Synergistic Effect of Ni/Ni(OH)2 Core-Shell Catalyst Boosts Tandem Nitrate Reduction for Ampere-Level Ammonia Production

Electrocatalytic reduction of nitrate to ammonia provides a green alternate to the Haber-Bosch method, yet it suffers from sluggish kinetics and a low yield rate. The nitrate reduction follows a tandem reaction of nitrate reduction to nitrite and subsequent nitrite hydrogenation to generate ammonia, and the ammonia Faraday efficiency (FE) is limited by the competitive hydrogen evolution reaction. Herein, we design a heterostructure catalyst to remedy the above issues, which consists of Ni nanosphere core and Ni(OH)2 nanosheet shell (Ni/Ni(OH)2). In-situ Raman spectroscopy reveals Ni and Ni(OH)2 are interconvertible according to the applied potential, facilitating the cascade nitrate reduction synergistically. Consequently, it attains superior electrocatalytic nitrate reduction performance with an ammonia FE of 98.50% and a current density of 0.934 A cm-2 at -0.476 V versus reversible hydrogen electrode, and exhibits an average ammonia yield rate of 84.74 mg h-1 cm-2 during the 102-hour stability test, which is highly superior to the reported catalysts tested under similar conditions. Density functional theory calculations corroborate the synergistic effect of Ni and Ni(OH)2 in the tandem reaction of nitrate reduction. Moreover, the Ni/Ni(OH)2 catalyst also possesses good capability for methanol oxidation and thus is used to establish a system coupling with nitrate reduction.

Light-driven thermocatalytic CO2 reduction by CH4 on alumina-cluster-modified Ni nanoparticles with excellent durability and high light-to-fuel efficiency promoted by the photoactivation effect

Using inexhaustible solar energy to drive efficient light-driven thermocatalytic CO2 reduction by CH4 (DRM) is an attractive approach that can synchronously reduce the greenhouse effect and convert solar energy into fuels. However, it is often limited by the intense light intensity required to produce high fuel production rates, and the catalyst deactivation due to severe carbon deposition generated from side reactions. Herein, a nanostructure of alumina-cluster-modified Ni nanoparticles supported on Al2O3 nanorods (ACM-Ni/Al2O3) was synthesized, displaying good catalytic performance under focused UV-vis-IR illumination. By light-driven thermocatalytic DRM on ACM-Ni/Al2O3 at a reduced light intensity of 76.9 kW m-2, the high fuel production rates of H2 (rH2, 65.7 mmol g-1 min-1) and CO (rCO, 78.8 mmol g-1 min-1), as well as an efficient light-to-fuel efficiency (η, 26.3 %) are achieved without additional heating. The rH2 and rCO of light-driven thermocatalysis are 2.9 and 1.9 times higher, respectively, compared to conventional thermocatalysis at the same temperature. We have discovered that high light-driven thermocatalytic activity originates from the photoactivation effect, significantly reducing the apparent activation energy and facilitating C* oxidation as a decisive step in DRM. ACM-Ni/Al2O3 possesses excellent durability and exhibits an extremely low coking rate of 4.40 × 10-3 gc gcatalyst-1 h-1, which is 26.8 times lower than that of the reference sample without Al2O3 cluster modification (R-Ni/Al2O3). This is owing to a decrease in activation energies (Ea) of C* oxidation and an increase in Ea of C* polymerization by the surface modification of Ni nanoparticles with Al2O3 clusters, effectively inhibiting carbon deposition.

Core-Shell Design of Metastable Phase Catalyst Enables Highly-Performance Selective Hydrogenation

Highly selective semihydrogenation of alkynes to alkenes is a highly important reaction for catalytic industry. Developing non-noble metal based catalysts with platinum group metal-like activity and selectivity is extremely crucial yet challenging. Metastable phase catalysts provide a potential candidate to realize high activity, yet the control of selectivity remains an open question. Here, this work first reports a metastable phase core-shell: face-centered cubic (fcc) phase Ag (10 at%) core-metastable hexagonal closest packed (hcp) phase Ni (90 at%) shell catalyst, which represents high conversion rate, high selectivity, and remarkable universality for the semihydrogenation of phenylacetylene and its derivatives. More impressively, a turnover frequency (TOF) value of 8241.8 h-1 is achieved, much higher than those of stable phase catalysts and reported platinum group metal based catalysts. Mechanistic investigation reveals that the surface of hcp Ni becomes more oxidized due to electron transfer from hcp Ni shell to fcc Ag core, which decreases the adsorption capacity of styrene on the metastable phase Ni surface, thus preventing full hydrogenation. This work has gained crucial research significance for the design of high performance metastable phase catalysts.

Optimizing biogas methanation over nickel supported on ceria-alumina catalyst: Towards CO2-rich biomass utilization for a negative emissions society

Biogas methanation emerges as a prominent technology for converting biogas into biomethane in a single step. Furthermore, this technology can be implemented at biogas plant locations, supporting local economies and reducing dependence on large energy producers. However, there is a lack of comprehensive studies on biogas methanation, particularly regarding the technical optimization of operational parameters and the profitability analysis of the overall process. To address this gap, our study represents a seminal work on the technical optimization of biogas methanation obtaining an empirical model to predict the performance of biogas methanation. We investigate the influence of operational parameters, such as reaction temperature, H2/CO2 ratio, space velocity, and CO2 share in the biogas stream through an experimental design. Based on previous research we selected a nickel supported on ceria-alumina catalyst; being nickel a benchmark system for methanation process such selection permits a reliable data extrapolation to commercial units. We showcase the remarkable impact of studied key operation parameters, being the temperature, the most critical factor affecting the reaction performance (ca. 2 to 5 times higher than the second most influencing parameter). The impact of the H2/CO2 ratio is also noticeable. The response surfaces and contour maps suggest that a temperature between 350 and 450 °C and an H2/CO2 ratio between 2.5 and 3.2 optimize the reaction performance. Further experimental tests were performed for model validation and optimization leading to a reliable predictive model. Overall, this study provides validated equations for technology scaling-up and techno-economic analysis, thus representing a step ahead towards real-world applications for bio-methane production.

Improved hydrogen evolution performance of Ni-based nanoporous catalyst with Mo and B co-addition

The exploration of efficient and stable noble-metal-free electrocatalysts for hydrogen evolution reaction (HER) is of great interest for the development of electrochemical hydrogen production technologies. Herein, nanoporous Ni-based catalyst with Mo and B co-addition (NiMoB) prepared by dealloying is reported as an efficient electrocatalysts for HER. The nanoporous NiMoB achieves an overpotential of 31 mV at 10 mA cm-2, along with exceptional catalytic stability in alkaline electrolyte. Density functional theory (DFT) calculations reveal that the incorporation of Mo and B can synergistically optimize the electronic structure and regulate the adsorption of HER intermediates on the Ni active site, thus accelerating the HER kinetics. This study provides a new perspective for the development of non-precious Ni-based catalysts towards efficient hydrogen energy conversion.

Engineering the Quaternary Hydrotalcite-Derived Ce-Promoted Ni-Based Catalysts for Enhanced Low-Temperature CO2 Hydrogenation into Methane

Ce-promoted NiMgAl mixed-oxide (NiCex-C, x = 0, 1, 5, 10) catalysts were prepared from the quaternary hydrotalcite precursors for CO₂ hydrogenation to methane. By engineering the Ce contents, NiCe5-C showed its prior catalytic performance in low-temperature CO₂ hydrogenation, being about three times higher than that of the Ce-free NiCe0-C catalyst (turnover frequency of NiCe5-C and NiCe0-C: 11.9 h^-1 vs. 3.9 h^-1 @ 225 °C). With extensive characterization, it was found that Ce dopants promoted the reduction of NiO by adjusting the interaction between Ni and Mg(Ce)AlOx support. The highest ratio of surface Ni⁰/(Ni²⁺ + Ni⁰) was obtained over NiCe5-C. Meanwhile, the surface basicity was tailored with Ce dopants. The strongest medium-strength basicity and highest capacity of CO₂ adsorption was achieved on NiCe5-C with 5 wt.% Ce content. The TOF tests indicated a good correlation with medium-strength basicity over the NiCex-C samples. The results showed that the high medium-strength and Ce-promoted surface Ni⁰ species endows the enhanced low-temperature catalytic performance in CO₂ hydrogenation to methane.

Removal of Bisphenol A via peroxymonosulfate activation over graphite carbon nitride supported NiCx nanoclusters catalyst: Synergistic oxidation of high-valent nickel-oxo species and singlet oxygen

In this work, a g-C₃N₄ supported NiCx nanoclusters catalyst (NiCx-CN) was developed, and its performance in activating peroxymonosulfate (PMS) was evaluated. Mechanism investigation stated that although singlet oxygen (¹O₂) was formed in the catalytic process, its contribution to BPA elimination was weeny. Interestingly, through the experiment with dimethyl sulfoxide as the probe, it was considered that the high-valent nickel-oxo species (Ni^&+=O), generated after the interaction of NiCx-CN and PMS, was the dominating reactive oxygen species (ROS). Theoretical calculations (DFT) implied that NiCx-CN might lose electrons to generate high-valent Ni, which was consistent with the detection of Ni³⁺ on the surface of the used NiCx-CN. Besides, the prepared NiCx-CN showed advantages in resisting the interference of inorganic anions. Meanwhile, three BPA degradation routes had been proposed based on the transformation intermediates. This study will establish a new protocol for PMS activation using heterogeneous Ni-based catalysts to efficiently degrade organic pollutants via a nonradical mechanism.

Alloying effect of Ni-Mo catalyst in hydrogenation of N-ethylcarbazole for hydrogen storage

Liquid organic hydrogen storage with N-ethylcarbazole (NEC) as a carrier is a very promising method. The use of precious metal hydrogenation catalysts restricts the development in industrial grade. Efficient and low-cost hydrogen storage catalysts are essential for its application. In this work, a Ni-Mo alloy catalyst supported by commercial activated carbon was synthesized by impregnation method, and the Ni-Mo ratio and preparation conditions were optimized. The catalyst was characterized by XRD, XPS, H₂-TPR, SEM, and TEM. The results showed that the doping of Mo could dramatically promote the catalytic hydrogenation of N-ethylcarbazole by the Ni-based catalyst. More than 5.75 wt% hydrogenation could be achieved in 4 h using the Ni-Mo catalyst, and the selectivity of the fully hydrogenated product 12H-NEC could be effectively improved. This result reduces the cost of hydrogenation catalysts by more than 90% and makes liquid organic hydrogen storage a scaled possibility.

Flash hydropyrolysis of cotton stalks: Role of temperature, metal loading, pressure for enhancement of aromatics

Hydropyrolysis of underutilized cotton stalks with catalytic upgradation was examined at different temperatures (500 to 800 °C) in the presence of nickel impregnated HY-zeolite (Ni/HY) catalysts using pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS). The effects of different metal loading (10, 15, 20 and 25 wt%) and its size were investigated to understand their impact on product distribution, mainly aromatic and aliphatic hydrocarbons. Aromatic hydrocarbons increased with an increase in metal content and optimum metal loading was 20 wt%. The pyrolysis temperature and hydrogen pressure had significant effect on product distribution. Aromatic hydrocarbon area% increased from 1.5% to 48% with an increase in temperature from 500 to 800 °C in non-catalytic hydropyrolysis. Aromatic hydrocarbon area% reached 75.5% with 20 wt% Ni/HY at 10 bar H₂ pressure at 800 °C.

Bimetallic Ni-Ru and Ni-Re Catalysts for Dry Reforming of Methane: Understanding the Synergies of the Selected Promoters

Designing an economically viable catalyst that maintains high catalytic activity and stability is the key to unlock dry reforming of methane (DRM) as a primary strategy for biogas valorization. Ni/Al₂O₃ catalysts have been widely used for this purpose; however, several modifications have been reported in the last years in order to prevent coke deposition and deactivation of the samples. Modification of the acidity of the support and the addition of noble metal promoters are between the most reported strategies. Nevertheless, in the task of designing an active and stable catalyst for DRM, the selection of an appropriate noble metal promoter is turning more challenging owing to the lack of homogeneity of the different studies. Therefore, this research aims to compare Ru (0.50 and 2.0%) and Re (0.50 and 2.0%) as noble metal promoters for a Ni/MgAl₂O₄ catalyst under the same synthesis and reaction conditions. Catalysts were characterized by XRF, BET, XRD, TPR, hydrogen chemisorption (H₂-TPD), and dry reforming reaction tests. Results show that both promoters increase Ni reducibility and dispersion. However, Ru seems a better promoter for DRM since 0.50% of Ru increases the catalytic activity in 10% and leads to less coke deposition.

A highly active Ni-based anode material for urea electrocatalysis by a modified sol-gel method

A highly electroactive Ni-based catalyst for urea oxidation is prepared by a sol-gel method with bubbling of gel mixture. It is observed that the conditions for the gel formation strongly affect the morphology and electrochemical properties of the catalyst materials. As synthesized Ni-catalysts are characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The Ni-based catalyst prepared at optimum conditions in the scope of this study exhibits the urea oxidation activity of 570 mA mg^-1 (at 0.54 V). In a single urea/hydrogen peroxide fuel cell test, the Ni-catalyst provides maximum power densities of 19.6 and 36.4 mW cm^-2 at 30 and 70 °C, respectively. Additionally, the cell catalyst shows a stable voltage for 3 days. Thus, this work suggests that a novel Ni-based catalyst derived from a facile method can be used for urea oxidation and as an efficient anode material for urea fuel cells.

Recent Progresses in Constructing the Highly Efficient Ni Based Catalysts With Advanced Low-Temperature Activity Toward CO2 Methanation

With the development and prosperity of the global economy, the emission of carbon dioxide (CO2) has become an increasing concern. Its greenhouse effect will cause serious environmental problems, such as the global warming and climate change. Therefore, the worldwide scientists have devoted great efforts to control CO2 emissions through various strategies, such as capture, resource utilization, sequestration, etc. Among these, the catalytic conversion of CO2 to methane is considered as one of the most efficient routes for resource utilization of CO2 owing to the mild reaction conditions and simple reaction device. Pioneer thermodynamic studies have revealed that low reaction temperature is beneficial to the high catalytic activity and CH4 selectivity. However, the low temperature will be adverse to the enhancement of the reaction rate due to kinetic barrier for the activation of CO2. Therefore, the invention of highly efficient catalysts with promising low temperature activities toward CO2 methanation reaction is the key solution. The Ni based catalysts have been widely investigated as the catalysts toward CO2 methanation due to their low cost and excellent catalytic performances. However, the Ni based catalysts usually perform poor low-temperature activities and stabilities. Therefore, the development of highly efficient Ni based catalysts with excellent low-temperature catalytic performances has become the research focus as well as challenge in this field. Therefore, we summarized the recent research progresses of constructing highly efficient Ni based catalysts toward CO2 methanation in this review. Specifically, the strategies on how to enhance the catalytic performances of the Ni based catalysts have been carefully reviewed, which include various influencing factors, such as catalytic supports, catalytic auxiliaries and dopants, the fabrication methods, reaction conditions, etc. Finally, the future development trend of the Ni based catalysts is also prospected, which will be helpful to the design and fabrication of the Ni catalysts with high efficiency toward CO2 methanation process.

Facile use of coal combustion fly ash (CCFA) as Ni-Re bimetallic catalyst support for high-performance CO2 methanation

The industrial waste coal combustion fly ash (CCFA) was used as a cheap catalyst support and by facile co-impregnation method, the active component Ni and promoter Re was loaded to form a bimetallic catalyst for high-performance CO₂ methanation. The physico-chemical properties of the prepared catalyst were further measured by a series of characterizations such as X-ray fluorescence (XRF), N₂ isothermal adsorption-desorption, X-ray diffraction (XRD), H₂ temperature-programmed reduction (H₂-TPR) et al. The effect of reaction temperature and gas hourly space velocity (GHSV) on the catalytic performance was carefully investigated on a continuous fixed-bed reactor. The results showed that in comparison with the non-promoted monometallic Ni/CCFA catalyst, the bimetallic Ni-Re/CCFA catalyst displayed a superior activity, which could achieve 99.55% of CO₂ conversion and 70.27% of CH₄ selectivity under the condition of 400 °C, 2000 h^-1, 1 atm and H₂:CO₂:N₂ = 4:1:0.5, possibly owing to the higher Ni dispersion and more active sites in Ni-Re/CCFA. Besides, the addition of Re promoter was beneficial to enhance the catalyst anti-sintering and anti-coking abilities as reflected by the smaller Ni particle size growth and less carbon deposition amount in Ni-Re/CCFA. The in-situ diffuse reflection infrared Fourier transform spectrum (in-situ DRIFTS) was finally carried out to determine the CO₂ adsorption state and its methanation intermediates, from which a loop mechanism of CO₂ methanation process was proposed and depicted.

Template-Assisted Wet-Combustion Synthesis of Fibrous Nickel-Based Catalyst for Carbon Dioxide Methanation and Methane Steam Reforming

Efficient capture and recycling of CO₂ enable not only prevention of global warming but also the supply of useful low-carbon fuels. The catalytic conversion of CO₂ into an organic compound is a promising recycling approach which opens new concepts and opportunities for catalytic and industrial development. Here we report about template-assisted wet-combustion synthesis of a one-dimensional nickel-based catalyst for carbon dioxide methanation and methane steam reforming. Because of a high temperature achieved in a short time during reaction and a large amount of evolved gases, the wet-combustion synthesis yields homogeneously precipitated nanoparticles of NiO with average particle size of 4 nm on alumina nanofibers covered with a NiAl₂O₄ nanolayer. The as-synthesized core-shell structured fibers exhibit outstanding activity in steam reforming of methane and sufficient activity in carbon dioxide methanation with 100% selectivity toward methane formation. The as-synthesized catalyst shows stable operation under the reaction conditions for at least 50 h.