Nonthermal plasma (NTP) activated metal–organic frameworks (MOFs) catalyst for catalytic CO
 2
 hydrogenation

[The copyright line in this article was changed on 19 June 2020 after original online publication.] Abstract A systematic study of Ni supported on metal–organic frameworks (MOFs) catalyst (i.e., 15Ni/UiO-66) for catalytic CO2 hydrogenation under nonthermal plasma (NTP) conditions was presented. The catalyst outperformed other catalysts based on conventional supports such as ZrO2, representing highest CO2 conversion and CH4 selectivity at about 85 and 99%, respectively. We found that the turnover frequency of the NTP catalysis system (1.8 ± 0.02 s) has a nearly two-fold improvement compared with the thermal catalysis (1.0 ± 0.06 s). After 20 hr test, XPS and HRTEM characterizations confirmed the stability of the 15Ni/UiO-66 catalyst in the NTPactivated catalysis. The activation barrier for the NTP-activated catalysis was calculated as ~32 kJ mol, being lower than the activation energy of the thermal catalysis (~70 kJ mol). In situ DRIFTS characterization confirmed the formation of multiple carbonates and formates on catalyst surface activated by NTP, surpassing the control catalysts (e.g., 15Ni/α-Al2O3 and 15Ni/ZrO2).


| INTRODUCTION
Valorization of carbon dioxide (CO 2 ) by catalytic hydrogenation based on the power-to-gas (P2G) concept is a promising technique that can not only convert the renewable energy to high energy density fuels (e.g., CH 4 ), but also reduce the anthropogenic CO 2 emissions. 1,2 P2G process involves the conversion of electricity into synthetic natural gas (SNG), typically consisting of two steps: (a) hydrogen (H 2 ) production by water electrolysis (using the electricity generated from renewable sources such as wind) and (b) hydrogenation of CO 2 (captured from various industrial sources such as biogas and flue gas) into CH 4 . 3 As a proof of concept, a production of SNG (ca., 1,000 tons per year) has been achieved by the e-gas plant of Audi Motor Company located in Werlte (Germany) through utilizing the concentrated CO 2 from a nearby biogas plant and renewable hydrogen. 2 Indeed, CO 2 methanation is the crucial reaction to determine the effectiveness and efficiency of P2G processes. 4 Catalytic CO 2 methanation (i.e., Sabatier reaction, Equation 1) is thermodynamically favorable (ΔG 0 298 K = −113.5 kJ mol −1 ), however, it suffers from kinetic limitations due to the high stability of CO 2 molecule. 5,6 Normally, high temperatures between 200 and 450 C are required to enable the activation and transformation of CO 2 , depending on the catalyst and operating conditions employed. [7][8][9] The reaction is highly exothermic (Equation 1), being prone to form local hotspots which leads to the catalyst sintering and deactivation.
Generally, conventional catalysts such as the supported nickel (Ni) catalysts present low activity at temperatures below 300 C with low CO 2 conversions which cannot match the corresponding thermodynamic equilibrium value. 10 Additionally, at temperatures higher than 320 C, the reverse water gas shift (WGS) reaction can occur and, therefore, inevitably produce by-products such as CO. 11 Therefore, to overcome the limitations and issues experienced by the conventional thermal catalysis, either new catalytic systems or highly active and stable catalysts are needed to activate CO 2 molecule efficiently at relatively lower temperatures (e.g., <300 C).
Nonthermal plasma (NTP) is a promising alternative to the conventional thermal system for activating catalysts at comparatively low temperatures (<200 C), enabling various challenging reactions. 12,13 Specifically, NTP activation has been demonstrated as an efficient technique for promoting WGS and CH 4 oxidation, without an external heat source. [14][15][16] Recently, catalytic CO 2 hydrogenation was also enabled by NTP at low temperatures of <150 C. 13,17,18 For instance, hydrogenation of CO 2 to CH 4 over Ni- Although the limited stability of MOFs has been deemed to be a major issue of these materials, 25

| Catalysts evaluation for catalytic CO 2 hydrogenation under NTP conditions
The NTP-assisted CO 2 hydrogenation over Ni/UiO-66 catalysts was error. Condensable vapors (e.g., water) generated from the reaction was condensed from the outlet stream in a water trap cooled by an ice bath. The dry gas flow rate was also measured using a bubble-flow meter in order to determine the CO 2 conversion and selectivity to CH 4 and CO. Control experiments of NTP-assisted gas phase reaction (i.e., empty reactor without a catalyst) and the thermally activated reaction over the 15Ni/UiO-66 catalyst were also performed for comparison. CO 2 (X CO2 ) conversion, selectivity towards CH 4 (S CH4 ) and CO (S CO ), as well as turnover frequency (TOF) were determined accordingly to evaluate the catalytic performance, these are detailed in Supporting Information.

| In situ DRIFTS characterization of catalytic CO 2 hydrogenation
To Tables S1-S3). Figure 2 (Figure 1a,b), while CO was found to be main product ( Figure 1c) due to the NTP-induced CO 2 dissociation. 27 Figure S5, the 20Ni/ UiO-66 catalyst presented a relatively poor catalytic activity, which could be the result of the segregation of Ni particles to the surface due to the overloading of Ni. 29 Hence, the 15Ni/UiO-66 catalyst was selected for further investigation in this work.
Under the thermal condition, at least 200 C was needed to activate CO 2 over the 15Ni/UiO-66 catalyst, as evidenced by the lightoff curve in Figure 3a. At 350 C, the CO 2 conversion was about 75%, which is~15% lower than the corresponding theoretical thermodynamic equilibrium conversion (red dashed line, calculated using Aspen Plus 8.0). At least 300 C was required to enable the high selectivity to In comparison, the changes measured for the used catalyst from the NTP-catalysis are less significant. Figure 5a shows the main peak of HRTEM analysis also confirmed the effect of NTP and thermal activation on the 15Ni/UiO-66 catalyst, as shown in Figure 6. The UiO-66 MOF exhibits the characteristic octahedral morphology (Figure 6a,b). 19 After the Ni dispersion, the as-synthesized catalyst (as shown in Figure 6c,d) retains the octahedral morphology. Specifically, Ni particle sizes are determined by counting different particles for multiple TEM images taken in different sample regions. Additionally, Ni nanoparticles were formed nonuniformly on the support with large Ni particles possibly on the external surfaces of UiO-66 crystals (with the average particle size of 12 ± 2.6 nm). After the NTPactivated catalysis, as shown in Figure 6e,f, the MOF support was largely intact. Interestingly, the dispersion of Ni particles was improved significantly due to the NTP activation, showing the presence of highly dispersed small Ni particles at 4 ± 0.5 nm. In contrast, the decomposition of the MOF support, as shown in Figure 6g,h, was caused by the conventional thermal activation of the catalysis. Additionally, metal sintering was also observed after the thermal catalysis, leading to the presence of relatively large Ni particle sizes of 13 ± 1.4 nm in the used catalyst.
F I G U R E 5 X-ray photoelectron spectra spectra of (a) the fresh 15Ni/UiO-66 catalyst, (b) the used 15Ni/UiO-66 catalyst after the nonthermal plasma-activated catalytic CO 2  and CO 2 ) must firstly adsorb on the catalyst surface, and subsequently, the surface reaction between the surface dissociated H and adsorbed CO 2 proceeds via the Langmuir-Hinshelwood mechanism. 13 Therefore, the input energy of thermal system (i.e., heat) is the key factor to enable the dissociation of reactants and the following surface reaction. Therefore, the activation energy of thermal catalytic system was determined according to the Arrhenius law as a function of temperature. Conversely, unlike the catalysis under the thermal conditions, in an NTP-catalyst system, NTP-created vibrationally-excited CO 2 and dissociated H species could interact readily with the Ni active sites, leading to a reduced energy barrier (i.e., the dissociation of molecules on the catalyst surface might not be necessary). Therefore, under the NTP conditions, both the residence time of the total feed gases and the density of high-energy electron (which is determined by the input plasma power) are the key parameters in determining the apparent activation energy of NTP catalysis system. 36 Accordingly, a modified Arrhenius expression, which depends on the plasma power and total feed flow rate, was developed to extract the kinetic parameters such as the energy barrier from the NTP-activated catalysis (exemplified by dry reforming of CH 4 with CO 2 over Ni catalysts supported on γ-Al 2 O 3 ). 36 As shown in Figure 7b, the linear fit of the plot of the reaction rate of NTP-catalysis (ln r NTP-cat ) against the inverse DBD discharge power (1/power DBD ) was performed, giving E A, NTP =~32 kJ mol −1 , which is 55% lower than the counterpart of the thermal catalysis. Additionally, as shown in Figure 7c, the reaction rate of the NTP-catalysis can be improved significantly by increasing the specific input energy (SIE), indicating that the generation of vibrationally-excited gas species (CO 2 and H 2 ) could be improved by NTP activation with higher input energy. This phenomenon is in line with the finding reported by Kim et al. 36

| In situ DRIFTS characterization of catalytic CO 2 hydrogenation
The comparatively easy activation of CO 2 over the 15Ni/UiO-66 catalyst by NTP was also confirmed by in situ DRIFTS study of the NTP and thermal systems. As shown in Figure 8, under the thermal activation, the 15Ni/UiO-66 catalyst was inactive at relatively low temperatures of <150 C with the insignificant change of the surface species.
By increasing the temperature above 150 C, the intensity of the gas phase CO 2 IR band (at 2,340 cm −1 , as shown by the shaded region) decreased gradually, suggesting the conversion of CO 2 over the catalyst. Additionally, multiple surface species with IR signals at 1,000-2,000 cm −1 were observed by increasing the reaction temperature, which could be attributed to surface carbonates [37][38][39]   for NTP conditions and also the initial preliminary design, configuration and operation of NTP reactors for potential practical usages. However, the practical application of NTP-catalysis is still challenging, especially at large-scales. Therefore, future multi-/inter-disciplinary efforts (involving physics and engineering) are needed to enable the development of stable yet durable NTP reactors/processes to be developed for the practical scenarios.