Formic Acid Dehydration Rates and Elementary Steps on Lewis Acid-Base Site Pairs at Anatase and Rutile TiO2 Surfaces

Formic acid (HCOOH) decomposition is often used to assess the acid-base properties of oxide surfaces. Its reverse reaction forms HCOOH and formate species that can act as intermediates in CO2/CO/H-2/H2O reactions that are important in C1 conversions. This study describes the mechanism of HCOOH dehydration on acid-base pairs at anatase and rutile TiO2 surfaces through spectroscopic, desorption-reaction, kinetic, isotopic, and theoretical methods. HCOOH dehydration turnover rates are measured at coverages that allow bound intermediates to interact directly with Ti-5c-O-2c pairs. Such rates then reflect their acid-base properties without interference from a refractory bidentate formate adlayer that acts as the catalytic surface at lower temperatures, as evident from infrared and desorption reaction data. HCOOH dehydration elementary steps involve the concurrent activation of C-O and C-H bonds in a molecularly bound HCOOH (HCOOH*) by a Ti-5c-O-2c pair at the kinetically relevant step. The transition state mediating this step involves the OH group and the H-atom of the C-H group in HCOOH* that are almost fully transferred to the Ti-5c and the vicinal O-2c center, respectively. Such concerted interactions with the acid and base centers and the late character of the transition state render the H2O dissociation energy at Ti-5c-O-2c pairs a more suitable descriptor of HCOOH reactivity than the respective strengths of each Lewis center. These mechanistic conclusions allow quantitative inferences of the rate and kinetic parameters for HCOOH synthesis from CO-H2O reactants on TiO2 surfaces through the tenets of microscopic reversibility extended to the sequence of elementary steps. The results also illustrate how acid-base pairs act in concert to stabilize the relevant transition states, thus making the balance between acid and base strengths, instead of their independent properties, the rigorous arbiters of reactivity, as shown by the similar reactivities and H2O dissociation energies on Ti-5c-O-2c pairs at anatase and rutile surfaces in spite of their very different acid and base strengths.