Selective catalytic reduction of NO_x with hydrocarbons (CH-SCR-NO_x) under oxygen-rich condition has been widely studied for its potential to clean up exhausts from lean-burn engines. Hydrocarbons are readily available reducing agents which can provide non-polluting processes. The research advances in reaction mechanism of selective catalytic reduction of NO_x by hydrocarbons and the properties of copper-free metal (Fe, Co, In, Mn, Pd, etc.) catalysts have been reviewed in this article. The technical trends of copper-free metal supported zeolite are prospected. The supported metal, the zeolite and the reductant correspond to their own active sites. The active sites of selective catalytic reduction of NO_x with propane on Fe-ZSM-5 zeolite are isolated Fe²⁺ and Fe³⁺, yet the active sites are isolated Fe³⁺ and oligonuclear Fe_xO_y on Fe-AlPO-5 zeolite with methane as reductant. Methane which is the preferred catalyst for removing nitrogen oxides is present in almost all combustion exhausts, while hydrocarbons such as ethene and propane have also been extensively studied due to their lower reduction temperature. The formation of key intermediates such as isocyanate and cyanide species is a necessary process during catalytic reaction. Study on reaction mechanism can help us to identify the key factors that influence the reaction and then optimize the catalytic processes. Efficient and environmentally friendly denitration catalyst system can be developed according to the specific characteristics of the supported metal, the zeolite and the reductant.

Contents
1 Introduction
2 CH-SCR-NO_x reaction mechanism over copper-free metal supported zeolite
2.1 Reaction mechanism over iron supported zeolite
2.2 Reaction mechanism over Co-ZSM-5 zeolite
2.3 Reaction mechanism over Mn-ZSM-5 zeolite
3 Catalytic properties of copper-free metal supported zeolite with different reductant systems
3.1 CH₄-SCR-NO_x reaction
3.2 C_yH_z-SCR-NO_x reaction (y≥2)
4 Conclusion and outlook