Rational design of bimetallic alloys for effective hydrodechlorination of 4-chlorophenol

Abstract

A combined density functional theory (DFT) and microkinetic model (MKM) based approach is applied to design Pd-based A₃B type bimetallic alloys – Pd₃Cu, Pd₃Ag and Pd₃Au for hydrodechlorination (HDC) reaction of 4-chlorophenol (4-CP). The DFT results predict that the binding energy trend of 4-CP is Pd₃Cu (111) [−257 kJ mol⁻¹] > Pd (111) [−165 kJ mol⁻¹] > Pd₃Au (111) [−151 kJ mol⁻¹] ≈ Pd₃Ag (111) [−151 kJ mol⁻¹]. The adsorbed 4-CP then undergoes C–Cl bond dissociation followed by hydrogenation to form phenol. Of all the surfaces, Pd₃Ag (111) exhibits the least activation barrier for Cl dissociation from 4-CP (44 kJ mol⁻¹), followed by Pd₃Au (111) (66 kJ mol⁻¹), Pd (111) (71 kJ mol⁻¹) and Pd₃Cu (111) (76 kJ mol⁻¹) surfaces. The DFT generated data are utilized to construct an ab initio microkinetic model to calculate the turnover frequencies (TOFs) of 4-CP HDC. The Pd₃Ag (111) surface displays the highest TOF, which could be attributed to the ease of Cl dissociation from the 4-CP molecule. However, the low TOF over the Pd₃Cu (111) surface could be due to surface poisoning as a result of high 4-CP binding energy on the surface. For the C–Cl bond dissociation step, a positive degree of rate control is observed, which suggests that this step is crucial in ascertaining the HDC rate.