Paper HC+SS-ThA6
Spectroscopic Techniques for Identifying Reactive Intermediate Structures during Decomposition of Formic Acid over Metals and Metal Oxides
Thursday, October 25, 2018, 4:00 pm, Room 201A
Identifying reactive intermediates within an “organometallic zoo” of species that form on surfaces of metal nanoparticles and metal oxides during reactions is a long standing challenge in heterogeneous catalysis.1 We use a combination of spectroscopic techniques and data analysis methods to isolate and extract spectral features of the reactive intermediates present under relevant operating conditions. Modulation excitation spectroscopy coupled with phase sensitive detection selectively filters spectra to show only reactive intermediates and suppress features of spectating species.2 The combined spectra of all reactive species are deconvoluted using singular value decomposition techniques that provide distinct spectra and estimate surface coverages for independent species.3 Comparisons of spectra obtained at different modulation frequencies exploit differences in intrinsic free energy barriers between reactive intermediates by removing species that are unable to form at higher stimulated frequencies. We combine these techniques to molecularly interpret infrared spectra to identify compositionally similar surface intermediates formed in situ during decomposition of formic acid (HCOOH) over anatase TiO2 and Au-TiO2. Operando and transient measurements determine the sequence of elementary steps that decompose HCOOH and identify surface species that do not form CO and CO2. Transient measurements show that HCOOH adsorbs and deprotonates to form bidentate formates over TiO2 while monodentate intermediates do not lead to CO or CO2 formation. Selective inhibition of Au sites with co-fed CO over Au-TiO2 indicate monodentate reactive intermediates evolve into bidentate formates on Au at higher coverages. Distinguishing the structure, composition, and orientation of reactive intermediates provides complimentary evidence to measured rates and DFT to depict reaction mechanisms and provide insight into how surface properties dictate rate and selectivity. References:
(1) Burwell, R. L. The mechanism of heterogeneous catalysis, C&EN Magazine, 22 August, 1966, 56.
(2) Urakawa, A.; Burgi, T.; Baiker, A. Chem. Eng. Sci. 2008, 63, 4902.
(3) Jaumot, J.; de Juan, A.; Tauler, R. Chemometr. Intell. Lab. 2015, 140, 1.