Synthesis of Hydroxylamine in the Nitric Oxide Hydrogen Fuel Cell

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1 Journal of New Materials for Electrochemical Systems 9, (2006) J. New Mat. Electrochem. Systems Synthesis of Hydroxylamine in the Nitric Oxide Hydrogen Fuel Cell W. Lewdorowicz 1,2,W.Tokarz 1,2,P.Piela 2 and P. K. Wrona 1 Laboratory of Electrochemical Power Sources, Department of Chemistry, Warsaw University, Pasteura 1, Warsaw, Poland. 2 Industrial Chemistry Research Institute, Rydygiera 8, Warsaw, Poland. Received: February 14, 2006, Accepted: May 27, 2006 Abstract: A method for the synthesis of hydroxylamine in a laboratory scale nitric oxide-hydrogen fuel cell with 3.5 M sulfuric acid electrolyte, nanocrystalline platinum gas diffusion electrodes and a proton exchange membrane is described. The measurements were performed for different voltages and different NO flow rates. The highest current efficiencies of NH 2 OH production obtained were above 80% and were NO flow rate dependent. The maximum current density of NO reduction was about 5 ma cm 2 in the fuel cell voltage region from 0.2 V down to 0.0 V, in which the highest selectivity of hydroxylamine formation was also obtained. Keywords: hydroxylamine, nitric oxide, electro-reduction, fuel cell. 1. INTRODUCTION Hydroxylamine (NH 2 OH) plays an important role in the chemical industry. More than 95% of world capacity of hydroxylamine, estimated to be tons per year, is used to produce cyclohexanone oxime, which is then converted mainly to caprolactam. Hydroxylammonium salts constituting stable form of hydroxylamine are used in many branches of the chemical industry. Industrial production of hydroxylamine is based on chemical reduction of higheroxidation-state compounds of nitrogen. Among them, the hydroxylamine-phosphate-oxime (HPO) process, involving catalytic hydrogenation of the nitrates, is the most common, but other processes are also used. These are: the Raschig process employing the reaction of ammonium carbonate with sulfur dioxide, the catalytic hydrogenation of nitric oxide and acid hydrolysis of nitroalkanes [1]. The main disadvantage of the methods listed above, with the exception of the catalytic hydrogenation of nitric oxide, is the formation of unwanted byproducts, mainly nitrous oxide and ammonium sulfate. Other disadvantages of the methods include: use of toxic electrode materials (mercury and amalgams in catalytic hydrogenation of nitrates) and effectiveness at high production capacity (catalytic hydrogenation of nitric oxide). Therefore, there is need for a process that would provide pure hydroxylamine and would To whom correspondence should be addressed: lewdor@chem.uw.edu.pl, Phone: +48 (22) , Fax: +48 (22) be easily scalable. Electrochemical generation of hydroxylamine has, in our minds, potential to become such a process. In an electrochemical process, control of reaction potential often enables control over the selectivity of desired product hydroxylamine in this case. It is possible to reduce or even to eliminate the formation of other species, thus obtaining a pure chemical. Moreover, an electrochemical process can be easily scaled from bench-top production to large manufacturing. Unit electrochemical reactors could be multiplied to satisfy a particular hydroxylamine demand. We propose to generate hydroxylamine in a nitric oxidehydrogen fuel cell. Based on the literature describing NO electroreduction on platinum [2-10] electrode reactions in the NO-H 2 fuel cell are as follows. On the anode oxidation of hydrogen takes place: H 2 2H + +2e E 0 =0.00V (1) Protons are transported through the electrolyte, while electrons are transported through the external electric circuit to the cathode. On the cathode, NO is reduced according to the following general reactions: 2NO+2H + +2e N 2 O( )+H 2 O E 0 =1.59V (2) 2NO+4H + +4e N 2 ( )+2H 2 O E 0 =1.68V (3) NO+3H + +3e NH 2 OH E 0 =0.50V (4) 339

2 340 W. Lewdorowicz et al. / J. New Mat. Electrochem. Systems NO+5H + +5e NH 3 +H 2 O E 0 =0.84V (5) Since we use sulfuric acid as electrolyte, NH 2 OH and NH 3 react with it forming corresponding sulfates. It is also thermodynamically possible that hydrazine will form, but it has never been observed among the NO electroreduction products by any investigator. Indeed, we see from the standard reaction potentials that coupling NO reduction according to any of the reactions 2-5 with hydrogen oxidation 1 potentially leads to production of both electricity and nitrogen compounds, of which hydroxylamine has value. The idea to generate hydroxylamine in a fuel cell is not new. There are reports of Langer and Pate on investigations of nitric oxide reduction in electrogenerative mode [9,10]. They investigated NO reduction on Pt and Ru electrodes coupled with hydrogen oxidation at a Pt anode using various mineral acids as the electrolyte. They found that relatively high selectivity (above 50%) towards hydroxylamine formation can be achieved for NO reduction on the Ru electrode using 6 M HCl or 2 M HNO 3 as electrolyte. On the Pt electrode, NH 2 OH formed with substantial selectivity only when the cathode feed was a mixture of NO and CO. However, their work lacks an analysis of fuel cell voltage influence on hydroxylamine selectivity for the combination of the Pt cathode and sulfuric acid electrolyte. In this paper we present our preliminary research results on hydroxylamine production in a laboratory scale nitric oxide-hydrogen fuel cell that are both in contrast with or perhaps complementary to the results of Langer and Pate [10]. Figure 1. Working principle of nitric oxide-hydrogen fuel cell (unwanted byproducts in parentheses). 2. EXPERIMENTAL A working principle scheme and a cross-section of the hardware for the fuel cell used in our investigations are shown in Figures 1 and 2, respectively. The fuel cell had the typical parallel plate and frame design. Two E-TEK ELAT carbon cloth gas diffusion electrodes, each of 57 cm 2 working geometric area were used as cathode and anode. Both electrodes were identical with 40% Pt on Vulcan XC- 72 catalyst and Pt loading of 2.0 mg cm 2. Current collectors, against which the electrodes were pressed, were made of NO-resistant and H 2 SO 4 -resistant stainless steel. Anode and cathode liquid compartments were separated with a Nafion TM 117 (Du Pont) proton-exchange membrane, and aqueous 3.5 M sulfuric acid solution prepared from analytical grade concentrated H 2 SO 4 (POCh S.A., Poland) was used as electrolyte both in the anode and cathode liquid compartments. The liquid compartments were filled with expanded PVC spacers to assure compression of the gas diffusion electrodes to the current collectors and to hold the Nafion TM 117 membrane straight between the compartments. The cell used acrylic plates and frames and it was sealed with silicone rubber gaskets. Catholyte and anolyte were delivered to the cell with a peristaltic pump (Pharmacia LKB Pump P-1) and a membrane pump (Cole Figure 2. Cross-section of nitric oxide-hydrogen fuel cell hardware.

3 Synthesis of Hydroxylamine in the Nitric Oxide Hydrogen Fuel Cell / J. New Mat. Electrochem. Systems 341 Parmer LR 31741), respectively. The anolyte was recycled, while the catholyte was removed for analysis after leaving the cathode compartment as a product solution. The cathode was fed with nitric oxide (99.9 %, Air Products Polska sp. z o. o., Poland) at slight backpressure of cm of water. Traces of NO 2 present in NO were scrubbed with 4 M KOH solution prior to directing the gas to the cell. Nitric oxide flow rate was determined using a U-tube manometer (measurement of pressure drop in a capillary). The anode was fed with pure hydrogen (analytical grade, Multax s.c., Poland) at slight backpressure of cm of water. Also pure argon (analytical grade, Multax s.c., Poland) was used for polarization experiments. All experiments were conducted at ambient temperature, i.e. 25 ± 5 o C. The electrical measurements were conducted with a Hewlett Packard 6031A system power supply coupled with an Agilent 6051A/60504B DC electronic load. Both instruments were controlled with a personal computer running in-house data acquisition software. The software allowed for recording constant voltage/current/resistance polarization data as well as automatic steady-state polarization curve recording. The steady-state polarization curve was acquired in constant voltage mode, the open circuit fuel cell voltage being the starting voltage. Voltage increment was 50 mv. Current was recorded 10 minutes after applying a given voltage value. Catholyte solution was sampled during constant voltage fuel cell polarization. All catholyte samples were collected not earlier than 4.5 hours after the beginning of each experiment, since it was experimentally established that for the catholyte flow rates used ( cm 3 min 1 )this delay time was needed for the NH 3 OH + concentration to become steady at the catholyte outlet. The concentration of NH 3 OH + was determined using a procedure based on the reaction of NH 2 OH with K 3 [Fe(CN) 6 ] in alkaline solution, which is described elsewhere [11]. From the concentration of NH 3 OH + and the charge obtained by current integration, the current efficiency of hydroxylamine synthesis, W %, was calculated: W 3 F vc Δt C = Δt i( t) dt m % 0 100% F is the Faraday constant (C mol 1 ), v c is the catholyte volumetric velocity (dm 3 s 1 ), t isthetimeelapsedfrom the beginning of electrolysis to sample collection (s), C m is the molar concentration of hydroxylamine in the sample (M), i(t) is the instantaneous fuel cell current (A), and t is experiment time (s). 3. RESULT AND DISCUSSION The open circuit voltage of the fuel cell was ca V. Figure 3 presents the steady-state polarization curves of the fuel cell for the voltage range from 0.9 V to -0.2 (6) Figure 3. Steady-state polarization curves of nitric oxide-hydrogen fuel cell. Conditions: temperature 25 ± 5 o C, NO flow rate 0.5 dm 3 h 1,H 2 flow rate 1.5 dm 3 h 1, anolyte and catholyte 3.5 M H 2 SO 4. V. The electrochemical reactor worked as a fuel cell in the positive voltage region, whereas in the negative voltage region in electrolyser mode. Curves A and B were obtained with cathode feed being Ar and NO, respectively. Curve C was obtained by subtraction of curve A from curve B. A substantial current density associated with the reduction process was recorded only for voltages lower than 0.65 V. Two regions in the polarization curve with NO could be distinguished: the first, wave-like in shape, from 0.65 to 0.2 V corresponding to reduction of nitric oxide to nitrous oxide and the second from 0.2 V down, characterized by increasing current density, in which also hydroxylamine and ammonia formed and, below 0.1 V, hydrogen evolution occurred, too. Analysis of the curves from 0.1 V down to -0.2 V suggests two facts: (i) hydrogen evolution is a competitive process for NO reduction and (ii) hydrogen evolution is hindered by the presence of NO (cf. curve C goes negative below -0.1 V). The latter may be evidence of the presence of adsorbing species during NO reduction. Indeed, adsorbed reaction intermediates exist in all NO reduction mechanisms proposed to date [5,8,12]. Figure 4 presents the dependence of current efficiency of hydroxylamine production on the fuel cell voltage. Each current efficiency value is a mean value obtained for several samples collected during a single constant voltage polarization experiment. Every constant voltage experiment was conducted for several hours and the nitric oxide flow rate in the course of each such experiment was maintained not to exceed the range between 0.5 and 0.7 dm 3 h 1 (filled circles) or between 1.4 and 1.6 dm 3 h 1 (empty squares). The ranges are given, because of limited control over the flow of NO for such low flow rates. It can be seen that the reduction of nitric oxide to hydroxylamine starts oc-

4 342 W. Lewdorowicz et al. / J. New Mat. Electrochem. Systems employed flow rates of NO were much in excess relative to the achievable current densities. E.g. assuming threeelectron reduction, the current density of 5 ma cm 2 and NO flow rate of 0.5 dm 3 h 1 leads to the stoichiometric ratio of NO to the reduction current equal 6.3. Two explanations can be offered for the dependence. Firstly, there is a possibility of a consecutive reaction between forming hydroxylamine and bulk NO, favoring N 2 O formation: 2NH 2 OH (ads) +4NO 3N 2 O( )+3H 2 O (7) Secondly, NO from solution can react with HNO + adsorbed at the catalytic sites ultimately directing the reaction towards N 2 O, as proposed by de Vooys et al. [8]: HNO + (ads) +NO+e HN 2O 2(ads) (8) Figure 4. Dependence of hydroxylamine current efficiency on fuel cell voltage. Each efficiency value is a mean value for several samples taken during a single constant voltage experiment, in which the nitric oxide flow was maintained either between 0.5 and 0.7 dm 3 h 1 (filled circles) or between1.4and1.6dm 3 h 1 (empty squares). Other conditions as for Figure 3. curring at voltages lower than 0.2 V and peaks about 0 V. Moreover, the current efficiency of hydroxylamine production strongly depends on nitric oxide flow rate through the cathode compartment of the fuel cell. (In view of this dependence the obtained efficiencies should be considered as average for the NO flow rate ranges indicated in Figure 4.) The highest values of current efficiency were over 80% and close to 30% for the low and high NO flow, respectively. These values are at variance with the selectivity of hydroxylamine obtained by Langer and Pate under similar experimental conditions [10]. They found no hydroxylamineformationonptin3mh 2 SO 4. Weattributethis discrepancy to two differences between theirs and our experiments. Firstly, they did not report for low cell voltages and secondly, their fuel cell lacked a separator membrane. Without the membrane hydroxylamine can reach the hydrogen anode, where it can be reduced electrochemically or hydrogenated catalytically to ammonia. The fact that hydroxylamine actually forms on Pt in strong H 2 SO 4 at low cathode potentials has been reported by others [3,5]. The decrease of current efficiency for voltages lower than 0 V can be evidence either for electrochemical reduction of hydroxylamine to ammonia that can occur at the cathode of this fuel cell at sufficiently low potential or for vigorous hydrogen evolution preventing NO reduction altogether. Without analysis of products other than hydroxylamine discrimination between the two options was impossible. Another interesting observation is that the yield of hydroxylamine strongly depends on the NO flow rate through the cathode gas chamber. It must be kept in mind that the HN 2 O 2(ads) +H + +e N 2 O ( ) +H 2 O (9) In view of these possible mechanisms one can understand why hydroxylamine current efficiency was higher in our experiments when NO reduction was accompanied by hydrogen evolution (cf. Figure 3, curve C maximum NO reduction current density at 0.1 V vs. Figure 4 maximum current efficiency at 0.0 V). We consider it possible, that the evolving hydrogen inhibits the consecutive reaction between nitric oxide from solution and the hydroxylamineyielding adsorbed intermediate, which results in higher current efficiencies of hydroxylamine production in the hydrogen evolution voltage region. Assuming uncontrolled access of NO to the cathode surface and/or hydroxylamine-containing catholyte is detrimental for the yield of hydroxylamine, one can state that operation of our fuel cell under flooded cathode conditions (vide small current densities) was helping the high current efficiencies of hydroxylamine obtained by us. Liquid electrolyte filling the porous cathode was a considerable diffusion barrier for incoming NO, which limited the extent of the consecutive chemical reaction. 4. CONCLUSION It is possible to produce hydroxylamine with high current efficiency in the nitric oxide-hydrogen fuel cell with Pt electrodes and H 2 SO 4 electrolyte, but the control over the process needs improvement. The current efficiencies of hydroxylamine production were NO flow rate dependent and reached maximum values of over 80%. They could perhaps be even higher if NO flow rates below 0.5 dm 3 h 1 could be employed. (This flow rate was the lowest we could achieve with our experimental setup.) We consider it possible, that the reduction of NO to hydroxylamine is accompanied by the consecutive reaction between bulk NO and hydroxylamine-yielding adsorbed reaction intermediate, which results in decrease of current efficiency of hydroxylamine formation. To experimentally confirm that such reaction proceeds, modification of the experimental system is needed. For fuel cell hydroxylamine production to be industrially acceptable, extensive optimization of the

5 Synthesis of Hydroxylamine in the Nitric Oxide Hydrogen Fuel Cell / J. New Mat. Electrochem. Systems 343 process conditions is needed involving both mechanistic studies of NO reduction in strong acid media as well as better reactor design. As to the latter, a likely path to increasing hydroxylamine selectivity in the studied system would be to engineer the reactor such that there would be immediate separation of forming hydroxylamine from the reaction substrate, i.e. bulk NO. REFERENCES [1] J. Ritz, H. Fuchs, H. G. Perryman, Hydroxylamine, in: Ullmann s Encyclopedia of Industrial Chemistry, Sixth Edition, 2002 Electronic Release. [2] N. N. Savodnik, V. A. Shepelin, Z. I. Zalkind, Elektrokhimiya, 7, 424 (1971). [3] N. N. Savodnik, V. A. Shepelin, Z. I. Zalkind, Elektrokhimiya, 7, 583 (1971). [4] D. Dutta, D. Landolt, J. Electrochem. Soc., 119, 1320 (1972). [5] L.J.J.Janssen,M.M.J.Pieterse,E.Barendrecht, Electrochim. Acta, 22, 27 (1977). [6] J. F. E. Gootzen, R. M. van Hardeveld, W. Visscher, R. A. van Santen, J. A. R. van Veen, Recl. Trav. Chim. Pays-Bas, 115, 480 (1996). [7] K. Hara, M. Kamata, N. Sonoyama, T. Sakata, J. Electroanal. Chem., 451, 181 (1998). [8] A.C.A.deVooys,M.T.M.Koper,R.A.vanSanten, J. A. R. van Veen, Electrochim. Acta, 46, 923 (2001). [9] S. H. Langer, K. T. Pate, Nature, 284, 434 (1980). [10] S. H. Langer, K. T. Pate, Ind. Eng. Chem. Process Des. Dev., 22, 264 (1983). [11]B. Piela, P. K. Wrona, J. Electrochem. Soc., 151 (2), E69-E79 (2004). [12] I. Paseka, React. Kinet. Catal. Lett., 11, 85 (1979).

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