Understanding the CO Preoxidation and the Intrinsic Catalytic Activity of Step Sites in Stepped Pt Surfaces in Acidic Medium
Understanding the CO Preoxidation and the Intrinsic Catalytic Activity of Step Sites in Stepped Pt Surfaces in Acidic Medium
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DOI：
10.1021/acs.jpcc.5b05386
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发表时间：
2015-09-03
影响因子：
3.7
通讯作者：
Feliu, Juan M.
中科院分区：
文献类型：
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作者：
Farias, Manuel J. S.;Camara, Giuseppe A.;Feliu, Juan M.
为了加深对CO预氧化过程的起源和铂表面步骤对CO氧化的内在催化活性的认识，在酸性介质中对阶梯状铂电极进行了一系列的CO反萃实验。对于CO预氧化的发生，发现只要同时满足四个相互关联的条件：(1)CO在低于0.2V的电位下吸附；(2)在饱和的CoAds表面上；(3)在溶液中存在微量CO的情况下；(4)在存在表面台阶的情况下，发生CO预氧化。如果这四个条件中的任何一个不满足，即使电极表面的台阶完全被CO覆盖，也不会出现CO预氧化途径。通过控制CO附着层的去除(伏安法)，我们发现，一旦CO附着层被部分氧化，台阶表面的(111)梯形位比(110)台阶位更早被释放。此外，如果选择性地用CO修饰(110)晶面，则其氧化仅在比CO预氧化峰高150 mV的电位下发生。我们的结果系统地证明了阶跃中心对CO的氧化活性低于那些负责CO预氧化过程的中心。一旦负责CO预氧化的位置被释放，剩余的吸附的CO层没有明显的运动，这表明表面的激活控制了整个过程，而不是Coads向假设的“最活跃的位置”扩散。在部分覆盖的CO附着层上进行的伏安和计时电流法实验表明，吸附的CO在其氧化过程中表现为一种静止的物种，其中CO附着层被逐段移除。通过原位FTIR实验，考察了CO选择性吸附在(110)台阶位上的伸缩频率。谱带频率结果证实，当表面达到全覆盖时，这些吸附在台阶上的分子与吸附在(111)平台上的CO完全耦合。
In order to deepen the knowledge about the origin of the CO preoxidation process and the intrinsic catalytic activity of Pt superficial steps toward CO oxidation, a series of CO stripping experiments were performed on stepped Pt electrodes in acidic medium. For the occurrence of CO preoxidation, it was found that it arises (reproducibly) whenever four interconnected conditions are simultaneously fulfilled: (1) CO adsorption at potentials lower than about 0.2 V; (2) on surfaces saturated with COads; (3) in the presence of traces of CO in solution; (4) in the presence of surface steps. If any of these four conditions is not satisfied, the CO preoxidation pathway does not appear, even though the steps on the electrode surface are completely covered by CO. By controlling the removal of the CO adlayer (voltammetrically), we show that once the CO adlayer has been partially oxidized, the (111) terrace sites of stepped surfaces are released earlier than the (110) step sites. Moreover, if (110) steps are selectively decorated with CO, its oxidation occurs only at potentials similar to 150 mV higher than the CO preoxidation peak. Our results systematically demonstrate that step sites are less active to oxidize CO than those ones responsible for the CO preoxidation process. Once the sites responsible for the CO preoxidation are made free, there is no apparent motion of the remaining adsorbed CO layer, suggesting that the activation of the surface controls the whole process, rather than the diffusion of COads toward hypothetically "most active sites". Voltammetric and chronoamperometric experiments performed on partially covered CO adlayers suggest that adsorbed CO behave as a motionless species during its oxidation, in which the CO adlayer is removed piece by piece. By means of in situ FTIR experiments, the stretching frequency of CO selectively adsorbed on (110) step sites was examined. Band frequency results confirm that those molecules adsorbed on steps are fully coupled with the adsorbed CO on (111) terraces when the surface reaches full coverage.