High rate CO2 photoreduction using flame annealed TiO2 nanotubes

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This document describes a study that investigated the use of flame annealed titanium dioxide (TiO2) nanotubes for high-rate photoreduction of carbon dioxide (CO2). Flame annealing improved the crystallinity, phase composition, and light absorption properties of the TiO2 nanotubes. Photocatalytic testing showed that flame annealed TiO2 nanotubes produced methane (CH4) from CO2 photoreduction at higher rates than regular TiO2 nanotubes. A mechanism is proposed to explain the enhanced activity of the flame annealed nanotubes based on their material properties.

High rate CO2 photoreduction using flame annealed TiO2
nanotubes

Fig. 1. Schematic of synthesis of square shaped flame annealed
TiO2 nanotubes

Fig. 2. Cross-section FESEM images of LANT-eg (a), FANT-eg (b), LANT-aq (c) and
FANT-aq (d). Insets are top-view FESEM images of the same.

Fig. 3. HR-TEM images
showing fringe patterns
and 2d spacing of FANT-
eg (a), FANT-aq (b),
LANT-eg (e) and LANT-aq
(f); SAED patterns of
FANT-eg (c), FANT-aq (d),
LANT-eg and (g) LANT-aq
(h).

Fig. 4. (a) Glancing angle XRD patterns of
FANT-eg, LANT-eg, FANT-aq, and LANT-
aq (b) Glancing angle XRD pattern of
FANT-eg specifically, highlighting the
mixed phase nature by separating the
signal from each phase.

Fig. 5. Raman spectra of:
(a) FANT-eg, and LANT-
eg; and (b) FANT-aq, and
LANT-aq.

Fig. 6. UV–vis spectra of the nanotubes.

Fig. 7. High resolution XPS spectra of FANT-eg, LANT-eg, FANT-aq and LANT-aq in
(a–d) Ti 2p region and (e–h) in O 1s region respectively.

Fig. 8. (a) FTIR spectra of gas
sample generated by photoreduction
of 13CO2 using FANT-eg. Shifts to
lower wavenumbers (between 7 and
10 cm−1) of peaks in v3 bands of the
gas sample, from 12CH4 standard,
confirms presence of 13CH4 as a
product. (b) GS MS ion-
chromatogram at m/z = 17,
confirming presence of 13CH4 by
photoreduction of 13CO2.

Fig. 9. Poynting vector plots for (a) LANT and (b) FANT-eg for λ = 450 nm.
Electric field intensity plots for (c) LANT and (d) FANT-eg for λ = 400 nm.

Fig. 10. Plausible mechanism of photoreduction of CO2 by mixed phase
FANT-eg nanotubes. The mechanism is arrived at by closely considering
pertinent reactions and material properties reported in the literature
[98,103,113].

Figure S1. (a) Lorentz fits of anatase (101) and rutile (110) XRD peaks, from which
the peak widths at half of maximum intensities were obtained. Here: (a), (b), (c) and
(d) are for LANT-aq, FANT-aq, LANT-eg and FANT-eg, respectively.

Figure S3. Fluorescence lifetime
imaging lifetime decay plots FLIM
decay plots showing fluorescence
lifetime distributions for the different
nanotube types. Flame annealed TiO2
nanotubes (FANT-eg and FANT-aq)
have longer lifetimes than regular
nanotubes (LANT-eg and LANT-aq).

Figure S4. 2-photon fluorescence images of the nanotubes at excitation wavelength
of 750 nm. Emission was between 394 and 750 nm. Flame annealed TiO2 nanotubes
(FANT-eg and FANT-aq) have a brighter emission than regular nanotubes (LANT-eg
and LANT-aq).

Figure S5. XPS survey scan of FANT-eg (red), LANT-eg (navy blue), FANT-aq
(purple) and LANT-aq (black).

$Figure S6. X band EPR spectra of FANT-eg sample recorded at 100 K under dark The Insets show magnified view of regions and their respective g values. The signal intensity was not optimal (loss of signal occurred) due to the following reasons: (i) The spectra were collected in the presence of O2 and (ii) At a temperature of 100 K, electrons have enough energy to exchange vacancy site which may lead to pairing of some fraction of unpaired electrons resulting in reduced signal intensity.$

Figure S7. Plots of CH4 yield (in terms of µmol g catalyst–1.hr–1) for FANT-eg and LANT-
eg, using LEDs of different wavelengths (400, 450, 500 and 600 nm). LED lights were
operated with a power density of 10 mW cm-2 and the photocatalytic reactions were
performed for 2 hours

Figure S8. CH4 yield (in terms of µmol cm-2
illuminated. area) of CO2 photoreduction using
FANT-aq and LANT-aq under different conditions of illumination. Experiments were
performed in presence of water vapor at 80 ºC.

Figure S9. Plots of the absorption and
scattering intensity of single nanotubes for (a)
LANT (b) FANT-eg and (c) FANT-aq.

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Multifunctional carbon nitride nanoarchitectures for catalysis

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Catalysis is at the heart of modern-day chemical and pharmaceutical industries, and there is an urgent demand to develop metal-free, high surface area, and efficient catalysts in a scalable, reproducible and economic manner. Amongst the ever-expanding two-dimensional materials family, carbon nitride (CN) has emerged as the most researched material for catalytic applications due to its unique molecular structure with tunable visible range band gap, surface defects, basic sites, and nitrogen functionalities. These properties also endow it with anchoring capability with a large number of catalytically active sites and provide opportunities for doping, hybridization, sensitization, etc. To make considerable progress in the use of CN as a highly effective catalyst for various applications, it is critical to have an in-depth understanding of its synthesis, structure and surface sites. The present review provides an overview of the recent advances in synthetic approaches of CN, its physicochemical properties, and band gap engineering, with a focus on its exclusive usage in a variety of catalytic reactions, including hydrogen evolution reactions, overall water splitting, water oxidation, CO2 reduction, nitrogen reduction reactions, pollutant degradation, and organocatalysis. While the structural design and band gap engineering of catalysts are elaborated, the surface chemistry is dealt with in detail to demonstrate efficient catalytic performances. Burning challenges in catalytic design and future outlook are elucidated.

Production of Renewable Fuels by the Photocatalytic Reduction of CO2 using Ma...

Pawan Kumar

The photo-reductive performance of natural ilmenite was boosted and the production of renewable fuels from the reduction of CO2 was enhanced by doping the natural mineral with magnesium. The doping was achieved by high energy ball milling in the presence of MgO and Mg(NO3)2. The photo-reduction of CO2 in aqueous solution led to the evolution of H2, CH4, C2H4, and C2H6, and the insertion of Mg in the structure of ilmenite enabled increases of up to 1245% in the fuel production yield, reaching total production of 210.9 µmol h-1 gcat-1. Displacements of the conduction band to more negative potentials were evidenced for the samples doped with magnesium. Indirect effects such as increases in the valence band maximum, and the introduction of intermediate energy levels were also evidenced through the measurement of the crystallite size and the determination of the band structure of the materials. Mott-Schottky analyses of the samples showed the n-type nature of the semiconductor materials and enabled the estimation of the density of charge carriers, which strongly influenced the photocatalytic performance. The strong potential of the application of natural ilmenite in gas phase artificial photosynthesis was proved by the evaluation of CO2 reduction in gas conditions, which allowed the enhancement in the selectivity and significantly increased the production of CH4 as compared to aqueous solution, reaching an important yield of CH4 of 16.1 µmol h-1 gcat-1.

Nanoengineered Au-Carbon Nitride Interfaces Enhance PhotoCatalytic Pure Water...

Pawan Kumar

Photocatalytic pure water splitting using solar energy is one of the promising routes to produce sustainable green hydrogen (H2). Tuning the interfacial active site density at catalytic heterojunctions and better light management are imperative to steer the structure-activity correlations to enhance the photo-efficiency of nanocomposite photocatalysts. Herein, we report the decoration of nitrogen defects-rich carbon nitride CN(T) with metallic Au nanostructures of different morphologies and sizes to investigate their influence on the photocatalytic hydrogen evolution reactions (HER). The CN(T)-7-NP nano-heterostructure comprises Au nanoparticles (NPs) of ~7 nm and thiourea-derived defective CN exhibits an excellent H2 production rate of 76.8 µmol g–1 h–1 from pure water under simulated AM 1.5 solar irradiation. In contrast to large-size Au nanorods, the high activity of CN(T)-7-NP was attributed to their strong localized surface plasmon resonance (LSPR) mediated visible absorption and interfacial charge separation. The surface ligands used to control Au nanostructures morphology were found to play a major role in the stabilization of NPs and improve interfacial charge transport between Au NPs and CN(T). First-principles calculations revealed that defects in CN and Au-CN interfacial sites in these nanocomposites facilitate the separation of e-/h+ pairs after light excitation and provide lower energy barrier pathways for H2 production by photocatalytic water splitting.

Nanoengineered Au-Carbon Nitride Interfaces Enhance Photo-Catalytic Pure Wate...

Pawan Kumar

Cooperative Copper Single Atom Catalyst in Two-dimensional Carbon Nitride for...

Pawan Kumar

Renewable electricity powered carbon dioxide (CO2) reduction (eCO2R) to high-value fuels like methane (CH4) holds the potential to close the carbon cycle at meaningful scales. However, this kinetically staggered 8-electron multistep reduction still suffers from inadequate catalytic efficiency and current density. Atomic Cu-structures can boost eCO2R-to-CH4 selectivity due to enhanced intermediate binding energies (BEs) resulting from favorably shifted d-band centers. Herein, we exploit two-dimensional carbon nitride (CN) matrices, viz. Na-polyheptazine (PHI) and Li-polytriazine imides (PTI), to host Cu-N2 type single atom sites with high density (∼1.5 at%), via a facile metal ion exchange process. Optimized Cu loading in nanocrystalline Cu-PTI maximizes eCO2R-to-CH4 performance with Faradaic efficiency (FECH4) of ≈68% and a high partial current density of 348 mA cm−2 at a low potential of -0.84 V versus RHE, surpassing the state-of-the-art catalysts. Multi-Cu substituted N-appended nanopores in the CN frameworks yield thermodynamically stable quasi-dual/triple sites with large interatomic distances dictated by the pore dimensions. First-principles calculations elucidate the relative Cu-CN cooperative effects between the two matrices and how the Cu-Cu distance and local environment dictate the adsorbate BEs, density of states, and CO2-to-CH4 energy profile landscape. The 9N pores in Cu-PTI yield cooperative Cu-Cu sites that synergistically enhance the kinetics of the rate-limiting steps in the eCO2R-to-CH4 pathway.

Bioinspired multimetal electrocatalyst for selective methane oxidation

Pawan Kumar

Selective partial electrooxidation of methane (CH4) to liquid oxygenates has been a long-sought goal. However, the high activation energy of C–H bonds and competing oxygen evolution reaction limit product selectivity and reaction rates. Inspired by iron (IV)-oxo containing metalloenzymes’ functionality to activate the C–H bond, here we report on the design of a copper-iron-nickel catalyst for selective oxidation of CH4 to formate via a peroxide-assisted pathway. Each catalyst serves a specific role which is confirmed via electrochemical, in situ, and theoretical studies. A combination of electrochemical and in situ spectroelectrochemical studies revealed that H2O2 oxidation on nickel led to the formation of active oxygen species which trigger the formation of iron (IV) at low voltages. Density functional theory analysis helped reveal the role of iron (IV)-oxo species in reducing the activation energy barrier for CH4 deprotonation and the critical role of copper to suppress overoxidation. Our multimetal catalyst exhibits a formate faradaic efficiency of 42% at an applied potential of 0.9 V versus a reversible hydrogen electrode.

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