An Application of Steady‐state Isotopic‐transient Kinetic Analysis (SSITKA) in DeNOx Process

This Minireview presents an overview of the advancement and capabilities of the steady‐state isotopic transient kinetic analysis in the selective catalytic reduction of NOx. Firstly, a brief overview of the method and the kinetic parameters of catalyst‐surface reaction intermediates, including concentration and coverage of surface intermediates, surface residence time and intrinsic turnover frequency (TOF), is provided. Furthermore, the focus is on the application of SSITKA or a unique combination of SSITKA‐DRIFTS for the identification of active and/or inactive (spectators) species in NH3−SCR, H2−SCR and HCs−SCR. Different forms of adsorbed species and their formation rates revealing the main elementary steps on the catalyst's surface involving labeled molecules are discussed. The emphasis is laid on the optimization and design of industrial catalysts.


Introduction
Steady-state isotopic transient kinetic analysis (SSITKA) was developed in large part by Biloen, [1] Bennett, [2] Happel [3] and Bell [4] and is applied in the heterogeneous catalysis for the understanding of the reaction pathway of surface-catalyzed reactions. Extensive details (i. e., a typical system for isotopic transient kinetic study, SSITKA modeling, limitations and corrective techniques, advanced mathematical analysis and applications in different chemical reactions) about SSITKA (for measurements under adsorption/desorption reaction equilibrium) or ITKA (for measurements that are not at steady state) are given in references. [5][6][7][8] The technique consists of allowing the reaction to reach steady-state using an unlabeled reaction mixture and abruptly switching from the unlabeled mixture to the labeled one (without perturbation of the steady-state catalytic process). Consequently, the total rate of formation of any product (i. e., the chemical composition of the surface) does not change due to the abrupt replacement of one reactant by its isotope. The accurate in situ determination of kinetic parameters, such as surface residence time (t P ), concentration (N P ) and coverage (q P ) of the main intermediates (not of any inactive (spectator) species) or their turnover frequency ( TOF ITK ) by SSITKA allows to determine the reaction mechanism.
For a given reversible heterogeneous catalytic reaction at steady-state, a constant amount of reactant (R) is transformed into a product (P) through an adsorbed intermediate (X) [Eq. (1)]: RðgÞ þ other reactantsðgÞ $ XðadsÞ $ PðgÞ þ other productsðgÞ (1) By continuously monitoring the relaxation and evolution of the unlabeled and labeled species via mass spectrometry (with high resolution), isotopic transient curves can be obtained. Figure 1 shows typical normalized isotopic-transient responses of a species F(t) as a function of time after the isotope switch. [5] The surface residence time (t P ) of the intermediates leading to product is determined from the area between inert tracer (Ar or Kr that does not adsorb on the catalysts' surface, F I ) and product (F P ) transient response curves [Eq. (2)]: [5] t P ¼ Based on the surface residence time (t P , s) and the formation rate of the unlabeled product (r P , μmol g À 1 s À 1 ), the concentration of the most active surface intermediates (N P , μmol g À 1 ) can be determined [Eq. (3)]: where F -molar flow rate of gas, (μmol s À 1 ), X -conversion to the product, (%), W -catalyst's weight, (g). [9] Furthermore, the surface coverage (q P ) is calculated based on the number of adsorbed species (N P ) and the total number of the active sites (N S , mainly determined by H 2 chemisorption) [Eq. (4)]: The turnover frequency on the average site participating in product formation (TOF ITK ), can be estimated by taking the reciprocal of t P [Eq. (5)]: The values of TOF ITK are higher than TOF Site (turnover frequency based on the concentration of true active sites) and also TOF Chem (turnover frequency based on chemisorption), and follows the relationship: TOF ITK � TOF Site � TOF Chem . Thus, SSITKA allows us to determine the number of active centers on the working catalyst. SSITKA provides on the homogeneity or heterogeneity of the catalyst's surface and insight into the possible mechanism. The simplest way is to analyze the shape of the curves showing the change in product concentration as a function of time that has elapsed since switching between reactants with different isotope composition. E.g., Figure 2 presents the transient responses for an irreversible-reaction mechanism consisting of a single pool, two serial pools or two parallel pools. [5] Different applications -in ammonia synthesis, [10,11] CO hydrogenation (methanation and Fischer-Tropsch synthesis), [12,13] methane activation (coupling, partial/total oxidation), [14,15] methane reforming, [16,17] water-gas-shift (WGS), [18,19] selective catalytic reduction of NO x (discussed below) and many other catalytic processes, have demonstrated the versatility of the SSITKA technique. Effects of temperature, reactant partial pressure and metal promotions on the catalysts or cause of deactivation (e. g., [13,20,21] ) have also been explored using SSITKA. It has been shown that in many cases there is an increase of active surface intermediates rather than the increase of intrinsic activity. Thus, a better understanding of reaction at the site level is provided, resulting in better catalyst design. SSITKA experiments on DeNO x reactions have been published for example in NH 3 À SCR over V 2 O 5 /TiO 2 , [22][23][24] CH 4 À SCR over Pt/ TiO 2 , [25] La 2 O 3 À CaO [26] or Co/ZSM-5, [27,28] C 3 H 6 À SCR over Pt/ SiO 2 [29,30] and H 2 À SCR over Pt/SiO 2 , [31,32] Pt/LaÀ CeÀ MnÀ O, [33] and Pt/SiO 2 , [33,34] etc. (Table 1).

Selective NO Reduction
Nitrogen oxides (NO, NO 2 and N 2 O) released from both mobile and stationary sources are considered major air pollutants that contribute considerably to the formation of photochemical smog, acid rain, ozone depletion, ground-level ozone and greenhouse effect, etc. The selective catalytic reduction with NH 3 (NH 3 À SCR) is the currently commercialized aftertreatment system for NO x removal in coal-fired power plants and diesel engine vehicles. [47] However, many problems are encountered in the use of NH 3 À SCR technology, namely catalysts deterioration, NH 3 -slip (emission of unreacted toxic NH 3 ), ash odor, air heaters fouling and high running cost). Thus, SCR of NO x with H 2 or hydrocarbons (HCsÀ SCR, CH 4 À SCR or C 3 H 6 À SCR) has been considered as an alternative technology. [48][49][50] In the literature, the application of SSTTKA or its combination with diffuse reflectance infrared Fourier transform spectroscopy (SSITKA-DRIFTS) studies focus mainly on NO reduction by CH 4 or H 2 , which are presented in the following sections.

NO reduction by NH 3 and NH 3 oxidation
The selective catalytic reduction of NO x with NH 3 under strongly oxidizing conditions (NH 3 À SCR) was first introduced in the early 1970s by Hitachi Zosen in Japan over V 2 O 5 -TiO 2 . [51,52] Currently, the investigated catalysts in NH 3 À SCR can be divided into three categories: noble metal-based catalysts (e. g., Ag/Al 2 O 3 [53] ), transition metal oxides (e. g., Cu(Fe)-MgÀ Al, Mn/Ti-Si [54,55] ) and transition metal ion-exchanged zeolites (e. g., Cu(Fe)-ZSM-5, Cu-SSZ-13 [56][57][58] ). The studies can be found in numerous review articles. [59][60][61][62][63] The SSITKA studies over selective catalytic NO x reduction by NH 3 (NH 3 À SCR) or selective catalytic NH 3 oxidation to N 2 (NH 3 -SCO) are rather rare. E.g., Otto et al. [35]   The responses have been corrected for the gas-phase holdup. Reprinted from [5] with the permission of ACS Publications.  [25] ChemCatChem Minireviews doi.org/10.1002/cctc.202001317 The formation of (unmixed) N 2 was more pronounced over CuO [64] than Pt/Al 2 O 3 . They excluded the reaction between two NH 3 molecules due to the absence of 15 N 2 and/or 15 N 2 O. Janssen et al. [22] claimed that oxidation of 15 NH 3 to 15 N 2 takes place in the absence of NO (at 400°C). However, as they also mentioned 15 N 2 (m/e 30) coincide with the mass of NO, which can also be formed during NH 3 oxidation. Otherwise, there was no mention of NO and/or N 2 O formation in their study. In NH 3 À SCR, N 2 and/ or N 2 O arise form one molecule of NO and one molecule of NH 3 (cross-labeled N 15 [36] Overall, the authors analyzed mainly isotope transfer with time without a detailed analysis of experimental data. Kinetic parameters as the surface residence time and the surface concentration of reaction intermediates were not fully explored in such studies. Ozkan et al. [37] 3 oxidation using lattice oxygen. The authors claimed the existence of more than one type of adsorbed NH 3 species on V 2 O 5 surfaces, i. e., namely V-ONH 2 , V-ONH 3 and V-ONH 4 and pointed out sites responsible for NH 3 oxidation to NO and sites responsible for NH 3 À SCR. The V-ONH 2 species were believed to convert to NO [Eq. (10)]: The V-ONH 3 species lead to the formation of N 2 and/or N 2 O [Eqs. (11)(12)(13)]: During NH 3 À SCR reaction the V-ONH 4 species can react with NO to form N 2 [Eqs. (14)]: (14) The transient isotopic studies strongly suggest that replenishment of oxygen vacancies takes place by gas-phase O 2 and to a lesser extent by diffusion from the lattice oxygen. [37,38] Together with NH 3 À SCR, they investigated NH 3 oxidation at 400°C. [38] N 2 and/or N 2 O were believed to be formed from two NH 3 , which must remain on the V 2 O 5 surface for a considerable length of time after the gas-phase has been replaced by 15 [23,24] studied NH 3 oxidation over (3.6, 9.0 wt.%) V 2 O 5 /TiO 2 . Figure 3 shows the obtained responses of 14 N, 14 N 15 N and 15 N 2 isotopic species. They claimed that the difference in the time of appearance of the maximum in the rate formation of 14 N 15 N (0.67 or 6.4 min) demonstrate that the intrinsic site reactivity associated with adsorbed NH 3 , e. g., V-ONH 3 , is influenced by the V 2 O 5 loading (3.6 or 9.0 wt.%) deposited on TiO 2 . [23] According to the proposed reaction scheme, [37,38] direct ammonia oxidation takes place on the basal planes of the V 2 O 5 crystals. Thus, the surface coverage and turnover frequency values obtained for N 2 and/or N 2 O were remarkably similar for the applied catalysts -V 2 O 5 -D (having preferential exposure of the side planes) and V 2 O 5 -M (having preferential exposure of the (010) basal plane). The samples varied in the NO transient values (not quantified for V 2 O 5 -D due to low values obtained for the NO formation). The sites that lead to NO formation appear to be different (active in a rapid insertion of lattice oxygen once the NH 3 adsorbs on these sites; residence time of 0.1044 s) from the ones that lead to N 2 and/or N 2 O formation (residence time of 13.57-17.42 s). Furthermore, NO produced from NH 3 oxidation using the lattice oxygen could further react with NH 3 , according to nowadays widely accepted the internal (in-situ) selective catalytic reduction (i-SCR) mechanism for NH 3 oxidation. [65,66] Besides the application of SSITKA, also, the temporal analysis of products (TAP) has been applied in a limited number of studies concerning the ammonia oxidation. In particular, Pérez-Ramírez et al. [67,68] investigated the sequence of steps in NH 3 oxidation at high temperatures to give NO, which is one of the stages in nitric acid production over PGM (Pt, Pd, Rh) wires or oxides (Fe 2 O 3 , Cr 2 O 3 and CeO 2 ). The authors studied primary ( 15 NH 3 + O 2 ) and secondary ( 15 NH 3 + NO) interactions over PGMs at 900°C using different C(NH 3 ):C(O 2 ) and C(NH 3 ):C(NO) ratios. NO was found as a primary reaction product, while N 2 and N 2 O originated from consecutive NO transformations. The decrease in the C(NH 3 ):C(O 2 ) ratio effectively minimized the formation of N 2 O and N 2 over the PGMs. [68] Furthermore, they found that NO was the primary product of NH 3 oxidation also for oxides. [67] Multi-pulse NH 3 experiments in the absence of gas-phase O 2 revealed that surface lattice oxygen of the studied oxides participated in NH 3 oxidation to NO. However, the formation of NO depended on the nature of the oxide. On the other hand, Olofsson et al. [69] investigated the reaction mechanism of NH 3 -SCO over (20. Over Pt/CuO/Al 2 O 3 most N 2 was formed by the NH 3 oxidation involving two NH x species, while reaction takes place at the Pt/CuO phase boundary between NH x species being adsorbed on CuO and oxygen on Pt sites. Additionally, the alternate pulsing of NH 3 and NO over preoxidized samples excluded NO as an intermediate, which is in contrast to studies of Burch and Southward [70] or Gang et al. [71] over Pt/CuO/Al 2 O 3 or Ag/CuO/Al 2 O 3 , respectively. Certainly, SSITKA applicationcombined with other techniques (e. g., DRIFTS), over a broad Reprinted from [23] with the permission of Elsevier.

ChemCatChem
Minireviews doi.org/10.1002/cctc.202001317 range of catalysts could provide mechanistic insight of NH 3 -SCO in real-time and conditions.

NO reduction by H 2
The effective NO x reduction by H 2 in slight excess of O 2 over Pt/ Al 2 O 3 was firstly published in 1971 by Jones et al. [72] Research progress in the selective catalytic reduction of NO x by H 2 in the presence of O 2 was given by Liu et al., [49,50] Royer et al. [73] or Nova and Tronconi. [74] Especially Pt-based catalysts, including Pt/SiO 2 , Pt/Al 2 O 3 , Pt/mesoporous silica (MCM-41, Al-MCM-41, etc.), Pt/zeolite, Pt/perovskite, Pt/TiO 2 -ZrO 2 , Pt/MgO-CeO 2 , etc. (also promoted with Na, Mo and/or W), are reported to be active at relatively low-temperatures (65-200°C) while indicating also a high yield of N 2 O. Burch et al. [31,32,34,40] studied H 2 À SCR over (5.0 wt.%)Pt/SiO 2 by SSITKA experiments. They reported that N 2 O and/or N 2 appeared as the isotopically first product (with the lowest residence time on the catalysts relative to all products) and the last product, respectively. [31,34] The increase in temperature from 60 to 83°C resulted in a higher N 2 production due to an increase in the concentration of sites (from 5.1 to 7.6 μmol g À 1 ) active for the production of N 2 together with a decrease of their mean surface residence time (from 64 to 42 s � 0.5 s). The surface coverage of intermediates leading to N 2 was higher (0.06-0.10) than that of intermediates leading to N 2 O (0.01-0.02). [34] 14 N 15 N was still formed for a considerable time after all traces of gaseous 14 NO was removed (Figure 4). The production of N 2 O required gas-phase or weakly adsorbed NO. [31] The promotion of Pt/SiO 2 (Al 2 O 3 ) with Na 2 O delayed the desorption of NO and causes a parallel increase in the decay time for the labeled N 2 O. Furthermore, the addition of MoO 3 to these catalytic systems induced substantial increases in the concentration of surface species leading to N 2 . [40] Further, a mathematical analysis (Temporal Redistribution of Isotopically Labeled Molecules of Product -TRIMP, i. e., the function y(i), the deviation from statistical isotopic distribution within the product molecules) of the profile shape allowed the recognition of the possible paths of the intermediates to the final products ( Figure 5). [32,34] The authors suggested that formation of N 2 (in an extent of 85 %) was the result of the interaction of two different NO x species (a weakly chemisorbed NO named as NO preads and NO' ads ), while that of N 2 O was the result of two similar in nature NO x species [Eqs. (15)(16)]: This mechanism was supported by the density functional theory (DFT) studies. [75] The surface coverage of the active NO x intermediate species that lead to N 2 formation is considerably larger than that leading to N 2 O. However, the chemical Reprinted from [31] with the permission of Elsevier.  [32,34] with the permission of Elsevier. structure of the active and/or inactive (spectator) NO x species was unidentified, as an effect of limitation of SSITKA. [34] The two different in the chemical structure active NO x species were identified by Costa and Efstathiou. [33] Thus, NO preads and NO' ads named by Burch et al. [34] were recognized as nitrosyls (Pt-NO δ + , IR band at 1900 cm À 1 ) and unidentate nitrates (and Pt-NO 3 , IR band at 1480 cm À 1 ) adsorbed on Pt, respectively, through a combination of SSITKA and in situ DRIFTS over Pt/ SiO 2 . The unidentate nitrates on Pt (IR band at 1620 cm À 1 ) were found to be spectators. Contrary to the results reported by Burch et al., [34] they suggested that the same active NO x precursor species must be involved in both N 2 and N 2 O formation pathways [33]. For Pt/La 0.5 Ce 0.5 MnO 3 , the active NO x species (M-NO 2 + , nitrosyls, IR band at 2220 cm À 1 and MÀ O-(NO)-OÀ M, bidentate nitrates, IR band at 1540 cm À 1 ) are populated on the metal-support inter-phase region. 8.10 μmol g À 1 (θ = 1.74) of these active NO x species (81.5 % leading to the formation of N 2 ) was found for Pt/La 0.5 Ce 0.5 MnO 3 (compared to 3.04 μmol g À 1 (θ = 0.65) obtained for Pt/SiO 2 ). [33] The active precursor intermediate NO x species (14.4 μmol g À 1 , θ = 3.1) that lead to the formation of N 2 and N 2 O over (0.1 wt.%)Pt/MgO-CeO 2 , [41] are located in the vicinity of Pt-CeO 2 (nitrosyls coadsorbed with nitrates on adjacent Ce 4 + -oxygen anion site pair) (IR band at 2220 cm À 1 ) and Pt-MgO support interface region (IR band at 1540 cm À 1 ). The chemical structure of the second active NO x species depended on the temperature and appeared as bidentate or monodentate nitrates (NO 3 À ; below 200°C) or chelating nitrites (NO 2 À ; above 200°C). The inactive adsorbed NO x appeared at the Pt metal surfaces and the MgO-CeO 2 support. [42,43] Chelating nitrite species (7.2-17.5 μmol g À 1 , θ = 1.8-4.4 at 200-300°C) were found as the active intermediate over Pt/Ce 0.5 Zr 0.5 O 2-δ , whereas nitrosyls, monodentate and bidentate nitrates were considered as spectators. [44] The presence of H 2 O in the feed resulted in a 25 % decrease in the active NO x intermediate species and thus, partly explaining the significant drop in the catalytic activity during H 2 À SCR. Based on the 18 O 2 SSITKA-MS experiments they found that for Pt/SiO 2 and Pt/La 0.5 Ce 0.5 MnO 3 , [33] H 2 À SCR toward N 2 O formation involves a reaction path of NO oxidation on Pt with the participation of gaseous O 2 . However, it was not the case over Pt/MgO-CeO 2 . [41][42][43] Concluding, mainly Pt-containing catalysts were investigated in H 2 À SCR by the combination of SSITKA-DRIFTS. The chemical structure of NO x adsorbed precursor intermediates as well as their abundance (i. e., the concentration of surface intermediates and their site coverage on the surface) significantly varied among applied catalysts compositions and applied temperatures.

NO reduction by CH 4 or C 3 H 6
Starting from the pioneering work of Held et al. [76] and Iwamoto et al. [77] over Cu-ZSM-5, the reduction of NO x by hydrocarbons under strongly oxidizing conditions (HCsÀ SCR) has been extensively studied. [78][79][80] Methane being the most abundant hydrocarbon in the form of natural gas offers a very attractive alternative for NO x reduction. CH 4 À SCR was investigated mainly over zeolites (e. g., CoÀ CHA, CoÀ RTH, CoÀ UFI, [81] supported noble metals (e. g., Ag/Al 2 O 3 , Pt/Al 2 O 3 [82,83] ), and metal oxides (e. g., CoO x /γ-alumina, CoO/ZrO 2 (SO 4 2À ) [84,85] ). Kumthekar and Ozkan [25,86] investigated CH 4 À SCR over Pd/TiO 2 with palladium primarily in metallic form. They claimed that CH 4 À SCR likely consists of NO-CH 4 reaction, CH 4 oxidation and NO decomposition proceeding simultaneously on the surface. The extent of each reaction was determined by the oxidation state of Pd on the surface and of the reaction parameters. Based on the 14  and CO 2 were found to be mainly formed through direct participation of CH 4 with a methyl dinitrosyl species acting as an intermediate. [25] Sadovskaya et al. [27,45,46,87,88] focused on the formation of NO x adsorbed species in the absence and the presence of O 2 over Co-ZSM-5. Firstly, they investigated the dynamics of isotopic exchange between the NO + O 2 without CH 4 (  [46] Based on previously reported in situ DRIFTS analysis, [27] mononitrosyls, NO 2 δ + species and nitrite complexes were distinguished on Co sites. The rate of 15 N exchange with gaseous NO decreased as follows: mononitroysls @ NO 2 δ + species > nitrite complexes. The authors proposed that CH 4 À SCR in the presence of O 2 proceed with the participation of NO 2 δ + (formed at the interface between oligonuclear cobalt oxide species and hydroxyl groups of the zeolite; r = 0.020 · 10 À 19 molec g À 1 s À 1 ) and NO 2 À nitrite complexes (on larger cobalt oxide particles located outside the zeolite channels; r = 0.008 · 10 À 19 molec g À 1 s À 1 ), Figure 6. [88] Mononitrosyl species possibly participated in the overall reaction by interconversion to nitrate species. [46,87] The active species vary among applied catalysts. E.g., Anastasiadou et at. [26] found an enhanced intrinsic reaction rate of NO reduction by CH 4 over (5.0 wt.%)La 2 O 3 À (95 wt.%)CaO (TOF = 8.5 · 10 À 3 s À 1 ) compared to pure CaO (TOF = 3.6 · 10 À 3 s À 1 ). Based on 15 NO SSITKA/DRIFTS, they revealed the formation of oxygen vacant sites in CaOÀ La 2 O 3 , which promoted the formation of active chemisorbed NO x species (NO 2 À ), thus facilitating CH 4 À SCR. Bridged Figure 6. Two main parallel pathways for CH 4 À SCR over Co-ZSM-5. Note that CH 4 and NO + O 2 addition along the proposed pathways are likely to proceed via adsorbed species: for CH 4 = [*CH 4 ], [z-*C] 1 and [z-*C] 2 and NO + O 2 = mononitrosyl and activated oxygen over Co nanoclusters, respectively. Reprinted from [88] with the permission of Elsevier.