(718b) Revisiting Formic Acid Decomposition On Bulk Metal Catalysts

Formic acid decomposition on bulk metal catalysts has been employed as a catalytic reactivity probe and used to establish the so-called “volcano curve” in heterogeneous catalysis more than 50 years ago.  A close examination of this early catalysis work reveals that the activity parameter (50% conversion of HCOOH) and catalyst property (bulk heat of metal-formate formation) are not the appropriate scientific descriptors of this catalytic reaction. For example, the reaction rates are qualitative and were not normalized per unit surface area of the catalyst or number of catalytic active sites. The bulk heat of metal-formate formation rather than the appropriate surface heat of metal-formate formation was employed in the correlation. Furthermore, this classic research was never repeated in the catalysis literature. Thus, the objective of the present investigation is to revisit this classic study from a modern catalysis perspective to examine if the “volcano plot” still holds up.

The bulk metal catalysts (Au, Au, Cu, Pt, Ru, Pd, Ni, W, Fe) were synthesized by reducing the corresponding metal oxides with H₂ at elevated temperatures. The number of catalytic active sites per gram of catalyst was determined with HCOOH chemisorption. The reaction rate for HCOOH decomposition per gram of catalyst was determined with steady-state reaction studies. The turnover frequencies, TOF=HCOOH molecules decomposed per catalytic active site per second, were determined by dividing the reaction rate/g with number of catalytic active sites/g. The nature of the surface formate intermediates on the different metal surfaces was determined by in situ IR spectroscopy.  The thermal stability and decomposition kinetics of the surface formate intermediates was determined with temperature programmed decomposition (TPD). The surface heats of metal-formate formation were determined from Density Functional Theory (DFT) calculations.

                The findings from this modern analysis of the classic “volcano curve” for formic acid decomposition on the surface of bulk metal catalysts do not result in a volcano plot. The reasons for the different findings between the classic work and the modern work will be discussed.