Johnson Grant E, Reveles J Ulises, Reilly Nelly M, Tyo Eric C, Khanna Shiv N, Castleman A W
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
J Phys Chem A. 2008 Nov 13;112(45):11330-40. doi: 10.1021/jp805186r. Epub 2008 Oct 15.
Cationic and anionic cobalt oxide clusters, generated by laser vaporization, were studied using guided-ion-beam mass spectrometry to obtain insight into their structure and reactivity with carbon monoxide. Anionic clusters having the stoichiometries Co2O3(-), Co2O5(-), Co3O5(-) and Co3O6(-) were found to exhibit dominant products corresponding to the transfer of a single oxygen atom to CO, indicating the formation of CO 2. Cationic clusters, in contrast, displayed products resulting from the adsorption of CO onto the cluster accompanied by the loss of either molecular O 2 or cobalt oxide units. In addition, collision induced dissociation experiments were conducted with N 2 and inert xenon gas for the anionic clusters, and xenon gas for the cationic clusters. It was found that cationic clusters fragment preferentially through the loss of molecular O 2 whereas anionic clusters tend to lose both atomic oxygen and cobalt oxide units. To further analyze how stoichiometry and ionic charge state influence the structure of cobalt oxide clusters and their reactivity with CO, first principles theoretical electronic structure studies within the density functional theory framework were performed. The calculations show that the enhanced reactivity of specific anionic cobalt oxides with CO is due to their relatively low atomic oxygen dissociation energy which makes the oxidation of CO energetically favorable. For cationic cobalt oxide clusters, in contrast, the oxygen dissociation energies are calculated to be even lower than for the anionic species. However, in the cationic clusters, oxygen is calculated to bind preferentially in a less activated molecular O 2 form. Furthermore, the CO adsorption energy is calculated to be larger for cationic clusters than for anionic species. Therefore, the experimentally observed displacement of weakly bound O 2 units through the exothermic adsorption of CO onto positively charged cobalt oxides is energetically favorable. Our joint experimental and theoretical findings indicate that positively charged sites in bulk-phase cobalt oxides may serve to bind CO to the catalyst surface and specific negatively charged sites provide the activated oxygen which leads to the formation of CO 2. These results provide molecular level insight into how size, stoichiometry, and ionic charge state influence the oxidation of CO in the presence of cobalt oxides, an important reaction for environmental pollution abatement.
利用激光蒸发产生阳离子和阴离子钴氧化物团簇，通过导向离子束质谱对其进行研究，以深入了解它们的结构以及与一氧化碳的反应活性。发现化学计量比为Co2O3(-)、Co2O5(-)、Co3O5(-)和Co3O6(-)的阴离子团簇表现出主要产物，对应于单个氧原子向CO的转移，表明形成了CO2。相比之下，阳离子团簇显示出的产物是由于CO吸附到团簇上，同时伴随着分子O2或钴氧化物单元的损失。此外，对阴离子团簇进行了与N2和惰性氙气的碰撞诱导解离实验，对阳离子团簇进行了与氙气的碰撞诱导解离实验。发现阳离子团簇优先通过分子O2的损失而碎片化，而阴离子团簇倾向于同时失去原子氧和钴氧化物单元。为了进一步分析化学计量比和离子电荷状态如何影响钴氧化物团簇的结构及其与CO的反应活性，在密度泛函理论框架内进行了第一性原理理论电子结构研究。计算表明，特定阴离子钴氧化物与CO反应活性增强是由于其相对较低的原子氧解离能，这使得CO的氧化在能量上是有利的。相比之下，对于阳离子钴氧化物团簇，计算出的氧解离能甚至比阴离子物种的还要低。然而，在阳离子团簇中，计算表明氧优先以活性较低的分子O2形式结合。此外，计算得出阳离子团簇的CO吸附能比阴离子物种的更大。因此，通过CO在带正电的钴氧化物上的放热吸附，实验观察到的弱结合O2单元的取代在能量上是有利的。我们联合的实验和理论研究结果表明，体相钴氧化物中的带正电位点可能用于将CO结合到催化剂表面，而特定的带负电位点提供活性氧，从而导致CO2的形成。这些结果为尺寸、化学计量比和离子电荷状态如何影响钴氧化物存在下CO的氧化提供了分子水平的见解，这是减少环境污染的一个重要反应。
