Maximizing the catalytic function of hydrogen spillover in platinum-encapsulated aluminosilicates with controlled nanostructures

Abstract

Hydrogen spillover has been studied for several decades, but its nature, catalytic functions and even its existence remain topics of vigorous debate. This is a consequence of the lack of model catalysts that can provide direct evidences of the existence of hydrogen spillover and simplify the catalytic interpretation. Here we use platinum encapsulated in a dense aluminosilicate matrix with controlled diffusional properties and surface hydroxyl concentrations to elucidate the catalytic functions of hydrogen spillover. The catalytic investigation and theoretical modelling show that surface hydroxyls, presumably Brønsted acids, are crucial for utilizing the catalytic functions of hydrogen spillover on the aluminosilicate surface. The catalysts with optimized nanostructure show remarkable activities in hydro-/dehydrogenation, but virtually no activity for hydrogenolysis. This distinct chemoselectivity may be beneficial in industrially important hydroconversions such as propane dehydrogenation to propylene because the undesired hydrogenolysis pathway producing light hydrocarbons of low value (methane and ethane) is greatly suppressed.

Revisiting hydrogen spillover in Pt/LTA: Effects of physical diluents having different acid site distributions

“…3a). The NMR spectra reveal that the amorphization is due to partial framework dealumination [6]. TEM investigation (Fig.…”

“…It should be noted that diluents mentioned above generally possess Brønsted acid sites as well as other ''protonic'' surface hydroxyls (-OH), whose proton (H + ) can be exchanged with the -O À Na + and -O À K + groups of Pt/NaA and Pt/KA. If surface hydroxyls provide an important H spillover pathway as we proposed in our recent work [6], H spillover may not be possible in pure Pt/NaA or Pt/KA, but becomes possible only after solid-state H + exchange with other oxide diluents. Alternatively, too harsh mechanical grinding can also lead to sintering of the encapsulated Pt clusters on the external surface of the zeolite crystallites, which may increase overall catalytic activity even without the catalytic contribution of H spillover.…”

“…Because Pt clusters were still encapsulated in the solid matrix after the decationization, the remarkably increased catalytic activities strongly supported the conclusion that H spillover can be used for hydrogenation only in the presence of abundant surface hydroxyls. DFT calculation suggested that the activated hydrogen can migrate by relaying along the chain of surface hydroxyl groups, while Lewis acid sites at the external surface of the zeolites act as the activation sites for unsaturated organic reactants [6]. It is noteworthy that Pt/LTA (NaA and KA) model catalysts were similarly investigated in earlier works for various catalytic hydrogenations [7][8][9][10].…”

“…Recently, we reported strong evidence for the existence of H spillover on aluminosilicates by demonstrating significant hydro-/ dehydrogenation catalytic activities via H spillover and reverse spillover, respectively [6]. In that work, we synthesized a series of Pt/ NaA model catalysts containing $1 nm Pt clusters selectively in the micropores of NaA zeolite (LTA structure).…”

Exfoliated Pd/HNb3O8 nanosheet as highly efficient bifunctional catalyst for one-pot cascade reaction

“…In the case of acidor base-metal bifunctional catalysts, on the other hand, ensemble or synergistic catalysis of the metal site with the acid or base site is advantageous for accelerating the reaction rate or improving the selectivity for a desired product. There have been examples showing that the surface properties of supports play important roles in determining the catalytic performance [23][24][25][26][27][28].…”