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. 2025 Jan 16;2(3):199–209. doi: 10.1021/cbe.4c00154

Hydrogen Peroxide Producing Titania-Silica Supraparticles as Tailorable Photocatalysts for Flow Chemistry Reactions in Microfluidic Reactors

Bettina Herbig †,*, Egzon Cermjani ‡,§, Doris Hanselmann †, Angelika Schmitt †, Christoph Deckers ‡, Thomas H Rehm ‡, Karl Mandel †,∥, Susanne Wintzheimer †,∥
PMCID: PMC11955856  PMID: 40171125

Abstract

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The development of hybrid catalysts for cascade reactions that demonstrate high efficiency and longevity strongly relies on the precise arrangement of the individual components within such a material. This guarantees both their proximity for enhanced interaction and, at the same time, sufficient separation avoiding mutual harm. Before the acutal design of a usually very complex hybrid catalyst, it is essential to study and understand the impact of structural characteristics on catalytic activities of each catalytically active constituent separately. This study thus focuses on a comprehensive structure–activity analysis of the component within a highly customizable TiO2-SiO2 material, which produces H2O2 photocatalytically. The tailorable design of the hybrid material is achieved through the utilization of the spray-drying process. The H2O2 productivity of the obtained so-called TiO2-SiO2 supraparticles is demonstrated in both a batch and a flow reactor, marking a crucial step toward their future application as hybrid catalysts in photoassisted cascade reactions.

Keywords: Supraparticles, photocatalyst, titania, hydrogen peroxide production, microfluidic reactor, flow chemistry, cascade reaction

1. Introduction

In an effort to emulate the biosynthetic efficacy observed in nature and to identify more environmentally conscious and sustainable alternatives to current chemical production methods, cascade reactions have attracted increasing attention over the past two decades.1 Such reactions are usually promoted by a hybrid catalyst. Prominent examples are combinations of photocatalysts and enzymes,2 as well as of organo- and metal catalysts.3 Given the incompatibility of the majority of catalysts, compartmentalization techniques, i.e., the spatial separation of the two components within a hybrid catalyst, are usually required to prevent catalyst deactivation.1−3 In this context, systematic studies on structure-dependent catalytic activities are highly needed, focusing not only on the often very complex, hard-to-understand complete hybrid system but also on each component on its own. Such more easily investigable model systems will pave the way to better understanding and designing future hybrid systems.

The study presented herein therefore accounts for a detailed structure–activity correlation of photocatalytic hydrogen peroxide-producing TiO2 nanoparticles within a highly tunable TiO2-silica (SiO2) material, which can in the future be extended by the addition of a second catalyst, such as an enzymatic or metal component. H2O2 is widely required as an oxidant in the chemical and biotechnology industries as it facilitates the oxygenation reactions of many enzymes4 or metal-catalyzed reactions.5,6 Although the anthraquinone cycle displays the predominant process for the industrial production of H2O2ex situ, there are multiple benefits to generating hydrogen in situ such as a significant process intensification and decarbonization.6 Photocatalytic in situ production of H2O2 can be facilitated by continuous flow technology and the utilization of micro reactors.7−9 This approach takes advantage of an enhanced multiphase reaction between oxygen and water due to increased mixing in microreactors, together with an efficient irradiation of the photocatalyst, which is enabled by a high surface-to-volume ratio. Furthermore, continuous flow processing enables a streamlined and automated steady production, excellent reaction control possibilities10−14 and good handling of heat development and system pressure, thereby increasing safety.15 In the case of capillary photoreactors utilizing fluorinated ethylene-propylene tubing (FEP), even and efficient irradiation of the reaction medium within the capillary is enabled, thereby providing optimal conditions for photocatalytic approaches.16,17

This study exploits for the TiO2–SiO2 material creation an approach that is ideally suited for the design of multicomponent materials with high freedom in component variations, namely, the spray-drying-assisted assembly of nanoparticles, yielding micrometer-scaled porous supraparticles.18 This assembly method allows for structure control and tuning, direct processing of fragile moieties such as enzymes, and subsequent post-treatments such as bio- or chemofunctionalizations, and is a fast as well as scalable process.19 The H2O2-productivity of the developed TiO2–SiO2 supraparticles is showcased in both batch and flow reactors as an essential step for their future use as a hybrid catalyst in photoassisted cascade reactions.

2. Materials and Methods

The chemicals were purchased from the following companies: tetraethoxysilane (TEOS, Sigma–Aldrich, Germany), (3-aminopropyl)triethoxysilane (Sigma-Aldrich, Germany), aqueous ammonia solution (Sigma-Aldrich, Germany), ethanol (CSC Jäkle Chemie GmbH & Co. KG, Germany), glutaraldehyde (Sigma-Aldrich, Germany, 25% in water), titanium(IV) ethoxide (Ti(OEt)4, 93.1%; CSC Jäkle Chemie GmbH & Co. KG, Germany), acetylacetone (2,4-pentandione, 99.3%, Sigma-Aldrich, Germany), p-toluenesulfonic acid monohydrate (Sigma-Aldrich, Germany). All chemicals were used without further purification. All syntheses were performed in deionized water.

Nanoparticle Syntheses

TiO2 nanoparticles were synthesized using a hydrothermally assisted sol–gel process.20,21 First, an amorphous TiO2 precursor powder was synthesized by complexing titanium(IV) ethoxide (1.5 mol) drop by drop with acetylacetone (1 mol). Then, p-toluenesulfonic acid monohydrate (0.05 mol) was dissolved in deionized water, and the PTSA/H2O mixture was added to the above-mentioned solution. Without further delay, all volatiles were removed from the reaction mixture by rotational evaporation. The product is a fine yellowish powder which can easily be removed from the reaction vessel. The precursor powder is soluble in water and results in a transparent, stable sol (12 wt % TiO2), which was transferred into stainless steel Teflon-lined autoclaves (100 mL) and heat treated in a convection oven (Type LHT6/60/E201/OTC; Carbolite GmbH, Ubstadt-Weiher, Germany) at 160 °C at autogenous pressure for 4 h. The autoclaves were taken out of the hot oven and cooled down to room temperature before opening them. The products obtained were of gel-like consistency and showed brownish-red color. The gels were removed from the liners, and a dispersion was prepared by redispersing the gels containing nanocrystalline TiO2 particles in water for spray-drying experiments.

SiO2 nanoparticles with a diameter of 80 nm were synthesized using a recently published method that is based on the Stöber process.22 In a typical experiment, 300 mL of ethanol was mixed with 15 mL of aqueous ammonia solution (25 wt %). The mixture was vigorously stirred while 15 g (0.072 mol) of TEOS was added and subsequently stirred overnight. To purify the nanoparticles, the ethanol/ammonia solution was removed by rotary evaporation, and the mixture was dialyzed for 24 h with multiple water changes using a dialysis membrane with a molecular weight cutoff of 10 kDa (cellulose hydrate membrane, Nadir-dialysis tubing, Roth, Germany). The functionalization of the SiO2 nanoparticles with (3-aminopropyl)triethoxysilane and glutaric acid was conducted following the protocol described elsewhere and the functionalized nanoparticles were subsequently redispersed in water.23

Supraparticle Fabrications

Either plain TiO2 or mixed TiO2–SiO2 nanoparticle dispersions at weight ratios of 1:6, 1:3, 1:2, 1:1, 2:1, 3:1, and 6:1 were spray-dried using a B-290 mini spray-dryer (Büchi, Switzerland). Upon mixing the TiO2 with the SiO2 dispersion, the obtained binary nanoparticle dispersion displays pH values around 5 due to the high acidity of the TiO2 precursor solutions (yielding core–shell structures upon spray-drying due to electrostatically stabilized nanoparticles, as mentioned in the Results and Discussion). For the creation of intermixed supraparticles, the pH value of the precursor dispersion was adjusted to a pH of 7.5 with the dropwise addition of 25% ammonia solution while stirring the dispersion (for a destabilization and preagglomeration of the nanoparticles). The spray-dryer was equipped with a two-fluid nozzle, and the inlet temperature was set at 30 °C, with an outlet temperature of about 24 °C at 1 mL/min velocity.

Characterization Methods

The obtained SiO2 and TiO2 nanoparticles were characterized as described in SI. The spray-dried supraparticles were characterized using scanning electron microscopy (SEM, Zeiss Supra 25, Germany). The acceleration voltage for SEM was 2 kV. To prepare cross section samples, 10 mg of supraparticles was dispersed in 500 μL of lacquer. The mixture was then cured at 100 °C and cut using a scalpel. These samples were fixed onto a sample holder using conductive silver and sputtered with platinum for 5 s at 50 mA using the MED 010 instrument from Balzers Union. Laser diffraction (Microtrak Bluewave) was used to measure the supraparticle size distributions. Further supraparticle characterization methods (Nitrogen sorption and SEM-EDS) are listed in SI.

Photocatalytic Experiments in Batch Mode

The amount of photocatalytically produced hydrogen peroxide was determined by using the Eisenberg method.24 Here 0.125 g of plain TiO2 or TiO2–SiO2 supraparticles was suspended in 250 mL deionized water and stirred at 375 rpm and 25 °C in an in-house built reactor (see Figure S1a,b). Air was continuously bubbled in (250 l/h), and irradiation was performed through a borosilicate glass window on one side of the reactor by adjusting four 365 nm LEDs (Nichia NCSU276A UV SMD-LED, 780 mW) in front. Before irradiation (t = 0 min) and after different irradiation times (5 to 20 min), an aliquot of the solution was removed from the reactor. To 5 g of the solution, 0.5 mL of TiOSO4 solution (1.9–2.1%, Sigma-Aldrich) was added and filtered through a 0.2 μm syringe filter. The resulting yellow-colored titanium peroxo-complex was analyzed photometrically using a Specord 50, Analytik Jena GmbH & Co.KG spectrophotometer. The absorption at 408 nm was chosen to plot a calibration curve using standard H2O2 concentrations (20 to 1000 μmol/L, via dilution of a 30% hydrogen peroxide solution, Carl Roth) and to estimate the concentrations of photocatalytically produced H2O2. The obtained calibration curve is shown in Figure S1c (in the Supporting Information). The error bars in the catalytic tests are obtained via performing 3 reproductions of both the test and the material preparation.

Photocatalytic Experiments in Continuous-Flow Mode

A capillary photoreactor was used for continuous flow synthesis of H2O2 (Figure S2, Table S1). The LED array (Table S2, Figure S3) is cooled by water to approximately 10 °C, and the reactor setup is equipped with an additional thermostat, which maintains the desired temperature within the reactor. The reaction suspension is introduced into the reactor via syringe pumps, which are equipped with an integrated external magnetic stirring motor, thereby ensuring an even and continuous agitation of the aqueous suspension within the syringe. A gas bottle containing synthetic air and a mass flow controller for providing oxygen as a reactant and setting precise gas flow rates are connected to the system by a three-way valve (PTFE, 1.6 mm bore). The supraparticles are dispersed in water and subsequently transferred to a syringe. Subsequently, the suspension is fed into the capillary photoreactor under constant stirring at specified flow rates (Tables S3–S5). The reaction is initiated by irradiating the reaction suspension at a wavelength of 365 nm for varying periods of time, thereby providing data for time-dependent generation of H2O2. Subsequently, the suspension is collected, and the concentration of H2O2 is determined following the filtration of the supraparticles and applying the Eisenberg method.24

3. Results and Discussion

The synthesized TiO2 nanoparticles utilized for assembling H2O2-producing supraparticles via spray-drying, provide sizes of around 7 nm, anatase phase, and a charged surface with its isoelectric point around pH 7 (determined via transmission electron microscopy, dynamic light scattering, X-ray diffraction, and zeta potential measurements; Figure S4 in the Supporting Information and studied in detail elsewhere20,21,25−27). The process of spray-drying a TiO2 nanoparticle dispersion is depicted in Figure 1a. Initially, the dispersion is pumped through a nozzle, resulting in the release of a fine mist into a heated chamber. In this chamber, the solvent present in the individual droplets is evaporated, forcing the contained nanoparticles together to form supraparticles that can be retrieved as a powder sample.

Figure 1.

Figure 1

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(a) Schematic representation of the supraparticle fabrication during spray-drying via droplet evaporation using TiO2 nanoparticles dispersed in water as precursors (adapted from Reichstein et al.28 under terms of the CC-BY license). (b) SEM micrographs of (b1) a TiO2 supraparticle and (b2) its cross section. (c) Cumulated concentration of H2O2 produced by TiO2 supraparticles throughout UV-light exposure in the presence of oxygen, fitted with the kinetic equation of the photocatalytic H2O2 production y = kf/kd*(1 – exp(−kdx) with kf and kd being the formation and decomposition rate constants and x being the exposure time,29 and (d) cumulated concentration of H2O2 produced by TiO2 supraparticles either after stirring in darkness, exposure to UV-light, exposure to UV-light and oxygen, as well as exposure to UV-light, oxygen, and methanol (1.0 vol %) for 20 min. (e) Schematic representation of the photocatalytic H2O2 production using a TiO2 nanoparticle as the photocatalyst.

The spherical, dense morphology of the obtained TiO2 supraparticles is shown in scanning electron micrographs of Figure 1b and Figure S5a (in the Supporting Information). They possess micrometer sizes in the range of 0.6 to 5 μm (determined via a laser diffraction measurement; Figure S5b, Supporting Information). To investigate the textural properties of the TiO2 supraparticles, an N2 sorption measurement was performed at 77 K (Figure S5c to e, Supporting Information). The obtained type IV isotherm (IUPAC classification) is typical of a mesoporous material. The pore size distribution (PSD) and cumulative pore volume obtained by applying nonlocal density functional theory (NLDFT) to the adsorption branches indicate an average pore size of around 5 nm. The surface area of the TiO2 supraparticles calculated using the Brunauer–Emmett–Teller (BET) model is 180 m2/g.

The H2O2-productivity of the TiO2 supraparticles was determined in a catalytic test setup based on a batch reactor, a UV light source, and a pressurized air inlet (see Materials and Methods and Figure S1a and b in the Supporting Information). The quantification of H2O2 was achieved using the Eisenberg method,24 which is based on the light absorbance at 408 nm of H2(TiO2(SO4)2) formed by the reaction of H2O2 with TiOSO4 and H2SO4. The obtained calibration curve is shown in Figure S1c (in the Supporting Information). The determined concentration of H2O2 over the UV light exposure duration is displayed in Figure 1c. After an initial almost continuous increase in H2O2, its production reaches a steady state after approximately 15 min of irradiation time (which will be discussed later on). In this context, it has to be mentioned that a significant amount of H2O2 may also adsorb on the (inner and outer) surface of the TiO2 supraparticles (Supporting Information Figure S6) and is thus not available in the solution and consequently is not quantifiable via the utilized method. As the H2O2 production rate is highly suppressed by removing O2 or UV light irradiation (Figure 1d), it can be concluded that the photocatalytic O2 reduction is the dominant route for H2O2 generation over TiO2 supraparticles (Figure 1e). Their productivity can be further enhanced by adding a hole scavenger like methanol to the reaction (Figure 1d and Figure S7) which decreases the electron–hole recombination probability as well as the undesired oxidation of H2O2.30,31 Due to the continuous air inlet, the O2 concentration is considered to be constant. Consequently, the formation rate of H2O2 can be considered as zero-order kinetic.29,32 At the same time, H2O2 as a metastable molecule undergoes photodegradation such as reduction to water and oxidation to O2 (the latter can partly be suppressed by the addition of methanol). Additionally, H2O2 is easily converted to Ti-OOH complexes when it is in touch with the Ti–OH groups on the TiO2 surface. Subsequently, Ti-OOH is once again reduced upon reaction with an electron (Ti-OOH + H+ + e– → Ti–OH + ·OH) competing with the oxygen reduction reaction.33−35 These degradation processes presumably follow first-order kinetics related to the concentration of H2O2. Consequently, the total H2O2 generation y should follow the kinetic equation y = kf/kd*(1 – exp(−kdx) with kf and kd being the formation and decomposition rate constants and x being the exposure time.29 Under these conditions, the concentration of H2O2 will reach a steady state after a certain amount of irradiation. The recorded data (Figure 1c) can be fitted very well by this equation (R2 = 0.999). In general, TiO2 is often claimed to be an inefficient photocatalyst for H2O2 mainly due to a high electron–hole recombination rate and a high decomposition rate of H2O2 on TiO2, especially on the commercial benchmark catalyst P25.31,36,37 Indeed, we also observed negligible activity of commercial P25 and P90 catalysts in our catalytic batch reactor setup (Figure S8 in the Supporting Information). The significantly higher activity of the herein-developed TiO2 supraparticles can potentially be attributed to the wet-chemical nanoparticle synthesis (compared to flame-pyrolyzed P25 and P90). Organic molecules from the synthesis, such as acetylacetonate (and its decomposition product acetate), may still be adsorbed on the TiO2 surface and consequently decrease the rate of photodegradation of H2O2. This may be attributed to either a steric effect in which these molecules block the sites where H2O2 is degraded or their efficient competition with H2O2 for valence band holes.32 Another reason for a significantly higher activity of the developed supraparticles compared to P25 as well as P90 could be the presence of mixed anatase–rutile phases in the commercial sample compared to pure anatase in the supraparticles.38 Rutile is attributed to decompose H2O2 to a higher extent than anatase due to a different reaction mechanism (on rutile H2O2 reduction is dominant, on anatase its oxidation).38 In general, the obtained H2O2 productivity of the TiO2 supraparticles lies within the expected broad activity range of photocatalytic TiO2 materials (Table S6).

While the H2O2 productivity of the synthesized TiO2 supraparticles has been confirmed, for the envisaged aim to develop a dual-component particle for future photoassisted cascade reactions, the introduction of a second, inert material is crucial. Such material is not meant to interfere with the light interaction of TiO2 or provide energy transfers between the two materials but serves for the immobilization of enzymes or organocatalysts to create hybrid catalysts and allows for spatial separation of these components from the photocatalyst to avoid, for example, their photodeactivation. The addition of a second component, e.g., functionalized SiO2 nanoparticles, comes along with the challenge of directly influencing the distribution of TiO2 nanoparticles within a supraparticle. This affects the interaction of the photocatalyst component with UV light and, consequently, the H2O2 productivity of the system. This structure–reactivity correlation for the photocatalyst component within a supraparticle is the main focus of the hereafter-described study.

As the second nanoparticle building block, glutaraldehyde-modified SiO2 nanoparticles ∼100 nm in size were chosen (see Figure S9 in the Supporting Information for SiO2 nanoparticle characterization). SiO2 nanoparticles are probably the most frequently used material for the creation of hybrid catalysts as they have been proven to provide outstanding properties required for the immobilization of catalysts, such as high surface area and inertness. Additionally, SiO2 can be easily organo-functionalized (e.g., with glutaraldehyde) to control interactions between enzymes or organocatalysts and support surface and to ultimately yield a covalent attachment without catalyst deactivation.39−43 Using a binary (aqueous) dispersion of TiO2 and SiO2 nanoparticles with a fixed TiO2:SiO2 weight ratio of 1:2, the spray-drying-assisted assembly can either yield a core–shell or an intermixed arrangement of both components (Figure 2a) dependent on the dispersion stability and electrostatic surface charges of the precursor nanoparticles.25,44 Below pH values of 6, both nanoparticles are positively charged and form an excellently stabilized dispersion due to mutual electrostatic repulsions. This results in a core–shell structure upon spray-drying, where the larger (SiO2) nanoparticles display the core and the smaller (TiO2) ones display the shell of a supraparticle, as the forces acting on the nanoparticles during solvent evaporation depend on their sizes. The evaporation of the solvent at the air/liquid interface causes a drag force on the nanoparticles toward the interface. Small nanoparticles migrate more easily and quickly toward the interface, while larger nanoparticles tend to remain in the droplet center. Upon destabilization of the dispersion due to a pH value around 7 to 8, which approximates the point-of-zero-charge of the nanoparticles (see zeta potential measurements of both nanoparticle types in Figures S4d and S9b), both nanoparticle types already preagglomerate statistically in the dispersion before spray-drying. This results in an intermixed supraparticle composition where both nanoparticle types are more or less homogeneously distributed throughout the supraparticle. The obtained samples indeed display the envisaged core–shell and intermixed structures as shown in scanning electron micrographs (Figure 2b, c and Figure S10 in the Supporting Information). While they both provide mean diameters similar to those of the pure TiO2 supraparticles (Figure S11a), their isotherms obtained in nitrogen sorption measurements show an abrupt and sharp hysteresis at p·p0–1 close to 1 without plateau, indicating the presence of larger meso- and macropores whose complete pore filling cannot entirely be resolved by adsorption, which is also indicated by the PSD and the cumulative pore volume. The intermixed supraparticles display a higher amount of mesopores than the core–shell ones (as derived from the PSD and cumulative pore volume plots). The core–shell structure provides a higher BET surface area of 145 m2/g compared to that of 102 m2/g of the intermixed TiO2–SiO2 supraparticles. Consequently, despite the same amount of TiO2 and SiO2 nanoparticles within both samples, their differing internal structure significantly affects their accessible surface area and their cumulative pore volume.

Figure 2.

Figure 2

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(a) Scheme showing the fabrication of either core–shell or intermixed supraparticles dependent on the stability of the TiO2–SiO2 nanoparticle precursor dispersion (displaying a TiO2 to SiO2 weight ratio of 1:2). Stable dispersions yield core–shell supraparticles, while destabilized dispersions result in intermixed supraparticles. SEM micrographs of (b) a core–shell supraparticle: its surface and its cross section and (c) an intermixed supraparticle: its surface and its cross section. White arrows indicate the areas of increased TiO2 nanoparticle accumulation within the supraparticle cross sections. (d) Cumulated concentration of H2O2 produced by TiO2, as well as TiO2–SiO2 core–shell and intermixed supraparticles over the UV-light exposure duration (dashed lines are only for guiding the eye), and (e) the same values referenced to the weight of contained photocatalyst within each supraparticle sample (dashed lines are only for guiding the eye).

Regarding the H2O2 productivity of these TiO2–SiO2 supraparticles, it was expected that the introduction of the inert SiO2 component would decrease the overall activity of the catalyst (Figure 2d). However, the extent of the decline in catalytic activity is strongly dependent on the position of the TiO2 nanoparticles within a supraparticle. The core–shell supraparticles produce significantly higher H2O2 amounts compared to the intermixed sample, which is due to a higher number of TiO2 nanoparticles located at the outside of the supraparticles. They can thus directly interact with the UV light that approaches the supraparticle45 or that is reflected from the inner SiO2-based core of the supraparticle. In the case of intermixed supraparticles, TiO2 presumably cannot be approached by UV irradiation due to the strong scattering of light by SiO2 nanoparticles. If the H2O2 productivity is displayed as per gram of photocatalyst component contained in a supraparticle sample (Figure 2e), this phenomenon becomes even more evident. TiO2 nanoparticles located in the outer shell of a supraparticle are more involved in photocatalysis compared with photocatalyst nanoparticles in the supraparticle core. Thus, the TiO2–SiO2 core–shell sample provides even higher activity per gram of contained TiO2 than pure TiO2 supraparticles. In this context, it has to be mentioned that the H2O2 formation pathway is not affected by the SiO2 component due to its amorphous/inert nature.

After the core–shell supraparticle structure was identified as the most preferred option, the determination of the most appropriate (weight) ratio of TiO2 to SiO2 within a supraparticle was of high importance for designing the most appropriate catalyst structure. Supraparticles were thus synthesized with varying TiO2 to SiO2 contents ranging from 6:1 to 1:6 TiO2:SiO2 as displayed in Figure 3a. The obtained supraparticle structures are shown in Figure 3b and Figure S12 (in the Supporting Information). With increasing SiO2 content, TiO2 shell thickness steadily decreases until a complete coverage of the SiO2 core is no longer given and single SiO2 nanoparticles in the case of TiO2:SiO2 = 1:1 or even clusters of SiO2 in the case of TiO2:SiO2 = 1:2, 1:3 or 1:6 stick out of the TiO2 shell and create a rougher (outer) supraparticle surface. With increasing SiO2, the supraparticle diameters slightly increase (Figure S13a in the Supporting Information) and the supraparticles display a higher amount of macro and a lower amount of mesopores (as derived from the nitrogen sorption isotherms, the PSD, and the cumulative pore volume plots, Figure S13b and c in the Supporting Information). BET surface area continuously decreases from 169 to 64 m2/g for supraparticles with TiO2:SiO2 ratios of 6:1 to 1:6.

Figure 3.

Figure 3

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(a) Scheme showing the surface composition of core–shell supraparticles dependent on the TiO2 to SiO2 weight ratio. An increase in the SiO2 nanoparticle content leads to their increased presence on the supraparticle surface. (b) SEM micrographs of core–shell supraparticles with increasing SiO2 nanoparticle content, i.e. rising from a TiO2:SiO2 weight ratio of 6:1 to 1:6. (c) Number-weighted particle size distribution of TiO2 supraparticles and core–shell supraparticles with increasing SiO2 nanoparticle contents, i.e., rising from a TiO2 to SiO2 weight ratio of 1:1 to 1:6; measured as prepared and after strong stirring (at 250 rpm) as a suspension in water for 24 h (pH = 7, using a magnetic stirring bar).

As for the application of such supraparticles in batch and flow reactors, their mechanical stability is a crucial requirement, and the stability of the obtained supraparticle samples was examined by measuring their particle size distribution before and after stirring as a suspension in water for 24 h (Figure 3c and Figure S14 in the Supporting Information). It is evident that with increasing SiO2 nanoparticle content, the stability of the supraparticles significantly increases as the original supraparticle diameters are retained after 24 h of stirring in suspension. A small decrease in diameters after 24 h of stirring as detected for pure SiO2 supraparticles (Figure S14 in the Supporting Information) and supraparticles with TiO2:SiO2 weight ratios of 1:2 and 1:6 is attributed to the separation of small agglomerates of supraparticles into single supraparticles. The stabilization effect is attributed to the ability of glutar aldehyde to self-polymerize46 and of SiO2 to form Si–O–Si bridges between nanoparticles during spray-drying,47 which presumably also stabilizes the contained TiO2 nanoparticles within the supraparticle shell.

The H2O2 productivity of these samples after 5 and 15 min of UV light irradiation is displayed in Figure 4 and Figure S15. In general, without considering the internal structure of the supraparticle, a continuous decrease in activity is expected with increasing SiO2 and thus decreasing TiO2 contents. Supraparticles with a TiO2–SiO2 ratio of 6:1 up to 2:1 however provide approximately the same H2O2 productivity after a 5 min exposure time. At this point, the samples have not reached their steady state, and the reaction follows a first-order kinetic as H2O2 production is dominant while its decomposition does not yet play a significant role in the overall reaction. It is expected that for the efficient oxygen reduction to H2O2 a high interaction of the photocatalyst with UV light is crucial. This is why only the TiO2 nanoparticles located in the outer shell of a supraparticle participate in this reaction, while the ones located inside a supraparticle do not play a role in it due to no interaction with UV light (as already mentioned before). The composition of the outer shell of the supraparticles stays the same for all supraparticles with a TiO2–SiO2 ratio of 6:1 up to 2:1, i.e. displays exclusively TiO2 nanoparticles. This consequently explains why they all provide the same concentration of produced H2O2 after a 5 min exposure time.

Figure 4.

Figure 4

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Cumulated concentration of H2O2 after 5 or 15 min of UV-light exposure produced by core–shell supraparticles with increasing SiO2 nanoparticle contents, i.e., rising from a TiO2 to SiO2 weight ratio of 6:1 to 1:6, in comparison to pure TiO2 supraparticles (1:0 TiO2:SiO2 ratio).

Regarding the H2O2 concentration after 15 min of irradiation time, the supraparticle samples with a TiO2–SiO2 ratio of 6:1 up to 2:1 now show the expected decreasing activity with declining TiO2 content within a supraparticle sample. In this context, it must be mentioned that all samples have reached their steady state in H2O2 production at this point in time. It is assumed that for the steady-state concentration, the overall amount of present TiO2 catalyst and thus catalytically active surface area combined with the local light intensity plays a significant role.31 Additionally, the porous structure of the supraparticle enables adsorption, desorption, and diffusion processes of reactant and product molecules throughout the entire supraparticle.48,49 The studied samples significantly differ in the total amount of surface area, the macro- and mesopore volumes, and the overall pore volumes. All these parameters may also affect the steady-state concentration of the samples (as already observed for other reactions with supraparticle catalysts50).

In the case of supraparticles with a TiO2–SiO2 ratio of 1:1 up to 1:6, the shell composition changes continuously, i.e., the amount of TiO2 in the outer shell(s) decreases, with increasing SiO2 amount while no further TiO2 nanoparticles are enclosed inside the supraparticle. This results in the expected decreasing activity with declining TiO2 content both after 5 and 15 min, while it has to be considered that the reactions have reached steady state already at 5 min irradiation time for these samples. At this state, other supraparticle properties such as the overall amount of the photocatalyst component, mesopore-macropore concentrations, or the overall surface area of a supraparticle, seem to play in general a more dominant role than the supraparticle structure itself (as discussed before). The evaluations of both the mechanical stability and the H2O2 productivity of the produced supraparticles and their correlation to the TiO2:SiO2 nanoparticle ratios as well as the overall supraparticle structures showed that both parameters evolve in opposite ways. Consequently, the supraparticle sample with a TiO2–SiO2 ratio of 1:2 appears to be the most promising candidate for use as a future hybrid catalyst. It is expected to provide sufficient mechanical stability and adequate H2O2 productivity. Regarding the impact of the supraparticle structure on a potential postsynthetic grafting of an enzyme or organocatalyst to the silica nanoparticle components, the TiO2–SiO2 ratio of 1:2 is also expected to be favorable. While the TiO2-based supraparticles display relatively narrow pore sizes (especially for the presented core–shell compositions) which potentially hinder the diffusion of large organic or biological molecules into the center of the supraparticles, the interruption of the outer TiO2 shell of a supraparticle by zones of silica nanoparticles as shown in Figure 3 ensures an accessible surface for the successful grafting of such large molecules. Additionally, these silica nanoparticle “islands” on the outer supraparticle surface may also allow a certain spatial separation of an attached organo- or biocatalyst from the photocatalytically active nanoparticles.

Regarding the stability and reusability of the studied supraparticles, several additional experiments were conducted. On the one hand, it was determined if both TiO2 and TiO2–SiO2 (1:2 ratio) supraparticles retain their H2O2 productivity after 24 h of stirring (Figure S16), which indeed was the case. The partial destruction of the TiO2 supraparticles upon stirring for 24 h (Figure 3c) seemed to only slightly affect the overall productivity of this sample, while the TiO2–SiO2 conserved as expected their activity due to no structural supraparticle changes. On the other hand, the morphology, chemical composition, and productivity of these catalyst supraparticles were conserved after a photocatalytic batch run (for more details, see Figure S17 and the corresponding discussion).

While the first study of H2O2-producing supraparticles was conducted in batch reactions, for further upscaling, reactions involving gases such as oxygen are severely limited in batch due to mass transfer limitations and safety concerns when high system pressures are applied.16 The use of light for photochemical reactions is similarly complicated in batches due to the large reactor sizes and difficulties regarding efficient irradiation of reaction suspension.16 This is why this study aimed to achieve the continuous flow synthesis of H2O2 through the use of the developed supraparticles with a capillary photoreactor comprising FEP tubing and a centrally positioned LED array for the irradiation of the reaction suspension (Figure 5a and Materials and Methods section). Synthetic air was introduced into the capillary reactor, together with the aqueous particle suspension, to create a three-phase slug flow (Figure 5b and c). The continuous flow movement of the suspension enabled by the slug flow ensured an evenly distributed catalyst for the reaction and prevented clogging.51

Figure 5.

Figure 5

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(a) Scheme for the photocatalytic generation of H2O2 in continuous flow mode and (b) photograph of the slug flow consisting of air segments and segments of aqueous TiO2/SiO2 (1:2) suspension. (c) Photograph of the continuous flow photoreactor with syringe pumps illustrating the main plant setup and (d) cumulated concentration of H2O2 produced by TiO2, as well as TiO2–SiO2 (1:2) core–shell supraparticles in the continuous flow setup over the UV-light exposure duration (dashed lines are only for guiding the eye).

The H2O2 production by using TiO2 and TiO2-SiO2 supraparticles with a 1:2 weight ratio was investigated in the capillary photoreactor (Figure 5d) using similar amounts of photocatalyst material as in batch (Table S3). The equilibrium between the production and degradation of H2O2 on the surface of TiO2 (observed as a plateau) was achieved at a residence time of 10 min, due to the degradation of H2O2 on the TiO2 surface at higher concentrations. Once again, a decrease in H2O2 generation was observed with a lower fraction of photocatalyst components, while the supraparticle mass fraction was kept constant. The overall achieved H2O2 concentrations were significantly higher in continuous flow compared to those in the initial batch experiments. In this context, it also must be mentioned that the supraparticles maintained their stability throughout the continuous flow reaction (Figure S18).

For obtaining further insights, the concentrations of produced H2O2 after 5 min of reaction time are compared in the following of both batch and flow synthesis for all supraparticle samples with TiO2:SiO2 weight ratios of 6:1 to 1:6 and pure TiO2 supraparticles either with the same amount of supraparticles (Figure 6a and b, as well as Figure S19a, b) or with the same amount of the photocatalytically active TiO2 component (Figure 6c and d, as well as Figure S19c, d) in each reaction (Table S4 and S5). Once again, it is evident that the overall achieved H2O2 concentrations are significantly higher in continuous flow compared to the batch experiments (especially, upon referencing the obtained H2O2 to the amount of contained TiO2 catalyst weight as shown in Figure S19). The enhanced productivity in continuous flow is attributed to an improved mass transfer due to the generated slug flow and the increased surface-to-volume ratio of the catalyst suspension. This consequently leads to a higher availability of oxygen as the reactant for the photocatalytic reduction reaction.

Figure 6.

Figure 6

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Cumulated concentration of H2O2 after 5 min UV-light exposure produced by core–shell supraparticles with increasing SiO2 nanoparticle contents, i.e., rising from a TiO2 to SiO2 weight ratio of 6:1 to 1:6, in comparison to pure TiO2 supraparticles (1:0 TiO2:SiO2 ratio): obtained in (a) batch reactions and (b) continuous flow reactions keeping the total supraparticle amount per batch constant, as well as in (c) batch reactions and (d) continuous flow reactions keeping the total TiO2 photocatalyst amount per batch constant.

In the case of a maintained amount of photocatalyst components (Figure 6c and d), the overall amount of supraparticles per reaction increases with higher SiO2 contents. In the batch reactions, the produced H2O2 concentration decreases with higher SiO2 contents in the case of supraparticles with a TiO2–SiO2 ratio of 1:1 up to 1:6. This is attributed to enhanced scattering of the UV light due to a high supraparticle concentration in the reaction. The overall light interaction is thus reduced, while the contained catalytically active component in a single supraparticle should still be able to easily interact with light due to the supraparticle structure. The same effect can be observed in continuous flow (reduction in the H2O2 concentration by roughly 50%).

4. Conclusion

Supraparticles consisting of either pure TiO2 or TiO2 and SiO2 nanoparticles in varying weight ratios were assembled by using the spray-drying process. H2O2 is generated by them via an oxygen reduction reaction, and its production reaches a steady state after a certain UV light irradiation time, which is typical for anatase-based photocatalysts. The synthesized supraparticles show significantly higher productivity compared to commercial P25 and P90. This is attributed to organic surface molecules from the wet-chemical TiO2 nanoparticle synthesis that prevent photodegradation of H2O2 to a considerable extent. Studying two different arrangements of the TiO2 and SiO2 nanoparticles within the supraparticle, namely a core–shell and an intermixed structure, it became evident that core–shell supraparticles produce significantly higher H2O2 amounts. This is caused by the higher number of TiO2 nanoparticles located at the outside of a supraparticle, which can directly interact with the UV light that approaches the supraparticle. An increase in SiO2 nanoparticle content within the TiO2–SiO2 core–shell supraparticles results in their enhanced mechanical stability, while their overall catalytic activity declines. Increasing concentrations of supraparticles result both in batch and in flow reactors in enhanced light scattering and thus a decrease in overall productivity, while the catalysts’ productivity is generally higher in flow reactions due to improved mass transfer and higher oxygen availability. The in situ synthesized H2O2 of the developed supraparticles can potentially be provided for cascade or downstream reactions, paving the way for efficient and sustainable syntheses by a controlled, safe, and needs-oriented production of H2O2.52,53 Additionally, this study displays a first step toward the future design of supraparticle-based hybrid catalysts for photoassisted cascade reactions.

Acknowledgments

The authors thank Gabriele Ulm and Johannes Prieschl for their support in the sample analyses (catalytic batch tests, SEM, and laser diffraction measurements). Valentin Müller is acknowledged for nitrogen sorption measurements. Furthermore, Ivette Herzberg, Julian Höth and Knut Welzel (all Fraunhofer IMM), are to be commended for their contributions to the electronics design of the hardware and software components. Hans-Joachim Kost (Fraunhofer IMM) is acknowledged for the design of the photoreactor and the lab plant.

Glossary

Abbreviations

BET

Brunauer–Emmett–Teller

FEP

fluorinated ethylene propylene

H2O2

hydrogen peroxide

NLDFT

nonlocal density functional theory

PSD

pore size distribution

SiO2

silica

TiO2

titania

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/cbe.4c00154.

  • Nanoparticle characterizations including materials and methods description, transmission as well as scanning electron microscopy, dynamic light scattering, X-ray diffraction, and zeta potential measurements; supraparticle characterizations including scanning electron microscopy, laser diffraction, nitrogen sorption, and measured hydrogen concentrations; photocatalysis setup photographs and calibration curve for the Eisenberg method (PDF)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. CRediT authorship contribution statement: Bettina Herbig: Conceptualization, Methodology, Formal analysis, Writing - Original Draft, Writing - Review & Editing, Visualization, Supervision, Project administration, Funding acquisition; Egzon Cermjani: Methodology, Validation, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Writing - Review & Editing; Doris Hanselmann: Methodology, Validation, Formal analysis, Investigation, Data Curation; Angelika Schmitt: Methodology, Validation, Formal analysis, Investigation, Data Curation; Christoph Deckers: Methodology, Validation, Writing - Review & Editing; Thomas Rehm: Conceptualization, Writing - Review & Editing, Supervision, Project administration, Funding acquisition; Karl Mandel: Conceptualization, Writing - Review & Editing; Susanne Wintzheimer: Visualization, Writing - Original Draft, Writing - Review & Editing.

This work was financially supported by the Fraunhofer Society and the BMBF/PTJ (ILLUMINATE, 031B1121), which is gratefully acknowledged.

The authors declare no competing financial interest.

Supplementary Material

be4c00154_si_001.pdf (2.7MB, pdf)

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