The dehydrogenation of butane on metal-free graphene.

The dehydrogenation of alkane feedstock to produce alkenes is a significant and energy intensive industrial process, generally occurring on metals and metal oxides. Here, we investigate a catalytic mechanism for the dehydrogenation of butane on single-layer, metal-free graphene using a combination of ab initio quantum chemical calculations and adsorption microcalorimetry. Dispersion-corrected Density Functional Theory (DFT) is employed to calculate transition states and energy minima that describe the reaction pathways connecting butane to the two possible products, but-1-ene and but-2-ene. The deprotonations occur with moderate energy barriers in the 0.54 eV-0.69 eV range. A strong agreement is observed between the results of the adsorption energies calculated by DFT (0.40 eV) and the measured differential heat of adsorption of n-butane on a graphitic overlayer. We conclude that the active-site for this catalytic reaction is a metal-free graphene vacancy, created by removing a carbon atom from a single-layer graphene sheet.

F-doping of nanostructured ZnO: a way to modify structural, electronic, and surface properties.

Polycrystalline ZnO is a material often used in heterogeneous catalysis. Its properties can be altered by the addition of dopants. We used gaseous fluorine (F2(g)) as direct way to incorporate fluoride in ZnO as anionic dopants. Here, the consequences of this treatment on the structural and electronic properties, as well as on the acidic/basic sites of the surface, are investigated. It is shown that the amount of F incorporation into the structure can be controlled by the synthesis parameters (t, T, p). While the surface of ZnO was altered as shown by, e.g., IR spectroscopy, XPS, and STEM/EDX measurements, the F2 treatment also influenced the electronic properties (optical band gap, conductivity) of ZnO. Furthermore, the Lewis acidity/basicity of the surface was affected which is evidenced by using, e.g., different probe molecules (CO2, NH3). In situ investigations of the fluorination process offer valuable insights on the fluorination process itself.

Ni Single Atom Catalysts for CO2 Activation.

We report on the activation of CO2 on Ni single-atom catalysts. These catalysts were synthesized using a solid solution approach by controlled substitution of 1-10 atom % of Mg2+ by Ni2+ inside the MgO structure. The Ni atoms are preferentially located on the surface of the MgO and, as predicted by hybrid-functional calculations, favor low-coordinated sites. The isolated Ni atoms are active for CO2 conversion through the reverse water-gas shift (rWGS) but are unable to conduct its further hydrogenation to CH4 (or MeOH), for which Ni clusters are needed. The CO formation rates correlate linearly with the concentration of Ni on the surface evidenced by XPS and microcalorimetry. The calculations show that the substitution of Mg atoms by Ni atoms on the surface of the oxide structure reduces the strength of the CO2 binding at low-coordinated sites and also promotes H2 dissociation. Astonishingly, the single-atom catalysts stayed stable over 100 h on stream, after which no clusters or particle formation could be detected. Upon catalysis, a surface carbonate adsorbate-layer was formed, of which the decompositions appear to be directly linked to the aggregation of Ni. This study on atomically dispersed Ni species brings new fundamental understanding of Ni active sites for reactions involving CO2 and clearly evidence the limits of single-atom catalysis for complex reactions.

Reactive oxygen species in iridium-based OER catalysts.

Tremendous effort has been devoted towards elucidating the fundamental reasons for the higher activity of hydrated amorphous IrIII/IV oxyhydroxides (IrO x ) in the oxygen evolution reaction (OER) in comparison with their crystalline counterpart, rutile-type IrO2, by focusing on the metal oxidation state. Here we demonstrate that, through an analogy to photosystem II, the nature of this reactive species is not solely a property of the metal but is intimately tied to the electronic structure of oxygen. We use a combination of synchrotron-based X-ray photoemission and absorption spectroscopies, ab initio calculations, and microcalorimetry to show that holes in the O 2p states in amorphous IrO x give rise to a weakly bound oxygen that is extremely susceptible to nucleophilic attack, reacting stoichiometrically with CO already at room temperature. As such, we expect this species to play the critical role of the electrophilic oxygen involved in O-O bond formation in the electrocatalytic OER on IrO x . We propose that the dynamic nature of the Ir framework in amorphous IrO x imparts the flexibility in Ir oxidation state required for the formation of this active electrophilic oxygen.

Dynamics of palladium on nanocarbon in the direct synthesis of H2O2.

This work aims to clarify the nanostructural transformation accompanying the loss of activity and selectivity for the hydrogen peroxide synthesis of palladium and gold-palladium nanoparticles supported on N-functionalized carbon nanotubes. High-resolution X-ray photoemission spectroscopy (XPS) allows the discrimination of metallic palladium, electronically modified metallic palladium hosting impurities, and cationic palladium. This is paralleled by the morphological heterogeneity observed by high-resolution TEM, in which nanoparticles with an average size of 2 nm coexisted with very small palladium clusters. The morphological distribution of palladium is modified after reaction through sintering and dissolution/redeposition pathways. The loss of selectivity is correlated to the extent to which these processes occur as a result of the instability of the particle at the carbon surface. We assign beneficial activity in the selective hydrogenation of oxygen to palladium clusters with a modified electronic structure compared with palladium metal or palladium oxides. These beneficial species are formed and stabilized on carbons modified with nitrogen atoms in substitutional positions. The formation of larger metallic palladium particles not only reduces the number of active sites for the synthesis, but also enhances the activity for deep hydrogenation to water. The structural instability of the active species is thus detrimental in a dual way. Minimizing the chance of sintering of palladium clusters by all means is thus the key to better performing catalysts.

Understanding CeO2 as a Deacon catalyst by probe molecule adsorption and in situ infrared characterisations.

CeO(2) has been identified as an efficient catalyst for HCl oxidation in the temperature range of 623-723 K provided that the oxygen content in the feed mixture was sufficiently high to avoid bulk chlorination and thus deactivation. Here we characterise ceria in its fresh and post-reaction states by adsorption of CO(2), NH(3) and CO. Micro-calorimetry, FTIR and TPD experiments are complemented by DFT calculations, which assess adsorption energies and vibrational frequencies. The calculations were performed on the lowest energy surface, CeO(2)(111), with perfect termination and with various degrees of hydroxylation and/or chlorination. Both experiments and calculations suggest that the basic character of the ceria surface has been eliminated upon reaction in HCl oxidation, indicating that most of the basic lattice O sites are exchanged by chlorine and that the OH groups formed are rather acidic. The density and the strength of surface acidic functions increased significantly upon reaction. An in situ FTIR reaction cell has been designed and constructed to study the evolution of OH group density of the ceria surface during HCl oxidation. The effect of experimental variables, such as pO(2), pHCl and temperature, has been investigated. We found that the OH group density positively correlated with the reactivity in the pO(2) and temperature series, whereas negative correlation was observed when pHCl was varied. Implications of the above observations to the reaction mechanism are discussed.

Tailoring the morphology of Pd nanoparticles on CNTs by nitrogen and oxygen functionalization.

The influence of N and O functionalization of CNT on the morphology of supported Pd-PVA nanoparticles is studied with respect to the catalytic activity in the liquid phase oxidation of benzyl alcohol to benzaldehyde. The impact of specific N and O sites on the carbon surface induced by the high temperature N-functionalization in the temperature range 673-873 K was observed by HRTEM as increased nanoparticles dispersion and enhanced metal wetting at the carbon surface. Those small nanoparticles that stabilized at the N-CNTs surface are beneficial for improving catalytic performance. The interaction of O(2) with the metal surface was studied by microcalorimetry. At 353 K, the PVA shell hinders the dissociative oxygen chemisorption at the surface of the fresh catalyst. Differently, a very high (maximum for Pd/N-CNT873K 750 kJ mol(-1)) and oscillating exothermic differential heat is registered for the washed samples. Such high differential heat on the "washed" sample is due to the sum of oxygen chemisorption and PVA oxidation. Thereby, it is demonstrated that the PVA overlayer suppresses the total combustion reaction pathway. This contribution has highlighted the impact of the dynamic change of morphology of these Pd nanoparticles under the reaction conditions on the catalytic performance and how this is modulated by the nature of the support as well as the PVA. The support with its varying ability to strongly bind Pd regulates the morphology of the nanoparticles on which the sub-surface penetration of O, H, C from the reactants depends, all modulating the electronic structure and thus the reactivity.

In situ generation of active sites in olefin metathesis.

The depth of our understanding in catalysis is governed by the information we have about the number of active sites and their molecular structure. The nature of an active center on the surface of a working heterogeneous catalyst is, however, extremely difficult to identify and precise quantification of active species is generally missing. In metathesis of propene over dispersed molybdenum oxide supported on silica, only 1.5% of all Mo atoms in the catalyst are captured to form the active centers. Here we combine infrared spectroscopy in operando with microcalorimetry and reactivity studies using isotopic labeling to monitor catalyst formation. We show that the active Mo(VI)-alkylidene moieties are generated in situ by surface reaction of grafted molybdenum oxide precursor species with the substrate molecule itself gaining insight into the pathways limiting the number of active centers on the surface of a heterogeneous catalyst. The active site formation involves sequential steps requiring multiple catalyst functions: protonation of propene to surface Mo(VI)-isopropoxide species driven by surface Brønsted acid sites, subsequent oxidation of isopropoxide to acetone in the adsorbed state owing to the red-ox capability of molybdenum leaving naked Mo(IV) sites after desorption of acetone, and oxidative addition of another propene molecule yielding finally the active Mo(VI)-alkylidene species. This view is quite different from the one-step mechanism, which has been accepted in the community for three decades, however, fully consistent with the empirically recognized importance of acidity, reducibility, and strict dehydration of the catalyst. The knowledge acquired in the present work has been successfully implemented for catalyst improvement. Simple heat treatment after the initial propene adsorption doubled the catalytic activity by accelerating the oxidation and desorption-capturing steps, demonstrating the merit of knowledge-based strategies in heterogeneous catalysis. Molecular structure of active Mo(VI)-alkylidene sites derived from surface molybdena is discussed in the context of similarity to the highly active Schrock-type homogeneous catalysts.

New insights from microcalorimetry on the FeOx/CNT-based electrocatalysts active in the conversion of CO2 to fuels.

Fe oxide nanoparticles show enhanced electrocatalytic performance in the reduction of CO(2) to isopropanol when deposited on an N-functionalized carbon nanotube (CNT) support rather than on a pristine or oxidized CNT support. XRD and high-resolution TEM were used to investigate the nanostructure of the electrocatalysts, and CO(2) adsorptive microcalorimetry was used to study the chemical nature of the interaction of CO(2) with the surface sites. Although the particles always present the same Fe(3)O(4) phase, their structural anisotropy and size inhomogeneity are consequences of the preparation method of the carbon surface. Two types of chemisorption sites have been determined by using microcalorimetry: irreversible sites (280 kJ mol(-1)) at the uncoordinated sites of the facets and reversible sites (120 kJ mol(-1)) at the hydrated oxide surface of the small nanoparticles. N-Functionalization of the carbon support is advantageous, as it causes the formation of small nanoparticles, which are highly populated by reversible chemisorbing sites. These characteristic features correlate with a higher electrocatalytic performance.

Tuning the acid/base properties of nanocarbons by functionalization via amination.

The surface chemical properties and the electronic properties of vapor grown carbon nanofibers (VGCNFs) have been modified by treatment of the oxidized CNFs with NH(3). The effect of treatment temperature on the types of nitrogen functionalities introduced was evaluated by synchrotron based X-ray photoelectron spectroscopy (XPS), while the impact of the preparation methods on the surface acid-base properties was investigated by potentiometric titration, microcalorimetry, and zeta potential measurements. The impact of the N-functionalization on the electronic properties was measured by THz-Time Domain spectroscopy. The samples functionalized via amination are characterized by the coexistence of acidic and basic O and N sites. The population of O and N species is temperature dependent. In particular, at 873 K nitrogen is stabilized in substitutional positions within the graphitic structure, as heterocyclic-like moieties. The surface presents heterogeneously distributed and energetically different basic sites. A small amount of strong basic sites gives rise to a differential heat of CO(2) adsorption of 150 kJ mol(-1). However, when functionalization is carried out at 473 K, nitrogen moieties with basic character are introduced and the maximum heat of adsorption is significantly lower, at approximately 90 kJ mol(-1). In the latter sample, energetically different basic sites coexist with acidic oxygen groups introduced during the oxidative step. Under these conditions, a bifunctional acidic and basic surface is obtained with high hydrophilic character. N-functionalization carried out at higher temperature changes the electronic properties of the CNFs as evaluated by THz-TDS. The functionalization procedure presented in this work allows high versatility and flexibility in tailoring the surface chemistry of nanocarbon material to specific needs. This work shows the potential of the N-containing nanocarbon materials obtained via amination in catalysis as well as electronic device materials.