Hydrogen boride (HB) sheets are metal-free two-dimensional materials comprising boron and hydrogen in a 1:1 stoichiometric ratio. In spite of the several advancements, the fundamental interactions between HB sheets and discrete molecules remain unclear. Here, we report the adsorption of CO₂ and its conversion to CH₄ and C₂H₆ using hydrogen-deficient HB sheets. Although fresh HB sheets did not adsorb CO₂, hydrogen-deficient HB sheets reproducibly physisorbed CO₂ at 297 K. The adsorption followed the Langmuir model with a saturation coverage of 2.4 × 10⁻⁴ mol g⁻¹ and a heat of adsorption of approximately 20 kJ mol⁻¹, which was supported by density functional theory calculations. When heated in a CO₂ atmosphere, hydrogen-deficient HB began reacting with CO₂ at 423 K. The detection of CH₄ and C₂H₆ as CO₂ reaction products in a moist atmosphere indicated that hydrogen-deficient HB promotes C–C coupling and CO₂ conversion reactions. Our findings highlight the application potential of HB sheets as catalysts for CO₂ conversion.