CAS-Climate: Atomically Resolved Single-Molecule Microscopy of Catalytic Intermediates in CO2 Reduction

Yale University, New Haven CT

Investigators

Abstract

With support from the Chemical Measurement and Imaging (CMI) Program in the Division of Chemistry, a research team led by Professors Udo Schwarz and Eric Altman at Yale University is using a combination of scanning probe microscopy (SPM) methods and theory to obtain a detailed picture of the interaction between carbon monoxide (CO) and the single-molecule catalyst cobalt phthalocyanine (CoPc). The project will examine how new imaging capabilities can be used to probe the roles of surface interactions and of substituent groups on the CoPc catalysts in order to adjust the adsorption strength of CO, a key intermediate in the reduction and conversion of CO2 to methanol. This research is designed to address issues related to climate change by contributing to the creation of a reliable, carbon-neutral energy supply based on the catalytic conversion of CO2 into methanol. Methanol is a commodity chemical that can readily be stored and transported and either used as fuel directly or converted into other liquid fuels such as diesel, gasoline, or aviation kerosene. Recent work at Yale has identified immobilized CoPc molecules as a promising platform for promoting selective CO2 conversion to methanol, but little is known about how to optimize the activity, selectivity, and stability of this potential catalyst. Therefore, the research team is developing a chemical imaging approach to show how individual CoPc molecules interact with the supporting surface and with key intermediate species in the CO2 reduction process, including CO. The work has the potential to provide new methods for optimizing these interactions by tuning substituent groups on the CoPc catalyst. The detailed molecular-level understanding that emerges from this effort could lead to improvements in electrocatalytic performance. In addition to broader impacts related to the mitigation of climate change, the project will provide advanced student training opportunities in state-of-the-art imaging methods and supports outreach activities for the general public. The reduction of CO2 involves generating CO as an intermediate, therefore efficient methanol production requires that CO binding to the cobalt atom in CoPc is neither too strong nor too weak. Although CO binding can likely be fine-tuned by changing the catalyst structure, current spectroscopic methods are inadequate for understanding CO binding strength on a single-molecule basis. To alleviate this shortcoming, the research team uses advanced scanning probe microscopy methods to locally measure the CO adsorption strength as a function of catalyst structure and support, with the goal of enabling rational catalyst optimization. Key to the project are recent advances in SPM that provide the ability to image molecular structures, distinguish bond orders, and measure small distortions in molecular structures as electrons are injected into molecules using non-contact atomic force microscopy (NC-AFM) with CO-functionalized tips. For complementary information, results from NC-AFM imaging are combined with the measurement of local work function variations obtained by Kelvin probe microscopy and the electronic structure obtained through tunneling spectroscopy, which allow mapping of the positions of the highest occupied and lowest unoccupied molecular orbitals, total charge densities, and electron transfer to and from the support. As the third major element of this effort, scanning tunneling microscopy-based action spectroscopy should enable quantification on an individual molecule basis of how supports, adsorption sites and geometries, and substituent groups influence the adsorption of reactive intermediates in the conversion of CO2 to liquid fuels This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.