Bromine-Sensitized Solar Photolysis of CO₂

Abstract

Direct photochemical reduction of CO(2) has generally been accomplished by using transition-metal compounds as electron transfer reagents. Here, we show that elemental bromine can function as an alternative photosensitizer. When sunlight is tightly focused on mixtures of CO(2) and Br(2), in the presence of a polar adsorbent such as silica gel, glass wool, alumina, or titania, a metastable red adduct is formed within seconds and concentrates at the point of illumination. Further illumination causes deposition of a stable black film on the polar adsorbent. Mass spectrometry of the cold-trapped red intermediate shows clusters of peaks corresponding to the expected distribution of isotopomers of C(2)O(4)Br(4)(+), as well as of C(2)O(4)Br(3)(+). DFT computations indicate that the lowest-energy species with the formula C(2)O(4)Br(4) is trans-2,4-dibromo-2,4-dihypobromo-1,3-dioxetane. Formation of this molecule from (2CO(2) + 2Br(2)) would require a minimum of 3 visible photons, two of which would hypothetically be used in formation of as-yet undetected CO(2)Br(2) and the third, in a subsequent photodimerization. By elemental analysis, the final amorphous solid product contains a C/Br atomic ratio >12, suggesting that Br(2) is acting photocatalytically. Even with a poorly optimized optical system, the reaction rate has reached as high as 1.6 mg reduced C with 40 s of solar collection using a 30 cm diameter paraboloid reflector. This rate is consistent with the storage of approximately 1% of incident solar energy.

Computations on the Primary Photoreaction of Br2 with CO2: Stepwise vs Concerted Addition of Br Atoms

“…The hypothetical primary photoproduct, Br(CO)­OBr, is also analogous to a known molecule, F(CO)­OF, that has been reported to be stable indefinitely at ambient temperatures. , Although Br(CO)­OBr is itself explicitly predicted to dissociate spontaneously back to CO 2 and Br 2 , we hypothesize that it has a sufficient lifetime to permit it to participate in further photochemical reactions, such as dimerization to the previously observed C 2 O 4 Br 4 …”

“…17,18 Although Br(CO)OBr is itself explicitly predicted to dissociate spontaneously back to CO 2 and Br 2 , we hypothesize that it has a sufficient lifetime to permit it to participate in further photochemical reactions, such as dimerization to the previously observed C 2 O 4 Br 4 . 3 Nevertheless, addition of X 2 across a CO bond has not been reported previously, either thermally or photochemically. We have made initial attempts at using DFT theory to simulate relaxation along the excited-state energy surface, by optimizing the geometry of the lowest singlet excited state starting from 7c,d or 7e.…”

“…An alternative photoreduction scheme was recently devised in which sunlight is used to produce solid (elemental) carbon from CO 2 , using inexpensive Br 2 as a photosensitizer, along with a polar adsorbent (such as silica gel, alumina, or glass wool) that serves as a requisite surface for some type of heterogeneous catalysis. 3 The mechanism of this Br 2 -sensitized solar photolysis of CO 2 is likely to be complicated, requiring multiple visible/UV photons per carbon atom in order to supply the requisite ∼400 kJ mol −1 of energy.…”

“…Initial mass spectrometry experiments on a metastable red photoproduct indicated that it has the molecular formula C 2 O 4 Br 4 , suggesting a mechanism in which there is a precursor photoproduct, as-yet unisolated, with the formula CO 2 Br 2 . 3 Bulk water is not required, unlike in photocatalytic processes in which Ti compounds have been used. 1,2 This feature may make Br 2 -sensitized photolysis of CO 2 more suitable for solar energy capture in desert regions, where it would not have to compete with agriculture for water.…”