Transmission Electron Microscopy and Infrared Spectroscopy

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Combining In-situ Transmission Electron Microscopy and Infrared Spectroscopy for Understanding Dynamic and Atomic Scale Features of Supported Metal Catalysts Joaquin Resasco, Sheng Dai, George W. Graham, Xiaoqing Pan, and Phillip Christopher J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03959 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

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The Journal of Physical Chemistry

Combining In-situ Transmission Electron Microscopy and Infrared Spectroscopy for Understanding Dynamic and Atomic Scale Features of Supported Metal Catalysts Joaquin Resasco1, Sheng Dai2, George Graham2,3, Xiaoqing Pan2,4, and Phillip Christopher1*

1. Department of Chemical Engineering University of California Santa Barbara Santa Barbara, California 93106, USA 2. Department of Chemical Engineering and Materials Science University of California Irvine Irvine, California 92697, USA

3. Department of Materials Science and Engineering University of Michigan Ann Arbor, Michigan 48109, USA

4. Department of Physics and Astronomy University of California Irvine Irvine, California 92697, USA Submitted to Journal of Physical Chemistry C

*To whom correspondence should be sent: [email protected]

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Abstract The elucidation of structure property relationships for supported metal catalysts requires atomic scale descriptions and quantitative measurements of the relative population of various exposed active sites under reaction conditions. The requirement of describing catalyst structures under reactions conditions stems from the potential physical and chemical reconstruction that can be induced by changes in environments. Here we highlight our recent work, where catalyst characterization via a combination of in-situ transmission electron microscopy and infrared spectroscopy is used to provide sample averaged, site-specific atomic level information of supported metal catalysts under reaction conditions. We show two illustrative examples using different reaction systems: the oxidation of CO over supported Pt, and the reduction of CO2 over supported Rh. Using these examples we demonstrate differentiation of the catalytic reactivity of small supported metal clusters from isolated atoms and interrogate the catalytic consequences of structural transformations such as adsorbate-induced surface reconstruction of metal particles, and adsorbate-mediated metal particle encapsulation by oxide supports. Insights gained from these techniques help elucidate structure-property relations for these reactions including structure sensitivity and dynamic reactivity changes. We expect that this combination of characterization techniques will be generally useful for understanding structurally dynamic catalytic systems with atomic resolution.


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1. Introduction Designing catalysts that are efficient for driving chemical transformations requires understanding relationships between catalyst structure and function. For heterogeneous supported metal catalysts, after the identity of the metal itself, the coordination of metal species to other metal atoms and the support plays the primary role in dictating catalytic properties, thus information on the identity and distribution of exposed sites and their bonding environment is crucial.1-7 Significant changes in catalyst structure can occur as a result of catalyst pretreatments or exposure to reaction environments.8 These structural changes are often driven by variations in adsorbate coverage or identity, which modify surface free energies, and occur under elevated temperatures where kinetic barriers for structural changes can be traversed. Therefore, characterization of functional catalysts at the atomic scale under realistic reaction temperatures and environments is required for understanding and optimizing catalytic performance.9-11 A variety of spectroscopy and microscopy-based tools exist for characterizing supported metal catalysts, however, each has its own strengths and limitations.12 Recently, we have sought to understand relationships between atomic scale structural characteristics of supported metal catalysts and catalytic reactivity through coupling kinetic measurements with catalyst characterization via a combination of atomic resolution transmission electron microscopy (TEM) and Fourier Transform-infrared spectroscopy (FTIR).13-18 The combination of these two characterization techniques provides a wealth of structural and electronic information about supported metal catalysts and changes that may be induced by pre-treatment or exposure to reaction conditions. TEM provides direct visualization of catalyst structures and spatially resolved spectroscopic information through electron energy loss spectroscopy (EELS). Recent developments in microscopy instrumentation have made it possible to obtain structural and spectroscopic information at the atomic scale under in-situ conditions relevant to catalysis – elevated temperature and atmospheric pressure with controlled composition.19 However, TEM analysis provides a statistically limited representation of a catalytic material, necessitating complementary sample-averaged characterization techniques. Probe molecule FTIR spectroscopy is a sample-averaged technique that is surface sensitive and site-specific due to changes in the vibrational frequency of the probe molecule when adsorbed to sites that have different local coordination environments. Probe molecule FTIR can be executed in a temperature-programmed manner to learn about the differences in chemical reactivity between various types of catalytic sites. Furthermore, by using known extinction coefficients for probe molecules adsorbed at different sites and working at known coverage this technique can be used quantitatively. FTIR can also be applied in-situ to identify adsorbed molecules on supported metal catalysts, providing important information about species that drive structural transformations and more broadly to understand catalytic mechanisms. The ability to perform insitu analysis of catalytic materials using a combination of TEM and IR under similar conditions opens new opportunities for understanding the dynamic behavior of heterogeneous catalysts. Here, rather than attempting to provide a comprehensive overview of this topic, we describe selected recent work from our groups highlighting the utility of these complementary techniques to obtain atomic level structural information of high surface area oxide supported metal catalysts under realistic operating conditions.13-18 These structural insights are related to kinetic data to answer questions of catalytic consequence. We focus on two illustrative examples: the oxidation of CO over supported Pt, and the reduction of CO2 over supported Rh. Using these examples we



demonstrate the differentiation of the catalytic reactivity of small supported metal clusters from atomically dispersed species and interrogate the catalytic consequences of structural transformations such as adsorbate-induced surface reconstruction of metal particles, and adsorbate-mediated metal particle encapsulation by oxide supports. Insights gained from these techniques help elucidate structure-property relations for these reactions including structure sensitivity and dynamic reactivity changes, paving a way for the design of more efficient catalyst systems. We expect that this combination of in-situ characterization techniques will be generally useful for characterizing structurally dynamic catalytic systems with atomic resolution.

2. Experimental Considerations Transmission Electron Microscopy A direct visualization of the structure of catalyst particles can be obtained using TEM.20-23 In TEM, a sample is illuminated with a beam of high energy electrons, a fraction of which pass through the material without suffering energy loss. These transmitted electrons form a two dimensional projection of the sample. As electrons have a very small wavelength, these images can provide atomic level information, unlike visible light microscopy.24-25 In addition to imaging information, the interaction of electrons with the sample yields additional signals which can be measured to provide a wealth of information on the composition, crystallography, and electronic structure of the material.26 Figure 1a shows a schematic of some of the interactions an electron beam has in the TEM and the characterization techniques associated with these processes. Diffracted electrons can be collected to provide crystallographic information. Inelastically scattered electrons, or electrons that lose energy as they pass through the sample, provide several signals that can provide spectroscopic information about the sample. The two most common techniques involve the direct measurement of the energy of these electrons in electron energy loss spectroscopy (EELS) or the collection of x-rays generated in the sample by the incident electrons in X-ray energy dispersive spectroscopy (EDS). As the surface structure of catalytic materials can be affected by its reaction environment, TEM studies performed in conditions which resemble as closely as possible those of the reaction in which the catalyst operates are essential. The use of TEM for these type of in-situ studies is complicated by the small mean-free path of the electrons in gases and the incompatibility of the microscope’s components with high pressures.27 Therefore, to allow operation of the microscope under these conditions, the number of gas molecules along the path of the electron beam must be minimized, typically by confining the high pressure atmosphere to the vicinity of the catalyst. One method of confining the gas environment is through the use of a differential pumping system.28-29 In this configuration, small apertures limit gas flow out of the high pressure region around the sample, while several stages of pumping keep the other microscope components under high vacuum. Unfortunately, the maximum achievable pressures in differentially pumped systems while maintaining atomic level resolution are on the order of 10 Torr, significantly lower than the pressures of relevance for catalytic processes.30 Additionally, differential pump apertures block electrons scattered through high angles by the sample, as required for high angle annular dark field scanning transmission electron microscopy (HAADF-STEM).


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A second method of operating a TEM in the presence of a gas environment is through the use of windowed cells.31-34 In this configuration, thin windows that are electron transparent but mechanically robust separate a high pressure environment near the sample from the high vacuum of the rest of the TEM. Amorphous silicon nitride (SiNx) is an excellent window material due to its high fracture strength and low electron diffraction contrast.35-37 Recent developments in the fabrication of sample holders with miniature cells which integrate SiNx windows and silicon carbide (SiC) heating elements into silicon (Si) chips allow for imaging with atomic resolution at atmospheric pressure and high temperature in an aberration-corrected TEM.32-34, 38 A schematic of a windowed gas cell for in-situ TEM is shown in Figure 1b. In the studies presented here, we used a windowed cell system made by Protochips to perform aberration-corrected STEM and EELS analysis of catalytic materials at relevant reaction conditions. Measurements were conducted in a JEOL 3100R5 with double Cs correctors operated at 300 kV or a JEM-ARM300F Grand ARM equipped with two spherical aberration correctors and a 300 kV cold field emission gun. To reduce beam-induced changes, relatively small beam currents (≤20 pA) were employed for imaging and the electron beam was turned on only during image collection. It is important to point out the main limitations of TEM as a characterization tool. As TEM provides atomic level information of individual nanoparticles, the information gained is statistically very limited, and should not be taken as representative of the entire catalyst sample without confirmation from sample averaged techniques. Further, as TEM provides projection images with no depth sensitivity, it can be difficult to directly obtain a three dimensional description of exposed active sites from these two dimensional images. Finally, the ionizing radiation of the electron beam can introduce artifacts during prolonged exposure time.41-42 During in-situ TEM studies, ionization of gas molecules must be considered in addition to electron beam damage of materials of interest. Currently, it is best practice to use as low of an electron dose as possible without sacrificing imaging quality, and imaging at a range of dose rates to verify that changing electron beam current density and total dose have no effect on observations. Understanding these limitations, it is important to corroborate information gained from TEM studies with complementary characterization techniques. For example, structural information can be verified in some cases with sample averaged diffraction and scattering techniques such as X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS), while chemical information gained from EELS can be corroborated by core level spectroscopies such as ambient pressure X-ray photoelectron spectroscopy (AP-XPS) or X-ray absorption near edge structure (XANES). Below we discuss how infrared spectroscopy can also be used to provide complementary information to information gained from TEM.



Figure 1: Transmission electron microscopy for studying catalytic materials. a) An overview of some of the interactions of an electron beam with a sample. Secondary signals produced and the techniques associated with their measurement are noted. b) Illustration of a windowed cell for in-situ TEM. Gas atmosphere is contained in the vicinity of the sample by silicon nitride windows, separate from the vacuum environment of the microscope.

Infrared Spectroscopy Infrared (IR) spectroscopy is a useful tool which can provide complementary sample averaged information to TEM for the characterization of catalytic materials.43 IR spectroscopy operates on the principle that molecules possess discrete vibrational energy levels, and transitions between these levels can occur by absorption of photons with frequency in the mid IR range (~200-4000 cm-1). Identification of specific bonds is possible as the characteristic frequency at which vibrational excitation occurs depends on the mass of the atoms involved and the bond strength. Changes in bond molecular bond strength or geometry as a result of adsorption onto a catalyst surface result in changes to intermolecular vibrational frequencies. This has made IR useful as a method of identifying adsorbed species and determining how these species are bound to the surface of the catalyst. Probe molecule IR spectroscopy takes advantage of the fact that the vibrational frequency of a probe molecule changes when adsorbed to different types of sites. Carbon monoxide (CO) is a particularly convenient molecule to study due to its strong dipole moment, sensitivity to binding environment and relatively strong adsorption strength to many catalytically active species of interest. The C-O stretch vibration in the gas phase is at 2143 cm-1. When CO adsorbs to a metal surface, electron density is transferred from the CO 5σ orbital to the metal and back-donation from the metal to the CO 2π* weakening the C-O bond and shifting its vibrational frequency to lower wavenumbers.44-47 The vibrational frequency can be used as a diagnostic for the binding geometry, as the wavenumber varies significantly as a function of COmetal stoichiometry: for linearly adsorbed CO (~2000-2130 cm-1), bridge bonded CO (~1880 and 2000 cm-1), three fold coordinated CO (1800-1880 cm-1), and four fold coordination (below 1800 cm-1).48 With increasing surface coordination, mechanical coupling to a larger mass and increased electron back donation from multiple metal atoms weakens the C-O bond and shifts the


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stretching frequency to lower wavenumbers. The precise frequency depends on the substrate metal, its surface structure, and oxidation state. An example of an IR spectrum for CO adsorbed on well-coordinated (WC) and under-coordinated (UC) sites on a Pt nanoparticle is shown in Figure 2.17 An additional factor that can influence the vibrational frequency is dipole-dipole interactions between adsorbed CO molecules, the extent of which depends on the CO coverage.49 The magnitude of this effect can be quantified using isotopic mixtures of 12CO and 13CO, as there is reduced resonance between the dipoles of the isotopes due to their differences in mass.50 NO is another useful probe molecule, however NO can dimerize, complicating site-specific analysis, and is more likely to dissociate on metal surfaces than CO.51 It is important to note what effects the probe molecule has on the surface under investigation. Probe molecules may induce reconstruction of metal surfaces, so the probe molecule may not always be considered a spectator species, which gives information on the nature of the surface in the absence of its adsorption.52-54 Low temperature IR studies are interesting for this reason, as thermal energy for reconstruction is minimized, and weakly adsorbing sites can be probed. IR spectroscopy can in principle be used quantitatively as the absorption intensity is dependent on the number of absorbing molecules.55 In practice, several requirements must be met for accurate quantification to be possible. Additionally, relative quantification of the fraction of two types of sites, as employed in these studies, is more feasible than absolute quantification. Quantitative measurements must be performed at fixed coverage, most practically at saturation coverage. As extinction coefficients are dependent on the molecule and its adsorption environment, these coefficients must be known for any quantification to be possible. Additionally, absorption intensity can change as a result of energy transfer from lower energy to higher energy vibrational modes. For example, IR measurements of CO bound to stepped Pt single crystals show that at high coverage vibrational energy transfer between CO bound to step sites and terrace sites can be significant, resulting in a decrease in the intensity of the absorption feature associated with adsorption on step sites.56-58 From our previous studies on Pt catalysts, it appears this effect is less pronounced on supported metal nanoparticles than what has been observed on single crystals.17 This could be attributed to the higher radius of curvature for nanoparticles compared to stepped single crystals, allowing adsorbed molecules to increase their separation from one another.59-60 Due to the cubic dependence of dipole interactions on distance and increased heterogeneity between the directions of the dipoles, interactions between CO bound at well-coordinated and under-coordinated sites are reduced. This is substantiated experimentally by the small shifts in vibrational frequency with changing CO coverage for CO at a given adsorption site. Measurements conducted at consistent saturation coverage, in which vibrational energy transfer does not influence the intensity of different absorption features on the different Pt adsorption sites allow the use of IR spectra to quantify the fraction of different types of sites using known extinction coefficients, as follows: =

/( × ) ∑[ /( × )]

where is the fraction of total adsorbed molecules bound to each type of site, is the absorption peak area associated with each type of site, is the relative extinction coefficient for each type of site, and is number of metal atoms coordinated to the adsorbate, for different types of sites. In addition to use as a tool for quantifying site fractions, probe molecule IR



spectroscopy can also be operated in a temperature-programmed way to gain information about the differences in reactivity between these various types of sites. In the studies reported here, IR spectroscopy was performed in the diffuse reflectance Fourier transform mode (DRIFTS). In DRIFTS, samples can be measured as powders, avoiding potential diffusion limitations associated with pressed transmission wafers. DRIFTS measurements can be reported in several different ways. Kubelka-Munk units relate the measured reflectivity to sample absorption and scattering coefficients: (1 ) = 2 Where is the frequency dependent absorption coefficient, is the scattering coefficient, and is the absolute reflectance as a function of frequency. Kubelka-Munk units provide a unit which is an approximately linear representation of adsorbate coverage for the case of highly absorbing adsorbates (samples with relative reflectance less than ~0.6). For poorly absorbing species, absorbance should be used instead measured as log(1/ ).55 DRIFTS experiments reported here were carried out in a high temperature reaction chamber (Harrick Scientific) equipped with ZnSe windows, mounted inside a Praying Mantis diffuse reflectance adapter (Harrick Scientific), and coupled to a Thermo Scientific Nicolet iS10 FTIR spectrometer with a liquid-nitrogen-cooled HgCdTe (MCT) detector. It should be noted that care must be taken regarding the temperature profile of in-situ DRIFTS reactors. All temperatures in our studies were calibrated using an optical pyrometer to represent the temperature of the catalyst surface being observed by the spectrometer.

Figure 2: Site specificity of IR spectroscopy. FTIR spectrum of a saturation coverage of CO adsorbed to a Pt catalyst at 298 K. The high-frequency vibrational mode is assigned to CO linearly bound to WC sites, and the and the low-frequency mode is assigned to CO linearly bound to UC sites. A schematic of


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CO adsorbed to these different sites is shown in the inset. Reprinted (adapted) with permission from Kale, M. J.; Christopher, P., Utilizing Quantitative in Situ FTIR Spectroscopy to Identify Well-Coordinated Pt Atoms as the Active Site for CO Oxidation on Al2O3-Supported Pt Catalysts. ACS Catalysis 2016, 6, 5599-5609. Copyright 2016 American Chemical Society.

3. Differentiating the reactivity of supported single Pt atoms from sub-nm clusters The synthesis of oxide supported Pt-group catalysts via impregnation approaches is typically assumed to produce metal particles with dimensions of a few nanometers due to their smaller surface free energy compared to atomically dispersed or site isolated Pt-group atoms on the support.61-62 Early studies using spectroscopic approaches and recent work combining spectroscopy and atomic resolution imaging in TEM or STEM have identified that on many functional catalysts, Pt-group species co-exist as nanoparticles and single atoms.63-70 Further, it has been shown that careful synthetic approaches can be used to produce materials with almost exclusively atomically dispersed species.66, 71-75 Significant interest in the reactivity of isolated Pt-group atoms on supports stems from the maximized utilization efficiency of expensive metals, potentially unique reactivity or selectivity imparted by these sites, and the connection to homogeneous orgametallic catalysts.76-77 However, kinetic characterization of the intrinsic activity of isolated Pt-group atom active sites remains a challenge. These challenges exist due to (1) the inherent small size of these species, (2) differences in their local environments on the support that cause heterogeneity in chemical and catalytic properties, and (3) the low stability of atomically dispersed species at elevated temperatures under reactive environments and during characterization. We recently developed an approach to synthesize stable atomically dispersed Pt existing in homogeneous local environments on TiO2 supports, and use CO oxidation as an illustrative example of a test of the catalytic behavior of these sites. The catalytic oxidation of CO is an extensively studied process due to its seeming mechanistic simplicity, and its practical importance in the treatment of automotive exhaust.78-80 The possibility of using isolated single site Pt catalysts (Ptiso) as a way to maximize metal utilization has drawn significant attention, as the largest worldwide demand for Pt is for this application.81 Interestingly, significant variations exist in reported adsorption energies of CO onto Ptiso, and their activity for CO oxidation.66, 71-73, 82-83 These variations suggest that the synthesis, low stability, or heterogeneity of produced Ptiso species caused inconsistent conclusions. To address these issues, we developed a synthetic approach to produce oxide supported Ptiso species that would not sinter to form Pt clusters through pre-treatment, characterization and reactivity analysis. The approach involves the use of small (~5 nm diameter) oxide nanoparticles as supports and carefully chosen synthesis conditions such that ~1 Pt atom was deposited per support particle.14 In the first example of this approach, anatase TiO2 was chosen as a support. Pt weight loadings were chosen (< 0.1 weight %) such that there was nominally less than 1 Pt atom/TiO2 particle. To promote repulsive interactions between Pt ions in solution and attractive interactions between Pt ions and the TiO2 surface, strong electrostatic adsorption (SEA) wet impregnation was used.84 This preparation method and catalytic architecture was designed so that agglomeration of Pt atoms was minimized. Even if Pt atoms have significant mobility on the oxide surface, by physically separating Pt atoms on different support particles Pt cluster formation would require Pt hopping between support particles or support sintering.14



Prior to characterization of Ptiso species by CO probe molecule FTIR, catalysts were pre-treated in H2 at 250 oC, which is known to reduce oxidized Pt clusters to the metallic state.85 Probe molecule IR showed that Pt deposited under highly dilute conditions, at high pH, and with weight loadings lower than that required to nominally produce 1 Pt atom/TiO2 particle, showed a single CO absorption band with a frequency of 2112 cm−1 and a full width half maximum of 6−10 cm−1. A representative spectrum is shown in Figure 3a.14 The retained stretching frequency characteristic of CO adsorption on cationic Pt atoms (>2100 cm-1), and the sharpness of this band, suggests an assignment of the 2112 cm−1 stretch to CO adsorbed to Ptiso on the TiO2 support. The small full width at half max of the CO stretch provides evidence that the Ptiso species were homogeneously distributed at predominantly the same site on TiO2.86-87 Although the narrow bandwidth of the CO stretching absorption band indicates that Ptiso sites exist in similar coordination environments, the nature of the Pt bonding to the support was not directly identified here. IR measurements with changing CO coverage showed no change in frequency, width, or symmetry of the Ptiso absorption feature, in contrast with the behavior observed on oxidized and metallic Pt nanoparticle sites. The lack of dipole-dipole interaction between CO molecules on Ptiso sites suggests that these sites are spatially isolated.88 Aberration-corrected STEM imaging, presented in Figure 3c, showed that the 0.05 wt % SEA catalyst consisted almost entirely of Ptiso species, even after calcination and reduction, consistent with FTIR measurements. As a comparative sample, ~1 nm Pt clusters were deposited on the TiO2 particles by increasing the Pt weight loading to 1%. Probe molecule IR spectra showed substantially different spectral features, and STEM analysis indicated the presence of Pt clusters and larger particles, Figure 3b and d. Having demonstrated the synthesis and characterization of Ptiso species on TiO2 that are homogeneous in their adsorption site on the support and stable through various conditions, questions regarding the adsorption properties and catalytic activity of these sites could be addressed. Temperature-programmed analysis indicated that the adsorption energy of CO on Pt species followed Ptiso ≪ Ptmetal < Ptox. Kinetic CO oxidation measurements showed that Ptiso sites exhibited an average turnover frequency ~2x higher than 1 nm Pt metal clusters at 200 °C. CO strongly adsorbed to oxidized Pt clusters was unreactive, suggesting that Ptox do not contribute to steady-state CO oxidation reactivity measurements on Pt/TiO2 catalysts. Assuming peripheral Pt atoms are the only active site, catalysts containing exclusively Ptiso species are predicted to be more active on an average TOF Pt basis compared to 1 nm Ptmetal clusters as all sites interface with the support, which is consistent with our experimental results. This study provides insight into the activity of Ptiso species on TiO2, enabled by a new architecture for synthesizing and unambiguously characterizing stable isolated sites through the combination of characterization by probe molecule IR spectroscopy and STEM.


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Figure 3. Reactivity of isolated Pt sites for CO oxidation. a,b) IR spectra of CO adsorbed on Pt at room temperature and saturation coverage for 0.05% and 1% weight loading Pt/TiO2 catalysts that were reduced at 240 °C in H2. (c,d) HAADF images of the same catalysts after reduction ex-situ. Ptiso species are circled in yellow, while Pt clusters are circled in red. e) Proposed scheme of the active site of Ptiso and Ptmetal clusters for CO oxidation. Reprinted (adapted) with permission from DeRita, L.; Dai, S.; LopezZepeda, K.; Pham, N.; Graham, G. W.; Pan, X.; Christopher, P., Catalyst Architecture for Stable Single Atom Dispersion Enables Site-Specific Spectroscopic and Reactivity Measurements of CO Adsorbed to Pt Atoms, Oxidized Pt Clusters, and Metallic Pt Clusters on TiO2. Journal of the American Chemical Society 2017, 139, 14150-14165. Copyright 2017 American Chemical Society.



4. Reconstruction of supported Pt catalysts by CO While the previous study helped to understand the reactivity of Ptiso on reducible supports, there remain questions regarding the reactivity of various types of metallic sites (terraces, steps, edges) on non-reducible oxide supported Pt nanoparticles for low temperature CO oxidation. At low temperatures, chemisorbed CO saturates the surface of metallic Pt particles. Under these conditions with metallic Pt particles on irreducible oxide supports, the reaction occurs by a modified Langmuir-Hinshelwood-type mechanism, with the kinetically relevant step involving CO assisted O2 dissociation.95 For this mechanism, measured rate constants depend on the ratio of the activation energy for O2 activation and reaction enthalpy for CO adsorption. The adsorption energy of CO is strongly affected by the coordination of the metal atoms on which it adsorbs, with adsorption on under-coordinated (UC, 6 and 7-fold coordinated steps/edges) sites being 0.5-1.0 eV stronger than on well-coordinated (WC, 8 and 9-fold coordinated terraces) sites.5 As a result, turnover rates for this reaction are expected to be strongly dependent on catalyst particle size due to the expected variation in population of well- and under-coordinated sites as a function of particle size.96-97 Instead, CO oxidation remains a classic example of a structure insensitive reaction, as experimental turnover rates are found to minimally depend on metal dispersion.98-106 One plausible explanation for the observed apparent structure insensitivity is that CO induces surface restructuring, which may lead to a different fraction of UC and WC sites than what would be expected from geometric models for a given nanoparticle size. Studies on single crystal surfaces have shown evidence for surface reconstructions of Pt by CO that transform WC Pt sites into UC sites, but this process is known to be facet dependent and a description of this process on supported metal nanoparticles has not been resolved with atomic scale or quantitative detail.52-54 We recently used combined information gained from in-situ IR and STEM and catalytic activity measurements to more fully understand the behavior of Pt nanoparticles on non-reducible oxide supports for low temperature CO oxidation.17 To identify which surfaces are susceptible to CO-induced reconstruction on supported Pt nanoparticles and how much reconstruction of Pt nanoparticle surfaces is expected at saturation CO coverage, surface energies of different crystallographic Pt facets were calculated as a function of CO coverage using Density Functional Theory (DFT).16 Adsorbate-induced surface reconstruction is driven by a change in the relative surface energy of competing facets upon the adsorption of gas phase species.107-108 Wulff constructions were derived from the adsorbate free and CO-saturated surfaces energies of the Pt facets, showing the energetically most favorable shape for a nanoparticle of a given size.109-110 The Wulff constructions for an adsorbate free and CO saturated 9.2 nm Pt nanoparticle are shown in Figure 4a and 4b (note that the CO is not shown in Figure 4b for clarity). The Wulff constructions were visually compared to in-situ STEM measurements, shown in Figure 4c and 4d for a 9 nm Pt particle under 500 Torr of N2 at 423K (adsorbate free) and 25 Torr of CO at 423 K (CO saturated surface). It was seen that the insitu HAADF images and DFT based Wulff constructions were in excellent agreement. Both analyses concluded that the bare Pt truncated octahedron nanoparticle would undergo a reversible and facet-specific reconstruction induced by the adsorption of saturation CO coverage, resulting in reconstruction of WC [100] facets into vicinal stepped high Miller index facets dominated by UC sites, while WC [111] facets remain largely unchanged.16


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While agreement between the in-situ STEM and DFT calculations is strong evidence supporting the proposed reconstruction, the sample size on the STEM images is very small (2 different particles were observed) and the Wulff construction only provides the thermodynamic energy minimum of the system. There could exist almost degenerate low energy states of this system, which do not agree with the proposed reconstruction. To address this issue a quantitative comparison of the extent of reconstruction was made by comparing the predicted amount of reconstruction of WC into UC sites from the DFT derived Wulff construction and quantitative measurements made by in-situ FTIR, which provides sample averaged information. CO adsorbed to WC and UC Pt sites are distinguishable in FTIR with a lower frequency vibrational stretch features (2060−2075 cm−1) assigned to vibrational stretching modes of CO molecules linearly adsorbed on UC Pt sites, while a higher frequency stretch (2080−2098 cm−1) is assigned to CO linearly adsorbed on WC Pt sites.48, 56-57, 111 Further separation of features attributed to WC sites into contributions from different close packed crystallographic planes is generally not possible. To understand if FTIR measurements could corroborate the TEM and DFT studies, the relative fraction of CO adsorbed to WC and UC sites were quantified using known extinction coefficients at saturation CO coverage as temperature was raised from 298 to 363K.56-57 The experiment was designed such that the IR spectra at room temperature would represent the initial state of the Pt particles (i.e. not enough thermal energy to allow rapid reconstruction), while at somewhat elevated temperature kinetic energy for the reconstruction could be provided without driving CO desorption. It was found that CO adsorption at elevated temperature (363 K in this case) induces reconstruction consistent with the conversion of WC into UC sites, Figure 4c. Quantitative agreement was found between DFT calculated and FTIR measured fraction of WC sites converted into UC sites as a function of Pt particle size (not shown here), where the relative amount of surface reconstruction increased with increasing Pt particle size. Quantitative consistency between the FTIR and DFT derived amount of CO induced Pt nanoparticle reconstruction and visual agreement between the TEM and DFT derived measurements provide strong evidence that a CO induced WC [100] facet selective reconstruction to form UC Pt sites well-represents the behavior of the entire sample.

Figure 4. Atomic level insights into reaction induced surface reconstruction on Pt. a,b) Wulff constructions of a 9.2 nm Pt particle based on DFT calculated surface free energies for (a) bare surfaces and (b) CO-saturated surfaces. WC Pt atoms are depicted in green, and UC Pt edge and corner atoms are depicted in blue. c,d) Corresponding aberration-corrected HAADF images of a 9 nm Pt particle taken



along the ⟨110⟩ zone axis (c) at 423 K in 500 Torr N2, and (d) at 423 K in 500 Torr of 5% CO in Ar (25 Torr CO). Layers 0, 1, and 3 are labeled in each image for comparison. Below each image is the intensity for layer 1 of each corresponding image. Simulated HAADF image based on layers 0−6 of the [100] facets of the (e) clean and (f) CO-saturated 9.2 nm Wulff constructions, along with the corresponding intensity analysis of layer 1 for each particle model. (g) In-situ IR spectra associated with a time evolution of a prereduced 17 ± 9 nm Pt/Al2O3 catalyst (larger particle is chosen for clarity). The inset in shows example adsorption geometries of CO on a Pt nanoparticle on WC and UC atoms and the associated vibrational frequencies. Reprinted (adapted) with permission from Avanesian, T.; Dai, S.; Kale, M. J.; Graham, G. W.; Pan, X.; Christopher, P., Quantitative and Atomic-Scale View of CO-Induced Pt Nanoparticle Surface Reconstruction at Saturation Coverage Via DFT Calculations Coupled with in Situ Tem and Ir. Journal of the American Chemical Society 2017, 139, 4551-4558. Copyright 2017 American Chemical Society.

To relate CO-induced Pt surface reconstruction to the the minimal structure sensitivity observed for CO oxidation at CO saturated Pt surfaces, the activity of α-Al2O3-supported Pt nanoparticle catalysts with average Pt sizes ranging from ∼1.4 to 19 nm in diameter was measured under conditions that ensured strict kinetic control.112-113 Reaction orders in CO (-0.73) and O2 (0.90), as well as apparent activation energies (~ 85kJ/mol) were found to be similar for all particle sizes.17 Figure 5a shows Arrhenius plots for the range of particle sizes. These kinetic parameters were in agreement with previous literature reports for CO poisoned surfaces and LangmuirHinshelwood kinetics.78, 95, 98, 114-115 The TOF increased slightly with particle size, where differences in reactivity between the most active and least active catalysts were consistent with prior studies.95, 99, 102-104 The comparative extent of CO induced surface reconstruction increased with increasing Pt nanoparticle size, with the result that on the reconstructed Pt nanoparticles the fraction of WC and UC Pt sites changes less as a function of particle size than what would be expected from typically used geometric models. A comparison of the average TEM measured particle size, the effective particle size from FTIR measurements at room temperature (non-reconstructed surface) and effective particle size from FTIR measurements under reaction conditions (reconstructed surface), shown in Figure 5c, demonstrate that because of the CO-induced Pt surface reconstruction, the effective particle size is similar for all dispersions.17 The effective particle size estimation was done by comparing the relative fraction of UC and WC sites measured by IR, and correlating this to what would be expected for ideal nanoparticle geometries of different shapes. This analysis shows that in the CO induced reconstructed state the distribution of WC and UC sites does not depend as significantly on particle size as would be expected from geometric models. A strong correlation between the CO oxidation TOF and the measured fraction of WC sites under reaction conditions as a function of Pt particle size was observed, which suggests that the active site of CO oxidation on Pt nanoparticles is the WC Pt site, Figure 5c. Further, these results suggest that CO molecules adsorbed to UC Pt sites have no reactivity, although they could provide CO to active WC sites through equilibrated surface diffusion.97, 116 Using a combination of in-situ FTIR, TEM, DFT and kinetic measurements to gain quantitative sample averaged and atomic-level information, the apparent structure insensitivity of Pt for CO oxidation was rationalized. The lack of measured structure sensitivity in CO oxidation on


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irreducible oxide supported Pt nanoparticles under CO saturation conditions is a result of particle size dependent CO induced Pt surface reconstruction, which renders the fraction of WC and UC sites similar for all measured particle sizes. It is proposed that the CO induced surface reconstruction primarily influences WC [100] facets that significantly roughen to form UC sites, while [111] facets remain largely intact.

Figure 5: Structure sensitivity of Pt for low temperature CO oxidation a) Arrhenius plots for Pt nanoparticles of different sizes for CO oxidation (1% CO, 10% O2, balance He). b) Comparison of the average TEM particle size to the effective particle size from DRIFTS measurements (made at room temperature before and after reaction, and under reaction conditions at 442 K). c) Comparison of measured and calculated TOF from structure sensitivity model (both normalized to 1 for the 19 nm Pt catalyst) as a function of TEM-measured average catalyst particle size. Also shown are the measured and calculated apparent activation energies for all catalysts as a function of TEM-measured average particle size. Reprinted (adapted) with permission from Kale, M. J.; Christopher, P., Utilizing Quantitative in Situ FTIR Spectroscopy to Identify Well-Coordinated Pt Atoms as the Active Site for CO Oxidation on Al2O3Supported Pt Catalysts. ACS Catalysis 2016, 6, 5599-5609. Copyright 2016 American Chemical Society.

5. Quantifying the influence of single Rh atoms on CO2 reduction selectivity A second reaction system for which we analyzed structure−function relations is the selective reduction of CO2 by H2 over oxide supported Rh catalysts. Structure-function relations for this reaction must account for geometric changes to Rh under reaction conditions, as well as potential structural or electronic influences of the support.117-119 Using the combination of in-situ FTIR spectroscopy and TEM, we gained insights into the reactivity and dynamic behavior of supported Rh catalysts for CO2 reduction by H2, particularly elucidating the reactivity of isolated Rh atoms and effects of adsorbate-mediated encapsulation of Rh particles by reducible supports.13, 15 We first sought to understand the contributions of isolated Rh atom active sites to CO2 reduction reactivity.76, 120-123 The catalytic activity and selectivity was measured for five Rh weight loadings on TiO2 under varying CO2:H2 ratios.15 The dominant reaction pathways are the formation of CH4 and the competing reverse water gas shift reaction (r-WGS) to form CO. For a stoichiometric methanation feed (CO2:4H2), 0.5% Rh/TiO2 (the Rh lowest loading explored) exhibited a CH4 selectivity of 65%, while all higher Rh weight loadings produced nearly exclusively CH4 (>98% selectivity). Increasing the CO2:H2 feed ratio resulted in a decrease in



CH4 selectivity for all weight loadings.124 A positive correlation between CH4 selectivity and Rh weight loading was observed consistently for all CO2:H2 feed ratios. To understand the trend between weight loading and selectivity, CO probe molecule FTIR measurements were performed on the same five samples, an example of which is shown in Figure 6a. The CO stretches at ∼2097 and ∼2028 cm−1 can be ascribed to the symmetric and asymmetric stretching modes of Rh(CO)2 gem-dicarbonyl formed uniquely at isolated Rh atom active sites (Rhiso). The assignment of the gem-dicarbonyl to Rhiso species has been thoroughly substantiated by previous studies.70, 120, 125-126 The CO stretches at 2068 cm-1 and 1860 cm-1 can be attributed to CO adsorbed in on top and bridge geometries on the surface of Rh nanoparticles (Rhnp).70, 127 Using site-specific CO stretch intensities and known site-specific extinction coefficients, relative fractions of Rhiso and Rhnp sites could be quantified. This analysis, presented in Figure 6b, showed that the Rhiso site fraction decreased with increasing weight loading. The results are consistent with TEM analysis of a 2% Rh/TiO2 catalyst shown in Figure 6d that shows primarily Rhnp with some Rhiso identified as well. Examining the CO and CH4 production TOFs independently provides insight into the origin of the CO2 reduction selectivity dependence on Rh weight loading. The trend between CO production TOF and Rh weight loading mirrored the trend between Rhiso site fraction and weight loading for the three feed conditions tested, see for example Figure 6c. The agreement in the trends between the site fraction and TOF suggests that the relative fraction of Rhiso sites determines the TOF for CO production, while the fraction of Rhnp sites controls the TOF for CH4 production. The overall selectivity is then dictated by the ratio of these types of sites on a given catalyst.63 To validate this hypothesis, a treatment intended to selectively leach nanoparticles was executed, which resulted in a more significant decrease the rate of CH4 production as compared to CO production, providing support for the conclusion that CO production occurs exclusively at Rhiso sites while CH4 production occurs at Rhnp sites. These results raise questions about why Rhiso and Rhnp sites exhibit different selectivity for CO2 reduction. The active site on a Rhnp will bind CO strongly, while being in proximity to other Rh sites that can dissociate H2.128 Thus, reaction steps to hydrogenate CO, or directly CO2, will be favored over CO desorption.129-130 Rhiso sites on oxide supports interact more weakly with CO than Rhnp sites, suggesting that CO desorption before further hydrogenation should be more favorable on Rhiso sites compared to Rhnp sites.127 In addition, the lack of H2 dissociation on the oxide support surrounding a Rhiso site suggests that sequential hydrogenation of CO or CO2 on the Rhiso site would be less probable. These simple arguments describe why Rhiso sites areselective for r-WGS, while Rhnp sites are selective for CO2 methanation. Interestingly, it was observed that when operating under reaction conditions containing high CO2 to H2 ratios and high Rh weight loadings dynamic reactivity changes were observed where the rate of CH4 production decreased with time on stream, while the rate of CO production increased. It was hypothesized that the decrease in CH4 selectivity with time on stream was a result of fragmentation of Rhnp to form a higher concentration of Rhiso sites that are selective for CO production. However, there was a disproportionate magnitude of changes in CO and CH4 TOFs, where the decrease in CH4 production was much greater than the increase in CO production. The disparate change in TOFs was difficult to understand in the context of Rh


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fragmentation, where an increased number of Rhiso sites are formed for losses in Rhnp sites (because Rhnp exhibit

Transmission Electron Microscopy and Infrared Spectroscopy

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