Hydroxyl-Induced Modification of Oxygen Activation and Desorption Free Energy on Defective Tetragonal Zirconia Catalysts

Abstract

Describing the activation of O₂ on metal surfaces is crucial for understanding fundamental electrochemical processes, such as the oxygen reduction reaction (ORR) in hydrogen fuel cells. This study explores how defects influence O₂ adsorption mechanisms on a zirconia-based cathode. In the first step, we model O₂ adsorption on two defective surfaces: oxygen-deficient t-ZrO_2–x and oxynitride t-ZrO_2–xN_x, in an aqueous solution. We describe various O₂ adsorption states by analyzing charge transfer and cohesive energy changes in O₂ molecules, Zr active sites, and defects. The results suggest that O₂ adsorption mechanisms on the surfaces of t-ZrO_2–x and t-ZrO_2–xN_x occur through dissociative and associative pathways, respectively. Additionally, O₂ adsorption on t-ZrO_2–xN_x leads to the departure of N dopants from the surface, which is unfavorable for catalytic activity. In the second step, we modified the surfaces of t-ZrO_2–x and t-ZrO_2–xN_x with the hydroxyl (OH) group. Afterward, we simulate the O₂ activation process on these modified surfaces and identify the most probable active sites. Our findings reveal that OH groups stabilize N dopants on hydroxylated t-ZrO_2–xN_x, preventing their loss. Moreover, OH groups influence the O₂ adsorption mechanism on t-ZrO_2–x, shifting toward associative O–O bond breaking. Conversely, O₂ adsorption on hydroxylated t-ZrO_2–xN_x remains molecularly associative. Overall, on hydroxylated surfaces, O₂ adsorption involves stronger charge transfer among oxygen, defects, and Zr active sites. In the third step, we explored the trends of desorption of the O₂ from these surfaces. This entails analyzing O₂ desorption using steered molecular dynamics (SMD) to generate potential mean force (PMF) profiles and applying Jarzynski’s equality to calculate the free energy of desorption. Herein, we find that the free energy of the desorption of O₂ from hydroxylated surfaces is lower, indicating a more spontaneous process compared to t-ZrO_2–x and t-ZrO_2–xN_x. Moreover, we discover that oxygen has the highest tendency to desorb from the hydroxylated-ZrO_2–x surface, which is attributed to the lowest free energy involved in pulling oxygen from the surface, potentially influencing ORR acceleration. These findings offer valuable guidance for developing efficient nonplatinum-based cathode materials, particularly in catalysis applications.