Influence of Carbon and Oxygen Chemical Potentials on the Hydrogen Donor Identity During Methanation on Ni, Co, and Ni‐Co Clusters

Abstract

Rate measurements in the kinetically controlled regime and equilibrium carbon and oxygen chemical titrations show two distinct mechanistic paths during COx methanation reactions on first‐row transition metal clusters. On Ni and, for a limiting set of conditions, Ni−Co clusters, the reaction occurs via the addition of a hydrogen adatom into CH3* intermediates on clusters partially covered with carbon. On Co and, in a subset of conditions, Ni−Co clusters, it occurs via the donation of hydrogen from OH* to CH3* on clusters partially covered with reactive oxygen adatoms (O*). The [CO]2‐to‐[CO2] and [CO2]‐to‐[CO] operating ratios are the surrogates of carbon and oxygen chemical potentials, respectively, as a consequence of water‐gas shift equilibration. These ratios, together with the carbon and oxygen binding energies, determine the relative surface coverages of carbon and oxygen, the involvement of H* vs. OH* in the kinetically‐relevant step, and in turn, the rate dependencies. Stronger carbon and oxygen binding energies lead to more stabilized transition states of the kinetically relevant steps and larger methanation rates.

The Role of Protons and Hydrides in the Catalytic Hydrogenolysis of Guaiacol at the Ruthenium Nanoparticle–Water Interface

“…97 The mechanistic synergy extends to CO hydrogenation on Co and Ni−Co catalysts, where isolated surface CO is surrounded with neighboring surface hydroxyl groups, which serve Brønsted acid functions that assist the activation of the strong CO bond. 114 Thus, we emphasize that the direct introduction of external acid functions will not promote the hydrodeoxygenation reactivity, whereas a carefully engineered active site with acid and metal functions in proximity, as we have reported above, would result in high reactivity and selectivity toward the desired C−O bond rupture.…”

Concepts Relevant for the Kinetic Analysis of Reversible Reaction Systems

“…(4. 16) is truly a number. The condition of entire equilibrium therefore redresses the incongruity of Z i and Z ov 1/σ ̅ = Z ov 1/σd eq and awards to composite reactions intimate connections between kinetics, thermodynamics, and stoichiometry (Table 2).…”

“…Even with the correct assignment of σ̅, simplest in stoichiometrically regular reactions, precedent literature has, in large part, misapprehended quantification of forward rate because of inexact alliance of the De Donder equation with governing reactor mass balances. ,,,− The De Donder equation is, crucially, a statement relating the reaction reversibility, a thermodynamic state function, to instantaneous, or differential, rates. The path- independent nature of Z ov and path- dependent nature of R and R⃗ complicates application of eq to experimental rate measurements in batch, semibatch, and flow reactors.…”

“…That is to say, that unless Δ Z eff ≪ 1, the forward rate cannot be calculated as R⃗ = − normalΔ F A / L false( 1 − Z eff false| l = L false) = ⟨ R ⟩ false( 1 − Z eff false| l = L false) because ⟨ R ⟩ = −Δ F A / L is an integral measure of reaction kinetics and Z eff | l = L is a measure of thermodynamics specific to the outlet of the reactor . Equation has, based on our survey of the literature, almost invariably been used for the assessment of forward rates in flow reactors in the past three decades. ,,,− We therefore take this opportunity to caution researchers against discordant comparison of length-averaged net rates and instantaneous, effluent reversibility for the calculation of unidirectional reaction rates. Instead, best practice dictates that forward rates ought be calculated either by (i) assumption of spatially invariant R⃗ and length-averaging of Z eff for judicious comparison to ⟨ R ⟩ or, more preferably, (ii) explicit calculation of d F A /d l by differentiation of contact-time-dependent data entirely analogous to the calculation of rates by numerical differentiation of clock-time-dependent data in batch reactors.…”

“…In exploring the posed questions, we will demonstrate that, appropriately understood and exploited, consequences of thermodynamics can edify ,,− rather than confound. The De Donder equality and inequality, for instance, afford quantitative criteria for falsification of mechanistic postulates operative even for reactions far from equilibrium.…”