Improved H2 Production Research Articles

Emerging Trends in CdS-Based Nanoheterostructures: From Type-II and Z-Scheme toward S-Scheme Photocatalytic H2 Production.

Cadmium sulfide (CdS) based heterojunctions, including type-II, Z-scheme, and S-scheme systems emerged as promising materials for augmenting photocatalytic hydrogen (H2) generation from water splitting. This review offers an exclusive highlight of their fundamental principles, synthesis routes, charge transfer mechanisms, and performance properties in improving H2 production. We overview the crucial roles of Type-II heterojunctions in enhancing charge separation, Z-scheme heterojunctions in promoting redox potentials to reduce electron-hole (e-/h+) pairs recombination, and S-scheme heterojunctions in combining the merits of both type-II and Z-scheme frameworks to obtain highly efficient H2 production. The importance of this review is demonstrated by its thorough comparison of these three configurations, presenting valuable insights into their special contributions and capability for augmenting photocatalytic H2 activity. Additionally, key challenges and prospects in the practical applications of CdS-based heterojunctions are addressed, which provides a comprehensive route for emerging research in achieving sustainable energy goals.

Er3+:Y3Al5O12/Pt/ZnO material for photocatalytic hydrogen production under UV–Vis light

Hydrogen is a promising energy source, requiring increasingly efficient photocatalysts for production under UV–Vis light. For the first time, this study evaluated the photocatalytic generation of H2 using the Er3+:Y3Al5O12/Pt/ZnO material. The effect of the Er3+:Y3Al5O12 garnet on the photocatalytic activity of Pt/ZnO under UV–Vis light was evaluated. Firstly, both the ZnO and the Er3+:Y3Al5O12 garnet were synthesized by a sol-gel method, and the Pt-coating of the ZnO was performed using a sonochemical method. The samples were characterized through XRD, SEM-EDX, Raman, UV–Vis-DRS, N2 adsorption-desorption, XPS, and fluorescence spectroscopy. The H2 production under UV–Vis light was measured using mass spectrometry, and the effect of methanol as a sacrificial agent was evaluated. The synthesis method allowed obtaining ZnO with a defined spherical morphology and high crystallinity, with the H2 generation of 676 μmolh−1g−1. In the meantime, the Pt-coating reduced oxygen vacancies and the recombination rate of e−/h+ pairs, and the garnet addition in a mass ratio of Pt/ZnO:garnet at 1.0:0.3 led to the highest H2 production (1304 μmolh−1g−1), with only a 20 % decrease after 5 production cycles. Finally, adding 10 % v/v methanol significantly enhanced the H2 generation (3026 μmolh−1g−1). Altogether, the Er3+:Y3Al5O12/Pt/ZnO material with the sacrificial agent (10 % v/v methanol) improved H2 production from ZnO by a factor of 4.5. Thus, this study demonstrates that the Er3+:Y3Al5O12 garnet is an efficient alternative for significantly increasing H2 production from Pt/ZnO.

Novel and noble-metal-free CdIn2S4/MoB Schottky heterojunction photocatalysts with efficient charge separation for boosting photocatalytic H2 production

The construction of Schottky heterojunctions by selecting applicable metalloids and semiconductors is highly effective for improving the activity of semiconductors. Herein, we deposited CdIn2S4 on MoB by hydrothermal method to construct a noble metal-free and unreported CdIn2S4/MoB Schottky heterojunction for achieving high photocatalytic H2 production. In ultraviolet photoelectron tests, the Fermi level of the CdIn2S4/MoB composites exhibited a certain shift compared with CdIn2S4 and MoB, manifesting the rearrangement of the Fermi level and the formation of Schottky heterojunction. The formed Schottky heterojunction greatly promoted the electron migration from CdIn2S4 to MoB (proved by XPS tests) to obtain high charge separation efficiency (confirmed by PL and electrochemical tests). CdIn2S4/MoB composites possessed smaller H2 evolution overpotential and Tafel slope, which greatly promoted the release of H2 in the kinetics. Besides, CdIn2S4/MoB composites had higher light absorption capacity and larger specific surface area, which were for more conducive to H2 production. Ultimately, CdIn2S4/MoB composites had significantly improved H2 production activity. Among all composites, the activity of CdIn2S4/20MoB was the highest (463.53 μmol·g−1·h−1), which was 3.5 times that of CdIn2S4-1 %Pt. Besides, CdIn2S4/MoB composites possessed excellent stability in five experiments. Simultaneously, the formation of the Schottky heterojunction and its mechanism of promoting H2 production reaction were also thoroughly investigated. This work is the first to report that MoB could construct Schottky heterojunctions with sulfides for efficient photocatalytic H2 evolution, which is easily extended to sulfides other than CdIn2S4.

Optimizing ammonia cracking in microwave argon plasma: Temperature control and ammonia delivery

Hydrogen has been conceived as an alternative to hydrocarbons to achieve a carbon–neutral society, with ammonia emerging as the most practical hydrogen carrier for long-distance transportation. Conventional thermo-catalytic cracking of ammonia is an energy-intensive process, leading to the consideration of low-temperature plasmas as potential alternatives. Recently, a surfatron-based Microwave Plasma Jet (MWPJ) was investigated, demonstrating the fundamental physical and chemical mechanisms of NH3 cracking. In this study, we present an optimization scheme for NH3 cracking using a surfatron-based argon MWPJ to improve H2 production rate and efficiency. Our approach included: (i) introducing downstream inlets of NH3 to decouple NH3 from plasma generation and (ii) adding additive (N2 or NH3) to the main Ar flow to elevate the gas temperature of the MWPJ. We found that both additives to the Ar flow raised the gas temperature from 2700 K up to 4200 K (N2) or 5400 K (NH3), showing a trade-off between gas temperature and the height of the MWPJ. The case with NH3 added to the main Ar flow showed better performance than the case with N2 due to the higher gas temperature and additional H2 production from the additive NH3. The main NH3 supply in the downstream showed no negative effect on the plasma generation and significantly increased the H2 production rate and efficiency, primarily due to the sufficient NH3 supply rate. Increased number of NH3 inlets also facilitated NH3 cracking, illustrating saturated performance with four inlets. To scale up this process, the obtained optimal geometry and operating conditions will be further explored using a high-power microwave system.

Fabrication of Ni and Cd co-doped Zn ZIF-L photocatalyst for enhanced solar hydrogen evolution

Recently, photocatalytic H2 production has gained significant attention for its potential to generate clean and renewable energy. In this study, the enhancement of H2 production was examined by introducing various transition metal elements in Zn/Ni/Cd zeolitic imidazole frameworks photocatalysts (ZIF). The photocatalysts were synthesized using a precipitation method in water, resulting in the formation of ZIF-L structures. Increasing the Ni content leads to large particle sizes and typically reduced surface areas, owing to the increased possibility of coordination at the six coordination sites of Ni atoms. The trimetallic Zn/Ni/Cd ZIF at its optimum composition exhibits the highest absorbance and narrowest band gap, consequently yielding the highest average H2 production rate. To address unstable reproducibility from restricted electron transfer in the optimal sample, CdS is added to form a heterojunction and to achieve stable reproducibility. Consequently, improved H2 production is achieved by incorporating imidazole organic ligands into the co-doped trimetallic ZIF-L photocatalyst. Overall, in this study, the synthesis and optimization of ZIF composite photocatalysts are elucidated, providing insights into the design of efficient materials for sustainable energy production.

Techno-economic analysis of the production of synthetic fuels using CO2 generated by the cement industry and green hydrogen

Cement industry, due to the decomposition of CaCO3 and the production of clinker, emits large amounts of CO2 into the atmosphere. This anthropogenic gas can be captured and through its synthesis with green hydrogen, methanol and finally synthetic fuels are achieved. By using e-fuel, Europe's climate neutrality objectives could be achieved. However, the energy transition still lacks a clear roadmap, and decisions are strongly affected by the geopolitical situation, the energy demand and the economy. Therefore, different scenarios are analysed to assess the influence of key factors on the overall economic viability of the process: 1) A business-as-usual scenario, EU perspectives 2) allowing e-fuels and 3) improving H2 production processes. The technical feasibility of the production of synthetic fuels is verified. The most optimistic projections indicate future production costs of synthetic fuels will be lower than those of fossil fuels. This is directly related to the cost of green hydrogen production.

Catalysis/sorption enhanced pyrolysis-gasification of biomass for H2-rich gas production: Effects of various nickel-based catalysts addition and the combination with calcined dolomite

The effects of addition of various Ni-based catalysts and their combination with calcined dolomite on H2-rich gas production were investigated during catalysis/sorption enhanced pyrolysis-gasification of sawdust. It was found that catalyst support greatly affected the catalytic performance of Ni-based catalysts, the highest H2 concentration and yield was obtained by NiO/SiO2, followed by NiO/γ-Al2O3 and NiO/ZSM-5. Additional active sites on Ni-based catalyst would be formed by introduction of secondary metals onto NiO/γ-Al2O3. Positive synergistic effect between secondary metals and Ni was observed, especially for Mg and Co, resulting in further improved H2 production. Calcined dolomite was further introduced into the process to enhance H2 production, simultaneous catalysis and sorption was found to be better for H2 production than sequential catalysis and sorption. The addition of calcined dolomite could offset the catalytic performance difference for varied Ni-based catalysts due to its catalysis/sorption enhancing effect. Comparatively, bimetallic catalysts were more effective than monometallic catalysts when combined use with calcined dolomite, the H2 concentration was significantly improved despite of the slight changes in H2 yield. Ni-Cu/γ-Al2O3 and Ni-Zn/γ-Al2O3 with good performance on WGS reaction showed better synergistic effect with calcined dolomite, a highest H2 concentration of 87.06 vol.% was achieved by Ni-Cu/γ-Al2O3 catalyst.

Fast carrier separation induced by the metal-like O-doped MoS2/CoS cocatalyst for achieving photocatalytic and photothermal hydrogen production

Full solar-spectrum-driven hydrogen (H2) production from water-splitting is highly desirable but remains a great challenge. A heterojunction composite photocatalyst with a full-spectrum absorption range of up to 1020 nm was constructed by introducing 2D/2D metallic oxygen-doped MoS2/CoS (O-MC) nanosheets onto 1D Zn0.1Cd0.9S nanorods (ZCS NR) for photocatalytic and photothermal H2 production. Consequently, the optimal ZMC1.5 heterojunction exhibited a significantly improved H2 production activity of 95.5 mmol g−1 h−1 under 420 nm monowavelength irradiation, which is 13.84 times higher than that of pristine ZCS NR. Even under 1020 nm monowavelength irradiation, the catalyst also shows an expressive H2 generation rate of 0.202 mmol g−1 h−1. Moreover, it gives an apparent quantum yield (AQY) of 39.5% at 420 nm. Femtosecond transient absorption spectroscopy (Fs-TAS) results reveal the multiple intraband and interband excited-charge transitions in the partially occupied band of O-MC and interfacial electron transfer between O-MC and ZCS NR result in the enhancement H2 evolution rate. This study highlights the extended light absorption and special partially occupied energy bands of metallic cocatalysts, which hold great potential for the design and synthesis of efficient full solar-spectrum (UV–vis-NIR light) responsive photocatalysts.

Enhanced H2 photoproduction in sodium sulfite-treated Chlamydomonas reinhardtii by optimizing wavelength of pulsed light

Photosynthetic hydrogen induced by Chlamydomonas reinhardtii presents a promising pathway for producing clean and renewable biofuel. Whereas, this process is transient owing to the repression of [FeFe]-hydrogenase by the oxygen co-evolved in photosystem II. Pulsed light is employed to prevent O2 accumulation and the activation of Calvin-Benson-Bassham cycle for sustainable H2. While light quality significantly influences photosynthesis, the optimal wavelength for pulsed light-induced H2 production remains not fully elucidated. Continuous irradiation at 660–670 nm performs poorly in H2 production due to the activation of PSII, which is responsible for water oxidation. However, in this study, O2 was removed using a combination of pulsed light and sulfite treatment. Among six light regimes with different wavelengths, pulsed light peaking at 660 nm resulted in the highest photo-H2 evolution, reaching approximately 180 mL L−1 within three days. This value was 1.4 times higher than that achieved under conventionally white pulsed light. Exposure to 660 nm red pulsed light not only sustained high activity of PSII and hydrogenase but also reduced the gene expression levels of ribulose-1,5-bisphosphate carboxylase/oxygenase and ferredoxin-NADP+ reductase, which was supposed to attenuate the competing effects of CBB cycle. This finding optimizes an efficient light source for pulsed light-induced H2 production and demonstrates a great potential for refining pulsed light parameters to improve H2 production in algae in the future.

Facilitated Unidirectional Electron Transmission by Ru Nano Particulars Distribution on MXene Mo2C@g-C3N4 Heterostructures for Enhanced Photocatalytic H2 Evolution.

Precious metals exhibit promising potential for the hydrogen evolution reaction (HER), but their limited abundance restricts widespread utilization. Loading precious metal nanoparticles (NPs) on 2D/2D heterojunctions has garnered considerable interest since it saves precious metal consumption and facilitates unidirectional electron transmission from semiconductors to active sites. In this study, Ru NPs loaded on MXenes Mo2C by an in-site simple strategy and then formed 2D/2D heterojunctions with 2D g-C3N4 (CN) via electrostatic self-assembly were used to enhance photocatalytic H2 evolution. Evident from energy band structure analyses such as UV-vis and TRPL, trace amounts of Ru NPs as active sites significantly improve the efficiency of the hydrogen evolution reaction. More interestingly, MXene Mo2C, as substrates for supporting Ru NPs, enriches photoexcited electrons from CN, thereby enhancing the unidirectional electron transmission. As a result, the combination of Ru-Mo2C and CN constructs a composite heterojunction (Ru-Mo2C@CN) that shows an improved H2 production rate at 1776.4 μmol∙g-1∙h-1 (AQE 3.58% at 400 nm), which is facilitated by the unidirectional photogenerated electron transmission from the valence band on CN to the active sites on Ru (CN→Mo2C→Ru). The study offers fresh perspectives on accelerated unidirectional photogenerated electron transmission and saved precious metal usage in photocatalytic systems.

Tailoring Second Coordination Sphere for Tunable Solid–Liquid Interfacial Charge Transfer toward Enhanced Photoelectrochemical H2 Production

AbstractThe recombination of photogenerated charge carriers severely limits the performance of photoelectrochemical (PEC) H2 production. Here, we demonstrate that this limitation can be overcome by optimizing the charge transfer dynamics at the solid–liquid interface via molecular catalyst design. Specifically, the surface of a p‐Si photocathode is modulated using molecular catalysts with different metal atoms and organic ligands to improve H2 production performance. Co(pda‐SO3H)2 is identified as an efficient and durable catalyst for H2 production through the rational design of metal centers and first/second coordination spheres. The modulation with Co(pda‐SO3H)2, which contains an electron‐withdrawing −SO3H group in the second coordination sphere, elevates the flat‐band potential of the polished p‐Si photocathode and nanoporous p‐Si photocathode by 81 mV and 124 mV, respectively, leading to the maximized energy band bending and the minimized interfacial carrier transport resistance. Consequently, both the two photocathodes achieve the Faradaic efficiency of more than 95 % for H2 production, which is well maintained during 18 h and 21 h reaction, respectively. This work highlights that the band‐edge engineering by molecular catalysts could be an important design consideration for semiconductor–catalyst hybrids toward PEC H2 production.

Tailoring Second Coordination Sphere for Tunable Solid-Liquid Interfacial Charge Transfer toward Enhanced Photoelectrochemical H2 Production.

The recombination of photogenerated charge carriers severely limits the performance of photoelectrochemical (PEC) H2 production. Here, we demonstrate that this limitation can be overcome by optimizing the charge transfer dynamics at the solid-liquid interface via molecular catalyst design. Specifically, the surface of a p-Si photocathode is modulated using molecular catalysts with different metal atoms and organic ligands to improve H2 production performance. Co(pda-SO3H)2 is identified as an efficient and durable catalyst for H2 production through the rational design of metal centers and first/second coordination spheres. The modulation with Co(pda-SO3H)2, which contains an electron-withdrawing -SO3H group in the second coordination sphere, elevates the flat-band potential of the polished p-Si photocathode and nanoporous p-Si photocathode by 81 mV and 124 mV, respectively, leading to the maximized energy band bending and the minimized interfacial carrier transport resistance. Consequently, both the two photocathodes achieve the Faradaic efficiency of more than 95 % for H2 production, which is well maintained during 18 h and 21 h reaction, respectively. This work highlights that the band-edge engineering by molecular catalysts could be an important design consideration for semiconductor-catalyst hybrids toward PEC H2 production.

High ratios of Ni3+ and Co3+ facilitated by Mn-addition for enhanced oxygen evolution reaction and ethanol oxidation reaction

The design of dual-functional catalyst for oxygen evolution reaction and ethanol oxidation reaction is essential for improving H2 production efficiency, and one strategy to improve catalytic performance is the incorporation of high-valence metal.

Spatially ordered NiOOH-ZnS/CdS heterostructures with an efficient photo-carrier transmission channel for markedly improved H2 production.

Spatially-ordered 1D nanocrystal-based semiconductor nanostructures possess distinct merits for photocatalytic reaction, including large surface area, fast carrier separation, and enhanced light scattering and absorption. Nevertheless, establishing a valid photo-carrier transmission channel is still crucial yet challenging for semiconductor heterostructures to realize efficient photocatalysis. In this work, spatially ordered NiOOH-ZnS/CdS heterostructures were constructed by sequential ZnS coating and NiOOH photo-deposition on multi-armed CdS, which consists of {112̄0}-faceted wurtzite nanorods grown epitaxially on {111}-faceted zinc blende core. Intriguingly, the surface photovoltage spectroscopy and PbO2 photo-deposition results suggest that the photogenerated holes of CdS were first transferred to the Zn-vacancy level of ZnS and then to NiOOH, as driven by the built-in electric field between ZnS and CdS and the hole-extracting effect of the NiOOH cocatalyst, leading to the efficient charge separation of NiOOH-ZnS/CdS. With visible-light (λ > 420 nm) irradiation, NiOOH-ZnS/CdS exhibited a distinguished H2-evolution rate of 152.20 mmol g-1 h-1 (apparent quantum efficiency of 40.9% at 420 nm), approximately 18 folds that of 3 wt% Pt-loaded CdS and much higher than that of ZnS/CdS and NiOOH-CdS counterparts as well as the most reported CdS-containing photocatalysts. Moreover, the cycling and long-term H2 generation tests manifested the outstanding photocatalyst stability of NiOOH-ZnS/CdS. The study results presented here may propel the controllable design of highly-active nanomaterials for solar conversion and utilization.

Strategic Ni-doping improved electrocatalytic H2 production by Bi3O4Br in alkaline water

A stable, efficient, and low-cost Ni-doped Bi3O4Br electrocatalyst for improved H2 production in alkaline conditions.

H2 production through aqueous phase reforming of ethanol over molybdenum carbide catalysts supported on zirconium oxide.

Molybdenum carbide catalysts supported on monoclinic and tetragonal zirconium oxide were studied for hydrogen production through aqueous phase reforming of ethanol. Catalysts were characterized by N2 physisorption, XRD, TPR and XPS. Results showed that 10%Mo2C/m-ZrO2 was less carburized and had a lower surface area than 10%Mo2C/t-ZrO2 and 10%MoC/t-ZrO2. Mo oxide was identified on the surface as well as two types of Mo oxycarbide and Mo oxynitride. The α crystalline phase of the carbide was more active than β phase and was ascribed to its higher relative superficial distribution. However, the α phase generated less H2 probably because there was less oxycarbide presence. 10%Mo2C/m-ZrO2 produced significantly more H2 and was stable for five consecutive reactions. This catalyst showed higher carburization degree after the reaction, which greatly enhanced the generation of H2, suggesting that carbides species improved H2 production compared to oxycarbides.

Evaluation of water splitting efficiency of g-C3N4 thermally etched/exfoliated under air and CO2 atmospheres

Graphitic carbon nitride synthetized from melamine was treated in air and CO2 at temperatures varying from 500° to 600°C for 2 and 6 h to fabricate a material with enhanced ability to photocatalytically produce H2 from water splitting. The materials were thoroughly characterized and H2 production was studied in a Pyrex reactor connected in-line with a Micro-GC. The results showed that the best HER (hydrogen evolution rate) were obtained in both atmospheres at a lower temperature with 6 h treatment rather than shorter treatments at higher temperatures. The thermal treatment produced an increase in surface area and band gap with a decrease in the crystallinity of the structure resulting in improved H2 production three times that of the bulk.

Photothermal-driven Reforming of Methanol Solution into Hydrogen over Ultra-stable Cr-MOF-embedded CuInS2 Heterostructure

Photothermal-driven reforming of methanol solution to hydrogen (PTRM) is an attractive way for sustainable in-situ hydrogen (H2) supply by solar energy. But the effective activation of MeOH and H2O to improve H2 production kinetics at low temperatures faces really challenge. As the cleavage energy barrier of CH bond in methanol is much higher than that of OH bond dissociation, here, we propose the use of CuInS2 with high interfacial hydroxyl activation capacity to integrate with MIL-101(Cr) matrix via in-situ encapsulation strategy. The pore confinement and site isolation of Cr-MOF matrix (MIL-101(Cr)) keep the high dispersion of CuInS2 (4.20 nm) and prohibit its agglomeration in PTRM. With synergistically photothermal effect, the insertion of CuInS2 not only provides a high-speed channel for photoexcited charge migration through the interface between CuInS2 and MIL-101(Cr) but also enhances the dehydrogenation activity of MeOH and H2O effectively as an electron-enriched tank. Moreover, the excellent ability of CuInS2 to dissociate H2O molecules at low temperature promoted the formation of abundant interfacial OH, which reinforces the CH bond cleavage of MeOH to decline the apparent activation energy (26%) and boost the H2 evolution kinetics (36233.0 μmolgcat-1h−1). Encouragingly, CuInS2@MIL-101(Cr) with an exceptional total turnover number (TON) climbing up to 16,775 in 65 h of run without catalyst deactivation. This work provides an important insight for the rational design of ultra-stable photo-thermal catalysts toward solar-driven reforming of methanol solution to hydrogen and conducive to the high activity performance in hydrogen-powered polymer electrolyte membrane (PEMFC) fuel cell.

Understanding how microbial electrolysis cell assisted anaerobic digestion enhances triclocarban dechlorination in sludge

Microbial electrolysis cell assisted anaerobic digestion (MEC-AD) has recently been considered as an efficient method for degradations of refractory pollutants. To date, however, knowledge about whether and how MEC-AD enhances the degradations of refractory pollutants in sludge remains largely unknown. This study therefore aims to fill this knowledge gap through investigating the transformation of triclocarban (TCC), a widely used antimicrobial agent, in MEC-AD reactors. Experimental results showed that over 83.3 % of TCC was dechlorinated to less toxic dichlorocarbanilide, monochlorocarbanilide and carbanilide in MEC-AD reactors. However, the mass loss of TCC in AD reactor (the electrodeless control) was merely 0.53 %. The presence of electrodes promoted TCC dechlorination in MEC-AD reactors, while the applied voltages (0.6 and 0.8 V) promoted hydrogenotrophic methanogenesis. H2-utilizing Nitrospira and homoacetogenic Acetobacterium were recognized as potential TCC dechlorinators, with their abundances in the planktonic sludge of MEC-AD reactors being 5.0–16.5 times higher than those in AD reactor. The carbon brush electrodes in MEC-AD reactors caused the enrichment of acetoclastic Methanothrix and the complete removal of acetic acid, which thereby thermodynamically accelerated homoacetogenesis and H2-producing acetogenesis in the planktonic sludge. Moreover, the direct interspecies electron transfer using hydrogenase as terminal electron acceptor was enhanced in the planktonic sludge of MEC-AD reactors, which could also improve H2 production rate and stimulate the growth and activity of TCC dechlorinators.

Application of modified olivine to a two-stage gasification process to evaluate the effects on hydrogen generation and retention of heavy metals

An environmentally-friendly and efficient waste-to-energy and heavy metal retention system was successfully developed in this study. The natural resource-olivine was applied to be the bed material of second-stage gasifier, and the effects of second-stage gasifier temperature, and steam/biomass ratio on the hydrogen (H2) production ratio of syngas and heavy metal retention were discussed in detail. The optimal hydrogen production ratio (30.3%) was achieved when second-stage temperature, steam/biomass ratio (S/B ratio) were respectively controlled at 900 °C, and 0.25. The characterization analysis results evidenced that the Fe and Mg from pristine olivine and calcined olivine can facilitate the waste-to-hydrogen during the gasification reaction. Moreover, the NiO and Ni loaded on the surface of the calcine olivine could not only facilitate the water–gas shift reaction for H2 production, but also provide strong binding sites for the linkage of ether and cleavage of the C–O bond, leading to formation of CH4. The formed CH4 would be further dissociated over the metallic Ni/olivine, resulting in a significantly improved H2 production efficiency. In general, the order based on the H2 ratio of the syngas at the second-stage was calcined Ni-loaded olivine > calcined olivine > olivine; the heavy metal retention efficiency of three bed materials was in the order of Cu > Pb > Zn.