Theoretical understanding on all-solid frustrated Lewis pair sites of C2N anchored by single metal atom

Designing all-solid heterogeneous catalysts with frustrated Lewis pairs (FLPs) has aroused great attentions recently because of its appealing low dissociation energy for H2 molecule and thus a promotion of hydrogenation reaction is expected. The sterically encumbered Lewis acid (metal site) and base (nitrogen site) in the cavity of single transition metal atom doped M/C2N sheet makes it potential candidate with FLP, while a comprehensive understanding of its intrinsic property and reactivity is still required. Calculations show that the complete dissociation of H2 molecule into two H* at the N sites requires two steps, i.e., heterolytic cleavage of H2 molecule and the transfer of H* from metal site to N site, which are highly related to the acidity of the metal site. The Ni/C2N and Pd/C2N, which outperform over the other 8 transition metal atom (M) anchored M/C2N candidates, possess low energy barriers for the complete dissociation of H2 molecule, with values of only 0.30 and 0.20 eV, respectively. Furthermore, both Ni/C2N and Pd/C2N catalysts can achieve semi-hydrogenation of C2H2 into C2H4, with overall barriers of 0.81 and 0.75 eV, respectively, lower than many reported catalysts. It is speculated that M/C2N catalysts with intrinsic FLPs may also find applications in other important hydrogenation reaction.

Achieving Theory-Experiment Parity for Activity and Selectivity in Heterogeneous Catalysis Using Microkinetic Modeling

Microkinetic modeling based on density functional theory (DFT) energies plays an essential role in heterogeneous catalysis because it reveals the fundamental chemistry for catalytic reactions and bridges the microscopic understanding from theoretical calculations and experimental observations. Microkinetic modeling requires building a set of ordinary differential equations (ODEs) based on the calculation results of thermodynamic properties of adsorbates and kinetic parameters for the reaction elementary steps. Solving a microkinetic model can extract information on catalytic chemistry, including critical reaction intermediates, reaction pathways, the surface species distribution, activity, and selectivity, thus providing vital guidelines for altering catalysts.

However, the quantitative reliability of traditional microkinetic models is often insufficient to conclusively extrapolate the mechanistic details of complex reaction systems. This can be attributed to several factors, the most important of which is the limitation of obtaining an accurate estimation of the energy inputs via traditional calculation methods. These limitations include the difficulty of using static DFT methods to calculate reaction energies of adsorption/desorption processes, often rate-controlling or selectivity-determining steps, and the inadequate consideration of surface coverage effects. In addition, the robust microkinetic software is rare, which also complicates the resolution of complex catalytic systems.

In this Account, we review our recent works toward refining the predictions of microkinetic modeling in heterogeneous catalysis and achieving theory–experiment parity for activity and selectivity. First, we introduce CATKINAS, a microkinetic software developed in our group, and show how it disentangles the problem that traditional microkinetic software has and how it can now be applied to obtain kinetic results for more sophisticated reaction systems. Second, we describe a molecular dynamics method developed recently to obtain the free-energy changes for the adsorption/desorption process to fill in the missing energy inputs. Third, we show that a rigorous consideration of surface coverage effects is pivotal for building more realistic models and obtaining accurate kinetic results. Following a series of studies on acetylene hydrogenation reactions on Pd catalysts, we demonstrate how this new approach can provide an improved quantitative understanding of the mechanism, active site, and intrinsic structural sensitivity. Finally, we conclude with a brief outlook and the remaining challenges in this field.

Discovery of a New Solvent Co-Catalyzed Mechanism in Heterogeneous Catalysis: A First-Principles Study with Molecular Dynamics on Acetaldehyde Hydrogenation on Birnessite

Heterogenous hydrogenation reactions are essential in a wide range of chemical industries. In this work, we find that the hydrogenation of acetaldehyde on birnessite cannot occur through the traditional mechanisms due to the strong adsorption of the aldehyde and hydrogen on the surface, using first-principles calculations. We discover that this reaction can occur feasibly via a solvent-cocatalyzed mechanism with molecular hydrogen in the liquid phase: a methanol solvent or a similar solvent is required for the reaction. Free energy calculations shows that the methanol solvent preferentially fills the oxygen vacancies of the catalyst surface and spontaneously dissociates on the surface, in which the resulting hydroxyl group then acts as the coordination site for the carbonyl bond and allows the reaction to proceed without adsorption of the reactants on the surface. The reasons this new mechanism is more favorable over the traditional mechanisms in the literature are scrutinized and discussed. The new mechanism may be followed in many other systems.