Elucidating the Role of Strain in Catalysis toward Modulating Surface-Adsorbate Interactions and Tuning Catalytic Activity

Strain has been shown to modulate adsorption and reactions on metal surfaces. While its effect on surface-adsorbate interactions has been rationalized, an understanding of the electronic factors that drive these interactions and their consequences on catalytic activity is lacking. In this work, we use ab initio density functional theory (DFT) and microkinetic modeling (MKM) to develop electronic descriptors that govern the effect of biaxial strain in the modulation of interactions between adsorbate and transition states with catalyst surface and report its significance in enhancing the activity of fcc Pd(111) in the synthesis of ammonia (NH3), an important renewable-energy and hydrogen (H2) vector. We established the p-band center (pcenter) of the adsorbates and transition states (TS) and the hybridized d-band center (dcenter) of the surface metal as key electronic descriptors for adsorbate and TS energy variations with strain. Specifically, the pcenter of the adsorbates is lowest for the sites with the strongest adsorption, and the upshift of the dcenter of the surface metal atoms is greatest for the adsorption site with the highest strain susceptibility (i.e., the change in adsorption energy per unit applied strain). Importantly, we showed significant deviations in scaling relations with strain compared to periodic scaling relationships, both for adsorption and reaction. Over a net 4% tensile strain (±2%), the dcenter of Pd(111) moved upward by 0.21 eV, enhancing N2 dissociation, the rate-determining step in NH3 synthesis by ~37×, and the pcenter in N bound to the catalyst surface moved downward in the adsorbed state and upward in the TS (i.e., electron density shifted toward the bonding and anti-bonding states, respectively). Thus, tensile strain played a dual role in enhancing N2 dissociation, strengthening the adsorption of atomic N and weakening the N-N bond in the TS. We then evaluated N2 dissociation at 3/4 ML H-coverage under industrial conditions (150 atm H¬2, 50 atm N2, and 723 K), revealing the effect of tensile strain on the rate enhancement to be nearly two orders of magnitude greater (~3273× vs. ~37×) at high surface coverages. Overall, this study highlights strain as a useful design tool to improve catalytic activity.