Highest Hydrogen Yield Research Articles

Hydrogen Production from Biogas: Development of an Efficient Nickel Catalyst by the Exsolution Approach

Hydrogen production from biogas over alumina-supported Ce1−xNixO2−x catalysts was studied in a temperature range of 600–850 °C with an initial gas composition of CH4/CO2/H2O of 1/0.8/0.4. To achieve a high and stable hydrogen yield, highly dispersed Ni catalysts were prepared through the exsolution approach. A solid solution of Ce1−xNixO2−x was firstly formed on the surface of Al2O3 and then activated in H2/Ar at 800 °C. The genesis and properties of the Ce1−xNixO2−x/Al2O3 catalysts were established using X-ray fluorescence analysis, thermal analysis, N2 adsorption, ex situ and in situ X-ray diffraction, Raman spectroscopy, electron microscopy, EDX analysis, and temperature-programmed hydrogen reduction. The performance of Ce1−xNixO2−x/Al2O3 catalysts in biogas conversion was tuned by regulation of the dispersion and reducibility of the active component through variation of content (5–20 wt.%) and composition (x = 0.2, 0.5, 0.8) of Ce1−xNixO2−x as well as the mode of its loading (co-impregnation (CI), citrate sol–gel method (SG)). For the 20 wt.% Ce1−xNixO2−x/Al2O3 catalyst, the rate of the coke formation decreased by a factor of 10 as x increased from 0.2 to 0.8. The optimal catalyst composition (20 wt.% Ce0.2Ni0.8O1.8/80 wt.% Al2O3) and preparation mode (citrate sol–gel method) were determined. At 850 °C, the 20 wt.% Ce0.2Ni0.8O1.8/Al2O3-SG catalyst provides 100% hydrogen yield at full CH4 conversion and 85% CO2 utilization.

Co3O4-CeO2 for enhanced syngas by low-temperature methane conversion with CO2 utilization via a catalytic chemical looping process

Fabricating effective transition metal-based oxygen carriers (OCs) in a low-temperature methane chemical looping conversion is of considerable importance but remains challenging. Herein, a series of Co3O4-CeO2 metal oxides were synthesized and used as OCs in a low-temperature methane conversion with CO2 utilization via chemical looping systems. Co3O4/CeO2–3 (Co/Ce ratios =7/3) showed the better performance with CH4 conversion above 70%, the highest hydrogen yield and CH4 conversion up to 55.73 mmol/g and 94.09% at 700 °C. The enhancement on the performance of Co3O4/CeO2–3 for hydrogen production is attributed to the interaction between Co3O4 and CeO2 particles at the interfaces, which could cause charge redistribution. Compared to pristine Co3O4, this structure endows Co3O4-CeO2 with higher CH4 conversion performance. Moreover, the resulting Co0 in the reduced OCs is beneficial for catalytic methane decomposition toward C and H2, followed by C gasification into CO during CO2 atmosphere with CO2 conversion as high as 81%. In the proposed chemical looping processes, catalytic methane decomposition step is active for hydrogen production, coupling with CO2 conversion toward CO to adjust the H2/CO ratio of syngas.

Dark fermentation of expired fruit juices for biohydrogen production followed by treatment and biotechnological exploitation of effluents towards bioplastics and microbial lipids

The treatment and biotechnological exploitation of expired fruit juices (EFJ) towards biohydrogen, polyhydroxyalkanoates (PHAs) and lipids production was investigated. The EFJ were initially fermented via an acidogenic consortium, during which the effect of the organic loading rate (OLR, 20–40 g sugars/L.d) on the sugars uptake and the yields of metabolites i.e. hydrogen, volatile fatty acids (VFAs) and lactate, was studied. The increase of the OLR resulted to lower bioconversion of sugars and metabolic shift to lactate production, whereas the highest hydrogen yield, 10.83 ± 0.41 L H2/m3 EFJ, was obtained for the OLR 30 g sugars/L.d. The acidogenic effluents were then utilized for the production of PHAs and microbial lipids, via bacterial heterotrophic and cyanobacterial consortia, respectively. The PHAs produced were in all cases copolymers, and the highest yield, 20.69 ± 1.01 kg PHAs/m3 EFJ was achieved from the effluent with the highest VFAs content. Microbial lipids were also successfully produced from all effluents. The results revealed that low VFAs and high sugars concentration, allowed cyanobacterial growth of up to 373.5 mg /L.d and maximum lipid content of up to 11.7 % d.w. (5.47 ± 0.49 kg biolipids/m3 EFJ), as well as significant substrate degradation (dissolved oxygen demand - d-COD, total nitrogen-ΤΝ, orthophosphates-PO43-, total sugars up to about 93 %, 95 %, 67 % and 96 %, respectively).

Evaluation of Engineered Biochar-Based Catalysts for Syngas Production in a Biomass Pyrolysis and Catalytic Reforming Process

Biochar, originating from biomass pyrolysis, has been proven a promising catalyst for tar cracking/reforming with great coke resistance. This work aims to evaluate various engineered biochar-based catalysts on syngas production in a biomass pyrolysis and catalytic reforming process without feeding extra steam. The tested engineered biochar catalysts include physical- and chemical-activated, nitrogen-doped, and nickel-doped biochars. The results illustrated that the syngas yields were comparable when using biochar and activated biochar as catalysts. A relatively high specific surface area (SSA) and a hierarchical porous structure are beneficial for syngas and hydrogen production. A 2 h physical-activated biochar catalyst induced the syngas with the highest H2/CO ratio (1.5). The use of N-doped biochar decreased the syngas yield sharply due to the collapse of the pore structure but obtained syngas with the highest LHVgas (18.5MJ/Nm3). The use of Ni-doped biochar facilitated high syngas and hydrogen yields (78.2 wt % and 26 mmol H2/g-biomass) and improved gas energy conversion efficiency (73%). Its stability and durability test showed a slight decrease in performance after a three-time repetitive use. A future experiment with a longer time is suggested to determine when the catalyst will finally deactivate and how to reduce the catalyst deterioration.

Microwave-carbon fiber cloth co-ignited catalytic degradation of waste plastic into high-yield hydrogen and carbon nanotubes

Environmental pollution and waste of resources caused by the massive accumulation and landfill of plastic waste has been prompting the research considering its reclamation. In this work, a microwave-carbon fiber cloth (CFC) co-ignited cracking strategy is proposed for highly-efficient conversion of polyethylene to hydrogen and high-quality carbon nanotubes conducted in a simple multimode microwave reactor. When FeAlOx@C is used as the catalyst, a hydrogen yield of 64.5 mmol g−1plastic is achieved while biochar powder as the ignitor only give yield to 30.6 mmol H2 g−1plastic, demonstrating the superiority of the CFC strategy where the carbon ignitor has little contact with the reactants. Further, FeAlOx@C treated by air plasma with rich oxygen vacancies is used as the catalyst. The yield is enhanced significantly, especially at low microwave power, resulting in a hydrogen yield of 67.3 mmol g−1plastic at a microwave power of 900 W, which was the highest as far as we know in the literature under direct catalytic cracking processes. Notably, the integrity of the CFC piece is maintained well, and is highly recyclable. Numerical simulation suggests that the position of CFC plays a key role in initiating the enhanced catalytic cracking process enabled by direct microwave heating.

Experimental Study of Bio-Hydrogen Production by Clostridium beijerinckii from Different Substrates

Glucose, alcohol stillage and glycerol were used as substrates for bio-hydrogen production by the newly isolated strain Clostridium beijerinckii 6A1 under batch conditions. High molar yields of hydrogen from the studied organic substrates were observed. There was a neat difference in the metabolic pathways of substrate digestion when hexose-based substrate or glycerol were used. The products of glycerol digestion showed that a pathway with no formic acid formation as intermediate was probable. In this case, considerable concentrations of acetic and propionic acid (up to 6 g dm−3) and small amounts of butanol were observed after 48 h. When glucose or hexose-based substrates were used, considerable amounts of formic acid (up to 6 g dm−3), i.e., the pathway proposed for Clostridia mixed cultures, were appropriate for the observed process of hydrogen release. For these substrates, considerable amounts of propionic acid in concentrations up to 1 g dm−3 were observed. That is why the pathway proposed for mixed cultures seemed more appropriate for our experiments carried out with hexose-based substrates. When hexoses were used, substrate digestion stopped the formation of acetic acid, propionic acid and ethanol. Probably, these intermediates are inhibitors to the further digestion to other products.

Sequential dark fermentation and microbial electrolysis cells for hydrogen production: Volatile fatty acids influence and energy considerations

Hydrogen production from food waste by coupling dark fermentation (DF) and microbial electrolysis cells (MEC) was studied. Metabolic patterns in DF, their effects on MECs efficiency, and the energy output of the coupling were investigated. Mesophilic temperature and acidic pH 5.5 resulted in 72 ± 20 mL H2/g CODin and a butyrate-enriched profile (C2/C4, 0.5–0.6) contrasting with an acetate-enriched profile (C2/C4, 1.8–1.9) and 36 ± 10 mL H2/g CODin at pH 7. Assessment in series of the DF effluents in MECs resulted in a higher hydrogen yield (566–733 mL H2/g CODin) and volatile fatty acids (VFAs) removal (84–95%) obtained from pH 7 effluents compared to pH 5.5 effluents (173–186 mL H2/g CODin and 29–59%). Finally, the output energy was lower in DF at pH 7, however, these effluents retrieved the highest energy in the MEC, showing the importance of process pH and VFAs profile to balance the coupling.

Microwave-assisted co-gasification of mixed plastics and corn stover: A synergistic approach to produce clean hydrogen

Co-gasification of waste plastic and corn stover into hydrogen is an effective way of reducing hydrogen cost and carbon footprint. However, current gasification technologies are energy intensive and cannot accommodate flexible feedstock. Microwave gasification offers several advantages including higher H2 yield and enhanced selectivity to syngas over tars. This work presents parametric optimization study of microwave co-gasification of mixed plastics and corn stover for hydrogen-rich syngas. High hydrogen yields of 30.5 mmolH2/gfeed at 700 °C were obtained using a microwave reactor, as opposed to 0.9 mmolH2/gfeed at 700–950 °C with conventional heating. Hydrogen yields are >98% of the theoretical extractable hydrogen from feedstock. Microwaves enhanced plastic and corn stover synergy and reduced tar yields. H-poor carbon formation resulting from the dehydrogenation of tar and coke, and light olefins decomposition, triggered by the external microwave electric fields, allowed further cracking of intermediate hydrocarbons to produce additional H2.

Hydrogen Production by Three-Stage (i) Pyrolysis, (ii) Catalytic Steam Reforming, and (iii) Water Gas Shift Processing of Waste Plastic.

The three-stage (i) pyrolysis, (ii) catalytic steam reforming, and (iii) water gas shift processing of waste plastic for the production of hydrogen have been investigated. The (i) pyrolysis and (ii) catalytic steam reforming process conditions were maintained throughout, and the experimental program investigated the influence of process conditions in the (iii) water gas shift reactor in terms of catalyst type (metal-alumina), catalyst temperature, steam/carbon ratio, and catalyst support material. The metal-alumina catalysts investigated in the (iii) water gas shift stage showed distinct maximization of hydrogen yield, which was dependent on the catalyst type at either higher temperature (550 °C) (Fe/Al2O3, Zn/Al2O3, Mn/Al2O3) or lower temperature (350 °C) (Cu/Al2O3, Co/Al2O3). The highest hydrogen yield was found with the Fe/Al2O3 catalyst; also, increased catalyst Fe metal loading resulted in improved catalytic performance, with hydrogen yield increasing from 107 mmol gplastic -1 at 5 wt % Fe loading to 122 mmol gplastic -1 at 40 wt % Fe/Al2O3 Fe loading. Increased addition of steam to the (iii) water gas shift reactor in the presence of the Fe/Al2O3 catalyst resulted in higher hydrogen yield; however, as further steam was added, the hydrogen yield decreased due to catalyst saturation. The Fe-based catalyst support materials investigated alumina (Al2O3), dolomite, MCM-41, silica (SiO2), and Y-zeolite; all showed similar hydrogen yields of ∼118 mmol gplastic -1, except for the Fe/MCM-41 catalyst, which produced only 88 mmol gplastic -1 of hydrogen yield.

CeO2‒x modified Ru/γ-Al2O3 catalysts for ammonia decomposition reaction

Developing high-performance ammonia decomposition catalysts for preparing COx-free hydrogen shows great practical significance. Herein, CeO2 is used as a promoter to modulate the metal-support interaction to enhance the catalytic performance of Ru/Al2O3 catalysts. A series of 1Ru/xCe-10Al (x = 0.5, 1, or 3) catalysts was prepared by a facile colloidal deposition method. We find that the optimized 1Ru/1Ce–10Al catalyst exhibits excellent activity for the decomposition of ammonia with a very high hydrogen yield of 7097 mmolH2/(gRu·min) at 450 °C. It is confirmed that Ru species are highly dispersed on the support surface as stable small clusters (∼1.3 nm). More importantly, due to the interaction between Ru species and partially reduced CeO2–x, the electron density of Ru species is increased, which is beneficial to the high activity of the 1Ru/xCe-10Al catalysts. This work paves a way to construct high-efficiency ammonia decomposition catalysts modified by CeO2.

Aluminum Scrap to Hydrogen: Complex Effects of Oxidation Medium, Ball Milling Parameters, and Copper Additive Dispersity

An effective combination of oxidation medium, ball milling parameters, and copper additive disperstiy ensuring fast aluminum scrap reaction with high hydrogen yield, was suggested. Different milling parameters (5, 10, and 15 mm steel balls; 1 and 2 h; unidirectional and bidirectional rotation modes) were tested for Al-10 wt.% Cu (50–70 μm) composition. The samples milled with 5 (2 h) and 10 mm (1 and 2 h) balls contained undesirable intermetallic phases Al2Cu and Cu9Al4, while those activated with 15 mm balls (1 h) provided the second-finest powder and best preservation of the original Cu and Al phases. Among the tested (at 60 °C) 2 M solutions NaCl, LiCl, KCl, MgCl2, ZnCl2, BaCl2, CaCl2, NiCl2, CoCl2, FeCl2, and AlCl3, the first six appeared to be almost useless (below 4% hydrogen yield), the following four provided better results, and the ultimate 91.5% corresponded to AlCl3. Samples with Cu dispersity of 50–100 nm, 1–19, 50–70, and 150–250 μm, and with no additive, were milled under the optimal parameters and tested in AlCl3. Their total yields were similar (~90–94%), while reaction rates differed. The highest rate was obtained for the sample modified with 50–70 μm powder.

Effect of Sr and Ti substitutions on optical and photocatalytic properties of Bi1-x Sr x Fe1-x Ti x O3 nanomaterials.

The potential use of down-sized BFO-xSTO systems (x ≤ 25%) as highly efficient photoanodes for photocatalytic water splitting is investigated. BFO-xSTO is prepared by a solid-state method and subsequently deposited by spray coating. The compounds possess rhombohedral symmetry for x ≤ 15% and phase coexistence for x > 15%, as demonstrated by Raman spectroscopy and transmission electron microscopy. Our findings revealed a drastic grain size decrease with increasing STO content, namely 260 nm for BFO to 50 nm for BFO with 25% STO. Moreover, BFO-xSTO, x > 10% exhibited high optical absorption (> 80%) in the full spectrum and interestingly a very promising band alignment with water redox potentials. Moreover, the photochemical measurements revealed a photocurrent density of ∼0.17 μA cm-2 achieved for x = 15% at 0 bias. Using DFT calculations, the substitution effects on the electronic, optical, and photocatalytic performances of the BFO system were investigated and quantified. Surprisingly, a high hydrogen yield (∼191 μmol g-1) was achieved by BFO-12.5%STO compared to 1 μmol g-1 and 57 μmol g-1 for BFO and STO, respectively. This result highlights the beneficial effects of both the downsizing and substitution of BFO on the photocatalytic water splitting and hydrogen production performances of Bi1-x Sr x Fe1-x Ti x O3 systems.

Performance of plasma-assisted chemical looping hydrogen generation at moderate temperature

Enhanced hydrogen production performance was achieved via plasma-assisted CLHG with the highest hydrogen yield at 400 °C.

Two-Step Gasification of Cattle Manure for Hydrogen-Rich Gas Production: Effect of Gasification Temperature, Steam Flow Rate, and Catalysts

Highlights Steam gasification parameters of cattle manure char were optimized. Effects of slow heating and fast heating modes on char gasification were analyzed. 1.92 m3/kg syngas yield and 88.72% C-conversion efficiency were obtained at 850 °C, with fast heating, 1.66 g/min of steam flow rate, and K2CO3 catalyst. Abstract. This study investigated the hydrogen-rich gas production from cattle manure char by steam gasification. The characteristics of char were studied by proximate analysis, ultimate analysis, thermogravimetric analysis, and XRD analysis. The effects of temperature, heating mode, steam flow rate, and catalyst on syngas yield, hydrogen yield, gas composition, heating value, and carbon conversion efficiency were explored. The results showed that the fixed carbon content in cattle manure char was 34.46%, an increase of 2.24 times compared with that of cattle manure, and the cattle manure char had a high fixed carbon content and thermal stability. The steam gasification indicated that the high temperature and fast heating could obtain high syngas and hydrogen yields. The steam flow rate played an important role in the char and steam reforming reactions. The SiO2 and ash catalysts had an inhibitory effect on the steam gasification of char. However, the K2CO3 catalyst improved the syngas and hydrogen yields. The optimal parameters in this study were 850 °C, fast heating, 1.66 g/min of steam flow rate, and K2CO3 catalyst. By comparison, the syngas and hydrogen production of char were better than those of biomass. As a result, the steam gasification of char was a promising method for producing hydrogen-rich gas. Keywords: Catalysts, Cattle Manure Char, Hydrogen, Steam Flow Rate, Steam Gasification.

Improving photocatalytic hydrogen production by switching charge kinetics from type-I to Z-scheme via defective engineering.

By providing the spatial separation of the active sites and retaining high oxidative and reducing capacity, the direct Z-scheme heterostructure is considered the most potential structure for yielding photo-electric response. However, challenges still exist in the directional transfer of charge carriers between two semiconductors in direct Z-scheme structures. In this regard, by constructing the Vzn defect and p-n junction, a direct Z-scheme ZnxCd1-xS@ZnS-NiS heterostructure was obtained for the regulated electronic structure, which ensured high-yield hydrogen properties. The Zn vacancy in the partially-coated ZnS shell led to the Vzn energy level, and the addition of NiS led to the p-n structure, which caused a drastic downshift of the band edge potentials in comparison to that of pristine CdS. This variation gave rise to a staggered band edge alignment between ZnxCd1-xS and NiS, resulting in the variation of charge transfer kinetics from type-I to direct Z-scheme. Through careful characterization, it was found that the optimal photocatalytic hydrogen precipitation activity reached 16 683.6 μmol g-1 h-1, which was 70 times that of CdS, and this improvement was considered to form a spatial barrier, providing a clear direction and path for carrier transmission.

Robust modelling development for optimisation of hydrogen production from biomass gasification process using bootstrap aggregated neural network

In this study, a robust model using bootstrapped aggregated neural network (BANN) was developed for optimising operating conditions of a two-stage gasification for high carbon conversion, high hydrogen yield and low CO2. The developed BAAN model predicted accurately (R2 of 0.999) the gas composition and the 95% confidence bounds for model predictions on unseen validation data indicated good prediction reliability for various feedstock. The BANN was also used to predict the optimum operating condition for hydrogen production from waste wood (1st stage temperature of 900 °C, 2nd stage temperature of 1000 °C, steam/carbon molar ratio of 5.7) to achieve high hydrogen (71–72 mol%), gas yield (98–99 wt%) and low CO2 (17–18 mol%). The optimal conditions were tested in the laboratory and the experimental results agreed well with the predicted data with an error of 0.01–0.05. Sensitivity analysis revealed that an increase in temperatures for both stages and high steam/carbon ratio favoured the H2 production and carbon conversion.

Effects of Different Hydrolysis Methods on the Hydrolysate Characteristics and Photo-Fermentative Hydrogen Production Performance of Corn and Sorghum Straw

The effects of hydrolysis methods (hydrothermal, acid, alkali, hydrothermal-enzyme, acid-enzyme, and alkali-enzyme) on hydrolysate characteristics and photo fermentative hydrogen production (PFHP) of corn straw (CS) and sorghum straw (SS) were investigated. The optimum production of reducing the sugar of straw in different solvent environments was studied by one-step hydrolysis and co-enzymatic hydrolysis pretreatment through a 3,5-dinitrosalicylic acid method. The hydrogen production process by photolytic fermentation of hydrolysates of Rhodobacter sphaeroides HY01 was further analyzed through a gas chromatograph, including the differences in accumulated PFHP yield, chemical oxygen consumption (COD), and volatile fatty acid (VFA) composition. The results showed that the highest reducing sugar yield was obtained by the acid method among one-step hydrolysis. In contrast, acid-enzyme hydrolysis can further increase the reducing sugar yield, which reached 0.42 g·g−1-straw of both straws. Both CS and SS had the highest hydrogen yield from acid-enzyme hydrolysate, 122.72 ± 3.34 mL·g−1-total solid of straw (TS) and 170.04 ± 4.12 mL·g−1-TS, respectively, compared with their acid hydrolysates with 40.46% and 10.53% higher hydrogen yields, respectively. The use of enzymatic hydrolysis showed a significantly higher hydrogen yield for CS compared to SS, indicating that acid hydrolysis was more suitable for SS and acid-enzyme hydrolysis was more suitable for CS.

Respiratory electrogen Geobacter boosts hydrogen production efficiency of fermentative electrotroph Clostridium pasteurianum

Electrogens and electrotrophs are microorganisms that generate and consume electricity in a bioelectrochemical system, respectively. However, the influence of respiratory electrogen Geobacter on the hydrogen production efficiency of fermentative electrotroph Clostridium pasteurianum remains unclear. Herein, cocultures with Geobacter sulfurreducens and C. pasteurianum were successfully established during glucose fermentation. Compared with those in C. pasteurianum monoculture alone, the maximum rate and yield of hydrogen production in the coculture increased by 122.2% and 28.92%, respectively. Meanwhile, substrate conversation efficiency elevated by 50.6%. The acetic and butyric acid fermentation pathways in the cocultures were also promoted relative to those in the monoculture. Carbon and electron balance analysis also unraveled that acetate production in syntrophic coculture accounted for approximately-two times more carbon and electron contributions than that in the monoculture. A direct manner mediated by pili-like structures or cell contacts was built between the two electroactive bacteria during the electric syntropy. Extracellular electron inputting from G. sulfurreducens shifted the fermentative pattern of C. pasteurianum, which resulted in an enhanced acetic acid fermentation pathway accompanied by a higher hydrogen yield. These findings provide new insights into electric syntrophy between respiratory electrogenic G. sulfurreducens and fermentative electrotrophic C. pasteurianum. A new strategy for hydrogen enhancement by coculturing hydrogen-producing electrotrophic bacteria with electrogenic microorganisms is also explored.

One dimensional nano-fiber structured Fe2O3/ZrO2 to enable efficient hydrogen production via water gas shift with chemical looping

Chemical looping provides a novel route to enable efficient hydrogen generation but is limited by lacking highly active and stable oxygen carriers. Here, we proposed the one-dimensional nano-fiber Fe2O3/ZrO2 for chemical looping water gas shift to produce hydrogen and studied the size effect on the redox chemistry. The results show that the reactivity and stability of nano-fibers is size dependent. Increasing the size can significantly strengthen the stability albeit reducing the activity. In particular, the nano-fiber with the diameter of ∼99 nm shows both high hydrogen yield (9.07 mmol.g−1) and stable performance over 60 redox cycles at 750 °C. Mechanism study manifests that the small size can enhance the reduction depth in the initial few cycles, leading to high hydrogen production, but at the expense of severe sintering after repetitive cycles. Therefore, the selection of an optimal size (99 nm in diameter) is key to simultaneously ensure high reactivity and stability. We anticipate that the size effect on the redox chemistry will have extensive implications for the development of efficient oxygen carriers.

Hydrogen Production and Carbon Capture by Gas-Phase Methane Pyrolysis: A Feasibility Study.

Using natural gas and sustainable biogas as feed, high-temperature pyrolysis represents a potential technology for large-scale hydrogen production and simultaneous carbon capture. Further utilization of solid carbon accruing during the process (i. e., in battery industry or for metallurgy) increases the process's economic chances. This study demonstrated the feasibility of gas-phase methane pyrolysis for hydrogen production and carbon capture in an electrically heated high-temperature reactor operated between 1200 and 1600 °C under industrially relevant conditions. While hydrogen addition controlled methane conversion and suppressed the formation of undesired byproducts, an increasing residence time decreased the amount of byproducts and benefited high hydrogen yields. A temperature of 1400 °C ensured almost full methane conversion, moderate byproduct formation, and high hydrogen yield. A reaction flow analysis of the gas-phase kinetics revealed acetylene, ethylene, and benzene as the main intermediate products and precursors of carbon formation.