Ideal Bifunctional Catalysis for Propane Dehydrogenation over Pt-Promoted Gallia-Alumina and Minimized Use of Precious Elements

Gallia-alumina (Ga_xAl_2-xO₃), promoted with a trace amount of Pt, exhibits excellent activity and selectivity in propane dehydrogenation (PDH) due to a bifunctional mechanism. Tetrahedrally coordinated Ga (Ga_IV) catalyzes C-H dissociation, while Pt facilitates H recombination into H₂. To maximize the utilization of precious Pt and Ga, it is crucial to understand the optimal balance between the two catalytic functions or determine the 'ideal regime.' Here, we developed techniques to quantify the catalytic functions of Pt and Ga_IV sites. H₂-D₂ exchange rates (r_HD) and propylene chemisorption (Q(Ga_IV,surf)) were used as effective measures of the catalytic functions of Pt and Ga_IV, respectively. When Pt is sufficient relative to Ga_IV (r_HD/Q(Ga_IV,surf) ratio >0.3 mol_HD mol_GaIV,surf^-1 s^-1), the catalysts exhibit ideal catalytic properties. During repeated reaction and regeneration cycles, the catalysts deactivated mainly due to Pt sintering, which leads to an imbalance between the two catalytic functions. Notably, catalysts with higher Pt contents lost catalytic activity faster than those with lower Pt contents, eventually exhibiting reversed activities. This is because increased Pt loading facilitates sintering. Doping a small amount of Ce³⁺ onto Ga_xAl_2-xO₃ effectively suppresses Pt sintering via strong metal-support interaction. Thus, optimal loadings of Ga and Pt, combined with Ce³⁺ doping to stabilize Pt, enabled the minimized use of precious elements while maintaining excellent catalytic properties. Even with 100 ppm Pt and 1 wt % Ga, the catalyst exhibited superior activity, selectivity, and stability compared to the benchmark catalyst, PtSn/γ-Al₂O₃, with 7000 ppm (0.7 wt %) Pt.