Mechanism of CO2 Hydrogenation on Pd/Al2O3 Catalysts: Kinetics and Transient DRIFTS-MS Studies
Mechanism of CO2 Hydrogenation on Pd/Al2O3 Catalysts: Kinetics and Transient DRIFTS-MS Studies
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DOI：
10.1021/acscatal.5b01464
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发表时间：
2015-11-01
期刊：
影响因子：
12.9
通讯作者：
Szanyi, Janos
中科院分区：
文献类型：
--
作者：
Wang, Xiang;Shi, Hui;Szanyi, Janos
用两种不同Pd负载量(5%和0.5%)和不同分散度(分别为11%和100%)的Pd/γ-Al_2O_3催化剂在多种反应条件下考察了CO_2加氢反应。5%Pd/Al_2O_3催化剂平均Pd粒径大于0.5%Pd/Al_2O_3平均粒径较小的Pd/Al_2O_3催化剂，CO和CH4的转化率均高于5%Pd/Al_2O_3催化剂。5%Pd/Al_2O_3催化剂的甲烷选择性(22~40%)比0.5%Pd/Al_2O_3催化剂高2~3倍。CO和CH4生成的速率表达式和表观活化能截然不同，这导致我们得出结论：逆水煤气变换和CO2甲烷化在Pd上不具有相同的限速步骤，这两条途径可能是在不同的表面位置上催化的。在两种催化剂上测得的CO2和H-2压力下的反应级数相似，表明每条反应途径的反应机理都不随颗粒大小而改变。与之相一致的是，漂移结果表明，在CO2、H-2和CO2+H-2之间切换进气的暂态和稳态实验中，两种催化剂上的主要表面物种及其演化规律是相似的。DRIFTS和MS结果还表明，CO2在两种催化剂上没有发生直接解离，CO2必须首先与氧化物载体上的表面羟基反应。这样形成的重碳酸盐与Pd颗粒上解离吸附的氢反应生成吸附的甲酸盐物种(双功能催化剂：氧化物载体上的CO2活化和金属颗粒上的H-2解离)。Pd颗粒附近的甲酸盐(最有可能在金属/氧化物界面)可以与吸附的H迅速反应生成CO，然后CO吸附在金属Pd颗粒上。确定了两种类型的Pd中心：一种与CO有弱的相互作用，在反应温度下很容易脱附到气相中；另一种与CO有较强的相互作用，主要以多键形式存在，在高温下在He流动中保持稳定，但与Pd上吸附的H原子反应，最终形成CH4。与0.5%Pd/Al_2O_3相比，5%Pd/Al_2O_3催化剂含有更多有利于形成更多结合态和更稳定CO物种的梯形中心。因此，我们认为，不同Pd颗粒大小的甲烷生成速率和选择性的差异源于Pd表面甲烷化反应中间体浓度的不同。
The hydrogenation of CO2 was investigated over a wide range of reaction conditions, using two Pd/gamma-Al2O3 catalysts with different Pd loadings (5% and 0.5%) and dispersions (similar to 11% and similar to 100%, respectively). Turnover rates for CO and CH4 formation were both higher over 5% Pd/Al2O3 with a larger average Pd particle size than those over 0.5% Pd/Al2O3 with a smaller average particle size. The selectivity to methane (22-40%) on 5% Pd/Al2O3 was higher by a factor of 2-3 than that on 0.5% Pd/Al2O3. The drastically different rate expressions and apparent energies of activation for CO and CH4 formation led us to conclude that reverse water gas shift and CO2 methanation do not share the same rate-limiting step on Pd and that the two pathways are probably catalyzed at different surface sites. Measured reaction orders in CO2 and H-2 pressures were similar over the two catalysts, suggesting that the reaction mechanism for each pathway does not change with particle size. In accordance, the DRIFTS results reveal that the prevalent surface species and their evolution patterns are comparable on the two catalysts during transient and steady-state experiments, switching feed gases among CO2, H-2 and CO2 + H-2. The DRIFTS and MS results also demonstrate that no direct dissociation of CO2 takes place over the two catalysts and that CO2 has to first react with surface hydroxyls on the oxide support. The thus-formed bicarbonates react with dissociatively adsorbed hydrogen on Pd particles to produce adsorbed formate species (bifunctional catalyst: CO2 activation on the oxide support and H-2 dissociation on the metal particles). Formates near the Pd particles (most likely at the metal/oxide interface) can react rapidly with adsorbed H to produce CO, which then adsorbs on the metallic Pd particles. Two types of Pd sites are identified: one has a weak interaction with CO, which easily desorbs into gas phase at reaction temperatures, whereas the other interacts more strongly with CO, which is mainly in multibound forms and remains stable in He flow at high temperatures, but is reactive toward adsorbed H atoms on Pd leading eventually to CH4 formation. 5% Pd/Al2O3 contains a larger fraction of terrace sites favorable for forming these more multibound and stable CO species than 0.5% Pd/Al2O3. Consequently, we propose that the difference in the formation rate and selectivity to CH4 on different Pd particle sizes stems from the different concentrations of the reactive intermediate for the methanation pathway on the Pd surface.