Selective Catalytic Oxidation Of NH3 Research Articles

Enhancing acidity of Ru/Ce/ZrO2 bifunctional catalysts for promoting N2 selectivity in selective catalytic oxidation of NH3

To solve the problem of low N2 selectivity of Ru/Ce/ZrO2 (R/C/Z) catalysts in NH3 selective catalytic oxidation (NH3-SCO) reaction, herein ZrO2 supports were treated with sulfuric acid and its effects on NH3-SCO performance were investigated systematically. The results showed that Ru/Ce/ZrO2-10S (R/C/Z-10S) catalyst, in which the mass ratio of sulfuric acid to ZrO2 was 10 %, exhibited N2 selectivity of 95.7 % at 350 °C, that was significantly higher than R/C/Z catalyst (57.8 %). NH3-TPD tests indicated that sulfate treatment of ZrO2 supports in R/C/Z catalysts led to more abundant acidic sites on catalyst surface, which was in favor of enhancing NH3 adsorption capacity remarkably. But when mass ratio of sulfuric acid to ZrO2 increased up to 20 %, an aggregation of metal oxides on the surface of R/C/Z-20S catalyst could be observed clearly, which might impose an adverse impact on NH3-SCO performance. In-situ DRIFTS results revealed that, compared to R/C/Z catalyst, there were much more Brönsted acid sites on the surface of R/C/Z-10S catalyst, and the corresponding NH3 species on acidic sites could be consumed quickly in intermediate reactions. More importantly, amide (-NH2) species rather than imide (-NH) and nitrosyl (-HNO) species, were the majority on the surface of R/C/Z-10S catalyst, implying that there might be a suppressing effect on further dehydrogenation of -NH2 species into -NH species owning to sulfate treatment of ZrO2 supports in R/C/Z catalysts. Therefore, internal selective catalytic reduction (i-SCR) mechanism rather than -NH mechanism played a vital role in NH3-SCO reaction over R/C/Z-10S catalyst.

Fe2O3/γ-Al2O3 and NiO/γ-Al2O3 catalysts for the selective catalytic oxidation of ammonia

Ammonia, a significant atmospheric pollutant, requires effective emission control due to its inherent toxicity and the generation of secondary pollutants like particulate matter. This control can be achieved through various methods, including catalytic processes. Therefore, our study focuses on evaluating the potential of catalysts based on iron oxide and nickel oxide supported on γ–Al2O3 for the selective catalytic oxidation of NH3 to N2 (NH3-SCO). The γ–Al2O3 was obtained by thermal decomposition of aluminum hydroxide, and 5 or 10 wt% of Fe or Ni was added through wetness incipient impregnation. XRD diffractograms confirmed the formation of the γ–Al2O3 phase. XRD, H2-TPR, and UV–vis DRS data showed the presence of Fe2O3, NiO, and NiAl2O4 in the catalysts. Introducing metal oxides onto the support led to a drop in the specific area, pore size, pore volume, and NH3 desorption, which was higher for the catalysts containing Fe. The catalysts were active in NH3-SCO, and the insertion of Fe or Ni was essential because it promoted a significant increase in the NH3 conversion (∼75 % Fe and ∼55 % Ni), compared to pure support (∼8 %), mainly from 400 °C. However, doubling the metal content has not resulted in a considerable increase in NH3 conversion. The N2 selectivity was higher for the catalysts containing Ni (∼85 %) from 400 °C compared to catalysts containing Fe (∼76 %). Such behavior was due to the larger surface area of the Ni-containing catalysts. Despite that, the 5Fe/γ–Al2O3 catalyst emerged as the most effective option for NH3-SCO applications, combining higher NH3 conversion and good N2 selectivity.

Design Strategies for High-Performance NH3-SCO Catalysts: Identifying and Modulating Direct Anchoring Sites for Ag on TiO2.

Ammonia (NH3) slip from diesel vehicle aftertreatment systems and internal combustion engines fueled by NH3 or NH3/H2 poses serious environmental problems. Ag-based catalysts are widely used for the selective catalytic oxidation of NH3 to N2 (NH3-SCO), and their performance is greatly dependent on the state of Ag, which is influenced by the anchoring sites on the support. Despite efforts to identify the direct anchoring sites of metal atoms on TiO2, conflicting views persist. Here, we compared the correlation between Ag dispersion and the content of hydroxyl (OH) groups or defects on TiO2 and conducted density functional theory (DFT) calculations, and the results confirmed that the surface OH groups of TiO2 serve as the direct anchoring sites for Ag. By modulating the OH group content through thermal induction, the optimal OH group content on TiO2-800 resulted in more metallic Ag nanoparticles (Ag0 NPs) in larger sizes, leading to the development of an excellent NH3-SCO catalyst. Moreover, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), kinetic studies, and DFT calculations suggested that more Ag0 NPs in larger sizes on 10Ag/TiO2-800 were conducive to O2 activation and NH3 dissociation. Our findings provide new insights for designing efficient NH3-SCO catalysts, and OH groups as direct anchoring sites could be extended to other metals and supports for the rational design of catalysts.

Thermally stable high-loading single Cu sites on ZSM-5 for selective catalytic oxidation of NH3

Rigorous comparisons between single site- and nanoparticle (NP)-dispersed catalysts featuring the same composition, in terms of activity, selectivity, and reaction mechanism, are limited. This limitation is partly due to the tendency of single metal atoms to sinter into aggregated NPs at high loadings and elevated temperatures, driven by a decrease in metal surface free energy. Here, we have developed a unique two-step method for the synthesis of single Cu sites on ZSM-5 (termed CuS/ZSM-5) with high thermal stability. The atomic-level dispersion of single Cu sites was confirmed through scanning transmission electron microscopy, X-ray absorption fine structure (XAFS), and electron paramagnetic resonance spectroscopy. The CuS/ZSM-5 catalyst was compared to a CuO NP-based catalyst (termed CuN/ZSM-5) in the oxidation of NH3 to N2, with the former exhibiting superior activity and selectivity. Furthermore, operando XAFS and diffuse reflectance infrared Fourier transform spectroscopy studies were conducted to simultaneously assess the fate of the Cu and the surface adsorbates, providing a comprehensive understanding of the mechanism of the two catalysts. The study shows that the facile redox behavior exhibited by single Cu sites correlates with the enhanced activity observed for the CuS/ZSM-5 catalyst.

Pt/Al2O3@Ce/ZrO2-S bifunctional catalysts prepared by mechanically milling for selective catalytic oxidation of high-concentration ammonia.

The selective catalytic oxidation (SCO) is an effective method for removing slipped high-concentration ammonia from NH3-fueled engine exhaust gas. Herein a novel bifunctional catalyst was synthesized by mechanically mixing sulfated Ce/ZrO2 (Ce/ZrO2-S) with a small fraction of Pt/Al2O3 (Pt 0.1 wt.%) for SCO of NH3. As expected, the introduction of a small amount of Pt/Al2O3 significantly improved NH3 conversion ability of Ce/ZrO2-S catalysts toward low-temperature direction. When the mass ratio of Pt/Al2O3 to Ce/ZrO2-S was 7.5% (the corresponding mixed catalyst was denoted as P@CZS-7.5), T90 temperature was 312°C. More importantly, P@CZS-7.5 catalyst exhibited a much better N2 selectivity (> 96%) in a wide temperature range (320 ~ 450°C). H2-TPR results revealed that the addition of a trace amount of Pt/Al2O3 significantly led to a distinct shift of reduction temperature peak toward low-temperature direction, thereby greatly improved the low-temperature redox performance of mixed catalysts. Furthermore, NH3-TPD and BET results showed that P@CZS-7.5 catalyst exhibited a similar NH3 adsorption capacity to Ce/ZrO2-S catalyst, while the former had a relatively higher specific surface area than the latter. It was considered as a crucial factor for P@CZS-7.5 catalyst maintaining superior N2 selectivity in high-concentration NH3 (5000ppm) removal processes. In situ DRIFTS results indicated that P@CZS-7.5 catalyst followed the internal selective catalytic reduction (i-SCR) mechanism in NH3-SCO reactions.

Designing ammonia oxidation catalysts via physical mixing of Pt/SiO2 and Cu/ZSM-5

Ammonia (NH3) is emerging as a carbon-neutral fuel alternative; however, its use poses environmental and health risks due to NH3 slip. The NH3-selective catalytic oxidation (NH3-SCO) process, which converts NH3 into N2 and H2O, offers a solution. However, it faces challenges with respect to the formation of byproducts, particularly N2O and NOx. This study focuses on designing NH3-SCO catalysts to enhance low temperature NH3 conversion efficiency while minimizing undesired byproducts. Pt/SiO2 catalysts were synthesized with adjusted Pt particle sizes to improve the NH3-SCO activity and were physically mixed with Cu/ZSM-5 to overcome the low N2 selectivity of Pt-based catalysts by utilizing the selective catalytic reduction (SCR) ability of Cu/ZSM-5. Pt1Cu3 catalyst, a physical mixture of Pt/SiO2 and Cu/ZSM-5 in a 1:3 mass ratio, was found to be the most effective, achieving complete NH3 oxidation at 250 ℃ while maintaining a high N2 selectivity of approximately 80%. Further investigation revealed that the performance of the Pt1Cu3 catalyst was significantly influenced by the arrangement and physical proximity of the Pt/SiO2 and Cu/ZSM-5 catalysts, with a closely contacted mixture showing synergistic effects that enhanced activity and selectivity. These results suggest that the Pt1Cu3 catalyst offers a promising solution for NH3 emission control by utilizing a balanced approach that combines SCO and SCR reactions.

Modulating the anchoring states of Ag on TiO2 by SiO2 to boost the NH3-SCO activity over Ag-based catalysts

Ag-based catalysts show high activity in NH3-selective catalytic oxidation (NH3-SCO). The anchoring states of Ag species determines its valence and dispersion, thereby influencing the performance of Ag-based catalysts. Herein, we elucidated that the content and anchoring strength of the hydroxyl (OH) groups on support together affect the anchoring states of Ag. TiO2 has few OH groups but high anchoring strength, the anchored Ag is in a highly dispersed oxidized state, while the unanchored Ag is aggregated in a metallic state (Ag0), so it presents an unevenly sized dispersion and mainly in oxidized state. SiO2 has abundant OH groups but weak anchoring, and can excessively agglomerate into large Ag0 nanoparticles (Ag0 NPs). Therefore, the introduction of SiO2 can regulate the anchoring states of Ag on the TiO2 support, making Ag presents highly dispersed Ag0 NPs, thus following a reaction mechanism with lower energy barriers and significantly improving the NH3-SCO performance.

Insight into hydroxyl groups in anchoring Ir single–atoms on vacancy–deficient rutile TiO2 supports for selective catalytic oxidation of ammonia

High–performance catalysts are extremely required for controlling NH3 emission via selective catalytic oxidation (SCO), and the anchoring structural feature of active sites is a key prerequisite for developing them. This study confirms the importance of hydroxyl groups on vacancy–deficient reducible oxides as active groups. On the one hand, spontaneous atomic dispersion of active metal Ir is promoted by the abundant terminal hydroxyl groups. On the other hand, Ir cations anchor on the TiO2 surface through exchange with H+ in Ti–OH groups, and thus occupy the Brönsted acid sites. The adsorption strength of NH3 is another key factor affecting the reaction rate–determining step, namely NH3 dehydrogenation, which occurs at a faster rate in the coordinated L–NH3 rather than the ionic B–NH4+. Meanwhile, the coordinated L–NH3 significantly avoids the competitive adsorption of water vapor in the NH3–SCO reaction by reducing the number of hydrogen bonding. The TOF of preferred 0.8Ir/TiO2 sample is significantly higher than 0.2Ir/TiO2 sample, although Ir is almost always atomic dispersed. Finally, NH3 conversion is 85% in a wet circumstance (5% H2O) at 240 °C (GHSV = 85 000 h–1), with a N2 selectivity of up to 65% on 0.8Ir/TiO2 sample.

Reasonable design of Sm-modified Cu-based catalyst for NH3-SCO: Role of the amide intermediates

The selective catalytic oxidation of ammonia (NH3-SCO) is currently the most effective method used to eliminate NH3. However, one of the major challenges for NH3-SCO is the development of catalyst capable of completely converting NH3 into harmless N2 and water vapor. In this paper, a high N2 selective catalyst prepared by the sol-gel method using TiO2 as the support, Cu as the active species, and Sm as the auxiliary agent is presented. Compared to traditional Cu/TiO2 catalyst, the 4SmCu/TiO2 catalyst has higher catalytic activity and N2 selectivity at low temperatures. The NH3 conversion can reach 100%, and the selectivity of N2 can be maintained at 100% at 275 °C. The excellent catalytic activity is attributed to the highly dispersed active species, abundant Lewis acid sites (LASs), and the generation of large amounts of surface-adsorbed oxygen. In addition, doping Sm species will cause TiO2 lattice distortion, and at the same time load the distorted TiO2 with more active material. Moreover, in-situ DRIFTS analysis suggests that both the 4SmCu/TiO2 and Cu/TiO2 catalysts follow the "internal" selective catalytic reduction (iSCR) mechanism during NH3-SCO reactions. The 4SmCu/TiO2 catalyst generates more amides (-NH2), which reduces the non-selective oxidation of the catalysts and promotes the formation of N2. This provides a new idea and method for the selective catalytic oxidation of NH3 to N2 and water vapor using Cu-based catalysts.

The enhancement effect of Cu modification on N2 selectivity of V0.5/Pt0.04/TiO2 catalyst for selective catalytic oxidation of NH3

BackgroundCurrently selective catalytic oxidation (SCO) is considered as an effective approach to remove slipped ammonia from exhaust gas of NH3-fuel engines. But it is still challenging to achieve high NH3 conversion and N2 selectivity simultaneously. MethodsCu was introduced to modify V0.5/Pt0.04/TiO2 catalysts and the effects of Cu addition step and amounts on NH3 removal performance of the composite catalysts were investigated. Significant findingsCompared to V0.5/Pt0.04/TiO2 catalyst, Cu0.5V0.5/Pt0.04/TiO2 catalyst exhibited a similar NH3 conversion rate but a much better N2 selectivity. When reaction temperature was higher than 225 °C, a complete NH3 removal could be achieved for Cu0.5V0.5/Pt0.04/TiO2 catalyst, while N2 selectivity was higher than 80 % in a wide temperature range of 225–375 °C. The introduction of Cu resulted in the formation of CuVOx species with strong catalytic oxidation activity due to double redox couples of V5+/V4+ and Cu2+/Cu1+. There was a synergistic effect between CuVOx species and Pt species, which significantly facilitated i-SCR reactions, thus improving N2 selectivity. As main intermediates, -NH2 species actively participated in NH3-SCO reactions over Cu0.5V0.5/Pt0.04/TiO2 catalyst.

High throughput screening of noble Metal-Free High-Entropy alloys catalysts for selective catalytic oxidation of NH3

The high price and scarcity of Pt require the development of alternative low-cost catalysts for selective catalytic oxidation of ammonia (NH3-SCO). This work uses first-principles-based high-throughput calculations to evaluate noble metal-free high-entropy alloys (NMF-HEAs) as potential NH3-SCO candidate catalysts to replace Pt. Firstly, by employing empirical rules to determine the formation probability of single-phase solid solution, 350 NMF-HEAs are found to be single-phase solid solution among the vast configuration space (about 104) consisting of 20 non-noble metals. Among these, NMF-HEAs containing Cu and Zn elements are identified as promising catalysts based on the reactivity and selectivity rules of the NH3-SCO process. Further analysis of elemental composition and distribution on configurations with N-atom adsorption energies reveals that the co-existence of Cu and Zn elements and the symmetric distribution of surface atoms is conducive to tuning N-binding ability in the optimal region. Finally, full reaction paths of NH3-SCO processes on the optimal CuZnNiMoCo HEA are calculated and confirm their activity comparable to that of Pt catalysts and better selectivity. This study gains insight into the effects of the composition and surface configuration of HEAs on the NH3-SCO process and provides a theoretical framework for designing and optimizing efficient catalysts of the NH3-SCO process.

Enhancement of acidic sites in layered MnO2 for the highly efficient selective catalytic oxidation of gaseous ammonia

Ammonia (NH3), as a typical harmful gaseous pollutant in atmosphere, poses a serious threat to human health and ecological environment. It is of great significance to develop the cost-effective catalysts with both excellent selective catalytic oxidation (SCO) activity and N2 selectivity. In this study, the acidic sites of layered MnO2 were regulated and its NH3-SCO performance was investigated. The enhancement of acidic sites in layered MnO2 significantly improved the SCO of NH3, and the resulting MnO2 could achieve complete conversion of NH3 at as lower as 100 °C, even better than the reported noble-metal catalysts. Furthermore, the improvement in NH3-SCO activity can be attributed to the abundant active oxygen species and enhancement of acidic sites (especially stronger Brønsted acidic sites). On the one hand, more active oxygen species can enhance the catalytic oxidation of NH3. On the other hand, Brønsted acidic sites acted not only as adsorption sites but also reaction sites, the increase in the content and strength of acidic sites facilitated the adsorption and activation of NH3. This research reveals the functional role of acidic sites in NH3-SCO, and provides a suggestive for the rational design of catalysts with excellent N2 selectivity and low-temperature NH3-SCO activity.

Elimination of NH3 by Interfacial Charge Transfer over the Ag/CeSnOx Tandem Catalyst

Selective catalytic oxidation of NH3 is the most promising method for removing low-concentration NH3. However, achieving high activity and N2 selectivity remains a great challenge. A Ag/CeSnOx tandem catalyst with dual active centers was designed and synthesized, which couples the NH3 over-oxidation on the noble metal active sites with NOx reduction on the support. The tandem catalyst exhibited excellent NH3 selective catalytic oxidation (NH3–SCO) performance at 200–400 °C. Based on various characterization techniques and DFT calculations, it was identified that the silver species on the Ag/CeSnOx catalysts existed as AgO nanoparticles (AgO NPs), and the electrons on the support were more easily transferred to AgO NPs, which promoted the oxidation activity of AgO and the reduction performance of the CeSnOx support. The coupling between the AgO NPs and CeSnOx helped balance the NH3 oxidation rate and the NOx reduction rate. In addition, the uniform adsorption of gaseous NH3 on the oxidation and reduction sites was also demonstrated by theoretical calculations, which is a prerequisite for tandem catalysis. By in situ DRIFTS, we revealed that the NH3–SCO reaction over Ag/CeSnOx catalysts mainly follows the internal selective catalytic reduction mechanism. It was characterized by excessive oxidation of NH3 to NOx on AgO NPs. At a temperature lower than 200 °C, NOx was reduced to N2 by the adsorbed NH3 on the AgO. When the temperature was higher than 200 °C, NOx was reduced to N2 by NH3 or NH4+ adsorbed on the CeSnOx support. Therefore, the charge transfer at the Ag/CeSnOx catalyst interface and the coordination of atomic scale catalytic sites have realized the conversion of NH3 to N2 through NOx in a tandem catalytic mode.

Multifunctional interfaces for multiple uses: Tin(II)-hydroxyapatite for reductive adsorption of Cr(VI) and its upcycling into catalyst for air protection reactions

Evidence collected to date by our group has demonstrated that tin(II)-functionalized hydroxyapatites (Sn/HAP) are a newly discovered class of ecofriendly reductive adsorbents for Cr(VI) removal from wastewaters.In this work an upgraded series of Sn/HAP materials assured a maximum removal capacity of ≈ 20 mgCr/g, doubling the previously reported value for Sn/HAP materials, thanks to higher Sn-dispersion as proved by X-ray photoelectron spectroscopy and electron microscopy.Insights on kinetics and thermodynamics of the reductive adsorption process are provided and the influence of pH, dosage, and nature of Cr(VI) precursors on chromium removal performances have been investigated. Pseudo-second-order kinetics described the interfacial reductive adsorption process on Sn/HAP, characterized by low activation energy (21 kJ mol−1), when measured in the 278–318 K range.Tests performed in the 2–6 pH interval showed similar efficiency in terms of Cr(VI) removal.Conventional procedures of recycling and regeneration resulted ineffective in restoring the pristine performances of the samples due to surface presence of both Sn(IV) and Cr(III). To overcome these weaknesses, the used samples (Sn + Cr/HAP) were upcycled into catalysts in a circular economy perspective.Used samples were tested as catalysts in gas-phase catalytic processes for air pollution remediation: selective catalytic reduction of NOx (NH3-SCR), NH3 selective catalytic Oxidation (NH3-SCO), and selective catalytic oxidation of methane to CO2. Catalytic tests enlightened the interesting activity of the upcycled Sn + Cr/HAP samples in catalytic oxidation processes, being able to selectively oxidize methane to CO2 at relatively low temperature.

The Unique CO Activation Effects for Boosting NH3 Selective Catalytic Oxidation over CuOx-CeO2.

Slip NH3 is a priority pollutant of concern to be removed in various flue gases with NOx and CO after denitrification using NH3-SCR or NH3-SNCR, and the simultaneous catalytic removal of NH3 and CO has become one of the new topics in the deep treatment of such flue gases. Synergistic catalytic oxidation of CO and NH3 appears to be a promising method but still has many challenges. Due to the competition for active oxidizing species, CO was supposed to hinder the NH3 selective catalytic oxidation (NH3-SCO). However, it is first found that CO could significantly promote NH3-SCO over the CuOx-CeO2 catalyst. The NH3 conversion rates increased linearly with CO concentrations in the range of 180-300 °C. Specifically, it accelerated by 2.8 times with 10,000 ppm CO inflow at 220 °C. Mechanism studies found that the Cu-O-Ce solid solution was more active for CO oxidation, while the CuOx species facilitated the NH3 dehydrogenation and mitigated the competition of NH3 and CO, further stabilizing the promotion effects. Gaseous CO boosted the generation of active isolated oxygen atoms (Oi) by actuating the Cu+/Cu2+ redox cycle. The enriched Oi facilitated oxidation of NH3 to NO and was conducive to the NH3-SCO via the i-SCR approach. This study tapped the potential of CO for promoting simultaneous catalytic oxidation of coexisting pollutants in the flue gas.

Ce–Ti catalysts modified with copper and vanadium to effectively remove slip NH3 and NOx from coal-fired plants

In this work, V/Ce–Ti catalysts were modified with different kinds of transition metals (Cu, Fe, Co, Mn) by sol–gel and impregnation methods. The NH3 oxidation performance of them was tested to select the most active catalyst in NH3-selective catalytic oxidation (NH3–SCO). The effect of NO, SO2 and H2O was also investigated. The experimental results indicate that 1% Cu–V/Ce–Ti catalyst exhibits the most significant ability to remove slip ammonia discharged from coal-fired plants and its NH3 conversion efficiency reaches 90% at 300 °C. In addition, 97% NOx can be removed when NO is introduced in the gas. Cu–V/Ce–Ti catalyst also obtains good resistance to H2O and SO2. Based on the characterization experiment, the introduced Cu and V are highly dispersed on Ce–Ti catalyst and they can increase the redox properties and the number of acidic sites. Besides, the redox cycles among Cu, V and Ce species on Cu–V/Ce–Ti catalyst surface are conducive to generating more active oxygen and promoting the oxidation capacity of the catalyst.

Modulate the superficial structure of La2Ce2O7 catalyst with anchoring CuO species for the selective catalytic oxidation of NH3

• The abundant intrinsic defects in La 2 Ce 2 O 7 pyrochlore are beneficial to NH 3 -SCO reaction. • Electrostatic adsorption improves the dispersion and stability of anchoring cuo x species. • The mass-transfer efficiency of surface free o is promoted by the [Ce-O-Cu] connection. • The anchoring cuo x species enhance the low-temperature activity of La 2 Ce 2 O 7 catalytic. Air contamination caused by the ammonia slip phenomenon has gradually captured the researcher's extensive attention. An effective strategy for controlling fugitive NH 3 is critical to improving the air quality and living environment. In the present work, CuO x /La 2 Ce 2 O 7 composite as a potential candidate catalyst is synthesized through the electrostatic adsorption method for the selective catalytic oxidation (SCO) of NH 3 to N 2 . The 5% CuO x /La 2 Ce 2 O 7 exhibits the best catalytic activity ( T 90 =243 ℃) and ammonia conversion efficiency. The improvement of performance is mainly attributed to the superficial connection of [Ce-O-Cu], which enhances the capturing ability of ammonia molecule and accelerates the dissociating efficiency of N H bonding for N 2 evolution, simultaneously. This work provides a facile method to synthesis pyrochlore-like composite catalyst of NH 3 -SCO for solving the problem of ammonia slip pollution in the future.

Deterioration mechanism of selective catalytic oxidation of NH3 to N2 over CeZrO4 by incorporation of alkali metals and calcium

Alkali metals and calcium were added to CeZrO4 mixed oxide by wet impregnation. The CeZrO4 catalyst poisoned by alkali metals and calcium could inhibit the SCO performance. In particular, Na poisoned catalyst showed the lowest NH3 conversion. The structure analysis proved that this poisoning of metal ions could weaken the interaction between cerium and zirconium, particularly K and Na ions. Moreover, the low surface Brønsted acidity might directly reduce the NH3 oxidation performance. The reaction pathway by in situ DRIFTS results at low temperatures (<350 °C) over poisoned catalyst was different from the temperatures above 350 °C, which was followed the -NH and i-SCR mechanism, respectively.

Single–atom Ir1 supported on rutile TiO2 for excellent selective catalytic oxidation of ammonia

Gaseous ammonia (NH3) in the atmosphere is potentially harmful to both human health and the environment. The selective catalytic oxidation of NH3 (termed as NH3–SCO) into N2 and H2O is a promising method for decreasing NH3 emissions. A highly efficient catalyst is required for controlling NH3 emissions by this method in practice. In this study, we prepared Ir/TiO2 catalysts using different crystal structures of TiO2 (rutile, P25 or anatase) as supports by a simple impregnation method and evaluated their performance in the NH3–SCO. We found that the Ir/TiO2–R (rutile) catalyst performed better than the Ir/TiO2–P25 (mixed–phase) and Ir/TiO2–A (anatase) catalyst. High–angle annular dark–field images of the aberration–corrected scanning transmission electron microscopy revealed that the Ir species were mainly atomically dispersed on the TiO2 support in Ir/TiO2–R with 1 wt% Ir loading, whereas the Ir species agglomerated to form clusters or nanoparticles in Ir/TiO2–P25 and Ir/TiO2–A. The combined results of X–ray absorption fine structure, H2–temperature–programmed reduction, and in situ diffuse reflectance for infrared Fourier Transform spectroscopy studies suggested that atomically dispersed Ir species had stronger electronic metal–support interaction with rutile TiO2, which resulted in easier to adsorb and activate O2 at the interface and thus, better low–temperature activity of the Ir/TiO2–R catalyst.

Effects of IrO2 nanoparticle sizes on Ir/Al2O3 catalysts for the selective catalytic oxidation of ammonia

Gaseous ammonia (NH3), especially that emitted from selective catalytic reduction with NH3 (NH3–SCR) units in the atmosphere is potentially harmful to both human health and the environment. The noble metal catalytic oxidation technology placed behind the SCR unit has been considered to be a promising method for decreasing NH3 slip. In this study, we prepared Ir/Al2O3 catalysts using two different crystal structures of Al2O3 (α–Al2O3 or γ–Al2O3) as supported by an impregnation method and evaluated the performance of the selective catalytic oxidation of NH3 (NH3–SCO). We found that the Ir/γ–Al2O3 catalyst performed better than the Ir/α–Al2O3 catalysts, with 99% of NH3 converted at 260 °C. The X–ray absorption fine structure revealed that the active species on the Ir/Al2O3 catalysts are mainly oxidized IrO2. High–angle annular dark–field images of aberration–corrected scanning transmission electron microscopy revealed that IrO2 species over 1 nm in diameter are beneficial for activity. In situ diffuse reflectance for infrared Fourier transform spectroscopy studies suggested that the differences in NH3–SCO activity were caused by the activity of reactive oxygen species, and the formation of –NH2 may be the decisive step of the NH3 oxidation reaction.