CeO_(2)负载的PdO与Ce_(1‒x)Pd_(x)O_(2‒δ)物种的甲烷催化燃烧性能

Catalytic Methane Combustion over CeO_(2) Supported PdO and Ce_(1‒x)Pd_(x)O_(2‒δ) Species

通过调控Pd前驱物在CeO_(2)上的沉积方式,分别制备了以PdO和离子态的Ce_(1‒x)Pd_(x)O_(2‒δ)物种为主的Pd/CeO_(2)催化剂,并采用X射线光电子能谱(XPS)和拉曼(Raman)光谱确证了这两种Pd物种的存在.氧气程序升温脱附(O_(2)-TPD)和氢气程序升温还原(H_(2)-TPR)的表征结果显示,相比于与载体相互作用较弱的PdO物种,与CeO_(2)相互作用较强的Ce_(1‒x)Pd_(x)O_(2‒δ)物种具有更加稳定的Pd—O键.催化剂的甲烷燃烧反应起燃活性测试结果显示,以PdO物种为主的催化剂表现出了良好的低温催化性能,在原料气配比为1%CH_(4)/4%O_(2)-Ar,空速为60000 mL·g^(-1)_(cat)·h^(‒1)的条件下,T_(10)和T_(90)分别为275和367℃,而两种以Ce_(1‒x)Pd_(x)O_(2‒δ)物种为主的催化剂的T_(10)均超过420℃.催化剂的甲烷程序升温还原(CH_(4)-TPR)表征结果表明,在升温过程中只有当PdO或Ce_(1‒x)Pd_(x)O_(2‒δ)物种被CH_(4)还原后,催化活性才开始上升.由于PdO物种的Pd—O键强度较弱,有利于Pd物种上的晶格氧在较低温度下参与CH_(4)的氧化过程.而Ce_(1‒x)Pd_(x)O_(2‒δ)物种的Pd—O键较稳定,且在反应条件下离子态Pd^(2+)与反应体系中氧物种的作用较强,进而抑制了其与CH_(4)的反应,因此反应的起燃温度较高.以γ-Al_(2)O_(3)为载体采用相同的方法制备了Pd/γ-Al_(2)O_(3)催化剂,相关的表征结果进一步证实,与载体相互作用较弱的PdO物种更容易被CH_(4)还原,进而具有较高的催化活性.据此可以认为Pd/CeO_(2)上氧化态的Pd物种被CH_(4)的还原性能是决定其甲烷催化燃烧反应活性的重要因素之一.

Pd/CeO_(2) catalysts containing mainly PdO and Ce_(1‒x)Pd_(x)O_(2‒δ)species were prepared by modulating the depo⁃sition method of Pd species on CeO_(2).The Raman and XPS characterizations of the catalysts confirmed the presence of these two types of Pd species on the prepared catalysts.The characterization results of O_(2)-TPD and H_(2)-TPR showed that the Ce_(1‒x)Pd_(x)O_(2‒δ)species with stronger interactions with CeO_(2) featured more stronger Pd—O bonds compared to the PdO species.The results of catalytic performance testing for the CH_(4) combustion reaction showed that,the Pd/CeO_(2) catalyst dominated with PdO species showed a good activity during the light off process with a T_(10) and T_(90) of 275 and 367℃,respectively,at a feed gas ratio of 1%CH_(4)-4%O_(2)-Ar and gaseous hourly space velocity(GHSV)of 60000 mL·g^(-1)_(cat)·h^(‒1),while the catalysts dominated with Ce_(1‒x)Pd_(x)O_(2‒δ)species possessed poor activity with T_(10) in excess of 420℃under the same condition.The CH_(4)-TPR characterization of the catalysts showed that the catalytic activity started to increase only when the PdO or Ce_(1‒x)Pd_(x)O_(2‒δ)species started to be reduced by CH_(4) during the light off process.For the PdO species,the relatively weak Pd—O bonding allows the lattice oxygen on the Pd species to participate in the oxidation of CH_(4) at lower temperatures,and CeO_(2) support contributes to the reoxidation of the PdO_(x) through oxygen migration,which in turn completes the process of catalysis by PdO/Pd0 cycle.The low activity of the ionic Ce_(1‒x)Pd_(x)O_(2‒δ)should attribute to its strong Pd—O—Ce linkages which are difficult to be reduced by CH_(4) during the reaction.Furthermore,we prepared Pd/γ-Al_(2)O_(3) catalysts by the same method usingγ-Al_(2)O_(3) as the support,and the characterization results further confirmed that the PdO species with weaker interaction withγ-Al_(2)O_(3) are more easily be reduced by CH_(4) and thus exhibit better catalytic activity.It can be concluded that the reducibility of the oxidized Pd species on Pd/CeO_(2) by CH_(4) is one of the important factors determining the activity of the catalyst for the methane combustion reaction.