Wenbin
Li‡
ad,
Junhao
Ding‡
b,
Xiao
Chen
c,
You
Wang
d,
Xu
Song
*b and
Sai
Zhang
*ad
aResearch & Development Institute of Northwestern Polytechnical University in Shenzhen, Shenzhen 518057, China. E-mail: zhangsai1112@nwpu.edu.cn
bDepartment of Mechanical and Automation Engineering, Chinese University of Hong Kong, Shatin, Hong Kong 999077, China. E-mail: xsong@mae.cuhk.edu.hk
cSchool of Materials Science and Engineering, North China University of Water Resources and Electric Power, Zhengzhou 450045, China
dSchool of Chemistry and Chemical Engineering, Northwestern Polytechnical University, Xian 710072, China
First published on 18th November 2023
The efficient enhancement of mass and heat transfer, as well as mechanical stability, has attracted particular interest for fixed-bed reactors in practical applications. Herein, a monolithic CuNi alloy catalyst with ordered microchannels composed of a triply periodic minimal surface (TPMS) was designed and fabricated using three-dimensional (3D) printing technology, which boosted the highly efficient and robust reverse water-gas shift (RWGS) reaction. The unique TPMS lattice structure enabled the monolithic CuNi catalyst to enhance mass and heat transfer efficiencies, resulting in a significantly improved catalytic performance for the RWGS reaction compared with the monolithic catalyst with a honeycomb structure or the traditional CuNi/Al2O3 catalyst. Furthermore, the 3D-printed monolithic CuNi catalyst exhibited excellent catalytic and mechanical stability at high reaction temperatures. The simple and cost-effective fabrication of conductive metal catalysts with tunable 3D multichannel architectures opens new opportunities in developing heterogeneous catalysts for fixed-bed reactors.
To solve the aforementioned problem, a monolithic catalyst with ordered microchannels is considered a potential choice.8–12 Benefiting from the submillimetre dimensions, ordered microchannels facilitate both heat and mass transport, enabling reactions to occur faster, more selectively, and with greater energy efficiency in a confined space.7,13–15 Meanwhile, the integrated structural features further ensure heat transfer through the catalyst itself, thereby enhancing the efficiency of heat transfer. In addition, monolithic catalysts generally exhibit significantly enhanced mechanical strength compared to classical catalysts (such as powders, pellets, and extrudates).9,16 Therefore, a monolithic catalyst with a honeycomb structure is the most common one with ordered microchannels.17–20 However, these straight microchannels can still be considerably improved in terms of mass and heat transfer, as well as mechanical strength, via the design of geometric structures using manufacturing technology.
Triply periodic minimal surface (TPMS)-based lattice structures have received considerable attention in various industrial sectors owing to their unique geometric structure of smooth surfaces with zero mean curvature and periodic distribution in three perpendicular directions.14,21–23 Compared with the honeycomb structure, these surfaces can split the space into two interlocked channels through a smooth transition rather than an angular transition.24,25 The smooth and ordered microchannels with a TPMS can decrease the pressure drop and enhance the heat transfer effects and flow efficiency, thereby providing significant potential for the design of monolithic fixed-bed catalysts. In addition, the exceptional mechanical strength derived from the unique geometry of a TPMS can effectively improve the structural stability of a catalyst during actual operation.23,26
Herein, we have fabricated a monolithic CuNi alloy catalyst with ordered microchannels, composed of a typical TPMS, using a three-dimensional (3D) printing technology known as selective laser melting (SLM) to investigate the influence of geometrical structure on the catalytic performance of reverse water-gas shift (RWGS) reactions. The intrinsic catalytic performance of CuNi alloys has been extensively demonstrated to surpass that of their respective pure metals in various reactions.27–30 Also, the RWGS reaction as a model reaction can directly convert CO2 into CO to further synthesise various value-added chemicals.31–34 Combined with computational fluid dynamics (CFD) for gas flow behaviours, we demonstrated that the monolithic catalyst with a TPMS architecture exhibits significantly improved mass and heat transfer compared with the monolithic catalyst with a honeycomb structures. Benefiting from the TPMS-based architectures, the monolithic CuNi alloy catalyst yielded a CO generation rate with a turnover frequency (TOF) of 277 min−1 (based on each exposed active site) at 500 °C, which was 6.4 times higher than that of the CuNi nanoparticle supported on Al2O3 (43 min−1). Additionally, the monolithic CuNi alloy catalyst exhibited remarkable catalytic and mechanical stability even under high temperature conditions of up to 500 °C.
Parameters | Honeycomb | DL3-RD20 | DL2-RD20 |
---|---|---|---|
Surface area (mm2) | 1336.1 | 957.1 | 1395.6 |
Relative density | 20.5% | 20.4% | 20.6% |
Porosity | 79.5% | 79.6% | 79.3% |
Solid volume (mm3) | 73.1 | 72.8 | 73.6 |
Channel diameter (mm) | 1 | 1 | 0.6 |
Wall thickness (μm) | 110 | 150 | 105 |
Weight (mg) | 954 | 957 | 704 |
Subsequently, velocity magnitude fields were employed to show the flow patterns in different structures. The inlet velocity was set as 0.014 m s−1, corresponding to the volume speed of 20 mL min−1. As air flowed independently through the separated channels in the honeycomb catalyst, the velocities and flow directions remained almost unchanged (Fig. 1c). The maximum speed of 0.04 m s−1 is achieved for the honeycomb catalyst with a turbulence intensity of 0.037%. In contrast, the velocity-field pattern in the TPMS architecture significantly differed from that in the honeycomb structure (Fig. 1a). Particularly, both the velocity magnitude and trends fluctuated periodically in the TPMS lattice structure, which was directly related to the continuously changing cross-sectional areas. Owing to the interconnected and curved flow channels, the air was repeatedly mixed and redistributed, resulting in periodic acceleration and deceleration. Highly curved streamlines show intense recirculation in the turning regions. Although the turbulence intensity of 0.036 in DL3-RD20 is similar to that in the honeycomb catalyst, the discrete spots with a maximum speed increase to 0.06 m s−1, which is 3.3 times higher than the initial value (0.014 m s−1) at the inlet. Furthermore, the strong helical-motion streamwise flow can be further demonstrated from the view in the longitudinal direction, which squeezes the boundary layer in the channels. Therefore, catalysts with a TPMS structure can cause violent turbulent flow, enhancing mass transport for the catalytic reaction.
When the unit cell size of the TPMS architecture is decreased, the fluctuation frequency is further improved in the DL2-RD20 catalyst (Fig. 1b). Compared with the DL3-RD20 catalyst, the turbulence intensity for DL2-RD20 enhances significantly from 0.036% to 0.15% with a maximum velocity of 0.09 m s−1. The further enhanced flow state results in a higher mass transfer efficiency. Therefore, the DL2-RD20 catalyst, featuring a small unit cell size of the TPMS structure, can transform the random and uncontrollable flow state into a controlled periodic turbulent flow, thereby significantly enhancing mass transfer in a traditional fixed-bed reactor.
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Fig. 2 Heat transfer of various monolithic catalysts. Temperature distribution along the longitudinal direction when gases flow through different catalysts. |
The microscopic components of the printed catalyst were analysed by X-ray diffraction (XRD). As observed in XRD patterns, the three catalysts exhibited the same single-phase solid solution (Fig. 3c). Also, the peaks at 2θ of 43.7, 50.8, and 74.6° corresponded to the (111), (200), and (220) lattice faces of the FCC crystal structure, respectively. Compared to standard copper (PDF # 04-0836) and nickel (PDF # 04-0850), the shift in the XRD peaks of the as-printed catalyst suggested the formation of CuNi alloys.37,38
Moreover, X-ray photoelectron spectroscopy (XPS) analysis of the as-printed catalyst further indicated the co-existence of Cu and Ni (Fig. S4a†). However, owing to exposure to air, both the surface Cu and Ni elements were partially oxidised, as shown by the analysis of the Cu 3p and Ni 3p peaks (Fig. S3b and c†).
Subsequently, scanning electron microscopy (SEM) was used to observe the microscopic morphologies of the as-printed catalysts. As shown in Fig. 3d, the DL2-RD20 catalyst exhibits typical microchannels, similar to the theoretically constructed geometric channel. The DL3-RD20 and honeycomb catalysts also exhibited the designed geometric channels (Fig. S5†). Energy-dispersive X-ray spectroscopy (EDS) further revealed that the surface mass ratio of Cu:
Ni was close to 1
:
1 (Fig. S6a†), which was maintained on the surface of the catalysts via the line sweep analysis of the EDS data (Fig. S6b†). These results corroborated the formation of a uniform Cu–Ni alloy for the as-printed catalysts.
Kinetic experiments were performed to confirm the structure-determined catalytic performance in the RWGS reaction. The CO generation rate constant (k) of each catalyst was obtained based on kinetic experiments at 250–400 °C (Fig. S7†, note: the conversions of CO2 were lower than 25%). As shown in Fig. 4c, all catalysts exhibited good linearity between lnk and 1/T, and the corresponding slope of the plot yields the activation energy (Ea). It was seen that the Ea of the RWGS reaction was in the range of 54.1–59.2 kJ mol−1 (Fig. 4d), which implied that the Ea value was not very sensitive to the catalyst structure. The approximately unchanged Ea for various catalysts revealed that the active sites in DL2-RD20, DL3-RD20, and honeycomb are similar owing to their same fabrication process with the same print instrument. After excluding the possible influence of the microstructure, these kinetic results further suggest that the improved catalytic performance of DL2-RD20 was derived from the enhanced efficiency of heat and mass transfer via violent turbulent flow in the TPMS structure.
Subsequently, commercial Al2O3-supported CuNi nanoparticles (CuNi/Al2O3) were also prepared to further investigate the catalytic performance of DL2-RD20. The metal loading on Al2O3 was 3 wt% with a Cu to Ni molar ratio of 1:
1. In addition, the clear metal nanoparticles with an average size of 3.2 ± 0.7 nm could be observed from TEM images (Fig. S8†). Under identical reaction conditions, the DL2-RD20 catalyst afforded similar CO2 conversion with 100 mg of traditional CuNi/Al2O3 catalysts (Fig. 5a). To further highlight the advantages of DL2-RD20 for the RWGS reaction, the TOF values based on each of the explored Cu and Ni atoms were calculated. The total surface metal atom of DL2-RD20 was measured as 0.5 μmol g−1 by CO chemisorption. However, the dispersion of the CuNi nanoparticles was 3.0% for the CuNi/Al2O3 catalysts. At 350 °C, the calculated TOF value based on each exposed Cu and Ni atom for the DL2-RD20 catalyst was as high as 77 min−1 (Fig. 5c), which was 12.8 times higher than the value for the CuNi/Al2O3 catalyst (6 min−1). When the reaction temperature was raised to enhance the mass transfer, the conversion of CO2 for CuNi/Al2O3 was higher than that for DL2-RD20 at 500 °C. However, the TOF value of 43 min−1 was still 6.4 times lower than that for DL2-RD20 (277 min−1). In addition, the TOF value of the DL2-RD20 catalyst was also higher than that of previous supported metal catalysts (Table S1†), thereby further revealing its excellent catalytic performance for the RWGS reactions.
Meanwhile, the DL2-RD20 catalyst exhibited a higher selectivity for CO than that of the CuNi/Al2O3 catalyst, especially in the case of relatively high CO2 conversion at high reaction temperature (Fig. 5a). Considering the RWGS reaction at 500 °C as an example, the CuNi/Al2O3 catalyst yielded 79.7% selectivity for CO, similar to the previously reported CuNi catalysts.39–42 Notably, with a similar CO2 conversion, 93.5% selectivity for CO was obtained by DL2-RD20 at 500 °C. A more efficient mass transfer for the DL2-RD20 catalysts could promote the desorption of CO from the catalyst surface, improving CO selectivity.43,44 Consequently, the CO yield of DL2-RD20 was close to the thermodynamic equilibrium yield of the RWGS reaction at this temperature (Fig. 5b).
Catalytic stability is another important criterion for the evaluation of the current catalysts, especially at relatively high reaction temperatures.45,46 The catalytic stability of DL2-RD20 and CuNi/Al2O3 catalysts was investigated at a relatively high temperature of 500 °C using the H2:
CO2 ratio of 3
:
1 and flow velocity of 20 mL min−1. As shown in Fig. 5d, the conversion of CO2 catalysed by CuNi/Al2O3 decreased from 50.1% to 47.7% after 10 h, suggesting poor catalytic stability. In contrast to CuNi/Al2O3 and the previously reported supported metal catalysts,32,40,47,48 the conversion of CO2 catalysed by DL2-RD20 did not decrease but slowly increased from the initial 40.0% to the final 47.3% after 100 h. The enhanced activity of the DL2-RD20 catalysts can be attributed to the formation of a more uniform CuNi alloy through the reconstruction of the surface atoms at high reaction temperatures. The selectivity of CO was maintained in the range of 93.4–94.6%. After the stability test, the TPMS structural features remained unchanged as is evident from the optical photographs of the spent DL2-RD20 catalysts (Fig. S9†). The DL2-RD20 catalyst exhibits a microscopic morphology and surface structure similar to those of the as-printed catalysts. Therefore, the slightly increased conversion of CO2 and maintained selectivity for CO, as well as the unchanged macroscopic and microscopic structural features revealed the excellent catalytic stability of the DL2-RD20 catalysts for the RWGS reaction at high temperatures.
This high strength in the vertical direction was mainly attributed to the full membrane stress states, whereas the stress state became bending-dominated when the compression was performed in the horizontal direction, showing a low strength with a smooth curve. By continuously increasing the loading in the vertical direction of the honeycomb, localised plastic buckling occurred, which was associated with a sudden decrease in stress. Compared with the honeycomb catalysts, the DL2-RD20 catalysts exhibit similar and stable structures in various directions. The smooth curves without any drastic stress dropped further demonstrating the suppression of localised deformation, such as layer-by-layer collapse, shear band, or fracture, during the entire compression process (Fig. 6c). This uniform deformation behaviour indicates the excellent ductility and mechanical stability of the DL2-RD20 catalysts at room temperature.
We investigated the mechanical performance of DL2-RD20 at high temperatures. Under hot compression, the stress–strain curve remained smooth with a minor decrease in strength (Fig. 6d), indicating that the deformation behaviour of the catalyst did not change significantly. When the temperature increased from 25 to 400 °C, the yield strength decreased only by 18.9% (Fig. 6e), suggesting the high thermal stability and mechanical robustness of the catalysts. Subsequently, the specific strength of DL2-RD20 was comparable with that reported previously. As shown in Fig. 6f and Table S2,† all the previously reported architected structures exhibited either brittle failure with relatively low compressive strains or ductile failure with low strength. These results demonstrate the capability of the 3D-printed catalyst with tunable geometries and an exceptional combination of strength and ductility.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ta05845j |
‡ These authors contributed equally: Wenbin Li and Junhao Ding. |
This journal is © The Royal Society of Chemistry 2024 |