Abstract
Transition metals (Fe, Co, or Ni) modified K/Mo2C catalysts were prepared and investigated as catalysts for CO hydrogenation. The addition of Fe, Co, or Ni to K/Mo2C catalyst led to a sharp increase in both the activity and selectivity of , but the promotion effects were quite different and followed the sequence: Ni > Co > Fe for the activity and Fe > Co > Ni for the alcohol selectivity. For the products distributions, it also displayed some differences; Co promoter showed much higher hydrocarbon selectivity than Fe or Ni promoter, but Fe or Co promoter gave lower methane selectivity than Ni promoter, and Fe promoter showed the highest selectivity.
1. Introduction
The production of fuels from coal via syngas is one of the most industrial challenges for near future. As for Fischer-Tropsch reaction, reaction proceeds on transition metal supported catalysts, the catalysts currently investigated being Co, Fe, or Ru supported on oxides, such as Al2O3, SiO2, TiO2, and ZrO2 [1–3]. However, the application of such an attractive synthesis still suffers from the lack of high performance catalysts. Thus, great efforts are still being made to improve both activity and selectivity of catalysts. Since 1973, when Levy and Boudart [4] reported that WC displayed reactivity similar to Pt for hydrogenation reactions, there has been considerable interest in the catalytic properties of metal carbides, particularly of the group VI transition metals. Synthesis of hydrocarbons from CO hydrogenation over Co or Ru promoted molybdenum carbides has been evidenced by Griboval-Constant et al. [5], and the results reveal that molybdenum carbide gives light hydrocarbons and alcohols; the addition of Ru decreases the alcohol production yet Co increases formation of heavy hydrocarbons.
As reported elsewhere [6], promotion of molybdenum carbide by potassium has been found to greatly enhance the selectivity to alcohols. For MoS2-based catalysts, the addition of transition metals, such as Co, Ni, Rh, and Pd, is found to be able to improve the catalytic activity and selectivity of alcohols [7–9]. In this regard, it also can be speculated that the transition metals may have positive effect on both alcohols and heavy hydrocarbons formation from CO hydrogenation over Mo2C-based catalysts, and still much less information is available.
Thus, the aim of this work is to study the influence of addition of transition metals, such as Fe, Co, or Ni, on K/Mo2C catalyst in terms of catalytic activity and selectivity in CO hydrogenation reaction.
2. Experimental
2.1. Sample Preparation
The molybdenum carbide was prepared through temperature-programmed-reaction (TPRe) method, and it could be found elsewhere [10]. In detail, the Mo2C and M-Mo2C (M = Fe, Co, Ni) with the hexagonal close packed (HCP) structure were prepared by direct carburization of the MoO3 and MMo oxide precursors, respectively. The oxide precursors for bimetallic carbides were prepared by mechanically mixing a stoichiometric amount of the corresponding nitrate, for example, Fe(NO3)3·9H2O, Co(NO3)2·6H2O or Ni(NO3)2·6H2O with (NH4)6Mo7O24·4H2O; the mixture was then calcined at 673 K for several hours. TPRe was carried out under atmospheric pressure in a flow of CH4/H2 gas mixture. The temperature was linearly increased from room temperature (RT) to 973 K, where it was maintained for additional hours. Then the samples were quenched to RT and gradually passivated with O2/N2 before exposure to air. K2CO3 modification was accomplished by a postdoping procedure after synthesis of the final carbide [6].
2.2. Characterization Methods
X-ray powder diffraction (XRD) patterns of the tested catalysts were obtained on a Rigaku D/Max 2500 powder diffractometer using Cu Kα radiation as the X-ray source.
XPS was recorded on a VG MultiLab 2000 spectrometer using an Mg Kα X-ray source. The catalysts were pretreated at 633 K under syngas (H2/CO = 1.0) for several hours then quenched to RT under N2 atmosphere (see Figure 3). The catalyst was removed from the reactor under N2 atmosphere in a glovebox and secured onto the XPS holder with carbon tape for testing.
2.3. CO Hydrogenation
CO hydrogenation was carried out using a stainless fixed-bed reactor with 4 mL (about 6 gram) of catalyst. The effluents were analyzed by 1790-GC and among these H2, CO, CH4, and CO2 were analyzed by thermal conductivity detector (TCD) equipped with a TDX-101 column; C1–C4 alkanes were detected by flame ionization detector (FID) with a Chromosorb 101 column; C1–C4 alkenes were detected by FID with a packed column C-18; the water and methanol in liquids were also detected by TCD with a GDX-401 column; the alcohols and hydrocarbons were analyzed by FID with a Porapack-Q column. hydrocarbons were detected by FID with a capillary column. The mass balance was based on carbon, and the error of the balance of oxygen and hydrogen was within 5%.
3. Results and Discussion
3.1. Catalytic Performances
3.1.1. Catalytic Performance of K/Mo2C
The results of CO hydrogenation over K/Mo2C catalyst are listed in Table 1. Clearly, the catalytic performance was readily affected by the reaction conditions employed. As the pressure enhanced, both the CO conversion and hydrocarbon selectivity increased, whereas the alcohols selectivity rapidly reduced. According to the literature [11], the higher pressure led to higher alcohol selectivity. However, it was noted that the low pressure favored the alcohol synthesis over K/Mo2C catalyst. In all cases, K/Mo2C catalyst presented almost a constant carbon dioxide yield (~35%), which reflected high water-gas shift activities of molybdenum carbide as frequently reported by others [12, 13]. As the pressure increased, the distribution of alcohols nearly remained the same, but the hydrocarbon distribution changed remarkably, the methane was suppressed, and hydrocarbons was formed. Anyway, the methane selectivity was very high, which might be another feature of Mo2C-based catalyst as reported elsewhere [10].
3.1.2. Catalytic Performance of Transition Metals Modified K/Mo2C
The catalytic performance of CO hydrogenation over transition metals modified K/Mo2C catalyst is shown in Table 2. It can be seen that Fe, Co, or Ni promoter induced similar trends in activity and products distribution over K/Mo2C catalyst. An increase of pressure reduced the alcohol selectivity, whereas the hydrocarbon formation was greatly promoted. However, it was obvious to see that Fe, Co, or Ni promoter led to a sharp increase in both the activity and selectivity of . Compared with K/Mo2C catalyst, the proportion of methanol was further suppressed, and became the major alcohol product. This indicated that transition metals had strong ability to promote the carbon chain growth. Also, in K/Mo2C catalyst modified by Fe, Co, or Ni, the fraction of methane was decreased and the production of hydrocarbons was greatly enhanced.
Undoubtedly, Fe, Co, or Ni had positive effect on catalytic performance of CO hydrogenation over K/Mo2C catalyst; however, it also presented a different promoting effect (see Figure 1). Under the conditions of 613 K, 8.0 MPa, H2/CO = 1.0, and 1000 h−1, Ni promoter showed the highest catalytic activity, but Fe promoter gave the highest alcohol selectivity. As a result, the promotion effects of transition metals were different and followed the sequence: Ni > Co > Fe for the activity and Fe > Co > Ni for the alcohol selectivity. For the products distributions, it also displayed some differences. Co promoter showed much higher hydrocarbon selectivity than Fe or Ni promoter, but Fe or Co promoter gave lower methane selectivity than Ni promoter, which might be attributed to Ni that was an excellent methanation component. Among them, Fe promoter showed the highest selectivity due to its strong ability of carbon chain propagation.
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3.2. Structural Properties
The XRD patterns of the samples are shown in Figure 2; the as-prepared catalysts all had definitive phase of the β-Mo2C [6] ( = 34.4°, 38.0°, 39.4°, 52.1°, 61.5°, 69.6°, 74.6°, and 75.6° for β-Mo2C [100], [002], [101], [102], [110], [103], [112], and [201], resp.). When modified by Ni, the diffraction peaks corresponding to the metallic Ni were detected ( values was at 44.5° and 51.3°) [14, 15]. Modified by Fe, the Fe3C or metallic Fe phase appeared at the value of 44.7°, which might be accounted for the promotional effect of Fe [16]. Also, modified by Co promoter, the signals at 42.6° and 46.5° might correspond to Co3Mo3C, and weak signals at 44.2° might correspond to Co2C [17, 18], which ascribed to the strong synergistic effect of Co and Mo.
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3.3. XPS Studies
In order to study the interaction between Mo and promoters, first the surface chemistry as well as electronic environment of K/Mo2C catalyst was investigated. It showed the presence of Mo 3d in several oxidation states located at 228.0 eV, 231.5 eV, and 235.0 eV. According to the literature, the peak at about 228.2 eV was unambiguously attributed to Mo atoms in the carbide form, metallic molybdenum being detected at about 227.2 eV [19]; the peak at 235.0 eV was attributed to Mo6+, which was characteristic of oxidized phases, resulting from the passivation step [20], and the peak at 231.5 eV was probably attributed to Mo4+ and Mo5+ species. The C 1s spectrum primarily showed three peaks. The peak at 284.6 eV was attributed to amorphous carbon as reported by several authors [21, 22]. The peak at 288.3 eV was attributed to the carbon atoms involved in carbonate species such as C = O or C–O species and was due to contamination. Further investigation revealed that the weak signal at 283.3 eV might correspond to a carbon bonded to a metal in a carbide form [21, 22]. Very likely due to the deposition of excess carbon in the form of coke, the signal of C 1s at 284.6 eV was very strong. Thus, the peak for the carbon atoms in the carbide form was very weak.
Modified by transition metals (Fe, Co, or Ni), dramatic changes of the Mo 3d spectra took place compared to those of K/Mo2C catalyst, and the signals assigned to low value of Mo became strong, but the middle value of Mo showed a slight decrease. The electron interaction between transition metals and molybdenum resulted in the increase of the d-orbital occupation in molybdenum atoms, which further promoted the intrinsic activity. Laniecki et al. reported that the presence of transition metals enhanced the reducibility of Mo species and was responsible for the improvement of the catalytic performance [23]. However, the Fe, Co, or Ni promoter displayed a different promotional effect. Modified by Fe, the signals at about 228.0 eV and 230.7 eV were the strongest, whereas modified by Ni, the signal at about 231.8 eV ascribed to middle value of Mo was the strongest. Also, from the C 1s spectra, it was noted that the intensity of peaks at 284.6 eV was dramatically strong when K/Mo2C catalyst modified by Ni, which revealed the ability of CO dissociation that became strong owing to the Ni was a methanation promoter. These different changes might be related to the different interactions between transition metals and Mo, which further influenced the catalytic performances of CO hydrogenation over the K/Mo2C catalyst.
4. Conclusions
Fe, Co, or Ni had a positive effect on catalytic activity of CO hydrogenation over K/Mo2C catalyst, and they made the products over K/Mo2C shift remarkably from methanol to , and from methane to hydrocarbons at the expense of alcohol selectivity. However, Fe, Co, or Ni also displayed a different promoting effect on catalytic performance of CO hydrogenation. In terms of characterization results, the different interactions between transition metals (Fe, Co, or Ni) and Mo might account for the different catalytic performances over K/Mo2C catalyst.