Adsorption Energy Shifts for Oxygen and Hydroxyl on 4-atom Metal-Decorated Graphene Catalysts Via Solvation, pH, and Substrate Dopants: Effects on ORR Activity

Abstract

Recent literature results have highlighted the role of small transition metal and intermetallic nanoparticles supported on graphene as catalysts for many key applications in energy and commodity chemicals industries. Specifically, metal nanoparticle catalysts down to sizes of 4 and even 1 (single atom catalysts) on graphene have been studied for the Oxygen Reduction Reaction (ORR). A recent study showed that 4-atom transition metal intermetallic nanoparticles (NP) on graphene (metal-decorated graphene (MDG)) even generate a predictive Volcano Plot for ORR activity. Initial results from that study were not completely explained, and an expanded analysis and discussion built from that work is presented in this manuscript. Specifically, in this new work, the original Volcano Plot for 4-atom MDG NP catalysts for the ORR is analyzed for its counter-intuitive thermodynamic inversion between the rate limiting steps of O* hydrogenation and OH* hydrogenation. The Volcano Plot is then further studied for dependence on solvent correction energy, system pH, and with an initial probe on the sensitivity of descriptor values on doping of the graphene support via B and N atoms. Recommendations for optimum 4-atom MDG NP catalyst operation for the ORR are provided, and directions for future work and study are provided.

Size, Composition, and Support-Doping Effects on Oxygen Reduction Activity of Platinum-Alloy and on non-Platinum Metal-Decorated-Graphene Nanocatalysts

“…Previous work focused on identifying the thermodynamically stable 4-atom graphenesupported nanoparticle structures and calculating the adsorption energies of the intermediate species for the ORR (O*, OH*, OOH*, and O 2 *). [1,2] Following the approach from, these adsorption energies were transformed into catalytic activities and an electrochemical volcano plot was created, allowing to identify the best graphene-supported 4-atom intermetallic nanoparticles. [1][2][3][4][5][100][101][102] In this work, those best candidates were further studied for size and composition dependence, by increasing the number of atoms to 7 and 19 (considered to have higher stability than other particle sizes).…”

“…[1,2] Following the approach from, these adsorption energies were transformed into catalytic activities and an electrochemical volcano plot was created, allowing to identify the best graphene-supported 4-atom intermetallic nanoparticles. [1][2][3][4][5][100][101][102] In this work, those best candidates were further studied for size and composition dependence, by increasing the number of atoms to 7 and 19 (considered to have higher stability than other particle sizes). [1,2,6,[103][104][105] The composition of these particles was kept close to the original 4-atom ones, by using a ratio of 5:2 in the 7 atom particles, and 14:5 in the 19 atom nanoparticles.…”

“…The use of advanced, specifically-engineered paired catalyst-support systems is likely the key to advancing the field in pursuit of several transformational technologies related to reduction of small molecules such as O 2 , CO 2 , and N 2 . [1][2][3][4][5][6][7][8][9][10][11][12] Such reactions are critical to meet rising global concerns in energy, the environment, and sustainability/green chemistry all driven by rising populations and associated demands for increases in the standard-ofliving. In recent years graphene and graphene-metal hybrid catalyst systems have gathered growing attention in the literature.…”

“…[1, 2, 32, 34, 35, 37, 42-44, 50, 52, 53, 55, 58, 69, 70, 72] This manuscript presents striking new results that extend our initial work from the last 2 years to examine the role of sub-nanometer transition metal nanoparticles (TMNP) on graphene as metal-decorated-graphene (MDG). [1,2] TMNP-MDG serves as a conceptual physical regime bridge between single-atom-catalysts (typically embedded in graphene/etc) and traditional small TMNP with large degree of faceting related to low Miller Index surfaces. Specifically, in this new work presented in this manuscript, we show the ability of dopants in the graphene support to significantly modulate (upshift or downshift) the adsorption energy of mono-atomic oxygen on 4-atom, 7-atom, and 19-atom TMNP of various binary alloy/intermetallic compositions.…”

“…Oxygen (and hence, Hydorxyl) are key intermediates in the ORR and the adsorption of mono-atomic oxygen, O*, on the catalyst surface is typically given as the descriptor necessary to predict the performance of a TMNP for the ORR. [1][2][3][4][5] Our prior work identified a Volcano Plot for the ORR on undoped TMNP-MDG where none of the 4-atom binary intermetallic systems fell on the Volcano maximum, and no systems close to the maximum used constituent elements that are significantly more abundant /affordable/sustainable than platinum or the other platinum-group-metals (PGM). Our prior work showed that there is an unanticipated inversion in the thermodynamics of OH* formation at the Volcano crossing from the "base metals" to the "precious metals".…”