Engineered Nanocomposites for Capturing and Converting Carbon Dioxide
into Useful Chemicals Michael Ashley1, 2 †, Punnamchandar Ramidi3, †, Timothy Bontrager1, Charles Magiera2, Anindya Ghosh3,*, Alexandru S. Biris4, Ilker S. Bayer5 and Abhijit Biswas1,* 1Center for Nano Science and Technology, Department of Electrical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA. 2Department of Chemical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA. 3Department of Chemistry, University of Arkansas, Little Rock, AR 72204, USA. 4Nanotechnology Center, University of Arkansas, Little Rock, AR 72204, USA. 5Center for Biomolecular Nanotechnologies, Smart Materials Platform, Italian Institute of Technology, Lecce 73010, Italy. † Equal Contributions *Corresponding authors: [email protected] (Abhijit Biswas); [email protected] (Anindya Ghosh) ABSTRACT
We describe a simple drop-cast processing method to synthesize multicomponent polymer-based nanocomposites for carbon dioxide (CO2) capture and conversion into stable carbonates. These multicomponent nanocomposites are made of combination of different metal oxide nanoparticles and catalysts in a porous polymer matrix. The formulation includes the combination of titanium dioxide and magnesium oxide, ruthenium oxide, and iron oxide where each metal oxide exhibits its own catalytic function of trapping carbon dioxide. Such a material system provides numerous localized catalytically active hot reaction spots generated by the dispersed multifunctional oxide nanoparticles that react with CO2 when exposed to the gas stream and instantaneously convert the captured carbon into carbonates. Finally, we discuss our ongoing work on the possibility of converting captured-carbon-formed-carbonate into useful products/commodities such as methane, methanol and formic acid. The integration of polymer materials with catalytically active nanomaterials shows a promising strategy for CO2 capture and conversion into useful products towards achieving a sustainable energy future. INTRODUCTION
The Green House Effect has established that placing enormous amounts of carbon products into atmosphere has a warming effect on the earth resulting in catastrophic weather and unhealthy regional air quality. Hence, the development of economically viable and/or energy-efficient pathways/approaches for carbon dioxide (CO2) capture/adsorption is critically important to mitigate climate change and preserve the eco system. The combustion of carbon-containing fuels i.e. hydrocarbons (natural gas, oil and coal, biomass) is the main factor in the creation of CO2. The existing technological processes used to capture CO2 include cryogenic distillation of air, condensation to remove condensable organic vapors from gas mixtures, and amine absorption together with biological, oceanic, chemical, and geological sequestration by underground injection [1-6]. Because current carbon capture methods are expensive and energy
inefficient, this technology must be refined [7]. Furthermore, CO2 is a renewable feedstock which can serve both as a reagent in chemical synthesis and as an environmentally benign solvent system. At present, given the depleting petroleum stockpiles that we heavily depend on to produce industrially important chemicals, it is of utmost importance that we develop new and novel uses for CO2 in chemical synthesis and purification in order to curb global petroleum consumption [8, 9]. Thus, for a sustainable energy future, new materials and processes need to be investigated and developed that, in addition to capturing CO2, would also convert it into useful chemicals/products. The major challenge, however, lies in activating stable CO2.
Nanocomposites have become more frequently used in engineering as a result of their unique characteristics. Nanocomposites provide opportunities to incorporate several desirable material properties concurrently in one material, which results in new phenomena and novel functional properties that are unachievable with traditional materials. Past carbon capture research and developmental projects have studied the use of nanocomposites for gas separation or capture [7, 10, 11]. For instance, brominated poly(2,6-diphenyl-1,4-phenylene oxide) has been used as a flexible membranous material with high permeability and high selectivity [12]. These nanocomposites offer larger surface area, higher surface reactivity, and more design flexibility than conventional bulk composites or polymer membranes [13-15].
Here, we present synthesis and characterization of multicomponent polymer-based nanocomposites. Prepared by a drop-casting process [7], these nanocomposites contain combinations of MgO, CuO, SiO2, TiO2, and Fe2O3 nanoparticles dispersed in a polyethylene glycol (PEG) matrix. Such a nanocomposite design allows the catalytic actions of each of these components to occur synergistically in a single system. Together in the PEG matrix, these nanocomposites display the effects of each of the metal oxides in order to successfully capture and store carbon dioxide. Stable carbonate minerals can then be obtained. EXPERIMENTAL DETAILS
Fabrication and characterization of nanocomposites
Figure 1 schematically illustrates the drop-cast synthesis process (a) and a scanning electron microscope (SEM) image of the fabricated nanocomposite that shows nanoparticles of metal oxides dispersed in a PEG matrix (b) and a Fourier Transformed Infrared Spectrum (FTIR) of the carbonate byproduct formation after the reaction with CO2(c) [7]. Synthesis of the nanocomposites involved thorough sequential mixing of the polymer matrix with the inorganic components (Figure 1a). The sequential mixing in solution allowed organic and inorganic building blocks (molecules, atoms, ions etc.) of the bulk components to self-organize and bind together based on their natural affinity to form physical and chemical cross-links and bonds. As a result, inorganic components/minerals find suitable nucleation sites in the macromolecular domains that incorporate mineral precursor particles (various ions or even ionic clusters) that eventually help grow metal oxide nanocrystals into the layers of the nanocomposites. Initially, about 10 mL of PEG solution (Sigma-Aldrich – BioUltra line for molecular biology, 20 000, ~50% in H2O) was poured into a 50 mL beaker. Then, each appropriate metal oxide powder was added to the solution. The most complex sample contained five metal oxides (SiO2, Sigma-Aldrich – nanopowder (spherical, porous) 5-15 nm particle size, 99.5%, CuO, Alfa Aesar – copper (II) oxide, NanoArc®, 97.5%; 23-37 nm particle size, MgO, SkySpring Nanomatrials –
magnesium oxide nanopowder, 99.9%, 10-30 nm particle size, TiO2, Sigma-Aldrich – titanium (IV) oxide nanopowder, anatase, 99.7%, <25 nm particle size, and Fe2O3, Sigma-Aldrich – iron (III) oxide, >99%, <5µm particle size), which were all added individually. After one metal oxide component was added, the solution was sonicated and mechanically shaken until homogenous, at which another metal oxide component could be added. The combination of shaking and applying ultrasonic energy allowed the nanoparticles to evenly disperse in the polymer matrix, which is essential to the multifunctionality of the nanocomposites. Variation of experimental parameters could have an effect on the final structure of the nanocomposite. Once all of the particles dispersed and the solution was homogeneous, a sample of the mixture was poured onto a glass slide and heated to 50°C for 20 minutes. Cooling then helped the sample to dry [7].
The scanning electron microscope (SEM) used for analysis of the bionanocomposites was a JEOL 7000 FE (resolution 1.2 nm at 30 KV) coupled with an energy dispersive spectroscopy (EDS) system for elemental analysis. Nanocomposite films were dissolved in tetrahydro furan (THF) and pressurized with CO2 in a 100 mL Parr high pressure reactor equipped with a Parr 4843 controller. After the reaction, a little aliquot of THF solution was evaporated to dryness and mixed with solid potassium bromide (KBr). The mixture was grounded thoroughly and a transparent pellet was made using a pellet press. The pellet was then used to record the FTIR spectra using a Shimadzu IRAffinity-1 Fourier transform infrared spectrophotometer. Nanocomposites were exposed to CO2 gas at the pressure of 300 psi (20 atm) and temperature of 60oC for several hours.
Figure 1: A schematic representation of the drop-casting process (a), an SEM image of a nanocomposite and a FTIR spectrum of the nanocomposite exposed to the CO2 stream showing the appearance of a carbonate peak after the reaction with CO2 (c) [7]. DISCUSSION The SEM image in Figure 1b displays a nearly homogeneous dispersion of different metal oxides nanoparticles. We observe faceted nanoparticles. Faceted two-dimensional
nanoparticles are better as they offer a high surface-to-volume ratio for enhanced catalytic actions for the reaction with CO2. The EDS analysis/data (not shown here) of the elemental profile confirmed the presence of different metal oxides. The PEG polymer provides an excellent matrix to disperse the metal oxides. The polymer contains several ether oxygen atoms, which can potentially weakly coordinate with the metal oxides and thus disperse and hold the metal oxides in the polymer matrix [7]. The conversion of CO2 adsorption into predominantly carbonate species (CO3
2-) is observed by a strong peak at the wavenumber 1797 cm-1 (Figure 1c) The band at 1797 cm-1 for carbonate species matches with the earlier report on the observation of (CO3
2-) species assigned to 1800 cm-1 [7, 16]. The carbonate formation is the most thermodynamically favored process, and hence the most stable. The catalytic functions of individual metal oxide nanoparticles possibly may have promoted the reactions with CO2 leading to the direct conversion into carbonate species. However, the mechanism of catalytic functions is still a matter of further investigations. The large-area approach for the sprayable nanocomposite coatings allows for implementation compatible with virtually any geometry and dimension. Thus, the method can be readily customized and scaled-up for automated industrial level production.
Figure 2 illustrates our ongoing work involving photocatalytic conversion into other useful products. It shows the nanocomposite materials for CO2 capture, photocatalytic production of H2 from water and catalytic reduction of CO2 into useful products such as formic acid, methanol and methane (equation in Figure 2a) and an example of the production route for methane (Figure 2b).
Figure 2: A schematic illustration of the CO2 reduction process into products. CONCLUSIONS
We have described a simple top-down method based on a drop-cast process to synthesize polymer-based multicomponent nanocomposites for CO2 adsorption and conversion into carbonate products. Such nanocomposites provide numerous localized catalytically active hot reaction spots generated by the dispersed multifunctional nanoparticles. This results in CO2 capture properties when the surface of the nanocomposite is exposed to the gas stream. We expect that this work will open new avenues for low-cost, energy efficient CO2 capture using these nanocomposites that can be sprayed over any surface as coatings for a variety of
applications. The ability to recover the captured carbon in the form of commercially useful gasses/chemicals provides an opportunity to develop pathways towards a sustainable energy future.
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