A New Computational Interface for Catalysis

Kravchenko, Pavlo; Plaisance, Craig; Hibbitts, David

doi:10.26434/chemrxiv.8040737.v3

Cited by 19 publications

(30 citation statements)

References 23 publications

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“…Periodic DFT calculations were completed using the Vienna ab initio simulation package (VASP) − in the computational catalysis interface (CCI) . The revised Perdew–Burke–Ernzerhof (RPBE) form of the generalized gradient approximation (GGA) was used for all calculations, , and plane waves were composed of projector augmented-waves (PAWs) , with an energy cutoff of 400 eV, unless otherwise noted.…”

Section: Methodsmentioning

confidence: 99%

Theoretical and Experimental Characterization of Adsorbed CO and NO on γ-Al₂O₃-Supported Rh Nanoparticles

et al. 2021

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Rh active sites are critical for NO x reduction in automotive three-way catalysts. Low Rh loadings used in industrial catalysts lead to a mixture of small nanoparticles and single-atom Rh species. This active-site heterogeneity complicates the interpretation of characterization and reactivity, making the development of structure–function relationships challenging. Density functional theory (DFT) investigations of Rh catalysts often employ flat, periodic surfaces, which lack the curvature of oxide-supported Rh nanoparticle surfaces, raising questions about the validity of periodic surface model systems. Here, we combine DFT with probe molecule Fourier transform infrared (FTIR) spectroscopy and high-resolution scanning transmission electron microscopy of supported Rh catalysts synthesized to insure against the in situ formation of single-atom Rh species to compare periodic and nanoparticle DFT models for describing the interaction of CO and NO with supported Rh nanoparticles. We focus on comparing the behavior of model systemsRh(111) and a 201-atom cubo-octahedral Rh nanoparticle (Rh201; ∼1.7 nm diameter)to explain the behavior of CO and NO bound to Rh nanoparticles with an average particle diameter of ∼2.6 nm. Our DFT calculations indicate that CO* occupies a mixture of threefold and atop modes on Rh(111), saturating at 0.56 ML CO* (473 K, 1 bar), while CO* saturates Rh201 near 1 ML. Similarly, NO* binds to threefold sites and saturates the Rh(111) surface at 0.67 ML but saturates the Rh201 particle surface at 1.38 ML, indicating that more NO* binds than there are Rhsurf atoms. Moreover, the adlayers on the Rh201 particle contain predominantly atop-bound CO*, with bridge CO* possible on particle edges and predominantly threefold NO* with bridge- and atop-bound NO* bound to edges and corners. These binding modes and higher coverages are made possible by the curvature of these nanoparticles and by the expansion of surface metal–metal bondsneither of which can occur on Rh(111)which together permit the adlayer to laterally relax, reducing internal strain. FTIR data for CO* on 10 wt % Rh/γ-Al2O3 show predominantly atop binding modes (2067 cm–1) with small broad peaks near bridge (1955 cm–1) and threefold (1865 cm–1) regions. Meanwhile, NO* FTIR spectroscopy also shows a mixture of atop (1820 cm–1) and threefold (1685 cm–1) NO* features, with similar features observed at reaction conditions (5 mbar NO, 1 mbar CO, 478 K), indicating that NO* dominates Rh surfaces during catalysis. Frequency calculations on these adlayers of Rh201 particles yield dominant frequencies that more closely resemble those observed in FTIR spectra and demonstrate how coverage and dipole–dipole coupling affect vibrational frequencies with surface curvature. Taken together, these results indicate that the Rh surface curvature alters the structure and spectral characteristics of NO* and CO* for Rh nanoparticles of ∼2.6 nm diameter, which must be accurately reflected in DFT models.

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Section: Methodsmentioning

confidence: 99%

Theoretical and Experimental Characterization of Adsorbed CO and NO on γ-Al₂O₃-Supported Rh Nanoparticles

et al. 2021

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“…For particle calculations, the Brillouin zone was sampled at the Γ-point in all calculations and the geometries were (as before) converged in a two-step scheme. These multistep optimization schemes implemented in CCI are 4–7× faster than analogous one-step calculations …”

Section: Methodsmentioning

confidence: 99%

Mechanism and Effects of Coverage and Particle Morphology on Rh-Catalyzed NO–H₂ Reactions

2020

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Three-way catalysts, which typically include Rh, are used to treat automotive exhaust and reduce nitric oxide (NO) with a combination of CO and H2, although few kinetic and theoretical investigations have studied NO–H2 reactions on Rh. Here, we examine NO activation, which is believed to control the rate of NO reduction, through direct, NO-assisted, and H2-assisted dissociation pathways on NO*-covered Rh(111) surfaces and Rh nanoparticle models using density functional theory (DFT) and contrast these results with previously reported data on Pt(111) surfaces. Saturation coveragesdetermined by incrementally adsorbing NO*were determined to be 5/9 ML NO* on Pt(111), 6/9 ML on Rh(111), and 1.38 ML on a 201-atom Rh nanoparticle (∼2 nm). Free energies of activation and reaction were calculated by DFT for the pathways at these coverages and interpreted through maximum rate analyses over a wide range of NO and H2 pressures to predict NO activation mechanisms and kinetics. Rates are inhibited by NO at all relevant NO pressures and to similar extents on all catalyst models. On Pt(111) surfaces, NO is activated through NOH* formation and dissociation (to N* and OH*) at low H2 pressures (<0.5 bar) and through HNOH* (to HN* and OH*) at high H2 pressures (>0.5 bar), resulting in a shift in the H2 dependency from half order to first order. NO is activated through NOH* formation and dissociation on Rh(111) at all relevant H2 pressures, with all other pathways being >1000 times slower. NO activation occurs with similar rates through either NOH* or HNO* on Rh particles at 1.38 ML NO*, indicating that these high coverages can shift mechanistic preferences. Predicted NO consumption rates are half order in H2 on Rh particles and surfaces and are similar in magnitude to one another, despite shifts in the mechanism; these rates on Rh are 106 times slower than Pt, consistent with the prior reports that demonstrate that equal turnover rates for Pt at 60 °C occur for Rh at 200 °C. This work demonstrates that strong NO bonds activate through bimolecular (assisted) pathways and that particle models of catalysts enable high coverages of strongly bound species, which can then influence relative rates and mechanistic predictions.

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“…Periodic, dispersion-corrected density functional theory (DFT) calculations were carried out using the Vienna ab initio simulation package (VASP) [44][45][46][47] as implemented in the Computational Catalysis Interface (CCI). 48 Planewaves were constructed using the projector augmented-wave (PAW) potentials with an energy cutoff of 400 eV. The Perdew-Burke-Ernzerhof (PBE) form of the generalized gradient approximation (GGA) was used to determine exchange and correlation energies.…”

Section: Methodsmentioning

confidence: 99%

“…All reactant, product, and transition state structures were modeled on the three accessible O-sites of T11 in MFI (O14, O16, and O24) and at all four unique O-sites in CHA (O1, O2, O3, and O4). Each reactant, product, and transition state structure was systematically reoriented as implemented in CCI 48 in an attempt to identify the global minimum, rather than local minima. These reorientations can find configurations of guest species with energies as much as 50 kJ mol −1 , as discussed in previous work.…”

Section: Reorientation Of Reactant Product and Transition State Spementioning

confidence: 99%

Mechanisms of Alkene and Diene Hydrogenation Reactions in H-MFI and H-CHA Zeolite Frameworks During MTO

DeLuca

Janes²,

Hibbitts³

2019

Preprint

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Co-feeding H2 at high pressures increases zeolite catalyst lifetimes during methanol-to-olefin (MTO) reactions while maintaining high alkene-to-alkane ratios; however, the mechanisms and species hydrogenated by H2 cofeeds to prevent catalyst deactivation remain unknown. This study uses periodic density functional theory (DFT) to examine hydrogenation mechanisms of MTO product C2-C4 alkenes, as well as species related to the deactivation of MTO catalysts such as C4 and C6 dienes, benzene, and formaldehyde in H-MFI and H-CHA zeolite catalysts. Results show that dienes and formaldehyde are selectively hydrogenated in both frameworks at MTO conditions because their hydrogenation transition states proceed via allylic and oxocarbenium cations which are more stable than alkylcarbenium ions which mediate alkene hydrogenation. Diene hydrogenation is further stabilized by protonation and hydridation at α,δ positioned C-atoms to form 2-butene from butadiene and 3-hexene from hexadiene as primary hydrogenation products. This α,δ-hydrogenation directly leads to selective hydrogenation of dienes; pathways which hydrogenate dienes at the α,β-position (e.g., forming 1-butene from butadiene) proceed with barriers 20 kJ mol -1 higher than α,δ-hydrogenation and with barriers nearly equivalent to butene hydrogenation, despite α,β-hydrogenation of butadiene also occurring through allylic carbocations. Hydrogenation of formaldehyde, a diene precursor, occurs with barriers that are within 15 kJ mol -1 of diene hydrogenation barriers, indicating that it may also contribute to increasing catalyst lifetimes by preventing diene formation. Benzene, in contrast to dienes and formaldehyde, is hydrogenated with higher barriers than C2-C4 alkenes despite proceeding via stable benzenium cations because of the thermodynamic instability of the product which has lost aromaticity. Carbocation stabilities predict the relative rates of alkene hydrogenation and in some cases shed insights into the hydrogenation of benzene, dienes, and formaldehyde, but cation stabilities alone cannot account for the poor hydrogenation of benzene or the facile hydrogenation of dienes, boosted by stabilization conferred by a,δ-hydrogenation. This work suggests that the main mechanisms of catalyst lifetime improvement with high H2 co-feeds is reduction of diene concentrations through both their selective hydrogenation and hydrogenation of their precursors to prevent formation of deactivating polyaromatic species.

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A New Computational Interface for Catalysis

Abstract: This manuscript outlines the utility and power of our computational catalysis interface. This interface has been developed by our group and used extensively to study metal, ceramic, and zeolite catalyst systems.

Cited by 19 publications

References 23 publications

Theoretical and Experimental Characterization of Adsorbed CO and NO on γ-Al₂O₃-Supported Rh Nanoparticles

Theoretical and Experimental Characterization of Adsorbed CO and NO on γ-Al₂O₃-Supported Rh Nanoparticles

Mechanism and Effects of Coverage and Particle Morphology on Rh-Catalyzed NO–H₂ Reactions

Mechanisms of Alkene and Diene Hydrogenation Reactions in H-MFI and H-CHA Zeolite Frameworks During MTO

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A New Computational Interface for Catalysis

Abstract: This manuscript outlines the utility and power of our computational catalysis interface. This interface has been developed by our group and used extensively to study metal, ceramic, and zeolite catalyst systems.

Cited by 19 publications

References 23 publications

Theoretical and Experimental Characterization of Adsorbed CO and NO on γ-Al2O3-Supported Rh Nanoparticles

Theoretical and Experimental Characterization of Adsorbed CO and NO on γ-Al2O3-Supported Rh Nanoparticles

Mechanism and Effects of Coverage and Particle Morphology on Rh-Catalyzed NO–H2 Reactions

Mechanisms of Alkene and Diene Hydrogenation Reactions in H-MFI and H-CHA Zeolite Frameworks During MTO

Contact Info

Product

Resources

About

Theoretical and Experimental Characterization of Adsorbed CO and NO on γ-Al₂O₃-Supported Rh Nanoparticles

Theoretical and Experimental Characterization of Adsorbed CO and NO on γ-Al₂O₃-Supported Rh Nanoparticles

Mechanism and Effects of Coverage and Particle Morphology on Rh-Catalyzed NO–H₂ Reactions