Role of Oxygen Vacancies in Catalytic SO₃ Decomposition over Cu₂V₂O₇ in Solar Thermochemical Water Splitting Cycles

We report the structure–activity relationship of copper pyrovanadate (Cu₂V₂O₇) as an efficient catalyst for SO₃ decomposition in solar thermochemical water splitting cycles. Of the α, β, and γ polymorphs of Cu₂V₂O₇, the α-phase, which has a blossite-type structure, was stable under the catalytic reaction conditions. Spontaneous oxygen desorption accompanied by charge compensation through the reduction of Cu²⁺ to Cu⁺ produced an oxygen deficiency corresponding to Cu₁₆V₁₆O₅₅ at 600 °C. Density functional theory calculations based on these results showed that oxygen vacancy formation is more favorable on the Cu–O–V bridging sites than on the V–O–V site in the pyrovanadate unit. The oxygen vacancy formation energy of the (100) surface is considerably less than that of bulk Cu₁₆V₁₆O₅₆. The reaction, Cu₁₆V₁₆O₅₅ + SO₃ → Cu₁₆V₁₆O₅₆ + SO₂, is exothermic, suggesting that oxygen vacancies play a key role in catalytic SO₃ decomposition over a Cu₂V₂O₇ catalyst