Reverse Microemulsion Synthesis as a Method for Production of Catalytic Nanomaterials for CO₂ Conversion to Renewable Fuels and Chemicals

In a reverse microemulsion (RME) system, water nano-droplets are surrounded by surfactant molecules and dispersed in a continuous oil phase. Reverse microemulsions are clear, thermodynamically stable isotropic liquid mixtures that form by self-assembly upon mixing their components in appropriate ratios. Water nanodroplets can contain reactants. If two (or more) reverse microemulsions containing different reactants are mixed, the reaction happens when aqueous nanodroplets collide and merge, resulting in a slow, gradual process of self-assembly of nanoparticles. The resulting nanoparticles are extracted by changing the RME composition in such a way that it is not thermodynamically stable anymore. Upon segregation of aqueous and organic phases, nanoparticles undergo aggregation within the aqueous phase, followed by sedimentation allowing for their easy separation. RME method can be used to generate nanomaterials with high specific surface area, since aqueous nano-droplets act as nano-reactors, restraining amounts of reactants and therefore limiting the resulting nanoparticle size.
We present implementation of the RME method for generation of catalytic nanomaterials for converting CO₂ to renewable fuels and chemicals. Catalytic conversion of CO₂ to value-added products is one of the promising ways to reduce our dependence on fossil fuels, while also creating economic benefits. More specifically, thermocatalytic hydrogenation of CO₂ using renewable H₂ can produce a variety of renewable fuels, including renewable natural gas and aviation fuels, and chemicals such as renewable methanol.
Various catalytic nanomaterials were synthized via the RME method using aluminum oxide (Al₂O₃) and cerium oxide (CeO₂) as structural support materials and several transition metals (Cu, Fe, Co, Ni and Mo) as highly dispersed active phases in the form of either metal oxide or metal carbide. All nanomaterials generated had high specific surface areas ranging from 200-400 m²/g and were composed of nanoparticles typically sized less than 20 nm. The resulting catalytic materials were characterized by a number of analytical techniques (ICP-OES, XPS, XRD, BET, TEM) to investigate their chemical composition, crystallinity, and morphology. Temperature programmed reduction (TPR) and temperature programmed desorption (TPD) were employed to investigate reducibility and adsorption properties of catalytic surfaces, and in situ Fourier transform infrared spectroscopy (FTIR) was used to investigate surface reactivity. In addition, density functional theory (DFT) computations were conducted to investigate the effect of incorporation of foreign transition metals into CeO₂ lattice on CO₂ adsorption and cleavage properties.
To assess the implementability of synthesized materials, reaction tests were conducted in a continous flow reactor, using a CO₂/H₂ mixture as a feed and a wide range of operating conditions such as temperature, pressure and flow rate. The outlet from the reactor was analyzed vi infrared (IR) and Fourier transform infrared (FTIR) spectroscopy, gas chromatography (GC), and mass spectrometry (MS). Synthesized catalytic nanomaterials exhibited different levels of catalytic activity and selectivity to various products, ranging from CO and CH₄ to light hydrocarbons. For certain materials, CO₂ conversions approaching the chemical equilibrium levels were achieved (95% CO₂ conversion and 100% selectivity to desired product generation in some cases), and materials stability was also investigated comprehensively (more than 100 h). The results of catalytic perfomance evaluation were correlated with the results of materials characterization, while focusing on the structure-property relationship. Our work provides an avenue for developing highly efficient methods for CO₂ conversion to renewable fuels, while also providing fundamental insights into the relationship between the material synthesis method and its reactive properties.