Rutger A. van Santen

Reactivity theory of transition-metal surfaces: a Brønsted-Evans-Polanyi linear activation energy - free-energy analysis

The exponential increase in computational processor speed, the development of novel computational architectures, together with the tremendous advances in ab initio theoretical methods that have emerged over the past two decades have led to unprecedented advances in our ability to probe the fundamental chemistry that occurs on complex catalytic surfaces. In particular, advances in density functional theory (DFT) have made it possible to elucidate the elementary steps and mechanisms in surface-catalyzed processes that would be difficult to explore experimentally. The advanced state of plane wave DFT has made it possible to rapidly examine systematic changes to the metal or the reactant in order to establish structure-property relationships. As a result, extensive data based on the energetics for various different surface-catalyzed reactions has been generated. This invites a detailed theoretical analysis of the factors that control reaction paths and corresponding potentialenergy surfaces of surface reactions. Such a theoretical analysis will not only provide interesting new insights into the intricate relationship between the chemical bonding features, structure, and energies of transition states but also serve as a basis for the development of analytical expressions that relate transitionstate properties to more easily accessible thermodynamic properties. The Brønsted-Evans-Polanyi (BEP) relationship is one such example which has been widely applied in the analysis of surface elementary reaction steps.1-8 δEact )RδEr (1)

Concepts in Theoretical Heterogeneous Catalytic Reactivity

Introduction A. General The heart of many commercial catalytic processes involves chemistry on transition metal particles and surfaces. The success in designing active surface ensembles, promoters, and selective poisons is inevitably tied to our knowledge of the fundamental principles which control transition metal surface chemistry. One extreme would be the rigorous description and energetic predictions for each elementary reaction step of an entire catalytic cycle from first-principle theoretical methods. While desirable, this has to date been an unattainable goal due to the limitations in both raw computer (CPU) requirements and the accuracy of the available computational methods. Recent advances in both quantum-chemical methods and computational resources, however, are driving this goal closer to reality. Theoretical treatments of adsorbate-surface interactions have rapidly advanced to the stage where detailed understandings of the governing structural and electronic features are readily available. In...

Imaging the Assembly Process of the Organic-Mediated Synthesis of a Zeolite

The formation and consumption of precursors during zeolite crystallisation has been followed by small-angle X-ray scattering techniques. The nanometer-scale precursors are specific for the zeolite topology formed, and their aggregation is an essential step in the nucleation process shown here.

Mechanism for vibrational relaxation in water investigated by femtosecond infrared spectroscopy

We present a study on the relaxation of the O–H stretch vibration in a dilute HDO:D2O solution using femtosecond mid-infrared pump-probe spectroscopy. We performed one-color experiments in which the 0→1 vibrational transition is probed at different frequencies, and two-color experiments in which the 1→2 transition is probed. In the one-color experiments, it is observed that the relaxation is faster at the blue side than at the center of the absorption band. Furthermore, it is observed that the vibrational relaxation time T1 shows an anomalous temperature dependence and increases from 0.74±0.01 ps at 298 K to 0.90±0.02 ps at 363 K. These results indicate that the O–H⋯O hydrogen bond forms the dominant accepting mode in the vibrational relaxation of the O–H stretch vibration.

Elementary Reaction Steps in Heterogeneous Catalysis

Heterogeneous catalysis is rapidly developing from a black box technology employed by the chemical engineer and becoming a frontier area of modern physical science. Progress is encouraged by parallel developments in surface science, in in situ methods suitable for studies of the working catalyst, in our understanding of reactivity, kinetics and mechanism, and of relevant and cognate theoretical studies. This improved insight into catalysis at the molecular level has increased the rate of catalyst development and is changing catalytic practise. This is therefore an appropriate time to review the field and to set the agenda for future progress.

Glucose Activation by Transient Cr2+ Dimers

By combining in situ X-ray absorption spectroscopy, DFT calculations and kinetic measurements we demonstrate that the unique ability of CrCl2 in ionic liquid media to catalyze glucose dehydration to 5-hydroxymethylfurfural relates to the transient self-organization of Cr2+ dimers, which promotes the isomerization of glucose to fructose. The molecular details of the active site environment during the rate controlling step resemble those in hexose isomerase enzymes.

Direct versus Hydrogen-Assisted CO Dissociation

The mechanism of CO dissociation is a fundamental issue in the technologically important Fischer-Tropsch (F-T) process that converts synthesis gas into liquid hydrocarbons. In the present study, we propose that on a corrugated Ru surface consisting of active sixfold (i.e., fourfold + twofold) sites, direct CO dissociation has a substantially lower barrier than the hydrogen-assisted paths (i.e., via HCO or COH intermediates). This proves that the F-T process on corrugated Ru surfaces and nanoparticles with active sixfold sites initiates through direct CO dissociation instead of hydrogenated intermediates.

Zeosil nanoslabs: building blocks in nPr(4)N(+)-mediated synthesis of MFI zeolite

Tetrapropylammonium (TPA)-containing precursors are the building blocks in the crystallization of silica. In the first steps slab-shaped silicalite nanoparticles are formed by ordered combination of the precursors. These nanoslabs have MFI-type zeolite framework topology and play a key role in TPA-ion-mediated zeolite crystallization from monomeric and polymeric silica sources.

Field-dependent chemisorption of carbon monoxide and nitric oxide on platinum-group (111) surfaces: Quantum chemical calculations compared with infrared spectroscopy at electrochemical and vacuum-based interfaces

Density Functional Theory (DFT) is utilized to compute field-dependent binding energies and intramolecular vibrational frequencies for carbon monoxide and nitric oxide chemisorbed on five hexagonal Pt-group metal surfaces, Pt, Ir, Pd, Rh, and Ru. The results are compared with corresponding binding geometries and vibrational frequencies obtained chiefly from infrared spectroscopy in electrochemical and ultrahigh vacuum environments in order to elucidate the broad-based quantum-chemical factors responsible for the observed metal- and potential-dependent surface bonding in these benchmark diatomic chemisorbate systems. The surfaces are modeled chiefly as 13-atom metal clusters in a variable external field, enabling examination of potential-dependent CO and NO bonding at low coverages in atop and threefold-hollow geometries. The calculated trends in the CO binding-site preferences are in accordance with spectral data: Pt and Rh switch from atop to multifold coordination at negative fields, whereas Ir and Ru e...

Interaction of H, O and OH with metal surfaces

Abstract The interaction of the primary water dissociation products H, O and OH with various (111) metal surfaces is studied by density functional theory (DFT) calculations using clusters. It is found that H forms an essentially covalent bond with the metal, whereas O and OH form a largely ionic bond. The O and OH adsorbates prefer the high coordination three-fold hollow site on all metals: no such clear trend for H is found, the adsorption energy for on-top and hollow sites being comparable for most metals, especially on transition metals. The O and OH adsorbates are attracted towards, and donate some electronic charge to, the surface when a positive electric field (electrode potential) is applied, whereas the effect of an applied field on H adsorption is much smaller. We also show how the trends in the OH adsorption energies on different metals, as compared with O adsorption, can be explained by a weaker covalent interaction and a stronger Pauli repulsion of the OH with the metal d electrons.