Thermal Migratory Alloying in Vacuum-Stabilized Porous Silica Nanoreactor.

High-temperature nanocrystal (NC) transformations in porous silica (pSiO2) are typically limited by densification, pore collapse, and sintering. We show that HVA-enabled opens a distinct thermal window at 1000°Cwhere unusually preserved pSiO2 confines metal species to gain sufficient mobility for NC-alloying. Continuous removal of volatile SiOx and condensates suppresses viscous flow, enabling nanoscopic migratory solid-state reactions inside intact pSiO2. Migration pathways are metal-dependent: noble metals (Rh, Pd, Pt, Ru) diffuse inward to form concentric alloy NC, whereas non-noble metals such as Fe, Co, and Ni proceed through transient silicide formation that drives outward NC migration, yielding an eccentric configuration via a melt-migrate-freeze mechanism. The resulting nanoreactors are compositionally tunable, thermally robust, and solution-dispersible. Confinement-programmed architectures generate catalytic microenvironments that tune activity and chemoselectivity across multiple hydrogenation reactions. HVA strategy converts migration from a high-temperature failure mode into a controllable synthetic handle for designing functional confined alloys.