Noble metal ionic catalysts

Abstract

Because of growing environmental concerns and increasingly stringent regulations governing auto emissions, new more efficient exhaust catalysts are needed to reduce the amount of pollutants released from internal combustion engines. To accomplish this goal, the major pollutants in exhaust-CO, NO_x, and unburned hydrocarbons-need to be fully converted to CO₂, N₂, and H₂O. Most exhaust catalysts contain nanocrystalline noble metals (Pt, Pd, Rh) dispersed on oxide supports such as Al₂O₃ or SiO₂ promoted by CeO₂. However, in conventional catalysts, only the surface atoms of the noble metal particles serve as adsorption sites, and even in 4-6 nm metal particles, only 1/4 to 1/5 of the total noble metal atoms are utilized for catalytic conversion. The complete dispersion of noble metals can be achieved only as ions within an oxide support. In this Account, we describe a novel solution to this dispersion problem: a new solution combustion method for synthesizing dispersed noble metal ionic catalysts. We have synthesized nanocrystalline, single-phase Ce_1-xM_xO_2-δ and Ce_1-x-yTi_yM_xO_2-δ (M = Pt, Pd, Rh; x = 0.01-0.02, δ ≈ x, y = 0.15-0.25) oxides in fluorite structure. In these oxide catalysts, Pt²⁺, Pd²⁺, or Rh³⁺ ions are substituted only to the extent of 1-2% of Ce⁴⁺ ion. Lower-valent noble metal ion substitution in CeO₂ creates oxygen vacancies. Reducing molecules (CO, H₂, NH₃) are adsorbed onto electron-deficient noble metal ions, while oxidizing (O₂, NO) molecules are absorbed onto electron-rich oxide ion vacancy sites. The rates of CO and hydrocarbon oxidation and NO_x reduction (with >80% N₂ selectivity) are 15-30 times higher in the presence of these ionic catalysts than when the same amount of noble metal loaded on an oxide support is used. Catalysts with palladium ion dispersed in CeO₂ or Ce_1-xTi_xO₂ were far superior to Pt or Rh ionic catalysts. Therefore, we have demonstrated that the more expensive Pt and Rh metals are not necessary in exhaust catalysts. We have also grown these nanocrystalline ionic catalysts on ceramic cordierite and have reproduced the results we observed in powder material on the honeycomb catalytic converter. Oxygen in a CeO₂ lattice is activated by the substitution of Ti ion, as well as noble metal ions. Because this substitution creates longer Ti-O and M-O bonds relative to the average Ce-O bond within the lattice, the materials facilitate high oxygen storage and release. The interaction among M⁰/Mⁿ⁺, Ce⁴⁺/Ce³⁺, and Ti⁴⁺/Ti³⁺ redox couples leads to the promoting action of CeO₂, activation of lattice oxygen and high oxygen storage capacity, metal support interaction, and high rates of catalytic activity in exhaust catalysis.