Mechanism of CO2 Hydrogenation on Pd/Al2O3 Catalysts: Kinetics and Transient DRIFTS-MS Studies,ACS Catalysis

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Mechanism of CO2 Hydrogenation on Pd/Al2O3 Catalysts: Kinetics and Transient DRIFTS-MS Studies
ACS Catalysis ( IF 11.3 ) Pub Date : 2015-09-29 00:00:00 , DOI: 10.1021/acscatal.5b01464
Xiang Wang ₁ , Hui Shi ₁ , Ja Hun Kwak _{1,

2} , János Szanyi ₁

Affiliation

The hydrogenation of CO₂ was investigated over a wide range of reaction conditions, using two Pd/γ-Al₂O₃ catalysts with different Pd loadings (5% and 0.5%) and dispersions (∼11% and ∼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 H₂ 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, H₂, and CO₂ + H₂. 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 H₂ 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.

中文翻译：

Pd / Al ₂ O ₃催化剂上CO ₂加氢机理：动力学和瞬态DRIFTS-MS研究

CO的氢化₂进行了研究在宽范围的反应条件下，使用两个钯/γ-Al系₂ ö _3种催化剂（分别为〜11％和〜100％）的Pd不同负载（5％和0.5％）和分散体。在平均Pd粒径较大的5％Pd / Al ₂ O ₃上，CO和CH _4的生成率都比在平均粒径较小的0.5％Pd / Al ₂ O ₃上的转变率更高。在5％Pd / Al ₂ O ₃上对甲烷的选择性（22–40％）比在0.5％Pd / Al ₂ O ₃上的甲烷的选择性高2–3倍。。形成CO和CH _4的速率表达和活化能明显不同，使我们得出结论，反向水煤气变换和CO ₂甲烷化对Pd的速率限制步骤不相同，并且这两种途径可能在不同的催化条件下发生。表面部位。在两种催化剂上测得的CO ₂和H ₂压力下的反应顺序相似，这表明每种途径的反应机理都不会随粒径而变化。因此，DRIFTS结果表明，在瞬态和稳态实验中，在CO ₂，H ₂之间切换进料气体时，两种催化剂上的普遍表面物种及其演化方式是可比的。和CO ₂ + H ₂。DRIFTS和MS结果还表明，在两种催化剂上没有发生CO _2的直接离解，并且CO ₂必须首先与氧化物载体上的表面羟基反应。如此形成的碳酸氢盐与Pd颗粒上的解离吸附的氢反应，生成吸附的甲酸盐类（双功能催化剂：氧化物载体上的CO ₂活化和H ₂在金属颗粒上解离）。Pd颗粒附近的格式（最可能在金属/氧化物界面处）可以与吸附的H快速反应生成CO，然后CO吸附在金属Pd颗粒上。鉴定出两种类型的Pd位点：一种与CO的相互作用弱，在反应温度下很容易解吸到气相中，而另一种与CO的相互作用更强，CO主要以多键形式存在，并且在高温的氦气中保持稳定，但对Pd上吸附的H原子具有反应性，最终导致CH ₄形成。的5％Pd / Al的₂ ö ₃包含露台站点的用于形成大于0.5％的Pd / Al的这些更multibound和稳定CO物种有利的较大部分₂ ö ₃。因此，我们认为，在不同的Pd粒径下，CH ₄的形成速率和选择性的差异是由于Pd表面甲烷化途径的反应性中间体的浓度不同所致。

更新日期：2015-09-29

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