Loh Andrea S, Davis Scott W, Medlin J Will
Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309-0424, USA.
J Am Chem Soc. 2008 Apr 23;130(16):5507-14. doi: 10.1021/ja711013n. Epub 2008 Mar 28.
High-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations were used to study the adsorption and reaction of 1-epoxy-3-butene (EpB) on Pt(111). These investigations were conducted to help elucidate mechanisms for improving olefin hydrogenation selectivity in reactions of unsaturated oxygenates. EpB dosed to Pt(111) at 91 K adsorbs molecularly on the surface through the vinyl group with apparent rehybridization to a di-sigma-bound state. By 233 K, however, EpB undergoes epoxide ring opening to form an aldehyde intermediate, which further decomposes upon heating to yield gas phase products CO, H2, and propylene. Comparison of the HREELS and TPD data to experiments performed with 2-butenal (crotonaldehyde) shows that EpB and 2-butenal decompose through related pathways. However, the EpB-derived aldehyde intermediate clearly has a unique structure, features of which have been elucidated by DFT calculations. In conjunction with previous surface science studies of EpB chemistry, these results can help explain selectivity trends for reactions of EpB on Pt catalysts and bimetallic PtAg catalysts, with indications that the enhanced olefin hydrogenation selectivity of PtAg catalysts likely originates from a bifunctional effect.
采用高分辨电子能量损失谱(HREELS)、程序升温脱附(TPD)和密度泛函理论(DFT)计算研究了1-环氧-3-丁烯(EpB)在Pt(111)上的吸附和反应。进行这些研究是为了帮助阐明在不饱和含氧化合物反应中提高烯烃加氢选择性的机制。在91 K下向Pt(111)上通入的EpB通过乙烯基以分子形式吸附在表面,并明显重新杂化形成双σ键合状态。然而,到233 K时,EpB发生环氧环开环形成醛中间体,该中间体在加热时进一步分解产生气相产物CO、H2和丙烯。将HREELS和TPD数据与用2-丁烯醛(巴豆醛)进行的实验结果进行比较表明,EpB和2-丁烯醛通过相关途径分解。然而,源自EpB的醛中间体显然具有独特的结构,其特征已通过DFT计算得以阐明。结合先前关于EpB化学的表面科学研究,这些结果有助于解释EpB在Pt催化剂和双金属PtAg催化剂上反应的选择性趋势,表明PtAg催化剂烯烃加氢选择性增强可能源于双功能效应。