Water-Gas Shift Reaction on a Highly Active Inverse CeOx/Cu(111) Catalyst: Unique Role of Ceria Nanoparticles
Water-Gas Shift Reaction on a Highly Active Inverse CeOx/Cu(111) Catalyst: Unique Role of Ceria Nanoparticles
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DOI：
10.1002/anie.200903918
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发表时间：
2009-01-01
影响因子：
16.6
通讯作者：
Hrbek, Jan
中科院分区：
文献类型：
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作者：
Rodriguez, Jose A.;Graciani, Jesus;Hrbek, Jan
水煤气变换（WGS，CO+ H2O！CO2+ H2）是化学工业中经常用于生产清洁H2的重要反应。[1]氧化物负载的铜催化剂显示出显著的水煤气变换活性，但它们的性能尚未完全了解，并且高度依赖于合成条件或氧化物载体的性质。[1-4]金属Cu或Cu+阳离子已被提议作为WGS的活性位点。[1-8]此外，氧化物载体可能不是简单的旁观者，并且可能在催化过程中发挥直接作用。[1，4，9]金属铜的扩展表面和纳米颗粒能够催化WGS [3，5，6，10，11]，Cu（111）已成为研究WGS的基准表面。[3，5，11-14]比速率、活化能和反应级数与ZnO负载的Cu催化剂报道的数据一致。[5，13] Cu（111）上WGS的限速步骤似乎是水的解离，[11-13]并且它受到表面改性剂如S，O或Cs的存在的影响。[13]在此，我们研究了部分被氧化铈纳米颗粒覆盖的Cu（111）表面上的WGS反应。块状二氧化铈是用于WGS催化剂的公知氧化物载体。[1，4，8，9] CeOx/Cu（111）系统使我们能够详细研究氧化物在催化过程中的作用，此外，具有反氧化物/金属构型的催化剂在实际应用中具有一些优势。[15]图1示出了在650 K下在O2气氛（p% 5 1017 Torr）下向Cu（111）中加入Ce之后获得的扫描隧道显微镜（STM）的图像。O2与Cu（111）衬底的反应导致形成氧化铜层，该氧化铜层表现出Cu 2 O（111）表面的不同重构域。[16]在氧化铜层的顶部，有两种类型的二氧化铈特征。这些特征在Cu（111）吸附O2的空白实验中没有观察到。在图1中，小的二氧化铈岛（尺寸为2-5 nm，CeOx-I）出现在铜衬底的台阶上，而大的二氧化铈岛（尺寸为30-50 nm，三角形形状，CeOx-II）嵌入在衬底台阶边缘中。在大的二氧化铈岛内部，可以看到波纹图案，其中凹陷之间的间隔为约5 nm。大的二氧化铈岛的形态与块状二氧化铈的两个最稳定表面CeO 2（111）和CeO 2（110）不同。对这些岛的结构的分析表明，它们基本上具有0.31 nm的高度，这与夹在两层氧之间的单层铈一致。[17]可能存在O-Ce-O-Cu（界面）堆叠。CeOx/Cu（111）中的氧化物纳米颗粒显示出与之前使用相同的沉积方法在Au（111）衬底上生长二氧化铈纳米颗粒所发现的形态完全不同的形态。[17]在CeOx/Au（111）系统中，在台阶边缘处仅存在大的氧化物岛，并且这些氧化物岛暴露了块状二氧化铈的（111）面。[17]的相互作用
The water-gas shift (WGS, CO+ H2O! CO2+ H2) is an important reaction frequently used in the chemical industry for the production of clean H2.[1] Oxide-supported copper catalysts show significant water-gas shift activity but their performance is not fully understood and is highly dependent on the synthesis conditions or the nature of the oxide support.[1–4] Either metallic Cu or Cu+ cations have been proposed as active sites for the WGS.[1–8] In addition, the oxide support may not be a simple spectator and may play a direct role in the catalytic process.[1, 4, 9] Extended surfaces and nanoparticles of metallic copper are able to catalyze the WGS [3, 5, 6, 10, 11] and Cu (111) has become a benchmark surface for studying the WGS.[3, 5, 11–14] Specific rates, activation energies, and reaction orders are consistent with data reported for ZnO-supported Cu catalysts.[5, 13] The rate-limiting step for the WGS on Cu (111) seems to be the dissociation of water,[11–13] and it is affected by the presence of surface modifiers such as S, O, or Cs.[13] Herein, we investigate the WGS reaction on Cu (111) surfaces partially covered with ceria nanoparticles. Bulk ceria is a well known oxide support for WGS catalysts.[1, 4, 8, 9] The CeOx/Cu (111) system allows us to study in detail the role of the oxide in the catalytic process and, furthermore, catalysts with an inverse oxide/metal configuration have some advantages for practical applications.[15] Figure 1 shows images of scanning tunneling microscopy (STM) acquired after dosing Ce to Cu (111) at 650 K under an atmosphere of O2 (p% 5 10À7 Torr). The reaction of O2 with the Cu (111) substrate leads to formation of a layer of copper oxide which exhibits domains of different reconstructions of a Cu2O (111) surface.[16] On top of the layer of copper oxide, there are two types of ceria features. These features were not seen in blank experiments for the adsorption of O2 on Cu (111). In Figure 1, small islands of ceria (2–5 nm in size, CeOx-I) appear on the terraces of the copper substrate, whereas large islands of ceria (30–50 nm in size, triangular shape, CeOx-II) are embedded in the substrate step edges. Inside the large ceria islands, a moirØ pattern can be seen with a separation of approximately 5 nm in between the depressions. The large ceria islands have a morphology different from that seen for the two most stable surfaces of bulk ceria: CeO2 (111) and CeO2 (110). An analysis of the structure of these islands reveals that they essentially have a height of 0.31 nm, which is consistent with a single layer of cerium sandwiched in between two layers of oxygen.[17] An O-Ce-O-Cu (interface) stacking is likely. The oxide nanoparticles in CeOx/Cu (111) display a morphology quite different from that found before for the growth of ceria nanoparticles on a Au (111) substrate using the same deposition methodology.[17] In the CeOx/Au (111) systems there were only large oxide islands at the step edges and these were exposing the (111) face of bulk ceria.[17] The interactions of