Beyond the Reverse Horiuti–Polanyi Mechanism in Propane Dehydrogenation over Pt Catalysts

Reactive Force Field Development for Propane Dehydrogenation on Platinum Surfaces

Propane dehydrogenation (PDH) is an on-purpose catalytic technology to produce propylene from propane that operates at high temperatures, 773-973 K. Several key industry players have been active in developing new catalysts and processes with improved carbon footprint and economics, where Pt-based catalysts have played a central role. The optimization of these catalytic systems through computational and atomistic simulations requires large-scale models that account for their reactivity and dynamic properties. To address this challenge, we developed a new reactive ReaxFF force field (2023-Pt/C/H) that enables large-scale simulations of PDH reactions catalyzed on Pt surfaces. The optimization of force-field parameters relies on a large training set of density functional theory (DFT) calculations of Pt-catalyzed PDH mechanism, including geometries, adsorption and relative energies of reaction intermediates, and key C-H and C-C bond-breaking/forming reaction steps on the Pt(111) surface. The internal validation supports the accuracy of the developed 2023-Pt/C/H force-field parameters, resulting in mean absolute errors (MAE) against DFT data of 14 and 12 kJ mol-1 for relative energies of intermediates and energy barriers, respectively. We demonstrated the applicability of the 2023-Pt/C/H force field with reactive molecular dynamics simulations of propane on different Pt surface topologies and temperatures. The simulations successfully model the formation of propene in the gas phase as well as competitive, unproductive reactions such as deep dehydrogenation and C-C bond cleavage that produce H, C1 and C2 adsorbed species responsible of catalytic deactivation of Pt surface. Results show the following reactivity order: Pt(111) < Pt(100) < Pt(211), and that for the stepped Pt(211) surface, propane activation occurs on low-coordinated Pt atoms at the steps. The measured selectivity as a function of surface topology follows the same trend as activity, the Pt(211) facet being the most selective. The 2023-Pt/C/H reactive force field can also describe the increase of reactivity with the temperature. From these simulations, we were able to estimate the Arrhenius activation energy, 73 kJ mol-1, whose value is close to those reported experimentally for PDH catalyzed by large, supported Pt nanoparticles . The newly developed 2023-Pt/C/H reactive force field can be used in subsequent investigations of different Pt topologies and of collective effects such as temperature, propane pressure, or H surface coverage.

Ethane and Propane Dehydrogenation on Small Platinum Clusters Supported on Silica: An Ab Initio Molecular Dynamics and DFT Study

The size-dependent activity of catalysts has been researched for a long time in the field of catalysis. Positively charged small Pt clusters enhance catalytic activity than bigger clusters and bulk for propane dehydrogenation. We performed DFT calculations on small Pt clusters adsorbed on silica support. The planar structure of Pt clusters is present till 4 Pt atoms, after which three-dimensional structures are observed. AIMD and DFT calculations for silica showed that it has a high surface area and thermal stability suitable to conduct dehydrogenation reactions. The adsorption of Pt cluster on silica results in the formation of directional bonds which affects the properties of the adsorbed Pt catalysts by changing the redox properties. In the bulk phase, ethane and propane molecules undergo dehydrogenation reactions with 0.133 eV atom-1 and 0.244 eV atom-1 energies, respectively. NEB calculations showed that except for Pt-2/SiO2 , all the even Pt clusters require less activation energy than the neighboring odd Pt clusters. Ethane molecule interacting with Pt-4/SiO2 , Pt-5/SiO2 , Pt-6/SiO2 , and propane with Pt-3/SiO2, Pt-4/SiO2 , Pt-5/SiO2 , Pt-6/SiO2 , follows the reverse Horiuti-Polanyi mechanism during dehydrogenation, whereas non-reverse Horiuti-Polanyi mechanism (which requires comparatively lower activation energy) is followed for smaller Pt clusters.

Surface oxygen vacancy-dependent molecular oxygen activation for propane combustion over α-MnO2

Oxygen vacancies (OV), as the sites of molecular oxygen adsorption and activation, play an important role in the catalytic combustion process of volatile organic compounds (VOCs). Revealing the relationship between OV concentration and molecular oxygen activation behavior is of significance to construct the efficient catalysts. Herein, α-MnO2 with different OV concentrations was prepared to investigate the molecular oxygen activation for C3H8 combustion. It is disclosed that the enhanced OV concentration in α-MnO2 induced the reconfiguration of surface metal atoms, resulting in the transformation of oxygen activation configuration from end-on mode to side-on mode. Oxygen molecules in side-on mode possessed more localized electron density and weaker coordination bond strength with surrounding Mn atoms, which were more favorable to adsorb C3H8 molecules and activate C-H bond for the improved combustion performance. This work provides a new understanding to reveal that the increased OV concentration contributes to more efficient VOCs combustion.

Kinetic Promotion Effect of Hydrogen and Dimethyl Disulfide Addition on Propane Dehydrogenation over the Pt-Sn-K/Al2O3 Catalyst

The kinetic effects of co-feeding of dimethyl disulfide (DMDS) and hydrogen on propane dehydrogenation (PDH) over the Pt-Sn-K/Al₂O₃ catalyst were investigated by the response surface method. The 3-level Box-Behnken design for 4 factors (reaction temperature, propene, hydrogen, and DMDS flow rate) was used to design the experiment. The initial propane conversion, propene selectivity, and coking amount were chosen as responses and the results were fitted by quadratic models. The fresh and coked catalysts were characterized by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS), thermogravimetry (TG), N₂ physisorption, and Fourier-transform infrared spectroscopy (FT-IR). Analysis of variance (ANOVA) results showed that the DMDS flow rate is significant for propane conversion and coking amount while hydrogen flow rate is only significant for the conversion. By using the fitted model for the response surface, it is found that DMDS can significantly reduce the coking amount at the expense of propane conversion, and hydrogen weakly affects the selectivity and coking amount. The optimal conditions to achieve maximum conversion and selectivity and minimum coking amount are not consistent. The DMDS and hydrogen flow rate should be optimized to obtain the maximum economic profit out of the propane dehydrogenation (PDH) process.

On-purpose design of dual active sites in single V atom anchored C2N nanosheet for propane dehydrogenation catalysis

As an emerging propylene production technology, propane dehydrogenation (PDH) has attracted much attentions from researchers. In this work, we designed the single transition metal atom anchored C2N nanosheets (TM1/C2N) with...

Aligning time-resolved kinetics (TAP) and surface spectroscopy (AP-XPS) for a more comprehensive understanding of ALD-derived 2D and 3D model catalysts

Spectro-kinetic characterization of complex catalytic materials, i.e. relating the observed reaction kinetics to spectroscopic descriptors of the catalyst state, presents a fundamental challenge with a potentially significant impact on various...

C-H bond activation in light alkanes: a theoretical perspective

Alkanes are the major constituents of natural gas and crude oil, the feedstocks for the chemical industry. The efficient and selective activation of C-H bonds can convert abundant and low-cost hydrocarbon feedstocks into value-added products. Due to the increasing global demand for light alkenes and their corresponding polymers as well as synthesis gas and hydrogen production, C-H bond activation of light alkanes has attracted widespread attention. A theoretical understanding of C-H bond activation in light hydrocarbons via density functional theory (DFT) and microkinetic modeling provides a feasible approach to gain insight into the process and guidelines for designing more efficient catalysts to promote light alkane transformation. This review describes the recent progress in computational catalysis that has addressed the C-H bond activation of light alkanes. We start with direct and oxidative C-H bond activation of methane, with emphasis placed on kinetic and mechanistic insights obtained from DFT assisted microkinetic analysis into steam and dry reforming, and the partial oxidation dependence on metal/oxide surfaces and nanoparticle size. Direct and oxidative activation of the C-H bond of ethane and propane on various metal and oxide surfaces are subsequently reviewed, including the elucidation of active sites, intriguing mechanisms, microkinetic modeling, and electronic features of the ethane and propane conversion processes with a focus on suppressing the side reaction and coke formation. The main target of this review is to give fundamental insight into C-H bond activation of light alkanes, which can provide useful guidance for the optimization of catalysts in future research.

First-principles microkinetic simulations revealing the scaling relations and structure sensitivity of CO2 hydrogenation to C1 & C2 oxygenates on Pd surfaces

CO₂ hydrogenation to alcohols and other oxygenates on Pd(211) and Pd(111) surfaces was studied by microkinetic modelling. Energy scaling relations on two surfaces were established. Activity plots as a function of reaction conditions were identified.