Shape Effect Undermined by Surface Reconstruction: Ethanol

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Shape Effect Undermined by Surface Reconstruction: Ethanol Dehydrogenation over Shape-Controlled SrTiO3 Nanocrystals Guo Shiou Foo, Zachary D. Hood, and Zili Wu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03341 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017

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Shape Effect Undermined by Surface Reconstruction: Ethanol Dehydrogenation over Shape-Controlled SrTiO3 Nanocrystals Guo Shiou Foo, † Zachary D. Hood, ‡ Zili Wu†*

† Chemical Sciences Division and Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡ School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States *Email: [email protected]

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Abstract To gain an in-depth understanding of the surface properties relevant for catalysis using ternary oxides, we report the acid-base pair reactivity of shape-controlled SrTiO3 (STO) nanocrystals for the dehydrogenation of ethanol. Cubes, Truncated Cubes, Dodecahedra, and Etched Cubes of STO with varying ratios of (001) and (110) crystal facets were synthesized using a hydrothermal method. Low-energy ion scattering (LEIS) analysis revealed that the (001) surface on Cubes of STO is enriched with SrO due to surface reconstruction, resulting in a high ratio of strong base sites. Chemical treatment with dilute nitric acid to form Etched Cubes of STO resulted in a surface enriched with Ti cations and strong acidity. Furthermore, the strength and distribution of surface acidic sites increase with the ratio of (110) facet from Cubes to Truncated Cubes to Dodecahedra for STO. Kinetic, isotopic, and spectroscopy methods show that the dehydrogenation of ethanol proceeds through the facile dissociation of the alcohol group, followed by the cleavage of the Cα-H bond, which is the rate determining step. Co-feeding of various probe molecules during catalysis, such as NH3, 2,6-di-tert-butylpyridine, CO2, and SO2, reveals that a pair of Lewis acid site and basic surface oxygen atom is involved in the dehydrogenation reaction. The surface density of acid-base site pairs was measured using acetic acid as a probe molecule, allowing initial acetaldehyde formation turnover rates to be obtained. Comparison among various catalysts reveals no simple correlation between ethanol turnover rate and the percentage of either surface facet ((001) or (110)) of the STO nanocrystals. Instead, the reaction rate is found to increase with the strength of acid sites but reversely with the strength of base sites. The acid-base property is directly related to the surface composition as a result from different surface reconstruction behaviors of the shaped STO nanocrystals. The finding in this work underscores the importance of characterizing


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the top surface compositions and sites properties when assessing the catalytic performance of shape-controlled complex oxides such as perovskites. Keywords: strontium titanate, ethanol, dehydrogenation, (001) facet, (110) facet, surface termination


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1. Introduction Strontium titanate (SrTiO3) is a common perovskite that has been gaining interest due to its optical, electrical and chemical properties.1-6 Interestingly, the surface structure of strontium titanate is complex because it has an additional degree of freedom associated with ternary oxides. There is a wide variety of reconstructions on the (001), (110), and (111) surfaces,7-13 making it possible for controlling selectivity and activity in catalysis.14 On a SrTiO3 (001) surface, which is the most thermodynamically stable facet, it can be terminated by a SrO or TiO2 plane with a similar surface energy of 6.85 eV/nm2 (Figure 1A-1D).15-17 For the (110) facet, it consists of alternating layers of SrTiO4+ and O24-, resulting in an unbalanced dipole with a high surface energy of 20.02 eV/nm2 (Figure 1E and 1F).17

(001) SrO Termination

(001) TiO2 Termination

(110) facet

(A)

(C)

(E)

(B)

(D)

(F)

Figure 1. (A) Side view and (B) top view of SrTiO3 (001) SrO terminated surface, (C) side view and (D) top view of SrTiO3 (001) TiO2 terminated surface, (E) side view and (F) top view of SrTiO3 (110) surface. Green, blue, and red spheres represent Sr, Ti, and O atom, respectively. Surface science studies on strontium titanate single crystals allow atomic arrangements to be elucidated, which are distinctly different from the bulk structure. Most of the reconstructions


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reported up to date are known to be rich with TiO2. For the (2 × 1) and c(4 × 2) reconstruction of the SrTiO3 (001) surface, it is characterized by a single overlayer of TiO, in which the Ti cations are arranged into edge-sharing truncated octahedra.7-8 For the (110) crystal facet of strontium titanate, the surface would undergo reconstruction to form a stable structure, which also has a single overlayer of TiO. It was reported that the (3 × 1) SrTiO3 (110) surface structure composed of rings of six or eight corner-sharing TiO4 tetrahedra.9, 11 However, all of these surfaces appear to be vastly different compared to powder perovskites that are used in catalysis.14, 18 Recently, we have shown that for a series of selected powder perovskites (ABO3: SrTiO3, SrZrO3, BaTiO3, and BaZrO3), the surface is preferentially exposed with AOx (either SrO or BaO) after pretreatment in oxygen at 550 °C, resulting in a high selectivity to acetone (> 70%) in the conversion of 2-propanol.18 Furthermore, we have also reported that the reaction selectivity can be controlled by tuning the surface termination (composition) of SrTiO3 nanocubes (dominated with (001) facet) via different pretreatments.14 Specifically, the surface termination of SrTiO3 (001) can be modified by chemical etching with nitric acid, leading to a surface enrichment of Ti cations and consequently high selectivity to propene (87%) in the conversion of 2-propanol, in contrast to the thermally treated case where SrO termination dominates and leads to high selectivity to acetone. As such, it is possible to modulate the surface composition by using different pretreatment methods of perovskites dominated with a single facet. Furthermore, the surface termination can be controlled by changing the morphology/facet of strontium titanate particles. To this end, the hydrothermal method has been widely used to control the particle morphology by using various precursors, solvents, and concentration. Different morphologies of strontium titanate crystallites have been synthesized, such as cubes,19 spheres,2021

nanowires,22 and dodecahedra.23 Depending on the morphology of the crystallite, the surface of 5 ACS Paragon Plus Environment

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the strontium titanate particle could be terminated with (001) or (110) surfaces. Mu et al. synthesized SrTiO3 cubes and truncated cubes nanocrystals to study the effect of different crystal facets on the photocatalysis of water splitting.24 Dong et al. also synthesized similar SrTiO3 nanocrystals to study the adsorption of proteins on different crystal facets.25 While the (001) facet was exposed on all 6 surfaces of the SrTiO3 cubes, the truncated cubes were terminated by (001) and (110) crystal facets. The surface of dodecahedra SrTiO3 nanocrystals was mainly dominated by the (110) crystal facet.11, 23 Although these shape-controlled SrTiO3 nanocrystals have been used in various applications, to the best of our knowledge, there are no reports in the literature that systematically compare the acid-base activity between the (001) and the (110) crystal facets. In this work, we used the conversion of ethanol as a probe reaction to study the acid-base pair reactivity of various shape-controlled SrTiO3 nanocrystals, such as cubes, truncated cubes, and dodecahedra. In addition, the SrTiO3 cubes were subjected to chemical etching vs thermal treatment to study the catalytic effect of surface-enriched Ti or Sr cations. Rigorous turnover rates, obtained through kinetic measurements and chemical titration of active sites, allow the comparison of activity between the various shape-controlled SrTiO3 nanocrystals. It was found that the nature of the acidbase pairs, which are critical for ethanol dehydrogenation, is determined by the surface compositions of the SrTiO3 nanocrystals, a consequence largely resulting from surface reconstruction rather than surface faceting. These findings suggest that not only the shape but also the surface reconstruction plays an important role in controlling the acid-base catalysis over complex oxide nanocrystals such as SrTiO3.


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2. Experimental Section Synthesis. Shape-controlled SrTiO3 nanocrystals were prepared using the hydrothermal synthesis method as reported elsewhere.23-25 Initially, 25 mL of deionized water was cooled in an ice bath. For the synthesis of cubes, truncated cubes, or dodecahedra SrTiO3 nanocrystals, 20 g ethanol (200 proof, Decon Laboratories), 12 g 1,2-popanediol (99.5%, Sigma Aldrich), or 1.2 g pentaerythritol (99%, Sigma Aldrich) was added, respectively. Subsequently, 0.26 mL of titanium (IV) chloride (99.9%, Sigma Aldrich) was added dropwise into the solution. After stirring for 5 min, 30 mL of LiOH aqueous solution containing 3.78 g of LiOH·H2O (> 98% Sigma Aldrich) was added. After stirring for 30 min, 10 mL of SrCl2 aqueous solution containing 0.63 g of SrCl2·6H2O (> 99%, Alfa Aesar) was added. After further stirring for 30 min, the resultant mixture was transferred to a Teflon-lined stainless-steel autoclave (Parr 5000 Multi Reactor Stirrer System). The autoclave was heated at 180 °C for 48 h. After cooling down to room temperature, the sample was centrifuged at 5000 rpm for 8 min. The collected sample was then centrifuged 5 times in deionized water and 3 times in ethanol. After drying the sample in an oven at 100 °C for 12 h, it was calcined in air at 550 °C for 4 h. The samples are herein referred to as Cubes, Truncated, and Dodecahedra STO. To tune the surface properties of the SrTiO3 nanocrystals, a similar procedure reported in the literature was used.26 Briefly, 400 mg of Cubes STO was immersed in 40 mL of 0.2 M HNO3 aqueous solution at room temperature. After stirring for 24 h, the sample was centrifuged at 5000 rpm for 8 min. The collected sample was then centrifuged 5 times in deionized water and 3 times in ethanol. After drying the sample in an oven at 100 °C for 12 h, it was calcined in air at 550 °C for 4 h. The catalyst is herein referred to as Etched Cubes STO.


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Nitrogen Physisorption. Physisorption was performed using a Micromeritics Gemini 2375 Surface Area and Pore Size Analyzer at -196 °C. Each sample was degassed for 1 h prior to measurement. The BET method27 was used for calculating surface areas. X-ray Diffraction. Powder X-ray diffraction (XRD) patterns were obtained using a PANalytical X’Pert Pro system with Cu Κα radiation. Diffractograms were collected at incident angles for 2θ = 10‒80°. SEM and TEM. Scanning electron microscopy (SEM) was performed using a Zeiss Merlin system operating at 1 kV. Transmission electron microscopy (TEM) images were collected on a Zeiss Libra 120 TEM operated with an acceleration voltage of 120.0 kV. Samples were dropcasted onto carbon-coated copper grids after dispersing each sample in ethanol. Low Energy Ion Scattering. The samples were sent to Lehigh University for Low Energy Ion Scattering (LEIS) analysis to determine the composition of the top surface layer.28-29 LEIS spectra were collected using an IONTOF Qtac100 spectrometer (ION-TOF GmbH, Mìnster, Germany). Briefly, each sample was pressed into a self-supported wafer and transferred into a sample holder. The sample holder was then transferred into a high-vacuum LEIS spectrometer system, and was exposed to ambient temperature O atoms for 30 min. Subsequently, LEIS analysis of the sample surface was performed using a 3 keV He+ ion probe and 0.5 keV Ar+ for sputteretching. The sample was then transferred to the LEIS preparation chamber where it was exposed to 100 mbar of O2 at 550 °C for 1 h. After cooling down to room temperature, the sample was transferred back into the spectrometer system for analysis using 3 keV He+ probe ions. Adsorption Microcalorimetry. To determine the strength and concentration of various adsorption sites, the chemisorption of ammonia, carbon dioxide, ethanol, and acetic acid were 8 ACS Paragon Plus Environment

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performed using a Micromeritics 3Flex Characterization Analyzer coupled with a Setaram Sensys Evo DSC microcalorimeter.30 Each catalyst was loaded into one side of a custom-made quartz bitube.31 The bi-tube was inserted into the aluminum block of the microcalorimeter, while the tube opening was attached to the chemisorption port of the 3Flex instrument for degassing and dosing. Each catalyst was evacuated and heated at 550 °C for 1 h, dosed with 500 mmHg of O2 and evacuated for 30 min each for two cycles, and then cooled down to 150 °C (ammonia and acetic acid adsorption) or 30 °C (carbon dioxide and ethanol adsorption). Subsequently, the temperature was held for 1 h to reach thermal equilibrium. Each catalyst was then exposed to doses of acetic acid, ammonia, carbon dioxide, or ethanol. Prior to adsorption, acetic acid and ethanol in a stainless-steel reservoir was subjected to three cycles of freeze-pump-thaw. After the target pressure was reached, the sample was degassed for 1 h at the same temperature, and dosed with the same adsorbate again. The concentration of adsorption sites was determined by the amount of irreversibly adsorbed probe molecules. A control experiment using an empty bi-tube showed negligible amount of irreversible adsorption of the probe molecule. FTIR Spectroscopy. To determine the nature of acid and base sites, ammonia and carbon dioxide adsorption followed by FTIR spectroscopy were performed using a Thermo Nicolet Nexus 670 FTIR spectrometer equipped with an MCT detector. Ethanol adsorption was also performed to probe its surface interaction, while acetic acid adsorption was performed to determine its interaction with acid-base site pairs. Each spectrum was collected with 32 scans at a resolution of 4 cm-1. Each catalyst was loaded into a porous ceramic cup, and the cup was inserted into a Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) cell (HC-900, Pike Technologies). The sample was pretreated at 550 °C for 1 h under 30 mL/min of 5% O2/He and cooled down to 150 °C (ammonia), 25 °C (carbon dioxide and ethanol) or 300 °C (acetic acid). 9 ACS Paragon Plus Environment

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The gas was switched to 30 mL/min of He and a background spectrum was collected. For ammonia and carbon dioxide adsorption, 30 mL/min of 2% NH3/He and 30 mL/min of 2% CO2/He was introduced into the DRIFTS cell for 10 min, respectively. Subsequently, the sample was purged with 30 mL/min of He for 10 min before a spectrum was collected. For ethanol and acetic acid adsorption, 30 mL/min of He was bubbled through a saturator at 25 °C and a pulse of saturated gas (0.5 mL) was introduced into the DRIFTS cell. A spectrum was collected after purging the cell for 15 min. After the adsorption of ethanol, the temperature was increased step-wise to 300 °C and a spectrum was collected at 100, 200, and 300 °C using the background spectrum taken at its respective temperature. Kinetic Measurements. Kinetic experiments for the conversion of ethanol were performed using an Altamira Instruments system (AMI-300). Each catalyst (0.050 g) was sieved (177 – 250 µm), mixed with silicon carbide (0.200 g), and placed inside a quartz u-tube. The catalyst bed was held in place by placing quartz wool at both ends of the bed. Ethanol (200 proof, Decon Laboratory) was fed and vaporized into the reactor system using a Chemyx Nexus 3000 syringe pump with 40 mL/min of argon. All of the lines were heated to 120 °C to prevent condensation. Experiments were performed at 300 °C under low conversion (< 5%). Each catalyst was pretreated at 550 °C for 1 h under 40 mL/min of 5 % O2/He (ultra-high purity, Air Liquide) before ethanol was introduced into the reactor at various feed rates. This method was also used to regenerate the catalysts. For co-feeding experiments using NH3, CO2, or SO2, 2% NH3/He, CO2 (99.98%), or 200 ppm SO2/He was fed into the reactor system at various flow rates with argon, respectively, while ensuring the total gas flow was constant at 40 mL/min. To determine the role of Brønsted acid sites, ethanol was co-fed with 2,6-di-tert-butylpyridine (DTBP) after achieving steady-state formation rates. The mixture was prepared by dissolving DTBP (> 97%, Sigma 10 ACS Paragon Plus Environment

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Aldrich) in ethanol with a molar ratio of 1000 (ethanol to DTBP). For the inhibition of acid-base site pairs, ethanol was co-fed with acetic acid after achieving steady-state formation rates. The mixture was prepared by dissolving acetic acid (> 99.7%, Fisher Chemical) in ethanol with a molar ratio of 200 (ethanol to DTBP). The products were analyzed and quantified using an Agilent 6890N gas chromatograph equipped with a Restek 4% carbowax-20M column and a flame ionization detector. 3. Results and Discussion 3.1 Morphology and surface properties of shape-controlled SrTiO3 Figure 2 shows the XRD patterns of the shape-controlled SrTiO3 nanocrystals. Each catalyst displayed major peaks at 32.5, 40.1, 46.6, 57.9 and 67.9°, which are assigned as the (110), (111), (200), (211) and (220) planes of its cubic crystal structure, respectively.24-25 No other phases such as TiO2 or SrO were observed from the XRD patterns.

(110) (111)

(200)

(211) (220)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Etched Cubes

Dodecahedra

Truncated

Cubes

20

30

40

50

60

70

80

2θ (°)

Figure 2. XRD pattern of shape-controlled SrTiO3 nanocrystals.


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Figure 3 shows the SEM and TEM images of the shape-controlled SrTiO3 nanocrystals. The inset shows the schematic drawing of each catalyst. For Cubes STO, the general morphology of the nanocrystals is cube bounded by (001) facets (Figure 3A), while the morphology of Truncated STO is cubic SrTiO3 nanocrystals with its edges truncated (Figure 3B). As shown in the schematic drawing, the truncated edges are bounded by (110) facets.24-25 For Dodecahedra STO, the nanocrystals have a high degree of truncation, resulting in a dodecahedral morphology with predominantly (110) facet being exposed (Figure 3C).11, 23 For Etched Cubes STO, the nanocrystals retained its cubic shape with slight truncation on the edges after chemical treatment with nitric acid (Figure 3D). The average particle size for each catalyst is approximately 120 nm.

(A)

(B)

(110)

(001)

(001)

200 nm

200 nm

(C)

(D)

(110)

(001)

200 nm

200 nm

Figure 3. SEM and TEM images of (A) Cubes, (B) Truncated, (C) Dodecahedra, and (D) Etched Cubes STO. The inset shows the schematic drawing of each SrTiO3 nanocrystal.


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The specific surface area measured using nitrogen physisorption (Table 1) shows that the shapecontrolled SrTiO3 nanocrystals displayed a similar surface area of approximately 12 m2/g. Table 1. Surface area of shape-controlled SrTiO3 nanocrystals and surface density of sites accessible by ammonia, carbon dioxide, ethanol, and acetic acid. Catalyst Cubes Truncated Dodecahedra Etched Cubes

Surface Area (m2/g) 12 12 14 12

Ammonia (μmol/m2) 0.05 0.34 1.37 0.60

Carbon Dioxide (μmol/m2) 2.08 0.48 2.66 0.14

Ethanol (μmol/m2) 3.03 3.19 4.14 4.00

Acetic Acid (μmol/m2) 4.70 2.02 2.38 2.50

Figure 4 shows the ratio of the integrated scattering intensity of the surface Sr to Ti cations on the SrTiO3 nanocrystals as function of depth determined using low energy ion scattering (LEIS), which is a surface sensitive technique and it is suitable to probe the composition of the top atomic layer.28-29 The ratio of surface cations for each sample is normalized to unity at higher depths. Prior to pretreatment, the surface of each SrTiO3 sample is enriched with Ti cations (Figure 4A). This result is similar to surface science studies on (001) and (110) SrTiO3 single crystals,7-12 which show that there is an overlayer of TiO on top the perovskite. After pretreatment at 550 °C in the presence of molecular oxygen for 1 h (similar to the pretreatment condition used before reaction test), the surface of Cubes STO is enriched with Sr cations to the highest extent, followed by Dodecahedra STO (Figure 4B). This indicates that the surface termination/composition of Cubes and Dodecahedra STO has changed. The surface of Truncated and Etched Cubes STO remained enriched with Ti cations.


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(A)

(B)

Figure 4. Ratio of integral of surface Sr to Ti cation on shape-controlled SrTiO3 nanocrystals as a function of depth probed using low energy ion scattering (LEIS). (A) Before pretreatment, (B) after pretreatment at 550 °C in 100 Torr O2 for 1 h. Adsorption microcalorimetry was used to measure the strength and distribution of surface sites using ammonia, carbon dioxide, and ethanol. For the adsorption of NH3 on acidic sites (Figure 5A), Dodecahedra STO displayed a distribution of strongest acid sites, followed by Etched Cubes, Truncated, and Cubes STO. The differential heat of adsorption decreased sharply with surface coverage for Cubes STO compared to the other SrTiO3 nanocrystals. The distribution of strongest acid sites on Dodecahedra STO could be due to the presence of TiOx microfacets on the surface as a result of surface reconstruction of the SrTiO terminated (110) surface, similar to the 14 ACS Paragon Plus Environment

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phenomenon observed on SrTiO3 (110) single crystals annealed in oxygen at 900 °C and 1000 °C.32 A general comparison between Dodecahedra, Truncated, and Cubes STO indicates that a higher ratio of (110) to (001) crystal facet results in stronger acidic sites, while the surface acidity

(A)

150

Heat of Adsorption (kJ/mol)

of the (001) facet can be enhanced by chemical etching with dilute nitric acid (Etched Cubes STO).

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0.5

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1.5

2.0

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Figure 5. Heat of adsorption of (A) NH3 and (B) CO2 on shape-controlled SrTiO3 nanocrystals. For the adsorption of CO2 on base sites, Cubes STO displayed the strongest distribution of basic sites up to a surface coverage of 1.5 µmol/m2, followed by Dodecahedra STO (Figure 5B). 15 ACS Paragon Plus Environment

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Truncated and Etched Cubes STO have a similar distribution of weaker basic sites, and the differential heat of adsorption decreased sharply with surface coverage relative to Cubes and Dodecahedra STO. In contrast to the adsorption of NH3, a trend in basicity from adsorption microcalorimetry at 30 °C was not observed among the shape-controlled SrTiO3 nanoshapes. For the adsorption of ethanol, Dodecahedra STO displayed the highest initial heat of adsorption (182 kJ/mol), followed by Cubes STO, while Truncated and Etched Cubes STO had a similar value of 130 kJ/mol (Figure S1). The differential heat of adsorption also decreased as a function of surface coverage, and the nanocrystals possessed a similar distribution of adsorption sites for ethanol. The surface density of sites determined by the amount of irreversibly adsorbed titrants is shown in Table 1. Dodecahedra STO possessed the highest surface density of base and acid sites, as determined from the adsorption of CO2 and NH3, respectively. Both Cubes and Dodecahedra STO displayed a high ratio of base to acid sites, which is due to the high ratio of Sr to Ti cations as evidenced by LEIS analysis. This is consistent with previous reports, which show that a high ratio of surface alkaline cations resulted in a high ratio of base to acid sites, and vice versa.14, 18 However, after chemical treatment with nitric acid, the surface of Etched Cubes STO is enriched with acidic sites and Ti cations compared to Cubes STO, indicating that acidic sites are due to surface Ti cations. Polo-Garzon et al. reported that chemical etching with nitric acid on a commercial SrTiO3 led to the presence of single and double TiO2 enrichment on the surface, with an initial heat of adsorption of NH3 of approximately 110 kJ/mol,14 which is similar to Etched Cubes STO (Figure 5A). As such, the surface of Etched Cubes STO could also be enriched with a single or double layer of TiO. Truncated STO displayed a similar surface density of acid and


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base sites. For the adsorption of ethanol, Dodecahedra and Etched Cubes STO had a higher surface density of adsorption sites compared to Cubes and Truncated STO. The surface density of sites accessible by NH3 and CO2 shows no correlation with the surface facets among the catalysts, suggesting that surface reconstruction (i.e., surface composition) on the (001) and (110) facets is not similar between different SrTiO3 nanocrystals, which is likely due to the effect of various capping agents used in the synthesis process that may result in different concentrations of defects on the surface and/or in the bulk of SrTiO3.13 As such, the effect due to different surface facets on the SrTiO3 nanocrystals is undermined and the kinetic measurements for ethanol dehydrogenation would have to be correlated to surface composition or termination of the perovskites. This correlation has a general implication for understanding catalysis over complex oxides such as perovskites, where surface reconstruction or segregation often occurs and results in deviation of surface composition from the ideally truncated surface. Therefore, careful characterization of the top surface compositions and sites are needed for complex oxide catalysts in establishing a reliable structure-catalysis relationship. To determine the nature of the acid and base sites, the adsorption of NH3 and CO2 on the shape-controlled SrTiO3 nanocrystals was monitored by DRIFTS. For the adsorption of NH3 (Figure 6A), the peak observed at 1452-1455 cm-1 on Cubes, Truncated, and Dodecahedra STO is assigned as the δNH4+ vibration of ammonium ion protonated by Brønsted acid sites,33 whereas Brønsted acid sites were not observed on Etched Cubes STO. Additional peaks observed at 15941626 cm and 1114-1186 cm-1 on all of the catalysts are attributed to the νasymNH3 and νsymNH3 vibration of ammonia coordinated to Lewis acid sites, respectively.34


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1220 1675

1400 1353

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1675

1291

1280

Etched Cubes

Truncated

1277 Dodecahedra 1800


1200

Figure 6. FTIR spectra of (A) NH3 and (B) CO2 adsorption on shape-controlled SrTiO3 nanocrystals. Interaction of CO2 with medium and strong basic surface oxygen atoms results in the formation of bidentate and unidentate carbonates, respectively, while interaction with weakly basic hydroxyl groups leads to the formation of bicarbonate surface species.35 For the adsorption of CO2 (Figure 6B), peaks observed at 1655-1675 and 1277-1304 cm-1 on all of the catalysts are attributed 18 ACS Paragon Plus Environment

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ACS Catalysis

to the νasymOCO and νsymOCO vibrations of bidentate carbonate, respectively.35 Additional peaks observed at 1580 and 1350-1353 cm-1 on Cubes and Etched Cubes STO are assigned as the νasymOCO and νsymOCO vibrations of unidentate carbonate, respectively. Furthermore, the presence of weak bicarbonate on Etched Cubes STO is observed at 1617 (νasymOCO), 1400 (νsymOCO), and 1220 cm-1 (δCOH).36 The intensity of these bicarbonate peaks vanished at higher temperatures (< 100 °C), indicating the weak strength of the basic hydroxyls. Evidently, most of the SrTiO3 nanocrystals only possess medium to strong basic surface oxygens. 3.2 Kinetics and mechanism of ethanol dehydrogenation Ethanol dehydrogenation at low conversion was performed over the shape-controlled SrTiO3 nanocrystals and acetaldehyde was observed to be the major product (> 95% selectivity) with CO and CH4 as the minor products. For all of the catalysts, significant deactivation was observed at ethanol pressures of 1 kPa or greater at 300 °C (Figure S2). No deactivation was observed at 0.53 kPa of ethanol pressure and steady-state rates were measured at this partial pressure. To examine the influence of ethanol pressure on dehydrogenation rate in the absence of deactivation, initial rates were estimated by extrapolating the rate curves to zero time on stream. The dehydrogenation of ethanol over the shape-controlled SrTiO3 nanocrystals was performed at multiple ethanol partial pressures and the results are shown in Figure 7.


ACS Catalysis

10 Acetaldehyde Formation Rate (µmol/µmolpair.min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Dodecahedra

8 6

Etched Cubes

4

Truncated

2 0

Cubes 0

1

2 3 4 5 Ethanol Pressure (kPa)

6

Figure 7. Initial acetaldehyde formation rate as a function of ethanol pressure over shapecontrolled SrTiO3 nanocrystals. Solid lines represent the model fit from eq 1. Reaction conditions: 300 °C, 50 mg catalyst, 40 mL/min argon. The initial acetaldehyde synthesis rates are normalized by the number of active sites of each catalyst (for determination of density of active sites, see Section 3.3), which is essential for comparing the various shape-controlled SrTiO3 nanocrystals. Between 0.53 and 5.05 kPa of ethanol pressure, acetaldehyde formation rates for Dodecahedra STO displayed the highest values, followed by Etched Cubes, Truncated, and Cubes STO. To explore the rate limiting step for ethanol dehydrogenation over the shape-controlled SrTiO3 nanocrystals, various types of deuterated ethanol was used to investigate the kinetic isotope effect. In the presence of C2H5OD and CD3CH2OH (Table 2), the ratio of 𝑟𝑟𝐻𝐻 to 𝑟𝑟𝑑𝑑1 and 𝑟𝑟𝐻𝐻 to 𝑟𝑟𝑑𝑑3

is close to unity, respectively. A kinetic isotope effect (~1.5) was observed only in the presence of C2D5OD for all SrTiO3 nanocrystals, indicating that the cleavage of the Cα-H bond is kinetically relevant, rather than the cleavage of the Cβ-H bond to form a surface enolate. This result is in good


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ACS Catalysis

agreement with the literature, which reported that the cleavage of the Cα-H bond is also rate limiting in the dehydrogenation of ethanol over γ-Al2O3,37 Ni/γ-Al2O3,38 and Cr2O3.39 Table 2. Measured kinetic isotope effects for the dehydrogenation of ethanol over shapecontrolled SrTiO3 nanocrystals at 300 °C, 50 mg catalyst, 0.5 kPa ethanol, and 40 mL/min Ara Catalyst

𝒓𝒓𝑯𝑯 /𝒓𝒓𝒅𝒅𝒅𝒅



Cubes STO 1.01 0.98 1.51 Truncated STO 1.01 1.03 1.59 Dodecahedra STO 1.08 1.00 1.46 Etched Cubes STO 1.04 1.02 1.48 a 𝑟𝑟𝐻𝐻 , 𝑟𝑟𝑑𝑑1 , 𝑟𝑟𝑑𝑑3 , and 𝑟𝑟𝑑𝑑6 represents acetaldehyde synthesis rate using C2H5OH, C2H5OD, CD3CH2OH, and C2D5OD, respectively. Co-feeding experiments were performed to assess site requirements for the dehydrogenation of ethanol. Ammonia and 2,6-di-tert-butylpyridine (DTBP) were used to inhibit acidic sites, while CO2 and SO2 were used to inhibit basic sites. Acetaldehyde synthesis rate for all catalysts decreased after 1 kPa of NH3 was co-fed with ethanol (Figure 8A), indicating that acid sites are involved in dehydrogenation. For Cubes STO, the formation of acetaldehyde was completely suppressed, while the normalized rates decreased to less than 20% for the other SrTiO3 nanocrystals. After the flow of NH3 was stopped, acetaldehyde synthesis rates for Dodecahedra and Etched Cubes STO were fully restored, while the rates were restored to approximately 65% for Cubes and Truncated STO, indicating that a fraction of these acid sites can still bind NH3 strongly at 300 °C. As shown in Figure 5A, Cube STO displayed a high heat of adsorption of NH3 at low surface coverage, indicating that it possessed a small concentration of strongly acidic sites. This could be similar for Truncated STO under the reaction conditions. Since NH3 binds to both Lewis and Brønsted acid sites (Figure 6A), ethanol was co-fed with DTBP to determine the role of Brønsted acid sites on Cubes, Truncated, and Dodecahedra STO. DTBP can selectively inhibit 21 ACS Paragon Plus Environment

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Brønsted acid sites as the sterically hindered nitrogen atom is unable to coordinate to Lewis acid sites.40-42 Upon introduction of 0.14 Pa of DTBP (Figure 8B), acetaldehyde synthesis rates remained constant for all samples, indicating that Brønsted acid sites are not involved in ethanol dehydrogenation. This result suggests that Lewis acidic sites (Ti cations) are involved in ethanol dehydrogenation.

(A)

(B)

Figure 8. Transient acetaldehyde synthesis rate over shape-controlled SrTiO3 nanocrystals during introduction and removal of (A) ammonia and (B) 2,6-di-tert-butylpyridine (DTBP). Rates are normalized to initial steady-state values. Reaction conditions: 300 °C, 50 mg catalyst, 0.53 kPa ethanol, total gas flow rate 40 mL/min. 22 ACS Paragon Plus Environment

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To determine the role of base sites in the dehydrogenation of ethanol, 25 kPa of CO2 was co-fed with ethanol over the shape-controlled SrTiO3 nanocrystals (Figure 9A). In the presence of CO2, acetaldehyde synthesis rates over Cubes and Truncated STO decreased by approximately 3040%, while the rate of formation of acetaldehyde was not perturbed for Dodecahedra and Etched Cubes STO. After the removal of CO2, acetaldehyde synthesis rate was partially restored for Cubes STO, whereas the rate was restored to its initial steady-state value for Truncated STO. Since CO2 is a weak titrant, SO2 was co-fed with ethanol over Dodecahedra and Etched Cubes STO. Upon introduction of 10 Pa of SO2 (Figure 9B), acetaldehyde synthesis rate for both catalysts decreased by approximately 50-65%. After SO2 was removed from the system, acetaldehyde synthesis rate was restored to its initial steady-state value for Dodecahedra STO, while a fraction of base sites remains inhibited on Etched Cubes STO. On the basis of these results, Cubes STO possessed the strongest base sites under reaction conditions, followed by Truncated, Etched Cubes, and Dodecahedra STO. This trend is slightly different from that measured from room temperature CO2 adsorption microcalorimetry (Figure 5B and Table 1), cautioning that care needs be taken when utilizing low temperature adsorption result to predict high temperature behavior of oxide catalysts.


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(A)

(B)

Figure 9. Transient acetaldehyde synthesis rate over shape-controlled SrTiO3 nanocrystals during introduction and removal of (A) CO2 and (B) SO2. Rates are normalized to initial steadystate values. Reaction conditions: 300 °C, 50 mg catalyst, 0.53 kPa ethanol, total gas flow rate 40 mL/min. The adsorption and temperature-programmed desorption (TPD) of ethanol was performed in the DRIFTS cell to probe the surface interaction of the reactant with the shape-controlled SrTiO3 nanocrystals. The FTIR spectra for Cubes STO are shown in Figure 10, while similar results for Truncated, Dodecahedra, and Etched Cubes STO are shown in Figure S3. 24 ACS Paragon Plus Environment

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1377 δCH3 Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1280 δOH

νC-C-O 1115 1066

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25 °C

1605 100 °C 1608 νC=O 200 °C

300 °C 1800

1600


1000

Figure 10. FTIR spectrum of ethanol adsorption and TPD on Cubes STO. Upon adsorption at 25 °C, peaks observed at 1377 and 1280 cm-1 are assigned as the νCH3 and δOH vibrations of the methyl and alcohol group of ethanol, respectively.43-44 The two peaks at 1115 and 1066 cm-1 are attributed to the νC-C-O vibration of dissociated and molecularly adsorbed ethanol,45-46 respectively, while the negative band around 1605 cm-1 is likely due to the perturbation of molecularly adsorbed water. At higher temperatures, the relative peak intensity of the δOH vibration decreased, indicating further dissociation of the alcohol group to form adsorbed ethoxide species. At 200 °C, the peak intensity of the ethoxide bands decreased considerably with the rise of a new peak at 1608 cm-1 (νC=O), which is likely due to the formation of adsorbed acetaldehyde,47-49 or its coupling product such as crotonaldehyde.50-51 The new feature of the product became more evident on the surface of SrTiO3 nanocrystals up to 300 °C with the disappearance of the IR bands from ethoxides, suggesting that the strong binding or condensation of acetaldehyde on the active sites during catalysis resulted in catalyst deactivation (Figure S2).52


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FTIR spectroscopy revealed that ethanol adsorbs both molecularly and dissociatively on the surface of the shape-controlled SrTiO3 nanocrystals at 25 °C (Figure 10 and S3), and the alcohol group is completely dissociated at higher temperatures. The molecular adsorption of ethanol is in good agreement with our recent study on the adsorption of 2-propanol on a series of perovskites, such as SrTiO3, BaTiO3, SrZrO3, and SrTiO3.18 On the basis of FTIR spectroscopy and DFT calculations, the adsorption configuration of 2-propanol is dependent on the surface termination of the perovskite. On an A-cation-terminated surface, the alcohol group is facilely dissociated, forming an OH group with a basic surface oxygen atom, while the oxygen atom of the 2-propoxide molecule forms a hydrogen bond with the newly formed OH group. On a B-cationterminated surface, the oxygen atom of the undissociated alcohol group engages in a Lewis acidbase interaction with a surface B cation. Therefore, the adsorption of ethanol can take place on both a surface Ti cation and basic surface oxygen atom, resulting in a higher accessible surface density of sites compared to that measured via adsorption of NH3 or CO2 (Table 1), which only selectively binds to acid or base sites, respectively. Since the surface sites of the shape-controlled SrTiO3 nanocrystals are heterogeneous from ethanol adsorption microcalorimetry measurement (Figure 5), the ethoxide species observed at high temperatures is proposed to adsorb on surface Ti cations via a Lewis acid-base interaction, which is the active surface intermediate under reaction conditions.45-46, 53-54 Since both Lewis acid site and basic surface oxygen atom are required for dehydrogenation, and surface ethoxide is the observed intermediate, the proposed reaction mechanism is shown in Scheme 1.


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H

H3C

CH3CHO

C C2H5OH O

Ti

O

H O

O

H

Ti

O

Cα-H cleavage

H2 O

Ti

O

Scheme 1. Proposed reaction mechanism for ethanol dehydrogenation over SrTiO3 nanocrystals. The adsorption of ethanol on the perovskite surface under reaction condition results in the formation of a surface ethoxide intermediate and a dissociated hydrogen atom. Subsequently, interaction between a Hα and an adjacent basic surface oxygen atom results in the cleavage of the Cα-H bond (rate determining step), whereas the surface Lewis acid site (Ti cation) is involved in stabilizing the intermediate by interacting with the oxygen atom of the alcohol group. This leads to the formation of acetaldehyde and molecular hydrogen. 3.3 Density and property of acid-base site pairs Since both acid and base sites are required for ethanol dehydrogenation, carboxylic acids can be used as a suitable probe molecule to titrate acid-base site pairs.55-57 This would allow comparison of turnover rates among surfaces with different chemical compositions and crystal facets. Figure 11 shows the FTIR spectra of acetic acid adsorption on the shape-controlled SrTiO3 nanocrystals at 300 °C. Major peaks observed at 1607-1653 and 1277-1343 cm-1 are assigned as the asymmetric and symmetric νCOO- vibration of monodentate acetic acid, respectively (see inset of Figure 11).57-61 This result is in good agreement with studies reporting the formation of acetate species on various metal oxides such as TiO2, CeO2, and ZrO2.57, 62 For Etched Cubes STO, additional peaks observed at 1583 and 1455 cm-1 are attributed to the asymmetric and symmetric νCOO- vibration of monodentate or bidentate acetate on a TiOx terminated surface.57, 62 On the basis of the adsorption configuration, the binding of acetic acid on the shape-controlled SrTiO3 27 ACS Paragon Plus Environment

ACS Catalysis

nanocrystals is assumed to lead to a stoichiometric ratio of one acetic acid per one pair of acidbase site.

CH3 νCOOC O νCOOO H 1634

O

Ti

1343

1607 Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1313

1653

1648

O Cubes

Etched Cubes

1277

Truncated

1294 1800


Dodecahedra 1200

Figure 11. FTIR spectrum of acetic acid adsorption on shape-controlled SrTiO3 nanocrystals at 300 °C. Inset: adsorption configuration of monodentate acetate. Co-feeding ethanol with 2.7 Pa of acetic acid over Truncated STO resulted the rate to decrease by approximately 40% (Figure S4A), indicating that acetic acid can only inhibit the active sites to a certain extent due to reversible adsorption on weak acid-base pairs at 300 °C. Likely products such as acetone related to the conversion of acetic acid was not observed. A lower reaction temperature of 240 °C resulted in a full suppression of dehydrogenation rate (Figure S4B). However, the surface density of acid-base pairs cannot be determined accurately via in situ titration due to catalyst deactivation using 0.5 kPa of ethanol at lower reaction temperatures. As such, the strength and distribution of acid-base site pairs was obtained via chemisorption of acetic 28 ACS Paragon Plus Environment

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acid at 150 °C to maximize titration of all active sites and to avoid physisorption at low surface coverage (Figure 12).

210

Heat of Adsorption (kJ/mol)

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ACS Catalysis


180 150 120 90 60 30 0 0

1

2

3

4

5

6

7

8

9

2


Figure 12. Heat of adsorption of acetic acid on shape-controlled SrTiO3 nanocrystals at 150 °C. Dodecahedra STO displayed the highest heat of adsorption of acetic acid, while Cubes STO, Truncated STO, and Etched Cubes STO have a similar distribution of acid-base pair sites up to 1.5 μmol/m2. The high initial heat of adsorption (165 – 210 kJ/mol) is likely due to the dissociative binding of acetic acid to form monodentate acetate species. The differential heat of adsorption of the STO catalysts decreased to low values (< 60 kJ/mol) at higher surface coverage, indicating physisorption at higher equilibrium pressures. While the strength of acid and base sites is different between Cubes, Truncated, and Etched Cubes STO (Figure 5), the three catalysts have a similar strength and distribution of acid-base pairs up to 1.5 µmol/m2 as shown by the heat of adsorption of acetic acid (Figure 12). This result suggests that the strength of an acid-base pair is likely an average between the strength of the acid and base site.


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The surface density of irreversibly adsorbed acetic acid was obtained (Table 1). Cubes STO possessed the highest surface density of acid-base site pairs, while Truncated, Dodecahedra, and Etched Cubes STO have values ranging from 2.0 to 2.5 μmol/m2. The surface density of sites accessible by acetic acid is used to normalize acetaldehyde synthesis rate to obtain turnover frequency (Figure 7), assuming that all sites are involved in ethanol dehydrogenation. In addition, the surface density of sites accessible by acetic acid is higher compared to NH3 or CO2 (Table 1), suggesting that NH3 and CO2 are not able to irreversibly adsorb on these additional sites. The kinetic model presented in eq. 1 is derived based on the fact that the cleavage of the Cα-H bond of the surface ethoxide intermediate is the rate limiting step, ethoxide is the major surface species, and it is assumed that the surface intermediate is in quasi-equilibrium with gaseous ethanol.37, 63

𝑟𝑟𝐴𝐴𝐴𝐴𝐴𝐴 =

𝑘𝑘1 𝐾𝐾𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 𝑃𝑃𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸

(1)

1+𝐾𝐾𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 𝑃𝑃𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸

From eq 1, 𝑟𝑟𝐴𝐴𝐴𝐴𝐴𝐴 is the turnover rate, 𝑘𝑘1 represents the intrinsic rate constant, 𝐾𝐾𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 is the

adsorption equilibrium constant of ethanol, and 𝑃𝑃𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 is the pressure of ethanol. The values of 𝑘𝑘1

and 𝐾𝐾𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 were estimated using the kinetic measurements in Figure 7 and are shown in Table 3.

A parity plot comparing measured and predicted turnover rates for acetaldehyde is shown in

Figure 13. The data points are clustered along the diagonal, which indicates a good description of the data by eq 1.


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Table 3. Estimated kinetic parameters of acetaldehyde formation over shaped-controlled SrTiO3 nanocrystals at 300 °C using kinetic data from Figure 6 and model presented in eq 1. Catalyst Cubes STO Truncated STO Dodecahedra STO Etched Cubes STO

Predicted Acetaldehyde Formation Rate (µmol/µmolpair.min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

𝒌𝒌𝟏𝟏 (μmol/μmolpair.min) 5.92 ± 1.17 6.25 ± 0.64 19.21 ± 2.20 6.94 ± 0.33

𝑲𝑲𝑬𝑬𝑬𝑬𝑬𝑬𝑬𝑬 (kPa-1) 0.15 ± 0.04 0.41 ± 0.10 0.14 ± 0.02 0.54 ± 0.07

10 Cubes Truncated Dodecahedra Etched Cubes

8 6 4 2 0

0

2 4 6 8 10 Observed Acetaldehyde Formation Rate (µmol/µmolpair.min)

Figure 13. Parity plot for acetaldehyde formation rates derived from kinetic data and eq 1. Reaction conditions: 300 °C, 50 mg catalyst, 40 mL/min argon. Table 3 shows that the intrinsic rate constant is the highest for Dodecahedra STO, followed by Etched Cubes STO, while Truncated and Cubes STO have similar values of 𝑘𝑘1 . However,

between 0.53 and 5.03 kPa of ethanol pressure, acetaldehyde formation rates for Truncated STO are greater than Cubes STO as shown in Figure 7. This difference is due to higher values of 𝐾𝐾𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸

for Truncated and Etched Cubes STO as compared to Cubes and Dodecahedra STO, suggesting that the surface has a higher affinity for ethanol adsorption under the reaction condition. In general,


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the dehydrogenation turnover rate is positively correlated with the strength and distribution of acid sites displayed from the heat of adsorption of NH3 (Figure 5A). In contrast, the presence of weak base sites, as evidenced by the co-feeding of CO2 and SO2 during catalysis (Figure 9), leads to a higher turnover rate. The results suggest that the strength of Lewis acid sites has a greater effect in stabilizing the transition state compared to base sites during dehydrogenation, as the acetaldehyde formation turnover rate is dependent on the free energy difference between the transition state and its precursor state,18 which is the surface ethoxide intermediate. As such, while both Lewis acid and base sites are required for dehydrogenation, SrTiO3 surfaces exhibiting strong surface Ti cations and weak basic surface oxygen atoms results in a higher dehydrogenation turnover rate. Although Lewis acid sites are associated with surface Ti cations, and basic surface oxygen atoms are associated with Sr cations, ethanol dehydrogenation turnover rate does not depend on the ratio of Sr to Ti cations, as this only illustrates the ratio of base to acid sites, not the strength and distribution of active sites. Similarly, the turnover rate of ethanol does not show a clear correlation with the proportion of either facet, (001) or (110), of SrTiO3 nanocrystals. Instead, it is more related to the surface acid-base property as a result of the surface reconstruction of the different facets of SrTiO3 nanocrystals. This finding cautions that the correlation between the surface facet (shape) and the catalytic behaviors often observed for single component oxides51, 6465

cannot be simply applied to complex oxide catalysts.

4. Conclusion The dehydrogenation of ethanol is used as a probe reaction to understand the shape effect on the acid-base pair reactivity over SrTiO3 (STO) nanocrystals, such as cubes, truncated cubes, 32 ACS Paragon Plus Environment

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ACS Catalysis

dodecahedra, and etched cubes. Low energy ion scattering analysis reveals that the surface termination of Cubes and Dodecahedra STO changes after pretreatment at 550 °C in the presence of molecular oxygen due to surface reconstruction. Surface acidity probed via adsorption of NH3 indicates that Dodecahedra STO displayed the strongest acid sites, followed by Etched Cubes, Truncated Cubes, and Cubes STO. However, the strength of base sites probed via co-feeding of CO2 and SO2 during catalysis shows a reverse trend. Co-feeding of various titrants with ethanol, such as NH3, 2,6-di-tert-butylpyridine, CO2, and SO2, indicates that both Lewis acid sites (surface Ti cations) and basic surface oxygen atoms are involved in ethanol dehydrogenation. FTIR spectroscopy and kinetic isotope effect reveal that surface ethoxide intermediate, which is formed from the dissociation of ethanol, undergoes Cα-H bond cleavage over an acid-base pair to form acetaldehyde and hydrogen. Initial dehydrogenation turnover rates, which are normalized by the surface density of acid-base pair sites accessible by acetic acid via chemisorption, were obtained as a function of ethanol pressure between 0.53 and 5.05 kPa for comparison between catalysts. Dodecahedra STO exhibited the highest turnover rates, followed by Etched Cubes, Truncated, and Cubes STO. This trend is not directly in line with the proportion of either surface facet, (001) or (110), on the SrTiO3 nanocrystals. Instead, the reaction rate is correlated with the strength and distribution of acid and base sites on the shape-controlled SrTiO3 nanocrystals, which is governed by the exact surface compositions resulting from surface reconstruction of different facets. Our work reveals that the facet effect in catalysis by the shape-controlled SrTiO3 nanocrystals is clearly undermined by the surface reconstruction behaviors of different facets. As such, we suggest that the characterization of top surface composition and property is a prerequisite for understanding catalysis by complex oxides. ASSOCIATED CONTENT 33 ACS Paragon Plus Environment

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Supporting Information. Heat of adsorption of ethanol, catalyst deactivation in the dehydrogenation of ethanol over Cubes STO, FTIR spectra of ethanol adsorption on shapecontrolled STO, co-feeding acetic acid with ethanol over Truncated STO. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This research is sponsored by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. Part of the work including XRD, SEM, FTIR spectroscopy, and kinetic measurements were conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. ZDH gratefully acknowledges a graduate fellowship from the National Science Foundation under Grant No. DGE-1650044 and the Georgia Tech-ORNL Fellowship. The authors thank Victor Fung (University of California Riverside) for providing figures of SrTiO3 (001) and (110) surfaces. Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC0500OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy


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will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

REFERENCES 1. 2.

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Table of Content

Shape-Controlled SrTiO3 Nanocrystals SrO Termination

TiO2 Termination (110)

(001)

(001)

(110)

(001)

Stronger Acid Sites, Weaker Base Sites

Ethanol Dehydrogenation Turnover Rate


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Shape Effect Undermined by Surface Reconstruction: Ethanol

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