Gas-phase phosphorous poisoning of a Pt/Ba/Al2O3 NOx storage catalyst

: The effect of phosphorous exposure on the NO x storage capacity of a Pt/Ba/Al 2 O 3 catalyst coated on a ceramic monolith substrate has been studied. The catalyst was exposed to phosphorous by evaporating phosphoric acid in presence of H 2 O and O 2 . The NO x storage capacity was measured before and after the phosphorus exposure and a signiﬁcant loss of the NO x storage capacity was detected after phosphorous exposure. The phosphorous poisoned samples were characterized by X-ray photoelectron spectroscopy (XPS), environmental scanning electron microscopy (ESEM), N 2 -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 P 4 O 10 . However, in the middle section of the monolith, the XPS analysis revealed the presence of more metaphosphate (PO 3 − ). 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.


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 CO 2 , which is a greenhouse gas.However, it is critical to remove NO x , 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 NO x 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 NO 2 , which facilities the NO x 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 N 2 and H 2 O.The reducing agents in the exhaust gas are hydrocarbons (HC), carbon monoxide (CO) and hydrogen (H 2 ), which react with the NO x to produce CO 2 , H 2 O and N 2 .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 (NH 3 ) and nitrous oxide (N 2 O) [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 SO 2 , 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/Al 2 O 3 catalyst was impregnated with (NH 4 ) 2 HPO 4 , showed that the amount of phosphorus in the catalyst correlated with the loss of NO x 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/Al 2 O 3 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 NO x storage capacity, NO x 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.

Activity Measurements
Lean/rich cycling experiments were conducted in which the system was exposed to 400 vol.-ppmNO, 5 vol.-%CO 2 , 5 vol.-%H 2 O and 8 vol.-%O 2 during the lean phase.In this phase, NO is first oxidized over the platinum sites, and thereafter storage of NO x 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 NO 2 formation seen in Figure 1.In the initial part of the lean phase, all NO x is stored and after a certain amount of time, breakthrough of NO x , due to saturation of the storage sites, is seen.Most NO x is stored onto the barium sites in form of Ba(NO 3 ) 2 , but some NO x is also stored onto alumina sites [3].At lower temperature, storage of NO x occurs over both the barium and alumina sites, but at higher temperature the barium sites dominate the NO x storage [25], due to the lower stability of alumina nitrates.As the system is switched from lean to rich phase, a NO x peak is visible, which is related to release of stored NO x in the catalytic material [26].Directly after the switch from the lean to the rich (1 vol.-%H 2 , 5 vol.-%CO 2 and 5 vol.-%H 2 O) phase, minor formation of N 2 O is seen and it is during this short period of time, that most of the N 2 formation is suggested to occur [27][28][29].After the N 2 O peak a large ammonia peak is observed, which is consistent with the work by Lindholm et al. [25].Most of the formed NH 3 , early in the rich phase, is consumed by a Selective Catalytic Reduction (SCR) reaction with the stored NO x , according to Lietti et al. [27] and Lindholm et al. [6].Hence, the NH 3 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/Al  2. The results show that exposure to phosphorus causes a significant loss of NO x storage capacity for the Pt/Ba/Al 2 O 3 catalyst.The loss of NO x 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 NO x storage capacity of Pt/Ba/Al 2 O 3 was studied by impregnation of the catalyst with (NH 3 ) 3 PO 4 dissolved in water.The authors also observed a decreased NO x storage capacity after phosphorous exposure.At 300 • C, the NO x reduction properties of C 3 H 6 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 NO x storage capacity is still observed for this case after phosphorus exposure.For exposure at 400 • C, the phosphorus poisoning is similar for both H 2 and C 3 H 6 , which indicates that as long as the reductant can remove the NO x , 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 NO x storage and therefore the effect of phosphorus poisoning is not seen equally clear.The resulting N 2 O formation during these experiments is shown in Figure 3, and it is observed that the selectivity towards N 2 O is higher at low temperatures, which is in accordance with the literature [31].Most of the N 2 O 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 N 2 O peak relates to reactions with NH 3 .When shifting from rich to lean phase, the formation of NH 3 ceases due to the replacement of H 2 with O 2 in the feed.Therefore, it is plausible that the second N 2 O peak relates to consumption of stored NH 3 to form N 2 O.In addition, more N 2 O is formed when using propene as the reducing agent compared to hydrogen.The N 2 O formation was previously studied by Olsson et al. [30] where a kinetic model for the reduction of NO x by C 3 H 6 was developed.The reason for the N 2 O formation is the reaction between the hydrocarbon and NO x to form N 2 O [33].Furthermore, in the beginning of the lean phase after propene reduction, there is an increase in the N 2 O formation.This is due to reaction between NO x and HC species, stored during the rich phase.In addition, the results clearly show that the formation of N 2 O in the rich phase decreases after exposure to phosphoric acid.The decrease in N 2 O formation occurs at both 300 and 400 • C for both H 2 and C 3 H 6 as the reducing agent.When comparing the ratio of (N 2 O formed)/(NO x 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.-ppmP, and for second sample before and after exposure for 50 vol.-ppmP, respectively).Thus, it is clear that the N 2 O formation not decreases only due that less NO x 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 N 2 O. 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 N 2 O formation is not likely.Therefore, the reduced N 2 O formation after phosphorous exposure is more likely to relate to poisoning of the platinum sites.
With H 2 as the reducing agent, the formation of NH 3 is higher compared to C 3 H 6 as reductant, which can be seen in Figure 4.At 300 • C, the formation of NH 3 is even negligible for the C 3 H 6 case.For both samples, the NH 3 formation is slightly higher after the exposure to phosphorus when H 2 is used as the reducing agent.The ammonia formed over the LNT depends on several reactions occurring simultaneously: (i) ammonia production from inlet NO x ; (ii) ammonia production from stored NO x ; and (iii) reaction of the formed ammonia with stored NO x 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 NO x storage capacity depicted in Figure 2. Since less NO x 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 NO x , 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 N 2 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, NO 2 , NH 3 , N 2 O or N 2 , the remaining N 2 can be determined.The quantities of NO x , NO, NO 2 NH 3 and N 2 O 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 N 2 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 N 2 .Moreover, the results clearly show a minor N 2 O formation in all conditions except with C 3 H 6 as reducing agent at 300 • C. Hence, the main difference between the results from the different experimental conditions is the formation of NH 3 , N 2 and release of NO x .The phosphorous-exposed samples tend to release higher amounts of NO x and NH 3 at the cost of formation of the desired product; N 2 .According to the ICP-AES results shown in Table 2, the amount of accumulated phosphorous is similar for both H 3 PO 4 concentrations.Thus the H 3 PO 4 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 H 2 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 N 2 selectivity.However, for C 3 H 6 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 NH 3 and N 2 O formed in these experiments are low, so most of the species are NO x and N 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 P 4 O 10 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, PO 3 − , 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 P 4 O 10 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 P 4 O 10 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/Al 2 O 3 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.-ppmphosphorous 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/Al 2 O 3 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.-ppmphosphorous for 34 h are presented in Figures 8 and 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 P 4 O 10 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 NO x 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 P 4 O 10 .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.

Catalyst Synthesis
The Pt/BaO/Al 2 O 3 catalyst was synthesized using incipient wetness impregnation.To enhance the NO x 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 NO x 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/Al 2 O 3 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.

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.

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, NO 2 , N 2 O, NH 3 and H 2 O, 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 -% H 2 O).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.-%H 2 , 5 vol.-%CO 2 and 5 vol.-%H 2 O for 15 min at 400 • C.This pre-treatment was also conducted for the phosphorus-exposed catalysts.

Phosphorous Exposure
The NO x 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.-%H 2 O, 8 vol.-%O 2 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.-ppmphosphoric 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.

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 PO 3 − , P 4 O 10 and PO 4 3− , respectively.

N 2 -Physisorption and ICP-AES
The specific surface area and pore volume of the samples were measured by N 2 -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 N 2 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.

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.

Conclusions
In the present study, a Pt/Ba/Al 2 O 3 NO x storage catalyst coated on a ceramic monolith substrate was exposed to two different phosphorus containing atmospheres for 34 h at 200 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 NO x storage capacity of Pt/BaO/Al 2 O 3 decreased with presence of phosphorous species on the catalyst.The results also showed that the formation of N 2 decreased after the exposure to phosphorous, in favour of ammonia production.Furthermore, the formation of N 2 O 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 NO x ; 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 P 4 O 10 while further along the axial direction metaphosphates (PO 3 − ) 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.

Figure 5 .
Figure 5. Balance over outlet nitrogen atoms over a lean/rich cycle before and after exposing Pt/Ba/Al 2 O 3 to 50 and 100 vol.-ppmH 3 PO 4 for 34 h, assuming that all nitrogen leaving the catalyst is either in form of NO x , NH 3 , N 2 O or N 2 .(A) 300 • C with 1 vol.-%H 2 in the rich phase, (B) 300 • C with 0.1 vol.-%C 3 H 6 in the rich phase, (C) 400 • C with 1 vol.-%H 2 in the rich phase, (D) 400 • C with 0.1 vol.-%C 3 H 6 in the rich phase.

Figure 6 .
Figure 6.(A) Normalized X-ray photoelectron spectroscopy (XPS) spectra from the inlet, middle and outlet section of the Pt/Ba/Al 2 O 3 sample exposed to 50 vol.-ppmH 3 PO 4 for 34 h.(B) Corresponding XPS spectra for the P 2p region from the inlet and middle section of the sample.

Figure 7 .
Figure 7. Deconvolution of the P 2p XPS spectra from the inlet (A) and middle (B) section of the Pt/Ba/Al 2 O 3 sample exposed to 50 vol.-ppmH 3 PO 4 for 34 h.The considered phosphorous species are P 4 O 10 , PO 3 − and PO 4 3− .

Figure 8 .
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/Al 2 O 3 sample exposed to 50 vol.-ppmH 3 PO 4 for 34 h.

Figure 9 .
Figure 9. ESEM and EDX images from the cross-section 2 mm from the back of the Pt/Ba/Al 2 O 3 sample exposed to 50 vol.-ppmH 3 PO 4 for 34 h.

Table 1 .
NOx storage capacity for fresh and phosphorous exposed samples.

Table 2 .
Phosphorous content, specific surface area and pore volume of the fresh Pt/Ba/Al 2 O 3 sample and of the Pt/Ba/Al 2 O 3 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).