Atomic Layer Deposition (ALD) as a Way to Prepare New Mixed-Oxide Catalyst Supports: The Case of Alumina Addition to Silica-Supported Platinum for the Selective Hydrogenation of Cinnamaldehyde

The case is made here for the power of using atomic layer deposition (ALD) as a way to induce changes in the nature of the oxides used as supports in many catalytic processes. ALD provides a route to grow thin films in a conformal way and with submonolayer thickness control, affording the creation of unique mixed-oxide structures with new reaction sites. This approach is exemplified here for the case of the hydrogenation of unsaturated aldehydes with platinum-based catalysts. Silica-supported catalysts were modified with thin alumina films, grown by ALD using trimethylaluminum(III) (TMA) and water, and their performance contrasted with pure Pt/SiO₂ and Pt/Al₂O₃ samples as well as with catalysts previously reported by us made by silica ALD on Pt/Al₂O₃. The quality of the alumina films grown on Pt/SiO₂ was first evaluated by using N₂ adsorption–desorption isotherms in conjunction with SBA-15 as the support, a mesoporous material with well-defined 1D cylindrical pores. An initial deposition of approximately 1.5 Å of the alumina film per ALD cycle was estimated from those measurements, with retention of the narrow distribution of pore diameters indicative of homogeneous coverage throughout the length of the pores. The catalytic hydrogenation of cinnamaldehyde was then determined to be slower but more selective with silica supports compared to alumina. Addition of a half of a monolayer of alumina to Pt/SiO₂ reduces the total activity, but only marginally. In exchange, the new mixed-oxide catalysts exhibit a higher selectivity toward the production of the desirable unsaturated alcohol at high conversions, and a lower activity for its subsequent hydrogenation to the saturated alcohol. These trends were associated with the formation of new Brønsted and Lewis acidic sites, possibly based on mixed Si–O–Al surface structures.