Direct visualisation of the surface atomic active sites of carbon-supported Co3O4 nanocrystals via high-resolution phase restoration

The atomic arrangement of the terminating facets on spinel Co3O4 nanocrystals is strongly linked to their catalytic performance. However, the spinel crystal structure offers multiple possible surface terminations depending on the synthesis. Thus, understanding the terminating surface atomic structure is essential in developing high-performance Co3O4 nanocrystals. In this work, we present direct atomic-scale observation of the surface terminations of Co3O4 nanoparticles supported on hollow carbon spheres (HCSs) using exit wavefunction reconstruction from aberration-corrected transmission electron microscopy focal-series. The restored high-resolution phases show distinct resolved oxygen and cobalt atomic columns. The data show that the structure of {100}, {110}, and {111} facets of spinel Co3O4 exhibit characteristic active sites for carbon monoxide (CO) adsorption, in agreement with density functional theory calculations. Of these facets, the {100} and {110} surface terminations are better suited for CO adsorption than the {111}. However, the presence of oxygen on the {111} surface termination indicates this facet also plays an essential role in CO adsorption. Our results demonstrate direct evidence of the surface termination atomic structure beyond the assumed stoichiometry of the surface

Post doped nitrogen-decorated hollow carbon spheres as a support for Co Fischer-Tropsch catalysts

In this study the outer surface of porous hollow carbon spheres (HCSs) materials were functionalized by N-doping using a post-synthesis method and they were used as a Fischer-Tropsch catalyst support. Melamine was used as the nitrogen source, and carbonization was performed at different temperatures (600 and 900 °C) to introduce variable levels of N into the HCSs, with different bonding configurations. This procedure allowed for the incorporation of up to 13% N. Our results show that post-synthesis N-doping introduced marginal defects into the carbon framework and this did not affect the thermal stability of the materials. XPS studies revealed that the surface content on these materials varied and provided evidence for temperature-tunable bonding configurations. Effects associated with post-synthesis N-doping were apparent on the Co catalyst (˜10 wt.%) properties such as the inhibited reduction caused by a metal-support interaction observed by the H2-TPR and in situ PXRD techniques. As a consequence the Fischer-Tropsch performance was influenced as both the activity and stability were improved on the catalysts supported on the N-doped materials. TEM analysis of the spent catalysts demonstrated the influence of N-doping on the sintering characteristics of Co, with particles > 30 nm measured on the N-free catalyst while N-doped samples had sizes < 15 nm

Effect of a titania covering on CNTS as support for the Ru catalysed selective CO methanation

One of the major set-backs in the selective CO methanation process, as the final clean-up step in removing residual CO from reformate gas feed, is the reverse water gas shift (RWGS) reaction. This reaction is an undesired reaction because, it runs parallel with the selective CO methanation reaction. This increases the CO outlet concentration. The catalytic performance of ruthenium supported on carbon nanotubes (CNTs), nitrogen doped carbon nanotubes (NCNTs), titania coated carbon nanotubes (NCNT-TiO2 and CNTs-TiO2) and TiO2 anatase (TiO2-A) for selective CO methanation was investigated. The feed composition relevant to reformate gas was used but in the absence of steam. The experiments were conducted within a temperature range of 100 °C and 360 °C. It was observed that carbon dioxide methanation was suppressed until CO methanation attained a maximum conversion for all the catalysts studied. The Ru/NCNT showed higher activity than Ru/CNT at all temperatures examined due to the nitrogen incorporation in the carbon domains. Both Ru/CNT and Ru/NCNT however promoted the RWGS reaction at temperatures above 250 °C. The Ru/CNT-TiO2 catalyst recorded the highest activity for both the CO and selective CO methanation followed by Ru/TiO2-A. The presence of titania on the carbon nanotubes significantly retarded the RWGS reaction from about −120% CO conversion to about 80% CO conversion, while selectivity towards methane increased in all catalysts with increasing temperature

A sinter resistant Co Fischer-Tropsch catalyst promoted with Ru and supported on titania encapsulated by mesoporous silica

One of the pathways responsible for the deactivation of Fischer-Tropsch catalysts is the loss of active metal surface area due to nanoparticle agglomeration. To combat this effect efforts have been made to increase the interaction between the metal nanoparticles and the support using materials like silica. In this study, the supported metal particles were covered with a highly porous layer of silica to stabilize the Co nanoparticles on a titania support both during reduction and under reaction conditions. Co3O4 nanoparticles (size range: 8–12 nm) supported on titania were stabilized by coating them with a thin layer of mesoporous silica ( ∼ 4 nm) to make Fischer-Tropsch catalysts that are less prone to sintering (Co/TiO2@mSiO2). To mitigate the strong metal support interactions brought about by the titania and silica a Ru promoter was loaded together with the cobalt nanoparticles onto the titania (CoRu/TiO2@mSiO2). Temperature programmed XRD studies on the evolution of the Co metal nanoparticles showed that there was no significant particle size growth under reduction conditions in the temperature range from 30 to 600 °C. Chemisorption studies following reduction under hydrogen at 350 °C and 450 °C gave results consistent with the in situ XRD data when compared to the Co/TiO2. Fischer-Tropsch synthesis on the Co/TiO2@mSiO2 and CoRu/TiO2@mSiO2 catalysts encapsulated inside the mesoporous silica shell exhibited good catalytic performance without any display of significant mass transport limitations that might arise due to a silica shell coating of the active sites. For these two catalysts the Fischer-Tropsch activity increased with reduction temperature without any significant negative changes in their selectivity due to sintering, while the activity on Co/TiO2 decreased due to Co nanoparticle sintering

The vapour phase hydrogenation of cinnamaldehyde using cobalt supported inside and outside hollow carbon spheres

The hydrogenation of cinnamaldehyde is usually performed in the liquid phase in batch mode. In this study, a vapour phase flow system has been used to evaluate the use of cobalt catalysts supported inside and outside hollow carbon spheres (HCSs). The influence of temperature, hydrogen flow rate and catalyst mass on the hydrogenation reaction was investigated. The catalysts generally showed modest conversion to the required products, hydrocinnamaldehyde, 3-phenyl propanol, cinnamyl alcohol together with formation of various decomposition products. The data revealed that the Co@HCS showed better conversion and product selectivity compared to the Co/HCS. The catalysts with smaller particle sizes (ca. 6 nm) were more efficient than big particles (30 – 40 nm). An increase in reaction temperature (200 – 300C) resulted in a lower cinnamaldehyde conversion and a poor product selectivity. TPR studies revealed that the Co@HCSs had a stronger metal-support interaction than the Co/HCSs catalysts. Catalyst recycling studies revealed that only the Co/HCSs could be regenerated (4 cycles) and post reaction analysis of the catalysts revealed that this was due to HCS pore blockage and not Co sintering.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Ruthenium nanoparticles encapsulated inside porous hollow carbon spheres: A novel catalyst for Fischer-Tropsch synthesis

Two novel Ru Fischer–Tropsch (FT) catalysts were made that were supported on the inside of two hollow carbon spheres that differed in terms of their shell porosity. Mesoporous Stober spheres were made and Ru deposited on the silica. The Ru/silica spheres were encapsulated with carbon deposited by CVD (toluene) or from resorcinol/formaldehyde. Removal of the silica gave Ru@HCS (dRu = 5.5) and Ru@MHCS (3.2 nm) that had carbon shells (d = ca. 20 nm) with different physicochemical properties as evidenced by the TEM, nitrogen adsorption-desorption, TGA, Raman spectroscopy and XRD measurements. FT studies were performed on the two catalysts (10 bar; 190/220/250 °C; 2/1 ratio H2/CO). Classical Fischer–Tropsch data was obtained indicating that the catalysts could access the reactants and that FT products could escape from the inside of the spheres (acting as a nanoreactor). Activity data indicated diffusion control of CO/H2 into the nanoreactor and selectivity data indicated an alpha value of 0.74–0.78 (220 °C). Typical product selectivity associated with small Ru particles was observed and the methane content increased with reaction temperature. No substantial Ru sintering occurred below 220 °C. It is thus seen that the porosity of the two hollow carbon architectures is suitable for the FT polymerization reaction

Direct Visualisation of the Surface Atomic Active Sites of Carbon-Supported Co3O4 Nanocrystals via High-Resolution Phase Restoration

The atomic arrangement of the terminating facets on spinel Co3O4 nanocrystals is strongly linked to their catalytic performance. However, the spinel crystal structure offers multiple possible surface terminations depending on the synthesis. Thus, understanding the terminating surface atomic structure is essential in developing high-performance Co3O4 nanocrystals. In this work, we present direct atomic-scale observation of the surface terminations of Co3O4 nanoparticles supported on hollow carbon spheres (HCSs) using exit wavefunction reconstruction from aberration-corrected transmission electron microscopy focal-series. The restored high-resolution phases show distinct resolved oxygen and cobalt atomic columns. The data show that the structure of {100}, {110}, and {111} facets of spinel Co3O4 exhibit characteristic active sites for carbon monoxide (CO) adsorption, in agreement with density functional theory calculations. Of these facets, the {100} and {110} surface terminations are better suited for CO adsorption than the {111}. However, the presence of oxygen on the {111} surface termination indicates this facet also plays an essential role in CO adsorption. Our results demonstrate direct evidence of the surface termination atomic structure beyond the assumed stoichiometry of the surface

Co inside hollow carbon spheres as a Fischer-Tropsch catalyst: spillover effects from Ru placed inside and outside the HCS

Hollow carbon spheres (HCSs) containing Co nanoparticles placed inside the HCS were synthesized for the first time using polystyrene spheres as a template. The encapsulated Co nanoparticles, after reduction, showed Fischer-Tropsch (FT) activity indicating syngas accessibility through the HCS porous shell. Two Co catalysts promoted by Ru, placed either inside or outside the HCS (CoRu@HCS and Co@HCS@Ru), were also synthesized and characterized. The location of the Co and Ru was confirmed by SEM and TEM analyses. In-situ XRD studies indicated enhanced H2 reduction of the Co oxide to Co, inside the HCS, in the order CoRu@HCS > Co@HCS@Ru > Co@HCS. The CoRu@HCS catalyst had the highest FT activity, and this was ascribed to a primary spillover effect associated with the direct contact of the Co and Ru inside the HCS. A small secondary spillover effect was also noted for Co@HCS@Ru

SSelective CO Methanation Over Ru Supported on Carbon Spheres: The Effect of Carbon Functionalization on the Reverse Water Gas Shift Reaction

Mesoporous carbon spheres (CSs-H) were hydrothermally synthesised using sugar as carbon source. The as-synthesized CSs-H was microporous but after thermal treatment at 900 °C for 4 h it became mesoporous (surface area of 463 m2 g−1). Further treatment of the annealed CSs-H with HNO3 gave a functionalized CSs-H with a high defect content in the carbon matrix which resulted in an increase in surface area (509 m2 g−1). The functionalized and un-functionalized CSs-H were used to support nano Ru particles for CO, CO2 and selective CO methanation reactions. The Ru supported catalysts were prepared using both impregnation and microwave polyol synthesis methods. It was evident from the reduction studies that the functional groups on the surface of the CSs-H influenced the reduction of the RuO2 to Ru. The catalyst with smaller and well dispersed RuO2 particles (d = 2.7 nm) (prepared by the microwave polyol technique) gave a high activity in both CO and selective CO methanation studies. The larger Ru particles observed on the un-functionalized CSs-H showed poor activity for CO and selective CO methanation reactions and also did not promote the reverse water gas shift reaction.Chemical and Civil Engineerin

Effect of Support Particle Size in Fischer–Tropsch Synthesis: the Use of Natural Clinoptilolite as Support

In the past, Fischer–Tropsch (FT) coal/biomass-to-liquids projects have required a significant initial investment. The high price of the catalysts used is one area where costs could be reduced. This research explored the possibility of using clinoptilolite as a catalyst to reduce costs without sacrificing performance. The as-received clinoptilolite was ground and sieved to yield different size classes. For this study, three size classes were investigated as the support for an FT catalyst: −75 to +53 μm; −53 to +38 μm; less than 25 μm. Using a fixed bed reactor, 10% cobalt supported on these various supports was synthesized and evaluated. The maximum CO conversion obtained was 44.97% when using the −53 to +38 μm size class with the experiments carried out at 220 °C, 2 L(NTP)/(gcat h) and 10.85 bar(abs). A one-way analysis of variance was performed. Then a posthoc Bonferroni adjustment test was carried out to determine whether or not the utilization of different support size classes affected CO conversion. The results indicated a significant difference in CO conversion, with P(T ≤ t) two-tail values ranging from 6.08 × 10–5 to 2.37 × 1027. At 220 °C and 10.85 bar(abs), methane selectivity ranged between 14.95 and 16.97% for the support class sizes studied, while C2–C4 selectivity ranged between 14.55 and 19.01%, and C5+ selectivity ranged between 66.04 and 70.29%. The acquired product selectivity results using this cheaper support are comparable to those reported in the literature. These discoveries might have valuable implications for the design of a catalyst that can be used in the coal/biomass to liquid process