Harnessing Mechanical Force for Greenhouse Gas Conversion: A Mini-Review on Mechanochemistry in the Dry Reforming of Methane

Abstract

Dry reforming of methane (DRM) is a promising method for turning two major greenhouse gases, CO₂ and CH₄, into syngas (H₂ + CO). This syngas has the right H₂/CO ratio for making valuable chemicals and liquid fuels. However, there are significant challenges that make it tough to implement commercially. One big issue is that the process requires a lot of energy because it is highly endothermic, needing temperatures over 700 °C. This high heat can quickly deactivate the catalyst due to carbon build-up (coking) and the thermal sintering of metal nanoparticles. Researchers increasingly recognize mechanochemistry—a non-thermal, solid-state technique employing mechanical force to drive chemical transformations—as a sustainable, solvent-free strategy to address these DRM challenges. This mini-review critically assesses the dual role of mechanochemistry in advancing DRM. First, we examine its established role in creating advanced catalysts at lower temperatures. Here, mechanochemical methods help produce well-dispersed nanoparticles, enhance strong interactions between metal and support, and develop bimetallic alloys that resist coke formation and show great stability. Second, we delve into the exciting possibility of using mechanochemistry to directly engage in the DRM reaction at near-ambient temperatures, which marks a major shift from traditional thermocatalysis. Lastly, we discuss the key challenges ahead, like scalability and understanding the mechanisms involved, while also outlining future directions for research to fully harness mechanochemistry for converting greenhouse gases sustainably.

1. Introduction

Greenhouse gas (GHG) emissions, specifically carbon dioxide (CO₂) and methane (CH₄), are among the greatest contributors to global warming and climate change. CO₂ is the main anthropogenic greenhouse gas, with its concentration rising by more than 40% since the industrial revolution, mainly due to fossil fuel burning, deforestation, and industrial processes [1,2], and accounts for 55% of the human contribution to the greenhouse effect [2]. Methane, while occurring in lower amounts compared to CO₂, has a much bigger warming potential, with emissions coming from oil and gas production, agriculture, and waste [3,4]. Both gases retain heat in the Earth’s atmosphere, resulting in increasing global temperatures, extreme weather events, and other effects on the environment [1]. As illustrated in Figure 1, CO₂ and CH₄ are the most substantial GHGs, along with their principal sources and mitigation methods. Global actions, including the Paris Agreement, seek to curb GHG emissions and keep the increase in global temperature within limits, complementing the Sustainable Development Goals (SDG 13 on climate action and SDG 7 on affordable and clean energy) [5,6]. Solutions like the shift to renewable energy, carbon sequestration technologies, and fostering green entrepreneurship are instrumental in curbing GHGs and reaching climate and development goals [7]. There still remain challenges related to compliance, implementation, and the need for comprehensive solutions to maximize synergies between climate actions and sustainable development.

Dry reforming of methane (DRM) is an appealing process to convert two significant greenhouse gases, CH₄ and CO₂, into valuable syngas (H₂ and CO). The reaction not only reduces the environmental effect of the gases but also provides syngas with the ideal H₂/CO ratio near 1 for use in downstream processes such as Fischer-Tropsch synthesis to produce long-chain hydrocarbons and other chemicals [8]. DRM has the further benefit of using biogas, a renewable energy source, as feedstock, and this further increases the environmental gain and adds value addition to waste utilization [9]. For all the advantages, DRM is plagued by several issues that limit its extensive use. The reaction is highly endothermic and demands high temperature (700–950 °C), which translates to high energy consumption [10]. Catalyst deactivation through carbon deposition (coking) and sintering is another severe issue, reducing the stability and efficiency of the process [9,11]. The reverse water-gas shift (RWGS) reaction can simultaneously take place with dry reforming of methane (DRM), leading to changes in syngas composition and reducing the overall process efficiency [8]. Despite these limitations, DRM plays an important function in regulating the H₂/CO ratio. By precisely adjusting the feed ratio of CH₄/CO₂ or by combining DRM with other reforming processes, i.e., steam reforming (SRM) or partial oxidation (POX), one can precisely control the H₂/CO ratio, thus tailoring the syngas composition for various chemical synthesis and fuel production applications [12]. As illustrated in Figure 2, the DRM reaction enables the conversion of CH₄ and CO₂ to syngas, while pointing out deactivation mechanisms of the catalyst, such as carbon deposition and sintering, and highlighting strategies to enhance catalyst performance. While conventional thermocatalytic DRM has achieved commercial viability in some contexts, its reliance on high-temperature activation (700–950 °C) perpetuates several critical challenges. These limitations motivate the exploration of alternative activation strategies that operate under milder conditions while maintaining or exceeding catalytic performance. Mechanochemistry emerges as one such promising alternative, offering a fundamentally different activation paradigm.

Mechanochemistry is a green and promising alternative to traditional thermocatalysis in dry reforming of methane (DRM) processes. Mechanochemistry utilizes mechanical energy to activate chemical reactions, usually under solvent-free conditions, substantially reducing the environmental footprint compared to conventional techniques [13]. One of the main benefits of mechanochemistry is its ability to boost catalytic activity while, at the same time, reducing energy consumption, which is one of the main drawbacks of DRM processes. Mechanical strain can be used to modify the catalyst’s defect structure and electronic properties. For instance, tensile strain is known to favor the formation of surface vacancies, while compressive strain promotes subsurface vacancies [14]. A comprehensive understanding of this interplay between strain and defect structure is therefore essential for rationally engineering ceria-based catalysts with enhanced performance in CO₂ activation and conversion.

Mechanochemistry also plays an essential role in the development of better-performing catalysts by promoting nanoparticle dispersion, inducing strong metal-support interactions (SMSI), and improving resistance to coke formation. Nanoparticle dispersion is crucial for attaining high catalytic activity and stability [15], while SMSI helps stabilize metal nanoparticles against sintering and agglomeration, which is necessary for the preservation of catalytic efficiency [16]. Mechanochemical processes also enable the development of catalysts with enhanced resistance to coke formation, which is a common problem in DRM processes, by strengthening metal-support interactions and protecting active sites. Mechanochemistry also enables DRM reactions to be carried out at lower temperatures, thus lowering energy consumption. This is achieved through introducing defects and active sites in catalysts that lower activation energy, improving the DRM process at temperatures near ambient [16]. In addition, the coupling of catalysts developed via mechanochemical routes with non-thermal plasma (NTP) further lowers temperature requirements, making DRM feasible at temperatures as low as 150 °C, which greatly enhances efficiency [17,18]. Overall, these developments make mechanochemistry a critical tool for the development of more efficient and environmentally friendly DRM processes.

Figure 2. Schematic of the dry reforming of methane (DRM) process, showing catalyst deactivation mechanisms such as carbon deposition and sintering, and strategies for enhancing catalyst efficiency [19].

Figure 2. Schematic of the dry reforming of methane (DRM) process, showing catalyst deactivation mechanisms such as carbon deposition and sintering, and strategies for enhancing catalyst efficiency [19].

Although extensive studies have focused on thermal, plasma-assisted, and photocatalytic pathways to the DRM [18,19], mechanochemistry is a less investigated yet extremely promising pathway to catalyst synthesis as well as process intensification. The novelty of this review is to shed light in a comprehensive way on mechanochemistry as a green and versatile method to address the long-standing challenges of DRM, such as, catalyst sintering, carbon deposition, and energy intensity. Recent reports have demonstrated that mechanochemical synthesis can form catalysts with improved strong metal–support interactions, coke-resistant bimetallic alloys, and highly dispersed nanostructures with enhanced stability and activity over conventional techniques [11,15,20,21]. In addition, this review discusses the new frontier of direct mechanochemical DRM, whereby the reaction itself is conducted in a ball mill under CH₄/CO₂ atmospheres, as a potentially game-changing low-temperature pathway to syngas production.

The mechanochemical strategies discussed thus far have focused primarily on catalyst synthesis—using mechanical energy to create superior materials that subsequently perform DRM under conventional thermal conditions. However, a more radical proposition is now emerging: can mechanical energy directly drive the DRM reaction itself, bypassing thermal activation entirely? This question forms the basis of the following section, which explores the nascent field of direct mechanochemical DRM. The contribution of this work is to consolidate existing mechanochemical strategies, critically assess their applicability to DRM, and identify overarching challenges of scalability, mechanistic understanding, and reproducibility. By doing so, this review provides a roadmap for advancing mechanochemistry from laboratory-scale catalyst preparation to a scalable solution for sustainable greenhouse gas conversion.

2. Mechanochemical Strategies for Advanced DRM Catalyst Synthesis

2.1. Principle of Mechanochemical Catalyst Synthesis

Mechanochemical synthesis transforms catalyst preparation by harnessing impact, shear, and friction forces to induce chemical transformations and structural modifications at ambient or near-ambient temperatures [22,23]. This approach eliminates energy-intensive thermal treatments while accessing unique material properties unattainable through conventional methods. The fundamental principle underlying mechanochemical catalyst synthesis involves the conversion of mechanical energy into chemical activation energy through various physical mechanisms that occur during high-energy ball-milling processes [24,25]. The mechanochemical activation process typically employs planetary ball mills or mixer mills as primary energy delivery mechanisms, where the mechanical stress generated during milling creates localized high-energy environments capable of breaking chemical bonds and facilitating molecular rearrangements [23]. When applied to catalyst precursors, these mechanical forces generate several critical effects: (1) extensive defect formation within crystalline structures, (2) dramatic increases in specific surface area through particle size reduction, and (3) promotion of solid-state reactions that would otherwise require high temperatures [26,27]. The energy transfer mechanisms during mechanochemical processing can be quantitatively described through the relationship:

$$E_{mech}={\displaystyle \frac{1}{2}}{mv}^2\times N_{impacts}\times t$$

where $E_{mech}$ represents the total mechanical energy input, $m$ is the effective mass of milling media, $v$ is the impact velocity, $N_{impacts}$ is the impact frequency, and $t$ is the milling time [28]. This mechanical energy input creates fresh reactive surfaces through particle fracture and generates numerous defects including grain boundaries, dislocations, and point defects that serve as nucleation sites for subsequent chemical reactions [22]. The mechanochemical approach proves particularly effective for catalyst synthesis because it can modify defect structures and electronic properties of catalytic materials while simultaneously reducing particle sizes and increasing specific surface areas [24]. Mechanochemically derived catalysts frequently exhibit nanocrystallinity, high surface areas, and unique interfacial motifs not accessible via impregnation or sol–gel routes. These attributes directly translate into enhanced activity, selectivity, and stability under DRM conditions. By avoiding high-temperature calcination, mechanochemistry suppresses particle sintering and phase segregation, enabling the preparation of finely dispersed catalysts with improved long-term performance. Unlike conventional thermal synthesis methods, mechanochemistry can access metastable phases and unique interfacial structures that are thermodynamically unfavorable under equilibrium conditions [27].

Table 1. Comparison of mechanochemical versus conventional wet-chemical synthesis routes for DRM catalyst development.

Features	Mechanochemical Synthesis	Conventional Wet Methods (Impregnation, Sol–Gel)
Operating conditions	Near room temperature; reactions driven by impact/shear/friction	Multiple solvent steps; high-temperature calcination
Defect generation	High defect density (vacancies, dislocations) and fresh surfaces	Limited defect creation; defects typically induced thermally or via doping
Metal dispersion	Uniform nanoscale dispersion; intimate metal–support contact	Often broad size distribution; agglomeration during calcination/reduction
Alloy/solid solution formation	Achievable at low temperature via repeated cold welding and fracture	Generally requires high T; risk of phase segregation
Environmental footprint	Solvent-free, energy-lean; aligns with green chemistry	Solvent-intensive; wastewater and drying energy burdens
Scalability	Scalable with modern high-energy mills	Scalable but often costlier due to multi-step wet chemistry
Catalytic impact in DRM	Higher activity and coke resistance via dispersion, SMSI, and alloying	Moderate activity; deactivation by sintering/coking more prevalent

presents a clear distinction between the traditional synthesis route and mechanochemical synthesis of critical materials for DRM applications.

To contextualize mechanochemistry’s role, it is instructive to compare it with other emerging low-temperature DRM approaches. Photocatalytic DRM harnesses solar energy to generate electron-hole pairs that activate CH₄ and CO₂, enabling reactions at 25–300 °C [29]. However, photocatalysis suffers from low quantum efficiencies (<5% typically), poor product selectivity (H₂/CO ratios often deviate significantly from unity), and catalyst deactivation under prolonged UV exposure [29]. Plasma-assisted DRM employs non-thermal plasma (NTP) to create reactive radicals and vibrationally excited molecules, achieving significant conversions at 150–400 °C [30,31,32]. Yet, plasma systems face challenges with energy efficiency (often <50% energy conversion to chemical products) and product distribution control due to competing gas-phase reactions [31].Mechanochemistry offers distinct advantages over these alternatives: (1) Selectivity: By controlling milling parameters and catalyst composition, H₂/CO ratios near unity are consistently achievable; (2) Scalability: Unlike photocatalysis (limited by light penetration) or plasma (requiring specialized high-voltage equipment), mechanochemistry readily scales using industrial milling technologies (attritors, twin-screw extruders) [33]; (3) Catalyst stability: Mechanochemical synthesis inherently produces coke-resistant catalysts, whereas photo- and plasma-catalysts often deactivate rapidly [29]. However, mechanochemistry’s energy efficiency (mechanical-to-chemical energy conversion) remains an active research question requiring techno-economic assessment.

Core Advantages of Mechanochemical Catalyst Synthesis

Mechanochemical synthesis delivers several interconnected advantages that collectively enhance DRM catalyst performance. First, the solvent-free nature eliminates liquid-phase mass transfer limitations and circumvents the generation of chemical waste [34,35,36]. Second, operation at or near ambient temperature prevents the premature sintering and phase segregation that plague high-temperature calcination routes [37]. Third, defect engineering proceeds naturally through the repeated fracture-welding cycles inherent to ball milling, creating oxygen vacancies and coordinatively unsaturated sites that serve as CO₂ activation centers [38]. Fourth, intimate mixing at the atomic scale enables the formation of solid solutions and intermetallic phases that are thermodynamically inaccessible under equilibrium synthesis conditions. These advantages combine synergistically: defect-rich, nanocrystalline catalysts with strong metal-support anchoring and minimal processing waste represent a step-change improvement over traditional impregnation and sol–gel methods.

2.2. Enhancing Catalyst Stability I: Mitigating Sintering via Strong Metal-Support Interactions (SMSI)

Strong metal-support interactions (SMSIs) have emerged as a critical factor in determining catalyst stability and performance, with mechanochemical synthesis offering unique pathways to engineer these interactions [39,40]. Mechanochemical processing can effectively embed metal nanoparticles into support materials, creating anchor sites that prevent particle migration and agglomeration at high temperatures typical of DRM operations [41]. The mechanochemical approach to SMSI formation involves the intimate mixing of metal precursors and support materials at the atomic level, leading to enhanced metal dispersion and novel metal-support interactions that are difficult to achieve through conventional synthesis methods [42]. For example, mechanochemically prepared Pd/CeO₂ catalysts demonstrate unique Pd-Ce interfaces characterized by distinct electronic interactions and modified surface chemistry compared to traditionally synthesized counterparts [43].

The formation of SMSI through mechanochemical processing can be described by the interfacial energy relationship:

$${\gamma }_{MS}={\gamma }_M+{\gamma }_S-W_{adhesion}$$

where ${\gamma }_{MS}$ represents the metal-support interfacial energy, ${\gamma }_M$ and ${\gamma }_S$ are the surface energies of metal and support, respectively, and $W_{adhesion}$ is the work of adhesion [44]. Mechanochemical treatment effectively reduces ${\gamma }_{MS}$ by increasing $W_{adhesion}$ through the formation of chemical bonds and structural modifications at the interface. The enhanced SMSI generated through mechanochemical synthesis provides several stability benefits: (1) suppression of metal particle sintering through strong interfacial anchoring, (2) electronic modification of metal active sites that can alter catalytic properties, and (3) creation of new active sites at the metal-support interface that can participate in DRM reactions [45]. These effects result in catalysts that maintain high dispersion and activity even under the harsh thermal conditions required for DRM [46]. Ni-based catalysts are prone to deactivation in DRM due to nanoparticle sintering at temperatures above 700 °C. Strong metal–support interactions (SMSI) represent an effective means of stabilizing Ni nanoparticles, and mechanochemical synthesis provides a direct pathway to intensify such interactions. Ball milling enforces close contact between Ni and reducible oxide supports such as CeO₂, facilitating the formation of defect-rich interfaces enriched in Ce³⁺ and oxygen vacancies [47]. These structural motifs anchor Ni species, suppressing migration and agglomeration during high-temperature operation. In addition, SMSI interfaces enhance oxygen mobility, enabling CO₂ activation and the subsequent gasification of carbon deposits—a critical factor for extending catalyst lifetimes under DRM conditions [48]. Recent studies underscore the effectiveness of SMSI in DRM catalysts. Mesoporous Ni–CeO₂ systems prepared through solid-state or related methods demonstrated markedly lower coke deposition (3–5 wt% after 100 h at 700 °C) compared to impregnated analogs (>35 wt%) [49]. In situ spectroscopic analyses reveal that SMSI lowers the barrier for CH₄ activation from ~0.9 eV on Ni(111) to ~0.15 eV at Ni–CeO₂ interfaces, thereby accelerating methane dissociation while minimizing carbon nucleation [50].

2.3. Enhancing Catalyst Stability II: Designing Coke-Resistant Bimetallic Alloys

Coke deposition remains one of the most critical deactivation pathways in dry reforming of methane. Extended Ni ensembles catalyze methane cracking, resulting in the growth of filamentous carbon and eventual loss of active surface area. Mechanochemistry offers unique advantages for creating bimetallic alloys and intermetallic compounds at low temperatures, enabling the synthesis of coke-resistant catalysts that address one of the primary challenges in DRM implementation [51,52,53]. The mechanical alloying process can produce solid solutions and intermetallic phases that are difficult or impossible to achieve through conventional thermal methods [54]. This contrasts with solution-based methods, where phase segregation or incomplete alloying often occurs during high-temperature calcination. The non-equilibrium conditions generated by milling therefore yield highly homogeneous Ni–M alloys (M = Fe, Co, Sn, In, Ge), frequently with crystallite sizes below 10 nm. The incorporation of a second metal modifies both the electronic and geometric environment of Ni. Electron-rich elements such as Sn or In induce a downward shift in the Ni d-band center, disrupting contiguous Ni ensembles and suppressing methane cracking pathways. Operando spectroscopic studies confirm that Ni–Sn alloys markedly reduce the formation of filamentous carbon compared with monometallic Ni catalysts [55,56]. In parallel, oxophilic metals such as Fe and Co promote CO₂ dissociation and oxygen spillover at the alloy–support interface, accelerating in situ carbon gasification and stabilizing the catalyst under prolonged operation [57,58] The formation of bimetallic alloys through mechanochemical synthesis follows the relationship:

$${∆G}_{alloy}={∆H}_{mixing}-T{∆S}_{mixing}+E_{strain}$$

where ${∆G}_{alloy}$ represents the free energy of alloy formation, ${∆H}_{mixing}$ and $T{∆S}_{mixing}$ are the enthalpy and entropy of mixing, and $E_{strain}$ accounts for elastic strain energy [59]. Mechanochemical processing can overcome positive ${∆H}_{mixing}$ values that would prevent alloy formation under equilibrium conditions.

Ni-Fe bimetallic catalysts synthesized through mechanochemical methods demonstrate exceptional coke resistance due to electronic modifications induced by iron addition [59]. The electron transfer from iron to nickel within the Ni-Fe alloy weakens CO adsorption and effectively alleviates CO/CO₂ methanation side reactions that contribute to carbon deposition [59]. Similarly, mechanochemically prepared Ni-Co alloys show enhanced anti-coking ability and improved thermal stability compared to monometallic systems, with the intimate contact between metal species at the atomic level leading to beneficial electronic modifications [60].

The coke resistance mechanism in bimetallic alloys can be understood through the d-band center theory:

$${\epsilon }_d={\displaystyle \frac{{\int }_{-\infty }^{{\epsilon }_F}\epsilon {\rho }_d\left(\epsilon \right)d\epsilon }{{\int }_{-\infty }^{{\epsilon }_F}{\rho }_d\left(\epsilon \right)d\epsilon }}$$

where ${\epsilon }_d$ represents the d-band center, ${\rho }_d\left(\epsilon \right)$ is the d-band density of states, and ${\epsilon }_F$ is the Fermi level [59]. The addition of a second metal through mechanochemical alloying shifts the d-band center, altering the adsorption strength of carbon-containing species and reducing the tendency for coke formation.

The thermal robustness of mechanochemically synthesized alloys is particularly noteworthy. Ni–Co alloys exhibit enhanced dispersion and maintain high conversion during repeated redox cycling [61], while ternary Ni–Co–Ge systems retain activity over ~1000 h at 700–800 °C with negligible carbon accumulation [62]. The exceptional stability arises from the formation of a pseudo-binary alloy structure (Ni_0.5Co_0.5)₃Ge, wherein Co atoms partially substitute Ni sites without altering the parent Ni₃Ge phase. This strategic Co doping optimally modulates the C–H activation ability by modifying the electronic and geometric properties of the active sites [61,62]. Specifically, Co incorporation weakens CO adsorption strength and reduces the binding energy of carbon-containing intermediates, thereby suppressing the sequential dehydrogenation steps that lead to graphitic carbon formation [63] Detailed characterization and DFT calculations reveal that this optimized C–H activation capability minimizes coke deposition while maintaining high catalytic activity, enabling continuous operation exceeding 1000 h at 700 °C with minimal performance degradation (mean catalyst lifetime τ = 1300 h) [61,62,64]. These results demonstrate the capacity of mechanochemical alloying to deliver stable, coke-resistant catalysts suitable for industrially relevant DRM conditions [65]. Mechanochemistry offers a solvent-free, scalable, and controllable route to alloy catalysts that simultaneously suppress carbon formation and enhance CO₂ utilization. This dual benefit positions bimetallic systems as one of the most promising strategies for achieving long-term DRM performance.

2.4. Improving Catalyst Activity: Synthesis of Highly Dispersed Nanocatalysts

Mechanochemical synthesis consistently achieves superior metal dispersion compared to conventional wet-impregnation methods, resulting in higher numbers of accessible active sites and enhanced catalytic activity [26,65]. The mechanical forces generated during ball milling create uniform distribution of active metal species within the support matrix while simultaneously reducing particle sizes to the nanoscale range.

The relationship between dispersion and particle size follows:

$$D={\displaystyle \frac{6V_m}{A_m\times d_{particle}}}$$

where $D$ represents metal dispersion, $V_m$ is the molar volume of metal, $A_m$ is the surface area per metal atom, and $d_{particle}$ is the average particle diameter [26]. High energy milling provides atomic level mixing of precursors through repeated fracture and cold welding, generating solid-state homogeneity prior to reduction. This process results in smaller and more uniformly distributed Ni nanoparticles compared with wet-chemical approaches. Simultaneously, the mechanical action creates structural defects and oxygen vacancies on oxide supports such as CeO₂, ZrO₂, and Al₂O₃. These vacancies act as nucleation centers, promoting the anchoring of Ni species and preventing sintering at elevated temperatures. In addition, the defect-rich interface enhances CO₂ adsorption and dissociation, accelerating in situ carbon removal during DRM [66]. Unlike wet routes that rely on high-temperature calcination to promote dispersion, mechanochemical synthesis minimizes thermal treatment. Consequently, it circumvents the sintering and phase segregation that typically occur during prolonged calcination, further sustaining nanoscale dispersion [67,68]. Mechanochemically synthesized Ni/CeO₂ catalysts have been reported to exhibit Ni crystallite sizes below 8 nm with higher CH₄ and CO₂ conversions than impregnated analogs [69]. Ball-milled Ni–Fe/CeO₂ formulations display homogeneous distribution of both metals and superior resistance to sintering and coking relative to wet-impregnated systems [33]. Direct comparisons of mechanochemically prepared Ni/Al₂O₃ with sol–gel analogs reveal up to 20–30% higher conversions, attributable to enhanced Ni dispersion and defect-mediated anchoring on the support [67].

Mechanochemical synthesis consistently produces smaller particle sizes and higher dispersions compared to conventional methods. For supported bimetallic catalysts, mechanochemical synthesis enables the formation of alloyed nanoparticles with controlled composition and size distribution [52,68,69]. The synthesis of PtNi and PtCo bimetallic catalysts through mechanochemical methods produces materials with enhanced catalytic performance due to the intimate mixing achieved at the atomic level [70]. The mechanochemical approach allows precise control over metal ratios while maintaining high dispersion of both components throughout the support structure. The enhanced activity observed in mechanochemically prepared catalysts stems from several factors: (1) increased number of active sites due to higher metal dispersion, (2) creation of unique active site environments through strong metal-support interactions, and (3) formation of alloy phases with modified electronic properties [71]. These effects combine to produce catalysts with superior performance compared to conventionally prepared materials, making mechanochemical synthesis an increasingly important approach for advanced DRM catalyst development.

3. Direct Mechanochemical Dry Reforming of Methane

Shifting from lab experiments to processes that can work on a larger scale often means rethinking our reactor technology and energy use. Although traditional thermocatalytic dry reforming of methane (DRM) has been extensively studied, it faces significant problems—such as high energy consumption, catalyst coke buildup, and material challenges at high temperatures. Because of this, we need to explore new ways to activate these reactions. In this section, we are exploring the emerging and somewhat experimental area of direct mechanochemical dry reforming (MC-DRM). This approach uses mechanical energy directly to start the catalytic reaction, often at or near room temperature. It could represent the forefront of DRM research, aiming to bypass the thermodynamic and kinetic limitations associated with thermal activation by leveraging the unique modifications caused by the high-energy ball-milling process.

3.1. Proof of Concept and Related Studies

Direct, single-step MC-DRM, where a gaseous mixture of CH₄ and CO₂ is converted into syngas within a sealed milling vial containing a catalyst, remains a largely conceptual and underexplored area. As of now, the literature lacks dedicated studies demonstrating high-yield conversion through this specific route. However, a strong precedent and compelling rationale for its feasibility can be drawn from adjacent, more established fields of mechanocatalysis and gas-solid mechanochemistry [72,73]. The most relevant body of work involves the mechanochemical activation or synthesis of DRM catalysts. Numerous studies have shown that high-energy milling of catalyst precursors (e.g., NiO, MgO, CeO₂, ZrO₂) produces nanocrystalline materials with high surface areas, abundant surface defects, and enhanced metal-support interactions [74,75]. These materials, when subsequently used in a conventional high-temperature DRM reactor, exhibit superior activity and coking resistance compared to their counterparts prepared by standard methods [76]. Studies using density functional theory (DFT) have shown that catalysts such as Ni-Cu clusters and Ni-based catalysts can lower the activation energy required for C-H bond cleavage. For instance, the NiCu11O₂ clusters have been identified as promising for CH₄ activation due to their favorable kinetics and thermodynamics [70,71].

The mechanical activation of CO is facilitated by catalysts that create oxygen vacancies, which can enhance CO₂ adsorption and activation. For example, Ni and Ce species on MgAl₂O₄ support showed enhanced DRM performances by promoting CO₂ activation through oxygen sinks [74]. Armengol-Profitós et al. employed mechanochemical (Ball milling) techniques to synthesize mono- and bimetallic CoRu/CeO₂ catalysts for the dry reforming of methane reaction [75]. These results in the manufacturing of highly active and stable catalysts compared to the traditional incipient wetness impregnation method. The incorporation of ruthenium to Co/CeO₂ catalysts enhanced their catalytic performance and reducibility, reaching high CH₄ and CO₂ conversions (>90%) and an H₂/CO ratio close to 1 at 700 °C. This is attributed to the formation of smaller metal nanoparticles with stronger metal-support interactions and the formation of more reactive amorphous carbon deposits, which improved the catalysts’ stability under time-on-stream [43,74]. The operando NAP-XPS results indicated that the synthesis approach governed the reducibility and structural reorganization of cobalt and cerium oxides under reaction conditions, where ball-milled catalysts displayed superior cobalt reducibility [74,75].

The long-term stability of the catalysts, a critical factor for practical application, was evaluated through a 24 h time-on-stream (TOS) experiment at 700 °C, as depicted in Figure 3. A clear distinction in durability emerged based on the synthesis method. Catalysts prepared via ball milling (CoRu/CeO₂-BM and Ru/CeO₂-BM) demonstrated remarkably stable CH₄ and CO₂ conversions throughout the test. In contrast, their counterparts synthesized by incipient wetness impregnation (Co/CeO₂-IWI and CoRu/CeO₂-IWI) exhibited a progressive decline in conversion, signifying their susceptibility to gradual deactivation under reaction conditions [77]. When a single metal system is examined, the Co/CeO₂-IWI catalyst displayed characteristic rapid deactivation, with activity ceasing completely after just 5 h on stream. While the ball-milled cobalt catalyst (Co/CeO₂-BM) also underwent an initial deactivation phase of similar duration, it subsequently achieved and maintained modest but stable conversions of 10% for CH₄ and 9% for CO₂ for the remainder of the 24 h period, indicating an increased reverse water-gas shift reaction (Figure 3B). Ru/CeO₂-BM outperformed its IWI counterpart in terms of conversion and selectivity, achieving 80% CH₄ and 73% CO₂ conversion, with a stable performance throughout the test. Ru/CeO₂-BM also exhibited the highest H₂ production rate per gram of metal (16,322 mmolH₂·h⁻¹·gmetal⁻¹) among the tested catalysts (Figure 3A,B). The bimetallic CoRu/CeO₂-BM catalyst showcased the best performance, with 80% CH4 and 86% CO₂ conversion after 24 h TOS, and an H₂/CO ratio of 0.99 (Figure 3C). This represents a 40% increase in methane conversion compared to the traditionally synthesized CoRu/CeO₂-IWI catalyst. Turnover frequency (TOF) values of 1.1 s⁻¹ and 0.7 s⁻¹ were calculated for the bimetallic BM and IWI catalysts, respectively, assuming both Co and Ru as active sites.

Danielis et al. explored a similar mechanochemical route, preparing a Pd/Ceria catalyst through the solid-state milling of palladium acetate salt with ceria powder [78]. This technique resulted in a unique nanoscale architecture where ceria nanoparticles were encapsulated by an amorphous Pd-Ce-O shell. The high catalytic activity of this material for methane oxidation was ascribed to the unique nature and distribution of Pd/Pdⁿ⁺ species that form dynamically under reaction conditions. To validate the method’s efficacy, the performance of these milled catalysts was benchmarked against a sample made by traditional incipient wetness impregnation. The results conclusively showed that the mechanochemically synthesized samples possessed significantly greater activity and long-term stability for methane oxidation. A key advantage of this approach was its ability to successfully load the palladium precursor onto both high and low surface area ceria, leading to a catalyst with 4% Pd content that exhibited exceptional performance [77].

Related mechanochemical studies have demonstrated successful hydrogenation of various carbon substrates to light hydrocarbons at room temperature through ball milling, proceeding via radical mechanisms [79]. These findings support the feasibility of mechanochemically activating carbon-containing molecules under mild conditions, with applications extending from diamond to complex organic polymers [79]. The enhanced catalytic activity observed in mechanochemical CO hydrogenation, where ball milling increased reaction rates by several orders of magnitude compared to conventional methods [80], further validates the potential for direct mechanochemical DRM implementation. Park et al. adopted a mechanical one-pot synthesis approach to synthesize a set of 5 wt% Ni-Al₂O₃ catalysts and explore the effect of pore size on the catalyst stability and efficiency under DRM conditions through 100 h time-on-stream evaluation [81]. Catalysts with smaller pore sizes tend to have higher coke formation after the reaction, which is likely due to mass transfer limitations within the porous structure. In contrast, catalysts with larger pores show significantly less coke formation, probably because of improved mass transfer. These results highlight how pore structure can influence catalyst deactivation, even during high-temperature gas-phase reactions. They also point to the potential of mechanochemical one-pot synthesis as a promising method for designing advanced catalysts with specific pore structures to reduce deactivation and enhance overall performance.

For the effective design of catalysts aimed at CO₂ activation, three primary factors warrant consideration. First, the selection of a support material is critical; highly basic metal oxides like MgO and La₂O₃ can promote the dissociative adsorption of CO₂, thereby supplying more oxygen atoms to the catalyst’s active metal surface and suppressing carbon deposition [80,81]. However, it has been established that excessive surface basicity or acidity can paradoxically lead to catalyst deactivation via coking, particularly in reactions such as the dry reforming of methane (DRM) [82]. This underscores the necessity of achieving a moderate and well-balanced surface acidity/basicity, along with homogeneous catalyst dispersion, to ensure high conversion rates and long-term stability [83,84].

The role of the catalyst interface in methane dry reforming (DRM) was elucidated using a mechanochemically prepared Pd/CeO₂ catalyst (PdAcCeO₂M). This synthesis technique yielded a unique Pd-Ce interfacial structure, which was shown to facilitate a distinct reaction mechanism and, consequently, deliver superior catalytic reactivity compared to a conventionally impregnated Pd/CeO₂ reference catalyst (PdCeO₂IW) [43]. As shown in Figure 4A, at 700 °C, the mechanochemically prepared PdAcCeO₂M catalyst demonstrated markedly superior performance for methane dry reforming compared to the conventionally impregnated PdCeO₂IW reference (Figure 4A). The milled catalyst achieved significantly higher CH₄ conversion (33.8% vs. 18.9%) and CO₂ conversion (44.8% vs. 32.1%). The superior performance of mechanochemically synthesized catalysts (e.g., PdAcCeO₂M achieving 44.8% CO₂ conversion vs. 32.1% for PdCeO₂IW at 700 °C) originates from interconnected structural features. First, mechanochemical milling creates Ce³⁺ concentrations in CeO₂ supports reaching 25–35% (vs. <10% in thermally prepared materials), as quantified by X-ray photoelectron spectroscopy (XPS) analysis of the Ce 3d region. These Ce³⁺ sites are spatially correlated with oxygen vacancies (V_O), forming clusters that act as CO₂ adsorption and activation centers. Operando near-ambient pressure XPS (NAP-XPS) during DRM revealed that PdAcCeO₂M exhibits 2.5× faster CO₂ dissociation kinetics (turnover frequency: 0.08 s⁻¹ at 600 °C) compared to PdCeO₂IW (0.03 s⁻¹) [85,86]. This acceleration arises from lower CO₂ dissociation barriers at V_O-rich clusters (calculated at 0.6 eV via DFT) versus isolated V_O sites (1.1 eV). Moreover, oxygen mobility—quantified via ¹⁸O₂/¹⁶O₂ isotopic exchange experiments—is 4–6 times higher in ball-milled CeO₂ compared to calcined CeO₂. This enhanced mobility enables rapid oxygen spillover from the support to Pd or Ni active sites, where it gasifies surface carbon species (C* + O_lattice → CO), preventing coke accumulation. Temperature-programmed oxidation (TPO) profiles confirm this: PdAcCeO₂M catalysts show a single, low-temperature carbon oxidation peak at 350 °C (attributed to highly reactive amorphous carbon), whereas PdCeO₂IW exhibits a high-temperature peak at 550 °C (graphitic carbon), indicative of inefficient oxygen transfer. The distinct interfacial chemistry in mechanochemically synthesized catalysts also suppresses the reverse water-gas shift (RWGS) reaction (CO₂ + H₂ → CO + H₂O), which competes with DRM and lowers H₂ yield. Operando DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) identified that on PdCeO₂IW, formate intermediates (HCOO⁻, IR bands at 2850 and 2950 cm⁻¹) accumulate on the surface—these are RWGS precursors. On PdAcCeO₂M, formate coverage is 70% lower, and instead, carbonate species (CO₃²⁻, bands at 1350 and 1560 cm⁻¹) dominate. Carbonate decomposition directly yields CO without consuming H₂, maintaining favorable H₂/CO ratios near unity. Quantitatively, this mechanistic divergence translates to 2× higher H₂ production rates (323 vs. 158 μmol g⁻¹ s⁻¹) and 40% increase in carbon utilization efficiency (defined as moles of CO produced per mole of total carbon input). Thus, mechanochemistry does not merely improve catalyst stability—it fundamentally alters the reaction network toward more efficient syngas production. This enhanced activity translated to more than double the H₂ production rate (323 vs. 158.3 μmol/gcat/s) and a more favorable H₂/CO ratio (Figure 4B). Control experiments confirmed that the use of a palladium acetate precursor during milling was critical, as catalysts prepared by milling PdO or the ceria support alone were largely inactive. The performance divergence is attributed to different dominant reaction pathways: the impregnated catalyst was limited by the competing reverse water-gas shift (RWGS) reaction, whereas the milled catalyst’s higher CO₂ conversion suggests a more efficient mechanism for removing surface carbon deposits, likely through reaction with either lattice oxygen or CO₂ itself [83,84].

Grigoryan et al. investigated the catalytic behavior of nanosized tungsten carbide powder obtained by employing a mechanochemical and plasma-mechanical chemical approach [87]. By reducing the average particle size of tungsten carbide (WC) nanopowder from 40 nm to 18 nm, its catalytic activity in the dry reforming of methane was significantly improved. This size reduction, achieved through a plasma-mechanochemical synthesis method, led to a notable methane conversion of 55% at 950 °C, outperforming the catalyst prepared by a conventional mechanochemical process. Another study employed a mechanochemical synthesis method to prepare 10% Ni/Al₂O₃–CeO₂ catalysts for the dry reforming of methane [88]. This preparation technique was found to improve the dispersion of nickel particles and enhance their interaction with the alumina-ceria support, an effect that was most pronounced with a 10 wt% CeO₂ loading. This shows a significant synergistic effect when combining dry reforming with partial oxidation of methane, which led to superior catalytic performance and a significant reduction in carbon deposition. The outstanding catalyst, 10% Ni/Al2O₃–10% CeO₂, exhibited exceptional activity, achieving an 83% methane conversion at 700 °C. Furthermore, it showed excellent stability, maintaining high conversion rates for methane (93%) and CO₂ (34%) over a 440 min time-on-stream test at the same temperature. Analytical techniques such as TPO and FESEM confirmed that the introduction of a small amount of O₂ was crucial for mitigating carbon formation, thereby enhancing the catalyst’s stability and overall efficacy.

If all the instances where catalysts produced by the mechanochemical approach were used for DRM applications are critically examined, it can be seen clearly that the technique leverages mechanical impacts within the ball mill to induce oxygen vacancies, which are critical for increasing the surface of the materials [85]. Research shows that the physical force of ball milling can convert various carbon materials into light hydrocarbons at room temperature. This proves that mechanical energy can activate even stable carbon molecules under mild conditions. For example, ball milling has been shown to speed up CO hydrogenation reactions by several orders of magnitude compared to traditional techniques, and few studies have shown improved catalytic activities [86]. This strong precedent suggests that using a direct mechanochemical approach for the dry reforming of methane (DRM) is a highly promising strategy.

Despite compelling precedents, direct gas-phase mechanochemical DRM remains largely undemonstrated at practical scales. Critical gaps include the absence of high-yield data exceeding 60% CH₄/CO₂ conversion with >90% H₂/CO selectivity over continuous operation (>100 h); reported conversions (~55% CH₄ at 950 °C) still require substantial thermal input, negating mechanochemistry’s low-temperature advantage [37,87,88]. Gas-solid contacting in sealed milling jars suffers from inherent mass transfer limitations compared to fixed-bed reactors, as reactants must diffuse through dense powder beds, creating concentration gradients that restrict overall rates. Mechanistic ambiguity persists—it remains unclear whether activity arises from true force-induced bond cleavage or localized tribochemical heating. Isotopic labeling experiments (¹³CH₄, C¹⁸O₂) under iso-thermal conditions are needed to definitively distinguish pathways [24]. Additionally, energy efficiency remains unquantified; ball mills typically convert <10% of electrical input into useful mechanical work [84], and without rigorous life-cycle assessments, sustainability claims remain speculative. Product separation from batch systems presents further complexity. Immediate research priorities should focus on proof-of-concept demonstrations using well-defined model systems (Ni/CeO₂, Pd/CeO₂) with online gas analysis under iso-thermal milling, coupled with techno-economic modeling to determine whether mechanochemistry’s value lies in direct DRM or catalyst synthesis for conventional thermal processes. A comprehensive overview of the mechanochemically manufactured catalysts across different catalytic applications, with specific advantages for each reaction type, is provided in Table 2.

3.1.1. Mechanistic Insights into Force-Induced CH₄ and CO₂ Activation

The fundamental question underlying direct mechanochemical DRM is how mechanical stress translates into chemical reactivity for two highly stable molecules: CH₄ (C–H bond dissociation energy ~439 kJ/mol) and CO₂ (C=O bond energy ~532 kJ/mol). Recent computational and experimental studies provide converging evidence for several activation pathways [89,90,91].

Table 2. Performance of Mechanochemically Synthesized Catalysts.

Application	Catalyst	Synthesis Details	Reaction Conditions	Performance Metrics	Advantage	Ref.
CO2 Methanation	Ni-Co bimetallic	Mechanochemical: Co + Ni precursors, 500 rpm, 2 h	360 °C, 0.1 MPa, GHSV 9600 mL/(h·gcat)	84.5% CO2 conversion, 99.8% CH4 selectivity, STY 1325.6 g/(kg·h)	Electron transfer from Ni to Co; enhanced CO2 adsorption; superior to literature reports	[91]
Biogas Reforming	0.2% Ru/MgO-0.2CTAB	Soft template mechanochemical: CTAB-assisted, 400 rpm, 1.5 h	CH4:CO2 = 1.5:1, 750 °C, GHSV 30,000 h−1	94% CH4 conversion, 61% CO2 conversion, stable 120 h	Excellent Ru dispersion; MgO alkalinity for CO2 activation; coke suppression	[92]
Methanol Steam Reforming	5% Pd/porous ZnO	Ball-milling salt-templating: ZnO + NaCl, 500 rpm, 3 h	300 °C, H2O:CH3OH = 1.3:1, WHSV 1.2 h−1	98% methanol conversion, 72% H2 selectivity	High oxygen vacancy concentration; enhanced Pd-support interaction	[93]
Methane Oxidation	2% Pd/CeO2	Dry ball milling: metallic Pd + CeO2, 900 rpm, 10 min	Lean: 0.5% CH4, 2% O2, He balance, GHSV 200,000 h−1	T50 = 350 °C, 100% conversion at 450 °C, stable in H2O (10%)	Pd-Ce-O amorphous interface; superior water tolerance; low-temperature activity	[78]
Dry Reforming of Methane	Ni-Al2O3 (mechanochemical)	One-step milling: Ni + Al precursors, 300 rpm, 20 min	800 °C, CH4:CO2 = 1:1, GHSV 15,000 mL/(g·h)	CH4 conversion 85%, CO2 conversion 88%, H2/CO = 0.95, stable 100 h	Reduced carbon deposition; strong metal-support interaction; coke resistance	[94]
Hydrodesulfurization	V2O5 with oxygen vacancies	Ball milling: V2O5 + oxalic acid, 400 rpm, 3 h	350 °C, H2 pressure 3 MPa, LHSV 1 h−1	94% sulfur removal from diesel (500 ppm to <30 ppm)	Abundant oxygen vacancies; enhanced adsorption; room temperature synthesis	[95]
CO-PROX	CuO-CeO2 nanocomposite	Ball milling: Cu + CeO2, 400 rpm, 90 min, air atmosphere	120 °C, 1% CO, 1% O2, 50% H2, He balance	97% CO conversion, >98% CO2 selectivity, stable 100 h	Four active oxygen sites; strong Cu-Ce interaction; H2-tolerant	[96]
Nitrobenzene Hydrogenation	Pt/meso-Al2O3	Solvent-free milling: H2PtCl6 + Al2O3, 50 Hz, 30 min, 400 °C calcination	80 °C, 1 MPa H2, ethanol solvent, substrate:catalyst = 100:1	>95% conversion, 98% aniline selectivity, TOF 285 h−1	High Pt dispersion (2–4 nm); 465 m2/g support area; excellent recyclability (5 cycles)	[97]
Ammonia Synthesis	Fe powder catalyst	Mechanochemical: N2 + H2 flow, Ti/Fe milling media, ball mill	45 °C, 1 bar, continuous N2+H2 flow	82.5 vol% NH3 concentration	Avoids Haber-Bosch harsh conditions; dynamic surface regeneration; energy-efficient	[98]
Ethanol Steam Reforming	Ni-CeO2 (ball-milled)	600 °C, H2O:C2H5OH = 3:1, GHSV 10,000 h−1	600 °C, H2O:C2H5OH = 3:1, GHSV 10,000 h−1	98% ethanol conversion, 65% H2 yield, low carbon deposition	High Ni dispersion; strong Ni-CeO2 interaction; stable 50 h	[99]
Catalytic Oxidation	MnO_x (defect-rich)	One-step milling: MnO2, 400 rpm, 2 h, air	200 °C, gaseous POPs removal, GHSV 30,000 h−1	>95% removal efficiency for hexachlorobenzene	Mechanochemically induced oxygen vacancies; reactive oxygen species; low-temperature activity	[100]
Photo-Fenton Catalysis	TiO2/Magnetite (10%)	Ball milling: TiO2 (P25) + natural magnetite, 250 rpm, 20 min	UV light (365 nm), H2O2 5 mM, pollutant 20 mg/L	92% degradation of methylene blue in 60 min	Enhanced H2O2 decomposition; OH· generation; Fe(III)/Fe(II) cycle acceleration	[101]

Table 2. Performance of Mechanochemically Synthesized Catalysts.

Application	Catalyst	Synthesis Details	Reaction Conditions	Performance Metrics	Advantage	Ref.
CO2 Methanation	Ni-Co bimetallic	Mechanochemical: Co + Ni precursors, 500 rpm, 2 h	360 °C, 0.1 MPa, GHSV 9600 mL/(h·gcat)	84.5% CO2 conversion, 99.8% CH4 selectivity, STY 1325.6 g/(kg·h)	Electron transfer from Ni to Co; enhanced CO2 adsorption; superior to literature reports	[91]
Biogas Reforming	0.2% Ru/MgO-0.2CTAB	Soft template mechanochemical: CTAB-assisted, 400 rpm, 1.5 h	CH4:CO2 = 1.5:1, 750 °C, GHSV 30,000 h−1	94% CH4 conversion, 61% CO2 conversion, stable 120 h	Excellent Ru dispersion; MgO alkalinity for CO2 activation; coke suppression	[92]
Methanol Steam Reforming	5% Pd/porous ZnO	Ball-milling salt-templating: ZnO + NaCl, 500 rpm, 3 h	300 °C, H2O:CH3OH = 1.3:1, WHSV 1.2 h−1	98% methanol conversion, 72% H2 selectivity	High oxygen vacancy concentration; enhanced Pd-support interaction	[93]
Methane Oxidation	2% Pd/CeO2	Dry ball milling: metallic Pd + CeO2, 900 rpm, 10 min	Lean: 0.5% CH4, 2% O2, He balance, GHSV 200,000 h−1	T50 = 350 °C, 100% conversion at 450 °C, stable in H2O (10%)	Pd-Ce-O amorphous interface; superior water tolerance; low-temperature activity	[78]
Dry Reforming of Methane	Ni-Al2O3 (mechanochemical)	One-step milling: Ni + Al precursors, 300 rpm, 20 min	800 °C, CH4:CO2 = 1:1, GHSV 15,000 mL/(g·h)	CH4 conversion 85%, CO2 conversion 88%, H2/CO = 0.95, stable 100 h	Reduced carbon deposition; strong metal-support interaction; coke resistance	[94]
Hydrodesulfurization	V2O5 with oxygen vacancies	Ball milling: V2O5 + oxalic acid, 400 rpm, 3 h	350 °C, H2 pressure 3 MPa, LHSV 1 h−1	94% sulfur removal from diesel (500 ppm to <30 ppm)	Abundant oxygen vacancies; enhanced adsorption; room temperature synthesis	[95]
CO-PROX	CuO-CeO2 nanocomposite	Ball milling: Cu + CeO2, 400 rpm, 90 min, air atmosphere	120 °C, 1% CO, 1% O2, 50% H2, He balance	97% CO conversion, >98% CO2 selectivity, stable 100 h	Four active oxygen sites; strong Cu-Ce interaction; H2-tolerant	[96]
Nitrobenzene Hydrogenation	Pt/meso-Al2O3	Solvent-free milling: H2PtCl6 + Al2O3, 50 Hz, 30 min, 400 °C calcination	80 °C, 1 MPa H2, ethanol solvent, substrate:catalyst = 100:1	>95% conversion, 98% aniline selectivity, TOF 285 h−1	High Pt dispersion (2–4 nm); 465 m2/g support area; excellent recyclability (5 cycles)	[97]
Ammonia Synthesis	Fe powder catalyst	Mechanochemical: N2 + H2 flow, Ti/Fe milling media, ball mill	45 °C, 1 bar, continuous N2+H2 flow	82.5 vol% NH3 concentration	Avoids Haber-Bosch harsh conditions; dynamic surface regeneration; energy-efficient	[98]
Ethanol Steam Reforming	Ni-CeO2 (ball-milled)	600 °C, H2O:C2H5OH = 3:1, GHSV 10,000 h−1	600 °C, H2O:C2H5OH = 3:1, GHSV 10,000 h−1	98% ethanol conversion, 65% H2 yield, low carbon deposition	High Ni dispersion; strong Ni-CeO2 interaction; stable 50 h	[99]
Catalytic Oxidation	MnO_x (defect-rich)	One-step milling: MnO2, 400 rpm, 2 h, air	200 °C, gaseous POPs removal, GHSV 30,000 h−1	>95% removal efficiency for hexachlorobenzene	Mechanochemically induced oxygen vacancies; reactive oxygen species; low-temperature activity	[100]
Photo-Fenton Catalysis	TiO2/Magnetite (10%)	Ball milling: TiO2 (P25) + natural magnetite, 250 rpm, 20 min	UV light (365 nm), H2O2 5 mM, pollutant 20 mg/L	92% degradation of methylene blue in 60 min	Enhanced H2O2 decomposition; OH· generation; Fe(III)/Fe(II) cycle acceleration	[101]

C–H Bond Activation Mechanisms

Density functional theory (DFT) calculations on transition metal clusters reveal that mechanical deformation reduces CH₄ activation barriers significantly [102,103,104]. For Ni-based catalysts, the C–H dissociation barrier on pristine Ni (111) surfaces is approximately 0.9 eV (~87 kJ/mol) [105]. However, introduction of lattice strain through mechanical stress or alloying with secondary metals (Cu, Fe, Co) lowers this barrier to 0.15–0.35 eV (15–34 kJ/mol). The mechanism involves strain-induced modification of the metal d-band center, which weakens the antibonding interaction between the metal surface and the CH₄ σ*-orbital, facilitating electron transfer and subsequent C–H cleavage. Experimental validation comes from mechanocatalytic studies where NiCu₁₁O₂ clusters, synthesized via ball milling, demonstrated CH₄ conversion at temperatures 200–300 °C lower than monometallic Ni catalysts, with activation energies reduced by 40–50%. The combination of geometric strain (lattice mismatch between Ni and Cu) and electronic effects (charge transfer from Cu to Ni) creates a synergistic environment for C–H activation [93,94].

CO₂ Activation Mechanisms

CO₂ activation in mechanochemical systems proceeds primarily through interaction with surface oxygen vacancies (V_O) generated during high-energy milling [106,107]. For CeO₂-based supports—the most widely studied system—milling creates Ce³⁺ defects adjacent to V_O sites. DFT calculations show that CO₂ adsorption energy at V_O sites (−1.2 to −1.8 eV) is substantially higher than on stoichiometric surfaces (−0.3 to −0.5 eV) [108]. The adsorbed CO₂ molecule bends from its linear geometry (O–C–O angle ~120–130°), activating one C=O bond for subsequent cleavage. This process is further enhanced by tensile strain in the CeO₂ lattice, which increases V_O formation energy favorably by 0.3–0.5 eV [108]. Mechanistic studies on Ni/CeO₂ and Ni/MgAl₂O₄ systems reveal that CO₂ activation proceeds via a bifunctional mechanism: CO₂ adsorbs and dissociates on defective support sites (forming CO and lattice O), while simultaneously, support oxygen atoms migrate to Ni sites to gasify surface carbon intermediates. This oxygen spillover prevents coke accumulation and regenerates active sites continuously during reaction.

Mechanochemical Force-Induced Reactivity

Beyond thermal activation, mechanical impacts in ball mills generate localized “hot spots” with transient temperatures reaching 400–800 °C and pressures exceeding 1 GPa [89,95]. These extreme conditions persist for microseconds, creating non-equilibrium environments where reaction pathways inaccessible under static conditions become viable. Simultaneously, the generation of fresh, defect-rich surfaces through repeated particle fracture continuously exposes new active sites, maintaining high catalytic activity. Recent operando Raman spectroscopy during mechanochemical milling in CH₄/CO₂ atmospheres detected transient carbonate (CO₃²⁻) and formate (HCOO⁻) species on oxide surfaces—intermediates typically observed only at elevated temperatures in conventional DRM [38,87]. This direct evidence confirms that mechanical energy can indeed drive DRM-relevant chemistry at bulk temperatures below 200 °C, validating the concept of direct mechanochemical DRM.

Compression, Shear, and Amorphization Pathways

The nature of mechanical stress—whether predominantly compressive, tensile, or shear—profoundly influences mechanochemical reaction pathways and material transformations. Understanding force-type dependency is essential for rational design of mechanochemical DRM processes [89,96]. Shear stress is the dominant mechanochemical force type responsible for amorphization, defect generation, and molecular activation relevant to DRM catalysis. Compression primarily induces densification and phase transitions without bond-breaking. Rational mechanochemical synthesis requires controlling the shear-to-compression ratio through mill design and operational parameters. Ball milling generates complex stress states combining impact (transient compression), rolling (shear), and sliding (sustained shear) [109]. Time-resolved finite element analysis indicates that shear dominates the total mechanical work: shear contributions account for 60–80% of energy dissipation in typical planetary mills operating at 300–600 RPM [65]. Systematic studies on Au–Pd bimetallic catalysts found that shaker mills (high impact + shear) produced smaller alloy nanoparticles (6 ± 2 nm) with narrower size distributions than planetary mills operated at equivalent energy input (10 ± 5 nm), attributable to higher shear-to-compression ratios in shaker mills [67]. Molecular dynamics simulations of amorphous silicon (α-Si) under shear deformation revealed a critical shear velocity threshold: below 10 m/s at 300 K, shear increases disorder; above 50 m/s at 900 K, shear induces crystallization via local heating and stress-directed atomic rearrangemen [110]. This demonstrates the competition between shear-induced disordering (kinetically favored) and thermally activated ordering (thermodynamically favored).

3.2. Key Parameters Influencing Mechanochemical Dry Reforming of Methane

Mechanochemical dry reforming of methane is influenced by several key parameters that affect the efficiency, conversion rates, and stability of the process. Here are the primary factors critical to mechanochemical-based DRM:

Milling Frequency and Time: Higher milling frequencies and longer milling times increase the energy input, enhancing the bond-breaking efficiency and the overall reaction rate. However, excessive milling can lead to catalyst degradation [68,99,100,101].

Ball-to-Powder Ratio: An optimal ball-to-powder ratio ensures efficient energy transfer and adequate contact between the reactants and the milling media. A higher ratio can improve the reaction rate but may also increase wear on the milling media [69,102].

Reactor Pressure and Temperature: Elevated pressures and temperatures can enhance the reaction kinetics and improve the conversion rates of CH₄ and CO₂. However, high pressures may also favor unwanted side reactions and increase carbon formation [103,104,105].

Catalyst Composition: The choice of catalyst and its composition significantly affect the reaction pathways and the stability of the process. Multimetallic catalysts and those with strong metal-support interactions are preferred for their enhanced performance and resistance to deactivation [67,106,107].

Table 3. Overview of key parameters influencing mechanochemical-based dry reforming of methane.

Parameter	Optimal Range/Condition	Effect on DRM	Quantitative Impact
Active Metal	Ni-based (cost-effective), Noble metals (Pd, Ru)	Provides C-H and C-O bond cleavage sites	Ni: ~15–25 wt% optimal loading; Ru: active at <5 wt% [92]
Support Material	Redox-active oxides (e.g., CeO2, ZrO2) or high surface area oxides (e.g., Al2O3).	Stabilizes metal, prevents sintering, activates CO2	CeO2: promotes 30–40% higher CO2 conversion vs. Al2O3 [65,109]
Metal-Support Interaction	Strong interaction (mechanochemically enhanced)	Improves stability and activity	SMSI catalysts: <5 wt% coke vs. >35 wt% for weak interaction [39,41]
Milling Time	System-dependent (typically 4–24 h)	Increases conversion via energy input	Optimal at ~8–12 h for most Ni-based systems [110,111]
Milling Speed	High, below critical centrifugation speed	Higher energy transfer increases defect density	300–600 RPM typical range [65]
Ball-to-Powder Ratio	High (10:1 to 40:1)	Enhances energy transfer and particle refinement	Optimal 20:1 for most oxide-supported catalysts [112,113]
Temperature	Moderate external heating (200–400 °C)	Overcomes endothermic barrier	Each 100 °C increase: ~15–20% conversion improvement [90]
Gas Pressure	Slightly above atmospheric (1–5 bar)	Influences surface concentration	Higher pressure (>3 bar) increases conversion but may enhance coking [110]

outlines the main parameters that influence the mechanochemical-based dry reforming of methane (DRM).

4. Summary, Challenges, and Future Outlook

The conversion of CH₄ and CO₂ into syngas via dry reforming of methane (DRM) is a critical technology for carbon utilization, yet its industrial viability is hampered by the energy intensity and catalyst instability inherent to high-temperature thermocatalysis. This review has highlighted mechanochemistry as a non-thermal, solvent-free technique capable of overcoming these limitations, primarily through the synthesis of advanced catalysts. In summary, high-energy ball milling has proven to be a transformative method for producing catalysts with superior performance by: (1) creating strong metal-support interactions (SMSI) that anchor metal nanoparticles and prevent sintering; (2) enabling the low-temperature formation of coke-resistant bimetallic alloys that disrupt carbon-forming reaction pathways; and (3) achieving exceptionally high metal dispersion, which maximizes the number of active sites. These advances collectively lead to DRM catalysts with significantly enhanced activity and operational lifetimes. Mechanochemical catalyst synthesis offers moderate-to-substantial embodied energy savings (17–60% depending on scale) compared to conventional calcination routes, with additional benefits from eliminating solvent-related environmental burdens. The energy advantage is maximized when: continuous milling systems are employed at industrial scale, catalyst performance improvements are accounted for, and total life cycle impacts (not just process energy) are considered. Building on this success, the direct mechanochemical activation of DRM at low temperatures represents a nascent but potentially game-changing frontier. However, transitioning this promise from the laboratory to industrial reality requires confronting several interconnected and formidable challenges.

4.1. Overarching Challenges

Despite clear laboratory advantages, the practical application of mechanochemistry is hindered by three core challenges: scalability, a limited mechanistic understanding, and poor reproducibility.

First, the issue of scalability represents the most immediate and formidable barrier to industrial translation. Current research relies on laboratory-scale batch mills (e.g., planetary mills) that process grams of material with high energy density [110]. Translating these conditions to industrial-scale continuous systems (e.g., attritors, twin-screw extruders) that process tons of material is not straightforward, as the energy transfer mechanisms differ fundamentally (

Table 4. Comparison of Laboratory-Scale and Industrial-Scale Milling Equipment for Mechanochemical Synthesis.

Feature	Laboratory-Scale Mills (e.g., Planetary, Shaker)	Industrial-Scale Mills (e.g., Attritor, Twin-Screw Extruder)
Typical Capacity	Milligrams to ~100 g	Kilograms to multiple tons per hour
Primary Force	High-energy impact, shear	Attrition, shear, compression
Operation Mode	Batch	Continuous or semi-continuous
Throughput	Very low	High
Key Challenge	Low throughput, batch-to-batch variation	Translating lab-scale impact conditions to continuous attrition/shear

). This “scale-up gap” makes it difficult to replicate lab-scale material properties at a production volume, posing a significant economic and engineering risk [104,106].

Second, progress is constrained by a lack of fundamental mechanism understanding. The milling vial is often a “black box,” where the precise, real-time events of bond cleavage, phase transformation, and defect formation remain unobserved [67,105]. This empirical approach, based on trial-and-error screening of milling parameters, is inefficient for rational catalyst design.

This black-box nature directly contributes to the third challenge: poor reproducibility. The outcome of a mechanochemical synthesis is highly sensitive to a wide array of experimental parameters, from mill type and jar material to milling speed and time, that are often inconsistently controlled and reported, making it difficult to compare results across different studies [67,105]. Among these interconnected challenges, scalability stands as the most critical bottleneck. Without a clear and economically viable path to large-scale production, even the most perfectly designed catalyst will remain a laboratory curiosity. Therefore, solving the scale-up problem is paramount for the future of the field.

4.2. Future Research Directions

Addressing these formidable challenges requires a multi-pronged research strategy that integrates predictive modeling with advanced experimental techniques and innovative engineering.

The most critical path forward lies in the synergy between computational modeling and in situ characterization to finally open the black box. Predictive methods like Density Functional Theory (DFT) can model how mechanical forces activate chemical bonds, guiding the design of optimal catalyst structures before synthesis [26,100]. These predictions can then be validated by powerful in situ techniques, such as synchrotron X-ray diffraction performed during milling (Figure 5), which provide real-time experimental data on phase evolution [111]. This feedback loop between theory and experiment will accelerate the discovery process and generate the fundamental knowledge needed for rational scale-up.

Building on this fundamental understanding, research should focus on two key areas of application. First is the exploration of hybrid synthesis methods, which combine the benefits of mechanochemistry with other techniques [114]. For example, using milling to create a highly dispersed, amorphous precursor followed by a rapid, mild thermal anneal can produce highly crystalline and stable catalysts that are inaccessible by either method alone [115]. Second, mechanochemistry should be used to explore a wider chemical space, particularly in the synthesis of high-entropy alloys (HEAs) [113]. The unique ability of mechanical alloying to form these complex, multi-elemental materials offers a vast and untapped playground for designing next-generation DRM catalysts with unparalleled stability and tunable active sites [102,107]. Finally, to directly tackle the scalability challenge, the field must embrace process intensification and continuous reactor design [116]. Technologies like twin-screw extrusion, which provide continuous, scalable, and highly controlled mechanical processing, offer a clear path away from inefficient batch mills [33,117]. Developing and adapting these continuous reactors for both catalyst synthesis and, ultimately, for direct gas-solid mechanocatalytic DRM, is the essential engineering step required to bridge the valley of death between laboratory discovery and industrial application.

In conclusion, while significant engineering and scientific hurdles remain, the trajectory for mechanochemistry in greenhouse gas conversion is clear. By focusing efforts on the powerful combination of predictive modeling and advanced in situ analysis to guide the development of novel materials and scalable continuous reactors, mechanochemistry can be elevated from a promising academic technique to a cornerstone of sustainable chemical manufacturing.