Quantitative Studies of the Key Aspects in Selective Acetylene Hydrogenation on Pd(111) by Microkinetic Modeling with Coverage Effects and Molecular Dynamics

Abstract

Selective acetylene hydrogenation as a vital industrial reaction has been studied for decades. However, some key issues remain elusive. Herein, a detailed microkinetic model using density functional theory energies is developed to comprehensively investigate the key aspects of acetylene hydrogenation on Pd(111). The coverage-dependent kinetic simulation, factoring in both self and cross-interactions of adsorbates as well as the coverage effects on the transition states of each elementary step, is compared with the coverage-independent kinetic calculation derived from energies obtained at low coverages. The accurate determination of the free-energy barrier of ethylene desorption using ab initio molecular dynamics (AIMD) with umbrella sampling adds further crucial insights into the microkinetic model. By combining the coverage-dependent calculations and AIMD results, we achieve a full first-principles kinetic simulation with all the kinetic parameters being systematically calculated to understand the acetylene hydrogenation. We show that the coverage-dependent microkinetic model gives a much more reasonable turnover frequency result of 1.41 s −1 at 300 K than that calculated using the coverage-independent model (3.16 × 10 −24 s −1 ). The microkinetic results, including activity and selectivity, are tested against the experimental data, yielding a good agreement. A comprehensive set of kinetic analyses is performed. It is found that the activity is mainly determined by the barriers of the first two hydrogenation steps. The step with the major effect on the reaction activity is gradually transitioned from the second hydrogenation step, C 2 H 3 * + H* ↔ C 2 H 4 * + *, to the first hydrogenation step of C 2 H 2 * + H* ↔ C 2 H 3 * + * as the temperature increases. Both the desorption barrier of ethylene and the hydrogenation barrier of C 2 H 4 * cause significant impacts on the selectivity of ethylene, but the former is found to be the most prominent physical quantity affecting the ethylene selectivity.

Influence of surface defects on activity and selectivity: a quantitative study of structure sensitivity of Pd catalysts for acetylene hydrogenation

“…The results of Pd(111) were taken from our previous work. 38 The favorable adsorption geometry of C 2 H 2 is on the B5 site under the step edge, and C 2 H 4 is found to adsorb on the step edge with a di-σ configuration, which is consistent with previously reported results. 5,61 For C 2 H 2, the adsorption energy of -2.33 eV on Pd(211) suggests a strong chemisorption, and the binding energy is slightly larger than that on Pd(111), which has an adsorption energy of -2.01 eV.…”

“…Recall our previous coverage-dependent study on Pd(111), 38 the reason for choosing both C 2 H 2 and C 2 H 3 as environmental species was that the C 2 H 3 hydrogenation is kinetically hindered and the barrier is as high as the first hydrogenation step, which is the rate-limiting step on Pd(111). With the C 2 H 3 hydrogenation barrier dropped significantly on Pd(211), the…”

“…The self-interaction of the H atom on the step Pd(211) is found to be similar to that on Pd(111), which makes it negligible unless a very high coverage is achieved. 38 In this work, a total of 8 possible cross-interactions are listed in Table 3. Three trends worth mentioning here: (i) Among all the adsorbate-adsorbate interactions, the differential chemisorption energy of C 2 H 2 was most affected by the selfinteraction of C 2 H 2 with a slope of 2.47 in low coverage region and 6.502 in high coverage region.…”

“…Taking the coverage effect into account and using methods such as ab initio molecular dynamics (AIMD) to obtain energetic details that are not readily available in traditional calculations brings new insights to the reaction kinetics. 30,37,38 This framework has included self and cross adsorbate-adsorbate interactions, in order to provide a quantitative description of differential chemisorption energies on different structured surfaces. The microkinetic model achieves agreement with experimental results on the Pd(111) and lays the foundation for our study of structural sensitivity.…”

“…To achieve more accurate kinetic results, the ethylene desorption energy barrier was obtained using the AIMD method based on the steady-state results obtained from coverage-dependent calculations. 38 The activity and selectivity of ethylene were explicitly investigated using microkinetic simulations on Pd(211). Our simulations finds the Pd(211) surface to be much more active than the Pd(111) surface, while a strong differential chemisorption energy and lower further hydrogenation barrier of ethylene limit the selectivity.…”