Syngaschem (Netherlands)

Photocatalysis provides an eco-friendly route for the hydrogenation of aromatic carbonyls to O-free aromatics, which is an important refining process in the chemical industry that is generally carried out under high pressure of hydrogen at elevated temperatures. However, aromatic carbonyls are often only partially hydrogenated to alcohols, which readily desorbs and are hardly further deoxygenated under ambient conditions. Here, we show that by constructing an oxide surface over the Pd cocatalyst supported on graphitic carbon nitride, an alternative hydrogenation path of aromatic carbonyls becomes available via a step-wise acetalization and hydrogenation, thus allowing efficient and selective production of O-free aromatics. The PdO surface allows for optimum adsorption of reactants and intermediates and rapid abstraction of hydrogen from the alcohol donor, favoring fast acetalization of the carbonyls and their consecutive hydrogenation to O-free hydrocarbons. The photocatalytic hydrogenation of benzaldehyde into toluene shows a high selectivity of >90% and a quantum efficiency of ∼10.2% under 410 nm irradiation. By adding trace amounts of HCl to the reaction solution, the PdO surface remains stable and active for long-term operation at high concentrations, offering perspective for practical applications.

Understanding the kinetics of oxygen removal from catalytically active metal surfaces by hydrogen is important for several catalytic reactions such as Fischer–Tropsch synthesis, methanation of CO or CO2, and the reverse-water–gas-shift reaction. Motivated by FTS, a Co(0001) single crystal model catalyst was used to study the kinetics of oxygen removal through reaction with hydrogen. Kinetic studies in the 10–7–10–4 mbar H2 pressure regime show that water formation is first order in the surface hydrogen concentration while the order in oxygen concentration changes from one at low oxygen coverage to zero at high oxygen coverage. In situ XPS shows that the hydrogen surface concentration saturates around 10–1 mbar and on this basis the typical temperature of 450 K needed for water formation in this pressure regime can be considered as typical for high pressures as well. The absence of OH buildup during the reaction points to O + H as the rate-limiting step, with a barrier of ∼120 kJ mol–1. Such a high barrier shows that slow removal of adsorbed oxygen from the surface of reactive metal catalysts such as cobalt may be rate-limiting for the overall reaction.

Hydroxyl adsorbates generally appear as transient species during water formation from adsorbed oxygen and hydrogen atoms on a metal surface, a reaction that is part of the catalytic cycle in various important surface-catalyzed reactions such as Fischer–Tropsch synthesis. In the present work, temperature-programmed desorption and in situ synchrotron XPS were used to study water adsorption and OH reactivity on a flat and a stepped cobalt single crystal surface. Water adsorbs intact on the flat Co(0001) surface and desorbs around 160 K. Electrons induce dissociation of water and produce OH species at low temperature. Hydroxyl species can also be formed by the reaction between Oad and H2O, but only for high initial oxygen coverage while low coverage Oad appears largely unreactive. Reactive hydrogen species (H atoms) produced by a hot tungsten filament hydrogenate adsorbed oxygen atoms at low temperature already and both OHad and H2O are formed. In all cases, hydroxyl adsorbates react around 190 K to form water via 2 OHad → H2O (g) + Oad associated with an activation barrier of 40–50 kJ mol–1. Water readily dissociates on the step sites exposed by vicinal Co(10–19). A part of the OHad species recombine to form water and oxygen between 200 and 300 K, while decomposition of OHad into Oad and Had dominates above 370 K. For catalysis, the high reactivity of step sites for water dissociation and the high stability of OHad at these sites implies that O removal from these sites may be difficult and may limit the overall rate of Fischer–Tropsch synthesis on cobalt catalysts.

The adsorption and dissociation of O2 on Ir(100) has been studied with X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy together with density functional theory calculations. At low coverage, XPS shows a single O 1s peak at 530.6 eV, associated with bridged O atoms in an Ir(100)–(3 × 1)–O structure. At saturation coverage, an additional peak at 529.5 eV appears, accompanied by an Ir(100)–(2 × 1)–O structure. The O 1s peak at 530.6 eV is assigned to oxygen atoms adsorbed on bridge sites of Ir atoms, and the peak at 529.5 eV to oxygen atoms on fourfold hollow sites of Ir. Based on our detailed experimental and computational study, oxygen diffusion into the subsurface region can be ruled out. The present study provides new findings on an additional oxygen state of the O/Ir(100) system. It also provides evidence for the absence of subsurface oxygen in Ir, implying that some discussions on subsurface oxygen in the literature may be misleading.

[This corrects the article DOI: 10.1021/acsomega.1c01130.].

Zeolitic imidazolateframeworks, like ZIF-8 and related structures,have shown great potential for the capture of carbon dioxide. Modifyingtheir structure by exchanging part of the constituent organic ligandsis a proven method for enhancing the capacity to absorb CO2. In this work, we performed solvent-assisted ligand exchange (SALE)on nanosized ZIF-8 (nZIF-8) with a series of functionalized imidazolederivatives (exchange percentages, after 24 h): 2-bromoimidazole (19%),2-chloroimidazole (29%), 2-trifluoromethylbenzimidazole (4%), 2-mercaptobenzimidazole(4%), and 2-nitroimidazole (54%). The sodalite topology and porosityof nZIF-8 were maintained with all SALE modifications. Low-pressureCO2 adsorption of nZIF-8 (38.5 cm3 g–1) at STP was appreciably enhanced with all mixed-linker SALE products.Using halogenated (−Cl, −Br, and −CF3) imidazole derivatives in a 24 h SALE treatment resulted in increasesbetween 11 and 22% in CO2 adsorption, while the thiol (−SH)-and nitro (−NO2)-functionalized SALE products ledto 32 and 100% increases in CO2 uptakes, respectively.These CO2 uptakes were further optimized by varying theSALE treatment time. The SHbIm- and NO2Im-exchanged SALEproducts of nZIF-8 show 87 and 98 cm3 g–1 of CO2 uptakes after 60 and 120 h of SALE, respectively.These are record high CO2 adsorptions for all reportedZIF derivatives at low-pressure conditions.

The identity of the surface intermediates involved in chain growth during Fischer–Tropsch synthesis remains a topic of ongoing debate. In the present work we use a combination of temperature programmed reaction spectroscopy and high resolution X-ray photoemission spectroscopy to study the reactivity of C3Hx adsorbates on a Co(0001) single crystal surface in order to explore the stabilities of the different C3Hx surface intermediates and to study elementary reaction steps relevant to chain growth and chain termination. Propene (H3C–CH=CH2) and propyl (H3C–CH2–CH2–) adsorbates react below 200 K already, either by desorption of propene or by dehydrogenation to adsorbed propyne (H3C–C≡CH). Co-adsorbed Had and COad do not affect the temperature at which propyl and propene react, but they do suppress the dehydrogenation pathway in favour of propene desorption. Their high reactivity under simulated FTS conditions disqualifies them as feasible intermediates for FTS, which requires long-lived intermediates to match the low monomer formation rate. Propyne, the most stable C3Hx adsorbate in the absence of COad, is hydrogenated to propylidyne (H3C–CH2–C≡) > 230 K when both COad and Had are present. Propylidyne dimerization occurs around 313 K and produces a 3-hexyne (H5C2–C≡C–C2H5) surface intermediate which is hydrogenated to 3-hexene (H5C2–CH=CH–C2H5) above 350 K. These findings are of direct relevance to FTS: they show that the high coverage of COad and Had present during the reaction influence the reactivity of CxHy adsorbates involved in chain growth and ultimately steer product selectivity. The findings provide further experimental support for the previously proposed alkylidyne chain growth mechanism on close-packed cobalt terraces: CO stabilizes CxHy growth intermediates in the alkylidyne form and growth proceeds via coupling of a long chain alkylidyne and methylidyne (CH).

Catalytic glycerol conversion by means of either photon or thermal energy is of great importance and can be realized by metal supported on TiO2 systems. Although various procedures have been employed to synthesize efficient metal/TiO2 catalysts, the promotional mechanisms for both reactions are still unclear due to the lack of well-defined systems. Here, we have deposited gold nanoparticles on a series of highly crystalline anatase TiO2 substrates with different crystallite sizes (7, 12, 16, 28 nm) by both direct precipitation and sol-immobilization methods to examine the effect of metal deposition methods and TiO2 sizes on both photo- and thermal catalytic glycerol reforming. For photocatalytic H2 evolution from glycerol, optimum performance was observed for the Au supported on 12 nm TiO2 for both deposition methods. For thermal catalytic glycerol oxidation, all catalysts show a similar selectivity to glycerate (>70%) regardless of the TiO2 size and metal deposition method; however, the metal deposition method significantly influences the catalytic activity. In situ UV–vis spectrometry reveals that the optimized photocatalytic performance originates from enhanced charge transfer kinetics and a more negative Fermi level for proton reduction, whereas electrochemical analysis reveals that the promoted glycerol oxidation is caused by the enhanced oxygen reduction half-reaction.

Undercoordinated (defect) sites on cobalt surfaces favor NH3 decomposition and dehydrogenated products adsorb strongly around these defect sites.

Cobalt-catalyzed low temperature Fischer–Tropsch synthesis is a prime example of an industrially relevant reaction in which CxHy intermediates involved in chain growth react in the presence of a large quantity of COad. In this study, we use a Co(0001) single-crystal model catalyst to investigate how CO, adsorbed alongside CxHy adsorbates affects their reactivity. Temperature-programmed reaction spectroscopy was used to determine the hydrogen content of the CxHy intermediates formed at different temperatures, and infrared absorption spectroscopy was used to obtain more specific information on the chemical identity of the various reaction intermediates formed. Ethene, propene, and but-1-ene precursors decompose below 200 K. The 1-alkyne adsorbate is identified as a major product, and some alkylidyne species form as well when the initial alkene coverage is high. The surface hydrogen atoms produced in the low temperature decomposition step start leaving the surface >300 K. When an alkyne/Had-covered surface is heated in the presence of CO, the alkyne adsorbates are hydrogenated to the corresponding alkylidyne at temperatures <250 K. This finding shows that CxHy surface species react differently in the presence of COad, a notion of general importance for catalytic reactions where both CO and CxHy species are present. In the context of Fischer–Tropsch synthesis, the observed CO-induced reaction is of specific importance for the alkylidyne chain growth mechanism. In this reaction, scheme hydrocarbon chains grow via coupling of CHad with a (Cn) alkylidyne adsorbate to produce the (Cn+1) alkyne. A subsequent hydrogenation of the alkyne product to the corresponding alkylidyne is required for further growth. The present work shows that this specific reaction is promoted by the presence of CO. This suggests that the influence of CO spectators on the stability of CxHy surface intermediates is beneficial for efficient chain growth.