An Attenuated Total Reflection Fourier Transform Infrared (ATR FT-IR) Spectroscopic Study of Gas Adsorption on Colloidal Stearate-Capped ZnO Catalyst Substrate Ian P. Silverwood,a Colin W. Keyworth,b Neil J. Brown,b Milo S.P. Shaffer,b Charlotte K. Williams,b Klaus Hellgardt,a Geoff H. Kelsall,a Sergei G. Kazariana,* a b

Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom

Attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy has been applied in situ to study gas adsorption on a colloidal stearate-capped zinc oxide (ZnO) surface. Infrared spectra of a colloidal stearate-capped ZnO catalyst substrate were assigned at room temperature using zinc stearate as a reference compound. Heating was shown to create a monodentate species that allowed conformational change to occur, leading to altered binding geometry of the stearate ligands upon cooling. CO2 and H2 adsorption measurements demonstrated that the ligand shell was permeable and did not cover the entire surface, allowing adsorption and reaction with at least some portion of the ZnO surface. It has been demonstrated that stearate ligands did not prevent the usual chemisorption processes involved in catalytic reactions on a model ZnO catalyst system, yet the ligand-support system is dynamic under representative reaction conditions. Index Headings: ZnO; Zinc stearate; Colloidal catalyst; Catalysis; Gas adsorption; ATR FT-IR; Attenuated total reflection Fourier transform infrared spectroscopy.

INTRODUCTION Metal-carboxylate complexes, or metal soaps, are of interest in a number of fields, such as paint additives,1,2 polymer composites,3 and coatings for lubrication, superhydrophobicity, or wear resistance.4–7 Recent publications have considered the use of stearate ligands in the manufacture of metal oxide nanoparticles8 and colloidal catalysts.9–11 Unfortunately such materials have proved difficult to characterize using traditional diffraction methods. The low symmetry of the carboxylate group coupled with the disorder in long hydrocarbon chains complicates crystallographic measurement. As an alternative, the subtle changes in vibrational frequency of the carboxylate group caused by bonding geometry can be studied using infrared and Raman spectroscopy.2,12,13 Broadly, there are three bonding modes (Fig. 1), producing chelating, bridging, and monodentate forms. Most publications do not trouble themselves with further distinction, but it is worth acknowledging that this is not the limit of all possibilities. Ligands can adopt a number of geometries in bridging (syn—syn, syn—anti, anti— anti) as well as bond in mixed bridging and chelating forms, as shown by Deacon and Phillips.14 Received 12 June 2013; accepted 18 September 2013. * Author to whom correspondence should be sent. E-mail: s.kazarian@ imperial.ac.uk DOI: 10.1366/13-07174

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In 1998, Ishioka et al. determined that the coordination geometry of the zinc atom in zinc stearate was tetrahedral, with the carboxylate in bridging geometry.12 This assignment was made by comparison of the carboxylate rocking mode with that of monoclinic zinc acetate, which X-ray diffraction showed to form twodimensional polymeric sheets with the ligand in bidentate bridging mode.15 A previous report by Mesubi assigned the spectrum of the stearate to the chelating form at room temperature,13 with formation of a monodentate species upon melting. Ishioka, in a later publication, assigned the change in spectra upon heating to geometrical distortion of the carboxylate groups with the assistance of an X-ray absorption fine structure (XAFS) study.16 Ambiguity regarding the nature of the ligand geometry apparent in the spectra, and their changes upon heating, persists in the literature. Commercial heterogeneous catalysts for methanol synthesis from synthesis gas are a mixture of Cu, zinc oxide (ZnO), and aluminum oxide (Al2O3).17 Utilization of CO2 as an alternative carbon source for production of methanol could offer an alternative to fossil fuel exploitation, while satisfying the planet’s increasing energy need.18 While this would require a renewable energy source, it would allow production of a liquid fuel and consume CO2. Novel nano-structured catalysts suited to methanol synthesis from CO2 are therefore highly relevant research targets, with Cu,10,17 ZnO19, and mixed Cu/ZnO20 nanomaterials all reported in the literature. Structural targets for optimized catalytic activity would include a high surface area available for adsorption, stabilization of highly dispersed copper particles, strong metal-support interaction, and selection of exposed crystal faces that favor adsorption and reaction. This work comprises a Fourier transform infrared (FTIR) spectroscopic study of stearate-capped colloidal ZnO manufactured by the hydrolysis of diethyl zinc with zinc stearate, after catalytically relevant thermal treatments and gas sorption experiments; simple zinc stearate was used to inform spectral assignment. This preparation method allows nanoparticulate ZnO to be created, with varying stearate-ZnO ratios and without the use of excess ligands. Surface species found on ZnO have relevance to both heterogeneous and quasi-homogenous catalytic reactions where the colloidal catalyst is dispersed in solution as a sol. Modification of a reactant bond structure upon adsorption to a catalyst is fundamental to catalytic mechanism, and thus investigation of

0003-7028/14/6801-0088/0 Q 2014 Society for Applied Spectroscopy

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FIG. 1.

Major bonding geometries in metal carboxylates.

surface species may identify activated reaction intermediates. With colloidal materials it may also provide structural information about the interfacial bonding between the catalyst and surfactant.

EXPERIMENTAL Materials and Synthesis. The preparation of stearatecapped ZnO is described in detail elsewhere.21 Briefly a 5:1 mixture of diethyl zinc and zinc stearate in toluene was hydrolyzed to produce 3–4 nm crystalline ZnO particles. Zinc stearate was synthesized from potassium stearate and zinc chloride in ethanol.22 All gases were from BOC Industrial Gases and used without further purification.

Infrared Spectroscopy. Infrared spectra were recorded with the use of a Bruker Tensor 27 FT-IR spectrometer equipped with a liquid nitrogen cooled MCT detector. A heated Golden Gate ATR accessory (Specac) with a single reflection diamond crystal in the shape of an inverted prism was used for all measurements. Gas Flow. Gas flow was controlled using Bronkhorst EL-FLOWt Select mass flow controllers and a custommade stainless steel high-pressure, gas-handling manifold attached to a small volume stainless steel reactor. The reactor was a specially designed ‘‘covering-cap’’ high-pressure cell compatible with the Golden Gate accessory,23,24 sealed against the top plate with a Viton O-ring. Stainless steel tubes measuring .0625 in (1.5875 mm) outside diameter, with their bore at right angles to that of the reactor and opposite to each other, carried the reactant and exhaust gas flow. The internal volume of the reactor was 0.55 cm3. Sample Film Preparation. Solid zinc stearate was measured by compression of the material against the diamond crystal using the clamp of the Golden Gate accessory. Molten films were obtained by heating a quantity of zinc stearate contained by an O-ring atop the diamond plate, and post-melting samples were measured using the adhered film after cooling. Stearate-capped ZnO films were cast by evaporating suspensions of the material in ethanol onto the surface of the diamond of the Golden Gate accessory at room temperature, allowing a small volume to evaporate before adding another. Casting was complete when a film was visible to the naked eye as a milky cast, and inspected using an optical microscope to ensure total coverage of the diamond.

RESULTS AND DISCUSSION Zinc Stearate. Figure 2 shows the FT-IR spectra of zinc stearate at 323 K obtained before and after melting,

FIG. 2. ATR FT-IR spectra of zinc stearate recorded (A) before melting at 323 K, (B) after melting at 323 K, and (C) while molten at 473 K. Spectra are offset for clarity.

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TABLE I. Assignment of vibrational bands in zinc stearate before melting. Vibrational strengths denoted S, strong; M, medium; W, weak; sh, shoulder. Wavenumber/ cm 2954 2916 2873 2847 1534 1455 1397 949 742 721 579 549

1

Assignment

Strength

masCH3 masCH2 msCH3 masCH2 masCO2 dCH2 scissors msCO2 cCH2 wag dCO2 bend dCH2 rock cCO2 out of plane dCO2 rock

W, sh S W, sh S S S S W M, sh M W W

with that of the molten material, collected at 473 K. The spectrum of the unmelted material was assigned as follows:2, 12,1 3,16 The C–H stretching region displays bands due to the asymmetric (2955, 2916 cm 1) and symmetric (2873, 2847 cm 1) C–H stretches of CH3 and CH2 groups of the hydrocarbon tail, respectively. The band corresponding to the CH2 scissoring vibration was visible at 1455 cm 1, with the bands corresponding to the wagging mode at 949 cm 1 and a rocking mode at 721 cm 1 also apparent. As the electrons in the carboxylate group are delocalized, in the bidentate geometry the two C–O bonds are equivalent. Asymmetric and symmetric stretches are seen at 1534 and 1397 cm 1. The OCO bending mode is at 742 cm 1, with a deformation at 579 cm 1 and a rocking mode at 549 cm 1. Band assignments are shown in Table I. After melting and cooling, the spectrum of the solid zinc stearate (Fig. 2B) shows a number of changes below 1700 cm 1. These effects may be explained after a brief reference to the molten substance (Fig. 2C). Upon heating, bands in the infrared broaden, lowering the ability to resolve some peaks. Clearly, however, new bands have arisen above the asymmetric CO2 stretch. The highest wavenumber band, at 1631 cm 1, was assigned to the asymmetric COO stretch of a monodentate carboxylate species in agreement with Mesubi et al.13 Another band was resolved in this envelope, with its maximum at 1594 cm 1, which suggested a second monodentate species. A weak, broad peak at 1316 cm 1 corresponds to the symmetric stretch of these two monodentate species. This interpretation requires that the thermal energy supplied to the system was sufficient to overcome the energy of binding one of the two oxygen atoms in the carboxylate group to the zinc ions. As the absorbance of the bands does not significantly decrease, and all bands were assigned to bound carboxylate species, it suggests that the energy was not sufficient to break both Zn–O bonds simultaneously, which would lead to detachment of the stearate ligand. Detachment of the ligand would be likely to be through regeneration of the carboxylic acid or formation of an ester or alcohol. Both the ester and carboxylic acid contain a carbonyl group that would absorb strongly around 1700 cm 1. A primary alcohol would display the C–O stretch around 1050 cm 1. Neither of these bands was visible in the spectrum.

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Upon cooling to 323 K and solidification of the zinc stearate (Fig. 2B), the peak at 1631 cm 1 disappeared, demonstrating that it corresponds to a species that is stable only at elevated temperature. The heat-treated material shows additional peaks to those of the unmelted material, demonstrating that conformational change has occurred. Although not definitive evidence for the presence of a monodentate species13 over a distortion of the ligand geometry 16 in the liquid state, the irreversible nature of the change favors this interpretation. A band at 1593 cm 1 due to the monodentate species was seen in the CO stretching region. The bands due to the bidentate species at 1534 and 1397 cm 1 before melting have shifted to 1528 and 1399 cm 1, and shoulders have appeared at 1546 and 1468 cm 1. The shoulders at 1546 and 1468 cm 1 are assigned to the asymmetric and symmetric CO stretches of a further carboxylate geometry of undetermined type. Both chelating and bridging species are compatible with this separation of bands.25 Stearate-Capped Zinc Oxide. The ATR FT-IR spectra of stearate-capped ZnO nanoparticles collected during heating in air are shown in Fig. 3. Similarities to the zinc stearate spectra in Fig. 2 are to be expected, and the vibrations due to the CH3 and CH2 groups in the hydrocarbon chain appeared largely unchanged. Surface hydroxyl groups were apparent on the unheated sample as a broad, weak OH stretching peak at 3400 cm 1. This is visible as a negative feature in an elevated baseline in Fig. 3C upon heating, demonstrating that their presence was due to the dissociative adsorption of water, and suggesting that some fraction of the ZnO surface was free to interact with the atmosphere. Although this demonstrates that the surface of ZnO is accessible to gaseous reactants, it does not preclude that the stearate ligand is present at its maximum capacity. Steric inhibition by the hydrocarbon chain may prevent higher coverage. After cooling (Fig. 3B) the hydroxyls partially reformed, having reacted with moisture in the atmosphere. ZnO also displays strong absorption below 600 cm 1 , which is a common characteristic of metal oxides. This unfortunately masks the CO2 out of the plane-bending and rocking modes previously used to define the bonding geometry around the zinc.12 The other bands arising from carboxyl group vibrations, however, displayed certain differences. The OCO bending mode at 742 cm 1 in the stearate was not observed at all in the capped ZnO after heating and was considerably weaker than seen in the zinc stearate before heating. A possible explanation is that the zinc atoms bound to the carboxylate are held within a rigid ZnO lattice and are thus not as free to move as in the zinc stearate, frustrating the OCO bend in a bridging Zn– O–C–O–Zn species. Although the unheated material shows three well-resolved bands at 1539, 1462, and 1398 cm 1 due to the carboxylate stretches and the CH bend, a broad shoulder is visible at 1595 cm 1. This spectrum is reminiscent of the zinc stearate spectrum after heating, which showed a peak at 1593 cm 1, and suggested a monodentate carboxylate. These features demonstrate that although the majority of the carboxylate ligands have the same bridging structure as the zinc stearate before heating, the synthesis of the capped ZnO

FIG. 3. ATR FT-IR spectra of stearate-capped ZnO nanoparticles in air (A) at 323 K before heating, (B) 323 K after heating, and (C) at 473 K. Spectra are offset for clarity.

has caused some to be present in a different geometry. This suggests that the ZnO surface displays environmental heterogeneity and encourages a mixture of binding modes. After heating, the sharp bands due to the tetrahedral geometry were reduced, demonstrating that a shift to a less defined structure had occurred, as carboxylate-zinc bonds were broken upon heating, although without stearate detachment, and then rearranged before cooling. With the change of the ligand binding from a bidentate to monodentate form, the nature of adsorption sites for the reacting gases will be affected,

possibly freeing up a greater number of adsorption sites at reaction temperature and increasing catalytic activity. In the zinc stearate, the change in ligand binding occurs as part of the melting process. For the colloidal material, while ZnO remains solid, it is likely that the ligand shell ‘‘melts’’ at a similar temperature to that of the zinc stearate, as the hydrocarbon chains become mobile. This phase change is likely to enhance gas permeation and further improve catalyst performance under reaction conditions (typically 100 bar, 523 K for methanol synthesis).26 Over the short time scales studied within

FIG. 4. ATR FT-IR spectra of H2 adsorption on stearate-capped ZnO nanoparticles at room temperature using background spectra of capped ZnO under hydrogen at atmospheric pressure. Pressure increased from 1.8 bar (lowest) to 19.9 bar (highest). Spectra are offset for clarity.

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FIG. 5. ATR FT-IR spectra of stearate-capped ZnO nanoparticles under H2 at (A) 294 K, 0 bar before heating, (B) 294 K, 19.9 bar before heating, (C) 473 K, 0 bar, and (D) 323 K, 0 bar after heating. Spectra are offset for clarity.

this work, loss of the stearate is inconsequential up to 473 K. Hydrogen Adsorption on Stearate-Capped ZnO. Hydrogen pressure was increased to approximately 20 bar at ambient temperature, before the sample was heated to 473 K, where it was held for 5 min before cooling. The effect of increasing hydrogen pressure is shown in Fig. 4. Absorption spectra were recorded using a background spectrum of ZnO under H2 at atmospheric pressure. A small increase of absorbance due to the aliphatic chain is visible (2920, 2849, 729 cm 1). A possible explanation is that improved contact between the capped ZnO and the surface of the diamond of the ATR accessory was obtained with the increased pressure. Also of note is the increase of absorbance in the region of the broad OH stretching mode between 3500 and 3150 cm 1, accompanied by a band corresponding to the OH deformation at 950 cm 1. The strength of these bands relative to the CH stretching mode indicates that they are due to chemisorption of the hydrogen and not improved contact due to increased pressure. New surface OH groups on ZnO have therefore formed without displacement of the bound stearate, through either dissociative hydrogen adsorption or some other mechanism. Heating the samples under hydrogen (Fig. 5) produced spectra similar to those seen when heating under air. The intensity of the sharp features between 1700 and 1200 cm 1 decreased, and broader features indicating a range of geometries evolved. A loss of absorbance due to the surface hydroxyls at 3400 cm 1 upon heating was apparent, as was the decrease in the OCO bend at 742 cm 1. Carbon Dioxide Adsorption on Stearate-Capped ZnO. Carbon dioxide adsorption was studied spectroscopically in a similar manner to adsorption of hydrogen, by increasing pressure at room temperature, followed by heating to 473 K. Strong bands of gaseous CO2 in the

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regions of antisymmetric stretching (not shown) and bending modes (667 cm 1) were apparent as the pressure was increased. The most prominent change besides the increasing gas phase contribution is the range of overlapping bands from 1700 to 1300 cm 1, shown enlarged in Fig. 6. Three maxima were identified at 1522, 1447, and 1365 cm 1, which were assigned to the asymmetric (1522 cm 1) and symmetric (1365 cm 1) OCO

FIG. 6. ATR FT-IR spectra of CO2 adsorption on stearate-capped ZnO nanoparticles at room temperature using background spectra of ZnO under carbon dioxide at atmospheric pressure. Pressures of 1.8 (lowest), 6.3, 15.0, 19.1, and 20.1 bar (highest). Spectra are offset for clarity.

FIG. 7. ATR-FTIR spectra of stearate-capped ZnO nanoparticles under CO2 using background spectrum of air (A) 294 K, 1.8 bar before heating, (B) 294 K, 20.1 bar before heating, (C) 20.8 bar, 473 K, and (D) 323 K, 0 bar after heating. Spectra are offset for clarity.

stretch of a monodentate carbonate and the asymmetric OCO stretch (1447 cm 1) of an non-coordinated carbonate.25 It must be acknowledged that these features coincide with the bands due to the stearate ligand, both the OCO stretches of the carboxylate head and the CH2 bend of the aliphatic tail. However, no change in the CH stretching region is observed as well as a lack of features above 1550 cm 1 that would indicate carboxylate detachment, suggesting that the ligand was unaffected by CO2 adsorption. A weak band at 1070 cm 1 corresponds to both the CO stretch of the monodentate species and the symmetric bend of the non-coordinated carbonate ion. The band at 836 cm 1 was assigned to the out-of-plane carbonate-bending mode, providing evidence for CO2 adsorption on ZnO to form surface carbonate species. Figure 7 shows how the spectra of the capped ZnO are affected by heat and pressure under an atmosphere of CO2. A loss of surface hydroxyls is obvious around 3400 cm 1, and the sharp, zinc stearate-like bands become less prominent, as previously seen when heated under both air and hydrogen. The ligands have rearranged upon heating, and the portion of the surface uncovered by the stearate has reacted with the gas phase. The system has demonstrated itself to be rather dynamic, with the carboxylate changing its bonding to the ZnO surface, and the permeability of the ligand shell allowing diffusion of gases to the reaction sites, which remain accessible even at room temperature.

ambient temperature, showing that these gases permeated the ligand shell in the solid state and are free to react at the ZnO surface. Access of the reactants to the catalyst surface is therefore not prevented by the partial ligand coverage. The evidence suggests that adsorption occurred upon the ZnO surface not covered by the stearate, as changes to the adsorption bands of the ligand were not observed without heating. Upon heating, it was demonstrated that the stearate ligands in zinc stearate and stearate-capped ZnO can rearrange and adopt different binding geometry between the oxygen atoms in the carboxylate group and the zinc. This study has shown that stearate ligands used to stabilize colloidal catalyst nanoparticles do not prevent the usual chemisorption processes involved in catalytic reactions on a model ZnO catalyst system, yet the ligand-support interaction is dynamic under representative reaction conditions. Spectroscopic characterization has been demonstrated as a powerful tool in studying these materials and their reactions and has applications for colloidal particles beyond catalysis. Further work to exploit the benefits of this synthetic procedure and generate catalysts with well-defined and subtly varying properties should allow measurement of structureactivity relationships to a greater degree than conventional methods allow. ACKNOWLEDGMENTS This work was supported by an Engineering and Physical Science Research Council (EPSRC) grant (EP/H046380/1).

CONCLUSIONS The presence of surface hydroxyl groups identified upon the stearate-capped ZnO before heating proved that the ligand capping did not cover the entire metal oxide surface. Chemisorption of hydrogen and carbon dioxide as hydroxyls and carbonates was apparent at

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An attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopic study of gas adsorption on colloidal stearate-capped ZnO catalyst substrate.

Attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy has been applied in situ to study gas adsorption on a colloidal steara...
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