POST-TREATMENT STRATEGY: MOLECULAR SIEVE-CONFINED BIFUNCTIONAL METAL-SITE CATALYSTS FOR HYDROGEN PRODUCTION FROM FORMIC ACID DECOMPOSITION

Abstract

Formic acid, as an important liquid organic hydrogen carrier, is considered a highly promising chemical medium for hydrogen storage due to its high hydrogen storage density, safety, non-toxicity, and ease of storage and transportation. It can also catalytically decompose to release hydrogen under mild conditions. However, traditional supported noble metal catalysts face issues such as metal nanoparticle migration, agglomeration, and leaching during the reaction process, leading to reduced catalyst activity and instability. To address these challenges, this thesis utilizes the microporous confinement effect of molecular sieves to design and prepare a series of Pd-based catalysts confined within molecular sieves. The catalytic performance, kinetic behavior, and cycle stability of these catalysts were systematically investigated. Various characterization techniques were used to elucidate the relationship between catalyst structure and performance. Using a commercial Beta molecular sieve as the carrier, acidic sites were removed through aluminophosphoric acid treatment. Then, highly dispersed Pd nanoclusters confined within the molecular sieve pores were successfully synthesized using a combination of ligand protection and hydrogen reduction after calcination. A second metal, Ce, was subsequently introduced to create the PdCe-Beta-DeAL-C-H bimetallic catalyst. Characterization results showed that aluminophosphoric acid treatment and the addition of Ce effectively improved the dispersion and stability of the Pd particles. The Pd nanoparticles had a small size of 2–5 nm and were uniformly distributed on the surface of the carrier and near its pores. Ce was in close contact with Pd, and the interfacial interactions altered the surface electronic structure of Pd, thereby promoting the formation and stabilization of metallic Pd⁰. Catalytic tests demonstrated that the optimal Pd₁.₀Ce₀.₂-Beta-DeAL-C-H catalyst achieved a conversion rate of 3199 molH₂·molPd⁻¹·h⁻¹ at 60 °C during formic acid decomposition. This value was approximately 127% higher than that of the single-metal catalyst. Additionally, the catalyst exhibited excellent cycle stability: after five cycles, its conversion rate remained above 1800 molH₂·molPd⁻¹·h⁻¹. Kinetic studies revealed that the apparent activation energy for formic acid decomposition over this catalyst was 55.93 kJ·mol⁻¹, significantly lower than that of the catalyst without Ce (84.5 kJ·mol⁻¹). Moreover, the catalyst performed well in the reaction of CO₂ hydrogenation to formic acid, with a reaction rate of 62 molformate·molPd⁻¹·h⁻¹. This study demonstrates that the incorporation of Ce through post-treatment effectively enhances the performance of Pd-based catalysts for formic acid decomposition. It provides valuable insights for the design of efficient bimetallic catalysts.