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

Abstract

Describing the activation of O 2 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 O 2 adsorption mechanisms on a zirconia-based cathode. In the first step, we model O 2 adsorption on two defective surfaces: oxygen-deficient t-ZrO 2−x and oxynitride t-ZrO 2−x N x , in an aqueous solution. We describe various O 2 adsorption states by analyzing charge transfer and cohesive energy changes in O 2 molecules, Zr active sites, and defects. The results suggest that O 2 adsorption mechanisms on the surfaces of t-ZrO 2−x and t-ZrO 2−x N x occur through dissociative and associative pathways, respectively. Additionally, O 2 adsorption on t-ZrO 2−x N 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−x N x with the hydroxyl (OH) group. Afterward, we simulate the O 2 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−x N x , preventing their loss. Moreover, OH groups influence the O 2 adsorption mechanism on t-ZrO 2−x , shifting toward associative O−O bond breaking. Conversely, O 2 adsorption on hydroxylated t-ZrO 2−x N x remains molecularly associative. Overall, on hydroxylated surfaces, O 2 adsorption involves stronger charge transfer among oxygen, defects, and Zr active sites. In the third step, we explored the trends of desorption of the O 2 from these surfaces. This entails analyzing O 2 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 O 2 from hydroxylated surfaces is lower, indicating a more spontaneous process compared to t-ZrO 2−x and t-ZrO 2−x N 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.