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. 2024 Jan 13;1(6):541–547. doi: 10.1021/cbe.3c00072

Oxidation–Reduction Treatment Initiated In Situ Redispersion of the Supported Cu/Al2O3 Catalyst

Didi Li 1, Zhen Wang 1, Shiqing Jin 1, Minghui Zhu 1,*
PMCID: PMC11835255  PMID: 39974604

Abstract

graphic file with name be3c00072_0006.jpg

Achieving a high dispersion of the supported catalyst is crucial for heterogeneous catalysts. However, such a goal is difficult to attain for copper-based catalysts with conventional preparation methods, especially at higher yet industrial-related loadings. In this study, we explore an oxidation–reduction treatment for in situ reconstruction of supported copper catalysts, promoting the redispersion of large Cu nanoparticles. Oxidation of the reduced Cu/Al2O3 catalyst turns large Cu nanoparticles into a hollow structure, which is attributed to the Kirkendall effect. The following reduction step produces reduced Cu nanoparticles with smaller diameters and an increased number of active sites. Such an oxidation–reduction treatment results in a remarkable two-fold increase in the activity of the Cu/Al2O3 catalyst, as compared to the untreated one, toward the methanol steam reforming reaction. Our findings present an innovative approach for the in situ improvement of the catalyst dispersion, holding great promise for heterogeneous catalytic applications.

Keywords: Cu/Al2O3 catalyst, oxidation−reduction treatment, in situ redispersion, methanol steam reforming

1. Introduction

Supported copper-based catalysts are the most commonly used catalysts in methanol steam reforming (MSR) due to their low price, high activity, and low CO selectivity.1 In addition, copper-based catalysts are also widely used in many other catalytic processes, such as methanol synthesis and the water–gas shift reaction.2−5 Cu dispersion influences catalytic activity by affecting the number of both metallic and metal–support interfacial sites.6−8 However, metallic Cu has a low cohesive energy of 3.48 eV and a Tammann temperature of 407 °C, making it difficult to stabilize as ultrasmall nanoparticles under elevated temperatures, particularly with high-weight loadings.9 While reducing the loading of the active metal can partly overcome the sintering attributed to the identical chemical potential of all nanoparticles surface,10 it also diminishes the number of active sites, significantly compromising catalytic performance.11 Therefore, achieving a high dispersion with sufficient active sites presents a significant challenge.

Current strategies that have been proposed to improve the performance of supported copper catalysts focus on synthesis, for example, developing new preparation methods,12 modifying the support,13,14 and doping with additional elements.15,16 However, these strategies are usually not favorable for high Cu loadings. Recent studies have also revealed that the gas environments have an impact on the surface morphology of copper-based catalysts, thus inspiring new design methodologies.17−20 For instance, supported Cu nanoparticles on ZnO exhibit a spherical shape after treatment with H2/H2O mixture and a disklike shape upon exposure to a H2/CO mixture.18 Introducing a mild oxidant N2O to Cu/SiO2 (30.0 wt % CuO) during alcohol dehydrogenation was found to result in the formation of more coordinatively unsaturated Cu sites without altering Cu dispersion, whereas the Cu nanoparticle size increased significantly after stronger oxidant O2 treatment.21 Exposing the commercial Cu/ZnO/Al2O3 catalyst with 40.0 wt % CuO loading to an H2/H2O/CH3OH/N2 mixture is found to accelerate the migration of ZnOx species onto the surface of metallic Cu0 nanoparticles without altering Cu dispersion.22 Although in situ manipulation of oxidation-prone supported metal catalysts without changing their composition has garnered significant attention, developing a feasible method for supported copper-based catalysts, however, remains a challenge.22,23

Herein, we develop an in situ strategy by utilizing an oxidation–reduction treatment to reconstruct the copper-based catalyst, leading to the redispersion of large Cu nanoparticles. Such a strategy is substantiated by the 2-fold increase in activity observed in a Cu/Al2O3 catalyst prepared by the traditional co-precipitation method. Combining various characterization techniques, including high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), in situ X-ray diffraction (XRD), H2 temperature-programmed reduction (H2-TPR), quasi in situ X-ray absorption spectroscopy (XAS), in situ diffuse reflectance Fourier transform infrared spectroscopy with CO as a probing molecule (CO-DRIFTS), and pulsed N2O chemisorption, the structural changes of the catalyst during oxidation–reduction treatment and their correlation with reactivity were revealed.

2. Experimental Details

2.1. Catalyst Preparation

The Cu/Al2O3 catalyst with 40 wt % CuO loading was prepared using the co-precipitation method. Briefly, 5.8 g of Cu(NO3)2·3H2O (Aladdin, 99.99%) and 14.2 g of Al(NO3)3·9H2O (Aladdin, 99.99%) were dispersed in 100 mL of deionized water with continuous stirring. A mixed solution of Na2CO3 (12.9 g, Aladdin, ≥99.5%) and NaOH (8.0 g, Aladdin, 99.9%) was gradually added at 60 °C under magnetic stirring. The resulting mixture was stirred for 1 h and aged for 18 h at this temperature. After aging, the residue was filtered and thoroughly washed with ultra-pure water until the pH was around 7, and the final residue was dried at 100 °C for 12 h. The catalyst powders were finally calcined in static air at 450 °C (heating rate of 2 °C min–1) for 2 h. The fresh Cu/Al2O3 catalysts were labeled as 40CuAl. Both 10CuAl and 30CuAl were synthesized by the same the co-precipitation method.

2.2. Catalyst Characterizations

A transmission electron microscope (TEM, ThermoFisher Talos F200X), equipped with an EDS detector, was used to analyze the morphology and elemental distribution of the catalyst.

X-ray diffraction (XRD) measurements were conducted on a Bruker D8-Advance X-ray powder diffractometer equipped with a Cu Kα ray source (λ = 0.154 nm), accelerating voltage of 40 kV, and detector current of 40 mA. A homemade reaction cell was used for the in situ XRD measurements. XRD patterns were collected continuously during the treatments with the 2θ values ranging from 30 to 80° and a scanning rate of 0.02°.

H2-TPR was carried out using a Tianjin Xianquan TP-5080B chemisorption apparatus. Initially, the sample (20–30 mg) underwent pretreatments. Specifically, for the H2-TPR of the fresh catalyst, it was pretreated by outgassing in N2 at 150 °C for 30 min. Subsequently, the sample was cooled to room temperature in a N2 flow. Finally, the temperature was ramped to 300 °C at a rate of 10 °C min–1 in 10%H2/N2 (30 mL min–1), and kept for 60 min, with the consumption of H2 monitored by the online thermal conductivity detector (TCD).

Quasi in situ X-ray absorption spectroscopy (XAS) experiments were conducted at BL14W1 at the Shanghai Synchrotron Radiation Facility. The samples were pretreated with a fixed-bed reactor and then transferred to a glove box without air exposure, where the sample was sealed within an Ar-protected quartz capillary.

In situ diffuse reflectance infrared Fourier transform spectroscopy of CO-adsorption (in situ CO-DRIFTS) was measured on a PerkinElmer Frontier Spectrum 3 equipped with an MCT detector and low void-volume cell (DR-A01, Jiaxing Puxiang Tech. Ltd.). Spectra were collected with a spectral resolution of 4 cm–1, and each spectrum was an average of 64 scans. After pretreatments, the catalyst was purged with Ar flow and cooled down to room temperature. The cell was fed with 0.05%CO/He at a rate of 30 mL min–1, with the spectrum continuously collected for 30 min.

Pulsed N2O chemisorption was conducted on a homemade fixed-bed apparatus. 30 mg of the catalyst was placed in a quartz tube reactor. After pretreatment, the catalyst was cooled to 50 °C under a He flow of 30 mL min–1. Then, a series of 90 μL of N2O pulses were injected into the He flowing via a six-port valve at intervals of 6 min until no N2O was consumed. The outlet gases (N2 and N2O) were analyzed by GC (Ruimin GC 2060, GC), equipped with a 13x column and a TCD.

2.3. Catalytic Performance Tests

The methanol steam reforming reaction was carried out in a plug-flow fixed-bed reactor. Before the test, 20 mg of the catalyst and 80 mg of quartz sand with 60–80 mesh were loaded into a 4 mm inner diameter quartz tube and pretreated in situ. An aqueous methanol solution with a specific water/methanol ratio of 1.3 was injected into the heated chamber by a syringe pump at a rate of 0.0084 mL min–1 to evaporate the liquid. The evaporated mixture and 30 mL min–1 carrier gas (Ar, 99.999%) were introduced into the reactor. The reactor effluent was separated in a phase separator and analyzed by online gas chromatography equipped with a TCD and a flame ionization detector (FID). It is speculated that H2, CO, and CO2 were the primary products of methanol steam reforming. Other possible products such as CH4 were not detected above the FID detection limit. The methanol conversion and hydrogen productivity were calculated as follows:

2.3. 1
2.3. 2

where F(CH3OH, in) is the inlet molar flow rate of CH3OH, F(x, out) represents the outlet molar flow rate of x (x = CO, CO2, or H2), and mcat is the mass of loaded catalyst.

3. Results and Discussion

3.1. MSR Activity of Catalyst with Oxidation–Reduction Treatment

The Cu/Al2O3 catalyst containing 40 wt % CuO, denoted as 40CuAl, was prepared by a traditional co-precipitation method with NaOH and Na2CO3 mixture solution as the precipitation agent. The 40CuAl catalyst was reduced at 300 °C (denoted as 40CuAl-R) and then evaluated for the MSR at 200 °C for 2 h, where it showed hydrogen productivity of 40–37.8 mmol/gcat/h with the methanol conversion of 2.95 ± 0.05% to 2.75 ± 0.05% (Figure 1a and Figure S1). It is worthwhile to mention that the reaction condition (temperature and weight hour space velocity (WHSV)) was chosen to reduce the effect of transport limitation. After 2 h of steady-state MSR reaction, 4 vol % O2 was added into the MSR reactant gases for another 2 h, which completely stopped the hydrogen production. This is due to the occurrence of the complete oxidation of methanol.24 Surprisingly, after O2 is removed, the hydrogen productivity increases to 80 mmol/gcat/h, demonstrating the promotional effect of the oxidation–reduction treatment on the catalyst. We further examined the influence of O2 + H2O, O2 + H2, O + MeOH, O2 + MeOH, O2 + CO, O2 + H2, and O2 treatments on the reduced 40CuAl catalyst (Table S1). It turns out that compared to the untreated one, the catalyst exhibits higher activity as long as the treatment environment involves O2 (Figure 1b and Figure S2). Therefore, we simplified the treatment procedure to an oxidation–reduction sequence, involving 4% O2/Ar as the oxidizing agent and 10% H2/N2 as the reducing agent.

Figure 1.

Figure 1

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(a) Hydrogen productivity for the 40CuAl catalyst in the MSR reaction during which 4 vol % O2 was added to the gas feed. (b) Hydrogen productivity of the 40CuAl catalyst subjected to various atmosphere treatments. (c) H2 productivity of the 40CuAl-R, 40CuAl-ROR200, and 40CuAl-ROR300 catalysts in MSR. (d) In situ XRD patterns of the 40CuAl catalysts during the oxidation–reduction treatment.

We also investigated the effect of reduction temperature on the catalytic performance. 40CuAl-R was first subjected to an oxidative treatment using 4% O2/Ar at 200 °C, referred to as 40CuAl-RO. Subsequently, the catalyst was re-reduced with 10% H2/N2 at either 200 °C or 300 °C, with the catalyst denoted as 40CuAl-ROR200 or 40CuAl-ROR300, respectively. There is a 3-fold increase in hydrogen productivity for 40CuAl-ROR200 (141 mmol/gcat/h), surpassing the original untreated 40CuAl-R catalyst (40 mmol/gcat/h) (Figure 1c and Figure S3). However, the hydrogen productivity of 40CuAl-ROR300 with a higher reduction temperature, although still presented to be higher than that of 40CuAl-R, decreases to ∼80 mmol/gcat/h. The decrease in activity with increased reduction temperature is likely due to the sintering of copper nanoparticles.25

3.2. Structure of Catalyst with Oxidation–Reduction Treatment

In situ XRD was utilized to investigate the evolution of the bulk structure during oxidation–reduction treatment (Figure 1d and Figure S4). The XRD pattern of the fresh 40CuAl catalyst displays a single diffraction peak at 2θ = 35.2°, corresponding to CuO (111). After reduction (40CuAl-R), Cu (111), (200), and (220) peaks appear. The average crystalline size of the metallic Cu nanoparticles was estimated to be 11.3 ± 0.6 nm by using the Scherrer equation. Upon exposure to 4% O2/Ar at 200 °C, metallic Cu peaks vanish, concomitant with the appearance of Cu2O peaks, confirming the transformation of metallic Cu mainly into Cu2O during the oxidation process. Surprisingly, the average diameter of the newly formed Cu2O is only 6.1 ± 0.5 nm, which is much smaller than that of metallic Cu nanoparticles before oxidation. This intriguing observation can be due to the Kirkendall effect during the oxidation of Cu nanoparticles.26 Afterward, the introduction of 10%H2/N2 results in the re-reduction of Cu2O back into metallic Cu, as confirmed by the reappearance of metallic Cu peaks. The average size of Cu nanoparticles of 40CuAl-R after oxidation–reduction treatment (40CuAl-ROR200) (8.7 ± 0.6 nm) is slightly lower than that of the untreated catalyst (40CuAl-R) (11.3 ± 0.6 nm). Furthermore, a higher re-reduction temperature results in an increase in Cu particle size (12.8 ± 0.9 nm), suggesting that higher re-reduction temperatures lead to the sintering of Cu nanoparticles.

The formation of Cu2O after oxidation treatment can also be demonstrated by H2-TPR. Fresh 40CuAl shows a reduction peak at 297 °C, but this shifts to approximately 206 °C for the 40CuAl-RO catalyst (Figure S5). Notably, the 40CuAl-R mildly oxidized by O2 exhibits a peak temperature marginally higher than when oxidized by N2O, the latter typically oxidizing the near-surface of metallic Cu into Cu2O.17,27 Quantification of the H2-TPR results shows that the hydrogen consumption of 40CuAl-RO (2.3 mmol gcat–1) is slightly higher than half of that of the fresh catalyst (4.1 mmol gcat–1) (Table S3), further confirming that Cu predominantly exists as Cu2O with traces of CuO for the 40CuAl-RO catalyst.

We further analyzed the morphology of the 40CuAl catalyst at different stages during the oxidation–reduction sequence by HAADF-STEM. The 40CuAl-R catalyst exhibits a broad Cu particle size distribution, encompassing both small and large copper nanoparticles, with an average size of 11.7 ± 1.9 nm (Figure 2a and Figure S6a). For the oxidation-treated catalyst (40CuAl-RO), the Cu species exhibit a higher degree of dispersion based on energy-dispersive X-ray spectroscopy (EDS) (Figure 2b and Figure S6b). Interestingly, it can be seen that the large Cu nanoparticles form a hollow structure, which confirms the occurrence of the Kirkendall effect (Figure 2b and Figure S6b).28 For the 40CuAl-ROR200 catalyst, we observe hollow structures primarily consisting of smaller Cu nanoparticles (Figure 2c and Figure S6c), likely from the breakage of the aforementioned hollow-structured Cu nanoparticles after re-reduction. In addition, the Cu particle size distribution is narrower than that of 40CuAl-R, with a mean size of 9.6 ± 0.6 nm, which is consistent with the in situ XRD results.

Figure 2.

Figure 2

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HAADF-STEM images of the (a) 40CuAl-R, (b) 40CuAl-RO, and (c) 40CuAl-ROR200 catalysts and corresponding elemental map of Cu, Al, and O. Scale bar, 10 nm.

3.3. Surface Properties of Catalyst with Oxidation–Reduction Treatment

The electronic structure and coordination state of 40CuAl after the oxidation–reduction treatment were investigated with quasi in situ XAS, which involves vacuum transfer of the sample after pretreatment without exposure to air. X-ray absorption near-edge structure (XANES) spectrum of 40CuAl-R and 40CuAl-ROR200 closely resembles Cu foil (Figure 3a), showing that Cu mainly exists as metallic copper (Cu0). The extended X-ray absorption fine structure (EXAFS) spectrum of the 40CuAl-R catalyst features a single Cu–Cu scattering at ∼2.2 Å with a coordination number (CN) of 4.3 (Figure 3b and Table S4). For the 40CuAl-ROR200 catalyst, the CN of Cu–Cu scattering decreases slightly to 3.7, indicating that the oxidation–reduction sequence redisperses Cu nanoparticles, which is consistent with the in situ XRD and HAADF-STEM results.

Figure 3.

Figure 3

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Quasi in situ (a) XANES spectra and (b) Fourier-transformed EXAFS of the Cu K-edge for the 40CuAl catalyst after different treatments. (c) Pulsed N2O chemisorption and (d) in situ CO-DRITFS results of the 40CuAl catalyst after different treatments.

In situ XRD, HAADF-STEM, and quasi in situ XAS analyses suggest that oxidation–reduction treatment induces the redispersion of large Cu nanoparticles. The formation of a hollow structure, resultant from the Kirkendall effect during oxidation, undergoes fragmentation, yielding smaller copper nanoparticles or clusters upon re-reduction (Scheme 1). Pulsed N2O chemisorption was further employed to quantitatively compare the change in the number of Cu surface sites resulting from the oxidation–reduction treatment (Figure 3c). The N2O consumption of the 40CuAl-R catalyst is determined to be 41.7 μmol/gcat. Remarkably, after the oxidation–reduction treatment (40CuAl-ROR200), the N2O consumption increases to 152.1 μmol/gcat. Increasing the re-reduction temperature results in the decrease of N2O consumption to 90.0 μmol/gcat for 40CuAl-ROR300, again showing that the reduction temperature should not be too high. It is worthwhile to mention that the order of N2O consumption is in excellent agreement with their catalytic activities. In situ CO-DRIFTS was also used to investigate the electronic states of the topmost surface of Cu in the 40CuAl catalyst after oxidation–reduction treatment (Figure 3d). For the 40CuAl-R catalyst, the CO adsorption peak can be fitted into two peaks (Figure 3d and Table S5), one at 2100 cm–1 belongs to CO adsorbed on Cu0 sites (86%),29 and the other at 2132 cm–1 is associated with larger Cu nanoparticles.17 When compared with 40CuAl-R, 40CuAl-ROR200 shows a markedly enhanced CO adsorption intensity, with the peak related to large Cu nanoparticles dropping from 14% to 5%. This finding indicates that oxidation–reduction treatment is beneficial for the redispersion of larger Cu nanoparticles. In addition, we also noticed that the CO adsorption peak of the 40CuAl-ROR200 catalyst exhibits the smallest full width at half-maximum (FWHM) (24 cm–1) among these samples, which is another indication of the presence of small and uniform Cu nanoparticles.30,31

Scheme 1. Schematic Diagram of Catalyst Structure Evolution during the Oxidation–Reduction Treatment.

Scheme 1

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3.4. In Situ Redispersion of Catalyst with Oxidation–Reduction Treatment

Overall, the above characterization techniques reveal that the enhanced Cu dispersion in the reduced 40CuAl after oxidation–reduction treatment is attributed to the redispersion of large Cu nanoparticles. Although the Kirkendall effect has been frequently mentioned for the oxidation of Co-based catalyst in Fischer–Tropsch synthesis,28,32 the application in the supported copper-based catalyst has never been reported to the best of our knowledge. To further verify the structural modification, we synthesized 10 and 30 wt % Cu/Al2O3 (denoted as 10CuAl and 30CuAl) using the same co-precipitation method. Lowered Cu loading leads to smaller average particle sizes of Cu,33 which also decreases after the oxidation–reduction sequence. The average size of Cu nanoparticles during in situ XRD for 10CuAl-R (4.1 ± 0.5 nm) and 30CuAl-R (5.7 ± 0.3 nm) after the oxidation–reduction treatment show slightly lower than those of 10CuAl-R (4.6 ± 0.5 nm) and 30CuAl-R (6.4 ± 0.4 nm) without treatments (Figure S7). Besides, the intensity of the CO adsorption peak during in situ CO-DRIFTS for 10CuAl-ROR200 and 30CuAl-ROR200 also appears to be slightly higher than that of 10CuAl-R and 30CuAl-R, respectively (Figure 4b). It is worth noting that the Cu nanoparticle size of 40CuAl-R (11.3 ± 0.6 nm) is higher than those of 10CuAl-R (4.6 ± 0.5 nm) and 30CuAl-R (6.4 ± 0.4 nm). Correspondingly, oxidation–reduction treatment results in a large difference in the average size of Cu nanoparticles for 40CuAl (from 11.3 ± 0.6 nm to 8.7 ± 0.6 nm), which also shows a large improvement in hydrogen production rates. In contrast, both 10CuAl-R and 30CuAl-R show similar Cu nanoparticle size. Thus, oxidation–reduction treatment results in similar extent of reduction in Cu nanoparticle size (10% for 10CuAl and 11% for 30CuAl) and similar extent of improvement in hydrogen production rates. These findings reveal a size-dependent Kirkendall effect, where larger Cu nanoparticles form more readily,34 inducing the redispersion of large Cu nanoparticles during the oxidation–reduction treatment.

Figure 4.

Figure 4

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(a) Hydrogen productivity of the Cu/Al2O3 catalyst with different loadings and treatments and increment in activity comparison. (b) In situ CO-DRITFS of 10CuAl and 30CuAl after various treatments. (c) Hydrogen productivity and (d) in situ CO-DRITFS of the 40CuAl-R(OR200)n catalysts.

Based on the observation that the oxidation–reduction sequence redisperses large Cu nanoparticles, we conducted multiple oxidation–reduction treatments on the 40CuAl-R catalyst, denoted as 40CuAl-R(OR200)n, where n represents the treatment cycles. It is shown that the 40CuAl-R(OR200)2 and 40CuAl-R(OR200)3 catalysts exhibit higher activity than the 40CuAl-ROR200 catalyst, attaining a hydrogen productivity of 160 mmol/gcat/h (Figure 4c). Accordingly, the disappearance of the CO adsorption peak assigned to large Cu nanoparticles (2132 cm–1) and the decrease in the FWHM were observed for the 40CuAl-R(OR200)2 and 40CuAl-R(OR200)3 catalysts (Figure 4d and Table S5). Therefore, these findings conclusively indicate that the oxidation–reduction treatment enhances the redispersion of large Cu nanoparticles, making this method more feasible and promising for practical applications.

4. Conclusions

In conclusion, we propose an in situ catalyst reconstruction method, involving oxidation–reduction treatment, to redisperse large Cu nanoparticles. Oxidation of the metallic Cu catalyst yields a hollow structure in the larger Cu nanoparticles via the Kirkendall effect. This effect is more pronounced in larger Cu nanoparticles. Upon re-reduction at mild temperature, the catalyst acquires a much-increased number of Cu surface active sites. As a result, such an oxidation–reduction treatment results in a remarkable 3-fold increase in the hydrogen productivity of the Cu/Al2O3 catalyst, as compared to the untreated Cu/Al2O3 catalyst, toward the MSR reaction. Besides, multiple oxidation–reduction treatments further enhance the hydrogen productivity of the Cu/Al2O3 catalyst. This in situ reconstruction method, without altering the composition of the catalyst, offers the potential for both activation and regeneration of copper-based and other metal-based catalysts.

Acknowledgments

This work was supported by the National Key R&D Program of China (2022YFB3805504), the National Natural Science Foundation of China (22078089), the China Postdoctoral Science Foundation (2023M731081), the Shanghai Special Program for Fundamental Research (22TQ1400100-7), the Basic Research Program of Science and Technology Commission of Shanghai Municipality (22JC1400600), SINOPEC (No. 421056), the Open Foundation of Shanghai Jiao Tong University Shaoxing Research Institute of Renewable Energy and Molecular Engineering (Grant No. JDSX2022046), and the Shanghai Super Postdoctoral Fellow program.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/cbe.3c00072.

  • Methanol conversion plots and bar charts, XRD patterns, H2-TPR spectra, HAADF-STEM images; tables of various atmospheres, pretreatment conditions, quantification of the H2-TPR results, fitted structural parameters, and fitting results (PDF)

Author Contributions

# D.L. and Z.W. contributed equally.

The authors declare no competing financial interest.

Supplementary Material

be3c00072_si_001.pdf (515.5KB, pdf)

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