An aerogel-based photocatalytic microreactor driven by light guiding for degradation of toxic pollutants

Highlights

• A new type of photocatalytic microreactor based on composite aerogel was developed.

• Light can be efficiently guided along the reactor by total internal reflection.

• Toxic phenol was successfully converted into harmless CO₂ and water.

• A quantitative model of the reactor/light guide was devised and verified.

• The photoreactor is scalable and compatible with both polar and non-polar solvents.

Abstract

Efficient utilization of light in photocatalytic chemical processes requires careful optimization of the photocatalytic reactor layout to maximize the interaction between the incident light, photocatalyst and reactant molecules. Herein, we report a new type of photocatalytic flow microreactor with an integrated light guide, formed by a channel fabricated inside a hydrophobic composite aerogel monolith made of silica and titania (TiO₂). The liquid-filled channel simultaneously acts as a reaction vessel and as a liquid-core optofluidic waveguide, distributing the incident light over the whole reaction volume. Anatase TiO₂ nanoparticles embedded in the channel walls then serve as a photocatalyst that can efficiently interact with both the guided light and the reactant solution along the channel length. Composite aerogels were synthesized with TiO₂ content between 1 and 50 wt %, retaining their interconnected mesoporous network, low refractive index, and waveguide propagation losses below −3.9 dB/cm over this range of compositions. Using photocatalytic degradation of phenol – an organic compound with harmful environmental effects – as a model chemical reaction, the performance of the microreactor was systematically investigated. Reactant conversion was observed to increase with increasing incident light power, decreasing reactant flow rate and increasing mass fraction of TiO₂ in the composite. An analytical model of the reactor/light guide system was developed that predicted successfully the scaling of the reactant conversion with the incident light power and reactant flow rate. The presented concept of aerogel-based optofluidic photocatalytic microreactors is readily scalable and possesses great potential for carrying out other photocatalytic reactions in both polar and non-polar solvents.

Introduction

Photocatalytic reactions [1] underlie a number of technologically relevant chemical processes including splitting of water into hydrogen and oxygen for “clean” energy production [2], [3], fixation of atmospheric carbon dioxide for mitigating the consequences of combustion of fossil fuels [4], and synthesis or degradation of complex organic molecules [5], [6], [7]. Optofluidic microphotoreactors [8], [9] integrate optical and photonic circuits with small-volume microfluidic channels, in which light-matter interaction is greatly enhanced. This unique feature provides the foundation for implementing highly efficient photochemical and photocatalytic processes [10], [11], [12], [13], [14], [15]. The majority of systems demonstrated in the literature have been based on planar architectures consisting of a single wide channel or an array of channels fabricated in a solid substrate [16], [17], [18], [19] and covered by a transparent plate that seals the channels from the top and provides access to external irradiation [20], [21], [22], [23]. Such planar microphotoreactors can be utilized with heterogeneous photocatalysts that can be dispersed in the form of nanoparticles in the reactant solution. However, this approach typically leads to a largely non-uniform distribution of illumination intensity within the reactor resulting from the absorption and scattering of light by the suspended particles. Furthermore, the catalyst particles must be separated from the reaction mixture downstream from the reactor [10], [11], [13], [24]. Packed-bed microphotoreactors circumvent the issue of photocatalyst recovery. Still, the problems associated with the non-uniform intensity of activation light within the reaction volume remain, since the photons arriving from outside of the reactor are predominantly absorbed in the outer layer of the photocatalyst particle bed and/or scattered away from the reactor [24], [25]. Immobilization of the photocatalyst on the inner reactor walls in the form of a transparent film coating is often considered to be a more effective choice for carrying out photocatalytic reactions [14], [15], [19], [26], [27], [28], [29]. However, smooth surfaces of the reactor walls may lead to poor adhesion of photocatalyst particles and, hence, the photocatalyst layer may not be sufficiently stable to withstand extended periods at the reaction conditions, especially when the reactor is operated continuously [30], [31].

In addition to irradiating the reaction mixture through the transparent reactor walls, optical fibers have emerged as a promising alternative for improving the light delivery. Tugaoen et al. [32] developed a flow-through photoreactor, into which a bundle of optical fibers coated with titania (TiO₂) photocatalyst film was inserted. UV light coupled into the fibers was delivered to the photocatalyst surface by partial refraction from the fiber cladding and, subsequently, used for photocatalytic oxidation of liquid organic pollutants flowing along the fibers. However, only a very small fraction of the activation light propagating through the fibers could actually reach the catalyst surface, which limited the overall efficiency of the photoconversion process. Hollow-core photonic crystal fibers (HC-PCF) [33] have also been employed for carrying out various photocatalytic reactions [9]. Here, the photocatalyst particles were selectively deposited on the inner walls of the HC-PCF core. Subsequently, the reactant solution was flown through the core, interacting strongly with the guided light propagating in the core. The main challenges of this approach lie in the selective, uniform deposition of the catalyst particles on the inner surfaces of the fiber and in the limited volume available for the reactant solution inside the fiber cores with micrometer-scale diameters.

Recently, aerogels have been introduced as promising materials for fabricating optofluidic waveguides with liquid cores that operate on the principle of total internal reflection (TIR) from an interface between dielectric media with a sufficient contrast of refractive indices [34], [35], [36], [37], [38]. Due to their highly porous nanostructure, aerogels have very low refractive indices in the visible part of the spectrum (~1.05) [37], [38]; thus, virtually all common liquids have refractive indices exceeding those of aerogels. TIR-based optofluidic waveguides can be constructed by simply fabricating channels inside monolithic aerogel blocks and filling these channels with a suitable core liquid. In principle, the core liquid can be formed by a photosensitive reactant solution. In such an approach, the liquid-filled channel is transformed into a microphotoreactor that simultaneously confines the reaction medium and also serves as the waveguide cladding whereas the reaction volume resides within the liquid core of the waveguide [37], [39]. Using UV light guided in an aerogel-based optofluidic microphotoreactor with an aqueous core, we have demonstrated a highly efficient photodegradation of methylene blue dye dissolved in water [36].

In this study, we introduce novel continuous photocatalytic microreactors based on light-guiding optofluidic channels fabricated in monolithic SiO₂–TiO₂ composite aerogels, which provide both structural framework and photocatalytic activity to the reactor. To obtain monolithic composite aerogels that are stable in contact with aqueous reactant solutions, we embed photocatalytically active anatase TiO₂ nanoparticles in the SiO₂ matrix during the gelation phase of the aerogel synthesis and apply hydrophobic treatment to the skeleton of the resulting aerogel monoliths. The synthesized hydrophobic composites containing as much as 50 wt % of TiO₂ maintain the structural integrity and desirable optical characteristics of native SiO₂ aerogels and integrate both photocatalytic and light-guiding properties. Unlike in conventional microphotoreactors illuminated through the walls, in this system, the activation light directly propagates along the liquid-filled channel, sharing the same space with the flowing reactant solution, which results in a highly efficient light-matter interaction and low light propagation losses. The light guided in the channel by TIR from the channel walls is gradually absorbed by the photocatalyst particles immobilized in the walls and drives oxidation–reduction reactions of dissolved organic reactant molecules at the catalyst surface along the full channel length.

We systematically investigate the effects of the incident light power, flow rate of the reactant solution, and mass fraction of TiO₂ in the composite aerogel on the performance of the photoreactor for a model photocatalytic reaction – degradation of phenol in aqueous solutions. We complement our experimental findings by a quantitative theoretical model of the reactor performance, which provides additional insight into the scaling of reactant photoconversion with changing parameters of the photoconversion process. Overall, we demonstrate that our photocatalytic microreactor is well suited for photocatalytic degradation of toxic organic pollutants dissolved in water under a wide range of operating conditions.