Bond Order Conservation Strategies in Catalysis Applied to the NH₃ Decomposition Reaction

On the basis of an extensive set of density functional theory calculations, it is shown that a simple scheme provides a fundamental understanding of variations in the transition state energies and structures of reaction intermediates on transition metal surfaces across the periodic table. The scheme is built on the bond order conservation principle and requires a limited set of input data, still achieving transition state energies as a function of simple descriptors with an error smaller than those of approaches based on linear fits to a set of calculated transition state energies. We have applied this approach together with linear scaling of adsorption energies to obtain the energetics of the NH₃ decomposition reaction on a series of stepped fcc(211) transition metal surfaces. This information is used to establish a microkinetic model for the formation of N₂ and H₂, thus providing insight into the components of the reaction that determines the activity

Examining the Linearity of Transition State Scaling Relations

The dissociation of strong bonds in molecules shows large variations in the geometric structure of the transition state depending on the reactivity of the surface. It is therefore remarkable that the transition state energy can be accurately described through linear relations such as the Brønsted-Evans–Polanyi relations. Linear scaling relations for adsorbates with fixed structure can be understood in terms of bond order conservation but such arguments should not apply to transition states where the geometric structure varies. We have investigated how to relate the concepts from linear adsorption energy scaling to transition state energies. We expect that strong deviations from linearity only occur for very early or very late transition states. According to the Sabatier principle, the rate-limiting step of the best catalysts is not expected to be in either of these regions. Our results therefore support the use of linear transition state scaling relations for the optimization of catalysts

Sintering of Pt Nanoparticles via Volatile PtO₂: Simulation and Comparison with Experiments

It is a longstanding question whether sintering of platinum under oxidizing conditions is mediated by surface migration of Pt species or through the gas phase, by PtO₂(g). Clearly, a rational approach to avoid sintering requires understanding the underlying mechanism. A basic theory for the simulation of ripening through the vapor phase has been derived by Wynblatt and Gjostein. Recent modeling efforts, however, have focused entirely on surface-mediated ripening. In this work, we explicitly model ripening through PtO₂(g) and study how oxygen pressure, temperature, and shape of the particle size distribution affect sintering. On the basis of the available data on α-quartz, adsorption of monomeric Pt species on the support is extremely weak and has therefore not been explicitly simulated, while this may be important for more strongly interacting supports. Our simulations clearly show that ripening through the gas phase is predicted to be relevant. Assuming clean Pt particles, sintering is generally overestimated. This can be remedied by explicitly including oxygen coverage effects that lower both surface free energies and the sticking coefficient of PtO₂(g). Additionally, mass-transport limitations in the gas phase may play a role. Using a parameterization that accounts for these effects, we can quantitatively reproduce a number of experiments from the literature, including pressure and temperature dependence. This substantiates the hypothesis of ripening via PtO₂(g) as an alternative to surface-mediated ripening

Data Available for 'Generic Approach to Access Barriers in Dehydrogenation Reactions'

Data sources for plotting the figures in the article.<div>In the file name, term 'TSS' means transition state scaling relation; term 'GX' means the Gamma (slope) and Xi (intercept) functions.</div><div><br></div><div>Please contact Liang Yu at [email protected] or Frank Abild-Pedersen at [email protected] to access the unzipping password.</div

Theoretical Insights into Methane C–H Bond Activation on Alkaline Metal Oxides

In this work, we investigate the role of alkaline metal oxides (AMO) (MgO, CaO, and SrO) in activating the C–H bond in methane. We use Density Functional Theory (DFT) and microkinetic modeling to study the catalytic elementary steps in breaking the C–H bond in methane and creating the methyl radical, a precursor prior to creating C₂ products. We study the effects of surface geometry on the catalytic activity of AMO by examining terrace and step sites. We observe that the process of activating methane depends strongly on the structure of the AMO. When the AMO surface is doped with an alkali metal, the transition state (TS) structure has a methyl radical-like behavior, where the methyl radical interacts weakly with the AMO surface. In this case, the TS energy scales with the hydrogen binding energy. On pure AMO, the TS interacts with AMO surface oxygen as well as the metal atom on the surface, and consequently the TS energy scales with the binding energy of hydrogen and methyl. We study the activity of AMO using a mean-field microkinetic model. The results indicate that terrace sites have similar catalytic activity, with the exception of MgO(100). Step sites bind hydrogen more strongly, making them more active, and this confirms previously reported experimental results. We map the catalytic activity of AMO using a volcano plot with two descriptors: the methyl and the hydrogen binding energies, with the latter being a more significant descriptor. The microkinetic model results suggest that C–H bond dissociation is not always the rate-limiting step. At weak hydrogen binding, the reaction is limited by C–H bond activation. At strong hydrogen binding, the reaction is limited due to poisoning of the active site. We found an increase in activity of AMO as the basicity increased. Finally, the developed microkinetic model allows screening for improved catalysts using simple calculations of the hydrogen binding energy

Configurational Energies of Nanoparticles Based on Metal–Metal Coordination

Nanoparticle sintering remains a fundamental problem in heterogeneous catalysis, motivating mechanistic studies toward mitigating deactivation of precious metal catalysts. We present a model based on the local coordination environment of metal atoms that can be used to provide total energy estimates for metal nanoparticles in a space of generic configurations. All energies are based only on a small set of density functional theory calculations of single metal atom adsorption on metal slabs. A model that can provide accurate nanoparticle energies is an important step toward the goal of understanding their sintering behavior in practical catalytic contexts

A Theoretical Study of Methanol Oxidation on RuO₂(110): Bridging the Pressure Gap

Partial oxidation catalysis is often fraught with selectivity problems, largely because there is a tendency of oxidation products to be more reactive than the starting material. One industrial process that has successfully overcome this problem is partial oxidation of methanol to formaldehyde. This process has become a global success, with an annual production of 30 million tons. Although ruthenium catalysts have not shown activity as high as the current molybdena or silver-based industrial standards, the study of ruthenium systems has the potential to elucidate which catalyst properties facilitate the desired partial oxidation reaction as opposed to deep combustion due to a pressure-dependent selectivity “switch” that has been observed in ruthenium-based catalysts. In this work, we find that we are able to successfully rationalize this “pressure gap” using near-ab initio steady-state microkinetic modeling on RuO₂(110). We obtain molecular desorption prefactors from experiment and determine all other energetics using density functional theory. We show that, under ambient pressure conditions, formaldehyde production is favored on RuO₂(110), whereas under ultrahigh vacuum pressure conditions, full combustion to CO₂ takes place. We glean from our model several insights regarding how coverage effects, oxygen activity, and rate-determining steps influence selectivity and activity. We believe the understanding gained in this work might advise and inspire the greater partial oxidation community and be applied to other catalytic processes which have not yet found industrial success

Tuning the MoS₂ Edge-Site Activity for Hydrogen Evolution via Support Interactions

The hydrogen evolution reaction (HER) on supported MoS₂ catalysts is investigated using periodic density functional theory, employing the new BEEF-vdW functional that explicitly takes long-range van der Waals (vdW) forces into account. We find that the support interactions involving vdW forces leads to significant changes in the hydrogen binding energy, resulting in several orders of magnitude difference in HER activity. It is generally seen for the Mo-edge that strong adhesion of the catalyst onto the support leads to weakening in the hydrogen binding. This presents a way to optimally tune the hydrogen binding on MoS₂ and explains the lower than expected exchange current densities of supported MoS₂ in electrochemical H₂ evolution studies