Abstract
Palladium (Pd), a typical platinum-group transition metal, possesses unique d-band electronic structure, adjustable valence states (Pd⁰, Pd⁺, Pd²⁺, Pd⁴⁺), excellent hydrogen dissociation capacity and moderate redox potential, which makes it one of the most versatile noble metal catalytic materials. Palladium-based catalysts are divided into homogeneous molecular Pd catalysts and heterogeneous supported Pd catalysts, showing unparalleled catalytic performance in organic synthesis, petrochemical refining, environmental pollution control and electrocatalytic energy conversion. Homogeneous Pd catalysts feature precise active site regulation and high reaction selectivity, while heterogeneous Pd catalysts have prominent recyclability, easy separation and industrial scalability. This article systematically summarizes the classification, mainstream synthesis strategies, core catalytic reaction mechanisms, structure-activity relationship, typical applications and existing deactivation mechanisms of palladium catalysts. Meanwhile, the modification methods including bimetallic doping, carrier defect engineering and ligand regulation are elaborated to improve anti-sintering, anti-poisoning and catalytic stability of Pd active sites. Finally, the development trends of low-loading Pd catalysts, single-atom Pd catalysts and green ligand-free Pd catalytic systems are prospected, providing theoretical basis for the design and industrial application of high-efficiency low-cost palladium catalytic materials.
Keywords: Palladium catalyst; Supported noble metal catalyst; Catalytic mechanism; Organic coupling reaction; Environmental catalysis; Electrocatalysis; Catalyst deactivation
1. Introduction
Noble metal catalysts dominate high-selectivity catalytic reactions owing to favorable orbital hybridization and reactant adsorption activation ability. Among platinum-group metals, palladium has lower market price than platinum (Pt) and rhodium (Rh), weaker sulfur poisoning sensitivity than nickel (Ni), and superior catalytic activity for bond cleavage and reconstruction reactions. Since Pd-catalyzed Suzuki cross-coupling reaction was established in 1979, palladium catalysis has gradually become the core technology of fine organic synthesis, winning the Nobel Prize in Chemistry in 2010 for classic cross-coupling reactions.
According to existing morphology and dispersion state, palladium catalysts are categorized into homogeneous Pd complex catalysts and heterogeneous supported Pd catalysts. Homogeneous Pd catalysts mostly adopt chelated Pd²⁺ or zero-valent Pd⁰ complexes coordinated with phosphorus-containing, nitrogen-containing organic ligands, which can realize atomic-level active site regulation. Heterogeneous Pd catalysts fix Pd nanoparticles, Pd clusters or single Pd atoms on porous carriers, including carbon materials, metal oxides, zeolites, silica and metal-organic frameworks (MOFs). Combined with the previous research system of mordenite zeolite carrier, zeolite-supported Pd catalysts have become a hotspot in petrochemical catalysis, due to the shape-selective confinement effect of zeolite pore channels and strong metal-support interaction (SMSI).
In practical industrial reactions, monometallic Pd catalysts suffer from inherent defects: high noble metal consumption, nanoparticle sintering under high temperature, sulfur/halogen poisoning, and over-hydrogenation side reactions. Current research focuses on reducing Pd loading, constructing bimetallic Pd-alloy active sites, modifying carrier surface defects, and developing recyclable single-atom Pd catalysts to balance catalytic activity, selectivity and economic cost. This review matches the research framework of porous zeolite-based catalysts, unifies academic terminology, and comprehensively analyzes the whole research system of palladium catalytic materials.
2. Classification and Basic Properties of Palladium Catalysts
2.1 Classification by Dispersion State
2.1.1 Homogeneous Palladium Complex Catalysts
Homogeneous Pd catalysts dissolve uniformly in liquid reaction solvents, mainly divided into zero-valent Pd⁰ complexes and divalent Pd²⁺ precursors. Common commercial precursors include Pd(PPh₃)₄, Pd(OAc)₂, PdCl₂ and Pd(acac)₂. Organic ligands (phosphines, N-heterocyclic carbenes) can adjust the electron density and steric hindrance of central Pd atoms, so as to control reaction pathway and product selectivity. This type of catalyst has ultra-high catalytic activity under mild conditions, but faces unavoidable drawbacks: difficult product separation, poor recyclability, residual metal pollution in pharmaceutical intermediates, and high ligand synthesis cost.
2.1.2 Heterogeneous Supported Palladium Catalysts
Supported Pd catalysts are composed of Pd active components and functional carriers, which are the main types used in industrial production. Classified by carrier types:
(1) Carbon-based carriers: Activated carbon, carbon nanotubes, graphene, porous carbon spheres, with large specific surface area and strong electron transfer ability, suitable for liquid-phase hydrogenation reactions;
(2) Oxide carriers: Al₂O₃, SiO₂, TiO₂, CeO₂, ZnO, where CeO₂ provides abundant oxygen vacancies to optimize Pd redox performance;
(3) Microporous zeolite carriers: Na-MOR, H-MOR, ZSM-5, Y zeolite. Zeolite pore confinement limits Pd particle growth, and acid sites of zeolites construct metal-acid bifunctional catalytic sites for hydrocarbon conversion;
(4) Crystalline porous carriers: MOFs, COFs, with adjustable pore size for size-selective catalytic reactions.
2.2 Valence-Dependent Catalytic Characteristics of Pd Active Sites
1) Pd⁰: Zero-valent metallic palladium, the core active site for hydrogen dissociation, oxidative addition and catalytic hydrogenation reactions;
2) Pd²⁺: Ionic palladium stabilized by carriers or ligands, dominant in selective oxidation, C-H activation and electrophilic catalytic reactions;
3) Pd⁴⁺: High-valence palladium, usually generated by in-situ oxidation, applied in high-efficiency exhaust gas deep oxidation;
4) Bivalent Pd⁰/Pd²⁺ synergistic sites: The optimal active structure for most industrial reactions, realizing cyclic electron transfer to reduce reaction activation energy.
