Identifying higher oxygenate synthesis sites in Cu catalysts promoted and stabilized by atomic layer deposited Fe2O3

The conversion of syngas to higher oxygenates is an important step in the pathway towards the long-term sustainable production of fuels and chemicals. Mixed Cu/Fe catalysts combine metals with different CO activation characteristics, a property that makes them promising candidates for higher oxygenate production. However, Cu/Fe catalysts suffer from instability due to sintering and phase separation, and the active sites for higher oxygenate formation on these materials are not well understood. In this work, we use atomic layer deposition (ALD) to modify silica-supported Cu nanoparticles with monolayer precise amounts of Fe₂O₃ and study the effects on catalyst structure, reactivity, and stability. We demonstrate that the Fe₂O₃ ALD process is inherently selective towards deposition on the SiO₂ surface versus the Cu nanoparticles. As a result, a sufficiently thick Fe₂O₃ layer surrounding the Cu nanoparticles acts as a barrier against Cu migration, preventing catalyst deactivation through sintering without covering the Cu. In syngas conversion reactions, we find that higher oxygenate selectivity increases with Fe₂O₃ loading at low and intermediate ALD cycle numbers. Furthermore, using in situ X-ray absorption spectroscopy, we demonstrate that the Fe near the Cu nanoparticles is reduced to a metallic state under reaction conditions. From these measurements, we identify these metallic Cu/Fe interfaces as the active sites for higher oxygenate formation, which are most abundant at intermediate Fe₂O₃ ALD cycle numbers. As the Fe₂O₃ loading increases, higher oxygenate selectivity falls, resulting from encapsulation of the Cu nanoparticles by thick Fe₂O₃ layers. The results demonstrate how thickness-controlled ALD Fe₂O₃ can be used as both a promoter and stabilizer of Cu-based higher oxygenate synthesis catalysts.