Enhancing CO₂ gasification-reforming of municipal solid waste with Ni/CeO₂ and Ni/ZrO₂ catalysts

Abstract

The global energy crisis and environmental sustainability challenges are exacerbated by the rapid increase in population and industrialization, necessitating effective management of municipal solid waste. The CO₂ gasification-reforming of municipal solid waste with Ni/CeO₂ and Ni/ZrO₂ catalysts was conducted in a two-stage fixed-bed reactor. A significant increase in gas production from various waste samples (cabbage, poplar leaves, printed paper, PET, and HDPE) was observed, with the 5% Ni/CeO₂ demonstrating higher efficiency than the 5% Ni/ZrO₂ catalyst. The structural characterization of the catalysts revealed that Ni was more uniformly dispersed on the CeO₂ support compared to ZrO₂, resulting in enhanced activity of the 5% Ni/CeO₂ catalysts. Further exploration into the optimal nickel loading and the ideal reforming temperature was conducted to maximize the efficiency of the CO₂ gasification-reforming. The application of 5% Ni/CeO₂ catalysts in the CO₂ gasification-reforming of simulated municipal solid waste notably increased CO and total gas yields by 223% and 106%, respectively. This advancement holds promise for new technical approaches in resource utilization and the environmentally friendly processing of municipal solid waste.

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Hui Zhou

Dr Hui Zhou is an Associate Professor and the Director of the Solid Waste and Carbon Cycle Utilization Research Center at Department of Energy and Power Engineering, Tsinghua University, China. His research primarily focuses on heterogeneous catalysis for the thermochemical conversion of solid waste and renewable carbon resources. He has received the Leading Young Scientist of the Nation and the ACS Sustainable Chemistry & Engineering Lectureship Awards. He has served as the Chair for the International Conference on Carbon Capture Science and Technology in 2023. Currently, he is the Executive Editor for the journal Carbon Capture Science & Technology.

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1. Introduction

The escalation in population and rapid industrialization are precipitating a global energy crisis and challenges in environmental sustainability.^1,2 The management of municipal solid waste (MSW) is increasingly becoming a critical issue. It is estimated that over two billion tonnes of municipal solid waste are generated globally each year, and this amount is expected to rise sharply in the future.³ Alarmingly, at least a third of this waste is not processed in an environmentally benign manner.⁴ Comprising predominantly hydrocarbons, municipal solid waste represents a source of renewable energy.⁵ Traditional waste management methods, such as landfilling and incineration, offer partial solutions by reducing waste volume but often result in energy wastage and environmental pollution.^6,7 In contrast, the transformation of municipal solid waste into syngas through gasification emerges as a more sustainable and eco-friendlier alternative.⁸

Gasification is a process that transforms organic matter into carbon monoxide and hydrogen under high temperatures and a controlled oxygen environment.⁹ The gasification of municipal solid waste not only reduces the total volume of waste but also enables the recovery of energy, thereby alleviating urban energy pressures.¹⁰ Employing CO₂ as a gasifying agent facilitates the production of CO-rich syngas, while simultaneously offering a viable method for CO₂ utilization.¹¹ To achieve the global objective of reducing CO₂ emissions by 15–20% by 2050, the development of CO₂ utilization technologies is imperative.^12,13 A primary challenge in the CO₂ gasification of municipal solid waste is the incomplete cracking and re-polymerization of hydrocarbons, leading to the formation of tar.¹⁴ The presence of tar degrades the quality of the syngas and limits its applications.¹⁵ Additionally, tar can clog gasifier pipelines, reducing gasification efficiency, and increasing maintenance costs.¹⁶

Enhancing the efficiency of tar CO₂ reforming through the addition of catalysts is identified as the most effective method for tar removal, and it also increases the yield of syngas.¹⁷ Nickel-based catalysts have attracted widespread interest due to their high catalytic activity and cost-effectiveness.¹⁸ Nickel-based catalysts can facilitate the breaking of C–C, C–H, and O–H bonds in tar.¹⁹ The supports of catalysts play a crucial role in the catalytic performance by improving the dispersion of metal.^20,21 Common supports of nickel-based catalysts include Al₂O₃, SiO₂, ZrO₂, and CeO₂.^22–25 ZrO₂ exhibits excellent thermal stability and redox properties and can generate strong interaction with loaded metal, making Ni/ZrO₂ demonstrate superior catalytic performance in various thermochemical reactions, such as CO₂ methanation and CH₄ reforming.^26,27 It was reported that the reduction of ZrO₂ by H₂ led to the formation of oxygen vacancies, thus enhancing the catalytic activity of ZrO₂-supported catalysts.²⁸ In addition, ZrO₂ improved the activity of the nickel-based catalyst in the methane dry reforming at low temperatures.²⁹ CeO₂, distinguished by its high oxygen storage capacity and effective oxygen mobility has emerged as an efficient catalyst support in the realm of heterogeneous catalysis.^30,31 Liu et al.³² reported that Ni/CeO₂ showed good activity and stability during methane dry reforming, which was due to the strong metal-support interactions and the ease of gasification of carbon deposited on the catalysts.

In this work, a comparative analysis was conducted on the catalytic performance of Ni catalysts supported on different supports (CeO₂ and ZrO₂) in the CO₂ gasification-reforming process of municipal solid waste. Through the structural characterization of catalysts, the relationship between the structure and activity of catalysts was revealed. Additionally, the optimal nickel loading on the catalysts and the ideal reforming temperature for the reaction were further explored to maximize the catalyst's effectiveness in the CO₂ gasification-reforming process. This study will provide new technical pathways for the resource utilization and environmentally friendly treatment of municipal solid waste.

2. Materials and methods

2.1. Materials

PET and HDPE were both procured from Yangli Electromechanical Technology Co., Ltd. Poplar leaves were gathered from the trees on the Tsinghua University campus in Beijing, China, and the cabbages and paper were sourced from the supermarket at Tsinghua University. Municipal solid waste predominantly comprises approximately 62.6% food waste, 16.9% plastics, 12.8% paper, 4.7% textiles, and 3.0% leaves.³³ In this study, cabbage was selected to represent food waste, HDPE was chosen as a surrogate for plastics, and PET (Polyester) stood in for textiles. The simulated municipal solid waste consists of 62.6% cabbage, 16.9% HDPE, 12.8% printed paper, 4.7% PET, and 3.0% poplar leaves. Before the experiments, each sample underwent a thorough preparation process, including drying at 105 °C overnight, followed by grinding and sieving. Table S1† displays the approximate and ultimate analyses of all samples.

2.2. Preparation of catalysts

The CeO₂ and ZrO₂ supports were synthesized by the precipitation method. For ZrO₂ supports, 17.5 mmol of Zr(NO₃)₄·6H₂O (99.99%, Macklin) was dissolved in 350 mL of deionized water and stirred at 70 °C for 10 min. A 25–28% ammonia solution was then added dropwise to the Zr(NO₃)₄ solution until the pH reached 10, and the mixture was stirred at 70 °C for 4 h. The CeO₂ support was synthesized using a similar method, substituting Zr(NO₃)₄·6H₂O with Ce(NO₃)₃·6H₂O (99.99%, Sigma-Aldrich). The resultant solution was vacuum-filtered to yield the solid, which was thoroughly washed with deionized water until the pH stabilized at 7. The samples were then dried overnight at 105 °C, followed by calcination at 700 °C for 3 h with a ramp rate of 5 °C min⁻¹.

Ni/CeO₂ and Ni/ZrO₂ catalysts were synthesized utilizing the wet impregnation method. Specifically, for the 5% Ni/CeO₂ catalyst, 52 mg of Ni(NO₃)₃·6H₂O (99.999%, Sigma-Aldrich) was dissolved in 100 mL of deionized water and stirred for 10 min. Subsequently, 2.00 g of CeO₂ support was incorporated, and the suspension was stirred at 90 °C until complete evaporation of the water. The resulting solid was calcined in a muffle furnace at 700 °C for 3 h with a heating rate of 5 °C min⁻¹. Following calcination, the catalysts were reduced under a hydrogen atmosphere at 500 °C for 2 h (5 °C min⁻¹).

2.3. Catalyst characterization

The nickel content in the catalysts was analyzed using inductively coupled plasma spectrometry-atomic emission spectrometry (ICP-AES), employing the Agilent 725 instrument for precise measurement. Comprehensive XRD analysis was conducted on a Bruker AXS GmbH D8 Advance X-ray diffractometer, employing Cu Kα radiation across a broad range of angles from 10° to 90°, which facilitated detailed phase identification and crystal structure analysis. To evaluate the textural properties of the catalysts, N₂ adsorption–desorption isotherms were meticulously recorded using a Micromeritics ASAP2012 analyzer. This precise measurement allowed for the accurate determination of specific surface area, average pore diameter, and total pore volume, providing valuable insights into the porosity of the materials. High-resolution SEM (Zeiss, Merlin) was employed for a meticulous investigation of surface morphology to reveal the detailed surface features of catalysts. Furthermore, HAADF-STEM-EDS on the JEOL JEM-2100F instrument was utilized to gain a deeper understanding of the catalysts' elemental distribution and structural characteristics at the nanoscale. The coke deposition on catalysts was studied by the combustion experiments in the air atmosphere from room temperature to 800 °C at a heating rate of 10 °C min⁻¹ in a thermogravimetric analyzer (NETZSCH STA 409C/3/F).

2.4. CO₂ gasification-reforming of municipal solid waste

Experimental studies on CO₂ gasification-reforming of solid waste samples were conducted in a two-stage fixed-bed reactor connected to a micro-gas chromatograph (Micro-GC) as shown in Fig. 1. In a typical experiment, 1 g of sample was loaded in a crucible for the gasification stage, and 0.1 g of Ni/CeO₂ or Ni/ZrO₂ catalyst was fixed with quartz wool for the reforming stage. The CO₂ flow rate was set at 50 mL min⁻¹. Initially, the temperature in the reforming stage was gradually increased to a predefined target of 600, 700, or 800 °C at a heating rate of 10 °C min⁻¹. This was followed by a steady heating of the gasification stage from room temperature to 600 °C, progressing at a consistent rate of 4 °C min⁻¹, and kept at 600 °C for 30 min. The whole gasification process would last for 3 h. The liquid by-products were gathered by a condenser submerged in an icy water blend. Online gas composition analysis was efficiently conducted using the Micro-GC (Fusion 2-Module System, Inficon), where argon and helium were utilized as carrier gases for Module A (Rt-Molsieve 5A) and Module B (Rt-U-Bond), respectively. Micro-GC was programmed to automatically sample every 2 min, utilizing N₂ at a flow rate of 10 mL min⁻¹ as the internal standard for accurate quantification of gas production. Micro-GC extracted only 10 μL volume of gas per analysis. At the end of experiments, total gas production was quantified through analysis of the sample bags with Micro-GC.

Fig. 1 Schematic diagram of the fixed bed reactor.

3. Results and discussion

3.1. CO₂ gasification-reforming of municipal solid waste

3.1.1. CO₂ gasification-reforming without catalysts. The CO₂ gasification of various municipal solid waste samples, including cabbage, poplar leaves, printed paper, PET, and HDPE, was carried out in the two-stage fixed-bed reactor connected to a Micro-GC. The samples initially underwent gasification to produce volatiles, followed by a secondary reforming with CO₂. The reforming segment of the reactor was preheated to 700 °C, after which the municipal solid waste samples were subjected to a gradual heating process, reaching 600 °C at a rate of 4 °C min⁻¹. The online gas production rates of samples during CO₂ gasification-reforming are illustrated in Fig. 2. The gas production rates for cabbage, poplar leaves, printed paper, PET, and HDPE peaked at approximately 270, 295, 360, 440, and 485 °C, respectively. Upon examining the initial temperatures for thermal decomposition, it was observed that the synthetic polymers PET and HDPE exhibited markedly superior thermal stability compared to the three other samples, aligning with results related to thermal stability in the pyrolysis process.³⁴

Fig. 2 The gas production during CO₂ gasification-reforming of municipal solid wastes without catalysts at a reforming temperature of 700 °C: (a) cabbage, (b) poplar leaves, (c) printed paper, (d) PET, and (e) HDPE.

Fig. 3 presents the gas yields for various samples throughout the experimental process. Notably, the CO yields from cabbage, poplar leaves, printed paper, and PET exceeded the yields of H₂ and CH₄. However, HDPE's CO yield was almost negligible, yet it exhibited the highest CH₄ yield among all samples, attributed to the cleavage of chain hydrocarbons.³⁵ In scenarios lacking a catalyst, the total gas yields of these samples were relatively low, following a descending order: PET (8.2 mmol g_sample⁻¹) > HDPE (6.5 mmol g_sample⁻¹) > poplar leaves (5.6 mmol g_sample⁻¹) > cabbage (3.5 mmol g_sample⁻¹) > printed paper (1.1 mmol g_sample⁻¹).

Fig. 3 The gas yields of cabbage, poplar leaves, printed paper, PET, and HDPE during non-catalytic and catalytic CO₂ gasification-reforming (blue: H₂, purple: CH₄, pink: CO).

3.1.2. CO₂ gasification-reforming with Ni/CeO₂ and Ni/ZrO₂ catalysts. Incorporating catalysts into the CO₂ gasification-reforming process of municipal solid waste can significantly elevate gas generation, as delineated in Fig. 3. The deployment of 5% Ni/CeO₂ catalysts resulted in a substantial increase in the total gas yields from cabbage, poplar leaves, printed paper, PET, and HDPE, augmenting yields by 246%, 145%, 455%, 254%, and 563%, respectively. Similarly, the use of 5% Ni/ZrO₂ catalysts enhanced the total gas production for the same materials by 146%, 107%, 345%, 211%, and 555%, respectively. The efficacy of these nickel-based catalysts in boosting gas production was attributed to their role in promoting the cleavage of C–H and C–C bonds within the volatiles.³⁶ Comparative analysis revealed that the 5% Ni/CeO₂ catalysts demonstrated superior catalytic activity compared to 5% Ni/ZrO₂ catalysts, a phenomenon likely influenced by the support's impact on the dispersion of nickel.³⁷

In the CO₂ gasification-reforming processes for various municipal solid waste components, catalysts exhibited distinct levels of activity. Typically, for HDPE, the CO yield in the absence of catalysts was negligible. However, the introduction of the catalyst led to a dramatic increase in CO production, reaching approximately 31 mmol g_sample⁻¹, a yield significantly higher than that of the other four samples. Proximate analysis revealed that HDPE's volatile matter content was 100 wt% (Table S1†), however, the gas yield without the catalysts was a mere 6.5 mmol g_sample⁻¹. This suggested a substantial presence of condensable components in the volatiles. Catalysts markedly enhanced the reforming efficiency of tar, leading to a significant increase in the gas production of HDPE.

The online gas production rates from municipal solid waste during CO₂ gasification-reforming with 5% Ni/CeO₂ and 5% Ni/ZrO₂ catalysts are illustrated in Fig. S1–S5.† The peak temperatures aligned with those observed in the absence of catalysts, as the catalysts did not come into contact with the samples, thereby not influencing their decomposition. Nevertheless, the introduction of these catalysts significantly elevated the maximum values and peak areas of CO, H₂, and CH₄ production. Moreover, the composition of cabbage, poplar leaves, and printed paper was complex, including cellulose, lignin, and hemicellulose.³⁸ Consequently, their gasification processes were intricate, manifesting multiple peaks in gas generation. In contrast, synthetic polymers such as HDPE and PET, synthesized from monomers, exhibited a simpler structure, thereby resulting in a singular, prominent peak in gas production.³⁹

3.2. Optimization of Ni loading and reforming temperature for the CO₂ gasification-reforming

Given its highest gas yields among the five samples, HDPE was chosen to investigate the ideal Ni loading for Ni/CeO₂ catalysts and the optimal reforming temperature. First, the pivotal role of CO₂ was underscored by a comparative experiments between N₂ and CO₂ atmospheres. Gas yields for HDPE in both atmospheres with and without the catalysts are depicted in Table S2.† The introduction of the 5% Ni/CeO₂ catalyst in a N₂ atmosphere had a minimal impact on gas production, suggesting negligible influence on the cracking of HDPE volatiles. Conversely, the catalyst's presence in a CO₂ atmosphere markedly enhanced gas production. This boost was credited to the catalyst's facilitation of CO₂ reforming of volatile components, leading to substantial syngas generation. The absence of a catalyst in both N₂ and CO₂ atmospheres showed no notable variation in gas yields, indicating the CO₂ reforming of volatiles was difficult to occur in the absence of a catalyst. These findings underscore the significant role of CO₂ and the high activity and effectiveness of the Ni/CeO₂ catalyst in facilitating the gasification-reforming process of municipal solid waste.

Catalysts with different Ni loadings (2, 5 and 10 wt%) were assessed at CO₂ flow rate of 50 mL min⁻¹ and a reforming temperature of 700 °C. As shown in Table S3,† ICP-AES analysis verified the accuracy of Ni/CeO₂ catalyst loadings. As the Ni loadings increased, there was a notable decline in the gas production from HDPE, a trend clearly illustrated in Fig. 4 a. The yields of H₂, CH₄ and CO decreased from 1.9, 2.7 and 7.4 mol g_sample⁻¹ g_Ni⁻¹ with 2% Ni/CeO₂ catalysts to 0.7, 0.3 and 2.4 mol g_sample⁻¹ g_Ni⁻¹ with 10% Ni/CeO₂ catalysts, respectively. It was reported that with a Ni loading of 2 wt%, the Ni species were predominantly atomic or nanocluster forms, with just 8% as nanoparticles.⁴⁰ In contrast, Ni nanoparticles constituted 32% at 10 wt% Ni loading.⁴⁰ Hence, it was surmised that the reaction's active sites predominantly comprised Ni atoms or nanoclusters and the aggregation of Ni led to diminished catalytic effectiveness.

Fig. 4 Effects of (a) Ni loadings (2, 5 and 10 wt%) and (b) reforming temperatures (600, 700, and 800 °C) on gas production during the CO₂ gasification-reforming of HDPE.

The impact of varying reforming temperatures (600, 700, and 800 °C) on gas production from HDPE is clearly demonstrated in Fig. 4b. As the reforming temperature increased from 600 to 700 °C, a marked increase in both CO and total gas yields was observed, rising from 13.4 and 21.4 mmol g_sample⁻¹ to 31.7 and 43.1 mmol g_sample⁻¹, respectively. This phenomenon was ascribed to the endothermic properties inherent in tar's CO₂ reforming.⁴¹ Necessary heat uptake for the reaction implied a propensity for improved gas production at higher temperatures. Nevertheless, a further rise in the reforming temperature to 800 °C yielded no significant enhancement in gas production, with only a minor increase in CO yield of 1.4 mmol g_sample⁻¹. At elevated temperatures, the accelerated tar cracking resulted in the char formation, thereby diminishing the gas output from the CO₂ reforming of tar.⁴² When the temperature was raised from 600 to 800 °C, the H₂ yield initially increased and then declined. The H₂ production was not only associated with tar CO₂ reforming but also closely linked to the reverse water-gas shift reaction (CO₂ + H₂ → CO + H₂O), where an increase in reforming temperature shifted the reaction equilibrium to the right.⁴³

3.3. Stability of the catalyst

The stability experiments were conducted under the reforming temperature of 700 °C and Ni loading of 5 wt%. The stability of 5% Ni/CeO₂ catalyst was evaluated over five cycles of CO₂ gasification-reforming of HDPE (Fig. 5). The CO yield decreased with the increase of the number of cycles, dropping by 13.8% after five cycles—from 31.7 to 27.3 mmol g_sample⁻¹. The H₂ and CH₄ yields decreased from 8.5 and 2.9 mmol g_sample⁻¹ to 6.5 and 1.2 mmol g_sample⁻¹, respectively. The catalyst's ability to retain a significant portion of its initial activity, even after five cycles, underscores its potential for long-term application in relevant industrial processes. The deactivation mechanism of catalysts will be discussed in the following session.

Fig. 5 The stability of 5% Ni/CeO₂ catalyst through five cycles of CO₂ gasification-reforming of HDPE.

3.4. Characterization of fresh and used Ni/CeO₂ and Ni/ZrO₂ catalysts

Scanning electron microscopy (SEM) analysis of the CeO₂ and ZrO₂ supports revealed an irregular block-like structural morphology as shown in Fig. S6.† CeO₂ and ZrO₂ displayed similar specific surface areas, approximately 137 and 136 m² g⁻¹ respectively, as detailed in Table 1. However, the incorporation of 5 wt% Ni led to a significant decrease in the specific surface area, dropping to 52 m² g⁻¹ for 5% Ni/CeO₂ and 53 m² g⁻¹ for 5% Ni/ZrO₂. This decrease primarily stemmed from the clogging of support pores by Ni species or from sintering of the catalyst during calcination.⁴⁴ Concurrently, both the average pore diameter and total pore volume experienced a decline. Specifically, 5% Ni/CeO₂ exhibited an average pore diameter and total pore volume of 12.3 nm and 0.18 cm³ g⁻¹, respectively, which were obviously lower than those of pure CeO₂ support. Similarly, 5% Ni/ZrO₂ showed a reduction in both average pore diameter and total pore volume compared to ZrO₂ support. The N₂ adsorption–desorption isotherms for all the materials revealed Type IV isotherms, featuring typical H1 type hysteresis loops as shown in Fig. S7,† highlighting their mesoporous characteristics.⁴⁵

Table 1 Specific surface area and pore characteristics of the supports and catalysts

Material	Specific surface area (m^2 g^−1)	Average pore diameter (nm)	Total pore volume (cm^3 g^−1)
CeO2	137	22.1	0.77
ZrO2	136	21.2	0.76
5% Ni/CeO2	52	12.3	0.18
5% Ni/ZrO2	53	8.0	0.13

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Prominent XRD diffraction peaks at 28.5, 33.1, 47.5, and 56.3° were identified for CeO₂ and 5% Ni/CeO₂, corresponding accurately to the cubic fluorite structure typical of CeO₂, as shown in Fig. 6. In the case of ZrO₂ and 5% Ni/ZrO₂, the detection of both tetragonal and monoclinic phases was consistent with results reported in previous studies.⁴⁶ The Ni diffraction peak, located around 44.6°, was evident in the 5% Ni/CeO₂ and 5% Ni/ZrO₂ catalysts. Significantly, the Ni diffraction peak in the 5% Ni/ZrO₂ catalyst exhibited greater prominence and sharper definition relative to its counterpart in the 5% Ni/CeO₂ catalyst. This observation implied a more efficient dispersion and a diminution in the size of Ni nanoparticles on the CeO₂ support.⁴⁶

Fig. 6 XRD patterns of CeO₂, ZrO₂, 5% Ni/CeO₂, and 5% Ni/ZrO₂ catalysts.

High-resolution transmission electron microscopy (HR-TEM) images of the 5% Ni/CeO₂ and 5% Ni/ZrO₂ revealed that the catalysts' structures comprised agglomerations of numerous particles, each measuring between 10–20 nm in diameter (Fig. S8†). Given that both cerium and zirconium possessed higher atomic numbers than nickel, Ni nanoparticles were not readily discernible on the CeO₂ and ZrO₂ using HR-TEM due to contrast limitations. To overcome this, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) coupled with energy-dispersive X-ray spectroscopy (EDS) was employed to examine Ni distribution on these supports. The HAADF-STEM-EDS analysis clearly showed a more uniform dispersion of Ni on the CeO₂ support compared to ZrO₂ as shown in Fig. 7. This enhanced dispersion correlated with the findings from XRD analysis and likely contributed to the superior catalytic activity observed in the 5% Ni/CeO₂ catalyst during CO₂ gasification-reforming of municipal solid wastes due to the increased surface area for reactions and better interaction between the Ni particles and the CeO₂ surface.⁴⁷

Fig. 7 HAADF-STEM-EDS images of (a and b) 5% Ni/CeO₂ and (c and d) 5% Ni/ZrO₂ catalysts.

TGA and SEM analyses were performed on the used catalysts after 5 cycles to reveal the deactivation mechanism. TGA experiments were performed under air atmosphere to evaluate the type and amount of the deposited coke, as shown in Fig. S9a.† The two peaks of the DTG curve at about 400 and 600 °C correspond to the combustion of amorphous carbon and carbon nanotubes, respectively.^48,49 Predominantly, the carbon deposits identified in this study were in the form of carbon nanotubes. The total carbon deposit was around 40 wt%. This observation was in line with findings from other research, which had highlighted the role of Ni-based catalysts in fostering the formation of carbon nanotubes during the pyrolysis and gasification of plastics.^50,51 The morphology of the carbon nanotubes were further examined using SEM (Fig. S9b†).

3.5. CO₂ gasification-reforming of simulated municipal solid wastes with Ni/CeO₂ catalysts

The 5% Ni/CeO₂ catalysts were employed for the CO₂ gasification-reforming of simulated municipal solid waste. It was reported that municipal solid waste predominantly comprises approximately 62.6% food waste, 16.9% plastics, 12.8% paper, 4.7% textiles, and 3.0% leaves.³³ The simulated municipal solid waste used in this study was composed of 62.6% cabbage, 16.9% HDPE, 12.8% printed paper, 4.7% PET, and 3.0% poplar leaves. As demonstrated in Table 2, in the absence of catalysts, the production of CO and total gas were 2.0 and 4.4 mmol g_sample⁻¹, respectively. The introduction of the 5% Ni/CeO₂ catalyst led to a significant uptick in gas yields, with CO and total gas outputs rising by 223% and 106%, achieving 6.8 and 9.1 mmol g_sample⁻¹, respectively. A comparison between the experimental and theoretical gas yields calculated by the sum of individual components of the municipal solid waste highlighted a significant discrepancy. The theoretical predictions substantially outstripped the experimental results. This disparity underscored the complexity of the CO₂ gasification-reforming process of actual municipal solid waste due to the intricate interactions among various components, and the gas yields cannot be accurately determined by summing the results of individual component gasification.⁵²

Table 2 The experimental and theoretical gas yields of simulated municipal solid waste^a

Condition		Gas yield (mmol gsample^−1)
		H2	CH4	CO	Total gas
a Remark: reaction conditions: reforming temperature of 700 °C.
Without catalyst		1.1	1.3	2.0	4.4
With 5% Ni/CeO2 catalyst	Experimental values	1.1	1.3	6.8	9.1
	Theoretical values	2.4	1.4	13.6	17.4

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4. Conclusions

The CO₂ gasification-reforming of municipal solid waste was investigated in a two-stage fixed bed reactor. In scenarios lacking a catalyst, the total gas yields of these samples were relatively low, following a descending order: PET > HDPE > poplar leaves > cabbage > printed paper. In addition, the synthetic polymers PET and HDPE exhibited markedly superior thermal stability compared to the three other samples. Incorporating 5% Ni/CeO₂ and 5% Ni/ZrO₂ catalysts into the CO₂ gasification-reforming process of municipal solid waste significantly elevated gas generation, and the 5% Ni/CeO₂ catalysts demonstrated superior catalytic activity. XRD and HAADF-STEM-EDS analysis clearly showed a more uniform dispersion of Ni on the CeO₂ support compared to ZrO₂, leading to enhanced activity of the 5% Ni/CeO₂ catalysts. Comparative studies of catalysts with different Ni loadings (2, 5 and 10 wt%) demonstrated that Ni aggregation led to a decrease in catalytic activity. Furthermore, stability testing revealed that after five reaction cycles, the catalyst experienced a certain level of deactivation, primarily due to coke deposition. Using 5% Ni/CeO₂ catalysts in CO₂ gasification-reforming of simulated municipal solid waste significantly enhanced CO and total gas production, with increases of 223% and 106%, respectively.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52276202), National Key R&D Program of China (2023YFC3905701), Huaneng Group Science and Technology Research Project (KTHT-U23GCZH01, KTHT-U22YYJC12), Tsinghua-Jiangyin Innovation Special Fund (TJISF), Tsinghua-Toyota Joint Research Fund, and State Key Laboratory of Chemical Engineering (SKL-ChE-22A01).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta00665h

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