Aminovinylidene: A Stable Surface Intermediate in the Dehydrogenation of Ethylamine on Pt(1 1 1)
Aminovinylidene: A Stable Surface Intermediate in the Dehydrogenation of Ethylamine on Pt(1 1 1)
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氨基亚乙烯基：乙胺在 Pt(1 1 1) 上脱氢的稳定表面中间体
DOI：
10.1002/cctc.201200170
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发表时间：
2012
期刊：
影响因子：
4.5
通讯作者：
M. Trenary
中科院分区：
文献类型：
--
作者：
I. Waluyo;Joel D. Krooswyk;Jun Yin;Yuan Ren;M. Trenary
胺脱氢制腈和腈加氢制胺是化学工业中的关键过程。后者是伯胺商业生产中常用的一种工艺，而伯胺在各种化学品的制造中都很重要。在过去的二十年里，使用负载过渡金属催化剂催化乙腈加氢生产乙胺一直是人们非常感兴趣的课题。胺脱氢制腈更常在好氧条件下进行。然而，在Ni（111）和W(1 0 0)-(5 1)- c上，用卤化物簇催化剂可将乙胺脱氢为乙腈。在非均相催化的基础研究中，表征表面催化反应中形成的中间物质是至关重要的。本研究利用反射吸收红外光谱（RAIRS）和密度泛函理论（DFT）计算，确定了乙胺部分脱氢生成的Pt（111）表面稳定中间体为氨基偏乙胺（CCHNH2）。过去，人们对含有CCN或CNC骨架的吸附分子的表面化学性质进行了广泛的研究。通过对比实验RAIR光谱和DFT计算，发现NH3和C2在Pt（111）上的反应形成了CHCNH2表面中间体，通过其C原子与表面成键，且C - C键平行于表面。此外，RAIRS还测定了一种稳定的甲基氨基羰基（CNHCH3）表面中间体可以通过甲基异氰化物的质子化或二甲胺的部分脱氢形成，该中间体具有一个垂直于表面的C-N键和一个横跨其C- nc单元的p键。高分辨率电子能量损失谱（HREELS）研究表明，乙胺在Ni（111）上的部分脱氢产生了一个CH3CNH表面中间体，通过N和C原子与表面成键，且cn键平行于表面。我们提出的表面中间体，其dft优化结构如图1所示，通过末端C原子与表面结合，并且C - C键或多或少垂直于表面。该物种类似于偏乙烯基（CCH2），其结构在Pt35簇上计算得到，b[8]但碳-碳键更直立。由于其中一个H原子被一个NH2基团所取代，这种物质被命名为氨基偏二烯。虽然该物种含有一个CCN单元，类似于NH3和C2在Pt（111）上反应形成的中间体，但它的表面成键方案实际上与甲胺羰基相似，它含有一个跨三个中心的离域p键，具有单键和双键混合特征的C - N键。图2为样品在85 K条件下暴露于0.3 L CH3CH2 NH2 （1 L = 1 10 6 Torr s）后的RAIR光谱。随后将样品加热，在300 K和330 K温度下保持30秒，然后冷却回85 K，在此温度下获得所有RAIR光谱。在85 K时和加热至300 K后的光谱特征大致相似，说明在300 K时乙胺仍被分子吸附；更尖锐和更强烈的峰表明一个更有序的覆盖层。2800 ~ 3000 cm 1之间的峰对应于n（C-H）模式。将Ni（111）上乙胺的HREELS光谱与气相乙胺的实验和计算光谱进行比较，发现1446、1383、1343和1145 cm 1处的峰分别属于d（CH3）、w（CH2）、d（CH）和1（CH3）模式，而1038和1052 cm 1处的强峰可能对应于n（C n）和n（C C n）模式。当进一步加热到330 K时，光谱发生了巨大变化，表明在表面上形成了新的物种。温度程序解吸（TPD）数据（未显示）表明乙胺在该温度下发生部分脱氢，乙胺在Ni（111）和二甲胺在Pt（111）上也观察到这种情况。在加热到330 K后，光谱中最显著的变化是在3488 cm 1处出现了一个相对强烈的峰，在3342和3434 cm处出现了另外两个较弱的n（n H）峰。此外，2800 ~ 3000 cm 1之间的大部分n（碳氢）峰已不复存在，见图1。侧视图（左）和俯视图（右）在Pt4簇的三层中空位点上所提出的氨基乙烯基表面物质的结构。除Pt外，所有原子都不受约束，允许优化。优化后的碳碳键长为1.40，碳氮键长为1.36。
The dehydrogenation of amines into nitriles and the hydrogenation of nitriles into amines are crucial processes in the chemical industry. The latter is a commonly used process in the commercial production of primary amines, which are important in the manufacture of a variety of chemicals. The catalytic hydrogenation of acetonitrile using supported transition metal catalysts to produce ethylamine, in particular, has been a subject of great interest in the past two decades. The dehydrogenation of amines to nitriles is more commonly performed in aerobic conditions. However, it was observed that ethylamine can be dehydrogenated to acetonitrile with a halide cluster catalyst, on Ni(111), and on W(1 0 0)-(5 1)-C. The characterization of the intermediate species formed during a surfacecatalyzed reaction is crucial in the fundamental study of heterogeneous catalysis. Here we used reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations to identify aminovinylidene (CCHNH2) as a stable surface intermediate on Pt(111) from the partial dehydrogenation of ethylamine. The surface chemistry of adsorbate molecules containing a CCN or CNC backbone has been studied extensively in the past. Based on the comparison between experimental RAIR spectra and DFT calculations, the reaction between NH3 and C2 on Pt(111) was found to result in the formation of a CHCNH2 surface intermediate bonded to the surface through its C atoms with the C C bond parallel to the surface. In addition, RAIRS was used to determine that a stable methylaminocarbyne (CNHCH3) surface intermediate, with a C N bond perpendicular to the surface and a p-bond delocalized across its C-NC unit, could be formed through either the protonation of methyl isocyanide or the partial dehydrogenation of dimethylamine. A high-resolution electron energy loss spectroscopy (HREELS) study concluded that the partial dehydrogenation of ethylamine on Ni(111) produced a CH3CNH surface intermediate bonded to the surface through the N and C atoms with the C N bond parallel to the surface. Our proposed surface intermediate, whose DFT-optimized structure is shown in Figure 1, is bonded to the surface through the terminal C atom and the C C bond is more or less perpendicular to the surface. This species is analogous to vinylidene (CCH2), the structure of which was calculated on a Pt35 cluster, [8] but the C C bond is more upright. As one of the H atoms has been replaced by an NH2 group, this species is named aminovinylidene. Although this species contains a CCN unit like the intermediate formed through the reaction of NH3 and C2 on Pt(111), its surface bonding scheme is actually similar to that of methylaminocarbyne in that it contains a p-bond delocalized across three centers with C N bonding of mixed single and double bond character. Shown in Figure 2 are the RAIR spectra of the sample after exposure to 0.3 L of CH3CH2 NH2 (1 L = 1 10 6 Torr s) at 85 K. The sample was subsequently heated and the temperature was held at 300 K and 330 K for 30 seconds before it was cooled back to 85 K, at which all RAIR spectra were obtained. The spectral features at 85 K and after heating to 300 K are generally similar, indicating that ethylamine is still molecularly adsorbed at 300 K; the sharper and more intense peaks suggest a more ordered overlayer. The peaks between 2800 and 3000 cm 1 correspond to n(C-H) modes. Comparison with the HREELS spectrum of ethylamine on Ni(111) and the experimental and calculated spectra of gas-phase ethylamine revealed that the peaks at 1446, 1383, 1343, and 1145 cm 1 can be assigned to d(CH3), w(CH2), d(C H), and 1(CH3), respectively, while the intense peaks at 1038 and 1052 cm 1 likely correspond to n(C N) and n(C C N) modes. Upon further heating to 330 K, the spectrum changes dramatically, indicating that a new species was formed on the surface. Temperature programmed desorption (TPD) data (not shown) indicate that ethylamine undergoes partial dehydrogenation at this temperature, which is also observed for ethylamine on Ni(111) and dimethylamine on Pt(111). The most significant change in the spectrum after heating to 330 K is the appearance of a relatively intense peak at 3488 cm 1 and two other weaker n(N H) peaks at 3342 and 3434 cm . In addition, most of the n(C H) peaks between 2800 and 3000 cm 1 are no longer present, Figure 1. Side view (left) and top view (right) of the structure of the proposed aminovinylidene surface species on a threefold hollow site of a Pt4 cluster. All atoms except for Pt were unconstrained and were allowed to optimize. The optimized C C and C N bond lengths are 1.40 and 1.36 , respectively.
影响因子：
7.3
作者：
Kamiguchi, S;Nakamura, A;Chihara, T
通讯作者：
Chihara, T