Reversed Hysteresis during CO Oxidation over Pd75Ag25(100)
Reversed Hysteresis during CO Oxidation over Pd75Ag25(100)
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DOI：
10.1021/acscatal.6b00658
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发表时间：
2016-07-01
期刊：
影响因子：
12.9
通讯作者：
Borg, Anne
中科院分区：
文献类型：
--
作者：
Fernandes, Vasco R.;Van den Bossche, Maxime;Borg, Anne
用近常压X射线光电子能谱、四极杆质谱、密度泛函理论计算和微观动力学模拟相结合的方法研究了Pd(100)和Pd75Ag25(100)上的CO氧化反应。对于这两个表面，在加热和冷却循环中观察到了二氧化碳生成的滞后现象。Pd(100)存在起燃温度高于消光温度的正常磁滞，而Pd75Ag25(100)则出现反向磁滞。反转磁滞可以用表面成分的动态变化来解释。在加热坡道开始时，表面富含钯，这提供了一种CO覆盖，使表面有毒，直到解吸率变得足够高。在没有吸附物的情况下，对富银表面的热力学偏好促进了作为CO解吸的银从体相向表面的扩散。在冷却坡道期间，当温度太低时，表面覆盖率很高，无法有效地将银扩散回块体中。表面银的浓度过高导致了较高的消光温度，从而产生了反向磁滞。
CO oxidation over Pd(100) and Pd75Ag25(100) has been investigated by a combination of near-ambient pressure X-ray photoelectron spectroscopy, quadrupole mass spectrometry, density functional theory calculations, and microkinetic modeling. For both surfaces, hysteresis is observed in the CO2 formation during the heating and cooling cycles. Whereas normal hysteresis with light-off temperature higher than extinction temperature is present for Pd(100), reversed hysteresis is observed for Pd75Ag25(100). The reversed hysteresis can be explained by dynamic changes in the surface composition. At the beginning of the heating ramp, the surface is rich in palladium, which gives a CO coverage that poisons the surface until the desorption rate becomes sufficiently high. The thermodynamic preference for an Ag-rich surface in the absence of adsorbates promotes diffusion of Ag from the bulk to the surface as CO desorbs. During the cooling ramp, an appreciable surface coverage is reached at temperatures too low for efficient diffusion of Ag back into the bulk. The high concentration of Ag in the surface leads to a high extinction temperature and, consequently, the reversed hysteresis.