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Crystal-plane effects on surface and catalytic properties of Cu2O nanocrystals for NO reduction by CO
Accepted Manuscript Title: Crystal-plane effects on surface and catalytic properties of Cu2 O nanocrystals for NO reduction by CO Author: Weixin Zou Lichen Liu Lei Zhang Lulu Li Yuan Cao Xiaobo Wang Changjin Tang Fei Gao Lin Dong PII: DOI: Reference:
S0926-860X(15)30116-2 http://dx.doi.org/doi:10.1016/j.apcata.2015.08.021 APCATA 15515
To appear in:
Applied Catalysis A: General
Received date: Revised date: Accepted date:
22-4-2015 13-8-2015 14-8-2015
Please cite this article as: Weixin Zou, Lichen Liu, Lei Zhang, Lulu Li, Yuan Cao, Xiaobo Wang, Changjin Tang, Fei Gao, Lin Dong, Crystal-plane effects on surface and catalytic properties of Cu2O nanocrystals for NO reduction by CO, Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2015.08.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Revised Manuscript Submitted to Applied Catalysis A: General
Crystal-plane effects on surface and catalytic properties of Cu2O nanocrystals for NO reduction by CO Weixin Zou,a,b† Lichen Liu,a,b† Lei Zhang,a,b Lulu Li,a,b Yuan Cao,a,b Xiaobo Wang,a,b Changjin Tang,a,b Fei Gao,*a,b Lin Dong*a,b a
Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093, PR China b
Jiangsu Key Laboratory of Vehicle Emissions Control, Center of Modern Analysis,
Nanjing University, Nanjing 210093, PR China † These authors contribute equally.
Corresponding Author *E-mail: [email protected], [email protected] Tel: +86-25-83592290 Fax: +86-25-83317761 1
ABSTRACT: In this work, Cu2O rhombic dodecahedrons {110}, octahedrons {111} and cubes {100} were synthesized to study the crystal-plane effects on surfaces and catalytic properties in NO+CO reaction. TPR results demonstrated the reducibility was ranked by {110} > {111} > {100}. Interestingly, the activity order in NO+CO reaction was well consistent with the reducibility order. Moreover, the in-situ formed metallic Cu nanoparticles affected by Cu2O exposed crystal planes were found on surfaces during the process. In-situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) studies revealed that the Mars van Krevelen mechanism worked in this system and the adsorption/dissociation of NO was the key step. Dodecahedral Cu2O {110} with better reducibility was more sensitive to the NO+CO reaction, and the generated interface between Cu0 and Cu2O{110} was beneficial to the effective decomposition of adsorbed NO species. This work provided the scientific basis over Cu-based catalysts.
KEYWORDS: crystal-plane effect, Cu2O, metallic Cu, in-situ DRIFTS, NO reduction by CO
2
1. Introduction In recent years, with the sharp increase of vehicles, the issue of vehicle exhaust pollution has become more and more serious, which attracts a lot of public attention. NOx is one of the main vehicle exhaust pollutants, partially removed by NO+CO reaction [1-3]. The catalytic reduction of NO by CO belongs to heterogeneous catalytic reaction, which is widely involved in many important processes such as adsorption of reactants, the conversion of adsorbed intermediates (surface reaction), and desorption of products. It is found that the use of copper-based catalysts is a suitable one for the catalytic reduction of NO by CO [4-6]. In recent years, our group have reported some work on catalytic removal of NO by CO with supported copper-based catalysts and achieved some meaningful results [7,8]. However, the supported Cu-based catalysts are not ideal model catalysts to study the relationships between structures and activities, due to the complicated interaction between Cu species and the supports. It has been reached a general consensus that the catalytic properties are crystal-plane-dependent, thus, well-defined metal oxides nanocrystals are the perfect model catalysts to study the relationships between structural factors and catalytic properties [9-12]. Cu2O, as an important semiconductor, has appeared in many reports about controllable synthesis and catalytic applications. In addition, some work have reported that the catalytic properties of Cu2 O are significantly influenced by electronic and geometric factors. Jing et al. proposed that Cu2O with more high-index facets had better performance in photocatalytic H2 production, for the reason that photo-generated electrons preferred on high index planes, while holes tended to migrate to low index planes [13]. Huang et al. studied the electrical conductivity on three low-index facets of Cu2O crystals, and found different facets had different surface band structures and barrier heights, resulted in different photocatalytic performance [14]. Xiong et al. loaded Pd nanoparticles on {111} and {100} facets of Cu2O for photocatalysis, and discovered that electron transfer between Pd nanoparticles and {100} planes were more facilitated, due to the formation of Schottky junction between Pd and Cu2O {100} planes, which indicated that the 3
electron-transfer process between Pd and Cu2O had close relationships with the crystal planes [15]. On the other hand, the geometric structures of different Cu2O crystal planes also have significant influences on the catalytic behaviors. Huang et al. reported that Cu2O catalysts with different morphologies (cubes and octahedrons) showed surface reconstruction during CO oxidation. CuO shells would be formed on Cu2O nanocrystals during the CO oxidation reaction. The re-constructed CuO/Cu2O octahedrons showed higher activity than CuO/Cu2O cubes [16]. This work suggested that the real active structures might be in-situ formed during the reaction, which was related with the primary crystal planes. Huang et al. investigated different-shaped Cu2O on the catalytic multicomponent reactions, and found that rhombic dodecahedral Cu2O with Cu atoms fully exposed on {110} crysal planes had the superior catalytic activity [17]. Based on the discussion above, it is clearly that crystal-plane effect has the close relationship with catalytic properties. However, until recently, there are few reports about the crystal-plane effects of Cu2O on the dynamic changes during NO reduction by CO. Therefore, in this work, uniform Cu2O nanocrystals, including rhombic dodecahedrons {110}, octahedrons {111} and cubes {100} were prepared, and the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), and in-situ diffuse reflectance infrared Fourier transform spectra (DRIFTS). The study was mainly focused on: (1) the crystal-plane effects on the surface and catalytic properties in NO reduction by CO; (2) investigating the mechanism of NO+CO reaction by in-situ DRIFT studies; (3) exploring the key factors which had the significant influence on the reactivity.
2. Experimental 2.1. Catalyst preparation The cubic and octahedral Cu2O nanocrystals were prepared following an approach 4
proposed by Kuo et al. [18] with a minor modification of reaction temperature and ascorbic acid contents. CuCl2.2H2O (0.085 g) and different
contents of
polyvinylpyrrolidone (PVP, MW = 30000) (cubes: 0 g; octahedrons: 4.44 g) were dissolved in 50 mL distilled water, 5.0 mL NaOH aqueous solution (2.0 mol/L) was added dropwise at 60 oC for cubes, 55 oC for octahedrons. After 0.5 h, 5.0 mL ascorbic acid (Vc) aqueous solution (cubes: 0.7 g; octahedrons: 0.6 g) was dripped. The above mixture reacted for different time (cubes: 5 h; octahedrons: 3 h) at the mentioned reaction temperatures, respectively. Finally, the production was collected by centrifugation, washed with excessive distilled water, absolute ethanol in ultrasound for several times to remove the ions and surfactant, dried in vacuum at 50 o
C for 12 h. The process of rhombic dodecahedral Cu2O nanocrystals synthesis followed a
report by Zhang et al. [19] with a minor modification of oleic acid volume. 0.25 g CuSO4.5H2O was dissolved well in 40 mL distilled water, 5 mL oleic acid and 20 mL absolute ethanol were added successively into the above solution with vigorous stirring. Once the above mixture was heated to 100 oC, 10 mL NaOH solution (0.8 mol/L) was added dropwise. The 30 mL aqueous solution with 3.42 g D-(+)-glucose was dropped after 5 min. These reagents reacted for 60 min. The product was collected by centrifugation, washed by excessive cyclohexane and absolute ethanol in ultrasound for several times to remove oleic acid, and finally dried in vacuum at 50 oC for 12 h. 2.2. Catalyst characterization Scanning electron microscopy (SEM) experiments were performed on Philips XL30 electron microscope operated at beam energy of 10.0 kV. Transmission electron microscopy (TEM) images were taken on a JEM-2100 instrument at an acceleration voltage of 200 kV. The samples were crushed and dispersed in A.R. grade ethanol and the resulting suspensions were allowed to dry on carbon film supported on copper grids. The crystal structure of three Cu2O catalysts were identified by X-ray diffraction (XRD) with a Philips X’Pert Pro diffractometer using Ni-filtered Cu Kα 5
radiation ( = 0.15418 nm). The X-ray tube was operated at 40 kV and 40 mA. X-ray photoelectron spectroscopy (XPS) analysis were performed on a PHI 5000 VersaProbe high performance electron spectrometer, using monochromatic Al Kα radiation (1486.6 eV), the sample was outgassed at room temperature in a UHV chamber (< 5 × 10–7 Pa). The sample charging effects were compensated by all binding energies (BE) referenced to the C 1s peak at 284.6 eV. This reference gave BE values with an error within ±0.1 eV. H2-temperature programmed reduction (H2-TPR) experiments were performed with H2 as a reduced agent in a quartz U-type reactor, and about 25 mg sample was used for each measurement. H2-Ar mixture (7% H2 by volume) was switched on, the temperature increased with a ramp of 3 oC/min. The consumption of H2 was monitored on line by a thermal conductivity detector. CO-temperature programmed reduction (CO-TPR) experiments were simulated in the equipment of NO+CO reactor with the only CO gas, about 35 mg sample was used for each measurement. CO-Ar (5% of CO by volume) was switched on, the temperature increased with a ramp of 3 oC/min. In-situ diffuse reflectance infrared Fourier transform spectra (in-situ DRIFTS) were collected from 1000 to 4000 cm–1 at a spectral resolution of 4 cm–1 (number of scans, 32) on a Nicolet 5700 FT-IR spectrometer equipped with a high-sensitive MCT detector cooled by liquid N2. The DRIFTS cell (Harrick) was fitted with a ZnSe window and a heating cartridge that allowed sample to be heated to 400 oC. The fine catalyst powder placed on a sample holder was carefully flattened to enhance IR reflection. The sample was purified with a high purified N2 stream from room temperature to 400 oC at 5 oC/min to eliminate the physisorbed water and other impurities. The sample background of each target temperature was collected during the cooling process. At ambient temperature, the sample was exposed to a controlled stream of NO-Ar (5% of NO by volume) or/and CO-Ar (5% of CO by volume) at a rate of 5.0 ml/min for 30 min to be saturated. Reaction studies were performed by heating the adsorbed species and the spectra were recorded at various target temperatures at a rate of 5 oC/min from room temperature to 350 oC by subtraction of the corresponding background reference. The Cu2O crystal structures were drawn using the Materials Studio 6.0 software. 6
2.3. Catalytic performances measurement The catalytic performances of these catalysts for NO reduction by CO model reaction were determined under steady state, involving a feed stream with a fixed composition, 5% NO, 5% CO and 90% He by volume as diluents. The sample (35 mg) was fitted in a quartz tube, and the mixed gases were switched on. The reactions were carried out at different temperatures with a space velocity of 15000 ml g–1 h–1. Two columns (length, 1.75 m; diameter, 3 mm) and two thermal conductivity detectors (T = 100 oC) were used to analyze the products. Column A with Paropak Q was used for separating CO2 and N2O, column B packed with 5A and 13X molecule sieve (40–60 M) was applied for separating N2, NO and CO.
3. Results and discussion 3.1. Morphology and structural characterizations XRD patterns of three Cu2O samples were presented in Fig. 1. Obviously, all of them showed typical diffraction pattern of Cu2 O. The diffraction peaks (d-Cu2O, o-Cu2O, c-Cu2O) could be ascribed perfectly to the pure cubic-phase Cu2O (JCPDS No. 65-3288, space group Pn3m, lattice constant a=0.4260 nm) [20]. No other copper species (CuO or Cu) peaks were detected in this pattern, suggesting the phase composition of the prepared samples were quite pure. The relative intensities of the peaks in the diffraction patterns were different due to the percentages of different crystal planes [21], which were showed in Table S1. It was observed that the percentage of (110) plane in dodecahedral Cu2O sample was 10.1 %, higher than the other two samples. Similarly, octahedral and cubic Cu2O samples had the highest percentages of (111) and (200) planes among the three shapes, respectively. Therefore, d-Cu2O, o-Cu2O and c-Cu2O samples were mainly enclosed by {110}, {111} and {100} crystal planes, respectively. The morphologies of as-synthesized Cu2O crystals were characterized by SEM and TEM. As shown in Fig. 2, these particles had grain sizes of about 500 nm with uniform shapes and well dispersion. The detailed formation mechanism of 7
different-shaped Cu2O was elucidated in the view of structure and dynamic theory. For cuprite Cu2O, cubic Cu2O {100} planes with 100% oxygen terminated surfaces had a minimum energy state, and thus the cubic shape of Cu2O crystals could be synthesized without surfactants. Octahedral Cu2O {111} planes possessed a high energy status, which were minimized the {111} surface energy by the PVP surfactants adsorption, leading to the stability of {111} planes and the accelerated growth of {100} planes [22]. As for dodecahedral Cu2O crystals, glucose, a weak reducer, made the reduction process slow and suppressed the nanoparticles to aggregate. In addition, oleic acid played an important role in controlling the dodecahedral shape by the face-selective adsorption [23]. The above synthesis resulted in the Cu2O nanocrystals with rhombic dodecahedral, octahedral and cubic shapes, which were denoted as d-Cu2O, o-Cu2O and c-Cu2O, respectively. TEM and HRTEM characterizations were further introduced to check the morphology and their corresponding exposed crystal planes. As shown in Fig. S1a (in the Supplementary Information), the projection image also confirmed the dodecahedral shape. HRTEM image demonstrated the preferred {110} planes with a lattice spacing of 0.308 nm in Fig. S1b. Similarly, the octahedral Cu2O could also be confirmed in Fig. S1c. These octahedral nanocrystals showed a plane spacing of 0.238 nm corresponding to {111} planes (Fig. S1d). Moreover, the cubic Cu2O was seen in Fig. S1e, which was enclosed by {100} planes with a plane spacing of 0.214 nm (Fig. S1f). The above morphology characterizations indicated that the d-Cu2O, o-Cu2O and c-Cu2O crystals were enclosed by twelve {110}, eight {111} and six {100} crystal planes, respectively. Meanwhile, the as-prepared Cu2O nanocrystals had uniform shapes and well-defined exposed crystal planes, which were ideal model catalysts to study the crystal-plane effects. Generally, capping ligands residual on crystal surface had great influences on the catalytic properties of nanocrystals [24-26]. In our preparation, the surfactants (oleic acid for d-Cu2O, PVP for o-Cu2O) were inevitably brought in to control the morphologies. Before testing the catalytic activities of d-Cu2O and o-Cu2O, we need to make sure that the surfactants had already been totally removed. Cu2O samples after the washing process (details could be seen in experimental section) were 8
checked by FT-IR, XPS and SEM-EDAX techniques. From Fig. S2a, the prepared o-Cu2O and d-Cu2O without washing were denoted as o-Cu2O (PVP) and d-Cu2O (OA), respectively; the prepared o-Cu2O and d-Cu2O with washing treatment were denoted as o-Cu2O and d-Cu2O, respectively. The FT-IR spectra demonstrated that only Cu-O vibration of Cu2O was in the spectra of o-Cu2O and d-Cu2O [27], no other characteristic peaks of polyvinylpyrrolidone (PVP) and oleic acid (OA) were observed, which confirmed o-Cu2O and d-Cu2O catalysts were free of PVP and OA surfactants, respectively. XPS spectrum (Fig. S2b) further indicated that PVP was removed from surface, due to no peak of N 1s [17,28]. SEM-EDAX results (Fig. S2c,d) demonstrated that there were only Cu, O, Si elements (Silicon film was used to support Cu2O samples for SEM analysis). While, carbon element considered to be contained in organics was not found in o-Cu2O and d-Cu2O samples. Therefore, it was identified that the o-Cu2O and d-Cu2O catalysts for catalytic performance measurement were free of PVP and OA surfactants, respectively. 3.2 Reduction properties (H2 or CO-TPR) The reduction properties of Cu2O with different morphologies were determined by hydrogen temperature-programmed reduction (H2-TPR). The profiles were shown in Fig. 3a. For all the three samples, there was only one peak belonging to the reduction of Cu+ to Cu0 [29]. Notably, the reduction temperatures of dodecahedral, octahedral and cubic Cu2O were dramatically distinct. The reduction of dodecahedral, octahedral and cubic Cu2O nanocrystals started at ca. 190, 220 and 250 °C, respectively. Therefore, from their reduction temperatures, the reducibilities were ranked by d-Cu2O {110} > o-Cu2O {111} > c-Cu2O {100}. CO-TPR profiles (Fig. 3b) displayed the similar tendency. The reduction of dodecahedral, octahedral and cubic Cu2O nanocrystals started at ca. 200, 270 and 300 °C, respectively. TPR results clearly demonstrated that Cu2O dodecahedrons were much easier to be reduced than octahedrons and cubes. Generally, the reduction of oxides always initially took place on the surface or near surface layer. For temperature-programmed reduction, it was actually the gas-solid 9
interfacial reaction, the reducibility of Cu2O crystals was completely determined by the chemisorption and activation of H2 or CO molecules on Cu2O crystal planes and the surface Cu-O bond strength. The schematic illustrations of the Cu2O (110), (111), (100) planes were present in Fig. S3a. On d-Cu2O (110) plane, the first layer consisted of both coordinated unsaturated O (OCUS) and coordinated saturated Cu (CuCSA), and only CuCSA were on the second layer. While for the o-Cu2O (111) and c-Cu2O (100), the first layer only consisted of OCUS; the second layer was made up of 75% CuCSA and 25% coordinated unsaturated Cu (CuCUS) for o-Cu2O (111); 100% CuCSA for c-Cu2O (100) [30]. The schematic illustrations suggested that d-Cu2O nanocrystal had more exposed Cu atoms on the first layer of the surface. Furthermore, the DRIFTS results of saturated CO adsorption at 35 oC were shown in Fig. S3b. There were two vibration bands at 2180 and 2110 cm–1. According to the literature, the band above 2130 cm−1 was ascribed to CO bonded to ionic species of metal [8], herein, the band at 2180 cm–1 was the signal of Cu+-CO, and the other vibration band at 2110 cm–1 was gaseous CO [7]. The relative area ratio of peaks centered at 2180 cm–1 and 2110 cm–1 were 1.46, 1.39, 1.40 for d-Cu2O, o-Cu2O, c-Cu2O, respectively, which showed that more CO molecules were adsorbed on d-Cu2O samples than that on the two others, and d-Cu2O nanocrystal has more exposed Cu atoms on the surfaces, because CO facilely chemisorbed on the CuI site instead of O site [28]. The number of terminal copper atoms per unit surface area on the d-Cu2O {110} facet was calculated by Huang et al., which was roughly 1.5 times higher than that on the {111} facet, and that on {100} facet was the lowest [31]. It was inferred that d-Cu2O crystal had significant reducibility, owing to its exposed {110} plane terminated by Cu. Compared with O, Cu not only had rich d-electrons, but also possessed empty orbits, which were inclined to chemical adsorption of H2 and CO molecules [32,33], and thus d-Cu2O {110} was easier to be reduced. Conversely, because of insensitivity to H2 and CO molecules for O-terminated {100} plane, c-Cu2O {100} was difficult to be reduced. For o-Cu2O, 25% CuCUS were present on the second layer of {111} plane and displayed a much stronger
10
chemisorption ability of H2 and CO than CuCSA, therefore, the reducibility of o-Cu2O {111} was poorer than d-Cu2O {110}, but better than c-Cu2O {100}. In a word, the crystal-plane-effects made significant influence on the reducibility of Cu2O nanocrystals. 3.3. Catalytic performances (NO+CO model reaction). The NO reduction by CO reaction was used to investigate crystal-plane effects on the catalytic performances of Cu2O crystals (d-Cu2O, o-Cu2O and c-Cu2O). For the NO+CO reaction, the reaction products were N2O, N2, and CO2. Fig. 4a displayed the NO conversion of different-shaped Cu2O crystals. Herein, T20 (the temperature at which the NO conversion was 20%) was used to evaluate catalytic performances at low-temperature range. For the d-Cu2O, the T20 was 316 oC. While for o-Cu2O and c-Cu2O, the T20 were 327 oC and 344 oC, respectively. The activity order was as following: d-Cu2O > o-Cu2O > c-Cu2O. It was observed that the NO conversion didn’t reach 100%, and thus it was deduced that the phenomenon of coke deposition might happen on the catalyst. The similar phenomenon had been reported on Cu-zeolite catalysts for the selective catalytic reduction of NO [34,35] and the detailed investigation on coke deposition would be carried out in our future research. Furthermore, the N2 selectivity of NO+CO reaction was further demonstrated that d-Cu2O was the best for the catalytic reduction of NO to N2, and the order was: d-Cu2O {110} > o-Cu2O {111} > c-Cu2O {100} (Fig. 4b). Since the BET specific surface area values revealed those of Cu2O crystals were almost equivalent (in Table S2), their difference in catalytic performances should originate from their distinct surface structures on crystal-planes. In order to unveil the essence of crystal-plane-controlled reaction, these Cu2O crystals after NO+CO reaction had been determined by SEM, TEM, XRD and XPS characterizations. The SEM results of the used Cu2O crystals were showed in Fig. 5. The shapes were preserved after the reaction, but the surfaces were changed. Some particles were generated on the surface, indicating that the componential and morphological evolution took place on different exposed crystal planes. Especially, the surface 11
changes were more obvious in d-Cu2O sample than o-Cu2O and c-Cu2O, implying d-Cu2O was more sensitive to the NO+CO reaction. In addition, the nanoparticles generated on the used Cu2O surfaces were determined by HRTEM (Fig. S4), it was suggested that the lattice fringes of the nanoparticles were 0.212 nm, which was ascribed to Cu0 (111) crystal plane. XRD patterns of the used Cu2O crystals were also collected and were demonstrated by Fig. 6. It could be found that besides the diffraction peaks of cubic-phase Cu2O, another diffraction peaks corresponding to Cu0 species (JCPDS No. 03-1005, space group Fm3m) were observed. Moreover, the amount of the in-situ generated Cu0 species were varied with the exposed crystal planes. Based on the diffraction intensity ratio of Cu0 (111) peak/Cu2O (111) peak, the content of Cu0 could be roughly estimated and ranked as following: d-Cu2O (1.39) > o-Cu2O (0.82) > c-Cu2O (0.35). XPS was used as a more direct characterization tool to analyze the surface Cu species. The XPS spectra of the fresh and used Cu2O nanocrystals were shown in Fig. 7. Generally, the Cu-LMM XAES spectra were used to distinguish the low values of copper species (Cu+ and Cu0), and XPS spectra of Cu 2p peak were adopted to determine Cu2+ species for the reason that the binding energies of low values of copper species were too close to depart in Cu 2p XPS spectra [36-38]. It was accepted that the peaks with kinetic energy of ca. 916.7 eV and 918.4 eV were assigned to Cu+ and Cu0 species in Cu-LMM XAES spectra, respectively [38]. Fig. 7a showed that only peak corresponding to Cu+ species was in the fresh samples. While, there was a broad peak composed of two peaks in the used samples (Fig. 7b), one peak was centered at ca. 918.4 eV which could be attributed to Cu0 species, and the other peak located at ca. 916.7 eV was ascribed to Cu+ species. Cu0 species generated with the sacrifice of partial Cu+ species. The relative area of peaks centered at 918.4 eV and 916.7 eV were in comparison, the results were displayed in Table S3, corresponding to the relative content of Cu0/Cu+ on the surface. Obviously, the generated Cu0 on the d-Cu2O {110} surface was more than others, which was in accordance with roughness degree from the SEM images and XRD results of the used catalysts. Based on the results of the used Cu2O catalysts, it was indicated that the 12
dodecahedral Cu2O with the advantageous {110} surface was more sensitive to NO+CO reaction conditions. Furthermore, the kinetic test was carried out in NO+CO reaction. The TOF (turnover frequencies) of different-shaped Cu2O crystals were calculated, and experimental details were shown in the supplementary information. The related TOF data at different temperatures for the d-Cu2O, o-Cu2O and c-Cu2O were in Table S4. It was observed that the TOF number of d-Cu2O {110} facet was roughly 1.5 times higher than that of the o-Cu2O {111} facet at 330 oC, and was roughly 2.5 times higher than that of the c-Cu2O {100} facet at 350 oC, which suggested that d-Cu2O had the superior catalytic performance. In addition, the Arrhenius plots for NO conversion over different-shaped Cu2O nanocrystals were displayed in Fig. S5. The activation energy was ranked by d-Cu2O (83 kJ/mol) < o-Cu2O (104 kJ/mol) < c-Cu2O (130 kJ/mol). Both of the two kinetic results were confirmed that the catalytic performance of NO+CO reaction was greatly affected by the crystal structures, and the surface reaction process was further recognized by in-situ DRIFTS. 3.4 NO or/and CO adsorption in-situ DRIFTS with different-shaped Cu2O NO and CO adsorption in-situ DRIFTS were recorded at various temperatures in the simulated reaction conditions, and the corresponding results were displayed in the Fig. 8. The band at about 1600, 1400 and 1300 cm-1 were attributed to carbonate species, the band at 2180 cm-1 was assigned to copper carbonyl (Cu+-CO) species; whereas, the bands at 1490, 1220 and 1080 cm-1 were vibrations of the nitrate species [39-41]. Obviously, neither nitrate species nor adsorbed NO could be detected below 200 oC, indicating that CO was preferentially adsorbed on Cu+ sites of the surface, because Cu+–NO species was not stabilized by the σ-π synergistic effect [42]. With the increase of temperatures, the intensity of Cu+–CO and carbonate bands were decreased, which indicated that Cu+ species was reduced by CO [40] and then the interface of Cu0 and Cu+ was created. In addition, the bands of adsorbed NO species were appeared. Afterward, the adsorbed NO species were dissociated on the generated Cu0/Cu+ interface, confirmed by the appearance of N2O band (2240 cm-1). Simultaneously, the partial Cu0 species was reoxidized by NO. With the further 13
increase of temperatures, more low-state copper species were generated and then the band of N2O was decreased, indicating that N2O was reduced to N2 on the surface Cu0 species, because Cu0 species was facilitate to N2O decomposition [43]. Compared with the three shaped Cu2O samples, it was observed that the intermediate N2O was first to appear and disappear on d-Cu2O {110}, consistent with the results of NO conversion and N2 selectivity. Why does d-Cu2O {110} have the excellent catalytic performance? In order to answer the question, NO adsorption in-situ DRIFTS experiments were carried out on the fresh and H2-pretreated Cu2O crystals (the pretreated temperatures were according to H2-TPR), the corresponding results were presented in Fig. 9. N2O species dissociated by NO could be observed on the H2-pretreated Cu2O crystals, while, that band was not obvious on the fresh Cu2O crystals, except for more other nitrates species [40,41]. The results demonstrated that the low-valence copper was conducive to the dissociation of the adsorbed NO species, because the low-valence copper with extra electrons could donate the electrons back to the anti-bonding orbital of the adsorbed NO species, which weakened the N–O bond and promoted the dissociation, this process was considered of the key step for NO reduction [36,44,45]. Therefore, in the reaction conditions, d-Cu2O {110} was more sensitive to CO, consistent with the reducibility results (CO-TPR), and then the adsorbed nitrate species was decomposed on the interface of Cu0 and d-Cu2O {110}, which resulted in the superior catalytic activity. Furthermore, the intensity of N2O peak on the H2-pretreated Cu2O crystals was ranked by d-Cu2 O (H2) < o-Cu2O (H2) < c-Cu2O (H2), which was seemed to contradict with the activity result. For NO molecule, it could dissociate to N2O and N2. The intensity of N2O band on d-Cu2O (H2) was weaker than that of the two others, because from the N2 selectivity results in Fig. 4b, the interface between Cu0 and d-Cu2O {110} made N2O rapidly dissociate to N2, and N2 had no IR active mode [46]. Therefore, the crystal-plane effects on catalytic properties might result from their different activation process in the in-situ structural transformation during the reaction. 3.5 Proposed reaction mechanism for NO+CO reaction over Cu2O catalysts 14
For the reaction mechanism of NO reduction by CO, it was not reached a consensus. Our previous study researched the influence of CO-pretreatment on the performance of NO catalytic reduction by CO on the supported binary metal oxides CuO-Mn2O3/γ-Al2O3 catalyst, and proposed that NO preferentially adsorbed on Mn2+ and
CO
was
on
Cu+
during
the
in
situ
FT-IR,
which
indicated
a
Langmuir-Hinshelwood (L-H) mechanism for two kinds of active sites (Mn2+ and Cu+) [40]. In addition, the Mars van Krevelen (redox) mechanism was proposed. Fernάndez-García et al. investigated the state of copper species on CuO/γ-Al2O3 catalyst during the NO+CO reaction by in-situ XANES, suggesting that the valence states of surface copper species on the catalyst were related with its catalytic performance [47]. In our present work, in-situ DRIFT demonstrated that the Mars van Krevelen mechanism worked in our system and the rapid change in valence state of copper species was a crucial part, which was related to the reducibility of catalysts. In addition, the key step for NO reduction by CO was the dissociation of adsorbed nitrogen oxide species. A schematic illustration about the reaction pathways was presented in Fig. 10. Firstly, CO molecules were preferentially adsorbed on the Cu2O surface. With the increase of the temperature, the low-state copper species were generated by CO reduction; the adsorbed carbonate species was disappeared, and nitrate species was adsorbed, afterward, the adsorbed NO species was decomposed to intermediate N2O; meanwhile, metallic Cu was partially oxidized to Cu2O. With the reaction proceeding, more low-state copper was generated, leading to the intermediate N2O to the final product N2, resulting in a catalytic cycle.
4. Conclusions In this work, the catalytic performance for NO+CO reaction by different-shaped Cu2O nanocrystals was demonstrated. Dodecahedral Cu2O {110} showed the best NO conversion, octahedral Cu2O {111} were followed, and cubic Cu2O {100} were the poorest. In-situ DRIFT suggested that the Mars van Krevelen mechanism worked in our system and the rapid change in valence state of copper species was a crucial part. 15
Thus, the reactivity was well correlated with their surface properties, especially the reducibility. Furthermore, NO adsorption in-situ DRIFT on the fresh and H2-pretreated Cu2O crystals indicated that the interface of Cu0/Cu+ was beneficial for the dissociation of adsorbed nitrogen oxide species, which was the key step. Therefore, d-Cu2O {110} had the superior catalytic performance, because {110} surface was more sensitive to NO+CO reaction conditions and then Cu0 nanoparticles were generated, leading to the decomposition of NO and the migration of the dissociated oxygen atoms on the Cu0/Cu+ interface. In conclusion, the crystal-plane-effects made significant influence on the catalytic behaviors, and in-situ structural transformation. Through this study, it was helpful to provide the scientific basis for Cu-based catalyst.
Acknowledgements Financial supports from the Natural Science Foundation of Jiangsu Province (BK2012298) and National Natural Science Foundation of China (Nos.21273110, 21203091) are gratefully acknowledged.
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Fig. 1. XRD patterns of as-prepared d-Cu2O, o-Cu2O and c-Cu2O.
20
Fig. 2. SEM images of different morphologies of d-Cu2O (a, b), o-Cu2O (c, d) and c-Cu2O (e, f).
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Fig. 3. H2-TPR profiles (a) and CO-TPR profiles (b) of Cu2O with different morphologies.
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Fig. 4. Catalytic performance: NO conversion (a) and N2 selectivity (b) over Cu2O with different morphologies.
23
Fig. 5. SEM images of different-shaped Cu2O after NO+CO reaction: d- Cu2O (a, b), o-Cu2O (c, d) and c-Cu2O (e, f).
24
Fig. 6. XRD patterns of d-Cu2O, o-Cu2O, c-Cu2O after NO+CO reaction, respectively.
25
Fig. 7. Cu-LMM XAES spectra of Cu2O with different morphologies before reaction (a) and after reaction (b).
26
Fig. 8. In-situ DRIFTS of NO and CO (5% and 5% in volume, respectively) co-adsorption on d-Cu2O (a), o-Cu2O (b) and c-Cu2O (c).
27
Fig. 9. In-situ DRIFTS of NO adsorption on d-Cu2O (a), o-Cu2O (b), c-Cu2O (c), H2-pretreated d-Cu2O (d), H2-pretreated o-Cu2O (e) and H2-pretreated c-Cu2O (f).
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Fig. 10. Proposed reaction mechanism for NO+CO reaction over Cu2O (110) plane. Graphical abstract
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S0926-860X(15)30116-2 http://dx.doi.org/doi:10.1016/j.apcata.2015.08.021 APCATA 15515
To appear in:
Applied Catalysis A: General
Received date: Revised date: Accepted date:
22-4-2015 13-8-2015 14-8-2015
Please cite this article as: Weixin Zou, Lichen Liu, Lei Zhang, Lulu Li, Yuan Cao, Xiaobo Wang, Changjin Tang, Fei Gao, Lin Dong, Crystal-plane effects on surface and catalytic properties of Cu2O nanocrystals for NO reduction by CO, Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2015.08.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Revised Manuscript Submitted to Applied Catalysis A: General
Crystal-plane effects on surface and catalytic properties of Cu2O nanocrystals for NO reduction by CO Weixin Zou,a,b† Lichen Liu,a,b† Lei Zhang,a,b Lulu Li,a,b Yuan Cao,a,b Xiaobo Wang,a,b Changjin Tang,a,b Fei Gao,*a,b Lin Dong*a,b a
Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093, PR China b
Jiangsu Key Laboratory of Vehicle Emissions Control, Center of Modern Analysis,
Nanjing University, Nanjing 210093, PR China † These authors contribute equally.
Corresponding Author *E-mail: [email protected], [email protected] Tel: +86-25-83592290 Fax: +86-25-83317761 1
ABSTRACT: In this work, Cu2O rhombic dodecahedrons {110}, octahedrons {111} and cubes {100} were synthesized to study the crystal-plane effects on surfaces and catalytic properties in NO+CO reaction. TPR results demonstrated the reducibility was ranked by {110} > {111} > {100}. Interestingly, the activity order in NO+CO reaction was well consistent with the reducibility order. Moreover, the in-situ formed metallic Cu nanoparticles affected by Cu2O exposed crystal planes were found on surfaces during the process. In-situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) studies revealed that the Mars van Krevelen mechanism worked in this system and the adsorption/dissociation of NO was the key step. Dodecahedral Cu2O {110} with better reducibility was more sensitive to the NO+CO reaction, and the generated interface between Cu0 and Cu2O{110} was beneficial to the effective decomposition of adsorbed NO species. This work provided the scientific basis over Cu-based catalysts.
KEYWORDS: crystal-plane effect, Cu2O, metallic Cu, in-situ DRIFTS, NO reduction by CO
2
1. Introduction In recent years, with the sharp increase of vehicles, the issue of vehicle exhaust pollution has become more and more serious, which attracts a lot of public attention. NOx is one of the main vehicle exhaust pollutants, partially removed by NO+CO reaction [1-3]. The catalytic reduction of NO by CO belongs to heterogeneous catalytic reaction, which is widely involved in many important processes such as adsorption of reactants, the conversion of adsorbed intermediates (surface reaction), and desorption of products. It is found that the use of copper-based catalysts is a suitable one for the catalytic reduction of NO by CO [4-6]. In recent years, our group have reported some work on catalytic removal of NO by CO with supported copper-based catalysts and achieved some meaningful results [7,8]. However, the supported Cu-based catalysts are not ideal model catalysts to study the relationships between structures and activities, due to the complicated interaction between Cu species and the supports. It has been reached a general consensus that the catalytic properties are crystal-plane-dependent, thus, well-defined metal oxides nanocrystals are the perfect model catalysts to study the relationships between structural factors and catalytic properties [9-12]. Cu2O, as an important semiconductor, has appeared in many reports about controllable synthesis and catalytic applications. In addition, some work have reported that the catalytic properties of Cu2 O are significantly influenced by electronic and geometric factors. Jing et al. proposed that Cu2O with more high-index facets had better performance in photocatalytic H2 production, for the reason that photo-generated electrons preferred on high index planes, while holes tended to migrate to low index planes [13]. Huang et al. studied the electrical conductivity on three low-index facets of Cu2O crystals, and found different facets had different surface band structures and barrier heights, resulted in different photocatalytic performance [14]. Xiong et al. loaded Pd nanoparticles on {111} and {100} facets of Cu2O for photocatalysis, and discovered that electron transfer between Pd nanoparticles and {100} planes were more facilitated, due to the formation of Schottky junction between Pd and Cu2O {100} planes, which indicated that the 3
electron-transfer process between Pd and Cu2O had close relationships with the crystal planes [15]. On the other hand, the geometric structures of different Cu2O crystal planes also have significant influences on the catalytic behaviors. Huang et al. reported that Cu2O catalysts with different morphologies (cubes and octahedrons) showed surface reconstruction during CO oxidation. CuO shells would be formed on Cu2O nanocrystals during the CO oxidation reaction. The re-constructed CuO/Cu2O octahedrons showed higher activity than CuO/Cu2O cubes [16]. This work suggested that the real active structures might be in-situ formed during the reaction, which was related with the primary crystal planes. Huang et al. investigated different-shaped Cu2O on the catalytic multicomponent reactions, and found that rhombic dodecahedral Cu2O with Cu atoms fully exposed on {110} crysal planes had the superior catalytic activity [17]. Based on the discussion above, it is clearly that crystal-plane effect has the close relationship with catalytic properties. However, until recently, there are few reports about the crystal-plane effects of Cu2O on the dynamic changes during NO reduction by CO. Therefore, in this work, uniform Cu2O nanocrystals, including rhombic dodecahedrons {110}, octahedrons {111} and cubes {100} were prepared, and the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), and in-situ diffuse reflectance infrared Fourier transform spectra (DRIFTS). The study was mainly focused on: (1) the crystal-plane effects on the surface and catalytic properties in NO reduction by CO; (2) investigating the mechanism of NO+CO reaction by in-situ DRIFT studies; (3) exploring the key factors which had the significant influence on the reactivity.
2. Experimental 2.1. Catalyst preparation The cubic and octahedral Cu2O nanocrystals were prepared following an approach 4
proposed by Kuo et al. [18] with a minor modification of reaction temperature and ascorbic acid contents. CuCl2.2H2O (0.085 g) and different
contents of
polyvinylpyrrolidone (PVP, MW = 30000) (cubes: 0 g; octahedrons: 4.44 g) were dissolved in 50 mL distilled water, 5.0 mL NaOH aqueous solution (2.0 mol/L) was added dropwise at 60 oC for cubes, 55 oC for octahedrons. After 0.5 h, 5.0 mL ascorbic acid (Vc) aqueous solution (cubes: 0.7 g; octahedrons: 0.6 g) was dripped. The above mixture reacted for different time (cubes: 5 h; octahedrons: 3 h) at the mentioned reaction temperatures, respectively. Finally, the production was collected by centrifugation, washed with excessive distilled water, absolute ethanol in ultrasound for several times to remove the ions and surfactant, dried in vacuum at 50 o
C for 12 h. The process of rhombic dodecahedral Cu2O nanocrystals synthesis followed a
report by Zhang et al. [19] with a minor modification of oleic acid volume. 0.25 g CuSO4.5H2O was dissolved well in 40 mL distilled water, 5 mL oleic acid and 20 mL absolute ethanol were added successively into the above solution with vigorous stirring. Once the above mixture was heated to 100 oC, 10 mL NaOH solution (0.8 mol/L) was added dropwise. The 30 mL aqueous solution with 3.42 g D-(+)-glucose was dropped after 5 min. These reagents reacted for 60 min. The product was collected by centrifugation, washed by excessive cyclohexane and absolute ethanol in ultrasound for several times to remove oleic acid, and finally dried in vacuum at 50 oC for 12 h. 2.2. Catalyst characterization Scanning electron microscopy (SEM) experiments were performed on Philips XL30 electron microscope operated at beam energy of 10.0 kV. Transmission electron microscopy (TEM) images were taken on a JEM-2100 instrument at an acceleration voltage of 200 kV. The samples were crushed and dispersed in A.R. grade ethanol and the resulting suspensions were allowed to dry on carbon film supported on copper grids. The crystal structure of three Cu2O catalysts were identified by X-ray diffraction (XRD) with a Philips X’Pert Pro diffractometer using Ni-filtered Cu Kα 5
radiation ( = 0.15418 nm). The X-ray tube was operated at 40 kV and 40 mA. X-ray photoelectron spectroscopy (XPS) analysis were performed on a PHI 5000 VersaProbe high performance electron spectrometer, using monochromatic Al Kα radiation (1486.6 eV), the sample was outgassed at room temperature in a UHV chamber (< 5 × 10–7 Pa). The sample charging effects were compensated by all binding energies (BE) referenced to the C 1s peak at 284.6 eV. This reference gave BE values with an error within ±0.1 eV. H2-temperature programmed reduction (H2-TPR) experiments were performed with H2 as a reduced agent in a quartz U-type reactor, and about 25 mg sample was used for each measurement. H2-Ar mixture (7% H2 by volume) was switched on, the temperature increased with a ramp of 3 oC/min. The consumption of H2 was monitored on line by a thermal conductivity detector. CO-temperature programmed reduction (CO-TPR) experiments were simulated in the equipment of NO+CO reactor with the only CO gas, about 35 mg sample was used for each measurement. CO-Ar (5% of CO by volume) was switched on, the temperature increased with a ramp of 3 oC/min. In-situ diffuse reflectance infrared Fourier transform spectra (in-situ DRIFTS) were collected from 1000 to 4000 cm–1 at a spectral resolution of 4 cm–1 (number of scans, 32) on a Nicolet 5700 FT-IR spectrometer equipped with a high-sensitive MCT detector cooled by liquid N2. The DRIFTS cell (Harrick) was fitted with a ZnSe window and a heating cartridge that allowed sample to be heated to 400 oC. The fine catalyst powder placed on a sample holder was carefully flattened to enhance IR reflection. The sample was purified with a high purified N2 stream from room temperature to 400 oC at 5 oC/min to eliminate the physisorbed water and other impurities. The sample background of each target temperature was collected during the cooling process. At ambient temperature, the sample was exposed to a controlled stream of NO-Ar (5% of NO by volume) or/and CO-Ar (5% of CO by volume) at a rate of 5.0 ml/min for 30 min to be saturated. Reaction studies were performed by heating the adsorbed species and the spectra were recorded at various target temperatures at a rate of 5 oC/min from room temperature to 350 oC by subtraction of the corresponding background reference. The Cu2O crystal structures were drawn using the Materials Studio 6.0 software. 6
2.3. Catalytic performances measurement The catalytic performances of these catalysts for NO reduction by CO model reaction were determined under steady state, involving a feed stream with a fixed composition, 5% NO, 5% CO and 90% He by volume as diluents. The sample (35 mg) was fitted in a quartz tube, and the mixed gases were switched on. The reactions were carried out at different temperatures with a space velocity of 15000 ml g–1 h–1. Two columns (length, 1.75 m; diameter, 3 mm) and two thermal conductivity detectors (T = 100 oC) were used to analyze the products. Column A with Paropak Q was used for separating CO2 and N2O, column B packed with 5A and 13X molecule sieve (40–60 M) was applied for separating N2, NO and CO.
3. Results and discussion 3.1. Morphology and structural characterizations XRD patterns of three Cu2O samples were presented in Fig. 1. Obviously, all of them showed typical diffraction pattern of Cu2 O. The diffraction peaks (d-Cu2O, o-Cu2O, c-Cu2O) could be ascribed perfectly to the pure cubic-phase Cu2O (JCPDS No. 65-3288, space group Pn3m, lattice constant a=0.4260 nm) [20]. No other copper species (CuO or Cu) peaks were detected in this pattern, suggesting the phase composition of the prepared samples were quite pure. The relative intensities of the peaks in the diffraction patterns were different due to the percentages of different crystal planes [21], which were showed in Table S1. It was observed that the percentage of (110) plane in dodecahedral Cu2O sample was 10.1 %, higher than the other two samples. Similarly, octahedral and cubic Cu2O samples had the highest percentages of (111) and (200) planes among the three shapes, respectively. Therefore, d-Cu2O, o-Cu2O and c-Cu2O samples were mainly enclosed by {110}, {111} and {100} crystal planes, respectively. The morphologies of as-synthesized Cu2O crystals were characterized by SEM and TEM. As shown in Fig. 2, these particles had grain sizes of about 500 nm with uniform shapes and well dispersion. The detailed formation mechanism of 7
different-shaped Cu2O was elucidated in the view of structure and dynamic theory. For cuprite Cu2O, cubic Cu2O {100} planes with 100% oxygen terminated surfaces had a minimum energy state, and thus the cubic shape of Cu2O crystals could be synthesized without surfactants. Octahedral Cu2O {111} planes possessed a high energy status, which were minimized the {111} surface energy by the PVP surfactants adsorption, leading to the stability of {111} planes and the accelerated growth of {100} planes [22]. As for dodecahedral Cu2O crystals, glucose, a weak reducer, made the reduction process slow and suppressed the nanoparticles to aggregate. In addition, oleic acid played an important role in controlling the dodecahedral shape by the face-selective adsorption [23]. The above synthesis resulted in the Cu2O nanocrystals with rhombic dodecahedral, octahedral and cubic shapes, which were denoted as d-Cu2O, o-Cu2O and c-Cu2O, respectively. TEM and HRTEM characterizations were further introduced to check the morphology and their corresponding exposed crystal planes. As shown in Fig. S1a (in the Supplementary Information), the projection image also confirmed the dodecahedral shape. HRTEM image demonstrated the preferred {110} planes with a lattice spacing of 0.308 nm in Fig. S1b. Similarly, the octahedral Cu2O could also be confirmed in Fig. S1c. These octahedral nanocrystals showed a plane spacing of 0.238 nm corresponding to {111} planes (Fig. S1d). Moreover, the cubic Cu2O was seen in Fig. S1e, which was enclosed by {100} planes with a plane spacing of 0.214 nm (Fig. S1f). The above morphology characterizations indicated that the d-Cu2O, o-Cu2O and c-Cu2O crystals were enclosed by twelve {110}, eight {111} and six {100} crystal planes, respectively. Meanwhile, the as-prepared Cu2O nanocrystals had uniform shapes and well-defined exposed crystal planes, which were ideal model catalysts to study the crystal-plane effects. Generally, capping ligands residual on crystal surface had great influences on the catalytic properties of nanocrystals [24-26]. In our preparation, the surfactants (oleic acid for d-Cu2O, PVP for o-Cu2O) were inevitably brought in to control the morphologies. Before testing the catalytic activities of d-Cu2O and o-Cu2O, we need to make sure that the surfactants had already been totally removed. Cu2O samples after the washing process (details could be seen in experimental section) were 8
checked by FT-IR, XPS and SEM-EDAX techniques. From Fig. S2a, the prepared o-Cu2O and d-Cu2O without washing were denoted as o-Cu2O (PVP) and d-Cu2O (OA), respectively; the prepared o-Cu2O and d-Cu2O with washing treatment were denoted as o-Cu2O and d-Cu2O, respectively. The FT-IR spectra demonstrated that only Cu-O vibration of Cu2O was in the spectra of o-Cu2O and d-Cu2O [27], no other characteristic peaks of polyvinylpyrrolidone (PVP) and oleic acid (OA) were observed, which confirmed o-Cu2O and d-Cu2O catalysts were free of PVP and OA surfactants, respectively. XPS spectrum (Fig. S2b) further indicated that PVP was removed from surface, due to no peak of N 1s [17,28]. SEM-EDAX results (Fig. S2c,d) demonstrated that there were only Cu, O, Si elements (Silicon film was used to support Cu2O samples for SEM analysis). While, carbon element considered to be contained in organics was not found in o-Cu2O and d-Cu2O samples. Therefore, it was identified that the o-Cu2O and d-Cu2O catalysts for catalytic performance measurement were free of PVP and OA surfactants, respectively. 3.2 Reduction properties (H2 or CO-TPR) The reduction properties of Cu2O with different morphologies were determined by hydrogen temperature-programmed reduction (H2-TPR). The profiles were shown in Fig. 3a. For all the three samples, there was only one peak belonging to the reduction of Cu+ to Cu0 [29]. Notably, the reduction temperatures of dodecahedral, octahedral and cubic Cu2O were dramatically distinct. The reduction of dodecahedral, octahedral and cubic Cu2O nanocrystals started at ca. 190, 220 and 250 °C, respectively. Therefore, from their reduction temperatures, the reducibilities were ranked by d-Cu2O {110} > o-Cu2O {111} > c-Cu2O {100}. CO-TPR profiles (Fig. 3b) displayed the similar tendency. The reduction of dodecahedral, octahedral and cubic Cu2O nanocrystals started at ca. 200, 270 and 300 °C, respectively. TPR results clearly demonstrated that Cu2O dodecahedrons were much easier to be reduced than octahedrons and cubes. Generally, the reduction of oxides always initially took place on the surface or near surface layer. For temperature-programmed reduction, it was actually the gas-solid 9
interfacial reaction, the reducibility of Cu2O crystals was completely determined by the chemisorption and activation of H2 or CO molecules on Cu2O crystal planes and the surface Cu-O bond strength. The schematic illustrations of the Cu2O (110), (111), (100) planes were present in Fig. S3a. On d-Cu2O (110) plane, the first layer consisted of both coordinated unsaturated O (OCUS) and coordinated saturated Cu (CuCSA), and only CuCSA were on the second layer. While for the o-Cu2O (111) and c-Cu2O (100), the first layer only consisted of OCUS; the second layer was made up of 75% CuCSA and 25% coordinated unsaturated Cu (CuCUS) for o-Cu2O (111); 100% CuCSA for c-Cu2O (100) [30]. The schematic illustrations suggested that d-Cu2O nanocrystal had more exposed Cu atoms on the first layer of the surface. Furthermore, the DRIFTS results of saturated CO adsorption at 35 oC were shown in Fig. S3b. There were two vibration bands at 2180 and 2110 cm–1. According to the literature, the band above 2130 cm−1 was ascribed to CO bonded to ionic species of metal [8], herein, the band at 2180 cm–1 was the signal of Cu+-CO, and the other vibration band at 2110 cm–1 was gaseous CO [7]. The relative area ratio of peaks centered at 2180 cm–1 and 2110 cm–1 were 1.46, 1.39, 1.40 for d-Cu2O, o-Cu2O, c-Cu2O, respectively, which showed that more CO molecules were adsorbed on d-Cu2O samples than that on the two others, and d-Cu2O nanocrystal has more exposed Cu atoms on the surfaces, because CO facilely chemisorbed on the CuI site instead of O site [28]. The number of terminal copper atoms per unit surface area on the d-Cu2O {110} facet was calculated by Huang et al., which was roughly 1.5 times higher than that on the {111} facet, and that on {100} facet was the lowest [31]. It was inferred that d-Cu2O crystal had significant reducibility, owing to its exposed {110} plane terminated by Cu. Compared with O, Cu not only had rich d-electrons, but also possessed empty orbits, which were inclined to chemical adsorption of H2 and CO molecules [32,33], and thus d-Cu2O {110} was easier to be reduced. Conversely, because of insensitivity to H2 and CO molecules for O-terminated {100} plane, c-Cu2O {100} was difficult to be reduced. For o-Cu2O, 25% CuCUS were present on the second layer of {111} plane and displayed a much stronger
10
chemisorption ability of H2 and CO than CuCSA, therefore, the reducibility of o-Cu2O {111} was poorer than d-Cu2O {110}, but better than c-Cu2O {100}. In a word, the crystal-plane-effects made significant influence on the reducibility of Cu2O nanocrystals. 3.3. Catalytic performances (NO+CO model reaction). The NO reduction by CO reaction was used to investigate crystal-plane effects on the catalytic performances of Cu2O crystals (d-Cu2O, o-Cu2O and c-Cu2O). For the NO+CO reaction, the reaction products were N2O, N2, and CO2. Fig. 4a displayed the NO conversion of different-shaped Cu2O crystals. Herein, T20 (the temperature at which the NO conversion was 20%) was used to evaluate catalytic performances at low-temperature range. For the d-Cu2O, the T20 was 316 oC. While for o-Cu2O and c-Cu2O, the T20 were 327 oC and 344 oC, respectively. The activity order was as following: d-Cu2O > o-Cu2O > c-Cu2O. It was observed that the NO conversion didn’t reach 100%, and thus it was deduced that the phenomenon of coke deposition might happen on the catalyst. The similar phenomenon had been reported on Cu-zeolite catalysts for the selective catalytic reduction of NO [34,35] and the detailed investigation on coke deposition would be carried out in our future research. Furthermore, the N2 selectivity of NO+CO reaction was further demonstrated that d-Cu2O was the best for the catalytic reduction of NO to N2, and the order was: d-Cu2O {110} > o-Cu2O {111} > c-Cu2O {100} (Fig. 4b). Since the BET specific surface area values revealed those of Cu2O crystals were almost equivalent (in Table S2), their difference in catalytic performances should originate from their distinct surface structures on crystal-planes. In order to unveil the essence of crystal-plane-controlled reaction, these Cu2O crystals after NO+CO reaction had been determined by SEM, TEM, XRD and XPS characterizations. The SEM results of the used Cu2O crystals were showed in Fig. 5. The shapes were preserved after the reaction, but the surfaces were changed. Some particles were generated on the surface, indicating that the componential and morphological evolution took place on different exposed crystal planes. Especially, the surface 11
changes were more obvious in d-Cu2O sample than o-Cu2O and c-Cu2O, implying d-Cu2O was more sensitive to the NO+CO reaction. In addition, the nanoparticles generated on the used Cu2O surfaces were determined by HRTEM (Fig. S4), it was suggested that the lattice fringes of the nanoparticles were 0.212 nm, which was ascribed to Cu0 (111) crystal plane. XRD patterns of the used Cu2O crystals were also collected and were demonstrated by Fig. 6. It could be found that besides the diffraction peaks of cubic-phase Cu2O, another diffraction peaks corresponding to Cu0 species (JCPDS No. 03-1005, space group Fm3m) were observed. Moreover, the amount of the in-situ generated Cu0 species were varied with the exposed crystal planes. Based on the diffraction intensity ratio of Cu0 (111) peak/Cu2O (111) peak, the content of Cu0 could be roughly estimated and ranked as following: d-Cu2O (1.39) > o-Cu2O (0.82) > c-Cu2O (0.35). XPS was used as a more direct characterization tool to analyze the surface Cu species. The XPS spectra of the fresh and used Cu2O nanocrystals were shown in Fig. 7. Generally, the Cu-LMM XAES spectra were used to distinguish the low values of copper species (Cu+ and Cu0), and XPS spectra of Cu 2p peak were adopted to determine Cu2+ species for the reason that the binding energies of low values of copper species were too close to depart in Cu 2p XPS spectra [36-38]. It was accepted that the peaks with kinetic energy of ca. 916.7 eV and 918.4 eV were assigned to Cu+ and Cu0 species in Cu-LMM XAES spectra, respectively [38]. Fig. 7a showed that only peak corresponding to Cu+ species was in the fresh samples. While, there was a broad peak composed of two peaks in the used samples (Fig. 7b), one peak was centered at ca. 918.4 eV which could be attributed to Cu0 species, and the other peak located at ca. 916.7 eV was ascribed to Cu+ species. Cu0 species generated with the sacrifice of partial Cu+ species. The relative area of peaks centered at 918.4 eV and 916.7 eV were in comparison, the results were displayed in Table S3, corresponding to the relative content of Cu0/Cu+ on the surface. Obviously, the generated Cu0 on the d-Cu2O {110} surface was more than others, which was in accordance with roughness degree from the SEM images and XRD results of the used catalysts. Based on the results of the used Cu2O catalysts, it was indicated that the 12
dodecahedral Cu2O with the advantageous {110} surface was more sensitive to NO+CO reaction conditions. Furthermore, the kinetic test was carried out in NO+CO reaction. The TOF (turnover frequencies) of different-shaped Cu2O crystals were calculated, and experimental details were shown in the supplementary information. The related TOF data at different temperatures for the d-Cu2O, o-Cu2O and c-Cu2O were in Table S4. It was observed that the TOF number of d-Cu2O {110} facet was roughly 1.5 times higher than that of the o-Cu2O {111} facet at 330 oC, and was roughly 2.5 times higher than that of the c-Cu2O {100} facet at 350 oC, which suggested that d-Cu2O had the superior catalytic performance. In addition, the Arrhenius plots for NO conversion over different-shaped Cu2O nanocrystals were displayed in Fig. S5. The activation energy was ranked by d-Cu2O (83 kJ/mol) < o-Cu2O (104 kJ/mol) < c-Cu2O (130 kJ/mol). Both of the two kinetic results were confirmed that the catalytic performance of NO+CO reaction was greatly affected by the crystal structures, and the surface reaction process was further recognized by in-situ DRIFTS. 3.4 NO or/and CO adsorption in-situ DRIFTS with different-shaped Cu2O NO and CO adsorption in-situ DRIFTS were recorded at various temperatures in the simulated reaction conditions, and the corresponding results were displayed in the Fig. 8. The band at about 1600, 1400 and 1300 cm-1 were attributed to carbonate species, the band at 2180 cm-1 was assigned to copper carbonyl (Cu+-CO) species; whereas, the bands at 1490, 1220 and 1080 cm-1 were vibrations of the nitrate species [39-41]. Obviously, neither nitrate species nor adsorbed NO could be detected below 200 oC, indicating that CO was preferentially adsorbed on Cu+ sites of the surface, because Cu+–NO species was not stabilized by the σ-π synergistic effect [42]. With the increase of temperatures, the intensity of Cu+–CO and carbonate bands were decreased, which indicated that Cu+ species was reduced by CO [40] and then the interface of Cu0 and Cu+ was created. In addition, the bands of adsorbed NO species were appeared. Afterward, the adsorbed NO species were dissociated on the generated Cu0/Cu+ interface, confirmed by the appearance of N2O band (2240 cm-1). Simultaneously, the partial Cu0 species was reoxidized by NO. With the further 13
increase of temperatures, more low-state copper species were generated and then the band of N2O was decreased, indicating that N2O was reduced to N2 on the surface Cu0 species, because Cu0 species was facilitate to N2O decomposition [43]. Compared with the three shaped Cu2O samples, it was observed that the intermediate N2O was first to appear and disappear on d-Cu2O {110}, consistent with the results of NO conversion and N2 selectivity. Why does d-Cu2O {110} have the excellent catalytic performance? In order to answer the question, NO adsorption in-situ DRIFTS experiments were carried out on the fresh and H2-pretreated Cu2O crystals (the pretreated temperatures were according to H2-TPR), the corresponding results were presented in Fig. 9. N2O species dissociated by NO could be observed on the H2-pretreated Cu2O crystals, while, that band was not obvious on the fresh Cu2O crystals, except for more other nitrates species [40,41]. The results demonstrated that the low-valence copper was conducive to the dissociation of the adsorbed NO species, because the low-valence copper with extra electrons could donate the electrons back to the anti-bonding orbital of the adsorbed NO species, which weakened the N–O bond and promoted the dissociation, this process was considered of the key step for NO reduction [36,44,45]. Therefore, in the reaction conditions, d-Cu2O {110} was more sensitive to CO, consistent with the reducibility results (CO-TPR), and then the adsorbed nitrate species was decomposed on the interface of Cu0 and d-Cu2O {110}, which resulted in the superior catalytic activity. Furthermore, the intensity of N2O peak on the H2-pretreated Cu2O crystals was ranked by d-Cu2 O (H2) < o-Cu2O (H2) < c-Cu2O (H2), which was seemed to contradict with the activity result. For NO molecule, it could dissociate to N2O and N2. The intensity of N2O band on d-Cu2O (H2) was weaker than that of the two others, because from the N2 selectivity results in Fig. 4b, the interface between Cu0 and d-Cu2O {110} made N2O rapidly dissociate to N2, and N2 had no IR active mode [46]. Therefore, the crystal-plane effects on catalytic properties might result from their different activation process in the in-situ structural transformation during the reaction. 3.5 Proposed reaction mechanism for NO+CO reaction over Cu2O catalysts 14
For the reaction mechanism of NO reduction by CO, it was not reached a consensus. Our previous study researched the influence of CO-pretreatment on the performance of NO catalytic reduction by CO on the supported binary metal oxides CuO-Mn2O3/γ-Al2O3 catalyst, and proposed that NO preferentially adsorbed on Mn2+ and
CO
was
on
Cu+
during
the
in
situ
FT-IR,
which
indicated
a
Langmuir-Hinshelwood (L-H) mechanism for two kinds of active sites (Mn2+ and Cu+) [40]. In addition, the Mars van Krevelen (redox) mechanism was proposed. Fernάndez-García et al. investigated the state of copper species on CuO/γ-Al2O3 catalyst during the NO+CO reaction by in-situ XANES, suggesting that the valence states of surface copper species on the catalyst were related with its catalytic performance [47]. In our present work, in-situ DRIFT demonstrated that the Mars van Krevelen mechanism worked in our system and the rapid change in valence state of copper species was a crucial part, which was related to the reducibility of catalysts. In addition, the key step for NO reduction by CO was the dissociation of adsorbed nitrogen oxide species. A schematic illustration about the reaction pathways was presented in Fig. 10. Firstly, CO molecules were preferentially adsorbed on the Cu2O surface. With the increase of the temperature, the low-state copper species were generated by CO reduction; the adsorbed carbonate species was disappeared, and nitrate species was adsorbed, afterward, the adsorbed NO species was decomposed to intermediate N2O; meanwhile, metallic Cu was partially oxidized to Cu2O. With the reaction proceeding, more low-state copper was generated, leading to the intermediate N2O to the final product N2, resulting in a catalytic cycle.
4. Conclusions In this work, the catalytic performance for NO+CO reaction by different-shaped Cu2O nanocrystals was demonstrated. Dodecahedral Cu2O {110} showed the best NO conversion, octahedral Cu2O {111} were followed, and cubic Cu2O {100} were the poorest. In-situ DRIFT suggested that the Mars van Krevelen mechanism worked in our system and the rapid change in valence state of copper species was a crucial part. 15
Thus, the reactivity was well correlated with their surface properties, especially the reducibility. Furthermore, NO adsorption in-situ DRIFT on the fresh and H2-pretreated Cu2O crystals indicated that the interface of Cu0/Cu+ was beneficial for the dissociation of adsorbed nitrogen oxide species, which was the key step. Therefore, d-Cu2O {110} had the superior catalytic performance, because {110} surface was more sensitive to NO+CO reaction conditions and then Cu0 nanoparticles were generated, leading to the decomposition of NO and the migration of the dissociated oxygen atoms on the Cu0/Cu+ interface. In conclusion, the crystal-plane-effects made significant influence on the catalytic behaviors, and in-situ structural transformation. Through this study, it was helpful to provide the scientific basis for Cu-based catalyst.
Acknowledgements Financial supports from the Natural Science Foundation of Jiangsu Province (BK2012298) and National Natural Science Foundation of China (Nos.21273110, 21203091) are gratefully acknowledged.
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Fig. 1. XRD patterns of as-prepared d-Cu2O, o-Cu2O and c-Cu2O.
20
Fig. 2. SEM images of different morphologies of d-Cu2O (a, b), o-Cu2O (c, d) and c-Cu2O (e, f).
21
Fig. 3. H2-TPR profiles (a) and CO-TPR profiles (b) of Cu2O with different morphologies.
22
Fig. 4. Catalytic performance: NO conversion (a) and N2 selectivity (b) over Cu2O with different morphologies.
23
Fig. 5. SEM images of different-shaped Cu2O after NO+CO reaction: d- Cu2O (a, b), o-Cu2O (c, d) and c-Cu2O (e, f).
24
Fig. 6. XRD patterns of d-Cu2O, o-Cu2O, c-Cu2O after NO+CO reaction, respectively.
25
Fig. 7. Cu-LMM XAES spectra of Cu2O with different morphologies before reaction (a) and after reaction (b).
26
Fig. 8. In-situ DRIFTS of NO and CO (5% and 5% in volume, respectively) co-adsorption on d-Cu2O (a), o-Cu2O (b) and c-Cu2O (c).
27
Fig. 9. In-situ DRIFTS of NO adsorption on d-Cu2O (a), o-Cu2O (b), c-Cu2O (c), H2-pretreated d-Cu2O (d), H2-pretreated o-Cu2O (e) and H2-pretreated c-Cu2O (f).
28
Fig. 10. Proposed reaction mechanism for NO+CO reaction over Cu2O (110) plane. Graphical abstract
29