Academic Editor: Michalis Konsolakis Received: 11 January 2016; Accepted: 17 February 2016; Published: 1 March 2016

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1 catalysts Article Study N 2 O Formation over Rh- and Pt-Based LNT Catalysts Lukasz Kubiak 1, Roberto Matarrese 1, Lidia Castoldi 1, Luca Lietti 1, *, Marco Daturi 2 and Pio Forzatti 1, * 1 Laboratory Catalysis and Catalytic Processes, Dipartimento di Energia, Politecnico di Milano, Via La Masa, 34, Milano, Italy; lukaszmaciej.kubiak@polimi.it (L.K.); roberto.matarrese@polimi.it (R.M.); lidia.castoldi@polimi.it (L.C.) 2 Laboratoire Catalyse et Spectrochimie, ENSICAEN, Université de Caen, CNRS, 6 Bd du Maréchal Juin, Caen, France; Marco.daturi@ensicaen.fr * Correspondence: pio.forzatti@polimi.it (P.F.); luca.lietti@polimi.it (L.L.); Tel.: (P.F.); (L.L.) Academic Editor: Michalis Konsolakis Received: 11 January 2016; Accepted: 17 February 2016; Published: 1 March 2016 Abstract: In this paper, mechanistic aspects involved in formation N 2 O over Pt-BaO/Al 2 O 3 and Rh-BaO/Al 2 O 3 model NO x Storage-Reduction (NSR) catalysts are discussed. The reactivity both gas-phase NO and stored nitrates was investigated by using H 2 and NH 3 as reductants. It was found that N 2 O formation involves presence gas-phase NO, since no N 2 O is observed upon reduction nitrates stored over both Pt- and Rh-BaO/Al 2 O 3 catalyst samples. In particular, N 2 O formation involves coupling undissociated NO molecules with N-adspecies formed upon NO dissociation onto reduced Platinum-Group-Metal (PGM) sites. Accordingly, N 2 O formation is observed at low temperatures, when PGM sites start to be reduced, and disappears at high temperatures where PGM sites are fully reduced and complete NO dissociation takes place. Besides, N 2 O formation is observed at lower temperatures with H 2 than with NH 3 in view higher reactivity hydrogen in reduction PGM sites and onto Pt-containing catalyst due to higher reducibility Pt vs. Rh. Keywords: lean NO x trap; N 2 O formation; NO x storage reduction; Pt-BaO/Al 2 O 3 ; Rh-BaO/Al 2 O 3 1. Introduction The removal NO x under lean conditions according to NO x Storage-Reduction (NSR) technique is a viable approach for both diesel and lean gasoline fueled engines [1]. In this process NO x conversion is based on a cyclic engine operation alternating between fuel lean and rich conditions: NO x (NO+NO 2 ) are adsorbed on catalyst surface during lean operations, while surface trap is regenerated under fuel rich environment. The NO x adsorption step involves oxidation sites (generally provided by noble metals like Pt, Pd or Rh) and alkali or alkaline-earth oxides storage sites (Ba, K, Sr), whereas reduction stage involves reduction sites provided by noble metals. All se components are deposited over a high surface area support like γ-al 2 O 3. The operation NSR catalysts (also known as Lean NO x Traps, LNTs) reduces NO x mainly to N 2 although undesired by-products like NH 3 or N 2 O may also be generated. NH 3 is not generally a concern since an ammonia Selective Catalytic Reaction (SCR) catalyst can be placed downstream NSR system, such as in case combined NO x storage and NH 3 -SCR catalytic systems [2]. At variance, N 2 O is a highly undesired by-product in view its very high global warming potential (nearly 300 times that CO 2 ). Over fully formulated catalysts, it has been shown that N 2 O formation Catalysts 2016, 6, 36; doi: /catal

2 Catalysts 2016, 6, is apparent both upon switch from lean to rich mode but also during alternation from rich to lean regime [3 8]. It has been suggested that N 2 O formation at lean to rich transition occurs at regeneration front, upon reduction stored NO x over not fully reduced Platinum-Group-Metal (PGM) sites. On or hand, N 2 O formation upon rich to lean transition originates from reaction between residual surface NO x with reductive species (like NCO, CO or NH 3 ) in an adsorbed state [3,4,7]. This N 2 O peak can be reduced if a neutral or slightly lean phase is inserted between rich and lean phase, since a more complete regeneration catalyst can be attained [9]. In order to provide furr insights on pathway involved in N 2 O formation, a fundamental study was carried out on a model Pt-BaO/Al 2 O 3 catalyst under simplified and controlled reaction conditions [10]. Accordingly reactivity gaseous NO alone and in presence various reducing agents like H 2, NH 3, and CO was analyzed by micro-reactor transient reactivity experiments and operando Fourier transform infrared (FT-IR) spectroscopy, to provide a complementary overview on pathway N 2 O evolution. It has been shown that N 2 O is formed from gaseous NO provided that a proper oxidation degree Pt sites is attained [10]. Accordingly N 2 O evolution was observed only in a narrow temperature range corresponding to PtO to Pt transformation, determined by reactivity reducing agents (H 2 > NH 3 > CO). In this study investigation previously carried out over model Pt-BaO/Al 2 O 3 sample has been extended to a Rh-containing sample. Since Rh is generally considered to play an important role in reduction NO x [11], it is interest to analyze role this component in pathways involved in N 2 O formation. For this purpose reduction gaseous NO in presence various reductants (H 2, NH 3 ) was investigated by transient microreactor experiments and operando FT-IR spectroscopy to combine gas-phase data with surface species analysis. To point out role NO x surface species (nitrates) in formation N 2 O, experiments were also performed with same reductants after adsorption NO x on catalyst surface. 2. Results and Discussion 2.1. NO-Temperature Programmed Reaction (NO-TPR) The reactivity NO over reduced Pt-BaO/Al 2 O 3 and Rh-BaO/Al 2 O 3 was investigated at first. The results gas phase analysis are shown in Figure 1A,B for Rh-BaO/Al 2 O 3 and Pt-BaO/Al 2 O 3 catalyst, respectively. In both cases a NO consumption peak is observed, with a minimum in NO concentration centered near 250 and 200 C for Rh-BaO/Al 2 O 3 and Pt-BaO/Al 2 O 3, respectively. The NO consumption is more evident in case Pt-BaO/Al 2 O 3 catalyst sample; in both cases it is accompanied by evolution N 2 O and minor amounts N 2. Notably, NO consumption is not balanced by evolution N 2 O (and N 2 ), indicating build up N-containing species on catalyst surface. The results surface analysis are shown in Figure 2. In case Rh-BaO/Al 2 O 3 catalyst (Figure 2A) upon exposure NO at 50 C (spectrum a) a band appears at 1210 cm 1 corresponding to ν asym (NO 2 ) vibration chelating nitrites on barium phase. The presence ν sym (NO 2 ) mode nitrites may also be observed near 1365 cm 1. Upon heating, band located at 1210 cm 1 increases, reaches a maximum intensity around C (spectra e g) and n decreases with temperature. The decrease bands nitrites is accompanied by an increase in bands located near 1327 and 1405 cm 1 associated with ionic nitrates (ν asym (NO 3 ) at 1327 and 1405 cm 1 ). The presence an isosbestic point suggests occurrence transformation nitrites into nitrates on catalyst surface. The appearance nitrate species is also confirmed by presence a low intensity peak at 1030 cm 1 (ν sym (NO 3 )). Furrmore, a shoulder near 1545 cm 1, associated with bidentate nitrates (ν(n=o)) is also detected. At end temperature ramp (350 C) surface catalyst is dominated by presence nitrate species, however nitrites are still present (band at 1210 cm 1 ).

3 A similar picture was observed over Pt BaO/Al2O3 catalyst (Figure 2B), with only minor differences. Also in this case formation chelating nitrites (1218 cm 1 ) is seen at initial stage, reaching a maximum in same temperature range ( C) observed for Rh based catalyst. Then bands decrease in favor nitrate formation, as pointed out by isosbestic point and by growth bands near 1350 and 1405 cm 1. At end run (350 C), final amount Catalysts 2016, 6, Catalysts 2016, 6, Figure1. 1. NO TPR NO-TPR experiments experiments with with NO (1000 NO ppm) (1000 in ppm) He from in He from C (10 C/min) C (10 over C/min) pre reduced over Rh BaO/Al2O3 pre-reduced Rh-BaO/Al (A) and Pt BaO/Al2O3 2 O 3 (A) and (B) Pt-BaO/Al catalysts 2 O(rig 3 (B) #1). catalysts (rig #1). Catalysts 2016, 6, The results surface analysis are shown in Figure 2. In case Rh BaO/Al2O3 catalyst (Figure 2A) upon exposure NO at 50 C (spectrum a) a band appears at 1210 cm 1 corresponding to νasym(no2) vibration chelating nitrites on barium phase. The presence νsym(no2) mode nitrites may also be observed near 1365 cm 1. Upon heating, band located at 1210 cm 1 increases, reaches a maximum intensity around C (spectra e g) and n decreases with temperature. The decrease bands nitrites is accompanied by an increase in bands located near 1327 and 1405 cm 1 associated with ionic nitrates (νasym(no3) at 1327 and 1405 cm 1 ). The presence an isosbestic point suggests occurrence transformation nitrites into nitrates on catalyst surface. The appearance nitrate species is also confirmed by presence a low intensity peak at 1030 cm 1 (νsym(no3)). Furrmore, a shoulder near 1545 cm 1, associated with bidentate nitrates (ν(n=o)) is also detected. At end temperature ramp (350 C) surface catalyst is dominated by presence nitrate species, however nitrites are still present (band at 1210 cm 1 ). Figure 2. Operando FT IR FT-IR spectra recorded during NO-TPR NO TPR experiments with NO (1000 ppm) in He Rh BaO/Al2O3 (A) Pt BaO/Al2O3 over Rh-BaO/Al (B) from 50 to 350 C C/min), 2 O 3 (A) and Pt-BaO/Al 2 O 3 (B) from 50 to 350 C (10 C/min), spectra recorded every 1 min, displayed at selected temperatures: (a) 50 C; C; (b) 100 C; C; (c) 120 C; C; (d) 150 C; C; (e) 200 C; C; (f) 220 C; C; (g) 250 C; C; (h) 300 C; C; (i) (i) 320 C; C; (l) (l) C C (rig#2).

4 Catalysts 2016, 6, A similar picture was observed over Pt-BaO/Al 2 O 3 catalyst (Figure 2B), with only minor differences. Also in this case formation chelating nitrites (1218 cm 1 ) is seen at initial stage, reaching a maximum in same temperature range ( C) observed for Rh-based catalyst. Then bands decrease in favor nitrate formation, as pointed out by isosbestic point and by growth bands near 1350 and 1405 cm 1. At end run (350 C), final amount nitrites on Pt containing catalyst is higher if compared to Rh-based sample, as pointed out by relative intensity bands at 1218 cm 1 (nitrites) and at cm 1 (nitrates). The above results clearly show that upon contacting NO with reduced catalyst surface, NO is consumed leading to N 2 O and nitrites adsorbed over barium surface. In particular results are in line with occurrence, below 250 C, following NO disproportion reaction Equation (1): 4 NO ` BaO Ñ N 2 O ` BapNO 2 q 2 (1) In fact ratio between NO consumption and N 2 O formation is not very far from oretical value 1:4 expected from stoichiometry overall reaction Equation (1). At higher temperature, above 250 C, nitrites are transformed into nitrates, in line with previous results [10]. The overall stoichiometry NO disproportion reaction Equation (1) can be explained by considering occurrence following reactions Equations (2) (4): NO ` 2 Me Ñ Me-N ` Me-O (2) NO ` Me-N Ñ N 2 O ` Me (3) 2 NO ` Me-O ` BaO Ñ Me ` BapNO 2 q 2 (4) where Me is metal site (Pt or Rh). According to this scheme, reaction is initiated at low temperature by decomposition NO over reduced Me sites, leading to formation N- and O-adspecies (reaction Equation (2)). The formation oxidized metal sites (Me-O species) prevents dissociation or NO molecules and this results in formation N 2 O (reaction Equation (3)) and nitrites on Ba (reaction 4)). The occurrence reaction Equation (4) removes O-adatoms on metal sites leaving vacant metal sites accessible for furr NO dissociation. Reactions Equations (2) (4) lead to overall stoichiometry reaction Equation (1). Worth noting is that reactions Equations (2) (4) require presence reduced metal sites, to initiate NO dissociation reaction. In fact NO disproportion reaction is observed to a much lower extent over an oxidized Pt-based catalyst sample (data not reported). Along similar lines, storage component BaO is also important in that it provides adsorption sites for nitrite (and nitrate) surface species. In fact, experiments carried out over a bare Pt/Al 2 O 3 sample pointed out a much lower N 2 O (and nitrite) formation. However, presence BaO may also increase activity metal sites in dissociative chemisorption NO, as reported elsewhere [12] NO/H 2 -TPR To investigate effect presence a reductant in N 2 O formation, experiments were carried out in presence H 2 and results are shown in Figure 3A,B in case Rh-BaO/Al 2 O 3 and Pt-BaO/Al 2 O 3 catalysts, respectively. In case Rh-based catalyst, NO starts to be converted near 150 C, with formation N 2 O and N 2 in nearly equal amounts. Then, above 200 C, NO is fully consumed and N 2 O formation rapidly drops. Accordingly above 200 C only N 2 and NH 3 are seen in reaction products. Very similar results are observed in case Pt-BaO/Al 2 O 3 catalyst (Figure 3B). However, in this case NO consumption is observed at much lower temperatures (onset temperature near 50 C), and NH 3 selectivity at high temperatures is much higher if compared to Rh-BaO/Al 2 O 3 sample.

5 metal sites in dissociative chemisorption NO, as reported elsewhere [12] NO/H2 TPR To investigate effect presence a reductant in N2O formation, experiments were Catalysts carried 2016, out 6, 36 in presence H2 and results are shown in Figure 3A,B in case Rh BaO/Al2O and Pt BaO/Al2O3 catalysts, respectively. Figure 3. NO/H2 TPR experiments with NO (1000 ppm) + H2 (2000 ppm) over (A) Rh BaO/Al2O3 and Figure 3. NO/H 2 -TPR experiments with NO (1000 ppm) + H 2 (2000 ppm) over (A) Rh-BaO/Al 2 O 3 (B) Pt BaO/Al2O3 from C (4 C/min), (rig#3). and (B) Pt-BaO/Al 2 O 3 from C (4 C/min), (rig#3). In case Rh based catalyst, NO starts to be converted near 150 C, with formation N2O The and correspondent N2 in nearly equal surface amounts. analysis Then, is shown above 200 for C, NO Pt-BaO/Al is fully consumed 2 O 3 sample and inn2o Figure formation 4. Upon NOrapidly admission, drops. Accordingly formationabove a very 200 C weak only band N2 and atnh3 1218are cm 1 seen isin seen, reaction related products. to formation chelating Very nitrites similar onresults Ba are phase. observed This in band case slightly increases Pt BaO/Al2O3 withcatalyst temperature, (Figure 3B). reaches However, a weak Catalysts maximum in this 2016, case near 6, NO 36 85consumption C and nis disappears observed at at much higher lower temperatures, along (onset with temperature N 2 O. The near spectra 50 C), 6 also 16 show and complex NH3 selectivity bands in at high range temperatures is cm 1 much, with higher positive if compared and negative to Rh BaO/Al2O3 contributions. sample. These bands are The related correspondent to to modifications surface analysis a a covalent is shown carbonate for Pt BaO/Al2O3 species, species, initially initially sample present present in Figure on on 4. surface, Upon surface, NO into into more admission, more symmetric, symmetric, ionic formation ionic species species upon a very upon interaction weak interaction band with at 1218 with water cm water formed 1 is formed seen, during related during reduction to reduction formation NO with NO with H 2 chelating and H2 upon and nitrites upon temperature on temperature Ba phase. increase. increase. This band slightly increases with temperature, reaches a weak maximum near 85 C and n disappears at higher temperatures, along with N2O. The spectra also show complex bands in range cm 1, with positive and negative contributions. These FT IR NO/H2 TPR with NO H2 Figure 4. FT-IR spectra obtained during 2 TPR with NO (1000 ppm) + H 2 (2000 ppm) over Pt BaO/Al2O3 Pt-BaO/Al 2 O 3 from from C C (4 (4 C/min) C/min) at: at: (a) (a) C; C; (b) (b) C; C; (c) (c) C; C; (d) (d) C; C; (e) (e) 252 C; C; (f) 354 C; C; (g) 400 C C (rig#3). Very similar results were obtained in case Rh based sample (not shown): also in this Very similar results were obtained in case Rh-based sample (not shown): also in this case we observed formation weak nitrite band in temperature range where N2O is formed. case we observed formation a weak nitrite band in temperature range where N 2 O is formed. Upon comparing se results with those obtained in absence H2 (i.e., NO TPR runs, Figures 1 and 2), it clearly appears that N2O formation is seen at much lower temperatures in case Pt BaO/Al2O3 sample, and at slightly higher temperatures in case Rh based sample. In both cases N2O formation is accompanied by nitrite evolution, although band nitrites is much lower in presence H2. As suggested above (see reactions Equations (2) (4)), N2O (and nitrite) formation requires

6 Catalysts 2016, 6, Upon comparing se results with those obtained in absence H 2 (i.e., NO-TPR runs, Figures 1 and 2), it clearly appears that N 2 O formation is seen at much lower temperatures in case Pt-BaO/Al 2 O 3 sample, and at slightly higher temperatures in case Rh-based sample. In both cases N 2 O formation is accompanied by nitrite evolution, although band nitrites is much lower in presence H 2. As suggested above (see reactions Equations (2) (4)), N 2 O (and nitrite) formation requires presence reduced Me (Pt, Rh) sites. Hence reaction is initiated by reduction PGM sites by H 2 : lower onset temperature for NO reduction illustrates much higher reducibility Pt-based sample if compared to Rh-BaO/Al 2 O 3 catalyst. As shown in Figure 3, NO reduction reaction leads at low temperatures to formation N 2 and N 2 O. Then, at higher temperatures, N 2 O formation becomes nil and selectivity shifts towards N 2 and NH 3. It is speculated that upon reduction PGM sites by H 2, N 2 O (and nitrite) formation occurs according to reactions Equations (2) (4). The presence H 2 leads also to formation Rh-H and Pt-H species, according to reaction Equation (5). H 2 ` 2 Me Ñ 2 M-H (5) where Me is metal site (Pt or Rh). The so formed Me-H species scavenge O-adatoms, reaction Equation (6): 2 Me-H ` Me-O Ñ 3 Me ` H 2 O (6) The scavenging O-adatoms according to reaction Equation (6) limits formation nitrites according to reaction Equation (4), as indeed is observed in presence H 2. Besides, N 2 formation is also observed along with N 2 O, coming from coupling N-adatoms (reactions Equations (2) and (7)): 2 Me-N Ñ 2 Me ` N 2 (7) Since O-removal efficiency by H 2 increases with temperature, upon increasing temperature NO completely dissociates and N-adatoms lead to formation N 2 (reaction Equation (7)) or NH 3 (reaction Equation (8)), thus shifting selectivity towards N 2 and NH 3 : M-N ` 3 M-H Ñ 4 M ` NH 3 (8) Notably, in case Rh-containing catalyst, NH 3 concentration decreases above 275 C. This is due to occurrence NH 3 decomposition reaction, which is efficiently catalyzed over Rh, as it is shown below NO/NH 3 -TPR Figure 5 shows results obtained during reduction NO with NH 3 in case Rh-BaO/Al 2 O 3 (Figure 5A) and Pt-BaO/Al 2 O 3 catalysts (Figure 5B). The onset temperature for NO reduction is seen near 200 and 100 C for Rh- and Pt-based catalysts, respectively. These temperature thresholds are higher than those measured in case H 2, but show same order reactivity investigated samples, being Pt-BaO/Al 2 O 3 catalyst more reactive than Rh-BaO/Al 2 O 3. Over both catalytic systems initial formation N 2 O and N 2 is observed; n, when complete NO consumption is attained (i.e., near 295 C and 185 C over Rh-BaO/Al 2 O 3 and Pt-BaO/Al 2 O 3, respectively) N 2 O concentration drops to zero resulting in complete N 2 selectivity. In case Rh-based sample, above 300 C an increase in N 2 concentration is observed, along with H 2 and a correspondent decrease NH 3 concentration. This is due to occurrence NH 3 decomposition reaction: 2 NH 3 Ñ N 2 ` 3 H 2 (9) which is catalyzed by Rh component.

7 In case Rh based sample, above 300 C an increase in N2 concentration is observed, along with H2 and a correspondent decrease NH3 concentration. This is due to occurrence NH3 decomposition reaction: 2 NH3 N2 + 3 H2 (9) Catalysts 2016, 6, which is catalyzed by Rh component. Figure 5. NO/NH3 TPR experiments with NO (1000 ppm) NH3 (1000 ppm) over (A) Rh BaO/Al2O3 Figure 5. NO/NH 3 -TPR experiments with NO (1000 ppm) + 3 (1000 ppm) over (A) Rh-BaO/Al 2 O 3 and (B) Pt BaO/Al2O3 and (B) Pt-BaO/Al from C (4 C/min) (rig#3). 2 O 3 from C (4 C/min) (rig#3). The results FT IR analysis carried out over Rh BaO/Al2O3 and Pt BaO/Al2O3 The results FT-IR analysis carried out over Rh-BaO/Al catalysts are shown in Figures 6A,B respectively. In case 2 O 3 and Pt-BaO/Al Rh BaO/Al2O3 (see Figure 6A), upon 2 O 3 catalysts are shown in Figure 6A,B respectively. In case Rh-BaO/Al NH3 admission, bands at 1095 and 1647 cm 1 (low intensity) immediately appear 2 O 3 (see Figure 6A), upon (spectrum a). These NH bands 3 admission, bands at 1095 and 1647 are related to symmetric and cm 1 (low intensity) immediately appear (spectrum a). These antisymmetric bending modes ammonia. Then, upon NO bands are related to symmetric and antisymmetric bending modes ammonia. Then, upon NO admission, a weak band nitrites appears at 1212 cm 1. Upon heating, bands associated with admission, a weak band nitrites appears at 1212 NH3 decrease and eventually disappear. Besides, cm 1. Upon heating, bands associated with nitrite band slightly increases up to 275 C and NH 3 decrease and eventually disappear. Besides, nitrite band slightly increases up to 275 C and n disappears. In addition to se bands, positive and negative bands are evident in spectra due to distortion carbonate species, as previously discussed. The Pt-BaO/Al 2 O 3 catalyst shows similar features (see Figure 6B) but a much higher amount nitrites is formed upon NO admission, with respect to Rh-BaO/Al 2 O 3. Nitrites readily disappear near 200 C (spectrum g). The picture obtained from analysis with NH 3 is consistent with that discussed in case H 2. Upon reduction PGM sites by NH 3, NO is dissociated into N- and O-adatoms and formation N 2 O and nitrites is observed (reactions Equations (2) (4)). Formation se species is observed at lower temperatures over Pt-based sample in view its higher reactivity, in line with data obtained during NO/H 2 -TPR. By comparing NO/NH 3 -TPR and NO/H 2 -TPR data, it is apparent that onset N 2 O formation is shifted towards higher temperatures (compare Figures 3A and 4A) and also higher amounts nitrites are formed. This can be explained considering that NH 3 is a much less efficient reductant (i.e., a less efficient O-scavenger) compared to H 2. This results in a higher onset temperature for reaction and a higher temperature for complete NO dissociation.

8 Catalysts 2016, 6, n disappears. In addition to se bands, positive and negative bands are evident in spectra due to distortion carbonate species, as previously discussed. Catalysts 2016, 6, A 1413 Absorbance (a.u.) Absorbance (a.u.) h g f e d c B i h g f e d c Wavenumber (cm -1 ) b b a Wavenumber (cm -1 ) a 1090 Figure Figure Operando FT-IR FT IR spectra duringno/nh3 TPR 3 TPRwith with NO NO (1000 (1000 ppm) ppm) + + NH NH3 ( (1000 ppm) ppm) from from C (4 C C/min) (4 C/min) over (A) over (A) Rh-BaO/Al Rh BaO/Al2O3 at: 2 O(a) 3 40 at: C (a) with 40 C NH3; with(b) NH40 3 ; C (b) with 40 C NH3+NO; with NH(c) 3 +NO; 40 C (c) before 40 C temperature before temperature ramp; (d) ramp; 50 C; (d) (e) C; C; (e) (f) C; C; (f) (g) C; C; (h) (g) C. C; Panel (h) 400 (B) C. Panel (B) Pt BaO/Al2O3 Pt-BaO/Al at: (a) 40 2 C O 3 with at: (a) only 40 C NH3; with (b) only 40 C NHNH3 3 ; (b) + 40 NO; C(c) NH C NO; before (c) 40temperature C before temperature ramp; (d) 104 ramp; C; (d) (e) C; C; (e) (f) C; C; (f) (g) C; C; (g) (h) C; C; (h) (i) C; C (rig#3). (i) 400 C (rig#3). InThe Pt BaO/Al2O3 case NO/NH catalyst 3 -TPR shows experiments, similar features N 2 O(see andfigure N 2 formation 6B) but a much involves higher coupling amount N-containing nitrites is formed species upon coming NO admission, from NO and/or with respect NH 3 molecules. to Rh BaO/Al2O3. To investigate Nitrites such readily aspects, disappear isotopic near labeling 200 C (spectrum experiments g). were carried out with NH 3 and 15 NO over both Pt- [13] and Rh-BaO/Al 2 O 3 [14]. In both The cases picture obtained low-t NO from consumption analysis with (i.e., NH3 belowis consistent maximum with that in Ndiscussed 2 O concentration) in case is accompanied H2. Upon reduction by evolution PGM sites double by NH3, labeled NO is 15 dissociated N 2 O and into single N and labeled O adatoms N 2 ( 15 and N 14 N), formation with only N2O minor and amounts nitrites is single observed labeled (reactions N 2 O ( 15 Equations N 14 NO). Then, (2) (4)). at higher Formation temperatures se (near species is complete observed NO at lower consumption) temperatures N 2 O over is no more Pt based detected sample among in view reaction its higher products reactivity, and all in line N 2 with isotopes data are obtained observed. during NO/H2 TPR. These By comparing results indicate NO/NH3 TPR that N 2 Oand formation NO/H2 TPR involves data, primarily it is apparent NOthat molecules onset only, N2O according formation to reactions is shifted Equations towards higher (2) and temperatures (3). In this pathway, (compare Figures role 3A NHand 3 is hence 4A) and that also keeping higher amounts noble metal nitrites sites are informed. a reduced This state, can be with explained formation considering Me-Hthat andnh3 Me-NH is a much 2 species less according efficient reductant to reaction (i.e., Equation a less efficient (10): O scavenger) compared to H2. This results in a higher onset temperature for reaction and a higher temperature NHfor 3 `complete 2 Me Ñ Me-NH NO dissociation. 2 ` Me-H (10) In case NO/NH3 TPR experiments, N2O and N2 formation involves coupling Me-H scavenges O-adatoms, resulting in H N containing species coming from NO and/or 2 O formation while Me-NH NH3 molecules. To investigate 2 reacts with (labelled) such aspects, isotopic nitrite species leading to formation single labelled N labeling experiments were carried out with NH3 and 15 NO over both 2 ( 15 N Pt 14 N), according to reaction [13] and Rh BaO/Al2O3 [14]. Equation (11), where O In both cases low T = is lattice oxygen: NO consumption (i.e., below maximum in N2O concentration) is accompanied by evolution double labeled 15 N2O and single labeled N2 ( 15 N 14 N), with only Me-NH 2 ` 15 NO 2 Ñ Me ` 15 N 14 N ` 3{2 H 2 O ` ½ O = (11) minor amounts single labeled N2O ( 15 N 14 NO). Then, at higher temperatures (near complete NO This accounts for formation single labelled N 2 that accompanies N 2 O evolution. At higher temperatures, near complete NO consumption where no N 2 O is seen, all N 2 isotopes are detected due to statistical coupling N-adatoms coming from NO- and NH 3 -decomposition [13].

9 Me NH NO2ˉ Me + 15 N 14 N + 3/2 H2O + ½ O = (11) This accounts for formation single labelled N2 that accompanies N2O evolution. At higher temperatures, near complete NO consumption where no N2O is seen, all N2 isotopes are detected due to Catalysts statistical 2016, 6, 36 coupling N adatoms coming from NO and NH3 decomposition [13] H2 Temperature programmed surface reaction (H2 TPSR) 2.4. H 2 -Temperature Programmed Surface Reaction (H 2 -TPSR) Figure 7 shows results HH2 TPSR 2 -TPSR experiments performed over Rh-BaO/Al Rh BaO/Al2O3 2 O 3 and Pt BaO/Al2O3 Pt-BaO/Al 2 Oafter 3 after NOx NOadsorption x at 350 at 350 C. C. The The gas gas phase phase results results are are shown shown in Figure in Figure 7A,B 7A,B for for Rh Rh- and and Pt based Pt-based catalyst catalyst samples, samples, respectively. In In case case Rh containing Rh-containing sample, temperature threshold for hydrogen consumption is seen near 170 C. C. Above 200 C C NH3 3 evolution is seen, followed by that NN2 2 above 300 C. C. The initial H2 H 2 consumption is isprobably related to reduction Rh sites, since it is not accompanied by release reaction products. Figure7. 7. H2 TPSR H 2 -TPSR experiments with with H2 (2000 H 2 (2000 ppm) ppm) from from C (4 C/min) C (4 after C/min) NO/O2 after adsorption NO/O 2 at adsorption 350 C over at 350 (A) C Rh BaO/Al2O3 over (A) Rh-BaO/Al and (B) Pt BaO/Al2O3 2 O 3 and (B) Pt-BaO/Al (rig#3). 2 O 3 (rig#3). The corresponding FT-IR spectra are shown in Figure 8A. The spectrum recorded prior to H 2 consumption (spectrum a) corresponds to surface ionic nitrate (ν asym (NO 3 ) mode split into two peaks at 1400 and 1330 cm 1 ; ν sym (NO 3 ) mode at 1035 cm 1 ). The band near 1470 cm 1 is an artefact due to presence carbonate species, which have been partially displaced upon adsorption NO x and refore lead to formation negative peaks in subtraction spectra. Consequently, FT-IR analysis confirms that NO x have been stored at 350 C in form nitrate species. Upon heating, reduction nitrates occurs from 200 C, as it is obvious from corresponding decrease bands. The nitrate bands decrease in intensity with temperature, and no new bands are apparent. At 388 C (spectrum j) surface is clean and only a negative band at 1440 cm 1 is visible, due to carbonates that have been displaced upon nitrate adsorption from NO/O 2 at 350 C. These results indicate that, over Rh-BaO/Al 2 O 3, stored nitrates are reduced by hydrogen selectively into NH 3 (and minor amounts N 2 ), without any formation NO or N 2 O.

10 The corresponding FT IR spectra are shown in Figure 8A. The spectrum recorded prior to H2 consumption (spectrum a) corresponds to surface ionic nitrate (νasym(no3) mode split into two peaks at 1400 and 1330 cm 1 ; νsym(no3) mode at 1035 cm 1 ). The band near 1470 cm 1 is an artefact due to presence carbonate species, which have been partially displaced upon adsorption NOx and refore lead to formation negative peaks in subtraction spectra. Consequently, FT IR Catalysts 2016, 6, analysis confirms that NOx have been stored at 350 C in form nitrate species. Figure 8. Operando FT IR FT-IR spectra obtained during H2 TPSR 2 -TPSR experiments with H2 H 2 (2000 ppm) from C C (4 (4 C/min) C/min) after after NO/O2 NO/O adsorption 2 adsorption at 350 at C 350 over C(A) over Rh BaO/Al2O3 (A) Rh-BaO/Al at: (a) 2 O157 3 at: C; (a) (b) C; C; (c) (b) C; C; (c) (d) C; C; (d) (e) C; C; (e)(f) C; C; (f) (g) C; C; (g) 281 (h) C; 322 (h) C; 322 (i) C; 355 (i) C; 355 (j) C; 388 (j) C, 388and C; (B) and Pt BaO/Al2O3 (B) Pt-BaO/Al at: 2 O(a) 3 at: 40 (a) C; 40(b) C; 49 (b) C; 49 C; (c) 107 (c) 107 C; C; (d) (d) C; C; (e) (e) C; C; (f) (f) 132 C; C; (g) 140 C; C; (h) 148 C; C; (i) 157 C; C; (j) 165 C; C; (k) 182 C; C; (l) (l) 198 C C (rig#3). Upon Similar heating, results were reduction obtained in nitrates case occurs Pt-containing from 200 C, catalyst as it is (Figures obvious 7B from and 8B). corresponding Gas phase analysis decrease (Figure 7B) bands. shows The that nitrate H bands decrease in intensity with temperature, and 2 consumption is seen starting from 100 C, i.e., at no lower new temperatures bands are apparent. than Rh-BaO/Al At 388 C (spectrum j) surface is clean and only a negative band at 2 O 3. The H 2 consumption is accompanied by formation 1440 cm 1 is visible, due to carbonates that have been displaced upon nitrate adsorption from NO/O2 ammonia with traces nitrogen; no or reaction products (e.g., NO, N 2 O) are formed. Also in this at case 350 C. H 2 consumption is seen before evolution reaction products, likely indicating reduction These results catalyst indicate metal that, sites. over Rh BaO/Al2O3, stored nitrates are reduced by hydrogen selectively into NH3 Surface analysis (and (Figure minor 8B) amounts showed N2), presence without any formation ionic nitrates NO (ν or N2O. asym (NO 3 )) mode split into two Similar peaks results at 1417 were and obtained 1320 cm 1 in ; ν case Pt containing catalyst (Figures 7B and 8B). Gas sym (NO 3 ) mode at 1035 cm 1 ) and minor amounts bidentate phase analysis (Figure 7B) shows that H2 nitrates (ν(n=o) mode at 1555 cm 1 ) on consumption catalyst surface is seen at starting beginning from 100 run C, i.e., (spectrum at lower a). temperatures than Rh BaO/Al2O3. The H2 The reduction nitrates is observed above consumption 115 C, in line is accompanied with gas-phase by results. formation Upon increasing ammonia with temperature traces nitrogen; reduction no or nitrates reaction proceeds products faster (e.g., and complete NO, N2O) reduction are formed. is attained Also in above this case 200 C, H2 at a consumption much lower is temperature seen before with evolution respect to Rh-BaO/Al reaction products, likely indicating reduction 2 O 3. Also in this case, residual carbonates, present catalyst on metal surface sites. before NO x storage, were removed and this results in weak negative bands Surface analysis (Figure 8B) showed presence ionic nitrates (νasym at 1440 and 1380 cm 1. (NO3)) mode split into two H peaks at 1417 and 1320 cm 1 ; νsym(no3) mode at 1035 cm 1 ) and minor amounts bidentate 2 -TPSR experiments hence indicate that reduction with H 2 nitrates stored over both nitrates Pt- and (ν(n=o) Rh-containing mode at catalyst 1555 cm samples 1 ) on does catalyst not lead surface to at formation beginning N run (spectrum a). 2 O. The Pt-BaO/Al 2 O 3 The sample reduction is more active nitrates than is observed Rh-containing above 115 catalyst C, in line but in with both gas phase cases only results. NH Upon increasing 3 was observed as reduction product, along with traces N 2. These results are in line with previous data obtained over same Pt-containing sample used in this study and published elsewhere [15].

11 H2 TPSR experiments hence indicate that reduction with H2 nitrates stored over both Pt and Rh containing catalyst samples does not lead to formation N2O. The Pt BaO/Al2O3 sample is more active than Rh containing catalyst but in both cases only NH3 was observed as reduction product, along with traces N2. These results are in line with previous data obtained over Catalysts same 2016, Pt containing 6, 36 sample used in this study and published elsewhere [15] NH3 TPSR 2.5. NH 3 TPSR Figures 9 and 10 10show results obtained during TPSR experiments performed with NH3 3 after NOx x adsorption at at C. C. The Thegas gasphase results are shown in Figures 9A,B 9A,B for for Rh-BaO/Al Rh BaO/Al2O3 2 O 3 and Pt BaO/Al2O3, Pt-BaO/Al 2 O 3, respectively. In In case case Rh-catalyst Rh catalyst (Figure (Figure 9A), ammonia 9A), ammonia desorption desorption is observed is observed at beginning at beginning heating ramp. heating Then ramp. temperature Then onset temperature for ammoniaonset consumption for ammonia is seen consumption near 250 C, i.e., is seen at higher near 250 temperatures C, i.e., higher if compared temperatures to H 2 (see if compared Figure 7A). to H2 The NH (see 3 Figure consumption 7A). The is NH3 accompanied consumption by is accompanied formation nitrogen, by formation whose concentration nitrogen, whose showsconcentration a peak with maximum shows a peak near with 280 C. maximum At temperatures near 280 C. above At temperatures 300 C ammonia above is furr 300 C ammonia consumedis leading furr to consumed evolution leading Nto 2 andevolution H 2. The initial N2 Nand 2 formation, H2. The initial N2 with maximum formation, near with 280 C, maximum is due tonear reduction 280 C, is due stored reduction nitrates, whereas stored nitrogen nitrates, formation whereas above nitrogen 300 Cformation is due to occurrence above 300 C is due NHto 3 decomposition occurrence NH3 reaction. decomposition Notably, reduction reaction. Notably, stored nitrates reduction with ammonia stored is fully nitrates selective with to ammonia N 2, andis neir fully selective N 2 O norto NON2, formation and neir wasn2o observed. nor NO formation was observed. Figure 9. NH3 TPSR 3 -TPSR experiments with NH3 3 (1000 ppm) from C C (4 (4 C/min) C/min) after NO/O2 2 adsorption at 350 C C over: (A) Rh BaO/Al2O3 Rh-BaO/Al 2 Oand 3 and (B) (B) Pt BaO/Al2O3 Pt-BaO/Al 2 O(rig#3). 3 (rig#3). FT-IR spectra recorded during NH 3 -TPSR are shown in Figure 10A. Ionic nitrates (bands at 1402, 1338, and 1036 cm 1 ) and adsorbed ammonia (bands at 1095 and 1640 cm 1 ) are initially present on catalyst surface at 40 C (spectrum a). The increase temperature leads at first to disappearance 1095 and 1640 cm 1 bands, in line with desorption NH 3 (spectrum b). At furr temperature increase, nitrates are reduced and at 400 C (spectrum i) surface is free from nitrates.

12 Catalysts 2016, 6, Catalysts 2016, 6, Figure 10. Operando FT IR FT-IR spectra obtained during NH3 TPSR 3 -TPSR experiments with NH3 3 (1000 ppm) from C C (4 C/min) C/min) after NO/O2 2 adsorption at 350 C C over (A) Rh-BaO/Al Rh BaO/Al2O3 2 O 3 at: (a) 40 C; C; (b) 275 C; C; (c) 283 C; C; (d) 291 C; C; (e) 300 C; C; (f) 308 C; C; (g) 316 C; C; (h) 333 C; C; (i) 400 C; C, and (B) Pt-BaO/Al Pt BaO/Al2O3 2 O 3 at: at: (a) (a) C; C; (b) (b) 171 C; C; (c) (c) 196 C; C; (d) 204 C; C; (e) 213 C; C; (f) 221 C; C; (g) 229 C; C; (h) 237 C; C; (i) 246 C; C; (j) 254 C; C; (k) 303 C C (rig#3). FT IR In spectra case recorded Pt-catalyst during (Figure NH3 TPSR 9B), temperature are shown in threshold Figure 10A. for ammonia Ionic nitrates consumption (bands at 1402, is observed 1338, and near cm C, 1 ) i.e., and at adsorbed lower temperatures ammonia (bands if compared at 1095 and to 1640 Rh-containing cm 1 ) are initially sample, present and on leads tocatalyst N surface at 40 C (spectrum a). The increase temperature leads at first to 2 formation with a maximum near 220 C. At 270 C nitrogen evolution is completed disappearance 1095 and 1640 cm 1 bands, in line with desorption NH3 and NH (spectrum b). At 3 concentration reaches inlet concentration value. Also in this case reaction is furr fully selective temperature towards increase, nitrogen, nitrates and noare N reduced and at 400 C (spectrum i) surface is free 2 O formation was observed. Above 350 C Pt-catalyst from showsnitrates. a limited activity in dissociation NH 3, as pointed out by small evolution H 2 and nitrogen. In Concerning case Pt catalyst surface phase (Figure (Figure 9B), 10B), temperature ionic nitrates threshold (bands atfor 1415, ammonia 1348, and consumption 1035 cm 1 ), is bidentate observed nitrates near 150 (1555 C, cm 1 i.e., ) at and lower adsorbed temperatures ammonia if (band compared at 1090to cm 1 Rh containing ) are initially present sample, (40and C, leads to N2 spectrum a). formation Upon increasing with a maximum temperature, near 220 initially C. At bidentate 270 C nitrogen nitratesevolution are transformed is completed into ionic and NH3 nitrates (spectrum concentration b) and reaches n ionic inlet nitrates concentration decrease. Above value. 250 Also C in nitrates this case are completely reaction removed is fully selective (spectra jtowards and k). nitrogen, and no N2O formation was observed. Above 350 C Pt catalyst shows a limited activity in dissociation NH3, as pointed out by small evolution H2 The results hence indicate that over both catalyst samples NH and 3 is an effective reductant for nitrogen. stored nitrates, Concerning although it issurface less efficient phase than (Figure H 10B), ionic nitrates (bands at 1415, 1348, and 2. The Pt-containing sample is more reactive than 1035 Rh-based cm 1 ), catalyst. bidentate Notably, nitrates no(1555 N cm 1 ) and adsorbed ammonia (band at 1090 cm 1 ) are initially 2 O is formed upon reduction stored nitrates, reaction present being very (40 selective C, spectrum towardsa). N Upon increasing temperature, initially bidentate nitrates are 2. This apparently rules out involvement surface nitrates and/or transformed nitrate containing into ionic compounds nitrates (e.g., (spectrum ammonium b) and nitrate) n ionic in nitrates formation decrease. NAbove 250 C nitrates 2 O, at least under are investigated completely experimental removed (spectra conditions. j and k). The results hence indicate that over both catalyst samples NH3 is an effective reductant for stored nitrates, although it is less efficient than H2. The Pt containing sample is more reactive than Rh based catalyst. Notably, no N2O is formed upon reduction stored nitrates, reaction being very selective towards N2. This apparently rules out involvement surface nitrates

13 Catalysts 2016, 6, Pathways for N 2 O Formation The results reactivity gas-phase NO and adsorbed NO x species with H 2 and NH 3 clearly illustrated that N 2 O is formed only during run with presence gas-phase NO. In fact no nitrous oxide formation has been observed both over Pt- and Rh-BaO/Al 2 O 3 catalysts upon reduction stored nitrates, where no gas-phase NO is fed to catalyst samples. Notably, N 2 O is formed even in absence reductants (i.e., from NO alone), provided that reduced PGM sites are present over catalyst surface. Reduced Pt or Rh sites catalyze NO dissociation reaction into N- and O-adatoms; N-species furr interact with undissociated NO molecules (eir in gas-phase or from an adsorbed state) leading to formation N 2 O. This pathway is represented by reactions Equations (2) and (3) previously discussed. In absence any reductant, reaction is self-inhibited by formation oxidized PGM sites (i.e., O-adatoms), formed upon NO dissociation. In this respect, NO itself acts as a weak reductant, being able to scavenge O-adatoms and resulting in formation nitrite species adsorbed over storage component (Ba). The overall stoichiometry process is n well described by following NO disproportion reaction Equation (1). Hence also storage element participates in reaction, providing adsorption sites for nitrites. It may also increase reactivity PGM sites in NO dissociative chemisorption, as suggested in literature [10,12]. In fact, experiments carried out over a Ba-free sample showed a much lower N 2 O formation if compared to Ba-containing catalysts [14]. Reduced Pt- and Rh-based catalysts show similar reactivity in N 2 O formation from NO, N 2 O being formed at same temperature, although in case Rh-containing catalyst much lower amounts N 2 O were detected. This is likely related to lower metal dispersion Rh-BaO/Al 2 O 3 catalyst if compared to Pt-containing sample (6.8% vs. 48%, see Table 1), and hence to lower amounts surface reduced sites. Table 1. Morphological characterization catalysts. Catalyst Surface Area, m 2 /g Pore Volume, cm 3 /g Pore Radius, Å Metal Dispersion, % Pt-BaO/Al 2 O Rh-BaO/Al 2 O When a reductant (H 2 or NH 3 ) is co-fed with NO, N 2 O always appears as first NO reduction product, along with nitrogen. The onset temperature for NO reduction (and N 2 O evolution) depends on nature reductant (with H 2 showing a greater reactivity than NH 3 ) and on catalyst as well (Pt-based catalyst being more active than corresponding Rh-containing sample). The pathways involved in N 2 O formation are those already depicted for NO alone: reductant leads to reduction PGM sites, leading to NO dissociation into N- and O-adatoms. However in this case O-adatoms are scavenged by reductant, and nitrite formation is reduced with respect to reaction pure NO. When reactivity reductant is not very high (i.e., near onset temperature for NO reduction) PGM sites are not fully reduced and hence re is chance for N-adspecies to react with undissociated NO molecules, leading to N 2 O evolution. At variance, when reductant maintains PGM sites in a fully reduced state, NO is readily dissociated and reduction products are N 2 and NH 3 (when H 2 is used as reductant) or N 2 (reductant NH 3 ). It is concluded that formation N 2 O occurs only in presence undissociated NO molecules, i.e., when PGM sites are not fully reduced. Accordingly, temperature onset for N 2 O formation depends on reactivity reductant: it is seen at lower temperature when H 2 is used as a reductant instead NH 3, due to higher reactivity H 2. Notably, no N 2 O formation was observed under our experimental conditions upon reduction with both H 2 and NH 3 nitrates stored over both Pt- and Rh-BaO/Al 2 O 3. Notably, intermediacy species like ammonium nitrate in N 2 O formation is apparently ruled out since no N 2 O was observed

14 Catalysts 2016, 6, in NH 3 /nitrate reaction over both Pt- and Rh-based catalytic materials. It has been suggested that reduction stored NO x species is involved in release NO from stored NO x as first step, followed by reduction released NO over PGM sites [16 19]. The reduction process is initiated by reduction PGM sites by reductant, which precedes NO release. The observation that no N 2 O is formed in this case may be explained by considering that rate determining step in reduction stored NO x is NO release step [16]; once NO is released, it is readily converted to reduction products. Accordingly gas-phase concentration NO is small and this prevents N 2 O formation. However under real operating conditions, where significant temperature effects may take place, significant release NO may take place and accordingly N 2 O formation is likely to occur. 4. Materials and Methods Model Pt-BaO/Al 2 O 3 (1/20/100 w/w/w) and Rh-BaO/Al 2 O 3 (0.5/20/100 w/w/w) samples were prepared by incipient wetness impregnation a commercial γ-alumina (Versal 250 UOP, Des Plaines, IL, USA) support with Pt(NO 2 ) 2 (NH 3 ) 2 (Strem Chemicals, 5% w/w, Newburyport, MA, USA) and Rh(NO 3 ) 3 (Sigma Aldrich, 10% w/w, St. Louis, MO, USA) aqueous solutions, respectively, followed by impregnation with Ba(CH 3 COO) 2 (Sigma Aldrich, 99% w/w) aqueous solution. After each impregnation step samples were dried at 80 C overnight and calcined at 500 C for 5 h. The obtained catalysts were characterized by Brunauer-Emmett-Teller (BET) analysis for specific surface area and H 2 chemisorption for metal dispersion and results are reported in Table 1. The reactivity NO alone and with different reductants (H 2, NH 3 ) was investigated via temperature programmed reaction (TPR) experiments. In case NO alone, catalysts were heated in temperature range C (heating rate 10 C/min) under a flow NO (1000 ppm in He, NO-TPR). Before performing experiments, catalysts were reduced with H 2 (2000 ppm in He) at 350 C for 30 min and cooled to 40 C in He. Such experiments were carried out in a flow micro-reactor apparatus consisting an home made quartz tube reactor (7 mm I.D.) connected to a mass spectrometer (Omnistar 200, Pfeiffer Vacuum, Asslar, Germany), a micro-gas chromatograph (Agilent 3000A, Santa Clara, CA, USA) and a ultraviolet (UV) analyzer (Limas 11HW, ABB, Zurich, Switzerland) for on-line analysis outlet gases (rig #1). An amount 60 mg catalyst was used with a flow rate 100 cc/min (at 0 C and 1 atm). Information on surface species formed during experiments was collected in this case by performing NO-TPR experiments under same experimental conditions inside an IR reactor cell (ISRI Infrared Reactor, Granger, IN, USA) by using catalysts in form self-supported wafers (15 mg, diameter = 13 mm, thickness = 0.1 mm) (rig #2). The IR measurements were collected with a FT-IR Vertex 70 (Bruker, Billerica, MA, USA) spectrometer with 4 cm 1 spectral resolution and accumulation 64 scans using a mercury-cadmium-telluride (MCT) detector. The reactivity NO with H 2 and NH 3 (NO/H 2 -TPR and NO/NH 3 -TPR, respectively) was investigated by heating catalyst samples in a flow NO (1000 ppm) with H 2 (2000 ppm) or NH 3 (1000 ppm) in Ar in temperature range C (heating rate 4 C/min). These experiments were carried out in an operando FT-IR spectroscopy setup (rig #3) consisting in a Sandwich IR cell containing catalyst (15 mg) in form a self-supported wafer; gases leaving cell were analyzed by a mass spectrometer (ThermoStar TM GSD 301, Pfeiffer Vacuum) and a chemiluminescence analyzer (42i-HL MEGATEC, Thermo Fisher Scientific, Waltham, MA, USA). Total flow was 25 cm 3 min 1 (at 1 atm and 0 C). Surface FT-IR spectra were collected with a FT-IR Nicolet Nexus spectrometer (Thermo Fisher Scientific) with 4 cm 1 spectral resolution and accumulation 64 scans using a deuterated-triglycine sulfate (DTGS) detector. The same apparatus was used to perform temperature programmed surface reaction (H 2 - and NH 3 -TPSR) experiments. In this case catalysts were at first saturated with NO x species at 350 C by feeding NO (1000 ppm) in O 2 (3% v/v) + Ar. This resulted in formation surface nitrate species. Then catalyst was cooled under He at 40 C and subsequently heated in flowing H 2 (2000 ppm in

15 Catalysts 2016, 6, Ar, H 2 -TPSR) or NH 3 (1000 ppm in Ar, NH 3 -TPSR) up to 400 C (heating rate 4 C/min). Notably, due to low reductant concentration, experiments were carried out under nearly isormal conditions, i.e., in absence significant temperature effects during experiments. Furr details concerning apparatuses and procedures can be found in [10,15]. Since fresh catalyst samples were covered by carbonate species, before experiments few adsorption-reduction cycles under isormal conditions at 350 C, (i.e., NO/O 2 (1000 ppm NO + O 2 3%, v/v) and H 2 (2000 ppm)) were performed. This procedure partially replaces originally present BaCO 3 species with BaO/Ba(OH) 2 species [20]. 5. Conclusions In this paper, mechanistic aspects involved in formation N 2 O over Pt-BaO/Al 2 O 3 and Rh-BaO/Al 2 O 3 model NSR catalysts were investigated. In particular, reactivity both gas-phase NO and stored nitrates were investigated by using H 2 and NH 3 as reductants. Transient experiments with simultaneous gas phase and surface analysis using FT-IR spectroscopy were used to correlate concentration gas-phase species with adsorbed surface species. The data herein obtained provides clear indication fact that over both Rh- and Pt-BaO/Al 2 O 3 catalysts, N 2 O formation involves presence gas-phase NO. In fact under our experimental conditions no nitrous oxide formation was observed upon reduction stored nitrate species. The data show that N 2 O formation involves coupling undissociated NO molecules with N-adspecies formed upon NO dissociation onto reduced PGM sites. The reductant keeps PGM sites in a reduced state by scavenging O-adatoms formed upon NO dissociation, initial step process. However when complete dissociation gas-phase NO occurs, N 2 O formation is prevented. In fact N 2 O formation is observed at temperatures where reductants start to be active in reduction PGM sites, whereas at higher temperatures, where reductants effectively keep PGM sites in a fully reduced state, complete dissociation NO takes place and N 2 O formation is not effective. This explains why N 2 O formation is observed at low temperatures and n disappears at high temperatures. Over both Rh- and Pt-BaO/Al 2 O 3 catalysts N 2 O formation is observed at lower temperatures with H 2 than with NH 3, in view higher reactivity hydrogen in reduction PGM sites. Also, N 2 O formation occurs at lower temperatures onto Pt-containing catalyst if compared to Rh-based sample, due to higher reducibility Pt vs. Rh. In absence reducing agents, N 2 O can be formed from NO provided that reduced PGM sites are present. In such a case O-adatoms formed upon NO decomposition are scavenged by NO itself leading to formation nitrite species so that formation N 2 O occurs according to stoichiometry NO disproportionation reaction Equation (1). Finally, fact that N 2 O formation was not observed upon reduction stored nitrates with both H 2 and NH 3 is due to slow rate NO release process from stored species. This limits NO gas-phase concentration during reduction stored species thus preventing N 2 O formation. Acknowledgments: R. Matarrese gratefully acknowledges Fondazione Banca del Monte di Lombardia for financial support for FT-IR equipment. Author Contributions: The experimental study was conducted by L.K. and R.M. who were also responsible for preparation manuscript. The supervision during catalytic activity was provided by M.D. and L.C. The analysis and interpretation data was carried out by all authors in particular M.D., L.L., and P.F. Conflicts Interest: The authors declare no conflicts interest. References 1. Forzatti, P.; Lietti, L.; Castoldi, L. Storage and Reduction NO x Over LNT Catalysts. Catal. Lett. 2015, 145, [CrossRef]

16 Catalysts 2016, 6, Castoldi, L.; Bonzi, R.; Lietti, L.; Forzatti, P.; Morandi, S.; Ghiotti, G.; Dzwigaj, S. Catalytic behaviour hybrid LNT/SCR systems: Reactivity and in situ FTIR study. J. Catal. 2011, 282, [CrossRef] 3. Bártová, S.; Kočí, P.; Mráček, D.; Marek, M.; Pihl, J.A.; Choi, J.S.; Toops, T.J.; Partridge, W.P. New insights on N 2 O formation pathways during lean/rich cycling a commercial lean NO x trap catalyst. Catal. Today 2014, 231, [CrossRef] 4. Choi, J.S.; Partridge, W.P.; Pihl, J.A.; Kim, M.Y.; Kočí, P.; Daw, C.S. Spatiotemporal distribution NO x storage and impact on NH 3 and N 2 O selectivities during lean/rich cycling a Ba-based lean NO x trap catalyst. Catal. Today 2012, 184, [CrossRef] 5. Clayton, R.D.; Harold, M.P.; Balakotaiah, V. Performance Features Pt/BaO Lean NO x Trap with Hydrogen as Reductant. AIChE J. 2009, 55, [CrossRef] 6. Dasari, P.; Muncrief, R.; Harold, M.P. Cyclic lean reduction NO by CO in Excess H 2 O on Pt-Rh/Ba/Al 2 O 3 : Elucidating mechanistic features and catalyst performance. Top. Catal. 2013, 56, [CrossRef] 7. Kočí, P.; Bártová, Š.; Mráček, D.; Marek, M.; Choi, J.S.; Kim, M.Y.; Pihl, J.A.; Partridge, W.P. Effective Model for Prediction N 2 O and NH 3 Formation During Regeneration NO x Storage Catalyst. Top. Catal. 2013, 56, [CrossRef] 8. Kumar, A.; Harold, M.P.; Balakotaiah, V. Isotopic studies NO x storage and reduction on Pt/BaO/Al 2 O 3 catalyst using temporal analysis products. J. Catal. 2010, 270, [CrossRef] 9. Mráček, D.; Kočí, P.; Choi, J.S.; Partridge, W.P. New operation strategy for driving selectivity NO x reduction to N 2, NH 3 or N 2 O during lean/rich cycling a lean NO x trap catalyst. Appl. Catal. B 2016, 182, [CrossRef] 10. Kubiak, L.; Righini, L.; Castoldi, L.; Matarrese, R.; Forzatti, P.; Lietti, L.; Daturi, M. Mechanistic aspects N 2 O formation over Pt-based Lean NO x Trap catalysts. Top. Catal submitted. 11. Taylor, K.C. Nitric Oxide Catalysis in Automotive Exhaust Systems. Catal. Rev. 1993, 35, [CrossRef] 12. Konsolakis, M.; Yentekakis, I.V. The Reduction NO by Propene over Ba-Promoted Pt/γ-Al 2 O 3 Catalysts. J. Catal. 2001, 198, [CrossRef] 13. Lietti, L.; Artioli, N.; Righini, L.; Castoldi, L.; Forzatti, P. Pathways for N 2 and N 2 O Formation during Reduction NO x over Pt Ba/Al 2 O 3 LNT Catalysts Investigated by Labelling Isotopic Experiments. Ind. Eng. Chem. Res. 2012, 51, [CrossRef] 14. Kubiak, L.; Righini, L.; Castoldi, L.; Matarrese, R.; Forzatti, P.; Lietti, L.; Daturi, M. In-depth insights into N 2 O formation over Rh-Ba/Al 2 O 3 catalyst under operando FT-IR conditions and micro reactor study. Unpublished Lietti, L.; Nova, I.; Forzatti, P. Role ammonia in reduction by hydrogen NO x stored over Pt-Ba/Al 2 O 3 lean NO x trap catalysts. J. Catal. 2008, 257, [CrossRef] 16. Castoldi, L.; Righini, L.; Matarrese, R.; Lietti, L.; Forzatti, P. Mechanistic aspects release and reduction NO x stored on Pt Ba/Al 2 O 3. J. Catal. 2015, 328, [CrossRef] 17. Epling, W.S.; Campbell, L.E.; Yezerets, A.; Currier, N.W.; Parks, J.E. Overview fundamental reactions and degradation mechanisms NO x storage/reduction catalysts. Catal. Rev. Eng. 2004, 46, [CrossRef] 18. Kabin, K.S.; Muncrief, R.L.; Harold, M.P. NO x storage and reduction on a Pt/BaO/alumina monolithic storage catalyst. Catal. Today 2004, 96, [CrossRef] 19. Muncrief, R.L.; Kabin, K.S.; Harold, M.P. NO x storage and reduction with propylene on Pt/BaO/alumina. AIChE J. 2004, 50, [CrossRef] 20. Lietti, L.; Forzatti, P.; Nova, I.; Tronconi, E. NO x Storage Reduction over PtBa/γ-Al 2 O 3 Catalyst. J. Catal. 2001, 204, [CrossRef] 2016 by authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under terms and conditions Creative Commons by Attribution (CC-BY) license (