3D-Printed Fe-doped silicon carbide monolithic catalysts for wet peroxide oxidation processes
Graphical abstract
Introduction
The treatment of the wastewater coming from industrial processes is increasingly becoming an environmental and health priority. Catalytic Wet Peroxide Oxidation (CWPO) is one of the most promising technologies to remove non-biodegradable contaminants in wastewater. CWPO operates under mild conditions (temperatures and pressures in the range of 25–100 °C and 0.1–0.5 MPa, respectively) [1] and employs hydrogen peroxide (H2O2) as oxidant, which is considered an environmentally friendly agent. This process requires a solid catalyst with redox properties to generate hydroxyl (HO) and hydroperoxy (HOO) radicals from H2O2 decomposition. These radical species easily react with the pollutants, oxidizing them.
The catalyst par excellence is iron (Fe) due to its low cost and high-organic oxidation activity. Fe is usually anchored to different materials as iron oxide in powder or granular form, viz. alumina [2], silica [3], titania [4], zeolites [5], mesoporous molecular sieves [6], pillared clays [7] and activated carbon [8]. However, the poor resistance offered by Fe containing materials to leaching under the acidic conditions of the CWPO process is limiting its industrial application. Therefore, different approaches are being explored, among others, the employ of alternative catalysts either metal-based ones, such as copper [9], manganese [10], cobalt [11], gold [12] or metal-free carbon catalysts [13]. When metal leaching is not significant or even null, as it is the case of carbon-based catalysts, the contribution of the homogeneous reaction is avoided and, thus, modest catalytic activities are achieved [14]. The high-cost gold nanoparticles are an exception but in spite of a high catalytic activity, they are susceptible to reversible poisoning by oxidation intermediates [15]. On the other hand, the industrial implementation of the CWPO catalysts that implies their conformation in suitable morphological characteristics with the appropriate mechanical strength remains scarcely explored and just few works can be found. In this way, Taran et al. [16] studied the wet peroxide oxidation of phenol in a flow reactor using different geometric forms (cylinders, trefoils, spheres and monoliths) and sizes of copper-doped zeolites. These authors found that the highest efficiency toward phenol destruction was observed for the catalysts with the greatest geometric surface area that corresponded to trefoils and monoliths with high channel density.
The above highly demanding requirements could be accomplished by employing additive manufacturing techniques [17] to design with a precise control of the size and shape, rigid structures with embedded catalysts adapted to the requirements of the CWPO process. Herein, we propose the use of cost-effective direct ink writing techniques such as robocasting [18,19] to produce three-dimensional (3D) architected ceramics by colloidal processing employing an aqueous-based ink containing the catalytic material. At present, scarce works can be found on 3D catalysts directly obtained by robocasting [[20], [22]]. However, we expect upcoming interest in the application of this technique to the catalyst manufacturing because it can overcome the intricacies of the scale-up of heterogeneous catalyst (large sizes, scale-up formulation and structuring). The recent study by Tubio et al. [22], the only reported work up to now that focus on metal-based catalysts with a woodpile porous structure achieved by robocasting, provided promising results by demonstrating the good recyclability of these 3D printed Cu/Al2O3 catalysts in Ullmann reactions without leaching contamination of the reaction products.
The mechanical integrity of the 3D printed structures that allows manipulating them when using as catalysts is also an important advantage. Although that integrity should increase with the density of the scaffold [23], at the same time, it could compromise their catalytic activity by decreasing the porosity and, hence, the number of available catalytic active sites.
This work will accomplish the robocasting of 3D Fe/silicon carbide (SiC) catalysts by architecting colloidal suspensions of Fe-doped (0.52 wt.%) SiC powders into porous monoliths. SiC is selected as the catalyst support due to its excellent corrosion resistance to most chemicals, in addition to other attractive properties like low density, good thermal stability and high strength [24,25]. The challenge in the fabrication of this 3D Fe incorporated monoliths for the CWPO process is to trade-off a good catalytic properties (activity and stability) –promoted by the embedded Fe active sites accessible in the SiC porous structure- and a certain mechanical strength. Both requirements could be met by tailoring the porosity of the solid structure through heat treatments of the SiC support at temperatures that only produce a partial sintering, presently from 1000 to 1500 °C in a spark plasma furnace. The catalytic activity and stability of Fe/SiC monoliths thus obtained are studied in the CWPO of phenol, a common pollutant in wastewaters [26]. The results describing the effect of process variables including temperature, initial phenol, total organic carbon (TOC) and H2O2 concentrations are modelled according to some proposed reaction kinetics. The effect of mass transfer limitation on the CWPO of phenol is also reported. To the best of our knowledge, no reports can be found on 3D printed catalyst for CWPO reactions hitherto.
Section snippets
Monolith fabrication
Nanosized β-SiC powders containing 0.52 wt.% of Fe (Nanostructured & Amorphous Materials Inc. -NanoAmor-, polytype 3 C with mean SiC particle size of 45–55 nm) were employed as starting material. The apparent viscosity (η) and the shear storage (G’) and loss (G’’) moduli of the Fe/SiC inks were determined with a rheometer (CVO 100 D, Bohlin Instruments) equipped with a cone-and-plate geometry (diameter: 40 mm; cone angle: 4°). For η, the shear stress (τ) was recorded as shear rate () was
Fabrication and characterization of Fe/SiC monoliths
The manufacturing process of the Fe/SiC scaffolds first required the development of a printable ink, which must exhibit an adequate pseudoplastic behaviour. To achieve this goal, different organic additives were sequentially added to an Fe/SiC containing aqueous ink, starting with H-PEI and L-PEI that were employed as dispersant agents to improve the solids content, followed by the addition of MC as viscosifying agent. The ink composition was formed by 38.3 wt.% of deionized water, 55.7 wt.% of
Conclusions
The feasibility of the robocasting method for fabricating robust 3D Fe/SiC monolithic catalysts for CWPO processes using pseudoplastic Fe-doped SiC inks is demonstrated. This cost-effective method fully integrates the Fe catalyst into the ceramic scaffold and by further heat treatments, porosity (or apparent density) of the 3D structures can be tuned, thus modifying the access of the reactants to the Fe sites. The decrease of the porosity with temperature produces a detrimental effect in the
Acknowledgments
This work was supported by the following projects: MAT2015-67437-R (MINECO, FEDER, UE) and CTM2016-76454-R (MINECO). The authors thank Dr. Daniel Gamarra for their help in the characterization studies by XPS.
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