Tuning basicity of dual function materials widens operation temperature window for efficient CO2 adsorption and hydrogenation to CH4

Enhancement of the CO2 adsorption and hydrogenation to CH4 capacity of Ru-Na-Ca/γ-Al2O3 dual function material by controlling the Ru calcination atmosphere

Integrated CO2 capture and utilization (ICCU) technology requires dual functional materials (DFMs) to carry out the process in a single reaction system. The influence of the calcination atmosphere on efficiency of 4% Ru-8% Na2CO3-8% CaO/γ-Al2O3 DFM is studied. The adsorbent precursors are first co-impregnated onto alumina and calcined in air. Then, Ru precursor is impregnated and four aliquotes are subjected to different calcination protocols: static air in muffle or under different mixtures (10% H2/N2, 50% H2/N2 and N2) streams. Samples are characterized by XRD, N2 adsorption-desorption, H2 chemisorption, TEM, XPS, H2-TPD, H2-TPR, CO2-TPD and TPSR. The catalytic behavior is evaluated, in cycles of CO2 adsorption and hydrogenation to CH4, and temporal evolution of reactants and products concentrations is analyzed. The calcination atmosphere influences the physicochemical properties and, ultimately, activity of DFMs. Characterization data and catalytic performance discover the acccomodation of Ru nanoparticles disposition and basic sites is mostly influencing the catalytic activity. DFM calcined under N2 flow (RuNaCa-N2) shows the highest CH4 production (449 µmol/g at 370°C), because a well-controlled decomposition of precursors which favors the better accomodation of adsorbent and Ru phases, maximizing the specific surface area, the Ru-basic sites interface and the participation of different basic sites in the CO2 methanation reaction. Thus, the calcination in a N2 flow is revealed as the optimal calcination protocol to achieve highly efficient DFM for integrated CO2 adsorption and hydrogenation applications.

Propylene Production via Oxidative Dehydrogenation of Propane with Carbon Dioxide over Composite MxOy-TiO2 Catalysts

The CO2-assisted oxidative dehydrogenation of propane (ODP) was investigated over titania based composite metal oxides, 10% MxOy-TiO2 (M: Zr, Ce, Ca, Cr, Ga). It was found that the surface basicity of composite metal oxides was significantly higher than that of bare TiO2 and varied in a manner which depended strongly on the nature of the MxOy modifier. The addition of metal oxides on the TiO2 surface resulted in a significant improvement of catalytic performance induced by a synergetic interaction between MxOy and TiO2 support. Propane conversion and propylene yield were strongly influenced by the nature of the metal oxide additive and were found to be superior for the Cr2O3-TiO2 and Ga2O3-TiO2 catalysts characterized by moderate basicity. The reducibility of the latter catalysts was significantly increased, contributing to the improved catalytic performance. This was also the case for the surface acidity of Ga2O3-TiO2 which was found to be higher compared with Cr2O3-TiO2 and TiO2. A general trend was observed whereby catalytic performance increased significantly with decreasing the primary crystallite size of TiO2. DRIFTS studies conducted under reaction conditions showed that the adsorption/activation of CO2 was favored on the surface of composite metal oxides. This may be induced by the improved surface basicity observed with the MxOy addition on the TiO2 surface. The Ga2O3 containing sample exhibited sufficient stability for about 30 h on stream, indicating that it is suitable for the production of propylene through ODP with CO2 reaction.

Integrated CO2 capture and reverse water-gas shift reaction over CeO2-CaO dual functional materials

Achieving carbon neutrality is one of the most important tasks to meet the environmental challenges due to excessive CO₂ emissions. Integrated CO₂ capture and utilization (ICCU) represents an effective process for direct utilization of CO₂-contained exhaust gas (e.g. flue gas), in which converting the captured CO₂ into CO via reverse water-gas shift (RWGS) reaction is a promising route. The dual functional materials (DFMs), containing CO₂ adsorbents and catalysts, are widely applied to achieve ICCU. The conventional active metals (Ni, Fe, etc.)-based DFMs and non-transition metal DFMs (e.g. CaO) are restricted by low CO selectivity, catalytic efficiency or CO generation in the CO₂ capture step. To address the above obstructs in the application of DFMs, the metal oxides-based DFMs, MO_x-CaO (M = Al, Ce, Ti or Zr), are synthesized and evaluated. The CeO₂-CaO outperformed the other metal oxides-based DFMs and possessed significantly improved catalytic performance. It is found that 33% CeO₂-CaO DFM displayed approximately 49% CO₂ conversion and approximately 100% CO selectivity in integrated CO₂ capture and reverse water-gas shift reaction (ICCU-RWGS) at 650°C, while CaO-alone only achieved approximately 20% CO₂ conversion at the same condition. The surface basicity of CeO₂ is revealed to contribute to the improved catalytic performance by enhancing CO₂ chemisorption and activation in the hydrogenation step. Furthermore, CeO₂-CaO material possessed excellent cycle stability in 20 cycles ICCU-RWGS, achieving a sustainable and high-efficient performance in CO₂ conversion and CO selectivity.

Understanding the intermediates and carbon dioxide adsorption of potassium chloride-incorporated graphitic carbon nitride with tailoring melamine and urea as precursors

In this study, we demonstrated the synthesis of potassium chloride (KCl)-incorporated graphitic carbon nitride, (g-C₃N₄, CN) with varying amounts of N-vacancies and pyridinic-N as well as enhanced Lewis basicity, via a single-step thermal polymerization by tailoring the precursors of melamine and urea for carbon oxide (CO₂) capture. Melamine, as a precursor, undergoes a phase transformation into melam and triazine-rich g-C₃N₄, whereas the addition of urea polymerizes the mixture to form melem and heptazine-rich g-C₃N₄ (CN11). Owing to the abundance of pyridinic-N and the high surface area, CN11 adsorbed higher amounts of CO₂ (44.52 μmol m^-2 at 25 °C and 1 bar of CO₂) than those reported for other template-free carbon materials. Spectroscopic analysis revealed that the enhanced CO₂ adsorption is due to the presence of pyridinic-N and Lewis basic sites on the surface. The intermediates of CO₂adsorption, including carbonate and bicarbonate species, attached to the CN samples were identified using in-situ Fourier-transform infrared spectroscopy (FTIR). This work provides insights into the mechanism of CO₂ adsorption by comparing the structural features of the synthesized KCl-incorporated g-C₃N₄ samples_. CN11, with an excellent CO₂ uptake capacity, is viewed as a promising candidate for CO₂ capture and storage.

Synergistic promotions between CO2 capture and in-situ conversion on Ni-CaO composite catalyst

The integrated CO₂ capture and conversion (iCCC) technology has been booming as a promising cost-effective approach for Carbon Neutrality. However, the lack of the long-sought molecular consensus about the synergistic effect between the adsorption and in-situ catalytic reaction hinders its development. Herein, we illustrate the synergistic promotions between CO₂ capture and in-situ conversion through constructing the consecutive high-temperature Calcium-looping and dry reforming of methane processes. With systematic experimental measurements and density functional theory calculations, we reveal that the pathways of the reduction of carbonate and the dehydrogenation of CH₄ can be interactively facilitated by the participation of the intermediates produced in each process on the supported Ni-CaO composite catalyst. Specifically, the adsorptive/catalytic interface, which is controlled by balancing the loading density and size of Ni nanoparticles on porous CaO, plays an essential role in the ultra-high CO₂ and CH₄ conversions of 96.5% and 96.0% at 650 °C, respectively.