile in ethanol adsorbed on Pt/Al2O3/CaF2. The data are given by circles and the modeled fits are given by lines. (B) Relative intensity trend of the primary CtN peak developed from the modeled fits. (C) Relative intensity trend for the secondary CtN peak developed from the modeled fit.

Figure 5. (A) Concentration-dependent SF spectra in the cyano stretching region for acetonitrile in methanol adsorbed on Pt/Al2O3/CaF2. The data are given by circles and the modeled fits are given by lines. (B) Relative intensity trend of the primary CtN peak developed from the modeled fits. (C) Relative intensity trend for the secondary CtN peak developed from the modeled fit.

TABLE 1: Peak Assignments for Acetonitrile Adsorbed on Pt Films in Four Different Environmentsa AN/EtOH (cm-1) mode νCtN (primary) νCtN (secondary) ν(A)C-H a

AN/MeOH (cm-1)

AN/H2O (cm-1)

neat AN (cm-1)

90%

60%

20%

90%

60%

20%

90%

60%

20%

2239

2236 2938

2229 2212 2932

2234 2229 2936

2231 2219 2933

2230 2211 2933

2238 2225 2939

2236 2220 2936

2234

2939

2234 2227 2934

2934

The percentages refer to the volume percentage of acetonitrile in each solvent.

To compliment this data, the methyl stretching region of the spectra was also examined to explore solvent-dependent vibrational trends. Selected concentrations of acetonitrile in ethanol, methanol, and water were chosen, with the SF spectra shown in Figures 7-9, respectively. In the case of ethanol and methanol, the peaks from adsorbed acetonitrile are convoluted by contributions from solvent that is also adsorbed on the surface. The SFG spectrum of neat ethanol and methanol

adsorbed on Pt is shown in Figure 10 and can be compared with the spectrum of neat acetonitrile adsorption on the same Pt (Figure 3, circles). The spectra of ethanol and methanol are much different than that observed for acetonitrile due to the relative phases of the molecular resonances. Although there are positive peaks in the acetonitrile spectrum, small negative peaks appear in the ethanol and methanol spectra. These small negative peaks seem to indicate that the phase difference for ethanol and

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Figure 6. (A) Concentration-dependent SF spectra in the cyano stretching region for acetonitrile in water adsorbed on Pt/Al2O3/CaF2. The data are given by circles and the modeled fits are given by lines. (B) Relative intensity trend of the primary CtN peak developed from the modeled fits. (C) Relative intensity trend for the secondary CtN peak developed from the modeled fit.

Figure 7. Concentration-dependent SF spectra in the methyl stretching region for acetonitrile in ethanol. The data are given by circles and the model fits are shown as lines.

methanol is different than that for acetonitrile by about 180°. This then has some ramifications on the adsorption geometry of the methyl groups.38 Since acetonitrile has been shown to be adsorbed through the nitrogen to the Pt surface in this case, then the observed phase difference with ethanol and methanol suggests that they are adsorbed with their methyl groups pointed toward the surface. The peaks associated with ethanol occur at 2894, 2934, and 2974 cm-1 and are assigned to the CH3 and CH2 symmetric, CH3 + CH2 Fermi resonance, and CH3 asymmetric stretches, respectively. In the case of methanol, three

Figure 8. Concentration-dependent SF spectra in the methyl stretching region for acetonitrile in methanol. The data are given by circles and the model fits are shown as lines.

peaks are again observed at 2825, 2905, and 2957 cm-1 and are assigned to the ν(CH3) vibrations and Fermi resonances. Despite the peak overlap in the CH stretching region, SF modeling using eq 6 was able to deconvolute the various peaks observed in the acetonitrile/ethanol and acetonitrile/methanol systems. For acetonitrile in ethanol, a single peak can be attributed to adsorbed acetonitrile, located at 2938, 2937, and 2932 cm-1, for 90, 50, and 20 vol % acetonitrile, respectively. This red-shift of the methyl stretch is analogous to (although smaller than) that seen for the CtN stretch. Similarly, when

Acetonitrile Adsorption on Polycrystalline Platinum

Figure 9. Concentration-dependent SF spectra in the methyl stretching region for acetonitrile in water. The data are given by circles and the model fits are shown as lines.

J. Phys. Chem. C, Vol. 112, No. 1, 2008 225 The observed downward-shifting frequency trends are likely a combination of a variety of phenomena, including increased interfacial solvation, reduced dipole-dipole coupling, and charge-transfer effects. The effect of solvation on liquid acetonitrile νCtN stretching frequencies has been examined previously.39,40 In the present case, ATR-IR was used to examine νCtN frequencies at various concentrations of AN in EtOH, MeOH, and H2O. The results are shown in the Supporting Information in Figure S3, revealing that bulk solvation clearly results in a frequency blue-shift. In the case of the νCH stretch, a similar analysis was hindered by overlap with solvent vibrational bands in the case of EtOH and MeOH; however, for water, no shift was observed. Although it is possible that interfacial solvation might result in different effects, this is unlikely to explain the large shifts in the CtN stretching region. It is also well-known that a decrease in dipole-dipole coupling can cause a red-shift to occur as is often observed for CO on metal surfaces.41-46 Likewise, it is possible for solvent molecules that adsorb on the surface to cause changes of frequencies of adsorbates at interfaces by electronic charge transfer between solvent, adsorbate, and metal. Such an effect changes the electronic nature of the interaction between the adsorbate and the surface causing frequency shifting to occur.36,47-51 For example, Weaver et al. observed that the νCO stretching frequency of adsorbed carbon monoxide downshifts with increasing dosage of different solvents on Pt(111) in UHV.36 These red-shifts correlated with changes in the work function of the metal surface, which has been directly related to interfacial potential. Based on these results, adsorption configurations for acetonitrile on polycrystalline Pt are envisioned. First, clusters of acetonitrile and solvent can form discrete islands of each species on the surface. In the AN clusters, there are two different species. First, there is a more weakly bound species which exhibits a frequency closer to bulk values, and this is greatly affected by dipole coupling. Second, there is a more strongly bound species that seems to be equally populated over the full concentration range. As the surface begins to become more populated with solvent molecules, interfacial acetonitrile molecules gradually can become solvated. These solvated AN species would exhibit increasingly lower frequencies then their grouped counterparts since the breakup of discrete islands would decrease the local coverage and thus decrease dipole coupling. The increased amount of solvent on the surface may also tune the work function in such a way that it increases the red-shift observed. Therefore, the apparent red-shift of the primary νCtN band between 2239 and 2229 cm-1 is attributed to a loss of dipole-dipole coupling and solvent-tuning effects. Conclusions

Figure 10. SFG spectrum of neat methanol (squares) and ethanol (circles) adsorption on Pt/Al2O3/CaF2. The data are shown as open points and the fit is shown as a lines.

methanol and water are present as the solvent, the peak shifts follow a similar trend: 2939, 2937, and 2933 cm-1 for the former and 2939, 2937, and 2936 cm-1 for the later.

In this work, the various surface vibrations of acetonitrile adsorbed on a polycrystalline Pt film have been observed using in situ TIR-SFS. The spectra observed for neat acetonitrile adsorption exhibit two main peaks in the cyano stretching region corresponding to a weakly bound νCtN and a more strongly bound downshifted νCtN stretch. Examination of the methyl stretching region yielded a peak attributed to the ν(A)C-H stretching vibration of AN. The spectra observed for various concentrations of acetonitrile in ethanol, methanol, and water showed that the primary νCtN stretch was dependent on the acetonitrile concentration in solution. The experiments showed as much as a 10 cm-1 red-shift of the primary νCtN and drastic decreases in peak intensities with an 80 vol % change in liquidphase concentration. This peak was attributed to weakly bound

226 J. Phys. Chem. C, Vol. 112, No. 1, 2008 clusters of AN molecules that are adsorbed on the Pt surface. In addition, a red-shifted secondary νCtN band was observed and has been attributed to a more strongly adsorbed AN species on the surface due to the invariance of the peak strength with AN concentration. Similarly, the ν(A)C-H stretch of AN in both alcohol and water showed dependence on the liquid-phase concentrations of solvent. A decrease of dipole-dipole coupling and the presence of charge-transfer effects are hypothesized as the main causes of these effects. To our knowledge, this is the first surface spectroscopic study of solvent-dependent acetonitrile adsorption on platinum at elevated concentrations. From these results, it is clear that the liquid-phase environment has a significant effect on the adsorptive electronic properties of interfacial species. The ability of SFS to distinguish between adsorbed acetonitrile in different configurations is encouraging for future catalytic hydrogenation studies of this system. Of particular interest is the reactivity of these different species to form surface amine species as a function of concentration and temperature in solution. Studies of amine adsorption as well as time-dependent in situ SF measurements are currently underway in our laboratory. Acknowledgment. The authors thank the National Science Foundation for its generous funding of this project through the CAREER award (CTS-0093695). The authors also thank Dr. Hongsheng Gao of the Department of Chemical Engineering for his help with the AFM imaging. Likewise, we thank Dr. Andy Stamps of the Department of Chemical Engineering with his help in developing the Matlab code used to model the spectra. Supporting Information Available: SF spectrum of adsorption of acetonitrile on Pt from the gas-phase bulk-phase solvation effects on the CN stretching frequency examined by ATR-IR spectroscopy. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kauffman, G. B. J. Chem. Educ. 1988, 65 (9), 803. (2) Huang, Y.; Sachtler, W. M. H. Appl. Catal. A 1999, 182, 365. (3) Huang, Y.; Sachtler, W. M. H. Stud. Surf. Sci. Catal. 2000, 130, 527. (4) Huang, Y.; Sachtler, W. M. H. J. Catal. 2000, 190, 69. (5) Huang, Y.; Sachtler, W. M. H. J. Catal. 1999, 188, 215. (6) Sexton, B. A.; Avery, N. R. Surf. Sci. 1983, 129, 21. (7) Krtil, P.; Kavan, L.; Nova´k, P. J. Electrochem. Soc. 1993, 140, 3390. (8) Hubbard, A. T.; Cao, E. Y.; Stern, D. A. Electrochim. Acta 1994, 39 (8/9), 1007. (9) Morin, S.; Conway, B. E.; Edens, G. J.; Weaver, M. J. J. Electrochem. 1997, 421, 213. (10) Weaver, M. J.; Zou, S. In AdVances in Spectroscopy; Clark, R. J., Hester, R. E., Eds.; Wiley: Chichester, U.K., 1998; Vol. 26, p 219. (11) Lee, I.; Zaera, F. J. Phys. Chem. B 2005, 109 (26), 12920.

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