Catalytic Oxidation of NO Over Laco1-Xbxo3 (B = Mn

catalysts

Article Catalytic Oxidation of NO over LaCo1−xBxO3 (B = Mn, Ni) Perovskites for Nitric Acid Production

Ata ul Rauf Salman 1 , Signe Marit Hyrve 1, Samuel Konrad Regli 1 , Muhammad Zubair 1, Bjørn Christian Enger 2, Rune Lødeng 2, David Waller 3 and Magnus Rønning 1,* 1 Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), Sem Sælands vei 4, NO-7491 Trondheim, Norway; [email protected] (A.u.R.S.); [email protected] (S.M.H.); [email protected] (S.K.R.); [email protected] (M.Z.) 2 SINTEF Industry, Kinetic and Catalysis Group, P.O. Box 4760 Torgarden, NO-7465 Trondheim, Norway; [email protected] (B.C.E.); [email protected] (R.L.) 3 YARA Technology Center, Herøya Forskningspark, Bygg 92, Hydrovegen 67, NO-3936 Porsgrunn, Norway; [email protected] * Correspondence: [email protected]; Tel.: +47-73594121

Received: 31 March 2019; Accepted: 3 May 2019; Published: 8 May 2019

Abstract: Nitric acid (HNO3) is an important building block in the chemical industry. Industrial production takes place via the Ostwald process, where oxidation of NO to NO2 is one of the three chemical steps. The reaction is carried out as a homogeneous gas phase reaction. Introducing a catalyst for this reaction can lead to significant process intensification. A series of LaCo1 xMnxO3 − (x = 0, 0.25, 0.5 and 1) and LaCo1 yNiyO3 (y = 0, 0.25, 0.50, 0.75 and 1) were synthesized by a sol-gel − method and characterized using N2 adsorption, ex situ XRD, in situ XRD, SEM and TPR. All samples had low surface areas; between 8 and 12 m2/g. The formation of perovskites was confirmed by XRD. The crystallite size decreased linearly with the degree of substitution of Mn/Ni for partially doped samples. NO oxidation activity was tested using a feed (10% NO and 6% O2) that partly simulated nitric acid plant conditions. Amongst the undoped perovskites, LaCoO3 had the highest activity; with a conversion level of 24.9% at 350 ◦C; followed by LaNiO3 and LaMnO3. Substitution of LaCoO3 with 25% mol % Ni or Mn was found to be the optimum degree of substitution leading to an enhanced NO oxidation activity. The results showed that perovskites are promising catalysts for NO oxidation at industrial conditions.

Keywords: NO oxidation; catalytic oxidation; nitric oxide; perovskite; nitric acid; ostwald’s process; in situ; LaCoO3; LaMnO3; LaNiO3

1. Introduction Oxidation of nitric oxide (Equation (1)) is one of the few known third order reactions. The reaction is unusual, as the rate of reaction increases with a decrease in the temperature [1]

2NO + O NO ∆rH = 113.8 kJ/mol (1) 2 2 298 −

NO oxidation is a key reaction in lean NOx abatement technologies and in the Ostwald process for nitric acid production. In Ostwald’s process, NO oxidation is carried out as a non-catalytic process and the forward reaction is favored by the removal of heat and by providing sufficient residence time. Typical gas stream concentrations are 10% NO, 6% O2 and 15% H2O[2]. Using a catalyst for NO oxidation may lead to significant process intensification of the nitric acid plant. In addition to speeding up the oxidation process, it may reduce capital costs and increase heat recovery. Efforts have been made to find a catalyst effective under industrial conditions; but success so far has not been

Catalysts 2019, 9, 429; doi:10.3390/catal9050429 www.mdpi.com/journal/catalysts Catalysts 2019, 9, 429 2 of 11 achieved [2]. To date, the process is carried out as a homogenous process in modern nitric acid plants. To the best of the authors knowledge, apart from an earlier patent [3], only two recent studies [4,5] report catalytic oxidation of NO at nitric acid plant conditions. However, catalytic oxidation of NO has been extensively studied with regards to lean NOx abatement technologies and reviewed by Russel and Epling [6] and Hong et al. [7]. In these studies, oxidation of NO is carried out at very lean concentrations of NO ranging from 100–1000 ppm of NO [8–10]. The huge difference in NO concentration between NOx abatement and nitric acid production makes the extrapolation of these findings to nitric acid plant conditions, questionable. Although it has been demonstrated that platinum has significant catalytic activity for oxidation of NO to NO2 at nitric acid plant conditions [4,5] the high-cost and scarcity of platinum motivates the search for potential non-noble metal based catalysts. Perovskites have gained particular interest as catalytic materials due to their high thermal stability, ease of synthesis and good catalytic activity [11]. Perovskites are represented by a general formula ABO3, where A represents a rare earth or alkaline earth cation and B represents a transition metal cation. The activity of perovskites can be tuned by partial substitution of A and/or B site cations with another element to obtain the desired properties [11]. The lanthanum-based perovskite LaBO3 (B = Co, Mn, Ni) have been found to be active for NO oxidation at lean NO conditions [12,13]. It has been demonstrated for LaCoO3, that partial doping of the A-site with strontium or cerium or doping of the B-site with manganese or nickel enhances NO oxidation activity of the perovskite [13–16]. In this work, lanthanum-based perovskites with cobalt, nickel and manganese (LaCoO3, LaNiO3 and LaMnO3) were synthesized by the sol-gel method using citric acid. Catalytic tests were performed using a dry feed (10% NO, 6% O2) at atmospheric pressure; partially simulating nitric acid plant conditions. The effect of B-site substitution was studied by preparing a series of LaCo1 xMnxO3 and − LaCo1 yNiyO3 catalysts. The catalysts were characterized using N2 adsorption, X-ray diffraction − (XRD), Scanning electron microscopy (SEM) and temperature programmed reduction (TPR). Catalyst structure during pretreatment and oxidation of NO was monitored using in situ XRD.

2. Results and Discussion

2.1. Catalyst Characterisation Speciﬁc surface areas are summarized in Table1. All samples have a relatively low surface area, in the range of 8–12 m2/g, which is a typical of perovskites prepared by the citrate method [14,17,18]. The surface area remains unaﬀected by the degree of substitution of Mn (x). However, an irregular change in surface area is observed with the degree of substitution of Ni (y). For y = 0.25 and 0.75, the surface area increased by 25% (from 8 to 10 m2/g) and 50% (from 8 to 12 m2/g) respectively. However, for y = 0.50 the surface area remains the same as for LaCoO3.

Table 1. Brunauer–Emmett–Teller (BET) surface area and crystallite size (d) calculated using the Scherrer equation.

Surface Area d Sample (m2/g) (nm)

LaCoO3 8 28 LaCo0.75Mn0.25O3 8 24 LaCo0.50Mn0.50O3 7 18 LaMnO3 9 21 LaCo0.75Ni0.25O3 10 22 LaCo0.50Ni0.50O3 8 17 LaCo0.25Ni0.75O3 12 8 LaNiO3 8 13 Catalysts 2019, 9, 429 3 of 11

The XRD patterns shown in Figure1 reveal the formation of phase-pure perovskite structure for Catalysts 2019, 9, x FOR PEER REVIEW 3 of 11 all samples, and no peaks characteristic of the metallic oxides or carbonates were seen. The main characteristicexpansion peakcan be at explained 2θ = 33 ◦byfor comparing LaCoO3 theslightly ionic shiftsradii of towards the species lower in the 2θ perovskitevalues with structure. an increase in x Theand highery, indicating ionic radii an of increase Ni3+ (0.56 in Å) the than lattice Co3+ parameters(0.52 Å) responsible conﬁrming for lattice previous expansion ﬁndings in LaCo [131-,19,20]. ThisyNi expansionyO3 [20]. In can case be of explainedLaCo1-xMnxO by3, the comparing average trivalent the ionic metal radii site ofis conserved the species by inadjusting the perovskite the structure.ratioCatalysts between The 2019, higher 9, xMn FOR4+ /Mn ionicPEER3+ REVIEW and radii Co of 3+/Co Ni32++ [21].(0.56 Therefore, Å) than Cothe3 +relative(0.52 Å)amount responsible of Mn4+ (0.52 for lattice Å) present, expansion3 of 11 Mn3+ (0.645 Å), Co2+ (0.82 Å), Co3+ (0.52 Å) dictates the overall lattice parameters. in LaCoexpansion1 yNiy canO3 be[20 explained]. In case by of comparing LaCo1 xMn thex ionicO3, theradii average of the species trivalent in the metal perovskite site is structure. conserved by Among− undoped perovskites,4+ LaCoO3+ − 3 and3+ LaMnO2+ 3 belong to the rhombohedral phase in 4+ adjustingThe higher the ratio ionic between radii of Ni Mn3+ (0.56/Mn Å) thanand Co Co3+ (0.52/Co Å) [responsible21]. Therefore, for lattice the relative expansion amount in LaCo of1- Mn agreement with the3+ results reported2 +in literature [22,23].3+ LaNiO3 belongs to the cubic phase, which is (0.52consistenty Å)Niy present,O3 [20]. with In Mn caseprevious of(0.645 LaCo studies Å),1-xMn Co [24].xO3, (0.82the average Å), Co trivalent(0.52 Å)metal dictates site isthe conserved overall by lattice adjusting parameters. the 4+ 3+ 3+ 2+ 4+ ratio between Mn /Mn and Co /Co [21]. Therefore, the relative amount of Mn (0.52 Å) present, Mn3+ (0.645 Å), Co2+ (0.82 Å), Co3+ (0.52 Å) dictates the overall lattice parameters. Among undoped perovskites, LaCoO3 and LaMnO3 belong to the rhombohedral phase in agreement with the results reported in literature [22,23]. LaNiO3 belongs to the cubic phase, which is consistent with previous studies [24].

(a) (b)

Figure 1. XRD patterns of: (a) LaCo1 – xMnxO3; (b) LaCo1 – yNiyO3 perovskites. Figure 1. XRD patterns of: (a) LaCo1 xMnxO3;(b) LaCo1 yNiyO3 perovskites. − − Crystallite sizes calculated using the Scherrer equation are summarized in Table 1. Figure 2 Among undoped perovskites, LaCoO3 and LaMnO3 belong to the rhombohedral phase in shows crystallite size as a function of the degree of substitution. The highest crystallite size of 28 nm agreement with the results(a reported) in literature [22,23]. LaNiO3 belongs(b to) the cubic phase, which is was observed for LaCoO3 and a linear decrease was observed with an increase in x and y for partially consistent with previous studies [24]. substituted samplesFigure indicating 1. XRD patterns that of:partial (a) LaCo substi1 – xMntutionxO3; ( beffectively) LaCo1 – yNi restrainsyO3 perovskites. the crystal growth. Crystallite sizes calculated using the Scherrer equation are summarized in Table1. Figure2 However, other contributions to XRD peak broadening such as strain cannot be ruled out. Partially showssubstituted crystalliteCrystallite nickel size sizes perovskites as calculated a function had using smaller of the crystallite Scherrer degree size ofequation substitution.compared are summarized to their The manganese highest in Table counterparts. crystallite 1. Figure size2 of 28 nmshows was crystallite observed size for as LaCoO a function3 and of athe linear degree decrease of substitution. was observed The highest with crystallite an increase size inof 28x and nm y for partiallywas substitutedobserved for samplesLaCoO3 and indicating a linear decrease that partial was substitution observed with eﬀ anectively increase restrains in x and they for crystal partially growth. However,substituted other samples contributions indicating to XRD that peakpartial broadening substitution such effectively as strain restrains cannot the be crystal ruled out.growth. Partially However, other contributions to XRD peak broadening such as strain cannot be ruled out. Partially substituted nickel perovskites had smaller crystallite size compared to their manganese counterparts. substituted nickel perovskites had smaller crystallite size compared to their manganese counterparts.

Figure 2. Crystallite size as a function of the degree of substitution for LaCo1 – xMnxO3 and LaCo1 –

yNiyO3 perovskites.

Figure 3 shows the XRD pattern recorded in situ during pretreatment of LaCoO 3 and LaMnO3. No change in structure is observed apart from lattice expansion with an increase in temperature. The Figure 2. Crystallite size as a function of the degree of substitution for LaCo1 xMnxO3 and structuralFigure stability 2. Crystallite of the size perovskites as a function is inof accordancethe degree of with substitution the fact for that LaCo they1 – xareMn xcalcinedO3 and− LaCo at higher1 – LaCoy1NiyyONi3 yperovskites.O3 perovskites. −

Catalysts 2019, 9, 429 4 of 11

Figure3 shows the XRD pattern recorded in situ during pretreatment of LaCoO 3 and LaMnO3. No change in structure is observed apart from lattice expansion with an increase in temperature. The Catalysts 2019,, 9,, xx FORFOR PEERPEER REVIEWREVIEW 4 of 11 structural stability of the perovskites is in accordance with the fact that they are calcined at higher temperaturestemperatures in comparisoncomparison withwith the the pretreatment pretreatment temperature temperaturetemperature of ofof 500 500500◦C. °C.°C. No NoNo change changechange instructure inin structurestructure was wasobserved observed for LaCoOfor LaCoO3 and3 andand LaMnO LaMnOLaMnO3 during3 duringduring steady steadysteady state statestate oxidation oxidationoxidation ofNO ofof NONO at 350 atat 350350◦C indicating°C°C indicatingindicating that thatthat the thebulk bulk structure structure of perovskite of perovskite remains remains unaﬀ ectedunaffect duringed during the catalytic the catalytic process. process. However, However, minor changes minor changesbeyond thebeyond detectable the detectable range of ra XRDnge cannotof XRD be cannot ruled be out. ruled out.

(a) (b)

3 FigureFigure 3. 3. XRDXRD patterns patterns (λ ( λ== 0.49324=0.493240.49324 Å)Å) Å)recordedrecorded recorded inin situ insitu situ duringduring during pretreatmentpretreatment pretreatment of:of: of:((a) LaCoO (a) LaCoO3;; ((b)3 ; LaMnO(b) LaMnO33 perovskites.perovskites.3 perovskites.

FigureFigure 4 showsshows thethe SEMSEM imagesimages ofof LaCoOLaCoO33 withwithwith varyingvarying contentcontent ofof MnMn andand pure pure LaNiO LaNiO3... TheThe The presencepresence of of agglomerated agglomerated non-spheri non-sphericalcal particles particles is is observed observed for for all all samples. samples. A A significant signiﬁcant change change in in morphologymorphology is is observed observed with with the the substitution substitution of of Co Co with with Mn Mn along along with with an an increase increase in in the the extent extent of of agglomerationagglomeration (Figure (Figure 44(b),b–d). (c) and (d)).

FigureFigure 4. 4.SEM SEM images: images: (a ()a LaCoO) LaCoO3;(33;; b((b)) LaCo LaCo0.750.750.75MnMn0.250.250.25OO333;; ;( ((cc)) LaCo LaCo0.500.500.50MnMn0.500.500.50O33O;; ((3d;() dLaMnO) LaMnO33;; (e)(e)3;( LaNiOLaNiOe) LaNiO33.. 3.

The H2-TPR profiles of LaCo1 – xMnxO3 perovskitesperovskites areare givengiven inin FigureFigure 5(a).5(a). ThreeThree reductionreduction peaks at 334, 373 and 526 °C are observed in the TPR profile of LaCoO3.. TheThe firstfirst twotwo peakspeaks areare attributed to the reduction of Co3+ toto CoCo2+,, whilewhile thethe peakpeak atat 526526 °C°C representsrepresents thethe reductionreduction ofof CoCo2+ 0 to Co0 leadingleading toto thethe destructiondestruction ofof thethe perovskiteperovskite structurestructure [25].[25]. TPRTPR ofof LaMnOLaMnO3 showsshows twotwo mainmain reduction peaks at 383 and 818 °C. The first peak represents the reduction of Mn4+ toto MnMn3+,, whilewhile thethe

Catalysts 2019, 9, 429 5 of 11

The H2-TPR profiles of LaCo1 xMnxO3 perovskites are given in Figure5a. Three reduction peaks − at 334, 373 and 526 ◦C are observed in the TPR profile of LaCoO3. The first two peaks are attributed to 3+ 2+ 2+ 0 the reduction of Co to Co , while the peak at 526 ◦C represents the reduction of Co to Co leading Catalyststo the destruction2019, 9, x FOR of PEER the REVIEW perovskite structure [25]. TPR of LaMnO3 shows two main reduction peaks5 of 11 at 4+ 3+ 3+ 383 and 818 ◦C. The first peak represents the reduction of Mn to Mn , while the reduction of Mn 2+ reductionto Mn occurs of Mn at3+ elevatedto Mn2+ temperaturesoccurs at elevated (above 700temperatures◦C), forming (above MnO 700 and °C) simultaneous, forming MnO collapse and of simultaneousthe perovskite collapse structure of [ 26the]. perovskite For x = 0.25 structureand 0.5, broad[26]. For peaks x = overlapping0.25 and 0.5, reductionbroad peaks peaks overlapping of Co3+ to 2+ 4+ 3+ reductionCo and peaks Mn toof MnCo3+ toare Co observed2+ and Mn below4+ to Mn 5503+ are◦C. observed below 550 °C.

(a) (b)

FigureFigure 5.5. TPR proﬁlesprofiles of:of: (a) LaCo1 – xMnxOO33; ;((bb) )LaCo LaCo1 1– yyNiNiyOyO3 perovskites.3 perovskites. − −

FigureFigure 55(b)b shows shows H H2-TPR2-TPR profilesprofiles ofof LaCoLaCo11 y– NiyNiyOyO33 perovskites.perovskites. Reduction Reduction of of LaNiO 33 alsoalso − proceedsproceeds via via three three peaks peaks at at 304 304 °C,◦C, one one at at 334 334 °C◦C and and one one at at 465 465 °C.◦C. A A similar similar three-step three-step reduction reduction processprocess for for LaNiO LaNiO3 3hashas been been reported reported [27] [27 ]and and in in situ situ XRD XRD revealed revealed that that reduction reduction proceeds proceeds via via 0 formationformation of of the the La22NiONiO4 4phasephase [27]. [27 ].The The highest highest peak peak is is associated associated with with the the formation formation of of Ni Ni0 andand LaLa22OO3 3resultingresulting in in the the destruction ofof thethe perovskiteperovskite structure. structure. Four Four reduction reduction peaks peaks were were observed observed for fory = y0.25 = 0.25. Comparison. Comparison with with LaCoO LaCoO3 and3 and LaNiO LaNiO3 indicates3 indicates that that the the first first two two peaks peaks at 303at 303 and and 342 342◦C °Cmatch match with with the the reduction reduction peaks peaks of of Ni 3Ni+ 3+to to Ni Ni2+2+and and Co Co3+3+ toto CoCo22++.. Whereas Whereas the the former former two two peaks peaks at at 0 0 459,459, 495 495 °C◦C corresponds corresponds to to further further redu reductionction to to form form the the metallic metallic phases (Ni 0 andand Co Co0).). This This reduction reduction profileprofile matches matches well well with with the the previous previous findings findings [20]. [20]. In In contrast contrast to to distinct distinct reduction reduction peaks peaks for for cobalt cobalt andand nickel nickel for for LaCo 0.750.75NiNi0.250.25OO3, 3only, only two two broad broad peaks peaks were were observed observed for for yy == 0.500.50 andand 0.750.75. .The The first first 3+ 2+ 3+ 2+ peakpeak overlaps overlaps with with the the reduction reduction of ofNi Ni3+ to Nito2+ Ni and andCo3+ Coto Co2+to and Co increasesand increases from 313 from °C for 313 y◦ =0.50C for 0 0 toy =3460.50 °Cto for 346 y ◦=C 0.75 for. ySimultaneous= 0.75. Simultaneous reduction reduction to metallic to metallic phases (Ni phases0 and (Ni Co0and) was Co observed) was observed at 418 °Cat 418and ◦488C and °C 488for y=0.50◦C for andy = 0.500.75,and respectively.0.75, respectively.

2.2.2.2. NO NO Oxidation Oxidation Activity Activity TheThe oxidationoxidation ofof NO NO occurs occurs as a as homogeneous a homogeneou gas-phases gas-phase reaction reaction with a second with ordera second dependency order dependencyin NO concentration. in NO concentration. For this reason, For this contributions reason, contributions from gas phase from conversion gas phase haveconversion been detected have been for detectedstudies performedfor studies atperformed lean NO at concentrations lean NO concentrations [28–30]. This [28–30]. necessitates This necessitates the quantiﬁcation the quantification of the gas ofphase the contributiongas phase contribution to NO oxidation to NO in oxidation the current in study, the current which involvesstudy, which such ainvolves high concentration such a high of concentrationNO (10%). The of blank NO run(10%). performed The blank without run perfor catalystmed is given without in our catalyst previous is given publication in our [5 ].previous The gas publicationphase conversion [5]. The gradually gas phase decreases conversion with gradually increasing decreases temperature with in increasing the studied temperature temperature in range the studied(150–450 temperature◦C); with a conversionrange (150–450 of 6.1% °C); atwith 350 a◦ C.conversion of 6.1% at 350 °C. TheThe conversion conversion of NONO overover LaCo LaCo1 1 x–Mn xMnxOxO33as as a a function function of of temperature temperature is shownis shown in Figurein Figure6. The 6. − TheNO NO conversion conversion and and rate rate of reaction of reaction are summarized are summarized in Table in Table2. At low2. At temperature, low temperature, only gas only phase gas phaseconversion conversion is observed is observed for all for perovskites. all perovskites. The catalytic The catalytic activity activity for LaCoO for LaCoO3 starts3at starts about at 270about◦C 270 and °C and increases gradually until it becomes limited by the thermodynamic equilibrium. This is in contrast to studies performed at lean NOx conditions, where the catalytic activity starts at significantly lower temperatures (150–200 °C); increases with a steeper slope, and becomes thermodynamically limited at ca 300 °C [14,15]. The catalytic conversion starts at a lower temperature (240 °C) for x = 0.25 in comparison to other perovskites and a significant increase in conversion was observed. A further increase in x to 0.5 leads to a substantial decrease in NO conversion to conversion levels even lower than what is observed for LaMnO3. Manganese is stable in valence states +III and

Catalysts 2019, 9, 429 6 of 11 increases gradually until it becomes limited by the thermodynamic equilibrium. This is in contrast to studies performed at lean NOx conditions, where the catalytic activity starts at significantly lower temperatures (150–200 ◦C); increases with a steeper slope, and becomes thermodynamically limited at ca 300 ◦C[14,15]. The catalytic conversion starts at a lower temperature (240 ◦C) for x = 0.25 in comparison to other perovskites and a significant increase in conversion was observed. A further Catalystsincrease 2019 in, x9,to x FOR0.5 PEERleads REVIEW to a substantial decrease in NO conversion to conversion levels even lower6 of 11 than what is observed for LaMnO3. Manganese is stable in valence states +III and +IV while cobalt is +IVstable while in valence cobalt statesis stable+II in and valence+III. Ghiasi states et+II al. an [21d ]+III. used Ghiasi X-ray absorptionet al. [21] used spectroscopy X-ray absorption (XAS) to spectroscopystudy the valence (XAS) state to study of Mn the and valence Co in state a series of Mn of LaCoand Co1 x Mnin ax seriesO3 perovskites of LaCo1-x andMnxO found3 perovskites that the − andaverage found trivalent that the metal average site is conservedtrivalent metal by shifting site is the conserved balance between by shifting Mn4+ /theMn 3balance+ in combination between Mnwith4+/Mn Co3+3+/ Coin combination2+. The ratio with between Co3+/Co Mn42++. /TheMn3 ratio+ and between Co3+/Co Mn2+ decreases4+/Mn3+ and with Co3+ a/Co sequential2+ decreases increase with ain sequential manganese increase content, in themanganese highest value content, being the observed highest value for LaCo being0,75 observedMn0,25O3 .for The LaCo best0,75 activityMn0,25O of3. TheLaCo best0.75 Mnactivity0.25O 3ofin LaCo the series0.75Mn0.25 ofO LaCo3 in the1 x MnseriesxO 3ofperovskites LaCo1-xMn mayxO3 perovskites be attributed may to be the attributed presence to of 4+ 4+− 4+ 4+ thehighest presence content of ofhighest Mn ascontent amongst of diMnfferent as amongst valence states different of manganese, valence states Mn of(MnO manganese,2) exhibits Mn the 3+ 2+3+ 2+ (MnOhighest2) activityexhibits forthe NO highest oxidation activity followed for NO by oxidation Mn (Mn followed2O3) and by Mn Mn (Mn (Mn3O2O43)[) and31]. Mn (Mn3O4) [31].

FigureFigure 6.6. Conversion of NO over LaCo11 – xxMnxOO33 asas a a function function of of temperature. temperature. − Table 2. NO oxidation activity of perovskites at 350 C. Table 2. NO oxidation activity of perovskites at 350 ◦°C. X r 1 NOXNO rNO 1 NO Sample 1 1 Sample (%) (µmolgcat− s− ) (%) (µmolgcat-1s-1) LaCoO3 24.9 5.14 LaCoO3 24.9 5.14 LaCo0.75Mn0.25O3 29.7 6.55 LaCo0.75Mn0.25O3 29.7 6.55 LaCo0.50Mn0.50O3 15.9 2.72 0.50 0.50 3 LaMnOLaCo 3 Mn O 18.015.9 2.72 3.27 LaCo0.75NiLaMnO0.25O3 3 30.318.0 3.27 6.70 LaCo0.50LaCoNi0.500.75ONi3 0.25O3 23.230.3 6.70 4.74 LaCo0.25Ni0.75O3 16.4 2.86 LaCo0.50Ni0.50O3 23.2 4.74 LaNiO3 20.8 4.05 LaCo0.25Ni0.75O3 16.4 2.86 1 Catalytic activity obtained by subtracting the gas phase conversion of NO. LaNiO3 20.8 4.05 1 Figure7 shows Catalytic the conversion activity obtained of NO by over subtra LaCocting1 theyNi gasyO phase3 as a conversion function ofof temperature.NO. Minor − difference in gas phase conversion is observed at lower temperatures due to the differences in the Figure 7 shows the conversion of NO over LaCo1 – yNiyO3 as a function of temperature. Minor differencepacking of in the gas catalyst phase bed.conversion The substitution is observed with at lower 25 mol% temperatures Ni in LaCoO due3 had to the a significant differences positive in the impact on the activity exhibiting a conversion of 30% at 350 C. With a further increase in y, the activity packing of the catalyst bed. The substitution with 25 mol%◦ Ni in LaCoO3 had a significant positive impact on the activity exhibiting a conversion of 30% at 350 °C. With a further increase in y, the activity gradually decreased until 75% nickel content. Similar conversion curves are exhibited by y = 0,5 and y = 1 perovskites. Although our results differ from Zhong et al. [13], who reported 70 mol% nickel substitution in LaCoO3 to yield the best results for NO oxidation, it can be argued that they used a co-precipitation method for preparation of perovskites and the activity was tested at substantially different feed concentration (400 ppm NO and 6% O2) compared to the present study. The increase in activity for LaCo0.75Ni0.25O3 can in part be attributed to the 25% increase in surface area. However, for LaCo0.25Ni0.75O3 the surface area increased with 50% compared to LaCoO3 and 33%

Catalysts 2019, 9, 429 7 of 11 gradually decreased until 75% nickel content. Similar conversion curves are exhibited by y = 0.5 and y = 1 perovskites. Although our results diﬀer from Zhong et al. [13], who reported 70 mol% nickel substitution in LaCoO3 to yield the best results for NO oxidation, it can be argued that they used a co-precipitation method for preparation of perovskites and the activity was tested at substantially diﬀerent feed concentration (400 ppm NO and 6% O2) compared to the present study. The increase in Catalystsactivity 2019 for, LaCo9, x FOR0.75 PEERNi0.25 REVIEWO3 can in part be attributed to the 25% increase in surface area. However,7 of 11 for LaCo0.25Ni0.75O3 the surface area increased with 50% compared to LaCoO3 and 33% compared to comparedLaNiO3, while to LaNiO the catalytic3, while activity the catalytic decreased. activity Thus, decreased. it seems likelyThus, thatit seems the change likely inthat catalytic the change activity in catalyticis more related activity to is the more chemical related oxygen to the chemical dynamics oxygen and to dynamics the redox propertiesand to the thanredox to properties changes in than the tosurface changes area in in the this surface range. Ivanovaarea in this et al. range. [32] reported Ivanova a et maximum al. [32] reported in defect a structures maximum for inx =defect0.25 structuresin a series for of LaCo x = 0.251 x Niin xaO series3, detected of LaCo by1-x EPRNixO due3, detected to the presence by EPR due of magnetic to the presence Ni clusters. of magnetic However, Ni − clusters.this depends However, on the this method depends of preparation on the method and of pre-treatment preparation procedure.and pre-treatment The highest procedure. activity The of highestLaCo0.75 activityNi0.25O 3ofmay LaCo thus0.75Ni be0.25 relatedO3 may to thus lattice be defects related not to lattice detectable defec byts bulknot detectable XRD. The by NO bulk oxidation XRD. Theactivity NO followsoxidation the activity order LaCoOfollows3 the> LaNiO order 3LaCoO> LaMnO3 > LaNiO3 for the3 > LaMnO undoped3 for perovskites. the undoped perovskites.

1 – y y 3 FigureFigure 7.7. Conversion of NONO overover LaCoLaCo1 yNiyOO3 asas a a function function of of temperate. temperate. −

Figure 88 shows the rate rate of of reaction reaction as as a function a function of ofcrystallite crystallite size size for LaCo for LaCo1-xMn1xOx3Mn andxO LaCo3 and1- − yLaCoNixO13 perovskites.yNixO3 perovskites. The rate The of reaction rate of reaction increases increases linearly linearly with crystallite with crystallite size with size withx = 0.25x = 0.25andand y = − 0.25y = 0.25 samplessamples being being the exception the exception andand not notincluded included in the in thelinear linear fit. It fit. should It should be kept be kept in mind in mind thatthat the crystallitethe crystallite sizes sizes were were calculated calculated using using the the Scherr Scherrerer equation equation assuming assuming sp sphericalherical geometry. geometry. Though, Though, the crystallites are not spherical as revealed throughthrough SEM images. The crystallite size estimates from the Scherrer equation might not be 100% accurate butbut theythey provideprovide aa fairfair comparison.comparison. Partial substitution of LaCoO3 with either manganese or nickel leads to a modification in the redox properties, morphology, structure, crystallite size and valence state of the metal site. Therefore, it is difficult to pinpoint one governing factor determining the catalytic activity. In fact, it is a combination of several factors which contribute to dictating catalytic activity.

(a) (b)

Figure 8. Rate of reaction as a function of crystallite size: (a) LaCo1 – xMnxO3; (b) LaCo1 – yNiyO3 perovskites.

Catalysts 2019, 9, x FOR PEER REVIEW 7 of 11 compared to LaNiO3, while the catalytic activity decreased. Thus, it seems likely that the change in catalytic activity is more related to the chemical oxygen dynamics and to the redox properties than to changes in the surface area in this range. Ivanova et al. [32] reported a maximum in defect structures for x = 0.25 in a series of LaCo1-xNixO3, detected by EPR due to the presence of magnetic Ni clusters. However, this depends on the method of preparation and pre-treatment procedure. The highest activity of LaCo0.75Ni0.25O3 may thus be related to lattice defects not detectable by bulk XRD. The NO oxidation activity follows the order LaCoO3 > LaNiO3 > LaMnO3 for the undoped perovskites.

Figure 7. Conversion of NO over LaCo1 – yNiyO3 as a function of temperate.

Figure 8 shows the rate of reaction as a function of crystallite size for LaCo1-xMnxO3 and LaCo1- yNixO3 perovskites. The rate of reaction increases linearly with crystallite size with x = 0.25 and y = 0.25 samples being the exception and not included in the linear fit. It should be kept in mind that the crystallite sizes were calculated using the Scherrer equation assuming spherical geometry. Though, theCatalysts crystallites2019, 9, 429are not spherical as revealed through SEM images. The crystallite size estimates from8 of 11 the Scherrer equation might not be 100% accurate but they provide a fair comparison.

(a) (b)

FigureFigure 8. 8. Rate ofof reaction reaction as aas function a function of crystallite of crystallite size: (a )size: LaCo 1(a)x MnLaCoxO13 ;(– bxMn) LaCoxO3;1 (ybNi) LaCoyO3 perovskites1 – yNiyO3 . − − perovskites. 3. Materials and Methods

3.1. Catalyst Preparation All catalysts were prepared using the citrate method [14]. Nitrate salts were used as starting materials: La(NO ) 6H O (Alfa Aesar, 99.9%, Kandel, Germany), Mn(NO ) 4H O (Alfa Aesar, 99.98%, 3 3· 2 3 2· 2 Kandel, Germany), Co(NO ) 6H O (Acros Organics, 99%, Geel, Belgium) and Ni(NO ) 6H O (Acros 3 3· 2 3 2· 2 Organics, 99%, Geel, Belgium). An appropriate amount of nitrate salts of the desired A and B sites were dissolved in deionized water with 10 wt.% excess citric acid (Sigma Aldrich, 99.5%, Munich, Germany). The solution was stirred for 1 h at room temperature and further stirred in an oil bath at 80 ◦C until a viscous gel was obtained. The gel was dried in static air overnight at 90 ◦C and then heated to 150 ◦C for 1 h. The black, spongy material was crushed and calcined at 700 ◦C for 5 h in static air.

3.2. Catalyst Characterisation

N2 adsorption was used to measure the specific surface area. The sample (200 mg) was degassed at 200 ◦C overnight in VacPrep 061 Degasser (Norcross, GA, USA). Nitrogen adsorption was performed with a Miromeritics TriStar II 3020 Surface Area and Porosity Analyzer (Norcross, GA, USA) at 196 C. − ◦ The specific surface areas were calculated using the BET desorption branch of the isotherm [33]. Ex situ XRD patterns of as prepared samples were obtained with a Bruker D8 Advanced X-ray Diffractometer (Cambridge, United Kingdom) with a copper anode with an X-ray wavelength of 1.5418 Å. The measured angles (2θ) were scanned from 10◦ to 75◦ in 30 min; at a fixed divergence angle of 0.2◦. The PDF database was used for phase identification. The average crystallite sizes were calculated according to the Scherrer equation [34] assuming spherical crystals (K = 0.9) using the diffraction peak between 2θ = 45–50◦. In situ powder XRD experiments were carried out at BM31 of the Swiss-Norwegian beam lines (SNBL) at the European Synchrotron Radiation Facility (ESRF). Catalyst sample (30mg, sieve fraction 53–90 µm) was fixed between two quartz wool plugs in a quartz capillary of 1 mm internal diameter (bed length: 10 mm). The capillary was then mounted in a custom cell [35] and exposed to X-rays for diffraction measurements. The temperature of the capillary reactor was controlled by a calibrated hot air blower. Powder X-ray diffraction patterns were collected with a 2D plate detector (Mar-345) using monochromatic radiation of wavelength 0.49324Å. Note that is different from the Cu Kα wavelength, which is used for acquiring ex situ XRD patterns. The instrumental peak broadening, wavelength calibration, and detector distance corrections were performed using a NIST 660a LaB6 standard. Mass flow controllers were used to feed NO, O2 and He in to produce the desired gas-feed 3 composition (15 Ncm /min; 1% NO, 6% O2, He balance). XRD patterns were recorded in situ during the Catalysts 2019, 9, 429 9 of 11

3 pretreatment step (flowing 15 Ncm /min of 6% O2/He by heating at 10 ◦C/min from ambient to 500 ◦C and holding for 1 hr) and steady state NO oxidation (1%NO, 6% O2 and balance He) at 350 ◦C for 2 h. A lower concentration of NO (1%) was used for the in situ studies due to experimental constraints at the beamline. The morphological analysis of as-synthesized perovskites was performed by using an in-lens cold field emission electron microscope FE-S(T)EM, (Hitachi S-5500) in scanning electron microscopy (SEM) mode. Temperature programmed reduction by H2 was performed with an Altamira BenchCAT Hybrid 1000 HP (Pittsburgh, PA, USA). The samples (100 mg) were pretreated in 50 Ncm3/min flow of Ar at 150 ◦C for 30 min, with a heating rate of 10 ◦C. TPR was conducted by heating at a rate of 5 ◦C/min 3 from 50 to 850 ◦C with a 50 Ncm /min flow of 10% H2/Ar.

3.3. Catalyst Activity Testing Details of the reactor and experimental setup can be found in our previous publication [5]. Briefly, the activity of the catalysts was measured in a vertical stainless steel tubular reactor (internal diameter = 9.7 mm) operated at atmospheric pressure. The temperature was measured and controlled by a thermocouple, protected by a stainless steel jacket, inserted into the catalyst bed. A MKS MultiGas 2030-HS FTIR Gas Analyzer (path length 5.11 m, Cheshire, United Kingdom), calibrated at 1 bar and 191◦C was used to analyze the composition of NO and NO2 in the product stream. For activity measurements, 0.5 g of catalyst diluted with 2.75 g SiC was loaded into the reactor and held in place by quartz wool plugs. Prior to the activity tests, the catalysts were pretreated at 500 ◦C 3 for 1 h in 200 Ncm /min flow of 10% O2/Ar and subsequently cooled down in inert argon atmosphere. The activity of the catalysts was investigated by heating from 150 to 450 ◦C at a rate of 5 ◦C/min under 3 a flow of 200 Ncm /min of feed gas (10% NO, 6% O2 in balance Ar). Conversion of NO to NO2 was calculated by the following equation:

NO conversion = α [NO ] /[NO] (2) × 2 outlet inlet where [NO]inlet and [NO2]outlet are concentrations of NO at inlet and NO2 at the outlet of the reactor. Volume changes arising from the reaction is taken into account by the constant “α”[36] where α = 0.99. The closure of nitrogen balance across the reactor (99.5–100%) conﬁrmed that all nitrogen is present as NO and NO2. Comparison of catalyst activity is performed at 350 ◦C where the reaction is in the kinetic regime, away from equilibrium. The reaction rate (rNO) calculations were performed by subtracting the homogeneous gas phase conversion of NO; hence, reﬂecting only the activity provided by the catalyst.

4. Conclusions A series of lanthanum-based perovskites have been investigated for oxidation of NO using a feed containing 10% NO and 6% O2, thus partially simulating nitric acid plant conditions. Among the undoped perovskites, the NO oxidation activity follows the order LaCoO3 > LaNiO3 > LaMnO3.A signiﬁcant increase in NO oxidation activity was achieved by partial substitution of cobalt in LaCoO3 with 25 mol% of either nickel or manganese. Further increase in the degree of Co substitution had a negative impact on activity. From this work, perovskites are shown to be promising catalysts for oxidizing NO to NO2 at conditions representative of nitric acid plant operation. Low cost, ease of production and signiﬁcant catalytic activity make perovskites attractive candidates as alternatives to noble metal catalysts.

Author Contributions: Conceptualization, A.u.R.S., D.W., B.C.E., R.L. and M.R.; methodology, A.S., S.M.H.; formal analysis, A.u.R.S., S.M.H., S.K.R. and M.Z.; investigation, A.u.R.S., S.M.H., S.K.R. and M.Z.; writing—original draft preparation, A.u.R.S.; writing—review and editing, S.M.H., S.K.R, M.Z., B.C.E., R.L, D.W. and M.R.; visualization, A.u.R.S.; supervision, B.C.E., R.L., D.W. and M.R.; project administration, M.R.; funding acquisition, B.C.E., R.L., D.W. and M.R. Catalysts 2019, 9, 429 10 of 11

Funding: This research was funded by iCSI (industrial Catalysis Science and Innovation) Centre for Research-based Innovation, which receives financial support from the Research Council of Norway, grant number 237922. The Research Council of Norway is also acknowledged for financial support to the Swiss-Norwegian Beamlines at ESRF, grant number 273608. Conflicts of Interest: The authors declare no conflict of interest.

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