Hegde M S, Madras Giridhar, Patil K C
Solid State and Structural Chemistry Unit, Indian Institute of Science Bangalore 560012, India.
Acc Chem Res. 2009 Jun 16;42(6):704-12. doi: 10.1021/ar800209s.
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.
由于对环境问题的日益关注以及针对汽车尾气排放的法规日益严格，需要新型的更高效的尾气催化剂来减少内燃机排放的污染物量。为实现这一目标，尾气中的主要污染物——一氧化碳（CO）、氮氧化物（NO(x)）和未燃烧的碳氢化合物——需要完全转化为二氧化碳（CO₂）、氮气（N₂）和水（H₂O）。大多数尾气催化剂包含分散在氧化铝（Al₂O₃）或二氧化硅（SiO₂）等氧化物载体上的纳米晶贵金属（铂（Pt）、钯（Pd）、铑（Rh）），并由二氧化铈（CeO₂）促进。然而，在传统催化剂中，只有贵金属颗粒的表面原子作为吸附位点，即使在4 - 6纳米的金属颗粒中，也只有1/4到1/5的总贵金属原子用于催化转化。只有当贵金属以离子形式存在于氧化物载体中时，才能实现其完全分散。在本综述中，我们描述了一种解决这种分散问题的新方法：一种用于合成分散的贵金属离子催化剂的新型溶液燃烧法。我们已经合成了萤石结构的纳米晶单相Ce(1 - x)M(x)O(2 - δ)和Ce(1 - x - y)Ti(y)M(x)O(2 - δ)（M = Pt、Pd、Rh；x = 0.01 - 0.02，δ≈x，y = 0.15 - 0.25）氧化物。在这些氧化物催化剂中，Pt(2+)、Pd(2+)或Rh(3+)离子仅取代Ce(4+)离子的1 - 2%。CeO₂中低价贵金属离子的取代产生了氧空位。还原分子（CO、H₂、NH₃）吸附在缺电子的贵金属离子上，而氧化分子（O₂、NO）吸附在富电子的氧化物离子空位位点上。在这些离子催化剂存在下，CO和碳氢化合物的氧化速率以及NO(x)的还原速率（N₂选择性>80%）比使用相同量负载在氧化物载体上的贵金属时高15 - 30倍。钯离子分散在CeO₂或Ce(1 - x)Ti(x)O₂中的催化剂远优于铂或铑离子催化剂。因此，我们证明了在尾气催化剂中不需要更昂贵的铂和铑金属。我们还在陶瓷堇青石上生长了这些纳米晶离子催化剂，并在蜂窝状催化转化器上重现了我们在粉末材料中观察到的结果。CeO₂晶格中的氧通过钛离子以及贵金属离子的取代而被活化。因为这种取代相对于晶格内平均Ce - O键产生了更长的Ti - O和M - O键，这些材料有利于高氧存储和释放。M(0)/M(n+)、Ce(4+)/Ce(3+)和Ti(4+)/Ti(3+)氧化还原对之间的相互作用导致了CeO₂的促进作用、晶格氧的活化和高氧存储容量、金属载体相互作用以及尾气催化中的高催化活性速率。
