The Synergistic Effect of CeO₂ and Micron-Cu Enhances the Hydrogenation of CO₂ to CO

Abstract

The catalytic applications of micron Cu powder are limited due to its large particle size and small specific surface area. Modifying micro-Cu powder to achieve a high catalytic performance is a challenge in the application of micron copper. In this work, micro-Cu was used to synthesize a CeO₂–Cu catalyst, and the phase composition and surface pore structure were analyzed using XRD, BET, etc. The CO₂ hydrogenation performance of the CeO₂–Cu catalyst was analyzed in comparison with CeO₂ and Cu, and we found that the CeO₂–Cu catalyst exhibited a synergistic effect between Cu and cerium, resulting in a much higher hydrogenation performance at 500 °C than CeO₂ or Cu alone. H₂-TPR and TEM characterization revealed that the CeO₂–Cu catalyst formed interfacial interactions with a relatively large Ce–Cu interface, where cerium oxide could promote the reduction of CuO and lower the reduction temperature. Additionally, cerium oxide formed a confinement structure for Cu, and the CeO₂–Cu catalyst exhibited a higher oxygen vacancy concentration, thereby promoting the CO₂ hydrogenation performance. Cu–CeO₂ interaction provides valuable insights into the catalytic application of micron Cu powder.

1. Introduction

As industrialization accelerates, the demand for energy from humans is increasing, leading to the consumption of a large amount of fossil energy and the emission of a large amount of CO₂ into the atmosphere, disrupting the atmospheric carbon balance [1,2]. The increase in CO₂ concentration triggers a series of environmental problems, such as ocean acidification and the greenhouse effect [3,4,5,6]. The rising global temperature not only accelerates the melting of the Arctic glaciers and the rise in sea levels but also increases the frequency of extreme weather events, causing damage to the human living environment [7]. Therefore, reducing CO₂ emissions and in doing so lowering the concentration of CO₂ in the atmosphere is an urgent challenge that needs to be addressed [8,9,10].

The hydrogenation of CO₂ is currently one of the most researched and effective methods for CO₂ reduction [9,11,12]. This involves using unstable electricity generated from renewable energy sources (such as solar and wind power) to electrolyze water for hydrogen production, and then obtaining high-value-added products such as CO, methane, and ethylene through the hydrogenation of CO₂ [13,14,15,16]. CO₂ hydrogenation reduces the concentration of carbon dioxide in the atmosphere and converts CO₂ into high-value-added products, generating economic benefits [17]. Among various CO₂ hydrogenation methods, the process of converting CO₂ to CO via an RWGS reaction is widely studied due to the high application flexibility of the resulting CO [18]. CO, as the main component of syngas, can be utilized not only for methanol synthesis but also for the Fischer–Tropsch (F-T) reaction [19,20,21]. Additionally, CO can be applied in chemical reactions to synthesize acetic acid, phosgene, and other substances [18]. Due to the endothermic nature of the reverse water–gas shift (RWGS) reaction under atmospheric pressure, the reaction requires high temperatures [22]. However, high temperatures can lead to catalyst deactivation and hydrogenation side reactions, resulting in reduced CO selectivity [23]. Therefore, it is necessary to design efficient catalysts to lower the reaction temperature and regulate product selectivity.

Cu-based catalysts have been widely applied in CO₂ hydrogenation reactions in recent years due to their low reduction temperature and excellent hydrogenation CO selectivity [24]. However, the CO₂ hydrogenation conversion activity in the low-temperature conditions is relatively low [25,26,27]. Chen’s research revealed that the RWGS reaction mainly occurs at the metal–support interface, where the metal activates the hydrogen molecule and the metal–support interface adsorbs and activates CO₂ [11,28]. Increasing the metal–support interface will thus be beneficial for the RWGS reaction [29,30]. Currently, catalyst design research has primarily focused on nano-Cu catalysts, while micron-sized Cu powder produced from ball milling of industrial waste copper has received less application due to its large particle size and small specific surface area, resulting in poor CO₂ hydrogenation activity [31]. Therefore, modifying the micron-sized Cu powder to improve its catalytic capability presents a current challenge in applying industrial waste copper in CO₂ catalytic reactions.

The interfacial structure of the metal and oxide support plays a pivotal role in the reverse water–gas shift (RWGS), and reconstructing the Cu–CeO₂ interface will improve CO₂ hydrogenation performance [30,32,33]. Zhang and co-workers found that the Cu–CeO₂ interface is related to the RWGS activity, and increasing the Cu–CeO₂ interface benefits CO generation [24]. Furthermore, Zhou et al. found that Cu can be doped into the CeO₂ lattice, increasing CeO₂ lattice spacing, resulting in more Ce³⁺ formation, and inducing the generation of more oxygen vacancies, and that the oxygen vacancies together with Ce³⁺ can serve as active sites to activate CO₂ and enhance the CO₂ hydrogenation reaction [34]. Our previous work also demonstrated that Cu interacted with CeO₂ to form a Cu–O–Ce interface and induced more oxygen vacancy formation [31]. The oxygen vacancies around the Cu–CeO₂ interface enhanced CO₂ adsorption and promoted CO₂ conversion. CO₂ reacted with active hydrogen to form COOH, and then COOH species dissociated into CO and OH adsorbed on the surface of Cu–CeO₂. Therefore, constructing a Cu–CeO₂ interface may be advantageous for enhancing micro-Cu CO₂ hydrogenation performance. However, little work has investigated the relationship between Cu and CeO₂, though it may play a vital role in understanding the CO₂ hydrogenation performance of Cu–Ce-based catalysts.

In this work, the CeO₂–Cu catalyst was synthesized with micron-sized Cu, and the ceria–Cu interface was constructed for CO₂ utilization. The phase composition and surface pore structure information of the CeO₂–Cu catalyst were analyzed with XRD, BET, etc. The catalytic performance of CO₂ hydrogenation was analyzed and compared with CeO₂ and Cu alone. A synergistic effect appeared with Cu and cerium oxide. H₂-TPR and TEM characterization were measured to reveal the CeO₂–Cu interfacial interactions and CeO₂ effect on micro-Cu. Additionally, the CeO₂–Cu structure and oxygen vacancy information were investigated to reveal the enhancement of CeO₂ for CO₂ hydrogenation performance.

2. Experimental Section

2.1. Synthesis of Catalysts

All the chemicals (micro Cu, Ce(NO₃)₃·6H₂O) were purchased from Aladdin Chemistry Co., Ltd., Shanghai, China without any further purification. The CeO₂–Cu catalyst was synthesized using a precipitation method. Firstly, a certain amount of Ce(NO₃)₃·6H₂O was dispersed and dissolved in ethanol, and then 0.5 g of micro-Cu powder was added, followed by stirring at room temperature for 30 min. Subsequently, a grey solid was obtained by adding ammonia water for precipitation, which was washed three times with water by centrifugation, and then dried to obtain the grey–black sample named CeO₂–Cu. The CeO₂ catalyst was also synthesized using a precipitation method. Initially, a certain amount of Ce(NO₃)₃·6H₂O was dispersed and dissolved in water, and then a grey solid was obtained by adding ammonia water for precipitation; the solid was washed three times with water by centrifugation, and then dried to obtain the light yellow sample named CeO₂.

2.2. Characterization

X-ray powder diffraction (XRD) was employed using a D8 X-ray diffractometer (from Bruker AXS, Karlsruhe, Germany) to conduct crystal structure analysis of CeO₂–Cu, CeO₂, and Cu samples. Cu K_α (λ = 0.15418 nm) was utilized with parameters set at 40 kV, 40 mA, a testing range from 20 to 80°, and a scan step size of 0.01313°. The final spectrum was compared with the JCPDS card library to determine the composition of the powders. The BET surface area analyzer 3H-2000PS (Beishide Instrument Technology (Beijing) Co., Ltd., Beijing, China) was used to investigate differences in pore size distribution and specific surface area of the CeO₂–Cu, CeO₂, and Cu samples. Before the BET testing, all the materials were pre-heated at 180 °C for 180 min to decrease the absorbed gases. The scanning transmission electron microscope (STEM) Talos F200X (from FEI, Eindhoven, The Netherlands) was utilized to analyze the elemental composition of the CeO₂–Cu, CeO₂, and Cu samples, as well as to select a line for energy-dispersive X-ray spectroscopy (EDS) analysis. Prior to the TEM analysis, the CeO₂–Cu, CeO₂, and Cu samples were pretreated under ultra-sonication for 0.5 h, to achieve a uniform dispersion in an ethanol solvent, and then with the evaporation of three suspensions dropped on a gold grid.

Hydrogen Temperature Programmed Reduction (H₂-TPR) was performed using a chemisorption analyzer (AutoChem II, Micromeritics, Norcross, GA, USA), and the hydrogen signal was monitored and analyzed online using a TCD detector. CeO₂–Cu, CeO₂, and Cu samples were first pretreated at 400 °C with pure He gas (30 mL/min) for 20 min to degas the adsorbed molecules (such as oxygen, nitrogen, carbon dioxide, or water), and then cooled to room temperature before introducing 10% H₂/Ar (30 mL/min). Subsequently, the samples underwent programmed temperature ramping from 30 °C to 500 °C at a rate of 10 °C/min. The H₂ consumption during the temperature ramping was determined with a TCD detector. LabRAM HR800 (LabRAM Odyssey, Longjumeau, France) was measured to collect Raman spectroscopy of the CeO₂–Cu, CeO₂, and Cu samples.

2.3. Evaluation of Catalytic Performance

The performance of CeO₂–Cu, CeO₂, and Cu catalysts was evaluated with a microreactor furnace (PH950, Apera Instruments, Shanghai, China) under atmospheric pressure. First, 50 mg of the catalyst was loaded into a U-shaped tube (d = 8 mm) and subjected to a pre-reduction treatment at 400 °C in a reducing atmosphere. After cooling to room temperature, the reactants (1%CO₂ + 4%H₂ + 95%Ar, Ar was balanced gas) were introduced, and the temperature was ramped up at a rate of 10 °C/min for activity testing in the range of 500−700 °C. Online analysis of CO₂ hydrogenation products was performed using gas chromatography (GC-2020, Hengxin, Jiangsu, China), which was equipped with packed columns (ZKAT-Z13 PLOT, ATEO) and a flame ionization detector with mechanized nickel, which exhibited the separation and quantification of CO₂, CO, and CH₄. The formula for calculating the CO₂ conversion rate and CO selectivity are listed as follows:

CO₂ Conversion = ([CO₂]_in − [CO₂]_out)/[CO₂]_int ∗ 100%

CO Selectivity = [CO]_out/([CO₂]_in − [CO₂]_out) ∗ 100%

3. Results and Discussion

3.1. Characterization

In order to study the effect of cerium addition on the structure of micro-copper powders, XRD characterization was performed on CeO₂–Cu, CeO₂, and micro-Cu, and the results are shown in Figure 1. The peaks located at 28.549°, 33.077°, 47.483°, 56.342°, 59.09°, 69.416°, 76.704°, and 79.077° could be assigned to the fluorite cubic structure of CeO₂ (JCPDS#34-0394). The peaks located at 43°, 51°, and 74° were consistent with the Cu metallic phase (CPDS#04-0836). It can be observed that in the XRD spectrum of the CeO₂–Cu sample synthesized with cerium nitrate, the signal for CeO₂ is weak, the peak for Cu is strong, and there is essentially no peak for CuO. Micro-Cu exhibited a strong Cu peak, while the signal intensity for the cerium oxide sample was relatively weak, consistent with the weak CeO₂ signal for the CeO₂–Cu sample, indicating poor crystallinity of CeO₂ prepared by the cerium nitrate precipitation method. The weak signal peak for CeO₂ in CeO₂–Cu may be attributed to the strong interaction between Ce and Cu. The Cu signal peaks in the CeO₂–Cu synthesized from the cerium nitrate precursor and in the micro-Cu both exhibited strong signals, with the Cu signal peak in micro-Cu was stronger than that in CeO₂–Cu, indicating a possible interaction between cerium oxide and Cu, leading to a weakening of the metal Cu signal peak. The difference in the signal peaks of Cu substances suggests an interaction between CeO₂ and micrometer-sized Cu. The interaction between CeO₂ and micrometer-sized Cu may lead to the formation of more oxygen vacancies in CeO₂, which could potentially promote reactivity.

To study the effect of cerium addition on the surface pore structure of micro-Cu powder, a specific surface area analysis was conducted on CeO₂–Cu, CeO₂, and micro-Cu. The results of nitrogen adsorption–desorption isotherms and pore size distribution are shown in Figure 2, while the specific surface area and average pore size results are presented in

Table 1. Specific surface areas and pore information of CeO₂–Cu, CeO₂, and Cu catalysts.

Catalyst	gCeO2/gCu (Wt%)	SBET (m2/g)	Pore Volume (mL/g)	Average Pore Diameter (nm)
Cu		2.68	0.22	12.36
CeO2		149.87	0.407	5.02
CeO2–Cu	20	13.54	0.0277	6.03

. After loading with ceria species, the specific surface area of CeO₂–Cu was larger than that of micro-Cu but smaller than that of ceria, and the pore size distribution shifted towards that of CeO₂. There was a significant difference in the pore size distribution between CeO₂–Cu and micro-Cu, indicating that ceria addition has an impact on the pore size distribution of Cu and that the difference in the pore size distribution of CeO₂–Cu may result from the interaction between CeO₂ and micro-Cu.

3.2. CO₂ Hydrogenation Performance

To analyze the effect of cerium addition on the catalytic performance of micro-Cu, the CO₂ hydrogenation activities of CeO₂–Cu, CeO₂, and Cu micro-powders were studied. Activity tests were conducted in the 500−700 °C range, and the CO₂ hydrogenation activities of CeO₂–Cu, CeO₂, and Cu catalysts are shown in Figure 3. At 500 °C, the CO₂ hydrogenation activity of the CeO₂–Cu catalyst reached 49.82%, which is more than 204 times higher than that of the Cu catalyst (0.244%). In contrast, the CeO₂ catalyst showed minimal CO₂ hydrogenation activity at this temperature (4.627%), and the CO selectivity of all three catalysts for CO₂ hydrogenation was 100%. The results of CO₂ hydrogenation activities for CeO₂–Cu, CeO₂, and Cu catalysts indicated a synergistic effect between Cu and CeO₂ in CeO₂–Cu. While the CO₂ conversion activity of individual Cu or CeO₂ was poor at 500 °C, when Cu was combined with ceria, Cu–CeO₂ supported more active sites for the RWGS reaction, exhibiting a higher CO₂ conversion rate. These results suggested that both ceria and copper are involved in the CO₂ hydrogenation process, with Cu playing a key role in activating hydrogen at moderate temperatures, while ceria provides active sites to promote CO₂ activation and form carbonates for further hydrogenation conversion [11,24].

Increasing the temperature can promote the activation of CO₂ and H₂ molecules, thereby enhancing the CO₂ hydrogenation activity. At high temperatures, the CO selectivity of the CO₂ hydrogenation products for CeO₂–Cu, CeO₂, and Cu catalysts is 100%, indicating that the ceria addition does not affect the hydrogenation selectivity of Cu under atmospheric pressure conditions. These results were consistent with the results of previous studies on Cu-catalyzed hydrogenation [31,35]. Under atmospheric pressure, CuO species were reduced to a metallic state and exhibited high CO selectivity during the CO₂ hydrogenation process.

The hydrogenation performance of the cerium oxide was reported to be mainly determined by oxygen vacancies on the CeO₂ surface, and oxygen vacancies were key to activating hydrogen molecules [36]. Pure cerium oxide exhibited relatively poor activity, and increasing the temperature could significantly promote the hydrogenation activity of cerium oxide. In the Cu–CeO₂ catalyst system, the metal Cu can activate hydrogen molecules at low temperatures, producing active hydrogen, which then interacts with activated carbonates to produce formate or carboxylate and further hydrogenates to generate CO and water. During the RWGS reaction, the CO₂ conversion activity of Cu–CeO₂ at 500 °C is nearly 10.7 times higher than that of cerium oxide, possibly due to the reduction properties of the metal Cu, and the active Cu sites are beneficial for hydrogen activation. At 700 °C, the CO₂ hydrogenation activity of CeO₂–Cu is similar to that of CeO₂ catalysts, and it was 8.9 times higher than that of micro-Cu. Increasing the temperature induced a similar hydrogenation behavior between Cu–CeO₂ and cerium oxide, indicating that Cu metal has little effect on hydrogenation activity. CeO₂ could provide a function of H₂ activation and CO₂ activation, and the CO₂ conversion rate was no longer limited by hydrogen activation at high temperatures. Previous studies revealed that hydrogen activation is no longer the main limitation of cerium oxide under high-temperature conditions; instead, the activation of CO₂ molecules becomes the main factor affecting its activity [36,37,38]. Therefore, the micro-Cu catalyst exhibited relatively poor hydrogenation activity at 700 °C. On the surface of Cu, the surface charge is not conducive to the activation of CO₂ molecules, hence the poor catalytic activity of Cu [39]. The construction of Cu–CeO₂ can induce Cu–CeO₂ interface formation, thereby significantly enhancing the CO₂ hydrogenation activity via the interface oxygen vacancies and achieving efficient CO₂ conversion [40]. Metal Cu and oxygen vacancies around the Cu–CeO₂ interfacial area could serve as active sites for the RWGS reaction. Therefore, a higher CO₂ conversion rate appeared for Cu–CeO₂ than Cu or CeO₂ solely, and there was a synergistic effect between Cu and cerium oxide with Cu–CeO₂, enhancing the RWGS reaction performance.

3.3. CeO₂ Effect on Micro-Cu

In order to study the influence of CeO₂ addition on the microstructure of micro-Cu, the morphologies of CeO₂–Cu, CeO₂, and Cu were characterized, and the results obtained from transmission electron microscopy (TEM) are shown in Figure 4. The micro-Cu particles exhibited spherical shapes with relatively large sizes, with an average particle size distribution of 1.88 μm (Cu powders were dispersed in water, and the size distribution of micron Cu powders was measured using a mastersizer 2000, Malvern Instruments, Malvern, UK), and there could be some CuO_x species on the surface of micron Cu (Figure 4a) [31].

CeO₂ prepared by ammonia precipitation displayed relatively small particles, mainly showing the (110)-crystal facet (Figure 4b). The CeO₂–Cu catalyst mainly presents a morphology where CeO₂ wraps around micron Cu, while some cerium oxide is in a dispersed state. The synthesized CeO₂–Cu appeared similar in morphology to the micron Cu material. In Figure 4c, the black area within the large spherical particles represents the metal Cu particles, while the surrounding white shadows indicate the presence of cerium oxide. Micro-Cu was confined with CeO₂ species, and the CeO₂ shell was relatively thin. To further reveal information on the CeO₂–Cu interface, TEM energy-dispersive X-ray spectroscopy (EDS) was used to investigate the elemental distribution on the surface of CeO₂–Cu. It was found that the edge of the spherical CeO₂–Cu is mainly composed of cerium oxide. Figure 4d,e illustrate that the cerium oxide distribution is relatively uniform, with CeO₂ being the main component on the surface of the catalyst. When the Ce peak reaches its maximum, there is a localized increase in the Cu signal peak, indicating a higher local content of Cu elements in the cerium oxide region, possibly due to the interaction between CeO₂ and Cu.

CeO₂ addition on the surface of micro-Cu resulted in CeO₂–Cu interactions, which impacted the microstructural porosity and surface elemental distribution of Cu. In order to understand the influence of cerium oxide addition on the reduction performance of Cu catalysts, H₂-TPR tests were conducted on CeO₂–Cu, Cu, and CeO₂ catalysts, as shown in Figure 5. The reduction peak of micrometer-sized Cu appeared at 276 °C, mainly stemming from the reduction peak of the surface copper oxide on micrometer-sized Cu [31]. The reduction peak of cerium oxide appeared at 460 °C, primarily originating from the reduction peak of the oxygen adsorbed on the surface of cerium oxide [24]. For CeO₂–Cu, two reduction peaks were observed between 100 and 300 °C, attributed to the reduction peaks of Cu₂O and CuO_x, which strongly interact with CeO₂. The addition of cerium oxide led to a shift in the reduction peak position of Cu, indicating that the addition of cerium oxide facilitated the reduction of CuO, consistent with the conclusion that CeO₂ promotes the reduction of CuO_x in the Cu–Ce system [11].

In order to further analyze the enhancement effect of CeO₂ addition on Cu, Raman spectroscopy was conducted to confirm the role of oxygen vacancies of CeO₂–Cu and CeO₂ catalysts, and the results are shown in Figure 6. Two peaks appeared in the region of 200−800 cm⁻¹. The strong peak that appeared at 456 cm⁻¹ could correspond to the F_2g vibration mode of local octahedral symmetry in CeO₂. The broad Raman peak that appeared at around 600 cm⁻¹ could be ascribed to the lattice-defect-induced (D) mode resulting from oxygen defects. The presence of oxide peaks on the surface of micron Cu indicated the existence of CuO_x species [41]. After adding cerium oxide to micro-Cu, the F_2g vibration peak of cerium oxide significantly weakened, possibly due to the interaction between Cu and CeO₂. A reduction in the F_2g peak and a low Raman shift to the D peak appeared on CeO₂–Cu, indicating the presence of a Cu–O–CeO₂ structure [40]. The value of I_D/I_F2g was calculated to reveal the concentration of oxygen vacancies in the CeO₂–Cu and CeO₂ catalysts. It was found that the oxygen vacancy concentration (I_D/I_F2g = 0.387) on the surface of CeO₂–Cu was higher than that of pure CeO₂ (I_D/I_F2g = 0.062). The oxygen vacancy concentration was regarded as the leading active site for the CO₂ hydrogenation reaction, capable of activating CO₂ molecules to produce carbonates for further hydrogenation to produce CO [36,42]. Therefore, Cu–CeO₂ exhibited higher hydrogenation activity than that of cerium oxide at 700 °C during the hydrogenation process.

Metal sintering at high temperatures under a reduced atmosphere was the leading cause of deactivation during the RWGS reaction. A long-lifetime reaction test is a crucial indicator to evaluate the CeO₂–Cu catalyst. The stability test of the RWGS reaction was operated at 700 °C, and it was found that the CeO₂–Cu catalyst maintained good stability (Figure 7). The microstructure of Cu–CeO₂ after the RWGS reaction was analyzed to reveal the elemental distribution of Cu–CeO₂. It was observed that after high-temperature reactions, cerium oxide particles underwent sintering and increased in size, while micro-Cu was enveloped by a shell formed by cerium oxide (Figure 8). Therefore, Cu–CeO₂ exhibits good CO₂ hydrogenation stability at high temperatures.

4. Conclusions

This work focused on the synthesis of a CeO₂–Cu catalyst, by introducing cerium salt on the surface of micron Cu, and applied the RWGS reaction. The experimental results indicated that the modified CeO₂–Cu catalyst demonstrated efficient activity in the CO₂ hydrogenation process. Specifically, CeO₂–Cu exhibited a superior catalytic performance, reaching a conversion rate of 49.82% at 500 °C, which was 204 times higher than that of micro-Cu and 10.9 times higher than that of CeO₂. A synergistic effect appeared between CeO₂ and Cu species within the CeO₂–Cu catalyst. Cu species and oxygen vacancies formed around the Cu–CeO₂ interface, which enhanced the CO₂ hydrogenation performance. Furthermore, results from TEM and BET analysis confirmed the CeO₂-confined structure in the CeO₂–Cu catalyst, as well as the existence of a significant Ce–Cu interface. These structural characteristics contribute to the catalyst’s excellent CO₂ hydrogenation performance.

Micron-sized Cu powders are generated through ball-milling from industrial waste copper. The application of micron Cu is usually limited due to its large particle size and small specific surface area. Modified micro-Cu powder, designed to overcome this limitation and achieve a high catalytic performance, offers an insight into the application potential of industrial waste copper.