The Barrier for CO₂ Functionalization to Formate on Hydrogenated Pt

Abstract

Understanding heterogeneous catalysis is based on knowing the energetic stability of adsorbed reactants, intermediates, and products as well as the energetic barriers separating them. We report an experimental determination of the barrier to CO2 functionalization to form bidentate formate on a hydrogenated Pt surface and the corresponding reaction energy. This determination was possible using velocity resolved kinetics, which simultaneously provides information about both the dynamics and rates of surface chemical reactions. In these experiments, a pulse of isotopically labeled formic acid (DCOOH) doses the Pt surface rapidly forming bidentate formate (DCO*O*). We then record the (much slower) rate of decomposition of DCO*O* to form adsorbed D* and gas phase CO2. We establish the reaction mechanism by dosing with O2 to form adsorbed O*, which efficiently converts H* or D* to gas phase water. H2O is formed immediately reflecting rapid loss of the acidic proton associated with formation of formate, while D2O formation proceeds more slowly and on the same time scale as the CO2 production. The temperature dependence of the reaction rate yields an activation energy that reflects the energy of the transition state with respect to DCO*O*. The derived heat of formation for DCO*O* on Pt(111) agrees well with results of microcalorimetry. The maximum release of translational energy of the formed CO2 provides a measure of the energy of the transition state with respect to the products and the barrier to the reverse process, functionalization of CO2. The comparison between the results on Pt(111) and Pt(332) shows that the barrier for CO2 functionalization is reduced by the presence of steps. The approach taken here could provide a method to optimize catalysts for CO2 functionalization.

Adsorption and Absorption Energies of Hydrogen with Palladium

“…The intervening four decades have seen remarkable experimental as well as computational advances in our ability to investigate surface reaction kinetics. Velocity-resolved kinetics (VRK) is a recently developed variant of MBRS that initiates a surface reaction with a temporally narrow pulsed molecular beam and subsequently provides the flux of desorbing products as a function of reaction time at the surface, also known as the kinetic trace. − VRK can provide surface site-specific reaction rate constants as well as surface diffusion rate constants. , Computational methods have also improved dramatically. For example, density functional theory (DFT) has become a workhorse for computing fundamental adsorbate–surface interactions, , which can be helpful in modeling thermal rate coefficients, providing key parameters needed for implementing transition state theory .…”

Velocity map images of desorbing oxygen from sub-surface states of Rh(111)

“…We extract the image from the raw data by subtracting thermal background and converting density to flux by multiplying the signal with the corresponding velocity. [27] Figure 3: Top view of the ionization region. The dashed circle in the center illustrates the area from which ions are accelerated towards the detector.…”

“…The VMI image is fitted with a thermal Maxwell-Boltzmann distribution providing the pixel to velocity scaling factor of the detector as explained in detail in a previous publication. [27] Additionally, we record conventional TPD spectra by linearly ramping up the sample temperature in front of a mass spectrometer (RGA-200, Stanford-Research-Systems).…”

“…[24] This new technique has enabled a variety of studies resolving surface reaction kinetics with unprecedented resolution including CO oxidation on Pt, desorption kinetics of chiral molecules, H2 oxidation on Pt, and CO2 functionalization to formate on hydrogenated Pt. [25][26][27][28] In this paper, we present velocity distributions of recombinatively-desorbing oxygen from Rh (111) obtained during TPD experiments in combination with VMI. Such distributions are valuable dynamical fingerprints of the microscopic processes involved in desorption of sub-surface oxygen, to which theory can be tested and refined.…”

Velocity‐resolved Laser‐induced Desorption for Kinetics on Surface Adsorbates

“…VRK is capable of providing surface site specific rate constants; [2b] for example, reactions at terraces may appear with different product‐velocities than reactions at steps [3] and kinetic traces may depend dramatically on surface step‐density [2f] . Taking advantage of its kinetic competition with desorption, adsorbate diffusion has also been detected with VRK [2e,f] . While TPD, MBRS and VRK have proven highly useful, they cannot be used to monitor the concentrations of surface adsorbates directly, as they rely on desorption for their signals.…”