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 CO(2), N(2), and H(2)O. Most exhaust catalysts contain nanocrystalline noble metals (Pt, Pd, Rh) dispersed on oxide supports such as Al(2)O(3) or SiO(2) promoted by CeO(2). 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-x)M(x)O(2-delta) and Ce(1-x-y)Ti(y)M(x)O(2-delta) (M = Pt, Pd, Rh; x = 0.01-0.02, delta approximately x, y = 0.15-0.25) oxides in fluorite structure. In these oxide catalysts, Pt(2+), Pd(2+), or Rh(3+) ions are substituted only to the extent of 1-2% of Ce(4+) ion. Lower-valent noble metal ion substitution in CeO(2) creates oxygen vacancies. Reducing molecules (CO, H(2), NH(3)) are adsorbed onto electron-deficient noble metal ions, while oxidizing (O(2), NO) molecules are absorbed onto electron-rich oxide ion vacancy sites. The rates of CO and hydrocarbon oxidation and NO(x) reduction (with >80% N(2) 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 CeO(2) or Ce(1-x)Ti(x)O(2) 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 CeO(2) 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 M(0)/M(n+), Ce(4+)/Ce(3+), and Ti(4+)/Ti(3+) redox couples leads to the promoting action of CeO(2), activation of lattice oxygen and high oxygen storage capacity, metal support interaction, and high rates of catalytic activity in exhaust catalysis.

Surface sites on Pt–CeO2mixed oxide catalysts probed by CO adsorption: a synchrotron radiation photoelectron spectroscopy study

“…[1][2][3][4][5][6][7] The atomically dispersed supported noble metal catalysts represent a new generation of cost-effective catalytic materials. [1][2][3][4][5][6][7] The efficient use of noble metals requires the noble metal atoms to be anchored at the surface of the catalyst. The thermal stability of such systems must be high enough to prevent sintering and dissolution of the noble metal into the bulk.…”

“…Pt-CeO 2 mixed oxides that contain ionic Pt species can be prepared by several methods, including solution combustion, 3,4,9,10 r.f. magnetron sputtering, [11][12][13] and co-deposition of Pt and Ce in the oxygen atmosphere.…”

Noble metal ion-substituted ceria as efficient oxygen storage catalysts for clean combustion of fuels

“…The rates of conversion are 20-30 times higher than the corresponding metal particles impregnated catalysts [10] and their activation energies could be obtained from Arrhenius plots of the temperature dependence of reaction rates. These catalysts have been tested for NO x reduction also [11]. Using first principle density functional theory (DFT), it is shown that CO adsorption in these catalysts occurs at the ionic sites of Pd 2? , with a decrease in the net energy of the system [12].…”

Synthesis of metal oxides with improved performance using a solvothermal method

“…CeO 2 colloidal particles obtained by the solvothermal method at 250°C were coagulated with 1 mol/L solutions of various bases: NaOH, Na 2 CO 3 and NaHCO 3 . After calcination at 300°C of the coagulated products, the CeO 2 powders so-obtained had almost the same crystallite sizes (45 nm), while their BET surface areas varied considerably in the range from 61122 m 2 /g.…”