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Ultrathin, polycrystalline, two-dimensional Co3O4 for low-temperature CO oxidation Yafeng Cai, Jia Xu, Yun Guo, and Jingyue Liu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04064 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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Ultrathin, Polycrystalline, Two-Dimensional Co3O4 for Low-Temperature CO Oxidation Yafeng Caia,b, Jia Xua, Yun Guob,*, Jingyue Liua,* a Department

of Physics, Arizona State University, Tempe, Arizona 85287, United States Laboratory for Advanced Materials and Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China. b Key

*Corresponding Authors: [email protected]; [email protected] ABSTRACT Free-standing, hierarchical, ultrathin, and two-dimensional (2D) polycrystalline Co3O4 flowers were synthesized by a hydrothermal and topotactic transformation process. Aberration-corrected electron microscopy study of both the CoOx precursor structure and the subsequent topotactic transformation processes revealed the nucleation and growth mechanisms of the 2D polycrystalline Co3O4 nanosheets. The free-standing flower-shaped CoOx powders (1-5 µm) consist of numerous self-assembled nanocrystallites (average size ~1.8 nm). After the topotactic transformation, via a rapid calcination process, the powders maintained their hierarchical flower-like shape but the CoOx nanocrystallites structurally transformed into ultrathin 2D Co3O4 nanoplates with thicknesses ranging from 1-5 nm (average thickness ~2.4 nm). The final free-standing, ultrathin 2D polycrystalline Co3O4 flowers possess a BET surface area of 138 m2/g. Statistical structural analyses revealed that the exposed surfaces of the Co3O4 flowers are dominated by the Co3O4{112} (~70%). The hierarchical Co3O4 flowers contain many grain boundaries, pockets, surface steps and other types of surface defects. CO oxidation on the as-synthesized hierarchical Co3O4 flowers showed a specific activity (normalized to the surface area) of 0.377μmol∙m-2∙s-1, about five times that of the most active Co3O4 at 70C reported in literature. Furthermore, even under moisture saturated condition (~3% H2O) the ultrathin 2D Co3O4 catalyst demonstrated high specific rate and stable for at least 40 hours at 90C and 150C. The abundance of accessible coordinatively unsaturated Co3+, active oxygen species and surface defects on the polycrystalline Co3O4{112} nanosheets are responsible for the experimentally observed high catalytic activity.

KEYWORDS: Catalysis, CO oxidation, two-dimensional materials, grain boundaries, Co3O4, electron microscopy

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1. INTRODUCTION Two-dimensional (2D) nanomaterials are sheet-like nanostructures with thicknesses ranging from a monolayer to a few unit cells and large-area-to-thickness aspect ratios. The geometric confinement in the third dimension can result in physicochemical properties distinctly different from those of their bulk counterparts.1-5 For applications in heterogeneous catalysis, in addition to the potential quantum confinement effects, the high total surface area, the presence of many types of surface defects (e.g., steps, kinks, vacancies, grain boundaries, etc.) and the better-defined surface crystallographic structures are especially valuable.2, 6-7 Many synthesis approaches to fabricate ultrathin 2D nanomaterials have been reported, especially by exfoliation of anisotropic layer-structured bulk compounds.4-5, 8 Synthesis of nonlayer-structured 2D materials cannot, however, be accomplished by the exfoliation method. Instead, direct fabrication of 2D nanostructures can be obtained by wet chemistry approach, especially with the use of appropriate surfactants and post-synthesis processing.3, 5 To develop robust heterogeneous catalysts, however, requires that both the supports and active phases can withstand harsh reaction conditions such as high reaction temperatures, long-term structural stability under specific gas environment, and the assurance of accessible active sites during catalytic reactions. Although atomically thin 2D materials theoretically possess the highest surface area and unique electronic structure, few of these atomically thin 2D materials can maintain their structural integrity during a desired long-term catalytic reaction without the use of other types of support materials.6 Therefore, free-standing hierarchical architectures constructed from robust ultrathin 2D nanosheets are desirable for applications in heterogeneous catalysis. Catalytic CO oxidation is of both fundamental interest and practical applications in environmental remediation. Although noble metals are generally used for such reactions, the search for a sustainable and cost-effective CO oxidation catalyst which can function effectively at ambient temperatures and under relevant environmental conditions is becoming increasingly important. The spinel Co3O4, due to its versatile redox properties, has long been investigated for CO oxidation9 and is considered to be the most active material, among all the base metal oxides, for low temperature CO oxidation.10-11 It has been reported that Co3O4 nanorods with preferentially exposed {110} surfaces are extremely active even at cryogenic temperatures.10 The specific active sites on the Co3O4 surfaces are, however, still not fully understood.9, 11-20 The Co3O4 nanostructures with controlled facets can expose surfaces of specific coordination combinations between surface metal cations and oxygen anions, resulting in desirable redox properties for the selected catalytic reaction. The octahedrally coordinated Co3+ is generally considered to be the active site while the tetrahedrally coordinated Co2+ is assumed to be almost inactive.10, 13, 21 Therefore, the presence and amount of Co3+ on the exposed Co3O4 surfaces as well as the high Co3+/Co2+ ratio is proposed to be critical to low-temperature CO oxidation.13, 22-23 Recent DFT calculations suggest that the lowcoordinate oxygen (O2f/O3f) sites on the Co3O4 {110} should be responsible for the observed high CO oxidation activity as well.17-18, 24 Surface adsorbed active oxygen species are suggested to enhance low-temperature CO oxidation.16, 25-26 The recent DFT calculations elucidated that both 2

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the presence of Co3+ and O2f and their close proximity may play a major role in controlling the CO oxidation activity.19 Experimental studies have clearly demonstrated that the exposed crystal planes of the Co3O4 catalysts influence CO oxidation: The activity follows the order Co3O4{110} > Co3O4{100} > Co3O4{111}.10, 27 Morphology-dependent activity of Co3O4 was observed for methane combustion: Co3O4 nanosheets and nanotubes showed superior activity on the Co3O4{112} surfaces, presumably due to the open surface structure that possesses more coordinatively unsaturated bonds.28-29 There are barely detailed studies on CO oxidation on the more open and interesting Co3O4 {112} surfaces although the presence of the Co3O4 {112} side facets on thick Co3O4{111} plates was mentioned in one publication to explain the experimentally observed CO oxidation activity.30 Although Co3O4 is extremely active for CO oxidation under dry reaction conditions the activity decreases dramatically, especially at ambient temperatures, when the reactant gas mixture contains even minute amount of H2O.31-33 The presence of H2O molecules and their interactions with the various types of reactive species on the Co3O4 surfaces can significantly influence their capability for CO adsorption and/or O2 activation, and subsequently degrade the performance of the Co3O4 catalysts.23, 34 Under dry reaction conditions, adsorbed CO molecules can easily couple with labile reactive O species to form CO2 molecules and then desorb from Co3O4 surfaces. With reaction gases that contain H2O molecules the dissociative adsorption of H2O on the Co3O4 surfaces will produce hydroxyl or carbonate/formate intermediates which may persist on the Co3O4 surfaces to block the active sites and deactivate the Co3O4 catalyst.23, 26, 32, 34-36 DFT calculations suggest that the H2O adsorption at the Co3+ sites, via hydrogen bonding to the O2f sites, inhibits CO interaction with the O2f species and thus hinders the CO2 formation.34, 36 The rate of moistureinduced deactivation may increase proportionally, especially for low levels of H2O, with the amount of H2O in the reactant gas mixture. Tuning of surface structures10, 37 or the use of functional groups to make the catalyst surface hydrophobic38 may alleviate the susceptibility of Co3O4 catalysts to moisture poisoning. Development of a highly active, moisture-resistant and stable Co3O4 catalyst for ambient temperature CO oxidation has been a grand challenge. In this work, we use Co(acac)3 as precursor and ethylene glycol (EG) as both surfactant and reductant (Scheme 1a) to solvothermally grow small CoOx clusters (Scheme 1b) which selfassemble into interconnected 2D nanosheets (Scheme 1c). During the later stages of the solvothermal process the 2D nanosheets first grow laterally (Scheme 1d) and then hierarchically assemble into free-standing, flower-like microstructures. The topotactic transformation, via a rapid calcination process, of the CoOx flowers directly converts CoOx into Co3O4 (Scheme 1e) without changing the morphology of the hierarchically assembled flowers. The final free-standing Co3O4 flowers consist of hierarchically assembled ultrathin 2D Co3O4 nanosheets (average thickness ~2.4 nm) which are primarily composed of Co3O4{112} surfaces (~70%). The open {112} surfaces of the spinel Co3O4 nanosheets (Scheme 1f) possess abundant Co3+ and O2f species as well as many grain boundaries (Scheme 1e) which provide numerous coordinatively unsaturated bonds and oxygen vacancies. CO oxidation over the synthesized ultrathin 2D Co3O4 nanosheets at 55oC yielded a specific rate of 0.303μmol∙m-2∙s-1 (normalized to the total surface area), ~ 6 times higher 3

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than that on the Co3O4 nanobelts27. At a reaction temperature of 70oC, the specific rate was 0.377μmol∙m-2∙s-1, about 5 times that of the most active Co3O4 catalyst ever reported in literature at this temperature.39 The dominant Co3O4{112} surfaces and the presence of many grain boundary sites and steps on the polycrystalline 2D Co3O4 nanosheets are responsible for the observed high activity. Furthermore, the synthesized polycrystalline 2D Co3O4 nanosheets are relatively resistant to moisture and the activity remained stable (at 90oC and 150oC) for more than 40 hours under moisture saturated reaction condition.

Scheme 1. Schematic diagrams illustrate the solvothermal and topotactic transformation processes for synthesizing free-standing, hierarchically assembled flower-like structures that consist of ultrathin, polycrystalline 2D Co3O4 nanosheets which are primarily composed of Co3O4{112} and Co3O4{100} surfaces. Description of the synthesis processes: (a) mixing of cobalt(III) acetylacetonate with ethylene glycol; (b) formation of small CoOx nanostructures during the early stages of the solvothermal reaction; (c) self-assembly of CoOx nanostructures into templated 2D nanosheets; (d) final assembly of CoOx into large 2D nanosheets with formation of pockets during the final stages of the solvothermal reaction; (e) formation of ultrathin, polycrystalline and porous 2D Co3O4 nanosheets via a topotactic transformation process (rapid calcination). The scheme 1f depicts the structural arrangement of the various types of cobalt and oxygen species on the exposed {112} surface and subsurface of the spinel 2D Co3O4 nanosheets. O1f, O2f, and O3f refer to 1-fold, 2-fold and 3-fold coordinated oxygen species, respectively. Co2+ and Co3+ refer to cobalt atoms in the tetrahedral and octahedral hole with different oxidation states, respectively.

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2. EXPERIMENTAL SECTION Cobalt(III) acetylacetonate (Co(acac)3, 99.99% trace metals basis, Sigma-Aldrich), ethylene glycol (EG, ≥99.0%, VWR International) and dehydration alcohol (90.3% ethanol, 4.7% methanol, 5.0% isopropanol, Harleco) were purchased from the corresponding vendors and were directly used as received without further purification. The 2D CoOx flowers were synthesized by a modified solvothermal method.40 Briefly, various volume ratios of EG and deionized water with a total volume of 24ml were mixed with 100 mg of Co(acac)3 under vigorous stirring at ambient temperature for 2 hours. The resulting green liquid was then heated to 190oC for a designated period of time in a 50 mL Teflon-lined autoclave. After the autoclave was naturally cooled down to room temperature, green colored CoOx precipitates were collected by centrifugation, thoroughly washed with deionized water and dehydration alcohol, and then dried at 60oC for 12 hours. The 2D Co3O4 flowers were produced, via a rapid calcination method, by inserting the CoOx flower powders into an air-filled furnace at a designated temperature for 5 min. The Co3O4 nanoparticles (NPs) were obtained by thermal decomposition of 5 g Co(NO3)2·6H2O at 600oC for 4 hours in an air-filled furnace. Aberration-corrected electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy, and Raman spectroscopy techniques were used to characterize the synthesized catalysts. The instrumentation and characterization details are provided in the Supporting Information. CO oxidation was conducted in a plug-flow fixed-bed reactor at atmospheric pressure. Typically, 50 mg catalyst was mixed with 200 mg SiO2 and then the whole mixture was immobilized by quartz wool in a quartz tube. The catalyst bed temperature was controlled by a thermocouple. The feed gas was composed of 1 vol.%CO, 4 vol.%O2 and He balance. Since CO oxidation on Co3O4 is extremely sensitive to the presence of trace amounts of moisture in the feed gas we conducted the CO oxidation experiments under three different reaction conditions. The normal gas condition refers to the use of feed gas without any pre-treatments (3~10 ppm of H2O in the feed gas mixture). The dry gas condition refers to the use of feed gas that has passed through a cold trap (ethanol/liquid nitrogen) before being fed into the reactor (70%), Co3O4{100} ( {110} > {100} > {111}. Bond breaking has been recognized to cause distortion of crystal structure in ultrathin 2D materials, and the distortion may lead to enhanced electron density around cobalt atoms.47 The ratios and binding energies of surface Co3+, Co2+ and O2- species of the 2D Co3O4 are different from those of Co3O4 NPs. The high number density of dangling bonds on the Co3O4 {112} surfaces implies that the as-synthesized ultrathin 2D Co3O4 nanosheets possess unique surface electronic structures. Furthermore, the topmost layer of the {112} surfaces of the spinel Co3O4 is more open than any other low-index surfaces (Figures S17 and S18) and therefore even the sublayer Co3+ cations can be accessible by small reactant molecules. For the {110} surface (Figure S18b), it has been reported14 that the effect of Pauli repulsion among the neighboring oxygen atoms renders the sublayer Co3+ inaccessible. The sublayer Co3+ and O2- ions located on the more open Co3O4{112} can, however, be accessible to the small CO molecules since not only the {112} top surface is more open but the distance between the sublayer atoms to the outmost layer atoms is only ~ 1 Å (Table S6). Even though the total accessible number of Co3+ cations on the Co3O4{112} surfaces are comparable to that on the Co3O4{110} surfaces the total number of dangling bonds of the Co3+ cations are significantly larger on the Co3O4{112} surfaces than that on the Co3O4{110} surfaces. The predominantly Co3O4{112} surfaces, which possess a large number of topmost and sublayer Co3+ active sites as well as adjacent labile O2- sites, and the presence of a large number of strain-induced oxygen vacancies at the grain boundaries are responsible for the observed high activity of CO oxidation on the hierarchically assembled polycrystalline Co3O4 flowers. Since the thicknesses of many of the Co3O4{112} nanoplates are only about 2 unit cells and the reactant molecules can interact with both side surfaces of the Co3O4{112} nanoplates reaction-induced surface restructuring may occur. The effect of such reaction-induced surface restructuring of ultrathin 2D nanosheets on their catalytic properties is not understood yet. Further experiments on well-defined 2D Co3O4 surfaces are needed to understand the dependence of CO oxidation on the thickness of the ultrathin 2D Co3O4 nanosheets.

4. CONCLUSIONS We have synthesized, via a solvothermal and topotactic transformation process, free-standing, hierarchical Co3O4 flowers which consist of ultrathin 2D Co3O4 nanosheets with thicknesses ranging 1-5 nm. Aberration-corrected STEM imaging reveals that the ultrathin 2D Co3O4 nanosheets are composed primarily of Co3O4{112} surfaces (>70%) with abundant grain boundaries, surface steps and other types of surface defects. The as-synthesized high-surface-area 15

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hierarchical Co3O4 flower powders proved to be extremely active for low temperature CO oxidation with a specific activity (normalized to the surface area) of 0.377μmol∙m-2∙s-1, about five times that of the most active Co3O4 at 70oC ever reported in literature. The 2D Co3O4 flowers are more resistant to moisture and are relatively stable during CO oxidation at low (90C) and high reaction (150C) temperatures. The high total surface area, the open structure of the Co3O4{112} surfaces, and the presence of abundant strain-induced oxygen vacancies at grain boundaries and surface steps on the polycrystalline ultrathin 2D Co3O4 nanosheets are responsible for the observed better performance. Although we used CO oxidation as an example, the strategy of employing hierarchically assembled ultrathin 2D polycrystalline nanostructures as catalysts or catalyst supports can be broadly applied to a plethora of efficient catalytic transformations of important molecules. Supporting Information Determination of activation energy Ea; identification of exposed surfaces on 2D Co3O4; SEM images of 2D CoOx; FT-IR, Raman and XPS spectra of 2D CoOx and Co3O4; XRD patterns; catalytic performance data on 2D Co3O4; and structural models of different Co3O4 surfaces are available in Supporting Information. This information is available free of charge on the ACS Publications website.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Science Foundation under CHE-1465057. The authors gratefully acknowledge the use of facilities within the Eyring Materials Center and the John M. Cowley Center for High Resolution Electron Microscopy at Arizona State University. Yafeng Cai gratefully acknowledges financial support from the China Scholarship Council.

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