For complex reactions with more than one key surface intermediate, bifunctional multiphase catalysts have a significant advantage over conventional monophase catalysts since each phase can potentially be adjusted independently to activate a key reaction step. Unfortunately, our understanding of multiphase catalysts whose activity and selectivity is primarily determined by the interfaces and adjacent surroundings remains relatively poor. Andreas Heyden of the University of South Carolina in this Faculty Early Career Development (CAREER) Program Award proposes utilizing computational studies to establish the underlying science of such bifunctional heterogeneous catalysts whose activity is largely determined by the three-phase boundary (TPB) of a gas-phase, a reducible oxide support, and a noble metal cluster or nanoparticle. The work will focus on identifying the origin or descriptors of the unique activity of Au and Pt catalysts supported on ceria and titania for the water-gas shift (WGS) reaction. The desire to better understand chemical reactions at the TPB is motivated by the fact that most heterogeneous catalysts consist of several solid phases and that although the overall catalyst activity and selectivity is often known to be a result of multiphase effects, the understanding of the chemical function of multiphase systems is relatively poor.
Given the complexity of catalyst systems, connecting theoretical calculations to experimental observations becomes extremely challenging for multiphase systems. Heyden proposes to use modern density functionals and to quantify uncertainty in the computational predictions based on a network of reactions. As a result, realistic and meaningful probabilistic predictions can be made which significantly facilitates connecting theoretical calculations with experimental observations. Heyden plans to use modern Bayesian statistical tools to validate reaction site models and to identify a required level of accuracy for a given prediction. Experimental information for comparison is secured through collaborations with scientists at Purdue University and the University of Southern California.
Broader Impact Understanding the origin and identifying descriptors for the unique activity of oxide supported noble metal catalysts for the WGS has the potential to lead to the development of improved WGS catalysts for mobile and stationary applications. Furthermore, insights obtained from this study can likely be applied to various chemical reactions since the combination of reducible oxide supports and noble metals catalyze many reactions. In addition, the application of a computational strategy that quantifies uncertainty in computational catalysis predictions is important not only for reactions occurring at TPBs but for most complex reactions occurring at lower temperatures where even small errors in reaction energies lead to large uncertainties in predicted turnover frequencies, apparent activation barriers, and reaction orders.
The research results of the proposed project will be integrated into a joint graduate and undergraduate elective ?Multiscale Modeling: From Electrons to Chemical Reactors? as part of the core chemical engineering curriculum to promote active, inquiry based learning. A continuous outreach program will be established with the Engineering Academy of a local urban high school (92% African American students) to increase the participation of underrepresented minorities in the study of engineering. Key components of this program include guest lectures, hands-on learning experiences of Academy students on the USC campus, and a mentoring program for Engineering Academy students.
