摘要
化石燃料的日渐枯竭和环境污染的日趋加重，促使人们寻找新型可替代能源。氢气作为一种洁净的可再生能源，符合未来能源的发展趋势。二甲醚水蒸气重整（DME SR）是制氢的有效途径之一，该过程的核心技术是催化剂的开发。目前报道的催化剂需要克服的主要困难是如何提高催化剂的低温活性及选择性。本文分别对CuZnAl和CuFe_2O_4与固体酸γ-Al_2O_3组成的复合催化剂进行了研究，考察了催化剂的焙烧温度、活性组分担载量、重整反应工艺条件、催化剂的取代或掺杂改性等对催化剂性能的影响，采用BET、XRD、H_2-TPR、N_2O化学吸附、XAFS（包括XANES和XAFS）等技术对催化剂进行了详细表征，并对催化剂的结构与性能进行了关联。
通过对DME SR过程的热力学分析可知：在温度大于200oC，S/C>1.5时，二甲醚水蒸气重整不存在热力学平衡限制，DME可以完全转化；CH_4和C是热力学上容易生成的产物，但是CH_4和C的产生会大大降低产物氢气的收率。因此，要求催化剂对反应产物具有优良的选择性，抑制CH_4和C的形成。
采用共沉淀法通过水滑石前驱体制备了CuZnAl催化剂，对其进行了活性测试和表征，考察了焙烧温度、活性组分含量对催化剂性能的影响。根据XRD和TPR结果，500oC焙烧样品中Cu与载体相互作用适中，无CuAl_2O_4相生成，CuO物种在还原气氛或反应条件下较易被还原为活性物种（Cu~0和Cu~+），活性较高。活性组分Cu含量为40%时，能够形成结晶较好的水滑石前驱体，焙烧后为晶粒较小的CuO和ZnO物种，其对DME SR反应活性最好。
为了进一步提高催化剂的活性和选择性，对金属催化剂进行取代或掺杂改性，主要分为两部分工作。（1）以Zr取代部分Al。少量Zr的加入可以削弱Cu-Al之间的相互作用，提高Cu物种的还原性能，但过量Zr的引入会造成Cu物种的聚集长大，降低Cu的分散度。ZrO_2/(Al_2O_3+ZrO_2)的重量比为20wt%时，对应的催化剂CuZnAl_(0.8)Zr_(0.2)O具有最高的Cu分散度，最小的Cu晶粒尺寸以及最佳的催化性能。另外，通过对XANES图的线性拟合发现，反应后的CuZnAl0.8Zr0.2O催化剂含有最多的Cu~+物种，在DME SR反应中可以有效地抑制副产物CO的生成，提高CO_2选择性。（2）在CuZnAl催化剂中掺杂少量Na，K助剂。碱性组份的引入可以中和催化剂表面的部分酸中心，提高了产物中CO_2的选择性。从表征结果来看，在CuZnAl中引入助剂K，促进了Cu物种在载体表面的分散，使其具有较小的粒径，同时降低Cu-Al之间的相互作用，但K负载量太大会过度中和催化剂表面的酸中心，降低催化剂对DME水解反应的活性，从而使DME转化率下降。
最后对CuFe_2O_4和Al_2O_3复合催化剂用于DME SR反应进行了考察。发现催化剂较适宜的还原条件为350oC还原1h。还原时间过长或还原温度过高会导致Fe_3O_4被还原为金属Fe，削弱Cu与Fe_3O_4之间的相互作用，使Cu晶粒长大，引起活性下降。同时金属Fe的生成会促进甲醇分解反应的发生，导致CO选择性升高。在CuFe_2O_4中掺杂第二种金属组分Co后，促进了分解反应的发生，使DME转化率有一定程度的提升，低温下H_2收率也相应有所提高；高温下掺杂Co的样品比未掺杂的样品DME转化率高，但此时副反应较严重，导致H_2收率变化不大。掺杂Ni后，无论是对DME转化率还是对CO_2的选择性都有不利影响。45h的连续活性测试结果表明，CuFe_2O_4结构有利于Cu物种的稳定，使催化剂保持较高的活性。
The rapid consumption of fossil fuels and as-induced many kinds of environmentpollution have driven extensive exploration of new energy alternatives. Hydrogen is akind of clean and renewable energy, which is more suitable for future energydevelopment. Dimethyl ether steam reforming (DME SR) is an effective way toproduce hydrogen. For the production of hydrogen via DME SR, the development ofhighly efficient catalysts is crucial. Currently, the greatest challenge is to improve thecatalytic performance of the catalysts, including low-temperature activity andselectivity. In this work, a kind of complex catalysts consisting of metal (CuZnAl orCuFe_2O_4) and solid acid (γ-Al_2O_3) is employed for DME SR reaction, andsystematically studied from several aspects, including the effect of calcinationtemperature, contents of active components, reaction conditions and active phasemodifications. Multiple techniques such as BET, XRD, H_2-TPR, N_2O chemisorptionand XAFS (including XANES and EXAFS) were employed for catalystcharacterization. The structures and the properties of the catalysts are well correlated.
Firstly, we performed thermodynamics analysis and drew a conclusion that DMESR reaction is not controlled by thermodynamics equilibrium and DME can beconverted totally when reaction temperature is higher than200oC and S/C>1.5.Besides, CH_4and C are thermodynamically favorable products; however, theappearance of them will decrease the hydrogen yield. Therefore, high selectivity isrequired for the catalysts to inhibit the formation of CH_4and C.
Coprecipitation method was employed to prepare CuZnAl catalysts viahydrotalcite precursor, the effect of calcination temperature and contents of activecomponents are investigated based upon catalytic test and characterization results.According to XRD and TPR results, the sample calcined at500oC which is moreactive has a proper interaction between Cu and carrier, besides, no CuAl_2O_4wasidentified and the CuO species in it can be easily reduced to active spieces (Cu~0andCu~+). When CuO content reached40wt.%, hydrotalcite precursor was well formed,and the particle size of CuO and ZnO in the calcined catalyst is quite small from theXRD results, as a result, the catalyst shows the highest DME conversion.
Metal catalysts are often modified to achieve better catalytic activity andselectivity. In the first step, Al was partially substituted by Zr. The partial substitution of Al by Zr can decrease the Cu-Al ineration and improve the reducibility of Cuspeices. But the presence of excessive amount of Zr results in the aggregation of Cuspecies, decreasing the dispersion of copper species and the catalytic performance ofthe catalysts. The optimal content of Zr is20wt%according to ZrO_2/(Al_2O_3+ZrO_2)ratio. The catalyst CuZnAl_(0.8)Zr_(0.2)O with the highest Cu dispersion and smallest Cucrystallite size exhibits the best performance for DME SR reaction. In addition, it isrevealed by XANES that among all catalyst CuZnAl0.8Zr0.2O contains the highestamount of Cu~+species, which effectively inhibits the formation of CO and increasesthe selectivity of CO_2during dimethyl ether steam reforming. In the second step, Naand K were introduced to CuZnAl catalyst, neutralized a part of acid sites on catalystsurface and improved CO_2selectivity. From the characterization results, theintroduction of K, decreased the Cu-Al interation, made Cu speices more dispersed;however, excessive amount of K would neutralize more acid sites, decrease the activesites of DME hydrolysis, and then decrease DME conversion.
Finally, a study on the complex catalyst of CuFe_2O_4and Al_2O_3for DME SR wascarried out. Too long time or too high temperature of reduction will transform Fe_3O_4to metallic Fe and decrease the interation between Cu and Fe_3O_4, leading to increaseof Cu particles size and decrease of DME conversion. Meanwhile, the generation ofmetallic Fe will promote methanol decomposition reaction and increase CO selectivity.When the catalyst was doped with Co, the DME conversion increased due to thepromotion of methanol decomposition reaction. Although DME conversion on thecatalyst doped with Co is higher than that on the undoped one, H_2yield of the twocatalysts does not show much difference. Ni as the dopant has a negative effect onDME conversion and CO_2selectivity. The results of long-term DME SR reaction test(45h) shows that the complex catalyst of CuFe_2O_4and Al_2O_3exhibited high stability.
引文
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