Unveiling the Mechanism of Plasma-Catalyzed Oxidation of Methane to C2+ Oxygenates over Cu/UiO-66-NH2

Nonthermal plasma (NTP) offers the potential for converting CH4 with CO2 into liquid products under mild conditions, but controlling liquid selectivity and manipulating intermediate species remain significant challenges. Here, we demonstrate the effectiveness of the Cu/UiO-66-NH2 catalyst in promising the conversion of CH4 and CO2 into oxygenates within a dielectric barrier discharge NTP reactor under ambient conditions. The 10% Cu/UiO-66-NH2 catalyst achieved an impressive 53.4% overall liquid selectivity, with C2+ oxygenates accounting for ∼60.8% of the total liquid products. In situ plasma-coupled Fourier-transform infrared spectroscopy (FTIR) suggests that Cu facilitates the cleavage of surface adsorbed COOH species (*COOH), generating *CO and enabling its migration to the surface of Cu particles. This surface-bound *CO then undergoes C–C coupling and hydrogenation, leading to ethanol production. Further analysis using CO diffuse reflection FTIR and 1H nuclear magnetic resonance spectroscopy indicates that in situ generated surface *CO is more effective than gas-phase CO (g) in promoting C–C coupling and C2+ liquid formation. This work provides valuable mechanistic insights into C–C coupling and C2+ liquid production during plasma-catalytic CO2 oxidation of CH4 under ambient conditions. These findings hold broader implications for the rational design of more efficient catalysts for this reaction, paving the way for advancements in sustainable fuel and chemical production.


In situ FTIR characterization of the catalyst surface under plasma discharge
In situ Fourier-transform infrared (FTIR) spectroscopy was used to characterize the catalyst surface using a custom-designed integrated DBD/gas cell, enabling the determination of various surface reactions and key intermediate species within the plasma-catalytic reaction system.Firstly, the catalyst surface was pretreated with argon plasma for 30 min (Ar flow rate 100 mL/min, discharge power 15 W, temperature 25 °C).Afterward, the catalyst sample was exposed to a mixture of CO2 and CH4 for 30 min (total flow rate: 100 mL/min, CO2/CH4 = 1:1), and then to a mixture of CO2, CH4 and argon for 30 min at a lower flow rate to collect the background FTIR spectrum (total flow rate: 20 mL/min, CO2/CH4/Ar = 1:1:2).Under the condition of continuous flow (total flow rate: 20 mL/min, CO2/CH4/Ar = 1:1:2), the plasma was switched on for 15 min, and the FTIR spectra were collected every 3 min for 15 min.
To understand the formation of surface intermediates species under CO2 plasma exposure, the following test was conducted.The catalyst surface was firstly pretreated with argon plasma for 30 min (Ar flow rate 100 mL/min, discharge power 15 W, temperature 25 °C).After Ar plasma pretreatment, the catalyst sample was exposed to pure CO2 (total flow rate: 100 mL/min) for 30 min and then to a mixture of argon and CO2 at a lower flow rate for 30 min (total flow rate: 20 mL/min, CO2/Ar = 1:1).Under the condition of continuous flow (total flow rate: 20 mL/min, CO2/Ar = 1:1), the plasma was switched on for 15 min, and the FTIR spectra were collected every 3 min for 15 min.
To understand the reaction between CO2 and CH4 on the catalyst surface, CO2 was firstly absorbed onto the catalyst surface, and then the plasma was switched on under CH4 atmosphere.The test procedure was as follows.After Ar plasma pretreatment, the catalyst was exposed to CO2 (total flow rate: 100 mL/min) for 30 min and then to a mixture of argon and CH4 at a lower flow rate for 30 min (total flow rate: 20 mL/min, CH4/Ar = 1:1).Under the condition of continuous flow (total flow rate: 20 mL/min, CH4/Ar = 1:1), the plasma was switched on for 15 min, and the FTIR spectra were collected every 3 min for 15 min to monitor the evolution of the reaction intermediates and surface species.

Supplementary characterization of fresh catalysts
Characterization of UiO-66: Catalyst characterization confirmed the successful synthesis of UiO-66 with an octahedral structure and a particle size of ~250 nm.(b) 1800 cm -1 -1000 cm -1 .
In Figure S4, the -NH2 peak of UiO-66-NH2 originally located at ~1620 cm -1 shifts to 1650 cm -1 , 1660 cm -1 and 1670 cm -1 with increasing Cu loading from 0% to 15%, (Figure S4a and S4b).These shifts indicate a coordination interaction between Cu and the amino group on the surface of the catalyst.

Characterization of spent catalysts
Thermogravimetric analysis (TGA) was used to quantitatively determine carbon deposition on the catalysts.Since UiO-66-NH2 exhibits limited thermal stability and its framework tends to decompose at elevated temperatures, TGA measurements were conducted in both N2 and air atmospheres with four repeated experiments (80 min each) on both fresh and spent 10%Cu/UiO-66-NH2 catalysts.
In the N2 atmosphere, weight loss began around 400 °C, mainly due to the collapse of the UiO-66-NH2 framework.In contrast, in the air atmosphere, weight loss occurred between 300 °C and 400 °C, likely due to catalyst decomposition.The weight loss of the spent catalyst included both catalyst degradation and carbon deposition.
After subtracting the weight loss attributed to water desorption at 100 °C, the weight loss for fresh and spent catalysts in the N2 atmosphere was 21.1% and 24.3% between 30 and 600 °C, respectively.Similarly, the weight loss of the catalysts in air was 30.5% and 39.8%.These data indicate a carbon deposition of 6.6%, corresponding to an actual carbon deposition selectivity of 3.7%.

CO-DRIFTS characterization results
Both 10% Cu/UiO-66-NH2 and UiO-66-NH2 exhibited strong physical adsorption of CO(g), consistent with the results of in situ transmission FTIR characterization.The desorption profiles of CO showed that the physical absorption peaks of CO(g) at 2170 cm -1 of the two catalysts gradually disappeared with N2 purging.Notably, the absorption peak of 10% Cu/UiO-66-NH2 at 2119 cm -1 gradually shifted towards low wavenumber    a The Brunauer-Emmett-Teller (BET) method was used to determine the specific surface area of samples.
b The single point method (at p/p 0 = 0.99) was used to determine the total pore volume.c The BJH method was used to determine the average pore diameter.

2 .
Scheme S1.Schematic diagram of the experimental setup.
with purge time.Eventually, it stabilized at 2110 cm -1 , corresponding to the linear adsorption of Cu + -CO.In contrast, the absorption peak of UiO-66-NH2 at 2119 cm -1 completely disappeared, indicating the superior chemical adsorption capacity of Cu for *CO.

Table of contents 1 .
In situ FTIR characterization of the catalyst surface under plasma discharge 2. Schematic diagram of experimental setup and plasma reactor

Table S2 .
SBET, pore volume and pore diameter of fresh catalysts.

Table S3 .
Effect of catalysts on the conversion and total liquid selectivity.

Table S4 .
Effect of catalysts on the selectivity of gas products.

Table S5 .
Effect of catalysts on the selectivity of liquid products.

Table S8 .
Summary of conversions and liquid selectivities reported in the literature.