Article pubs.acs.org/IECR
Development of a Dual-Stage Continuous Flow Reactor for Hydrothermal Synthesis of Hybrid Nanoparticles Henrik L. Hellstern, Jacob Becker, Peter Hald, Martin Bremholm, Aref Mamakhel, and Bo Brummerstedt Iversen* Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark S Supporting Information *
ABSTRACT: This paper provides a comprehensive description of the design and commissioning of a dual-stage flow reactor for hydrothermal synthesis, notably heterogeneous nanomaterials such as core−shell particles or nanocomposites. The design is based on the hypothesis that the next frontier of studies within continuous, hydrothermal synthesis lies as much with scalability as it does with the materials properties and performance in applications. Therefore, this reactor belongs to the up-scaled end of a laboratory system with a synthesis capacity of up to 50 g/h. Commissioning was accomplished with TiO2 nanoparticles as a model material. Results comply with earlier ones obtained from single-stage reactors. Dual-stage synthesis of a TiO2@SnO2 nanocomposite was performed by adding a SnCl4 solution to newly formed 9 nm TiO2 nanoparticles, yielding deposition of 2 nm rutile SnO2. Synthesis of pure SnO2 produced much larger nanocrystals, indicating that TiO2 nanoparticles provide the nucleation sites for SnO2 and impede the growth beyond 2 nm. single-stage hydrothermal flow systems, this is not possible as all the formation chemistry of the materials takes place in a single mixing point. The only way around is to add dissolved precursor to a presynthesized NP suspension. Dual-stage flow systems have previously been used in microfluidics.24 However, one of the main problems of continuous hydrothermal synthesis is the product clogging up the reactor system. The issue prohibits small scale laboratory reactors from using concentrated precursors and hence limits their production capacity significantly. Although a large synthesis capacity is of little importance in fundamental studies of synthesis, the aspect cannot be ignored for functional materials. The tacit assumption is often made that a synthesis will perform identically with an up-scaled reactor. Unfortunately, high capacity laboratory reactors are few and far between. This is especially true for dual-stage reactors, which is the latest generation of flow reactors for hydrothermal synthesis of heterogeneous nanomaterials and nanocomposites. One main reason is a lack of scientific literature detailing their construction, discussing design, advantages, etc. The purpose of this paper is to provide such details. The subject is development and commissioning of a novel, continuous, dual-stage reactor for hydrothermal flow-synthesis, with the potential of inspiration to other research groups considering similar constructions. The combination of dual-stage with deliberate departure from small-scale production is the novel aspect and value of the present reactor system; the targeted capacity is >10 g/h of NPs (dry-mass basis). Commissioning was based on parametric, single-stage syntheses of anatase TiO2 nanoparticles. Sub-
1. INTRODUCTION Continuous flow-reactors for solvothermal and hydrothermal synthesis have attracted much attention in recent decades. This is largely due to their ability to provide rapid heating and short reaction times, which is suitable for synthesis of a wide variety of metal oxide nanoparticles (NPs) with narrow size distributions.1−5 A metal precursor is rapidly heated by mixing with a superheated solvent stream to induce rapid nucleation and then led into a reactor for growth and/or further crystallization of the NPs. The performance of different mixing geometries has been evaluated by a range of methods including computational fluid dynamics,6,7 product characteristics,8 light absorption imaging,9 neutron radiography10,11 and X-ray diffraction tomography.12 A tee mixing geometry as used in the original work by Adschiri et al.13,14 is still the most common geometry15−17 likely due the simple design, but reactors with other mixing geometries have been explored; for instance, the nozzle reactor developed by Lester et al.18 is based on a concentric tube-in-tube counter-flow geometry, whereas Kawasaki et al.19 reported a swirl mixer. A supercritical pulsed-flow reactor was used by Eltzholtz et al.20 In recent years, hybrid nanoparticles in which a secondary material is deposited onto primary nanoparticles have attracted much attention as it allows further functionalization of the nanoparticles. The secondary material can protect the core particle from oxidation or agglomeration, enhance catalytic or optical properties, combine properties into a multifunctional material, etc.21,22 Using a closed autoclave, Pang et al. showed how the deposition of SnO2 on TiO2 could enhance the performance of a dye-sensitized solar cell.23 Advanced heterogeneous materials can be synthesized in a single reaction vessel when the primary precursor nucleates and grows at a lower temperature than the secondary precursor, but in most cases two separate steps are required. In conventional © XXXX American Chemical Society
Received: August 6, 2015 Accepted: August 11, 2015
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DOI: 10.1021/acs.iecr.5b02899 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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2. Prior to construction, 3D designs were modeled in Autodesk Inventor Professional 2013 (see the Supporting Information).
sequently, the dual-stage capacity was demonstrated with successful up-scale production of hybrid nanoparticles in the form of a TiO2@SnO2 composite.
2. EXPERIMENTAL SECTION 2.1. Design Background. The design background for the dual-stage reactor was the single-stage system shown in Figure 1. Constructed in 2008, it too adopted the up-scale approach by
Figure 1. Diagram of the original single stage flow reactor. Note the injector vessel. A single, internal thermocouple (TC) measures mixing temperature while pressure is shown on a conventional dial manometer.
employing 3/8″ tubes (4.3 mm I.D.) in a module-based design. Solvent heating consisted of a preheater (1/8″ O.D., Tmax = 300 °C) and a main heater (3/8″ O.D., Tmax = 450 °C), with a reactor heater identical to the latter. Independent PID controllers (Cal9400) were used to control the temperature of all the components. Inconel-625 tubes and fittings (Butech) were used for all heated components, with the exception of the preheater, which was Hastelloy C-276. A medium pressure cone and threaded connections were used to allow frequent assembly and disassembly for cleaning the reactor. To aid the latter, only straight tube segments were employed. Different pumps have been employed since 2008. For the synthesis of TiO2, HPLC pumps (LabAlliance Prep 24, max flow rate 24 mL/min) were used.25 In the final version of the single-stage reactor, flow was maintained by pneumatic metering pumps (Milton Roy, models P250 V225 and P125 V125, MK-II Oscillamatic Controller), granting control of the pumping frequency (≤45 strokes/min) as well as stroke volume (≤0.1 mL and ≤0.8 mL). Pressure was maintained by a Swagelok proportional-relief valve (PRV). To protect the pumps from wear, precursor solutions were not pumped directly; instead, they were loaded into a tubular injector vessel containing a movable piston (O-ring sealed). The piston was shifted hydraulically by liquid from the high-pressure pump, thus displacing precursor solution into the reactor system. The advantages of this reactor system are (i) its modular construction, (ii) simple and robust pumps, (iii) ease of operation and (iv) great versatility with respect to precursors that could be solutions, suspensions, gels, etc. The appreciable tubing I.D. minimized the risk of clogging while also easing the cleaning of the reactor between syntheses. The main disadvantage was the injector system, where the price of versatility was paid in terms of a semicontinuous and labor intensive production. 2.2. Dual-Stage Flow Reactor. Overview. A diagram of the new dual-stage supercritical flow reactor is shown in Figure
Figure 2. Diagram of the dual-stage flow reactor. For single-stage synthesis, the ball valve downstream of the primary reactor is set to the collection point Product #1 (deselects remaining system).
As with the previous reactor system, a modular design was adopted utilizing straight tubing sections with no reducing unions in the heated sections of the system (constant I.D. of 5.0 mm), facilitating cleaning of these tubes. In “cold” sections, smaller tube diameters were employed. The system was designed for a Tmax in heated sections of 300 °C for preheaters and 450 °C for main heaters; the operative Pmax was set to 400 bar with burst diaphragms rupturing at 500 bar. Table 1 summarizes these features. Two different types of pumps were selected for the system. For the pure solvents, double-acting HPLC-pumps (LabAlliance Prep 36) were chosen as they yielded a very smooth (nonpulsating) and constant flow, each up to 36 mL/min; precursors were introduced directly, i.e., without an injector vessel, by particle-tolerant Milton Roy, Milroyal D pumps (93 strokes/min, stroke volume ≤ 0.71 mL). These are suited for handling of corrosive chemicals and suspensions and are robust as few moving parts are in contact with the medium. The tradeoffs by using this combination of pumps were (i) that all pure solvents had to be degassed (ultrasonication, vacuum) in order to comply with the HPLC pumps and (ii) that the Milroyal D pumps were not suitable for thick gels and concentrated suspensions. In such cases, precursors are pumped by separate pumps and mixed inside the system (see Table 1). The preheaters were constructed from 1/8″ Hastelloy C-276 tube whereas 3/8″ Inconel-625 was chosen for solvent tubing (see Supporting Information for heater description). For the reactors (downstream of mixing points), standard SS316 tubing was chosen based on cost considerations. First and foremost, B
DOI: 10.1021/acs.iecr.5b02899 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Table 1. Comparison of Specifications, Single-Stage Reactor (Model/Predecessor) vs the New Dual-Stage Reactor reactor specifications
single-stage reactor25
dual-stage reactor
length (cm) inner diameter (mm) volume (mL) max. working pressure (bar) safety limit, pressure (bar) max. temperature (°C) max. flow rates, solvent (mL/min, per pump) max. flow rates, precursor (mL/min, per pump) pumps, solvent pumps, reactant material, cold sections material, solvent heaters material, reactor zones
50 4.3 7.3 500 600 450 4.5 or 36 4.5 or 36 Milton Roy P125 V125 or P250 V225 Milton Roy P125 V125 or P250 V225 SS316 Inconel 625 and Hastelloy C-276 Inconel 625
100 5.0 19.6 400 500 450 36 66 LabAlliance Prep 36 Milton Royal D SS316 Inconel 625 and Hastelloy C-276 SS316
set independently from the synthesis temperature of the primary one. A distinct difference in the new dual-stage reactor compared to its predecessor is automatic monitoring of the system and synthesis conditions. Thermocouples monitor temperature of solvent downstream of the final heater, inside the mixing block and at the reactor outlet. Pressure transmitters and manometers (Armaturenbau) were installed to monitor pressure and the data collected by a single control unit (CRIO) (CompactRIO 9076, National Instruments). The precursors and solvents are placed on individual balances (Kern, 440-47N) in order to monitor mass-flow through the system. The balances are all connected to a serial server (Moxa, NPort 5610-8-DTL), and the data sent via an ethernet connection to the CRIO unit. Flow rates are determined by linear regression of the mass data from the scales sampled over 30 s. Flowmeters were deliberately avoided due to the extra dead-volume, connections and cost. LabVIEW software constantly monitors pressure and an internal alarm is triggered if the pressure exceeds 325 bar. In response, the software would automatically turn off power to all pumps and heaters by electronically operated switches. The PID controllers remain close-circuited. Only the PID output circuit is opened in order for the temperature of the heaters to be monitored. This kind of automatic monitoring and fast response is advantageous for a research facility with many users. General Operation. In normal operation, system pressure typically fluctuates by ±5 bar. Higher fluctuations are seen for viscous suspensions formed at high concentrations as particles first sediment in the PRV and are randomly eluted by the resulting pressure build-up. When using the secondary reactor, pressure fluctuations are usually lower. This is due to the larger total reactor volume. It is a general feature that a cold reactor system will exhibit a higher degree of pressure fluctuations due to incompressibility of the cold solvent; they significantly diminish (or disappear) at high temperature due to the much higher compressibility of near- and supercritical fluids. Additionally, the PRV is more open at high flow rates than at low flow rates, which further decreases particle sedimentation in the valve. General Considerations. Adding extra pumps to a system is tempting in terms of flexibility (added mixing options), but it also increases the total flow-rate and sequentially dilutes asformed suspensions in the downstream direction. This is a matter to consider even at low flow-rates, with the added issue
the choice of SS316 enabled a whole array of reactor tubes: one for each material being synthesized. The modular design enabled direct exchange of reactor tubes, and this made their cleaning an off-line operation that could be undertaken while other syntheses commenced immediately. Single-Stage Mode. As with the previous reactor model, Tpiece mixing was adopted. The reactor zone was extended and constructed as a 1.1 m SS316 3/8″ tube heated by two separate heating modules (each 45 cm long). A 35 cm straight cooler reduces temperature in the flow upstream of a ball-valve that directs the flow either to a PRV for direct product recovery or into the second reactor stage. In this manner, single- or dualstage was made optional on the system. Dual-Stage Mode. The secondary reactor zone (second mixing point and reactor) was made identical to the primary one. Instead of product recovery from the primary reactor, the primary product suspension would be cooled to RT and mixed with new precursor in a T-piece. The blend of nanoparticles and fresh precursor is then led into the secondary reactor for crystallization of the secondary phase. A second superheated solvent stream (T-piece mixing) was installed to provide a faster heating of the blend if necessary. Cooling to RT prior to mixing with new precursor for the dual-stage was a deliberate choice. It was motivated by the expectation that introducing a new precursor directly into a hot stream would inevitable cause some primary nucleation, i.e., formation of separate nanoparticles yielding a two-phase system rather than nucleation directly onto the primary material. A good mixing between the solution of the new precursor and the primary particles already formed was held to be essential, hence a static mixer was added (StaMixCo). Also, experience shows that cold streams do not cause clogging or fouling in tubes even at reduced diameters (here 1/ 4″, 2.8 mm I.D.), and the higher solvent density combined with increased flow-velocity are capable of carrying NP suspensions in any direction (vertical/horizontal) without sedimentation. This is useful as heated sections carrying particles should preferably be vertical in order to avoid sedimentation (eventual blockage). It requires a laboratory of unusual ceiling height to allow for a dual-stage reactor with no upward-going or horizontal particle flows; in the present case, it was indeed necessary to “fold” the entire setup around itself. A steel grid wall is used to suspend the primary reactor on one side and the secondary reactor as its mirror image on the opposite side. Finally, the choice of intermediate cooling between reactors enabled the reaction temperature of the second reactor to be C
DOI: 10.1021/acs.iecr.5b02899 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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SnO2 was synthesized from analytical grade SnCl4·5H2O (Reactant #2 in Figure 3) dissolved in demineralized water and pumped into the system. Reactant #3 in Figure 3 was demineralized water added to simulate flow rates at full system capacity and evaluate heating and cooling performance. The concentrations of both metal precursors were 0.1 M. The flow rates of the three cold reactant streams were all 4 mL/ min. Water was used as the hot solvent streams in both reactor sections. The flow rates of the two solvents were both 10 mL/ min. Consequently, the flow rate of the cold stream in the secondary reactor is 22 mL/min. For the synthesis reference of SnO2 without TiO2, the Ti-precursor was replaced with isopropyl alcohol; in this way, the main reactor only contained a water−alcohol mixture. All syntheses were performed at 250 bar. Temperature in the primary reactor was 350 and 300 °C in the secondary reactor. 2.4. Characterization. NPs were collected from suspension by centrifuging and decanting the supernatant. The products were then repeatedly washed with ethanol, centrifuged again and finally dried in an oven at 80 °C under vacuum overnight. The nanomaterials were characterized with powder X-ray diffraction, PXRD (Rigaku “SmartLab”, Ni-filtered Cu Kα radiation in parallel beam), scanning transmission electron microscopy, STEM (FEI Talos F220A operated at 200 kV) and X-ray fluorescence spectroscopy, XRF (Spectro Xepos-II, 4 secondary targets, TQ-powder software). The dissolution of metals from the reactor was monitored by inductively coupled plasma optical emission spectroscopy, ICP-OES (Spectro Arcos). STEM analysis included bright field, high angle annular dark field (HAADF) and energy dispersive X-ray spectroscopy (EDX). Rietveld refinements of as-produced nanomaterials were carried out using the Fullprof software.26 Silicon powder was mixed with TiO2 powders and used for crystallinity calculations. Structural models from the ICSD database (TiO2 anatase ICSD-92363; silicon ICSD-51688; SnO2 rutile ICSD-9163) were used as the initial models for Rietveld refinements of the data on a linearly interpolated background. A LaB6 standard was used to characterize instrumental broadening and to prepare an instrumental resolution file (IRF). A ThompsonCox-Hastings function was used to model peak profiles. The Lorentzian profile parameter X was kept at 0 as strainbroadening for small nanoparticles is usually negligible when compared to size-broadening, and it did not significantly improve the fit. Peak broadening was therefore attributed exclusively to size-broadening and only the Lorentzian Y parameter was used.
that pump performance (flow accuracy, constancy) typically suffers at the lowest flow-rates. The overall concern is residence time as well as heating and cooling efficiency. Additive mass flows lead to increasing flowvelocities, which in turn limit the achievable residence times in downstream reactor sections. The way around this is to use large I.D. tubing for such sections; this, however, increases wallthickness in the tube, which in turn sacrifices heat transfer through the metal into the fluid. High flow-velocities, either from a high overall flow rate or pressure build-up and release, may cause inadequate cooling in which case near- or supercritical fluid may reach the PRV and damage the O-ring seals. This is a particular concern in systems which use straight jacket coolers in order to enable cleaning of the reactor tube; here cooling efficiency scales with cooler length. A coiled heat-exchanger would not suffer from this problem but conversely introduce the risk of cross-contamination between syntheses, unless each material in the portfolio was assigned its own cooler (to be exchanged between syntheses). 2.3. Syntheses. TiO2 was synthesized from titanium tetraisopropoxide (TTIP) (97%, Sigma-Aldrich) dissolved in isopropyl alcohol (99.9%, Sigma-Aldrich). The solvent was mixtures of isopropyl alcohol and demineralized water ranging from 3 to 100 mol % water. The flow rates used were 10 mL/ min for both precursor and solvent. TTIP concentrations ranged from 0.125 to 1.0 M. Reactor temperatures were varied from 250 to 400 °C. Solvent temperatures were kept identical to reactor temperatures. In the synthesis of TiO2@SnO2 hybrid nanoparticles with SnO2 deposited on a TiO2 core, the secondary used the reactor configuration shown in Figure 3. The adaptation was straightforward due to the modular construction of the system, affirming this to be a sound choice of design.
3. RESULTS AND DISCUSSION 3.1. Synthesis of TiO2, Commissioning. TiO2 is an excellent “model material” for testing flow-reactors, conveniently produced via hydrolysis and condensation of TTIP to form the oxide. The parameter matrix spanned by (i) synthesis temperature, (ii) TTIP concentration in reactant and (iii) water concentration in solvent has previously been shown to affect crystallite size and NP crystallinity in a systematic way.25 Also, utilizing this particular synthesis allows for very high concentrations of TTIP (>1 M), which yields a dense paste of TiO2 NPs. For an up-scale laboratory system such as the one presented here, this feature enables “extreme testing” of the production capability and investigation of clogging tendencies. Such studies have been carried out using a single-stage reactor, which was the model and predecessor of the system
Figure 3. Diagram of the dual-stage flow reactor a configured for TiO2@SnO2 synthesis. D
DOI: 10.1021/acs.iecr.5b02899 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 5a shows data obtained from synthesis at 350 °C. A trend is observed of decreasing particle size as a function of increased water content in the solvent. The same trend was also observed in the previous work,25 which yielded crystallite sizes of ∼13−18 nm for low water contents whereas the new reactor yielded ∼15 nm under similar conditions. These sizes are comparable to those obtained by Kawasaki et al.27 Correspondingly, at the highest water contents, the new reactor also yields slightly smaller crystallites of ∼6 nm compared to ∼8 nm on the predecessor. Small nanoparticles at high water contents were also observed by Zhang et al.28 Generally, the higher water content means water is more available for hydrolysis; therefore, more crystals are formed and the average crystallite size decreases. The difference between systems may be a consequence of the pneumatic pumps driving the model reactor. These pumps have a pulsed stroke that in practice injects unit volumes of precursor into the heated solvent stream. Hence, a local depletion of water may occur with each stroke, which leads to the observed extra crystallite growth. By comparison, the new reactor system exhibits a greatly reduced pulsation in the precursor flow. Figure 5b shows the crystallite size as the precursor concentration increases from 0.125 to 1.0 M. It was found that the concentration in this range does not affect crystallite size. These results are also in good agreement with the previous study.25 At a concentration of 1.0 M TTIP, the pressure was
presented here.25 Hence, the new commissioning was based on an identical synthesis protocol in order to compare the results. For this reason, the new reactor was operated in a single-stage mode during commissioning. As with the previous study, all TiO2 samples were found to be phase-pure anatase. A representative Rietveld refinement of one of the samples is shown in Figure 4.
Figure 4. Rietveld refinement of TiO2 synthesized at 300 °C using 100% water and 0.25 M TTIP. The solid black line is the Rietveld refinement and it shows that pure anatase is formed. Parameters from refinement are included in Table S1.
Figure 5. TiO2 crystallite size in response to water content in the solvent (3, 5, 10, 50, 100 mol %) and different precursor concentrations, synthesized at 350 °C. (b) Crystallite size in response to precursor concentration for different water concentrations in the solvent. (c) Crystallite size in response to temperature for different water concentrations in the solvent. (d) Crystallinity of TiO2 in reponse to temperature for different water concentrations in the solvent. E
DOI: 10.1021/acs.iecr.5b02899 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research unstable due to the high viscosity of the paste-like product and at one point the reactor clogged. An increase in temperature was found to increase the crystallite size as well as the crystallinity as shown on Figure 5c,d for a TTIP concentration of 0.25 M. In comparison with the earlier study of Mi et al.,25 the overall crystal growth is similar in the respect that high H2O contents yield the smallest particles. In this study, remaining trends are less clear than in the previous one which showed clear inverse relationships between crystallite size and water contents. Accepting these observations, the 10% H2O data in this study are the hardest to explain. Another interesting fact is that the new reactor system seems able to produce smaller NPs, starting at ∼4.5 nm (250 °C), which under similar conditions yields ∼6.0 nm on the predecessor reactor. This could be attributed to the calculation method: the crystal sizes found in by Mi et al.25 are based on the Scherrer equation applied to a single peak fitted by a Lorentzian function. The current data sets were all Rietveld refined with all peaks included in the refinement and crystal size calculation. An increase in the water content of the solvent from 50% to 100% (Figure 5d) decreases the crystallinity by about 15% independently of synthesis temperature. The higher crystallinity at 50% water content is estimated to be due to faster heating of the reactor medium as the heat capacity of isopropyl alcohol is less than that of water. The study confirms the trend from Mi et al.25 However, although the curves are very similar, absolute crystallinities are not directly comparable. CaF2 was used as a standard in the previous work and the assumption of 100% crystallinity of the standard may not have been justified as the hygroscopic powder could have absorbed moisture from the air. This would have the effect to seemingly increase the crystallinity of the measured TiO2 powder. On the basis of the above observations, the commissioning was considered successful and in good agreement with the previous work on the model, single-stage predecessor. The trends of crystallite size with TTIP concentration and water contents in the solvent were reaffirmed, and the influence of temperature on both crystal size and crystallinity were comparable to the previous study. TiO2 powder synthesized from 1.0 M TTIP at 10 mL/min confirms a capacity of up to 50 g/h, but for most syntheses, the capacity will be around 10 g/h. 3.2. Synthesis of TiO2@SnO2. SnO2 was found to crystallize in the rutile structure. In the absence of TiO2, the product contains both SnO2 crystallites with an average size of 34 nm and smaller crystals of 2.4 nm (Figure 6 and Figure 7). This indicates that nucleation is not confined to the mixing point but is also occurring further down the reactor. The large particles are the first to nucleate and keep growing in the presence of excess unreacted Sn precursor in the reactor. Further downstream, as the precursor is heated, the small SnO2 particles nucleate, however, at a lower Sn concentration. A potentially competing growth mechanism is by Ostwald ripening in which the small SnO2 particles dissolve into precursor for the larger SnO2 particles. This was observed in situ at temperatures above 250 °C by Jensen et al.29 However, because of the lower concentration and shorter reaction time scale in our experiment, Ostwald ripening is likely not a major contributor to particle growth inside the dual-stage reactor. When SnO2 crystallizes in the presence of TiO2, the large SnO2 particles are no longer present as seen in Figure 8. Instead, the SnO2 crystallites remain at 2.1 nm. The presence of TiO2 is seen to stabilize the small SnO2 particles, and this
Figure 6. Rietveld refinement of rutile SnO2 synthesized at 300 °C and 250 bar from 0.1 M SnCl4 in the secondary reactor. Data were Rietveld refined with SnO2 rutile structures of two different crystal sizes, converging at 2.4 and 34 nm. Parameters from refinement are included in Table S2.
Figure 7. TEM image of SnO2 nanoparticles synthesized at 300 °C and 250 bar from 0.1 M SnCl4 in the secondary reactor showing a bimodal size distribution.
Figure 8. Rietveld refinement of TiO2@SnO2 at 300 °C. TiO2 was synthesized in the primary reactor at 350 °C from 0.1 M TTIP. The TiO2 suspension was mixed with 0.1 M SnCl4 and SnO2 was grown on the TiO2 particles in the secondary reactor. Rietveld refinement reveals 2.1 nm SnO2 and 9.2 nm TiO2. Parameters from refinement are included in Table S3. F
DOI: 10.1021/acs.iecr.5b02899 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Sn concentration, but both led to the formation of a Sn0.4Ti0.6O2 solid solution at the interface. The present results show no indications of such solid solution formation. This is likely due to the brief residence time in the secondary reactor, approximately 40 s. 3.3. Corrosion in Reactor Tubes. The degree of corrosion was investigated and selected TiO2 samples and supernatants were analyzed with XRF and ICP (Table 2). No significant reactor metals concentrations were found for the TiO2 sample.
indicates that SnO2 nucleates directly on TiO2. The anatase crystallite size is 9.2 nm, somewhat larger than the 6.2 nm found using only the primary reactor (single-stage). This is due to the lower precursor flow rate which causes a more rapid heating of the reactor contents. The observation is in agreement with previous results,25 where 8.2 nm TiO2 particles were obtained by increasing the ratio of solvent flow to precursor flow. Substitution of Sn for Ti in the anatase structure leads to an expansion of the unit cell.30 The anatase TiO2 unit cell in the TiO2@SnO2 composite was a = 3.7873(6) Å, c = 9.485(2) Å. These values are within the same range as those obtained from the TiO2 synthesized using only the single-stage reactor. On this basis, it is concluded that substitution of Sn into the anatase crystal lattice is insignificant. Unit cell data are given in the Supporting Information. Figure 9 confirms deposition of SnO2 on the TiO2 particles. Suspicion that particles are separate and merely adsorbed
Table 2. Elemental Composition of a Representative TiO2 Sample and the TiO2@SnO2 Compositea TiO2, 0.25M, 50% H2O, 350 °C
Ti Sn Fe Cr Ni
TiO2@SnO2, 300 °C
XRF [w/w %] (solid product)
ICP [mg/kg] (supernatant)
XRF [w/w %] (solid product)
ICP [mg/kg] (supernatant)
60.84(4) 0.00054(4)
