Figures (9)  Tables (5)

    • Figure 1. 

      Schematic illustrating the use of catalytic reforming technology to address the tar issue and enhance the yield of valuable syngas in the biomass gasification process.

    • Figure 2. 

      Schematic diagram of the catalytic mechanism of carbon-based catalysts in biomass tar conversion.

    • Figure 3. 

      Comparative schematic of coke conversion pathways over different catalyst systems.

    • Figure 4. 

      Structural advantages and applications of MOFs-derived carbon materials.

    • Figure 5. 

      (a) Schematic illustration of the fabrication of Co-SA/3DOM-NC; (b) The comparison of the FFA conversion between Co-SA/3DOM-NC and other catalysts; (c) Reusability tests of Co-SA/3DOM-NC for the oxidative esterification of FFA[183].

    • Figure 6. 

      (a) jMeOH of CoPc/SWCNT, CoPc/15 and CoPc/50 catalysts in a flow cell under CO2 atmosphere; (b) Schematic diagram of the synthetic process of CoPc/SWCNT catalyst and the structure of CoPc on different-diameter CNTs[210].

    • Figure 7. 

      Synthesis, applications, and prominent structural advantages of graphene-based catalysts.

    • Figure 8. 

      (a) Effect of the GO loading on aerobic oxidation of HMF into DFF; (b) Proposed reaction pathway for selective oxidation of HMF into DFF with molecular oxygen as the terminal oxidant in GO/TEMPO catalytic system[235].

    • Figure 9. 

      Modification strategies for conventional carbon-based catalysts based on the structure-activity relationship of advanced carbon materials, and applications of the modified catalysts in various fields.

    • Catalyst categories Typical representatives Key advantages Critical limitations Optimal application scenarios
      Non-carbon-based catalysts Natural minerals (dolomite, olivine)
      Metal oxides (Al2O3, MgO)
      Molecular sieves (ZSM-5, HZSM-5)      		 High intrinsic reforming activity
      Strong C–C and C–H bond cleavage ability
      Well-established industrial experience      		 Severe coke deposition and metal sintering
      High operating temperature
      Diffusion limitations for bulky tar molecules
      Frequent regeneration required      		 High-temperature secondary tar reforming
      Centralized and industrial-scale gasification systems
      Carbon-based catalysts Biochar, coal char, activated carbon
      CNTs, graphene, MOFs-derived carbons      		 Developed and tunable pore structures
      Excellent metal dispersion
      Low-temperature activity
      Strong metal-support interaction
      Unique coke management via directional carbon transformation      		 Lower mechanical strength (especially conventional chars)
      Surface heterogeneity
      Higher cost and scalability issues for advanced carbons      		 Low-temperature tar reforming
      Distributed biomass gasification
      Long-term stable operation and advanced catalytic systems

      Table 1. 

      Comparison of carbon-based and non-carbon-based catalysts for biomass tar reforming[25−29]

    • Catalyst Feedstock Reaction condition Conversion (%) Ref.
      Ni/liginte char Toluene 650 °C, S/C = 3.4 83.0 [87]
      Ni/ZSM-5 Toluene 600 °C, S/C = 2 83.1 [88]
      Fe-Ni/AC Toluene 600 °C, S/C = 2 93.8 [89]
      Ni-Pt/Ce0.8Zr0.2O2 Toluene 600 °C, S/C = 3 96.0 [90]
      Ni/biochar Naphthalene 900 °C, S/C = 2 92.0 [91]
      Ni/biochar Wheat straw 800 °C, S/B = 4 90.0 [92]
      Ni/Al2O3 Wood residue 550 °C, S/B = 0.6 85.2 [93]
      Ni/CeO2/Al2O3 Wood residue 550 °C, S/B = 0.6 92.4 [93]
      Ni/La2O3/Al2O3 Pine wood 600 °C, S/B = 4 96.4 [94]
      Ni-Co/Al2O3 Pine wood 600 °C, S/B = 4 99.0 [95]
      a S/C: Steam-to-carbon molar ratio; b S/B: Steam-to-biomass mass ratio.

      Table 2. 

      Comparison of catalytic activities of different Ni-based catalysts for the conversion of tar model compounds and biomass tar

    • Support Advantages Challenges
      Biochar Low-temperature high efficiency
      Excellent pore structure and surface functional groups
      Unique coke conversion mechanism
      Sustainability and low cost      		 Poor mechanical strength
      Poor surface chemical stability
      Interference from alkali metal ash
      Difficulty in coke control
      Coal char Excellent pore structure
      Good metal dispersibility
      Directional coke conversion
      Low cost and recyclable      		 Metals are prone to sintering
      Insufficient mechanical strength
      Prominent coke issue
      High variability in raw materials

      Table 3. 

      General advantages and challenges of catalysts with biochar or coal char as the support[125−130]

    • Features of advanced carbon materials Deficiencies of conventional carbon Mechanistic insights revealed Transferable modification strategies
      for conventional carbon
      Ordered hierarchical pores Random pore networks with high tortuosity and diffusion limitations Directional mass transfer suppresses pore blockage and random coke accumulation Template-assisted activation; pore-channel ordering via soft/hard templates
      Spatial confinement of metal nanoparticles Metal sintering and uneven dispersion Confined spaces stabilize metal size and inhibit migration Encapsulation strategies; carbon shell or mineral-carbon composites
      Single-atom or uniformly coordinated metal sites Broad distribution of metal active environments Uniform coordination lowers C–C activation barriers and suppresses coke precursors Heteroatom (N, S) doping to construct stable metal-Nx sites
      Highly graphitized frameworks Low conductivity and poor control of coke evolution Enhanced electron transfer directs carbon toward graphitic structures Partial graphitization treatments; conductive carbon additives
      Tunable surface chemistry and hydrophobicity Steam-induced functional group loss Controlled wettability stabilizes active sites under reforming conditions Surface modification to balance hydrophilicity/hydrophobicity

      Table 4. 

      Mapping key features of advanced carbon materials to deficiencies and modification strategies of conventional carbon catalysts[73,179,180]

    • Catalyst type Key features Cost Performance (activity/stability) Scalability Controllability (structure/sites) Recommended scenarios
      Biochar-based Abundant, low-temperature active, porous, surface-functionalized Low Moderate activity; limited stability in steam High (abundant biomass feedstocks) Low (heterogeneous structure) Distributed, low-temperature tar reforming; pilot-scale systems
      Coal char-based Tunable porosity, good metal dispersion, coke-convertible Low to moderate Good low-temperature activity; prone to sintering and coke High (coal resources available) Moderate (pore structure tunable) Industrial gasification with integrated char recycling
      MOFs-derived carbons Ordered hierarchical pores, single-atom sites, high SSA Fairly high Excellent activity and coke resistance; stability under study Low (complex synthesis, ligand cost) High (precise pore and site design) High-value syngas production; fundamental mechanism studies
      CNTs-based High conductivity, spatial confinement, oriented coke growth High High activity and long-term stability; good coke management Low to moderate (CVD scale-up challenging) High (diameter, doping, metal confinement tunable) Advanced catalytic systems requiring high conductivity and coke tolerance
      Graphene-based Ultra-high SSA, excellent conductivity, tunable surface chemistry High Superior activity and coke restructuring; stacking issues Low (CVD/GO production costly) High (surface chemistry and doping controllable) High-efficiency, low- temperature reforming; hybrid catalysts with conventional carbons

      Table 5. 

      Trade-off analysis of different types of carbon-based catalysts for biomass tar reforming[62,258−262]