Phase Transformations, Microstructural Effects and Photocatalytic Activity of Tungsten Trioxide Geopolymer Composites

Water and air pollution are pervasive issues, impacting the majority of global citizens. In response, geopolymers functionalizedwith photocatalytic metal oxides have emerged as sustainable materials for environmental remediation. This study examineshow the Si/Al molar ratio (1 and 2) and photocatalytic WO 3 loading (WO3 /Al molar ratios of 0.10, 0.15, and 0.20) impactthe microstructural, optical, and photocatalytic properties of WO 3 -geopolymer composites. XRD analysis showed the completeconversion of WO3 into Na2 WO4 , impairing the intended photocatalytic function under visible light. Solid-state 27 Al NMR showedincomplete geopolymerization, owing to NaOH consumption due to the Na 2 WO4 conversion. With WO3 loading, the BET surfacefor the Si/Al = 1 series stabilized at ∼ 11.7 m 2 /g after a significant initial decline; whereas the Si/Al = 2 series showed a substantialinitial reduction and further reductions from 14.5 to 2.5 m 2 /g. The photocatalytic activity was evaluated by the decolorization ofaqueous methylene blue (160 mg/L) and degradation of gaseous α-pinene (1 ppm) under UV irradiation. The Si/Al = 1 seriesshowed increased decolorization with WO3 loading, whereas the Si/Al = 2 series showed a decrease. Surprisingly, the pristinesamples with no WO3 addition outperformed the WO3 -geopolymers in both series, as assessed by the decolorization of methyleneblue, attributed to naturally occurring photocatalysts within the metakaolin feedstock and a larger surface area. This shows thatthe photocatalytic performance of geopolymers is not solely dependent on external photocatalytic WO 3 loading but governed by theunderlying geopolymer chemistry and network connectivity. These findings demonstrate the complexity in designing advancedceramic photocatalytic systems while highlighting their potential for environmental remediation.