The development of active, cost-effective and stable oxygen-evolving catalysts is one of the major challenges for solar-to-fuel conversion towards sustainable energy generation. Iridium oxide ...exhibits the best available compromise between catalytic activity and stability in acid media, but it is prohibitively expensive for large-scale applications. Therefore, preparing oxygen-evolving catalysts with lower amounts of the scarce but active and stable iridium is an attractive avenue to overcome this economical constraint. Here we report on a class of oxygen-evolving catalysts based on iridium double perovskites which contain 32 wt% less iridium than IrO2 and yet exhibit a more than threefold higher activity in acid media. According to recently suggested benchmarking criteria, the iridium double perovskites are the most active catalysts for oxygen evolution in acid media reported until now, to the best of our knowledge, and exhibit similar stability to IrO2.
Understanding the atomistic details of how platinum surfaces are oxidized under electrochemical conditions is of importance for many electrochemical devices such as fuel cells and electrolysers. Here ...we use in situ shell-isolated nanoparticle-enhanced Raman spectroscopy to identify the intermediate stages of the electrochemical oxidation of Pt(111) and Pt(100) single crystals in perchloric acid. Density functional theory calculations were carried out to assist in assigning the experimental Raman bands by simulating the vibrational frequencies of possible intermediates and products. The perchlorate anion is suggested to interact with hydroxyl phase formed on the surface. Peroxo-like and superoxo-like two-dimensional (2D) surface oxides and amorphous 3D α-PtO2 are sequentially formed during the anodic polarization. Our measurements elucidate the process of the electrochemical oxidation of platinum single crystals by providing evidence for the structure-sensitive formation of a 2D platinum-(su)peroxide phase. These results may contribute towards a fundamental understanding of the mechanism of degradation of platinum electrocatalysts.
The modern chemical industry uses heterogeneous catalysts in almost every production process. They commonly consist of nanometre-size active components (typically metals or metal oxides) dispersed on ...a high-surface-area solid support, with performance depending on the catalysts' nanometre-size features and on interactions involving the active components, the support and the reactant and product molecules. To gain insight into the mechanisms of heterogeneous catalysts, which could guide the design of improved or novel catalysts, it is thus necessary to have a detailed characterization of the physicochemical composition of heterogeneous catalysts in their working state at the nanometre scale. Scanning probe microscopy methods have been used to study inorganic catalyst phases at subnanometre resolution, but detailed chemical information of the materials in their working state is often difficult to obtain. By contrast, optical microspectroscopic approaches offer much flexibility for in situ chemical characterization; however, this comes at the expense of limited spatial resolution. A recent development promising high spatial resolution and chemical characterization capabilities is scanning transmission X-ray microscopy, which has been used in a proof-of-principle study to characterize a solid catalyst. Here we show that when adapting a nanoreactor specially designed for high-resolution electron microscopy, scanning transmission X-ray microscopy can be used at atmospheric pressure and up to 350 °C to monitor in situ phase changes in a complex iron-based Fisher-Tropsch catalyst and the nature and location of carbon species produced. We expect that our system, which is capable of operating up to 500 °C, will open new opportunities for nanometre-resolution imaging of a range of important chemical processes taking place on solids in gaseous or liquid environments.
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DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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•Resulfidation restores (Ni/Co)MoS2 slabs lost after oxidation.•Mo and S are completely resulfided; Co and especially Ni are not.•Oxidation-resulfidation increases thiophene HDS ...performance.•Smallest slabs are lost upon oxidation-resulfidation.•Thiophene HDS.
The effect of a sequential oxidation and resulfidation treatment on γ-Al2O3 supported (Ni/Co)MoS2 catalyst nanoparticles was investigated using (HR)TEM, XPS, and thiophene HDS catalytic performance experiments. Analysis of the HRTEM images revealed that, after initial sulfidation and oxidation, the resulfidation treatment restored the original slab length or increased it. The chemical composition of the samples, as determined by XPS, also slightly changed: the concentration of oxidic species increased, especially for the Ni promoter atoms. Comparing the catalytic HDS activity of the samples before and after the oxidation-resulfidation treatment showed that the catalysts were more than 20% more active after resulfidation. This increase in HDS activity is ascribed to a redistribution of the (Ni/Co)MoS2 slabs during the second sulfidation treatment, indicating a size effect.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
K-promoted MoS2 is an active catalyst for the synthesis of CH3SH, a valuable chemical intermediate, from synthesis gas (CO/H2) and H2S. The promotion of MoS2 by K increases the CO conversion rate and ...the CH3SH selectivity and is usually linked to the stabilization of the 1T-MoS2 polymorph as compared to the 2H-MoS2 in K-free samples. Sulfided catalysts were characterized using TEM, XPS, and EXAFS. TEM showed that MoS2 particles became larger and more stacked when the K/Mo ratio increased above unity. XPS and EXAFS evidenced the formation of the 1T-MoS2 phase at higher K/Mo ratio in addition to 2H-MoS2. The addition of K to MoS2 led to increased CH3SH productivity due to both increased CO conversion and CH3SH selectivity (T = 300–400 °C, P = 10 bar). The performance of the most active catalyst with a K/Mo ratio of 2 was found to be stable, despite the observation that the initially present 1T-MoS2 phase slowly converted to the 2H-MoS2 phase during the reaction at high pressure. There is no correlation between the CH3SH productivity and the amount of 1T-MoS2 in K-promoted MoS2 catalysts.
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Using a MEMS nanoreactor in combination with a specially designed
in situ
Transmission Electron Microscope (TEM) holder and gas supply system, we imaged the formation of multiple layers of graphene ...encapsulating a cobalt nanoparticle, at 1 bar CO : N
2
(1 : 1) and 500 °C. The cobalt nanoparticle was imaged live in a TEM during the Boudouard reaction. The
in situ
/operando TEM studies give insight into the behaviour of the catalyst at the nanometer-scale, under industrially relevant conditions. When switching from Fischer-Tropsch syngas conditions (CO : H
2
: N
2
1 : 2 : 3 at 1 bar) to CO-rich conditions (CO : N
2
1 : 1 at 1 bar), we observed the formation of multi-layered graphene on Co nanoparticles at 500 °C. Due to the high temperature, the surface of the Co nanoparticles facilitated the Boudouard reaction, causing CO dissociation and the formation of layers of graphene. After the formation of the first patches of graphene at the surface of the nanoparticle, more and more layers grew over the course of about 40 minutes. In its final state, around 10 layers of carbon capped the nanoparticle. During this process, the carbon shell caused mechanical stress in the nanoparticle, inducing permanent deformation.
The shape of metal nanoparticles can dramatically depend on reaction conditions. While Pt nanoparticles are known to dynamically respond to the partial pressure of CO, in situ TEM images show that, ...surprisingly, Co nanoparticles do not change their shape under a CO atmosphere despite going through several reconstructions. Detailed DFT calculations attribute this contrasting behavior to two factors: (1) CO adsorption has a higher stabilization effect on the high-index facets of Pt than on those of Co; (2) the Co surface energy is more sensitive to the coordination number, making high-index surfaces less stable relative to Pt. These two factors combined can affect the stability of high-index surfaces as is the case for Pt nanoparticles, which reconstruct already at low CO pressures. In the case of Co nanoparticles, the low-index surface remains the most stable even at high CO partial pressures. The robustness of the shape of Co nanoparticles challenges recent proposals that high-index facets, which facilitate direct CO dissociation, are present on Co nanoparticle catalysts under Fischer–Tropsch conditions.
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Metal–support interactions in the cobalt–alumina system are evaluated using an inverse model system generated by impregnating Co3O4 with a solution of aluminum sec-butoxide in n-hexane. This results ...in the formation of nano-sized alumina islands on the surface of cobalt oxide. The activated model systems were kinetically evaluated for their activity and selectivity in the Fischer–Tropsch synthesis under industrially relevant conditions (220 °C, 20 bar). The kinetic measurements were complemented by H2-chemisorption, CO-TPR, and pyridine TPD. It is shown that the introduction of aluminum in the model system results in the formation of strong acid sites and enhanced CO dissociation, as evidenced in the CO-TPR. The incorporation of aluminum in the model systems led to a strong increase in the activity factor per surface atom of cobalt in the rate expression proposed by Botes et al. (2009). However, the addition of aluminum also resulted in a strong increase in the kinetic inhibition factor. This is accompanied by a strong decrease in the methane selectivity, and an increase in the desired C5+ selectivity. The observed activity and selectivity changes are attributed to the increase in the coverage of the surface with carbon with increasing aluminum content, due to the facilitation of CO dissociation in the presence of Lewis acid sites associated with the alumina islands on the catalytically active material.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
MFI type zeolites have been known for decades for their strong tendency toward gas formation in hydrocracking. Our strategy for selectivity control in hydrocracking turns strong adsorption in ...micropores, responsible for secondary cracking, to an advantage. Enhancing diesel yield is a challenging goal in hydrocracking catalysis. Additionally, linear alkanes increase the diesel fuel cetane number and, consequently, effect a dramatic reduction in exhaust emissions. This study demonstrates for a set of Pt on mesoporous MFI zeolite catalysts that the gains in activity and selectivity attributed to enhanced mass transport are but modest by comparison to the effects of the competitive adsorption of water. Water suppresses secondary cracking, and primary cracking is now reported over MFI zeolites. Furthermore, competitive adsorption of water in shape-selective MFI zeolites facilitates desorption of the primary n-C16 cracking products and suppresses subsequent isomerization resulting in high yields of linear alkanes. The selectivity for linear alkanes from hydrocracking of n-hexadecane over Pt/MFI nanosheets reaches 80% at 80% conversion.
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