Elucidating the NanoparticleâMetal Organic Framework Interface of Pt

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Elucidating the Nanoparticle-MOF Interface of Pt@ZIF-8 Catalysts Cassandra L Whitford, Casey Justin Stephenson, Diego A Gomez-Gualdron, Joseph T. Hupp, Omar K. Farha, Randall Q. Snurr, and Peter C. Stair J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06773 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 15, 2017

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The Journal of Physical Chemistry

Elucidating the Nanoparticle-MOF Interface of Pt@ZIF-8 Catalysts Cassandra L. Whitford,a Casey J. Stephenson,b Diego A. Gómez-Gualdrón,a,c Joseph T. Hupp,b Omar K. Farha,b,d Randall Q. Snurra* and Peter C. Stairb,e* a

Department of Chemical & Biological Engineering, bDepartment of Chemistry, Northwestern

University, Evanston, Illinois 60208, USA.

c

Department of Chemical and Biological

Engineering, Colorado School of Mines, Golden, Colorado 80401, USA dDepartment of Chemistry, King Abdulaziz University, Jeddah, Saudi Arabia. eChemical Sciences & Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, USA. *

Corresponding authors: [email protected], 847-467-2977; [email protected],

847-491-3835

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Abstract Composites of metal nanoparticles encapsulated in metal-organic frameworks (NP@MOFs) have emerged as heterogeneous catalysts for regioselective reactions. While numerous NP@MOF composite combinations have been synthesized, characterization of the nanoparticle-MOF interface and the encapsulated nanoparticle surface have yet to be determined. In this work, Pt@ZIF-8 synthesized by the controlled encapsulation method was chosen as a representative NP@MOF, and in situ characterization methods coupled with DFT calculations were used to probe the nanoparticle surface. CO adsorption DRIFTS reveals that Pt@ZIF-8 exhibits redshifted linear- and bridge-bound CO peaks and a linear peak associated with cationic Pt. DFT calculations and 1H NMR suggest that these sites arise from the binding and electronic donation of the MOF linker, 2-methylimidazole, to the Pt surface. DRIFTS under argon reveals that linker fragments may be present on the Pt nanoparticle surface, suggesting a reaction between the nanoparticle and the MOF linker during controlled encapsulation synthesis. Finally, CO oxidation reveals via DRIFTS that the redshifted linear CO and bridging CO sites are active sites, while the cationic Pt is not. Overall, these results show that Pt@ZIF-8 contains unique Pt surface sites and indicate that the nanoparticle-MOF interface contains a heterogeneous mixture of framework 2-methylimidazole, free-standing 2-methylimidazole, and linker fragments. These findings expose the complex nature of the nanoparticle surface in NP@MOF composites and demonstrate the importance of characterizing their surface in order to understand their catalytic behavior.

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Introduction In the field of heterogeneous catalysis, there has been a recent emphasis on atom-efficient “green” reactions that minimize both the consumption of raw materials and the formation of byproducts.1 Of interest in the energy and chemicals industries, these reactions often target specific functional groups via regio- or chemo-selectivity. To achieve high selectivity, researchers often look to replicate the isolated active sites and the influence of tertiary structure found in enzymes.2,3 Homogeneous catalysts feature well-defined and isolated active sites but are difficult to separate and recycle, making them less desirable in industrial catalysis.4 Zeolites possess a size-selective pore structure somewhat analogous to an enzymatic crystal structure but possess limited chemical tunability.5,6 Thus, there is a search for synthetically tunable heterogeneous catalysts that effectively combine a known catalytic entity with a size-selective tertiary structure and well-designed local chemical environment. The catalytic properties of metal-organic frameworks (MOFs) have been investigated due to their well-defined, uniform pore structure, high surface area, and tunability.7–10 Catalytic active centers have been introduced into MOFs via functionalized linkers,11,12 coordinatively unsaturated or modified metal nodes,13–15 and by using the MOF external surface as a support for transition metal nanoparticles.16–18 Recently, composites of metal nanoparticles wholly encapsulated within MOFs have emerged as novel heterogeneous catalysts for regioselective reactions.7,19 These catalysts, designated NP@MOFs, exploit the crystalline structure and uniform pore size of the MOF to direct reactant access to the encapsulated catalytic nanoparticles. NP@MOFs were first synthesized by a ship-in-a-bottle method in which the MOF is synthesized and metal nanoparticles are loaded post-synthetically. Here, metal nanoparticles



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may be incorporated via chemical vapor deposition,20 gas phase infiltration,21 solution impregnation,22 solid grinding,23 or solution impregnation with microwave irradiation.24 However, these methods often require harsh pretreatment of the MOF and as a whole suffer from lack of control over placement of the metal nanoparticles.25 Recently, Lu et al. devised a controlled encapsulation synthesis technique for NP@MOFs in which ZIF-8 was crystallized in methanol in the presence of polyvinylpyrrolidone (PVP)-covered metal nanoparticles.26 This method allows for total encapsulation of nanoparticles and offers the ability to incorporate nanoparticles of almost any metal or size, including those larger than the characteristic ZIF-8 pore, provided that they are covered in PVP. Due to the high control and design capabilities of the controlled encapsulation synthesis method, it has been improved upon for ZIF-827 and extended to UiO-66,28 UiO-67,29 IRMOF-3,30 and other MOFs.31,32 NP@MOF composites have been applied to a variety of reactions, including selective hydrogenation,33–35 Suzuki-Miyaura coupling,36 and aerobic alcohol oxidation.21,37 In these studies, the NP@MOF composite is typically characterized by transmission electron microscopy (TEM), powder X-ray diffraction, and nitrogen sorption isotherms, with the goals of confirming MOF crystallization and observing complete encapsulation of the nanoparticles. However, the interface between the MOF and nanoparticle, and thus the surface of the metal nanoparticle, has not been characterized in detail. Reaction studies have cited steric restrictions by the MOF pore as the dominating factor influencing regioselectivity. Consequently, molecular modeling of reactions on these materials has thus far focused on steric effects.38,39 Thorough characterization of the nanoparticle-MOF interface is needed for improved calculations and for a full understanding of the experimentally observed regioselectivity in these systems.


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In this study, we selected Pt@ZIF-8 as a representative NP@MOF composite to probe the interface between the MOF and nanoparticle and to characterize the encapsulated Pt surface. ZIF-8 consists of 2-methylimidazolate linkers tetrahedrally coordinated to zinc nodes to yield a porous MOF having a sodalite topology.40 The stability of Pt@ZIF-8 has been previously established, and its ability to perform regioselective hydrogenations has been demonstrated.26,33 Here, we explore the influence of ZIF-8 on the encapsulated Pt surface using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO. This DRIFTS study was supplemented with density function theory (DFT) calculations to examine CO adsorption on Pt(111) and 1H NMR to identify interactions during Pt@ZIF-8 synthesis. By extending this DRIFTS investigation to CO oxidation on Pt@ZIF-8, we were able to determine which Pt active sites participate in the reaction. As a whole, this work aims to elucidate the nanoparticle-MOF interface of Pt@ZIF-8 and to establish a set of characterization tools that may be used to examine the nanoparticle surface within other NP@MOF composites in the future.

Methods 1. Synthesis a. PVP-Covered Pt Nanoparticles 2.5 nm PVP-covered Pt nanoparticles were prepared according to a previously reported procedure.26,41 A mixture of 533 mg PVP (MW = 29000, Sigma-Aldrich), 20 mL aqueous H2PtCl6 (6.0 mM, Sigma-Aldrich), and 180 mL methanol (99.99%, HPLC Grade, SigmaAldrich) was refluxed for 3 hours. Methanol was removed with a rotovap. The remaining PVPcovered nanoparticles were redispersed in methanol, and excess free PVP was removed with 30k-cutoff centrifuge filters (Pall Life Sciences).



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b. Pt@ZIF-8 Pt@ZIF-8 composites were prepared by scaling up a literature procedure.26 Separate 500 mL solutions of zinc nitrate hexahydrate (98%, Strem Chemicals) in methanol (0.1 M) and 2methylimidazole (99%, Sigma-Aldrich) in methanol (0.1 M) were prepared and combined in a 1 L Erlenmeyer flask. PVP-covered Pt nanoparticles were added immediately to the ZIF-8 reaction solution. The solution was left covered to react for 24 hours at room temperature. A gray precipitate formed and was isolated through centrifugation and washed three times with methanol. The composite was dried on a Schlenk line overnight to remove solvent trapped in the Pt@ZIF-8 pores. c. Supported Pt Nanoparticle Control Catalysts As a control, ZIF-8 was prepared with an identical procedure to Pt@ZIF-8 but without the addition of Pt nanoparticles. For some of the ZIF-8 sample, PVP-covered Pt nanoparticles were immobilized on the outer surface (designated Pt/ZIF-8) as follows. 4 mL PVP-covered Pt nanoparticles in methanol were added to 30 mg ZIF-8, sonicated, and allowed to physisorb over 3 hours. The resultant powder was washed three times with methanol and dried in air overnight. SBA-15 was prepared according to a procedure by Zhao et al.42 in which 10 g Plurionic P123 (Sigma-Aldrich) was dissolved in 75 mL H2O. 300 mL 2.0 M HCl (35%) was added, and the mixture was stirred for 24 hours to dissolve the P123. Next, 21 g tetraethyl orthosilicate (TEOS, Sigma-Aldrich) was added to the solution and stirred over 24 hours and then aged at 100 °C overnight. The resulting mixture was vacuum filtered and rinsed with deionized water, dried at 100 °C, and finally calcined in air at 550 °C for 5 hours. PVP-covered Pt nanoparticles were then immobilized on SBA-15 to give the control catalyst Pt/SBA-15. 30 mg SBA-15 were sonicated


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in 4 mL Pt nanoparticles for 1 minute and then left for 3 hours. The resultant Pt/SBA-15 was isolated via centrifugation and washed with methanol twice. 2. Characterization a. Conventional NP@MOF Characterization Inductively coupled plasma optical emission spectroscopy (ICP-OES) was performed on a Thermo iCAP 7600 to determine Pt loading of the composites. Powder X-ray diffraction (PXRD) patterns were taken on a Rigaku Smartlab thin-film diffractometer equipped with a 9 kW Cu rotating anode generator. Diffraction patterns were taken in the range 5° ≤ 2θ ≤ 50°. Nitrogen adsorption isotherm measurements were obtained from a Micromeritics Tristar II 3020 (Micromeritics, Norcross, GA) at -196 oC. Approximately 50 mg of catalyst was used for each measurement. Surface areas were estimated using the Brunauer-Emmett-Teller (BET) equation, with the pressure range chosen according to the consistency criteria of Rouquerol et al.43 Scanning transmission electron microscopy (STEM) images were captured using a Hitachi HD2300A STEM under phase contrast imaging at 200 kV accelerating voltage. Samples were made by adding a methanol suspension of catalyst dropwise to a lacey carbon-coated copper grid and allowing the solvent to evaporate. Particle size analysis was performed using ImageJ software on 200-300 nanoparticles per sample. b. 1H NMR NMR spectra were recorded on a Bruker AVANCE III 500 NMR spectrometer (500 MHz, 1

H). Chemical shifts (δ) for 1H are referenced to TMS, an internal standard. For each spectrum,

64 scans were collected with a delay time of 5 seconds. To examine the interaction between PVP-covered Pt nanoparticles and 2-methylimidazole, PtPVP nanoparticles were first synthesized and purified as previously stated and diluted to 0.2



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mg/mL Pt in methanol. 50.0 mL of the methanol solution of Pt-PVP nanoparticles (10 mg, 51 µmol Pt) was rotovapped to dryness at 40 °C. The brown paste was dissolved in 0.5 mL methanol-d4 and syringed into a J-Young NMR tube. In a separate 1 dram vial, 2methylimidazole (16 mg, 195 µmol, 4 equivalents) was dissolved in 0.25 mL methanol-d4. This was syringed into the J-Young NMR tube which was subsequently capped and mixed by inverting the tube. Approximately 15 minutes elapsed before the first spectrum was recorded. To examine the interaction between 2-methylimidazole and PVP, 2-methylidazole (17 mg, 20 µmol) and PVP (20 mg, 20 µmol, 1 equivalent based on monomer unit) were added to an NMR tube. 1 mL methanol-d4 was added, and approximately 15 minutes elapsed before the first spectrum was recorded. c. CO Chemisorption In Pt@ZIF-8 a portion of the NP surface is blocked by the MOF structure. To determine the percentage of available surface atoms, the dispersion obtained by CO chemisorption was compared to that obtained by STEM particle size measurements. Pulse CO chemisorption experiments were performed on an Altamira Instruments AMI-200 equipped with a thermal conductivity detector (TCD) in the CleanCat Core Facility at Northwestern University. Pt@ZIF8 and Pt/SBA-15 (100-200 mg) were loaded into a quartz U-tube reactor, which was weighed before and after sample loading. Temperature was controlled via a thermocouple located in the catalyst bed. The catalyst was reduced under 10% H2/N2 at 150 °C (30 cc/min, 10 °C min-1 ramp rate) for 1 hour and then purged with 100% He (30 cc/min) while the temperature was brought down to 40 °C and held for 5 minutes. A mixture of 5% CO/He and 100% He carrier gas was then pulsed into the 595 µL U-tube volume a total of 20 times to ensure the Pt surface was saturated with CO. The pulse outputs read by the TCD were integrated to quantify the amount of


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CO leaving the system. Dispersion based on CO chemisorption, Dchem, dispersion based on STEM, DSTEM, and available Pt surface sites, S, were calculated according to equations in Section S1. d. DRIFTS DRIFTS measurements were obtained on a Nicolet 6700 DRIFTS system equipped with a praying mantis cell. The sample was packed into the sample holder mounted to a sample heater. The sample was reduced at 150 °C for 1 hour with H2. The sample was further degassed with Ar at 80 °C for 3-5 hours and then left to equilibrate at 30 °C for one hour. Background spectra were taken under Ar at 30 °C. The sample was then saturated with CO for 25 minutes. CO was purged with Ar and spectra were taken 15-20 minutes after the purge began when there were no residual gas phase CO peaks. All spectra were measured at 30 °C using 64 scans and 4 cm-1 resolution. In order to analyze the full Pt@ZIF-8 spectrum under Ar, the sample was raised to various temperatures under Ar and brought down to equilibrate at 30 °C. Spectra were recorded with 64 scans and 4 cm-1 resolution using a KBr background taken at 30 °C. CO oxidation studies were performed using DRIFTS in situ. After the final Ar purge of gaseous CO from the cell, the temperature was increased to 50 °C and O2 was introduced into the system. After 5 minutes, the system was purged with Ar while the temperature equilibrated back to 30 °C. These steps were repeated three times total. All spectra were measured at 30 °C using 64 scans and 4 cm-1 resolution. 3. Theoretical Methods Plane-wave density functional theory (DFT) calculations were performed using VASP 5.3.1.44 The electron exchange and correlations were described with the Perdew-Burke-Ernzerhof (PBE) functional,45 which is based on the generalized gradient approximation (GGA). The D2 method



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of Grimme46 was used to explicitly consider dispersion interactions. Core electrons were described with the projector-augmented wave (PAW) method.47,48 An energy cutoff of 400 eV was used to construct the solutions to the Kohn-Sham equations from Bloch waves. Gaussian smearing49 with a 0.03 smearing parameter was used to accelerate energy convergence with respect to the k-mesh density. The electronic energy convergence was set to 10-6 eV, and geometries were considered converged when the energy difference between consecutive geometries was less than 10-5 eV. Using a 12 x 12 x 12 k-point mesh, the optimal lattice constant of bulk fcc platinum was calculated to be 3.915 Å, which is within 0.3% of the experimental value (3.924 Å). From optimized bulk platinum, we cut a 4-layer slab of Pt(111). The slab unit cell was reproduced along the a and b directions to obtain both 3 x 3 x 1 and 4 x 4 x 1 Pt(111) supercells. These supercells contain 36 and 64 Pt atoms, respectively, and are referred in the text as the Pt36 and Pt64 slabs. One and three CO molecules on Pt36 (nine surface atoms) correspond to coverages θ = 1/9 ML and θ = 1/3 ML, respectively. One and eight CO molecules on Pt64 (16 surface atoms) correspond to θ = 1/16 ML and θ = 1/2 ML, respectively. Note that in the latter four CO molecules are on atop sites and the rest are on bridge sites. The simulation supercells included a 15 Å vacuum space between slabs along the c direction. A 4 x 4 x 1 k-point mesh was used for the slab calculations. Unless otherwise specified, optimizations were done freezing the two bottom Pt layers, while optimizations for CO-Pt interactions were done freezing two bottom Pt layers and 2-methylimidazole. Calculations done to estimate CO vibrational frequencies were done with all atoms frozen but those of CO. Optimizations of gas phase molecules were done at the Γ points using a 10 x 15 x 12 Å orthorhombic unit cell.


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We calculated the electron density in optimized geometries using an 8 x 8 x 1 k-point mesh and the tetrahedron smearing method with Blochl corrections.50 The obtained electron density was used with the Bader splitting formalism51,52 to obtain atomic charges and in a subsequent non-self-consistent calculation to obtain the electronic density of states (DOS). Binding energies (∆E) of A to B were calculated as “EAB -EA -EB,” where EAB is the total energy of A and B together and EA and EB are the total energies of A and B separately. Structural models of the interface between Pt and ZIF-8 were constructed using the Crystal Builder module of Materials Studio.

Results and Discussion 1. Composite Formation, Nanoparticle Size, and Available Surface Pt The PXRD patterns and N2 adsorption isotherms for the Pt@ZIF-8 composites were compared to the parent MOF. The diffraction pattern for Pt@ZIF-8 matches that of ZIF-8 (Figure S2.1). Similarly, Pt@ZIF-8 has a gravimetric BET area of 1800 ± 24 m2/g, while that of ZIF-8 is 1760 ± 25 m2/g (Figure S2.2). STEM was used to confirm the encapsulation of the Pt nanoparticles in ZIF-8 and to determine the average nanoparticle diameter and size distribution for Pt@ZIF-8, Pt/SBA-15, and Pt/ZIF-8 (Figure 1). The average nanoparticle diameters for Pt@ZIF-8, Pt/SBA-15, and Pt/ZIF-8 were 2.7 nm (SD = 0.5 nm), 2.5 nm (SD = 0.5 nm), and 2.6 nm (SD = 0.5 nm), respectively. The Pt nanoparticles are thus larger than the characteristic pore size of ZIF-8. The amount of Pt in Pt@ZIF-8, Pt/SBA-15, and Pt/ZIF-8 was obtained with ICP, and the Pt loadings of these catalysts are 1 wt%, 0.25 wt%, and 0.7 wt%, respectively.



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Figure 1. STEM images of (a) Pt@ZIF-8, (b) Pt/SBA-15, and (c) Pt/ZIF-8 and (d) nanoparticle diameter distributions of each catalyst (Pt@ZIF-8 = top, Pt/SBA-15 = middle, Pt/ZIF-8 = bottom).

Pulse CO chemisorption was performed on Pt@ZIF-8 and Pt/SBA-15 to compare the available Pt surface atoms of encapsulated vs. supported catalysts. For 1 wt% Pt loading, Pt@ZIF-8 has a Pt dispersion of 4.2%. From Equation 1, this dispersion corresponds to a nanoparticle diameter of around 26 nm.53–55 From STEM, Pt@ZIF-8 contains 2.7 nm nanoparticles; using Equations 2-4, we can estimate that only 11% of the total surface Pt atoms are available for CO to adsorb. Agostini et al. examined the dispersion and particle diameter of Pd nanoparticles using multiple characterization techniques and found that Pd dispersion from CO chemisorption is systematically lower than Pd dispersion from TEM, and in turn CO chemisorption gives higher diameters than TEM.56 However, this systematic discrepancy between CO chemisorption and TEM dispersions is less than 5%. Our CO chemisorption gives a Pt@ZIF-8 dispersion fivefold lower than STEM, and since this exceeds systematic error, it is an 11 ACS Paragon Plus Environment

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indication of Pt surface site blockage. By comparison, Pt/SBA-15 dispersion is 20.1% which corresponds to approximately 5.4 nm diameter nanoparticles. Given the diameter of 2.5 nm from STEM for Pt/SBA-15, this leaves approximately 46% of the Pt nanoparticle surface available for CO to bind. The discrepancy between STEM and CO chemisorption dispersion for Pt/SBA-15 can be attributed to residual PVP that blocks the Pt surface.57,58 Since the nanoparticle diameters for Pt@ZIF-8 and Pt/SBA-15 measured from STEM are nearly the same, the difference in CO chemisorption indicates that more surface Pt is blocked in Pt@ZIF-8 than Pt/SBA-15. Thus, while it is unknown whether PVP is present in Pt@ZIF-8, we hypothesize that ZIF-8 components contribute to the additional surface blockage. We were unable to detect any chemisorbed CO when performing CO chemisorption on Pt/ZIF-8. The implication of this finding is discussed in Section 6. 2. Probing the Pt Nanoparticles with DRIFTS a. Establishing available Pt sites with CO adsorption The adsorption of CO onto metals provides information about the available sites on the metal surface.59,60 There are three common binding modes of CO to metals: linearly bound CO, bridging CO, and three-fold hollow-bound CO. Vibrational frequency assignments for these binding modes on Pt have been established through single crystal infrared reflection absorption spectroscopy (IRAS) studies61–64 and later expanded upon for supported Pt through in situ IR spectroscopy65–67 and density functional theory.68–70 For supported Pt nanoparticles, the most prominent binding mode is linearly bound CO in the region 2090-2065 cm-1.71,72 Bridge-bound73 and three-fold hollow-bound74 modes occur more rarely and can be found in the region 18701820 cm-1 and around 1740 cm-1, respectively. These frequency assignments may shift with changing coverage or a change in the Pt surface electron density. For example, as the CO



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coverage decreases from saturated to low coverage, the CO frequency may redshift 10-30 cm-1 due to reduced dipole-dipole coupling.75,76 Additionally, the surface electron density of Pt may be altered by electron-donating or electron-withdrawing adsorbates. Primet et al. studied CO adsorbed on Pt/Al2O3 and observed a 75 cm-1 redshift upon the adsorption of pyridine and trimethylamine and a 10 cm-1 blueshift after dissociative chemisorption of HCl.71 PVP-covered Pt nanoparticles were immobilized on SBA-15, an inert support, in order to establish CO binding modes on as-synthesized Pt nanoparticles preceding encapsulation (Figure 2a). We chose to leave PVP on the surface of the Pt nanoparticles to prevent size changes upon heating and CO introduction.77 PVP does not electronically affect CO adsorption on supported Pt nanoparticles78 but does decrease the number of surface sites available for CO adsorption.58 The strong peak at 2089 cm-1 is assigned to linearly-bound CO on Pt, in agreement with previous work on Pt/SBA-15.57 In contrast, Pt@ZIF-8 (Figure 2b) features three peaks: 2112 cm-1 assigned to linear CO on Ptδ+, 2017 cm-1 assigned to linear CO on Pt0, and 1797 cm-1 assigned to bridging CO on Pt0. The linear CO peak is significantly redshifted by 72 cm-1 from the corresponding peak for linear CO on non-encapsulated Pt (Pt/SBA-15). We attribute the redshift to binding of 2-methylimidazole (2-methylimidazolate) to the Pt nanoparticle. The lone electron pair of the free nitrogen of the ZIF-8 linker likely binds to Pt and donates electron density, changing the surface electron density of Pt.71,79,80


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Figure 2. DRIFTS spectra for CO on (a) Pt/SBA-15 and (b) Pt@ZIF-8.

Pt@ZIF-8 exhibits a shifted bridging peak that is absent from the Pt/SBA-15 spectrum. The bridge-bound CO peak must originate from either steric or electronic effects caused by encapsulation within ZIF-8, as both samples contain nanoparticles from a single batch. Atop surface sites may be blocked due to the coordination of 2-methylimidazole, decreasing the ratio of linear to bridging CO. Alternatively, Van der Eerden found that the ratio of linear to bridging site intensity decreases as electron density of Pt clusters increases.81,82 Thus, the extinction coefficient of bridging CO may increase with donated electron density from 2-methylimidazole. 14 ACS Paragon Plus Environment


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Pt@ZIF-8 displays an additional linear CO peak at 2112 cm-1 that does not appear for Pt/SBA15. Peaks above 2100 cm-1 are typically assigned to cationic or oxidized Pt83 and may be due to incomplete reduction or intrinsic to the composite as discussed below. b. Examining the Catalyst Structure with DRIFTS Figure 3a compares the DRIFTS spectra of ZIF-8 and Pt@ZIF-8 under argon at 30 °C. All peaks in the ZIF-8 structure are retained in Pt@ZIF-8. However, Pt@ZIF-8 features additional peaks at 2234 cm-1, 2220 cm-1, and 2213 cm-1 (Figure 3b). Peaks in this region are attributed to nitriles, and a triplet is suggestive of acetonitrile, as reported by gas and liquid phase IR data.84 Acetonitrile was not used in any step of catalyst preparation, yet this feature appeared across multiple batches of Pt@ZIF-8. We speculate this arises from an unexpected reaction between 2methylimidazole and Pt during nanoparticle encapsulation.


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Figure 3. DRIFTS spectra taken under Ar of (a) ZIF-8 (red), Pt@ZIF-8 (black) and (b) a comparison of the nitrile region.

To confirm this peak assignment, Pt@ZIF-8 was heated to a series of temperatures (Figure S3.1). As Pt@ZIF-8 is heated, a peak at 2195 cm-1 appears while the previously observed triplet decreases in intensity. According to Szilgáyi, when observing acetonitrile on Pt/SiO2, a peak centered at 2270 cm-1 corresponds to the CN stretch of physisorbed acetonitrile and a peak at 2195 cm-1 is indicative of end-on chemisorbed acetonitrile coordinated through the lone electron pair of the nitrogen.85 The C-N stretch of Pt@ZIF-8 is centered at 2220 cm-1, redshifted from that 16 ACS Paragon Plus Environment


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of Szilgáyi due to interaction with a more electron-dense Pt. The peak at 2195 cm-1 that appears with increasing temperature confirms the presence of acetonitrile. Because the sole source of nitrogen in Pt@ZIF-8 synthesis is the 2-methylimidazole, we hypothesize that 2-methylimidizole reacts with Pt during the ZIF-8 crystallization, producing fragments via ring-opening. This finding is significant, suggesting that in the controlled encapsulation synthesis of other NP@MOF catalysts, the MOF linkers may react with the metal nanoparticles. The presence of reactive fragments on the encapsulated nanoparticle surface could influence their catalytic activity. 3. Investigating 2-Methylimidazole Coordination with 1H NMR During Pt@ZIF-8 synthesis, a suspension of PVP-covered Pt nanoparticles, 2methylimidazole, and zinc nitrate hexahydrate in methanol are combined and allowed to crystallize. We utilized 1H NMR to investigate the individual and combined components of this synthesis mixture. In the 1H NMR spectrum for 2-methylimidazole (Figure 4a), the sharp peak at δ = 6.86 corresponds to hydrogen atoms on the ring carbons, while the distinct resonance at δ = 2.34 identifies the methyl group. For free 29000 MW PVP, signature resonances for the three different methylene (-CH2-) groups of the pyrrolidone are shown in Figure 4b, as well as resonances for the α- and β-carbons of the polymer backbone. PVP-covered Pt nanoparticles and 2-methylimidazole were combined and left to react in an NMR tube. This spectrum (Figure 4c) may be compared to those of 2-methylimidazole (Figure 4a) and free PVP (Figure 4b). While each PVP resonance remains unchanged, the 2-methylimidazole ring carbon resonance has broadened and shifted to δ = 7.01, indicating a loss of electron density. The strong methyl resonance of 2-methylimidazole is likely buried under the PVP methylene peaks. Free PVP and 2-methylimidazole were then mixed (Figure 4d) and exhibit unchanged resonances and thus no


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interaction. We can then attribute the shift and broadening of the 2-methylimidazole ring carbons to the binding of 2-methylimidazole to Pt nanoparticles via the lone electron pair of the molecule’s non-protonated nitrogen atom.

Figure 4. 1H NMR spectra of (a) 2-methylimidazole, (b) 29000 MW PVP, (c) PVP-covered Pt nanoparticles and 2-methylimidazole, and (d) PVP and 2-methylimidazole. The strong peak at 4.86 in (b) is due to residual water.

4. Exploring the Interaction Between 2-Methylimidazole, PVP, Acetonitrile and Pt(111) with DFT Density functional theory (DFT) was utilized to explore 2-methylimidazole interacting with Pt(111) through the methyl group, sitting flat on the surface, or interacting through a ring nitrogen atom. Figure 5 shows the most stable geometries for these three configurations, wherein interaction via the methyl group is significantly weaker than the other configurations (-74 kJ/mol, Figure 5a). The strongest interaction (-219 kJ/mol) occurs for the linker sitting flat ca. 3.0 Å from the surface (Figure 5c). Notably, the interaction via a nitrogen atom is only 8 kJ/mol 18 ACS Paragon Plus Environment


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weaker (-211 kJ/mol) despite significantly less contact between the 2-methylimidazole and the surface. This interaction occurs with the nitrogen atom located in an atop position (Figure 5b) at a distance of 2.1 Å from the Pt atom. The interaction energy and the bond distance suggest the formation of a N-Pt coordination bond. In this configuration, 2-methylimidazole exhibits a net +0.25 electronic charge, while the Pt atom in contact with 2-methylimidazole has a +0.14 electronic charge, suggesting distribution of donated electron density throughout the Pt surface beyond the Pt atom that binds the linker. See Section S3 for further details.

Figure 5. Top view (top row) and side view (bottom row) for DFT-optimized interaction configurations for 2-methylimidazole on a Pt(111) surface (a) via methyl group, (b) via N-Pt coordination bond, (c) sitting flat on the surface. Binding energies (∆E) and key distances are listed below each panel; C: gray, H: white, O: red, N: dark blue, Pt: blue.

In nanoparticle post-synthetic modification, a surfactant or capping agent may be replaced by a ligand with stronger binding affinity on a nanoparticle surface.86,87 Ligand exchange has been used to replace PVP with hexanethiol on Ag nanorods while maintaining their original size.88 To explore the possibility of this phenomenon occurring in our system during encapsulation, we


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examined various configurations of the PVP monomer, N-vinylpyrrolidone, on Pt(111) (Figure S5.1). All attempted initial configurations slowly converged toward N-vinylpyrrolidone sitting flat on Pt. However, since the vinyl group is part of the backbone of PVP, which restricts the vinyl orientation relative to the surface, this configuration is unlikely to occur with the polymer. The configurations in Figure S5.1a,b correspond to constrained optimizations where only the z coordinates of N-vinylpyrrolidone atoms were allowed to change, informed by the PVP/Pt interaction model proposed by Somorjai and coworkers.89 We found the configuration with O on an atop site to be more favorable (-106 kJ/mol, Figure S5.1b) than the configuration with O on a hollow site (-82 kJ/mol, Figure S4.1a). In this configuration, the overall charge on Nvinylpyrrolidone is -0.004 (Figure S5.2). In both cases, the N-vinylpyrrolidone interaction is significantly weaker than the analogous one for 2-methylimidazole (-211 kJ/mol, Figure 5b), which suggests that ligand exchange between PVP and 2-methylimidazole is plausible. Similar to 2-methylimidazole, acetonitrile coordinates to Pt(111) in an atop position via the nitrogen atom, with a binding energy of 145 kJ/mol (Figure S5.3). This coordination is weaker than 2-methylimidazole but stronger than N-vinylpyrrolidone and supports the existence of bound acetonitrile on the Pt surface of Pt@ZIF-8. 5. Examining CO Adsorption with DFT a. Establishing CO Frequencies on Pt(111) The frequencies of CO on linear, bridging, and three-fold hollow Pt(111) sites were established using DFT to confirm they match both the experimental DRIFTS results of Pt/SBA15 and the aforementioned literature values. Table S5.1 shows that our calculated CO stretching vibrational frequencies for CO binding on Pt(111) at low coverage using PBE agree well with



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experimental values reported in the literature. A discussion of the functional choice is provided in Section S5. Our calculations show that CO frequencies inversely correlate with C-O bond lengths and that changes in CO surface coverage produce noticeable shifts in vibrational frequencies (Figure S6.1). For instance, we found that as coverage decreases from 1/3 ML to 1/12 ML, there is a 10 cm-1 redshift in the highest calculated linear CO frequency (Figure S6.1a, g).90 Additionally, when considering coexisting linear and bridging CO, as coverage decreases from 1/4 ML to 1/12 ML, there is a 20 cm-1 redshift in linear CO (Figure S6.1a, h). Similarly in single crystal IR studies, a redshift of approximately 10-30 cm-1 for linear CO on Pt(111) occurs with decreasing coverage due to reduced dipole-dipole coupling of CO molecules.75,76 While we were unable to experimentally remove CO to test for coverage effects, the magnitude of our redshift between Pt/SBA-15 and Pt@ZIF-8 (72 cm-1) suggests that the shift is not caused by CO coverage effects71 but instead relates to the interaction between 2-methylimidazole and Pt. b. CO-Pt Interaction in the Presence of 2-Methylimidazole We explored the interaction of CO on Pt with 2-methylimidazole bound to the surface as presented in Figure 5b. This was done for both a Pt36 (Figure 6a-c) and a Pt64 slab (Figure 6d-g) to test different ratios of 2-methylimidazole to Pt atoms. As before, there is a good correlation between C-O bond length and CO frequency. For each case, there was a redshift in the CO vibrational frequencies relative to clean Pt(111) (Table S5.2). The magnitudes of the shifts for linear and bridging sites match those observed for Pt@ZIF-8 using DRIFTS. Specifically, the binding of one 2-methylimidazole on Pt36 causes a 64 cm-1 redshift for linearly bound CO, which agrees well with our experimental redshift for Pt@ZIF-8. Similarly, the binding of one 2-


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methylimidazole on Pt36 causes a 68 cm-1 redshift for bridging CO, which is similar to the ca. 55 cm-1 redshift of bridging CO on Pt@ZIF-8 relative to reported values.73

Figure 6. DFT-optimized configurations for adsorbed CO in the presence of 2-methylimidazole on Pt36 (a-c) and Pt64 (d-g). Calculated C-O stretching frequencies and C-O bond lengths are listed below each panel; C: gray, H: white, O: red, N: dark blue, Pt: blue.

The binding of one 2-methylimidazole molecule on Pt64 leaves a wider gap between linkers than on Pt36, which, in the case of linearly bound CO, results in different shift magnitudes depending on the proximity of CO to 2-methylimidazole (Figure 6d,e). Independently of this, the redshifts in CO frequencies caused by the binding of one 2-methylimidazole on Pt64 are somewhat smaller than those on Pt36 because the donation from one linker is distributed among more atoms. Nevertheless, the observed redshifts from DFT are consistent with those found in our DRIFTs experiments. c. Partial Density of States Analysis Figure S6.2 compares the partial density of states (DOS) for the d orbitals of the pristine Pt surface (Pt36) and of the Pt surface with bound 2-methylimidazole. The binding of 2methylimidazole to Pt(111) shifts the d band center closer to the Fermi energy by 0.11 eV. A



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higher d band center corresponds to stronger binding of adsorbates.91 According to the Blyholder model,80 during CO adsorption on transition metals, backdonation from the metal d orbitals to the CO antibonding 2π* orbital occurs, which weakens the C-O bond. A higher d band center can facilitate the backdonation from Pt d levels and weaken the C-O bond, which is consistent with the distances and vibrational frequencies reported in Figure 6. This is also apparent from the electron density maps illustrated in Figure 7.

Figure 7. Electron density mapped on strategically selected planes parallel to the (100) facet of the simulation supercell. (a) Pt36 slab, (b) linearly bound CO on Pt36, (c) 2-methylimidazole bound on Pt36, (d-e) linearly bound CO on Pt36 in the presence of 2-methylimidazole. In (d) the plane cuts through the C-O bond, and in (e) the plane cuts through the N-Pt bond (akin to (c)). Pt: blue; C: gray; N: blue; H: white. For electron densities, blue denotes absence of electron density (e. g. vacuum space), pink denotes intermediate electron density (e.g. at valence shells), and yellow donates the highest values of electron density (e. g. at atom cores).

The electron density on the topmost layer of the pristine Pt(111) surface (Figure 7a) decreases when CO binds to the surface (Figure 7b), showing electron backdonation from Pt to CO. When 2-methylimidazole binds the Pt(111) surface, however, this depletion of electron density does not


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occur (Figure 7c) and in fact, the increase in net negative charge of the slab indicates electron density is donated from 2-methylimidazole to the Pt surface. However, when CO is bound to the surface with 2-methylimidazole, the electron density is again depleted from the surface and high electron density appears instead on CO (Figure 7d,e), suggesting that part of the electron density donated by 2-methylimidazole to Pt is transferred to the CO. The bridge-bound and hollowbound cases are illustrated in Figures S5.3 and S5.4, respectively. 6. Encapsulation within ZIF-8 vs. Supporting on ZIF-8 PVP-covered Pt nanoparticles were physisorbed onto the external surface of ZIF-8 and the resulting CO adsorption spectrum is shown in Figure 8a. The 2106 cm-1 peak is assigned to linear CO on Ptδ+, the 2022 cm-1 peak is assigned to linear CO on Pt0, and the 1797 cm-1 peak is assigned to bridging CO on Pt0. This spectrum is nearly identical to that of Pt@ZIF-8 (Figure 2b). The difference between the catalysts, however, is that while CO adsorption on Pt/ZIF-8 is evident by DRIFTS, CO chemisorption was below the detection limit of pulse CO chemisorption for Pt/ZIF-8. Chizallet and coworkers used CO adsorption and DFT calculations to determine that the external surface of ZIF-8 contains numerous different surface sites, including Zn2+ ions, N-

groups

from

partially

ligated

ligands,

OH

and

NH

groups,

and

various

hydrogenocarbonates.92,93 Knowing this, Pt nanoparticles likely bind to exposed framework 2methylimidazolate nitrogen atoms on the external surface of ZIF-8 during Pt/ZIF-8 synthesis. While the ZIF-8 environment in contact with Pt is thus identical in Pt/ZIF-8 and Pt@ZIF-8, this does not explain why pulse CO chemisorption does not work for Pt/ZIF-8. DFT adsorption energies identify the possibility of removal of PVP by exchange with 2-methylimidazole in Pt@ZIF-8 (Figure S5.1). This is possible for Pt/ZIF-8 as well, but 2-methylimidazole is in contact with a much smaller fraction of the Pt nanoparticle. Consequently, the lack of CO



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chemisorption on Pt/ZIF-8 indicates that PVP is still on the Pt surface and is more thoroughly preventing CO chemisorption when compared to CO chemisorption on Pt/SBA-15. For Pt/ZIF-8, this leaves CO adsorption, as detected with DRIFTS, to proceed only on the portion of Pt nanoparticles in contact with ZIF-8, which is identical to the nanoparticle-MOF environment in Pt@ZIF-8.

Figure 8. (a) CO adsorption DRIFTS spectrum for Pt/ZIF-8 and (b) DRIFTS under Ar for ZIF-8 vs. Pt/ZIF-8. Pt/ZIF-8 was also examined with DRIFTS under Ar to compare its overall structure to that of ZIF-8 (Figure 8b). The only difference between Pt/ZIF-8 and ZIF-8 is a doublet in the 25 ACS Paragon Plus Environment

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nitrile region with peaks at 2235 cm-1 and 2223 cm-1. A doublet in this region is indicative of either propane nitrile or acrylonitrile.84 The appearance of a nitrile peak once again gives evidence that 2-methylimidazole reacts with the Pt nanoparticles. Interestingly, the two-carbon nitrile species found in Pt@ZIF-8 versus the three-carbon nitrile species found in Pt/ZIF-8 indicates different reactivity depending free versus framework 2-methylimidazole. 7. Investigation into the Cationic Pt The presence of cationic Pt in Pt@ZIF-8 (Figure 2b) was unexpected, as the preformed platinum nanoparticles appear fully reduced after identical reduction treatment in Pt/SBA-15 (Figure 2a). In order to determine whether Pt@ZIF-8 requires higher reduction temperatures to be fully reduced, we ran a series of in situ CO adsorption DRIFTS experiments after 1 hour H2 reduction treatments of 200 °C, 250 °C, and 300 °C as shown in Figure 9.

Figure 9. CO adsorption DRIFTS spectra of Pt@ZIF-8 reduced under H2 for 1 hour at (a) 200 °C, (b) 250 °C, and (c) 300 °C.

When Pt@ZIF-8 is reduced at 200 °C, there are three peaks similar to the original spectrum: the 2110 cm-1 peak is assigned to linear CO on Ptδ+, the 2040 cm-1 peak is assigned to linear CO on Pt0, and the 1806 cm-1 peak is bridging CO on Pt0. These assignments remain generally consistent for Pt@ZIF-8 reduced at 250 °C and 300 °C. However, after the reduction at 300 °C, 26 ACS Paragon Plus Environment


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the linear CO on Ptδ+ and bridging CO peaks have decreased in intensity with respect to linear CO on Pt0. The bridging peak at 300 °C also appears to have broadened, suggesting the presence of various types of bridging CO species. The linearly bound CO on Pt0 has blueshifted 39 cm-1 compared to Pt@ZIF-8 under 150 °C reduction. With higher reduction temperatures, the Pt@ZIF-8 spectrum begins to resemble that of Pt/SBA-15 (Figure 2a). Lu et al. reported that Pt@ZIF-8 begins to decompose slowly at 175 °C under N2.26 Considering this, Pt@ZIF-8 is likely degrading at 200 °C, 250 °C, and 300 °C under H2. Decomposition could destroy the coordination between the Pt nanoparticles and 2-methylimidazole, causing them to behave like free Pt nanoparticles or like Pt nanoparticles bound to an inert support. This degradation may also allow space for Pt nanoparticles to agglomerate, which could be the source of the blueshift in linear CO frequency.58,72 STEM images taken after in situ DRIFTS at higher reduction temperatures show some decomposition of the ZIF-8 edges and slight agglomeration at reduction temperatures above 150 °C (Figure S7.1). Reduction thus cannot be performed above 150 °C and the presence of Ptδ+ is inherent to Pt@ZIF-8. 8. Using CO Oxidation with in situ DRIFTS to Identify Active Sites Figure 10 shows spectra taken under argon after CO oxidation on Pt@ZIF-8, Pt/SBA-15, and Pt/ZIF-8. After 30 minutes of flowing O2 at 50 °C over Pt@ZIF-8, the peaks corresponding to linear and bridging CO on Pt0 disappear. The peak corresponding to linear CO on Ptδ+ remains and is shifted to 2128 cm-1. On Pt/SBA-15, the single peak decreases in intensity but does not disappear completely. For Pt/ZIF-8, the linear CO on Pt0 and bridging CO peaks decrease but do not disappear after 30 minutes in O2. The bridging CO peak has broadened and shifted to a center at approximately 1821 cm-1. Similarly to Pt@ZIF-8, the peak at 2107 cm-1 remains unchanged in intensity but has shifted to 2121 cm-1.


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Figure 10. CO adsorption DRIFTS spectra for (a) Pt@ZIF-8, (b) Pt/SBA-15, and (c) Pt/ZIF-8 after CO oxidation for times between 0 and 30 minutes of flowing O2 at 50 °C.

The disappearance of the linear and bridging CO peaks on Pt@ZIF-8 indicates that these sites are active for CO oxidation. Similarly on Pt/SBA-15, the linear CO site is active for CO oxidation, but does not disappear completely as it does for Pt@ZIF-8. This may indicate that the redshifted linear CO site on Pt@ZIF-8 is more active than the conventional linear CO site as represented by Pt/SBA-15. The linear CO peak redshift for Pt@ZIF-8 corresponds to increased π-backdonation from Pt to CO.80 This in turn weakens the C-O bond and makes CO more active toward the p electrons of O.94,95 Previous studies have found that electron-rich Pt nanoparticles enhance CO oxidation activity.81,95 From a mechanistic perspective, the higher surface electron density and blocked sites of Pt@ZIF-8 may also affect the reaction mechanism of CO oxidation by changing the CO coverage or altering the adsorption and dissociation of O2.81,95 Exploration of the CO oxidation mechanism, however, is outside the scope of this study. The spectra also indicate that the bridging site in Pt@ZIF-8 is an active site. Reports have shown that Pt catalysts with lower linear to bridging ratios have high CO oxidation activity, suggesting bridging CO is subject to oxidation.81 For Pt/ZIF-8, the linearly-bound CO and bridge-bound CO on Pt0 are active sites. The incomplete disappearance of these peaks is likely an artifact of slight 28 ACS Paragon Plus Environment


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temperature or O2 flow rate differences due to both experimental limitations and the qualitative nature of DRIFTS. In both Pt@ZIF-8 and Pt/ZIF-8, CO on Ptδ+ does not participate in CO oxidation. Recently, Ding et al. compared supported single atom Pt and supported Pt nanoparticles with DRIFTS and determined that single Pt atoms do not participate in CO oxidation.96 Zhang and coworkers have also shown that oxidizing Pt nanoparticles renders them inactive for CO oxidation.97 The cationic state of Pt, whether single atom, oxidic Pt, or PtOx, thus cannot be determined from its inactivity alone. Ding found that when desorbing CO from single atom Pt, Pt polycarbonyl peaks are visible in the spectrum. The spectrum of Pt@ZIF-8 taken immediately after switching from CO to Ar exhibits no polycarbonyl peaks (Figure S7.2). Additionally, Ding showed that as oxygen is introduced into the system, the CO peak on single atom Pt does not shift. In contrast, our Ptδ+ peak blueshifts, indicating that Pt is not in the form of single atoms. Because PtOx is not directly observed in the system, we assign the Ptδ+ to oxidic Pt within the Pt nanoparticle. 9. Proposed Model of the Pt@ZIF-8 Nanoparticle-MOF Interface Given the evidence of 2-methylimidazole coordination to Pt nanoparticles, the probability that 2-methylimidazole displaces PVP, and the observed CO binding sites, we set out to develop a plausible structural model for the interface between Pt and ZIF-8. One such model is presented in Figure 11. The model was obtained by cutting a slab of the experimentally obtained ZIF-8 crystal structure along the crystallographic (111) plane, making sure that both sides of the slab were terminated with 2-methylimidazolate “dangling” linkers. The obtained ZIF-8(111) slab is defined by a trigonal unit cell (a = b ≠ c, α = β = 90̊, γ=120̊). Subsequently, a monolayer Pt(111) slab was placed at either side of ZIF-8(111), expanding it 3.6% to match the ZIF-8(111) unit cell. The NZn-N angle, which is 109.5 ̊ throughout the bulk of a ZIF-8 crystal, took values ca. 115 ̊ near the


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interface, which suggest some minor strain at the interface. The crystallographic information file can be found in the SI.

Figure 11. Side-view of a plausible structural model for the interface between ZIF-8 and Pt in Pt@ZIF-8 based on the experiments and calculations discussed in this work. C: black, H: white, Zn: light purple, N: bright blue (N atoms forming coordination bonds with Pt shown as blue base).

We performed a cut of our model parallel to Pt(111) at the height of the 6-membered ring window of ZIF-8 closest to the Pt surface (5.8 Å) to reveal the view shown in Figure 12a. From this structure, we estimate that due to the presence of ZIF-8 in this interface model only ca. 45 % of Pt atoms at the surface are accessible. However, Figure 12a shows relatively wide areas on Pt(111) unoccupied by linkers. The ratio of 2-methylimidazolate linkers to surface Pt atoms is lower (0.074) than in our calculations on Pt36 (0.111) and closer to that in our calculation on Pt64 (0.065). Considering that i) the redshifts in CO adsorption experiments on Pt@ZIF-8 are closer to the redshifts in CO vibrational frequencies that we found in our calculations on Pt36, ii) the wide gap between 2-methylimidazolate linkers on Pt(111) is inconsistent with the sterically



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constrained environment at the Pt surface that it is inferred from the experimentally observed regioselective hydrogenation of 1,3 hexadiene in Pt@ZIF-8,33 iii) the CO chemisorption result that 11% of surface Pt atoms are free to bind CO, iv) the synthesis of Pt@ZIF-8 involves an extreme excess of 2-methylimidazole, and v) our DFT calculations showing that 2methylimidazole molecules can bind on their own to the Pt surface, it is likely that free-standing 2-methylimidazole not connected to ZIF-8 are present on Pt at the interface. One such scenario is illustrated in Figure 12b, where the 2-methylimidazole to surface Pt ratio is 0.099 and ca. 30% of Pt atoms are estimated to be accessible (crystallographic information file included in SI). This is closer to the value of 10% free Pt surface atoms calculated from CO chemisorption. Figure 3 suggested that chemisorbed acetonitrile likely also exists on the surface of the Pt nanoparticles, blocking additional CO adsorption sites. Thus, it is possible that the regioselectivity observed from catalytic testing of these NP@MOF materials26,33 is a result of 2-methylimidazole and smaller linker fragments blocking sites on the nanoparticle surface. Here, as in the work of Medlin and others, the organic fragments on the surface of the nanoparticle may prevent reactants from lying flat on the nanoparticle surface, forcing regioselectivity via end-on interactions.98,99 Finally, recent TEM work suggests that the most stable termination of ZIF-8 is its (110) plane.100 If the interface between Pt and ZIF-8 features the ZIF-8 (110) plane as opposed to the (111) plane, only 40% of surface Pt would be available to bind CO. In the presence of free-standing linkers, this value decreases to 16%, which is again very close to the value of 11% obtained from CO chemisorption. The model surface for this scenario is shown in Figure S8.1.


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(a)

(b)

Figure 12. Top-view of the suggested structural model of the interface between ZIF-8 and Pt obtained by a cut parallel to the Pt surface at 5.8 Å from it (i.e. at the height of ZIF-8 window closest to the surface) (a) without excess (free-standing) 2-methylimidazole linkers: ca. 45% accessible surface and (b) with excess 2-methylimidazole linkers: ca. 30% accessible surface.

Conclusions We have characterized the nanoparticle-MOF interface of Pt@ZIF-8, a prototype NP@MOF system that performs regioselective catalysis, using in situ DRIFTS, 1H NMR, DFT, and CO



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oxidation. CO adsorption DRIFTS revealed a redshifted linear CO peak on Pt0, 72 cm-1 from that of PVP-covered Pt nanoparticles physisorbed to SBA-15. This spectrum also shows a new bridging peak and a linear peak corresponding to CO on Ptδ+. DRIFTS under argon of Pt@ZIF-8 shows adsorbed acetonitrile on Pt, which we hypothesize results from reaction between 2methylimidazole and Pt during MOF synthesis. From 1H NMR, we observed the binding of the ring nitrogen atom of 2-methylimidazole to the PVP-covered Pt nanoparticles. DFT calculations of CO adsorption on Pt(111) in the presence of 2-methylimidazole support the hypothesis that the redshift found in the CO adsorption spectra comes from electron donation from 2methylimidazole to the Pt surface. A series of H2 reductions at higher temperatures reveal that Ptδ+ is intrinsic to Pt@ZIF-8. From CO oxidation experiments, we saw that the linear CO on Pt0 and bridging CO sites are active for CO oxidation, while the linear CO on Ptδ+ is not. This, as well as a blueshift in the linear CO on Ptδ+ peak during oxidation, leads us to believe that there is oxidic Pt within the Pt@ZIF-8 composite. Overall, the combination of experimental methods and calculations reveals a complex interface consisting of framework 2-methylimidazole, freestanding 2-methylimidazole, and reacted fragments bound to Pt in Pt@ZIF-8 and an electronically altered Pt surface. The approaches presented here thus substantiate the importanance of thoroughly examining and analyzing the unique metal surfaces within NP@MOF composite catalysts. This realization is essential to the planning and interpretation of future kinetic studies using NP@MOF catalysts, as the activity and selectivity of reactions depends on the surface environment of the nanoparticles. Acknowledgments We gratefully acknowledge financial support from the National Science Foundation (DMR1334928). The Clean Catalysis Facility of the Northwestern University Center for Catalysis and Surface Science is supported by a grant from the DOE (DE-SC0001329). The CleanCat Core facility acknowledges funding from the U.S. Department of Energy (DE-FG02-03ER15457) 33 ACS Paragon Plus Environment

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used for the purchase of the Altamira AMI-200. DFT calculations were made possible by the NERSC computational resources of the U.S. Department of Energy (DOE). This work made use of the J.B. Cohen X-Ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR-1121262) at the Materials Research Center of Northwestern University. This work made use of the EPIC facility of the NUANCE Center at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. Metal analysis was performed at the Northwestern University Quantitative Bio-element Imaging Center. C. L. W. would also like to thank Stephanie Kwon for SBA-15 synthesis, David Chen for his assistance in MOF characterization, Neil Schweitzer for helpful discussion, and Benjamin Schweitzer for his assistance with DFT calculations. Supporting Information. MOF characterization, DRIFTS under argon, charges on 2methylimidazole, binding of N-vinylpyrrolidone, CO on Pt(111) DFT calculations, cationic Pt reduction profiles, ZIF-8 (110) model surface (PDF); crystallization files. References (1) Hutchings, G. J. Heterogeneous Catalysts- Discovery and Design. J. Mater. Chem. 2009, 19, 1222–1235. (2) Corma, A. Attempts to Fill the Gap Between Enzymatic, Homogeneous, and Heterogeneous Catalysis. Catal. Rev. 2011, 46, 369–417. (3) Sanders, J. K. M. Supramolecular Catalysis in Transition. In Supramolecular Science: What Is It and Where Is It Going?; Springer Netherlands: Dordrecht, 1999; pp 273–286. (4) Cole-Hamilton, D. J. Homogeneous Catalysis- New Approaches to Catalyst Separation, Recovery, and Recycling. Science 2003, 299, 1702–1706. (5) Coronas, J. Present and Future Synthesis Challenges for Zeolites. Chem. Eng. J. 2010, 156, 236– 242. (6) Wang, Z.; Yu, J.; Xu, R. Needs and Trends in Rational Synthesis of Zeolitic Materials. Chem. Soc. Rev. 2012, 41, 1729–1741. (7) Corma, A.; Garcia, H.; Llabres i Xamena, F. X. Engineering Metal-Organic Frameworks for Heterogeneous Catalysis. Chem. Rev. 2010, 110, 4606–4655. (8) Gascon, J.; Corma, A.; Kapteijn, F.; Llabre i Xamena, F. X. Metal-Organic Framework Catalysis: Quo Vadis? ACS Catal. 2014, 4, 361–378. (9) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Metal–Organic Framework Materials as Catalysts. Chem. Soc. Rev. 2009, 38, 1450–1459. (10) Farrusseng, D.; Aguado, S.; Pinel, C. Metal–Organic Frameworks: Opportunities for Catalysis. Angew. Chemie - Int. Ed. 2009, 48, 7502–7513. (11) Vermoortele, F.; Ameloot, R.; Vimont, A.; Serre, C.; De Vos, D. An Amino-Modified ZrTerephthalate Metal–organic Framework as an Acid–base Catalyst for Cross-Aldol Condensation. Chem. Commun. 2011, 47, 1521–1523. (12) Kleist, W.; Jutz, F.; Maciejewski, M.; Baiker, A. Mixed-Linker Metal-Organic Frameworks as Catalysts for the Synthesis of Propylene Carbonate from Propylene Oxide and CO2. Eur. J. Inorg. Chem. 2009, 2009, 3552–3561. (13) Hong, D.; Hwang, Y. K.; Serre, C.; Ferey, G.; Chang, J.-S. Porous Chromium Terephthalate MIL101 with Coordinatively Unsaturated Sites: Surface Functionalization, Encapsulation, Sorption and Catalysis. Adv. Funct. Mater. 2009, 19, 1537–1552. (14) Nguyen, H. G. T.; Schweitzer, N. M.; Chang, C.-Y.; Drake, T. L.; So, M. C.; Stair, P. C.; Farha, 34 ACS Paragon Plus Environment


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