In this dissertation, we have employed a multi-faceted approach, combing well-controlled synthesis techniques, a variety of characterization techniques and Density Functional Theory calculations to optimize important heterogenous catalytic and photocatalytic process. The efforts were to investigate the supports effects on atomically dispersed metal electronic state, metal structure and pulsed illumination on methanol carbonylation, ethylene hydroformylation and methanol decomposition. In the first work, we successfully synthesize atomically dispersed Rh on a various of supports for ethylene hydroformylation. We utilized a combination of a series of characterization techniques and density functional theory calculations to illustrate that supports influence on the Rh electronic states resulting in different reaction barriers. This work demonstrates that the role of supports on atomically dispersed metal for ethylene hydroformylation and provides insights into how the electronic state of atomically dispersed metals can be controlled by the supports to drive important catalytic hydroformylation process.
In the second part of this work, we research how the metal structure influences on methanol carbonylation reactivity and selectivity. We hypothesized that a pair-site catalyst consisting of a Lewis acid site and an atomically dispersed Rh site could more selectively drive methanol carbonylation than Rh cluster through a mechanism where CO coordinated to Rh and methanol dissociated on the acid site to enable CO insertion at the interface. Through qualitative and quantitative characterization techniques including CO probe molecule FTIR and NH3-TPD along with rigorous kinetic measurements of methanol carbonylation reactivity, it was shown that atomically dispersed Rh species selectively promoted the AA formation, while Rh clusters unselectively decomposed methanol into CO/CO2. Further, decreasing the concentration of support acid sites reduced the rate of dimethyl ether (DME, the primary byproduct) formation. This work demonstrates how atomically dispersed Rh, support engineering, and active site pairing can be used to control selectivity in methanol carbonylation process. Additionally, effort was placed on more deeply understanding the role of Lewis acid sites in this process and developing approaches that minimize byproduct DME formation. We hypothesized that the formation of isolated Lewis’s acid sites on an inert oxide support could decrease the rate of DME formation, by minimizing the possibility of surface dimerization reactions. It is well known that isolated Rhenium oxide (ReOx) species can be formed on oxide supports and that they exhibit strong acidic sites. The structures of ReOx were characterized by using UV-vis spectroscopy, STEM, and XAS. Reactivity measurements surprisingly showed that atomically dispersed ReO4 species on SiO2 showed high selectivity (more than 93%) and rates of AA formation, while ReOx clusters were primarily selective for DME formation. With optimization of atomically dispersed ReOx species weight loading, and total catalyst loading in the reactor, methanol conversions > 60% with > 93% selectivity to AA were observed with stable performance over 60 hours. This is the best AA yield in a single pass heterogeneous catalytic system operating in a halide free process with industrially reasonable CO:methanol feed ratios that we are aware of. This work introduces a new class of promising heterogeneous catalysts based on atomically dispersed ReO4 on inert supports for alcohol carbonylation. We further modified the ReOx/SiO2 catalyst with Rh, forming atomically dispersed Rh-ReOx pair sites, which further promoted AA production rates 10-fold and AA selectivity to >96%, resulting in volumetric AA production rates comparable to homogeneous processes.
At last, photon illumination was utilized to promote reaction rate and selectivity through transient charge transfer to adsorbed species. In this project, we demonstrate that illumination of 2 nm diameter Pt nanoparticle catalysts with pulsed visible light enhances time-averaged rates of H2 production via methanol decomposition compared with static illumination. Our results suggest that using light pulses to dynamically control the energetics of elementary steps on catalytic surfaces may enable higher activity or selectivity than is possible with static illumination or dictated by linear free energy scaling relations.