Reaction Kinetics of CO and CO2 Methanation over Nickel

Rapid construction of nickel phyllosilicate with ultrathin layers and high performance for CO2 methanation

The traditional techniques for the synthesis of nickel phyllosilicates usually time-consuming and energy-intensive, which often lead to the formation of layers with excessive thickness due to uncontrolled crystal growth. In order to overcome these challenges, this work introduces a microwave-assisted synthesis strategy to facilitate the synthesis of Ni-phyllosilicate-based catalysts within an exceptionally short duration of only five minutes, attaining a peak temperature of merely 102 °C. To enhance the specific surface area and to increase the exposure of active sites, an investigation was conducted involving three surfactants. The employment of hexadecyl trimethyl ammonium bromide (CTAB) has yielded remarkable results, with an ultrahigh specific surface area reaching 535 m2 g-1 and an ultrathin lamellar thickness of 1.43 nm. The catalyst exhibited an impressive CO2 conversion of 81.7 % at 400 °C, 60 L g-1 h-1, 0.1 MPa. It also demonstrated a substantial turnover frequency for CO2 (TOFCO2) of 5.4 ± 0.1 × 10-2 s-1, alongside a relatively low activation energy (Ea) of 80.74 kJ·mol-1. Moreover, the catalyst maintained its high stability over a period of 100 h and displayed high resistance to sintering. To further elucidate growth temperature gradient of the catalyst and concentration gradient of the materials involved, COMSOL Multiphysics (COMSOL) simulations were effectively utilized. In conclusion, this work breaks the limitation associated with traditional, laborious synthesis methods for Ni-phyllosilicates, which can produce materials with high surface area and thin-layer characteristics.

Structural Heredity in Catalysis: CO2 Self-Selective CeO2 Nanocrystals for Efficient Photothermal CO2 Hydrogenation to Methane

The chemical inertness of CO2 molecules makes their adsorption and activation on a catalyst surface one of the key challenges in recycling CO2 into chemical fuels. However, the traditional template synthesis and chemical modification strategies used to tackle this problem face severe structural collapse and modifier deactivation issues during the often-needed post-processing procedure. Herein, a CO2 self-selective hydrothermal growth strategy is proposed for the synthesis of CeO2 octahedral nanocrystals that participate in strong physicochemical interactions with CO2 molecules. The intense affinity for CO2 molecules persists during successive high-temperature treatments required for Ni deposition. This demonstrates the excellent structural heredity of the CO2 self-selective CeO2 nanocrystals, which leads to an outstanding photothermal CH4 productivity exceeding 9 mmol h-1 mcat -2 and an impressive selectivity of >99%. The excellent performance is correlated with the abundant oxygen vacancies and hydroxyl species on the CeO2 surface, which create many frustrated Lewis-pair active sites, and the strong interaction between Ni and CeO2 that promotes the dissociation of H2 molecules and the spillover of H atoms, thereby greatly benefitting the photothermal CO2 methanation reaction. This self-selective hydrothermal growth strategy represents a new pathway for the development of effective catalysts for targeted chemical reactions.

Coal fly ash and bottom ash low-cost feedstocks for CO2 reduction using the adsorption and catalysis processes

Combustion of fossil fuels, industry and agriculture sectors are considered as the largest emitters of carbon dioxide. In fact, the emission of CO2 greenhouse gas has been considerably intensified during the last two decades, resulting in global warming and inducing variety of adverse health effects on human and environment. Calling for effective and green feedstocks to remove CO2, low-cost materials such as coal ashes "wastes-to-materials", have been considered among the interesting candidates of CO2 capture technologies. On the other hand, several techniques employing coal ashes as inorganic supports (e.g., catalytic reduction, photocatalysis, gas conversion, ceramic filter, gas scrubbing, adsorption, etc.) have been widely applied to reduce CO2. These processes are among the most efficient solutions utilized by industrialists and scientists to produce clean energy from CO2 and limit its continuous emission into the atmosphere. Herein, we review the recent trends and advancements in the applications of coal ashes including coal fly ash and bottom ash as low-cost wastes to reduce CO2 concentration through adsorption and catalysis processes. The chemical routes of structural modification and characterization of coal ash-based feedstocks are discussed in details. The adsorption and catalytic performance of the coal ashes derivatives towards CO2 selective reduction to CH4 are also described. The main objective of this review is to highlight the excellent capacity of coal fly ash and bottom ash to capture and selective conversion of CO2 to methane, with the aim of minimizing coal ashes disposal and their storage costs. From a practical view of point, the needs of developing new advanced technologies and recycling strategies might be urgent in the near future to efficient make use of coal ashes as new cleaner materials for CO2 remediation purposes, which favourably affects the rate of global warming.

Development of Ni-Mo carbide catalyst for production of syngas and CNTs by dry reforming of biogas

Biogas has been widely regarded as a promising source of renewable energy. Recently, the direct conversion of biogas over heterogeneous catalysts for the simultaneous production of syngas and carbon nanotubes exhibits a high potential for full utilization of biogas with great benefits. Involving the combined dry reforming of methane and catalytic decomposition of methane, the efficiency of process is strongly depended on the catalyst activity/stability, mainly caused by carbon deposition. In this study, Ni-Mo catalyst is engineered to provide a life-long performance and perform high activity in the combined process. The surface modification of catalysts by a controlled carburization pretreatment is proposed for the first time to produce a carbide catalyst along with improving the catalyst stability as well as the reactivity for direct conversion of biogas. The performance of as-prepared carbide catalysts is investigated with comparison to the oxide and metallic ones. As a result, the Ni-Mo2C catalyst exhibited superior activity and stability over its counterparts, even though the condensed nanocarbon was largely grown and covered on the surface. In addition, up to 82% of CH4 conversion and 93% of CO2 conversion could remain almost constant at 800 °C throughout the entire test period of 3 h under a high flowrate inlet stream of pure biogas at 48,000 cm3 g-1 h-1. The XPS spectra of catalysts confirmed that the presence of Mo2C species on the catalyst surface could promote the stability and reactivity of the catalyst, resulting in higher productivity of carbon nanotubes over a longer time.

Iterative multiscale and multi-physics computations for operando catalyst nanostructure elucidation and kinetic modeling

Modern heterogeneous catalysis has benefitted immensely from computational predictions of catalyst structure and its evolution under reaction conditions, first-principles mechanistic investigations, and detailed kinetic modeling, which are rungs on a multiscale workflow. Establishing connections across these rungs and integration with experiments have been challenging. Here, operando catalyst structure prediction techniques using density functional theory simulations and ab initio thermodynamics calculations, molecular dynamics, and machine learning techniques are presented. Surface structure characterization by computational spectroscopic and machine learning techniques is then discussed. Hierarchical approaches in kinetic parameter estimation involving semi-empirical, data-driven, and first-principles calculations and detailed kinetic modeling via mean-field microkinetic modeling and kinetic Monte Carlo simulations are discussed along with methods and the need for uncertainty quantification. With these as the background, this article proposes a bottom-up hierarchical and closed loop modeling framework incorporating consistency checks and iterative refinements at each level and across levels.

CO2 Activation on Ni(111) and Ni(110) Surfaces in the Presence of Hydrogen

The structure sensitivity of CO₂ activation in the presence of H₂ has been identified by ambient-pressure X-ray photoelectron spectroscopy (APXPS) on Ni(111) and Ni(110) surfaces under identical reaction conditions. Based on the APXPS results and computer simulations, we propose that, around room temperature, the hydrogen-assisted activation of CO₂ is the major reaction path on Ni(111), while the redox pathway of CO₂ prevails on Ni(110). With increasing temperature, the two activation pathways are activated in parallel. While the Ni(111) surface is fully reduced to the metallic state at elevated temperatures, two stable Ni oxide species can be observed on Ni(110). Turnover frequency measurements indicate that the low-coordinated sites on Ni(110) promote the activity and selectivity of CO₂ hydrogenation to methane. Our findings provide insights into the role of low-coordinated Ni sites in nanoparticle catalysts for CO₂ methanation.

CO2 Methanation over Nickel Catalysts: Support Effects Investigated through Specific Activity and Operando IR Spectroscopy Measurements

Renewed interest in CO2 methanation is due to its role within the framework of the Power-to-Methane processes. While the use of nickel-based catalysts for CO2 methanation is well stablished, the support is being subjected to thorough research due to its complex effects. The objective of this work was the study of the influence of the support with a series of catalysts supported on alumina, ceria, ceria–zirconia, and titania. Catalysts’ performance has been kinetically and spectroscopically evaluated over a wide range of temperatures (150–500 °C). The main results have shown remarkable differences among the catalysts as concerns Ni dispersion, metallic precursor reducibility, basic properties, and catalytic activity. Operando infrared spectroscopy measurements have evidenced the presence of almost the same type of adsorbed species during the course of the reaction, but with different relative intensities. The results indicate that using as support of Ni a reducible metal oxide that is capable of developing the basicity associated with medium-strength basic sites and a suitable balance between metallic sites and centers linked to the support leads to high CO2 methanation activity. In addition, the results obtained by operando FTIR spectroscopy suggest that CO2 methanation follows the formate pathway over the catalysts under consideration.

Carbon Dioxide Conversion on Supported Metal Nanoparticles: A Brief Review

The increasing concentration of anthropogenic CO2 in the air is one of the main causes of global warming. The Paris Agreement at COP 21 aims to reach the global peak of greenhouse gas emissions in the second half of this century, with CO2 conversion towards valuable added compounds being one of the main strategies, especially in the field of heterogeneous catalysis. In the current search for new catalysts, the deposition of metallic nanoparticles (NPs) supported on metal oxides and metal carbide surfaces paves the way to new catalytic solutions. This review provides a comprehensive description and analysis of the relevant literature on the utilization of metal-supported NPs as catalysts for CO2 conversion to useful chemicals and propose that the next catalysts generation can be led by single-metal-atom deposition, since in general, small metal particles enhance the catalytic activity. Among the range of potential indicators of catalytic activity and selectivity, the relevance of NPs’ size, the strong metal–support interactions, and the formation of vacancies on the support are exhaustively discussed from experimental and computational perspective.

Methanation of CO2 over High Surface Nickel/Aluminates Compounds Prepared by a Self-Generated Carbon Template

Catalytic gas-phase hydrogenation of CO2 into CH4 was tested under three different nickel/aluminate catalysts obtained from precursors of hexaaluminate composition (MAl16O19, M = Mg, Ca, Ba). These catalysts were prepared using a carbon template method, where carbon is self-generated from a sol-gel that contains an excess of citric acid and the Al and M salts (Ba2+, Ca2+, Mg2+) by two-step calcination in an inert/oxidizing atmosphere. This procedure yielded Ni particles decorating the surface of a porous high surface area matrix, which presents a typical XRD pattern of aluminate structure. Ni particles are obtained with a homogeneous distribution over the surface and an average diameter of ca 25–30 nm. Obtained materials exhibit a high conversion of CO2 below 500 °C, yielding CH4 as a final product with selectivity >95%. The observed trend with the alkaline earth cation follows the order NiBaAlO-PRx > NiCaAlO-PRx > NiMgAlO-PRx. We propose that the high performance of the NiBaAlO sample is derived from both an appropriate distribution of Ni particle size and the presence of BaCO3, acting as a CO2 buffer in the process.

Quo Vadis Dry Reforming of Methane?—A Review on Its Chemical, Environmental, and Industrial Prospects

In recent years, the catalytic dry reforming of methane (DRM) has increasingly come into academic focus. The interesting aspect of this reaction is seemingly the conversion of CO2 and methane, two greenhouse gases, into a valuable synthesis gas (syngas) mixture with an otherwise unachievable but industrially relevant H2/CO ratio of one. In a possible scenario, the chemical conversion of CO2 and CH4 to syngas could be used in consecutive reactions to produce synthetic fuels, with combustion to harness the stored energy. Although the educts of DRM suggest a superior impact of this reaction to mitigate global warming, its potential as a chemical energy converter and greenhouse gas absorber has still to be elucidated. In this review article, we will provide insights into the industrial maturity of this reaction and critically discuss its applicability as a cornerstone in the energy transition. We derive these insights from assessing the current state of research and knowledge on DRM. We conclude that the entire industrial process of syngas production from two greenhouse gases, including heating with current technologies, releases at least 1.23 moles of CO2 per mol of CO2 converted in the catalytic reaction. Furthermore, we show that synthetic fuels derived from this reaction exhibit a negative carbon dioxide capturing efficiency which is similar to burning methane directly in the air. We also outline potential applications and introduce prospective technologies toward a net-zero CO2 strategy based on DRM.

CO2 methanation reaction pathways over unpromoted and NaNO3-promoted Ru/Al2O3 catalysts

Catalytic CO2 sorbents, materials that adsorb and pre-concentrate CO2 on the catalyst surface prior to subsequent conversion, are becoming important materials in CO2 capture and utilization.

CO2 Methanation: Nickel-Alumina Catalyst Prepared by Solid-State Combustion

The development of solvent-free methods for the synthesis of catalysts is one of the main tasks of green chemistry. A nickel–alumina catalyst for CO₂ methanation was synthesized by solid-state combustion method using hexakis-(imidazole) nickel (II) nitrate complex. Using X-ray Powder Diffraction (XRD), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Hydrogen temperature-programmed reduction (H₂-TPR), it was shown that the synthesized catalyst is characterized by the localization of easily reduced nickel oxide on alumina surface. This provided low-temperature activation of the catalyst in the reaction mixture containing 4 vol% CO₂. In addition, the synthesized catalyst had higher activity in low-temperature CO₂ methanation compared to industrial NIAP-07-01 catalyst, which contained almost three times more hard-to-reduce nickel–aluminum spinel. Thus, the proposed approaches to the synthesis and activation of the catalyst make it possible to simplify the catalyst preparation procedure and to abandon the use of solvents, which must be disposed of later on.

Quantifying the Impact of Parametric Uncertainty on Automatic Mechanism Generation for CO2 Hydrogenation on Ni(111)

Automatic mechanism generation is used to determine mechanisms for the CO2 hydrogenation on Ni(111) in a two-stage process while considering the correlated uncertainty in DFT-based energetic parameters systematically. In a coarse stage, all the possible chemistry is explored with gas-phase products down to the ppb level, while a refined stage discovers the core methanation submechanism. Five thousand unique mechanisms were generated, which contain minor perturbations in all parameters. Global uncertainty assessment, global sensitivity analysis, and degree of rate control analysis are performed to study the effect of this parametric uncertainty on the microkinetic model predictions. Comparison of the model predictions with experimental data on a Ni/SiO2 catalyst find a feasible set of microkinetic mechanisms within the correlated uncertainty space that are in quantitative agreement with the measured data, without relying on explicit parameter optimization. Global uncertainty and sensitivity analyses provide tools to determine the pathways and key factors that control the methanation activity within the parameter space. Together, these methods reveal that the degree of rate control approach can be misleading if parametric uncertainty is not considered. The procedure of considering uncertainties in the automated mechanism generation is not unique to CO2 methanation and can be easily extended to other challenging heterogeneously catalyzed reactions.