Chen Qingjun, Svenum Ingeborg-Helene, Qi Yanying, Gavrilovic Ljubisa, Chen De, Holmen Anders, Blekkan Edd A
Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway.
Phys Chem Chem Phys. 2017 May 17;19(19):12246-12254. doi: 10.1039/c7cp00620a.
Potassium (K), an important impurity in syngas from biomass, can have a large influence on the activity and selectivity of cobalt-based Fischer-Tropsch synthesis (FTS) catalysts in Biomass to Liquids (BTL) processes. In this work, the potassium adsorption behavior on hcp cobalt was systematically studied using density functional theory. The surface energy calculations and Wulff construction of the equilibrium shape of hcp cobalt showed it is dominated by 10 facets. The interaction of K with these facets has been investigated. The results showed that the stepped facet (10-12) has the highest K adsorption energy of -2.40 eV. The facets (0001), (10-10), (10-11), (10-15), and (21-30) also showed relatively high K adsorption energies in the range of -2.28 to -2.34 eV. The corrugated facets exhibited comparatively lower K adsorption energies (-2.04 to -2.18 eV), and would be less favorable for K adsorption. It was also found that the adsorption properties depend on coverage, where the K adsorption energy decreased with increasing coverage. Diffusion energy barrier calculations indicated that K was mobile on typical facets (0001) and (10-11) with very low diffusion barriers (<0.15 eV). On stepped facets, although K could move freely along the same step (diffusion barrier <0.01 eV), diffusion from one step to another had a significantly higher barrier of 0.56 eV. This suggested that K atoms would be mobile to some extent during FTS reaction conditions, and tend to occupy the most favorable sites independent of their initial position. The results obtained in this work provide valuable information on the interaction of K with cobalt surfaces, relevant for practical cobalt catalysts and their application in BTL processes.
钾（K）是生物质合成气中的一种重要杂质，在生物质制液体燃料（BTL）过程中，它会对钴基费托合成（FTS）催化剂的活性和选择性产生很大影响。在这项工作中，利用密度泛函理论系统研究了钾在六方钴上的吸附行为。六方钴平衡形状的表面能计算和伍尔夫构造表明，它由10个晶面主导。研究了钾与这些晶面的相互作用。结果表明，阶梯晶面（10-12）具有最高的钾吸附能，为-2.40 eV。晶面（0001）、（10-10）、（10-11）、（10-15）和（21-30）的钾吸附能也相对较高，在-2.28至-2.34 eV范围内。波纹晶面的钾吸附能相对较低（-2.04至-2.18 eV），不利于钾的吸附。还发现吸附性能取决于覆盖度，钾吸附能随覆盖度增加而降低。扩散能垒计算表明，钾在典型晶面（0001）和（10-11）上具有流动性，扩散能垒非常低（<0.15 eV）。在阶梯晶面上，虽然钾可以沿着同一台阶自由移动（扩散能垒<0.01 eV），但从一个台阶扩散到另一个台阶的能垒明显更高，为0.56 eV。这表明在费托合成反应条件下，钾原子在一定程度上具有流动性，并且倾向于占据最有利的位置，而与其初始位置无关。这项工作获得的结果为钾与钴表面的相互作用提供了有价值的信息，这与实际的钴催化剂及其在BTL过程中的应用相关。
