Heterogeneous catalytic reactor for hydrogen production from formic acid and its use in polymer electrolyte fuel cells Igor Yuranov1,2, Nordahl Autissier2, Katerina Sordakis1, Andrew F. Dalebrook1, Martin Grasemann1, Vit Orava3, Peter Cendula3*, Lorenz Gubler4*, Gábor Laurenczy1* Laboratory of Organometallic and Medical Chemistry, Group of Catalysis for Energy and Environment, École Polytechnique Fédérale de Lausanne, EPFL, CH-1015 Lausanne, Switzerland; 2 GRT Operations SA, CH-1350 Orbe, Switzerland; [email protected]
(I.Y.); [email protected]
(N.A.) 3 ZHAW, ICP - Institute of Computational Physics, Zurich University of Applied Sciences, CH-8401 Winterthur 4 Electrochemistry Laboratory, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland * Correspondence: [email protected]
, [email protected]
, [email protected]
Received: 26 January 20187; Accepted: Published: date
Abstract A proof-of-concept prototype of a heterogeneous catalytic reactor has been developed for continuous production of hydrogen via formic acid (FA) dehydrogenation. A laboratory-type proton polymer electrolyte fuel cell (PEFC) fed with the resulting reformate gas stream (H 2 + CO2) was applied to convert chemical energy to electricity. To implement an efficient coupling of the reactor and FC, research efforts in interrelated areas were undertaken: 1) solid catalyst development and testing for H2 production; 2) computer modelling of heat and mass transfer to optimize the reactor design, 3) study of compatibility of the reformate gas fuel (H 2 + CO2) with a PEFC, and 4) elimination of carbon monoxide impurities via preferential oxidation (PROX). During the catalyst development, immobilization of the ruthenium(II) – metatrisulphonated triphenylphosphine, Ru-mTPPTS catalyst on different supports was performed and this complex, supported on phosphinated polystyrene beads, demonstrated the best results. A validated mathematical model of the catalytic reactor with coupled heat transfer, fluid flow and chemical reactions was proposed for catalyst bed and reactor design. Measured reactor operating data and characteristics were used to refine modelling parameters. In turn, catalyst bed and reactor geometry were optimised during an iterative adaptation of the reactor and model parameters. PEFC operating conditions and fuel gas treatment/purification were optimized to provide the best FC efficiency and lifetime. The low CO concentration (below 5 ppm) in the reformate was ensured by a preferential oxidation (PROX) stage. Stable performance of a 100 W PEFC coupled with the developed reactor prototype was successfully demonstrated.
Keywords: formic acid; dehydrogenation; hydrogen; heterogeneous catalyst; ruthenium; carbon monoxide elimination; preferential oxidation PROX; polymer electrolyte fuel cell,
Introduction Hydrogen could play an important role as energy carrier in future energetic scenarios because of its high gravimetric energy content and the conversion pathway (combustion or electrochemical oxidation) resulting in water as the only product.1–5 However, the success of a potential H2-based economy hinges upon crucial areas: H2 generation, storage and delivery to end-users. Conventional storage methods comprising compressed and liquefied H 2 are disadvantageous due to significant energy requirements and inherent safety risks. 6,7 Liquid organic hydrogen carriers (LOHC) where hydrogen is stored chemically provide a valuable alternative. For example, formic acid (FA), a low-hazardous liquid containing 4.4 wt.% or 53 g L-1 of hydrogen, can release H2 on demand in the presence of a catalyst.8–15 H2 generators based on FA can find applications in mobile or decentralised power sources, along with larger fixed-site installations for power grid backup generation. FA decomposition into H 2 and CO2 is a reversible reaction.16–21 In fact, CO2 is a recyclable energy vector as it can shuttle between the stable “charged” (FA) and “discharged” (H2 + CO2) states delivering H2 ready for use.22 It was shown CO2 can be hydrogenated catalytically in aqueous solution.23–29 The feasibility of the FA-H2 technological approach has been already validated, running continuous FA decomposition over a homogeneous Ru-mTPPTS catalyst.8,30,31 This complex demonstrated high activity and was completely stable over a 2-year period of intermittent use. An extensive characterization of the homogeneous catalytic system provided a detailed insight into the reaction mechanism as well its intrinsic limitations.32,33 However, in the developed FA-based hydrogen storage system a heterogeneous catalyst seems to be preferred to the homogeneous analogue. The homogeneous Ru-mTPPTS catalyst is active and stable only in solution. The “liquid catalyst” can leach out over an extended period of time and has a problem of water content: dilution/evaporation. A heterogeneous catalyst,
which could be shaped into a bed of any geometry optimized for multiphase flow conditions, solves all these problems; a step forward to a simple and safe process. Heterogeneous catalytic systems can be used also with diluted formic acid solutions and with HCOOH produced from biomass/biowaste.34,35 Therefore, a heterogeneous catalytic system based on immobilized homogeneous Ru-mTPPTS catalyst was used in this study. The main objective of this research was to develop a concept of a medium-scale heterogeneous catalytic reactor for continuous production of H2 from FA with subsequent use in a PEFC. To realise an efficient coupling of reactor and FC, the following interrelated research tasks were solved: 1) heterogeneous catalyst development; 2) computer modelling of precise flow dynamics, heat transfer mechanisms and effects of catalyst particle size, geometry, etc.; 3) study of compatibility of the reactor gas outflow (H 2+CO2) with single- and multi-cell high temperature (HT) and low temperature (LT) PEFC, with Pt and Pt-Ru electrocatalyst, and study of the effect of CO impurities on the FC performance.
1. Results and discussion 1.1.
Ru-mTPPTS catalyst supported on triphenylphosphinated polystyrene beads
Different active Ru complexes, supports (zeolite ZSM-5, mesoporous MCM-41 silica and functionalized polystyrene (PS)) as well as immobilization techniques were applied. The performance of the resulting catalysts was assessed using catalyst activity and stability measurements. Finally, the catalyst of choice was Ru-mTPPTS complex immobilized on triphenylphosphinated cross-linked polystyrene beads (Sigma-Aldrich) (SAFC). The catalyst support ( = 0.3-0.6 mm) is suitable for use in a continuous reactor. A large quantity (300 g) of solid catalyst; Ru-mTPPTS complex immobilized on triphenylphosphinated cross-linked polystyrene beads; was prepared by immobilization of Ru-mTPPTS complex within the
polystyrene porous structure through triphenylphosphine functionalities (cf. Supporting Information). The Ru content in the prepared batch was measured through an elemental analysis using an Inductively Coupled Plasma Mass Spectrometry (ICP-MS) method. Samples for the analysis were prepared as follows: ~0.1 g of the catalyst was calcined at 700 °C in air for 8 hours in order to remove the organic constituents and to convert Ru to RuO 2. RuO2 was then dissolved in an oxidative KNO3/KOH fusion (350 °C, 0.5 hour). The resulting mixture was diluted with 5% HCl up to 100 ml. The synthesized Ru-TPPTS/PS catalysts were found to contain 2.27 wt. % of Ru. Reactor setup (Figure 1, Figure S1 - SI):
Figure 1: A scheme of the experimental setup of the formic acid reformer unit.
A continuous feed of pure FA was provided by a HPLC pump (Kontron Instruments 422). A (H2 + CO2) gas flow evolving from the reactor through the top flange was cleaned from H 2O and FA vapors by passing through a condenser and adsorption column loaded with activated carbon extrudates and MgO granules (98%, ~30 mesh, Sigma-Aldrich). The pressure in the
system was regulated by a metering valve ( Figure 1, Figure S1 – SI) The gas flow rate was measured (20 °C, 1 atm) using a bubble flow meter. In special experiments, the gas mixture was treated in a PROX catalytic converter (Figure S2). The converter consisted of a commercial metal foil honeycomb monolith coated by a Pt/CeO 2 catalyst (Johnson Matthey, Ø = 25 mm, L = 75 mm), a mixer, a pre-heating loop and three thermocouples placed at the inlet, outlet and in the center of the monolith. A small amount of air was injected into the (H2+CO2) stream before the mixer. The air flow rate was controlled through a mass-flow controller (MFC). The composition of a gas mixture (H2, CO2 and CO) was monitored by in-line IR analyzers (Calomat 6 and Ultramat 23, Siemens) and a gas chromatograph (Agilent 7890B) equipped with a column (CarboPLOT P7, 25m x 0.53mm x 25µm) and a Nickel Catalyst Kit to convert CO (CO2) to CH4. It was detected that the presence of a large amount of CO 2 influenced the CO measurements by infrared (in ppm level) analysis. Thus, all CO measurements were carried out also through an Agilent 7890B gas chromatograph calibrated using standard (50% H2 + 50% CO2 + 40 ppm CO) and (50% H2 + 100 ppm CO + 50% N2) mixtures (Carbagas). The CO sensitivity of the used GC method was found to be 2-3 ppm. 1.1.1.
Catalytic tests: catalyst pretreatment
Before experiments, a pneumatic pressure test of the set-up was performed. The set-up was found to be leak tight at P = 7 bar. After loading all reactants: Ru-TPPTS/PS heterogeneous catalyst (60 g), 77 wt.% FA aqueous solution (1 L) and Na formate (156 g) into the reactor, the catalyst was pre-treated in the reacting mixture at Tr = 85-105 °C and Pr = 4 bar for 4 hours. It was observed that during the pre-treatment, the color of the catalyst turned from dark green to bright yellow (Figure S3) indicating the in-situ generation of catalytically active Ru species, the Ru-mTPPTS hydride complex bound in the triphenylphosphinated cross-linked polystyrene beads. It was clearly seen that catalyst particles driven by the (H 2+CO2) bubbles
and convection continuously rose up and then sank down providing a homogeneous distribution of the catalyst through the reaction volume. The reactor temperature measured at two points (20 and 130 mm from the reactor bottom) were found to be the same (±0.5 °C). It was observed that after the catalyst activation the gas outflow stabilized at ~60% of the initial value, Figure 2. The catalyst activity (turnover frequency, TOF) measured at 80-105 °C was in the range of 90-270 h-1. The catalyst activity remained stable over 120 h of intermittent operation under the experimental conditions used. No metal leaching was detected during the tests. 2.0 1.8
0.86 0.85 0.89
0.99 0.99 0.96 0.97
0.8 0.6 0.4 0.2 0.0 1
Figure 2 Gas production rate during the catalyst pre-treatment (Tr = 90 °C; Pr = 4 bar); and during the regular use (one run duration was ~6 hours).
Catalytic tests: temperature dependence
The reactor performance was studied in a continuous mode under following conditions: Tr = 85 – 110 °C; Pr = 1.5 - 5 bar; FA feed = 1-3 ml/min. All readings were taken at a steady state, allowing ~1 h for stabilization prior to measurement. The (H2+CO2) outflow as a function of reactor temperature is shown in Figure 3.
Figure 3: Temperature dependence of reformate gas flow (Pr = 4 bar).
The apparent activation energy, Ea, calculated from the Arrhenius plot (Figure 4) was 93.60 +/- 2.4 kJ/mol.
Figure 4: Arrhenius plot for catalytic HCOOH decomposition (Pr = 4 bar).
Catalytic tests: reactor performance: pressure dependence
The product gas flow as a function of working pressure was measured at Tr = 100 °C. The obtained results are presented in Figure 5. As can be seen, the gas flow rate is slightly dependent on the working pressure in the studied range. The H2 production of the reactor measured at Tr = 105 °C and Pr = 4 bar was found to be ~0.09 m3/h (20 °C, 1 atm).
Figure 5: Reformate gas production as a function of working pressure (Tr = 100 °C).
Catalytic tests: composition of gas mixture
It was found that under the studied conditions the produced gas mixture consisted of H 2 and CO2 at a molar ratio of 1:1. The CO concentration in the mixture was measured to be in the range of 400-2000 ppm. The amount of CO was dependent on both reaction temperature and FA residence time. Carbon monoxide was generated through an undesired dehydration pathway: HCOOH H2O + CO. The CO elimination from the reformate gas mixture was carried out by preferential catalytic oxidation of CO by oxygen/air (PROX).36 The PROX converter were placed after the adsorption column (Figure S2). The injected air flow rate (5-45 ml/min) was controlled
through a mass flow controller (MFC) to provide an O2 concentration in the gas mixture in the range of 0.24 – 0.72 vol.%. The converter temperature increased due to the exothermic oxidation reaction. Table 1 : Preferential CO oxidation in a reformate gas flow (~1 L/min, nominally 1:1 vol-% H 2 and CO2, initial CO conc. between 400 ppm and 2000 ppm)
O2 conc., % 0.24 0.36 0.48 0.6 0.72
PROX converter temperature, °C T1 65 74 85 95 107
T2 59 70 77 82 88
T3 42 50 50 50 51
Final CO conc., ppm 15 5 ˂3 ˂3 ˂3
As can be seen (Table 1), the PROX treatment decreases the CO concentration below 5 ppm, to the level acceptable for fuel cell applications (3 L/min (50 vol% H2 + 50 vol% CO2) to the PEFC. The next tests were performed using the experimental set-up described above (Figure S1 and S2) working at following conditions: reactor temperature – 100-110°C; pressure - 4 bar; (H 2 + CO2) gas outflow – 2-4 L/min. A (H 2 + CO2) gas mixture evolving from the reactor was passed through a condenser, adsorption column and PROX converter to eliminate FA, H 2O and CO. CO content in the treated gas mixture was ˂5 ppm. The reformate (H2 + CO2) evolved from the reactor and hence the amount of H 2 supplied to the PEFC was controlled by the reactor temperature (Figure S4). The obtained results are presented in Table 3.
14 Table 3 : PEFC cell test performed using a (H2 + CO2) mixture from the reactor
temp. (°C) 102 107 111
pressure (bar) 4 4 4
(L/min) 2 3 3.7
55 110 110
13.5 10.5 12.3
4.1 7.5 7.8
55.3 78.7 96.0
Continuous run tests (1 hour each) of the system were performed using a 3.7 L/min (H 2 + CO2) flow (reactor temperature – 111 °C). The performance of the fuel cell was found to be stable.
Modelling of multiphase flow dynamics and heat transfer
A multi-phase model of the gas and liquid ﬂow, the chemical reaction and heat transfer within the hydrogen generator was written down in form of coupled partial differential equations.39 1.3.1.
Validation of our model was possible only when we included simulation of the heating oil circulation inside the heating tubes. The gas flow rate follows the Arrhenius exponential law and enabled us to extract two parameters - the activation energy 93.60 2.4 kJ/mol and the frequency factor 1.24x1010 Hz. In doing so we proceeded by estimating the average temperature of the reactor by iteration between measurements and simulations in the following four steps: First, we derive the parameter values from the measurements using Treact as the average. Second, we perform simulations with the derived values and, consequently, compute the average temperature (Tav) within the reactor (using Tout as the heating boundary temperature.) Third, we use it to derive the corrected parameter values. Finally, we perform a simulation with corrected parameter values and compare the simulated and measured temperature within the reactor (Treact) and this is shown with experimental data in Figure 7. Considering the reaction rate equation, there should be no explicit dependence on the pressure. However, there is an indirect influence of the pressure caused by change of
interfacial area, thus, change of rate of evaporation for liquid. The (mass) rate of vaporization of dissolved gas, on the other hand, stays practically the same due to the very quick increase of dissolved gas concentration to equilibrate the chemical reaction rate. As a consequence of lower evaporation rate, the temperature slightly rises and subsequently the reaction rate somehow rises as well. The change is not radical, but we can gain about 10% more production by increasing the pressure from 2.5 to 5 atm, hence, reducing the interfacial area by 37%. If the liquid vapour consumes around 8% of the overall energy consumption, it reduces to 5% increasing the reaction rate by 3%. Therefore, we consider the pressure dependency as negligible. The comparison of actual reactor performance with an ideal reactor having uniform temperature shows the loss in efficiency by non-uniform temperature profile Figure 7. Despite a first intuition, the response of the reactor for temperature increase Tset is not exponential. This is due to the steeper temperature gradient for higher temperature. While for Tset = 100 °C is Treact = 86.9 °C, i.e. they differ by 13.1 °C, for Tset = 130 °C is Treact = 105.4 °C, i.e. they differ by 24.6 °C. This trends magniﬁes for higher temperatures (Figure S5). The similar nonlinear response was simulated in case of catalyst packing dependence (Figure S6). This brings us to conclude that heating by oil is insufficient to achieve higher output of hydrogen generator and we will identify alternative forms of heating in the scale-up section.
Figure 7: (top) Reactor simulations (catalyst loading 5% vol., pressure 4 bar). (bottom) Gas flow rate as a function of the temperature. The gas production at uniform reactor temperature is shown for comparison.
We conclude that our model is validated with the single temperature reading available from the experiments, and we estimate the model predictions are trustworthy within 5% production and 2 °C temperature error for temperatures between 100 °C and 130 °C and pressures between 1.5 and 4 atm.
Guidelines for the scaled-up reactor
To address the suboptimal heating of the reactor, we replace the heating by oil circulating in tubes with electrical heating body on the bottom of the reactor. Hence, the thermal convection becomes strong enough to sufficiently distribute the heat along the reactor and the magnitude of hot spots is rapidly reduced. We present the following up-scale of the reactor to obtain a desired net-power 5 kW: the reactor has a simple cylindrical shape with radius 75 mm and height 192 mm, i.e. the volume is 3350 ml. The heating body formed by 7 mm radius wire of length 656 mm is based on the bottom. It was a 85 W unit connected to a 220 V source of electrical power. The electric power of the heating wire is 593 W. For simulation, we have used 80 g, i.e. ca. 3 %[vol] of the solid catalyst which is, actually, a lower percentage than at the lab-scale prototype. However, the new reactor design and electrical heating allows to improve the performance easily by increasing its amount.
As we can see in Figure 8, the operating temperature is about 20 °C higher than in case of the lab-scale prototype and the liquid circulates with higher rate (max. 4.5 cm/s, average 1.8 cm/s). The average temperature of the heating is 142°C and the maximum is 148°C, thus, the heating possesses a relatively uniform temperature profile and we did not observe any significant hot-spots. The reactor produces approximately 50 l/min of the H 2–CO2 gas at a molar ratio of 1 : 1, which corresponds to 5 kW of theoretical chemical power of produced hydrogen. Using a PEFC, the efficiency drops to 50-60% or, with the recirculation of the exhausted gas and heat recapture, we can move towards the theoretical maximum efficiency of 83 %. For this reason, although the chemical energy output of the up-scale system is about 5 kW, we should expect a net power output, reduced by the heating system consumption, of around 2 kW.
Figure 8: (top) Simulation on the scaled-up reactor (5 kWel output) with electrical wire heating, catalyst packing 3% vol. and reactor volume 3.4 litres (height 192 mm, radius 75 mm). Mesh, temperature (deg C) and velocity profile [m/s] are shown. (bottom) Comparison of the simulated and ideal power output of the reactor as function of catalyst loading.
A detailed synthetic protocol of a heterogeneous Ru-TPPTS/PS catalyst was developed and a large quantity of active solid catalyst was synthesized. The catalyst support consisting of cross-linked polystyrene beads ( = 0.3-0.6 mm) is suitable for use in a continuous reactor. The specific catalyst activity (TOF) in FA dehydrogenation was measured to be 270 h-1 (105 °C). A small-scale (1 L) reactor prototype was built up on the base of a pressure resistant borosilicate glass column (Øint = 70 mm, L = 460 mm) with sealing flanges and was used in a continuous mode. The specific reactor productivity was found to be 5 kmol/m3h-1 of H2 and CO2 (at 105 °C). The reactor performance was stable during 120 h under operation conditions (2 months of intermittent use). The target CO concentration level (5 ppm) was achieved through a PROX oxidation using a Pt/CeO2 catalytic converter. It was shown in a special model test that polymer electrolyte fuel cells can be operated on (H2+CO2) mixtures with performance close to that obtained with neat hydrogen. Stable performance of a commercial 100 W fuel cell coupled with the developed reactor prototype was successfully demonstrated. A validated mathematical model of the heterogeneous catalytic reactor with coupled heat transfer, fluid flow and chemical reactions was developed. A scaled-up reactor design from the validated model, showing feasibility of achieving the target output capacity 2-5 kW el was proposed. These developments will certainly help to use formic acid as a hydrogen carrier medium and make possible the industrial application of this sustainable technology.
Notes The authors declare no competing financial interest. ■
DEDICATION This paper is dedicated to Prof. Dr. István T. Horvath on the occasion of his 65th birthday.
ACKNOWLEDGMENTS École Polytechnique Fédérale de Lausanne (EPFL), Swiss Competence Center for Energy Research (SCCER), Swiss Commission for Technology and Innovation (CTI) and Swisselectric Research are thanked for financial support.
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