Hydrogen Peroxide Generation in Divided-Cell Trickle Bed

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Hydrogen Peroxide Generation in DividedCell Trickle Bed Electrochemical Reactor Ghassan H. Abdullah, and Yangchuan Xing Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02890 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 13, 2017

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Industrial & Engineering Chemistry Research

Hydrogen Peroxide Generation in Divided-Cell Trickle Bed Electrochemical Reactor Ghassan H. Abdullah, †,1 and Yangchuan Xing †, * † Department of Chemical Engineering, University of Missouri, Columbia, Missouri 65211, United States. *Corresponding author: [email protected] ABSTRACT: A divided-cell trickle bed electrochemical reactor (TBER) with a porous cathode composed of carbon black and polytetrafluoroethylene was developed for generation of hydrogen peroxide. An important feature of the reactor is a divided cathode of different cells made with stainless steel meshes. This division into sectional cathode resulted in a concentration of hydrogen peroxide that is more than twice of that produced in an undivided cathode. The much improved performance was attributed to the even distribution of electrolyte and oxygen in the cathode bed, as well as an effective mass transport of oxygen from the gas phase to the electrolyte-cathode interface.

Hydrogen

peroxide generation

was

demonstrated

from

electrochemically reducing oxygen in concentrated alkaline electrolyte solutions using the TBER. Factors for the hydrogen peroxide electrosynthesis were systematically studied, including cell potential, electrolyte flow rates and concentrations, temperatures, and the number of cells.

Keywords: hydrogen peroxide, electrosynthesis, trickle bed electrochemical reactor, divided cell, composite cathode.

1

Permanent address: Department of Chemical Engineering, University of Tikrit, Saladin, Iraq.

1 ACS Paragon Plus Environment


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1. INTRODUCTION Trickle bed reactors (TBRs) are fixed bed reactors with multiphase flows of gases and liquids, in which the fluids can flow concurrently downward at a low superficial velocity and the catalyst particles are in constant contact with the gas-liquid stream.1,2 TBRs are widely used in chemical processing, such as hydrogen peroxide production from anthraquinone process3, hydrogenation of citral4, hydrodesulfurization of crude oil5, hydrogenation of pyrolysis gasoline6, waste water treatment7 and removal of volatile organic compounds.8 An electrochemical process for generation of hydrogen peroxide using graphite chips in a trickle bed was introduced by Oloman and Watkinson in 1979.9 This process was good for onsite production of hydrogen peroxide. However, the low current efficiency (60%) and high oxygen pressure (800 kPa) keep it from being commercially feasible. Yamada et al.10 in 1999 developed a practical trickle bed electrochemical reactor (TBER) using carbon felt as cathode for hydrogen peroxide production in concentrated alkaline electrolyte solution with fairly large cathodic surface area of 0.8 m2. However, sodium peroxide produced in this process could be crystalized within the cathode, making it fouling quickly. Most recently, Lei et al.11,12 developed a TBER for in situ generation of hydrogen peroxide for waste water treatment. They used polytetrafluoroethylene (PTFE) coated graphite chips in the cathode. This PTFE can form hydrophobic regions to provide better gas access to the reaction surface sites. An alternative option for electrogeneration of hydrogen peroxide was suggested by Pérez et al.13, which has the advantages of easily operation, mild conditions, but suitable for small scale application. Most recently, Lu et al.14 have investigated the feasibility of in situ hydrogen peroxide generation using a novel stacked electrosynthesis reactor (SER) which composed of gas diffusion cathode,


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titanium plate anode coated with mixed metal oxides of IrO2 and Ta2O5, however a rare elements were used in their study. Hydrogen peroxide is considered one of the most powerful and versatile chemical oxidants for many processes such as paper and pulp bleaching15; it is also used as an oxidant in waste water treatment,16 disinfection applications17 and degradation of organic contaminants.18 It is environmentally friendly, because its reaction leaves no harmful residuals in the reaction stream, unlike that of chlorine-based bleaches.10 A high concentration of hydrogen peroxide is produced in large scale through oxygen reduction by hydrogen gas. However, there are some limitations about this process, such as using H2 gas in large amount and the need for organic compounds extraction.19, 20 Thus, electrochemical technology for hydrogen peroxide generation could be a viable alternative to the conventional processes. In trickle bed reactor, hydrogen peroxide can be produced electrochemically based on oxygen reduction reaction on the cathode surface at low current densities.21 It can also be produced by direct reaction of hydrogen and oxygen, but there are two major issues in the direct synthesis of hydrogen peroxide are related to the process safety and the product selectivity.22 The low solubility of oxygen in aqueous electrolyte solution23 often limits the production rate and efficiency.21 This problem can be resolved by using a threedimensional high surface area electrode such as carbon felt or reticulated vitreous carbon,24 and gas diffusion electrode (GDE).25,26 In such cathode hydrogen peroxide is generated in situ at the cathode without the need to dissolve oxygen gas in the electrolyte solution and the electrogenerated hydrogen peroxide is accumulated in the solution.27,28 GDE electrodes have problems, however, such as continuous leakage of the electrolyte to the gas feeder at the bottom of the electrode, and that when the height of electrode exceeds 0.25 m the gases produce bubbles in the electrolyte at the top of the electrode.15 These problems are



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attributed to the imbalance between the gas phase and the pressure levels in the electrolyte. The TBER could avoid leaking and bubbling problems observed in GDE, but the reported data showed a very low concentration of the generated hydrogen peroxide.11 For this reason, we developed a TBER composed of Vulcan-XC72 carbon black and PTFE (or C-PTFE). More importantly, a divided cathode was designed and used in the TBER, which demonstrated a much higher production rate of hydrogen peroxide.

2. EXPERIMENTAL SECTION 2.1 Materials. Carbon black (Vulcan-XC72) was obtained from Cabot Corporation. PTFE aqueous suspension (60 wt.% dispersion in water), KOH (85%), and molybdic acid (NH4)6 Mo7O24·4H2O were purchased from Sigma Aldrich. H2SO4 (98%) was purchased from Alfa Aesar and HNO3 aqueous solution was purchased from Fisher Scientific. Carbon cloth was obtained from the Fuel Cell Store. High purity deionized water (Millipore, >18.2 MΩ·cm) was used to prepare the electrolyte solution in all experiments. 2.2 Trickle Bed Electrochemical Reactor. The experiments have been carried out in the TBER developed in our lab as shown in Figure 1. The components of the reactor are two stainless steel plates as the anode and cathode support, a Celgard diaphragm, and a cathode frame with dimensions of 60 mm x 50 mm x 14 mm (width, height, depth). Three stainless steel meshes of 54 mm long, 12 mm wide and 0.85 mm thick were placed in the frame to divide the cathode into four cells and re-distribute the electrolyte inside the cathode frame. A 1 mm gap was left between the cells, so that the active dimensions were 53.45 mm x 50 mm x 14 mm. This TBER was compared to a previous reactor design without divided cathode used by Lei et al.11, 12 with active dimensions of 60 mm x 50 mm x 14 mm. Three thin rubber gasket rings (60 mm x 4 ACS Paragon Plus Environment

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50 mm) were placed between the anode and cathode and the diaphragm to electrically isolate the components. The cathode frame has a fluid inlet with five holes on the top for distribution of the fluids in the cathode, and a single outlet. The entire assembly is tightly fitted and sandwiched between two Plexiglas plates. Composite of the C-PTFE cathode was prepared by mixing carbon black with an appropriate amount of PTFE (60 wt.% in solution) at a mass ratio of 2:1 of C: PTFE.29 The mixture was placed and pressed in a divided frame to make porous C-PTFE blocks. These blocks were taken out and dried overnight under vacuum at 80 °C and then sintered at 360 °C for 2 h. A second bed with undivided cathode following that of Lei et al.11,

12

was also

fabricated by coating C-PTFE on a carbon cloth until the appropriate thickness was obtained. Before use, the carbon black was treated ultrasonically in a mixture of H2SO4 and HNO3 acids for 2 h at 60 °C to increase its hydrophilicity30 and create more surface oxygen functional groups, considered to be beneficial for H2O2 production.31 2.3 H2O2 Generation Procedures. H2O2 generation in concentrated KOH solution was carried out in a continuous flow mode using the TBER with C-PTFE composite cathode as illustrated in Figure 2. Oxygen was bubbled into electrolyte and delivered together in a flow to the trickle bed reactor from the top of the cathode frame. The electrolyte that comes out from the outlet at the bottom of the reactor was circulated back in a continuous flow mode and unreacted oxygen was flowing out with the electrolyte from the outlet. A power supply that can be run either at constant voltage or at constant current, was connected to the reactor to provide required cell voltage, and the electrolyte tank was connected to a circulating bath for temperature control. Different voltages (cell potentials) were imposed to the TBER in both cathode designs, and the H2O2 concentrations were obtained corresponding to the applied potentials. The concentration of the produced H2O2 was evaluated using a peroxymolybdate method.32, 33 In this method a sample



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of 0.5 mL of the solution was added to 5 mL of (2.4 mM) (NH4)6Mo7O24 in (0.5 M) H2SO4 solution, and a Genesys 20 Spectrophotometer was used to quantify H2O2 concentration at wavelength of 350 nm. 3. RESULTS AND DISCUSSION 3.1 Effect of Cathode Bed Design on H2O2 Generation. The production of hydrogen peroxide occurs through oxygen cathodic reduction in a three-phase reaction. In alkaline electrolytes H2O2 exists predominantly as HO2- (hydroperoxyl anion)34, and the main chemical reactions in alkaline solution including generation and reduction are:9 O2 + H2O + 2e- → HO2- + OH-

(1)

HO2- + H2O + 2e- → 3OH-

(2)

Experiments were carried out in both divided and undivided cathodes and the results are shown in Figure 3(a). It can be seen that the concentration of H2O2 is much higher with the divided cathode and 31.79 mM at 2.0 V was recorded, a 2.58-fold increase from 12.32 mM produced in the undivided cathode. It is noted that this concentration produced in the undivided reactor is in a good agreement with that (9.43 mM) produced in a similar reactor.11 The much higher concentration produced in the divided-cell reactor implies that a good re-distribution of the electrolyte and oxygen gas in the C-PTFE cathode was achieved by placing stainless steel screens to divide the cathode. This important configuration would lead to higher wettability of carbon black layers and more oxygen mass transport that result in higher effective area. Carbon black provides active sites for oxygen reduction to generate H2O231, thereby more wetted active sites are needed to produce more H2O2. It is noted that the maximum concentration was achieved at applied cell potential of 2.0 V for both designs. This is in good agreement with the value of 1.8 V for optimal H2O2 production by Oloman et al.9 The concentration started to decrease slightly


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from 2.0 to 3.0 V and strongly decreases at potentials higher than 3.0 V. Increasing the applied potentials activates side reactions, such as H2O2 oxidation on the anode to O2 via the formation -

·

of perhydroxyl radicals HO2 .35 Moreover, a direct reduction of H2O2 to OH on the C-PTFE cathode via a two-electron process is also possible36, or by reduction of the hydroperoxide ion.9 Figure 3(b) shows the impact of time on the process at different applied potentials. It can be seen that at all applied potentials the H2O2 concentration increased linearly in the first 60 min of the electrolysis. On the other hand, H2O2 concentration did not increase linearly with reaction time; after 60 min, the H2O2 concentration reached its steady-state value and remained almost constant for the remainder of the reaction time.37 H2O2 could undergo chemical decomposition to O2 either on the anode (heterogeneous process) or in the medium (homogeneous process) as ·

described in Equation (3).38 H2O2 could also be anodically oxidized to yield intermediate HO 2 radicals as seen in Equations (4) and (5).39 At steady state, H2O2 was electrogenerated and simultaneously destroyed in the system. H2O2 → H2O + ½ O2

(3)

·

H2O2 → HO 2+ H+ + e-

(4)

·

HO 2 → O2 + H+ + e-

(5)

The cross section of C-PTFE composite was examined by scanning electron microscopy (SEM) to understand the morphology of C-PTFE composite cathode as shown in Figure 4. It can be seen from image (a) the homogenous distribution of C and PTFE particles which is due to the good mixing during the preparation; this could be beneficial for making more channels and binding the structure of C-PTFE. A high magnification image (b) shows the in-depth structure of the composite. The image reveals that PTFE is bound on the carbon black particles and



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interconnected matrix is obviously observed. It also displays that there are channels in the bed confirming the porous structure, favorable for O2 gas to pass through these channels to the electrode/electrolyte interface and thus prevent mass transport limitations.28 The reactor structures that can be correlated to the fluid flows (channel flows vs periodic redistribution in a trickling manner), which is illustrated in Figure 5. Channel flow is presented in schematic (a) which shows that the initial uniform distribution of the electrolyte in the bed forms full and partial wetted areas by the flowing liquid, but the surface tension will tend to reduce the total film area, and then channel it in one stream at the middle forming unwetted surface in large fraction. Unwetted surface does not contribute to reactions for the peroxide generation. It was observed that even with an excellent initial distribution, the liquid may still gather into small channels.1 By contrast, dividing the cathode into cells as shown in schematic (b) redistributes the electrolyte in the bed, preventing it from channeling. This leads to increased wetted area in the cathode and enhancing oxygen mass transfer to the liquid-solid interface. The flow regimes can be identified, because several different flow regimes may exist in trickle bed reactors due to different levels of interphase interactions, such as trickle flow, pulsing flow and spray flow regimes.40 In the trickle bed reactor gas and liquid loadings can be in the range of about 0.0l – l.0 kg m-2 s-1 and 0.1 – 10 kg m-2 s-1 respectively41. These values correspond to the superficial velocities of 1 – 100 cm s-1 and 0.01 – 1.0 cm s-1 for air and water, respectively1. In the present case, the calculated gas and liquid loading were 0.0457 kg m-2 s-1 and 0.392 kg m-2 s-1, respectively, which confirmed that our system is in the regime of trickle flow. 3.2 Effect of Oxygen Gas. A comparison between supplying air and pure oxygen on H2O2 production is presented in Figure 6(a). It is obvious that the maximum concentration was


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achieved if pure oxygen is used and the minimum concentration observed if air is used. Air has only 21% oxygen, which leads to mass-transfer limitation as a result of a low concentration of oxygen in the electrolyte solution. An insufficient supply of oxygen decreases the production of perhydroxyl ions, thus reducing the H2O2 production.42 H2O2 concentration increases with increasing oxygen flow rate at constant electrolyte flow rate of 15 mL/min and reaches a maximum concentration at O2 flow rate of 2.0 L/min as shown in Figure 6(b). However, the concentration decreased by increasing the O2 flow rate higher than 2.0 L/min. The O2 flow rate determines the inlet O2 amount and affects the distribution of the gas and liquid phase holdup or gas-liquid phase in the trickle bed cathode, which further affects the electro-generation of H2O2.12 Increasing O2 flow rate leads to a decrease in liquid holdup and thickness of the liquid film on the C-PTFE cathode surface, which accelerates O2 transfer from gas phase to electrolytecathode interface. On the other hand, decreasing the liquid holdup by raising O2 flow rate lowers the wettability of C-PTFE bed and active reactor area, leading to low production of H2O2.43 3.3 Effect of Electrolyte. For the effect of electrolyte concentration on H2O2 production, the experimental data are given in Figure 7(a). The H2O2 concentration increases with increasing KOH concentration, reaching the maximum concentration of 43.76 mM at 6.0 M KOH. This could be attributed to the increase in the electrolyte conductivity, which extends the electroactive area. As well as, the higher concentration could protect the peroxide from the subsequent side reactions and promote the current efficiency. However, by increasing KOH concentration to higher than 6.0 M the H2O2 concentration started to quickly decrease. As the concentration of perhydroxyl ion increases, the rate of oxidation will increase and the rate of peroxide formation will decrease.9 The oxygen reduction reaction on the cathode in alkaline solution progresses by two competitive and successive reactions43 as stated in Equations (1) and (2) above.



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The effect of electrolyte flow rates was investigated and the results are shown in Figure 7(b). Findings showed that H2O2 concentration slightly increases with increasing electrolyte flow rate at constant O2 flow rate (1.5 mL/min) and reaches a maximum concentration at flow rate of 17.5 mL/min. The flow rate of the electrolyte is an important parameter that can influence the electrogeneration of H2O2 due to the influence on the distribution of gas-liquid phase in the trickle bed cathode.12 Raising the electrolyte flow rate increases the liquid phase holdup in the trickle bed cathode and leads to increasing of the thickness of liquid film on the C-PTFE bed. However, it would hinder and delay the O2 transfer from the gas phase to the electrolyte-cathode interface. Insufficient O2 transfer decreases the H2O2 generation. On the other hand, raising the electrolyte flow rate increases the C-PTFE wetting and the effective reactor area.43 3.4 Effect of Temperature. It can be seen from Figure 8(a) that H2O2 concentration increases with decreasing the temperature and reaches the maximum concentration (54.51 mM) at 0 °C. The reason is that more oxygen was dissolved in the electrolyte, because the oxygen solubility is strongly dependent on temperatures, and the higher the temperature is the smaller the O2 solubility. At 0 °C it increases 50% higher than that at room temperature in pure water.44 O2 solubility in 6.0 M KOH is plotted in Figure 8(b), in which the data were obtained from previous studies.45,

46

The results showed a linear decrease in O2 solubility with increasing

temperatures. In addition to increased oxygen solubility, the higher production rate was attributed to increased current yield.47 Low temperatures suppress H2O2 self-decomposition by reducing the rate of the non-faradaic reaction below:48 -

H2O2 + HO2 → H2O + O2 + OH-

(7)

3.5 Effect of Divided Cathode. The number of cells in the TBER is an important parameter to study regarding H2O2 electrogeneration performance in the reactor. H2O2 electrogeneration in


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the divided-cell TBER was evaluated using different number of cells, i.e., 2, 4, 6, 8, 10 and 12, at otherwise constant operating conditions. Figure 9(a) shows the experimental data. The results showed that H2O2 electrogeneration increases as the number of cells increase, as expected. It is attributed to that more divided cells would reduce channel flows and increase the effective area for reactions, which means more surface sites for oxygen reduction reaction that leads to increase the production rate of H2O2. The H2O2 concentrations in terms of the number of cells are further shown in Figure 9(b). A curve fitting was generated and shows a nonlinear relationship, attributed to more oxygen consumption in the cathode bed. The oxygen flow rate is fixed at the inlet and each cell would consume part of it, which should be related to reaction kinetics. It is not difficult to predict that the maximum concentration would be reached when the number of cells approaches infinity. A theoretical model is currently being developed and will be reported in a future paper.

4. CONCLUSION This paper reported a study on a divided-cell electrochemical trickle bed reactor, which is composed of C-PTFE as cathode designed for the production of H2O2. The divided-cell TBER showed a much higher production of H2O2 than the undivided cathode design. The better performance was attributed to the better distribution of aqueous electrolyte in the divided cathode. Re-distribution of the electrolyte can significantly increase the effective wetted cathode area and oxygen mass transport from the gas phase to the cathode-electrolyte interface. The maximum concentration of H2O2 was obtained at 54.51 mM under optimized conditions of 2.0 V cell potential, 6.0 M KOH, 2.0 L/min O2 flow rate, 17.5 L/min electrolyte flow rate, and at 0 °C temperature in the four-cell reactor. Therefore, the newly developed reactor with divided cathode



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cells presents a promising design for electrogeneration of hydrogen peroxide. Further understanding of the TBER would lead to an optimized reactor design for even higher production rate of H2O2 that could become a feasible industrial process.


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Figure 1. Trickle bed electrochemical reactor assembly and components.


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Figure 2. Flow diagram of electrolysis in continuous mode for the production of hydrogen peroxide.



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Figure 3. (a) H2O2 generation as a function of applied potential for the divided design and undivided design of the TBER in 2.0 M KOH, 15 mL/min electrolyte flow rate, 1.5 L/min O2 flow rate, 1 h and 20 °C. (b) H2O2 concentration as a function of electrolysis time in 2.0 M KOH, 2.0 V, 15 mL/min electrolyte flow rate, 1.5 L/min O2 flow rate, 1 h and 20 °C.


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Figure 4. SEM cross section images for C-PTFE composite. (a) Low magnification, and (b) high magnification.



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Figure 5. Schematic illustration of reactor designs. (a) Channel flow mode in undivided reactor. (b) Sectional flow mode (2cells) with electrolyte redistribution to avoid channel flow.


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Figure 6. (a) H2O2 generation as a function of applied potential and type of gas supplied. (b) H2O2 concentration as a function of O2 flow rate at constant electrolyte flow rate of 15 mL/min in 6.0 M KOH at applied potential of 2.0 V for 1 h at 20 °C.



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Figure 7. (a) H2O2 generation as a function of KOH concentration at 2.0 V, 1.5 L/min O2 flow rate and 15 mL/min electrolyte flow rate for 1 h at 20 °C. (b) H2O2 concentration as a function of electrolyte flow rate at constant O2 flow rate of 1.5 L/min in 6.0 M KOH, 2.0 V, 1 h and 20° C.


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Figure 8. (a) H2O2 generation as a function of temperature at constant flow rates of O2 2 L/min and solution flow rate of 17.5 mL/min in 6.0 M KOH at applied potential of 2.0 V for 1 h. (b) O2 solubility in 6.0 M KOH solution as a function of temperature.



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Figure 9. (a) H2O2 generation as a function of cathode cells number in 2.0 M KOH, 2.0 V, 1.5 L/min O2 flow rate and 15 mL/min electrolyte flow rate for 2 h and 20 °C. (b) H2O2 generation as a function of number of cells.


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TOC graph


Hydrogen Peroxide Generation in Divided-Cell Trickle Bed

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