From α-Al₂O₃ Phase Purity to Surface Reactivity: A Causal Design Framework for Selective Ethylene Epoxidation over Ag/α-Al₂O₃ Catalysts

Silver supported on α-alumina remains the industrially dominant catalytic platform for direct ethylene epoxidation, yet the support is often treated as a passive mechanical carrier even though recent studies show that phase purity, hydroxyl chemistry, exposed support area and Ag/support geometry can directly alter ethylene oxide (EO) selectivity and secondary EO loss. This critical review integrates evidence from phase-controlled α-Al₂O₃ synthesis, Ag particle and surface-ratio studies, EO degradation experiments, promoter studies, operando oxygen-species characterization and kinetic modeling. Four original synthesis contributions are proposed: (i) a Phase–Surface Selectivity Envelope that separates bulk phase purity from surface EO-loss propensity; (ii) a Support-Exposure Penalty concept that treats uncovered α-Al₂O₃ as an independent selectivity variable; (iii) a Causal Control Matrix for disentangling phase, surface modifier and Ag morphology effects; and (iv) a Minimum Evidence Set for claiming support-controlled selectivity. These contributions are not presented as experimentally validated laws; they are falsifiable design frameworks intended to guide the PhD program on phase-pure and surface-controlled Ag/α-Al₂O₃ catalysts. The review concludes that the most defensible route to higher selectivity is not simply to maximize α-phase fraction, but to combine quantified phase purity with low secondary EO reactivity, controlled Ag coverage and promoter history under matched conversion conditions.

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