Nanofaceted Pd-O Sites in Pd-Ce Surface Superstructures: Enhanced Activity in Catalytic Combustion of Methane
Nanofaceted Pd-O Sites in Pd-Ce Surface Superstructures: Enhanced Activity in Catalytic Combustion of Methane
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DOI：
10.1002/anie.200903581
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发表时间：
2009-01-01
影响因子：
16.6
通讯作者：
Trovarelli, Alessandro
中科院分区：
文献类型：
--
作者：
Colussi, Sara;Gayen, Arup;Trovarelli, Alessandro
能源和环境方面的挑战要求开发高活性催化剂，以便更有效、更清洁地利用能源供应甲烷催化燃烧是预防和净化排放的领先技术与传统的火焰燃烧相比，它的主要优点是在低温下稳定燃料的完全氧化，同时控制NOx的排放。在低温下产生最高活性的催化剂是由分散在高表面积氧化物载体上的贵金属组成的。分散在氧化物载体上的PdO颗粒是最活跃的甲烷燃烧催化剂，但由于PdO颗粒在低温（低于673 K）时活性不足，在高温（高于973 K）时活性失活这种转变受反应条件下PdO生成和分解成Pd的复杂动力学调控，该动力学受温度和反应混合物的影响避免这种转变的一种可能性是将离子形式的钯分散在氧化物载体上。贵金属作为离子基团在可还原性载体如铈（CeO2）上的稳定作用已被证明对一些反应是有效的，如水气转移反应和总氧化反应，而铈在高度分散状态下稳定钯的能力也得到了广泛的认可将贵金属插入金属氧化物晶格将导致给定金属负载的最高分散程度，在几种催化应用中具有重要的后果。在贵金属/铈体系的早期研究中，曾报道过由于金属-载体之间的强相互作用，在铈中分离出包封的金属钯。[6, 7]张晓燕。基于Ce0组分PdO/CeO2的固溶体。99 pd0。01O2Àδ或Ce0。76 zr0。19 pd0。05O2Àδ最近被报道，并发现在CO/NO反应和甲烷燃烧中活跃；最近的密度泛函理论（DFT）计算也证实了这一发现，表明将Pd插入CeO2表面为甲烷的解离吸附提供了较低的能垒然而，钯取代的二氧化铈的稳定是困难的，并且在高温下通常可以观察到钯从氧化物中分离出来形成PdO或金属钯晶体本文报道了基于高分辨率（HR） TEM数据的DFT计算揭示的有序稳定的Pd-O-Ce表面上层结构。它是由（110）CeO2表面的复杂重建引起的，并导致宽的表面通道开放，暴露出高度欠配位的氧原子。采用一步溶液燃烧合成（SCS）法制备了两种Pd/CeO2催化剂。新催化剂的Pd含量在1 ~ 1.71 wt%之间，记为SCS1和SCS2（表1）。我们还采用初湿浸渍法制备了常规Pd/CeO2催化剂样品。这些催化剂是由两种不同的商业二氧化铈样品制备的，其额定负载为1%和1.75 wt% Pd。分别记为IWI1和IWI2（表1中的样本a和样本b）。所有材料均表现出优异的催化燃烧活性。得到的活化能值与文献中报道的不同钯基催化剂的活化能值相似在温度为553 K的条件下测定了所有样品的反应速率
Challenges in energy and the environment call for the development of highly active catalysts, allowing for a more efficient and cleaner use of energy supplies.[1] Catalytic combustion of methane is a leading technology in emission prevention and cleanup.[2] Its main advantage over traditional flame combustion is to stabilize complete oxidation of fuel at low temperature while simultaneously controlling NOx emissions. Catalysts yielding the highest activity at low temperatures consist of noble metals dispersed on high-surface-area oxide supports. PdO particles dispersed on oxide carriers are the most active methane combustion catalysts, but they still suffer from inadequate activity at low temperature (below 673 K) and deactivation at high temperature (above 973 K) owing to formation of metallic Pd from PdO particles.[3] This transformation is regulated by a complex dynamic of formation and decomposition of PdO to Pd under reaction conditions, which is affected by the temperature and the reaction mixture.[4] One possibility for avoiding this transformation is to disperse Pd already in the ionic form over an oxide support. Stabilization of precious metals as ionic moieties over reducible supports such as ceria (CeO2) has been shown to be effective for several reactions, such as the water–gas shift reaction and total oxidation,[5] and the ability of ceria to stabilize Pd in a highly dispersed state is wellrecognized.[6] Insertion of the precious metal into the metal oxide lattice would lead to the highest degree of dispersion for a given metal loading, with important consequences in several catalytic applications. Isolated encapsulated Pd metal in ceria as a result of a strong metal–support interaction was reported in early studies of noble-metal/ceria systems.[6, 7] Solid solutions based on PdO/CeO2 of composition Ce0. 99Pd0. 01O2Àδ or Ce0. 76Zr0. 19Pd0. 05O2Àδ were reported more recently and found to be active in CO/NO reaction and methane combustion;[8] this finding is also corroborated by recent density functional theory (DFT) calculations suggesting that insertion of Pd into CeO2 surfaces provides a lower energy barrier for dissociative adsorption of methane.[9] However, stabilization of Pdsubstituted ceria is difficult, and Pd segregation out of the oxide to form PdO or metallic Pd crystallites is commonly observed at high temperatures.[8] Herein we report an ordered and stable Pd-O-Ce surface superstructure as revealed by DFT calculations on the basis of high-resolution (HR) TEM data. It results from a complex reconstruction of the (110) CeO2 surface and leads to the opening of wide surface channels exposing highly undercoordinated oxygen atoms.We have prepared two Pd/CeO2 catalysts by one-step solution combustion synthesis (SCS). The new catalysts contain between 1 and 1.71 wt% Pd and are denoted SCS1 and SCS2 (Table 1). We also prepared samples of conventional Pd/CeO2 catalysts by incipient wetness impregnation (IWI). These catalysts were prepared from two different samples of commercial ceria and had a nominal loading of 1 and 1.75 wt% Pd. They are denoted as IWI1 and IWI2 (samples a and b in Table 1). All the materials showed excellent catalytic combustion activity. The values obtained for activation energies are similar to those reported in literature for different palladiumbased catalysts.[10] Reaction rates were measured for all samples at a temperature of 553 K under differential con-