A Study on the Photoreduction of Atmospheric CO₂ to C₂H₄ Using Photoionisation Mass Spectrometry for Trace Analysis

Applications: CO₂ conversion and utilisation; photocatalytic synthesis; low-carbon chemical industry; ethylene production

Introduction

Ethylene is a key global chemical product, serving as the base material for 75 per cent of petrochemical products. Currently, 99 per cent of ethylene production relies on the high-temperature cracking of hydrocarbons, which is accompanied by substantial CO₂ emissions. Converting atmospheric CO₂ (0.03 per cent) into ethylene via photoreduction represents a technological pathway that combines environmental and energy benefits. The core bottleneck of this process is the difficulty in achieving the C–C coupling reaction, resulting in low selectivity for high-value-added C₂ products. This application overcomes the limitations of C–C coupling by constructing charge-asymmetric heteroatom sites through interface engineering.

Vacuum ultraviolet photoionisation mass spectrometry (SVUV-PIMS) offers the advantages of being free from fragment interference, enabling real-time detection and precise isotopic tracing. To verify the authenticity of the carbon source in C₂H₄—the product of the photoreduction of atmospheric-concentration CO₂—and to rule out interference from other carbon impurities, this study employed vacuum ultraviolet photoionisation mass spectrometry (SVUV-PIMS) for key characterisation.

Experimental Materials and Methods

1. Catalyst preparation: Pd nanoclusters were loaded onto ZnO nanosheets to prepare Pd-ZnO composite catalysts; pure ZnO nanosheets were also prepared as a control.

2. Core characterisation techniques: In addition to SVUV-PIMS, ¹H nuclear magnetic resonance (NMR) spectroscopy was employed to detect liquid carbon-based products.

3. Catalytic performance testing: Photoreduction experiments were conducted in simulated air (0.03% CO₂) and pure water environments to evaluate the C₂H₄ formation rate and product selectivity.

‌Results and Conclusions

Product tracing and catalytic performance testing

Gas chromatography (GC) analysis of the products (Figure 1d) shows that pure ZnO produces only CO, whilst Pd-ZnO achieves C₂H₄ synthesis. Through heteroatom sites engineered at the interface, the Pd-ZnO composite catalyst successfully overcame the C–C coupling bottleneck that pure ZnO could not resolve, transforming the photoreduction product of CO₂ from low-value C₁ (CO) to high-value C₂ (C₂H₄).

¹H nuclear magnetic resonance (¹H NMR) (Figure 1e) detected no liquid carbon-based products, indicating that only gaseous products such as C₂H₄, CO and CH₄ were generated, with no formation of liquid carbon-based compounds, thereby avoiding a mixture of product states. The Pd-ZnO catalyst exhibits selectivity towards the ‘gaseous C₂ product (C₂H₄)’, rather than randomly generating a variety of carbon-based products.

In the ¹³CO₂ labelling experiment using SVUV-PIMS (Figure 1f), the peaks at m/z = 29 and 30 correspond to ¹³CO and ¹³C₂H₄, confirming that the carbon in the products originates from the photoreduction of CO₂, rather than from the catalyst itself or other carbon impurities in the experimental environment. These characterisation results confirm the authenticity of the reaction driven by the Pd-ZnO catalyst converting CO₂ into C₂H₄, eliminating uncertainties regarding the origin of the products, and provide evidence for the reaction mechanism whereby heteroatom sites engineered at the interface facilitate C–C coupling. Furthermore, in conjunction with the results from ¹H NMR, which detected no liquid carbon-based products, SVUV-PIMS further confirmed that the main gas-phase products of the reaction are C₂H₄, CO and a small amount of CH₄, providing precise data support for the evaluation of catalytic performance.

Figure 1: (d) Product formation rates on ZnO and Pd-ZnO nanosheets; (e) ¹H NMR spectra of products from Pd-ZnO and ZnO nanosheets; (f) SVUV-PIMS spectra of products during the photoreduction of ¹³CO₂-labelled CO₂ on Pd-ZnO nanosheets.

The charge-asymmetric Pd-Zn heteroatom sites engineered at the interface reduce the C–C coupling energy barrier by regulating the adsorption of intermediates and reaction pathways, thereby overcoming a key bottleneck in the photoreduction of atmospheric-concentration CO₂ to produce C₂H₄. Meanwhile, the SVUV-PIMS technique, utilising ¹³CO₂ isotope labelling experiments, precisely traces the carbon source of the products and effectively eliminates interfering factors, providing core evidence for the authenticity and validity of the reaction; it serves as a crucial technical underpinning for the conclusions of this study.

References

[1] Peijin Du, Jinyu Ding, Chengyuan Liu, Peipei Li, Wenxiu Liu, Wensheng Yan, Yang Pan, Jun Hu, Junfa Zhu, Xiaodong Li*, Qingxia Chen*, Xingchen Jiao*, Interface-Engineering-Induced C−C Coupling for C₂H₄ Photosynthesis from Atmospheric-Concentration CO₂ Reduction, Angew. Chem. Int. Ed., 2025, 64, e202421353