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Article

Electrochemical Removal of NOx on Ceria-Based Catalyst-Electrodes

1
Université de Lyon, CNRS, Université Claude Bernard Lyon 1, IRCELYON, UMR 5256, 2 Avenue A. Einstein, 69626 Villeurbanne, France
2
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Tsinghua University, 100084 Beijing, China
3
Ecole Nationale Supérieure des Mines, SPIN-EMSE, CNRS: UMR5307, LGF, F-42023 Saint-Etienne, France
*
Author to whom correspondence should be addressed.
Catalysts 2017, 7(2), 61; https://doi.org/10.3390/catal7020061
Submission received: 23 December 2016 / Revised: 3 February 2017 / Accepted: 8 February 2017 / Published: 16 February 2017
(This article belongs to the Special Issue Ceria-based Catalysts)

Abstract

:
This study reports the electrochemical properties for NOx reduction of a ceria-based mixed ionic electronic conducting porous electrode promoted by Pt nanoparticles, as efficient catalyst for NO oxidation, and BaO, as sorbent to store NOx. This catalytic layer was deposited by screen-printing on a dense membrane of gadolinia-doped ceria, an O2− ionic conductor. The targeted Ba and Pt loadings were 150 and 5 μg/cm2, respectively. The NOx selective electrochemical reduction was performed between 400 °C and 500 °C with and without oxygen in the feed. Variations of the open-circuit voltage with time were found to be a good sensor of the NOx storage process on the ceria-based catalyst-electrode. However, no N2 production was observed in the presence of O2 phase in spite of nitrates formation.

1. Introduction

Nitrogen oxides (NOx: NO + NO2) are pollutants mainly emitted by thermal engines using fossil fuels. They are at the origin of safety problems. The design of more efficient and stable catalysts to reduce NOx to nitrogen in atmospheres containing an excess of oxygen, such as exhaust gas emitted by diesel and lean-burn gasoline engines, has attracted much attention in the last years. Due to the more and more stringent emission standards, an efficient and clean technology to control the NOx emissions from automotive engines becomes an important demand. In Europe [1], the NOx emission limit is 80 mg/km for diesel passenger cars according to the Euro 6 standard in force since 2014. The two actual technologies on the market are using an additional reducing agent to remove NOx, i.e., diesel fuel post-injections for the NOx Lean Trap Catalyst (LTC) and urea for the Selective Catalytic Reduction (SCR) [2]. The former is containing Pt and BaO for the oxidation NO into NO2, and the storage of NO2 as nitrates, respectively. When the LTC surface is saturated by nitrates, a diesel post injection is triggered for few seconds to decompose nitrates and reduce NOx into N2. However, these periodic short rich phases provoke a fuel overconsumption. SCR catalysts are based on zeolite materials which are cheap and robust but the control of the urea injection to avoid NH3 release is tricky. In addition, recent studies on the on-road emissions of NOx from Euro 6 Diesel vehicles have clearly shown that these two technologies are not sufficiently effective to comply the standards. Therefore, alternative solutions have to be developed. One promising solution could be the Selective Electrochemical Reduction (SER) of NOx into N2 in a Solid Oxide Electrolysis Cell (SOEC). This latter process does not need an additional reducing agent to reduce NOx. The reduction is ensured by a cathodic polarization. Then, SER saves the large reducing agent storage system and avoids the emission of pollutants such as NH3 (SCR) or VOC (LTC) produced by the reducing agents [3,4,5]. It was first proposed by Pancharatnam et al. [6] in 1976 that NO can be electrochemically reduced into N2 on a Pt cathode interfaced on an oxygen ionic conductor. The reaction mechanism was investigated by Gür and Huggins [7] in 1979. However, diesel exhausts contain a large quantity of oxygen. The competitive electrochemical reduction of O2 into O2− is much faster than the SER of NOx on a Pt electrode [8]. To improve the selectivity of the NOx electrochemical conversion, some researchers have recently shown that the addition of a NOx sorbent (K, Ba) can promote SER of NOx into N2 [9,10,11,12]. Two approaches are proposed in the literature. The former deals with multilayers configurations with a NOx adsorption layer deposited at the top of a catalyst-electrode layer [9,11]. The latter seems to be more promising and refers to the infiltration of a NOx sorbent, such as BaO, into the porosity of the catalyst-electrode [12,13,14]. For instance, remarkable results have been obtained with the infiltration of both BaO particles and a Pt/Al2O3 catalyst into the porosity of symmetrical electrodes of (La0.85Sr0.15)0.99MnO3 (LSM) interfaced on a dense GDC membrane [12]. At 450 °C, NOx conversion and N2 selectivity as high as 65% and 86%, respectively, were achieved in an excess of oxygen.
The objective of this study was to infiltrate both BaO and Pt nanoparticles in the porosity of a ceria-based electrode layer to delocalize the SER of NOx into the whole volume of the electrode. The mixed ionic electronic conducting (MIEC) coating was a composite electrode between LSCF (La0.6Sr0.4Co0.8Fe0.2O3-δ) and GDC (Gd0.2Ce0.8O1.9), able to conduct both O2− and electrons. A dense membrane of GDC was used as the electrolyte due to its high ionic conductivity at low temperature [15,16]. The NOx electrocatalytic performances were investigated between 400 °C and 500 °C in NO/O2/He atmospheres.

2. Results and Discussion

2.1. Catalyst-Electrode Characterizations

The Pt-BaO/LSCF-GDC electrode structure and morphology were characterized by SEM (Scanning Electronic Microscopy). The thickness of the LSCF/GDC electrode was around 10 μm (Figure 1a). The morphology of the LSFC-GDC composite layer was shown on Figure 1b. The electrode is made of micrometric and agglomerated grains, probably due to the high calcination temperature used during the preparation (1200 °C). Pores size is around 1–3 μm. TEM (Transmission Electronic Microscopy) observations after the extractive replica preparation method of the samples, have revealed the presence of Pt nanoparticles. Figure 2 shows a typical Pt nanoparticle. The mean diameter of Pt nanoparticles was around 10 nm. Unfortunately, BaO particles were not detected neither with SEM, most probably due to small grain sizes, nor with TEM. This suggests that BaO particles are heterogeneously distributed in the electrode. Recently, the group of K. Kammer Hansen [3] has infiltrated BaO in the porosity of a similar cell made of 60 wt % of La0.85Sr0.15CoxMn1-xO3+δ and 40 wt % of Ce0.9Gd0.1O1.95. They have shown that BaO nanoparticles in the inner layers are only present on the LSM phase. Therefore, BaO in our case, could be preferentially localized on LSCF grains.

2.2. NOx Electrocatalytic Conversions without Oxygen in the Feed

The parameter ΔNOx difference between inlet (NOx,in) and outlet (NOx,out) concentrations of NOx (NO + NO2), was used to highlight the NOx storage process as nitrates on the catalyst-electrode. Upon open-circuit voltage (OCV), without any polarization and then electrochemical reaction, positive values of ΔNOx indicate that a part of NOx species is stored on the catalyst-electrode.
Figure 3 shows variations of OCV (Open-Circuit Voltage) and ΔNOx with time at 500 °C. Initial positive values of ΔNOx confirm that a part of NOx is stored on the catalyst-electrode, most probably on BaO sites as Ba(NO3)2. Similar experiments carried out with a Ba free composite electrode gave negligible values of ΔNOx, then confirming the active role of BaO to store NOx.
On the complete composite electrode, traces of oxygen in the feed or oxygens stored in GDC allow the catalytic production of NO2 on Pt nanoparticles. An NO2 production peak at around 40 ppm was detected 4 min after the reactants introduction, corresponding to the beginning of the ΔNOx plateau. The duration of this latter is approximately 3 min and then the ΔNOx value gradually decreases down to zero after 30 min on stream. This slow process corresponds to the saturation of the Ba sites. The most interesting point is the OCV variations with time. The OCV value corresponds to the potential difference between that of the catalyst-electrode and that of the Au counter-electrode. As reported in the literature [10,17,18], OCV values in a single chamber electrochemical cell refer to the difference in the thermodynamic activity of adsorbed atomic oxygen between the two electrodes. Variations of the potential of the Au counter-electrode prepared from Au paste are negligible in this temperature range [17]. Therefore, recorded variations of OCV are linked to modifications of the oxygen coverage on the catalyst-electrode. Before the introduction of the reactants, sample was only exposed to He and the OCV value was −50 mV at 500 °C, suggesting that the oxygen coverage in presence of traces of oxygen is lower on Pt-Ba/LSCF-GDC than on Au. After the introduction of the NOx reactant, OCV values rapidly drop to reach a plateau at −160 mV which exactly corresponds to that of ΔNOx, demonstrating that the catalyst-electrode potential gives a direct evidence of the NOx storage progress, as already reported on a Pt-Ba electrode [10]. The NOx storage process strongly decreases the oxygen coverage on the electrode most probably because NO2 and traces of oxygen are consumed to produce nitrates according to the reaction shown in Equation (1) whereas NO2 and O2 can be adsorbed on Au.
2 NO 2 + 1 2 O 2 + BaO Ba ( NO 3 ) 2
After the plateau, OCV gradually increases with time, in good concordance with the ΔNOx decay.
The NOx SER performance was investigated at 450 °C and 550 °C. Different negative polarizations were applied between the catalytic electrode and the counter electrode (Figure 4 and Figure 5) between −5 V and −7 V. Please note that the ohmic drop was not subtracted from these values. Without any oxygen in the feed, nitrogen oxides, mainly composed of NO, can be electrochemically reduced into N2 with a 100% selectivity. At 500 °C, NOx conversions are around 20% and only slightly vary with the negative potential (Figure 4b and Figure 5a) whereas produced negative currents strongly increase (from −2.4 mA to −4.2 mA). This indicates that the generated current is not only produced by the NOx SER and the N2 production. One can assume that, under these operating conditions, GDC can be concomitantly electrochemically reduced. On the opposite, at lower temperature, i.e., 450 °C, the NOx conversion linearly increases with the cathodic potential from 10.5% upon −5 V up to 15.5% upon −7 V. At 450 °C (Figure 4a), current intensities are lower and proportional to the NOx conversion, suggesting that there is no GDC reduction.
Figure 5 shows that the NO concentration decrease upon cathodic polarizations is gradual and slow. At 450 °C (Figure 5a), the NO concentration does not reach a plateau after more than 40 min polarization. Figure 6 displays variations of the NO concentration and the current with time at 450 °C upon −5 V. The decay of the NO concentration with time is slow, confirming the low kinetic rate of the process. The current variation shows a negative peak after 3 min on stream which corresponds to the ΔNOx value plateau (Figure 3). This demonstrates that the generated current is dependent on the NOx storage process and then on the NO2 production peak.

2.3. NOx Electrocatalytic Conversion in the Presence of 1% O2

Electrocatalytic performances were also investigated in the presence of 1% O2. At 500 °C, the inlet NO and NO2 concentrations were about 650 ppm and 40 ppm, respectively, due to the production of NO2 in the stainless steel pipes. The presence of O2 concentration increases the steady stable value of NO2 concentration, as NO is oxidized into NO2 onto the catalyst-electrode. The NO conversion into NO2 is around 9% at 500 °C. Values of ΔNOx are positive right after the reactive mixture introduction (Figure 7). Therefore, as expected, the NOx storage process is taking place on the catalyst-electrode. Let us note that without any BaO in the electrode, values of ΔNOx is closed to zero meaning that no NOx storage is taking place. In presence of BaO, contrary to the experiments without oxygen, ΔNOx variations do not exhibit a plateau but a sharp peak after 3 min on stream followed by a gradual decay down to −10 ppm. Features of the OCV variations are symmetrically inverted with an initial drop to negative values (−45 mV) followed by a progressive increase up to around 0 mV. The observed slight time shift between the ΔNOx positive peak and the OCV negative one is only due to the slow response time of the NOx analyzer. Therefore, these experiments in presence of oxygen confirm that the OCV value is a good sensor of the NOx storage process. Negative values of OCV indicate that the NOx storage process is running while a null value points that the catalyst-electrode surface is saturated by nitrates compounds.
Figure 8 gives the variations of NO and NO2 concentrations upon different voltages from ±3 V to ±1 V at 400 °C. At OCV, the NO conversion into NO2 is 13%. Whatever the polarization, no production of nitrogen was detected in spite of the NOx storage process (Figure 7) which proves the presence of Ba. Cathodic and anodic polarizations only induce the electrochemical reduction of NO2 into NO and the reverse reaction, respectively. Conversion of NO into NO2 can reach 15% upon +3 V. The most important point is the ability of the electrode to electrochemically reduce NO2 into NO for low potentials. At 400 °C, upon −1 V, the NO2 conversion into NO can reach 30%. However, this conversion is not enhanced by higher cathodic potentials, suggesting that the electrochemical reduction of oxygen becomes predominant. These results are in contradiction with those obtained by Shao et al. [12] with a LSM electrode infiltrated with both BaO and Pt/Al2O3. This study has evidenced high conversions of NOx into N2 in excess of oxygen upon cathodic potentials. Several causes can explain the different results obtained in this study, such as a quite heterogeneous distribution of the BaO particles in the electrode or a chemical reactivity between BaO and LSCF/GDC materials as proposed by the group of Kammer Hansen [3]. The localization of BaO nanoparticles is quite important since the production of N2 will depend on the efficiency of the selective electrochemical reduction of Ba(NO3)2.
Ba(NO3)2 + 10 e → BaO + N2 + 5 O2−
Ba(NO3)2, located near triple phase boundaries (gas/O2−/e) in the vicinity of the layer closed to the CGO electrolyte will be much more easily electrochemically reducible than nitrate nanoparticles only present on LSFC grains at the surface of the electrode. During this latter case, the electrochemical reduction of Ba(NO3)2 will be in strong competition with the oxygen electrochemical reduction. Therefore, if we consider that Ba(NO3)2 is preferentially formed on LSCF nanoparticles as suggested by the group of K. Kammer Hansen [3], it explains the low kinetic of the reduction process (Figure 5 and Figure 6) observed without oxygen in the feed as nitrates have to diffuse near TPBs (Triple Phase Boundary) and the non-production of N2 in presence of oxygen as oxygen becomes predominantly electrochemically reduced. Additional characterizations are clearly needed to elucidate the origin of the non-selective electrochemical reduction of NOx of the ceria-based catalyst electrode.

3. Materials and Methods

3.1. Electrode Preparation

The LSCF (70 wt %)/GDC(30 wt %) composite electrode was prepared by screen-printing (semin-automatic AUREL C890 machine, Modigliana (FC), Italy) [19] on a GDC dense disk (diameter 17 mm, thickness 1 mm) followed by a calcination step at 1200 °C. A gold film was deposited on the opposite side of the GDC pellet in order to act as counter electrode by using a gold paste (Metalor, Oullins, France) annealed at 500 °C for 2 h. Generated micrometric gold particles were found to be inactive for NOx activation as verified through blank experiments under our experimental conditions. Nanoparticles of Pt and BaO were impregnated in the porosity of the LSCF/GDC electrode. An aqueous solution containing both precursors of Pt(NH3)4(NO3)2 and Ba(NO3)2 has been prepared. The concentrations of the Pt and Ba solutions were 0.32 × 10−3 mol/L and 1.36 × 10−2 mol/L, respectively. In addition, PVP (polyvinylpyrrolidone) as the surfactant was also added to the solution (10 wt %) to ensure a suitable viscosity. A controlled volume of 40 μL of the Pt/BaO solution has been infiltrated four times successively into the porosity of the LSCF/GDC electrode by using a micropipette. Between each infiltration, the sample was dried for 1 h at 80 °C. The targeted loadings of the Pt and Ba into the porous electrode were 5 μg/cm2 and 150 μg/cm2, respectively. The sintering temperature of infiltrated electrodes was set at 600 °C for 1 h in air. The Pt nanoparticles in the LSCF/GDC film were characterized by TEM (JEOL 2010 LaB6, JEOL, Peabody, MA, USA) using an extractive replica technique (Figure 2) as described in [20]. A series of composite electrodes without BaO have been also prepared with the same loading of Pt to compare the NOx storage capacity.

3.2. Measurements of the Electrocatalytic Performances

The Pt-BaO/LSCF-GDC/GDC/Au samples were placed in a single chamber quartz reactor described elsewhere [21]. All the current collectors were made of gold. A gold mesh was placed on the top of the Pt-BaO/LSCF-GDC porous working electrode as current collector. The quartz reactor was installed in a tubular furnace. A potentiostat-galvanostat (Voltalab 80, Radiometer Analytical, Hach Company, Loveland, CO, USA) was used to polarize samples between the catalyst-electrode and the Au counter-electrode.
The reactive mixture gases were composed of NO (4000 ppm in He, LINDE France S.A., Saint-Priest, France, 99.95% purity) and O2 (LINDE France S.A. 99.95% purity). Helium (LINDE, 99.95% purity) was used as the carrier gas. The gas compositions were adjusted with mass flow controllers (Brooks) with an accuracy of 1%. The testing temperature range was varied from 400 °C to 500 °C. Concentrations of NO, N2O and NO2 were monitored by an online analyzer (EMERSON NGA 2000, Emerson Process Management, Bron, France) while N2 and O2 contents were determined with a micro-chromatograph (R3000 SRA Instruments, Marcy l’Etoile, France). The NOx conversion experiments were investigated in presence of 680 ppm NOx (670 ppm NO and 10 ppm NO2) without O2 and for a total flow rate of 42 mL/min. Based on above experiments, the NOx conversion experiments were also studied in presence of 1% O2.
The NOx conversion was calculated as follows:
NOx conversion = 100 × (NOx,in − NOx,out)/NOx,in
The selectivity to N2, S N 2 , was extracted from the Equation (3).
S N 2 = 100 × 2 N 2 , out / NO x , in
The production of N2O was never detected.

4. Conclusions

The NOx electrochemical catalytic performances of Pt-Ba impregnated LSCF-GDC catalyst-electrode was investigated between 400 °C and 500 °C with and without oxygen in the feed. Pt and BaO nanoparticles were finely dispersed in the catalyst-electrode. Variations of the OCV with time were found to be a good sensor of the NOx storage process on the electrode. Without oxygen in the feed, NOx can be selectively electrochemically reduced into N2, even if the electrochemical process is slow. In presence of O2, no N2 production was observed in spite of nitrate formation, showing the key role of Ba nitrate localization in the catalyst-electrode layer. However, the catalyst-electrode was found to be effective at electrochemically reducing NO2 into NO at 400 °C for low potentials.

Acknowledgments

The authors would like to acknowledge the China scholarship council for the grant of Xi Wang.

Author Contributions

Philippe Vernoux, Yi Xiang, Ning Sheng Cai, Mathilde Rieu and Jean-Paul Viricelle conceived and designed the experiments; Xi Wang, Alexandre Westermann and Mathilde Rieu performed the experiments; Xi Wang, Alexandre Westermann, Philippe Vernoux analyzed the data; Xi Wang and Philippe Vernoux wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. (a) Cross section SEM (Scanning Electronic Microscopy) image of the Pt-BaO/LSCF (La0.6Sr0.4Co0.8Fe0.2O3−δ)-GDC (Gd0.2Ce0.8O1.9)/GDC cell and (b) SEM image of the surface of the catalyst-electrode.
Figure 1. (a) Cross section SEM (Scanning Electronic Microscopy) image of the Pt-BaO/LSCF (La0.6Sr0.4Co0.8Fe0.2O3−δ)-GDC (Gd0.2Ce0.8O1.9)/GDC cell and (b) SEM image of the surface of the catalyst-electrode.
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Figure 2. TEM ((Transmission Electronic Microscopy) image of a Pt nanoparticle infiltrated in the catalyst-electrode.
Figure 2. TEM ((Transmission Electronic Microscopy) image of a Pt nanoparticle infiltrated in the catalyst-electrode.
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Figure 3. Variations of open-circuit voltage (OCV) and ΔNOx as a function of time at 500 °C. Reactive mixture: NO/NO2: 680 ppm/10 ppm in He.
Figure 3. Variations of open-circuit voltage (OCV) and ΔNOx as a function of time at 500 °C. Reactive mixture: NO/NO2: 680 ppm/10 ppm in He.
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Figure 4. Variations of NOx conversion and currents as a function of applied potentials (a) at 450 °C and (b) at 500 °C. Reactive mixture: NO/NO2: 680 ppm/10 ppm in He.
Figure 4. Variations of NOx conversion and currents as a function of applied potentials (a) at 450 °C and (b) at 500 °C. Reactive mixture: NO/NO2: 680 ppm/10 ppm in He.
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Figure 5. Variations of NO and N2 concentrations as a function of time at (a) 450 °C and (b) 500 °C. Reactive mixture: NO/NO2: 680 ppm/10 ppm in He.
Figure 5. Variations of NO and N2 concentrations as a function of time at (a) 450 °C and (b) 500 °C. Reactive mixture: NO/NO2: 680 ppm/10 ppm in He.
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Figure 6. Variations of NO concentration and the current as a function of time at 450 °C. Reactive mixture: NO/NO2: 680 ppm/10 ppm in He.
Figure 6. Variations of NO concentration and the current as a function of time at 450 °C. Reactive mixture: NO/NO2: 680 ppm/10 ppm in He.
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Figure 7. Variations of OCV and ΔNOx as a function of time at 500 °C. Reactive mixture: NO/NO2/O2: 650 ppm/40 ppm/1% in He.
Figure 7. Variations of OCV and ΔNOx as a function of time at 500 °C. Reactive mixture: NO/NO2/O2: 650 ppm/40 ppm/1% in He.
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Figure 8. Variations of NO and NO2 concentrations with time at 400 °C upon polarizations. Reactive mixture: NO/NO2/O2: 525 ppm/85 ppm/1% in He.
Figure 8. Variations of NO and NO2 concentrations with time at 400 °C upon polarizations. Reactive mixture: NO/NO2/O2: 525 ppm/85 ppm/1% in He.
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MDPI and ACS Style

Wang, X.; Westermann, A.; Shi, Y.X.; Cai, N.S.; Rieu, M.; Viricelle, J.-P.; Vernoux, P. Electrochemical Removal of NOx on Ceria-Based Catalyst-Electrodes. Catalysts 2017, 7, 61. https://doi.org/10.3390/catal7020061

AMA Style

Wang X, Westermann A, Shi YX, Cai NS, Rieu M, Viricelle J-P, Vernoux P. Electrochemical Removal of NOx on Ceria-Based Catalyst-Electrodes. Catalysts. 2017; 7(2):61. https://doi.org/10.3390/catal7020061

Chicago/Turabian Style

Wang, Xi, Alexandre Westermann, Yi Xiang Shi, Ning Sheng Cai, Mathilde Rieu, Jean-Paul Viricelle, and Philippe Vernoux. 2017. "Electrochemical Removal of NOx on Ceria-Based Catalyst-Electrodes" Catalysts 7, no. 2: 61. https://doi.org/10.3390/catal7020061

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