Investigation of process parameters assessment via design of experiments for CO2 photoreduction in two photoreactors

doi:10.1016/j.jcou.2019.10.009

Journal of CO2 Utilization

Volume 36, February 2020, Pages 25-32

https://doi.org/10.1016/j.jcou.2019.10.009 Get rights and content

Highlights

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CO₂ photoreduction to solar fuels was investigated.
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A design of experiments (DOE) approach was used.
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Two different photocatalytic rigs were considered.
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Irradiance and reaction time effects on CH₄ yield were analysed.

Abstract

CO₂ photoreduction with water to obtain solar fuels is one of the most innovative and sustainable processes to harvest light energy and convert it into hydrocarbons. Although photocatalytically active materials and photoreactors have been developed for this purpose, lack of standardisation in testing conditions makes the assessment of process parameters and the comparison of material performance a challenge. Therefore, this paper is aimed at investigating the effect of CO₂ photoreduction parameters irradiance and reaction time on production of methane from two photocatalytic rigs. This was pursued through a design of experiments (DOE) approach, which assessed the influence of experimental conditions between different setups. Using low irradiance (40–60 W m⁻²), reaction time and temperature significantly affected methane production, with a maximum production of 28.50 μmol g_cat⁻¹ (40 W m⁻², 4 h). When using high irradiance (60–2400 W m⁻²), only irradiance was found to significantly affect methane production, with a maximum production of 1.90 ∙ 10⁻¹ μmol g_cat⁻¹ (1240 W m⁻², 2 h). Considering proposed reaction mechanism for CO₂ photoreduction, this paper highlights that experimental results give different yet complementary information on the two most important steps of the process, i.e. photoexcitation and surface chemical reaction.

Graphical abstract

Introduction

Photoreduction is one of the most innovative, environmentally sustainable and promising technologies to convert carbon dioxide (CO₂) into hydrocarbons using water as a reducing agent and light as primary energy input [1,2]. Efforts have been placed on researching how to improve the effectiveness of this process [1,3,4]. For example, photocatalysts modification is one of the pursued strategies, where several semi-conductors have been investigated, including CdS [[5], [6], [7]], ZnO [8,9], ZrO₂ [10], WO₃ [11], SrTiO₃ [12], although TiO₂ remains the most investigated and promising material [[13], [14], [15]]. Another strategy to increase photoactivity is materials modification, aimed at suppressing electron-hole recombination, which can be pursued in several ways: use of co-catalyst [[16], [17], [18], [19]], metal doping [20], graphene encapsulation [21]; but one of the most promising options is the introduction of surface plasmonic resonance particles [[21], [22], [23], [24]]. Material engineering aside, reactor design and catalytic conditions for CO₂ photoreduction still need to be further investigated and standardised to compare materials photoactivity significantly. A wide variety of photocatalytic reactors has been reported [[25], [26], [27]], but due to a lack in standardisation in experimental procedures, reaction regimes and data collection and processing, it is difficult to compare results reported from different systems. Recently, gas phase systems have been preferred to liquid phase to overcome limitations by photon and mass transfer [[27], [28], [29]], focusing on gas phase systems operating at room temperature and atmospheric pressure [[30], [31], [32]]. A recent study reported that when reagents are in gas phase, CO₂ undergoes deoxygenation faster than hydrogenation, improving selectivity to methane (CH₄), which is the most desired solar fuel, due to its high hydrogen to carbon ratio [28].

In the field of CO₂ photoreduction, general catalysis parameters (e.g. catalyst amount, reaction time, reagents concentrations) have been investigated [[33], [34], [35]], whilst photons irradiation, which represents the primary energy input have not been thoroughly studied yet. Materials light harvesting considerably affects surface activation and, consequently, the number of active sites to catalyse CO₂ photoreduction [36]. In the case of photooxidation reactions, correlation between photons input and reaction rate varies with photons flux, indicating different activation mechanisms [37]. However, to the best of the authors’ knowledge, photons input effect on CO₂ photoreduction with water has not been investigated yet.

In this paper, the authors investigate the effect of reaction time and irradiance on conversion. The choice of these parameters is due to their correlation to photons input in the catalytic system, which is the energy source of the whole process [38]. To provide a rational approach to understanding the effect of these parameters on CO₂ photoreduction into methane, a Design of Experiments (DOE) was employed for the first time for two different reactor designs. DOE is a powerful tool that allows for generating highly efficient systematic experimental designs that can be used to screen and optimise parameters on selected responses [[39], [40], [41]]. The objective of DOE is to fit a function including only statistically significant parameters to a response. The systematic treatment of the data employed here allows for parameters irradiance and reaction time to be compared for the two studied different reactor systems on the responses to methane production. This study highlights how reactor design and experimental conditions can affect selectivity and conversion in CO₂ photoreduction.

Section snippets

Materials synthesis

Titanium dioxide was prepared by the precipitation method [28]. A 1.2 M titanyl sulphate solution (TiOSO₄∙xH₂O∙yH₂SO₄, Ti assay >29% Sigma Aldrich) and a 9.0 M NaOH solution (assay >97% Carlo Erba) were dropped simultaneously to 200 mL of distilled water under vigorous stirring, maintaining pH neutral. The Ti(OH)₄ suspension was then aged at 60 °C for 20 h, filtered and washed with distilled water to remove the sulphate ions, as verified by the barium chloride test [42]. Wet Ti(OH)₄ was dried

Materials characterisation

The synthesized Au-TiO₂ photocatalyst sample provided suitable surface and lattice properties for CO₂ photoreduction, with similar values to those previously reported in the literature [9,51,53]. The N₂ physisorption isotherm (Fig. S1) showed this sample is mesoporous and characterised by 110 m² g⁻¹ surface area, with wide pore size distribution between 5–25 nm. The XRD spectrum (Fig. S2) concluded that the only titanium dioxide crystal phase observed is anatase, which is the most suitable for

Conclusions

Assessing irradiance conditions is fundamental to understand results from photocatalytic CO₂ reduction. Photons represent the primary energetic input and a change in this parameter might have an effect on materials performance.

The different effect of photonic input was assessed by design of experiments approach, considering the experimental parameters affecting photons input, i.e. irradiance and reaction time. The effect of these variables proved to be different according to experimental

Declaration of Competing Interest

I declare there is not conflict of interest

Acknowledgements

The authors thank Tania Fantinel (Ca’ Foscari University Venice) and Richard Kinsella (Heriot-Watt University) for the excellent technical assistance. The authors acknowledge EU ERASMUS + Traineeship program for support of the collaboration between Research Centre for Carbon Solutions (RCCS) at Heriot Watt Universityand the Department of Molecular Sciences and Nanosystems at Ca’ Foscari University Venice. The authors also thank the financial support provided by the Engineering and Physical

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Investigation of process parameters assessment via design of experiments for CO2 photoreduction in two photoreactors

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials synthesis

Materials characterisation

Conclusions

Declaration of Competing Interest

Acknowledgements

Catal. Today

J. Photochem. Photobiol. C Photochem. Rev.

Appl. Surf. Sci.

Chem. Eng. J.

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Curr. Opin. Chem. Eng.

Chem. Eng. J.

Chemosphere

Appl. Catal. B: Environ.

Renew. Sustain. Energy Rev.

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Appl. Catal. B: Environ.

Appl. Catal. B: Environ.

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Appl. Catal. B: Environ.

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J. CO2 Util.

Chem. Eng. J.

Appl. Catal. A Gen.

Chem. Phys. Lett.

J. Electroanal. Chem.

J. Catal.

Investigation of process parameters assessment via design of experiments for CO₂ photoreduction in two photoreactors