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

Abstract

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 H 2 and in the temperature range of 291−315 K. The extracted kinetics were then used in conjunction with a complementary constant rate of H 2 input experimentation to model the performance of a DPB-catalysis/support system as a function of temperature and H 2 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 H 2 transport in the material. This work provides a good illustration of a rarely encountered chemical process with a negative activation energy barrier.

Hydrogen Uptake Kinetics of 1,4-Bis(phenylethynyl)benzene Rubberized O-Rings: Measurements, Modeling, and Comparison with Additional Forms of Organic Getters

“…In isoconversional analysis (or analysis at the same conversion level), the experiments performed at different temperatures or heating rates need to have the same starting and ending conversion levels. Usually, the range of 0–100% is academically preferred, but from a practical consideration of degradation in performance of organic getters with increasing consumption, , a much less than 100% conversion (like 80–90%) would suffice. In this report, one of the experiments was terminated early at 91% conversion due to a technical difficulty.…”

“…Isoconversional analysis has previously successfully modeled the complex hydrogen uptake reactions of hydrogen getter materials. 5,7,13 A more detailed description of isoconversional model-free analysis has been given previously. 7 Isoconversional analysis begins with the general reaction rate equation in eq 1 below.…”

“…Isoconversional kinetic analysis determines the kinetic parameters of a reaction without requiring the assignment of a specific mechanism; thus, it is described as a model-free kinetic analysis. Isoconversional analysis has previously successfully modeled the complex hydrogen uptake reactions of hydrogen getter materials. ,, A more detailed description of isoconversional model-free analysis has been given previously …”