Fine cubic Cu2O nanocrystals as highly selective catalyst for propylene epoxidation with molecular oxygen

Propylene epoxidation with O2 to propylene oxide is a very valuable reaction but remains as a long-standing challenge due to unavailable efficient catalysts with high selectivity. Herein, we successfully explore 27 nm-sized cubic Cu2O nanocrystals enclosed with {100} faces and {110} edges as a highly selective catalyst for propylene epoxidation with O2, which acquires propylene oxide selectivity of more than 80% at 90–110 °C. Propylene epoxidation with weakly-adsorbed O2 species at the {110} edge sites exhibits a low barrier and is the dominant reaction occurring at low reaction temperatures, leading to the high propylene oxide selectivity. Such a weakly-adsorbed O2 species is not stable at high reaction temperatures, and the surface lattice oxygen species becomes the active oxygen species to participate in propylene epoxidation to propylene oxide and propylene partial oxidation to acrolein at the {110} edge sites and propylene combustion to CO2 at the {100} face sites, which all exhibit high barriers and result in decreased propylene oxide selectivity.

P ropylene oxide (PO) is a platform chemical for numerous commodity chemicals 1 , such as polyols and glycol ethers. The current industrial production of PO from propylene involves uses of chlorohydrin or H 2 O 2 and is cost-ineffective and environment-unfriendly 2,3 .
Fundamental understanding of active sites for heterogeneous catalytic reactions is an efficient approach to explore novel catalysts. Successful examples have been only a few and they are all based on density functional theory (DFT) calculations [19][20][21][22][23] . Herein, we report a successful exploration of fine cubic Cu 2 O nanocrystals (NCs) enclosed with {100} faces and {110} edges as a highly selective catalyst for propylene epoxidation with O 2 to PO, guided by an experimental fundamental understanding of the active site. We previously used large rhombic dodecahedral NCs (denoted as d-Cu 2 O) enclosed with Cu 2 O{110} facets to identify the Cu 2 O{110} facets as the active facet for propylene epoxidation with O 2 17 , in which, however, reaction temperatures above 150°C were adopted due to the low density of the active site on the used large d-Cu 2 O NCs, favoring the combustion reaction and limiting the acquired PO selectivity. Later large d-Cu 2 O NCs with the Cl − dopant were reported to exhibit enhanced activity in catalyzing propylene epoxidation with O 2 and consequently high PO selectivity at low temperatures 9 . Thus, a reasonable strategy to explore highly selective catalysts for propylene epoxidation with O 2 to PO is to synthesize uniform fine d-Cu 2 O NCs with high densities of Cu 2 O{110} active site, which, unfortunately, has not been realized. Meanwhile, Cu 2 O cubes (denoted as c-Cu 2 O) are enclosed with the {100} faces and {110} edges. We found that the densities of {110} edges are high on Cu 2 O cubes (denoted as c-Cu 2 O) finer than 100 nm and the {110} edge sites on these fine c-Cu 2 O NCs, rather than the {100} face sites, are the dominant active site catalyzing the CO oxidation reaction 24 . Intrigued by these findings, we have investigated propylene oxidation with O 2 over c-Cu 2 O NCs with different sizes and report herein that fine c-Cu 2 O NCs with an average size of 27 nm selectively catalyze propylene epoxidation with O 2 to PO at temperatures below 110°C with the Cu 2 O{110} edge sites as the active site. Interestingly, the reaction mechanism for PO production at the Cu 2 O{110} active site was found to switch from weakly adsorbed O 2 -participating Langmuir-Hinshelwood (LH) mechanism at low temperatures to surface lattice oxygen-participating Mars-van Krevelen (MvK) mechanism at high temperatures.

Results
Synthesis and structural characterizations catalysts. Following previously established procedures 17,[25][26][27] , uniform surfactant-free c-Cu 2 O NCs with sizes of 27 ± 4.5, 106 ± 12, and 774 ± 147 nm were synthesized (Fig. 1a-c  NCs selectively catalyzed the propylene epoxidation with PO selectivity respectively of above 80 and 70% between 90 and 110°C, but barely catalyzed propylene partial oxidation to acrolein. As the temperature increased above 110°C, the CO 2 selectivity increased rapidly at the expense of PO selectivity, and the acrolein production emerged and grew. The catalytic performance of c-Cu 2 O-774 NCs is contributed by the Cu 2 O{100} face sites that selectively catalyze the propylene combustion reaction 17 , while the very different catalytic performances of c-Cu 2 O-27 and c-Cu 2 O-106 NCs arise from both the Cu 2 O{110} edge sites of enough high densities and the Cu 2 O{100} face sites. The Cu 2 O{110} facets were identified to selectively catalyze the propylene epoxidation reaction 17 . Therefore, the Cu 2 O{110} edge sites of c-Cu 2 O-27 and c-Cu 2 O-106 NCs are the dominant surface sites catalyzing propylene oxidation between 90 and 110°C, giving high PO selectivity, while the contribution from the Cu 2 O{100} face sites increases with the reaction temperature, leading to increased CO 2 selectivity at the expense of PO selectivity. These results, on one hand, demonstrate that the Cu 2 O{110} edge sites on c-Cu 2 O-27 and c-Cu 2 O-106 NCs are more active than the Cu 2 O{100} face sites, on the other hand, demonstrates that the Cu 2 O{110} site is active in selectively catalyzing the propylene epoxidation with O 2 at low temperatures. As far as we know, PO selectivity above 80% in propylene oxidation with O 2 catalyzed by c-Cu 2 O-27 NCs are much higher than all previously reported Cu-based catalysts except the recently-reported Cldoped d-Cu 2 O NCs 9 . It is noteworthy that the catalytic selectivity of   Probed by CO adsorption, the oxidation of Cu 2 O{110} edges of c-Cu 2 O-27 and c-Cu 2 O-106 NCs at 130 and 150°C also reduces the available surface Cu(I) sites ( Supplementary Fig. 10). We found that C 3 H 6 conversion rates of c-Cu 2 O-27 and c-Cu 2 O-106 NCs were proportional to the vibrational peak intensities of CO adsorbed at the surface Cu(I) sites at 90 and 130°C but not at 150°C ( Supplementary Fig. 11). Therefore, the catalytic performances of c-Cu 2 O-27 and c-Cu 2 O-106 NCs up to 130°C are dominantly contributed by the Cu 2 O{110} edges with the Cu(I) sites, and the observed decrease of PO selectivity and increase of CO 2 selectivity at 130°C should be due to the more extensive over-oxidation of PO. At 150°C, although less active than the Cu 2 O{110} edge sites, the Cu 2 O{100} face sites of c-Cu 2 O-27 and c-Cu 2 O-106 NCs also contribute to the catalytic performance, enhancing the overall CO 2 production and selectivity.
Stability of c-Cu 2 O-27 NCs at 90°C was further evaluated. C 3 H 6 conversion kept decreasing with the reaction time, while the PO selectivity gradually increased to almost 100% (Supplementary Fig. 12a). XPS spectra show that the surface of spent c-Cu 2 O-27 NCs is not oxidized (Supplementary Fig. 12b), while C-H species with the C 1s binding energy at 285.4 eV 12 emerges ( Supplementary Fig. 12c). Meanwhile, vibration features of carbonate species (1338 and 1537 cm −1 ) 29 , C-O-C (1046 cm −1 ) 30 , and C-H (~2928 cm −1 ) groups were observed on the spent catalyst ( Supplementary Fig. 12d). These observations indicate that oligomers likely form and accumulate to block the active surface sites on c-Cu 2 O-27 NCs during the catalytic reaction.
Reaction mechanism of C 3 H 6 oxidation with O 2 . Figure 3a, b compare C 3 H 6 and C 3 H 6 + O 2 temperature-programmed reaction spectra (TPRS) over c-Cu 2 O-27 and d-Cu 2 O-439 NCs. Over c-Cu 2 O-27 NCs (Fig. 3a), PO (m/z = 58 and 31) and CO 2 (m/z = 44) productions did not appear in the C 3 H 6 -TPRS profile but appeared at ∼80°C with similar traces in the C 3 H 6 + O 2 -TPRS profile. Similar acrolein (m/z = 56) production traces appeared at ∼100°C in both C 3 H 6 -TPRS and C 3 H 6 + O 2 -TPRS profiles, and the acrolein production decreased with the temperature increasing in the C 3 H 6 -TPRS profile but increased in the C 3 H 6 + O 2 -TPRS profile. Over d-Cu 2 O-439 NCs (Fig. 3b), acrolein, PO, and CO 2 productions were observed above 200°C to display similar traces in the C 3 H 6 -TPRS and C 3 H 6 + O 2 -TPRS profiles with more productions in the presence of O 2 . Thus, no matter at low temperatures over c-Cu 2 O-27 NCs or at high temperatures over d-Cu 2 O-439 NCs, the acrolein production by C 3 H 6 with O 2 follows the surface lattice oxygen-participated MvK mechanism, consistent with the previous results 31 . The observed decrease of acrolein with the temperature in the C 3 H 6 -TPRS profile over c-Cu 2 O-27 NCs likely arises from the insufficient supply of surface lattice oxygen species at the Cu 2 O{110} edges. The PO production by C 3 H 6 with O 2 at high temperatures over d-Cu 2 O-439 NCs also follows the MvK mechanism, whereas the PO production at low temperatures over c-Cu 2 O-27 NCs does not, instead, it should follow a LH mechanism involving surface reactions between co-adsorbed propylene and oxygen species. Therefore, propylene epoxidation with O 2 at the Cu 2 O{110} active site follows the LH mechanism at low temperatures and the MvK mechanism at high temperatures.
In-situ DRIFTS measurements of C 3 H 6 and C 3 H 6 + O 2 adsorption on c-Cu 2 O-27 NCs at different temperatures were carried out to explore the temperature-dependent reaction mechanisms of propylene epoxidation with O 2 (Fig. 3c- 32,33 . Therefore, C 3 H 6 + O 2 adsorption at 90°C involves surface reactions between co-adsorbed C 3 H 6 (a) and oxygen species following the LH mechanism, consistent with the above catalytic performance and TPRS results. Interestingly, few acrolein is produced although C 3 H 5 (a) and C 3 H 4 O(a) intermediates are formed, whereas PO is the dominant product but few relevant surface intermediates can be identified. This suggests that the desorption of C 3 H 4 O(a) to produce gaseous acrolein should exhibit a large barrier and barely occur at 90°C, whereas surface reactions producing PO can occur. As the temperature increased to 150°C, the vibrational features of all observed surface intermediates significantly grew, consistent with the enhanced C 3 H 6 conversion. Meanwhile, C 3 H 6 + O 2 adsorption gave the same vibrational bands as C 3 H 6 adsorption but with stronger intensities, supporting the above TPRS result that Cu 2 O{110}-catalyzed propylene oxidation with O 2 at high temperatures follows the MvK mechanism. Acrolein production was observed, demonstrating the occurrence of C 3 H 4 O(a) desorption.
DFT calculations of C 3 H 6 oxidation with O 2 . DFT calculations were performed to understand the mechanisms of propylene oxidation at the Cu 2 O{110} active site (Supplementary Table 3).  Fig. 14), and its dissociation into two oxygen adatoms exhibits an enthalpy of −0.12 eV but a barrier of 1.16 eV. It can thus be expected that O 2 dissociation on the perfect Cu 2 O{110} surface is unlikely at low temperatures. Propylene epoxidation with O 2 at the Cu 2 O{110} active site was found to occur via either a LH mechanism or a MvK mechanism (Fig. 4). The LH mechanism initiates via co-adsorption of C 3 H 6  Propylene partial oxidation to acrolein at the Cu 2 O{110} active site following the Mvk mechanism was also calculated (Supplementary Fig. 15). It initiates by an abstract of an α-H atom of C 3 H 6 (a) Cu to produce C 3 H 5 (a) Cu  The above DFT calculation results suggest that the largest barrier is 0.68 eV among elementary surface reactions of LH mechanism for Cu 2 O{110}-catalyzed propylene epoxidation, but is 0.99-1.77 eV of MvK mechanism for Cu 2 O{110}-catalyzed propylene epoxidation, LH and MvK mechanisms for Cu 2 O{110}-catalyzed propylene partial oxidation to acrolein, and MvK mechanism for Cu 2 O{100}-catalyzed propylene combustion. Meanwhile, due to the very small adsorption energy but very large dissociation barrier of O 2 (a), O 2 (a) is the dominant adsorbed oxygen species on the stoichiometric Cu 2 O{110} site and can only form at low temperatures. Thus, at low temperatures at which O 2 (a) forms, Cu 2 O{110}-catalyzed propylene epoxidation following the LH mechanism occurs and other reactions with large barriers barely, leading to the high PO selectivity; however, at high temperatures at which O 2 (a) is few, Cu 2 O{110}-catalyzed propylene epoxidation following the LH mechanism barely occurs although with low barriers, and Cu 2 O{110}-catalyzed propylene epoxidation and propylene partial oxidation to acrolein and Cu 2 O{100}-catalyzed propylene combustion, all following the MvK mechanism, occur to produce PO, acrolein and CO 2 , respectively. These DFT calculation results agree well with the experimental observations of temperature-dependent catalytic selectivity of our fine c-Cu 2 O NCs in propylene oxidation with O 2 . Thus, the reactivity and temperature-dependent coverages of various surface species at different surface sites of Cu 2 O NCs are responsible for the observed apparent catalytic activity and selectivity in propylene oxidation with O 2 .

Discussion
In summary, based on the fundamental understanding of the active site of Cu 2 O catalysts for propylene epoxidation with O 2 , we successfully explore finely-sized cubic Cu 2 O NCs with a high density of active Cu 2 O{110} edge sites as the highly selective catalyst to catalyze propylene epoxidation with O 2 . Over the c-Cu 2 O-27 NCs catalyst, Cu 2 O{110}-catalyzed propylene epoxidation with weakly adsorbed O 2 (a) species as the active oxygen species exhibits a low barrier and is the dominant reaction occurring at low temperatures, selectively producing PO with a selectivity of above 80%, whereas Cu 2 O{110}-catalyzed propylene partial oxidation and propylene epoxidation and Cu 2 O{100}catalyzed propylene combustion, all with surface lattice oxygen as the dominant active oxygen species and exhibiting large barriers, occur at high reaction temperatures, producing acrolein, PO and CO 2 , respectively. These results demonstrate the effectiveness of fundamental understanding in guiding the exploration of efficient catalysts for challenging heterogeneous catalytic reactions.

Methods
Chemicals and materials. All chemical reagents with the analytical grade were purchased from Sinopharm Chemical Reagent Co. C 3 H 6 (99.95%), O 2 (99.999%), CO (99.99%), and Ar (99.999%) were purchased from Nanjing Shangyuan Industrial Factory. All chemicals were used as received.
Synthesis of c-Cu 2 O-27 and c-Cu 2 O-106 NCs. c-Cu 2 O-27 and c-Cu 2 O-106 NCs were synthesized according to the method reported by Chang et al. 26 . To synthesize c-Cu 2 O-27 NCs, 1 mL CuSO 4 aqueous solution (1.2 mol L −1 ) was rapidly injected into 400 mL deionized water at 25°C. After stirring for 5 min, 1 mL NaOH aqueous solution (4.8 mol L −1 ) was poured into the solution. After stirring for another 5 min, 1 mL ascorbic acid aqueous solution (1.2 mol L −1 ) was injected. Then the solution was kept for another 30 min, and the resulting precipitate was collected by centrifugation, decanting, and washing with distilled water and absolute ethanol, and finally dried in vacuum at RT for 12 h. c-Cu 2 O-106 NCs were synthesized similarly, except that 0.26 g sodium citrate was added to the initial 400 mL deionized water at 25°C.
Synthesis of c-Cu 2 O-774 NCs. c-Cu 2 O-774 NCs were synthesized according to the following typical procedure 25 : 5.0 mL NaOH aqueous solution (2.0 mol L −1 ) was added dropwise into 50 mL CuCl 2 aqueous solution (0.01 mol L −1 ) at 60°C. After adequately stirring for 0.5 h, 5.0 mL ascorbic acid aqueous solution (0.6 mol L −1 ) was added dropwise into the solution. The mixed solution was adequately stirred at 60°C for 5 h. The resulting precipitate was collected by centrifugation, decanting, and washing with distilled water and absolute ethanol, and finally dried in vacuum at RT for 12 h. Typically, under vigorous stirring, 4 mL OA was mixed with 20 mL of absolute ethanol, and slowly added to 40 mL CuSO 4 aqueous solution (0.025 mol L −1 ). The mixture was heated to 100°C for 0.5 h. Then 10 mL NaOH aqueous solution (0.8 mol L −1 ) was added. After stirring for another 5 min, 30 mL D-(+)-glucose aqueous solution (0.63 mol L −1 ) was quickly added. The obtained mixture was stirred at 100°C for another 1 h, and its color changed from black to green, and finally to brick red. The resulting precipitate was collected by centrifugation, decanting, and washing with distilled water and absolute ethanol, and finally dried in vacuum at RT for 12 h.
Capping ligands on as-synthesized d-Cu 2 O-439-OA NCs were removed following Hua et al.'s procedure 17 . Typically, 150 mg d-Cu 2 O-439-OA NCs were placed in a U-shaped quartz microreactor and purged in the stream of C 3 H 6 + O 2 + Ar gas mixture (C 3 H 6 : O 2 : Ar = 2: 1: 22) with a flow rate of 50 mL min −1 at RT for 0.5 h, and then heated to 215°C at a heating rate of 5°C min −1 and kept for another 0.5 h. Then the sample was cooled down to room temperature to acquire d-Cu 2 O-439 NCs.
In-situ C 3 H 6 and C 3 H 6 + O 2 DRIFTS. Diffuse reflectance infrared spectroscopy (DRIFTS) measurements of chemisorption processes were performed on a Nicolet 6700 FTIR spectrometer equipped with an in-situ DRIFTS reaction cell (Harrick Scientific Products, INC) and a MCT/A detector. 50 mg catalyst was loaded onto the sample stage of the reaction cell. Prior to the experiments, the sample was heated at the desired temperatures at pressures better than 0.1 Pa, and the spectrum was measured and used as the background spectrum, then the adsorbed gas was admitted into the reaction cell to desirable pressures through a leak valve, and the DRIFTS spectra were recorded after the chemisorption processes reached a steady state.
In-situ C 3 H 6 + O 2 NAPXPS. Near-ambient pressure X-ray photoelectron spectroscopy (NAPXPS) measurements were carried out at BL02B01 of Shanghai Synchrotron Radiation Facility 35 . The bending magnet beamline delivered a soft X-ray with photon flux around 1 × 10 11 photons s −1 , energy resolution of E/ ΔE = 3700 and beam spot size of~200 µm × 75 µm on the sample. XPS spectra were calibrated using Au 4f 7/2 binding energy at 84.0 eV. During the NAPXPS experiments, 0.6 mbar C 3 H 6 and 0.3 mbar O 2 were introduced into the chamber, and the c-Cu 2 O NCs were heated and stabilized at desirable temperatures for 0.5 h, and then the NAPXPS spectra were measured.
Structural characterizations. Power X-ray diffraction (XRD) patterns were conducted on a Philips X'Pert PROS diffractometer using a nickel-filtered Cu Kα (wavelength: 0.15418 nm) radiation source with the operation voltage and operation current being 50 mA and 40 kV, respectively. X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB 250 high-performance electron spectrometer using monochromatized Al Kα (hν = 1486.7 eV) as the excitation source. The likely charging of samples was corrected by setting the binding energy of the adventitious carbon (C 1s) to 284.8 eV. Scanning electron microscope (SEM) images were obtained on a JEOL JSM-6700 field emission scanning electron microscope. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) were obtained on a JEM-2100F high-resolution transmission electron microscope. C 3 H 6 -temperature-programmed reaction spectra (C 3 H 6 -TPRS) and C 3 H 6 + O 2 TPRS were measured in a quartz tube microreactor equipped with an axial quartz sheathed thermocouple and connected to an online mass spectrometer (HIDEN QIC-20). In the C 3 H 6 -TPRS experiments, 50 mg catalyst was pretreated in Ar with a flow rate of 30 mL min −1 at 200°C for 0.5 h and then cooled to 30°C, then the gas stream was switched to 8% C 3 H 6 in Ar with a flow rate of 50 mL min −1 and the catalyst was heated at a heating rate of 5°C min −1 . In the C 3 H 6 + O 2 TPRS experiments, 50 mg catalyst was pretreated in Ar with a flow rate of 30 mL min −1 at 200°C for 0.5 h and then cooled to 30°C, then the gas stream was switched to 8% C 3 H 6 + 4% O 2 in Ar with a flow rate of 50 mL min −1 and the catalyst was heated to the desired temperature at a heating rate of 5°C min −1 .
In-situ CO adsorption after catalytic reactions at different temperatures was performed on a Nicolet 6700 FTIR spectrometer equipped with an in-situ lowtemperature and vacuum DRIFTS reaction cell (Harrick Scientific Products, Inc.) in order to enhance the chemisorption with minimum interference of gas-phase molecules. The DRIFTS spectra were measured with 256 scans and a resolution of 4 cm −1 using a MCT/A detector. 50 mg catalyst was loaded on the sample stage of the reaction cell. Prior to adsorption experiments, the sample was evacuated at 293 K for 1 h at a base pressure of 0.1 Pa and then cooled to 123 K, whose spectrum was taken as the background spectra. Then CO was admitted into the reaction cell to the desirable pressures via a leak valve, and the DRIFTS spectrum was recorded after the chemisorption reached the steady state.
Catalytic performance evaluation. Catalytic performance of Cu 2 O nanocrystals in propylene oxidation with O 2 without any pretreatments was evaluated in a quartz tube microreactor equipped with an axial quartz sheathed thermocouple. 200 mg catalyst was used and heated to the desired reaction temperatures at a rate of 2°C min −1 in a reaction gas mixture (C 3 H 6 : O 2 : Ar = 2: 1: 22, flow rate: 50 mL min −1 ). After the catalytic reaction reached a steady state, the composition of outlet gas was analyzed using an online Shimazu GC-2014 gas chromatograph equipped with two flame ionization detectors (FIDs) and one thermal conductivity detector (TCD). One FID was attached to a Stabilwax-DA capillary column (0.53 mm × 60 m) to detect propylene and oxygenates (acetaldehyde, PO, acetone, propionaldehyde, acrolein, acetic acid, and isopropanol) to a detection limit of 1 ppm, and the TCD was attached to a Porapak Q (3 mm × 3 m) and C13x compact column (3 mm × 3 m, Shimazu) to detect O 2 . A CH 4 conversion oven was connected to the end of the TCD to convert trace CO 2 to CH 4 , whose concentration was detected by the other FID. All the lines and valves between the exit of the reactor and the gas chromatographs were heated to 80°C to prevent condensation of the products. The activity and selectivity of the catalytic reaction were calculated as the following, in which X i represents conversion, S i selectivity, m i mass, and n i moles of substance i, FC3H6 represents the flow rate of C 3 H 6 : S Acrolein ¼ n Acrolein n oxygenates þ n CO2=3 ð3Þ DFT calculations. DFT calculations were performed by Vienna ab initio Simulation Package (VASP) 36,37 . The exchange-correlation interaction was described by the Bayesian error estimation functional with van der Waals correlation (BEEF-vdW) 38 . The Kohn-Sham equations were solved by a plane wave basis set with a kinetic energy cutoff of 400 eV. A Cu 2 O(110) surface with (2 × 2) unit cell was modeled by a slab model including four-layer O and seven-layer Cu atoms. To prevent the artificial interaction between the repeated slabs along z-direction, 15 Å vacuum was introduced with correction of the dipole moment. The (2 × 2 × 1) kpoint mesh was used to sample the Brillouin zone. During the optimization, the bottom two-layer O and four-layer Cu atoms were fixed in their bulk positions, while the remained atoms and adsorbates were relaxed until the residual forces were less than 0.02 eV Å −1 . DFT + U correction was used with U-J = 6 eV for Cu 3d-orbitals 39 . Adsorption energies were calculated by E ads = E ad/sub -E ad -E sub , where E ad/sub , E ad , and E sub were the total energies of the optimized adsorbate/ substrate system, the adsorbate in the gas phase, and the clean substrate, respectively. Transition states of the elementary steps were located by the climbing-image nudged elastic band (CI-NEB) method 40 .
Data availability