Reports: ND651290-ND6: Exploring Electronic-Transfer Pathways of Hot Electrons in Organically-Assisted Metal Catalysts

James P. Lewis, PhD, West Virginia University

Hong Wang, PhD, West Virginia University

Research Impact

Gold nanoparticles for novel nano-catalysis research is a new major research directions of the Lewis' (PI) research group. One senior research scientist and two undergraduate students have been working on the theoretical study of the fundamental properties of organic assisted gold nanoparticles. Our research direction has lead to two submitted articles and one pending proposal (for three years' research project) submitted to the National Science Foundation (NSF).

1.  We performed a theoretical study on the synthesized gold nanocatalyts for the aerobic oxidation of olefins. Different from homogeneous catalysts for olefin oxidation, which are difficult for separation and regeneration, pyrrolidone-modified gold nanoparticles present unique catalytic performance in olefins oxidations. Our experimental collaborators find that pyrrolidone modified Au/SBA-15 always exhibits superior catalytic properties in the oxidation of cyclohexene and styrene by molecular oxygen at atmospheric pressure, compared with the pyrrolidone-free SBA-15 supported Au catalyst (Au/SBA-15-N). We theoretically observed, through density functional theory (DFT) calculations that this phenomenon was strongly related to the interaction between Au nanoparticles and organic ligand pyrrolidone.

2.  In collaboration with NMR experimental studies, we investigated two possible attaching modes of L-cysteine molecules on the gold nanoparticle, Au55. We find that cysteine bonds strongly to the gold nanoparticle via a strong Au-S interaction. Furthermore, cysteine can arrange in either a monolayer or bilayer configuration around the nanoparticle, which determines whether or not it has a zwitterion structure. In the bilayer model, the cysteine zwitterion structure is stabilized via the H-bonding between inner layer cysteine and outer layer cysteine. With the outer layer as a charge balance shell, the inner layer cysteine tends to anchor to gold particle via stronger interaction. The three type carbon atoms (Cα, Cβ and Cγ) of inner layer and outer layer present the similar trend in terms of charge changes as what observed in solid NMR spectrum. The two carbons (Cβ and Cγ) closer to sulfur atoms exhibit higher charge changes, which corresponds to the chemical shift signal detected. While for the Cα, the chemical shift preserves the original peak, which is similar to the minor charge changes shown in our calculations. Overall, we propose that the bilayer model is the more energetically possible arrangment for cysteine anchoring to gold nanoparticles. This motif enables the outer layer sulfur atoms to serve as open portal for further bio-functional modification.

3.  As an extension of our proposed research, we also investigate gold nanoparticles as catalysts on variety of oxide supports, which include TiO2, SiO2, ZSM-5 (zeolites) and MAO (Mg-Al mixed oxide). Among the different oxide supported gold nanoparticles, we found MAO-supported gold nanocatalyts exhibit superior catalytic properties in the aerobic homo-coupling of phenylboronic acid. Through our DFT calculations, we find that the gold nanoparticle catalytically assists the reaction mechanism. More importantly, we are pursuing calculations to investigate the impact from different oxide supports; we believe the oxide support will play an important role in enhancing the catalytic performance of gold nanocatalyst. Our comprehensive study of gold nanoparticles on oxide supports, will enable us to reveal the reason of the unique catalytic activity presented by gold nanoparticles as a function of the oxide support. Further, we will be able to achieve a clear picture of how to engineer the catalytic activity by using different modifications techniques, such which oxide supports or organic ligands to utilize.

In summary, all these studies have been written as research articles, one of them has been published, and the other two have been submitted to prestigious research journals and currently under consideration. Also, gold nanocatalysis research has become one of the major research directions in Lewis' research group and as such one pending proposal has been submitted to NSF.

Educational Impact

Two undergraduate students have been working on in this research direction and they are playing a major role in delivering research results. Since joining Dr. Lewis' research group, Jessica Carr has been mentored by the senior scientist (Wang) in performing the computational investigation of the interface between ligands and gold nanoparticles. A recent investigation of monolayers and bilayers on Au nanoparticles was submitted for publication to J. Phys. Chem. C and she is first author on this manuscript. In May 2012, she was awarded a Goldwater Scholarship as a result of her remarkable academic record and research contributions related to this research. In her Goldwater Scholarship application, she noted that the research experience gained in Lewis' research group changed her perspective about chemistry research. As a result of her participation in this research, Jessica has changed her career plans from initially wanting to become a pharmacist to now wanting to pursue a professional career in chemistry academy.

Another student, Andrew Rice, has been working on understanding the fundamental properties of gold nanoparticles with different oxide supports. This current research project is also his capstone research thesis as well. After research months experience in the Lewis' research group, Andrew Rice also decided to change his initial four years' studying for B.S. into a Ph. D degree pursuit.