Molecular TiO₂ Modifications of Supported PPh₃-Capped Pd Nanocatalysts for CO₂ Hydrogenation into Formates

TiO₂-supported Pd NPs-based materials were prepared following two distinct approaches: For the first set of materials 1–8, modification of the TiO₂ support was performed prior to Pd NPs deposition, while the second set (9–15) was synthesized by deposition of modifiers over pre-synthesized Pd-PPh₃/TiO₂. These catalysts were applied in the hydrogenation of CO₂ to formate, and their performance was compared with that of the unmodified Pd-PPh₃/TiO₂. Modification of the TiO₂ support by organosilanes provided a beneficial effect in catalysis compared with the catalyst containing unmodified TiO₂ or TiO₂ modified by organophosphonic acids. In contrast, in most cases, the deposition of modifiers over previously synthesized Pd NPs supported on TiO₂ was not beneficial to the activity of the catalyst. Interestingly, upon recycling, the first set of catalysts suffered a rapid decrease in activity, while the anchoring of modifiers over previously formed Pd NPs showed an improved stability (TON > 500 after the third recycling).