Photocatalytic reduction of CO2 over noble metal-loaded and nitrogen-doped mesoporous TiO2

V. Jeyalakshmi07-07-2012

Xiukai Li et al., Applied Catalysis A: General 429 (2012) 31

Introduction

CO2 Photoreduction,

Titania • Photo-stable, non-toxic, ability to resist photo-corrosion, abundant availability at low cost. • Mesoporous materials possess unique pore character, larger surface area value, and generally higher catalytic activity.

Drawbacks

• Absorption of only a small fraction (<5%), UV region of the solar spectrum, in accordance with its band-gap of 3.0 to 3.2 eV. No absorption is observed in the visible region (λ > 400 nm) which constitutes major part of the solar spectrum.• Low interfacial charge-transfer rates of photo-generated charge carriers and consequently high recombination rate.

In the present study, nitrogen doped & noble metal loaded Mesoporous Titania was prepared for the application of CO2 Reduction.

20 ml of Ti(OBu)4 + aq. CTAB solution (4 wt.%, 100 ml).

Stirred at 40 ◦C for 4 h,Aged at RT for 24 h.

Slurry was thermally treated at 100 ◦C for 48 h.

white precipitateWashed with ethanol and distilled water, dried at 80 ◦C for 12 h.Calcined at 400 ◦C for 8 h

Mesoporous TiO2.Annealed at desired temp. for 4 h in a tube furnace under flowing NH3 (flow rate:100 ml min−1).

N doped mesoporous TiO2.

Purged with Ar at 400 ◦C for 1 h to remove the surface adsorbed ammonia species.

N doped mesoporous TiO2.

Noble metals (i.e., Pt, Au, and Ag) are loaded by Photodeposition Method.

Experimental Section

(a) non-doped mesoporous TiO2, and the samples nitridized at (b) 500 ◦C, (c) 525 ◦C, (d) 550 ◦C, and (e) 575 ◦C.

XRD patterns

(A) N2 adsorption–desorption isotherms and (B) the corresponding BJH pore size distribution curves of (a) non-doped mesoporous TiO2, and the samples nitridized at (b) 500 ◦C, (c) 525 ◦C, (d) 550 ◦C, and (e) 575 ◦C.

N2 adsorption–desorption isotherms

(A) The UV–vis diffuse reflectance spectra of (a) the non-doped mesoporous TiO2, and the samples nitridized at (b) 500 ◦C, (c) 525 ◦C, (d) 550 ◦C, and (e) 575 ◦C.

(B) The UV–vis diffuse reflectance spectra of (a) the unloaded mesoporous TiO2, and the samples loaded with (b) 0.2 wt.% Pt, (c) 0.2 wt.% Au, and (d) 0.2 wt.% Ag.

UV–vis diffuse reflectance spectra

The physical characteristics of the noble metal-loaded and unloaded mesoporous TiO2 samples.

The physical characteristics of the N-doped and non-doped mesoporous TiO2 samples.

a Estimated according to the Scherrer’s formula: D = (Kλ)/(βcosѲ ). λ = 0.1542 nm, K = 1.0.b Deduced from ICP analysis.

a The values in the parentheses indicate the band gap values related to N 2p state.

(a) Non-doped TiO2, (b) TiO2–N (500 ◦C), (c) TiO2–N (525 ◦C), (d) TiO2–N (550 ◦C),and (e) TiO2–N (575 ◦C).

FT-IR spectra of nitrogen-doped and non-doped mesoporous TiO2 samples.

(A) Non-doped mesoporous TiO2, (B) TiO2–N (525 ◦C), (C) and (D) 0.2 wt.% Pt/TiO2.

TEM images

XPS spectra of TiO2–N (525 ◦C). (A) Survey spectrum and (B) peak fitting ofN 1s spectrum. The spectrum was obtained after the surface of the powder wassputtered with Ar+ to remove the adsorbed contaminants.

XPS spectra

Photocatalytic reduction of CO2 to methane over the non-doped mesoporous TiO2 under full Xe-arc. (A) The effects of different noble metal loading.

Activity evaluation

Reaction conditions: Catalyst, 0.1 g; reaction temperature, 60 ± 2 ◦C; CO2:H2O = 0.06:1; irradiation intensity, 34.8 mW cm−2.

(B) The activity tests of CO2 photocatalytic reduction over 0.2 wt.%Pt/TiO2 under different conditions.

Photocatalytic reduction of CO2 to methane over 0.2 wt.% Pt-loaded TiO2 samples under UV light. (a) Mesoporous TiO2 and (b) common anatase without mesoporous structure.

Photocatalytic reduction of CO2 to methane over nitrogen-doped and nondopedTiO2 samples under visible light irradiation. 0.2 wt.% Pt was loaded as cocatalyst for all the samples.

Repeated reactions of CO2 photocatalytic reduction over 0.2 wt.% Pt loaded TiO2–N (525 ◦C) under visible light irradiation.

The schematic drawing of CO2 photoreduction to methane over Pt-loaded TiO2−x–Nx.

Conclusion

Noble metals loading generally improved the photocatalytic activity, and theefficiency follows the order: Pt > Au > Ag. Optimum loading amount of Pt was 0.2 wt.%.

The optimum nitridation temperature to achieve the best visible light activity was 525 ◦C, and the amount of doped N was 0.84% on the basis of the lattice oxygen atoms.

Methane is the major hydrocarbon product in CO2 photoreductionby means of nitrogen doped and noble metal loaded on mesoporous TiO2 samples.

The unique mesoporous structure, the optical property, and the synergism with noble metal determine the overall catalyst performance.