Gold/Substituted Hydroxyapatites for Oxidative Esterification: Control of Thin Apatite Layer on Gold Based on Strong Metal-Support Interaction (SMSI) Results in High Activity

Gold nanoparticles (Au NPs) deposited on various cation-and anion-substitutedhydroxyapatites (Au/sHAPs) show oxidative strong metal-supportinteraction (SMSI), wherein a thin layer of the sHAP covered the surfaceof the Au NPs by heat treatment in an oxidative atmosphere. Calcinationof Au/sHAPs at 300 & DEG;C caused a partial SMSI and that at 500 & DEG;Cgave fully encapsulated Au NPs. We investigated the influence of thesubstituted ions in sHAP and the degree of the oxidative SMSI on thecatalytic performance of Au/sHAPs for oxidative esterification ofoctanal or 1-octanol with ethanol to obtain ethyl octanoate. The catalyticactivity depends on the size of the Au NPs but not on the supportused, owing to the similarity of the acid and base properties of sHAPsexcept for Au/CaFAP. The presence of a large number of acidic siteson CaFAP lowered the product selectivity, but all other sHAPs exhibitedsimilar activity when the Au particle size was almost the same, owingto the similarity of the acid and base properties. Au/sHAPs_O-2 with SMSI exhibited higher catalytic activity than Au/sHAPs_H-2 without SMSI despite the fact that the number of exposedsurface Au atoms was decreased by the SMSI. In addition, the oxidativeesterification reaction proceeded even though the Au NPs were fullycovered by the sHAP layer when the thickness of the layer was controlledto be less than 1 nm. The substrate can access the surfaces of theAu NPs covered by the thin sHAP layer (<1 nm), and the presenceof the sHAP structure in close contact with the Au NPs resulted insignificantly higher catalytic activity compared with that for fullyexposed Au NPs deposited on the sHAPs. This result suggests that maximizingthe contact area between the Au NPs and the sHAP support based onthe SMSI enhances the catalytic activity of Au.

Automated summary

Gold nanoparticles (Au NPs) interact strongly with cation-and anion-substituted hydroxyapatites (Au/sHAPs) due to oxidative strong metal-support interaction (SMSI). The catalytic performance of Au/sHAPs for oxidative esterification depends on the size of the Au NPs but not on the support used. Even when fully covered by a thin sHAP layer (<1 nm), the Au NPs can still catalyze oxidative esterification. The contact area between Au NPs and the sHAP support, maximized through SMSI, enhances the catalytic activity of Au.