Anchoring highly dispersed metal nanoparticles by strong electrostatic adsorption (SEA) on a dealuminated beta zeolite for catalysis

Abstract

Zeolites with defects can be combined with appropriate synthetic protocols to beneficially stabilise metallic clusters and nanoparticles (NPs). In this work, highly dispersed Ni NPs were prepared on a defect-rich dealuminated beta (deAl-beta) zeolite through strong electrostatic adsorption (SEA) synthesis, which enabled strong interactions between the electronegative deAl-beta and cationic metal ammine complexes (e.g., Ni(NH₃)₆²⁺) via the framework silanol nests. Ni NPs with diameters of 1.9 ± 0.2 nm were formed after SEA and reduction in H₂ at 500 °C and showed good activity in CO₂ methanation (i.e., specific reaction rate of 3.92 × 10⁻⁴ mol s⁻¹ g_Ni⁻¹ and methane selectivity of 99.8% at 400 °C under GHSV of 30 000 mL g⁻¹ h⁻¹). The mechanism of the SEA synthetic process was elucidated by ex situ XAFS, in situ DRIFTS, and DFT. XAFS of the as-prepared Ni catalysts (i.e., unreduced) indicates that SEA leads to the exchange of anions in Ni precursors (e.g., Cl⁻ and NO₃⁻) to form Ni(OH)₂, while in situ DRIFTS of catalyst reduction shows a significant decrease in the signal of IR bands assigned to the silanol nests (at ∼960 cm⁻¹), which could be ascribed to the strong interaction between Ni(OH)₂ and silanol nests via SEA. DFT calculations show that metallic complexes bind more strongly to charged defect sites compared to neutral silanol nest defects (up to 150 kJ mol⁻¹), confirming the enhanced interaction between metallic complexes and zeolitic supports under SEA synthesis conditions. The results provide new opportunities for preparing highly dispersed metal catalysts using defect-rich zeolitic carriers for catalysis.