Functionalized Carbon Materials in Syngas Conversion

Functionalized carbon materials are widely used in heterogeneous catalysis due to their unique properties such as adjustable surface properties, excellent thermal conductivity, high surface areas, tunable porosity, and moderate interactions with guest metals. The transformation of syngas into hydrocarbons (known as the Fischer-Tropsch synthesis) or oxygenates is an exothermic reaction and is typically catalyzed by transition metals dispersed on functionalized supports. Various carbon materials have been employed in syngas conversions not only for improving the performance or decreasing the dosage of expensive active metals but also for building model catalysts for fundamental research. This article provides a critical review on recent advances in the utilization of carbon materials, in particular the recently developed functionalized nanocarbon materials, for syngas conversions to either hydrocarbons or oxygenates. The unique features of carbon materials in dispersing metal nanoparticles, heteroatom doping, surface modification, and building special nanoarchitectures are highlighted. The key factors that control the reaction course and the reaction mechanism are discussed to gain insights for the rational design of efficient carbon-supported catalysts for syngas conversions. The challenges and future opportunities in developing functionalized carbon materials for syngas conversions are briefly analyzed.

Synergy between Ni and Co Nanoparticles Supported on Carbon in Guaiacol Conversion

Nickel-cobalt bimetallic catalysts supported on high surface area graphite with different Ni:Co ratios (3:1, 2:1 and 1:1) and the monometallic Ni and Co were prepared by wetness impregnation method. The catalysts were tested in hydrodeoxygenation (HDO) of guaiacol in the liquid phase at 50 bar of H2 and 300 °C. The materials were characterized by N2 adsorption-desorption, XRD, TEM/STEM, H2-TPR, and CO-chemisorption to assess their properties and correlate them with the catalytic results. The activity was higher on the bimetallic catalysts and followed the trend NiCo2:1/G ∼ NiCo3:1/G > NiCo1:1/G > Co/G > Ni/G. Also, selectivity results showed that Ni was more active in the hydrogenation favoring cyclohexanol production from phenol, while this was inhibited on the Co-containing catalysts. Hence, the results showed that synergy was created between Ni and Co and that their interaction, properties, and catalytic performance depend on the metals' ratio.

Disk-Shaped Cobalt Nanocrystals as Fischer–Tropsch Synthesis Catalysts Under Industrially Relevant Conditions

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.

Preparation of Cobalt Nanocrystals Supported on Metal Oxides To Study Particle Growth in Fischer-Tropsch Catalysts

Colloidal synthesis of nanocrystals (NC) followed by their attachment to a support and activation is a promising route to prepare model catalysts for research on structure-performance relationships. Here, we investigated the suitability of this method to prepare well-defined Co/TiO₂ and Co/SiO₂ catalysts for the Fischer–Tropsch (FT) synthesis with high control over the cobalt particle size. To this end, Co-NC of 3, 6, 9, and 12 nm with narrow size distributions were synthesized and attached uniformly on either TiO₂ or SiO₂ supports with comparable morphology and Co loadings of 2–10 wt %. After activation in H₂, the FT activity of the TiO₂-supported 6 and 12 nm Co-NC was similar to that of a Co/TiO₂ catalyst prepared by impregnation, showing that full activation was achieved and relevant catalysts had been obtained; however, 3 nm Co-NC on TiO₂ were less active than anticipated. Analysis after FT revealed that all Co-NC on TiO₂ as well as 3 nm Co-NC on SiO₂ had grown to ∼13 nm, while the sizes of the 6 and 9 nm Co-NC on SiO₂ had remained stable. It was found that the 3 nm Co-NC on TiO₂ already grew to 10 nm during activation in H₂. Furthermore, substantial amounts of Co (up to 60%) migrated from the Co-NC to the support during activation on TiO₂ against only 15% on SiO₂. We showed that the stronger interaction between cobalt and TiO₂ leads to enhanced catalyst restructuring as compared to SiO₂. These findings demonstrate the potential of the NC-based method to produce relevant model catalysts to investigate phenomena that could not be studied using conventionally synthesized catalysts.