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Thiolate‐protected Cu clusters with well‐defined structures and stable low‐coordinated Cu+ species exhibit remarkable potential for the CO2RR and are ideal model catalysts for establishing structure‐electrocatalytic property relationships at the atomic level. However, extant Cu clusters employed in the CO2RR predominantly yield 2e− products. Herein, two model Cu4(MMI)4 and Cu8(MMI)4(tBuS)4 clusters (MMI=2‐mercapto‐1‐methylimidazole) are prepared to investigate the synergistic effect of Cu+ and adjacent S sites on the CO2RR. Cu4(MMI)4 can reduce CO2 to deep‐reduced products with a 91.0 % Faradaic efficiency (including 53.7 % for CH4) while maintaining remarkable stability. Conversely, Cu8(MMI)4(tBuS)4 shows a remarkable preference for C2+ products, achieving a maximum FE of 58.5 % with a C2+ current density of 152.1 mA⋅cm−2. In situ XAS and ex situ XPS spectra reveal the preservation of Cu+ species in Cu clusters during CO2RR, extensively enhancing the adsorption capacity of *CO intermediate. Moreover, kinetic analysis and theoretical calculations confirm that S sites facilitate H2O dissociation into *H species, which directly participate in the protonation process on adjacent Cu sites for the protonation of *CO to *CHO. This study highlights the important role of Cu−S dual sites in Cu clusters and provides mechanistic insights into the CO2RR pathway at the atomic level. [ABSTRACT FROM AUTHOR]

Metal–N–C single‐atom catalysts, mostly prepared from pyrolysis of metal‐organic precursors, are widely used in heterogeneous electrocatalysis. Since metal sites with diverse local structures coexist in this type of material and it is challenging to characterize the local structure, a reliable structure–property relationship is difficult to establish. Conjugated macrocyclic complexes adsorbed on carbon support are well‐defined models to mimic the single‐atom catalysts. Metal–N4 site with four electroneutral pyridine‐type ligands embedded in a graphene layer is the most commonly proposed structure of the active site of single‐atom catalysts, but its molecular counterpart has not been reported. In this work, we synthesized the conjugated macrocyclic complexes with a metal center (Co, Fe, or Ni) coordinated with four electroneutral pyridinic ligands as model catalysts for CO2 electroreduction. For comparison, the complexes with anionic quadri‐pyridine macrocyclic ligand were also prepared. The Co complex with the electroneutral ligand expressed a turnover frequency of CO formation more than an order of magnitude higher than that of the Co complex with the anionic ligand. Constrained ab initio molecular dynamics simulations based on the well‐defined structures of the model catalysts indicate that the Co complex with the electroneutral ligand possesses a stronger ability to mediate electron transfer from carbon to CO2. [ABSTRACT FROM AUTHOR]

Developing effective descriptors for perovskite oxygen evolution reaction (OER) electrocatalysts is crucial for advancing clean energy technologies, which are often constrained by incomplete theoretical calculations. Herein, using step‐wise strained (−3–3%) NdNiO3 as a model, the density functional theory (DFT) is employed, considering both bulk superlattice model (B‐model) and surface slab models (S‐model) to garner a set of electronic descriptors. The S‐model‐derived metal─oxygen orbital hybridization is found to agree with the X‐ray absorption spectroscopy (XAS) results, which, however, are disentangled from the observed OER activity trend. Further detailed analysis discovers that the alignment of the calculated O 2p band position with the OER redox potential on an absolute scale can serve as a hitherto unexplored descriptor for comprehending the activity of lattice oxygen and its bonding strength to OH*. Combined DFT and XAS data reveals that excessive strain hinders surface oxygen exchange, slows down the rate‐determining step of OH* adsorption, and impairs the durability of the electrocatalyst by altering the random electron occupancy in frontier orbitals. The work sheds light on the rational design of cost‐effective, high‐performance perovskite‐based electrocatalysts. [ABSTRACT FROM AUTHOR]

Abstract Metal–N–C single‐atom catalysts, mostly prepared from pyrolysis of metal‐organic precursors, are widely used in heterogeneous electrocatalysis. Since metal sites with diverse local structures coexist in this type of material and it is challenging to characterize the local structure, a reliable structure–property relationship is difficult to establish. Conjugated macrocyclic complexes adsorbed on carbon support are well‐defined models to mimic the single‐atom catalysts. Metal–N4 site with four electroneutral pyridine‐type ligands embedded in a graphene layer is the most commonly proposed structure of the active site of single‐atom catalysts, but its molecular counterpart has not been reported. In this work, we synthesized the conjugated macrocyclic complexes with a metal center (Co, Fe, or Ni) coordinated with four electroneutral pyridinic ligands as model catalysts for CO2 electroreduction. For comparison, the complexes with anionic quadri‐pyridine macrocyclic ligand were also prepared. The Co complex with the electroneutral ligand expressed a turnover frequency of CO formation more than an order of magnitude higher than that of the Co complex with the anionic ligand. Constrained ab initio molecular dynamics simulations based on the well‐defined structures of the model catalysts indicate that the Co complex with the electroneutral ligand possesses a stronger ability to mediate electron transfer from carbon to CO2.

Highlights Mechanistic understandings of metal-free carbon thermocatalysis and electrocatalysis from the viewpoint of model method are summarized. Active sites and reaction mechanisms are discussed with a focus on in-situ techniques and 2D structure–activity relationships. The real contribution of each alien species, defect and edge configuration to catalytic reactions are systematically highlighted at a molecular level.

Highlights: Mechanistic understandings of metal-free carbon thermocatalysis and electrocatalysis from the viewpoint of model method are summarized. Active sites and reaction mechanisms are discussed with a focus on in-situ techniques and 2D structure–activity relationships. The real contribution of each alien species, defect and edge configuration to catalytic reactions are systematically highlighted at a molecular level. Metal-free carbon, as the most representative heterogeneous metal-free catalysts, have received considerable interests in electro- and thermo-catalytic reactions due to their impressive performance and sustainability. Over the past decade, well-designed carbon catalysts with tunable structures and heteroatom groups coupled with various characterization techniques have proposed numerous reaction mechanisms. However, active sites, key intermediate species, precise structure–activity relationships and dynamic evolution processes of carbon catalysts are still rife with controversies due to the monotony and limitation of used experimental methods. In this Review, we summarize the extensive efforts on model catalysts since the 2000s, particularly in the past decade, to overcome the influences of material and structure limitations in metal-free carbon catalysis. Using both nanomolecule model and bulk model, the real contribution of each alien species, defect and edge configuration to a series of fundamentally important reactions, such as thermocatalytic reactions, electrocatalytic reactions, were systematically studied. Combined with in situ techniques, isotope labeling and size control, the detailed reaction mechanisms, the precise 2D structure–activity relationships and the rate-determining steps were revealed at a molecular level. Furthermore, the outlook of model carbon catalysis has also been proposed in this work. [ABSTRACT FROM AUTHOR]

Manganese oxide is an effective promoter for enhancing the activity and selectivity of cobalt-based Fischer-Tropsch synthesis (FTS) catalysts. Previous studies suggest that Co atoms located at the Co-MnO perimeter adsorb CO more strongly and undergo C-O bond cleavage more readily than CO adsorbed on Co sites, which leads to improved C5+ selectivity. Thus, direct characterization of the formation and structure of the active interface between Co nanoparticles and MnO is essential for a full understanding of the FTS promotion effect. Scanning transmission electron microscopic (STEM) methods are well-suited for direct characterization of nanoscale features through imaging, energy dispersive spectroscopy (EDS) elemental mapping, and spatially resolved electron diffraction, a technique known as scanning nanobeam diffraction. However, these techniques require that the catalytic nanoparticles are non-overlapping and well-defined, which is not typical for nanoparticles on porous metal oxide FTS supports. In the present study, we addressed these challenges by synthesizing catalysts composed of cobalt dispersed on nonporous alumina nanospheres (Co/ANS), which yielded isolated nanoparticles that are amenable to elemental and phase mapping by STEM-EDS and scanning nanobeam diffraction. Then, manganese oxide-promoted Co/ANS (Mn-Co/ANS) was characterized using these two techniques, which provided direct evidence for the transformation of a Co-Mn oxide spinel present in the as-prepared catalyst into closely associated Co3O4 and MnO2 nanoparticles in the passivated catalyst after its use for FTS.

Manganese oxide is an effective promoter for enhancing the activity and selectivity of cobalt-based Fischer-Tropsch synthesis (FTS) catalysts. Previous studies suggest that Co atoms located at the Co-MnO perimeter adsorb CO more strongly and undergo C-O bond cleavage more readily than CO adsorbed on Co sites, which leads to improved C5+ selectivity. Thus, direct characterization of the formation and structure of the active interface between Co nanoparticles and MnO is essential for a full understanding of the FTS promotion effect. Scanning transmission electron microscopic (STEM) methods are well-suited for direct characterization of nanoscale features through imaging, energy dispersive spectroscopy (EDS) elemental mapping, and spatially resolved electron diffraction, a technique known as scanning nanobeam diffraction. However, these techniques require that the catalytic nanoparticles are non-overlapping and well-defined, which is not typical for nanoparticles on porous metal oxide FTS supports. In the present study, we addressed these challenges by synthesizing catalysts composed of cobalt dispersed on nonporous alumina nanospheres (Co/ANS), which yielded isolated nanoparticles that are amenable to elemental and phase mapping by STEM-EDS and scanning nanobeam diffraction. Then, manganese oxide-promoted Co/ANS (Mn-Co/ANS) was characterized using these two techniques, which provided direct evidence for the transformation of a Co-Mn oxide spinel present in the as-prepared catalyst into closely associated Co3O4 and MnO2 nanoparticles in the passivated catalyst after its use for FTS.

Single-atom catalysts are a family of heterogeneous electrocatalysts widely used in energy storage and conversion. The determination of the local structure of the active metal sites is challenging, which limits the establishment of the reliable structure-property relationship of single-atom catalysts. A carbon black-conjugated complex can be used as the model catalyst to probe the intrinsic activity of metal sites with certain local structures. In this work, we prepared carbon black-conjugated [Co(phenanthroline)Cl2], [Co(o-phenylenediamine)Cl2] and [Co(salophen)]. In these catalysts, the Co complexes with well-defined structures are anchored on the edge of carbon black by pyrazine moieties. The number of electrochemical accessible Co sites can be measured from the area of the redox peaks of pyrazine linkers in the cyclic voltammetry curve. Then, the intrinsic electrocatalytic activity of one Co site can be obtained. The catalytic performances of the three catalysts towards oxygen reduction reaction in alkaline conditions were measured. Carbon black-conjugated [Co(salophen)] showed the highest intrinsic activity with the turnover frequency of 0.72 s−1 at 0.75 V vs. the reversible hydrogen electrode. The strategy developed in this work can be used to explore and verify the possible local structure of active sites proposed for single-atom catalysts. [ABSTRACT FROM AUTHOR]

Thiolate-protected Cu clusters with well-defined structures and stable low-coordinated Cu + species exhibit remarkable potential for the CO 2 RR and are ideal model catalysts for establishing structure-electrocatalytic property relationships at the atomic level. However, extant Cu clusters employed in the CO 2 RR predominantly yield 2e - products. Herein, two model Cu 4 (MMI) 4 and Cu 8 (MMI) 4 ( t BuS) 4 clusters (MMI=2-mercapto-1-methylimidazole) are prepared to investigate the synergistic effect of Cu + and adjacent S sites on the CO 2 RR. Cu 4 (MMI) 4 can reduce CO 2 to deep-reduced products with a 91.0 % Faradaic efficiency (including 53.7 % for CH 4 ) while maintaining remarkable stability. Conversely, Cu 8 (MMI) 4 ( t BuS) 4 shows a remarkable preference for C 2+ products, achieving a maximum FE of 58.5 % with a C 2+ current density of 152.1 mA⋅cm -2 . In situ XAS and ex situ XPS spectra reveal the preservation of Cu + species in Cu clusters during CO 2 RR, extensively enhancing the adsorption capacity of *CO intermediate. Moreover, kinetic analysis and theoretical calculations confirm that S sites facilitate H 2 O dissociation into *H species, which directly participate in the protonation process on adjacent Cu sites for the protonation of *CO to *CHO. This study highlights the important role of Cu-S dual sites in Cu clusters and provides mechanistic insights into the CO 2 RR pathway at the atomic level., (© 2024 Wiley-VCH GmbH.)

Post-synthesis treatment of bimetallic catalysts in different gas phases resulting in the adsorption-induced segregation is among promising approaches to enhance their activity not compromising selectivity towards a number of low-temperature reactions. Our recently published paper (M.A. Panafidin, A.V. Bukhtiyarov, I.P. Prosvirin, I.A. Chetyrin, A.Yu. Klyushin, A. Knop-Gericke, N.S. Smirnova, P.V. Markov, I.S. Mashkovsky, Y.V. Zubavichus, A.Yu. Stakheev, V.I. Bukhtiyarov, A mild post-synthesis oxidative treatment of Pd-In/HOPG bimetallic catalysts as a tool of their surface structure fine tuning. Appl. Surf. Sci.) reports on Pd-In intermetallic formation regularities and their evolution after storage in air as well as during treatment in oxygen at submillibar pressures. The current paper gives an extended representation of experimental ex situ/in situ synchrotron-based photoelectron spectroscopy (SRPES) and scanning tunnelling microscopy (STM) data used to derive scientific conclusions in the paper quoted above.

Tuning graphitic surface with nitrogen functionalities renders potential carbocatalysts for a wide range of reactions. Herein, g‐C3N4/graphite composite material (g‐C3N4@G) is reported for oxidation of styrene to styrene oxide. The detailed physiochemical properties of g‐C3N4@G are investigated by X‐ray photoelectron spectroscopy (XPS), Transmission electron microscopy (TEM), Scanning electron microscopy (SEM), X‐ray diffraction (XRD) and Fourier transmission infrared spectroscopy (FTIR) analysis. The TEM and SEM micrographs indicate the formation of g‐C3N4 sheets and N1s XPS results confirm the formation of g‐C3N4 on the graphitic surface. The role of specific catalytic sites is experimentally studied by the use of model catalysts those mimic the functional groups on the surface of g‐C3N4@G. It is suggested that the catalytic sites on the surface showcase a synergeistic effect, with sp2 hybridized N site being most selective for styrene oxide. [ABSTRACT FROM AUTHOR]

[Display omitted] • Isopropanol on Pt/Co 3 O 4 (1 1 1) decomposes to isopropoxide on Co 3 O 4 (1 1 1) and enolate species on Pt nanoparticles. • Reactive interaction of isopropanol yields metallic Co and Pt–Co alloy. • Selectivity depends on reduction degree of Co 3 O 4 (1 1 1) and chemical composition of nanoparticles. The structure and chemical composition of the catalyst may change under reaction conditions resulting in changes of the active sites and, thereby, loss of selectivity. In this work, we investigated the reactive interaction of isopropanol with well-defined Co 3 O 4 (1 1 1)/Ir(1 0 0) and Pt/Co 3 O 4 (1 1 1)/Ir(1 0 0) model catalysts by means of synchrotron radiation photoelectron spectroscopy (SRPES), near ambient pressure X-ray photoelectron spectroscopy (NAP−XPS), scanning tunneling microscopy (STM), and temperature programmed desorption (TPD). We found that adsorption at 150 K yields molecularly adsorbed isopropanol on both model catalysts accompanied by small fractions of isopropoxide and enolate species on Co 3 O 4 (1 1 1) and supported Pt nanoparticles, respectively. The reactive interaction of isopropanol-derived species with the model catalysts upon annealing in UHV and in 1×10-7 mbar and 1 mbar of isopropanol results in the reduction of Co 3 O 4 (1 1 1) followed by its conversion to CoO(1 1 1) and, finally, to metallic Co and Pt–Co alloy. The mechanism of isopropanol decomposition reveals remarkable sensitivity to the oxidation state and morphology of the model catalysts. On as-prepared Pt/Co 3 O 4 (1 1 1) catalyst, Pt particles densely cover the Co 3 O 4 (1 1 1) substrate steering isopropanol decomposition to acetone and hydrogen. Selective channels towards acetone and propene, both accompanied by water, open after temperature-driven sintering of the supported Pt particles. The selectivity of these channels is controlled by the degree of reduction of the Co 3 O 4 (1 1 1) substrate and the chemical composition of the supported nanoparticles. At high degrees of reduction of Co 3 O 4 (1 1 1), the formation of both propene and acetone through selective channels decline due to the strong preference for C–C bond scission. Under NAP conditions, the formation of acetone resumes after complete reduction of model catalysts to metallic Co and Pt–Co alloy. [ABSTRACT FROM AUTHOR]

The spatially resolved identification of active sites on the heterogeneous catalyst surface is an essential step toward directly visualizing a catalytic reaction with atomic scale. To date, ferrous centers on platinum group metals have shown promising potential for low-temperature CO catalytic oxidation, but the temporal and spatial distribution of active sites during the reaction and how molecular-scale structures develop at the interface are not fully understood. Here, we studied the catalytic CO oxidation and the effect of co-adsorbed hydrogen on the FeO 2- x /Ir(111) surface. Combining scanning tunneling microscopy (STM), isotope-labeled pulse reaction measurements, and DFT calculations, we identified both FeO 2 /Ir and FeO 2 /FeO sites as active sites with different reactivity. The trilayer O-Fe-O structure with its Moiré pattern can be fully recovered after O 2 exposure, where molecular O 2 dissociates at the FeO/Ir interface. Additionally, as a competitor, dissociated hydrogen migrates onto the oxide film with the formation of surface hydroxyl and water clusters down to 150 K.

Colloidal synthesis of metal nanocrystals (NC) offers control over size, crystal structure and shape of nanoparticles, making it a promising method to synthesize model catalysts to investigate structure-performance relationships. Here, we investigated the synthesis of disk-shaped Co-NC, their deposition on a support and performance in the Fischer–Tropsch (FT) synthesis under industrially relevant conditions. From the NC synthesis, either spheres only or a mixture of disk-shaped and spherical Co-NC was obtained. The disks had an average diameter of 15 nm, a thickness of 4 nm and consisted of hcp Co exposing (0001) on the base planes. The spheres were 11 nm on average and consisted of ε-Co. After mild oxidation, the CoO-NC were deposited on SiO2 with numerically 66% of the NC being disk-shaped. After reduction, the catalyst with spherical plus disk-shaped Co-NC had 50% lower intrinsic activity for FT synthesis (20 bar, 220 °C, H2/CO = 2 v/v) than the catalyst with spherical NC only, while C5+-selectivity was similar. Surprisingly, the Co-NC morphology was unchanged after catalysis. Using XPS it was established that nitrogen-containing ligands were largely removed and in situ XRD revealed that both catalysts consisted of 65% hcp Co and 21 or 32% fcc Co during FT. Furthermore, 3–5 nm polycrystalline domains were observed. Through exclusion of several phenomena, we tentatively conclude that the high fraction of (0001) facets in disk-shaped Co-NC decrease FT activity and, although very challenging to pursue, that metal nanoparticle shape effects can be studied at industrially relevant conditions. [ABSTRACT FROM AUTHOR]

Ru-core Pt-shell nanoparticles show an enhanced electrocatalytic activity for the methanol oxidation reaction (MOR) compared to the respective monometallic or most bimetallic alloy systems. Due to their ill-defined structure, however, a simple interpretation of these findings is challenging. In the present work we present results of a systematic study on the influence of the Pt film thickness of structurally well-defined Pt X-ML /Ru(0001) model electrodes with varying Pt film thickness (1.1–5.5 monolayers (ML)) on their MOR performance. The electrodes are prepared and structurally characterized by scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. The electrochemical/-catalytic properties of the electrodes are evaluated in an electrochemical flow cell, which is attached to the UHV system and combined with a mass spectrometer in a differential electrochemical mass spectrometry (DEMS) set-up, allowing for online detection of volatile reaction products/intermediates. We demonstrate that the MOR activity of Pt X-ML /Ru(0001) increases with increasing Pt film thickness and that thicker Pt films (>2.5 ML) are significantly more active than Pt(111) and thinner Pt films, whereas the selectivity for CO 2 formation remains essentially constant. We explain this trend in the activity via the Sabatier principle, where the higher activities of the Pt film electrodes with thicker Pt films compared to both Pt(111) and electrodes with thinner Pt films are due to a lower binding strength of adsorbed species than on Pt(111), due to vertical electronic ligand effects (Ru – Pt interactions) and compressive strain effects in the pseudomorphic films, and due to an increasing binding strength with increasing Pt film thickness, due to decreasing electronic ligand effects. In this picture the Pt 5.5-ML /Ru(0001) electrode exhibits the best value of the binding strength and thus the highest rates, while for electrodes with lower/higher binding strength the rates are lower. Consequences of these findings for bimetallic PtRu nanoparticle catalysts are discussed. Image 1 [ABSTRACT FROM AUTHOR]

Three structurally distinct interfaces, i. e. Au/α-Fe2O3-THB-Air, Au/α-Fe2O3-HS-Air, and Au/α-Fe2O3-QC-Air, with the major substrate facet being {113}, {001}, and {012} respectively, were controllably prepared. The low temperature water gas shift (WGS) reaction (100-260 °C) was applied to these unique systems to reveal how these structurally distinct interfaces can impact on the reaction behavior. With the detailed performance evaluation plus the characterizations of CO-TPSR, XPS, and in situ-FTIR in particular, the correlations between the evolution of different intermediates and the distinct catalytic behaviors have been established. The substrate facet structure and the Au entity largely determine the form and reactivity of surface OH/ H2O species which in turn determine the formation of different intermediates and eventually the product formation. The Au/α-Fe2O3-THB-Air is enriched with oxygen vacant sites, favorable for dissociative adsorption of H2O, carboxyl intermediate formation and H2 production. The Au/α-Fe2O3-HS-Air is enriched with surface lattice oxygen, favorable for molecular adsorption of H2O, formate intermediate formation, and CO2 production. The Au/α-Fe2O3-QC-Air interface is enriched with surface Fe sites, favorable for dissociative adsorption of H2O and evolution of stable carboxyl intermediate as well as isolated OH species at low reaction temperatures. The findings are informative to control the interfacial structure for the WGS and other reactions. [ABSTRACT FROM AUTHOR]

• Monitoring the stability of isolated Co nanoparticles on a graphitic support during reduction in situ. • High stability of well-defined model catalyst during co-feeding of water mimicking Fischer-Tropsch conversion levels. • Permanent deactivation observed upon simulating 92% conversion of CO. • Oxidation of Co followed by sintering and re-reduction captured by means of in situ magnetometry. • Experimental results in line with thermodynamic stability predictions for nanoparticles. Supported nano-sized metal crystallites as catalysts in the Fischer-Tropsch synthesis have become a major research focus due to their high mass specific surface area and resulting lower cost. Such small supported cobalt crystallites have been reported to show a very different resistance with regard to deactivation compared to larger cobalt particles. The Fischer-Tropsch product water is reported to have a severe effect on the deactivation of cobalt-based Fischer-Tropsch catalysts. Compared to other water-induced deactivation mechanisms, hydrothermal sintering of cobalt nanoparticles is fairly well established in literature. A previously hypothesised interconnection between oxidation of cobalt nanoparticles and hydrothermal sintering has – for the first time – been captured in situ in the presented study. High concentrations of water induce oxidation of the cobalt nanoparticles increasing their mobility and resulting in crystallite growth via particle migration and coalescence whilst in the oxidised state. A well-defined model catalyst comprising highly dispersed cobalt nanoparticles on a relatively inert exfoliated graphite support in combination with an in situ magnetometer allowed for these observations, which resulted in irreversible deactivation of the catalyst. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • STM images reveal pyridine and quinoline adsorption on HDS catalysts. • Pyridine and quinoline are adsorbed in the form of pyridinium and quinolinium ions. • Diffuse physisorbed molecules are related to partially reduced MoS 2 S-edges. • Pyridinium and quinolinium ions block two or three active sites along the S-edges. • Pyridine and quinoline adsorption may inhibit the HYD pathway of DBT and 4,6-DMDBT. Abstract Heterocyclic nitrogen compounds in crude oil, such as pyridine and quinoline, are known to have a strongly disabling effect on the hydrodesulfurization (HDS) reactions on MoS 2 -based catalysts. To shed light on the nature and distribution of the adsorption sites of such nitrogen-containing inhibitors, we use atom-resolved scanning tunneling microscopy (STM) on a gold-supported HDS model system to probe the detailed adsorption configurations and diffusion characteristics of pyridine and quinoline molecules on both unpromoted and Co-promoted MoS 2 (CoMoS) nanoparticles. We find that pyridine and quinoline molecules adsorb strongly on the S-edges of reduced MoS 2 and CoMoS nanoparticles through the formation of protonated pyridinium/quinolinium ions. The quinoline molecules adsorbed in a flat configuration on the S-edge are concluded to block significantly more active sites than the pyridine molecules, due to the larger molecular size and higher mobility. The experimental observations also illustrate how proton transfer to the adsorbed N-containing species may affects bonding of the terminal S atoms of the active edge to the Au substrate. The STM findings overall substantiate the notion that these N-bearing compounds compete with the adsorption of sulfur-containing molecules on the same active edge sites in hydrogenation reactions, but also indicate that corner sites may be less affected. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Two MgCl 2 -supported ZN catalysts with 0.1% and 1.5% Ti content were prepared. • Kinetics of ethylene homo- and copolymerization with 1-hexene were compared. • [C*] and k p were hardly changed by introducing 1-hexene in catalyst with 0.1% Ti. • [C*] was markedly increased by introducing 1-hexene for catalyst with 1.5% Ti. • Both physical and chemical factors play important role in the comonomer effects. Abstract Two MgCl 2 -supported Ziegler-Natta model catalysts were prepared by contacting activated MgCl 2 with deficient or excess amount of TiCl 4. Ethylene/1-hexene copolymerization with the catalysts was conducted under different 1-hexene feed, and active center concentration of the reaction system was determined by quench-labeling method using thiophene-2-carbonyl chloride as the quencher. The catalytic activity was only moderately enhanced (increment <50%) by adding 1-hexene in ethylene polymerization with the model catalyst (Cat-1) of 0.1 wt% Ti content, and the active center ratio ([C*]/[Ti]) was only slightly increased (from 64 to 72 mol%). The comonomer activation effect in Cat-1 catalyzed copolymerization was mainly attributed to increase of apparent propagation rate constant by the comonomer, which can be explained by reduction of mass transfer barrier in the polymer/catalyst particles because of larger monomer diffusion coefficients in the copolymer phase than in the more crystalline polyethylene phase. In contrast, the activity was nearly tripled by adding 1-hexene when the catalyst of 1.47% Ti content (Cat-2) was used, meanwhile its [C*]/[Ti] was also tripled (from 15 to 49 mol%). The strong comonomer activation effect in Cat-2 catalyzed copolymerization was caused by marked increase of active center concentration. In this case, fragmentation of the polymer/catalyst particles was intensified by adding the comonomer, and more active centers that were originally inaccessible to the cocatalyst and monomer were exposed through the particle fragmentation. According to the results of this work, the comonomer effect in conventional supported Ziegler-Natta catalysts can be explained by the combination of physical factor (increase in diffusion coefficient) and chemical factor (exposure of inaccessible active center precursors through particle fragmentation). [ABSTRACT FROM AUTHOR]

The growth, electronic structure and stability of Sm on ordered CeO2(111) thin films grown on Cu(111) were investigated by means of X-ray photoelectron spectroscopy (XPS), low energy electron diffraction, and scanning tunneling microscopy (STM). Metallic samarium was deposited on the CeO2(111) surface by thermal evaporation under ultrahigh vacuum conditions at room temperature. The XPS data suggest that metallic Sm is oxidized to Sm3+ upon the deposition of Sm on CeO2, accompanied by the reduction of Ce4+ to Ce3+. With increasing the Sm coverage, the concentration of Ce3+ increases monotonically. After depositing 6 ML of Sm, only Ce3+ is observed within the detection depth of XPS. The STM results indicate that Sm exhibits a two-dimensional growth on the CeO2(111) surface at low coverages. Annealing to higher temperatures leads to the agglomeration of Sm particles and concurrent diffusion of Sm into the ceria film. These results illustrate that Sm can modify both the electronic and structural properties of ceria. [ABSTRACT FROM AUTHOR]

Electronic adjustment is one of the most commonly used strategies to improve the catalytic performance of heterogeneous catalysts. We prepared hexagonal ultrathin Pd nanosheets with edges modified by gold nanoparticles (Au@Pd nanosheets) using galvanic replacement method. By virtue of the electronic interactions between the Pd nanosheets and Au nanoparticles, the Au@Pd nanosheets exhibited excellent catalytic performances in the carbonylation of iodobenzene by carbon monoxide. The novel nanocomposites could be applied as model catalysts to explore electronic effects in catalysis. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Ni/CeO 2 model catalyst is designed to study ethanol steam reforming. • Behavior of acetate, acetaldehyde and ethoxy groups is explored. • The key role of acetaldehyde is confirmed; acetate formation is a minor route. • Water provides OH groups for removal of carbon formed during C C bond breaking. • At the same time, water maintains the oxidation state of the catalyst. Abstract In this contribution a model system consisting of Ni particles grown onto epitaxial CeO 2 films deposited on CaF 2 -buffered Si(111) is presented for molecular level study of the transformations of ethanol under steam reforming conditions. The adsorption behaviour of ethanol and other stable compounds thought to be important in ethanol reforming is explored by means of photoelectron spectroscopy. Acetic acid adsorbs dissociatively and relatively strongly on both fully oxidized and slightly reduced ceria surfaces. Acetaldehyde, a key intermediate of ethanol reforming, interacts weakly with CeO 2 (111) while on the reduced surface stronger adsorption and a transformation towards acetate were observed. In case of ethanol, low temperature molecular adsorbates transform into ethoxy moieties around 200–250 K along with considerable surface reduction. A competitive behaviour between O 2 and ethanol emphasize the role of oxygen vacancies in the stabilization of the ethoxy groups. On the Ni-free ceria surface strongly bound ethoxy adsorbates show signs of oxidation at elevated temperatures. In the presence of Ni a certain part of the ethoxy adsorbates oxidizes towards acetate even at room temperature, which transform to carbonate upon annealing. At the same time, a significant amount of graphitic species appears on the surface, which are removed by either lattice oxygen during annealing in vacuum, resulting in heavy surface reduction or by hydroxyl groups upon annealing in water vapour. In the latter case the acetate-carbonate forming pathway becomes also suppressed, indicating that surface hydroxyl species from adsorbed water influence the decomposition route of ethanol, contribute to the elimination of carbonaceous deposits and maintain the oxidation state of ceria at the same time. [ABSTRACT FROM AUTHOR]

There is an ongoing debate on the structure of titania supported vanadia used for catalytic partial oxidation reactions as well as for abatement of nitrogen oxide (NO) by ammonia selective catalytic reduction (NH-SCR). To gain further insight into the essential features of the catalyst composition of the surface we introduce a new class of vanadia model catalysts based on monodispersed silica particles as a platform. The evaluation of the influence of different catalyst components (TiO, WO) and different vanadia synthesis approaches (incipient wetness, ion-exchange) is facilitated by the hierarchical structure of the catalysts as well as the controlled synthesis approach. Catalysts tested under industrial ammonia SCR conditions show a strong dependence on the catalyst composition. In particular, the presence of crystalline titania and tungsta leads to a strong increase in NO conversion. Detailed catalyst characterization by X-ray diffraction and by visible Raman, UV Raman, UV-Vis and X-ray photoelectron spectroscopies provides insight into the bulk structure, the surface composition as well as the vanadia surface structure of the catalysts. The potential of UV resonance Raman spectroscopy for structural analysis of vanadia model catalysts for SCR is highlighted. Differences in catalytic activity are discussed in light of the structural results and the available literature. [ABSTRACT FROM AUTHOR]

A well-defined model catalyst constituting a compromise between high surface area, porous, industrial catalysts and a planar model catalyst has been developed. It allows for measurements of catalytic activity in micro reactors, where diffusion problems are kept at a minimum, while characterization both by surface science techniques and by bulk techniques can be applied. Monodisperse, non-porous SiO2 microspheres with diameter 875 ± 25 nm have been synthesized, serving as the large area model support. These where then impregnated with pre-formed, monodisperse, colloidal Fe and FeMn nanoparticles resulting in a three-dimensional equivalent of a flat, Fe(-Mn)/SiO2 model catalysts. Characterization with electron microscopy (SEM and TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), before and after catalytic testing was performed. It was shown that the model catalysts can be used in Fischer-Tropsch synthesis experiments under industrially relevant conditions. The FTS experiments reveal that compared to the pure Fe catalyst, FeMn shows more stable activity, higher selectivity towards olefins and lower selectivity toward CH4 and CO2. Significant amounts of hydrocarbons on the catalyst surfaces and some minor indications of sintering were detected after the reaction. Formation of FeCx was detected for the Fe catalyst while no significant amounts could be seen on the Mn-promoted catalyst. [ABSTRACT FROM AUTHOR]

Large ZSM-5 zeolite crystals synthesized in fluoride medium show an astonishing activity, stability as well as selectivity towards light olefins in the Methanol-To-Olefins (MTO) reaction. By proper control of the synthesis parameters, ZSM-5 single crystals of unprecedented high quality are produced. The absence of usually uncontrollable variables such as structural defects, external non selective surface acid sites and extra-framework aluminium (EFAl) species was evidenced by SEM, HRTEM, CO-FTIR, 27 Al and 19 F MAS-NMR, Rietveld structure refinement and N 2 - and Ar-gas sorption measurements. Interestingly, diffusivity evaluation of different probe molecules (toluene, benzene and neopentane) has been carried out with PFG-NMR, allowing casting light on an interesting structure-diffusion-activity relationship. A “levitation” effect could be experimentally demonstrated and its impact on catalysis is highlighted in a rationalization attempt: Maxwell-Boltzmann based diffusion models properly predict product distributions for this counter-intuitively outstanding Methanol-To-Propylene (MTP) catalyst. [ABSTRACT FROM AUTHOR]