Metal nanoparticles can be used as catalyst in various chemical reactions, especially in the decomposition of the toxic such as dyes, pesticides and so on. It is significant to prevent the aggregation of metal nanoparticles caused by the higher surface energy. Hydrogels with 3-dimention network structure is a soft solid-like material. The utilization of environmentally benign hydrogel networks provides two advantages: one is a template in which metal nanoparticles prepared are prevented from aggregation, and the other one is the properties of recovery and reuse of metal nanoparticles are sharply enhanced by incorporating them into hydrogels. In this review, we mainly focus on the preparation methods and performances of hydrogel/metal nanoparticles composite catalysts from two aspects: natural hydrogels and synthetic hydrogels. Based on the work of our own laboratory, the effects of the performances of hydrogel/metal nanoparticles composite catalysts are summarized as well as the existing problems and further research directions are discussed.

Contents
1 Introduction
2 Application of natural hydrogels in catalytic reactions
2.1 Agarose hydrogel based catalysts
2.2 Chitosan hydrogel based catalysts
2.3 Alginate hydrogel based catalysts
3 Application of synthetic hydrogels in catalytic reactions
3.1 The copolymer of ethylene glycol hydrogel based catalysts
3.2 Polymers(copolymers) of acrylic acid (acrylamide) hydrogel based catalysts
4 Conclusion and outlook