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[Display omitted] • The CO coverage of the disputed '2 √ 3' structure is θ CO =0.58 ML as determined by TPD. • Two unknown high coverage phases of CO on Co(0001) were identified. • The c(nx2) structures contain (2x2)-3CO domains diluted by antiphase boundaries. • These high density structures do not form at FTS pressures and temperatures. • They should be taken into account in catalyst characterization using CO at RT. The adsorbate overlayer structures formed by CO on highly covered Co(0001) were investigated using temperature programmed desorption, low energy electron diffraction, reflection absorption infrared spectroscopy and synchrotron-based x-ray photoemission spectroscopy, with the purpose to clarify some recent confusion on CO adsorption at high coverages. TPD shows that the coverage of the (2√3 × 2√3)R30o structure (abbreviated as 2√3 structure) is 7/12 (0.583) ML. Its unit cell contains one CO top and six CO bridge species that form three 'bridge dicarbonyls' in which two CO bridge adsorbates share the same cobalt surface atom. Between 0.58 and 0.63 ML the 2√3 structure breaks up into small 2D domains separated by antiphase domain boundaries that consist of 'double bridge dicarbonyls' in which three CO bridge adsorbates share two cobalt surface atoms. A phase transition that occurs around 0.63 ML creates previously unknown c(8 × 2) and c(12 × 2) domain boundary structures which consist of high density strips with a (2 × 2)–3CO structure and top/hollow site occupation which are separated by antiphase domain boundaries with a lower local coverage. Up to 0.63 ML the structures found at low temperature in UHV are very similar to those observed by others using in-situ STM at 300 K under CO pressure. This changes above 0.63 ML, where STM shows that moiré structures form instead of the c(n × 2) domain boundary structures. We propose that the moiré and c(n × 2) structures have a very similar enthalpy of formation so that small variations of temperature and CO chemical potential can lead to different structures for the same CO coverage. We show that these high-density phases that exist above 0.5 ML do not form at the typical pressures and temperatures used in applied FTS, but they do need to be considered when CO adsorption at room temperature is used as a diagnostic tool to characterize the catalyst surface. [ABSTRACT FROM AUTHOR]

Formation of CH x (O) monomers and C 1 products (CH 4 , CH 2 O, and CH 3 OH) on C-terminated χ-Fe 5 C 2 (0 0 1) (Hägg carbide) surfaces of different carbon contents was investigated using periodic DFT simulations. Methane (CH 4 ) as well as monomer (CH x ) formation follows a Mars–van Krevelen-like cycle starting with the hydrogenation of surface carbidic carbon, which is regenerated by subsequent CO dissociation, while oxygen is removed as H 2 O. In cases where surface carbon is readily available, the apparent barrier for CH 4 formation was found to be ∼95 kJ/mol. However, different rate-determining steps show that different propagation mechanisms may be possible for actual chain growth, depending on the carbon content of the surface. Hydrogen addition to CO forms formyl (HCO), which is a precursor for both H-assisted CO activation and oxygenate formation. Further hydrogenation of HCO yields adsorbed formaldehyde and methoxy, rather than hydroxymethyl (HCOH) that would give C–O bond splitting. Full hydrogenation to gas-phase methanol faces a high barrier, suggesting that CH x O species may be involved in higher oxygenate formation in a full Fischer–Tropsch mechanism or that the C–O bond does not break until the CHO fragment has been incorporated in a C 2 species, a route for which precedents are available in the literature. [ABSTRACT FROM AUTHOR]

The present article summarizes experimental findings of the interaction of CO with single crystal surfaces of cobalt. We first provide a quantitative study of non-dissociative CO adsorption on Co(0001) and establish a quantitative correlation between θ CO and adsorption site occupation. In light of these findings we revisit the structure of previously reported ordered CO/Co(0001) adsorbate layers. Measurements of the CO coverage at equilibrium conditions are used to derive a phase diagram for CO on Co(0001). For low temperature Fischer-Tropsch synthesis conditions the CO coverage is predicted to be ≈0.5 ML, a value that hardly changes with p CO . The CO desorption temperature found in temperature programmed desorption is practically structure-independent, despite structure-dependent heats of adsorption reported in the literature. This mismatch is attributed to a structure-dependent pre-exponential factor for desorption. IR spectra reported throughout this study provide a reference point for IR studies on cobalt catalysts. Results for CO adsorbed on flat and defect-rich Co surfaces as well as particular, CO adsorbed on top sites, and in addition affect the distribution of CO ad over the various possible adsorption sites. [ABSTRACT FROM AUTHOR]

Abstract: The hydrogenation of HCN to methylamine on Co(111) was used as a model reaction to study the hydrogenation of nitriles to primary amines. Density functional theory was used to characterize the reaction mechanism, and the results obtained were compared with those for the same reaction on Ni(111). Hydrogen cyanide adsorbs more strongly on Co(111) than on Ni(111), with an adsorption energy of −1.72 eV. The hydrogenation product, methylamine, is weakly adsorbed on Co(111), with an adsorption energy of −0.53 eV, which is very similar to the adsorption energy calculated on Ni(111). The calculated adsorption energies were used to explain the differences in activity and selectivity observed between nickel- and cobalt-based catalysts; the stronger adsorption of HCN on cobalt explains both the lower activity and the higher selectivity observed on this metal. Regarding the reaction mechanism, the hydrogenation reaction implies an imine intermediate (H2CNH) independent of whether hydrogen reacts with the carbon atom or with the nitrogen atom of the hydrogen cyanide molecule in the first step. The imine intermediate subsequently reacts to form H3CNH, which is finally hydrogenated to yield methylamine. The overall surface reaction is endothermic. Remarkably, comparing the HCN hydrogenation reaction mechanism on Co(111) and Ni(111) revealed no significant differences. [Copyright &y& Elsevier]

Abstract: The hydrogenation of hydrogen cyanide to methylamine on Ni(111) has been studied as a model reaction for the hydrogenation of nitriles to primary amines, using density functional theory. Hydrogen cyanide adsorbs strongly on Ni(111) with an adsorption energy of with the CN bond parallel to the surface. The product of the hydrogenation reaction, methylamine, is relatively weakly adsorbed on the surface compared with the hydrogen cyanide, with an adsorption energy of , very similar to the adsorption energy of ammonia, . The hydrogenation reaction goes through an imine intermediate (H2CNH) independently of whether hydrogen reacts with the carbon atom or nitrogen atom of the hydrogen cyanide molecule in the first hydrogenation step. From the imine intermediate, the hydrogenation reaction is likely to proceed via a H3CNH species to methylamine (H3CNH2). On the other hand, if we explore the backward reaction, methylamine decomposition on Ni(111), our calculations show that Ceavage is slightly favored over Neaking, leading to the formation of H2CNH2. [Copyright &y& Elsevier]

Abstract: The polymerization properties of a well-defined flat CrO x /SiO2/Si(100) catalyst were investigated. To compare this model system with the conventional Phillips catalyst based on porous silica, polymerization reactions were carried out at various temperatures. In contrast to the conventional Phillips catalyst, our model showed constant activity from the start of polymerization. Both the activity and molecular weight were tenfold higher for the flat catalyst. As expected, an increase in polymerization temperature led to an increase in activity; however, the molecular weight distribution was only mildly affected. At a temperature around the melting point of polyethylene, a sudden drop in both activity and molecular weight was observed. Based on our observations, we propose that chromium sites produce polymer chains in short bursts of high activity, followed by a dormant period, and that ethylene polymerization of active chromium sites can result in local ethylene depletions that limit both activity and molecular weight. This can severely affect the polymerization properties in the event of high active site density and reduced mass transfer through the formed polymer layer. [Copyright &y& Elsevier]

[Display omitted] • a time- and space-resolved iron carbide-oxide Fe 5 C 2 @Fe 3 O 4 core–shell structure. • NAP-XPS and ETEM uncovering of phase transformation of a metallic iron catalyst exposed to carbon monoxide. • role of iron carbides in FTS chain growth and Fe 3 O 4 overlayer in the water–gas shift reaction. High resolution structure models of the spatial distribution of iron carbide cores with decorated oxide species are critical to the understanding and definition of active species for Fischer-Tropsch Synthesis and the water–gas shift reaction. In situ tools such as NAP-XPS and ETEM facilitated a layered uncovering of the dynamic phase transformation of a metallic iron catalyst exposed to carbon monoxide, leading to a time- and space-resolved iron carbide-oxide core–shell model, which can unravel the role of iron carbides in FTS chain growth and of an Fe 3 O 4 overlayer in the water–gas shift reaction, respectively. Such definitive knowledge on Fe 5 C 2 @Fe 3 O 4 core–shell structure can potentially boost iron catalyst stability and realize CO 2 selectivity mitigation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrogen dissociation is slightly activated on close-packed cobalt surface. • Step sites offer a barrierless pathway for H 2 dissociation. • Hydrogen adsorbs strongest on close-packed terraces and more weakly on more open surfaces. • Hydrogen adsorption on HCP cobalt particles is generally weaker than on FCC-cobalt particles. The interaction of hydrogen with cobalt surfaces is of fundamental interest for Fischer-Tropsch synthesis. In the present work, the adsorption and desorption of hydrogen was studied on various cobalt single crystal surfaces that together represent the surface structures exposed by FCC and HCP cobalt nanoparticles used in applied catalysis. Dissociative hydrogen adsorption is activated on flat Co(0001), especially for hydrogen coverages beyond 0.5 ML. A tungsten filament creates hydrogen atoms and hot hydrogen molecules that increase the dissociative sticking probability and make it possible to obtain hydrogen coverages above 0.5 ML. Hydrogen in excess of 0.5 ML binds more weakly and desorbs in a separate low temperature desorption peak, in line with theoretical predictions. A third desorption peak appears above 1 ML and is attributed to subsurface hydrogen, the formation of which is attributed to hydrogen atoms produced by the tungsten filament. Adsorbed hydrogen atoms form (islands) of an ordered (2 × 2)-2H honeycomb structure for coverages between 0.3 and 0.8 ML which points to a specific stability of this structure. Step and kink sites on vicinal close-packed surfaces provide a low energy path for both hydrogen adsorption and desorption which results in a much higher dissociative sticking probability and a lower desorption temperature. The hydrogen adsorption strength on various FCC and HCP cobalt surfaces varies between 30 and 45 kJ/mol H ad and is strongest on threefold hollow sites on the close-packed terraces while it is significantly lower on fourfold hollow sites on FCC-(100) and on threefold hollow sites on various open HCP surfaces. Under reaction conditions, the structure-dependent adsorption energy translates to a two to three orders of magnitude variation of the equilibrium constant for hydrogen. [ABSTRACT FROM AUTHOR]

Photocatalytic polyol conversion provides a green approach for the synthesis of value-added products. However, efficient and selective photocatalysts that can prevent unwanted full oxidation are still missing, mostly due to a lack of mechanistic understanding. Here we use ethylene glycol (EG) as model compound to study the reaction pathways in photocatalytic polyol dissociation under aerated conditions using in - situ vibrational spectroscopy coupled with mass spectrometry. On pristine TiO 2 , the presence of oxygen leads to the formation of formaldehyde via photocatalytic C C bond cleavage, where the removal of photo-generated surface adsorbed proton (H ads + ) in the form of water is the rate determining step (RDS). The photo-generated formaldehyde molecules subsequently convert into CO 2 via complete oxidation by oxygen, or into paraformaldehyde by polymerization with water. A promotion effect is observed when noble metal (Au, Pt, Ag) nanoparticles (NPs) are used as cocatalysts. While Ag and Au selectively promote the formation of paraformaldehyde, the addition of Pt facilitates the complete oxidation of EG into CO 2 . By performing the reaction under a low oxygen partial pressure, we rationalize that Ag and Au NPs accelerate the polymerization of formaldehyde by providing water rapidly through direct oxidation of H ads oxidation, whereas Pt NPs supply water indirectly, in a pathway via H 2 or formaldehyde oxidation. [ABSTRACT FROM AUTHOR]

Polyol conversion to value-added products is of great interest for the bio-diesel industry. Photocatalytic oxidation processes may offer a green approach for polyol conversion; however the lack of comprehensive mechanistic understanding from an interdisciplinary perspective limits or even misleads the design of highly selective and efficient photocatalysts for such process. Here we have studied the photocatalytic polyol conversion on pristine TiO 2 and metal (Au, Pd, and Pt) nanoparticles (NPs) decorated TiO 2 using ethylene glycol (EG) as the model compound. We have developed a mechanistic picture at molecular level by coupling in-situ surface science study on rutile (110) surface with in-situ vibrational-mass spectrometry study on TiO 2 nanopowders. The C C bond cleavage was found to be the only pathway in EG photo-conversion under deaerated conditions, leading to the formation of formaldehyde and hydrogen. We rationalized that the desorption of the surface adsorbed H (H ads ) to be the rate determining step (RDS), making pristine TiO 2 a poor photocatalyst that only catalyze the EG conversion at very low surface coverages. The addition of metal NPs on TiO 2 surface promotes the desorption of H ads significantly, thus leading to an enhanced C C bond cleavage performance at higher surface coverages that is more applicable. [ABSTRACT FROM AUTHOR]

Catalyst deactivation is an important topic for industrial catalyst development. Sintering of small cobalt crystallites is one of the deactivation mechanisms of cobalt-based Fischer–Tropsch synthesis (FTS) catalysts. This study investigates the mechanism of cobalt sintering at low-conversion FTS conditions. A Co/SiO 2 /Si(1 0 0) model catalyst is exposed to 20 bar dry synthesis gas (H 2 /CO: 2/1) at 230 °C for 10 h. Cobalt nanoparticles were characterized before and after treatment using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). TEM images of identical locations on the model catalyst showed a loss of some small crystallites and decrease in size of some crystallites. Sintering is dominated by an Ostwald ripening mechanism using our model catalyst under the present conditions. Complementary XPS measurements confirm the loss of Co dispersion. Therefore, the loss of small Co nanoparticles causes a rapid loss of metal surface area when exposed to model FTS conditions. [ABSTRACT FROM AUTHOR]

Sintering of nanoparticles is an important contributor to loss of activity in heterogeneous catalysts, such as those used for controlling harmful emissions from automobiles. But mechanistic details, such as the rates of atom emission or the nature of the mobile species, remain poorly understood. Herein we report a novel approach that allows direct measurement of atom emission from nanoparticles. We use model catalyst samples and a novel reactor that allows the same region of the sample to be observed after short-term heat treatments (seconds) under conditions relevant to diesel oxidation catalysts (DOCs). Monometallic Pd is very stable and does not sinter when heated in air ( T ⩽ 800 °C). Pt sinters readily in air, and at high temperatures (⩾800 °C) mobile Pt species emitted to the vapor phase cause the formation of large, faceted particles. In Pt–Pd nanoparticles, Pd slows the rate of emission of atoms to the vapor phase due to the formation of an alloy. However, the role of Pd in Pt DOCs in air is quite complex: at low temperatures, Pt enhances the rate of Pd sintering (which otherwise would be stable as an oxide), while at higher temperature Pd helps to slow the rate of Pt sintering. DFT calculations show that the barrier for atom emission to the vapor phase is much greater than the barrier for emitting atoms to the support. Hence, vapor-phase transport becomes significant only at high temperatures while diffusion of adatoms on the support dominates at lower temperatures. [ABSTRACT FROM AUTHOR]

Abstract: The oxidation of nanosized metallic cobalt to cobalt oxide during Fischer–Tropsch synthesis has long been postulated as a major deactivation mechanism apparently related to cobalt crystallite size. To establish a connection between cobalt crystallite size and oxidation behaviour, well-defined spherical Co/SiO2 model catalysts with average cobalt crystallites sizes of 4, 13, and 28 nm were synthesised. The crystallite size distribution of the spherical Co/SiO2 model catalysts was characterised with high-resolution transmission electron microscopy and in situ X-ray diffraction. The oxidation behaviour of the reduced spherical Co/SiO2 model catalysts of differing cobalt crystallite size was studied using in situ X-ray absorption fine structure under model oxidation conditions (H2O/He, ). Surprisingly, it was found that the spherical Co/SiO2 model catalyst with small cobalt crystallites (i.e., 4 nm) did not show oxidation under H2O/He mixtures () up to 400 °C, which is against bulk thermodynamic calculations for the oxidation of cobalt metal to cobalt oxide. This was attributed to the encapsulation of the cobalt crystallites with silica after reduction at 500 °C in hydrogen. The encapsulation was verified with high-resolution transmission electron microscopy. The spherical Co/SiO2 model catalysts with medium-sized cobalt crystallites (i.e., 13 nm) did oxidize at 100 °C and reached a maximum oxidation of 30% at 300 °C (H2O/He; ). The spherical Co/SiO2 model catalysts with large cobalt crystallites (i.e., 28 nm) was found to undergo very little oxidation, <2% at 300 °C under a H2O/He () environment. In general, it could be concluded that the oxidation of spherical Co/SiO2 model catalysts with water is difficult and is size-dependent. [Copyright &y& Elsevier]

Abstract: Density functional theory has been used to gain molecular understanding of various catalytic processes involving N species on Rh(111). These include CN, N2, and HCN formation and N2O decomposition. Our calculations substantiate the conclusion that, starting from chemisorbed C and N atomic species, CN formation is preferred over N2 formation, because of the lower activation energy of the former process (1.73 vs. 2.10 eV). HCN formation has been studied starting from adsorbed CH and N species, with a computed activation barrier of 1.35 eV. The process of binding CH to N is more favorable than recombination of C and N atoms into CN followed by hydrogenation. Concerning the adsorption and dissociation of N2O on Rh, two pathways have been investigated, leading to N2 or NO. From thermodynamic considerations, N2 can be concluded to be the preferred product resulting from N2O dissociation. Our results also support the participation of N2O as a reaction intermediate during reduction of nitric oxide to nitrogen over Rh surfaces by reaction of adsorbed NO and N atoms. [Copyright &y& Elsevier]

Abstract: In this in situ EXAFS and XANES study on the Phillips ethylene-polymerization Cr/SiO2 catalyst, two polymerization routes are investigated and compared. The first mimics that adopted in industrial plants, where ethylene is dosed directly on the oxidized catalyst, while in the second the oxidized catalyst is first reduced by CO at 623 K. On this reduced catalyst C2H4 polymerization has been investigated at room temperature and at 373 K. To allow experiments in transmission mode, a Cr loading of 4 wt% has been adopted. At this loading a fraction of clustered Cr2O3 particles has been observed and quantified. The use of a third-generation synchrotron radiation source has allowed us to improve the energy resolution and signal-to-noise ratio of the XANES data, allowing us to determine the fraction of Cr sites involved in the polymerization reaction. This number represents an upper limit of the active sites. Preliminary ReflEXAFS experiments have been performed on a model catalyst prepared by impregnation of Cr on a flat Si(100) substrate covered by a thin layer of amorphous silica. Experiments have been performed ex situ on the grafted catalyst (i.e., after impregnation and thermal activation) and at the end of the polymerization stage. [Copyright &y& Elsevier]

MoO3/ZrO2 catalysts with different MoO3 loadings (2–12 wt%) were prepared by the wet impregnation method. These catalysts were characterized by various techniques, such as X-ray diffraction (XRD), temperature-programmed reduction (TPR), laser Raman spectroscopy (LRS), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption of NH3 and the catalytic properties were evaluated for vapor-phase ammoxidation of toluene to benzonitrile. XRD patterns show the presence of crystalline MoO3 peaks above 6.6 wt% MoO3, which corresponds to the theoretical monolayer loading of MoO3 on the zirconia used in the present study. The TPR suggests that reduction of the catalysts occurs in two stages and indicates that the reducibility of the catalysts increases with increase in MoO3 loading up to 6.6 wt%. The acidity of the catalysts was also found to increase up to 6.6 wt% of molybdena loading and it does not increase much at higher loadings. Raman results show that the surface molybdate species are present in low-loading samples, while crystalline MoO3 bands are observed from 9 wt% of MoO3 and above loadings. XPS spectra showed that molybdenum was present at Mo6- on all fresh samples. The Mo/Zr atomic ratio shows that the dispersion of molybdena is high below 6.6 wt% MoO3 and dispersion decreases at higher molybdena loadings. The catalytic activity of the catalysts during ammoxidation of toluene was found to increase with loading up to 6.6 wt% and did not change appreciably beyond this loading. [Copyright &y& Elsevier]

A series of CrOx/SiO2/Si(100) model catalysts were tested for ethylene polymerization activity, varying chromium loading, and calcination temperature. Chromium coverage of the model catalyst, quantified by Rutherford backscattering spectrometry, decreases with increasing calcination temperature as some chromium desorbs from the silica support. The polymerization activity of the model catalysts is correlated to calcination temperature and chromium coverage. Based on the evidence presented, we propose that high local Cr coverage—short Cr–Cr distances—is detrimental to polymerization activity, possibly because it facilitates dimerization leading to inactive chromium sites. Calcination at high temperatures not only causes depletion of surface silanol groups, but may also facilitate the formation of isolated chromium sites, which can evolve into active polymerization centers. AFM images of nascent polymer films after short polymerization times offer a means to visualize the distribution of polymerization activity on the silica surface. They indicate that the catalytically active chromium forms islands on the silica surface. [Copyright &y& Elsevier]

The kinetics of thiophene hydrodesulfurization is studied on a planar surface science model of a sulfided NiMo/SiO2 catalyst under diffusion-limitation-free conditions in a batch reactor at atmospheric pressure and temperatures between 575 and 675 K. A Langmuir–Hinshelwood model is used to analyze the data and to derive activation energies and heats of adsorption for thiophene and hydrogen sulfide. The work illustrates the feasibility of kinetic and mechanistic studies using planar models of catalysts which can be prepared and manipulated with procedures available in surface science. [Copyright &y& Elsevier]