60th Annual Report on Research 2015

Burning petroleum based fossil fuels, while providing an efficient and convenient fuel source, has led to a dramatic increase in the atmospheric CO₂ content with associated well-established adverse environmental damage. Remediating this increased CO₂ concentration by direct sequestration or conversion into useful products is one of this century’s biggest challenges. This will in turn free up the planet’s limited petroleum resources to be used for other purposes, such as a feedstock for chemical transformations, which would arguably be a better use of this precious resource.

A large research effort sponsored by the ACS PRF and other funding agencies goes into developing molecular catalysts used for these remedial pathways. For CO₂ catalysis to be practical and economical, a heterogeneous system is needed. Immobilizing the catalyst on a high surface-area support improves the throughput and ease of separation of the reaction products from the catalytic system. However, adsorption of a molecular catalyst to a surface significantly changes the properties of the catalyst in a presently unknown and uncontrollable way. Colloidal or nanostructured TiO₂ has been a prototypical platform for catalyst immobilization due to the electrical conductivity in addition to its high surface area. While providing an electrocatalytic or photocatalytic support, colloidal TiO₂ possesses a very large structural heterogeneity through different exposed facets. This microscopic variability influences the catalyst binding geometry and molecular properties. This complex structure makes it challenging to systematically study the effect of the underlying surface on the properties of the catalyst.

We employ an alternative approach using surface-specific sum-frequency generation (SFG) spectroscopy to study molecular CO₂ catalysts bound to single-crystalline TiO₂ surfaces. SFG is a powerful method that measures the interfacial molecular vibrational spectrum with sub-monolayer sensitivity. To completely understand the effect of the TiO₂ surface on the catalyst, dynamical measurements on the time scale of the molecular motion, femtoseconds and picoseconds for molecular vibrations, are necessary in addition to characterizing the static structure. Our research efforts has increased the signal strength and decreased the acquisition time of these complex experiments, allowing us to systematically probe the properties of molecular catalysts bound to single-crystalline surfaces as a function of the specific TiO₂ surface.

With the ACS PRF grant, we have successfully implemented transient IR pump – SFG probe experiments to study the vibrational relaxation dynamics of catalyst monolayers on single-crystalline TiO₂ surfaces. Our initial efforts have focused on the Re(4,4’-dicarboxy-2,2’-bipyridine)(CO)₃Cl catalyst (referred to as ReCO3). We have examined the catalyst on both (110) and (001) single-crystalline rutile TiO₂ surfaces and found that both the structure and the vibrational relaxation dynamics differ at the two surfaces. We find that the catalyst orientation on rutile (001) is isotropic within the surface plane, whereas the catalyst on rutile (110) exhibit C_2V symmetry. This shows that the catalyst is oriented perpendicularly to the (001) surface, in agreement with the literature, but is tilted on the (110) surface, likely due to a difference in the binding geometry through the carboxylic acid groups.

We furthermore find that the vibrational relaxation time of the symmetric metal-CO stretch vibrational relaxation time is different at the two surfaces. This data proves that that the TiO₂ surface structure not only affects the static structure and orientation of the catalyst but also the dynamical properties. The difference in the vibrational relaxation is a result in a change in the catalyst electronic structure. By extension, the catalytic properties and efficiency of the catalyst will also depend on the specific TiO₂ surface the catalyst is bonded to. When considering that the catalytically useful nanocrystalline TiO₂ scaffold contains a random distribution of different exposed facets, correlating these properties to the surface structure is critical for further increasing the efficiency of these systems. We recently submitted a manuscript describing these findings.

We recently extended the transient SFG experiments to coherent surface-specific 2D-IR spectroscopy (2D-SFG), which we will soon apply to the catalytic monolayers. This advanced technique will provide a detailed insight into the ultrafast vibrational dynamics and couplings of the vibrational modes of the catalyst, offering further understanding of the effect of the TiO₂ surface structure on the electronic structure of the catalyst. A key feature of this technical improvement is the development of a new robust method of heterodyned detection of the SFG signal, which we are currently writing up.

The ACS PRF award has enabled my group to build the experimental infrastructure to perform static, transient, and 2D-SFG experiments. We have successfully shown that both the orientation and vibrational dynamics of the ReCO3 catalyst depend on the TiO₂ surface structure. It is well known that the catalytic properties of metallic surfaces greatly depend on the surface structure and a large research effort goes into discovering the optimal surface structure to drive specific catalytic processes. Our results show that the catalytic properties of molecular catalysts will also depend on the TiO₂ surface structure and will be important for optimizing chemical transformation of CO₂, especially as nanofabrication allows for increased control over the exposed surfaces of nanostructured materials. These initial results and proof of concept experiments were made possible by the ACS PRF award and will be instrumental in obtaining further funding of this project. By providing the experimental infrastructure and the initial results to launch this research direction in my laboratory, the ACS PRF award have had and will continue to have a significant impact on my own scientific career as well as that of my senior graduate student who has been funded on this award.