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Article

Synthesis of Well-Crystallized Cu-Rich Layered Double Hydroxides and Improved Catalytic Performances for Water–Gas Shift Reaction

by
Shicheng Liu
1,
Yinjie Hu
1,
Qian Zhang
1,
Xia Tan
1,
Haonan Cui
1,
Fei Li
1,2,
Huibin Lei
1,2 and
Ou Zhuo
1,2,*
1
College of Chemistry and Chemical Engineering, Jishou University, Jishou 416000, China
2
Hunan Province Key Laboratory of Mineral Cleaner Production and Green Functional Materials, Jishou University, Jishou 416000, China
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(6), 546; https://doi.org/10.3390/catal15060546
Submission received: 5 May 2025 / Revised: 26 May 2025 / Accepted: 29 May 2025 / Published: 30 May 2025
(This article belongs to the Special Issue Sustainable Catalysis for Green Chemistry and Energy Transition)
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Versions Notes

Abstract

Cu-based layered double hydroxides (LDH) have been extensively employed as catalyst precursors. However, due to the Jahn–Teller effect of copper ions, it is a challenge to synthesize well-crystallized LDH with a high Cu content, which usually contains considerable CuO impurity. By adding competitive ligands during the coprecipitation process, such as glycine, a well-crystallized Cu-rich LDH with less CuO impurity was successfully synthesized. The Cu-Mg-Al mixed oxides derived from the well-crystallized Cu-rich LDH have relatively high SBET, large pore volume, and well dispersion of Cu nanoparticles. The derived catalyst exhibited unexpectedly high catalytic activity in the water–gas shift (WGS) reaction, and the mass-specific reaction rate was reached as high as 33.5 μmolCO · g cat − 1 ·s−1 at 200 °C. The high catalytic activity of this catalyst may originate from the high SBET and well dispersion of Cu particles and metal oxides. Moreover, the derived catalyst also displayed outstanding long-term stability in the WGS reaction, which should benefit from the enhanced metal–support interaction.

1. Introduction

Layered double hydroxides (LDH) are excellent precursors for constructing highly dispersed supported-metal catalysts [1,2]. Cu-based LDHs have been extensively employed as precursors for synthesizing various catalysts, such as selective oxidation [2], selective hydrogenation [3,4,5], wastewater decontamination [6,7], hydrogen production [8,9], catalysts, etc. The water–gas shift (WGS) reaction is an industrially important reaction for transforming the CO contaminant of reforming gas into H2 production [9,10]. The Cu/ZnO/Al2O3 and Cu/ZnO catalysts have been applied in the hydrogen production industry since the 1960s [11]. However, these catalysts demonstrate low stability and tend to sinter [9,11]. Benefitting from well dispersion of Cu species and enriched metal-support interfaces, Cu-based LDH has focused considerable attention on developing high-performance WGS catalysts [12,13,14,15,16].
Cu-Zn-Al LDH has been proposed as an ideal precursor of the Cu/ZnO/Al2O3 catalyst [15,17,18]. The derived catalysts present a control microstructure with enhanced metal–oxide interactions and high dispersion of Cu nanoparticles [14,17,18]. Subsequent studies reported that the optimized catalysts derived from Cu-Zn-Al LDH exhibited better activity, thermal stability, and long-term stability than a commercial Cu/ZnO/Al2O3 catalyst [11,19]. Moreover, other types of Cu-based LDH have also been intensively studied for their catalytic performance in WGS reaction, such as Cu-Mg-Al LDH [20,21], Cu-Zn-Cr LDH [20], and Cu-Mn-Al LDH [16]. Our earlier studies found that the catalysts derived from Cu-Mn-Al LDH are rich in metal–support interfaces and oxygen vacancies [16], which are considered as intrinsic active centers for the WGS reaction [22,23,24]. Moreover, we have observed that the Cu content has an important influence on the catalytic activity of the catalysts [16]. Ahn and co-workers’ results also showed that the Cu content mainly affected the number of active Cu sites and was closely related to the WGS activity in the Cu/ZnO/Al2O3 catalyst derived from Cu-Zn-Al LDH [25].
Due to the Jahn–Teller effect of Cu2+ ions, it is difficult to synthesize LDH with a high Cu content, which usually contains considerable CuO impurity [14,26,27]. The CuO impurity comes from the thermal decomposition of the unstable copper hydroxide medium, which is formed due to local supersaturation of alkali concentration in the mixed solution during the coprecipitation process of the LDH synthesis period. By reducing the operating temperature and suppressing local alkali supersaturation of the coprecipitation process, the impurities of Cu-based LDH were obviously reduced, and the crystallinity was improved [16,27]. Even so, it is still a challenge to obtain Cu-based LDH with high copper content, high phase-purity, and a large specific surface area (SBET).
Based on the problem discussed above, we envision utilizing the coordination capability of organic ligands with Cu2+ to inhibit the formation of copper hydroxide media during the coprecipitation process of the preparation period and thereby reduce CuO impurity in the LDH products. The experimental results show that adding organic ligands during the coprecipitation process, such as glycine, oxalate anion, and iminodiacetate anion, can effectively improve the phase purity of Cu-based LDH and effectively reduce the content of CuO impurity. The mixed oxides derived from these Cu-based LDH samples have higher specific surface area, larger pore volume, and well dispersion of Cu particles, and they exhibit unexpectedly high catalytic activity and long-term stability in the WGS reaction.

2. Results and Discussion

In order to obtain well-crystallized Cu-rich LDH, we attempted to utilize the competitive coordination capability of organic ligands with Cu2+ to suppress the formation of CuO impurities during the coprecipitation process of the synthesis period. Glycine, disodium oxalate, trisodium citrate, disodium iminodiacetate, and ethylene diamine were chosen as competitive ligands and added into the mixed solution before the coprecipitation process, as shown in Figure 1a. A series of corresponding LDH samples were prepared and labelled as CuxMgyAlz-LDH-g, CuxMgyAlz-LDH-o, CuxMgyAlz-LDH-c, CuxMgyAlz-LDH-i, and CuxMgyAlz-LDH-e, respectively. The x, y, and z subscripts are the designed metal content (molar percent). For comparison, the control sample was prepared without adding organic ligands, using the same method, and labelled as CuxMgyAlz-LDH.
The X-ray diffraction (XRD) patterns of the Cu-Mg-Al LDH samples are shown in Figure 1b. Besides the diffraction peaks of the LDH phase (JCPDS No. 51-1525), there is a remarkable diffraction peak at 35.5° for CuO impurity ((-111) facet) in the Cu70Mg15Al15-LDH sample (the control sample) without adding a competitive ligand. However, for the Cu70Mg15Al15-LDH-g and Cu70Mg15Al15-LDH-i samples, no obvious diffraction signal for CuO impurity was observed. Moreover, the visibly lighter colors of the Cu70Mg15Al15-LDH-g and Cu70Mg15Al15-LDH-i samples compared to the control sample also indicate that the samples contain less CuO, since CuO is dark black (Figure S1). These results demonstrate that glycine and iminodiacetate can effectively reduce CuO byproduct, as expected. For the Cu70Mg15Al15-LDH-e and Cu70Mg15Al15-LDH-o samples, the intensities of the characteristic diffraction peak for CuO are distinctly lower than that of the control sample. The XRD pattern of the Cu70Mg15Al15-LDH-c sample does not show any apparent diffraction peak for LDH. Moreover, the thermogravimetry-mass (TG-MS) results illustrate that the thermal decomposition temperature of the Cu70Mg15Al15-LDH-g sample is considerably higher than that of the control sample (Figure S2). It indicates that the sample prepared by using glycine as a competitive ligand has been much improved in crystallinity. These results all showed that the competitive ligands remarkably affect the crystal formation and growth of Cu-Mg-Al LDH and the CuO byproduct during the coprecipitation process.
The specific surface area (SBET) and pore volume of the Cu-Mg-Al mixed oxides, obtained by calcination of the Cu-Mg-Al LDH samples, are presented in Figure 1c. The mixed oxide samples, prepared by using glycine, iminodiacetate, and oxalate anion as competitive ligands, show a significant improvement in SBET and pore volume. The mixed oxides derived from the Cu70Mg15Al15-LDH-g sample have a SBET as high as 102.8 m2·g−1, nearly three times that of the control sample. The mixed oxides derived from Cu-rich LDH rarely have high SBET [14,27,28,29] because CuO particles are prone to sintering in the calcination process due to low Hüttig (CuO: 243 °C) and Tammann temperatures (CuO: 526 °C) [16,30]. The SBET and pore volume of the mixed oxides derived from the Cu70Mg15Al15-LDH-e sample, in which more CuO impurities were observed (Figure 1b), are even lower than those of the control sample. These results indicate that the high SBET and pore volume should be closely correlated with less CuO impurity and/or the well-crystallinity of the LDH precursors. In addition, pore size distributions of the calcined Cu-Mg-Al LDH samples were also influenced by the competitive ligands (Figure S3).
Catalysts 15 00546 g001
Figure 1. Effects of different competitive ligands on the structures of Cu-Mg-Al LDH, the derived mixed oxides, and the catalytic activities of the derived catalysts. (a) Schematic illustration of the role of competitive ligands during the preparation process of Cu-based LDH; (b) XRD patterns for the LDH samples; (c) SBET and pore volume for the calcined samples; (d) CO conversion of WGS reaction for the activated samples; and (e) a summary of the mass-specific activities of Cu-based WGS catalysts in this study and previous references. The adding amount of the competitive ligands was 2.5% of the total amount of metal cations (nligand/(nCu + nMg + nAl) = 0.025). WGS reaction conditions: CO/H2/CO2/N2 = 14.6:27.0:9.2:49.2 (molar ratio), 50 mg catalyst, CO/H2O = 1/4. The weight hourly space velocity (WHSV) is 24,000 mL·g−1·h−1 (dry-gas base). The dotted line in (d) indicates the thermodynamic equilibrium CO conversion calculated by the reported method [31]. The references of the catalysts in (e): (1) CuMnAl-LDH [16], (2) CuZnAl-LDH [20], (3) ZnO/c-Cu [32], (4) CuMgHAlH-LDH [21], (5) CuZnAl-LDH [11], (6) 5Cu/Sm2O2CO3 [33], (7) CeO2/Cu [34], (8) CuO/1CeO2-1La2O3 [35], (9) Cu/CeO2-MOF [36], (10) Cu/Fe3O4 [37], (11) CuZnAl-LDH [38].
Figure 1. Effects of different competitive ligands on the structures of Cu-Mg-Al LDH, the derived mixed oxides, and the catalytic activities of the derived catalysts. (a) Schematic illustration of the role of competitive ligands during the preparation process of Cu-based LDH; (b) XRD patterns for the LDH samples; (c) SBET and pore volume for the calcined samples; (d) CO conversion of WGS reaction for the activated samples; and (e) a summary of the mass-specific activities of Cu-based WGS catalysts in this study and previous references. The adding amount of the competitive ligands was 2.5% of the total amount of metal cations (nligand/(nCu + nMg + nAl) = 0.025). WGS reaction conditions: CO/H2/CO2/N2 = 14.6:27.0:9.2:49.2 (molar ratio), 50 mg catalyst, CO/H2O = 1/4. The weight hourly space velocity (WHSV) is 24,000 mL·g−1·h−1 (dry-gas base). The dotted line in (d) indicates the thermodynamic equilibrium CO conversion calculated by the reported method [31]. The references of the catalysts in (e): (1) CuMnAl-LDH [16], (2) CuZnAl-LDH [20], (3) ZnO/c-Cu [32], (4) CuMgHAlH-LDH [21], (5) CuZnAl-LDH [11], (6) 5Cu/Sm2O2CO3 [33], (7) CeO2/Cu [34], (8) CuO/1CeO2-1La2O3 [35], (9) Cu/CeO2-MOF [36], (10) Cu/Fe3O4 [37], (11) CuZnAl-LDH [38].
Catalysts 15 00546 g001
The catalytic activities of the catalysts derived from these Cu-Mg-Al LDH samples for the WGS reaction have been tested, and the results are presented in Figure 1d. The catalyst prepared using glycine as a competitive ligand obtained the highest CO conversion, about 3.6 times that of the control sample at 200 °C. The mass-specific reaction rate for this catalyst reached as high as 33.5 μmolCO · g cat − 1 ·s−1 at 200 °C, much higher than other Cu-based catalysts without noble metals, to our knowledge (Figure 1e). The Cu70Mg15Al15-LDH-i catalyst also showed much higher catalytic activity than the control sample. The Cu70Mg15Al15-LDH-c catalyst without the LDH phase in the precursor exhibited the lowest catalytic activity. The high catalytic activities of the Cu70Mg15Al15-LDH-g and Cu70Mg15Al15-LDH-i catalysts should benefit substantially from the well-crystallinity and fewer CuO impurities of the LDH precursor.
The formation of complexes is determined by stability constants and reactant concentrations. The stability constant of the Cu(glycine)2+ complex (lgβ1 = 8.57) is higher than that of the CuOH+ complex (lgβ1 = 6.5), but it is much smaller than that of the Cu(OH)2 complex (lgβ2 = 11.8) [39]. Glycine is a bidentate ligand. The Cu-N and Cu-O coordination bands in the Cu(glycine)2+ complex should be less stable than the Cu-O coordination band in the Cu(OH)x2−x complex (Table S2). In the coprecipitation solution, the glycine concentration (0.44~1.5 × 10−2 mol·L−1 for the adding amount of 2.5%) is hundreds of times higher than the OH− concentration (0.1~1 × 10−5 mol·L−1, pH = 8~9). A certain number of Cu(glycine)x2+ complexes will be formed preferentially, which will inhibit the formation of Cu(OH)2 deposition. The unstable Cu(glycine)x2+ complexes will gradually transform into LDH crystals, which should be accelerated by the pH increase. Because the mixed salt solution (solution A) and the NaOH solution were pumped into a stoichiometric ratio, the transfer of metal ions into complexes led to an increase in the OH− concentration. Briefly, the Cu(glycine)x2+ complexes are favored in kinetics, while LDH is favored in thermodynamics, and the adding amount of glycine is the critical factor in controlling LDH crystal growth. Additionally, excessive glycine concentration will seriously change the precipitation environment, such as through exorbitant pH, which may be detrimental for LDH formation.
Based on the thought above, we further studied the effects of the adding amount of glycine on the structures and catalytic performances of Cu-based LDH and its derived materials. As shown in Figure 2a, when the adding amount of glycine is increased from 0.25 to 5.0%, the CuO diffraction signals of the XRD patterns for the LDH samples display a decreasing trend. This further demonstrates that using glycine as a competitive ligand during the coprecipitation process can effectively reduce the CuO byproduct in the Cu-Mg-Al LDH samples. Moreover, the SBET and pore volume of the mixed oxides derived from these LDH samples showed a volcano trend with the adding amount of glycine (Figure 2b). The highest SBET of 113.8 m2·g−1 was achieved at the adding amount of 3.3%. The highest pore volume of 1.12 mL·g−1 was achieved at the adding amount of 2.5%. In addition, the pore size distribution of the mixed oxides is also affected by the glycine-adding amount (Figure S4). Notably, the inductively coupled plasma optical emission spectroscopy (ICP-OES) results show that the mixed oxides prepared by adding glycine have higher contents of Cu and Mg elements than the control sample (Table S1). This indicates that glycine can promote the formation of Cu-based LDH crystals, rather than causing metal cation loss by forming soluble complexes.
The CO conversion of the catalysts derived from these Cu-Mg-Al LDH samples exhibited a volcano trend with the adding amount of glycine at the temperature range of 200–275 °C, and it reached the highest CO conversion at the glycine-adding amount of 2.5% (Figure 2c). These results show that the appropriate amount of glycine added during the coprecipitation process is extremely important for obtaining active catalysts. Figure 2d displays a long-term stability test for the WGS reaction. The CO conversion of the Cu70Mg15Al15-LDH-g catalyst decreased to 71.8% from 96.7% within 50 h, while it decreased to 28.6% from 92.1% for the control catalyst. This illustrates that adding a glycine ligand during the coprecipitation process of Cu-Mg-Al LDH can effectively improve the catalytic stability of the derived catalyst.
In addition, the improving effects of the glycine ligand on Cu-Mg-Al LDH are related to element content, and the Cu70Mg15Al15-LDH-g sample with a Cu content of 70 at.% showed the most significant improvement (Figure S5). Moreover, these improving effects were also observed in Cu-Zn-Al LDH and Cu-Mn-Al LDH (Figure S6). This strategy can be used to synthesize Cu-rich materials for other catalytic reactions and even other fields.
Transmission electron microscopy (TEM) images show that the Cu particles of the fresh Cu70Mg15Al15-LDH-g catalyst (reduced) were more uniformly dispersed than those of the control catalyst (Figure 3a,b). The average size of the Cu particles for the fresh Cu70Mg15Al15-LDH-g catalyst (adding 2.5% glycine) was smaller than that for the control catalyst, and the particle size range was also much narrower (Figure 3a,b,a’,b’). Moreover, the reduction temperature of the Cu70Mg15Al15-LDH catalyst was lower than that of the control catalyst, and the reduction temperature range was narrower (Figure S7). This also suggests that the Cu70Mg15Al15-LDH-g catalyst has a smaller average size and a narrower size distribution of Cu particles. The improved dispersion of Cu particles of the Cu70Mg15Al15-LDH-g catalyst is associated with the high SBET and high catalytic activity discussed above.
After 24 h on-stream in the WGS reaction, Cu particles for the Cu70Mg15Al15-LDH-g catalyst still maintained well dispersion, with only a slight increase in the average particle size (Figure 3c,c’). But for the control catalyst, the Cu particles were obviously sintered, and some large particles were observed (Figure 3d,d’). Moreover, the aberration-corrected high-angle annular dark-field scanning (HAADF) TEM and corresponding energy dispersive X-ray spectroscopy (EDX) mapping images also showed that the Cu particles of the spent Cu70Mg15Al15-LDH-g catalyst kept a uniform dispersion and were smaller than those of the spent Cu70Mg15Al15-LDH catalyst (Figure 3e–n). These results demonstrate that adding a glycine ligand during the coprecipitation process can significantly improve the sinter-resistant performance of the catalyst derived from Cu-Mg-Al LDH. The sinter-resistant performance suggests an enhanced metal-support interaction, which should be closely related to the outstanding catalytic stability (Figure 2d).
Figure 4 shows the X-ray photoelectron spectroscopy (XPS) of the spent catalysts. The O1s XPS were deconvolved into two fitted peaks representing two different kinds of oxygen species (OI and OII, Figure 4a and Table S3). The peak OI at 531.7 or 532 eV is mainly attributed to the adsorbed surface oxygen species, including the surface hydroxyl-like species [16,40], which are believed to be responsible for the conversion of surface-adsorbed CO to CO2 [22,23,33]. The peak OII at 530.4 eV is assigned to the lattice oxygen bound to metal cations [16,40]. The OI/OII ratio of the Cu70Mg15Al15-LDH-g (adding 2.5% glycine) catalyst is considerably higher than that of the control catalyst. It suggests that the Cu70Mg15Al15-LDH-g catalyst has more surface oxygen species, which is consistent with its high catalytic activity (Figure 1c and Figure 2c). Such a large number of surface oxygen species in the Cu70Mg15Al15-LDH-g catalyst may originate from the high SBET and well dispersion of metal oxides (MgO, Al2O3, and CuxO).
The Cu2p XPS of the spent catalysts was deconvolved into two types of fitted peaks in the Cu2p3/2 and Cu2p1/2 photoelectron peaks (Figure 4b and Table S3), which could be assigned to CuO and Cu0/Cu2O, respectively. This shows that Cu species exist mainly in the form of metallic Cu and Cu2O (Figure 4b and Table S3). Notably, the binding energy of Cu0/Cu2O for the Cu70Mg15Al15-LDH-g catalyst is 0.3~0.4 eV lower than that of the control catalyst, and the binding energy of Mg1s XPS is just 0.2 eV higher than that of the control catalyst (Figure 4b,c). It demonstrates electron transfer from MgO to Cu species in the Cu70Mg15Al15-LDH-g catalyst, which means an enhanced interaction between Cu species and MgO [22,41]. Moreover, the in situ diffuse reflectance infrared Fourier transform spectra of CO adsorption (CO-DRIFT) also corroborate a strong electron transfer between Cu species and oxide supports (Figure S8). This enhanced metal–support interaction endows the Cu70Mg15Al15-LDH-g catalyst with remarkable sinter-resistant performance (Figure 3) and thereby improves catalytic stability (Figure 2d). This enhanced metal–support interaction may originate from the improved dispersion of MgO and Cu species.
Because the binding energies of the Cu 2p electron for pure Cu and pure Cu2O are very close, it is hard to separate the peaks of Cu and Cu2O from each other in the Cu 2p XPS spectra [42,43,44]. The XRD patterns show that metallic Cu and Cu2O are both the main phases in the spent catalysts (Figure 4d), which is also supported by the Augur spectra of Cu LMM (Figure S9). Interestingly, in the reduced catalysts, metallic Cu is the main phase, and Cu2O was rarely observed, but a significant amount of Cu2O was found in the catalysts after 24 h on stream in the WGS reaction (Figure 4d). The Cu2O should be formed by the oxidation of metallic Cu with oxygen species derived from water dissociation [45]. The area ratio of Cu2O and Cu diffraction peaks for the Cu70Mg15Al15-LDH-g catalyst is higher than that of the control catalyst (Figure 4d). It indicates that a higher proportion of Cu in the Cu70Mg15Al15-LDH-g catalyst has transformed into Cu2O compared to the control catalyst. However, the Cu70Mg15Al15-LDH-g catalyst showed only a slight decrease in catalytic activity, while the control catalyst lost more than half of its activity within 24 h (Figure 2d). This implies that Cu2O is active for the WGS reaction, aligning with recent reports [45,46].

3. Experimental Section

3.1. Preparation of Samples

All LDH samples were prepared by the modified coprecipitation method. The operated processes were similar to our earlier reported method [16], except that a small number of competitive ligands were added to the three-neck flask before the coprecipitation process. Typically, solution A was obtained by dissolving a mixture of metal nitrates (Cu(NO3)2, Mg(NO3)2, and Al(NO3)3 with a total amount of 0.06 mol) in 120 mL of deionized water. Solution B was prepared by dissolving 0.12 mol of NaOH in 120 mL of deionized water. In a three-necked flask, a certain amount of glycine and 0.012 mol of NaHCO3 were dissolved in 100 mL of deionized water. Then, solution A and solution B were simultaneously pumped into the three-necked flask at 60 °C, and all of the flow rates were controlled at 5.0 mL·min−1. The slurry was stirred slowly at 60 °C for 12 h. Then, the resulting precipitate was filtered, washed with deionized water, and dried at 80 °C for 10 h. The obtained Cu-Mg-Al LDH sample was ground into a fine powder and labelled as CuxMgyAlz-LDH-g, in which the x, y, and z subscripts present the designed metal content (molar percent). About 0.5 g of the CuxMgyAlz-LDH-g sample was calcined at 450 °C for 2 h in a tube furnace under air flow (500 mL·min−1). After naturally cooling to room temperature, the Cu-Mg-Al mixed oxides sample was obtained and ready for testing.
Other Cu-Mg-Al LDH samples were prepared by the same method and using the same conditions, only replacing glycine with sodium citrate, sodium oxalate, disodium iminodiacetate, or ethylenediamine.

3.2. Catalyst Characterization

XRD measurements were carried out on a Rigaku Ultima IV diffractometer instrument (λCu Ka = 0.15418 nm, 40 kV and 40 mA). The N2 adsorption/desorption isotherm was measured on a surface area and pore size analyzer of Nova 2000e (Quantachrome, FL, USA). All samples were vacuum degassed before measurement at 200 °C for 12 h. The Brunauer-Emmett-Teller specific surface areas (SBET) were calculated using adsorption branch data, and the Barrett-Joyner-Halenda (BJH) model was utilized to calculate pore size distributions. The pore volumes were calculated by the last point of the adsorption branch (P/P0 ≈ 0.995). TEM images were obtained with a JEM-F200 instrument with an accelerating voltage of 200 kV (JEOL, Tokyo, Japan). The HAADF-TEM images and corresponding EDX mapping images were obtained on a JED-2300T detector. XPS measurements were obtained using a Thermo Scientific K-Alpha XPS spectrometer (Waltham, MA, USA). Binding energies were calibrated based on the graphite C1s peak at 284.8 eV. Before the XPS and TEM measurements, the catalyst samples were reduced for 0.5 h in H2 atmosphere at 300 °C and were then operated on stream for 24 h (reaction condition: 300 °C, CO:H2:CO2:N2 = 14.6:27.0:9.2:49.2, CO:H2O = 1:4).

3.3. Catalytic Testing

A fixed-bed reactor with an internal diameter of 8 mm was used for the catalytic performance tests of the WGS reaction. The reaction temperature was automatically controlled by a PID temperature controller with a thermocouple inserted into the center of a fixed catalyst bed. A total of 50 mg of the Cu-Mg-Al mixed oxides sample was fully mixed with 3.0 g of quartz sand (40–80 mesh), and then the mixed powder was loaded into the reactor. The catalyst was reduced in H2 gas flow (20 mL·min−1) for 0.5 h at 300 °C. After cooling to 200 °C, a gas mixture of CO/H2/CO2/N2 (molar ratio: 14.6:27.0:9.2:49.2) was introduced into the reactor with a flow rate of 20 mL·min−1 (WHSV = 24,000 mL·g−1·h−1, CO/H2O = 1/4). The deionized water was vaporized in a gasification chamber, and it was fully mixed with the feed gas flow before bringing it into the reactor. The tests were conducted at elevated temperatures, from 200 to 350 °C. The outlet gas was analyzed by a GC9860 gas chromatography device (Shanghai Qiyang Co., Shanghai, China) equipped with a thermal conductivity detector and a flame ionization detector.

4. Conclusions

Adding competitive ligands during the coprecipitation process of the preparation period significantly affects the formation of Cu-Mg-Al LDH, the structures of the derived Cu-Mg-Al mixed oxides, and the catalytic performances of the derived catalyst. Adding an appropriate amount of glycine is greatly helpful for synthesizing well-crystallized Cu-rich LDH with less CuO impurity. The derived Cu-Mg-Al mixed oxides, with added 2.5% glycine during the coprecipitation process, have relatively high SBET, large pore volume, and well dispersion of Cu nanoparticles. The derived catalyst exhibited unexpectedly high catalytic activity, and the mass-specific reaction rate reached as high as 33.5 μmolCO · g cat − 1 ·s−1 at 200 °C. The high catalytic activity may originate from high SBET and well dispersion of Cu particles and metal oxides. Moreover, benefiting from the enhanced metal–support interaction, this catalyst also exhibited outstanding long-term stability in the WGS reaction.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15060546/s1, Figure S1: images of the Cu-Mg-Al LDH samples prepared by using different competitive ligands; Figure S2: thermogravimetry-mass (TG-MS) profiles of the Cu70Mg15Al15-LDH-g and Cu70Mg15Al15-LDH samples; Figure S3: effect of different competitive ligands on pore size distribution of the calcined Cu-Mg-Al LDH samples; Figure S4: pore size distribution of the calcined Cu-Mg-Al LDH samples prepared by adding different amount of glycine during the precipitation process; Figure S5: effect of Cu content on the WGSR catalytic performance of the catalysts with/without adding glycine during the precipitation process; Figure S6: The XRD patterns and CO conversion of WGS for the Cu-Zn-Al LDH and Cu-Mn-Al LDH samples with/without adding glycine ligand in the prepared process; Figure S7: hydrogen temperature-programmed reduction (H2-TPR) of the Cu70Mg15Al15-LDH-g and Cu70Mg15Al15-LDH catalysts; Figure S8: CO- DRIFT spectra for the Cu70Mg15Al15-LDH-g (adding 2.5% glycine) and Cu70Mg15Al15-LDH catalysts [22,41,47]; Figure S9: the Augur spectra of Cu LMM for the catalysts after 24 h on stream at 300 °C in WGS reaction; Table S1: Metal contents of the calcined Cu-Mg-Al LDH samples prepared by adding different amount of glycine; Table S2: The stability constant of Cu2+ complexes [39,48]. Table S3: fitting parameters for O 1s-region and Cu2p3/2-region of XPS spectra.

Author Contributions

Conceptualization, O.Z., F.L. and H.L.; methodology, O.Z., F.L. and H.L.; synthesis and catalytic testing, S.L. and Y.H.; formal analysis, S.L., Y.H., Q.Z., X.T. and H.C.; data curation, S.L. and Y.H.; writing—original draft preparation, O.Z., S.L. and Y.H.; writing—review and editing, O.Z. and S.L.; supervision, O.Z.; project administration, O.Z.; funding acquisition, O.Z., F.L. and H.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Natural Science Foundation of China (grant number 22368021) and the Hunan Provincial Natural Science Foundation of China (grant numbers 2024jj7417 and 2023JJ40516).

Data Availability Statement

The data presented in this study are available in the article.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Figure 2. Effects of the glycine ligand on the structures of the Cu-Mg-Al LDH, the derived mixed oxides, and the catalytic performances of the derived catalysts. (a) XRD patterns of the LDH samples prepared by adding different amounts of glycine; (b) SBET and pore volume of the mixed oxides derived from the LDH samples; (c) CO conversion of WGS reaction for the derived catalysts; and (d) long-term stability test of WGS reaction at 300 °C. The adding amount of glycine is expressed as the molar percentage to the total amount of metal cations. WGS reaction conditions: CO/H2/CO2/N2 = 14.6:27.0:9.2:49.2 (molar ratio), 50 mg catalyst, WHSV = 24,000 mL·g−1·h−1 (dry-gas base), CO/H2O = 1/4. The long-term stability test was performed using the catalyst prepared by adding 2.5% glycine. For the Cu70Mg15Al15-LDH-g and Cu70Mg15Al15-LDH samples, the XRD patterns from Figure 1b, the SBET and pore volume from Figure 1c, and the CO conversions from Figure 1d are replotted in (a), (b), and (c) for comparison, respectively. The thermodynamic equilibrium CO conversion (the dotted line) is also replotted in (c).
Figure 2. Effects of the glycine ligand on the structures of the Cu-Mg-Al LDH, the derived mixed oxides, and the catalytic performances of the derived catalysts. (a) XRD patterns of the LDH samples prepared by adding different amounts of glycine; (b) SBET and pore volume of the mixed oxides derived from the LDH samples; (c) CO conversion of WGS reaction for the derived catalysts; and (d) long-term stability test of WGS reaction at 300 °C. The adding amount of glycine is expressed as the molar percentage to the total amount of metal cations. WGS reaction conditions: CO/H2/CO2/N2 = 14.6:27.0:9.2:49.2 (molar ratio), 50 mg catalyst, WHSV = 24,000 mL·g−1·h−1 (dry-gas base), CO/H2O = 1/4. The long-term stability test was performed using the catalyst prepared by adding 2.5% glycine. For the Cu70Mg15Al15-LDH-g and Cu70Mg15Al15-LDH samples, the XRD patterns from Figure 1b, the SBET and pore volume from Figure 1c, and the CO conversions from Figure 1d are replotted in (a), (b), and (c) for comparison, respectively. The thermodynamic equilibrium CO conversion (the dotted line) is also replotted in (c).
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Figure 3. Microscopic characterizations of the fresh and spent catalysts. (a–d) TEM images and (a’–d’) corresponding particle size histograms of the fresh and spent catalysts; (e–n) HAADF-TEM and corresponding EDX mapping images of the spent catalysts. The fresh catalysts were reduced in the H2 stream at 300 °C before measurements. For the spent catalysts before measurements, the samples were reduced in the H2 stream at 300 °C and then underwent 24 h WGS reaction at 300 °C.
Figure 3. Microscopic characterizations of the fresh and spent catalysts. (a–d) TEM images and (a’–d’) corresponding particle size histograms of the fresh and spent catalysts; (e–n) HAADF-TEM and corresponding EDX mapping images of the spent catalysts. The fresh catalysts were reduced in the H2 stream at 300 °C before measurements. For the spent catalysts before measurements, the samples were reduced in the H2 stream at 300 °C and then underwent 24 h WGS reaction at 300 °C.
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Figure 4. Chemical properties of the spent catalysts. (a–c) XPS profiles for the catalysts after 24 h on stream at 300 °C in WGS reaction, (d) XRD spectra for the reduced and spent catalysts. More details of fitting parameters of XPS spectra are shown in Table S3.
Figure 4. Chemical properties of the spent catalysts. (a–c) XPS profiles for the catalysts after 24 h on stream at 300 °C in WGS reaction, (d) XRD spectra for the reduced and spent catalysts. More details of fitting parameters of XPS spectra are shown in Table S3.
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Liu, S.; Hu, Y.; Zhang, Q.; Tan, X.; Cui, H.; Li, F.; Lei, H.; Zhuo, O. Synthesis of Well-Crystallized Cu-Rich Layered Double Hydroxides and Improved Catalytic Performances for Water–Gas Shift Reaction. Catalysts 2025, 15, 546. https://doi.org/10.3390/catal15060546

AMA Style

Liu S, Hu Y, Zhang Q, Tan X, Cui H, Li F, Lei H, Zhuo O. Synthesis of Well-Crystallized Cu-Rich Layered Double Hydroxides and Improved Catalytic Performances for Water–Gas Shift Reaction. Catalysts. 2025; 15(6):546. https://doi.org/10.3390/catal15060546

Chicago/Turabian Style

Liu, Shicheng, Yinjie Hu, Qian Zhang, Xia Tan, Haonan Cui, Fei Li, Huibin Lei, and Ou Zhuo. 2025. "Synthesis of Well-Crystallized Cu-Rich Layered Double Hydroxides and Improved Catalytic Performances for Water–Gas Shift Reaction" Catalysts 15, no. 6: 546. https://doi.org/10.3390/catal15060546

APA Style

Liu, S., Hu, Y., Zhang, Q., Tan, X., Cui, H., Li, F., Lei, H., & Zhuo, O. (2025). Synthesis of Well-Crystallized Cu-Rich Layered Double Hydroxides and Improved Catalytic Performances for Water–Gas Shift Reaction. Catalysts, 15(6), 546. https://doi.org/10.3390/catal15060546

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