Elementary steps and site requirements in formic acid dehydration reactions on anatase and rutile TiO2 surfaces

Abstract

Mechanistic details of HCOOH decomposition routes provide valuable insights into reactions involving bound formates as intermediates or spectators; these routes are also widely used as a probe of the acid-base properties of oxide surfaces. The identity and kinetic relevance of bound intermediates, transition states, and elementary steps are reported here for HCOOH dehydration on anatase and rutile TiO2 surfaces through complementary kinetic, isotopic, spectroscopic and theoretical assessments. Five-coordinate exposed Ti5c centers are saturated with bidentate formates (*HCOO*) at catalytic conditions (423–463 K; 0.1–3 kPa HCOOH), as evident from infrared spectra collected during catalysis and the amounts of HCOOH and CO evolved upon heating the TiO2 samples containing pre-adsorbed HCOOH-derived species. These *HCOO* species are inactive but form a stable “surface template” that contains stochiometric protons onto which HCOOH binds molecularly (HCOOH-H*) to form a coexisting adlayer. H2O elimination from HCOOH-H* is the sole kinetically-relevant step. DFT-derived barriers show that this step involves its reaction with Ti5c-O2c that acts as a Lewis acid-base pair. Such route, in turn, requires the access of HCOOH-H* to a Ti5c center, which is made available through a momentary reprotonation of a *HCOO*. This step is much less facile on rutile than on anatase due to stronger acid strength of its Ti5c centers that binds *HCOO* species more strongly and its shorter Ti5c-Ti5c distances that induce greater repulsions between co-adsorbed HCOOH* formed upon reprotonation step. These differences account for low dehydration reactivity of rutile at these temperatures. This mechanistic interpretation is in full accord with DFT-derived barriers, binding energies, and kinetic isotope effects that quantitatively agree with the values from regressed kinetic and thermodynamic parameters, with in-situ infrared spectra that identify HCOOH-H* species as the sole reactive intermediates, and with the differences in turnover rates between anatase and rutile catalysts. These dehydration routes are also consistent with the surface chemistry expected for Lewis acid-base pairs on stoichiometry TiO2 surfaces without requiring the presence or involvement of reduced centers or titanols in the catalytic cycle. The reaction routes described in this work show how strongly-bound species, evident in presence and unreactive nature from in-situ infrared spectra, provide an organic “permanent” template for reactions of weakly-bound species that are often invisible in spectroscopy.