545783 Nickel-Manganese Bimetallic Catalysts for Selective Hydrogenation of COx in the Presence of Light Hydrocarbons: Promotional Effect of Manganese

Monday, June 3, 2019: 2:18 PM
Republic ABC (Grand Hyatt San Antonio)
Vahid Shadravan1, Matthew Drewery2, Meng-Jung Li1, Eric M. Kennedy1 and Michael Stockenhuber1, (1)Chemical Engineering, The University of Newcastle, Newcastle, Australia, (2)University of Newcastle, Callaghan, Australia

Nickel-manganese bimetallic catalysts for selective hydrogenation of COx in the presence of light hydrocarbons: Promotional effect of manganese


Vahid Shadravan, Molly Meng-Jung Li, Eric Kennedy, Michael Stockenhuber*


Chemical Engineering, School of Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia

* michael.stockenhuber@newcastle.edu.au


1. Introduction

The worldwide demand for development of new energy storage and resources has reanimated research into carbon oxide hydrogenation reactions. Carbon oxide methanation reaction is a well-known process that has been used for various applications, such as purification of hydrogen used in ammonia synthesis and fuel cells, synthetic natural gas production, and chemical storage of electricity1. On the other hand, in the catalytic synthesis of hydrocarbons from oxidative processes, significant quantities of CO and CO2 usually present as undesirable by-products which hinders  commercialization for example in the production of C2 feedstock for the polymer industry Therefore, effectively removing CO and CO2 from a synthesized hydrocarbon stream via hydrogenation reaction, while maintaining or increasing the concentration of C2+ hydrocarbons is highly desirable for the industrial processes. In this paper, we report on the influence of Ni-Mn/Al2O3 catalyst structure and transition metal content on the C2+ yield in the hydrogenation of carbon oxides in the presence of hydrocarbons (CH4, C2H4, C2H6, C3H6, and C3H8).   

2. Results and discussion

Figure 1a and 1b disclose that the Ni-Mn/Al2O3 catalysts with Ni/Mn ratios of 5 and 2 show enhanced CO and CO2 conversion compared to the Ni/Al2O3 benchmark catalyst. However, by decreasing the Ni/Mn ratio to below 2, it is found that the activity was significantly reduced. The monometallic Mn/Al2O3 catalyst is inactive for CO conversion during the examined temperature range, instead, CO production is observed at above 300 °C presumably due to dry reforming of methane or reverse water gas shift reactions. On the other hand, Figure 1c shows that the catalysts with the Ni/Mn ratios of 5 and 2 give enhanced C2+ yield compared to the unmodified Ni/Al2O3 catalysts, whereas a decreased C2+ yield observed in the Ni-Mn/Al2O3 catalysts with Ni/Mn < 2. Besides, the yield of C2+ hydrocarbons increases with increasing temperature and reaches a maximum then subsequently decreases at elevated temperature due to the cracking of C2+ hydrocarbons to form methane.


      Figure 1. CO conversion (a), CO2 conversion, and C2-C4 yield (c) of the catalysts with varying Ni/Mn ratio.

The observation of the changes in the catalytic performance of Ni-Mn/Al2O3 catalysts with different Ni/Mn ratios can be supported by the calculated activation energy value. Table ‎1 shows the activation energy for the reaction of CO, CO2, and C2H6 with hydrogen over the selected Ni/Al2O3 and Ni-Mn/Al2O3 (with Ni:Mn = 12:0, 8:4, and 4:8) catalysts. It is found that by adding a moderate loading of manganese (Ni:Mn = 8:4) to the Ni/Al2O3 catalyst (Ni:Mn = 12:0), the activation barrier for both CO and CO2 hydrogenation is reduced. In contrast, adding more manganese (Ni:Mn = 4:8) results in an increase in the activation energy value for CO and CO2 hydrogenation. As for the ethane hydrocracking reaction, the activation energy value of each catalyst follows the order of Ni:Mn 4:8 > Ni:Mn 8:4 > Ni:Mn 12:0, suggesting that the activation barrier for catalyzing ethane hydrocracking increases as incorporating manganese to the Ni/Al2O3 catalyst.

Table 1. Calculated activation energy values for the reaction of CO, CO2 and C2H6 with H2 over Ni-Mn/Al2O3 catalysts.




Activation Energy value (kJ/mol)

Ni:Mn 12:0



Ni:Mn 8:4


Ni:Mn 4:8


Ni:Mn 12:0



Ni:Mn 8:4


Ni:Mn 4:8


Ni:Mn 12:0



Ni:Mn 8:4


Ni:Mn 4:8


In-situ infrared spectroscopy using nitric oxide as a probe molecule (NO-FTIR) was performed to understand the adsorption and structural properties of the transition metal sites with different Ni/Mn ratios. The coordinated nitric oxide molecules, bonded to the catalyst’s active sites, have properties which are similar to carbon monoxide on the active sites2, making NO-FTIR suitable for investigating COx hydrogenation over Ni-Mn/Al2O3 catalysts. Figure 2 demonstrates the NO-FTIR spectra of the Ni-Mn/Al2O3 catalyst with Ni:Mn = 8:4. Sections ‘a’ and ‘b’ show the adsorption and desorption spectra, respectively. The peaks presented in the spectra at around 1850 cm-1 wavenumbers (see the zoomed-in windows) can be assigned as mononitrosyls arising from the nonreactive adsorption of NO on the Ni-containing samples3. Other peaks appearing at wavenumbers around 1300 cm-1 and 1600 cm-1 arise from the reactive adsorption of NO molecules, which are most likely due to the formation of charged and neutral NxOy species such as NO2 and NO2- surface compounds2. It has been reported that mononitrosyl in the form of NOd+ coordination (linear geometry) is basically an electron donor to an electron accepting site, whereas the form of NOd- coordination (bent geometry) is an electron acceptor to an electron donating site4. Therefore, the peaks assigned as mononitrosyls were deconvoluted to separate the linear-NO (at higher wavenumber) and the bent-NO (at lower wavenumber) peaks, see Figure 3a. The changes of linear/bent ratio on the selected Ni-Mn/Al2O3 catalysts are summarized in Figure ‎3b. According to Figure 3b, decreasing the Ni/Mn ratio results in reducing the value of linear/bent ratio, suggesting that the numbers of the electron donating sites increases as incorporating Mn into Ni/Al2O3.  

Figure 2. In-situ FTIR spectra for a. NO adsorption over bi-metallic Ni-Mn/Al2O3 (Ni:Mn 8:4), followed by b. temperature programmed desorption.

Figure 3. (a) Peak deconvolution for the metal–mononitrosyl species formed on nickel-containing catalysts observed from the in-situ NO-FTIR analysis; (b) calculated value of linear/bent ratio of metal–mononitrosyl compounds for the catalysts with different Ni/Mn ratio.


Solid state surface electrostatic potential Vs(r) were computed based on KS-DFT (Kohn-Sham Density Functional Theory) performed at the PW91/pob-TZVP level of the theory. Figure 4 shows the surface electrostatic potential Vs(r) at 0.001 au isodensity level for Ni and Ni-Mn particles containing one conventional unit cell of each crystal (Figure ‎4a) and 64 crystallographic unit cell (Figure 4b). The redistribution of the electron density in multi-atomic systems leads to the formation of minima (Vs,min) and maxima (Vs,max) of the surface electrostatic potential Vs(r). The Vs,min and Vs,max or areas with different electrostatic values on the surface of the catalyst can be interpreted as different catalytic sites5. In this case, the sites with Vs,min values can act as electron donors and the ones with Vs,max can act as electron acceptors. The colour-coded maps for Ni and Ni-Mn particles are considerably different, indicating the changes in the electronic structure of the Ni/Al2O3 catalyst by the addition of manganese, which is consistent with our findings from in-situ NO-FTIR.




Figure 4. Surface electrostatic potential Vs(r) on 0.001 au isodensity for Ni and Ni-Mn particles with the size of (a) one crystallographic unit cell, and (b) 64 crystallographic unit cell.



Carbide-based mechanism is a widely accepted mechanism for carbon oxide hydrogenation reactions, in which COx hydrogenation initiates via the dissociation of C–O bonds. The cleavage of C–O bond generates active surface carbon species which can be followed by their hydrogenation (known as chain growth step) to different CxHy species6. It has been reported that the changes of the electronic properties of catalysts can affect both chain initiation (C–O cleavage) and chain growth (CxHy formation)7, thus leading to the changes in the catalytic performance. On the other hand, the alteration of electronic properties of catalysts induced by the addition of manganese can also affect the hydrocracking activity (e.g. ethane hydrocracking). It is reported that hydrocracking activity over nickel catalysts depends on the C–C bond rupture. In this case, the C–C bond cleavage happens more readily on catalysts with stronger carbon-metal bonding (more electron accepting sites)8. Therefore, the enhancement of COx hydrogenation activity and C2+ hydrocarbon selectivity by adding manganese to Ni/Al2O3 can be explained by the alteration of electronic properties induced by forming bimetallic Ni-Mn catalysts.



3. Conclusion

In summary, it is shown that the addition of manganese to Ni/Al2O3 catalyst significantly altered its catalytic performance for hydrogenation of CO and CO2 in the presence of light hydrocarbons. Adding an optimum amount of Mn to Ni/Al2O3 can enhance the catalyst activity and selectivity of C2-C4 production. According to our investigation of the catalysts’ electronic properties with different Ni and Mn contents, changes in catalytic activity (for COx hydrogenation) and selectivity (for light hydrocarbons formation) can be interpreted as being due to the effect of different electronic structure of the catalysts with variety of Ni/Mn ratios.



4. Reference

1.    Gao, J. et al., RSC Advances 2015, 5, 22759-22776.

2.    Hadjiivanov, K. I. et al., Catalysis Reviews 2000, 42, 71-144.

3.    Zecchina, A. et al., Catalysis Today 1996, 27, 403-435.

4.    Crabtree, R. H. et al.,  General Properties of Organometallic Complexes. John Wiley & Sons, Inc.: 2005; pp 29-52.

5.    Stenlid, J. H. et al., Journal of the American Chemical Society 2017, 139, 11012-11015.

6.    James, O. O. et al., RSC Advances 2012, 2, 7347-7366.

7.    Maitlis, P. M. et al., Chemical Communications 2009, 0, 1619-1634.

8.    Zeigarnik, A. V. et al., The Journal of Physical Chemistry B 2000, 104, 10578-10587.

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