Sustainable Hydrogen from Methanol: NiCuCe Catalyst Design with CO₂-Driven Regeneration for Carbon-Neutral Energy Systems

Abstract

This study addresses energy transition challenges through the development of NiCuCe catalysts for high-purity hydrogen production via methanol decomposition, with carbon deposition issues mitigated by CO₂-assisted regeneration. As fossil fuel depletion advances and the urgency of climate change increases, methanol-derived hydrogen (CH₃OH → CO + 2H₂) emerges as a carbon-neutral alternative to conventional fossil fuel-based energy systems. The catalyst’s dual Cu²⁺/Ni²⁺ active sites facilitate selective C–O bond cleavage, achieving more than 80% methanol conversion at temperatures exceeding 280 °C without the need for fossil methane inputs. Crucially, CO₂ gasification enables catalyst regeneration through the conversion of 90% carbon deposits into reusable media, circumventing energy-intensive combustion processes. This dual-function system couples carbon capture to hydrogen infrastructure, thereby stabilizing production while valorizing waste CO₂. This innovation minimizes reliance on rare metals through efficient regeneration cycles, mitigating resource constraints during energy crises.

1. Introduction

Methanol, serving as a crucial energy carrier and chemical feedstock, possesses significant strategic value in the sustainable energy transition [1,2]. Its molecular structure (CH₃OH) combines a high hydrogen-to-carbon ratio with liquid-state storage/transportation capabilities, overcoming the hydrogen storage and long-distance transportation bottlenecks [3]. Currently, approximately 75% of global methanol production depends on fossil energy sources (i.e., coal and natural gas). Nevertheless, green pathways—including biomass gasification and catalytic CO₂ synthesis using green hydrogen—achieve 65–95% life-cycle carbon reductions, positioning methanol as a critical medium that bridges renewable energy and end-use applications [4,5]. Notably, in maritime decarbonization, green methanol has been designated by the International Maritime Organization as a priority 2050 zero-carbon fuel candidate, due to its high energy density and infrastructure compatibility [6,7].

Nickel-based catalysts represent a technological breakthrough in methanol-to-energy conversion [8,9,10]. When benchmarked against traditional noble metal catalysts, they significantly reduce C–H and O–H bond dissociation energy barriers through strong metal–support interactions that modulate the electronic configuration of active sites. Their unique advantages are twofold: First, the suppression of C–O coupling side reactions maintains CO selectivity below industrial threshold levels, substantially cutting purification costs. Second, the homogeneity of nanoscale alloy particles enhances low-temperature activity, enabling the optimization of reaction conditions for ambient-pressure operation. Notably, in situ regeneration has emerged as a promising strategy to ensure sustained reaction continuity and economic feasibility [11,12]. Nevertheless, the interdependencies between catalyst properties, operating temperatures, and carbon deposition in relation to regeneration efficiency require further mechanistic investigation. Yusuf et al. [13] investigated the effects of promoters and trace amounts of heterogeneous catalysts in suppressing carbon deposition while enhancing oxygen storage capacity and basicity. They found that although high Ni loading exhibits exceptionally high initial CH₄ and CO₂ conversion rates, its long-term stability remains poor due to Ni aggregation at elevated temperatures; it forms large Ni crystallites that induce severe coke formation and catalyst deactivation within short time intervals. The optimal loading of active metals on catalyst supports depends on specific metal systems and the surface area of support materials. Bimetallic catalysts are comparable to metal-promoted catalysts but differ in terms of the magnitude of metal–support interactions and metal–metal interactions. The addition of alkaline earth metal promoters (e.g., Ca and rare earth metals, such as Sc and Y) enhances overall catalyst basicity and oxygen vacancies, thereby improving coke resistance and structural stability. Wang et al. [14] studied the reusability and deactivation mechanisms of Ni-Ca-Mg-Al catalysts, revealing that the interaction between active nickel species and methanol preferentially enhances catalytic activity. The impregnation method facilitated the formation of NiAl₂O₄ species, whereas the coprecipitation method suppressed their formation, yielding a higher specific surface area, pore volume, active metal oxide (Ni²⁺) content, and superior Ni dispersion at lower pyrolysis temperatures. Furthermore, NiCaMgAl catalysts demonstrated excellent reusability and hydrothermal stability, attributed to both reduced graphitic carbon deposition and suppressed sintering via lower coke accumulation.

This study systematically investigates the reaction mechanisms of NiCuCe catalysts in methanol conversion and evaluates the feasibility of regeneration strategies for Ni-based catalysts. By elucidating the regeneration mechanisms of NiCuCe catalysts during methanol decomposition, this work provides new insights into designing highly efficient and stable non-precious Ni-based catalysts. These findings advance the scalable application of clean hydrogen production technologies while reducing carbon emissions associated with conventional fossil fuel-derived hydrogen processes. This research demonstrates enhanced atomic efficiency in methanol conversion and extends catalyst service life through regenerable properties, offering innovative solutions to achieve resource circularity in the chemical industry.

2. Results and Discussion

2.1. Catalyst Reaction Mechanism

Figure 1 shows the XRD patterns of NiCuCe catalysts before and after the methanol conversion reaction. The post-reaction catalysts exhibit stronger CuO diffraction peaks accompanied by particle aggregation, a larger crystallite size, and enhanced crystallinity. Similarly, the NiO peak intensities increase after the reaction, indicating increased crystallinity. In contrast, NiAl₂O₃ peaks weaken, suggesting reduced phase content and crystallinity. The appearance of carbon diffraction peaks confirms carbon deposition as the primary deactivation mechanism. Figure 2 presents N₂ adsorption–desorption isotherms of NiCuCe catalysts pre- and post-reaction. All samples show Type IV isotherms with H1-type hysteresis loops (according to IUPAC classification), confirming mesoporous structures. Subsequent analyses focused on the specific surface area, pore volume, and average pore size within the mesoporous framework.

Figure 3 shows the time-dependent methanol cracking rate, H₂ selectivity, and CO selectivity profiles of the NiCuCe catalyst at 340 °C. The results indicate stable cracking rates and product selectivity over time, demonstrating preserved active sites and robust catalytic durability. However, H₂ selectivity shows a gradual decline after 16 h of operation, suggesting initial catalyst deactivation.

Table 1. BET data of NiCuCe catalyst before and after reaction.

Type	SSA (m2/g)	Vp (m3/g)	Dp (nm)
Post-reaction	127.95	0.16	5.93
Pre-reaction	149.04	0.21	6.77

summarizes the BET surface area, pore volume, and pore size of NiCuCe catalysts pre- and post-reaction. The reduced specific surface area may primarily result from the synergistic effects of metal particle agglomeration and pore blockage. The reduction in the specific surface area of the catalyst after the reaction reflects the potential for loss of catalyst active sites. Concurrent decreases in pore volume and pore size correlate with performance degradation, consistent with the trends observed in Figure 3. Understanding the particle size and morphology of catalysts is critical for evaluating their catalytic stability and activity. Figure 4 illustrates the pore size distributions of the NiCuCe catalyst before and after the reaction, obtained via the Barrett–Joyner–Halenda (BJH) method. As shown in Figure 4, the pre-reaction catalyst displays a smaller modal pore size (the peak position in the distribution curve), indicating that the predominant pore size range aligns more closely with the average pore size. This observation corroborates the diminished catalytic activity observed post-reaction. Importantly, the NiCuCe catalyst exhibits a smaller average metal particle size compared to the pure Ni catalyst, which stems from the synergistic interplay between the CeO₂ oxygen vacancy anchoring effect and Cu-mediated electronic modulation, both of which effectively inhibit metal particle sintering and aggregation [15]. Figure 5 presents the scanning electron microscopy (SEM) images of the NiCuCe catalyst before and after the reaction. As shown in Figure 5, the structural evolution of the catalyst is primarily reflected in the particle size, surface morphology, and pore dimensions, while the overall microscopic morphology remains largely consistent. The post-reaction catalyst displays a densely packed granular texture, whereas the pre-reaction catalyst surface exhibits a hybrid structure comprising both granular and partially flaky features. Notably, the post-reaction catalyst exhibits a reduction in pore size and an increase in surface particle size, suggesting enhanced crystallinity, which is detrimental to the improvement in catalytic performance.

2.2. Catalyst Active Center

The active centers of Ni–Cu-based catalysts, particularly the valence states of Ni and Cu, remain a subject of extensive debate. For Ni, potential active species may include Ni⁰, Ni⁺, and Ni²⁺, while Cu could exist as Cu⁰, Cu⁺, or Cu²⁺. Notably, these valence states exhibit dynamic evolution under varying reaction conditions. In the methanol cracking experiments, Ni²⁺ and Cu²⁺ were identified as the most plausible active centers. Importantly, the introduction of CeO₂ in the NiCuCe catalyst plays a dual role in enhancing methanol activation [16,17,18]: (1) CeO₂ stabilizes the Ni–Cu alloy through strong metal–support interactions (SMSIs), where oxygen vacancies in CeO₂ act as anchoring sites to inhibit particle sintering, thereby preserving the exposure of active Ni²⁺/Cu²⁺ sites; (2) The Ce³⁺/Ce⁴⁺ redox cycle facilitates electron transfer at the Ni–Cu–CeO₂ interface, weakening the C–O bond in the adsorbed methoxy (CH₃O*) intermediates and lowering the energy barrier for C–H bond cleavage. The experimental proof is as follows:

(1)

Methanol cracking products (H₂ and CO) exhibit strong reducing properties. Fresh catalysts predominantly contain CuO (Cu²⁺). Figure 1 shows enhanced CuO crystallinity with the reaction time, which coincides with catalytic deactivation, suggesting a progressive reduction of Cu⁺/Cu²⁺ to Cu⁰.

(2)

If Cu⁰ were the active species, then catalytic performance should stabilize or improve with Cu⁰ accumulation. However, Figure 3 demonstrates a gradual activity loss, which contradicts this hypothesis.

(3)

Choi et al. [19] report increased Cu²⁺ content during methanol reforming, correlating with enhanced H₂ selectivity, thereby supporting Cu²⁺ as the active species.

(4)

Figure 1 reveals intensified NiO (Ni²⁺) diffraction peaks post-reaction, despite the marginal performance decline. This indicates that Ni²⁺ → Ni⁰ reduction dominates over time. Furthermore, Kwon et al. [20] demonstrate that NiAl₂O₄ spinel enhances dispersion of active Ni species in Ni/Al₂O₃ systems. Ni²⁺ ions liberated from NiAl₂O₄ form smaller metallic Ni particles (<5 nm) while strengthening metal–support interactions, thereby improving the activity, selectivity, stability, and coking resistance.

2.3. Response Pathways

The methanol cracking process involves a complex reaction network governing the formation of byproducts (H₂, CO, and dimethyl ether). As illustrated in Figure 6, thirteen predominant reaction pathways were identified based on the reaction products observed in our experimental system and corroborated by the prior literature [21,22,23]. While the O-H bond cracking pathway (CH₃OH → CH₃O + H) is recognized as the primary initiation step in this system, alternative reaction routes warrant consideration. Notably, the C-O bond cracking pathway (CH₃OH → CH₃ + OH), despite its higher activation energy requirement, exhibits potential feasibility under elevated temperatures. CeO₂ may regulate reaction pathways by stabilizing C-O bonds through oxygen vacancies generated by Ce³⁺ species. These oxygen vacancies function as active sites for adsorbing oxygen-containing intermediates (e.g., CH₃O⁻ or CO), thereby extending their residence time via C-O bond stabilization. This mechanistic framework preferentially promotes subsequent oxidation toward CO₂ formation over deoxygenation pathways yielding CH₄. These steps can be rationalized as follows:

CH₃OH → CH₃O + H

(1)

2CH₃O → (CH₃O)₂ (DME)

(2)

CH₃O → CH₂O + H

(3)

CH₂O → CO + H₂

(4)

CH₃O + CH₂O → CH₃OCHO + H

(5)

CH₃OCHO → CH₃OCHO (MF)

(6)

CH₃OCHO → CO₂ + H₂

(7)

CH₃OCHO → CO + H₂

(8)

CO + H₂ → CH₄ + H₂O

(9)

3H₂ + 3CO → (CH₃O)₂ + CO₂

(10)

CH₄ → C + 2H₂

(11)

2CO → C + CO₂

(12)

C + CO₂ → 2CO

(13)

This study synthesized NiCuCe along with NiCuY, NiCuZn, NiCuLa, and NiCuIn catalysts for methanol cracking applications. Their catalytic performance (activity and selectivity) was systematically evaluated over the temperature range of 220–400 °C. The results reveal NiCuCe’s superior overall performance, confirming the system’s practical relevance. Figure 7 compares the temperature-dependent behaviors of these catalysts. Figure 7a shows that NiCuCe maintains >99% cracking efficiency at temperatures above 320 °C but exhibits limited low-temperature activity (33.1% conversion at 220 °C). As depicted in Figure 7b, NiCuCe achieves sustained H₂ selectivity (>80%) throughout the entire tested temperature range. Figure 7c reveals stable CO selectivity for NiCuCe, with a peak value observed below 380 °C.

Figure 7 reveals two distinct temperature regimes for NiCuCe catalyst performance: T < 280 °C and T > 280 °C. These regimes exhibit fundamental differences in methanol cracking rate dynamics and H₂/CO selectivity evolution. Below 280 °C, the cracking rate increases rapidly with temperature (activation-controlled regime), while above 280 °C, the rate plateaus near maximum conversion (diffusion-limited regime). The CO selectivity remains below 50% at T < 280 °C, with CO₂ and DME concentrations increasing proportionally. This proportional relationship persists above 280 °C, suggesting the occurrence of 3H₂ + 3CO → C₂H₆O + CO₂

Above 280 °C, decreasing CO₂ levels accompany rising CO selectivity, consistent with the carbon gasification reaction: C + CO₂ → 2CO (as evidenced by carbon deposition patterns in Figure 1). This carbon-mediated pathway represents a newly identified behavior for NiCuCe in methanol catalysis. In the low-temperature regime (<280 °C), minimal methyl formate formation, fluctuating H₂ selectivity, and stable CO selectivity were observed alongside accumulating CO₂/DME. These trends indicate Reactions (2), (7), and (10) as the rate-limiting steps. Conversely, at elevated temperatures (>280 °C), H₂/CO selectivity and methane yield are enhanced while CO₂/DME concentrations are reduced, with Reactions (3), (9), and (13) becoming dominant.

2.4. Regeneration of Ni-Based Catalysts

Numerous strategies—including promoter doping, support engineering, and nanostructural optimization—have been developed to enhance Ni-based catalysts’ performance. Nevertheless, their inherent deactivation under high-temperature operation remains a critical challenge. In situ regeneration through periodic carbon gasification emerges as a sustainable mitigation strategy. The mechanism leverages the Boudouard reaction:

C + CO₂ → 2CO

During methanol cracking, simply introducing excess CO₂ fails to prevent carbon deposition, rendering the in situ regeneration of Ni-based catalysts particularly challenging. Djinovic et al. [24] observed the regenerability of NiCo/Ce_0.8Zr_0.2O₂ aerogel catalysts during CO₂ reforming of methane, which inspired Moura-Nickel et al. [11] to conduct pioneering research on cyclic dry reforming of methane and in situ regeneration over Ni-Y₂O₃-Al₂O₃ aerogel catalysts. However, although the feasibility of in situ regeneration has been preliminarily demonstrated, the relationships between regeneration efficacy and catalyst properties, operating temperatures, and carbon deposition levels/types remain indeterminate without advanced techniques for the precise quantification and characterization of carbon deposits. This constitutes a primary limitation in current in situ regeneration strategies for Ni-based catalysts. The main pathways of carbon formation are as follows:

CH₄ → C + 2H₂

2CO → C + CO₂

Carbon deposits can be identified through XRD patterns, while their morphological types are partially observable via TEM imaging. However, the precise quantification and complete characterization of deposited carbon requires TPO analysis. This study employs in situ TPO measurements to accurately determine carbon content and speciation, thereby eliminating the material loss risks associated with catalyst transfer. During TPO analysis, the simultaneous monitoring of CO and CO₂ production enables a full carbon mass balance calculation. Notably, all TPO experiments exclusively detected CO₂ emissions, with no measurable CO release, confirming complete CO-to-CO₂ oxidation on Ni catalytic sites [25]. In situ CO₂ TPD measurements were conducted to evaluate carbon gasification behavior. Figure 8 shows the TPO profile of the spent catalyst, revealing three distinct temperature zones: below 100 °C, 100–300 °C, and above 300 °C, with maximum carbon consumption occurring between 100 and 300 °C. The TPO peak positions correlate with carbon species types and spatial distribution on the catalysts [25,26]. The peaks in the 100–300 °C range correspond to the oxidation of non-filamentous/encapsulated coke near active sites (catalyzed by metallic centers), while amorphous carbon on supports, filamentous carbon (away from active sites), and graphitic carbon require higher oxidation temperatures (>300 °C). Methane decomposition (CH₄ → C + 2H₂) and carbon gasification (C + CO₂ → 2CO) exhibit opposing temperature dependencies: elevated temperatures promote carbon deposition in the former but suppress it in the latter, resulting in maximum carbon accumulation at 200 °C for both catalysts.

In situ TPD-CO₂ measurements were conducted on spent catalysts, followed by in situ TPO tests to assess regeneration capacity. Figure 9 shows the TPD-CO₂ profile, where CO₂ consumption quantifies removable carbon types via gasification. Gasification initiates at approximately 50 °C with peaks at 120 °C and 320 °C, indicating temperature-dependent inhibition. Figure 10 displays post-TPD-CO₂ TPO profiles, revealing residual carbon characteristics. After TPD-CO₂ treatment, most of the non-filamentous/encapsulated carbon is removed, shifting the TPO peak from 200 °C (Figure 8) to 100 °C due to the irreversible carbon formed during high-temperature gasification.

Table 2. Specific data from in situ regeneration test.

Test Items	Peak Temperature(°C)	Peak Area	Oxygen Consumption (mmol/g)
TPO	208 °C	331	1.7212
TPO after TPD-CO2	93 °C	108	0.5616

summarizes the regeneration data: CO₂ gasification effectively removes deactivation-critical carbon species (non-filamentous/encapsulated/filamentous) from NiCuCe, though an optimal regeneration duration is essential for eliminating reversible deposits.

Table 3. TPD-CO₂ data for spent catalysts after 60 and 120 min gasification runs at 300 °C.

Test Items	Peak Temperature (°C)	Peak Area	CO2 Consumption (mmol/g)
TPD-CO2	120 °C/310 °C	4255	1.2996
TPD-CO2 after 60 min CO2 gasification	236 °C	2676	0.8173
TPD-CO2 after 120 min CO2 gasification	229 °C	1637	0.5266

compares 60 and 120 min TPD-CO₂ data at 300 °C (with CO₂ introduced after stopping methanol flow), confirming incomplete carbon removal despite prolonged treatment, which is correlated with the dual TPD peaks in Figure 9. The post-regeneration XRD analysis (Figure 11) shows carbon peaks in catalysts used for 22 h, confirming carbon-induced deactivation. After 120 min of CO₂ treatment, the carbon peaks disappear (ultralow content), with reduced NiO intensity and improved dispersion indicating partial activity recovery.

3. Experimental Details

CH₃OH and anhydrous ethanol were purchased from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China). Copper nitrate, nickel nitrate, zinc nitrate, indium nitrate, yttrium nitrate, lanthanum nitrate, and zirconium nitrate were purchased from Macklin Biochemical Technology Co., Ltd. (Shanghai, China). High-purity nitrogen (N₂, 99.999%) was purchased from Newrad Special Gases Co., Ltd. (Wuhan, China).

The xM–Ni–Al₂O₃ catalysts (M denotes any metal, and x denotes its loading content on the support) were prepared using a coimpregnation method. Initially, nickel nitrate and other metal nitrate precursors were coimpregnated onto the Al₂O₃ support to synthesize xM–Ni–Al₂O₃ catalysts. We assumed that Ni and M ions from the reaction solution were deposited onto the Al₂O₃ support with a fixed Ni loading of 10 wt.% (denoted as 10Ni) and the loading of other metals was 8 wt.% each (denoted as 8M). The resulting catalysts are represented as 8M–10Ni–Al₂O₃ (abbreviated as 10Ni8M). The metal nitrates were dissolved in 100 mL of deionized water, followed by the addition of 10 g of Al₂O₃. The suspension was impregnated at room temperature for 12 h, followed by drying at 120 °C for 16 h. The dried sample was then heated in air to 550 °C at a rate of 10 °C min⁻¹ and calcined in a muffle furnace for 4 h. The experiment for evaluating the catalyst activity was conducted as mentioned. An H₂ flame ionization detector (FID) and thermal conductivity detector (TCD) were initialized and self-tested. Purified H₂, air, and N₂ were used as carrier gases for the H₂ FID, while purified N₂ and air were used as carrier gases for the TCD. A catalyst tube furnace with a catalyst quality of 10 g was used for the CH₃OH cracking reaction. CH₃OH was delivered to the heating zone via a peristaltic pump, where it was heated and evaporated before entering the tube furnace for the reaction. The products generated during the reaction were monitored in real-time using two gas chromatographs. The FID was used to analyze organic products [CH₃OH, methane (CH₄), dimethyl ether (DME), and 2-methylfuran (MF)], while a dual TCD detector (TCD1 and TCD2) was used to analyze inorganic products. TCD1 was used to detect CO and CO₂, while TCD2 specifically analyzed H₂. All data were recorded using a workstation. The external standard method was used to calculate the concentrations of each product. The catalyst evaluation setup is depicted in Figure 12.

Methanol cracking rate (_XMeOH), CO selectivity (S_CO), and H₂ selectivity (S_H2) were calculated using C equilibrium and H equilibrium as follows:

$${\mathrm{C}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}={\mathrm{C}}_{\mathrm{M}\mathrm{e}\mathrm{O}\mathrm{H}}+{\mathrm{C}}_{{\mathrm{C}\mathrm{H}}_4}+{\mathrm{C}}_{\mathrm{C}\mathrm{O}}+{\mathrm{C}}_{{\mathrm{C}\mathrm{O}}_2}+2\times {\mathrm{C}}_{\mathrm{M}\mathrm{F}}+2\times {\mathrm{C}}_{\mathrm{D}\mathrm{M}\mathrm{E}}$$

(14)

$${\mathrm{H}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}={4\times \mathrm{C}}_{\mathrm{M}\mathrm{e}\mathrm{O}\mathrm{H}}+{4\times \mathrm{C}}_{{\mathrm{C}\mathrm{H}}_4}+4\times {\mathrm{C}}_{\mathrm{M}\mathrm{F}}+4\times {\mathrm{C}}_{\mathrm{D}\mathrm{M}\mathrm{E}}+2{\times \mathrm{H}}_{{\mathrm{H}}_2}$$

(15)

$${\mathrm{X}}_{\mathrm{M}\mathrm{e}\mathrm{O}\mathrm{H}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}-{\mathrm{C}}_{\mathrm{M}\mathrm{e}\mathrm{O}\mathrm{H}}}{{\mathrm{C}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}}}\times 100\mathrm{\%}$$

(16)

$${\mathrm{S}}_{\mathrm{C}\mathrm{O}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{C}\mathrm{O}}}{{\mathrm{C}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}}}\times 100\mathrm{\%}$$

(17)

$${\mathrm{S}}_{{\mathrm{H}}_2}={\displaystyle \frac{2{\mathrm{H}}_{{\mathrm{H}}_2}}{{\mathrm{H}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}}}\times 100\mathrm{\%}$$

(18)

Equation:

• C_total—Total C content in the product;
• C_MeOH—Content of CH₃OH in the product;
• C_CH4—Content of CH₄ in the product;
• C_CO—Content of CO in the product;
• C_CO2—Content of CO₂ in the product;
• C_MF—Content of MF in the product;
• C_DME—Content of DMF in the product;
• H_H2—Content of H₂ in the product;
• X_MeOH—Cleavage rate of CH₃OH;
• S_CO—The selectivity of CO;
• S_H2—The selectivity of H₂;
• M_MeOH—Molar mass of CH₃OH.

XPS measurements were performed using a Thermo Scientific K-Alpha X-ray photoelectron spectrometer (Puchuan Testing, Guangzhou, China) equipped with a neutralization gun to mitigate charge accumulation. Calibration was performed using the binding energy of 284.8 eV for the C1s orbital. Additionally, the instrument was fitted with an argon (Ar) ion beam etching attachment, which allowed the layer-by-layer etching of the sample by controlling the ion etching time. This enabled the determination of elemental and valence distributions at various depths. The XPS technique involves irradiating the sample surface and analyzing the spectrum of photoelectrons emitted to investigate the surface chemical composition and chemical states. BET surface area measurements were performed using a Micromeritics ASAP 2460/202 system (Micromeritics Instrument Corporation, Norcross, GA, USA) with a 3 kW X-ray generator using a Cu target. A horizontal goniometer was used with a minimum step size of 1/10,000 degree. Diffraction measurements were performed within a 2θ range of 0.5–5° with a small-angle attachment used for low-angle XRD analysis. SEM was performed using a Talos F200i S/TEM (Puchuan Testing, Guangzhou, China) with a resolution of ≤0.16 nm and scanning transmission electron microscopy magnification ranging from 310 million times to 330 million times. The maximum diffraction angle was 24° with a field emission gun, providing a total electron beam current of 150 nA. The sample stage was equipped with single-tilt and double-tilt holders, allowing for detailed surface morphology analysis at various resolutions for each sample.

TPD, TPR, and temperature-programmed oxidation experiments were conducted using the DAS-7000 (Puchuan Testing, Guangzhou, China) high-performance dynamic adsorption instrument. The instrument parameters were as mentioned: reactor temperature range, 0–1200 °C with long-term constant temperature operation limited to <900 °C; reactor temperature curve linearity, up to 99.999% with a repeatability deviation of <1 °C; programmed temperature ramp rate, from 0.5 °C min⁻¹ to 90 °C min⁻¹; and catalyst sample loading volume, from 0 mL to 1 mL. The instrument was equipped with a bidirectional mass flow meter with an accuracy of ±1% F.S, linearity deviation of ±1% F.S, precision of ±0.2% F.S, and response time of 2 s. Thermogravimetric–differential scanning calorimetry (TG-DSC) experiments were performed using a Mettler TGA/DSC3+ instrument (Puchuan Testing, Guangzhou, China) with a temperature range from room temperature to 1300 °C and a heating rate of 0.1–100 °C min⁻¹. The balance had a weighing range of 0–5000 mg with a precision of 0.001 mg. The maximum data acquisition rate was 10 samples s⁻¹. The instrument allowed simultaneous measurement of the DSC and TGA curves of the samples with a crucible volume of 70 μL and was capable of coupling with TG-DSC for integrated analysis.

4. Conclusions

The comparative XRD analysis of NiCuCe catalysts reveals post-reaction structural evolution: (i) intensified NiO diffraction peaks indicating enhanced crystallinity, (ii) attenuated NiAl₂O₄ peaks reflecting reduced phase content and crystallinity, and (iii) emergent carbon signatures. These changes correlate with catalytic performance degradation and a decreased specific surface area, pore volume, and average pore size. The characterization confirms NiAl₂O₄ as the structural promoter with Cu²⁺/Ni²⁺ dual-active centers governing methanol decomposition. The reaction demonstrates temperature-dependent bimodal behavior: below 280 °C, high conversion rates are associated with CO₂/DME accumulation, while above 280 °C, sustained conversion (>80%) accompanies progressive improvement in H₂/CO selectivity. Regeneration via sequential TPO–(TPD-CO₂)–TPO achieves 70% carbon removal through gasification. The time-dependent CO₂ treatment analysis shows incomplete carbon elimination at 60 min versus full removal at 120 min, establishing optimized regeneration via ≥120 min of CO₂ exposure. This work elucidates NiAl₂O₄–Cu²⁺/Ni²⁺ synergy for energy-efficient catalyst design, specifically enabling hydrogen-focused methanol decomposition (CH₃OH → CO + 2H₂) while aligning carbon-neutral conversion with closed-loop carbon management principles. The strategy combines targeted hydrogen production with carbon dioxide-mediated regeneration, which reduces industrial waste emissions and achieves a valorization shift in carbon deposition, thereby advancing the development of new energy systems.