Influence of catalysts on hydrogen production from wastewater generated from the HTL of human feces via catalytic hydrothermal gasification

The aqueous phase from the hydrothermal liquefaction of human feces is rich in organic matter. This study explored the usage of different catalysts on the catalytic hydrothermal gasification (CHG) of wastewater resulting from the hydrothermal liquefaction of human feces. The catalyst screening study revealed that NaOH (46.9%) and Raney Ni (41.2%) resulted in the highest H₂ composition, while Ru/AC resulted in the highest reduction in the liquid COD (97.7%). A catalyst mixture was then studied combining two heterogeneous catalysts, Raney Ni and Ru/AC, at different ratios to increase the H₂ composition. A weight ratio of 90% Raney Ni and 10% Ru/AC yielded a H₂ composition of 56.3%, a gas yield of 350.0 g/kg dry feed, and a H₂ yield of 10.61 mol/kg dry feed, indicating that catalyst synergy may exist between these two catalyst which further enhances H₂ production. Incorporation of a reaction coordinate diagram allowed for the direct comparison of energy recovery, COD_r, and H₂/COD value using a mixed Raney Ni and Ru/AC catalyst and varying temperatures and reaction times. Results showed that the optimal condition to maximize the H₂/COD content (9.5 mg H₂/g COD), the COD_r (64.3%), and energy recovery (30.4%) occurred at a temperature of 400 °C, a retention time of 60 min, and a catalyst to feedstock ratio of 0.1.     

Experimental study on the energy conversion of food waste via supercritical water gasification: Improvement of hydrogen production

In this study, the model food waste was gasified to hydrogen-rich syngas in a batch reactor under supercritical water condition. The model food consisted of rice, chicken, cabbage, and cooking oil. The effects of the main operating parameters including temperature (420–500 °C), residence time (20–60 min) and feedstock concentration (2–10 wt%) were investigated. Under the optimal condition at 500 °C, 2 wt% feedstock and 60 min residence time, the highest H₂ yield of 13.34 mol/kg and total gas yield of 28.27 mol/kg were obtained from non-catalytic experiments. In addition, four commercial catalysts namely FeCl₃, K₂CO₃, activated carbon, and KOH were employed to investigate the catalytic effect of additives at the optimal condition. The results showed that the highest hydrogen yield of 20.37 mol/kg with H₂ selectivity of 113.19%, and the total gas yield of 38.36 mol/kg were achieved with 5 wt% KOH addition Moreover, the low heating value of gas products from catalytic experiments with KOH increased by 32.21% compared to the non-catalytic experiment. The catalytic performance of the catalysts can be ranked in descending order as KOH > activated carbon > FeCl₃ > K₂CO₃. The supercritical water gasification (SCWG) with KOH addition can be a potential applied technology for food waste treatment with production of hydrogen-rich gases.     

CO2 methanation over ordered mesoporous NiRu-doped CaO-Al2O3 nanocomposites with enhanced catalytic performance

The ordered mesoporous NiRu-doped CaO-Al₂O₃ nanocomposites were synthesized via a facile evaporation-induced self-assembly method for CO₂ methanation. Metallic Ni and Ru species retained the single-component heterostructure rather than NiRu alloy over the 600 °C-reduced catalysts. Owing to the synergistic effect of bimetallic Ni–Ru as well as the improved H₂ and CO₂ chemisorption capacities after the addition of Ru and CaO promoters, the ordered mesoporous 10N1R2C-OMA catalyst exhibited enhanced catalytic activity and selectivity, which achieved the maximum CO₂ conversion of 83.8% and CH₄ selectivity of 100% at 380 °C, 0.1 MPa, 30000 mL g^?1 h^?1. In a 550 °C-109 h-lifetime test, the ordered mesoporous 10N1R2C-OMA catalyst showed high stability and superior anti-sintering property due to the confinement effect of the ordered mesostructure.     

Thermogravimetric characteristics and kinetics of sawdust pyrolysis catalyzed by potassium salt during the process of hydrogen preparation

Pyrolysis experiments of sawdust with KOH and K₂CO₃ catalysts were carried out under different heating rate in nitrogen atmosphere using thermogravimetric analyzer. The distributed activation energy model (DAEM) was used to analyze pyrolysis kinetics of sawdust. The results showed that both KOH and K₂CO₃ had strong catalytic effect on sawdust pyrolysis, which reduced the pyrolysis temperature of sawdust and increased the yield of char. There was only one main peak in DTG curve, which means that the pyrolysis behavior of cellulose and hemicellulose in sawdust was greatly changed. The catalytic performance of KOH was found to be more excellent in sawdust pyrolysis. Also KOH could catalyze the pyrolysis of sawdust at low temperature. The kinetic analysis results showed that the two kinds of catalysts could reduce the activation energy of sawdust pyrolysis and maintain a similar catalytic trend, but KOH had a more stable catalytic performance.     

Biogas steam reformer for hydrogen production: Evaluation of the reformer prototype and catalysts

This work aims to investigate a biogas steam reforming prototype performance for hydrogen production by mass spectrometry and gas chromatography analyses of catalysts and products of the reform. It was found that 7.4% Ni/NiAl₂O₄/γ-Al₂O₃ with aluminate layer and 3.1% Ru/γ-Al₂O₃ were effective as catalysts, given that they showed high CH₄ conversion, CO and H₂ selectivity, resistance to carbon deposition, and low activity loss. The effect of CH₄:CO₂ ratio revealed that both catalysts have the same behavior. An increase in CO₂ concentration resulted in a decrease in H₂/CO ratio from 2.9 to 2.4 for the Ni catalyst at 850 °C, and from 3 to 2.4 for the Ru catalyst at 700 °C. In conclusion, optimal performance has been achieved in a CH₄:CO₂ ratio of 1.5:1. H₂ yield was 60% for both catalysts at their respective operating temperature. Prototype dimensions and catalysts preparation and characterization are also presented.     

Co–Ni alloy supported on CeO2 as a bimetallic catalyst for dry reforming of methane

Dry reforming of methane (DRM) is an effective route to convert two major greenhouse gas (CH₄ and CO₂) to syngas (H₂ and CO). Herein, in this work, monometallic Ni/CeO₂ and a series of bimetallic Co–Ni/CeO₂ catalysts with Co/Ni ratios between 0 and 1.0 have been tested for DRM process at 600–850 °C, atmospheric pressure and a CH₄/CO₂ ratio of 1. The catalysts were characterized by X-ray diffraction, hydrogen-temperature programmed reduction, CO₂-Temperature programmed desorption, X-ray photoelectron spectroscopy, and thermogravimetric analysis. The catalyst with a Co/Ni ratio of 0.8 (labeled as 0.8 Co–Ni/CeO₂) exhibited the highest catalytic activity (CH₄ and CO₂ initial conversion for 80% and 85% at 800 °C, respectively) and the highest stability (less carbon deposition after 600min). This improved activity can be attributed to the Co–Ni alloy, which formed after reduction. Its weak chemisorption with hydrogen results in inhibition of reverse water gas shift reaction. In addition, Co-promoted the adsorption of surface oxygen enhances carbon removal, making it more stable.     

Glycerol steam reforming over Ru-Mg-Al hydrotalcite-derived mixed oxides: Role of the preparation method in catalytic activity

Ru-Mg-Al hydrotalcite-derived mixed oxides were synthesized using two preparation methods. The grafted catalyst was prepared by co-precipitation of the Ru/Mg/Al precursors at a constant pH. The impregnated catalyst was prepared by wet-impregnation of a calcined Mg-Al support with the Ru precursor (Ru(NO) (NO₃)₃). Both catalysts were calcined at 600 °C and characterized by X-ray diffraction, CO₂- temperature programmed desorption and H₂-temperature programmed reduction techniques. Catalytic activities were compared in the glycerol steam reforming reaction (400–700 °C, WGFR 9:1, flow rate: 0.075 ml/min) followed by a short-term stability test at 600 °C. The impregnated catalyst demonstrated a superior activity beyond 600 °C. This difference in activity was attributed to the easier accessibility of the active phase resulting from the preparation method. Short-term stability tests revealed deactivation of both catalysts as a result of coke formation.     

Biogas reforming over La-promoted Ni/SBA-16 catalyst for syngas production: Catalytic structure and process activity investigation

Biogas, a mixture of CO₂/CH₄, is reasonable for conversion to syngas (H₂/CO) by dry methane reforming (DMR) reaction. The modification of Ni/SBA-16 with a lanthanum promoter using the co-impregnation technique is investigated in this study. The temperature of reaction (600–750 °C), La loading (3.85–11.56 wt%), and Ni loading (10–30 wt%) are the parameters that are varied for maximizing reaction conversions. The synthesized catalysts and SBA-16 supporting material were characterized by several methods before and after reaction. According to the analysis, the existence of La₂O₃ particles on the catalyst's surface has decreased the particle sizes, as well as enhanced their dispersion. Therefore, the maximum CH₄ conversion of 94.21%, CO₂ conversion of 90.12%, H₂ yield of 90.53%, and H₂/CO molar ratio of 2.03 are achieved using 20Ni-5.78La/SBA16 at 700 °C. Besides, this catalyst showed lower deposited coke and higher stability compared with other synthesized catalysts.     

Screening of supported transition metal catalysts for hydrogen production from glucose via catalytic supercritical water gasification

In total 17 heterogeneous catalysts, with combinations of 4 transition metals (Ni, Ru, Cu and Co) and various promoters (e.g., Na, K, Mg, or Ru) supported on different materials (γ-Al₂O₃, ZrO₂, and activated carbon (AC)), were investigated with respect to their catalytic activity and stability for H₂ production from glucose via supercritical water gasification (SCWG). The experiments were carried out at 600 °C and 24 MPa in a bench-scale continuous-flow tubular reactor. Ni (in metallic form) and Ru (in both metallic and oxidized forms) supported on γ-Al₂O₃ exhibited very high activity and H₂ selectivity among all of the catalysts investigated for a time-on-stream of 5-10 h. With Ni20/γ-Al₂O₃ (i.e., γ-Al₂O₃ with 20 wt% Ni), a H₂ yield of 38.4 mol/kg glucose was achieved, approximately 20 times higher than that obtained during the blank test without catalyst (1.8 mol/kg glucose). In contrast, Cu and Co catalysts were much less effective for glucose SCWG reactions. As for the effects of catalyst support materials on activity, the following order of sequence was observed: γ-Al₂O₃ > ZrO₂ > AC. In addition, Mg and Ru were found to be effective promoters for the Ni/γ-Al₂O₃ catalyst, suppressing coke and tar formation.     

Assessment of sugarcane bagasse gasification in supercritical water for hydrogen production

Sugarcane bagasse is one of the major resources of agricultural biomass waste in the world. In this work, supercritical water gasification characteristics of sugarcane bagasse were investigated. The effect of temperature (600–750 °C), concentration (3–12 wt%), residence time (5–20 min) and catalysts (Raney-Ni, K₂CO₃ and Na₂CO₃) on bagasse gasification were studied. A kinetic study on the non-catalytic and Na₂CO₃ catalytic bagasse gasification was conducted to describe the kinetic information of the bagasse gasification reaction. The results showed that a higher reaction temperature, a lower bagasse concentration and a longer residence time could favor the gasification of bagasse, leading to a higher hydrogen yield. Bagasse was nearly completely gasified at 750 °C without using any catalyst and the carbon gasification efficiency could reach up to 96.28%. The addition of employed catalysts remarkably promoted the bagasse gasification reactivity. The maximum hydrogen yield (35.3 mol/kg) was achieved at 650 °C with the Na₂CO₃ loading of 20 wt%. The experimental data fitted well with a homogeneous model based on a Pseudo-first-order reaction hypothesis. The kinetic study showed that Na₂CO₃ catalyst could lower the activation energy E_a of bagasse gasification from 117.88 kJ/mol to 78.25 kJ/mol.     

Enhanced hydrogen production by the catalytic alkaline thermal gasification of cellulose with Ni/Fe dual-functional CaO based catalysts

A two-stage system involving alkaline thermal gasification of cellulose with Ca(OH)₂ sorbent and catalytic reforming with Ni/Fe dual-functional CaO based catalysts is proposed and applied to enhance H₂ production and in-situ CO₂ capture. The results show that the H₂ concentration is maximized at a considerably lower temperature (500 °C) than commercialized biomass gasification processes, reducing energy consumption. Sol-gel method is deemed better than impregnation method for its lower cost and higher-concentration H₂ production. Among the prepared catalysts, sol-NiCa catalyst exhibits the best performance in CO₂ absorption, resistance to carbon deposition, and cyclic stability, creating maximum H₂ concentration (79.22 vol%), H₂ yield (27.36 mmol g⁻¹ cellulose), and H₂ conversion (57.61%). Introduction of Ni rather than Fe on the CaO based catalyst promotes steam methane reforming at moderate temperature range of 400–600 °C, generating low contents of CH₄ (5.38 vol%), CO₂ (4.82 vol%), and CO (10.58 vol%).     

Effect of O2 and temperature on the catalytic performance of Ni/Al2O3 and Ni/MgAl2O4 for the dry reforming of methane (DRM)

Al₂O₃ and MgAl₂O₄ supported 10% (w/w) Ni catalysts having a dispersion of 1.5 and 2.0% are active for DRM at 600 and 750 °C. High temperature reduction of both the calcined catalysts resulted in metallic Ni being formed, suggesting strong support metal interactions. The CH₄ and CO₂ conversion during DRM are relatively constant with time-on-stream, and are higher for Ni/MgAl₂O₄ than Ni/Al₂O₃. Carbon-whiskers are also detected on both catalysts. O₂ co-feed of 2.6% (v/v) and increasing reaction temperature to 750 °C helped in decreasing the amount of carbon deposited, except for Ni/MgAl₂O₄ at 600 °C. Furthermore, higher conversions and H₂/CO ratios are achieved. It appears that on spent Ni/MgAl₂O₄ a different type of carbon species was formed, and this carbon species was difficult to remove by oxygen at 600 °C. Thus, co-feeding O₂, using an appropriate temperature, and choosing a suitable support can reduce the carbon present on the nickel catalysts during DRM.     

Ru incorporated Ni-MCM-41 mesoporous catalysts for dry reforming of methane: Effects of Mg addition, feed composition and temperature

Nickel incorporated MCM-41-like mesoporous materials, which were synthesized following a one-pot hydrothermal route, were promoted by Ru and Mg in order to improve their catalytic performances for dry reforming of methane. In this study, Ni-MCM-41 based catalysts (with a Ni/Si molar ratio of 0.2), containing different amounts of Ru (0.5-3.0 wt%) and Mg (1 and 5 wt%) were prepared by using sequential impregnation of Ru and Mg into Ni-MCM-41. Dry reforming of methane was studied in a tubular flow reactor in the temperature range of 500-600 °C with different CH₄/CO₂ ratios in the feed stream. Quite high hydrogen yield values and improved stability of these catalysts indicated the promoting effects of Ru for the Ni-MCM-41 type catalysts. Ru incorporation (1.0% Ru) was shown to improve H₂ yields. Mg impregnation into 1.0Ru@Ni-MCM-41 improved catalytic performance by increasing CH₄ conversion and decreasing the contribution of reverse water gas shift reaction, especially at initial times (first 60 min). Coke formation by decomposition of CH₄ contributed to the hydrogen selectivity, but did not cause significant change in catalytic performance, especially at longer reaction times.     

Novel LaNi intercalated Egyptian bentonite clay for direct conversion of methane using CO2 as soft oxidant

Natural Egyptian bentonite clay intercalated with both La and Ni having different molar ratio (La: Ni = 2:1, 1:1 & 1:2) were prepared, saving 5mmole pillar/gm clay, using ultrasonic assistance method. The prepared catalysts were calcined at 450 and then reduced at 400 °C & 600 °C.Characterization of the prepared LaNi-PILC was achieved by X-ray diffraction (XRD), Furrier transform infrared spectroscopy (FTIR), N₂ adsorption desorption isotherm (BET) and H₂-temperature programmed reduction (H₂-TPR). The data confirm the success of intercalation process for both La & Ni in the lamellar structure of bentonite clay. The La: Ni molar ratio affected the specific surface area, Ni crystal size, dispersion and reducibility of the prepared catalyst. The reduction temperature had a great effect on the reactivity and product selectivity during CO₂/CH₄ reforming at different reaction temperatures (600–800 °C). Where, reduction at 400 °C gives rise to CH₄ oxidation reaction (MOR) with formaldehyde as a main product. While reduction at 600 °C enhances the activity and stability for CO₂ reforming of methane (CRM) and syngas production (H₂/CO ~ 1.19). The most active and stable LaNi_1:2-PILC₅ catalyst (CO₂ and CH₄ conversions reached 85% and 90% respectively) is superior with respect to the performance of PILC based catalysts reported in the literatures.     

Oxidative catalytic steam gasification of sugarcane bagasse for hydrogen rich syngas production

Reactive Flash Volatilization (RFV) is an emerging thermochemical method to produce tar free hydrogen rich syngas from waste biomass at relatively lower temperature (<900 °C) in a single stage catalytic reactor within a millisecond residence time. Here, we show catalytic RFV of bagasse using Ru, Rh, Pd, or Re promoted Ni/Al₂O₃ catalysts under steam rich and oxygen deficient environment. The optimum reaction conditions were found to be 800 °C, steam to carbon ratio = 1.7 and carbon to oxygen ratio = 0.6. Rh–Ni/Al₂O₃ performed the best, resulting in highest hydrogen concentration in the synthesis gas at 54.8%, with a corresponding yield of 106.4 g-H₂/kg bagasse. A carbon conversion efficiency of 99.96% was achieved using Rh–Ni, followed by Ru–Ni, Pd–Ni, Re–Ni and mono metallic Ni catalyst in that order. Alkali and Alkaline Earth Metal species present in the bagasse ash and char, that deposited on the catalyst, was found to enhance its activity and stability. The hydrogen yield from bagasse was higher than previously reported woody biomass and comparable to the microalgae.     

Low-temperature catalytic conversion of greenhouse gases (CO2 and CH4) to syngas over ceria-magnesia mixed oxide supported nickel catalysts

Running dry reforming of methane (DRM) reaction at low-temperature is highly regarded to increase thermal efficiency. However, the process requires a robust catalyst that has a strong ability to activate both CH₄ and CO₂ as well as strong resistance against deactivation at the reaction conditions. Thus, this paper examines the prospect of DRM reaction at low temperature (400–600 °C) over CeO₂–MgO supported Nickel (Ni/CeO₂–MgO) catalysts. The catalysts were synthesized and characterized by XRD, N₂ adsorption/desorption, FE-SEM, H₂-TPR, and TPD-CO₂ methods. The results revealed that Ni/CeO₂–MgO catalysts possess suitable BET specific surface, pore volume, reducibility and basic sites, typical of heterogeneous catalysts required for DRM reaction. Remarkably, the activity of the catalysts at lower temperature reaction indicates the workability of the catalysts to activate both CH₄ and CO₂ at 400 °C. Increasing Ni loading and reaction temperature has gradually increased CH₄ conversion. 20 wt% Ni/CeO₂–MgO catalyst, CH₄ conversion reached 17% at 400 °C while at 900 °C it was 97.6% with considerable stability during the time on stream. Whereas, CO₂ conversions were 18.4% and 98.9% at 400 °C and 900 °C, respectively. Additionally, a higher CO₂ conversion was obtained over the catalysts with 15 wt% Ni content when the temperature was higher than 600 °C. This is because of the balance between a high number of Ni active sites and high basicity. The characterization of the used catalyst by TGA, FE-SEM and Raman Spectroscopy confirmed the presence of amorphous carbon at lower temperature reaction and carbon nanotubes at higher temperature.     

Valorization of sewage sludge through catalytic sub- and supercritical water gasification

Sub- and supercritical water gasification is applied to recover energy from sewage sludge in a batch reactor. The effects of reaction temperature and water-soluble additives as catalysts on gasification were examined. The resultant products, including syngas, hydrochar and liquid residues were characterized. The rise of temperature without the presence of catalysts increased the yield of H₂ (0.06 (350 °C) to 1.91 mol/kg (450 °C) and enhanced the gasification efficiency (1.29–19.61%), and decreased total organic carbon (TOC) by 68.50% in liquid residue. The changes in product distribution and characteristics of hydrochar and liquid residue implied that the organic matters in sewage sludge were dissolved and hydrolyzed in sub- and supercritical water, resulting in the production of syngas. The catalytic effect of different catalysts in relation to the H₂ gas yield was in the following order: KOH > NaOH > Na₂CO₃ ≈ K₂CO₃. In the case of catalytic supercritical water gasification at 400 °C, the highest molar fraction (37.28%) and yield of H₂ (1.60 mol/kg) were obtained in the presence of KOH. Furthermore, the scanning electron microscopy (SEM) analysis indicated that a conversion and dissolution of the organic matters in sewage sludge to liquid and gas, produced a porous, fragmented structure and disintegrated surface of hydrochar.     

Production of H2- and CO-rich syngas from the CO2 gasification of cow manure over (Sr/Mg)-promoted-Ni/Al2O3 catalysts

The valorization of cow manure (CM), as bio-waste, under a CO₂ atmosphere could be an attractive strategy for tackling the environmental problems related to waste management and CO₂ emission and producing valuable syngas. For this purpose, highly loaded Ni–Al₂O₃ catalysts with alkaline-earth metals (Mg and Sr) were synthesized and applied to the gasification of CM under CO₂. The lowest yields of bio-oil (16.98 wt %) and coke (0.34 wt %) and the highest yield of syngas (55.09 wt %) were obtained from the catalytic decomposition of hydrocarbons when Sr was incorporated into Ni/Al₂O₃ (SN-AO). The highest selectivity for H₂ (34.23 vol %) and CO (37.16 vol %) were obtained applying SN-AO followed by Mg-promoted Ni/Al₂O₃ (MN-AO) and Ni/Al₂O₃ (N-AO) catalysts. With increasing gasification temperature from 750 °C to 850 °C, the syngas yield (from 55.09 to 70.17 wt %) and H₂ concentration (from 34.23 to 38.03 vol %) increased considerably because of the endothermic gasification process. The yield and selectivity of syngas (H₂ and CO) increased under CO₂ compared to those obtained under N₂, indicating the high potential of CO₂ for the thermal decomposition and dehydrogenation of the volatile matter.     

Enhanced low-temperature activity for CO2 methanation over Ru doped the Ni/CexZr(1−x)O2 catalysts prepared by one-pot hydrolysis method

A series of 30 wt%Ni/Ce_xZr_1?xO₂ catalysts doped with Ru ranging from 0 to 5 wt% were prepared by one-pot hydrolysis of metal nitrates with ammonium carbonate for carbon dioxide methanation at low temperature range of 150–310 °C. The influences of Ce/Zr molar ratios and Ru contents on the physicochemical properties and catalytic activities of prepared catalysts were systematically investigated. The addition Ru can improve the Ni dispersion and the basicity of the yRu-30Ni/Ce_0.9Zr_0.1O₂ catalysts surface. As a result, their low-temperature catalytic activity had been enhanced over these doped Ru promoted catalysts. The optimal catalyst was 3Ru-30Ni/Ce_0.9Zr_0.1O₂ on which the CO₂ conversion reached theoretical equilibrium value as high as 98.2% with the methane selectivity of 100% at a reaction temperature as low as 230 °C. Moreover, there was almost no deactivation for the 3Ru-30Ni/Ce_0.9Zr_0.1O₂ catalyst during 300 h at 230 °C indicating excellent catalytic stability and coke resistence ability. It was also found that the low-temperature activity of 3Ru-30Ni/Ce_0.9Z_r0.1O₂ catalyst prepared by one-pot hydrolysis method was much higher than the one prepared by impregnation method.     

Evaluation of modified Ni/ZrO2 catalysts for hydrogen production by supercritical water gasification of oil-containing wastewater

Supercritical water gasification (SCWG) technology is a clean and cost-effective conversion technology due to its unique chemical and physical properties. However, the unique properties also lead to instability and inactivity for the pure Ni/ZrO₂ catalyst in SCWG process. In this work, we investigated the effect of second metal addition on the catalytic performance by modifying Ni/ZrO₂ catalysts with different promoters (Co, Ce, La, Y, Mg), which prepared by a single-step sol-gel method. The analysis results of catalysts by XRD, SEM and automatic micropore & chemisorption analyzer showed that Ce, Y, La may be helpful promoters to stabilize the structure of ZrO₂. Compared to the non-catalytic experiment, all the catalysts showed significantly higher activities in the SCWG reaction. Among all catalysts, Ni-Co/ZrO₂ exhibited excellent activity, which achieved the highest carbon gasification efficiency (CE) and highest hydrogen yield. Additionally, two key factors, concentration and temperature, were also investigated for the optimum conditions, and the maximum carbon gasification efficiency (CE) of 98.8% was achieved at 600 °C with the Ni-Co/ZrO₂ catalyst.