Adsorption and reaction of NO2 on ordered alumina films and mixed baria-alumina nanoparticles: Cooperative versus non-cooperative reaction mechanisms
Adsorption and reaction of NO2 on ordered alumina films and mixed baria-alumina nanoparticles: Cooperative versus non-cooperative reaction mechanisms
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DOI：
10.1016/j.jcat.2008.09.020
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发表时间：
2008-12-10
影响因子：
7.3
通讯作者：
Libuda, Joerg
中科院分区：
文献类型：
--
作者：
Desikusumastuti, Aine;Staudt, Thorsten;Libuda, Joerg
为了深入了解NO2在氧化铝和钡氧化物表面吸附和反应的机理和动力学，我们在单晶基模型材料上进行了系统的吸附实验。作为模型表面，我们使用在超高真空（UHV）条件下物理气相沉积（PVD） Ba在NiAl（110）上有序Al2O3薄膜上生长的BaO纳米颗粒，然后进行氧化和退火。利用扫描隧道显微镜（STM）和高分辨率光电子能谱（HR-PES）表征了通过该过程形成的三维混合钡铝氧化物颗粒（BaAl2xO1+3x）的生长行为、形貌和化学成分。为了监测这些系统的吸附和反应过程，我们在暴露于NO2分子束（MB）时进行了时间分辨红外反射吸收光谱（TR-IRAs）。第一步，研究了NO2与Al2O3/NiAl（110）模型支架的相互作用。在100 K时，从亚单层到多层的所有覆盖区域仅以D-2h二聚体（N2O4）的形式进行分子吸附。在300 K时，发生缓慢的表面反应，最初导致表面亚硝酸盐的形成，随后形成桥接吸附几何形状的表面硝酸盐。在BaAl2xO1+3x上，Al2O3/NiAl（110）上的NO2颗粒表现出非常不同的、强烈的温度依赖性行为。在100k下，D-2h二聚体（N2O4）的分子吸附伴随着一个高效的反应通道，导致表面亚硝酸盐和硝酸盐的形成。然而，当表面温度升高到300 K时，反应概率降低了几个数量级。与100 K下的反应相比，表面亚硝酸盐是唯一的初级产物，表明反应机制从合作到非合作的变化依赖于温度。随着暴露的增加，亚硝酸盐的覆盖率增加，最终表面亚硝酸盐转化为桥接和单齿表面硝酸盐。后一反应表现出复杂的动力学，包括初始诱导期。在400 K及以上的温度下，硝酸盐的形成变得更有效，最终导致离子硝酸盐的形成。这些离子的振动特性敏感地依赖于反应温度，表明在500 K左右的狭窄温度区间内形成明确的结构。在现有结果的基础上，对氮氧化物储存过程的机理和相关表面物质的振动分配进行了批判性的讨论。(C) 2008爱思唯尔公司版权所有。
In order to obtain insights into the mechanism and kinetics of adsorption and reaction of NO2 on aluminum and barium oxide surfaces, we have performed systematic adsorption experiments on single-crystal-based model materials. As a model surface, we use BaO containing nanoparticles grown by physical vapor deposition (PVD) of Ba under ultrahigh vacuum (UHV) conditions on an ordered Al2O3 film on NiAl(110) and subsequent oxidation and annealing. The growth behavior, the morphology and the chemical composition of the three-dimensional mixed barium aluminum oxide particles (BaAl2xO1+3x) formed via this procedure have previously been characterized by scanning tunneling microscopy (STM) and high-resolution photoelectron spectroscopy (HR-PES). In order to monitor adsorption and reaction processes on these systems, we perform time-resolved infrared reflection absorption spectroscopy (TR-IRAs) during exposure to a molecular beam (MB) of NO2. In a first step, the interaction of NO2 with the Al2O3/NiAl(110) model support is probed. At 100 K, only molecular adsorption occurs in form of the D-2h dimer (N2O4) in all coverage regions from the submonolayer up to multilayers. At 300 K, a slow surface reaction occurs, initially leading to the formation of surface nitrites and, subsequently, of surface nitrates in bridging adsorption geometry. On the BaAl2xO1+3x, particles on Al2O3/NiAl(110), NO2 shows a very different and strongly temperature-dependent behavior. At 100 K, molecular adsorption of the D-2h dimer (N2O4) is accompanied by a highly efficient reaction channel, leading to the formation of surface nitrites and nitrates. With the surface temperature increasing to 300 K, however, the reaction probability decreases by several orders of magnitude. In contrast to reaction at 100 K, surface nitrites in flat-lying adsorption geometry are the only primary product, indicating a temperature-dependent change in the reaction mechanism from a cooperative to a non-cooperative pathway. With increasing exposure, the nitrite coverage increases and, finally, the surface nitrites are converted into bridging and monodentate Surface nitrates. The latter reaction shows a complex kinetics, including an initial induction period. For temperatures of 400 K and above, nitrate formation becomes more efficient, eventually resulting in the formation of ionic nitrates. The vibrational properties of these ionic species sensitively depend on the reaction temperature, indicating the formation of well-defined structures in a narrow temperature interval around 500 K. The mechanism of the NOx storage process and the vibrational assignments of the related surface species are critically discussed on the basis of the present results. (C) 2008 Elsevier Inc. All rights reserved.