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Catalysts 2018, 8(4), 155; https://doi.org/10.3390/catal8040155

Article
Gas-Phase Phosphorous Poisoning of a Pt/Ba/Al2O3 NOx Storage Catalyst
1
Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
2
Volvo Car Corporation, SE-405 31 Gothenburg, Sweden
*
Author to whom correspondence should be addressed.
Received: 9 March 2018 / Accepted: 7 April 2018 / Published: 11 April 2018

Abstract

:
The effect of phosphorous exposure on the NOx storage capacity of a Pt/Ba/Al2O3 catalyst coated on a ceramic monolith substrate has been studied. The catalyst was exposed to phosphorous by evaporating phosphoric acid in presence of H2O and O2. The NOx storage capacity was measured before and after the phosphorus exposure and a significant loss of the NOx storage capacity was detected after phosphorous exposure. The phosphorous poisoned samples were characterized by X-ray photoelectron spectroscopy (XPS), environmental scanning electron microscopy (ESEM), N2-physisorption and inductive coupled plasma atomic emission spectroscopy (ICP-AES). All characterization methods showed an axial distribution of phosphorous ranging from the inlet to the outlet of the coated monolith samples with a higher concentration at the inlet of the samples. Elemental analysis, using ICP-AES, confirmed this distribution of phosphorous on the catalyst surface. The specific surface area and pore volume were significantly lower at the inlet section of the monolith where the phosphorous concentration was higher, and higher at the outlet where the phosphorous concentration was lower. The results from the XPS and scanning electron microscopy (SEM)-energy dispersive X-ray (EDX) analyses showed higher accumulation of phosphorus towards the surface of the catalyst at the inlet of the monolith and the phosphorus was to a large extent present in the form of P4O10. However, in the middle section of the monolith, the XPS analysis revealed the presence of more metaphosphate (PO3). Moreover, the SEM-EDX analysis showed that the phosphorous to higher extent had diffused into the washcoat and was less accumulated at the surface close to the outlet of the sample.
Keywords:
LNT; NSR; NOx storage; phosphorous; deactivation; poisoning

1. Introduction

In diesel and lean burn gasoline engines, the engine operates with a large excess of air, and this results in an increased fuel economy, and thereby reduced emissions of CO2, which is a greenhouse gas. However, it is critical to remove NOx, which can be done by different catalytic aftertreatment techniques. Emission standards have therefore been implemented in large parts of the world and the emission levels have been significantly reduced over the years [1].
Since the traditional Three-way catalyst (TWC), requires stoichiometric air:fuel ratio, the Lean NOx Trap (LNT) was introduced as a catalytic concept for exhaust gas treatment in diesel vehicles [2,3,4,5]. In the LNT, usually barium is used as a storage component, and the nitrogen oxides during lean conditions are stored on barium as barium nitrates. Moreover, noble metals, such as platinum, play a key role to oxidize the NO to NO2, which facilities the NOx storage [2,6,7,8,9,10]. Under short periods of time the engine is switched from operating under lean to rich conditions. Under rich conditions, barium nitrates decompose and are reduced over the noble metal sites to mainly N2 and H2O. The reducing agents in the exhaust gas are hydrocarbons (HC), carbon monoxide (CO) and hydrogen (H2), which react with the NOx to produce CO2, H2O and N2. Nitrogen is the desired product from the reduction of stored nitrates. However, there are other possible bi-products from the reduction process such as ammonia (NH3) and nitrous oxide (N2O) [3].
A vehicle is expected to be in service for a long time, which requires the catalyst to be durable. Over time, the catalytic properties of the catalyst deteriorate. There are a few mechanisms that contribute to degradation of catalysts, such as sintering of catalytic particles as a result of high-temperature exposure [11]. The catalytic washcoat can also undergo mechanical tear and poisoning of active sites on the catalyst caused by chemisorption or reactions of catalytically active sites with poisons in the exhaust gas [12]. The poisons are often introduced to the system through the gasoline or diesel fuel, which is the case for SO2, or through the lubricant of the engine. Lubricants, such as zinc dialkyldithiophosphate (ZDDP), are commonly used in vehicles and are a source for accumulation of both sulfates, zinc and phosphorus in the catalyst [13]. The deactivation effect of sulfur has extensively been studied over LNTs [12,14,15,16], and also how to regenerate the catalyst from sulfur [17].
There are only a few studies available in the open literature where phosphorus poisoning has been studied over noble metal catalysts used for emission control. Bunting et al. [13] studied a diesel oxidation catalyst (DOC) connected to an engine rig where the fuel was doped with ZDDP, which showed accumulation of foreign compounds such as phosphates, zinc and sulfates on the catalyst. Also, results from characterization of field-aged TWCs showed accumulation of elements found from ZDDP poisoning of catalysts [18,19,20]. Moreover, it has been shown that the distribution of phosphorus as it accumulates in the catalytic washcoat exhibits a gradient, with higher concentration at the inlet and lower along the washcoat length [19,20,21,22,23]. Compared to sulfur, which has a more even distribution over the washcoat and penetrates deep into the washcoat, phosphorus tends to be more concentrated close to the washcoat surface [13]. A study performed by Galisteo et al. [24], where a Pt/Ba/Al2O3 catalyst was impregnated with (NH4)2HPO4, showed that the amount of phosphorus in the catalyst correlated with the loss of NOx storage capacity.
However, there are, to our knowledge, no studies in the open literature that have examined phosphorus poisoning of LNT catalysts using vapour phase exposure that is closer to realistic poisoning conditions, which is the objective of the present work. In this study, we have examined a Pt/Ba/Al2O3 catalyst exposed to phosphorous in the gas phase using different phosphorous concentrations. The catalyst samples were studied in a flow reactor to determine changes in NOx storage capacity, NOx reduction activity and bi-product formation. The phosphorous-exposed samples were also characterized by X-ray photoelectron spectroscopy (XPS), nitrogen-physisorption, Inductive coupled plasma atomic emission spectroscopy (ICP-AES) and Environmental Scanning Electron Microscopy (ESEM) with energy dispersive X-ray (EDX) analysis.

2. Results and Discussion

2.1. Activity Measurements

Lean/rich cycling experiments were conducted in which the system was exposed to 400 vol.-ppm NO, 5 vol.-% CO2, 5 vol.-% H2O and 8 vol.-% O2 during the lean phase. In this phase, NO is first oxidized over the platinum sites, and thereafter storage of NOx species occurs as nitrates through a disproportionation mechanism, but also via nitrite formation at low temperature [5]. The NO oxidation capacity of the catalyst is clearly visible by the NO2 formation seen in Figure 1. In the initial part of the lean phase, all NOx is stored and after a certain amount of time, breakthrough of NOx, due to saturation of the storage sites, is seen. Most NOx is stored onto the barium sites in form of Ba(NO3)2, but some NOx is also stored onto alumina sites [3]. At lower temperature, storage of NOx occurs over both the barium and alumina sites, but at higher temperature the barium sites dominate the NOx storage [25], due to the lower stability of alumina nitrates.
As the system is switched from lean to rich phase, a NOx peak is visible, which is related to release of stored NOx in the catalytic material [26]. Directly after the switch from the lean to the rich (1 vol.-% H2, 5 vol.-% CO2 and 5 vol.-% H2O) phase, minor formation of N2O is seen and it is during this short period of time, that most of the N2 formation is suggested to occur [27,28,29]. After the N2O peak a large ammonia peak is observed, which is consistent with the work by Lindholm et al. [25]. Most of the formed NH3, early in the rich phase, is consumed by a Selective Catalytic Reduction (SCR) reaction with the stored NOx, according to Lietti et al. [27] and Lindholm et al. [6]. Hence, the NH3 peak is delayed and will dominate more as the rich phase proceeds and this is also seen in Figure 1. Indeed, spatially resolved MS measurements performed by Partridge et al. [10] showed a clear axial consumption of ammonia.
The effect of phosphorus exposure was studied by exposing the Pt/Ba/Al2O3 catalyst to 100 vol.-ppm H3PO4, 8 vol.-% O2 and 5 vol.-% H2O at 200 °C for 34 h and thereafter repeating the same experiment, see Figure 1. In addition, a second catalyst was used, which was exposed to 50 vol.-ppm H3PO4 instead. These catalysts were studied under lean/rich cycling experiments, which were conducted both at 300 and 400 °C, using 400 vol.-ppm NO, 5 vol.-% CO2, 8 vol.-% O2, and 5 vol.-% H2O for the lean conditions and 1 vol.-% H2 or 1000 vol.-ppm C3H6, 400 vol.-ppm NO, 5 vol.-% CO2 and 5 vol.-% H2O during the rich conditions, and the resulting outlet NOx concentrations are shown in Figure 2. The results show that exposure to phosphorus causes a significant loss of NOx storage capacity for the Pt/Ba/Al2O3 catalyst. The loss of NOx storage capacity is summarised in Table 1. It is clear that the loss in storage capacity is similar for both poisoning levels and, moreover, that the effect is more pronounced after exposure at 300 compared to 400 °C.
Our results are in line with those in the study by Galisteo et al. [24], where the effect of phosphorous on the NOx storage capacity of Pt/Ba/Al2O3 was studied by impregnation of the catalyst with (NH3)3PO4 dissolved in water. The authors also observed a decreased NOx storage capacity after phosphorous exposure. At 300 °C, the NOx reduction properties of C3H6 are poor compared to 400 °C (compare graph B and D in Figure 2), which previously has been observed by Olsson et al. [30]. However, loss in NOx storage capacity is still observed for this case after phosphorus exposure. For exposure at 400 °C, the phosphorus poisoning is similar for both H2 and C3H6, which indicates that as long as the reductant can remove the NOx, the phosphorus poisoning is related to poisoning of the reactions under lean conditions. For phosphorous exposure at 300 °C on the other hand, the reduction with propene is poor, which results in low NOx storage and therefore the effect of phosphorus poisoning is not seen equally clear.
The resulting N2O formation during these experiments is shown in Figure 3, and it is observed that the selectivity towards N2O is higher at low temperatures, which is in accordance with the literature [31]. Most of the N2O is formed in the first part of the rich period, but also a peak in the beginning of the lean phase is observed. Choi et al. [32] found that the second N2O peak relates to reactions with NH3. When shifting from rich to lean phase, the formation of NH3 ceases due to the replacement of H2 with O2 in the feed. Therefore, it is plausible that the second N2O peak relates to consumption of stored NH3 to form N2O.
In addition, more N2O is formed when using propene as the reducing agent compared to hydrogen. The N2O formation was previously studied by Olsson et al. [30] where a kinetic model for the reduction of NOx by C3H6 was developed. The reason for the N2O formation is the reaction between the hydrocarbon and NOx to form N2O [33]. Furthermore, in the beginning of the lean phase after propene reduction, there is an increase in the N2O formation. This is due to reaction between NOx and HC species, stored during the rich phase. In addition, the results clearly show that the formation of N2O in the rich phase decreases after exposure to phosphoric acid. The decrease in N2O formation occurs at both 300 and 400 °C for both H2 and C3H6 as the reducing agent. When comparing the ratio of (N2O formed)/(NOx stored) at 300 °C with propene as reductant, the ratio clearly decreases after phosphorous exposure (0.015, 0.0084, 0.020 and 0.011 for first sample before and after exposure for 100 vol.-ppm P, and for second sample before and after exposure for 50 vol.-ppm P, respectively). Thus, it is clear that the N2O formation not decreases only due that less NOx is stored, but also due to that the selectivity changes because of the poisoning. De Abreu Goes et al. [11] studied thermal exposure of LNT catalysts, which resulted in sintering of the catalytic particles that in turn resulted in decreased formation of N2O. Since the catalyst in the present study was not exposed to high temperature again after the degreening step, sintering as a cause for the loss of N2O formation is not likely. Therefore, the reduced N2O formation after phosphorous exposure is more likely to relate to poisoning of the platinum sites.
With H2 as the reducing agent, the formation of NH3 is higher compared to C3H6 as reductant, which can be seen in Figure 4. At 300 °C, the formation of NH3 is even negligible for the C3H6 case. For both samples, the NH3 formation is slightly higher after the exposure to phosphorus when H2 is used as the reducing agent. The ammonia formed over the LNT depends on several reactions occurring simultaneously: (i) ammonia production from inlet NOx; (ii) ammonia production from stored NOx; and (iii) reaction of the formed ammonia with stored NOx in an SCR process [6]. An increase in the ammonia production has also been observed for field-aged, oven-aged and sulphur-poisoned LNT catalysts [11,34]. The reason for the increase in ammonia formation after phosphorus exposure could be related to the loss of NOx storage capacity depicted in Figure 2. Since less NOx is available in the form of nitrates during the lean phase, less ammonia can in turn react in the SCR reaction with the stored nitrates [6], and thereby the ammonia release is higher. However, less ammonia will also be produced from the stored NOx, thus this means that the SCR reaction is more influenced by the phosphorous poisoning compared to the ammonia production. The increased ammonia production during rich conditions can be beneficial for the cases where an SCR catalyst is placed downstream of the LNT [8], in the so-called passive SCR technique.
The outlet N2 concentration was not measured due to IR spectroscopy was used for the gas phase analysis. By assuming that all nitrogen leaving the catalyst are either in form of NO, NO2, NH3, N2O or N2, the remaining N2 can be determined. The quantities of NOx, NO, NO2 NH3 and N2O were estimated by integration of the data from the FTIR analysis. The results from these calculations are shown in Figure 5 and it is clear that the formed amount of N2 is quite low. The reason for this is that we intentionally used long lean and rich cycles to come closer to saturation and also more complete regeneration. In a real application, the cycle times would be different, giving a high amount of N2. Moreover, the results clearly show a minor N2O formation in all conditions except with C3H6 as reducing agent at 300 °C. Hence, the main difference between the results from the different experimental conditions is the formation of NH3, N2 and release of NOx. The phosphorous-exposed samples tend to release higher amounts of NOx and NH3 at the cost of formation of the desired product; N2. According to the ICP-AES results shown in Table 2, the amount of accumulated phosphorous is similar for both H3PO4 concentrations. Thus the H3PO4 concentration does not seem to be such a critical factor for the poisoning at these levels. Therefore, the difference before and after phosphorous exposure between the two cases appears similar. For H2 as the reducing agent, exposure to phosphorus has a more pronounced effect at 300 than 400 °C. The reason for this could be that since the ammonia production is higher at lower temperature, and is to a large extent influenced by the phosphorous poisoning, it results in a more pronounced effect on the N2 selectivity. However, for C3H6 as the reducing agent, there is no clear trend regarding the selectivities between the fresh and phosphorous exposed samples at 300 and 400 °C. The reason for this could be that the amounts of NH3 and N2O formed in these experiments are low, so most of the species are NOx and N2.

2.2. Characterization of the Phosphorous Exposed Samples

One channel of the phosphorous-exposed monolith sample “50 ppm, 34 h” was cut out and the surface composition in the inlet, middle, and in the outlet section of the channel was analyzed by XPS. The overall spectra (see Figure 6A), show that the magnitude of the phosphorus peaks is highest from the surface of the inlet section and lowest from the outlet section of the phosphorous exposed sample. The P 2p binding energy region measured with higher resolution for the inlet and outlet sections for the phosphorous exposed sample are shown in Figure 6B. Due to different signal intensities from the two samples, the spectra are normalized to facilitate the interpretation of the peak positions when comparing different spectra with each other. The intensity of the P 2p peak from the middle section of the sample is significantly lower compared to the peak from the inlet section. In addition, the results show that the position of the P 2p peak shifts towards lower binding energy for the middle section compared to the inlet section of the sample. Deconvolution of the P 2p peak for the inlet and middle section of the sample can be seen in Figure 7. For the inlet section of the sample the deconvolution shows a higher presence of P4O10 with a binding energy ranging between 135.0 and 135.5 eV [35]. However, for the middle region of the sample, the deconvolution shows that metaphosphates, PO3, dominate, which have a binding energy ranging between 134.0 and 134.5 eV [35]. Formation of metaphosphates has previously been observed after exposing Cu/BEA [36] as well as Fe/BEA [37] for phosphoric acid in gas phase. In addition, formation of P4O10 was also found on Fe/BEA [37], which is in line with our observations. Our results show that the phosphorus-containing surface species mainly are in the form of P4O10 in the inlet section of the sample, where the phosphorus concentration and deposition is higher, while in the middle section more metaphosphates are seen.
In order to achieve further insight into the effect of phosphorous exposure in the axial direction of the Pt/Ba/Al2O3 catalyst, the specific surface area and pore volume, and elemental composition of the inlet, middle and outlet sections of the samples exposed for 100 and 50 vol.-ppm phosphorous for 34 h were analysed together with a degreened sample. In Table 2, the phosphorus content from ICP-AES analysis is presented along with the specific surface area and pore volume. The results show that phosphorus has a gradient distribution, from the inlet to the outlet section of the two phosphorous-exposed samples, which is in line with other studies of both field-aged catalysts and experiments in engine rigs [13,19,20,21,22,23,38,39]. Moreover, it is found that the effect of the phosphorus concentration is minor, which is in line with the flow reactor experiments. Glisteo et al. [24] impregnated a Pt/Ba/Al2O3 catalyst with aqueous solutions of phosphorous of different concentrations. Their results showed that the specific surface area decreased more by higher phosphorous concentration accumulated on the catalyst, which is in line with our results. We find that the phosphorus content is the highest, and the surface area and pore volume have lowest values, for the inlet section of both catalysts exposed to phosphorous. Close to the outlet section, the phosphorus content is much lower for both samples, whereas the specific surface area and pore volume are higher and more similar to the degreened sample.
The results from ESEM and EDX analyses acquired from two different positions; 2 mm from the front and 2 mm from the back of the monolith sample exposed for 50 vol.-ppm phosphorous for 34 h are presented in Figure 8 and Figure 9, respectively. Besides phosphorus, other elements (Al and Ba) that are present in the washcoat are depicted in the images. Moreover, silicon, which is one of the major compounds of the cordierite substrate, is also shown. The analysis 2 mm from the front of the sample (Figure 8) indicates that phosphorus is located towards the surface of the washcoat, which previously has been shown for catalysts exposed to ZDDP doped fuel in engine rigs [13]. Closer to the outlet of the sample, less phosphorus is present and the distribution of the phosphorus is more uniform in the washcoat. Accumulation of phosphorus towards the front of the catalyst has been shown for field-aged catalysts [21]. The ESEM-EDX results in the present study can be related to the XPS results (Figure 7), which showed more P4O10 in the inlet section of the sample, while more metaphosphates in the middle section of the phosphorous exposed sample. Finally, in the outlet section of the monolith sample, only small amounts of phosphorus are observed with EDX, but it should be noted that phosphorus is still visible, which means that in a catalytic aftertreatment system it is possible that a catalyst placed downstream the NOx storage catalyst also can be exposed to phosphorus.
To summarize, the combined XPS, EDX, BET and ESEM-EDX data suggest that during phosphorus exposure in the vapour phase more phosphorus is deposited in the inlet of the monolith sample and this phosphorous is mainly located at the surface of the washcoat and contains a large fraction of P4O10. While the middle section of the monolith contains less phosphorus and has a higher fraction of metaphosphates. Moreover, closer to the outlet the phosphorus is more evenly distributed.

3. Materials and Methods

3.1. Catalyst Synthesis

The Pt/BaO/Al2O3 catalyst was synthesized using incipient wetness impregnation. To enhance the NOx storage capacity of the chosen model catalyst, the impregnation of platinum on the support material was performed before incorporation of barium, which previously has shown beneficial to achieve high NOx storage capacity of the catalyst [25]. Prior to the impregnation, the γ-alumina (Sigma-Aldrich) support was calcined for 2 h at 600 °C. Thereafter the γ-alumina support was impregnated with a platinum nitrate precursor (Heraeus platinum (II) nitrate; solution type K, batch: 12207), diluted with Milli-Q water (18 MΩ·cm) of the estimated volume targeting 2 wt.-% Pt. The impregnated support material was dried in air at 100 °C for 2 h and subsequently calcined in air at 550 °C for 2 h, starting from room temperature with a temperature increase of 5 °C/min. The calcined Pt/Al2O3 sample was then impregnated with a barium precursor (Sigma-Aldrich barium acetate, 99% A.C.S reagent, batch: 11415KA), dissolved in Milli-Q water in two steps also by incipient wetness impregnation, targeting a total of 16 wt.-% of BaO. The catalytic material was dried and calcined after each impregnation step under same conditions as for the impregnation of platinum.

3.2. Monolith Preparation

The catalytic material was coated on honeycomb-shaped ceramic monolith substrates (20 mm in length, 21 mm in diameter, a wall thickness of 0.106 mm and with 400 cpsi). To improve the adhesiveness, 10 wt.-% boehmite (Sasol disperal P2, product code: 538116) was mixed with the catalyst in a solution of 50 vol.-% Milli-Q water and 50 vol.-% ethanol. The monolith substrates were stepwise dipped in the solution and dried at 90 °C until the target washcoat weight of 700 mg per monolith sample was reached. Thereafter, the catalysts were calcined at 550 °C for 2 h, starting from room temperature with a temperature increase of 5 °C/min.

3.3. Flow-Reactor Experiments

The flow-reactor experiments were carried out in a synthetic gas bench (SGB) reactor. The coated monolith samples were placed in a quartz tube, which was 750 mm in length and with an inner diameter of 22 mm. To prevent by-pass, quartz wool was wrapped around the monoliths. The gas flow and gas concentrations were regulated using mass flow controllers from Bronkhorst and water vapor was dosed using a Bronkhorst Controlled evaporation system (CEM) system. The quartz tube was placed in an electric heating coil and covered with insulation material, and the temperatures were measured by two thermocouples, one measuring and regulating the gas temperature before the catalyst and one measuring the catalyst temperature in a center channel of the monolith. An MKS Multigas 2030 FTIR spectrometer provided by MKS instruments, Andover, MA, USA was used to monitor the gas phase concentrations of NO, NO2, N2O, NH3 and H2O, and the total flow was 3500 mL/min, corresponding to 30,300 h−1 space velocity, with argon as carrier gas for all measurements. All lines and connections prior and after the reactor were heated to 200 °C, to prevent condensation.
To avoid sintering of platinum particles under the experiments, the coated monoliths were first H2-treated in 1 vol.-% H2, 5 vol.-% CO2 and 5 vol.-% H2O for 20 min at 500 °C and thereafter degreened at 600 °C, according to the following procedure. First the catalyst was exposed to the rich phase (400 vol.-ppm NO, 5 vol.-% CO2, 1 vol.-% H2 and 5 vol.-% H2O with argon as carrier gas) for 60 min, followed by exposure to the lean phase (400 vol.-ppm NO, 5 vol.-% CO2, 8 vol.-% O2, and 5 vol.-% H2O in argon) for 15 min. Subsequently, the catalyst was reduced in 1 vol.-% H2, 5 vol.-% CO2 and 5 vol.-% H2O in Ar for 20 min at 600 °C.
The NOx storage and reduction performance of the catalysts was studied at 300 and 400 °C using lean/rich cycling. At each temperature, two cycle segments consisting of five cycles in each segment were conducted. Cycle number 4 for all sequences is shown in the Figures. The gas composition of the lean phase was 400 vol.-ppm NO, 5 vol.-% CO2, 8 vol.-% O2, and 5 vol.-% H2O. In the first lean/rich phase cycle segment, hydrogen was used as the reductant (1 vol.-% H2, 400 vol.-ppm NO, 5 vol.-% CO2 and 5 vol.-% H2O) and in the second cycle segments propene was used as the reducing agent (0.1 vol.-% C3H6, 400 vol.-ppm NO, 5 vol.-% CO2 and 5 vol.-% H2O). For all cycles, the duration of the lean and rich phase were 4 and 1 min, respectively. Prior to the experiment at each temperature, the catalyst was pre-treated with 1 vol.-% H2, 5 vol.-% CO2 and 5 vol.-% H2O for 15 min at 400 °C. This pre-treatment was also conducted for the phosphorus-exposed catalysts.

3.4. Phosphorous Exposure

The NOx storage capacity of the coated monoliths was measured in the flow reactor before and after the exposure to phosphorus, using the procedure described in the previous section. The same reactor as described above was used for the phosphorous exposure, however using a separate reactor tube. In order to reduce surfaces for the evaporated phosphoric acid to stick to, the monolith was not wrapped in quartz wool during the phosphorous exposure. Phosphoric acid was introduced to the gaseous flow by a syringe pump connected to a Teflon pipe disposing the acid inside the quarts tube. The composition of the gas phase during the phosphorous exposure was 50 or 100 vol.-ppm evaporated phosphoric acid, 5 vol.-% H2O, 8 vol.-% O2 in argon as carrier gas, and the temperature was 200 °C. Two different phosphorous exposure conditions were studied in this work; in the first case the catalyst was exposed to 50 vol.-ppm phosphoric acid for 34 h, and in the second case the sample was exposed to 100 vol.-ppm for phosphoric acid for 34 h. This corresponds to an exposure of 1.7 and 3.4 g P/l catalyst, respectively. Note that high amounts of phosphorus passed through the catalyst without adsorption. These samples are denoted “50 ppm, 34 h” and “100 ppm, 34 h”, respectively. Note that new monolith samples were used for each poisoning condition.

3.5. XPS

To evaluate the oxidation state of phosphorus accumulated on the catalytic washcoat, an entire channel from the coated monolith was placed on a holder to measure the surface species of the washcoat. The use of a whole channel also enabled the identification of the axial position where measurements took place. The instrument used for the XPS analysis was a Perkin Elmer PHI 5000 ESCA system provided by PerkinElmer, Waltham, MA, USA equipped with an EDS elemental mapping system. The X-ray source for the XPS measurements was monochromatic Al Kα radiation at 1486.6 eV. Correction for charging was performed by normalizing the spectra using the C 1s peak at 284.6 eV [40] as reference. Furthermore, since normalizing will cause the largest peak being equal to 1, all spectra were divided by 1.1 for better visualization. Deconvolution of the P 2p peak from each sample was performed by fitting a Gaussian function to the experimental data, with subtraction of a linear baseline under both peaks. The peak positions were optimized according to the same procedure for both samples to achieve the lowest standard deviation and found to be 134.25, 135.5 and 132.5 eV for PO3, P4O10 and PO43−, respectively.

3.6. N2-Physisorption and ICP-AES

The specific surface area and pore volume of the samples were measured by N2-physisorption isotherms at −195 °C and were collected using a TriStar 3000 gas adsorption analyzer. The inlet-, middle- and outlet section of the coated monoliths were crushed and grinded to powder form. Approximately 300 mg sample from each section was thermally dried at 90 °C in N2 gas flow for 4 h and used for specific surface area and pore-volume measurements.
Powder samples from each section were used for elemental analysis by inductive coupled plasma atomic emission spectroscopy (ICP-AES), and the measurements were performed by ALS Scandinavia AB, Luleå, Sweden. The concentration of the measured phosphorus is reported as weight percentages.

3.7. ESEM

Environmental scanning electron microscopy (ESEM) was used to acquire an overview of the location of different elements over the catalytic washcoat. The instrument used to gain these images was a Quanta200 ESEM FEG from FEI, Hillsboro, OR, USA. An energy dispersive X-ray (EDX) system from Oxford Inca was equipped to the instrument to map the elements over scanned areas.

4. Conclusions

In the present study, a Pt/Ba/Al2O3 NOx storage catalyst coated on a ceramic monolith substrate was exposed to two different phosphorus containing atmospheres for 34 h at 200 °C; 50 vol.-ppm H3PO4 + 8 vol.-% O2 + 5 vol.-% H2O and 100 vol.-ppm H3PO4 + 8 vol.-% O2 + 5 vol.-% H2O. The catalysts were characterized by N2-physisorption, XPS, ESEM and ICP-AES, and the NOx storage capacity was measured in flow reactor experiments.
The specific surface area and pore volume measurements, together with the elemental analysis by ICP-AES, showed a clear gradient distribution of phosphorous along the axial direction of the monolith. The phosphorus concentrations close to the inlet section of the sample were significantly higher compared to the middle and outlet section. The specific surface area and pore volume were found to be lower in the sections where the concentration of phosphorus was higher. At the outlet section, both the specific surface area and the pore volume were found to be comparable with those for the fresh sample, and the accumulated amount of phosphorus was only about 0.1 wt.-%, which can be compared with 2 wt.-% at the inlet section of the phosphorous exposed catalyst. Moreover, there was no significant difference between the different phosphorous concentrations, which indicates that the phosphorus concentration is not limiting the deposition of phosphorous on the catalyst, due to long exposure time.
Flow-reactor experiments showed that the NOx storage capacity of Pt/BaO/Al2O3 decreased with presence of phosphorous species on the catalyst. The results also showed that the formation of N2 decreased after the exposure to phosphorous, in favour of ammonia production. Furthermore, the formation of N2O decreased after the phosphorus exposure. The increased ammonia production can be explained by the fact that the outlet ammonia concentration is dependent on three reactions over the catalyst: (i) the formation of ammonia from NO in the inlet gas; (ii) formation of ammonia from stored NOx; and (iii) the consumption of ammonia through the selective catalytic reduction by the reaction with the stored nitrates. Since the amount of stored nitrates is lower for the phosphorus exposed sample, it means that less ammonia is produced in reaction (ii), but simultaneously less ammonia is consumed in reaction (iii). Since there is more ammonia in total, it means that the SCR reaction is more influenced by the phosphorus poisoning than the ammonia formation from the stored nitrates.
The XPS analysis showed that phosphorus exposure resulted in the formation of different species depending on the axial location of the washcoat. At the inlet section of the catalyst, where most of the phosphorus was accumulated, the dominating phosphorous species was found to be P4O10 while further along the axial direction metaphosphates (PO3) was the dominating species. From the ESEM and EDX analysis it could be concluded that most phosphorus, close to the inlet of the catalyst, was located at the washcoat surface. At the outlet, only a small amount of phosphorus could be detected by SEM-EDX, however, it was more diffused into the washcoat. To summarize, the results from XPS and SEM indicate that the accumulation of phosphorus differs between the front and the back of the monolith catalyst.

Acknowledgments

This work was conducted at the Competence Centre for Catalysis (KCK) hosted by Chalmers University of Technology and financially supported by the Swedish Energy Agency, Chalmers and the member companies AB Volvo, ECAPS AB, Haldor Topsøe A/S, Scania CV AB, Volvo Car Corporation AB, and Wärtsilä Finland Oy. KCK is greatly acknowledged for their financial support. Lars Ilver is gratefully acknowledged for the assistance with XPS measurements.

Author Contributions

R.J. and L.O. conceived and designed the experiments; R.J. performed the experiments; R.J., O.M., M.S., E.O., M.B. and L.O. analyzed the data; L.I. and J.W. contributed analysis tools; R.J. wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Measured outlet NO, NO2, N2O and NH3 concentrations for the Pt/Ba/Al2O3 sample “100 ppm, 34 h” before and after exposure to phosphorous (100 vol.-ppm H3PO4, 8 vol.-% O2, 5 vol.-% H2O at 200 °C) in a flow-reactor experiment at 400 °C. Lean phase; 400 vol.-ppm NO, 5 vol.-% H2O, 5 vol.-% CO2 and 8 vol.-% O2. Rich phase; 400 vol.-ppm NO, 5 vol.-%H2O, 5 vol.-% CO2 and 1 vol.-% H2.
Figure 1. Measured outlet NO, NO2, N2O and NH3 concentrations for the Pt/Ba/Al2O3 sample “100 ppm, 34 h” before and after exposure to phosphorous (100 vol.-ppm H3PO4, 8 vol.-% O2, 5 vol.-% H2O at 200 °C) in a flow-reactor experiment at 400 °C. Lean phase; 400 vol.-ppm NO, 5 vol.-% H2O, 5 vol.-% CO2 and 8 vol.-% O2. Rich phase; 400 vol.-ppm NO, 5 vol.-%H2O, 5 vol.-% CO2 and 1 vol.-% H2.
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Figure 2. Measured outlet NOx before and after exposing Pt/Ba/Al2O3 to 50 or 100 vol.-ppm H3PO4, 8 vol.-% O2 and 5 vol.-% H2O for 34 h. The lean phase (400 vol.-ppm NO, 5 vol.-% H2O, 5 vol.-% CO2 and 8 vol.-% O2) starts at time = 0. The rich phase consist of 400 vol.-ppm NO, 5 vol.-% H2O, 5 vol.-% CO2 and H2 or C3H6, where (A) 300 °C with 1 vol.-% H2 in the rich phase, (B) 300 °C with 0.1 vol.-% C3H6 in the rich phase, (C) 400 °C with 1 vol.-% H2 in the rich phase, and (D) 400 °C with 0.1 vol.-% C3H6 in the rich phase.
Figure 2. Measured outlet NOx before and after exposing Pt/Ba/Al2O3 to 50 or 100 vol.-ppm H3PO4, 8 vol.-% O2 and 5 vol.-% H2O for 34 h. The lean phase (400 vol.-ppm NO, 5 vol.-% H2O, 5 vol.-% CO2 and 8 vol.-% O2) starts at time = 0. The rich phase consist of 400 vol.-ppm NO, 5 vol.-% H2O, 5 vol.-% CO2 and H2 or C3H6, where (A) 300 °C with 1 vol.-% H2 in the rich phase, (B) 300 °C with 0.1 vol.-% C3H6 in the rich phase, (C) 400 °C with 1 vol.-% H2 in the rich phase, and (D) 400 °C with 0.1 vol.-% C3H6 in the rich phase.
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Figure 3. Measured outlet N2O before and after exposing Pt/Ba/Al2O3 to 50 or 100 vol.-ppm H3PO4, 8 vol.-% O2 and 5 vol.-% H2O for 34 h. The rich phase (400 vol.-ppm NO, 5 vol.-% H2O, 5 vol.-% CO2 and H2 or C3H6) starts at time = 0. The lean phase consist of 400 vol.-ppm NO, 5 vol.-% H2O, 5 vol.-% CO2 and 8 vol.-% O2, where (A) 300 °C with 1 vol.-% H2 in the rich phase, (B) 300 °C with 0.1 vol.-% C3H6 in the rich phase, (C) 400 °C with 1 vol.-% H2 in the rich phase, and (D) 400 °C with 0.1 vol.-% C3H6 in the rich phase.
Figure 3. Measured outlet N2O before and after exposing Pt/Ba/Al2O3 to 50 or 100 vol.-ppm H3PO4, 8 vol.-% O2 and 5 vol.-% H2O for 34 h. The rich phase (400 vol.-ppm NO, 5 vol.-% H2O, 5 vol.-% CO2 and H2 or C3H6) starts at time = 0. The lean phase consist of 400 vol.-ppm NO, 5 vol.-% H2O, 5 vol.-% CO2 and 8 vol.-% O2, where (A) 300 °C with 1 vol.-% H2 in the rich phase, (B) 300 °C with 0.1 vol.-% C3H6 in the rich phase, (C) 400 °C with 1 vol.-% H2 in the rich phase, and (D) 400 °C with 0.1 vol.-% C3H6 in the rich phase.
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Figure 4. Measured outlet NH3 concentration before and after exposing Pt/Ba/Al2O3 to 50 or 100 vol.-ppm H3PO4, 8 vol.-% O2 and 5 vol.-% H2O for 34 h. The rich phase (400 vol.-ppm NO, 5 vol.-% H2O, 5 vol.-% CO2 and H2 or C3H6) starts at time = 0. The lean phase consist of 400 vol.-ppm NO, 5 vol.-% H2O, 5 vol.-% CO2 and 8 vol.-% O2, where (A) 300 °C with 1 vol.-% H2 in the rich phase, (B) 300 °C with 0.1 vol.-% C3H6 in the rich phase, (C) 400 °C with 1 vol.-% H2 in the rich phase, and (D) 400 °C with 0.1 vol.-% C3H6 in the rich phase.
Figure 4. Measured outlet NH3 concentration before and after exposing Pt/Ba/Al2O3 to 50 or 100 vol.-ppm H3PO4, 8 vol.-% O2 and 5 vol.-% H2O for 34 h. The rich phase (400 vol.-ppm NO, 5 vol.-% H2O, 5 vol.-% CO2 and H2 or C3H6) starts at time = 0. The lean phase consist of 400 vol.-ppm NO, 5 vol.-% H2O, 5 vol.-% CO2 and 8 vol.-% O2, where (A) 300 °C with 1 vol.-% H2 in the rich phase, (B) 300 °C with 0.1 vol.-% C3H6 in the rich phase, (C) 400 °C with 1 vol.-% H2 in the rich phase, and (D) 400 °C with 0.1 vol.-% C3H6 in the rich phase.
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Figure 5. Balance over outlet nitrogen atoms over a lean/rich cycle before and after exposing Pt/Ba/Al2O3 to 50 and 100 vol.-ppm H3PO4 for 34 h, assuming that all nitrogen leaving the catalyst is either in form of NOx, NH3, N2O or N2. (A) 300 °C with 1 vol.-% H2 in the rich phase, (B) 300 °C with 0.1 vol.-% C3H6 in the rich phase, (C) 400 °C with 1 vol.-% H2 in the rich phase, (D) 400 °C with 0.1 vol.-% C3H6 in the rich phase.
Figure 5. Balance over outlet nitrogen atoms over a lean/rich cycle before and after exposing Pt/Ba/Al2O3 to 50 and 100 vol.-ppm H3PO4 for 34 h, assuming that all nitrogen leaving the catalyst is either in form of NOx, NH3, N2O or N2. (A) 300 °C with 1 vol.-% H2 in the rich phase, (B) 300 °C with 0.1 vol.-% C3H6 in the rich phase, (C) 400 °C with 1 vol.-% H2 in the rich phase, (D) 400 °C with 0.1 vol.-% C3H6 in the rich phase.
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Figure 6. (A) Normalized X-ray photoelectron spectroscopy (XPS) spectra from the inlet, middle and outlet section of the Pt/Ba/Al2O3 sample exposed to 50 vol.-ppm H3PO4 for 34 h. (B) Corresponding XPS spectra for the P 2p region from the inlet and middle section of the sample.
Figure 6. (A) Normalized X-ray photoelectron spectroscopy (XPS) spectra from the inlet, middle and outlet section of the Pt/Ba/Al2O3 sample exposed to 50 vol.-ppm H3PO4 for 34 h. (B) Corresponding XPS spectra for the P 2p region from the inlet and middle section of the sample.
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Figure 7. Deconvolution of the P 2p XPS spectra from the inlet (A) and middle (B) section of the Pt/Ba/Al2O3 sample exposed to 50 vol.-ppm H3PO4 for 34 h. The considered phosphorous species are P4O10, PO3 and PO43−.
Figure 7. Deconvolution of the P 2p XPS spectra from the inlet (A) and middle (B) section of the Pt/Ba/Al2O3 sample exposed to 50 vol.-ppm H3PO4 for 34 h. The considered phosphorous species are P4O10, PO3 and PO43−.
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Figure 8. Environmental scanning electron microscopy (ESEM) and energy dispersive X-ray (EDX) images from the cross-section 2 mm from the front of the Pt/Ba/Al2O3 sample exposed to 50 vol.-ppm H3PO4 for 34 h.
Figure 8. Environmental scanning electron microscopy (ESEM) and energy dispersive X-ray (EDX) images from the cross-section 2 mm from the front of the Pt/Ba/Al2O3 sample exposed to 50 vol.-ppm H3PO4 for 34 h.
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Figure 9. ESEM and EDX images from the cross-section 2 mm from the back of the Pt/Ba/Al2O3 sample exposed to 50 vol.-ppm H3PO4 for 34 h.
Figure 9. ESEM and EDX images from the cross-section 2 mm from the back of the Pt/Ba/Al2O3 sample exposed to 50 vol.-ppm H3PO4 for 34 h.
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Table 1. NOx storage capacity for fresh and phosphorous exposed samples.
Table 1. NOx storage capacity for fresh and phosphorous exposed samples.
ConditionNOx Storage Capacity (μmol)Loss in NOx Storage Capacity (μmol)
NO+H2, 300 °C, fresh92.4-
NO+H2, 300 °C, 50 ppm P56.236.2
NO+H2, 300 °C, fresh88.8-
NO+H2, 300 °C, 100 ppm P51.437.4
NO+H2, 400 °C, fresh85.9-
NO+H2, 400 °C, 50 ppm P70.615.3
NO+H2, 400 °C, fresh70.3-
NO+H2, 400 °C, 100 ppm P51.518.8
Table 2. Phosphorous content, specific surface area and pore volume of the fresh Pt/Ba/Al2O3 sample and of the Pt/Ba/Al2O3 samples exposed to 50 and 100 vol.-ppm phosphorous for 34 h. The phosphorus contents are measured by inductive coupled plasma atomic emission spectroscopy (ICP-AES).
Table 2. Phosphorous content, specific surface area and pore volume of the fresh Pt/Ba/Al2O3 sample and of the Pt/Ba/Al2O3 samples exposed to 50 and 100 vol.-ppm phosphorous for 34 h. The phosphorus contents are measured by inductive coupled plasma atomic emission spectroscopy (ICP-AES).
P-content, (wt.-%)SBET (m2/(g washcoat)) 1VP (cm3/(g washcoat)) 1
Fresh-1340.46
50 ppm, 34 h, inlet2.2980.36
50 ppm, 34 h, middle0.401300.45
50 ppm, 34 h, outlet0.111340.45
100 ppm, 34 h, inlet2.01130.35
100 ppm, 34 h, middle0.381230.39
100 ppm, 34 h, outlet0.071320.39
1 Estimated based on washcoat amount.

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