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Catalysis at Multiple Length Scales: Crotonaldehyde Hydrogenation at Nanoscale and Mesoscale Interfaces in Platinum–Cerium Oxide Catalysts Yutichai Mueanngern, Xin Yang, Yu Tang, Franklin (Feng) Tao, and L. Robert Baker J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 05 Jun 2017 Downloaded from http://pubs.acs.org on June 10, 2017

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The Journal of Physical Chemistry

Catalysis at Multiple Length Scales: Crotonaldehyde Hydrogenation at Nanoscale and Mesoscale Interfaces in Platinum–Cerium Oxide Catalysts

Yutichai Mueanngern1, Xin Yang1, Yu Tang2,3, Franklin (Feng) Tao2,3, and L. Robert Baker1*

1

Department of Chemistry and Biochemistry, The Ohio State University, Columbus OH 43210 2

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Department of Chemistry, University of Kansas, Lawrence, Kansas 66045

Department of Chemical and Petroleum Engineering, University of Kansas, Lawrence, Kansas 66045

*

Address correspondence to [email protected]

ACS Paragon Plus Environment


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Abstract Crotonaldehyde hydrogenation is carried out on a series of Pt–CeO2 catalysts to investigate the mechanism of enhanced C=O bond hydrogenation by an active oxide–metal interface. We show that deposition of CeO2 nanoparticles onto a Pt surface leads to two types of Pt–CeO2 interfaces: 1) the nanoscale interface defined by the contact of individual CeO2 nanoparticles with the Pt surface and 2) a larger, mesoscale interface defined by the boundary between domains of a selfassembled nanoparticles and the surrounding clean Pt substrate. Surprisingly, although the nanoscale interface accounts for greater than 90% of the total number of 3 phase boundary sites in the catalysts tested as shown by TEM analysis, C=O bond hydrogenation kinetics scale exclusively with the larger, mesoscale interface. Using in situ ambient pressure XPS, we show that these kinetics are not the result of variations in the CeO2 oxidation state due to H spillover or the result of Pt decoration by the migration of reduced Ce atoms during reaction. Instead, we hypothesize that reaction is rate limited by the surface migration of crotyl-oxy intermediates as they form on CeO2 nanoparticles and subsequently diffuse and react on surrounding Pt.


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Introduction Understanding the mechanism of surface chemical reactions is necessary to inform the rational design of new materials for high efficiency applications in energy conversion and chemical processing. Surface reaction kinetics are sensitive to structural features of a catalyst, which can vary in size from the molecular regime up to the macroscale.1-3 Consequently, catalyst architecture must be controlled at multiple length scales in order to maximize reaction performance and catalyst selectivity.4-6 Considering the nanometer length scale, it has been demonstrated that tuning the size and shape of a nanoparticle catalyst can provide control over reaction rate and selectivity for a range of catalytic reactions.7-11 In addition to physical structure sensitivity, electronic factors also play an important role in determining the performance of nanoparticle systems. This is due both to the electronic effects of quantum confinement in small clusters12-14 as well as interfacial effects between a catalytic particle and its support.15-17 Support effects in heterogeneous catalysis in particular often dominate performance of a catalyst system, and certain catalysts have been shown to display order of magnitude changes in selective activity for specific reaction pathways depending on the electronic structure of the oxide support.18-21 Several mechanisms have been considered to explain the origin of support effects in heterogeneous catalysis including the ability of a support to alter the electronic structure of a supported nanoparticle via interface bonding and/or interfacial charge transfer22-24, the specific activity of 3 phase boundary sites where a molecular reactant can simultaneously interact with both phases of a metal–support catalyst system25-26, and spillover mediated reaction kinetics where a molecule can selectively adsorb on one phase and subsequently migrate and react on the other27-29. Due to the heterogeneous nature of most catalysts, it is usually impossible to accurately distinguish between the various roles that each of these effects has on the reaction


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kinetics of a particular catalyst system. However, the ability to understand the origin of interface effects at the catalyst–support interface is particularly important since designing custom interfaces for specific applications is becoming feasible through rapid advances in nanotechnology.30-32 The goal of this work is to investigate the scaling of catalytic activity for a supportmediated reaction pathway with respect to distance from the active metal–support interface. In this study, CeO2 was specifically selected because of its ability to selectively promote C=O bond hydrogenation on a Pt catalyst, and it has been shown that Pt not in contact with either CeO2 or TiO2 has a near zero rate for crotyl alcohol formation under the reaction conditions employed here.20, 33-34 In order to better control the metal–support interface, we further employ an inverse catalyst design where CeO2 nanocubes are used to decorate a Pt thin film catalyst. This system is selected because the cubic shape gives rise to a high degree of contact between the CeO2 nanoparticle and the Pt film in order to maximize the oxide–metal interface. Additionally, the greater thermal stability of CeO2 relative to Pt minimizes nanoparticle sintering during reaction, which occurs when, in the opposite case, Pt nanoparticles are dispersed on a planar oxide support. Depending on the exact workup of the colloidal suspension of synthesized nanocubes, we find that the CeO2 nanocubes can deposit on Pt either as rafts of uniformly packed nanocubes or as a homogenous distribution of individual particles. In either case, it is possible to tune the total coverage of CeO2 on Pt during Langmuir–Blodgett deposition of the CeO2 nanocubes. These surfaces are then used as catalysts for crotonaldehyde hydrogenation, and the kinetics of selective C=O bond hydrogenation to produce crotyl alcohol are monitored as a function of the packing density of CeO2 nanocubes. In the case where CeO2 nanocubes assemble into rafts of


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uniformly packed nanocubes on the Pt surface, we find that within a region of uniformly packed CeO2 nanocubes, little or no C=O bond hydrogenation occurs, while at the interface between domains of closely packed nanocubes, the rate of C=O bond hydrogenation is quite high. These results indicate that the selective enhancement of C=O bond hydrogenation on Pt–CeO2 catalysts extends well beyond the 3-phase boundary defined by the oxide–metal interface. In a related work, the authors have recently investigated the kinetics of crotonaldehyde hydrogenation on a series of Pt–TiO2 and Pt–CeO2 catalysts.33 In that work, we demonstrated that CeO2 behaves analogously to TiO2 in its ability to selectively promote C=O bond hydrogenation. We additionally showed that somewhat surprisingly, the rate of C=O bond hydrogenation does not scale with the density of Pt–oxide interface sites. Rather the ratelimiting step during crotyl alcohol formation occurs on the Pt side of the active Pt–CeO2 interface. The present work builds on this previous study by showing that there exists a dramatic difference in reactivity between nanoscale Pt–CeO2 interfaces defined by the contact of individual CeO2 nanoparticles on a Pt surface and mesoscale interfaces defined by the boundary between domains of self-assembled nanoparticles and the surrounding clean Pt substrate. Further this work presents results of in situ ambient pressure XPS experiments demonstrating that this difference is not the result of variations in the CeO2 oxidation state due to H spillover kinetics or Pt decoration by the migration of reduced Ce atoms during reaction. Instead, we hypothesize that reaction is rate limited by the surface migration of crotyl-oxy intermediates as they form on CeO2 nanoparticles and subsequently diffuse and react on surrounding Pt. Finally, correlation between reaction kinetics and electron microscopy enables us to quantify the length scale of bifunctional catalysis as ≥50 nm from the actual CeO2 interface. These results offer new


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insights into the molecular understanding of selective catalysis at an oxide–metal interface as well as provide important design parameters that have not been previously established. Methods Synthesis of CeO2 Nanoparticles The colloidal synthesis of the CeO2 nanoparticles closely followed a previously reported method.35 To summarize, 15 mL of 16.7 mM aqueous cerium (III) nitrate was transferred into a Teflon-lined stainless steel autoclave reactor. Next 1.5 mL of oleic acid was added to the container followed by the addition of 0.15 mL of tert-butylamine. Finally 15 mL of toluene was added, and the autoclave reactor was sealed and placed in an oven at 180 °C for 24 hours. Following reaction the crude solution was allowed to cool to room temperature and was then centrifuged at 3,000 rpm for 5 minutes. The upper organic layer was taken and then precipitated with twice the volume in ethanol. The supernatant was then discarded, and the remaining precipitate was dispersed in 7 mL of chloroform for Langmuir–Blodgett deposition. This synthesis resulted in CeO2 nanoparticles that deposit as a homogenous distribution of individual particles onto the surface of a Pt catalyst. To instead prepare CeO2 nanoparticles that deposit on Pt as discrete domains of selfassembled nanocubes, a similar synthesis was used with minor modifications. To summarize, 15 mL of 16.7 mM aqueous cerium (III) nitrate was transferred into a Teflon-lined stainless steel autoclave reactor. Next only 474.3 µL (instead of 1.5 mL) of oleic acid was added to the container followed by the addition of 0.15 mL of tert-butylamine. Finally 15 mL of toluene was added, and the autoclave reactor was sealed and placed into an oven at 180 °C for 48 (rather than 24) hours. Following reaction the crude solution was allowed to cool to room temperature and was then centrifuged at 3,000 rpm for 5 minutes. The upper organic layer was taken and then


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precipitated with twice the volume in ethanol. The supernatant was then discarded, and two additional wash steps were performed by dispersion of the pellet in 5 mL of hexane followed by addition of 5 mL of ethanol and centrifugation at 8,700 rpm for 15 minutes. Following the second wash, the particles were dispersed in 7 mL of chloroform for Langmuir–Blodgett deposition. Langmuir–Blodgett Deposition A Si (100) wafer with a 500 nm thermal oxide layer served as the catalyst support. A 20 nm Pt film was deposited onto this substrate via electron beam evaporation. Following preparation of the Pt films, Langmuir-Blodgett deposition was used to deposit the CeO2 nanocubes at well controlled coverages onto the Pt film.4,5 During LB deposition a suspension of CeO2 oxide nanocubes in chloroform was dropped onto a water surface. The chloroform solvent was allowed to evaporate for 30 minutes leaving only the CeO2 nanoparticles. The particle density at the water surface was controlled using two Teflon barriers located at the ends of the trough. Surface pressure was monitored as a function of film compression during nanoparticle deposition. These experiments used a NIMA 612D Langmuir-Blodgett trough. All Pt films were treated with UV irradiation for 30 minutes prior to LB deposition to render the surface hydrophilic. Following LB deposition, the catalysts were exposed to UV irradiation for 12 hours to remove the oleic acid capping agent as described in detail previously.33 TEM Imaging of CeO2 Nanoparticles The coverage of CeO2 nanocubes was characterized by transmission electron microscopy (TEM) and by X-ray photoelectron spectroscopy (XPS). Samples for TEM analysis were prepared by LB deposition onto lacy carbon film TEM grids. Prior to Langmuir–Blodgett deposition, the lacey carbon films were treated under UV irradiation for 20 minutes to render the


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surface hydrophilic. These grids were then placed directly next to the Pt thin film catalysts used in kinetic studies described above so that the TEM images reported here correspond directly to the nanoparticle films tested for crotonaldehyde hydrogenation. TEM imaging of the CeO2 nanocubes was performed using a Philips CM-200 TEM. Ambient Pressure XPS In situ studies of surface chemistry of the catalysts were performed on the lab-based ambient pressure XPS equipped with monochromated Al Kα X-ray source in the Tao group.36 In the AP-XPS studies, the sample was mounted in the reaction cell, which provides the gas environment of 0.3 Torr hydrogen (99.99%, Matheson). Photoemission features of Pt 4f, Ce 3d, O 1s, and C 1s were collected with the same pass energy of 200 eV at different reaction temperatures when hydrogen was continuously flowing through the reaction cell. The binding energies were calibrated by calibrating the peak position of C 1s to 284.5 eV. Processing and quantitative analyses of AP-XPS results were performed using CasaXPS software. The Ce3d feature was fit using 10 components corresponding to contributions from both Ce(III) and Ce(IV) states. The Ce(III) ratio in all the ceria species was calculated by equation below: =

+

1

The surface Ce to Pt ratio could be found by the equation below, where ASF stands for the atomic sensitive factors of Ce 3d and Pt 4f. 3/ 3 = 2 4/ 4


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Kinetic Measurements of Crotonaldehyde Hydrogenation Reaction kinetics were measured in a stainless steel batch mode reactor connected to an Agilent 7890B gas chromatograph (GC) equipped with a flame ionization detector. A fused silica Supelcowax capillary column was used for product separation (Supelcowax 10, 30m x 0.32mm x 0.5 µm). Injection sequences monitored the rate of product formation over the course of 3 hours. During reaction the catalyst temperature was fixed at 100 °C using a boron nitride substrate heater. The reaction mixture consisted of 1 Torr crotonaldehyde, 100 Torr H2, and 659 Torr He. A metal bellows recirculation pump mixed the gas inside the chamber to ensure that the measured kinetics were not diffusion limited. Reaction rates were determined from fits to peak areas versus time and normalized to sensitivity factors for each product. Total crotonaldehyde conversion was 10 fold more active for crotyl alcohol formation compared to any other surface coverage of the CeO2 nanocubes on Pt. Importantly, this is also the only sample that displayed a significantly increased concentration of mesoscale interface sites. This result indicates that within a region of uniformly packed CeO2 nanocubes, little or no C=O bond hydrogenation occurs, while the mesoscale interface between domains of nanoparticles is entirely responsible for responsible for the enhanced rate of C=O bond hydrogenation. In contrast to the kinetics of C=O bond hydrogenation, the rate of C=C


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bond hydrogenation to produce butyraldehyde and the rate of decarbonylation to produce propylene do not depend strongly on the morphology of CeO2 nanoparticles on Pt . Fig. 5B shows that the rate for these pathways is nearly constant with respect the number of Pt active sites as measured by ethylene hydrogenation. This is consistent with previous studies showing that these two reactions occur on Pt and do not require an active oxide–metal interface.20, 39 Comparison of Fig. 4F and Fig. 5A shows that the rate of C=O bond hydrogenation does not scale in direct proportion to the number of mesoscale interface sites. Rather a ~2 fold increase in the density of mesoscale interface produces a ~10 fold enhancement in the rate of C=O bond hydrogenation. This result is expected since we have recently shown that the kinetics of selective C=O bond hydrogenation are rate limited by Pt surface area near a CeO2 interface rather than by the density of interface sites directly.33 This can explain why almost no C=O bond hydrogenation occurs within a domain of closely spaced CeO2 nanoparticles because there is insufficient Pt surface area inside this region of tightly packed CeO2 particles, while large areas of clean Pt are in close proximity to the mesoscale interfaces located at the boundary of a tightly packed nanoparticle domain. From these observations we hypothesize that the measured rate of C=O bond hydrogenation is the result of surface diffusion limited reaction kinetics where a reaction intermediate must migrate across the Pt–CeO2 to facility selective C=O bond hydrogenation. In such a diffusion limited reaction mechanism, the actual diffusing species could be 1) a crotyl-oxy reaction intermediate that forms on CeO2 and subsequently spills over to Pt, 2) H atoms from H2 dissociation on Pt that subsequently diffuse to the CeO2 interface, or 3) reduced Ce3+ cations that diffuse to and decorate the Pt surface during reaction. Below we briefly discuss these possible


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reaction mechanisms and examine the plausibility of each to explain the observed activity scaling using in situ XPS. Recent studies of crotonaldehyde as well as furfuraldehyde hydrogenation have utilized sum frequency generation vibrational spectroscopy to study the reaction intermediates involved in selective C=O bond hydrogenation near the active oxide–metal interface.19-21 These spectroscopic studies showed that the C=O bond is activated by a charge transfer interaction of the aldehyde with the oxide surface. In this mechanism, the aldehyde molecule binds to redox active O-vacancy sites on the oxide surface. In the case of a Pt–TiO2 catalyst, this O-vacancy site represents a reduced Ti3+ cation, while in CeO2, reduced Ce3+ cations at O-vacany sites play an analogous role. DFT calculations showed that when an aldehyde molecule binds to an Ovacancy site, electron transfer occurs, such that the reduced 3+ cation is oxidized to the 4+ oxidation state, and the electron localizes on the carbonyl bond of the aldehyde molecule forming an anionic crotyl-oxy reaction intermediate.19 This negative charge density on the carbonyl bond is responsible for activating the adsorbed aldehyde for selective C=O bond hydrogenation. Although this reaction intermediate, which represents the surface precursor to crotyl alcohol was found to form on TiO2 without Pt, this species only turned over when Pt was present. It has also been shown that H spillover from Pt to the reducible oxide assists in generating redox active O-vacancy sites on the oxide surface.21, 40-41 From these spectroscopic studies, it was concluded that the reducible oxide is responsible for selectively activating the C=O bond, while Pt serves primarily to supply atomic H to the active oxide–metal interface. In this reaction mechanism, turnover occurs by subsequent spillover of active species (i.e. crotyl-oxy surface intermediates and/or atomic H) across the


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oxide–metal interface. Notably, these studies never determined the exact site for reaction between a crotyl-oxy surface intermediate and atomic H. We now consider three plausible reaction mechanisms, which are depicted schematically in Fig. 6: First, the observed crotyl-oxy intermediate actively diffuses from the CeO2 phase to Pt where it subsequently reacts with atomic H to form crotyl alcohol. In support of this mechanism, we note that spillover of alkoxy species across the oxide–metal interface have previously been observed on a Pt–TiO2 catalyst. Second, it is also possible that crotyl alcohol formation occurs directly at the 3-phase boundary defined by the Pt–CeO2 interface where crotyl-oxy intermediate from CeO2 react with H atoms from Pt. In this case, the observed reaction kinetics could be rate limited by the ability of Pt to supply sufficient atomic H to the CeO2 interface. H atoms supplied to CeO2 from Pt play two important roles in this reaction: First, atomic H is need for reaction with crotonaldehyde to produce crotyl alcohol. Second, H atoms from Pt assist in generating redox active O-vacancy sites on the oxide surface that are required for C=O bond activation. Consequently, it is possible that the observed length scale for C=O bond hydrogenation represents the area of Pt required to dissociate sufficient atomic H to perform these two critical steps in the formation of crotyl alcohol. Last, we note a third mechanism that could contribute to the observed scaling of C=O bond hydrogenation kinetics with respect to distance from the Pt– CeO2 interface. Specifically, it has been reported that metal cations from reducible oxides can decorate, or in some cases even encapsulate, a metal catalyst under reducing reaction conditions.42-44 Consequently, we consider the possibility that Ce3+ cations may migrate to the Pt surface, and these Ce3+ cations could represent an active site for selective C=O bond hydrogenation. Because this distribution of active sites would be defined by the migration or


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diffusion of reduced Ce cations on the Pt surface during reaction, this could explain the measured active sites scaling. To test these proposed mechanisms, we performed a series of in situ ambient pressure XPS experiments. First, we monitored the Ce oxidation state during reaction conditions at elevated temperature to determine if the observed reaction kinetics for crotyl alcohol formation correspond to enhanced H spillover at the mesoscale Pt–CeO2 interface (i.e. mechanism 2 above). Parts A and B of Fig. 7 compare the Ce 3d XPS spectra of the active catalyst prepared by Langmuir–Blodgett deposition of CeO2 nanoparticle at 150 cm2 compression area. Fig. 7A shows the spectrum obtained under ultra-high vacuum at 25 °C, while Fig. 7B shows the spectrum obtained in 0.3 Torr H2 at 100 °C. Each spectrum is fit by six peaks corresponding to Ce4+ states and four peaks corresponding to Ce3+ states as demonstrated previously.45-46 As

Fig. 6 Proposed reaction mechanisms for C=O bond hydrogenation on a Pt–CeO2 catalysts to explain enhanced activity at a mesoscale interface: 1) Crotyl-oxy intermediates that form during reaction on a reducible oxide migrate to Pt where they subsequently react with H to form crotyl alcohol or butanol; 2) Atomic H from Pt diffuses to a CeO2 interface where it reacts with crotyloxy reactions interemdiates on CeO2; or 3) New active sites are formed on the Pt side of a Pt– CeO2 interface by the migration of reduced CeOx during reaction. Each of these mechanisms is consistent with the observed diffusion limited reaction kinetics on the Pt side of a Pt–CeO2 interface. As described below, results of ambient pressure XPS exclude mechanisms 2 and 3.


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shown in the comparison of Fig. 7 A and B, there is a significant increase in the contribution of Ce3+ states when the Pt–CeO2 catalyst is heated in H2, reflecting the in situ reduction of CeO2 nanoparticles. Fig. 7C shows the Ce3+:Ce3+/4+ ratio of the three different catalysts prepared by Langmuir–Blodgett deposition of CeO2 nanoparticles at 450, 150, and 75 cm2 compression areas. Results are shown for spectra obtained first under ultrahigh vacuum at ambient temperature and then in 0.3 Torr H2 at gradually increasing catalyst temperatures. From Fig. 7C we observe that each of these three catalysts show the effects of CeO2 reduction in the presence of H2, which increases significantly with increasing catalyst temperature. However, comparing spectra at a given temperature shows only a subtle difference in Ce oxidation state between the three catalysts, with the most active catalyst actually showing a slightly lower degree of CeO2 reduction relative to the other two samples. From this experiment it is clear that the catalyst activity for selective C=O bond hydrogenation is not primarily dependent on the oxidation state of reduced CeOx during reaction. This result indicates that differences in the H spillover kinetics between these catalysts is not primarily responsible for the observed enhancement in C=O bond hydrogenation kinetics. Fig. 7D shows the Ce:Pt atomic fraction of these same three catalysts as a function of increasing catalyst temperature in H2. Comparing the three catalysts at a given reaction condition shows that the measured Ce:Pt ratio increases with increasing nanoparticle compression as expected. It has been reported that for Pt catalysts supported on reducible oxides, reduced metal centers from the oxide can migrate and subsequently decorate or even encapsulate the Pt catalyst under reducing conditions at elevated temperature, and this effect has sometimes been associated with strong metal–support interactions (SMSI).15 If this were to


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Fig. 7 Results of ambient pressure XPS experiments. A and B show fitting results of the Ce 3d spectrum for the Pt–CeO2 catalyst deposited at 150 cm2 compression area in ultrahigh vacuum at ambient temperature (A) and in 0.3 Torr H2 at 100 °C (B). The growth of the red and orange lines representing Ce3+ states in spectrum B reflects the reduction of the CeO2 nanoparticles under reaction-like conditions. C shows the Ce3+:Ce3+/4+ ratio for catalysts prepared with three different coverages of CeO2 nanoparticles measured first in ultrahigh vacuum at ambient temperature and then in 0.3 Torr H2 at increasing temperature. Red bars show results for the most active catalyst (see Fig. 5A above), while blue and black bars show results for catalysts prepared with lower and higher CeO2 nanoparticle coverage. All three catalysts show increasing CeO2 reduction at increasing temperature, but at any given temperature the active catalyst is actually the least reduced of the three. D shows the corresponding Ce:Pt atomic fraction for these same three catalysts measured as the area ratio of the Ce 3d and Pt 4f XPS lines followed by correction for the relative sensitivity factors.


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occur in the Pt–CeO2 catalysts employed in these studies, we would expect to see an increasing Ce:Pt atomic fraction at increasing reaction temperature indicating Pt decoration by reduced CeOx. However, we find no evidence for Pt decoration by CeOx at temperatures up to 200 °C. This is confirmed by the nearly constant Ce:Pt atomic fraction with respect to increasing temperature in H2 observed for each of the three catalysts tested (see Fig. 7D). Consequently, we also rule out the reversible formation of new Pt–CeO2 interface sites by the migration of reduced Ce atoms onto the Pt during reaction (i.e. mechanism 3 above). These results suggest that the crotyl-oxy intermediate identified previously by SFG spectroscopy, which forms at O vacancy sites on a reduced CeOx surface, subsequently diffuses onto Pt where it reacts to form crotyl alcohol. Assuming that the overall reaction rate is limited by kinetics occurring on the Pt side of the Pt–CeO2 interface, this proposed mechanism can explain the difference between the observed activity at nano and mesocale Pt–CeO2 interfaces. At the nanoscale interfaces present within a domain of tightly packed CeO2 nanoparticles, there is insufficient Pt surface area required to catalyze the final step in the conversion of crotonaldehyde to crotyl alcohol. However, at the mesoscale interface located at the boundary around the domain of closely packed CeO2 nanoparticles, there exists large areas of clean Pt and crotyl alcohol formation can proceed readily on this surface following the spillover of activated crotyl-oxy intermediates across this mesoscale interfaces. In an effort to quantify the length scale for bifunctional C=O bond hydrogenation, we performed experiments with an additional set of samples using separately prepared CeO2 nanocubes. In the sample preparation for these experiments, a modified nanoparticle synthesis and work up was used in order to prevent self-assembly of the nanoparticles into rafts as observed in the samples described above (see Experimental section for details). This difference


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between sample sets was verified by Langmuir–Blodgett compression isotherms as well as by electron microscopy, confirming that these CeO2 nanoparticles deposit homogenously on a surface rather than self-assembling into well-defined rafts. Fig. 8 shows a Langmuir–Blodgett compression isotherm of these CeO2 nanoparticles. Inset micrographs represent TEM images of nanoparticles deposited at various points along the compression isotherm. As shown, the spatial distribution of nanoparticles is much more uniform than in the previous set of samples. We also observe only a single plateau in the compression isotherm of these nanoparticles, indicating that there is no intermediate surface phase transition corresponding to break up of rafts as observed above. In the compression isotherm shown in Fig. 8, this plateau occurs at a surface pressure of 35 mN/m, slightly lower than the maximum surface pressure observed in the previous sample set (see Fig. 3). However, compression of these nanoparticles to even higher density still displayed only a single plateau, and we never observed an intermediate step in any of the numerous compression isotherms performed with these CeO2 nanoparticles.

Fig. 8 Langmuir–Blodgett compression isotherm of homogenously dispersed CeO2 nanoparticles. Inset micrographs represent TEM images of nanoparticles deposited onto a substrate at various points along the compression isotherm.


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Image processing of TEM micrographs allowed us to quantify the spacing between adjacent nanoparticles as a function of CeO2 nanoparticle coverage. This was done by plotting the image contrast as a function of length for lineouts from TEM images as shown in Fig. 9A and B. From these line plots showing the image contrast versus distance, it is straightforward to identify the location of particles (i.e. regions of high contrast) as well as the distance between adjacent particles (i.e. regions of low contrast). Similar plots were generated from lineouts taken

Fig. 9 Plot of contrast versus position along a lineout taken from a TEM image of CeO2 nanoparticles (A and B). From this type of plot it is possible to obtain the distribution of lengths between adjacent CeO2 nanoparticles for samples deposited at various points along a compression isotherm. Part C shows histograms of four nanoparticle compressions based on binning at 2 nm intervals. Approximately 200 nanoparticles were sampled to generate each distribution. The frequency axis was normalized to a total sampling length of 1 µm. The inset in part C shows the frequency of finding two adjacent particles spaced >10 nm apart, illustrating that the frequency of widely spaced particles decreases significantly as particles become closely packed at high coverage.


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from multiple images of a given sample and a distribution of interparticle spacing was determined for approximately 200 particles each on four different samples with varying coverage of CeO2 nanocubes. These distributions of interparticle lengths are plotted in Fig. 9C based on binning at 2 nm intervals. The frequency axis in this plot is normalized to reflect the frequency of a given distance between adjacent particles for a sampling length of 1 µm. The inset to Fig. 9C shows the frequency of finding two adjacent particles spaced >10 nm apart, illustrating that the frequency of widely spaced particles decreases significantly as particles become closely packed at high coverage. Using these measured distributions, it is possible to demonstrate a correlation between the activity of each sample for C=O bond hydrogenation and the area of free Pt surrounding CeO2 nanoparticles. In other words, we now ask at what distance on Pt from the CeO2 interface does the activity of selective C=O bond hydrogenation decay. This question is schematically depicted above in Fig. 1, where we consider what the activity profile of C=O bond hydrogenation may look like on Pt with increasing distance away from the CeO2 interface. At a molecular level, this scaling is expected to represent the mean free path diffusion of a crotyl-oxy reaction intermediate on Pt. Here we depict a hypothesized scenario where the activity for crotyl alcohol formation is described by a fixed 5 nm exponential rise followed by a variable exponential decay as a function of distance from a CeO2 nanoparticle. Below we demonstrate that this activity profile can accurately fit the kinetics of crotyl alcohol formation as a function of CeO2 nanoparticle coverage. To model reaction kinetics using this activity profile, we first calculate the radial integral of this exponential decay profile around the nanoparticle in all directions. To account for the effects of interparticle spacing, we truncate the integral at a radial distance corresponding to the


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length between adjacent nanoparticles. The value of this integration is then summed for numerous nanoparticles based on the measured interparticle spacing for each in order to predict the overall reaction rate on a given sample. Fig. 10 shows the predicted activity scaling as a function of CeO2 nanoparticle compression. To generate this plot, we used Equation 5 above, where t was fixed at 5 nm and the different colored lines represent various assumed values of λ. Since the actual spacing of nanoparticles on each sample was carefully characterized (see Fig.

Fig. 10 C=O bond hydrogenation activity for Pt–CeO2 catalysts prepared with increasing coverage of CeO2 nanoparticles. Colored points represent predicted activities based on various assumed exponential decay lengths and the measured distribution of interparticle spacings for each catalyst tested, while the black points represent the experimentally measured reaction rates. As shown exponential decay lengths ≥50 nm provide the best fit to the experimental data. The dark and light grey bars underneath the experimental data points show the normalized rates for crotyl alcohol and butanol formation, respectively.


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9C), the only adjustable parameter in this model is the exponential decay rate for C=O bond hydrogenation activity on Pt as a function of length from the CeO2 interface. Accordingly, the different colored lines in Fig. 10 represent the predicted activity scaling of these samples for C=O bond hydrogenation using assumed exponential decay lengths between 1 and 100 nm, and the black data points in Fig. 10 represents the measured kinetics for crotyl alcohol and butanol formation. Here we consider only the normalized activity rather than absolute TOF because in this analysis we treat the activity of individual Pt sites to be a function of distance from a CeO2 interface. As shown, the relative activity of these samples as a function of nanoparticle coverage is predicted to vary significantly for exponential decay lengths between 1 and 50 nm. Assuming short decay lengths (i.e. activity occurs at or near the actual 3 phase boundary), the highest activity is predicted for the densest nanoparticle coverage. Assuming longer decay lengths (i.e. activity occurs on Pt far from an actual CeO2 interface), the highest activity is predicted for an intermediate coverage followed by decreasing activity due to site blocking by excess CeO2 at higher coverage. For assumed exponential decay lengths above 50 nm this model does not predict any significant difference in the activity scaling of the given samples. In other words, the samples tested here are expected to show the same activity scaling for all assumed decay lengths above approximately 50 nm. This is due to our experimental inability to prepare catalysts with very dilute coverages of homogenously spaced nanoparticles. As shown in Fig. 10, the experimentally measured reaction kinetics closely match the predicted activity scaling for the longest decay lengths modeled (i.e. ≥50 nm). This result demonstrates that the kinetics of crotyl alcohol formation can be accurately predicted for Pt– CeO2 catalysts by treating the rate of C=O bond hydrogenation on Pt as an exponential decay with respect to distance from the CeO2 interface where the decay length is determined to be ≥50


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nm. Using decay lengths

Catalysis at Multiple Length Scales: Crotonaldehyde Hydrogenation at

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