Black indium oxide a photothermal CO2 hydrogenation catalyst

Abstract

Nanostructured forms of stoichiometric In₂O₃ are proving to be efficacious catalysts for the gas-phase hydrogenation of CO₂. These conversions can be facilitated using either heat or light; however, until now, the limited optical absorption intensity evidenced by the pale-yellow color of In₂O₃ has prevented the use of both together. To take advantage of the heat and light content of solar energy, it would be advantageous to make indium oxide black. Herein, we present a synthetic route to tune the color of In₂O₃ to pitch black by controlling its degree of non-stoichiometry. Black indium oxide comprises amorphous non-stoichiometric domains of In₂O_3-x on a core of crystalline stoichiometric In₂O₃, and has 100% selectivity towards the hydrogenation of CO₂ to CO with a turnover frequency of 2.44 s^-1.

Fig. 1. Structural and morphological information for In₂O_3−x/In₂O₃ materials S1, S2, S3, S4.

a PXRD patterns of S1−S4. b High-resolution O1s core level XPS spectrum of S4. c, d HRTEM and STEM images of S4 at different magnifications. A dashed green circle indicates an amorphous phase, yellow arrow indicates the measured lattice spacing and the red square indicates the imaged position.

Fig. 2. In situ high-resolution environmental transmission electron microscopy (HRETEM) observations of the In₂O₃ + xH₂ → In₂O_3−x + xH₂O process.

High-resolution images of a stoichiometric In₂O₃ nanocrystal (S1) at 400 °C a under an N₂ atmosphere and then switched to an H₂ atmosphere for the following times: b 5 min, c 10 min and d 20 min. Scale bars are the same for all images, green squares indicate the formation of an amorphous phase. Graphical representation of the e original and f treated In₂O₃, wherein blue region, pink dots, yellow dots and yellow circles represent amorphous phase, In atoms, O atoms and [O], respectively.

Fig. 3. Photocatalytic evaluation of black indium oxide.

a Photocatalytic CO₂ hydrogenation in a batch reactor. Conditions: H₂/CO₂ ratio = 1:1, light intensity = ~20 suns, no external heating and measurement time = 30 min. b Catalytic performance for S4 in a flow reactor at different temperatures, both with and without light irradiation; inset is the enlarged view of the catalytic performance for 200, 225 and 250 °C. c Stability test for S4 in a flow reactor at 300 °C with light irradiation for 70 h. Conditions for flow measurement: atmospheric pressure, H₂/CO₂ ratio = 1:1 with a flow rate of 1 mL min⁻¹ and light intensity of ~8 suns.

Fig. 4. Characterization of electronic properties and surface H₂ and H₂-CO₂ chemistry of the samples S1 and S4.

a Photocurrent saturation and decay plot acquired at ~200 °C with a 1:1 ratio of CO₂/H₂ and under a 100 W LED white lamp. b Corresponding in situ I−V plot. In situ DRIFTS spectra of S4 obtained c under H₂ at room temperature and d under both H₂ and CO₂ (1:1) with increased temperatures. The collected DRIFTS spectra are subtracted by the background signal of S4 obtained under He.

Fig. 5. Illustration of the electronic band structure.

The In₂O_3−x/In₂O₃ heterostructure showing the In(III)′, [O] electron-trapping and O′ hole-trapping mid-gap energy states near the CB and VB edges, respectively. Included also in the diagram is the outcome of photoexcitation and relaxation of electrons and holes involving valence, conduction and mid-gap energy states.