Ni/γ-Al_2O_3催化剂上CH_4-CO_2重整体系中积碳消碳的研究

Carbon Deposition and Carbon Elimination over Ni/γ-Al_2O_3 Catalyst for CH_4-CO_2 Reforming

用动态热重技术和色谱技术联用以及XRD和TEM等表征手段 ,研究了Ni/γ Al2 O3 催化剂上甲烷 二氧化碳重整体系的积碳和消碳及其与一氧化碳产率之间的关系 .结果表明 ,二氧化碳与甲烷有不同的活性中心 ,二氧化碳的存在不改变甲烷活化积碳的本质 ,但可通过及时将表面含碳物种转化为一氧化碳而降低积碳速率 .二氧化碳能消去活泼的须状碳 ,但不易消去惰性的石墨碳 .等摩尔条件下二氧化碳消碳能力弱于甲烷活化积碳 ,二氧化碳适度过量可有效抑制积碳 ,n(CO2 ) /n(CH4) ≈ 1 3为进料比最佳值 ,此时积碳 消碳达到动态平衡 ,可获得最高的一氧化碳产率和较低的积碳速率 ,且该值不受温度影响 .Ni/γ Al2 O3 催化剂有很强的再生能力 ,但反复积碳 消碳会使催化剂镍晶粒度增大 ,分散度下降 ,从而导致不可逆失活 .

The carbon deposition and carbon elimination during CH 4 reforming with CO 2 over Ni/γ Al 2O 3 catalyst were investigated by means of TGA, XRD and TEM techniques. The yield of CO was also detected by an on line chromatograph. The results showed that the activation of CH 4 over Ni/γ Al 2O 3 catalyst is different from that of CO 2, and its inherent law can not be substantially changed in the presence of CO 2. However, the rate of decomposition of CH 4 could be promoted by carbonaceous deposit from CH 4 converted to CO by the dissociation of CO 2. Correspondingly, the rate of carbon deposition dramatically decreased. When the feed stock of CH 4 is equal to CO 2, the rate of carbon elimination by CO 2 is less than that of carbon deposition by CH 4, but much more CO 2 would inhibit carbon deposition, as well as the activation of CH 4. Therefore, there is an optimum feed ratio of n (CO 2)/ n (CH 4)=1 3, which seems to be unchanged with temperature. In this way, carbon deposition and carbon elimination may be in balance and lead to maximum CO yield and low rate of carbon deposition. The Ni/γ Al 2O 3 catalyst has very high capacity of carbon deposition, and is easily regenerated by carbon elimination, but the regeneration for many times leads to the growth of Ni particles, as a result, the catalytic activity decreases irreversibly.