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Cyclohexene and 1,4-Cyclohexadiene Hydrogenation Occur through Mutually Exclusive Intermediate Pathways on Platinum Nanoparticles James M. Krier,*,†,§ Kyriakos Komvopoulos,‡ and Gabor A. Somorjai*,†,§ †

Department of Chemistry and ‡Department of Mechanical Engineering, University of California, Berkeley, Berkeley, California 94720, United States § Materials Sciences and Chemical Sciences Divisions, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: Platinum nanoparticles (NPs) capped with polyvinylpyrrolidone (PVP) were studied with sum frequency generation (SFG) vibrational spectroscopy under reaction conditions during cyclohexene (CH) and 1,4-cyclohexadiene (1,4-CHD) hydrogenation at 295 K. Despite similar vibrational features observed during reaction, CH and 1,4-CHD proceed through mutually exclusive pathways on 1.7 and 4.6 nm Pt-PVP NPs unlike Pt(111) studied previously. The intense red-shifted C−H stretch of adsorbed 1,4-CHD at 2770 cm−1 was monitored for both reactions. SFG and kinetic experiments show CH hydrogenation is active and reversible, while 1,4-CHD hydrogenation poisons the surface.

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INTRODUCTION The adsorption and hydrogenation of cyclohexene and other six-membered rings on Pt single crystals are the most wellcharacterized processes by surface vibrational spectroscopy.1−9 Cyclohexene (CH), 1,4-cyclohexadiene (1,4-CHD), and 1,3cyclohexadiene produce unique vibrational signatures on the surface of Pt(111), which make spectral interpretation in the aliphatic stretching range less ambiguous than other reactions. Because of this, CH hydrogenation serves as a model for sum frequency generation (SFG) studies on contemporary Pt NPs.10−14 On Pt(111) this reaction involves a dehydrogenated intermediate (adsorbed 1,4-CHD) under most conditions with excess H2 near 295 K. 1,4-CHD binds most stably through 3−4 Pt atoms and has higher adsorption energy than di-σ CH, which must bind to two Pt atoms.8 Above ∼5 Torr of CH, adsorbed 1,4-CHD reaches saturation coverage, and the rehydrogenation of this intermediate (to make cyclohexane (CA)) becomes rate determining. Surface intermediates found in ambient on Pt(111) were previously reasoned from the spectral similarities of 1,4-CHD and CH under ultrahigh vacuum (UHV).5 Under most temperature and pressure conditions CH and 1,4-CHD produce a red-shifted C−H stretch at 2770 cm−1.6 Molecular 1,4-CHD in the gas phase is expected to make a “shortcut” on Pt to the same boat adsorption achieved by dehydrogenating CH. The hydrogenation rates of CH and 1,4-CHD on Pt(111), in addition to SFG spectra, were reported to be similar near room temperature (RT).3 By performing SFG before, during, © 2016 American Chemical Society

and after reaction, it is shown here that CH and 1,4-CHD hydrogenation on PVP-capped Pt NPs occur through different pathways despite comparable vibrational signatures under reaction conditions. SFG and kinetic experiments described herein were performed on a fused silica prism with a compressed 2D NP Langmuir−Blodgett (LB) monolayer film (see representative images in ref 10). The 1.7 and 4.6 nm Pt-PVP NPs were synthesized according to previously published recipes.15−17 A recurring challenge with fundamental studies of colloidal Pt rests in the capping agent. Pt NPs are created in the presence of polyvinylpyrrolidone (PVP), which encapsulates and stabilizes 1−5 nm Pt clusters during high-temperature reduction of Pt4+/ Pt2+ in ethylene glycol solution. Because aliphatic groups of PVP (and other capping polymers) produce SFG signal in the 2800−2990 cm−1 range,18 a SFG alkyl reaction study would seem challenging to resolve without aggressive PVP removal. Recent work shows that PVP is disordered in the presence of H2 gas, which dissociates on Pt and results in a low SFG background.10 Leaving PVP intact inhibits aggregation in dense 2D films. Because SFG is coherent second-order nonlinear vibrational spectroscopy, disordering can be as effective as PVP capping removal in reducing background signal.19−21 Received: March 28, 2016 Published: March 31, 2016 8246

DOI: 10.1021/acs.jpcc.6b01615 J. Phys. Chem. C 2016, 120, 8246−8250

Article

The Journal of Physical Chemistry C

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EXPERIMENTAL METHODS

Nanoparticle Synthesis. Pt-PVP NPs of 1.7 nm average size were synthesized by combining 250 mg of chloroplatinic acid hexahydrate (H2Pt(IV)Cl6·6H2O, 37.5% metal basis, Sigma-Aldrich), 25 mL of ethylene glycol (ReagentPlus, Sigma-Aldrich), and 0.00625 g of NaOH at 433 K for 1 h under Ar with stirring. After synthesis, the NPs were precipitated using 1.0 M HCl, and PVP (Sigma-Aldrich) was introduced. Pt-PVP particles of 4.6 nm size were made using 100 mg of chloroplatinic acid hexahydrate (H2Pt(IV)Cl6·6H2O, Sigma-Aldrich) and 440 mg of PVP (∼29K, Sigma-Aldrich) with 20 mL of ethylene glycol (ReagentPlus, Sigma-Aldrich). The reaction flask was placed in a stirred silicone oil bath at 165 °C for 1 h under Ar flow. The Pt NPs were separated from the synthesis mixture by precipitation with acetone and centrifugation at 4000 rpm for 5 min. Repeated washing cycles with EtOH/hexane were done to remove excess PVP. Further details about Pt NP synthesis can be found elsewhere.10 Langmuir−Blodgett Film Deposition. Pt NPs dissolved in a 50/50 CHCl3/EtOH mixture were deposited onto an ultrapure water surface in a Kibron MTX automated trough. The film was then compressed, and the increase in surface pressure was monitored. When the desired surface pressure was reached, the SFG prism was pulled through the water at a rate of 3 mm/min, and a monolayer film of NPs was deposited onto the prism surface. For SFG and kinetic experiments, a surface pressure of >20 mN/m was used. Sum Frequency Generation Vibrational Spectroscopy. A Nd:YAG dye laser with 1064 nm fundamental output, 20 ps pulse width, and 20 Hz repetition rate was used in the SFG experiments. A frequency-doubling BBO crystal was used to generate a visible (532 nm) beam from 1064 nm. An optical parametric amplifier produced tunable infrared in the range 2680−3180 cm−1, corresponding to the C−H stretching modes of aliphatic and aromatic groups. Visible and infrared pulses of 150 μJ power were spatially and temporally overlapped on the SFG prism at angles of 63° and 48° from the surface normal to achieve total internal reflection. Experiments were performed in ppp polarization combination. Polished fused silica equilateral (60°) prisms were used in all the SFG experiments. A complete experimental description of SFG disordering under H2 is given elsewhere.10 Transmission Electron Microscopy. NP size and shape were measured with a 200 kV JEOL 2100 TEM microscope. Formvar films on copper grids (Electron Microscopy Sciences) were substrates for each NP sample. Kinetics. Turnover rates and selectivity were measured with a Hewlett-Packard 5890 Series II gas chromatograph connected to a circulating batch reactor. The Restek Haysep Q 80/100 column was used to monitor EH hydrogenation, while the Alltech 15% CBWX20M column was used to measure CH/1,4CHD hydrogenation, where cyclohexane, cyclohexene, benzene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene were readily separable. Benzene and 1,3-cyclohexadiene were not observed as products.

Figure 1. SFG spectra of 1.7 nm Pt-PVP NPs obtained during CH hydrogenation (A) followed by 1,4-CHD hydrogenation (B). The same film was exposed to a series of conditions (spectrum 1 → 9) at 295 K. The CH and 1,4-CHD were varied from 0 to 20 Torr with 200 Torr of H2 and background Ar to make the total pressure equal to 765 Torr. Dotted lines at 2770 cm−1 indicate the peak location of the 1,4CHD intermediate, whereas lines above 2800 cm−1 show the formation of molecular CH (spectrum 4) and molecular 1,4-CHD (spectrum 8) on the Pt-PVP NPs.

(spectrum 2) and the features of PVP (above 2800 cm−1) are reduced to nearly zero. The 1,4-CHD peak grows larger as CH pressure is increased to 10 and 20 Torr (spectra 3 and 4, respectively). Chemisorbed 1,4-CHD gradually saturates the surface at higher concentrations approximating a Langmuir adsorption isotherm, and the rehydrogenation of this intermediate becomes rate determining for the reaction. As H2 is reintroduced without CH, the 1,4-CHD peak disappears (spectrum 5), and the entire spectrum reverts back to the original background (spectrum 1). Within seconds of flushing with H2 again, all 1,4-CHD species react until the surface no longer contains reactive intermediates (CH hydrogenation is active on Pt at 295 K).13 When the same LB film is exposed to identical conditions with 1,4-CHD, stark differences are observed. First, a low background is again achieved by disordering PVP in H2 (spectrum 5). The same peak is observed new 2770 cm−1 when 2, 10, and 20 Torr of 1,4-CHD is introduced (spectra 6, 7, and 8, respectively), like CH hydrogenation. However, when H2 is reintroduced with 1,4-CHD removed, the peak of 1,4CHD remains at the same level observed during reaction (spectrum 9). In contrast to the reversible behavior of CH hydrogenation, 1,4-CHD irreversibly poisons Pt-PVP. Because of the inactivity, 2 Torr of 1,4-CHD immediately reaches saturation coverage as indicated by 10 and 20 Torr CH peak intensity at 2770 cm−1. The disappearance of adsorbed 1,4CHD occurs instantly (time-dependent data not shown) after circulating H2 in the absence of CH (spectrum 5). In contrast, the peak from “unreactive” adsorbed molecular 1,4-CHD persists for >1 h during H2 circulation without 1,4-CHD in the chamber (spectrum 9). Despite the same peaks near 2770 cm−1 obtained during reaction, CH and 1,4-CHD hydrogenation proceed through mutually exclusive reaction pathways. As pressure of 1,4-CHD is increased from 2 to 20 Torr, the peak of adsorbed 1,4-CHD subsides and new peaks appear (spectrum 8). Chemisorbed quatra-σ 1,4-CHD (2770 cm−1), i.e., the adsorption geometry with the strongest calculated

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RESULTS AND DISCUSSION Figure 1 shows a series of SFG spectra from a 1.7 nm Pt-PVP LB film to compare CH and 1,4-CHD hydrogenation. First, H2 is introduced alone to disorder PVP and obtain a low background (spectrum 1). When 2 Torr of CH is added, the distinct peak for adsorbed 1,4-CHD appears at 2770 cm−1 8247

DOI: 10.1021/acs.jpcc.6b01615 J. Phys. Chem. C 2016, 120, 8246−8250

Article

The Journal of Physical Chemistry C

Figure 2. (A) 1,4-CHD peak intensity at 2770 cm−1 on 1.7 nm Pt-PVP NPs. Following dosing with 2 Torr of 1,4-CHD and 200 Torr of H2, the reactor was flushed with only 200 Torr of H2 and Ar fill to produce 765 Torr total pressure. (B) Relative reaction rate (normalized to 20 Torr of CH) for 1.7 nm Pt-PVP NPs. Hydrogenation reactions were sequentially performed on the same film starting with 2 Torr of CH and ending with 20 Torr of 1,4-CHD at 295 K. The background Ar fill and 200 Torr of H2 were constant for all reactions. (C) Relative ethylene (EH) hydrogenation reaction rate (normalized to initial maximum rate) for 1.7 nm Pt-PVP NPs at 295 K. After the initial rate was measured, the sample was dosed with CH and 1,4-CHD hydrogenation conditions.

when H2 was reintroduced to glass, the peak intensity of 1,4CHD remained constant showing irreversible adsorption, with none of the dynamic behavior shown in Figure 1. All peaks in the SFG spectra shown here are attributed to organic species near the Pt surface. Figure 2A indicates the 1,4-CHD intermediate produced from 1,4-CHD hydrogenation is unreactive over a wide temperature range. As the temperature is increased and H2 circulates through the reactor, the 1,4-CHD peak at 2770 cm−1 persists above RT and diminishes to the baseline at ∼375 K as 1,4-CHD desorbs, reacts with H, or is dehydrogenated to benzene at the surface. On Pt(111) under UHV, the 1,4-CHD peak disappears between 250 and 300 K to make benzene, which also has a flat vibrational spectrum in SFG.8 (Because the C−H vibrations of benzene do not point away from the prism surface, they cannot produce SFG signal.) In the presence of H2 at atmospheric pressure, conversion processes of 1,4-CHD are hindered on Pt-PVP up to ∼375 K, even when CH is easily hydrogenated at RT.24 Kinetic tests of CH/1,4-CHD hydrogenation (Figure 2B) and EH hydrogenation (Figure 2C) were performed to compare activity and selectivity. 1,4-CHD hydrogenation yields two products, CH and CA (benzene and 1,3-cyclohexadiene were not detected). Even when considering both CH and CA from 1,4-CHD hydrogenation, total turnover for CH hydrogenation (which makes only CA) is ∼10 times higher compared to 1,4-CHD hydrogenation. Increasing from 2 to 20 Torr of 1,4-CHD, strongly chemisorbed 1,4-CHD is replaced by adsorbed molecular 1,4-CHD. The increase from 2 to 20 Torr of 1,4-CHD results in only a 10% increase in turnover frequency (TOF) and a shift in selectivity toward CA formation from 22 to 41%. As the surface contains less chemisorbed 1,4-CHD at 20 Torr, shown by a decrease of the intensity of the 2770 cm−1 peak (spectrum 8 in Figure 1), reactive species are less likely to be displaced as CH before achieving full hydrogenation. Finally, the EH hydrogenation TOF (a value routinely used to count active Pt sites at 295 K)25 is reduced by 90% following dosing with 1,4-CHD, further confirming irreversible poisoning (Figure 2C). In contrast, dosing with CH reduces EH hydrogenation rate by 10%. Both SFG experiments and kinetic results prove CH and 1,4-CHD hydrogenation proceed through mutually exclusive pathways despite similar vibrational features during reaction. Identical kinetic experiments on 4.6 nm Pt-PVP show the same major trends observed on 1.7 nm Pt-PVP (Figure S.2).

binding energy (145.6 kJ/mol) to Pt(111), may be by molecular 1,4-CHD because all four new peaks (2825, 2870, 2880, and 3030 cm−1) are consistent with the 1,4-CHD gas phase infrared spectrum.3 This is the first identification of molecular 1,4-CHD on the surface of a Pt catalytic material. To put this in context, Pt(111) must be cooled to 150 K to observe molecular vibrations similar to gas phase for crotonaldehyde.22 Even at 100 K in UHV, 1,4-CHD adsorbs exclusively as chemisorbed 1,4-CHD on Pt(111) as indicated one peak at 2763 cm−1 in reflection−adsorption infrared spectra.4 As 1,4CHD is increased from 2 to 20 Torr, the peak intensity of adsorbed 1,4-CHD decreases, and when the chamber is flushed with H2, the peak is highest of all conditions studied (spectrum 9). Two factors contribute to the formation of molecular 1,4CHD on the surface of 1.7 nm Pt-PVP at ambient conditions: (1) high-index/low-coordination sites destabilize quatra-σ 1,4CHD, which requires four flat close-packed surface atoms, and (2) PVP provides favorable van der Waals interactions to promote conversion to molecular 1,4-CHD near the Pt surface. Identical SFG experiments on 4.6 nm Pt-PVP (Figure S.1) indicate mutually exclusive reaction pathways among CH and 1,4-CHD, but not formation of molecular 1,4-CHD at 10 and 20 Torr. Because 4.6 nm NPs also are enveloped in PVP, (1) may be the strongest determinant. Recent work shows that the creation of high-index sites on Ru201 clusters alleviates repulsive intermolecular interactions among CO*−CO*, allowing supramonolayer coverage.23 High-index sites on 1.7 nm Pt-PVP destabilize 1,4-CHD adsorption, promoting transition back to molecular 1,4-CHD. The change from 20 to 0 Torr shifts the peak location of 1,4CHD from 2770 to 2780 cm−1 (on only 1.7 nm, not 4.6 nm PtPVP) providing further evidence that strong adsorbate− adsorbate interactions promote molecular 1,4-CHD at 20 Torr. In this context, the most effective way to liberate adsorbed 1,4-CHD from 1.7 nm Pt-PVP at 295 K may be to add competing organic adsorbates as opposed to reacting with H. There are also weak molecular features during CH hydrogenation at with 20 Torr (spectrum 4the new peaks match infrared spectra of molecular CH). The overall spectral change is small compared to spectrum 8, and new growth does not occur at the expense of 1,4-CHD intensity at 2770 cm−1. Simple adsorption of 1,4-CHD onto glass may in principle create the same molecular peaks observed at 20 Torr. However, in trials using a clean silica prism (data not shown), the strongest peak in the spectra was the CC−H stretch at 3030 cm−1 (for 1.7 nm Pt-PVP, this is the weakest peak). In addition, 8248

DOI: 10.1021/acs.jpcc.6b01615 J. Phys. Chem. C 2016, 120, 8246−8250

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The Journal of Physical Chemistry C Density functional theory simulations suggest there are two binding geometries on Pt(111) which allow the 1,4-CHD boat conformation to be strongly adsorbed (Figure 3),8 namely

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SFG spectra for CH and 1,4-CHD hydrogenation on 4.6 nm Pt-PVP (Figure S.1); kinetic results for 4.6 nm PtPVP (Figure S.2) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected], Tel 510-642-4053, Fax 510643-9668 (G.A.S.). *E-mail [email protected], Tel 510-642-4053, Fax 510643-9668 (J.M.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Director, Office of Basic Energy Sciences, Materials Science and Engineering Division of the U.S. Department of Energy under Contract No. DE-AC0205CH11231. Partial funding for J.M.K. was also provided by the UCB-KAUST Academic Excellence Alliance (AEA) Program.

Figure 3. Hallow HCP (above) and Bridge A (below) boat structures of 1,4-CHD chemisorbed on Pt(111) (adapted from ref 9) model catalyst. Black lobes represent adjacent CH2 groups with one C−H bond exactly perpendicular and the other parallel to the Pt surface. Each remaining C atom has a parallel C−H group and one Pt−C bond. CH hydrogenation is expected to proceed through Hallow HCP because this structure can start as a di-σ, forming two Pt−C bonds with C1 and C2. 1,4-CHD adsorbs as Bridge A, which forms stable quatra-σ bonds between four Pt atoms.

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“Bridge A” (adsorption energy: 145.6 kJ/mol) which is the only geometry with four C atoms binding with four separate surface Pt atoms (two di-σ-type bonds, “quatra-σ”) and “Hallow HCP” (one π bond and one di-σ bond), which is nearly as stable (adsorption energy: 141.6 kJ/mol). CH hydrogenation on PtPVP NPs may prefer the Hallow HCP mode because C1 and C2 atoms allow CH to first adsorb as a di-σ before being further dehydrogenated into Hallow HCP among three Pt atoms. 1,4CHD adsorption more likely involves Bridge A, which resists hydrogenation through four σ-type Pt−C bonds but is also prone to destabilizing adsorbate−adsorbate interactions, regenerating molecular 1,4-CHD at the Pt surface.

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CONCLUSIONS CH and 1,4-CHD hydrogenation are identical reactions on Pt(111) in terms of their surface vibrational fingerprint and kinetic turnover. It is shown here on Pt NP films CH hydrogenation is active and reversible, while 1,4-CHD hydrogenation poisons sites all the way up to 375 K. Unlike Pt(111), 1.7 and 4.6 nm Pt-PVP NPs may force cyclohexyl reactants to adopt Hallow HCP or Bridge A, and geometric factors and/or the presence of PVP eliminate conversion between the two. Previous work where PVP was gradually removed with UV light at RT (with other conditions held constant) showed dramatic shifts among π-allyl, 1,3-cyclohexadiene and 1,4-cyclohexadiene,10 i.e., three out of four intermediates ever observed on all conditions studied on Pt(111). These results provide further evidence that capped NPs can elicit reaction pathways which do not exist on single crystals. Although the structure of NPs is more heterogeneous, it is possible to create alkyl hydrogenation routes which are rigidly defined and exclusive from one another.

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REFERENCES

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ASSOCIATED CONTENT

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01615. 8249

DOI: 10.1021/acs.jpcc.6b01615 J. Phys. Chem. C 2016, 120, 8246−8250

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DOI: 10.1021/acs.jpcc.6b01615 J. Phys. Chem. C 2016, 120, 8246−8250