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NOx conversion in La0.85Sr0.15Co0.03Mn0.97O3+d-Ce0.9Gd0.1O1.95 porous cell stacks infiltrated with Pt

  • Anja Zarah Friedberg
  • Kent Kammer Hansen
Article
  • 35 Downloads

Abstract

Porous cell stacks with composite electrodes of La0.85Sr0.15Co0.03Mn0.97O3+d and Ce0.9Gd0.1O1.95 were characterized for the electrochemical reduction of NO in net oxidizing atmosphere in absence or presence of propene in the feed gas. No NOx was converted when the porous cell stacks were at OCV or when polarized. However, when the cells were infiltrated with Pt, an effect of this was observed. It was shown that Pt had a very positive effect on the NOx removal properties of the porous cell stacks, and that NOx could be removed both at OCV or when the porous cells stacks was polarized, both in the absence or presence of propene under net oxidizing conditions. The porous cell stacks was also investigated using electrochemical impedance spectroscopy. It was shown that the impedance data could be de-convoluted into four arcs. Each arc could be fitted with a constant phase elements in series with a resistor. The arcs could be attributed to physical processes.

Keywords

NO Propene Oxygen Perovskite Porous cell stack Electrochemical promotion 

1 Introduction

According to [1], platinum is the most active and selective metal out of platinum, palladium and rhodium for the selective catalytic reduction (SCR) of NO by propene. Platinum exhibits a typical volcano-type profile in the presence of excess oxygen that occurs on the Pt surface, which is in a reduced form containing hydro-carbonaceous residues [1].

The problem with a Pt catalyst is that it produces more N2O than N2 [2] and has a narrow operating temperature window for an effective NOx removal [3]. Several studies have investigated the electrochemical promotion of the NO reduction with CO or hydrocarbons in order to enhance the activity and selectivity of Pt [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19].

Many of these studies have simulated the exhaust gas with mixtures containing no oxygen or low concentrations of oxygen (1–5%) [3, 7, 12, 13, 14, 16].

Some of these have also investigated the effect of hydrocarbon concentration on the NO reduction [7, 8, 15, 19]. These investigations found that the formation of N2 and N2O obeys Langmuir-Hinshelwood-type kinetics, exhibiting a maximum of NOx conversion as a function of propene concentration.

Even though it has been reported by Tang et al. [20] that several authors have found a positive effect of the presence of hydrocarbons on the NOx removal, it has only been possible to find two papers where studies with and without hydrocarbons were made [15, 19]. In the work by Hibino et al. [19], a positive effect on the electrochemical promotion of NO decomposition on an yttrium stabilized zirconia (YSZ) cell with Pd/Rh electrodes was observed when adding CH4, C3H8 or C5H12. In this work, the oxygen concentration of the inlet gas was 3%. They attributed the effect of the HCs to a decrease in O2 concentration in the vicinity of the electrode by their perfect combustion.

Huang et al. [15] investigated an electrochemical-catalytic cell with an oxidation catalyst layer of palladium-infiltrated cerium-gadolinium oxide (CGO) and a copper-doped strontium-substituted lanthanum manganite/CGO cathode. They observed a very clear positive effect on the NO conversion when propene was added to the gas feed in the presence of 14% oxygen. They explained the effect by the reaction between propene and NOx to be similar to that or HC-deNOx over a catalyst.

Beguin et al. [14] and Vernoux et al. [16] investigated the electrochemical promotion of NO reduction by propene using an oxide ion conductor, YSZ, and a sodium ion conductor, NASICON, as electrolytes.

When platinum was deposited on the YSZ electrolyte, an enhancement of the NO reduction was observed when a negative current was applied but only in the presence of 1% O2, 2000 ppm NO and propene. When they used 5% O2, they observed an abrupt and total conversion of propene at 300 °C and no modification of the catalytic performance by the application of a current. When changing the electrolyte to NASICON, the application of a current was able to enhance not only both the NO reduction and the hydrocarbon oxidation but also the selectivity towards nitrogen. This was explained by the better ionic conductivity of NASICON at low temperatures.

Souentie et al. [18] were able to promote the SCR of NO by ethane with 450% in the presence of 10% O2. This was done on a monolith reactor with Rh/YSZ/Pt plate elements.

It has not been possible to find any investigations concerning the electrochemical promotion of the NO reduction with propene on a Pt catalyst in the presence of more than 5% oxygen.

In this work, the effect of platinum infiltration on a composite electrode of La0.85Sr0.15Co0.03Mn0.97O3 + d (LSCM) and Ce0.9Gd0.1O1.95 (CGO10) was investigated. The electrochemical reduction of NO in the presence of 10% oxygen was studied, and the effect of propene in the atmosphere on the enhancement ratio was examined.

2 Experimental

2.1 Cell fabrication and infiltration

The backbone cell was a porous cell stack with 11 layers (5 cells in series) consisting of a composite electrode of LSCM and CGO10 and a CGO10 electrolyte. The thickness of the layers was 20–30 μm. The cells are symmetrical. For fabrication of the porous cell stacks see [21]. The infiltration of the backbone porous cell stack was done using a 34 mM aqueous solution of Pt(NH3)4(NO3)2 (Sigma Aldrich) with 10 wt% Triton-45 (Sigma Aldrich) as the surfactant. The porous cell stack was soaked in the solution and placed in vacuum for 10 s and thereafter heated to 350 °C for 1 h after drying at 80 °C for several hours. The infiltration was only done one time, and the loading of Pt in the porous cell structure was too low to be determined by microscopy or weighting.

2.2 Electrochemical testing and conversion measurements

The electrochemical testing was performed in the same setup as described in [21]. The cell was first tested in a 1000 ppm NO +10% O2 atmosphere at 300, 350 and 400 °C. The electrochemical tests conducted at each temperature were electrochemical impedance spectroscopy (EIS) at OCV; polarization with 3, 5, 7 and 9 V; and cyclic voltammetry. The atmosphere was changed to also contain 1000 ppm C3H6, and the same electrochemical tests were performed at the same three temperatures.

Gas analysis was performed using a chemi-luminescence detector (Model 42i HL, Thermo Scientific, USA) for detection of NO and NO2. Propene, CO, CO2 and N2O concentrations were monitored by an Agilent 490 Micro gas chromatograph with Porapak Q and Molesieve 5X columns and two thermal conductivity detectors.

3 Results

3.1 Conversion measurements

The non-infiltrated cell stack showed no conversion of NOx or propene during the test. The Pt-infiltrated cell stack showed a very high catalytic conversion of propene at all three temperatures where the conversion was more than 99%. The catalytic conversion of NO was only seen in the propene-containing atmosphere where the activity was highest at 300 °C and 21% was removed. The activity decreased with increasing temperature, and only 8% was removed at 400 °C. The selectivity towards nitrogen was not 100% at 350 and 400 °C since some N2O was observed. The exact concentration could not be determined since it was below 100 ppm, which was below the calibration limit of the gas analysis equipment.

The NO and NO2 concentrations were different in the two atmospheres. In 1000 ppm NO +10% O2, the concentration of NO2 was higher than that of NO at all temperatures at OCV. This changed when propene was present; the NO concentration was higher than that of NO2.

The electrochemical activity of the cell was highest when no propene was present in the atmosphere, and the Pt infiltration increased this activity greatly (see Fig. 1). The activity increased with temperature.
Fig. 1

Cyclic voltammetry for a non-infiltrated and a Pt-infiltrated LSCM/CGO cell stack at 400 °C in 1000 ppm NO +1000 ppm C3H6 + 10% O2 in Ar

The effect of an applied potential to the porous cell stack can be seen in Fig. 2, where the NOx conversion is given in both atmospheres at three different temperatures.
Fig. 2

Percentage of NOx removed on porous cell stack infiltrated with platinum. Gas feed: 1000 ppm NO +10% O2 (open) or 1000 ppm NO +1000 ppm C3H6 + 10% O2 (solid)

More than 80% of NO was removed at 350 °C in the NO + O2 atmosphere when 9 V (equivalent to 1.8 V per cell) was applied. The effect of polarization increased with temperature, but the cell partially short-circuited at the highest polarization at 400 °C; therefore, this measurement was not included. In the propene containing atmosphere, the enhancement effect of the applied voltage was highest at 300 °C and lowest at 350 °C. Compared to the promoting effect in the atmosphere without propene, the propene had a positive effect on the NO decomposition under polarization at 300 °C, since more NO was removed relative to OCV. At 350 and 400 °C, the propene appeared to inhibit the electro-catalysis of NO reduction. Since the SCR of NO with propene was very high, the total removal of NO was still higher when propene was present at low potentials. The rate enhancement ratio (ρ, defined as the ratio between the rate at polarization and the rate at OCV) is shown in Fig. 3.
Fig. 3

Rate enhancement ratio vs applied voltage at different temperatures in atmospheres with and without propene. (a). Rate enhancement ratio in 1000 ppm NO +10% O2. (b). Rate enhancement ratio in 1000 ppm NO +1000 ppm C3H6 + 10% O2

The selectivity towards nitrogen was high when propene was present, and the amount of N2O detected could not be estimated because it was too low. However, more N2O was detected in the NO + O2 atmosphere, and only 50% was reduced to N2 when the 80% NO was removed. This was the lowest measured selectivity. The selectivity was 60% at a 7 V polarization, when more than 40 and 50% NO was removed at 350 and 400 °C, respectively.

The current efficiency (CE) for the decomposition of NO on the porous cell stack is plotted against the temperature in Fig. 4.
Fig. 4

Current efficiency on the Pt-infiltrated cell stack in the two atmospheres as a function of temperature. (a). Current efficiency in 1000 ppm NO +10% O2. (b). Current efficiency in 1000 ppm NO +1000 ppm C3H6 + 10% O2

The CE is calculated from the difference in NOx conversion at OCV and at polarization of the cell.

3.2 Electrochemical characterization by impedance spectroscopy

The impedance spectra of both non-infiltrated and platinum-infiltrated cell stacks were fitted with an equivalent circuit using the open source fitting software, Elchemea [22]. The equivalent circuit consisted of a resistance in series with four sub-circuits. These sub-circuits are in the form of a resistance in parallel with a constant phase element. Arrhenius plots of the polarization resistances of the cell stacks are given in Fig. 5. The infiltration with platinum has greatly increased the ease of which the cell is polarized. Both cells are more difficult to polarize when propene is present in the atmosphere, especially at low temperatures.
Fig. 5

Arrhenius plot of polarization resistance on a non-infiltrated cell and a Pt-infiltrated cell. Gas feed: 1000 ppm NO +10% O2 (open) or 1000 ppm NO +1000 ppm C3H6 + 10% O2 (solid)

In Fig. 6, the impedance spectra obtained on the two cells at 350 °C in 1000 ppm NO +1000 ppm C3H6 + 10% O2 is shown. In both cases, an arc at high frequency appears well separated from the rest of the spectrum. The activation energy of this is 1 eV for both cells in both atmospheres, but the resistance is less on the platinum-infiltrated cell stack. For both cells, the activation energy is independent of atmosphere composition, since the resistance, the near-equivalent capacitances, Cω, and the summit frequency do not change. The near equivalent capacity is calculated as:
$$ {C}_{\varpi }={\left({Y}_0\right)}^{1/n}\ {R}^{\left(1-n\right)/n}, $$
(1)
Where Y0, R and n is found from the fitting.
Fig. 6

Effect of Pt infiltration on impedance response (a). Nyquist plot of impedance spectrum of a non-infiltrated cell stack in 1000 ppm NO +1000 ppm C3H6 + 10% O2 in Ar at 350 °C normalized to reactor area. (b). Nyquist plot of impedance spectrum of a Pt-infiltrated cell stack in 1000 ppm NO +1000 ppm C3H6 + 10% O2 in Ar at 350 °C

The size of the Cω is approximately 10−8 Fcm−2, normalized to one electrode area.

In the middle frequency area, two arcs could be fitted and are denoted as Arc II and Arc III. A low frequency arc (Arc IV) was also found for both cells but only at 350 and 400 °C for the Pt-infiltrated cell. Arc III was dominating the spectrum of this cell, but Arc IV was the dominating arc in the spectrum obtained on the non-infiltrated cell. The fitting results for both types of cells are given in Table 1.
Table 1

Fitting parameters for Arc II, Arc III and Arc IV on non- and Pt-infiltrated cell stacks. Cω is given in Fcm2. Frequencies is given in Hz

  

Pt infiltrated

Backbone

Arc

 

NO + O2

NO + O2 + C3H6

NO + O2

NO + O2 + C3H6

Arc II

Ea

1 eV

0.83

0.76 eV

0.78 eV

Cω

1.5–1.9 × 10−5

1–1.6 × 10−5

2–3.3 × 10−5

2.2–3.5 × 10−5

fsummit

4–82

22–177

0.5–3

0.3–2.2

n

0.57

0.57

0.6

0.6

Arc III

Ea

0.75 eV

0.91 eV

0.83

0.9

Cω

2 × 10−5

2 × 10−5

1.4–2.4 × 10−4

1.6–2.4 × 10−4

fsummit

1–11

0.5–10

0.018–0.014

0.013

n

0.77

0.72

0.74

0.74

Arc IV

Ea

1.2 eV

1.4

0.72 eV

0.87 eV

Cω

6–15× 10−4

5–25× 10−4

2.5–3× 10−4

3× 10−4

fsummit

1–1.8

0.09–1.1

0.002–0.015

0.001–0.014

n

1

1

0.93

0.93

Even though the resistances of the different arcs in the spectra of the two types of cells are very different, the n-values are very close. This could indicate that the processes are of the same nature. Arrhenius plots of the resistance of the four arcs are given in Fig. 7. For the non-infiltrated cell stack, the processes responsible for Arcs II-IV in the spectrum increased in resistance when propene was introduced into the atmosphere. For the platinum infiltrated cell stack, the resistance of Arc II decreased when propene was added whereas the resistances of Arc III and Arc IV increased.
Fig. 7

Arrhenius plots. Gas feed: 1000 ppm NO +10% O2 (open) or 1000 ppm NO +1000 ppm C3H6 + 10% O2 (solid) normalized to reactor area. (a). Arc I (b). Arc II (c). Arc III (d). Arc IV

4 Discussion

4.1 Impact of Pt infiltration on conversion

The catalytic conversion of NO when propene is present is in very good agreement with literature, where Pt shows to be a good SCR catalyst at low temperatures [1, 2, 23]. This explains the decrease in NO conversion with increasing temperature.

Even though there was no catalytic conversion of NO in the atmosphere without propene, there was a much greater promotional effect of the polarization on the NO reduction than that in the atmosphere with propene. When a potential of 5 V and higher was applied, the electrochemical reduction of NO increased that of the SCR of NO with propene at 400 °C. The reduction of NO via the electrochemical reaction and via the catalytic reaction could not be distinguished. Therefore, it is difficult to conclude anything about the relatively high current efficiency that is seen on the Pt-infiltrated cell at 300 °C when propene is present. The current measured through the cell stack at 300 °C was lower when propene is present. A lower current means less oxygen ions are going through the electrolyte, which is not what one would expect if more NO is reduced. The oxide ions can come from the reduction of both NO and O2. If the current decreases, it means that one or both of these reactions are inhibited by the presence of propene. One explanation could be that the propene reacts with the oxygen at the cathode, prohibiting it from getting to the TPB and through the CGO electrolyte. Another explanation could be that the propene is taking up the active sites for the NO or the O2 on the catalyst surface.

The total conversion of propene already at 300 °C is in agreement with what Beguin et al. [14] observed in the presence of excess oxygen. They did a comparison of the electrochemical promotion in the presence of 1% and 5% oxygen. The increase in oxygen partial pressure causes an activation of C3H6 and NO at much lower temperatures. They did not see any oxidation of NO into NO2, which is consistent with what was observed in this study since much less NO2 was present in the gas outlet compared to the test done without propene. In this study, an enhancement ratio of 2.5 could be reached at 400 °C with a 9 V polarization. This ratio is more than what was observed by Beguin et al. [14], where no promotional effect was seen at the higher oxygen concentration. The enhancement of the NO reduction when propene is present is lower at 350 °C than at 300 and 400 °C. This cannot be explained on the basis of the current results.

In the reduction of NO on perovskite-type materials, NO2 is believed to be an intermediate [24]. The fact that almost no NO is oxidized into NO2 could explain the lower reduction of NO.

4.2 Identification of processes

In general, four arcs were identified in the impedance spectra of the two types of cell stacks. Compared to the non-infiltrated cell stack, the arcs observed for the Pt-infiltrated cell seem very similar based on the fact that the n-values of the constant phase element were very close for the two cells.

The independence of the temperature of Cω, with a value of 10−8 Fcm−2, and the independence of the resistance on the presence of propene together with the activation energy of 1 eV identify Arc I as being related to the of transfer of oxide ions between the electrolyte and electrode interface [25]. Arc II and Arc III for the non-infiltrated cell increase in resistance when propene is present. The Cω increases with temperature, and according to [25, 26], the values are in good agreement with the capacitances associated with adsorption and dissociation of oxygen on the composite electrode.

For the Pt-infiltrated cell, the resistance of Arc II behaves differently, and it decreases when propene is introduced into the atmosphere. If the arc represents the same process on the two types of cells in the two atmospheres, then it indicates that the propene is promoting this reaction on the platinum. This could suggest that the process related to Arc II is a step in the SCR of NO with propene. The n-values of this arc are almost the same for the two cells, but the activation energy has changed. On the Pt-infiltrated cell, the process is much more temperature sensitive. Arc III and Arc IV have different near equivalent capacitances, summit frequencies and activation energies. The n-value, however, is very similar. The NO reduction on LSM is believed to go through the intermediate NO2 [24]. This is not the case for the NO reduction on Pt where it has been proposed to involve adsorption and dissociation of NO to give Nads and Oads, with subsequent desorption of N2 and N2O and removal of Oads by C3H6 [3]. This can explain the difference in the arcs, but it cannot be determined based on the current results.

The low frequency arc on the Pt-infiltrated cell has the same characteristics as the one on the non-infiltrated cell. That is, the near equivalent capacitances are similar, as is the n-value. However, the n-value is much smaller, and the summit frequency was higher. The high n-value and the low frequencies have been observed on LSM/CGO electrodes and were related to the NOx species in the system [24, 26].

5 Conclusion

A rate enhancement ratio of 2.5 for the reduction of NO with propene was achieved in the presence of 10% O2 at 400 °C with a 9 V polarization. At 300 °C, the hydrocarbon seemed to have a positive effect on the electrochemical NO reduction. However, the rate enhancement ratio was lower than the results obtained in a propene-free atmosphere at all the tested temperatures. The highest total NOx conversion was 31% at a 9 V polarization at 300 °C compared to 80% at 350 °C when no propene was present. The high oxygen concentration caused the propene to fully oxidize at OCV, so no enhancement of this reaction was observed.

Notes

Acknowledgements

This study was funded by the strategic research council of Denmark (grant number2104-08-0009). The authors declare that they have no conflict of interest.

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© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Department of Energy Conversion and StorageTechnical University of Denmark – DTURoskildeDenmark

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