Kultapinnan S(CH2)xN3-adsorbaattien laskennallinen värähdysdynamiikka ja 2DIR-spektroskopia molekyylidynamiikkamenetelmin

Tehtiin MD-simulaatiot kultapinnan siltapaikkoihin kahdella eri peitolla (1/3 ja 1/4) yksikerrokseksi asetetuille pitkä- ja lyhytketjuisille S(CH2)xN3-adsorbaateille (x = 2, 11). Kukin systeemi simuloitiin sekä tyhjiössä että vesiympäristössä. Simulaatioista määritettiin atsidiryhmien FFCF:t ja CLS:t odotusaikavälille t2 ∈ [0, 200] ps. Näihin sovitettiin multieksponentiaaliset funktiot ja havaittiin FFCF:n hitaiden komponenttien vastaavan CLS-komponentteja. Lisäksi tarkasteltiin mallia, joka esittää CLS:n ja FFCF:n välille suoraviivaisen yhteyden ja mahdollistaa Lorentzin ja Gaussin leveyksien määrittämisen. Malli todettiin hyvin toimivaksi simulaatioista saatujen tulosten tapauksessa.MD simulations were ran for S(CH2)xN3 (x = 2, 11) monolayers of coverage 1/3 and 1/4 on the bridge sites of an Au(111) surface. Each system was simulated in both vacuum and water environments. The FFCF and CLS functions for the azide groups in the waiting time range t2 ∈ [0, 200] ps were determined from the simulations. These were fitted with multiexponential functions, and the slow components of the FFCF curves were found to correspond to the CLS components. In addition a model that straightforwardly connects the CLS to the FFCF and allows one to determine the Lorentzian and Gaussian linewidths was considered. The model was found to perform well in the case of the presented simulation results

Computational studies of catalytic active site properties and reactions at the metal–oxide interface

In this thesis work, the geometric and electronic structures of metal–oxide catalysts were studied using density functional theory. The studied systems were zirconia-supported metal atoms and clusters, and ReOx-modified rhodium. Various aspects of these metal–oxide systems were investigated, including the metal– oxide interaction and interfacial properties, the structural variation and dynamics of the supported clusters, and the reducibility and acidity of the oxide components. The dissociation of water over the metal–oxide interface and the hydrodeoxygenation of glycerol on ReOx-modified Rh were used as model reactions. It was shown that small Pt and Rh clusters on zirconia exhibit unique interfacial reaction sites, producing non-scaling behavior in the interfacial water splitting reaction. Less stable cluster isomers were found to dissociate water more exothermically due to the stronger binding of the dissociated fragments. The challenges of simulating the dynamics of such clusters using constant-temperature DFT-MD were investigated, highlighting the necessity of tight SCF convergence and proper thermostatting to avoid anomalies such as temperature gradients. The metal-enhanced reducibility of monoclinic zirconia was studied using a variety of adsorbed single transition metal atoms, with Ir and Pt providing the strongest enhancement. To account for the origin of the enhancement, the metal– oxide and metal–vacancy binding were investigated in detail, with a focus on the charge transfer and covalent interactions. Finally, the metal-acid bifunctional ReOx–Rh catalyst was found to acid-catalyze the dehydroxylation of glycerol, with a competitive metal-catalyzed pathway possibly explaining the experimentally observed poor selectivity. The same catalyst was found unable to acidcatalyze the ring opening of glycidol, pointing toward a ring-size effect in solid acid catalysis.Tässä väitöskirjatyössä tutkittiin tiheysfunktionaaliteorian avulla metalli–oksidikatalyyttien geometrisia ja elektronisia rakenteita. Tutkittuina systeemeinä olivat zirkoniatuetut metalliatomit ja -klusterit, ja ReOx-muokattu rodium. Työssä tarkasteltiin näiden metalli–oksidi-systeemien useita piirteitä, kuten metalli–oksidivuorovaikutusta ja rajapinnan ominaisuuksia, tuettujen klustereiden rakenteellista vaihtelua ja dynamiikkaa, sekä oksidikomponentin pelkistyvyys- ja happamuusominaisuuksia. Mallireaktioina käytettiin veden dissosiaatiota metalli– oksidi-rajapinnalla ja glyserolin hydrodeoksygenaatiota ReOx-muokatulla rodiumilla. Työssä osoitettiin, että pienten Pt- ja Rh-klusterien rajapinnat zirkonialla sisältävät ainutlaatuisia reaktiopaikkoja, mikä johtaa veden halkeamisreaktiossa skaalautumattomaan käytökseen. Epästabiilimpien klusteri-isomeerien osoitettiin dissosioivan vettä eksotermisemmin, johtuen dissosioituneiden fragmenttien voimakkaammasta sitoutumisesta niille. Tällaisten klustereiden vakiolämpötiladynamiikan DFT-MD-simuloinnin haasteita tarkasteltiin, ja tuloksissa korostuivat tiukan SCF-konvergenssin ja oikein valitun termostaatin merkitys anomalioiden kuten lämpötilagradienttien välttämiseksi. Monokliinisen zirkonian metalliavusteista pelkistymistä tutkittiin useiden adsorboituneiden yksittäisten siirtymämetalliatomien tapauksessa, joista Ir ja Pt avustivat pelkistymistä voimakkaimmin. Ilmiön alkuperän selvittämiseksi metalli–oksidi- ja metalli–vakanssivuorovaikutuksia tarkasteltiin yksityiskohtaisesti keskittyen varauksensiirtoon ja kovalenttisiin vuorovaikutuksiin. Bifunktionaalisen metalli-happokatalyytin, ReOx–Rh:n, todettiin happokatalysoivan glyserolin dehydroksylaatiota, joskin kilpaileva metallikatalysoitu reaktiopolku on läsnä ja saattaa selittää kokeellisesti havaitun heikon selektiivisyyden. Sama katalyytti todettiin kyvyttömäksi happokatalysoimaan glysidolin renkaanaukeamisreaktiota, mikä viittaa renkaankokoefektiin kiinteähappokatalyysissä

Reducing the Irreducible : Dispersed Metal Atoms Facilitate Reduction of Irreducible Oxides

Oxide reducibility is a central concept quantifying the role of the support in catalysis. While reducible oxides are often considered catalytically active, irreducible oxides are seen as inert supports. Enhancing the reducibility of irreducible oxides has, however, emerged as an effective way to increase their catalytic activity while retaining their inherent thermal stability. In this work, we focus on the prospect of using single metal atoms to increase the reducibility of a prototypical irreducible oxide, zirconia. Based on extensive self-consistent DFT+U calculations, we demonstrate that single metal atoms significantly improve and tune the surface reducibility of zirconia. Detailed analysis of the observed single atom induced reducibility allows us to attribute the enhanced reducibility to strong interactions between the metal atom and the electrons trapped in the vacancy and d–p orbital interactions between the metal atom and oxygen. This analysis enables transferring the obtained theoretical understanding to other irreducible oxides as well. The detailed understanding of how oxide reducibility can be tuned offers precise control over the catalytic properties of metal oxides.peerReviewe

Reducing the irreducible: Dispersed metal atoms facilitate reduction of irreducible oxides.

Oxide reducibility is a central concept quantifying the role of the support in catalysis. While reducible oxides are often considered catalytically active, irreducible oxides are seen as inert supports. Enhancing the reducibility of irreducible oxides has, however, emerged as an effective way to increase their catalytic activity while retaining their inherent thermal stability. In this work, we focus on the prospect of using single metal atoms to increase the reducibility of a prototypical irreducible oxide, zirconia. Based on extensive self-consistent DFT+U calculations, we demonstrate that single metal atoms significantly improve and tune the surface reducibility of zirconia. Detailed analysis of the observed single atom induced reducibility allows us to attribute the enhanced reducibility to strong interactions between the metal atom and the electrons trapped in the vacancy, and d-p orbital interactions between the metal atom and oxygen. This analysis enables transferring the obtained theoretical understanding to other irreducible oxides as well. The detailed understanding of how oxide reducibility can be tuned offers precise control over the catalytic properties of metal--oxides

ReOx as a Brønsted acidic modifier in glycerol hydrodeoxygenation: computational insight into the balance between acid and metal catalysis

A computational study for the competitive conversion of glycerol to 1,2-propanediol and 1,3-propanediol is presented, considering a two-step sequence of dehydration followed by hydrogenation. The elementary steps for dehydration, i.e., breaking of C–H followed by C–OH or vice versa, were studied computationally both on the Rh metal surface and the acid-modified ReOH–Rh surface in order to understand the role of the acid promoter. While the acid modifier can catalyze the C–OH cleavage, the activation energy for the C–H cleavage was found to be considerably smaller on both pure and acid-doped Rh(111) surfaces, and breaking the secondary C–H bond is kinetically favored over breaking the terminal C–H bond. This is in complete agreement with experimental protocols favoring the formation of 1,2-propanediol. Another potential feedstock, glycidol, was studied for the epoxide ring opening to yield 1,2-propanediol and 1,3-propanediol, and the reaction was found to be metal-catalyzed even in the presence of acid

ReO as a Brønsted acidic modifier in glycerol hydrodeoxygenation : Computational insight into the balance between acid and metal catalysis

A computational study for the competitive conversion of glycerol to 1,2-propanediol and 1,3-propanediol is presented, considering a two-step sequence of dehydration followed by hydrogenation. The elementary steps for dehydration, i.e., breaking of C–H followed by C–OH or vice versa, were studied computationally both on the Rh metal surface and the acid-modified ReOH–Rh surface in order to understand the role of the acid promoter. While the acid modifier can catalyze the C–OH cleavage, the activation energy for the C–H cleavage was found to be considerably smaller on both pure and acid-doped Rh(111) surfaces, and breaking the secondary C–H bond is kinetically favored over breaking the terminal C–H bond. This is in complete agreement with experimental protocols favoring the formation of 1,2-propanediol. Another potential feedstock, glycidol, was studied for the epoxide ring opening to yield 1,2-propanediol and 1,3-propanediol, and the reaction was found to be metal-catalyzed even in the presence of acid.peerReviewe

Escaping Scaling Relationships for Water Dissociation at Interfacial Sites of Zirconia-Supported Rh and Pt Clusters

Water dissociation is an important reaction involved in many industrial processes and a good model reaction for probing the activity of catalytic sites. In this computational study, the dissociation of water at interfacial sites of globally optimized ZrO2 sup- ported Pt and Rh clusters is investigated under the framework of density functional theory. Our findings demonstrate that the perimeter sites of these small clusters can activate water, but the dissociation behavior varies considerably between sites. It is shown that the studied clusters break scaling relationships for water dissociation, suggesting these catalysts may achieve activities beyond the maximum imposed by such relations. Furthermore, we observed large differences in the thermodynamics of the water dissociation reaction between global minimum and near-global minimum isomers of the clusters. Overall, our results highlight the uniqueness of interfacial sites in catalytic reactions, and the need for developing new concepts and tools to deal with the associated complexity.</p

Escaping scaling relationships for water dissociation at interfacial sites of zirconia-supported Rh and Pt clusters

Water dissociation is an important reaction involved in many industrial processes. In this computational study, the dissociation of water is used as a model reaction for probing the activity of interfacial sites of globally optimized ZrO2 supported Pt and Rh clusters under the framework of density functional theory. Our findings demonstrate that the perimeter sites of these small clusters can activate water, but the dissociation behavior varies considerably between sites. It is shown that the studied clusters break scaling relationships for water dissociation, suggesting that these catalysts may achieve activities beyond the maximum imposed by such relations. Furthermore, we observed large differences in the thermodynamics of the water dissociation reaction between global minimum and near-global minimum isomers of the clusters. Overall, our results highlight the uniqueness of interfacial sites in catalytic reactions and the need for developing new concepts and tools to deal with the associated complexity.peerReviewe

Addressing Dynamics at Catalytic Heterogeneous Interfaces with DFT-MD : Anomalous Temperature Distributions from Commonly Used Thermostats

Density functional theory-based molecular dynamics (DFT-MD) has been widely used for studying the chemistry of heterogeneous interfacial systems under operational conditions. We report frequently overlooked errors in thermostated or constant-temperature DFT-MD simulations applied to study (electro)catalytic chemistry. Our results demonstrate that commonly used thermostats such as Nose−Hoover, Berendsen, and simple velocity rescaling methods fail to provide are liable temperature description for systems considered. Instead, nonconstant temperatures and large temperature gradients within the different parts of the system are observed. The errors are not a “feature” of any particular code but a represent in several ab initio molecular dynamics implementations. This uneven temperature distribution, due to inadequate thermostatting, is well-known in the classical MD community, where it is ascribed to the failure in kinetic energy equipartition among different degrees of freedom in heterogeneous systems (Harvey et al. J. Comput. Chem. 1998, 726−740) and termed the flying ice cube effect. We provide tantamount evidence that interfacial systems are susceptible to substantial flying ice cube effects and demonstrate that the traditional Nose−Hoover and Berendsen thermostats should be applied with care when simulating, for example, catalytic properties or structures of solvated interfaces and supported clusters. We conclude that the flying ice cube effect in these systems can be conveniently avoided using Langevin dynamics.peerReviewe