61st Annual Report on Research 2016

With the ACS funding, we have made significant progress in the design and synthesis of new Fischer-Tropsch synthesis (FTS) catalysts with superior performance (high CO conversion, high selectivity towards liquid products, and long lifetime) than conventional catalysts (Fe, Co, or Ru and supported on SiO₂ or Al₂O₃). An important factor in the design of efficient heterogeneous catalysts is the catalyst support because the catalyst performance is strongly influenced by interactions between the active metal and support.¹ Our results demonstrate that the use of carbon nanotubes (CNTs) as catalyst supports improves catalyst dispersion (Figure 1) and mitigate the unfavorable catalyst-support interactions associated with oxide supports that often results in poor reducibility and catalyst deactivation. Key highlights of our research achievements in the last one year include: (1) scalable synthesis of CNTs using a gaseous product mixture from FTS (FTS-GP) as a feedstock,²(2) controlled UV-assisted oxidation of CNTs and deposition of Co or Fe nanoparticles on CNTs (Figure 1), and (3) evaluation of CO conversion and product selectivity on the synthesized catalysts during FTS.

Figure 1. TEM images of Co nanoparticles deposited on CNTs (A) and Fe nanoparticles deposited on CNTs (B).

1. UV-assisted Oxidation of CNTs and Deposition of Catalyst Nanoparticles

The synthesis of CNT-supported catalysts usually involves surface functionalization of CNTs followed by grafting of metal species to the surface of CNTs. However, surface functionalization of CNTs via acid treatment is well known to cut the CNTs, impart defects, and degrade CNT properties. To preserve the outstanding properties of CNTs during the catalyst synthesis process, we have developed an oxidation process that is less aggressive yet equally efficient as the conventional acid treatment. The process involves refluxing a solution of CNTs, H₂O₂ and catalyst precursor under UV illumination. The scanning electron microscope (SEM) images reveal that the HNO₃-treated CNTs were characterized by short and highly defective walls while the wall structure and CNT length were preserved for UV/H₂O₂-treated and H₂O₂-treated CNTs.

Figure 2. XPS C 1s spectra of (a) pristine CNTs, and CNTs oxidized with (b) HNO₃ mixture, (c) H₂O₂, and (d) UV light and H₂O₂.

The XPS C 1s spectra of pristine CNTs, and CNTs oxidized with HNO₃, H₂O₂, and H₂O₂ under UV illumination are presented in Figure 2. The XPS data reveal that UV-assisted oxidation using H₂O₂ achieves roughly the same level of oxidation as CNTs oxidized by refluxing in HNO₃. The deconvolution of the C 1s peak reveals the presence of five types of carbon species in the oxidized samples: C–C (284.5 eV), defect sites (285.4 eV), C–O (286.5 eV), C=O (288.7 eV) and carbonates (290.1 eV).³ The ratio between the functionalized carbon species and sp² graphitic carbon (C_f/sp²) is indicative of the degree of functionalization; values in the same range of 0.89 and 0.74 for HNO₃-treated and UV/H₂O₂-treated samples, respectively, were obtained. The TEM data (Figure 1) provide strong evidence that our new process results in well-dispersed Co or Fe nanoparticles on CNTs without the degradation of CNTs.

Figure 3. Schematic illustration of Fischer-Tropsch synthesis setup.

3. Evaluation of Catalyst Performance during FTS

The evaluation of FTS conversion and selectivity on CNT-supported Fe and Co catalysts was carried out in a fixed bed reactor shown schematically in Figure 3. For comparison, Fe and Co catalysts were also deposited on SiO₂ via incipient wetness. A catalyst sample of ~ 10g (SiC =80%, synthesized catalyst = 20%) was loaded into a ½ inch stainless steel tube and activated in hydrogen at 350°C for Co and 400°C for Fe. After activation, the temperature was lowered to ~250°C and the FTS reaction was carried out under 10 bar and H₂/CO = 2. The reaction products from the reactor were analyzed using an on-line gas chromatograph equipped with a flame ionization detector.

Figures 4a and b show the variations of CO conversion and product selectivity at steady state conditions with the time on-stream (TOS) for Co/SiO₂ and Fe/SiO₂ catalysts. CO conversion on Co/SiO₂ catalyst stays fairly constant ~65% during the 8 h duration of FTS process while Fe/SiO₂ catalyst shows a steady decrease in CO conversion for the same duration. The observed decrease in CO conversion with time on Fe/SiO₂ is attributed to the deactivation of the catalyst. Also, C5+ selectivity for FTS on Co/SiO₂ increases with TOS and reaches a maximum ~70% after 8h while Fe/SiO₂ shows an initial C5+ selectivity of ~85% after 1h, but quickly drops to ~50%. Further, the performances of Co/SiO₂ and Fe/SiO₂ were compared to Co/CNT and Fe/CNT catalysts to investigate the influence of CNTs as supports. From the plots of CO conversion and product selectivity at steady state conditions as a function of TOS for Co/CNT and Fe/CNT catalysts (Figures 4c and d), it is clear that CO conversion on the CNT-based catalysts is higher than conversion on SiO₂-supported catalysts. In fact, the CO conversion for Fe/CNT does not experience any substantial decrease as observed in Fe/SiO₂suggesting that the use of CNTs as supports improves the lifetime of Fe catalyst. We hypothesize that the use of CNTs as catalyst supports enhances the reducibility of Fe and Co catalysts, evidenced by the stable CO conversion.

Figure 4. Plots of CO conversion and product selectivity as a function of TOS for (a) Co/SiO₂, (b) Fe/SiO₂, (c) Co/CNT, and (d) Fe/CNT.

References Cited

1. Storsæter, S.; Tøtdal, B.; Walmsley, J. C.; Tanem, B. S.; Holmen, A., Characterization of alumina-, silica-, and titania-supported cobalt Fischer–Tropsch catalysts. J. Catal. 2005, 236(1), 139-152.

2. Almkhelfe, H.; Carpena-Nunez, J.; Back, T. C.; Amama, P. B., Gaseous product mixture from Fischer-Tropsch synthesis as an efficient carbon feedstock for low temperature CVD growth of carbon nanotube carpets. Nanoscale 2016, 8(27), 13476-13487.

3. Datsyuk, V.; Kalyva, M.; Papagelis, K.; Parthenios, J.; Tasis, D.; Siokou, A.; Kallitsis, I.; Galiotis, C., Chemical oxidation of multiwalled carbon nanotubes. Carbon 2008, 46 (6), 833-840.