(520f) Predictive Kinetics of the Water Gas Shift Reaction On Pt(111) Via Reaction Route Graph Analysis

Authors: 
Vilekar, S., Worcester Polytechnic Institute
Fishtik, I., Worcester Polytechnic Institute
Datta, R., WPI


New catalysts are sought for the water-gas shift (WGS) reaction in connection with distributed hydrogen generation. This quest is aided by an improved fundamental understanding of WGS catalysis. We will describe our systematic theoretical approach of Reaction Route (RR) Graphs for developing a comprehensive understanding of the WGS reaction based on a 17-step molecular mechanism with a priori DFT-predicted kinetics. Like electric circuits, the RR Graph approach is consistent with Kirchhoff's network laws of flux and potential, and in addition to flux analysis, thus, the problem of thermodynamic consistency of the reaction kinetics is naturally treated within the framework of the RR Graph approach. We show how to draw the RR Graph for the 17-step WGS mechanism. This is next utilized to show how all the 71 alternate reaction routes or pathways can be enumerated simply as walks between terminal nodes. These pathways are normally enumerated via a stoichiometric algorithm. Further, the flux analysis afforded by the RR graph allows the dominant reaction pathways and the rate-limiting steps to be identified in a transparent manner. The associative mechanism mediated via carboxyl species is shown to the dominant reaction pathway. The direct decomposition of carboxyl species and dissociation of water are identified as the slow or rate-limiting steps, based on the concept of step resistance without making any ad hoc assumptions. Moreover, our circuitry approach also allows us to derive a highly simplified rate expression agreeing well with the complete microkinetic model.

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