Contrasting Effects of Potassium Addition on M₃O₄ (M = Co, Fe, and Mn) Oxides during Direct NO Decomposition Catalysis

Abstract

While the promotional effect of potassium on Co₃O₄ NO decomposition catalytic performance is established in the literature, it remains unknown if K is also a promoter of NO decomposition over similar simple first-row transition metal spinels like Mn₃O₄ and Fe₃O₄. Thus, potassium was impregnated (0.9–3.0 wt.%) on Co₃O₄, Mn₃O₄, and Fe₃O₄ and evaluated for NO decomposition reactivity from 400–650 °C. The activity of Co₃O₄ was strongly dependent on the amount of potassium present, with a maximum of ~0.18 [(µmol NO to N₂) g⁻¹ s⁻¹] at 0.9 wt.% K. Without potassium, Fe₃O₄ exhibited deactivation with time-on-stream due to a non-catalytic chemical reaction with NO forming α-Fe₂O₃ (hematite), which is inactive for NO decomposition. Potassium addition led to some stabilization of Fe₃O₄, however, γ-Fe₂O₃ (maghemite) and a potassium–iron mixed oxide were also formed, and catalytic activity was only observed at 650 °C and was ~50× lower than 0.9 wt.% K on Co₃O₄. The addition of K to Mn₃O₄ led to formation of potassium–manganese mixed oxide phases, which became more prevalent after reaction and were nearly inactive for NO decomposition. Characterization of fresh and spent catalysts by scanning electron microscopy and energy dispersive X-ray analysis (SEM/EDX), in situ NO adsorption Fourier transform infrared spectroscopy, temperature programmed desorption techniques, X-ray powder diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) revealed the unique potassium promotion of Co₃O₄ for NO decomposition arises not only from modification of the interaction of the catalyst surface with NO_x (increased potassium-nitrite formation), but also from an improved ability to desorb oxygen as product O₂ while maintaining the integrity and purity of the spinel phase.

1. Introduction

Although the direct decomposition of nitric oxide (NO) to N₂ and O₂ is thermodynamically favorable, the reaction kinetics are hindered [1,2]. To overcome the slow kinetics, many and various catalyst materials have been studied, such as precious metals [3,4,5], transition metal ion-exchanged zeolites [6,7,8], perovskites [9,10,11], and rare earth oxides [12,13]. However, spinel oxides have emerged as one of the most widely studied catalyst groups for NO decomposition due to their versatile compositions and good redox properties [14,15,16,17,18,19,20,21].

Early reports of Co₃O₄ for NO decomposition describe it as one of the most promising single element oxides for NO decomposition, albeit with low activity [15,16]. A nickel–chromium spinel was also evaluated for NO decomposition, however it was oxidized by NO, leading to its deactivation [17]. This material could be regenerated via pulses of CO in the reactant feed, but such a result implies the necessity of a reductant to perform catalytic cycles, which is not truly direct NO decomposition. Similar reports of oxidation of the catalyst material via NO have been observed for other catalysts, particularly Pd-based materials [4,5].

A breakthrough in NO decomposition research on spinels occurred by evaluating Co₃O₄ synthesized via co-precipitation from a sodium carbonate aqueous solution [18]. It was deduced that residual Na was necessary to achieve good catalytic activity due to the tendency for alkali ions such as Na and K to promote nitrate formation. Later, a systematic optimization of the K concentration via impregnation onto Co₃O₄ led to a higher performance at ≥450 °C and confirmation that the presence of the alkali K was necessary to achieve the result [19]. However, alkali promotion is not the only path to improve NO decomposition performance over spinel oxides, as addition of other transition metals or dopants have yielded increases in activity. Very recently, studies from the authors have shown that doping Co₃O₄ with Cu, or impregnation of Cu on the surface of Co₃O₄, yields highly active NO decomposition catalysts at temperatures ≤500 °C, with optimization of the Cu loading during impregnation producing a catalyst with areal activity and N₂ selectivity exceeding that of the previous literature benchmark catalysts for NO decomposition, Cu-ZSM5 [20,21].

Potassium promotion and transition metal variation of spinel catalysts has also yielded success for N₂O decomposition [20,22,23,24,25]. The simple first-row transition metal spinels Co₃O₄, Mn₃O₄, and Fe₃O₄ spinels with K impregnation have been the focus of much research for N₂O decomposition, however to our knowledge, no work has been published regarding the performance of the latter two systems for NO decomposition [25]. Extension of these strategies for N₂O decomposition to improvement of NO decomposition performance is reinforced by reports of increased NO decomposition activity via addition of K to more complex spinels containing Co, Mn, and Al [26]. Activity for NO decomposition has been reported over Mn₃O₄ in the absence of K addition, but the activity was relatively low at 600 °C [27], thus the extension to Mn₃O₄ is natural. Spinels containing Fe are less frequently reported, however, perovskites based on La, Ti, Mg, and Fe are active from 300 to 550 °C. Characterization of the perovskites by Mössbauer spectroscopy revealed that the most active samples contained an Fe³⁺ sextet, which was able to form catalytically active complexes via exchange interaction with nearby Fe³⁺ doublet in octahedral coordination [10].

Thus, this study undertakes to determine the effect of alkali addition on the NO decomposition reactivity of Co₃O₄, Mn₃O₄, and Fe₃O₄ by impregnation of K ions at various loadings and to characterize the effect of K on the structure of the spinel and relevant mechanistic steps.

2. Results and Discussion

2.1. NO Decomposition Catalytic Performance

The NO conversion of Co₃O₄ based materials is strongly dependent on the presence of K. In the absence of K, NO conversion is below 5% at all temperatures from 400 to 650 °C (Figure 1a, Tables S1–S4). With the addition of K, the NO conversion is unaffected at 400 °C, but NO conversion begins to increase at 450 °C (~1%–2% increase). Conversion increases approximately by a factor of five at 550 °C, and reaches a maximum of ~60% at 650 °C. For all Mn₃O₄ and Fe₃O₄ samples without and with K promotion, the NO conversion is non-negligible, but falls below 5% at all temperatures (Figure 1b,c, Tables S1–S4).

Regarding catalytic activity for NO decomposition, it is emphasized that rates were calculated utilizing only the NO converted to the selective product of N₂ (calculated by mass-balance) and only from NO conversions of less than 25% to minimize the effect of reactant concentration gradients along the reactor bed length. The latter consideration was specifically addressed for the K Co₃O₄ catalysts at 650 °C, where conversions were significantly above 25% and additional testing was performed to achieve conversions below 10% (Table S5).

As calculation of activities incorporates the selectivity to N₂, additional insight into the catalyst reactivity for direct NO decomposition can be gained, i.e., NO conversion to nitrogen-containing products other than N₂ does not contribute to direct NO decomposition activity reported herein. Regarding the catalytic performance at 400 °C, some N₂ production was found over the Co₃O₄-based samples, however, this was only significant for the 3K Co₃O₄ catalysts (N₂ selectivity ~12%, Activity = 2.3 × 10⁻³ [(µmol NO to N₂) g⁻¹ s⁻¹], whereas the activity of catalysts with less K can be considered negligible, e.g., on the order of 10⁻⁴ [(µmol NO to N₂) g⁻¹ s⁻¹] (Figure 2a, Table S1). Moving to higher temperatures, the 0.9K and 2K Co₃O₄ catalysts proved most active, with the 0.9K Co₃O₄ sample yielding a maximum of ~0.18 [(µmol NO to N₂) g⁻¹ s⁻¹] at 650 °C. This result is in excellent agreement with earlier reports of K promotion of Co₃O₄ that determined impregnation of ~0.9 wt.% K was optimal to achieve maximum NO decomposition activity at ≥450 °C [19]. While the current study shows that the 0.9 K Co₃O₄ catalyst displayed the highest NO decomposition activity at both 550 and 650 °C, it was observed that the 2K Co₃O₄ catalyst was more active at 450 °C (Figure 2a). Higher loadings (2 and 3 wt.% K) were chosen for this study as the maximum N₂O decomposition for K promoted Mn₃O₄ and Fe₃O₄ occurred at higher loadings than the Co₃O₄ spinel [25], however, this variation in optimal K loading with temperature was not previously reported.

The selectivity to N₂ approached 90% at 550 °C and was nearly 100% at 650 °C for both 0.9K Co₃O₄ and 2K Co₃O₄ (Tables S3 and S4). For direct NO decomposition, additional non-selective nitrogen-containing products are typically identified as NO₂ and/or N₂O. For Co₃O₄-based catalysts, NO₂ was the only non-selective nitrogen-containing product detected (Tables S1–S4). Unfortunately, at the lower temperatures of 400 and 450 °C, the Co₃O₄-based catalyst produced more NO₂ with maximum selectivity to N₂ not exceeding 42% over the 2K and 3K Co₃O₄ catalysts (Tables S1 and S2). Despite this observation, the complete lack of N₂O production over Co₃O₄-based catalysts at these temperatures is of particular note as the undesired formation of this greenhouse gas has been reported as a potential selectivity issue for other direct NO decomposition catalysts exhibiting activity below 500 °C, such as Cu-ZSM5 [7].

Without K addition, both Mn₃O₄ and Fe₃O₄ did not display significant activity for NO decomposition to N₂ (Figure 2b,c), with calculated activities falling one–two orders of magnitude below those of Co₃O₄-based catalysts. There is activity for NO decomposition to N₂ at 650 °C over all Fe₃O₄ samples at all K loadings, however, the maximum activity of 3.3 × 10⁻³ [(µmol NO to N₂) g⁻¹ s⁻¹] over 2K Fe₃O₄ is more than 50× lower than that of 0.9 K Co₃O₄ (Figure 2c). Similarly, while some selectivity to N₂ over 0.9K Mn₃O₄ and 2K Mn₃O₄ was found at 650 °C (Table S6), the calculated activity values are even less than K Fe₃O₄ catalysts; corresponding to a reactor outlet concentration of less than 10 ppm N₂. At such a low production of N₂, the calculated rates are not reliably comparable (Figure 2b, Tables S1–S4). Most of the non-selective NO conversion is to NO₂, but unlike Co₃O₄, trace production of N₂O was detected over some of the K Mn₃O₄ and K Fe₃O₄ samples from 400 to 550 °C (Tables S1–S3). For N₂O decomposition, the maximum promotion effect was reported to occur at slightly higher K loadings for Mn₃O₄ and Fe₃O₄ as compared to Co₃O₄ [25], and indeed the current results agree that the 2 wt.% K loading was optimal compared to 0.9 wt.% for Co₃O₄. However, the current findings for NO decomposition over K-promoted Mn₃O₄ and Fe₃O₄ deviate from those of N₂O decomposition regarding the degree of promotion, which was strong for N₂O decomposition over Mn₃O₄ catalysts, and a similar, though milder, for Fe₃O₄ [25].

2.2. Comparison of Adsorbed Intermediates

Alkali addition is often utilized to increase NO adsorption of materials used as low temperature NO_x adsorbers, as nitrites and nitrates of alkalis are often thermally stable [28]. Therefore, it is likely that the addition of K to the spinel catalysts will affect the adsorption of surface NO_x species. The in situ NO adsorption Fourier transform infrared (FTIR) spectra at 300 °C of the spinel catalysts with and without K (2 wt.%) are presented in Figure 3. Without K, the spectra for Co₃O₄, Fe₃O₄, and Mn₃O₄ do not contain any strong features indicative of adsorbed NO_x species. With the addition of K to Co₃O₄, Mn₃O₄, and Fe₃O₄, several prominent features in the NO_x adsorption region 1600–1000 cm⁻¹ are observed and are indicative of the formation of stable nitrates and/or nitrites at 300 °C [28,29]. For K Co₃O₄, a single broad peak centered at 1375 cm⁻¹ is attributed to the ν_as(NO₃) bulk-like nitrate NO₃^- anions [29]. Such a species is likely the result of the formation of a potassium nitrate species. Indeed, similar features can be attributed to the FTIR spectra of both K Fe₃O₄ and K Mn₃O₄, which exhibit a broad peak overlapping similar wavenumbers. The spectra of K Fe₃O₄ and K Mn₃O₄ also contain two additional broad peaks centered at 1360 and 1250 cm⁻¹, which are assigned to the ν_s(NO₂) and ν_as(NO₂) modes of a bulk-like nitrite NO₂⁻ anion [29]. Given the absence of these features in the FTIR spectra of the alkali-free Fe₃O₄ and Mn₃O₄ catalysts, it is concluded that such species manifest as result of potassium nitrite formation.

Admittedly, the NO_x species identified by in situ FTIR during NO adsorption at 300 °C are not useful for identification of the molecular nature of the surface adsorption sites participating in direct NO decomposition as they were collected below the active temperatures for these materials. Thus, it would also be inappropriate to speculate that the NO_x adsorption species identified here are intermediates in the decomposition. Rather, the value of this result is to demonstrate and confirm that K addition changes the interaction of the catalyst surface with the NO reactant molecules. Alkali addition has been shown to improve the stability and quantity of adsorbed NO_x species over a variety of materials, especially those used as low temperature NO_x adsorption materials [28], and the FTIR spectra in Figure 3 strongly agree with those previous observations.

The observation of the surface NO_x species alone, however, is insufficient to explain the relatively high activity results only for K promotion of Co₃O₄ and not for Fe₃O₄ or Mn₃O₄. Given all three spinels show a significant increase in NO adsorption upon K addition, it is implied that additional mechanistic step(s) or catalyst properties are promoted when K is added specifically to Co₃O₄. The desorption of oxygen as product O₂ is well-established as a key step of the reaction mechanism for direct NO_x decomposition over a myriad of catalysts, including PGMs and spinel oxides [3,4,5,17]. Therefore, an investigation of the oxygen product formation and the effect of oxygen release on the catalyst structure and stability was performed.

2.3. O₂ Release and NO Decomposition and the Effect of K Loading

The above activity results emphasized the production of the selective product N₂ when assessing the reactivity of direct NO decomposition catalyst materials, but it is of equal importance to confirm that oxygen can desorb from the catalyst as product O₂ to complete the catalytic cycle. The effect of K on the release of O₂ from Co₃O₄, Fe₃O₄, and Mn₃O₄ was examined via oxygen temperature-programmed desorption (O₂-TPD). The release of O₂ was qualified by tracking increases in intensity of the O₂ mass spectrometer (MS) signal (m/z = 32) during the temperature ramp. Based on this metric, it is observed that Co₃O₄ releases O₂ across all relevant reaction temperatures, i.e., ≥400 °C (Figure 4a). The addition of K of various amounts to Co₃O₄ leads to some minor differences from ~220 to 400 °C in the O₂-TPD profile, but the general observations are largely similar, leading to the conclusion that K does not significantly affect O₂ release over Co₃O₄. Given the relatively large signal-to-noise compared to the O₂-TPD profiles of the Co₃O₄-based catalysts, very little O₂ release can said to be observed over the Mn₃O₄- and Fe₃O₄-based compounds (compare Figure 4a–c). Slight rises in intensity in the O₂-TPD profile can be observed for the Mn₃O₄ catalysts of all K loadings, but a relative comparison with varying K loading suggests that K actually can decrease O₂ release (Figure 4b). For Fe₃O₄, only the O₂-TPD profile of 3K Fe₃O₄ suggested any oxygen release (Figure 4c). Ultimately, the relatively greater O₂ release from Co₃O₄ compared to Mn₃O₄ is not unexpected and has been reported previously by others [27]. The O₂-TPD experiment is, however, conducted in the absence of the reactant NO, and the ability to release O₂ in its absence should not be interpreted as the ability to produce N₂ and O₂ under direct NO decomposition reaction conditions.

To address potential differences in O₂ release behavior in the presence of NO, the intensity of the O₂ MS signal (m/z = 32) relative to baseline was examined during NO decomposition under isothermal reactions conditions. These data are presented in Figure 5 with the data normalized to set the baseline intensity equal to unity for all catalyst materials. The baseline was determined by the m/z = 32 signal intensity during flow of the reactant gas mixture through the reactor bypass. Indeed, Figure 5 reveals that for Co₃O₄ and K Co₃O₄, O₂ release during NO decomposition at isothermal conditions only initiates at higher temperatures compared to O₂ release during O₂-TPD. Thus, not surprisingly, O₂ release is affected by the gas phase environment: at lower reaction temperature NO oxidation by surface oxygen is preferred leading to oxygen release as NO₂ (see Tables S1 and S2), and at higher reaction temperature oxygen release as product O₂ is preferred. Nonetheless, the increase in the relative intensity of the O₂ signal is in good agreement with the trends in activity to N₂ for the Co₃O₄ catalysts with varying K loading (Figure 2a), confirming these materials are active for direct NO_x decomposition to N₂ and O₂.

For Mn₃O₄, the highest O₂ release during NO decomposition is observed at 650 °C over the catalyst with 2 wt.% K at about four times the baseline concentration (Figure 5b). Once again, this is an indicator of NO decomposition catalysis and is in agreement with the activity data in the inset of Figure 2a, which indicated a low catalytic activity at 650 °C.

The promotion of O₂ release by K is perhaps best illustrated over Fe₃O₄. Without K, O₂ release is not observed in O₂-TPD or during isothermal NO decomposition (Figure 5c). A possible explanation for the lack of O₂ release can be found by examining the isothermal product formation over Fe₃O₄ as a function of time (Figure S1), which shows that the initial NO conversion and production of N₂ are quite high, but decrease to nearly zero with time on stream. The implication of this deactivation behavior is that Fe₃O₄ is likely reacting with the oxygen from the NO reactant molecule in a non-reversible chemical process, similar to the published work on NiCr₂O₄ [17]. For K Fe₃O₄, the isothermal NO decomposition results show some O₂ production by 650 °C, with the relative intensity of the MS signal increasing up to ~four times the baseline. In this case, it can be confirmed NO decomposition was able to proceed as a result of the K addition, preventing the chemical reaction that deactivated the unpromoted Fe₃O₄.

In summary, K does indeed promote O₂ release, and more importantly, is able to do so in the presence of an oxidizing reactant molecule (NO). The effect of K on the O₂ release was perhaps most significant for Fe₃O₄, which showed no appreciable oxygen release without K, even during O₂-TPD. It was observed, however, that only the K Co₃O₄ catalysts released oxygen in significant measurable quantities in both the O₂-TPD (Figure 4) and under reaction conditions in the presence of NO (Figure 5), suggesting additional factor(s) contribute to the inability of K-promoted Mn₃O₄ and Fe₃O₄ to release substantial amounts of product O₂. Therefore, it is necessary to determine how K affects the structure of the individual spinel catalysts to determine possible changes to stability that might impact O₂ release, and ultimately activity.

2.4. Catalyst Structure before and after Reaction—Influence of K Loading

2.4.1. X-ray Diffraction

The X-ray powder diffraction (XRD) patterns of the fresh and spent Co₃O₄ catalysts with varying K loadings presented in Figure 6 were collected to determine if structural changes occur as a result of K addition and/or during exposure to NO decomposition reaction conditions. For all loadings of K (0–3 wt.% K) on Co₃O₄, the reflections are indexed to the spinel Co₃O₄ (PDF# 00-043-1003). There are no detectable reflections for K₂O or potassium–cobalt mixed oxide species. Furthermore, no discernable phase change was observed after NO decomposition reaction. The observation that the Co₃O₄ phase is unaffected by K addition or reaction is consistent with the sustained activity and O₂ release characteristics observed during NO decomposition (see Figure 5), which would not be the case if additional stable oxide phases were formed. It is observed, however, that in the absence of K (0 wt.% K), the Co₃O₄ crystallite size as determined by the Scherrer equation (Co₃O₄ (103), shape factor = 0.89) dramatically increased from ~21 to 65 nm (Table S6). Thus, it is concluded K stabilizes the Co₃O₄ crystallite size at high temperature, since the crystallite size of samples with 0.9K, 2K, or 3K did not exceed ~31 nm after reaction (see Table S6).

The XRD patterns of the Mn₃O₄ catalysts with varying K loadings contain both Mn₃O₄ (PDF# 01-070-9110) and bixbyite Mn₂O₃ (PDF# 01-071-0636) reflections in all fresh samples (Figure 7), but the primary phase for all samples is Mn₃O₄. Unlike Co₃O₄, a monotonic increase in the Mn₃O₄ crystallite size is observed with increasing K loading (Table S6). After the reaction, the primary Mn₃O₄ and trace Mn₂O₃ remain unchanged, but an additional phase is also present (indicated by asterisks in Figure 7). The additional phase is first observed in the spent 0.9K Mn₃O₄ and the peaks intensify in the spent 2K and 3K Mn₃O₄. The reflections are likely due to the formation of a potassium–manganese mixed oxide. Based on previous literature reports of similar oxides, these peaks are tentatively indexed to cryptomelane KMn₈O₁₆ (PDF# 00-044-1386). Typically, pure-phase cryptomelane is synthesized at elevated pressures via hydrothermal methods, however, its formation has also been observed over CoAlMn-based oxides co-precipitated by KOH [25,30]. Cryptomelane also has tendency to oxidize to bixbyite above 600 °C in air, but this has not been observed under the current reaction conditions (~1% NO/He) as the XRD patterns of the spent K-containing samples displayed no notable increase in relative intensity of bixbyite reflections. Therefore, it appears Mn₃O₄ by itself is largely inert in NO gas, however the addition of K leads to a reaction between K and Mn₃O₄, forming potassium–manganese mixed oxides after exposure to the elevated temperatures of the reaction.

Figure 8 contains the XRD patterns of fresh and spent Fe₃O₄ catalysts with various K loadings. The fresh samples were primarily indexed to magnetite Fe₃O₄ (PDF# 04-015-3101). With respect to K loading, no change in crystallite size from Scherrer analysis was observed in the fresh or spent (see Table S6). However, K addition to Fe₃O₄ leads to the presence of additional phases in both fresh and spent samples. An additional XRD reflection is observed in the spent 0.9K Fe₃O₄, and at higher K loadings this peak intensifies and is accompanied by an additional peak in both fresh and spent samples (see asterisks in Figure 8). Similar to Mn₃O₄, the additional reflections in the K-containing Fe₃O₄ catalysts suggest the presence of a potassium–iron mixed oxide, which is tentatively indexed to K₂Fe₂₂O₃₄ (PDF# 00-031-1034). But while Mn₃O₄ did not undergo phase changes after reaction, significant phase changes were observed for Fe₃O₄, which were dependent on the K loading. In the absence of K, Fe₃O₄ was completely converted to α-Fe₂O₃ (PDF# 01-073-3825). With the addition of 0.9 wt.% K, the formation of α-Fe₂O₃ is mitigated, and 2 and 3 wt.% K appear to prevent the formation of detectable amounts of α-Fe₂O₃ in the XRD pattern of the spent samples. Instead, K addition leads to the formation of γ-Fe₂O₃ (PDF# 00-025-1402). Although an impurity phase, γ-Fe₂O₃ has a similar crystal structure to the spinel and is known to reversibly cycle between γ-Fe₂O₃, Fe₃O₄, and Fe_1-xO more readily than α-Fe₂O₃ [31]. However, the major phase by XRD remains that of Fe₃O₄ spinel. A more facile redox capability indicates greater propensity to release oxygen, and subsequently perform sustained cycles of catalytic NO decomposition, as observed at 650 °C in the γ-Fe₂O₃ containing K Fe₃O₄ samples (see Figure 2c). Therefore, XRD indicates both positive negative aspects of the K addition to Fe₃O₄, i.e., the suppression of a phase transition to inactive α-Fe₂O₃ and the formation of inactive potassium–iron mixed oxides, respectively.

2.4.2. Raman Spectroscopy

Raman spectroscopy was employed over the fresh and spent catalysts as an additional probe to examine any changes in local M–O bonding, and to corroborate phase assignments made via XRD. For Co₃O₄ based materials, as expected from the XRD pattern and good O₂ release characteristics during the reaction, there are no discernable M–O bonding environments detected in the Raman spectra other than those of Co₃O₄ (Figure 9). The bands from low to high wavenumbers are 476, 518, 615, and 682 cm⁻¹ labeled as the E_g, F_2g(2), F_2g(1), and A_1g modes, respectively, and all agree with those reported in the literature [32]. The spent Co₃O₄ catalyst without K displays a shift of ~16 cm⁻¹ to greater wavenumber at the A_1g mode (Figure 9). This shift is postulated to be the result of the increase in particle size after the reaction (see Table S6) [33]. For 0.9 and 2K Co₃O₄, there is very little difference in the A_1g mode position. The A_1g mode of the 3K Co₃O₄ sample shifted is at a slightly lower wavenumber (678 cm⁻¹) and the peak shape is visibly broader. It is possible the increase in K at the Co₃O₄ grain boundaries manifested as this red shift and peak broadening of the A_1g mode due to a greater population of unsaturated bonds, which has been similarly attributed with the addition of spectator nickel ions to Co₃O₄ [34]. This shift is not sustained, however, as after reaction the A_1g mode of 3K Co₃O₄ shifts to 692 cm⁻¹ (similar to spent Co₃O₄), and the peak shape returns to a width similar to the other K Co₃O₄ samples. Therefore, it follows that there are fewer unsaturated bonds on 3K Co₃O₄ after the reaction. Additionally, among the spent samples, the position of the A_1g mode shifts from low to high as 0.9K Co₃O₄ < 2K Co₃O₄ < 3K Co₃O₄. A decrease in the Raman shift of the A_1g position may suggest a greater population of unsaturated bonds of CoO₆ octahedra as a result of the K presence, establishing an explanation for the relative trend in NO decomposition activity of the Co₃O₄ series.

Major changes to the A_1g mode (658 cm⁻¹) position were not observed in Mn₃O₄ based samples with respect to K loading, nor the T_2g (285, 372, 474 cm⁻¹) and E_g (318 cm⁻¹) modes (Figure 10), and all positions are similar to those previously reports for Mn₃O₄ [34]. However, the Raman spectra of the K-containing Mn₃O₄ catalysts (both fresh and spent) display several new Raman modes. The positions of these modes are most easily identified in the Raman spectrum of the spent 3K Mn₃O₄ at approximately 398, 500, 551, and 576 cm⁻¹ (Figure 10), and are assigned to cryptomelane [35,36,37]. While some peaks, such as the peak at 551 cm⁻¹, might be ascribed as hollandite MnO₂ instead of cryptomelane, hollandite MnO₂ presence is unlikely because it also has an intense Raman mode at ~700 cm⁻¹ which is not observed in any of the sample sets [38]. Birnessite MnO₂ presence is also not observed by Raman, since Birnessite has a sharp peak at ~740 cm⁻¹ [39]. Thus, the Raman spectroscopy not only corroborates the XRD result that K addition to Mn₃O₄ facilitates the formation of cryptomelane, but is also more sensitive to smaller amounts of this impurity phase, identifying traces of cryptomelane even at 0.9 wt.% K and before reaction (fresh).

Oxides of iron can be difficult to distinguish via ambient Raman spectroscopy, especially Fe₃O₄ due to its inconsistent reporting of mode position throughout literature and due to the tendency of Fe₃O₄ and surface hydroxides of iron to oxidize to α-Fe₂O₃ via laser exposure [40]. Therefore, Raman spectroscopu is not expected to be useful in this study as a measurement of relative Fe₃O₄ and α-Fe₂O₃ content among the various fresh and spent Fe₃O₄-based samples. Indeed, Figure 11a shows the most intense features of the full Raman spectra of the Fe-based samples are attributed to α-Fe₂O₃, which was not observed in the XRD. A peak centered at 664 cm⁻¹ is typically assigned to the Fe₃O₄ spinel, and the current results are in agreement as this feature is most pronounced in the fresh Fe₃O₄ sample without K. Rather than identify iron-oxide phase changes, the Raman spectrum in the range of 550 to 750 cm⁻¹ (Figure 11b) was employed primarily to confirm the presence of potassium–iron mixed oxides observed in the XRD of fresh and spent Fe₃O₄ samples with K (Figure 11b). While it was difficult to discern additional Raman bands in the fresh samples, the spectra of the spent Fe₃O₄ catalysts with K exhibit several new features. First, the presence of γ-Fe₂O₃ in spent catalysts was confirmed by the features at ~655 and ~724 cm⁻¹ [41]. The spent samples also exhibit features at 636 and 716 cm⁻¹ that cannot be attributed to an iron oxide phase. These peaks are similar to those reported for the A_1g modes of K₂Fe₂₂O₃₄ and the presence of this phase is in agreement with the XRD [39].

2.4.3. X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) was employed to lend further support to the phase assignments made by XRD and Raman, and to aid interpretation of the O₂-TPD and the O₂ release profiles observed during NO decomposition. For Co₃O₄ without K addition, the peak at the lowest binding energy of the Co2p_3/2 spectrum maintained a similar first peak position before and after the reaction (Figure 12, Table S7). Both values were within ±0.3 eV of previously reported position at 779.6 eV, with sufficient peak area indicating surface Co²⁺ remains before and after reaction (Table S8) [42]. With increasing alkali addition (Figure 12a), the peak at low binding energy shifts to even lower energies, obtaining a minimum of 778.6 eV at 3K Co₃O₄. With respect to spent K Co₃O₄ (Figure 12b), a similar decrease in energy of the low binding energy peak is observed with increasing K, but no systematic differences in the trends are observed when comparing the spent and fresh spectra, indicating little change in the surface chemical state of Co occurred during NO decomposition. This finding, in conjunction with the observation that O₂ is detected in the product stream of the reaction at 650 °C (see Figure 5a), is a strong indicator of catalytic NO decomposition over Co₃O₄, i.e., no loss or change of the catalyst phase or surface structure. The slight decrease in binding energy resulting from K addition indicates a different electronic environment around Co than exists in Co₃O₄ without alkali and has been similarly interpreted in the literature [19]. Such a difference in electronic environment undoubtedly affects the ability to perform redox cycles between Co²⁺ and Co³⁺ during NO decomposition and is likely to play a critical role in the catalytic mechanism. Specifically, the release of product O₂ necessary to close the catalytic cycle [21]. Thus, K addition is established as a promoter of the NO decomposition over Co₃O₄.

The position of the peak at the lowest binding energy of the fresh and spent Mn₃O₄ varies from 640.7 to 640.2 eV, however the relative area of this peak (shaded red in Figure 13) remains the same (see numerical data for peaks in Tables S9 and S10). These results show that Mn₃O₄ without K maintains a similar oxidation state through the evaluation of catalytic activity for NO decomposition, corroborating the XRD patterns and Raman spectra that also showed similar fresh and spent results for this catalyst. The addition of K to Mn₃O₄ did not produce any change in the position of the low binding peak regardless of K loading (640.4–640.6 eV), however, it should be noted that the relative area of this peak is decreased upon K addition while the area of the high binding energy peak of the Mn2p_3/2 peak centered at ~643.2 eV (shaded green in Figure 13) is increased in both the fresh and spent samples (see numerical peak areas in Table S10). Based on peak positions and relative areas from previously reported XPS spectra for Mn³⁺ and Mn⁴⁺ containing materials [42], increases in the relative area of the higher binding energy peak is most likely an indication of increased contribution of Mn³⁺ and Mn⁴⁺. As suggested by the XRD and Raman spectroscopy, it is possible the potassium reacts with the Mn in the spinel and forms cryptomelane, which has both Mn³⁺ and Mn⁴⁺. This is particularly true of the spent 2K and 3K Mn₃O₄ samples, which show at least a ~19% higher contribution of the high binding energy peak; this is compared to only a 7% difference in relative area for the spent Mn₃O₄ without K (see Table S10). Thus, the XPS showing peaks related to Mn⁴⁺ [30,36,42] provides additional evidence of an impurity phase, likely cryptomelane as indicated in the XRD and Raman, that occurs as a result of oxidation of K and Mn and its formation is further accelerated in the NO decomposition reaction environment.

In the absence of K, the strongest contribution to the Fe2p_3/2 XPS peak of the Fe₃O₄ catalyst is the lowest binding energy peak centered at 709.8 eV (shaded in red in Figure 12). After reaction, the dominate feature of the Fe2p_3/2 peak shifts to 710.5 eV (shaded blue in Figure 12) in the spent Fe₃O₄ without K. This overall increase binding energy in consistent with transformation of Fe₃O₄ to α-Fe₂O₃ phase (Fe³⁺), which was also observed in the XRD patterns and Raman spectra of spent Fe₃O₄ catalysts (see Figure 8 and Figure 11). The presence α-Fe₂O₃ phase in the spent sample is also consistent with deactivation and inactivity of the same sample due to the inability to release oxygen (see Figure 4, Figure 5 and Figure S1). Upon addition of K, the position and contribution of the low binding energy peak shows some stabilization, especially for 0.9K and 2K Fe₃O₄ (Figure 14 and Tables S11 and S12). The high relative contribution of the lower binding energy feature to the Fe2p_3/2 peak in the K-containing spent Fe₃O₄ catalysts is indicative of the continued presence of Fe²⁺ even after reaction [42], which was also observed in the XRD (see Figure 8). This implies K presence on Fe₃O₄ may have provided some means to promote O₂ release by mitigating the oxidation to α-Fe₂O₃ (see Figure 5c inset and Figure 8). Nonetheless, there is a more significant contribution of the higher binding energy peak in the Fe2p_3/2 peak for all K-containing samples, even prior to reaction, and the presence of this peak coincides with low activity and oxygen release relative to the higher values of K Co₃O₄. Thus, even though some stabilization of the spinel Fe₃O₄ is observed, the γ-Fe₂O₃ and the potassium–iron mixed oxide detected by XRD and Raman likely contribute to the presence of this XPS feature and are concluded to be responsible for inactivity of K Fe₃O₄ catalysts.

When considering the myriad of characterization techniques with respect to the catalytic activity, a self-consistent set of conclusions emerge. The success of K as a promoter of Co₃O₄ is related to three aspects: (1) improved potential for interaction with NO at the catalysts surface (FTIR), (2) consistent and stable release of product O₂ from the catalysts surface (O₂-TPD), and (3) resistance to structural or phase changes as a result of K addition and/or reaction environment (XRD, Raman spectroscopy, XPS). While K improved NO adsorption for Mn₃O₄, it failed as a promoter of NO decomposition because either NO oxidation to NO₂ was dominant (lower temperature) or O₂ release was poor (higher temperature). The poor release of O₂ at high temperature could be linked to the formation of potassium–manganate mixed oxides both before (Raman spectroscopy) and after reaction (XRD, Raman spectroscopy, XPS). Addition of K to Fe₃O₄ improves the NO adsorption and O₂ release, however the O₂ release is at higher temperatures (≥550 °C for K Fe₃O₄ compared to ≥450 °C for K Co₃O₄) and is lower in relative magnitude than the K Co₃O₄ samples. At higher K loadings, some stabilization of the Fe₃O₄ spinel structure is observed preventing the formation of an excess of α-Fe₂O₃ (XRD and XPS), but potassium–iron oxide and γ-Fe₂O₃ impurities form as a result of the K addition (XRD and Raman spectroscopy), so ultimately the promoter effect of K for oxygen release was insufficient to overcome the deficiencies introduced by the impurity formation. Thus, maintaining a structure with labile redox pairs is essential for O₂ release, and this function is critical to maintain catalytic turnovers of the NO decomposition mechanism.

3. Materials and Methods

3.1. Catalyst Synthesis

The K Co₃O₄, K Mn₃O₄, and K Fe₃O₄ samples were synthesized by wet impregnation. Approximately 2 g of Co₃O₄, Mn₃O₄, and Fe₃O₄ (99.5%, 97%, and 99.5%, respectively, Sigma-Aldrich, St. Louis, MO, USA) were, placed in 500 mL beakers. Next, approximately 200 mL of millipure deionized water (18.2 megaohm and 3 ppb total organic carbon) were added to the beakers along with magnetic stirrers. Next, potassium carbonate monohydrate (99%, Sigma Aldrich) was added to the samples to yield loadings of 0.9, 2, and 3 wt.% K. The materials were stirred on hot plates under mild heating (~70 °C) until the water was evaporated. A more thorough drying step was conducted at 120 °C in an oven, and subsequently the materials were ground fine with a mortar and pestle and calcined at 400 °C for 4 h, which is consistent with reported calcination procedures [25]. Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) were collected utilizing a JEOL JSM 7800F (Peobody, MA, USA), and Aztec EDX Analyzer (Oxford Instruments, Concord, MA, USA), and confirmed the empirical K loading of the samples was within ±10% of the nominal K loading (Table S13). The EDX also provided a simple survey of the K distribution to confirm co-location with Co, Mn, or Fe (Figures S2–S4). Brunauer–Emmett–Teller (BET) surface area measurements were conducted utilizing a Micromeritics 3Flex instrument. Initially, 200 mg of each sample was degassed at 200 °C for 2 h, and then nitrogen physisorption was conducted at 76 K, calculating the BET utilizing 11 points between P/P₀ = 0.05–0.3 (Table S14).

3.2. Catalytic Evaluation

Evaluation of NO decomposition catalytic rates was performed using a fixed bed reactor system, (PID/Particulate Systems Effi Microreactor, Norcross, GA, USA) equipped with a quartz tubular reactor with 1 cm diameter. The NO, N₂O, and NO₂ concentrations were monitored by an online Fourier transform infrared detector (CAI 600 SC FTIR California Analytical Instruments, Inc., Orange CA, USA), and O₂ production (m/z = 32) was confirmed using an online Mass Spectrometer (MKS Instruments, Inc. Cirrus-2, Andover, MA, USA). The total flow rate was 27.8 mL/min of a reaction gas composed of approximately 9,700 ppm NO, 30–70 ppm NO₂, and 12–25 ppm N₂O in a ultra high purity (UHP) He balance. The catalyst mass was held constant at 500 mg of each sample, yielding a bed length of ~1 cm and a gas hourly space velocity (GHSV) of ∼2100 h⁻¹. Additional measurements were performed for the K Co₃O₄ samples using significantly less sample mass (80 mg) with 420 mg of quartz sand diluent to maintain constant GHSV in order to obtain differential conversions over these catalyst materials. The catalysts were first pretreated in 10% O₂/He for 60 min at 500 °C and cooled at 10 °C/min in He to the initial reaction temperature of 400 °C. The measurement at 400 °C was conducted for 6 h in an isothermal condition in order to obtain a near steady-state, and afterwards a 15 min He purge/ramp step to the next reaction temperature was performed. Additional reaction measurements were conducted at 450, 550, and 650 °C for two hours.

Steady-state NO conversion is taken as the average conversion over the final 5 min of time on stream. The NO conversions were determined by analyzing NO inlet feed concentration (bypass) and the NO outlet feed concentration after passing through the catalyst bed (reaction). The conversion was calculated using the following equation:

$$NO Conversion (\%)=\frac{\left[\mathrm{NO}\mathrm{bypass}\right]-\left[\mathrm{NO}\mathrm{reaction}\right]}{\left[\mathrm{NO}\mathrm{bypass}\right]}\times 100\%$$

All numerical values of NO conversions are reported in Figure 1 (as well as Tables S3–S6) and all are below 25% in order to maintain differential conditions to calculate and report catalytic activity. The N₂ (ppm) produced during the reaction was calculated by the mass balance of the total nitrogen species, and only N₂ solely produced from NO decomposition, rather than decomposition of the trace N₂O impurity in the feed, was considered in reporting N₂ production as described from the following equation:

$$N_2 Produced \left(ppm\right)=\frac{\left[NO converted\right] – 2\left[N_{2}O produced\left]-\right[N{O}_2 produced\right]}{2}-\left[N2O ppm converted\right]$$

The selectivity to N₂ is calculated as the percentage of NO converted to N₂. Rates for NO decomposition are therefore calculated only from the NO converted to the desired N₂ product using N₂ selectivity and, thus are displayed in units of μmol of NO converted to N₂ per second [(μmol NO to N₂) s⁻¹]. Specific activities [(μmol NO to N₂) g⁻¹ s⁻¹] are calculated by normalization of rates by the catalyst mass. All numerical values of specific activity and product selectivity are reported in Tables S3–S7.

3.3. In Situ FTIR

A Harrick Praying Mantis accessory equipped with a High Temperature Cell with gas flow and temperature control capabilities was used to collect in situ diffuse reflectance FTIR spectra of samples in powder form. A Thermo Scientific Nicolet 8700 Research FTIR Spectrometer with a liquid nitrogen cooled MCT detector was utilized to collect the spectra. Spectra were obtained with a resolution of 4 cm⁻¹ and by averaging 64 scans.

In situ diffuse reflectance FTIR spectra were collected during NO adsorption at 300 °C in a feed of 1% NO/He at 30 mL/min for 20 min with a spectrum obtained every minute in series collection mode. Prior to NO adsorption, the sample was first pretreated in 30 sccm of 10% O₂/He to 400 °C (calcination temperature). The background spectrum (64 scans) was of the catalyst after cooling to 300 °C in 30 sccm UHP He. The spectral intensities were normalized to the NO gas phase peak at ~1875 cm⁻¹ for comparison of relative peak intensity between samples.

3.4. Oxygen Temperature Programmed Desorption

Oxygen temperature-programmed desorption (O₂-TPD) profiles of the samples were collected utilizing a Micromeritics 3flex instrument equipped with a U-shaped quartz tube. Approximately 100 mg of each sample was loaded, pretreated to 500 °C to remove water and adventitious carbon, and cooled in UHP He to 100 °C. Subsequently, a purge utilizing UHP He was conducted for one hour to flush the system to remove physiosorbed oxygen and to obtain a suitable baseline for the mass spectrometer detector (MKS Instruments, Inc., Cirrus-2). Next, the temperature was ramped to 600 °C linearly at 10 °C/min in UHP He, and O₂ release was monitored via the intensity of the signal at m/z = 32.

3.5. X-ray Diffraction

X-ray powder diffraction (XRD) patterns of fresh and spent catalysts were collected with a Rigaku Smartlab (Auburn Hills, MI, USA) with a Cu Kα source over a 2θ range of 10–80° at 0.25 °/min with a step size of 0.01 °. XRD patterns were collected after flattening the powder onto a zero background quartz sample holder. Phase assignments were made using Rigaku PDXL2 Version 2.1.3.6. Crystallite size was determined using the Scherrer equation, holding the shape factor constant at 0.89.

3.6. Raman Spectroscopy

Raman spectra of fresh and spent catalysts were recorded utilizing a Horiba-Jobin Yvon LabRAM HR high-resolution Raman microscope instrument utilizing a 532 nm laser (Laser Quantum Ventus 532 Dedicated Raman laser, diode pumped laser, Kyoto, Japan), a confocal microscope with a 50× objective (Olympus MPlan N 50×, Center Valley, PA, USA) and a back illuminated CCD detector (Horiba-Jobin Yvon Synapse 1024 × 256, Kyoto, Japan). The spectrometer response was calibrated to 520.7 cm⁻¹ utilizing a silica reference with spectral resolution of 2 cm⁻¹ using a 600 grooves/mm grating, and with a holographic notch filter to reject Rayleigh scattering.

3.7. X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) analysis was carried out using a PHI Versaprobe II XPS system (Chanhassen, MN, USA) equipped with an Al Kα source (1486.7 eV) and a multichannel energy analyzer. High resolution spectra were collected using a pass energy of 29.35 eV at 10⁻⁶ Pa. The Co2p (805–760 eV), Mn2p (660–625 eV), and Fe2p (735–690 eV) peak envelopes were collected, and the binding energies were aligned to the signal for adventitious carbon at 284.6 eV (~0.1–0.2 eV error). The resulting peak positions and areas contributing to the Co2p_3/2, Mn2p_3/2, and Fe2p_3/2 peaks were calculated using Multipak (Version 9.3.03). The background was subtracted utilizing an Iterated Shirley method, and the spectral deconvolution was performed utilizing Gauss–Lorentz Function, with χ² < 5 achieved in most cases.

4. Conclusions

The investigation of K promotion of simple first-row transition metal spinels (Co₃O₄, Mn₃O₄, and Fe₃O₄) for NO decomposition was completed in this study. The promoter effect of K on the activity of Co₃O₄ was confirmed, and the 0.9 wt.% K Co₃O₄ catalyst was the most active at 550 and 650 °C. It was revealed that K addition led to increased interaction of the catalysts surface with the reactant NO molecule, stabilization of the Co₃O₄ crystallite size during reaction, and a greater population of unsaturated bonds of CoO₆ octahedra—all of which contribute to the increased reactivity over the unpromoted Co₃O₄. It is also critical that the K addition did not lead to any modification of the spinel phase or formation of any impurity phases during synthesis or after reaction. The addition of K was identified as a universal promoter of NO adsorption as additional NO_x surface species were also observed over Mn₃O₄ and Fe₃O₄, but this aspect alone is insufficient to establish K as an overall promoter for direct NO decomposition. The Mn₃O₄ catalysts were nearly inactive for NO decomposition across all temperatures and K loadings. This poor performance was attributed to an oxidation reaction between Mn and K at higher synthesis or reaction temperature to form a stable and inactive cryptomelane phase. Without K, the Fe₃O₄ phase oxidizes to α-Fe₂O₃ in a chemical interaction with NO, and a product stream lacking in O₂. The K presence on Fe₃O₄ facilitated some stabilization of the Fe₃O₄ phase, but was coupled by formation of γ-Fe₂O₃ and a reaction to form a potassium–iron mixed oxide. While the stabilization of the Fe₃O₄ spinel led to some trace detectable activity at 650 °C, the K Fe₃O₄ samples only released a small amount of O₂ at elevated temperatures.

Thus, this work provides a poignant demonstration of the major challenge in developing NO decomposition catalysts—sustainable release of oxygen from the surface as product O₂ in the presence of NO. While the NO adsorption and dissociation are obvious steps in the NO decomposition mechanism, the current results suggest that improving the interaction of the catalysts surface with the reactant NO molecule via alkali addition is, by itself, insufficient to increase activity. Additionally, development of improved direct NO decomposition catalyst materials remains challenging due to the lack of a consistent rational approach to design. Addition of K was generally well-suited to aid design of improved N₂O decomposition catalyst, but this is not universally true for direct NO decomposition. Nonetheless, it remains of interest to investigate other spinel compositions that might be well-suited to K impregnation, and this study establishes some criteria for the spinel suitability, e.g., it is largely phase stable after reaction with K, does not react with K to form mixed oxides, and releases the O₂ product during NO decomposition. Alternatively, one might consider strategies to improve the Fe₃O₄ spinel properties prior to K addition to improve O₂ release characteristics, possibly by doping the spinel structure directly. These results are, therefore, expected to help guide future design of spinel catalysts for direct NO decomposition, and it is expected that these strategies can be extended to other oxide catalyst systems such as perovskites.