Alveolar TiO2-β-SiC photocatalytic composite foams with tunable properties for water treatment

Porous Silicon Carbide (SiC): A Chance for Improving Catalysts or Just Another Active-Phase Carrier?

There is an obvious gap between efforts dedicated to the control of chemicophysical and morphological properties of catalyst active phases and the attention paid to the search of new materials to be employed as functional carriers in the upgrading of heterogeneous catalysts. Economic constraints and common habits in preparing heterogeneous catalysts have narrowed the selection of active-phase carriers to a handful of materials: oxide-based ceramics (e.g. Al₂O₃, SiO₂, TiO₂, and aluminosilicates-zeolites) and carbon. However, these carriers occasionally face chemicophysical constraints that limit their application in catalysis. For instance, oxides are easily corroded by acids or bases, and carbon is not resistant to oxidation. Therefore, these carriers cannot be recycled. Moreover, the poor thermal conductivity of metal oxide carriers often translates into permanent alterations of the catalyst active sites (i.e. metal active-phase sintering) that compromise the catalyst performance and its lifetime on run. Therefore, the development of new carriers for the design and synthesis of advanced functional catalytic materials and processes is an urgent priority for the heterogeneous catalysis of the future. Silicon carbide (SiC) is a non-oxide semiconductor with unique chemicophysical properties that make it highly attractive in several branches of catalysis. Accordingly, the past decade has witnessed a large increase of reports dedicated to the design of SiC-based catalysts, also in light of a steadily growing portfolio of porous SiC materials covering a wide range of well-controlled pore structure and surface properties. This review article provides a comprehensive overview on the synthesis and use of macro/mesoporous SiC materials in catalysis, stressing their unique features for the design of efficient, cost-effective, and easy to scale-up heterogeneous catalysts, outlining their success where other and more classical oxide-based supports failed. All applications of SiC in catalysis will be reviewed from the perspective of a given chemical reaction, highlighting all improvements rising from the use of SiC in terms of activity, selectivity, and process sustainability. We feel that the experienced viewpoint of SiC-based catalyst producers and end users (these authors) and their critical presentation of a comprehensive overview on the applications of SiC in catalysis will help the readership to create its own opinion on the central role of SiC for the future of heterogeneous catalysis.

Ti-Modified LaFeO3/β-SiC Alveolar Foams as Immobilized Dual Catalysts with Combined Photo-Fenton and Photocatalytic Activity

Ti-modified LaFeO₃/β-SiC alveolar foams were used as immobilized, highly robust dual catalysts with combined photocatalytic wet peroxide oxidation and photocatalytic activity under UV-A light. They were prepared by incipient wetness impregnation of a β-SiC foam support, by implementing a sol-gel Pechini synthesis at the foam surface in the presence of dried amorphous sol-gel titania as a titanium source. The physicochemical and catalytic features suggest the stabilization at the foam surface of a substituted La_1-xTi_xFeO₃ catalyst analogous to its powdery counterpart. Taking 4-chlorophenol removal in water as a model reaction, its dual nature enables both high reaction rates and full total organic carbon (TOC) conversion because of a synergy effect, while its macroscopic structure overcomes the drawback of working with powdery catalysts. Further, it yields photonic efficiencies for degradation and mineralization of ca. 9.4 and 38%, respectively, that strongly outperform those obtained with a reference TiO₂ P25/β-SiC foam photocatalyst. The enhancement of the catalyst robustness upon Ti modification prevents any Fe leaching to the solution, and therefore, the optimized macroscopic foam catalyst with 10 wt % catalyst loading operates through pure heterogeneous surface reactions, without any activity loss during reusability test cycles.

Coating-free TiO2@β-SiC alveolar foams as a ready-to-use composite photocatalyst with tunable adsorption properties for water treatment

Coating-free TiO₂@β-SiC photocatalytic composite foams gathered within a ready-to-use shell/core alveolar medium the photocatalytically active TiO₂ phase and the β-SiC foam structure were prepared via a multi-step shape memory synthesis (SMS) replica method. They were fabricated following a sequential two-step carburization approach, in which an external TiC skin was synthesized at the surface of a β-SiC skeleton foam obtained from a pre-shaped polyurethane foam during a first carburization step. The adsorption behaviour of the shell/core TiO₂@β-SiC composite foams towards the Diuron pollutant in water was tuned by submitting the carbide foams to a final calcination treatment within the 550–700 °C temperature range. The controlled calcination step allowed (i) the selective oxidation of the TiC shell into a TiO₂ crystallite shell owing to the β-SiC resistance to oxidation and (ii) the amount of residual unreacted carbon in the foams to be tuned. The lower the calcination temperature, the more pronounced the adsorption profiles of the composites and the higher the Diuron amount removed by adsorption on the residual unreacted carbon. The ready-to-use TiO₂@β-SiC composite foams were active in the degradation of the Diuron pesticide, without any further post-synthesis immobilization or synthesis process at the foam surface. They displayed good reusability with test cycles and benefitted from an enhanced stability in terms of the titania release to water.

Coating-free TiO₂@β-SiC photocatalytic composite foams gathering within a ready-to-use shell/core alveolar medium the TiO₂ photocatalyst and the β-SiC foam structure were prepared via a multi-step shape memory synthesis (SMS) replica method.