Phase Change-Driven Negative Activation Energies in Pd/Carbon-Based/Organic Getter Hydrogenation Reactions

The hydrogenation of 1,4-diphenylbutadiyne (DPB) blended with carbon-supported Pd (DPB–Pd/C) in the form of pellets was investigated by isothermal–isobaric experiments at 1333 Pa of H2 and in the temperature range of 291–315 K. The extracted kinetics were then used in conjunction with a complementary constant rate of H2 input experimentation to model the performance of a DPB-catalysis/support system as a function of temperature and H2 partial pressure. First-principles density functional theory (DFT) calculations were also performed to shed light on the molecular level energetics of DPB and its intermediate states. A seemingly puzzling formation of alternate positive activation energy barrier (higher reaction rate with higher temperature) and negative activation energy barrier (higher reaction rate with lower temperature) zones during the hydrogenation process was discovered. However, this observed phenomenon can be logically explained in terms of the associated phase changes and H2 transport in the material. This work provides a good illustration of a rarely encountered chemical process with a negative activation energy barrier.