The of mesopores in catalyst. The corresponding pore

The surface morphology of catalysts was examined by SEM. As shown in Fig. 1, ZCI (1,2,12) WC which was a without pyrolyzed catalyst shows the uniform formation of hollow polyhedral structure which exhibit uniform morphology, which are so import in the growth of carbon nanotube structure (10.1021/jp0142278). The ZCI (1,2,12) WC polyhedral are treated at 500 °C under reducing condition, after 4h of pyrolysis Surface are of ZCI (1,2,12) WC polyhedral become rough looks like CNT formation. This is because of breaking of some coordinate bonds between metal ions and organic ligands in ZCI(1,2,12)WC polyhedral breaks because their long binding length.( 10.1021/jacs.7b01942) When pyrolysis temperature increased up to 600 °C N-CNTs growing well length of NCNTs. At 700 °C of pyrolysis, sample (ZCI (1,2,12)-700), it appears as fully-grown N-CNTs form. The SEM image shows growing of N-CNTs with increasing its pyrolysis temperature.     The N2 adsorption and desorption of ZIF could be seen type II isotherm (IUPAC) and one H3-type hysteresis loop appeared 0.4-1 in the range of p/p0 shown in (Fig. 2), which indicates the presence of mesopores in catalyst. The corresponding pore distribution curve derived from the Barret-Joyner-Halenda model confirmed the large number of mesopores present in the ZIF. The BET specific surface area of ZIF gradually increased from 121.35cm3/g to 271.90 cm3/g with increasing pyrolysis temperature catalyst from 500 °C to 700 °C (Table 1). The physical properties of as-synthesized catalysts such as average pore diameter, pore volume and surface area listed in Table . The average pore sizes of ZCI (1,2,12)-500, ZCI (1,2,12)-600 and ZCI (1,2,12)-700 were 8.70nm, 8.18 nm and 51.86 nm Pore volume also increases with increasing the temperature of pyrolysis.XPS was employed to study the chemical composition and content of cobalt, zinc, nitrogen and carbon on the surface of ZIFs. It could be seen that in Fig. , the spectra of ZIFs shows the existence of C, O, Co, Zn and limited amount of nitrogen without other impurities. It could be seen that the Co and Zn were finally incorporated with imidazolic framework. The N 1s spectra ZCI-x shows that little amount of metal nitrite (~397), pyridinic N (N1, 398.8 eV) and pyrrolic N (N1, 400.4 eV) , after ZIFs was pyrolyzed in 5% H2/Ar atmosphere. The spectra of ZCI (1,2,12)-700 can be separated by three gaussian peaks 398.6, 400.2 and 401.2 eV as a pyridinic nitrogen, pyrrolic nitrogen and pyridine nitrogen respectively. The C 1s spectra of ZCI (1,2,12)-WC shows the major contribution of peak at 284.5 eV for sp2 hybridized carbon atoms. The low intensity peak at 282.2 and 287.8 eV attributed to sp1 hybridized carbon atoms and carbonyl group respectively. The peak located at 285.5 eV C-O, C-OH and/or sp3 carbon atoms. (DOI: 10.1186/1556-276X-9-182). The ZCI (1,2,12)-700 required four gaussian peaks for accurate fit to data. The largest peak at 284.5 eV assign to C-C means sp2 hybridized carbon atoms. The remaining three peaks at 285.5, 286.6 and 288.8 eV represent the C-OH or C-O, C=O and C-N-C bonds. The chemical state of Zn and Co metals are shown in fig.  there are two peaks observed in Zn 2p spectra matched with Zn 2p3/2 and Zn 2p1/2. Binding energy difference in between these two peaks is about 23.0eV. The Zn 2p core-level of ZnO nanoparticles has shown two peaks located at about 1021.53 and 1044.56 eV attributed to Zn 2p3/2 and Zn 2p1/2, respectively.Cobalt also show two Co 2p spectra of Co 2p3/2 and Co 2p1/2 with satellite peaks. The difference between peak position because reduction of catalyst at different temperature. The metallic cobalt having Co2p3/2 peak at 778.2 eV and Co+2 and Co+3 shows peak at 779.7 eV. Fig. displays the NH3-TPD results of reduced ZCI (1,2,12) catalysts, also the calculated amount of surface acidic sites. The ZCI (1,2,12)-500 shows the strong acidity peak at 537 °C, when the pyrolysis temperature increases NH3 desorption peak of catalyst shifted to moderately strong acid area. The surface area oxygenated groups (-OH) carbon support and Co-O are related to weak and strong acid cites respectively. Also, ZCI (1,2,12)-600 presents two types of high-temperature peaks at 507.86 °C and 345.29 °C and one peak appears at weak acid cite appears at 179.20 °C. ZCI (1,2,12)-700 shows one moderately strong peak at 459.44 °C and another peak at 330.22 °C. The shifting of acid strength from high region to low region because the reduction of cobalt from oxide form to metallic Cobalt. The TPR profile of ZCI (1,2,12)-x are shown in fig. The peaks obtained in the profile of ZCI (1,2,12)-500 can be divided into four groups. The 200-280 °C is first region assigned that the reduction of cobalt nitrate which remained after pyrolysis of catalyst. It appears due to nitrate group decomposition by zinc during calcination of catalyst. The second peak between 280 to 400 °C appears due to reduction of Co3O4 to CoO. The third group between 400 to 560 °C indicates the reduction of CoO to Co. the last one peak above 560 °C shows the reduction of Co interacting with ZnO. The thermal stabilities of synthesized Zn-Co CNx, and composites were studied by TGA. TGA revealed an almost no weight loss for the Zn-Co CNx and the composites up to 221 oC, which indicated N-CNTs are very stable in this temperature range. Results indicate that all samples have remarkable thermally stabilized with a decomposition temperature higher than 430 oC.The XRD pattern of ZIF-67 is identical and indicating similar crystal structure with as-synthesized ZCI (1,2,12)-WC (without thermal treated catalyst). XRD pattern of ZCI (1,2,12)-500 composites shows seven diffraction peaks with ZnCo2O4 (JCPDF 48-1719) shown in Fig. .  Also, ZCI (1,2,12)-600 could be shown five diffractions of peaks consist of metallic Co (JCPDF 15-0806) and Co3C (JCPDF 26-0450). Change in diffraction of peak with increasing pyrolysis temperature of ZIFs confirmed that some structural modification with reduction of cobalt species. ZCI (1,2,12)-700 shows only three peaks at 43.09o, 50.49oand 74.82o, which can be ascribed to non-stoichiometric phase of CoCx (JCPDF 44-0962). The absence of Zn or zinc oxide peaks in ZCI (1,2,12)-600 and ZCI (1,2,12)-700 containing some amount of zinc, indicates that existing of Zn content is in an amorphous phase.   Pyridine -FTIR spectroscopy (Py-IR) analysis tool was used for the determine acid types and types of acid sides in the as-synthesized catalysts shown in Fig. and by calculating peak areas listed the ratio of Lewis acid sites to Brønsted acid sites(L/B) in Table . The spectra differentiated into three different bands 1) weak band observed at 1557 cm-1 can be assigned to the strong  Brønsted sites.( 10.1016/j.jechem.2015.07.002) 2) The high intensity band at 1447 cm-1, which correspond to adsorption of pyridine on Lewis acid side and 3) The band observed  in between those two bands at 1487 cm-1 was due to adsorption of pyridine on both Lewis acid cites and Brønsted acid sites. Lewis acid cites and Brønsted acid sites increased their band intensity gradually with increasing pyrolysis temperature. 

x

Hi!
I'm Rita!

Would you like to get a custom essay? How about receiving a customized one?

Check it out