Heterogeneous catalysis offers one of the sustainable solutions to the global warming challenge by converting greenhouse gases, particularly carbon dioxide (CO2), into valuable chemicals, notably methane (CH4). The CO2 methanation is a thermodynamically favorable and scalable process, typically carried out using nickel (Ni) based catalysts. However, the intense exothermicity of the methanation reaction poses challenges, including sintering of Ni nanoparticles and carbon deposition, which critically undermine the catalyst's stability. Resilience of the catalyst can be enhanced by tuning the metal-support interaction (MSI) and regulating the dispersion of Ni nanoparticles. 

The aim of this thesis was to develop stable and robust Ni catalysts by tuning the MSI and dispersion of Ni nanoparticles supported over the hierarchical zeolite 13X (h13X). Preliminary experiments demonstrated that Ni loading, activity, and instability of the catalyst are correlated. Under optimized synthesis conditions, grafting functional groups onto the support resulted in a stronger MSI and preferentially deposited Ni nanolayers, thereby enhancing the activity and stability of the catalyst. The addition of the cobalt cocatalyst strengthened the MSI and stabilized the catalyst's performance, notably during the initial stages of CO2 methanation. The synergistic effect of surface modification and cocatalyst resulted in lower activation energy, higher activity, and increased stability of the catalyst. Additionally, the influence of oxide promoters (La, Ca, Mg, Ce) proved to be dependent on their specific characteristics, with a notable increase in surface basicity, MSI, and catalyst stability. 

Based on the outcomes of surface modification and traditional catalyst design, a metal-chelation strategy was explored to regulate the size of Ni nanoparticles. The Ni was coordinated to the amine-based ligands, including pyridine, bipyridine, diethylenetriamine, and oleylamine, followed by their impregnation on the support. Remarkable differences were observed in the characteristics of the catalysts depending upon the type of ligand. A broader Ni distribution was observed for the heterocyclic ligand, whereas better textural properties were achieved by the aliphatic amines, which were attributed to the coordination of Ni in the metal-chelate complexes and the strength of interaction with the support. Besides the catalyst’s design, the process parameters, including temperature, pressure, H2/CO2 ratio, gas hourly velocity, and gas composition, showed a profound impact on the CO2 conversion, CH4 selectivity, and stability of the catalysts. overall, this thesis offers insights into regulating the catalyst's MSI, dispersion, distribution, activity, and stability.

The development of robust nickel catalysts on porous substrates offers great potential for converting carbon dioxide (CO2) into methane, thereby helping to address the global warming and sustainability challenges. This study investigates the dispersion and stability of Ni nanolayers by grafting bifunctional groups over the hierarchical zeolite 13X (h13X) support using (3-aminopropyl)triethoxysilane (APTES). The Ni nanolayers, with a thickness of 1.5–7 nm, were deposited around the edges of h13X and analyzed using STEM imaging. A clear shift in the binding energies was observed by XPS analysis, substantiating the enhanced metal-support interaction (MSI) between NiO and h13X. The influence of reaction temperature on APTES incorporation into h13X was revealed by H2-TPR and CO2-TPD, with notable variations in the reducibility and surface basicity profiles of the catalysts. The optimized catalyst exhibited CO2 conversion of 61 % with CH4 selectivity of 97 % under GHSV of 60,000 mlgCat-1h-1 at 400 °C and 1 bar and demonstrated robust stability over a period of 150 h without discernible degradation. The enhanced performance could be attributed to the strengthened MSI and reduced size of Ni nanolayers over h13X. These findings highlight the development of robust heterogeneous catalysts by changing the surface chemistry of support material for various catalytic applications.

The alkaline earth metal halides (AEMHs), such as strontium chloride (SrCl2), are promising sorbents for hydrogen storage in the form of ammonia. However, these sorbents suffer from structural disintegration problems due to the extraordinary volume expansion during ammonia sorption. This study reports the fabrication of 3D-printed SrCl2 monoliths scaffolded with bentonite using the direct ink writing technique. The optimized monolith with a 60 % SrCl2 loading exhibited an ammonia storage capacity of 488 mg/g, maintaining remarkable structural integrity and effectively accommodating volumetric changes during sorption and desorption over 20 cycles. The kinetics data revealed that ammonia sorption followed a pseudo-second-order model, and intercrystalline diffusion was the rate-controlling step in the 3D-printed SrCl2 structures. High-pressure sorption isotherms were explained by the dual-site Langmuir-Freundlich model due to surface heterogeneity in terms of energies and binding sites for metal-amine complex formation. Thus, cognitively designed AEMHs monoliths present the potential for ammonia storage in various applications by effectively overcoming structural challenges.

The consequences of global warming due to increasing levels of greenhouse gas emissions stress the need to develop carbon capture technologies expeditiously. Metal-organic frameworks (MOFs) have been proven to be effective carbon dioxide (CO2) sorbents, but challenges lie in their integration into practical applications owing to the hurdles in processing the powder MOFs into usable structures. Herein, the Cu-MOFs nanocrystals were in situ grown over different cellulose substrates, including bacterial cellulose nanofibers lamellas (BCNFLs) and wood-derived cellulose nanofibers (WCNFs). The successfully prepared sorbents were evaluated for CO2 capture applications, along with their kinetic and diffusion dynamics. The loading of MOFs nanoparticles was confirmed via FESEM, showing the interconnected network of cellulose nanofibers (CNFs) and interwoven MOFs particles. The surface area and porosity of the samples, analyzed by the N2 sorption method, were proportional to the MOFs in the sorbents. The MOFs/BCNFLs and MOFs/WCNFs composites demonstrated CO2 uptake of approximately 1 and 1.19 mmol/g, respectively, and maintained stability over numerous cycles, highlighting the robustness of the developed structures. The CO2 sorption isotherms were explained by the Langmuir–Freundlich model, accounting for surface heterogeneity, and exhibited a selectivity ( ) of 49 with a heat of adsorption of 27 kJ/mol. The MOFs/BCNFLs exhibited 2.2 times higher sorption kinetics and a 25% greater diffusion coefficient than WCNFs, attributed to the thin MOFs layer that minimized mass transport limitations. Our findings underscore the significance of structural optimization and the potential of cellulose nanofiber-coated MOFs for practical carbon capture applications. 

Catalytic conversion of carbon dioxide (CO2) into useful chemicals such as methane (CH4) is a promising carbon utilization method that effectively mitigates CO2 and partially meets energy needs. The characteristics of commonly used nickel (Ni) supported meso/microporous catalysts for CO2 methanation can be tailored by tuning the structural properties of the support and adding promoters. This work investigated the Ni supported over hierarchical zeolite 13X (h13X) and incorporated with different promoters (Mg, Ca, Ce, and La) developed using the wet-impregnation method. The catalysts were thoroughly characterized using SEM, EDS, XRD, H2-TPR, CO2-TPD, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and N2 sorption and desorption techniques and evaluated for CO2 methanation. The impact of promoters on the characteristics of the catalysts was observed with improved surface basicity in CO2-TPD and metal-support interaction in H2-TPR analysis. Among the promoted catalysts, the NiLa/h13X catalyst exhibited the highest catalytic activity with a maximum conversion of 76% and CH4 selectivity of 98.5% at 400°C and 20 bar at GHSV of 60,000 mL gcat−1 h−1, respectively. Regarding stability, the Mg-promoted catalyst exhibited better stability during 24 h of reaction than other catalysts, demonstrating better resilience against deactivation. The enhanced performance of the NiLa/h13X catalyst could be credited to the increased surface basicity, high surface area, and dispersion. This study highlights the potential of hierarchical porous zeolites for CO2 methanation and other heterogeneous reactions. 

Nitrogen-doped porous carbons have been widely explored for CO₂ storage and separation, but expensive precursors and intricate synthetic approaches often limit their practical deployment. Here, we report a facile, one-step, solvent-free method to design nitrogen-doped microporous carbons (SBF-BC-KMx) for efficient CO₂ capture from sugarcane bagasse fibers (SBF) as a low-cost precursor. Melamine and KOH were used as a nitrogen-doping source and an activator, respectively. The specimen (SBF-BC-KM0.5), prepared with optimized melamine loading, possessed efficient textural features, including a specific surface area (SSA) of 1138 m² g⁻¹ , a micropore volume of 0.396 cm³ g⁻¹ , high concentration of ultra-micropores (<0.6 nm) (89 %) and high content of pyrrolic-N functionality (35 %). These properties enhanced the CO₂ capture performance, achieving 244.4 mg g⁻¹ at 273 K, 170.0 mg g⁻¹ at 293 K and 1 bar, and 351.5 mg g⁻¹ at 293 K and 10 bar. The optimized material exhibited a moderate isosteric heat of adsorption and an effective CO₂/N₂ selectivity at 293 K. The high ultra-micropore density significantly boosted CO₂ uptake and maintained stable CO₂ uptake over five adsorption cycles. Overall, this work devoted efforts to sustainable environment, biowaste management, and possible practical applicability of designed adsorbent for CO2 storage.

Implementing carbon capture and utilization (CCU) technologies is crucial to control the detrimental impact of increasing levels of greenhouse gases on the ecosystem. Among existing technologies for CO2 utilization, methanation stands out as an efficient method, capable of effectively mitigating CO2 concentration while also contributing to meeting energy requirements. The performance of commonly used Ni-based catalysts supported over meso/microporous materials depends upon active sites, metal dispersion, and resistance against deactivation due to coking and sintering. The mentioned characteristics can be tailored by modifying the structural properties of the support material and adding promotors to the catalyst. In this work, Ni-based catalysts were supported over hierarchical zeolite 13X (h13X) and incorporated with different promotors (Mg, Ca, Ce, and La) using the wet-impregnation method. The synthesized catalysts were characterized using XRD, TGA/DSC, SEM, and N2 sorption and desorption techniques to analyze the physical and chemical characteristics. The performance of the catalysts was evaluated for CO2 conversion to methane at different temperatures (250-500 oC), while stability analysis was carried out at 400 oC and 1 bar for 50 hours at GHSV of 60,000 ml.gcat.h-1. Among the promoted catalysts, the 1%La-Ni/h13X catalyst exhibited the highest catalytic activity across all temperatures, with a maximum conversion of 57.6% at 400 oC under atmospheric conditions. Unexpectedly, 1%Mg, Ce, and Ca-promoted catalysts exhibited no significant improvement in the activity of catalysts compared to containing La. The Mg-promoted catalyst showed better stability during 50 hours of methanation reaction compared to La, demonstrating better resilience against deactivation. Likewise, a catalyst containing 3%La displayed superior performance compared to catalysts promoted with 3%Mg, Ce, and Ca. The outstanding performance of 1%La-Ni/h13X catalysts could be ascribed to their high surface area, dispersion, and reducibility. This study highlights the potential of microporous zeolites for CO2 methanation and other heterogeneous reactions.

Engineering the interfacial interaction between the active metal element and support material is a promising strategy for improving the performance of catalysts toward CO2 methanation. Herein, the Ni-doped rare-earth metal-based A-site substituted perovskite-type oxide catalysts (Ni/AMnO3; A = Sm, La, Nd, Ce, Pr) were synthesized by auto-combustion method, thoroughly characterized, and evaluated for CO2 methanation reaction. The XRD analysis confirmed the perovskite structure and the formation of nano-size particles with crystallite sizes ranging from 18 to 47 nm. The Ni/CeMnO3 catalyst exhibited a higher CO2 conversion rate of 6.6 × 10−5 molCO2 gcat−1 s−1 and high selectivity towards CH4 formation due to the surface composition of the active sites and capability to activate CO2 molecules under redox property adopted associative and dissociative mechanisms. The higher activity of the catalyst could be attributed to the strong metal-support interface, available active sites, surface basicity, and higher surface area. XRD analysis of spent catalysts showed enlarged crystallite size, indicating particle aggregation during the reaction; nevertheless, the cerium-containing catalyst displayed the least increase, demonstrating resilience, structural stability, and potential for CO2 methanation reaction.

The selective catalytic reduction (SCR) system in automobiles using urea solution as a source of NH3 suffers from solid deposit problems in pipelines and poor efficiency during engine startup. Although direct use of high pressure NH3 is restricted due to safety concerns, which can be overcome by using solid sorbents as NH3 carrier. Strontium chloride (SrCl2) is considered the best sorbent due to its high sorption capacity; however, challenges are associated with the processing of stable engineering structures due to extraordinary volume expansion during the NH3 sorption. This study reports the fabrication of a novel structure consisting of a zeolite cage enclosing the SrCl2 pellet (SPZC) through extrusion-based 3D printing (Direct Ink Writing). The printed SPZC structure demonstrated steady sorption of NH3 for 10 consecutive cycles without significant uptake capacity and structural integrity loss. Furthermore, the structure exhibited improved sorption and desorption kinetics than pure SrCl2. The synergistic effect of zeolite as physisorbent and SrCl2 as chemisorbent in the novel composite structure enabled the low-pressure (<0.4 bar) and high-pressure (>0.4 bar) NH3 sorption, compared to pure SrCl2, which absorbed NH3 at pressures above 0.4 bar. Regeneration of SPZC composite sorbent under evacuation showed that 87.5% percent of NH3 was desorbed at 20 °C. Thus, the results demonstrate that the rationally designed novel SPZC structure offers safe and efficient storage of NH3 in the SCR system and other applications.

Polyacrylonitrile (PAN) nanofibers were prepared by electrospinning and coated with zeolitic imidazolate framework-8 (ZIF-8) by a phase conversion growth method and investigated for CO₂ capture. The PAN nanofibers were pre-treated with NaOH, and further coated with zinc hydroxide, which was subsequently converted into ZIF-8 by the addition of 2-methyl imidazolate. In the resulting flexible ZIF-8/PAN composite nanofibers, ZIF-8 loadings of up to 57 wt% were achieved. Scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDS) showed the formation of evenly distributed submicron-sized ZIF-8 crystals on the surface of the PAN nanofibers with sizes between 20 and 75 nm. X-ray photoelectron spectroscopy (XPS) and carbon-13 nuclear magnetic resonance (¹³C NMR) investigations indicated electrostatic interactions and hydrogen bonds between the ZIF-8 structure and the PAN nanofiber. The ZIF-8/composite nanofibers showed a high BET surface area of 887 m² g⁻¹. CO₂ adsorption isotherms of the ZIF-8/PAN composites revealed gravimetric CO₂ uptake capacities of 130 mg g⁻¹ (at 298 K and 40 bar) of the ZIF-8/PAN nanofiber and stable cyclic adsorption performance.