Precious metals mainly refer to gold (Au), silver(Ag), platinum (Pt) group : ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), platinum (Pt). Precious metals are widely used in industry due to their excellent physical and chemical properties (such as: high temperature oxidation resistance and corrosion resistance), electrical properties (excellent electrical conductivity, high temperature thermo-electric properties and stable resistance temperature coefficient etc.), high catalytic activity and strong coordination ability etc.. It is called the modern "industrial vitamin" because of the application of the "less, small, fine, wide".