Al Promotion of In2O3 for CO2 Hydrogenation to Methanol

Abstract

In₂O₃ is a promising catalyst for the hydrogenation of CO₂ to methanol, relevant to renewable energy storage in chemicals. Herein, we investigated the promoting role of Al on In₂O₃ using flame spray pyrolysis to prepare a series of In₂O₃-Al₂O₃ samples in a single step (0-20 mol % Al). Al promoted the methanol yield, with an optimum being observed at an Al content of 5 mol %. Extensive characterization showed that Al can dope into the In₂O₃ lattice (maximum ∼ 1.2 mol %), leading to the formation of more oxygen vacancies involved in CO₂ adsorption and methanol formation. The rest of Al is present as small Al₂O₃ domains at the In₂O₃ surface, blocking the active sites for CO₂ hydrogenation and contributing to higher CO selectivity. At higher Al content (≥10 mol % Al), the particle size of In₂O₃ decreases due to the stabilizing effect of Al₂O₃. Nevertheless, these smaller particles are prone to sintering during CO₂ hydrogenation since they appear to be more easily reduced. These findings show subtle effects of a structural promoter such as Al on the reducibility and texture of In₂O₃ as a CO₂ hydrogenation catalyst.

Figure 1

XRD patterns of the as-prepared (a) In₂O₃, (b) 98In₂O₃-2Al₂O₃, (c) 95In₂O₃-5Al₂O₃, (d) 95In₂O₃-5Al₂O₃-IM, (e) 90In₂O₃-10Al₂O₃, and (f) 80In₂O₃-20Al₂O₃ samples.

Figure 2

TEM images of the as-prepared In₂O₃–Al₂O₃ samples.

Figure 3

²⁷Al NMR of as-prepared In₂O₃–Al₂O₃ samples with different Al content and Al₂O₃.

Figure 4

²⁷Al MQMAS NMR patterns of the as-prepared (a) 95In₂O₃-5Al₂O₃, (b) 80In₂O₃-20Al₂O₃, and (c) 95In₂O₃-5Al₂O₃-IM samples.

Figure 5

H₂-TPR profiles of the In₂O₃–Al₂O₃ samples.

Figure 6

XPS spectra of the (a) In 3d and (b) O 1s regions of the as-prepared In₂O₃ and Al-doped In₂O₃ samples.

Figure 7

(a) CO₂ conversion and methanol selectivity. (b) Space-time yield of methanol as a function of the Al content on In₂O₃. Reaction conditions: 260 °C, 3.0 MPa, GHSV = 30000 mL·h^–1·g^–1 with a feed (H₂/CO₂/N₂ = 30/10/10) flow rate of 50 mL/min.

Figure 8

XRD patterns of used (a) In₂O₃, (b) 98In₂O₃-2Al₂O₃, (c) 95In₂O₃-5Al₂O₃, (d) 95In₂O₃-5Al₂O₃-IM, (e) 90In₂O₃-10Al₂O₃, and (f) 80In₂O₃-20Al₂O₃ samples.

Figure 9

HAADF images and STEM-EDX maps of used (a), (b) 95In₂O₃-5Al₂O₃, and (c), (d) 80In₂O₃-20Al₂O₃ samples (In red, Al green).

Figure 10

²⁷Al NMR spectra of the used In₂O₃–Al₂O₃ samples.

Figure 11

(a) CO₂-TPD profiles of In₂O₃–Al₂O₃ catalysts with different Al content and Al₂O₃. (b) The relation between methanol space-time yield (STY) over the In₂O₃–Al₂O₃ catalysts during CO₂ hydrogenation (Figure 7) and the amount of CO₂ desorbed, which was obtained by quantifying the TPD desorption peak in the range of 200 to 350 °C (Table S1).

Figure 12

IR spectra during CO₂ hydrogenation over (a) In₂O₃, (c) 95In₂O₃-5Al₂O₃, and (e) 80In₂O₃-20Al₂O₃. IR spectra after switching the reaction gas to He (1 bar) at 50 °C and rising temperature from 50 to 400 °C over (b) In₂O₃, (d) 95In₂O₃-5Al₂O₃, and (f) 80In₂O₃-20Al₂O₃. (g) Comparison of the peak area of HCOO* and CH₃O* collected from the spectra at 260 °C under a reaction atmosphere for 60 min over In₂O₃, 95In₂O₃-5Al₂O₃, and 80In₂O₃-20Al₂O₃. (h) Evolution of the CH₃O*/HCOO* ratio for In₂O₃, 95In₂O₃-5Al₂O₃, and 80In₂O₃-20Al₂O₃. Reaction conditions: CO₂:H₂ = 1:3, gas flow rate = 50 mL/min, and P = 10 bar. The intensity of the IR spectra is normalized by the weight of the catalyst pellet.

Scheme 1. Schematic Representation of the Evolution of the Bulk and Surface Structure of In₂O₃ upon Introduction of Al by FSP and Impregnation

The arrows qualitatively indicate the rate of the two main reactions, namely CO₂ hydrogenation to methanol (blue) and the reverse water–gas shift reaction (orange).