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

Nickel-manganese bimetallic catalysts for selective hydrogenation of CO_x 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.

 

Catalyst	Reactant	Activation Energy value (kJ/mol)
Ni:Mn 12:0	CO	98.3
Ni:Mn 8:4		93.3
Ni:Mn 4:8		113
Ni:Mn 12:0	CO2	80.3
Ni:Mn 8:4		75.7
Ni:Mn 4:8		99.4
Ni:Mn 12:0	C2H6	151
Ni:Mn 8:4		161
Ni:Mn 4:8		169

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 CO_x 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 N_xO_y species such as NO₂ and NO^2- surface compounds². It has been reported that mononitrosyl in the form of NO^d+ coordination (linear geometry) is basically an electron donor to an electron accepting site, whereas the form of NO^d- 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 V_s(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 V_s(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 (V_s,min) and maxima (V_s,max) of the surface electrostatic potential V_s(r). The V_s,min and V_s,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 V_s,min values can act as electron donors and the ones with V_s,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 V_s(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 CO_x 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 C_xH_y 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 (C_xH_y 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 CO_x 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 CO_x 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.

 

 

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