【作者】 王高伟；

【导师】 李永丹；

【作者基本信息】 天津大学 ， 工业催化， 2013， 博士

【摘要】 过渡金属碳化物作为一种新型的催化材料在众多反应过程中表现出类似贵金属的催化活性，从而受到了广泛关注。水煤气变换反应是工业制氢的重要组成部分。本组之前的结果表明，通过程序升温还原方法制备的Mo2C以及负载贵金属的Pt/Mo2C催化剂具有优异的低温变换反应活性。本文针对负载型Mo2C催化剂、碱金属添加对Mo2C和Pt/Mo2C活性的影响和Mo2C在变换反应中的稳定性展开研究。本文考察了负载型Mo2C/Al2O3催化剂以及负载贵金属的Pt-Mo2C/Al2O3催化剂的制备、表征以及活性评价。通过等体积浸渍的方法，将钼酸铵浸渍到Al2O3。再通过程序升温的方法制备了Mo2C/Al2O3。另外通过湿法浸渍的方法制备了不同Pt负载量的Pt-Mo2C/Al2O3催化剂。通过使用物理吸附、X射线吸收光谱(XAS)、X射线衍射(XRD)、CO化学吸附和程序升温脱附(COchemisorption和TPD)、扫描透视电镜(SETM-XEDS)等表征手段对Pt-Mo2C/Al2O3、Mo2C/Al2O3和Pt/Al2O3催化剂的结构进行了详细的表征并且评价了各催化剂在变换反应中的活性。Al2O3上Mo2C的存在能够促进Pt在其表面的吸附。Pt-Mo2C/Al2O3的活性远高于相近Pt含量的Pt/Al2O3催化剂。例如，3.8wt%Pt-Mo2C/Al2O3催化剂在240°C的转化频率(0.81s-1)高出Pt/Al2O3的转化频率(0.007s-1)两个数量级。Pt-Mo2C/Al2O3和Pt/Mo2C催化剂的活性随着Pt的负载表现出了相同的趋势。Pt-Mo2C/Al2O3体系上Pt选择性负载于Mo2C颗粒上并且具有相互作用，此作用改变了Pt的化学性质，因此导致了Pt在Pt-Mo2C/Al2O3和Pt/Al2O3上不同的化学吸附性质。以高比表面积的SiO2为载体，制备了负载型Mo2C/SiO2和Pt-Mo2C SiO2催化剂并用于变换反应。同样通过各种表征技术和方法对催化剂的结构和活性进行了考察。结果发现：在负载量为16.9和29.0wt.%的Mo2C/SiO2上，Mo2C颗粒粒径分布较窄，并且大部分颗粒在1-2nm之间。两种催化剂上单位重量的Mo2C具有相近的催化活性，并且高于未负载型Mo2C。通过湿法溶液浸渍后，负载了贵金属Pt。4.2wt.%Pt-16.3wt.%Mo2C/SiO2上Pt的粒径分布在5-10nm左右。和Al2O3上负载的Pt-Mo2C催化剂相比，Pt颗粒的形态没有明显的变化。Pt颗粒的组成分析结果表明，Pt和Mo2C之间紧密接触，具有较强的相互作用。这种相互作用同样提高了Pt的催化活性。通过过量浸渍的方法，在Mo2C和Pt/Mo2C催化剂上负载了碱金属K和Na。本章对催化剂进行了表征，并且比较了K/Mo2C、Na/Mo2C、Pt/Mo2C和Na-Pt/Mo2C之间的活性。碱金属的添加减少了催化剂的比表面积和化学吸附量，同时也降低了催化活性。在文献报导的结果中，碱金属物种的添加能够改善部分载体活化水的能力，例如A2O3和TiO2，从而提高了催化剂的变换反应活性。由于Mo2C活化水的能力高于碱金属物种，碱金属的添加覆盖了Mo2C的活性位和Pt-Mo2C之间的界面，从而导致了Mo2C和Pt/Mo2C催化剂活性的下降。另外本文制备了高比表面积的Mo2C催化剂(~120m2/g)，该催化剂在变换反应中具有和工业催化剂Cu/Zn/Al2O3相当的活性但失活速率较快。失活后的Mo2C的体相结构没有发生变化，因此失活主要在Mo2C的表面进行。本章比较了Mo2C在各反应组分中(H2、N2、H2O、CO和CO2)的失活。H2O在Mo2C表面上的活化生成了氢气和氧物种，氧物种在活性位上的不断累积造成了活性位减少从而导致失活。含碳物种在反应后的Mo2C中也被观测到但不是失活的主要原因。本章同时对Mo2C的再生条件也进行了考察分析，Mo2C在H2中240°C下还原5h即可恢复部分活性。更多还原

【Abstract】 The catalytic properties of early transition metal carbides have been the subject ofmany investigations since it was reported that some carbides possess catalyticproperties that resemble those of Pt group metals. Water gas shift (WGS) reaction isan important industrial process for hydrogen production. Previously we reported thatthe high surface area Mo2C prepared from the temperature programmed reaction andthe Pt deposited onto Mo2C (Pt/Mo2C) were active for this reaction. In this thesis, thesupported Mo2C catalysts, the influences of the alkali metal additions and the stabilityof the Mo2C catalysts were investigated.Chapter2describes the synthesis and characterization of Mo2C/Al2O3and Pt-Mo2C/Al2O3catalysts and their evaluation for the WGS reaction. Mo2C/Al2O3catalystwas prepared via the temperature programmed reaction from AM/Al2O3precursorwhich was prepared by the incipient wetness impregnation of Al2O3and AM. The Pt-Mo2C/Al2O3catalysts were prepared from the Mo2C/Al2O3catalyst using a wetimpregnation method. The materials were characterized using techniques includingphysical adsorption, X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD),CO chemisorption, temperature programmed desorption and transmission electronmicroscopy and the activities were evaluated for the WGS reaction under differentialconditions. The presence of Mo2C enhanced the deposition of Pt from H2PtCl6solutions. The Pt-Mo2C/Al2O3catalysts were much more active than thecorresponding Pt/Al2O3catalyst. For example, the turnover frequency for the3.8wt%Pt-Mo2C/Al2O3catalyst at240°C (0.81s-1) was two orders of magnitude higher thanthat for the3.9wt%Pt/Al2O3catalyst (0.007s-1). Trends with increasing Pt loadingfor the Pt-Mo2C/Al2O3catalysts were similar to those previously observed forPt/Mo2C catalysts. The results were consistent with Pt being co-located with Mo2C, aconsequence of the strong affinity of the Pt precursor for Mo2C. Interaction of the Ptwith Mo2C would account for the significant differences between the catalytic andsurface chemical properties of the Pt-Mo2C/Al2O3and Pt/Al2O3catalysts.In chapter3, high surface area SiO2was used as the support for the preparation ofthe Mo2C/SiO2and Pt-Mo2C/SiO2catalysts. These materials were also characterizedby physical adsorption, XRD and STEM-XEDS and evaluated for WGS reaction to identify the catalyst structures and compositions. The Mo2C particles supported on the16.9and29.0wt.%Mo2C/SiO2were in the range of the1-2nm and the loadingamount of the Mo2C had little influence on the particle size distribution. Thenormalized activities of the16.9and29.0wt.%Mo2C/SiO2catalyst were higher thanthe unsupported Mo2C catalyst because of the higher dispersion.4.2wt.%Pt-16.3wt.%Mo2C/SiO2catalyst was also prepared from the wet-impregnation method and the Ptparticle distribution was in the range of5-10nm. Pt and Mo2C were also co-locatedand the interaction enhanced the reactivity of the Pt.In chapter4, the influences of the alkali metal addition were investigated. Thesodium and potassium were loaded on the high surface Mo2C via the wetimpregnation method. The composition and structure of the catalysts werecharacterized and the activities were compared. The addition of the sodium andpotassium decreased the surface areas of the Mo2C catalysts, CO uptakes and also thereaction rates. The reaction rates of Na/Mo2C catalysts normalized by the surfaceareas were similar to each other and also silimar to the Mo2C catalyst indicating thatthe presences of the alkali metals didnot change the chemical properties of the activesites on the Mo2C catalyst while blocked the pores thus decreased the activities of thecatalysts. Alkali metals were reported as the promoters for the enhanced activities inthe WGS reaction which could modify the properties of the supports, such as TiO2andAl2O3. However, the Mo2C was a good catalyst for the water activation compared toalkali metal species. Thus, the addition of the alkali metals decreased the activities ofMo2C and Pt/Mo2C catalysts.In chapter5, the stability of the Mo2C in the WGS reaction was investigated. Thehigh surface area Mo2C (~120m2/g) manifested a higher deactivation rate comparedto the commercial Cu/Zn/Al2O3catalyst. The post reaction characterization showedthat the bulk structure of Mo2C was maintained. Thus, the deactivation was due to thesurface properties change of the Mo2C. The reaction rates of the Mo2C before andafter each reactant or reactants treatments were compared. The results showed thatrate of the Mo2C catalyst after the water treatment at240°C was much lower than thatof before the treatment indicating the water may be responsible for the rate decrease.Carbon species were also formed on the post reaction catalyst surface while may notbe the dominating deactivation reason. The water activation on the Mo2C can beprocessed at240°C to produce H2and the oxygen species. The oxygen species wereaccumulated on the Mo2C surface which resulted in the catalyst deactivation. The regeneration methods for the Mo2C catalyst were also investigated and the partialactivity of Mo2C catalyst could be restored after the treatment in H2at240°C.更多还原