Natalie Austin

Catalysis at the sub-nanoscale: complex CO oxidation chemistry on a few Au atoms

Au has been widely used as jewelry since ancient times due to its bulk, chemically inert properties. During the last three decades, nanoscale Au has attracted remarkable attention and has been shown to be an exceptional catalyst, especially for oxidation reactions. Herein, we elucidate a puzzle in catalysis by using multiscale computational modeling: the experimentally observed “magic number” CO oxidation catalytic behavior of sub-nanoscale Au clusters. Our results demonstrate that support effects (cluster charging), symmetry-induced electronic effects on the clusters, catalyst reconstruction, competing chemical pathways and formation of carbonate contribute to the marked differences in the observed catalytic behavior of Aun− clusters with n = 6, 8 and 10 atoms. This is the first demonstration of multiscale simulations on sub-nanoscale catalysts unraveling the magic number activity for the CO oxidation reaction on Au.

CO2 activation on Cu-based Zr-decorated nanoparticles

Density functional theory (DFT) calculations have been applied to investigate the electronic and CO2 adsorption properties of 55-atom Cu-based nanoparticles (NPs) decorated with Zr atoms (Cu55−xZrx, x = 0–12). Our results revealed that the Zr atoms preferably reside on the surface of the Cu NPs generating sites that chemisorb and activate CO2 (linear to bent geometry and elongation of CO bonds). Importantly, we demonstrate that while the CO2 formation of the activated state on the Cu NPs is endothermic, it becomes barrierless and exothermic on the Zr-decorated NPs. The CO2 activation and chemisorption was attributed to charge transferred from the NPs to the CO2 molecule. We identified the local-site d-band center and, interestingly, the ionization potential of the NP as descriptors correlating with the CO2 chemisorption. As a result, we demonstrate that one can tune the ionization potential of the NPs and, in turn the CO2 chemisorption energy, by varying the Zr content of the NPs. Additionally, we investigated the activity of CuZr NPs as catalysts for CO2 dissociation to CO and determined that Cu54Zr was a very efficient catalyst compared to Cu55. Overall, this work highlights how surface decoration can change the electronic properties of the NPs and result in CO2 activation, which are important steps for designing catalysts that capture and convert CO2 to fuels and chemicals.

Design of Copper‐Based Bimetallic Nanoparticles for Carbon Dioxide Adsorption and Activation

Cu-based nanoparticles (NPs) are promising candidates for the catalytic hydrogenation of CO2 to useful chemicals because of their low cost. However, CO2 adsorption and activation on Cu is not feasible. In this work we demonstrate a computational framework that identifies Cu-based bimetallic NPs able to adsorb and activate CO2 based on DFT calculations. We screen a series of heteroatoms on Cu-based NPs based on their preference to occupy a surface site on the NP and to adsorb and activate CO2 . We revealed two descriptors for CO2 adsorption on the bimetallic NPs, the heteroatom (i) local d-band center and (ii) electropositivity, which both drive an effective charge transfer from the NP to CO2 . We identified the CuZr bimetallic NP as a candidate nanostructure for CO2 adsorption and showed that although the Zr sites can be oxidized because of their high oxophilicity, they are still able to adsorb and activate CO2 strongly. Importantly, our computational results are verified by targeted synthesis, characterization, and CO2 adsorption experiments that demonstrate that i) Zr segregates on the surface of Cu, ii) Zr is oxidized to form a bimetallic mixed CuZr oxide catalyst, which iii) can strongly adsorb CO2 , whereas Cu NPs cannot. Overall our work highlights the importance of the generation of binding sites on a NP surface based on (catalyst) stability and electronic structure properties, which can lead to the design of more effective CO2 reduction catalysts.

Elucidating the active sites for CO2 electroreduction on ligand-protected Au25 nanoclusters

Using density functional theory (DFT) calculations, we investigated the electrochemical reduction of CO2 and the competing H2 evolution reaction on ligand-protected Au25 nanoclusters (NCs) of different charge states, Au25(SR)18q (q = −1, 0, +1). Our results showed that regardless of charge state, CO2 electroreduction over Au25(SR)18q NCs was not feasible because of the extreme endothermicity to stabilize the carboxyl (COOH) intermediate. When we accounted for the removal of a ligand (both –SR and –R) from Au25(SR)18q under electrochemical conditions, surprisingly we found that this is a thermodynamically feasible process at the experimentally applied potentials with the generated surface sites becoming active centers for electrocatalysis. In every case, the negatively charged NCs, losing a ligand from their surface during electrochemical conditions, were found to significantly stabilize the COOH intermediate, resulting in dramatically enhanced CO2 reduction. The generated sites for CO2 reduction were also found to be active for H2 evolution, which agrees with experimental observations that these two processes compete. Interestingly, we found that the removal of an –R ligand from the negatively charged NC, resulted in a catalyst that was both active and selective for CO2 reduction. This work highlights the importance of both the overall charge state and generation of catalytically active surface sites on ligand-protected NCs, while elucidating the CO2 electroreduction mechanisms. Overall, our work rationalizes a series of experimental observations and demonstrates pathways to convert a very stable and catalytically inactive NC to an active electrocatalyst.