Article pubs.acs.org/JPCC
Mechanistic Study of Gas-Phase Controlled Synthesis of Copper Oxide-Based Hybrid Nanoparticle for CO Oxidation Fu-Cheng Lee, Yi-Fu Lu, Fang-Chun Chou, Chung-Fu Cheng, Rong-Ming Ho, and De-Hao Tsai* Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, Republic of China S Supporting Information *
ABSTRACT: We report a systematic study of gas-phase controlled synthesis of copper oxides-based hybrid nanoparticles for catalytic CO oxidation. The complementary physical, spectroscopic, and microscopic analyses were conducted to obtain a better understanding of the material properties, including particle size, crystallinity, elemental composition, and oxidation state. Results showed that the synthesized nanoparticles exhibited highly durable catalytic activity and stability, also the particle size, crystallite size, and chemical composition were tunable by choosing suitable chemical compositions of precursors and temperatures. The crystallite size of CuO influenced the reducibility of CuO by CO and the subsequent catalytic activity of CO oxidation. The hybridization process of CeO2 and CuO induces the formation of new active sites at the Cu−Ce−O interface, which enhances reproducibility of CuO and the catalytic activity. However, the reproducibility of CuO and catalytic activity were considerably decreased when CeO2 was replaced with the inert Al2O3. This work describes a prototype method to form highly pure and well-controlled hybrid nanocatalysts, which can be used to establish the correlation of material properties versus reducibility and subsequent catalytic activity for energy and environmental applications.
1. INTRODUCTION Copper oxides (CuxO), especially in the form of particles with well-defined nanostructures, have shown to be an attractive alternative to noble metals.1−6 The CuxO-based nanoparticles (NPs) are cost-effective and also insensitive to nonselective poisoning, which has brought substantial interests for a variety of energy and environmental applications.2,5−12 The previous studies have shown that the interfacial contact could be greatly increased by decreasing physical size of conventional particle from microsized to nanoscales, which generates more active sites and reduces diffusion distance between the reactants.13−16 Hence, the accompanying catalytic performance has been proved to be enhanced by using CuxO nanocatalysts.16,17 Considering the further improvement in the catalytic activity of CuxO NPs, hybridization with functional components is a promising strategy.3,18−21 In principle, CuxO is recognized as an active site in the hybrid nanostructure. Other components (e.g., Al2O3 and CeO2 in this study) can be used to disperse CuxO to maximize the active surface area, to increase the number of active sites for the multiple types of reactants, and/or to improve stability in operation. The CuxO-based hybrid NPs have gained huge research potential in the areas of water−gas shift reactions22,23 and selective catalytic reduction of nitrogen oxides.24−27 The CuxO-based hybrid NPs can be obtained using the designed material, which enables a variety of catalysis in the nanostructure with high activity and stability.3 Since CuxO-based hybrid NPs are effective as nanostructures, yet they are of crucial in the optimization of the synthesis process to achieve a better performance. Hence, there is a necessity to develop suitable methods to fabricate hybrid NPs, © XXXX American Chemical Society
for which the physical and chemical properties can be both tunable. Solution-based approaches have been utilized extensively for the development of functional hybrid NPs.11,28 Although the solution-based approaches have demonstrated a facile capacity to synthesize the functional hybrid NPs in uniform size, shape, and elemental composition through a design in formulation chemistry, an obvious limitation is that solvent is used as a synthesis medium. The use of solvent is challengeable, as it is subject to have substantial limitations in physical and chemical properties (e.g., boiling point, solubility to precursor) upon the optimization of synthesis conditions (e.g., by tuning temperature and concentration of precursor). In addition, the crucial additives into formulation chemistry (e.g., using surfactants to control shape and colloidal stability of NPs) can further complicate the controlled synthesis and also the evaluation of corresponding performance. Therefore, it is essential to develop an approach without implementation of solution-based chemistry for fabricating functional hybrid NPs, optimizing their performance and also reducing their potential environmental hazards. In this work, we developed a gas-phase synthesis method for the systematic fabrication of CuxO-based hybrid NPs. In this method, the dilute solution of inorganic precursors was first nebulized into submicron droplets to create a number of liquid−air interfaces for the subsequent fast evaporation of droplets.29 The locally increased concentration due to fast Received: May 23, 2016 Revised: June 1, 2016
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DOI: 10.1021/acs.jpcc.6b05200 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. Schematic diagram of the gas-phase synthesis system equipped with a nebulizer, a drying unit, and an aerosol-based flow reactor.
2.2. Nanoparticle Synthesis with In Situ Differential Mobility Analysis (DMA). The experimental system, as depicted in Figure 1, includes a nebulizer, a drying unit, and a temperature-programmed flow reactor. The customized nebulizer was used to convert precursor solutions to become aerosolized droplets using a compressed filtered air at a flow rate of 1.5 L/min. Water in the aerosolized droplets were removed by the drying unit composed of a flow preheater (at 100 °C) and a diffusion dryer filled with the silica gel. Then aerosols of dried precursors were delivered to a flow reactor, which is made of quartz with an inner diameter (i.d.) of 2.2 cm, 60 cm in length. The flow reactor was placed in a tube furnace (HS-40, Huahsing, Taiwan, R.O.C.) with a heated length of 35 cm (i.e., corresponding to a heating time of 5.3 s), and the operating temperature of the tube furnace was ranging from 400 to 700 °C. In the flow reactor, the dried precursors were thermally decomposed to oxide NPs and then delivered downstream for sample collection and characterization. For the synthesis of CuxO-based NPs hybridized with CeO2 and Al2O3 (denoted as CuCeOx-NP and CuAlOx-NP, respectively), experiments were conducted using the molar ratios of Cu/(Ce + Cu) and Cu/(Al + Cu) at 33% and CCu = 5 wt %. The corresponding concentrations of Ce precursor (CCe) and Al precursor (CAl) were 14.1 and 9.5 wt %, respectively. Note that the particles synthesized were mainly singlet NPs (i.e., thermally decomposed from single droplets),40,41 and the peak droplet size was calculated as ≈375 nm (details of the calculation of droplet size distribution were shown in the Supporting Information, SI). Prior to sample collection, the diameter and the numberbased particle size distributions of aerosolized NPs were characterized by DMA. Briefly, the synthesized NPs were delivered to an electrostatic classifier (Model 3081, TSI Inc., MN, U.S.A.). As electric field was varied, NPs of a specific mobility diameter, dp,m, exited the electrostatic classifier, and were counted by a condensation particle counter (CPC, model 3775, TSI Inc.).42,43 The step size used in the particle size measurements was 2 nm, and the time interval between each step size was 10 s. Sheath flow rate and sample flow rate of the DMA were controlled at 10.0 and 1.2 L/min, respectively. 2.3. Ex Situ Nanoparticle Characterization. Particle size, crystallite size, and crystalline structure of NPs were imaged using a transmission electron microscope (TEM, JEM-2100HT, JEOL, Japan) at an acceleration voltage of 80 kV, a field emission high resolution TEM (HRTEM. JEM-3000F, JEOL) equipped with an energy dispersive spectrometer (EDS)
evaporation at the liquid−air interface induces precipitation and subsequent self-assembly of inorganic precursors to become a spherical cluster (dried precursor crystallites).30 After the gasphase thermal decomposition, the crystallites of dried precursors finally become an oxide NP with controlled size, chemical composition, and morphology.29−31 By using a multicomponent precursor solution, the dried precursors of each component were formed and self-arranged wherein, the evaporation-induced packing of dried precursor crystallites lead to formation of the particles with homogeneous distribution in chemical composition. The key advantages include the use of continuous flow gas-phase reactor, which effectively avoids the issues related to the restrictions arouse from solution-based chemistry described previously, and the initial homogeneous dispersions of the multiple components (i.e., at their molecular level homogeneity in solutions) can be maintained. Also, in gas phase synthesis approach, the hybrid nanoparticles with unique chemical compositions and structures can be prepared that can hardly be obtained in quasi-equilibrium condition.3,29 In addition, we choose a suite of suitable and multiple measurement techniques, including electron microscopy, differential mobility analysis, X-ray diffractometry, X-ray photoelectron spectroscopy, and CO-based temperature-programmed reduction analysis (CO-TPR), to provide a robust and comprehensive analysis of the formation process for CuxObased hybrid NPs. The CO oxidation is chosen as a model system, which is of considerable importance in the fields of gas purification, CO2 laser, gas detectors, fuel cells, and air pollution control in automobiles.2,3,10,18,32,33 CeO2 and Al2O3 are employed to be hybridized with CuxO in the present study, which have shown to be promising for improving thermal stability or catalytic performance of CO oxidation.2,4,10,18,34−39 Our objective is to fabricate CuxO-based hybrid NPs by design to effectively improve the catalytic performance in CO oxidation.
2. MATERIALS AND METHODS 2.1. Materials. 2.5-Hydrate cupric nitrate (Cu(NO3)2· 2.5H2O, as Cu precursor) and 9-hydrate aluminum nitrate (Al(NO3)3·9H2O, as Al precursor) were obtained from SigmaAldrich (98%. St. Louis, MO, U.S.A.). 6-Hydrate cerium nitrate (Ce(NO3)3·6H2O, as Ce precursor) were purchased from Showa Chemicals (99%. Showa Chemical Industry Co., Ltd., Tokyo, Japan). Biological grade 18.2 MΩ·cm deionized water (Millipore, Billerica, MA, U.S.A.) was used to prepare the precursor solutions. B
DOI: 10.1021/acs.jpcc.6b05200 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 2. Effect of temperature to the formation of the bare CuxO-based NPs. (a) Determination of Tdec required using TGA. (b) XRD patterns calcinated at Tdec = 400 °C, Tdec = 500 °C, and Tdec = 700 °C. (c) Representative TEM micrographs: (1) Tdec = 400 °C; (2) Tdec = 500 °C; (3) Tdec = 700 °C. Scale bars = 50 nm. (d) Differential mobility analysis (DMA) at Tdec = 400, 500, and 700 °C. (e-1) The cartoon depiction of the formation of bare CuxO-based NP: water evaporation, precipitation, and self-assembly of dried precursor crystallites, thermal decomposition [as shown in (e2)] and further sintering to form large crystallite of CuO (e-3) by the elevation of Tdec. The concentration of Cu precursor, CCu, was 5 wt %.
The X-ray photoelectron spectroscopy (XPS. PHI Quantera SXM, ULVAC-PHI, Chanhassen, MN, U.S.A.) was employed to obtain high-resolution spectra of Cu 2p, Al 2s, Ce 3d, and O 1s regions. The complete information on oxidation states and elemental ratios of CuCeOx-NP and CuAlOx-NP at the surface of hybrid structures were able to be analyzed by XPS. The CO-based temperature-programmed reduction (COTPR) was conducted using a customized gas-flow control system equipped with thermal conductivity detector (TCD, Model 2004A, China Chromatography Co., Taiwan, R.O.C.). A total of 30 mg of sample was placed in a temperatureprogrammed tube furnace using a U-shape quartz reactor (0.85 cm in i.d. and 20 cm in length). The reduction profiles were collected in the 10% CO/He gas mixture with a flow rate of 50 mL/min heated from ≈20 to 400 °C at a rate of 10 °C/min. The CO2 generated was removed from the gas flow by using a filter packed with molecular sieve 4 Å (Sigma-Aldrich) prior to enter TCD. 2.4. Catalytic Performance Tests. The catalytic CO oxidation was carried out in a fixed-bed reactor system using
operated at an acceleration voltage of 300 kV, and a scanning electron microscope (SEM, Hitachi SU8010, Hitachi, Japan) operated at 10 kV. The synthesized NPs were delivered to an electrostatic precipitator and deposited onto a copper grid, nickel grid, or silicon chip operated at a sample flow rate of ≈1.5 L min−1 and an electric field of −(2−5) kV cm−1. Due to electrostatic repulsion, the deposition-induced aggregation is negligible under these conditions.42,43 The thermogravimetric analysis (TGA, SDT Q600, TA Instruments, DE, U.S.A.) was employed to measure the change of sample mass versus heating temperature. First, the precursor solution used to form oxide NPs (e.g., Cu(NO3)2 aqueous solution for the bare CuxO-NP) was dried in a vacuum oven at 100 °C. Then 1.6−7.4 mg of dried precursors were measured by TGA. The operating temperature was from 25 to 800 °C at a constant heating rate of 10 °C/min. The air flow rate of TGA was 100 mL/min. The X-ray diffraction (XRD) was performed using a X-ray powder diffractometer (Ultima IV, Rigaku Cooperation, Japan), with Cu−Kα radiation (λ = 1.5406 Å) operated at 40 kV, 20 mA, and a scanning rate of 1.0°/min. C
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Figure 3. Effect of precursor concentration of Cu (CCu) on physical size and crystalline state of bare CuxO-based NPs. Tdec = 500 °C. (a) Particle size distributions measured by DMA. (b) Representative SEM images and histograms of primary diameter: (1) CCu = 0.1 wt %; (2) CCu = 1 wt %; (3) CCu = 5 wt %. Scale bars = 200 nm. (c) XRD patterns. CCu = 1 wt % and CCu = 5 wt %.
the temperature > 300 °C. XRD analysis (Figure 2b) shows a clear crystallinity of CuO, and dc of CuO (dc,CuO) was calculated as 13.7, 14.7, and 21.9 nm by heating at 400, 500, and 700 °C, respectively [i.e., using the CuO (111) peak at 2θ = 38.8° according to JPCDS file 80−0076)]. The XRD patterns confirm the effective conversion of the dried Cu precursor to CuO crystallites over the three operating temperatures of synthetic process (Tdec) studied, and dc,CuO increased with Tdec. Note that the concentration of Cu precursor (CCu) was 5 wt %. Figure 2c presents the representative TEM micrographs of bare CuxO-NPs synthesized over different Tdec and captured in situ (additional images available in SI). The concentration of Cu precursor (CCu) used was 5 wt %. It is clear that the morphology of bare CuxO-based NPs was spherical containing small crystallites of CuO. By increasing Tdec from 400 to 500 °C and 700 °C, the crystallites merged and the dc,CuO increased gradually from ≈10 to ≈15 nm and ≈25 nm, respectively. The trend is consistent with the results from XRD patterns (Figure 2b), indicating that CuO crystallites further sintered by the elevation of Tdec.
the same U-shaped quartz reactor described in section 2.3. The sample mass was (30−100) mg, and the total flow rate of the reaction gas was (50−100) cm3/min. The feed contained 17% CO, 17% O2, and 66% N2. An ice-cooled water condenser was used to trap excess water and other nonvolatile component downstream of reactor. Product and reactant analysis was carried out by a TCD-based gas chromatography (GC, Model GC1000TCD, China Chromatography Co., Taiwan, R.O.C.).
3. RESULTS AND DISCUSSION 3.1. Material Properties of the Bare CuxO NP. We begin by examining the formation of bare CuxO-based NP to study the key parameters governing material properties of CuxO in hybrid nanostructure (i.e., prior to the addition of CeO2 and Al2O3). TGA was employed to determine temperature required in the synthesis of NPs (i.e., to convert dried Cu precursor to CuxO-based NP). As shown in Figure 2a, a mass loss of ≈35% at the range of 200 to 300 °C was observed, showing that most of the dried Cu precursor was in the form of Cu2(OH)3NO3 and was capable of being thermally decomposed to CuO when D
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Figure 4. Analysis of primary particles of CuAlOx-NP and CuCeOx-NP vs the bare CuxO-based NP. CCu = 5 wt %. The molar ratios of Cu/(Cu + Al) and Cu/(Cu + Ce) were 0.33. CCu = 5 wt %. (a) TGA analysis: (1) CuCeOx-NP; (2) CuAlOx-NP. (b) Representative SEM images and histograms of primary diameter: (1) CuCeOx-NP; (2) CuAlOx-NP. Scale bars = 100 nm. (c) Particle size distributions measured by DMA. (d) HRTEM images with elemental mapping: (1) CuCeOx-NP; (2) CuAlOx-NP. Tdec used in (b)−(d) was 500 °C.
(>300 °C. A representative SEM image at Tdec = 400 °C is shown in Figure 2e-2 as an example). By further increasing Tdec, the crystallites of CuO NPs were simultaneously sintered, as we can see in Figure 2e-3 (Tdec = 700 °C). Figure 3 summarizes the effect of CCu on the physical properties of bare CuxO-based NPs. Tdec was constant at 500 °C. As shown in Figure 3a, the averaged dp,m increased from 59.6 to 64.2 and 66.3 nm by increasing CCu from 0.1 wt % to 1 and 5 wt %, respectively. From SEM images, the primary diameter of CuxO-NP also increased with CCu: 38.8 nm at CCu = 0.1 wt %, 52.3 nm at CCu = 1 wt %, and 62.8 nm at CCu = 5 wt % (Figure 3b). On the other hand, dc,CuO was shown to be less dependent on CCu based on XRD patterns (≈14.5 nm; Figure 3c) and TEM micrographs (≈16 nm; SI). 3.2. Material Properties of CuCeOx-NP and CuAlOx-NP. Material properties of CuCeOx-NP and CuAlOx-NP, the bicomponent hybrid NPs, were analyzed using complementary characterization techniques. As shown in Figure 4a-1, a threestage decline in the mass of precursor of CuCeOx-NP was measured by TGA, corresponding to complete transformation of [Cu2(OH)3NO3 + Ce2(OH) (NO3)5] to (CuO + CeO2) via thermal decomposition (details of calculations of TGA analysis were shown in SI). For CuAlOx-NP (Figure 4a-2), we observed
DMA was employed to measure particle size of synthesized NPs in situ, which has been reported as an important indicator to catalytic performance.3,14,16,44,45 As shown in Figure 2d, the average dp,m of bare CuxO-based NP were 71.9 nm at Tdec = 400 °C. When Tdec increased to Tdec = 500 and 700 °C, the average dp,m decreased to 64.6 and 52.0 nm, respectively. The trend of dp,m versus Tdec was consistent with the observation via SEM micrographs (SI). In fact, the number of crystallites per particle (N) decreased by 3 times with the increase of Tdec from 400 to 700 °C (SI), pointing to sintering of CuO crystallites. Note that N is calculated as dp,m3/dc,CuO3 by assuming both primary particle and crystallite of CuO are spherical. The results also indicate that DMA can be used to perform a fast, in situ characterization of nanocatalysts for the evaluation of diffusion resistance in the nanostructure. Figure 2e demonstrates a cartoon depiction of formation of bare CuxO-based NPs based on the results in Figure 2a−d. First, to prepare CuxO-based NPs crystallites, the precursor solution was nebulized to form submicron-sized droplets. The fast evaporation at the liquid−air interfaces of nebulized droplets induces precipitation (Cu2(OH)3NO3) in the form of a spherical cluster. Subsequently aerosols of Cu2(OH)3NO3 were converted to CuO in gas phase at a higher temperature E
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Figure 5. X-ray photoelectron spectroscopic (XPS) analysis of CuCeOx-NP and CuAlOx-NP vs the bare CuxO-based NP: (a) Cu 2p; (b) O 1s; (c-1) Ce 3d; (c-2) Al 2s; CCu = 5 wt %. The molar ratios of Cu/(Cu + Al) and Cu/(Cu + Ce) were 0.33. CCu = 5 wt %. Tdec = 500 °C.
HRTEM and XPS indicate that CuO was homogeneously distributed (i.e., no segregation at the surface of NPs), suggesting more active sites were created through the formation of hybrid nanostructures for subsequent catalysis. Note that no further segregation was found during the synthesis and the catalytic reaction (i.e., uniformly mixed in this study). The XPS results also provide information regarding the oxidation state of surface elements in the NPs. Figure 5a shows different peaks within the Cu 2p region typically identified as CuO (i.e., the presence of a higher Cu 2p peak at 933.3 eV, a shakeup peak between 940 and 944 eV, and peaks centered at 941.2 and 961.8 eV).2,10,33 As shown in Figure 5b, two peaks located at 529.2 and 531.3 eV in the O 1s region were assigned to lattice oxygen of copper oxides and other oxygen species adsorbed on surface, respectively. For CuAlOx-NP, the highintensity peak at 531.4 eV was attributed to the oxygen from Al2O3 (i.e., Al−O binding). The characteristic peaks in Ce 3d (Figure 5c-1) and Al 2s (Figure 5c-1) regions were close to the reported values of pure Al2O3 and CeO2, respectively. XPS results indicate surface composition of CuxO-based hybrid NPs can be adjusted directly by tuning the ratios of precursors in the solution, and the oxidation states of Cu, Ce, and Al in the hybrid nanostructures are identical to the states in the pure NPs (i.e., +2 for Cu, +4 for Ce, and +3 for Al). Figure 6a shows XRD patterns of the Cu-based NPs after hybridization with CeO2 and Al2O3. For CuCeOx-NP, the diffraction peaks located at 2θ = 28.68°, 34.61°, 47.55°, and
a 72% decline in total mass of dried precursor mainly by thermal decomposition of [Cu2(OH) 3NO 3 + Al 2(OH) (NO3)5] to (CuO + Al2O3). Note that the possibility of forming a subtle amount of CuAl2O4 during the employed conditions was found to be below the detection limit. This result indicates that hybrid precursors were able to completely transform into CuxO-based hybrid NPs through the thermal decomposition method when Tdec > 400 °C. Therefore, we choose Tdec = 500 °C for the synthesis of CuCeOx-NP and CuAlOx-NP. We also examined the morphology and the physical size of hybrid NPs using SEM. As shown in Figure 4b, both CuCeOxNP (Figure 4b-1) and CuAlOx-NP (Figure 4b-2) were spherical with a diameter of ≈62 nm, close to the value of bare CuxO-based NP. DMA results (Figure 4c) also confirm the average dp,m was relatively constant (≈63−67 nm) and close to the value of bare CuxO-NP, indicating that particle size was independent even after the hybridization with CeO2 and Al2O3 over the experimental conditions we studied. HRTEM images with elemental mapping were then used to examine the structure and the homogeneity at two different chemical compositions (i.e., CuCeOx-NP and CuAlOx-NP). As presented in Figure 4d, Cu atoms were homogeneously distributed in the hybrid nanostructure along with Ce (Figure 4d-1) or Al (Figure 4d-2). XPS results reported the molar ratios of Cu/(Cu + Al) and Cu/(Cu + Ce) were both ≈40%, close to the nominal value in the precursors (33%). The results of F
DOI: 10.1021/acs.jpcc.6b05200 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 6. Analysis of the crystallinity and the formation process of CuAlOx-NP and CuCeOx-NP vs the bare CuxO-based NP. CCu = 5 wt %. The molar ratios of Cu/(Cu + Al) and Cu/(Cu + Ce) were 0.33. (a) XRD patterns. (b) Representative HRTEM images of bare CuxO-based NP. Scale bar = 10 nm. (c) Representative HRTEM images of CuCeOx-NP. Scale bar = 10 nm. (d) Representative HRTEM images of CuAlOx-NP. Scale bar = 10 nm. (e) The cartoon depiction of the formation of CuAlOx-NP and CuCeOx-NP: water evaporation at a lower temperature (
