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The main goal of this work is to prepare, characterize, and determine the catalytic properties of molybdena supported γ-alumina extrudates with four Mo profiles. Concerning preparation, a procedure was established allowing us to prepare (both axially and radially) egg-shell, egg-white, egg-yolk, and uniform profiles with the same, relatively high, Mo loading. The preparation of the egg-shell and uniform profiles was achieved by impregnating γ-alumina extrudates with acidic and alkaline ammonium heptamolybdate solution, respectively. Moreover, it was found that an egg-white (egg-yolk) profile may be achieved from an egg-shell (uniform) profile by successive pore volume or simple nondry impregnations of the Mo-supported extrudates with NH4OH aqueous solutions. A qualitative mechanism, based on the relative rates of desorption and diffusion of the MoO42− ions occuring in the NH4OH impregnation, was developed to interpret the influence of desorption time, concentration of NH4OH solution, drying, and mode of impregnation on the characteristics of the egg-white and egg-yolk profiles obtained. The physicochemical characterization of the Mo/γ-alumina extrudates for which radial profiles have been achieved, done by using a number of techniques, showed a progressive increase of the “Mo supported-support” interactions and Mo dispersion following the order egg-yolk

The development of a novel, simple technique allowed us to investigate the influence of various impregnation parameters on the axial Mo profiles achieved on γ-alumina extrudates. The effects of pH, temperature, volume, and concentration of the molybdate solutions, as well as the impregnation time, the rate of drying, the modification of γ-alumina with F − ions, and the nature and concentration of various competitors, have been systematically studied. It was found that decreases in pH as well as increases in the concentration of the molybdate solutions, of the impregnation temperature, and of the rate of drying cause a progressive transformation of the Mo profile from uniform to eggshell type. Doping of γ-alumina with F − ions and the use of NH 4 F, H 3 PO 4 , and citric acid as competitors transformed the Mo profiles from eggshell to uniform. The change in volume of the impregnating solution had no effect on the Mo profiles. The effect of the impregnation time was found to be complicated. Increases in the impregnation time, until a critical value sufficient for the complete imbibition of the extrudates, allow for the transfer of great amounts of molybdate to the interior of the extrudates, leading to more uniform profiles. Further increases in the impregnation time caused a considerable increase in the sharpness of the Mo profile. Most of the above observations were explained on the basis of derived equations, adopting a very simple macrodistribution model. Finally, it was demonstrated that small chromatographic columns filled with powder supports may be used to study active ion profiles on catalytic supports.

Summarization: The adsorption of ethylene (C2H4) from a mixture simulating the products obtained during the oxidative coupling of CH4 using a commercial molecular sieve (MOS) is investigated. A joint experimental and theoretical approach has been employed. The adsorbent's trapping capacity decreases at higher temperatures, while the breakpoint time drops when the inlet C2H4 concentration increases. The Dubinin-Radushkevich isotherm model can better describe the adsorption of C2H4 onto the MOS. Pore diffusion is the rate-determining step given that the Bangham's kinetic model was the more suitable for this adsorption process. Finally, the low activation energy obtained corroborates the predominance of physical adsorption. Presented on: Chemical Engineering & Technology

The present work reports on the investigation of the catalytic performance for the methanation of CO2 over Ni catalysts based on CeO2, and for the first time, of Ni catalysts supported on binary CeO2-based oxides, namely, Sm2O3-CeO2, Pr2O3-CeO2 and MgO-CeO2. The supports were obtained using the microwave assisted sol-gel method under reflux, while the catalysts were prepared by the wet impregnation method. For the investigation of the morphological, textural, structural and other intrinsic properties of the catalytic materials a variety of characterization techniques were used, i.e., Raman spectroscopy, XRD, N2 physisorption-desorption, CO2-TPD, H2-TPR, H2-TPD, XPS and TEM. Carbon deposition and sintering were investigated using TEM. It was shown that the addition of Sm3+ or Pr3+, incorporated into the lattice of CeO2, generated oxygen vacancies, but the Ni/Pr-Ce catalyst was found to possess more surface oxygen vacancies (e.g. Ce4+-Ov-Pr3+ entities). Moreover, modification of CeO2 using Sm3+ or Pr3+ restricted the agglomeration of nickel active sites and led to the genesis of Lewis basic positions. These characteristics improved the hydrogenation reaction at lower temperature. On the other hand, the addition of Mg2+ resulted at strong metal support interactions reinforcing the resistance of the Ni/Mg-Ce catalyst against sintering. Furthermore, the addition of Sm3+, Pr3+ and Mg2+ cations increased the overall basicity and the moderate adsorption sites and led to the formation of smaller Ni nano particles; these physico-chemical properties enhanced the CO2 methanation reaction. Finally, the activity experiments (WGHSV = 25,000 mL g−1 h−1, H2/CO2 = 4:1, T =350 °C) showed that at lower reaction temperature the Ni/Pr-Ce had the highest catalytic performance in terms of CO2 conversion (54.5%) and CH4 yield (54.5%) and selectivity (100%). The TOF values were found to follow the order Ni/Pr-Ce >> Ni/Mg-Ce > Ni/Sm-Ce > Ni/Ce., GIS is grateful for financial support by Greece and ESF through the Operational Programme «Human Resources Development, Education and Lifelong Learning» in the context of the project “Strengthening Human Resources Research Potential via Doctorate Research” (MIS-5000432), implemented by the State Scholarships Foundation (IKY). NDC is grateful to the Research Committee of UOWM for financial support through grant no. 80304. IVY and MAG are grateful for financial support from the European Union and Greek national funds through the operational program Competitiveness, Entrepreneurship and Innovation, under the call Research-Create-Innovate (Project code: T1EDK-00782). SAK, AAK and KP acknowledge the financial support from the Abu Dhabi Department of Education and Knowledge (ADEK) under the AARE 2019-233 grant and support by the Khalifa University of Science and Technology under Award No. RC2-2018-024.

CO2 methanation has great potential for the better utilization of existing carbon resources via the transformation of spent carbon (CO2) to synthetic natural gas (CH4). Alkali and alkaline earth metals can serve both as promoters for methanation catalysts and as adsorbent phases upon the combined capture and methanation of CO2. Their promotion effect during methanation of carbon dioxide mainly relies on their ability to generate new basic sites on the surface of metal oxide supports that favour CO2 chemisorption and activation. However, suppression of methanation activity can also occur under certain conditions. Regarding the combined CO2 capture and methanation process, the development of novel dual‑function materials (DFMs) that incorporate both adsorption and methanation functions has opened a new pathway towards the utilization of carbon dioxide emitted from point sources. The sorption and catalytically active phases on these types of materials are crucial parameters influencing their performance and stability and thus, great efforts have been undertaken for their optimization. In this review, we present some of the most recent works on the development of alkali and alkaline earth metal promoted CO2 methanation catalysts, as well as DFMs for the combined capture and methanation of CO2.

CO2 methanation has recently emerged as a process that targets the reduction in anthropogenic CO2 emissions, via the conversion of CO2 captured from point and mobile sources, as well as H2 produced from renewables into CH4. Ni, among the early transition metals, as well as Ru and Rh, among the noble metals, have been known to be among the most active methanation catalysts, with Ni being favoured due to its low cost and high natural abundance. However, insufficient low-temperature activity, low dispersion and reducibility, as well as nanoparticle sintering are some of the main drawbacks when using Ni-based catalysts. Such problems can be partly overcome via the introduction of a second transition metal (e.g., Fe, Co) or a noble metal (e.g., Ru, Rh, Pt, Pd and Re) in Ni-based catalysts. Through Ni-M alloy formation, or the intricate synergy between two adjacent metallic phases, new high-performing and low-cost methanation catalysts can be obtained. This review summarizes and critically discusses recent progress made in the field of bimetallic Ni-M (M = Fe, Co, Cu, Ru, Rh, Pt, Pd, Re)-based catalyst development for the CO2 methanation reaction.

Summarization: The promoting impact of potassium (0–1 wt% K) on nitrous oxide (N2O) catalytic decomposition over Ir/Al2O3 is investigated under both oxygen deficient and oxygen excess conditions. All samples were characterized by means of X-ray powder diffraction (XRD), temperature-programmed reduction (H2-TPR), ammonia desorption (NH3-TPD) and Fourier Transform Infrared Spectroscopy of pyridine adsorption (FTIR-Pyridine). The results reveal that the K-free Ir/Al2O3 catalyst consists mainly of the IrO2 phase, exhibiting also significant Lewis acidity, which is gradually eliminated by the addition of K. Catalytic performance results showed that the deN2O performance in the absence of O2 in the feed mixture is negatively affected upon increasing potassium loading. However, under oxygen excess conditions, a pronounced effect of K is observed. Although the catalytic performance of the un-doped catalyst is drastically hindered by the presence of O2, the K-promotion notably prohibits the oxygen poisoning. The optimum deN2O performance under oxygen excess conditions is obtained with potassium loading of 0.5 wt% K, which offers complete conversion of N2O at 580 °C, instead of the corresponding 50 % N2O conversion achieved with the un-modified sample. On the basis of characterization results, it was concluded that alkali-doping in combination with oxygen excess conditions are required towards the formation of active Ir entities. Presented on: Topics in Catalysis

In order to produce a hydrogen stream for fuel cell utilization, the study of the ethanol steam reforming process over an alumina supported palladium commercial catalyst was carried out. The effect of the reaction temperature, the H2O/C2H6O molar ratios and the contact time on catalytic activity and stability was studied. It was found that even at very low temperature values complete ethanol conversion was possible. Hydrogen selectivities up to 98% were obtained at temperature values close to 625 °C. It was also observed that for different reactant molar ratios carbon monoxide concentration exhibits a minimum at a temperature value close to 450 °C. Furthermore, carbon formation was found to be negligible even for H2O/C2H6O molar ratios equal to the stoichiometric one.