Synergistic Effects of In-Situ Exsolved Ni-Ru Bimetallic Catalyst on High-Performance and Durable Direct-Methane Solid Oxide Fuel Cells

Direct-methane solid oxide fuel cells (CH4-SOFCs) have gained significant attention as methane, the primary component of natural gas (NG), is cheap and widely available and the natural gas infrastructures are relatively mature. However, at intermediate temperatures (e.g., 600-650 °C), current CH4-SOFCs suffer from low performance and poor durability under a low steam-to-carbon ratio (S/C ratio), which is ascribed to the Ni-based anode that is of low catalytic activity and prone to coking. Herein, with the guidance of density functional theory (DFT) studies, a highly active and coking tolerant steam methane reforming (SMR) catalyst, Sm-doped CeO2-supported Ni-Ru (SCNR), was developed. The synergy between Ni and Ru lowers the activation energy of the first C-H bond activation and promotes CHx decomposition. Additionally, Sm doping increases the oxygen vacancy concentration in CeO2, facilitating H2O adsorption and dissociation. The SCNR can therefore simultaneously activate both CH4 and H2O molecules while oxidizing the CH* and improving coking tolerance. We then applied SCNR as the CH4-SOFC anode catalytic reforming layer. A peak power density of 733 mW cm-2 was achieved at 650 °C, representing a 55% improvement compared to that of pristine CH4-SOFCs (473 mW cm-2). Moreover, long-term durability testing, with >2000 h continuous operation, was performed under almost dry methane (5% H2O). These results highlight that CH4-SOFCs with a SCNR catalytic layer can convert NG to electricity with high efficiency and resilience.

Resolving a structural issue in cerium-nickel-based oxide: a single compound or a two-phase system?

CeNiO3 has been reported in the literature in the last few years as a novel LnNiO3 compound with promising applications in different catalytic fields, but its structure has not been correctly reported so far. In this research, CeNiO3 (RB1), CeO2 and NiO have been synthesized in a nanocrystalline form using a modified citrate aqueous sol-gel route. A direct comparison between the equimolar physical mixture (n(CeO2) : n(NiO) = 1 : 1) and compound RB1 was made. Their structural differences were investigated by laboratory powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), transmission electron microscopy (TEM) with an energy-dispersive X-ray spectroscopy (EDS) detector, and Raman spectroscopy. The surface of the compounds was analyzed by X-ray photoelectron spectroscopy (XPS), while the thermal behaviour was explored by thermogravimetric analysis (TGA). Their magnetic properties were also investigated with the aim of exploring the differences between these two compounds. There were clear differences between the physical mixture of CeO2 + NiO and RB1 presented by all of these employed methods. Synchrotron methods, such as atomic pair distribution function analysis (PDF), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), were used to explore the structure of RB1 in more detail. Three different models for the structural solution of RB1 were proposed. One structural solution proposes that RB1 is a single-phase pyrochlore compound (Ce2Ni2O7) while the other two solutions suggest that RB1 is a two-phase system of either CeO2 + NiO or Ce1-xNixO2 and NiO.

Nanocomposite Catalyst for High-Performance and Durable Intermediate-Temperature Methane-Fueled Metal-Supported Solid Oxide Fuel Cells

CH₄-fueled metal-supported solid oxide fuel cells (CH₄-MS-SOFCs) are propitious as CH₄ is low-priced and readily available, and its renewable production is possible, such as biomethane. However, the current CH₄-MS-SOFCs suffer from either poor power density or short durable operation, which is ascribed to the low catalytic activity and poor coking tolerance of the metallic anode support. Herein, we have deliberately designed and synthesized a highly active nanocomposite catalyst, Sm-doped CeO₂-supported Ni, as the internal steam methane reforming catalyst, to optimize CH₄-MS-SOFCs. Both power densities and durability of optimized CH₄-MS-SOFCs have been dramatically enhanced compared to the pristine CH₄-MS-SOFCs. The optimized CH₄-MS-SOFCs deliver the highest performances among all zirconia-based CH₄-MS-SOFCs. Furthermore, the operating temperature has been reduced to 600 °C. At 600 °C, a viable peak power density of >350 mW/cm² is achieved, which is more than three times as high as the pristine CH₄-MS-SOFCs. Furthermore, the optimized CH₄-MS-SOFC achieves >1000 h of stable operation.

Density Functional Calculation of H2O/CO2/CH4 for Oxygen-Containing Functional Groups in Coal Molecules

To investigate the adsorption mechanism of H₂O, CO₂, and CH₄ molecules on oxygen-containing functional groups (OFGs) in coal molecules, quantum chemical density functional theory (DFT) simulations were performed to study the partial density of states and Mulliken bond layout of H₂O molecules bonded to different OFGs. The adsorption energy and Mulliken charge distribution of the H₂O, CO₂, and CH₄ molecules for each OFG were determined. The results showed that H₂O molecules form 2, 1, 1, and 1 hydrogen bonds with -COOH, -OH, -C=O, and -O-R groups, respectively. Double hydrogen bonds connected the H₂O molecules to -COOH with the smallest adsorption distances and highest Mulliken bond layout values, resulting in the strongest bonding between the H₂O molecules and -COOH. The most stable configuration for the adsorption of these molecules by the -OH group was when the O-H bond in the OFG served as a hydrogen bond donor and the O atom in the H₂O molecule served as a hydrogen bond acceptor. The order of the bonding strength between the OFGs and H₂O molecules was Ph-COOH > Ph-OH > Ph-C=O > Ph-O-R. The adsorption energy calculation results showed that H₂O molecules have a higher adsorption stability than CO₂ and CH₄ molecules. Compared with the -OH, -C=O, and -O-R groups, the -COOH group had a higher adsorption capacity for H₂O, CO₂, and CH₄ molecules. The adsorption stability of the CO₂ molecules for each OFG was higher than that of the CH₄ molecules. From the Mulliken charge layout, it was clear that after the adsorption of the H₂O molecules onto the OFGs, the O atoms in the OFGs tend to gain electrons, while the H atoms involved in bonding with the H₂O molecules tend to lose electrons. The formation of hydrogen bonds weakens the strength of the bonds in the H₂O molecule and OFGs, and thus, the bond lengths were elongated.

Enhanced CO2 Methanation Activity of Sm0.25Ce0.75O2-δ-Ni by Modulating the Chelating Agents-to-Metal Cation Ratio and Tuning Metal-Support Interactions

Highly active and selective CO₂ methanation catalysts are critical to CO₂ upgrading, synthetic natural gas production, and CO₂ emission reduction. Wet impregnation is widely used to synthesize oxide-supported metallic nanoparticles as the catalyst for CO₂ methanation. However, as the reagents cannot be homogeneously mixed at an atomic level, it is challenging to modulate the microstructure, crystal structure, chemical composition, and electronic structure of catalysts via wet impregnation. Herein, a scalable and straightforward catalyst fabrication approach has been designed and validated to produce Sm_0.25Ce_0.75O_2-δ-supported Ni (SDC-Ni) as the CO₂ methanation catalyst. By varying the chelating agents-to-total metal cations ratio (C/I ratio) during the catalyst synthesis, we can readily and simultaneously modulate the microstructure, metallic surface area, crystal structure, chemical composition, and electronic structure of SDC-Ni, consequently fine-tuning the oxide-support interactions and CO₂ methanation activity. The optimal C/I ratio (0.1) leads to an SDC-Ni catalyst that facilitates C-O bond cleavage and significantly improves CO₂ conversion at 250 °C. A CO₂-to-CH₄ yield of >73% has been achieved at 250 °C. Furthermore, a stable operation of >1500 hours has been demonstrated, and no degradation is observed. Extensive characterizations were performed to fundamentally understand how to tune and enhance CO₂ methanation activity of SDC-Ni by modulating the C/I ratio. The correlation of physical, chemical, and catalytic properties of SDC-Ni with the C/I ratio is established and thoroughly elaborated in this work. This study could be applied to tune the oxide-support interactions of various catalysts for enhancing the catalytic activity.

Catalytic Upgrading of Clean Biogas to Synthesis Gas

Clean biogas, produced by anaerobic digestion of biomasses or organic wastes, is one of the most promising substitutes for natural gas. After its purification, it can be valorized through different reforming processes that convert CH4 and CO2 into synthesis gas (a mixture of CO and H2). However, these processes have many issues related to the harsh conditions of reaction used, the high carbon formation rate and the remarkable endothermicity of the reforming reactions. In this context, the use of the appropriate catalyst is of paramount importance to avoid deactivation, to deal with heat issues and mild reaction conditions and to attain an exploitable syngas composition. The development of a catalyst with high activity and stability can be achieved using different active phases, catalytic supports, promoters, preparation methods and catalyst configurations. In this paper, a review of the recent findings in biogas reforming is presented. The different elements that compose the catalytic system are systematically reviewed with particular attention on the new findings that allow to obtain catalysts with high activity, stability, and resistance towards carbon formation.

Novel Ni/Ce(Ti)ZrO2 Catalysts for Methane Dry Reforming Prepared in Supercritical Alcohol Media

To achieve a high activity and coking stability of nickel catalysts in dry reforming of methane, materials comprised of ceria–zirconia doped by Ti were investigated as supports. Ceria–zirconia supports doped with titanium were prepared either via the Pechini method or by synthesis in supercritical alcohol media. Ni-containing catalysts were prepared by two techniques: standard incipient wetness impregnation and one-pot synthesis. The catalytic reaction of DRM to synthesis gas was carried out in the 600–750 °C range over 5% wt. Ni/Ce(Ti)ZrO2. Dried and calcined supports and catalysts were characterized by physicochemical methods including N2 adsorption, XRD, Raman, H2-TPR, and HRTEM. Both preparation methods led to formation of solid solution with cubic fluorite-like structure, as well as after addition of Ti. Introduction of Ti should provide improved oxygen storage capacity and mobility of support oxygen. The highest activity was observed with the catalyst of 5% wt. Ni/Ce0.75Ti0.2Zr0.05O2−δ composition due to optimized oxide support structure and support oxygen mobility.

Facile synthesis of homogeneous hollow microsphere Cu-Mn based catalysts for catalytic oxidation of toluene

There emerges an urgent stipulation towards the enhanced toluene catalytic combustion nanocatalysts for whittling down the footprint of toluene, a notorious air pollutant. Unfortunately, Few materials which are currently made accessible both present the high catalytic performance lower than 250 °C and keep durable at elevated temperatures. Herein, we demonstrate an expeditious salt hydrolysis-driven redox-precipitation protocol wherein H⁺ donated by the hydrolysis of copper salt was used to initiate the regioselective reduction of KMnO₄ by H₂O₂ under controlled redox kinetics in order to assemble the homogeneous mixed solid solution hollow microsphere Cu-Mn-based structure. Manifold characterization technologies unveil that in this unique nanbomicrosphere the abundant microscaled pores are successfully created across Cu-Mn bulks with fine-modulating the chemical properties. In sharp contrast with the compact counterparts without tailed porosity, the tuned crystallinity, accessed edge sites with the unsaturated coordination, fast redox chemistry, and boosted gaseous diffusion during reactions synergize to result in the signally good toluene oxidation, with the complete elimination activity at 252 °C, T₉₀ at 237 °C, and prominent long-term durability under the stringent reaction atmospheres. Our current study ushers in an alternative and tractable arena to excogitate the porous oxide materials for multifarious catalysis implementations.

Dry reforming of methane by stable Ni-Mo nanocatalysts on single-crystalline MgO

Large-scale carbon fixation requires high-volume chemicals production from carbon dioxide. Dry reforming of methane could provide an economically feasible route if coke- and sintering-resistant catalysts were developed. Here, we report a molybdenum-doped nickel nanocatalyst that is stabilized at the edges of a single-crystalline magnesium oxide (MgO) support and show quantitative production of synthesis gas from dry reforming of methane. The catalyst runs more than 850 hours of continuous operation under 60 liters per unit mass of catalyst per hour reactive gas flow with no detectable coking. Synchrotron studies also show no sintering and reveal that during activation, 2.9 nanometers as synthesized crystallites move to combine into stable 17-nanometer grains at the edges of MgO crystals above the Tammann temperature. Our findings enable an industrially and economically viable path for carbon reclamation, and the "Nanocatalysts On Single Crystal Edges" technique could lead to stable catalyst designs for many challenging reactions.

Al2O3-Coated Ni/CeO2 nanoparticles as coke-resistant catalyst for dry reforming of methane

To mitigate catalyst deactivation during the dry reforming of methane, Ni/CeO₂ catalysts composed of monodisperse Ni nanoparticles supported on CeO₂ nanorods are designed and coated with Al₂O₃ layers by atomic layer deposition.

Hydrotalcite-based CeNiAl mixed oxides for SO2 adsorption and oxidation

The impact of Ce on SO₂ adsoption and oxidation was studied over a series of flower-like hydrotalcite-based CeNiAl mixed oxides. Combined with XRD, BET, pyridine chemisorption, CO₂-TPD, XPS and H₂-TPR results, it revealed that introduction of Ce into NiAlO generates new centres for oxygen storage and release, promotes the enhancement of Lewis acid strength, increases weakly and strongly alkaline sites, and increases ability for SO₂ adsorption and oxidation. Furthermore, in situ Fourier transform infrared spectroscopy revealed that adsorbed SO₂ molecules formed surface bidentate binuclear sulfate. Taken together, we propose that the addition of Ce⁴⁺ to NiAlO acts to improve this compound as major adsorbent for SO₂.

Ceria-Based Materials in Hydrogenation and Reforming Reactions for CO2 Valorization

Reducing greenhouse emissions is of vital importance to tackle the climate changes and to decrease the carbon footprint of modern societies. Today there are several technologies that can be applied for this goal and especially there is a growing interest in all the processes dedicated to manage CO2 emissions. CO2 can be captured, stored or reused as carbon source to produce chemicals and fuels through catalytic technologies. This study reviews the use of ceria based catalysts in some important CO2 valorization processes such as the methanation reaction and methane dry-reforming. We analyzed the state of the art with the aim of highlighting the distinctive role of ceria in these reactions. The presence of cerium based oxides generally allows to obtain a strong metal-support interaction with beneficial effects on the dispersion of active metal phases, on the selectivity and durability of the catalysts. Moreover, it introduces different functionalities such as redox and acid-base centers offering versatility of approaches in designing and engineering more powerful formulations for the catalytic valorization of CO2 to fuels.

Regulation of Ni-CNT Interaction on Mn-Promoted Nickel Nanocatalysts Supported on Oxygenated CNTs for CO2 Selective Hydrogenation

Mn-promoted Ni nanoparticles (NPs) supported on oxygen-functionalized carbon nanotubes (CNTs) were synthesized for CO₂ hydrogenation to methane. This novel metal-carbon catalytic system was characterized by both experimental and computational studies. An anomalous metal-support interaction mode (i.e., a higher temperature would lead to a weakened Ni-CNT interaction) was observed. Deep investigation confirmed that surface oxygen groups (SOGs) on CNTs played a key role in tuning the Ni-CNT interaction. We proposed that high calcination temperature would firstly lead to the decomposition of SOGs (>400 °C), then causing a loss of anchoring sites and the anchoring effect of SOGs on Ni NPs, thus cutting off the connection between interfacial Ni atoms and CNT body, resulting in the migration and coalescence of fine flat Ni NPs into larger sphere ones at 550 °C (geometric effect). Density functional theory calculation study clarified that this kind of anchoring effect stemmed from the formation of covalent bonding between the interfacial Ni atom and C or O elements of SOGs, causing the electrons to be transferred from Ni atoms to CNT support because of the intrinsic electronegativity of -COOH (electronic effect). Besides, Mn promotion notably boosts the activity compared with unpromoted catalysts, which was irrelevant to the size effect, but enhanced CO₂ adsorption and conversion according to the result of CO₂-temperature programmed desorption and transient response experiment. The optimized NiMn350 catalyst endowed with Mn promotion and robust Ni-CNT interaction showed both high activity and sintering resistance for more than 140 h. Our findings paved the way to reasonably design the metal-carbon catalyst with both high activity and stability.

How Transparent Oxides Gain Some Color: Discovery of a CeNiO3 Reduced Bandgap Phase As an Absorber for Photovoltaics

In this work, we describe the formation of a reduced bandgap CeNiO₃ phase, which, to our knowledge, has not been previously reported, and we show how it is utilized as an absorber layer in a photovoltaic cell. The CeNiO₃ phase is prepared by a combinatorial materials science approach, where a library containing a continuous compositional spread of Ce _xNi_{1- x}O _y is formed by pulsed laser deposition (PLD); a method that has not been used in the past to form Ce-Ni-O materials. The library displays a reduced bandgap throughout, calculated to be 1.48-1.77 eV, compared to the starting materials, CeO₂ and NiO, which each have a bandgap of ∼3.3 eV. The materials library is further analyzed by X-ray diffraction to determine a new crystalline phase. By searching and comparing to the Materials Project database, the reduced bandgap CeNiO₃ phase is realized. The CeNiO₃ reduced bandgap phase is implemented as the absorber layer in a solar cell and photovoltages up to 550 mV are achieved. The solar cells are also measured by surface photovoltage spectroscopy, which shows that the source of the photovoltaic activity is the reduced bandgap CeNiO₃ phase, making it a viable material for solar energy.

Nanoparticles-in-concavities as efficient nanocatalysts for carbon dioxide reforming of methane to hydrogen and syngas

The ceria concavity-loaded Ni nanoparticle catalysts can lead to more active sites and promote CO₂ dissociative activation and CO desorption, thus enhancing significantly the catalytic performances for methane dry reforming with CO₂.

Crystal-plane effect of nanoscale CeO2 on the catalytic performance of Ni/CeO2 catalysts for methane dry reforming

The morphology and crystal-plane effects of CeO₂ materials (nanorods, nanocubes, nanooctas and nanoparticles) on the catalytic performance of Ni/CeO₂ in methane dry reforming were investigated.