An analytical scanning transmission electron microscopy study of the support effects on Mn-promoted Co Fischer–Tropsch catalysts

Yuan Zhao a, Theresa E. Feltes b, John R. Regalbuto b, Randall J. Meyer b and Robert F. Klie *a
aDepartment of Physics, University of Illinois at Chicago, Chicago, Illinois 60607, USA. E-mail: rfklie@uic.edu
bDepartment of Chemical Engineering, University of Illinois at Chicago, Chicago, Illinois 60607, USA

Received 22nd February 2011 , Accepted 27th July 2011

First published on 2nd September 2011


Abstract

The metal–support interaction in supported heterogeneous catalyst systems has been long known to affect the activity and selectivity of the catalyst. In this paper, we will combine scanning transmission electron microscopy with electron energy-loss spectroscopy to investigate the interaction between the Mn promoter, the Co nano-catalysts, and the SiO2 support. By comparing these results with similar catalysts on TiO2 supports, we will discuss the effect of metal–support interaction and interfacial oxygen vacancies on the catalyst morphology.


Introduction

The demand for alternative energy sources to crude-oil based fuels has been increasing over the last few decades, in particular at times of high petroleum prices, decreasing petroleum reserves and soaring petroleum consumption. Fischer–Tropsch (FT) synthesis could be one path to alleviate this looming energy crisis, by utilizing natural gas and converting it into synthetic diesel fuelvia heterogeneous catalysts. For this reason, extensive investigations have been conducted on the Co-based FT catalysts in order to improve the performance of the reaction by steering selectivity to longer chain hydrocarbons for use as diesel fuel, by using different supports or adding promoters. The roles of the TiO2, SiO2 and Al2O3 supports have been studied in previous work.1–8 Moreover, it is found that Mn is an effective promoter for increasing the selectivity of the Co-based catalysts on both TiO2 and SiO2 supports.9,10

Yet, the fundamental, atomic-scale mechanism of the promoter-element on the catalysts' activity and selectivity remains still unclear. Morales et al. proposed that the improved performance of Mn-promoted Co catalysts was due to a Mn–Co compound formed on the surface of the catalyst.1,11 As these Mn-oxide species cannot be completely reduced at the temperatures regularly used for the FT-reaction, it has been suggested that the generation of mixed Co–Mn oxides could potentially create active sites with better selectivity.12,13

It was previously proposed that the catalyst synthesis (i.e. dry impregnation or strong electrostatic adsorption) can play an important role in its final performance since the promoter should be closely associated with the active metal to be effective. Dry Impregnation (DI) is universally used to distribute the precursor onto the support.14 It is considered as a simple method for randomly dispersing metals onto the support and strong precursor–support interaction is not guaranteed. On the other hand, strong electrostatic adsorption (SEA) provides a strong interaction between the precursor and the support, which is based on the relative charges of the support and the metal precursor. The SEA method was shown to be very effective in selectively depositing precursors onto a specific site on the substrate.15

In particular, for the Mn promoted Co-based catalysts on TiO2 supports, we have previously shown that Mn only adsorbs onto the Co particles using SEA, while a random distribution of Mn was found in systems synthesized using DI.16 However, it was shown that after reduction the Mn promoter had migrated to the Co/TiO2 interfaces for both synthesis methods.16,17 The strong metal–support interaction (SMSI)18–20 was shown to be responsible for the formation of oxygen vacancies at the Co/TiO2 interface, which then act as anchoring sites for the mobile Mn atoms during the reduction process.17

In contrast, SiO2 is considered as an inert support and no SMSI is expected between the SiO2 supports and the supported catalyst particles. As a result, we do not expect to find oxygen vacancies at the Co/SiO2 interface, and therefore we do not expect preferential migration of Mn towards the Co/SiO2 interface. Mn-promoted Co/SiO2 synthesized using either DI or SEA will thus serve as important benchmark systems to examine the effect of metal–support interaction and the effect of oxygen vacancies on the elemental distribution and morphology of the Mn-promoted Co catalyst.

In this paper, we will utilize high-resolution Z-contrast imaging in a scanning transmission electron microscope (STEM) in combination with spatially resolved electron energy-loss spectroscopy (EELS) to study the migration of Mn in promoted Co-based catalysts on SiO2 supports after calcination and reduction. We will then compare the differences in particle morphology and elemental distribution between catalysts on SiO2 and TiO2 supports; in particular the presence of oxygen vacancies will be examined. The mechanism of Mn migration and its effect on the FT catalyst performance will be discussed.

Sample preparation and experimental methods

A Co3O4/SiO2 (∼5 wt% Co) supported material was prepared by DI of a Co(NO3)2·6H2O (Aldrich, ≥98%) precursor onto Aerosil 300 SiO2. Briefly, Co(NO3)2·6H2O was dissolved in an appropriate amount of deionized H2O to fill the SiO2 pore volume and added drop-wise onto the SiO2 support. This material was then dried at room temperature overnight and subsequently calcined at 400 °C for 4 h (5 °C/min ramp). The final Co3O4/SiO2 material was then divided and loaded with manganese (∼1.5 wt% Mn). The first promoted sample (MnSEA/Co/SiO2) was prepared via SEA of a KMnO4 (Aldrich, ≥99%) precursor from a solution adjusted to pH 1 following the method described previously.13 The second promoted sample (MnDI/Co/SiO2) was prepared by DI of a Mn(NO3)2·4H2O (Arcos, p.a.) salt with no pH adjustments. Following Mn addition, the samples were dried at room temperature overnight and calcined in air at 350 °C for 4 h (5 °C/min ramp). The reduced samples analyzed were treated in flowing H2 at 350 °C for 2 h (5 °C/min ramp).

Temperature Programmed Reduction (TPR) experiments of the calcined catalyst materials were performed on a Micromeritics Autochem 2920 using a Thermal conductivity detector (TCD). After stabilization of the baseline, the reduction was carried out at a heating rate of 10 °C min−1 in a 10% H2/Ar flow.

To characterize the catalyst samples, we utilize the UIC-JEOL2010F TEM/STEM,21,22 equipped with a Schottky field-emission electron gun operated at 200 kV, an ultra-high-resolution objective lens pole-piece (URP), and a post-column Gatan Imaging Filter (GIF). It has been set up for chemical analysis with high spatial resolution, resulting in an electron probe size of 1.8 Å at a convergence angle of 15 mrad and a spectrometer collection angle of 28 mrad. The core-loss and low-loss spectrum images were both acquired at the same location of each sample, with the acquisition time of 0.7 s pxl−1 (for core-loss) and 0.1 s pxl−1 (for low-loss) respectively. The dispersion for the spectra was set up to be 0.5 eV pxl−1 (using a 1k × 1k camera). The core-loss EELS spectrum includes the O K-edge at 532 eV, the MnL-edge at 640 eV and the CoL-edge at 779 eV. The low-loss EELS spectrum includes the zero-loss peak and the SiL-edge at 99 eV. The Co and MnL-edges were calibrated by using the O K-edge onset, while the SiL-edge was calibrated by the zero-loss peak.

Results

Fig. 1 shows the TPR profiles of the MnDI/Co/SiO2 and MnSEA/Co/SiO2 samples, as well as an unpromoted Co/SiO2 sample. In the profile of the unpromoted Co/SiO2 sample, two peaks at 241 °C and 284 °C can be seen. These two peaks represent the reductions of Co3O4 to CoO, and CoO to metallic Co.4,23,24 There is a small peak at 550 °C attributed to the reduction of Co silicate.6 For MnDI/Co/SiO2, a shoulder at 241 °C and a peak at 284 °C still can be found, which means a portion of the Co particles still contain Co-oxide. For MnSEA/Co/SiO2, the profile shape at temperatures less than 300 °C is similar to that of Co/SiO2. However, the first reduction peak shifts to lower energy (from 241 °C to 216 °C in MnSEA/Co/SiO2), which might be due to the addition of Mn to the Co/SiO2 sample leading to an increase in the number of oxygen vacancies and the oxygen ion mobility.10,25,26 The shift of the second peak to 300 °C is attributable to the fact that the Mn suppresses the reducibility of CoO to metallic Co.9,11
The TPR profiles of the Co/SiO2, MnDI/Co/SiO2 and MnSEA/Co/SiO2 samples.
Fig. 1 The TPR profiles of the Co/SiO2, MnDI/Co/SiO2 and MnSEA/Co/SiO2 samples.

Above 300 °C, both promoted Co/SiO2 samples show a peak and a broad feature extending up to 600 °C. More specifically, in MnDI/Co/SiO2, we find a peak at 328 °C and a subsequent broad shoulder extending to 600 °C. Liang et al have previously shown that the reduction of spinel type Co–Mn mixed oxide results in a H2 consumption peak in the temperature range between 300 °C and 600 °C.27 We therefore ascribe our measured peak extending to 600 °C with a maximum at 328 °C to the reduction of the spinel type Co–Mn mixed oxide formed between the Mn promoters and Co catalysts. Moreover, Morales et al. have found a H2 consumption peak at 550 °C due to the reduction of mixed Co–Mn spinel oxide materials.11 The Co silicate reduction peak is also located at 550 eV. Based solely on our TPR results, we therefore assign the shoulder at 550 °C in the MnDI/Co/SiO2 profile as arising from both Co–Mn spinel phase and Co silicate reduction. Similarly, for the MnSEA/Co/SiO2 sample, we attribute the broad shoulder from 328 °C to 600 °C to the reduction of Co–Mn oxides.

It is interesting to note here that the TPR profiles for both MnDI/Co/SiO2 and MnSEA/Co/SiO2 samples are very similar, although the Mn distribution should be significantly different. For the MnDI/Co/SiO2 sample, Mn should be randomly distributed over the Co/SiO2 support. The higher dispersion of Mn should result in less interaction between the Mn and the Co particles, and thus a smaller concentration of the Co–Mn spinel phase. On the other hand, the SEA method guarantees that all of the Mn is deposited onto the Co particles in the MnSEA/Co/SiO2 sample, resulting in a much larger concentration of the Co–Mn mixed oxide. However, the TPR profiles show that both methods generate comparable amounts of Co–Mn phase, indicating that Mn migrates to the Co particles and forms a compound with Co after deposition by DI.

In order to better understand the TPR results, we now use high-resolution Z-contrast imaging and EELS imaging to characterize the local chemical composition and electronic structures of the different catalyst samples. Around 20 images were acquired for each sample taken from regions that contain both Co and Mn. Fig. 2(a) shows the EELS spectrum image map of the Co, Mn and Si elemental distribution for the MnDI/Co/SiO2 sample after calcination. For each element (Co, Mn and Si), the integrated spectrum intensity after background subtraction over an energy range of 20 eV is plotted as a function of position. As a result, it can be clearly seen that the Mn particles do not associate with the Co particles using dry impregnation. In particular, we see that a Mn particle (approximately 30 nm in diameter) is separated about 30 nm from a Co particle (around 20 nm in diameter). However, Fig. 2(b) shows another elemental distribution for a different location in the same sample. Here, there are two Co particles of around 15 nm and 10 nm in diameters on the surface SiO2 support, while a 25 nm Mn particle covers both the Co particles and SiO2 support. In particular, we find that the Mn particle covers the entire larger Co particle and only parts of the smaller Co particle.


The elemental distribution of the MnDI/Co/SiO2 sample (a) and (b) after calcination; (c) and (d) after reduction. Red represents Co, green represents Mn, and blue represents Si.
Fig. 2 The elemental distribution of the MnDI/Co/SiO2 sample (a) and (b) after calcination; (c) and (d) after reduction. Red represents Co, green represents Mn, and blue represents Si.

These results highlight two important features of the MnDI/Co/SiO2 sample: (1) the initial distribution of Mn on the Co/SiO2 surface is random; (2) although the initial location of Mn after deposition should be separated from Co, some Mn can still move to the vicinity of the Co particles during the calcination.

Fig. 2(c) and (d) show the elemental distribution for the MnDI/Co/SiO2 sample after reduction. In both locations shown here, we can see that the Mn is in close proximity to the Co particles. Fig. 2(c) shows a smaller Co particle (around 8 nm in diameter) without any Mn, while parts of the larger Mn particle (around 10 nm × 20 nm) overlap with two Co particles around 10 nm in diameter. In Fig. 2(d), a Co particle of 10 nm in diameter sits on the edge of the SiO2 support, and a 15 nm Mn particle overlaps the position of the Co particle. Although the images shown here imply that Mn is always associated with Co after reduction, we can still find single Mn particles without any Co (not shown here). These spectrum images suggest that the catalyst reduction treatment does not lead to a fundamental change in the Co and Mn elemental distributions compared to the calcined sample. Yet, for Mn that is in close proximity to Co particles in the calcined sample, it appears that Mn migrates towards Co during calcination, and forms a Co–Mn oxide, which then leads to the high peak corresponding to a Co–Mn compound in the TPR profile of MnDI/Co/SiO2. Another interesting phenomenon shown in the spectrum images is that the Mn particles are larger than the Co particles, considering that the Mn is only 1.5 wt% compared to the 5 wt% Co. This could be due to the lack of reactivity of the SiO2 support such that Mn and SiO2 do not interact, facilitating Mn sintering on the SiO2 surface.

The elemental distribution for the MnSEA/Co/SiO2 sample after calcination is shown in Fig. 3(a) and (b), taken from different locations within the same sample. Fig. 3(a) shows a lower magnification spectrum image of a Co and Mn particle sitting at the edge of the SiO2 support. It does not appear that these are two individual particles, but rather a Co core covered by a Mn shell. Moreover, we do not find any evidence of isolated Mn on the surface of SiO2. Our analysis of different locations within the MnSEA/Co/SiO2 sample confirms that Mn is selectively adsorbed on Co and not on SiO2. In Fig. 3(b), a higher magnification spectrum image clearly shows that a Mn shell is formed on the surface of a 15 nm Co particle. The selective adsorption of Mn on Co creates a large Co/Mn interface, which leads to the large peak between 300 °C and 600 °C in our TPR profiles (Fig. 1).


The elemental distribution of the MnSEA/Co/SiO2 sample (a) and (b) after calcination; (c) after reduction. Red represents Co, green represents Mn, and blue represents Si.
Fig. 3 The elemental distribution of the MnSEA/Co/SiO2 sample (a) and (b) after calcination; (c) after reduction. Red represents Co, green represents Mn, and blue represents Si.

Fig. 3(c) shows the elemental distribution for the MnSEA/Co/SiO2 sample after reduction. Similar to the sample after calcination (Fig. 3(a) and (b)), Mn is only found on the Co particle; no Mn is on the SiO2 support after the reduction treatment. In the particular location shown in Fig. 3(c), a Mn-covered Co particle (15 nm diameter) sits on the edge of the SiO2 support. Although the relative intensity of Mn at the bottom of the Co particle is higher than at the top, a Mn signal could be detected over the entire surface of the Co particle. That means the Mn shell still covers the whole Co particle surface after the reduction treatment. These results indicate that if the Mn is initially on the Co particles, it prefers to stay on the Co surface, and does not migrate towards the Co/SiO2 interface during either the calcination or reduction treatment.

Next, we will examine the elemental composition and electronic structure of the Co/SiO2 interface by looking at the near-edge fine-structure of Si and CoL-edges.28Fig. 4 shows the SiL-edges acquired from the amorphous SiO2 at the Co/SiO2 interface for our four samples. For each spectrum, SiL3, L2 and L1 edges are located at 108 eV, 116 eV and 160 eV, respectively. The broad peak at 130 eV is due to the plural scattering events of the incident electrons, which usually occurs at larger sample thicknesses. The edge-onset of the SiL-edges is found to be 105.5 eV, using the second derivative method in Digital Micrograph. We cannot find any sign of the Si0 edge signal, which should have an edge onset of 99 eV. Therefore, we find that the fine structures of all SiL-edges indicate the presence of stoichiometric SiO229 at the Co/SiO2 interface. The absence of shift in the SiL-edge or the Si0 intensity indicates that there is no oxygen vacancy at the Co/SiO2 interface.



          Si
          L-edges acquired at the Co/Si interface from MnDI/Co/SiO2 and MnSEA/Co/SiO2 samples after calcination and after reduction. The dash line indicates the edge onset at 105.5 eV.
Fig. 4 Si L-edges acquired at the Co/Si interface from MnDI/Co/SiO2 and MnSEA/Co/SiO2 samples after calcination and after reduction. The dash line indicates the edge onset at 105.5 eV.

The near-edge fine structures of CoL2,3-edges were acquired at the same locations as those of the SiL-edges (Fig. 5). The change in the Co valence can be quantified using the change in the CoL2,3-edge onset, as well as the Co L3/L2-ratio.30 While it is customary to align the CoL2,3-edge onset with respect to the O K-edge onset at 532 eV, this will not be possible in our current experiment. Due to the presence of SiO2, the O K-edge shows a significant shift towards lower energies that is solely attributed to the SiO2 and not a shift in the Co valance. Therefore, we will only use the changes in the Co L3/L2-ratio to quantify the Co valence. In Fig. 5, the CoL-edge is aligned to the position of the maximum intensity of the Co L3 peak, in order to compare the changes in the L3/L2 ratio. Fig. 5(a) shows the CoL2,3-edges acquired from the calcined and reduced MnDI/Co/SiO2 samples. By using the second derivative method,31 the Co L3/L2 ratios of the calcined sample and reduced sample are found to be (2.2 ± 0.1) and (4.1 ± 0.1), respectively. According to our previous analysis of the Co L3/L2 ratio for different Co-oxide reference spectra,30 we find that the Co-oxide particles are Co3O4 after calcination, and CoO after the reduction treatment. Similarly, the analysis of the CoL-edges in the MnSEA/Co/SiO2 samples reveals a CoL3/L2 ratio in the calcined sample and reduced sample (2.1 ± 0.1) and (4.3 ± 0.1), respectively (Fig. 5(b)). Again, this indicates the presence of Co3O4 particles after calcination and CoO particles after reduction. It is important to note that all the particles analyzed are still oxides, even after reduction. This implies that either the samples were re-oxidized when exposed to air after reduction treatment, or formed Co–Mn or Co silicate compounds, as shown in Fig. 1, resulting in a reduction temperature higher than 350 °C used here.



          Co
          L2,3-edges acquired at the Co/Si interface after calcination and reduction from (a) MnDI/Co/SiO2 sample and (b) MnSEA/Co/SiO2 sample.
Fig. 5 Co L2,3-edges acquired at the Co/Si interface after calcination and reduction from (a) MnDI/Co/SiO2 sample and (b) MnSEA/Co/SiO2 sample.

Discussion

Our experimental results show that even in the MnDI/Co/SiO2 sample, there appears to be a driving force for the Mn and Co particles to agglomerate after the calcination treatment. Moreover, we have shown that the Mn/Co particle morphology (in both the MnDI/Co/SiO2 and MnSEA/Co/SiO2 samples) does not change during the reduction treatment, as opposed to the Mn/Co/TiO2 system, where a migration of Mn towards the Co/TiO2 interface was previously reported.17Table 1 shows a summary of the Mn location in Mn promoted Co catalysts on SiO2 and TiO2 supports.
Table 1 The Mn location in Mn promoted Co catalysts on SiO2 and TiO2 supports
SiO2 support TiO2 support
  MnDI/Co/SiO2 MnSEA/Co/SiO2 MnDI/Co/TiO2 MnSEA/Co/TiO2
Calcined Large Mn particles form on SiO2. Mn particles also form on Co surface. Mn forms shell covering the Co particle surfaces. Mn is randomly distributed on Co and TiO2 surfaces. Mn forms shell covering the Co particle surfaces.
Reduced No significant change is observed compared to calcined sample. No significant change is observed compared to calcined sample. Mn moves to the Co/TiO2 interface. Mn moves to the Co/TiO2 interface.


We propose the surface energies of the different oxides to be one reason to explain the agglomeration of the Mn–Co particles in the MnDI/Co/SiO2 sample. Although it is hard to determine the surface energies of all the possible exposed surface orientations for all the oxides present in our samples, we will utilize the following trends to support our claim: the Ramsdellite MnO2 (101) surface energy is 1.52 J m−2,32 that of Co3O4 (111) is 1.779 J m−2,33 and the SiO2 surface energy is 0.605 J m−2.34 During calcination of the MnDI/Co/SiO2 sample, MnO2 prefers to move onto the Co3O4 particle surface in order to decrease the surface energy of the system. In the MnSEA/Co/SiO2 sample, where Mn is already selectively deposited on the Co3O4 surface, MnO2 will remain there and stay away from the SiO2 surface in order to maintain the lower surface energy of the system. On the other hand, the formation of a Co–Mn spinel phase will result in an increased interaction energy at the Mn/Co interface35 that would act as an enthalpic driving force for the migration of Mn over the surface of the Co particles. Fig. 2(b) shows that the Co particle nearly completely overlaps with the Mn after calcination, and potentially covers the entire surface of the Co particles. Fig. 3 shows that for the MnSEA/Co/SiO2 sample, the Mn shell appears to cover the entire surface of the Co particles, after both the calcination and the reduction treatments.

The migration of Mn in the Co-based catalysts on TiO2 supports during reduction treatment compared to the calcined sample has been reported previously,1,3 resulting in an enhanced catalyst performance of the catalysts due to the so-called promotion effect. However, the mechanism for the Mn migration during reduction is still unclear. In our previous study using TiO2 supports, we had shown that by using the SEA method, Mn could form a homogeneous shell over the Co particle after calcination. The Mn shell ruptured after reduction treatment and the Mn migrated to the Co/TiO2 interface.16 By using DI as the synthesis method to deposit Mn on the Co/TiO2 sample, we found that the Mn also moved from the TiO2 support to the Co/TiO2 interface after reduction. Moreover, since the oxygen vacancies present on the TiO2 surface are known to act as anchoring sites for mobile the metal atoms,36 we speculated that the oxygen vacancies found at the Co/TiO2 interface promote the migration of Mn towards the Co/TiO2 interface.17

Although SiO2 does not partially reduce, the formation of interfacial compounds such as Co silicate in the SiO2 supported Co catalyst can still occur. It has been previously reported that the decomposition of Co(NO3)2·6H2O forms cobalt silicate on the silica support,37,38 and Ernst et al. reported that Co2SiO4 forms at reduction temperatures as low as 350 °C.5 It has also been shown that the Co silicate reduces above 500 °C.5,6,39Fig. 1 shows the TPR profile for a Co/SiO2 catalyst, and the peak at 550 °C is assigned to the reduction of Co2SiO4. Since the calcination and reduction treatments for the MnDI/Co/SiO2 and MnSEA/Co/SiO2 samples are all carried out at 350 °C, it is entirely possible that an interfacial Co2SiO4 phase is still present. Unfortunately, our TPR measurements cannot distinguish between the reduction peaks of the Co–Mn spinel phase and Co2SiO4, since they both overlap in the temperature range between 300 °C and 600 °C. Moreover, our EELS analysis would not be able to distinguish the SiO2 support from an interfacial Co2SiO4 or CoSiO3 phase,5,39,40 since the valence state for Si is 4+ in either phase, as measured by the SiL-edge EELS spectrum in Fig. 4. However, regardless of the formation of an interfacial Co silicate, we do not find any electron transfer between the silica support and the Co catalyst, and no oxygen vacancies are measured at the Co/SiO2 interface.

In order to verify that the oxygen vacancies provide the driving force for the formation of an interfacial Co–Mn catalyst, we can study the evolution of the Mn promoter in Co/SiO2 catalysts. The oxygen vacancy formation energy on SiO2 is nearly twice as high as that on TiO2 surfaces. More specifically, the oxygen vacancy formation energy on the SiO2 surface was calculated to be 8.5 eV,41 while that for TiO2 rutile and anatase surfaces was found at 3.77 eV and 4.94 eV, respectively.36 Therefore, the formation of oxygen vacancies is expected to be much less likely in the Co/SiO2 compared to the Co/TiO2 interface under similar reduction conditions. Thus, the different Co/Mn particle morphology changes during reduction on SiO2 supports (as shown in Fig. 2 and 3) compared to that on TiO2 supports can be attributed to the lack of interfacial oxygen vacancies, and anchoring sites for the mobile Mn atoms. As a result, we do not see any migration of the Mn on the Co particle towards the Co/SiO2 interface, which happens in the Co/TiO2 system as previously reported.13,16,17 Therefore, by comparing the results presented here with previous studies of similar catalysts on TiO2, where a strong metal–support interaction exists, we can show that the presence of interfacial oxygen vacancies is important for the formation of an interfacial Mn–Co catalyst, and thus a critical factor to the promotional effect of Mn/Co FT catalysts.

Conclusion

Temperature programmed reduction and Z-contrast imaging combined with electron energy-loss spectrum imaging is used to study the elemental distribution and morphology of the Mn-promoted Co nanoparticles on SiO2. By comparing the results with those of Mn-promoted Co particles on TiO2,13,16,17 we find that oxygen vacancies at the metal–support interface are important for understanding not only the migration effects of Mn on the Co particle surface, but also the promoter effects on the Mn promoted Co-based FT catalysts. More specifically, we find that the oxygen vacancies mediate formation of an interfacial Mn–Co phase with active sites that contribute to the higher selectivity found in Mn-promoted FT catalysts.

Acknowledgements

The authors Y.Z and R.F.K would like to acknowledge support for this research by the American Chemical Society-Petroleum Research Fund (grant number 47307-G). T.E.F, R.J.M and J.R.R would like to acknowledge support from the National Science Foundation (CBET-#0626505). The authors would acknowledge the Research Resource Center of UIC for providing instrumentation for the experiments.

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