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/H₂/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.