The IMASC research investigations, strategically organized into three Focus Areas driving the five Research Objectives, allows for seamless integration to achieve center-wide synergistic strategic objectives. An example of such integration (schematically shown) and some of the specific projects currently underway (described briefly below) is anticipated to lead to delineating principles for designing efficient catalytic processes for oxidation reactions. This strategy would be expanded to other platform reactions such as alcohol dehydrogenation reactions.
Project 1—O2 activation on Ag/Au alloys: Our recent theoretical studies have focused on Au crystals dilute in Ag as a means of understanding the roles of the Ag distribution in the material in the crucial O2 activation step. The activation of O2 to yield adsorbed oxygen atoms is the first and crucial step for selective oxidation on npAu. Without adsorbed O, there is no bond activation on single crystals or catalytic function of npAu. Our atomistic density functional theory (DFT) studies are exploring both O2 activation and the surface composition of the alloys as a function of temperature and oxygen pressure in order to probe the dynamic behavior of these alloys. We have found that single Ag atoms isolated in Au behave very similarly to pure Au, in agreement with previous work. Since the Ag is thought to play a very important role in nanoporous Au, and the Ag is present in a very low concentration (~1-3%), the exact role of Ag is yet to be elucidated; however, these results suggest that there is a critical ensemble of Ag required for O2 dissociation. This is a question under active study.
Project 2—Effect of molecular structure on bonding and reactivity. This aspect of our work is directed towards the goal of predicting how molecular structure affects reactivity so as to develop a molecular-scale model of selective catalysis. Prior studies of primary alcohols on O-covered Au established a general mechanism for selective oxidation and coupling reactions and also established a hierarchy of bonding strength that determined the competition for active sites during reactions. The hierarchy established for primary alcohols in model studies accurately predicted the conditions for controlling selectivity for methyl ester production under catalytic conditions using npAu. In order to probe the effect of molecular structure on bonding and reactivity, the reactivity of different alcohols are bieng investigated on Au(110) containing Oads. This is the first in a series of experimental studies that will probe competitive bonding of various molecules to the surface.
Project 3 – Ozone-activated npAu catalyst for selective oxidative coupling reactions across pressure regimes (UHV to atmospheric conditions): We have demonstrated that ozone-activated nanoporous gold (npAu) is a highly versatile catalyst for the production of various industrially relevant esters through self-coupling and cross-coupling of alcohols. The selectivity under realistic conditions corresponded directly to that on oxidized Au(111) and the catalyst can be recycled between various cross-coupling conditions with no permanent loss of activity. These investigations showed broader applicability across pressure regimes (UHV to atmospheric conditions) and the possibility to extend to unsaturated alcohols and aldehydes. We have also demonstrated the product selectivity for the cross-coupling of methanol and ethanol on oxygen-covered Au(111) as a function of the mole fraction of methanol deposited was demonstrated and for the cross-coupling of methanol and ethanol over npAu as a function of the gas phase mole fraction of MeOH, 150 °C, 50 mL/min, 20% O2.
Project 4 – Dynamics of restructuring in npAu catalysts: In order to investigate catalytically selective hydrogenation using np materials with engineered porosity and composition to improve activity and selectivity, we have demonstrated that ozone treatment of nanoporous gold (at 150 °C for 1 hour in 2% O3) resulted in high activity (and selectivity) for alcohol oxidation (i.e. coupling) reactions. The oxidation and reduction process appears to cause a restructuring of the catalyst); which appears to be responsible for the observed activity. The formation of metastable structures under reaction conditions appear to suggest a mechanism by which leaching of metals from the bulk occurs, resulting in metal-added intermediate formation. By taking advantage of both the high detection sensitivity and the controlled pulsing ofreactants provided by the temporal analysis of products reactor (TAP) technique, we found a clear evidence that O2 activation is due to residual silver atoms at the surface. These results showed that the apparent energy of O2 activation is comparable to the experimental results on a single-crystal Ag surfaces indicating that O2 activation occurs on clusters of Ag at the alloy surface. This is a clear experimental evidence that the residual Ag atoms are responsible for O2 activation and the investigations to date demonstrate that TAP reactor can be used to probe the activation of key reactants as well as to advance mechanistic understanding of oxidation reactions on npAu catalysts. We have utilized this knowledge for selective synthesis of methyl esters -methyl acetate and methyl butyrate - through catalytic O2-assisted cross coupling of methanol with ethanol or 1-butanol.