Fig. 4: Reaction product of CO₂ with hydrogen boride (HB) upon cycling.

a Amount of C₂H₆ produced upon cycling. The labels identify the reactants supplied to the system in each cycle. Before the measurements, the HB sample was heated at 523 K for 6 h in Ar to create hydrogen vacancies in the HB sheets (100 mg). For each cycle, the gas was changed to CO₂ or Ar at 300 K, and the HB sheets were heated at 373 K in a flow of CO₂ or Ar. In the second, fifth, and sixth cycles, when the temperature reached 373 K, 0.1 cm³ of H₂O was added to the system using a syringe. Thereafter, the reactor was closed, the gas flow was stopped, and the temperature was increased to 523 K. The sample was held at 523 K for 6 h. b, c Mass patterns of detected gas and standard gas. The reaction products were analyzed using gas chromatography-mass spectrometry (GC-MS). C₂H₆ was verified by comparing its GC-MS signal (b) with that of standard gas of 100% C₂H₆ (c). d–f Amounts of (d) H₂, (e) CH₄, and (f) C₂H₆ produced upon cycling by continuously using the same HB sample (100 mg). The atmospheres used for each cycle (6 h) were as follows. At 423 K: cycles 1–7, Ar gas; cycle 8, CO₂ gas; cycles 9 and 10, CO₂ + 0.1 cm³ H₂O. At 473, 523, 573, and again 523 K: cycles 11, 14, 17, and 20, Ar flow; cycles 12, 15, 18, and 21: Ar gas; cycles 13, 16, 19, and 22, CO₂ + 0.1 cm³ H₂O. The experiments were performed from left to right along the horizontal axis.