Despite the importance of subsoil carbon (C) deposition by deep-rooted crops in mitigating climate change and maintaining soil health, the quantification of root C input and its microbial utilization ...and stabilization below 1 m depth remains unexplored. We studied C input by three perennial deep-rooted plants (lucerne, kernza, and rosinweed) grown in a unique 4-m deep RootTower facility. 13C multiple pulse labeling was applied to trace C flows in roots, rhizodeposition, and soil as well as 13C incorporation into microbial groups by phospholipid fatty acids and the long-term stabilization of microbial residues by amino sugars. The ratio of rhizodeposited 13C in the PLFA and amino sugar pools was used to compare the relative microbial stability of rhizodeposited C across depths and plant species. Belowground C allocation between roots, rhizodeposits, and living and dead microorganisms indicated depth dependent plant investment. Rhizodeposition as a fraction of the total belowground C input declined from the topsoil (0–25 cm) to the deepest layer (360 cm), i.e., from 35%, 45%, and 36%–8.0%, 2.5%, and 2.7% for lucerne, kernza, and rosinweed, respectively, where lucerne had greater C input than the other species between 340 and 360 cm. The relative microbial stabilization of rhizodeposits in the subsoil across all species showed a dominance of recently assimilated C in microbial necromass, thus indicating a higher microbial stabilization of rhizodeposited C with depth. In conclusion, we traced photosynthates down to 3.6 m soil depth and showed that even relatively small C amounts allocated to deep soil layers will become microbially stabilized. Thus, deep-rooted crops, in particular lucerne are important for stabilization and storage of C over long time scales in deep soil.
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•Rhizodeposition strongly declined with depth across all plant species.•Microbial stabilization of rhizodeposits increases with depth.•Deep-rooted crops are important for storage and stabilization of rhizodeposition.
Addition of rare earth oxide, especially lanthanide oxide, was regarded as a promising strategy to improve the carbon resistance for Nickel-based catalysts in dry reforming of methane (DRM). In this ...work, Nickel-based catalysts containing lanthanide oxides (NiLa/SiO2, NiCe/SiO2, NiSm/SiO2, and NiGd/SiO2) were prepared and employed to catalyze DRM. Lanthanide oxide affected the formation of Ni nanoparticles in different size. In NiLa/SiO2 and NiCe/SiO2, Ni nanoparticles maintained relatively small size (4 nm), while in NiSm/SiO2 and NiGd/SiO2, nickel particles were in large size (8 nm). NiLa/SiO2 and NiCe/SiO2 exhibited better stability than the other two catalysts, with CH4 conversion decreasing from 64.6 to 57.6% and 61.6 to 60.3%, respectively in 10 h on stream. The kinetic study confirmed that adding lanthanide oxide significantly affected the activation energy of CH4 dissociation and CO2 dissociation. Compared to monometallic Ni/SiO2, the presence of Sm and Gd suppressed CO2 dissociation, and introduction of Ce and La effectively promoted CO2 dissociation. These characters contributed to the higher carbon resistance and good stability for NiLa/SiO2 and NiCe/SiO2 catalysts in DRM reaction.
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•Nickel-based silica catalysts containing lanthanide oxides were synthesized.•Addition of CeO2 and La2O3 showed high carbon resistance for CH4 dry reforming.•Introduction of CeO2 and La2O3 to nickel catalyst promoted the CO2 dissociation.•Nickel catalyst modified with Sm2O3 and CeO2 facilitated the CH4 dissociation.
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•Ni/SiO2 catalysts were prepared by wet impregnation and ammonia evaporation methods.•Ni phyllosilicate induced high dispersed Ni due to strong interaction with support.•Ni/SiO2@SiO2 ...core shell structure was successfully prepared by sol–gel method.•The highly dispersed Ni and confining effect cause excellent coke resistance.
Suppressing coke formation on nickel-based catalysts used in methane-dry reforming is one of the most important issues. We investigate the effect of nickel metal-support interaction and outer porous silica shells on the coke formation in methane dry reforming over Ni/SiO2@SiO2 core–shell catalysts. Using a simple ammonia evaporation method, the Ni/SiO2_AE catalyst, which has highly dispersed nickel nanoparticles with ca. 7 nm, was synthesized and coated with an outer porous silica shell to produce a Ni/SiO2_AE/SiO2 core shell catalyst. Based on the TEM and N2 isotherm results, a 20 nm thick SiO2 shell having pores with a size of about 2 nm was uniformly coated on the Ni/SiO2_AE catalyst. H2-TPR, and XPS analysis results suggest that the Ni/SiO2_AE and Ni/SiO2_AE@SiO2 catalysts have high nickel dispersion. This high dispersion is due to strong metal-support interactions and the formation of Ni phyllosilicate species. On the other hand, the ones synthesized by wet impregnation method have low nickel dispersion due to weaker metal-support interactions. The Ni/SiO2_AE@SiO2 core–shell catalyst showed stable catalytic activity under methane dry reforming conditions at 600 °C. On the other hand, the Ni/SiO2_WI and Ni/SiO2_AE catalysts showed rapid deactivation due to severe coke formation. It can be concluded that the strong metal-support interaction and the silica shell can suppress catalyst deactivation caused by coke formation very effectively.
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•Six classes of Ni catalysts for photothermal DRM reaction were reviewed.•Actives and photocarriers in photothermal MDR were illustrated.•Four types of photothermal mechanism were ...discussed.•The outlook of photothermal DRM for future study in proposed.
Dry reforming of methane (DRM) converts the greenhouse gases of methane and carbon dioxide to useful chemical intermediates of syngas, contributing to both environment and energy. However, the high bond energies of C–H in methane and C=O in carbon dioxide result in the endothermicity of DRM, requiring high temperature for considerable conversions of carbon dioxide and methane in conventional thermal DRM. This causes problems of carbon deposition and catalyst sintering. Recently, the photothermal DRM are demonstrated needing lower temperature than thermal DRM, significantly enhancing performance, reducing carbon deposition and minimizing sintering. These are owned to the synergistic contribution from both photo and temperature, which reduces activation energies of methane and carbon dioxide, increases surface temperature of catalyst and adjusts reaction path ways etc. in photothermal DRM. To have an overview achievement of photothermal DRM in recent years, this paper reviews catalyst developments and mechanism investigations for photothermal DRM. We firstly introduce the active sites and principles in photothermal DRM, then the latest development of catalysts including TiO2-, CeO2- C3N4-, SiO2-based catalysts, layered double hydroxide catalysts and single atom catalyst are summarized. After illustrating the reaction mechanism, the outlook of possible investigation is suggested in future. It is hoped that this paper could provide pioneers useful guidance developing catalyst for photothermal DRM, as well as other photo-sensitive reactions.
•H2 production systems coupled with CCS based on methane reforming are proposed.•CO2 reinjection is creatively integrated for high conversion of CH4 and CO2.•Carbon deposition risk has been eased by ...constructing a high CO2/CH4 reforming process.•From the angle of H2 production, new steam reforming one has an advanced performance.•New dry reforming technology has large potential to applied into multifunctional systems.
The combination of methane steam reforming technology and CCS (Carbon Capture and Storage) technology has great potential to reduce carbon emissions in the process of hydrogen production. Different from the traditional idea of capturing CO2 (Carbon Dioxide) in the exhaust gas with high work consumption, this study simultaneously focuses on CO2 separation from fuel gas and recycling. A new hydrogen production system is developed by methane steam reforming coupled with carbon capture. Separated and captured high-purity carbon dioxide could be recycled for methane dry reforming; on this basis, a new methane-dry-reforming-driven hydrogen production system with a carbon dioxide reinjection unit is innovatively proposed. In this study, the energy flow and irreversible loss in the two newly developed systems are analyzed in detail through energy and exergy balance analysis. The advantages are explored from the perspective of hydrogen production rate, natural gas consumption and work consumption. In addition, in consideration of the integrated performance, an optimal design analysis was conducted. In terms of hydrogen production, the new system based on dry reforming is better, with an advantage of 2.41%; however, it is worth noting that the comprehensive thermal performance of the new steam reforming system is better, reaching 10.95%. This study provides new ideas for hydrogen production from a low carbon emission perspective and also offers a new direction for future distributed energy system integration.
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•The effect of lanthanide metal oxides on the carbon deposition resistance was studied.•Lanthanide metal oxides increase the surface oxygen vacancies and basicity of the ...catalyst.•Lanthanide promoters reduce the degree of graphitization of carbon deposits.•The kinetics of DRM catalysts with different promoters were studied.•Catalysts added with lanthanide promoters follow a bifunctional reaction mechanism.
Dry reforming of methane (DRM) can well convert CO2 and CH4 into syngas to further synthesize valuable chemicals, thereby realizing the resource utilization of greenhouse gases. However, the sintering and carbon deposition vulnerabilities of Ni-based dry reforming catalysts prevent them from being commercialized. The ethylene glycol impregnation method was used in this study to create a group of Ni/MSS catalysts added with lanthanide metal oxides (CeO2、La2O3、Sm2O3、Nd2O3). A range of characterization techniques were employed to examine the impact on the resistance to carbon deposition and sintering. Studies have shown that lanthanide metal oxides enhance the metal support interaction, promote active metal Ni dispersion and reduce Ni particle size. Meanwhile, the catalyst's alkali strength and basic site count both rise, and a large number of surface oxygen vacancies are produced, among which CeO2 has the most obvious effect. As a result, the catalyst's activity and resistance to carbon deposition and sintering are greatly increased, while also inhibiting the degree of graphitization of carbon deposition. According to the kinetic studies, the catalyst containing lanthanide metal oxides had an apparent activation energy for CH4 and CO2 that was lower than the Ni/MSS catalyst.
Continuously rising concentrations of CO2 in the atmosphere has led to significant negative impacts on the environment which propelled research efforts to replace fossil fuel-based energy with clean ...energy sources. However, the reliance on fossil fuel as the main energy source is projected to stay for many years to come, and therefore developing cleaner pathways to utilize fossil fuels for energy generation is paramount. Chemical looping combustion (CLC) has emerged in recent years as a promising technology for CO2 capture in power plants and other CO2 intensive industries. A key cornerstone for CLC is the development of efficient oxygen carriers with high oxygen capacity, fast kinetics, cyclic stability, mechanical strength, and low cost. The research progress in this field is rapidly growing to tackle the major materials and operational challenges to advance the knowledge toward a commercial scale CLC application. This work provides an overview of the significant advancements achieved over the past seven years in the development of synthetic metal oxide-based oxygen carriers. A summary of key performance indicators used to evaluate oxygen carriers is presented in this study to enable for a systematic assessment of the oxygen carriers’ properties and performance. This work is focused on summarizing and critically reviewing the literature on the development of oxygen carriers such as Ni, Cu, Fe, Mn, Co-based metal oxides and combined/mixed oxygen carriers in the past seven years. Studies of naturally occurring materials and waste materials used as oxygen carriers are excluded from this review which is mainly focused on synthetically prepared materials. Significant efforts were dedicated to investigating the role of the supports, synthesis methods, promotors, and other factors affecting the CLC performance of synthetic oxygen carriers using solid, liquid, and gaseous fuels. This review also outlines the main challenges, research needs and opportunities for future progress to highlight potential pathways to develop synthetic oxygen carriers in an environmentally friendly, and cost-effective manner.
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•Extensive research has been performed on CLC recently 2016–2022.•CLC Performance indicators and equipment are briefly presented.•Oxygen carriers are keystone for the development and commercialization of CLC.•Use of single synthetic oxygen carriers in CLC over the last 5 years is reviewed.•Effects of different factors on CLC performance are comprehensively discussed.
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•Nickel (10 wt. %) catalysts supported on nanostructured ceria were prepared.•High temperature steam reforming of ethanol for hydrogen production was studied.•At 550 °C rod-shaped ...sample was the most stable with higher hydrogen yield.•Carbon removal was easier in rod-shaped catalysts due to its higher OSC.
Ceria nanostructures (particles, rods and cubes) were used as support of 10 wt. % Ni/CeO2 catalysts and tested in the steam reforming of ethanol (SRE) reaction at stoichiometric conditions. Supports were prepared by precipitation and hydrothermal methods. Nickel was incorporated by incipient wetness impregnation. Catalysts were characterized by N2 adsorption, EDS, X-ray diffraction, transmission electron microscopy, Raman spectroscopy, N2O chemisorption, temperature programed reduction and CO-oxidation. Characterization of materials showed differences in the nickel particle size, nickel dispersion and its interaction with the ceria support, all these features dependent on the morphology of ceria; trends observed in calcined samples are retrieved in the reduced catalysts. Nickel supported on ceria rods (Ni/Ce-r) exhibited the best activity and hydrogen yield in the SRE reaction at 550 °C for 24 h under stream. Moreover, temperature-programmed oxidation of spent catalysts showed that the Ni/Ce-r sample presented the lower amount of carbon deposits which were also removed at lower temperatures. These characteristics of the rod-shaped catalyst were ascribed to the enhanced oxygen storage capacity presented by ceria rods and the higher dispersion of nickel over this ceria nanostructure.
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•Fe-based catalysts have high selectivity towards C2H4 for CO2-ODH.•The addition of NiO to the support doubled catalytic activity.•Negligible carbon was deposited after 17 h ...time-on-stream.•Fe redox cycle occurs during CO2-ODH.
Ethylene is a very important raw material for chemical industry, which can be produced from the underutilized ethane, originating from shale gas, through the oxidative ethane dehydrogenation (EDH) using CO2 as a mild oxidant. Two 5 wt%Fe-based catalysts, supported on mixed MgO-ZrO2 and NiO-MgO-ZrO2 oxides, prepared by auto-combustion sol-gel method, were investigated for CO2-EDH. The crystallographic structure of the catalysts was examined by X-ray diffraction (XRD), while the surface composition and oxidation state were identified using X-ray photoelectron spectroscopy (XPS). The effect of catalyst pre-treatment, reduction by H2, on catalytic activity, selectivity and carbon-resistance was evaluated for the studied samples.
Both catalysts provided high ethylene selectivity and activity during CO2-oxidative dehydrogenation of C2H6 at 873 K, atmospheric pressure and equimolar CO2/C2H6 ratio. The addition of NiO in the support material had a promoting effect, since it resulted in a catalyst with a twofold higher activity. Similarly, the selectivity towards C2H4 remained at high levels, above 70%, slightly increased, as it was exemplified during an iso-conversion experiment. The best candidate lost 25% of its initial activity during the first 3 h of a stability test, while it remained unchanged from 3 to 17 h. Deposited carbon was negligible, whereas sintering was identified as the main deactivation reason. Systematic XPS analysis revealed that deposition of Fe on NiO-MgO-ZrO2 support significantly affected the local environment of Ni, contributing to the enhanced catalytic performance. Increased Fe2+ concentration was found for the “used” Fe-catalyst supported on NiO-MgO-ZrO2, compared to the “as-prepared”, implying partial reduction of Fe during CO2-EDH reaction, as it was also observed by HRTEM.
Dry reforming of methane (DRM) is an effective method for achieving carbon reduction and carbon neutrality by directly converting two greenhouse gases, CH4 and CO2, into value-added products. ...However, catalysts used in this process are prone to metal sintering and carbon deposition, which are major obstacles. Thus, the design of high-performance catalysts is crucial for the success of dry reforming technology. In this paper, a series of Co-based catalysts modified with different rare earth metals (Y, Tb, Gd) and γ-Al2O3 as the support were prepared using the conventional co-impregnation method. The effects of the introduction of rare earth metals additives on the activity and resistance to carbon build-up of the catalysts were investigated. The catalysts were analyzed using various characterization techniques. The results showed that Y and Tb as additives could effectively improve the activity and stability of the catalyst. However, the Gd modified Co-based catalyst had a large specific surface area but was less favorable for the DRM reaction. The best performance catalyst was achieved with the 10Co–5Y/γ-Al2O3 under 750 °C with CH4 and CO2 conversions of 91.79% and 98.33%, respectively. This is mainly due to the fact that the introduction of Y enhances the electron transfer between Co and metal oxides, generating more oxygen vacancies and base vacancies on the surface, thus enhancing the adsorption and activation of CO2. The 10Co–5Y/γ-Al2O3 catalyst demonstrated excellent DRM performance even after 20 h at 750 °C. Moreover, we proposed a hypothesis regarding the mechanism of resistance to carbon build-up. This study provides a reasonable idea to design Co-based catalysts with resistance to sintering and carbon accumulation to improve the performance of reforming catalysts.
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•The introduction of Y and Tb enhances the electron transfer ability of Co species.•The introduction of Y improves the cobalt dispersibility and oxygen vacancies.•30% enhancement in CH4 and CO2 conversion was achieved with 10Co–5Y/γ-Al2O3.•The mechanism of resistance to carbon build-up was proposed.
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