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

ABSTRACT Water and air pollution are pervasive issues, impacting the majority of global citizens. In response, geopolymers functionalized with photocatalytic metal oxides have emerged as sustainable materials for environmental remediation. This study examines how the Si/Al molar ratio (1 and 2) and photocatalytic WO 3 loading (WO 3 /Al molar ratios of 0.10, 0.15, and 0.20) impact the microstructural, optical, and photocatalytic properties of WO 3 ‐geopolymer composites. XRD analysis showed the complete conversion of WO 3 into Na 2 WO 4 , impairing the intended photocatalytic function under visible light. Solid‐state 27 Al NMR showed incomplete geopolymerization, owing to NaOH consumption due to the Na 2 WO 4 conversion. With WO 3 loading, the BET surface for 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 substantial initial reduction and further reductions from 14.5 to 2.5 m 2 /g. The photocatalytic activity was evaluated by the decolorization of aqueous methylene blue (160 mg/L) and degradation of gaseous α‐pinene (1 ppm) under UV irradiation. The Si/Al = 1 series showed increased decolorization with WO 3 loading, whereas the Si/Al = 2 series showed a decrease. Surprisingly, the pristine samples with no WO 3 addition outperformed the WO 3 ‐geopolymers in both series, as assessed by the decolorization of methylene blue, attributed to naturally occurring photocatalysts within the metakaolin feedstock and a larger surface area. This shows that the photocatalytic performance of geopolymers is not solely dependent on external photocatalytic WO 3 loading but governed by the underlying geopolymer chemistry and network connectivity. These findings demonstrate the complexity in designing advanced ceramic photocatalytic systems while highlighting their potential for environmental remediation.