Hopcalite nanoparticle catalysts with high water vapour stability for catalytic oxidation of carbon monoxide

•Alkali-free hopcalite nanoparticles were synthesised by flame spray pyrolysis.•Optimal synthesis parameters were examined.•Hopcalite particles exhibit outstanding performances in the CO oxidation.•The new catalysts show remarkably improved stability under humid conditions.•A hydrophobic carbonaceous barrier layer is responsible for improved moisture stability.