Recent Advances and Future Perspectives in Catalyst Development for Efficient and Sustainable Biomass Gasification: A Comprehensive Review

Abstract

Biomass gasification represents a pivotal technology for sustainable energy and chemical production, yet its efficiency and product quality are critically dependent on catalyst performance. This comprehensive review systematically synthesizes recent advancements in catalyst design, mechanistic insights, and process integration in biomass gasification. Firstly, it details the development and performance of catalysts in diverse categories, including metal-based catalysts, Ca-based catalysts, natural mineral catalysts, composite/supported catalysts, and emerging waste-derived catalysts. Secondly, this review delves into the fundamental catalytic reaction mechanisms governing key processes such as tar cracking/reforming, water–gas shift, and methane reforming. It further explores sophisticated strategies for catalyst structure optimization, focusing on pore structure/surface area control, strong metal–support interactions (SMSIs), alloying effects, nanodispersion, and crystal phase design. The critical challenges of catalyst deactivation mechanisms and the corresponding activation, regeneration strategies, and post-regeneration performance evaluation are thoroughly discussed. Thirdly, this review addresses the crucial integration of zero CO₂ emission concepts, covering in situ CO₂ adsorption/conversion, carbon capture and storage (CCS) integration, catalytic CO₂ reduction/valorization, multi-energy system synergy, and environmental impact/life cycle analysis (LCA). By synthesizing cutting-edge research, this review identifies key knowledge gaps and outlines future research directions towards designing robust, cost-effective, and environmentally benign catalysts for next-generation, carbon-neutral biomass gasification systems.

1. Introduction

The escalating global climate crisis, predominantly driven by anthropogenic CO₂ emissions from fossil fuel combustion, necessitates an urgent transition to sustainable, carbon-neutral energy systems. As the sole renewable source of fixed carbon, biomass offers a pivotal opportunity to decarbonize the energy and chemical sectors while enabling negative emissions when coupled with carbon capture technologies. Among thermochemical conversion pathways, gasification stands out for its versatility in transforming diverse biomass feedstocks into valuable syngas (CO + H₂) or hydrogen, which are key precursors for fuels, chemicals, and power generation [1].

However, the widespread adoption of biomass gasification is hindered by significant technical and environmental challenges. Chief among these is the formation of complex, recalcitrant tars during gasification, which cause operational issues such as equipment clogging and corrosion while degrading process efficiency and gas quality. Achieving optimal gas compositions (e.g., high H₂/CO ratios for targeted syntheses or low CH₄ content) requires precise process control and catalytic intervention. While catalyst design offers a promising pathway for tar abatement, the rapid evolution of this field necessitates a systematic assessment of research trends to guide future innovations. A recent bibliometric analysis by Arif Mansor et al. [2] synthesize two decades of advancements in transition bimetallic catalysts for biomass tar steam reforming, revealing a paradigm shift from monometallic systems toward alloyed Ni-Fe, Ni-Co, and Ru-Ni architectures that synergistically suppress coke deposition and enhance hydrogen selectivity—critical for sustainable syngas production. Catalyst deactivation from coking, sintering, and poisoning further undermines long-term operational stability and economic viability [3,4].

Recent mechanistic insights have revealed innovative approaches to mitigating deactivation. Li et al. [5] reveal that microwave-assisted catalytic cracking significantly suppresses coke deposition (a primary deactivation mechanism in tar reforming) by modifying carbon polymerization behavior. Their Ni-Ce@SiC catalyst achieved >90% tar conversion under high phenol concentrations while reducing coke formation by over 30% compared to conventional heating, attributed to the microwave-induced transformation of graphitic coke into less stable amorphous structures. Critically, conventional biomass gasification processes still generate substantial CO₂ emissions, falling short of the carbon neutrality or negativity essential for deep decarbonization.

The environmental sensitivity of biocatalytic enzyme activity—where operational parameters such as temperature and moisture synergistically regulate catalytic performance—provides a useful analogy for catalyst design in gasification systems. In the study by Li et al., soil extracellular enzymes exhibited differential responses to warming (+3 °C) and precipitation manipulation (−30% to +30%), with β-1,4-glucosidase (BG) activity decreasing under ambient precipitation but increasing under elevated precipitation conditions. Gasification catalysts may show divergent activity under fluctuating steam-to-biomass ratios or temperature gradients, highlighting that catalytic function is not static but a dynamic outcome of synergies in operational conditions [6]. Thus, innovative strategies integrating efficient catalysis with robust CO₂ management within gasification systems are imperative [7].

Catalysis plays a central role in addressing these challenges by enabling efficient tar destruction/reforming [8], enhancing gas yields (particularly H₂), and facilitating desirable shifts in gas composition. Recent research has increasingly focused on carbon-based catalysts (CBCs)—derived from biomass (e.g., biochar), activated carbon, or other carbonaceous materials—for biomass gasification. In modern materials science, techniques such as Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and atomic force microscopy (AFM) have become indispensable for the structural characterization of complex systems [9,10,11]. CBCs offer unique multifunctionality: their intrinsic catalytic activity drives tar cracking/reforming and water–gas shift reactions; tunable porous structures and surface chemistries enable in situ CO₂ adsorption [12]. Notably, recent advances in CBC design have further extended their functionality to synergistic process integration for comprehensive syngas purification.

Ding et al. [13] demonstrated such integration by coupling activated biochar (A-biochar) catalysts with a silicon carbide (SiC) membrane to concurrently remove tar and particulate matter (PM) from biomass gasification syngas at 800 °C. Their work revealed the following:

Hierarchical pores in A-biochar physically adsorbed heavy tar (e.g., fluorene), while inherent Ca/Al species catalytically reformed light tar (e.g., phenol), achieving 96.4% tar conversion; the SiC membrane (pore size 2.6 μm) captured > 95.9% of PM (>0.24–0.28 μm), preventing catalyst fouling. The synergistic reactor outperformed individual components, yielding syngas with tar (3.6 g/m³) and PM (0.03 g/m³) meeting solid oxide fuel cell requirements.

Their study exemplifies how structural design (porosity + active sites) and process integration (catalysis + separation) can jointly address multiple contaminants, advancing CBCs toward industrial implementation.

Analogously, biochar’s selective adsorption of aldehydes mirrors CBCs’ CO₂ capture mechanism, relying on its hierarchical porous architecture and surface oxygenated functional groups (e.g., carboxyl, carbonyl) to establish specific interactions with aldehyde carbonyl groups, as demonstrated in [14]; derivation from low-cost or waste resources (e.g., biomass residues, sewage sludge) aligns with circular economy principles [15,16]; and they often exhibit superior thermal stability and resistance to poisons (e.g., sulfur) compared to metal-based catalysts [17,18]. This dual capability to integrate catalysis with CO₂ capture positions CBCs as promising candidates for efficient, low-carbon/negative-carbon biomass gasification systems, such as sorption-enhanced gasification (SEG).

Advances in carbon material science have expanded the toolkit for CBC design. Beyond conventional biochars and activated carbons, emerging carbon architectures originally developed for electrochemical applications—such as metal–organic framework (MOF)-derived carbons with ultrahigh surface areas and tunable pore systems—show exceptional potential for dual-function catalysis and CO₂ capture [19]. Nitrogen-doped carbon matrices anchored with single-atom catalysts (SACs), renowned for maximizing atom efficiency and unique electronic structures in CO₂ electroreduction, offer new strategies to enhance active site density [20] and stability in thermochemical processes. Computational materials science, leveraging density functional theory (DFT) and machine learning (ML), is accelerating rational catalyst design by predicting optimal doping configurations, active sites [21,22], and adsorption energetics. The value of AI in catalyst design mirrors its transformative impact on enzyme engineering, as evidenced in Ref. [23] where AI-driven computational models enabled the precise optimization of enzymatic active sites for enhanced catalytic efficiency.

Significant progress has been made across multiple interconnected domains critical to CBC-enabled, CO₂-integrated gasification. In catalyst design, research has explored diverse formulations, including metal-doped carbons (e.g., Ni, Fe, K) to enhance tar reforming activity while utilizing carbon matrices for CO₂ adsorption; tailored biochars activated to serve as both catalysts and adsorbents; advanced materials like carbon nanotubes (CNTs) and ordered mesoporous carbons (OMCs) with improved performance but cost considerations; mineral-impregnated hybrids (e.g., CaO-CBC composites) for synergistic catalysis and high-temperature CO₂ capture; and waste-derived CBCs that valorize sewage sludge and industrial byproducts.

Elucidating reaction mechanisms under realistic gasification conditions requires sophisticated in situ and operando characterization techniques. Synchrotron-based X-ray absorption spectroscopy (XAS) probes electronic states and metal dopant coordination during catalysis, while in situ transmission electron microscopy (TEM) visualizes real-time morphological changes and deactivation processes. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) identifies surface intermediates, complemented by multiscale modeling—from DFT-based elementary step analysis to computational fluid dynamic (CFD) simulations of reactor-scale performance—that provides mechanistic insights and system optimization guidance.

The structural optimization of CBCs focuses on engineering porosity (micro/mesoporous), surface area, and functional group chemistry (e.g., O/N/S doping) to balance catalytic activity and CO₂ adsorption kinetics while exploring metal–carbon interactions (e.g., SMSI-like effects) and nanostructuring in doped systems. Deactivation mechanisms—such as pore blockage by coke/ash, oxidation, and attrition—are being studied to develop regeneration strategies (e.g., controlled combustion, steam activation) that restore performance.

In process integration, CBCs are central to sorption-enhanced gasification (SEG), where in situ CO₂ capture via CaO-hybridized or inherently adsorptive CBCs shifts reaction equilibria to boost H₂ yield and purity while concentrating CO₂ for capture. Integrated CO₂ capture and utilization (CCU) pathways leverage CBCs to convert captured CO₂ into value-added products [24] (e.g., via methanation or dry reforming) within or downstream of gasification systems. Moreover, pretreatment technologies like urea/KOH fractionation avoid washing steps, with recycled liquid containing N/K nutrients recyclable as fertilizers, closing the loop for carbon-neutral biorefineries [25]. System-level optimization involves integrating CBC-mediated gasification with renewable energy inputs (e.g., solar heat) and conducting rigorous life cycle assessments (LCAs) and techno-economic analyses (TEAs) [26] to validate carbon negativity and economic feasibility [27].

However, several challenges must be addressed for commercial deployment. Ensuring CBC stability under cyclic high-temperature, steam-rich SEG conditions remains difficult [28], while balancing catalytic activity, CO₂ adsorption, and mechanical strength poses trade-offs. The mechanistic understanding of synergies between catalytic and adsorptive sites on complex carbon surfaces remains incomplete. The scalable, cost-effective synthesis of high-performance CBCs—particularly from waste streams—and integration into large-scale gasification–CCUS systems require engineering innovation. Standardized LCA/TEA methodologies are needed to compare CBC-based pathways with alternative negative emission technologies, while bridging the gap between lab-scale novel materials (e.g., SACs, MOF-derived carbons) and industrial manufacturing remains a key hurdle.

This review synthesizes current progress in CBC design, mechanistic insights, process integration, and sustainability assessments to identify research gaps and guide the future development of scalable, carbon-negative biomass gasification technologies. By addressing these challenges, carbon-based catalysts hold promise to unlock a transformative pathway for decarbonizing energy systems while advancing circular economy goals.

2. Catalyst Types and Material Design

The core challenge in biomass gasification technology lies in achieving the efficient conversion of complex organic compounds while suppressing the formation of undesirable byproducts (e.g., tar), with the rational design of catalysts being pivotal to addressing this issue. Current research is predominantly focused on five major categories of catalyst systems, categorized by their active components, support types, and modification strategies. The material characteristics and performance variations among these catalytic systems significantly influence both gasification efficiency and product selectivity, establishing critical structure–activity relationships for process optimization in thermochemical conversion applications.

2.1. Metal-Based Catalysts

Metal-based catalysts, with transition metals (Ni, Fe, Co, etc.) as core active components, regulate catalytic performance by modulating the dispersion of metal nanoparticles, redox properties, and metal–support interactions. Nickel-based catalysts have emerged as the preferred material for biomass tar cracking and syngas reforming reactions due to their exceptional C-C bond cleavage capability. Studies demonstrate that when nickel active components are supported on Al₂O₃ [29], CaO [30], or biochar carriers [31], they significantly enhance hydrogen yield (as denoted by the “Ni” node in Figure 1). For instance, the Ni-CaO-C catalyst reported in Ref. [30] achieves a hydrogen yield of 80.36 mmol/g at 700 °C, while Ref. [31] confirms that biochar supports optimize nickel dispersion. Beyond traditional carriers, recent research on supercritical water gasification (SCWG)—a promising technology for wet biomass/waste conversion—highlights the role of metal oxide carriers in tailoring Ni-based catalyst performance. Azadvar et al. [32] synthesized Ni catalysts supported on Al₂O₃, MgO, ZnO, and ZrO₂ and evaluated their efficacy in co-gasifying canola meal (agricultural waste) and an N95 mask (plastic waste) under SCWG conditions (415 °C, 45 min). Their results showed the following:

Ni/MgO achieves the highest hydrogen yield (5.45 mmol/g) and selectivity (72.8%), attributed to the basicity of MgO which promoted water–gas shift (WGS) reactions and suppressed carbon deposition.

Ni/Al₂O₃ delivered the highest total gas yield (15.35 mmol/g) due to its superior ability to disperse Ni nanoparticles, enhancing C-C bond cleavage. Similarly, in the SCWG of bamboo, Cu/Al₂O₃ catalysts with optimized Al₂O₃ loading (7 wt.%) significantly enhanced hydrogen yield (7.72 mol/kg) and total gas production (42.06 mol/kg) by improving metal dispersion and suppressing coke formation, demonstrating the critical role of support composition in catalytic stability under harsh conditions. This study underscores that carrier properties (e.g., basicity, surface acidity, metal–support interaction) are critical for optimizing Ni-based catalysts for hydrogen production from diverse waste feedstocks, expanding the scope of carrier design beyond traditional Al₂O₃ or CaO.

However, monometallic nickel-based catalysts are prone to rapid deactivation in practical applications caused by carbon deposition coverage and sulfur poisoning, with performance degradation further exacerbated by nickel grain sintering at elevated temperatures.

To address these limitations, researchers have developed structural modulation strategies to enhance catalyst stability: (i) introducing promoters such as Ce or Mg to improve carbon deposition resistance, where adsorbed carbon species (denoted as C*) are effectively mitigated, as exemplified by the Ce-doped Pr₀.₄Ce₀.₆CoO₃/Dol catalyst reported in Ref. [33] (as shown in Figure 2, which presents relevant reaction pathways and catalyst-related processes), and (ii) constructing bimetallic synergistic systems, where the core–shell architecture Ni-Fe@CNF/PC designed in Ref. [34] effectively suppresses deactivation through intermetallic electronic interactions. Additionally, the Fe/olivine catalyst documented in Ref. [35] leverages the Fe³⁺/Fe²⁺ redox cycle to reduce tar concentration to 10.4 g/Nm³ at 850 °C, offering novel insights into carbon-tolerant catalyst design for industrial-scale applications.

Iron-based catalysts (e.g., red mud, Fe₂O₃) have garnered attention due to their cost-effectiveness and environmental compatibility. As depicted in Figure 3, these catalysts undergo a redox cycle between Fe₃O₄ and Fe₂O₃, concurrently mediating reactions like the water–gas shift (WGS) and steam methane reforming (SMR) during biomass gasification. In Ref. [36], acid mine drainage sludge (AMDS) was employed to catalyze woody biomass gasification, achieving an enhanced hydrogen concentration of 21.73 vol%. Cobalt-based catalysts such as Co/MgO (Ref. [37]) demonstrate superior hydrogen selectivity in supercritical water gasification systems, yet their practical implementation is constrained by inherent thermal instability, necessitating optimization through composite supports (e.g., ZrO₂, SiO₂) to improve structural robustness.

2.2. Calcium-Based Catalysts

Calcium-based catalysts, represented by CaO and dolomite (CaMg(CO₃)₂), serve dual functions in catalysis and CO₂ adsorption. These systems enhance hydrogen production efficiency through the “sorption-enhanced gasification” mechanism by promoting the water–gas shift (WGS) reaction. Refs. [38,39] demonstrate CaO’s bifunctional capability in simultaneous CO₂ capture and tar cracking catalysis during gasification processes. Recent studies show that Ca-doped La-Fe perovskite catalysts (e.g., La_1−xCa_xFeO₃) significantly boost CO₂ adsorption and tar cracking via tailored oxygen vacancy formation and Fe valence modulation. For instance, L6C4FO (La : Ca = 6:4) achieved a 3.66-fold increase in total syngas yield (938.03 mL/g) for lignin-rich biomass with near-complete carbon conversion (100%) [40]. For instance, the Fe-loaded CaO system investigated in Ref. [39]—whose morphological characteristics are visualized in Figure 4—achieved a hydrogen yield of 26.40 mol/kg biomass.

However, pure CaO suffers from sintering-induced deactivation at elevated temperatures, requiring stability improvements through metallic doping (e.g., Fe, Ni) or constructing composite porous architectures like CaO-Al₂O₃ (Ref. [29]). Ref. [41] further validated synergistic effects, where CaO-Al₂O₃ integration significantly increased the H₂/CO molar ratio from 0.5 to 3.3, highlighting structural optimization pathways for sustainable gasification systems.

Dolomite (transforming into CaO-MgO upon calcination) is widely employed in industrial practice due to its cost-effectiveness and availability. Ref. [42] demonstrated that calcined dolomite achieves up to 80% tar removal efficiency, though its low mechanical strength fundamentally limits industrial application. To mitigate this limitation, researchers have developed enhancement strategies through the following: (i) loading active metals (e.g., Ni/dolomite in Ref. [43]) and (ii) incorporating synthetic supports (e.g., ZSM-5 in Ref. [44]). Notably, the Ni/dolomite system significantly extended catalyst longevity through enhanced metal–support interactions, exemplifying rational interface engineering for durable catalytic performance under harsh gasification conditions.

2.3. Natural Mineral Catalysts

Natural minerals (e.g., olivine, red mud, ilmenite) have emerged as ideal candidates for green catalysts due to their abundant metal oxide constituents (Fe, Mg, K, etc.) and cost-efficient characteristics. Olivine, functioning simultaneously as a bed material and catalytic agent in fluidized bed systems, demonstrates exceptional multifunctional performance. Refs. [45,46] reveal that Ni/olivine catalysts reduce tar concentration from 27.71 g/Nm³ to 0.65 g/Nm³ while elevating hydrogen concentration to 48 vol% in fluidized bed reactors, with catalytic activity originating from the in situ reduction of surface Fe₂O₃ to active Fe⁰ sites. Red mud (containing Fe₂O₃/Al₂O₃) has been innovatively converted through carbothermal reduction (Ref. [47]) into Fe⁰/porous carbon composites, achieving 88.7% tar conversion efficiency. As shown in Figure 5, Ref. [48] further advances this approach via red mud–biomass co-gasification, enhancing methanol yield by 23.9% while establishing a sustainable waste-to-resource strategy through the synergistic valorization of industrial and agricultural residues.

2.4. Composite and Supported Catalysts

Hybrid and supported catalysts demonstrate exceptional performance in enhancing both activity and stability through the strategic design of multimetallic active sites and optimized support architectures. For example, bimetallic CuPd_x nanoparticles exhibit synergistic effects via alloying-induced electron transfer (e.g., Cu²⁺→Pd), enhancing active site density for glycerol dehydrogenation and lactic acid selectivity (up to 95.3%). This mechanism highlights the potential of bimetallic systems to overcome the limitations of monometallic catalysts, such as sintering and low selectivity [49]. Representative bimetallic systems such as Ni-Fe [34] and Ni-Co [50] exhibit enhanced catalytic functionality via synergistic electronic and geometric effects. As shown in Figure 6 (effect of catalysts on tar yield, gas yield, and distribution), the Ni-Fe@CNF/PCs bimetallic catalyst achieves a hydrogen yield of 33.66 mmol/g with minimal tar production (5.66 mg/g) at 700 °C, as documented in Ref. [34]. This performance enhancement arises from the tailored electronic structure of alloyed Ni-Fe nanoparticles and their stabilized dispersion on carbon nanofiber/polymer-derived carbon (CNF/PC) matrices, effectively suppressing metal agglomeration while promoting C-H bond activation in biomass-derived volatiles.

2.5. Waste-Derived Catalysts

Waste-derived catalysts utilizing industrial/agricultural residues such as biochar and fly ash demonstrate exceptional economic–environmental synergy by combining catalytic performance with circular economy principles. Beyond biomass, plastic waste represents another critical waste stream that can be valorized through hydrothermal processes (e.g., liquefaction and gasification) to produce fuels and chemicals, aligning with circular economy goals [51]. For instance, sugarcane pith—a low-value agro-industrial residue—is converted into high-performance CNC aerogels with 49.8 g/g oil absorption capacity, exemplifying the circular economy potential of waste-to-catalyst strategies in biomass gasification systems [52].

Biochar-based catalysts: Refs. [53,54] engineered Fe/Ni-loaded biochar through biomass pyrolysis, achieving >80% tar removal efficiency and 49.2 vol% H₂ concentration. As shown in Figure 7, the Ni-Fe-loaded biochar catalysts (e.g., Ni-Fe-BC@N₂, Ni-Fe-BC@CO₂) demonstrate markedly higher H₂ yields and total gas production across 600–750 °C relative to unloaded biochar or noncatalytic systems, visually validating their performance advantages. Their superior performance originates from hierarchically mesoporous structures (BET surface area: 330 m²/g) and oxygenated functional groups (e.g., carboxyl, carbonyl) that synergistically enhance tar adsorption and catalytic cracking. Similarly, Shao et al. prepared activated carbon (HBC) from a heavy fraction of bio-oil via carbonization and KOH activation, achieving a remarkable specific surface area of 2046.92 m²/g and microporosity of 87.19%. When used as a carrier for Mg-Al-O catalysts in cyclopentanone aldol condensation, HBC enhanced the yield of C9–C16 target products to 87.28%, highlighting the potential of waste-derived biochar for high-performance catalytic supports [55]. Fly ash applications: Acid-washed/calcined fly ash in Ref. [38] increased CaO content, elevating the H₂/CO ratio from 0.39 to 1.05. Recycled concrete (Ref. [56]) served as a cost-effective calcium source, boosting hydrogen yield by 77.4% in dual-bed reactors. Through rational structural modifications—including biochar mesopore engineering and fly ash CaO activation—these waste-derived catalysts achieved benchmark performance metrics, including >80% tar elimination and substantially improved H₂/CO ratios, conclusively validating the technical feasibility of “waste-to-catalyst” strategies for sustainable gasification processes.

The industrial scalability of waste-derived catalysts has been decisively demonstrated by Gurtner et al. [57] through their pilot-scale integration of a self-sustained physical activation reactor within a commercial 300 kW_el wood gasification plant. By converting on-site gasification char (GC) into activated carbon (AC), this system achieved a 150% porosity increase (661 m²/g) while concurrently generating combustible gas for electricity production—boosting overall plant efficiency by 12.5%. Crucially, the resulting AC exhibited a 400% enhancement in the removal efficiency of polycyclic aromatic hydrocarbons (PAHs), effectively transforming a waste byproduct into a high-performance purification material within an energy-positive circular framework.

Therefore, functionalization strategies for different catalyst systems must be meticulously tailored to their intrinsic material characteristics and operational requirements. For metal-based catalysts, performance enhancement is achieved through the following: (i) alloying strategies combining noble/non-noble metals (e.g., Pt-Ni, Pd-Fe), (ii) promoter doping (e.g., Ce/Mg incorporation for electronic structure modulation), and (iii) core–shell architecture design. These approaches enable the precise regulation of the electronic states and surface properties of active sites, thereby improving carbon deposition resistance and catalytic stability under cyclic operations. Supported catalyst systems prioritize multifunctional carrier optimization through the following: (i) the selection of high-temperature-resistant substrates (e.g., Al₂O₃) for structural integrity, (ii) adsorption-enhanced supports (e.g., CaO) for reactant enrichment, and (iii) conductive matrices (e.g., graphene) to accelerate charge transfer kinetics. Simultaneous surface functionalization (e.g., hydroxylation, sulfonation) enhances active phase dispersion and interfacial synergy. Waste-derived catalysts require targeted preprocessing (calcination activation, acid leaching) to remove impurities, followed by pore structure engineering [18] and controlled nanoparticle deposition [58] (e.g., Fe₃O₄@biochar) to transform industrial/agricultural residues into high-performance catalytic materials [59]. The systematic integration of these material-specific strategies—through electronic structure engineering, hierarchical architecture design, and circular economy principles—provides comprehensive solutions for developing advanced catalysts with scenario-adaptive performance in biomass gasification systems.

Table 1. Comparison of catalyst types: performance, cost, and environmental impact.

Catalyst Type	Typical Active Components	H2 Yield (mmol·g−1)	Tar Conversion (%)
Metal-based catalysts	Ni, Fe, Co	33.66 (Ni-Fe alloy)	93.3
Calcium-based catalysts	CaO, dolomite (CaMg(CO3)2)	26.40 (Fe-loaded CaO)	80
Natural mineral catalysts	Olivine, red mud (Fe2O3/Al2O3)	21.73 (Fe/olivine)	88.7
Composite/supported catalysts	Ni-CaO-C, Ni-Fe@CNF/PCs	80.36 (Ni-CaO-C)	95
Waste-derived catalysts	Fe-Ni-loaded biochar, fly ash	33.66 (Ni-Fe biochar)	>80

systematically compares the key performance indicators, application scenarios, and sustainability metrics of five major catalyst types in biomass gasification. It aims to provide a clear reference for researchers and engineers to select appropriate catalysts based on specific process requirements (e.g., gasifier type, feedstock composition) and sustainability goals (e.g., cost reduction, carbon footprint minimization). The data is synthesized from representative studies cited in this review, ensuring comparability across different catalyst systems.

3. Reaction Mechanisms and Structural Regulation

3.1. Catalytic Reaction Mechanisms

Catalysis in biomass gasification relies on active sites for tar cracking and gas-phase reactions [60]. The catalytic activity in biomass gasification processes is intrinsically linked to the catalyst’s regulatory capacity over critical reaction pathways. Fundamental mechanistic studies of tar cracking, water–gas shift (WGS), and methane reforming reactions—three pivotal processes governing gasification efficiency—provide theoretical foundations for catalyst design. Notably, distinct catalyst systems exhibit differentiated regulatory effects on reaction pathways.

3.1.1. Tar Cracking and Reforming

Tar, as a complex mixture of oxygenated polycyclic aromatic hydrocarbons (PAHs), undergoes multi-step cracking, including adsorption, activation, bond cleavage, and deoxygenation, where catalyst surface properties and acid/base functionalities play decisive roles (see Figure 8). Similarly, headspace solid-phase micro-extraction (HS-SPME) combined with GC-MS enables the detailed profiling of volatile compounds in biomass-derived liquids—e.g., 166 compounds identified in mulberry wine, including esters and aldehydes formed via nonthermal aging, demonstrating the utility of such techniques for characterizing reaction intermediates in gasification systems [61].

• Adsorption and Primary Cracking

Tar molecules are concentrated on catalyst surfaces through the following:

Physisorption: Mesoporous carriers (e.g., calcined dolomite CaO-MgO in Ref. [42]) trap PAHs via pore confinement.

Chemisorption: Alkaline sites (e.g., CaO in dolomite) selectively adsorb acidic tar components (phenolic compounds), initiating deoxygenation to CO/H₂ through proton abstraction mechanisms.

Ref. [62] further confirmed that natural zeolites (e.g., olivine) leverage their mesoporous structure (pore size: 2–10 nm) to retain macromolecular tar species (e.g., naphthalene), prolonging residence time at active sites by ~30–50% compared to nonporous catalysts. This hierarchical adsorption–cracking synergy effectively reduces tar content from 15.2 g/Nm³ to <1.0 g/Nm³ in fluidized bed reactors.

2.

Aromatic Ring Opening and Deoxygenation

Metallic active sites (e.g., Ni⁰, Fe³⁺) catalyze C–C bond cleavage through electron transfer mechanisms [63]. In Ref. [30], the Ni-CaO-C catalyst facilitates benzene ring opening via interactions between Ni⁰ d-orbital electrons and aromatic π-bonds [64], generating light olefins (C₂–C₄) as intermediates. Concurrently, CaO-mediated CO₂ adsorption drives the water–gas shift (WGS) reaction, converting CO to H₂ and achieving a hydrogen yield of 115.33 mmol/g, representing a 28.5% enhancement over non-adsorptive systems. This integrated mechanism—coupling tar cracking with ethane reforming—is schematically illustrated in Figure 8.

The Ni-Co bimetallic catalyst in Ref. [50] demonstrates complementary functionality: Co⁰ preferentially cleaves C–O bonds in oxygenated tars (e.g., furans, phenols), achieving 93.3% tar conversion efficiency with 91.52 g H₂/kg tar selectivity. This dual-action mechanism—Ni-driven C–C scission and Co-mediated deoxygenation—reduces polyaromatic hydrocarbon recalcitrance by 60–75% compared to monometallic analogs.

3.

Radical Chain Reactions

At elevated temperatures, catalytic surfaces generate reactive oxygen species (O, OH) that initiate the radical-mediated chain scission of tar molecules. In Ref. [36], red mud (Fe₂O₃) demonstrated exceptional redox activity at 850 °C, where Fe³⁺/Fe²⁺ cycles facilitated lattice oxygen release. This mechanism oxidized toluene—a recalcitrant tar component—into CO₂ and H₂O while simultaneously elevating hydrogen concentration to 21.73 vol%.

Figure 8. Schematic of reaction mechanism on integrated process of catalytic cracking of tar coupled with steam reforming of ethane [65].

Figure 8. Schematic of reaction mechanism on integrated process of catalytic cracking of tar coupled with steam reforming of ethane [65].

The efficient conversion of tar, a major byproduct of gasification, represents a central objective in catalyst design. Nickel-based catalysts facilitate the cleavage of C–C bonds in tar macromolecules (e.g., polycyclic aromatic hydrocarbons) via surface-active sites (Ni⁰), generating syngas components (H₂, CO) and intermediate products like CH₄ (see Figure 9 [66]). For instance, Ni/γ-Al₂O₃ catalysts reduce tar content from 27.71 g/Nm³ to 0.10–0.65 g/Nm³ at 800 °C [67], a performance attributed to the synergistic effects of highly dispersed Ni nanoparticles and surface acidic sites. However, industrial deployment is constrained by progressive deactivation from carbon deposition and Ni sintering during prolonged operation [68].

In contrast, bimetallic systems (e.g., Ni-Fe@CNF/PCs) demonstrate enhanced functionality through the formation of Fe₀.₆₄Ni₀.₃₆ alloy phases. This structural configuration not only improves deoxygenation capability for oxygenated tar components but also elevates H₂ selectivity to 89.7% [34]. Additionally, zinc-modified biochar catalysts have been shown to reduce the activation energy of lignin/LDPE co-pyrolysis from 49.10 kJ/mol to 30.26 kJ/mol, accelerating reaction kinetics and lowering decomposition temperatures [69]. Nevertheless, scalability challenges persist due to higher fabrication costs compared to monometallic analogs, necessitating a further optimization of synthesis protocols.

3.1.2. Water–Gas Shift and Methane Reforming

The water–gas shift (WGS) and methane reforming reactions are pivotal for regulating syngas composition (H₂/CO ratio) and enhancing hydrogen yield in biomass gasification systems.

• Water–Gas Shift (WGS) Reaction

The WGS reaction (CO + H₂O → CO₂ + H₂) relies on catalytic surface-active sites (e.g., metallic or alkaline oxide sites) to facilitate redox interactions between CO and H₂O. Calcium-based catalysts such as CaO enhance hydrogen production by in situ CO₂ adsorption, shifting the reaction equilibrium toward the product side. For example, the thermally treated G_BFA_treated catalyst, enriched with CaO, elevates H₂ concentration from 40.53% to 54.12% and increases the H₂/CO molar ratio by 140% [38]. This mechanism not only optimizes syngas composition but also indirectly reduces greenhouse gas emissions through CO₂ sequestration.

2.

Methane Reforming

Methane reforming encompasses two primary pathways:

Steam reforming:

CH₄ + H₂O → CO + 3H₂

(1)

Dry reforming:

CH₄ + CO₂ → 2CO + 2H₂

(2)

Nickel-based catalysts dominate this field due to their exceptional C–H bond activation capability. The Ni-CaO-C catalyst achieves a remarkable H₂ yield of 1805 mL/g biomass at 700 °C by synergizing Ni⁰-mediated CH₄ cracking with CaO-driven CO₂ adsorption [70]. Meanwhile, iron-based catalysts (e.g., Fe/olivine) leverage Fe³⁺/Fe²⁺ redox cycles to reduce CH₄ content from 3.4% to 2.6% at 850 °C while inhibiting carbon deposition [35], demonstrating the potential of non-noble metal catalysts for sustainable reforming processes.

For the regulation of the water–gas shift (WGS) reaction, the introduction of CaO successfully enhanced H₂ yield and syngas quality (see Figure 10). For instance, the Ni–Ce/CaO catalyst demonstrated the lowest CO₂ yield (2.31 mmol/g) and highest H₂ yield (12.8 mmol/g) at 600 °C [71], attributed to the synergistic effects of low-temperature efficiency and in situ CO₂ adsorption. However, such catalysts face challenges in long-term operation, including adsorption capacity decline caused by CaO carbonation [72], as well as sintering-induced deactivation at elevated temperatures (>800 °C), limiting their cyclic stability.

In methane reforming reactions, the Ni–Fe₂O₃–C catalyst reported in Ref. [73] utilizes electromagnetic induction heating to form an FeNi₃ alloy phase, achieving a 209.1% enhancement in H₂ yield. Its innovative feature lies in leveraging the dynamic heating characteristics of magnetic materials to suppress carbon deposition. However, this technology imposes higher demands on equipment, and the long-term thermal stability of the alloy phase remains unverified. Similarly, the Ni–Cu/γ-Al₂O₃ catalyst reduces the oxidation barrier of Ni through the electronic promoter effect of Cu [43], yet systematic studies on bimetallic ratio optimization are lacking, potentially leading to active site coverage or component segregation.

To address carbon deposition and regeneration issues, Huo [29] proposed that Ca doping reduces carbon accumulation by oxidizing filamentous coke (65.02% reduction), while Zhang [74] employed steam regeneration to restore the tar conversion rate of Ni/C catalysts to 90.41%. While effective, these strategies face limitations: CaO requires high-temperature calcination for regeneration after CO₂ adsorption saturation [75], increasing energy consumption, and steam regeneration may accelerate the structural aging of the support. This highlights the necessity of developing low-cost dynamic regeneration technologies.

3.2. Catalyst Structural Regulation and Optimization

The structural characteristics of catalysts (e.g., porosity, degree of dispersion, crystal phase composition) constitute pivotal determinants of their performance, necessitating precise control through tailored synthesis processes and post-treatment methodologies. Recent advancements in nanotechnology and alloy design paradigms have unveiled innovative pathways for structural optimization in catalytic systems.

3.2.1. Pore Structure and Specific Surface Area Regulation

The combination of high specific surface area and hierarchical pore architecture significantly enhances reactant diffusion kinetics and active site accessibility. A representative case is the hydrothermally synthesized Ni/Ca₃AlO catalyst with distinctive “foliated” morphology, whose meso/macroporous hierarchical structure demonstrates a 65.02% reduction in carbon deposition [29]. This breakthrough underscores the strategic value of morphological engineering in anti-coking catalyst design. Nevertheless, in supercritical water gasification (SCWG) systems, while the NaHCO₃-assisted synthesized HPC-Pt catalyst achieves remarkable hydrogen yield (23.81 mL/mg Pt) through its unique macroporous (1.8 μm)/microporous (2.3 nm) dual-channel configuration [76], the intricate synthesis protocol poses potential scalability challenges for industrial implementation. For a better understanding of how biomass, chicken feather hydrolysates, and NaHCO₃ affect the carbon morphology (see the marked regions therein) in related systems, refer to the SEM images in Figure 11.

3.2.2. Strong Metal–Support Interactions (SMSIs) and Alloy Effects

The strong metal–support interaction (SMSI) mechanism regulates the oxidation states and stability of metallic nanoparticles through interfacial electron transfer, a process by which electron transfer between metal sites and reactants is pivotal for tar reforming [77]. A representative demonstration involves the Ni-Fe₂O₃-C catalyst system leveraging SMSI effects through Fe₂O₃-Ni interactions, forming FeNi₃ alloys under electromagnetic induction heating. This suppresses carbon deposition and enhances H₂ yield by 209.1% [73]. Such innovative engineering not only addresses the persistent sintering challenge in conventional magnetic catalysts but also pioneers novel applications for dynamic thermal field control technologies. Nevertheless, in bimetallic catalytic systems (e.g., Ni-Cu/γ-Al₂O₃), while the electronic promoter effect of Cu significantly reduces the oxidation activation barrier of Ni [43], the precise optimization of elemental stoichiometry necessitates further systematic investigation through high-throughput combinatorial experimentation.

3.2.3. Nanoscale Dispersion and Crystal Phase Engineering

Nanostructuring and crystal facet engineering significantly enhance catalytic activity. Analogously, Camellia pollen extracts exhibit synergistic bioactivity through coexisting phenolic compounds (electron donors) and flavonoids (redox mediators), suggesting that multi-component carbon-based catalysts could leverage similar interactions to enhance tar cracking and CO₂ reduction [78]. In the Pr₀.₄Ce₀.₆CoO₃/Dol catalyst synthesized via the sol–gel method, the Ce³⁺/Ce⁴⁺ redox couple accelerates oxygen vacancy formation, resulting in a 40% higher H₂ yield compared to undoped systems [33]. However, the sensitivity of such catalysts to feedstock impurities (e.g., sulfur content) may limit their applicability in complex biomass systems. Additionally, the Fe-Mo@C catalyst forms an Fe₃C-Mo₂C heterojunction through carbothermal reduction, where its mesoporous structure (pore size: 4 nm) facilitates the cracking of long-chain hydrocarbons in tar [74]. Nevertheless, the introduction of Mo increases costs, necessitating careful cost–benefit analysis.

3.3. Catalyst Activation and Regeneration Strategies

3.3.1. Deactivation Mechanisms

Catalyst deactivation during biomass gasification results from the combined effects of carbon deposition, sintering, and chemical poisoning. Carbon deposition occurs when pyrolytically generated coke from hydrocarbons at elevated temperatures covers active sites, resulting in significant catalytic efficiency loss. For instance, Ni-based catalysts exhibit a 65.02% reduction in carbon accumulation after 10 cycles, yet the coke types (e.g., amorphous carbon and multi-walled carbon nanotubes) still exert complex influences on reaction kinetics [79]. Sintering arises from the aggregation of metal particles under high temperatures, as evidenced by conventional Ni-based catalysts suffering from drastic surface area reduction due to particle growth. In contrast, the Ni–Fe₂O₃–C catalyst maintains Ni particle sizes below 3.24 nm via electromagnetic induction heating technology, effectively suppressing sintering [73]. Chemical poisoning involves alkali metals (K, Na) from biomass ash migrating to catalyst surfaces to form inert layers, or gaseous H₂S—whose stable concentration (~3.5–4.0 ppm) within biomass gasification temperatures (600–1200 K) is confirmed by Figure 12—combining with active metals (e.g., Fe, Ni) to generate sulfides [80,81]. These mechanisms collectively contribute to active site loss, mass transfer obstruction, and selectivity reduction, representing a core bottleneck for large-scale gasification implementation.

3.3.2. Activation Strategies

In confronting catalyst deactivation challenges, strategic activation methodologies effectively restore or augment catalytic functionality through three principal approaches: pretreatment protocol optimization, structural engineering design, and advanced in situ reactivation technologies. Within the pretreatment phase, controlled exposure to reductive atmospheres (typically comprising H₂ or CO) facilitates the regeneration of metallic active phases. This is empirically substantiated by Ni-Co/Mg(Al)O catalysts, which exhibit substantially enhanced performance—specifically achieving a tar conversion efficiency of 93.3% following systematic H₂ prereduction protocols, as documented in reference [68].

Regarding structural innovation, architecturally engineered porous support matrices—exemplified by hierarchically porous carbons (HPCs)—leverage ultrahigh specific surface areas (quantified at 3200 m²/g) and well-defined mesoporous architectures to maximize active site dispersion density. Such structural advantages enable HPC/Pt catalyst systems to attain exceptional hydrogen production metrics, specifically 23.81 mL of H₂ per milligram of a platinum catalyst, as reported in [76]. Furthermore, bimetallic synergistic interactions, as manifested in Ni-Fe alloy systems, intrinsically modulate electronic properties to thermodynamically disfavor carbon deposition pathways. This electronic regulation mechanism contributes to a 40% elevation in hydrogen yield relative to conventional monometallic configurations, per the findings in reference [34].

Concerning in situ activation methodologies, the real-time monitoring of catalyst activity during activation is critical for optimizing process efficiency. Drawing inspiration from cross-disciplinary approaches, Wang et al. [82] demonstrated that acoustic wave propagation characteristics can serve as a non-intrusive indicator of dynamic changes in reaction systems—specifically, acoustic transit time variations correlate with temperature fluctuations and gas composition shifts (e.g., O₂ depletion and characteristic gas release) during coal oxidation. This principle can be adapted to biomass gasification: by integrating acoustic wave monitoring with gas composition analysis (e.g., CO, H₂), in situ activation processes can be dynamically adjusted based on the real-time feedback of catalyst bed status (e.g., porosity changes due to coking or active site regeneration). Cutting-edge techniques harness energy-dense microwave irradiation or supercritical fluid environments. The Ni-CaO-C catalyst system subjected to microwave-assisted activation demonstrates a remarkable 101.1% augmentation in H₂ productivity [83]. Concurrently, Ni/MgO catalysts operating within supercritical aqueous media exploit the high-pressure-induced scission of complex organic molecules, yielding a 3.8-fold amplification in hydrogen generation compared to conventional systems, as validated in reference [37]. Collectively, these multifaceted strategies establish synergistic physicochemical pathways that substantially prolong operational catalyst longevity while simultaneously elevating overall process efficiency indices.

3.3.3. Regeneration Methods

Catalyst regeneration technologies rejuvenate deactivated catalytic systems through physical, chemical, and biomass-assisted approaches, each presenting distinct mechanistic pathways and operational trade-offs. Physically mediated regeneration protocols employ high-temperature calcination (typically at 800 °C) to oxidatively eliminate carbonaceous deposits. This is exemplified by Ni/C catalysts achieving a 90.41% recovery of tar conversion efficiency post-calcination, as validated in reference [74].

Chemically driven regeneration leverages oxidative reactions or steam treatments to restore catalytic functionality. For instance, as illustrated in Figure 13, Ni catalysts supported on TiO₂-SiO₂ composites subjected to steam regeneration demonstrate significantly enhanced hydrogen production capacity, reaching 49.89 mmol/g post-reactivation according to [84].

Biomass-assisted regeneration represents an emerging paradigm utilizing carbon thermal reduction or ash-catalyzed mechanisms for cost-effective recovery. The red mud/biochar composite catalyst (RMBC) facilitates the thermochemical conversion of Fe₂O₃ to catalytically active metallic Fe⁰ during carbon thermal reduction, achieving 80.15% regeneration efficiency [47]. Concurrently, industrial-grade coal fly ash (BFA) exploits its intrinsic CaO content to catalyze coke gasification reactions, elevating hydrogen concentration to 54.12% in regenerated systems [85].

These multifaceted methodologies continuously negotiate critical balances between energy consumption and regenerative efficacy. Physical regeneration, while effective, incurs substantial thermal energy penalties; chemical approaches may induce structural degradation through repeated oxidation–reduction cycles; biomass-assisted techniques face feedstock variability challenges. Nevertheless, their collective development establishes a diversified technological portfolio for industrial-scale regeneration implementation, progressively addressing the trilemma of efficiency, sustainability, and economic viability in catalytic biomass gasification systems.

3.3.4. Post-Regeneration Performance Evaluation

The performance of regenerated catalysts requires comprehensive evaluation through both activity recovery rates and long-term stability metrics. Carbon clearance efficiency serves as a critical indicator, exemplified by Ni-Co/C composite catalysts maintaining 93.3% tar conversion post-regeneration [86]. The regulatory mechanism of catalytic systems for volatile organic compounds can draw reference from metabolic pathways in biotransformation. For instance, Tuly et al. revealed that microorganisms maintain 93.3% amino acid conversion after 10 cycles in mixed fermentation groups via a principal component analysis (PCA) of correlations between amino acids and vitamin C in fermentation products, whose stability assessment logic shares methodological consistency with the “active site retention rate” detection in catalytic systems [87]. Metal dispersion integrity is reflected in particle size evolution, where Ni catalysts retain high activity despite particle growth from 8 nm to 10 nm after regeneration [72].

Regarding stability, sintering-resistant designs (e.g., MgO-modified Ni/Al₂O₃) demonstrate < 10% activity loss after 10 operational cycles [88], while enhanced poisoning resistance (e.g., Fe₂-xNixTiO₅ catalysts in sulfur-containing environments) significantly extends service life [81]. Industrial viability necessitates balancing regeneration costs against operational complexity: steam regeneration offers efficiency but incurs high energy consumption, whereas biomass-assisted regeneration provides cost advantages yet requires optimization to mitigate ash impurity effects [47,85].

Current catalyst regeneration technologies face persistent challenges including noble metal dependency, excessive energy demands, and inadequate tolerance to variable operating conditions. Catalyst deactivation remains the primary barrier to industrial-scale biomass gasification, involving complex interactions among carbon deposition, sintering, and chemical poisoning mechanisms. While pretreatment activation, structural optimization, and in situ techniques effectively restore activity, physical, chemical, and biomass-assisted regeneration collectively offer diversified pathways for industrial implementation. Future advances must overcome cost and energy bottlenecks through integrated approaches combining AI-guided design, the utilization of waste-derived materials, and self-regenerative mechanisms—ultimately establishing efficient, economically viable, and environmentally sustainable catalyst systems for biomass gasification.

4. Carbon-Neutral CO₂ Conversion Mechanisms

Biomass gasification technology stands as a pivotal pathway toward achieving carbon neutrality goals, with its core challenge residing in enabling in situ CO₂ adsorption, efficient conversion, and systematic utilization through catalytic strategies. This section methodically elaborates on the scientific mechanisms and technological pathways for zero carbon emission and carbon cycle conversion during biomass gasification, structured across four interconnected dimensions—CO₂ adsorption and cyclic utilization, carbon capture and resource recovery, system integration, and environmental impact assessment—integrating recent advances in cutting-edge research.

4.1. In Situ CO₂ Adsorption and Conversion

Calcium-based materials (e.g., CaO, CaCO₃) serve as cornerstone adsorbents for in situ CO₂ capture in gasification systems, leveraging their exceptional CO₂ adsorption capacity (theoretical value: 0.78 g CO₂/g CaO) and cost-effectiveness. Research confirms that CaO dynamically adsorbs CO₂ through reversible carbonation–calcination cycles (CaO ↔ CaCO₃) under thermochemical conditions, thereby shifting the water–gas shift (WGS) reaction equilibrium toward hydrogen production. For instance, water-washed calcined fly ash (WCFA) catalysts increase cold gas efficiency from 53% to 72% and elevate the H₂/CO ratio from 0.39 to 1.05 by optimizing pore architecture (specific surface area increased to 3200 m²/g) [38].

Furthermore, bifunctional Ni-Ce/CaO catalysts achieve the lowest CO₂ yield (2.31 mmol/g) and highest H₂ yield (12.8 mmol/g) at 600 °C, with stability originating from the synergistic interplay between continuous CO₂ adsorption by CaO and Ni active sites [71]. Nevertheless, CaO sintering deactivation necessitates resolution through structural modifications (e.g., Mg/Fe doping) or composite support designs (e.g., CaO/Al₂O₃) [72,75].

Recent advancements propose integrated “calcium looping” processes, coupling gasification and combustion reactors to enable continuous CaCO₃ regeneration and CO₂ enrichment, achieving 86.6% carbon conversion efficiency [89].

4.2. Integration with Carbon Capture and Storage (CCS)

Conventional carbon capture and storage (CCS) technologies isolate CO₂ through physical adsorption or chemical absorption, yet their substantial energy requirements constrain large-scale deployment. Biomass gasification integrated with CCS significantly reduces carbon capture costs via in situ adsorption and catalytic conversion. For instance, Fe/CaO catalysts at 800 °C synergistically enhance CO₂ adsorption and tar cracking, elevating H₂ concentration to 21.73 vol% while improving CO₂ capture efficiency by 35% [39].

Supercritical water gasification (SCWG) technology leverages low-temperature reaction environments (430 °C) with in situ-synthesized metal oxide catalysts (e.g., Fe₂O₃), achieving 76.5% chemical oxygen demand (COD) reduction in wastewater and H₂ yields of 17 mol/kg—eliminating additional CO₂ compression energy [90]. Furthermore, red mud (Fe₂O₃/Al₂O₃) catalysts within gasification–electrolysis methanol coproduction systems boost methanol productivity by 23.9% through CO₂ hydrogenation while reducing production costs by 15% [31].

These integrated processes not only achieve carbon-negative emissions but also transform CO₂ into high-value chemicals such as methanol [91] and liquid fuels [92], thereby establishing a closed-loop carbon cycle economy.

4.3. Catalytic CO₂ Reduction and Valorization

The catalytic reduction of CO₂ fundamentally relies on the activation capability of metallic active sites toward C = O bond cleavage. Iron-based catalysts (e.g., Fe/olivine) utilize redox cycling (Fe³⁺/Fe²⁺) to reduce CO₂ to CO, subsequently generating H₂ via the water–gas shift (WGS) reaction, achieving 29.8 vol% H₂ concentration at 850 °C [39,45].

Nickel-based systems such as Ni-Fe@CNF/PCs enhance CO₂ methanation activity through alloying effects (Fe₀.₆₄Ni₀.₃₆ phase), yielding 33.66 mmol/g H₂ with tar content reduced to 5.66 mg/g at 700 °C [34]. Crucially, CaO’s dual function in CO₂ capture and in situ reduction is exemplified by bifunctional catalysts (e.g., Ni-CaO-C). Figure 14 visually confirms the following:

(1)

Ni-CaO-C dominates in H₂ enrichment (highest among all catalysts).

(2)

CO₂ is effectively suppressed (lowest concentration profile).

Significantly, bifunctional catalysts (e.g., Ni-CaO-C) demonstrate synergistic cooperation between CaO-mediated CO₂ adsorption and Ni-catalyzed cracking. At a steam injection rate of 5 mL/h, this system elevates H₂ production to 1805 mL/g while decreasing CO₂ concentration to 7.31 vol% [66].

Furthermore, biochar-supported Fe-Mo carbides (Fe₃C-Mo₂C) convert CO₂ to CH₄ through non-radical pathways, attaining 91.05% tar cracking efficiency—establishing a novel carbon-negative emission route [74].

4.4. System Integration and Multi-Energy Synergy

The deep integration of biomass gasification with renewable energy systems maximizes energy efficiency and carbon reduction benefits. Green pretreatment technologies, such as formic acid esterification at 80 °C, avoid hazardous waste generation and enable 99.4% substrate utilization, offering a sustainable alternative to conventional acid treatments [93]. In supercritical water gasification (SCWG) coupled with solid oxide electrolysis cells (SOECs), in situ-synthesized Fe₂O₃ nanocatalysts enhance hydrogen yield by 3.8-fold while enabling methanol synthesis without external hydrogen supply [48,90]. Recent advances in process integration have further demonstrated the scalability of supercritical water co-gasification systems. Rezende et al. [94] pioneered an integrated approach to hydrogen production through the co-gasification of ethanol–glycerol mixtures in supercritical water, achieving 85% carbon-to-gas conversion at 600 °C and 25 MPa. By combining thermodynamic analysis with computational fluid dynamic (CFD) simulations, their work designed a continuous reactor system capable of processing 10 L/h feedstock while maintaining energy self-sufficiency through homogeneous catalysis—providing a validated pathway for the industrial-scale implementation of biomass-to-hydrogen technologies.

Multi-energy synergistic systems (e.g., solar-assisted gasification) dynamically regulate reactor temperature and steam-to-biomass (S/B) ratios, elevating cold gas efficiency to 66.3% and carbon conversion efficiency beyond 86.6% [95]. Machine learning (ML) techniques employ artificial neural network (ANN) models to predict hydrogen yield (R² = 0.99) and optimize operational parameters (temperature, S/B ratio), revealing synergistic CO₂ reduction effects between temperature control (920–980 °C) and Ni-based catalysts [96,97]. Analogously, the k-means clustering of GC-IMS data has successfully partitioned rice mildew into four stages based on MVOC profiles, demonstrating the potential of chemometric models for real-time gasification process monitoring and stage discrimination [98]. These intelligent optimization strategies provide theoretical foundations for the real-time regulation of complex gasification systems.

4.5. Environmental Benefits and LCA

The carbon neutrality potential of biomass gasification requires validation through full LCA. Notably, environmental pollutants like CO₂ exhibit spatial diffusion, necessitating cross-regional policies—green finance reduces neighboring emissions by 2.874% through industrial spillover effects, mirroring the need for integrated LCA frameworks that account for regional emission interactions [99]. Studies demonstrate that Ni-Ca-Mn-Oₓ catalysts derived from waste eggshells reduce the carbon footprint of gasification processes by 21.05% while increasing H₂ proportion by 32.3% [100]. In methanol coproduction systems, red mud-based catalysts achieve 38% lower life cycle carbon emissions compared to conventional methods, with a 95% waste valorization rate [48].

Furthermore, the co-gasification of agricultural residues (e.g., straw) with plastics enables the directional conversion of coke into carbon nanotubes (CNTs), elevating net energy efficiency (NEE) to 55.7% [79]. Nevertheless, critical challenges persist in scaled applications: catalyst stability limitations (e.g., sulfur poisoning of Ni-based catalysts), selectivity constraints in CO₂ conversion, and suboptimal process economics.

Current research demonstrates that biomass gasification can achieve near-net-zero CO₂ emissions with resource recovery through calcium-based adsorption, catalytic CO₂ reduction, and multisystem integration. Future investigations should prioritize the following:

(1)

Developing multimetallic synergistic catalysts (e.g., Fe-Ni-Ca trimetallic systems) to enhance sintering resistance and poisoning tolerance;

(2)

Advancing gasification–electrolysis–chemical synthesis coproduct systems to maximize carbon utilization efficiency;

(3)

Implementing machine learning and digital twin technologies for dynamic process optimization;

(4)

Establishing standardized LCA methodologies to quantitatively assess carbon reduction benefits and economic viability.

Through interdisciplinary convergence and technological integration, biomass gasification is poised to become the central engine for synergistic energy–environment development under carbon neutrality objectives.

5. Conclusions

This comprehensive review systematically examined the critical aspects of developing efficient catalysts for biomass gasification, structured around three core pillars.

Firstly, we explored the diverse landscape of catalyst types and material design, encompassing metal-based catalysts, calcium-based catalysts, natural mineral catalysts, composite/supported catalysts, and catalysts derived from waste resources. Secondly, we delved into the fundamental catalytic reaction mechanisms, structure–property relationships, and activation/regeneration strategies, covering mechanisms like tar cracking/reforming and water–gas shift/methane reforming, alongside vital structural control elements (pore structure, SMSI, alloying, nanodispersion, crystal phase) and approaches to combat deactivation and ensure catalyst longevity. Thirdly, we addressed the paramount challenge of achieving zero CO₂ emissions, focusing on in situ CO₂ adsorption/conversion, integration with carbon capture and storage (CCS), catalytic CO₂ reduction pathways, system integration with renewable energy sources, and rigorous environmental life cycle assessment.

The selection of catalysts for biomass gasification should be tailored to specific operational goals, balancing efficiency, cost, and sustainability. For high hydrogen yield and tar reforming, Ni-Fe or Ni-Co bimetallic catalysts are recommended due to their superior activity and stability. To achieve carbon-neutral operations, CaO-based catalysts (e.g., Ni-CaO-C) offer dual functionality in CO₂ adsorption and catalytic cracking, enhancing hydrogen production while minimizing emissions. Waste-derived catalysts, such as biochar-supported Fe/Ni systems, provide an economical and environmentally friendly alternative, aligning with circular economy principles. For long-term stability in harsh conditions, SMSI-enhanced catalysts (e.g., Ni-Fe₂O₃-C) or MgO-modified Ni/Al₂O₃ demonstrates excellent resistance to sintering and poisoning. Future advancements should focus on multifunctional catalyst design, system integration with renewable energy, and standardized life cycle assessments to optimize both performance and sustainability. By strategically selecting and optimizing catalysts, biomass gasification can play a pivotal role in sustainable energy production and carbon mitigation.

Synthesizing the insights across these domains, composite catalysts featuring metals supported on carbon-based materials emerge as the most promising avenue for future advancement. These bifunctional catalysts uniquely combine high catalytic activity (e.g., for tar reforming or methane activation) with exceptional CO₂ adsorption capacity, enabling integrated processes where captured CO₂ is catalytically converted into valuable products (e.g., via methanation) rather than merely stored. Consequently, their development represents a pivotal pathway toward practical, carbon-neutral biomass gasification systems that contribute to global carbon cycle closure and climate change mitigation.

The development and optimization of such bifunctional composite catalysts, designed for simultaneous catalytic conversion and CO₂ management within a circular carbon framework, are therefore not only a key scientific frontier but also an imperative step towards practical, sustainable, and carbon-neutral bioenergy production, directly contributing to global carbon cycle closure and climate change mitigation goals.