Reactive Oxygen Species on the (100) Facet of Cobalt Spinel Nanocatalyst and their Relevance in 16O2/18O2 Isotopic Exchange, deN2O, and deCH4 Processes鈥擜 Theoretical and Experimental Account

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• 作者：Filip Zasada ; Witold Piskorz ; Janusz Janas ; Joanna Grybo艣 ; Paulina Indyka ; Zbigniew Sojka
• 刊名：ACS Catalysis
• 出版年：2015
• 出版时间：November 6, 2015
• 年：2015
• 卷：5
• 期：11
• 页码：6879-6892
• 全文大小：940K
• ISSN：2155-5435

文摘

Periodic spin unrestricted, gradient corrected DFT calculations joined with atomistic thermodynamic modeling and experiment were used to study the structure and stability of various reactive oxygen species (ROS) and oxygen vacancies produced on the most stable terminations of the cobalt spinel (100) surface. The surface state diagram of oxygen in a wide range of pressures and temperatures was constructed for coverage varying from 螛_O = 1.51 atom路nm^鈥? to 螛_O = 6.04 atom路nm^鈥?. A large variety of the unraveled surface ROS includes diatomic superoxo (Co^O鈥揙₂^{鈥?/sup>鈥揅oO), peroxo (CoT鈥揙₂2鈥?/sup>鈥揅oO), and spin paired (CoO鈥揙₂鈥揅oO) adducts along with monatomic metal-oxo (CoT鈥揙+, CoO鈥揙2+) species, where CoT and CoO stand for the tetrahedral and octahedral cobalt surface centers, respectively. There are also two kinds of peroxo species associated with surface oxygen ions connected with 3CoO or 2CoO and 1CoT cations ((O_2O,1T鈥揙)2鈥?/sup> and (O_3O鈥揙)2鈥?/sup>), respectively). The results revealed that in the oxygen pressure range of typical catalytic reactions (p_O2/p掳 from 鈭?.01 to 1), the most stable stoichiometric (100)-S surface accommodates the CoT鈥揙₂2鈥?/sup>鈥揅oO and CoO鈥揙₂鈥揅oO adducts at temperatures below 250鈥?00 掳C. In the temperature from 250 to 300 掳C and from 550 to 700 掳C, it is covered by the O species associated with the exposed tetrahedral cobalt sites (CoT鈥揙+) or remains in a bare state. In more reducing conditions (T > 550鈥?00 掳C), the (100)-S facet is readily defected due to trigonal oxygen (O_2O,1T) release and formation of surface oxygen vacancies. The reactivity of surface ROS was tested in 16O₂/18O₂ isotopic exchange, N₂O decomposition, and oxidation of CH₄ and CO model reactions, carried over Co₃O₄ and Co₃18O₄ nanocrystalline samples with the predominant (100) faceting revealed by high angle angular dark field STEM examination. The CoO鈥揙2+ adducts associated with octahedral cobalt sites, as well as the peroxo (O_2O,1T鈥揙)2鈥?/sup> and (O_3O鈥揙)2鈥?/sup> surface species being thermodynamically unstable are involved in surface oxygen recombination processes, probed by 16O₂/18O₂ exchange and N₂O decomposition. It was shown that at low temperatures CO is oxidized by the suprafacial CoO鈥揙₂鈥揅oO and CoT鈥揙₂鈥揅oO diatomic oxygen, whereas in CH₄ activation, the highly reactive cobalt-oxo species (CoT鈥揙+) are involved. Above 600 掳C at p_O2/p掳 = 0.01, due to the onset of oxygen vacancy formation, the suprafacial methane oxidation gradually changes into the intrafacial Mars-van Krevelen scheme. The constructed surface phase diagram was used for rationalization of the obtained catalytic data, allowing delineation of the specific role of the chemical state of the cobalt spinel surface in the investigated processes, as well as the range of the corresponding temperatures and oxygen pressures. It also provides a convenient background for molecular understanding of remarkable activity of Co₃O₄ in many other catalytic redox reactions.}