Catalysts Al-rich Cu-SSZ-39 and Cu-SSZ-13 are applied in ammonia selective catalytic reduction (NH3-SCR) reaction for NOx removal. Al-rich Cu-SSZ-39 maintained its excellent NH3-SCR activity even after hydrothermal aging at 850 ℃ for 16 h, whereas Al-rich Cu-SSZ-13 lost its deNOx activities completely when subjected to the same conditions. The main reason is that the active Cu2+ species are more stable in the structure of Cu-SSZ-39, because SSZ-39 zeolite contained more paired framework Al, leading to that Cu-SSZ-39 catalysts have more hydrothermally stable Cu2+-2Z species as compared to Cu-SSZ-13, and this makes Cu-SSZ-39 accumulate less CuOx during hydrothermal aging. In addition, the framework Al in SSZ-39 is more stable than that in SSZ-13, and a great amount of Cu2+-2Z species coordinated with paired framework Al in Cu-SSZ-39, which leaves fewer residual uncoordinated Al sites. Moreover, SSZ-39 has a more tortuous channel structure than SSZ-13 does. This inhibits the detached Al(OH)3 from exiting the pores of the AEI zeolite framework, which would result in reincorporation of Al(OH)3 into the framework when cooling down. Therefore, the Al-rich Cu-SSZ-39 with large amounts of stable paired Al shows a competitive potential candidate as an NH3-SCR catalyst for NOx elimination in practical applications.

A comparative study of the activity and hydrothermal stability of Al-rich Cu-SSZ-39 and Cu-SSZ-13

Platinum (Pt)-based-nanomaterials are currently the most successful catalysts for the oxygen reduction reaction (ORR) in electrochemical energy conversion devices such as fuel cells and metal-air batteries. Nonetheless, Pt catalysts have serious drawbacks, including low abundance in nature, sluggish kinetics, and very high costs, which limit their practical applications. Herein, we report the first rationally designed nonprecious Co-Cu bimetallic metal-organic framework (MOF) using a low-temperature hydrothermal method that outperforms the electrocatalytic activity of Pt/C for ORR in alkaline environments. The MOF catalyst surpassed the ORR performance of Pt/C, exhibiting an onset potential of 1.06 V vs RHE, a half-wave potential of 0.95 V vs RHE, and a higher electrochemical stability (ΔE1/2 = 30 mV) after 1000 ORR cycles in 0.1 M NaOH. Additionally, it outperformed Pt/C in terms of power density and cyclability in zinc-air batteries. This outstanding behavior was attributed to the unique electronic synergy of the Co-Cu bimetallic centers in the MOF network, which was revealed by XPS and PDOS.

Co-Cu Bimetallic Metal Organic Framework Catalyst Outperforms the Pt/C Benchmark for Oxygen Reduction.

Zeolites, as efficient and stable catalysts, are widely used in the environmental catalysis field. Typically, Cu-SSZ-13 with small-pore structure shows excellent catalytic activity for selective catalytic reduction of NO x with ammonia (NH3-SCR) as well as high hydrothermal stability. This review summarizes major advances in Cu-SSZ-13 applied to the NH3-SCR reaction, including the state of copper species, standard and fast SCR reaction mechanism, hydrothermal deactivation mechanism, poisoning resistance and synthetic methodology. The review gives a valuable summary of new insights into the matching between SCR catalyst design principles and the characteristics of Cu2+-exchanged zeolitic catalysts, highlighting the significant opportunity presented by zeolite-based catalysts. Principles for designing zeolites with excellent NH3-SCR performance and hydrothermal stability are proposed. On the basis of these principles, more hydrothermally stable Cu-AEI and Cu-LTA zeolites are elaborated as well as other alternative zeolites applied to NH3-SCR. Finally, we call attention to the challenges facing Cu-based small-pore zeolites that still need to be addressed.

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Selective catalytic reduction of NOx with NH3: opportunities and challenges of Cu-based small-pore zeolites

The development of high-efficiency catalysts is the key to the low-temperature NH3-SCR technology. The introduction of SO2 and H2O will lead to poisoning and deactivation of the catalysts, which severely limits the development and application of NH3-SCR technology. This review introduces the necessity of NOx removal, explains the mechanisms of H2O and SO2 poisoning on NH3-SCR catalysts, highlights the Mn-based catalysts of different active metals and supports and their resistance to H2O and SO2, and analyses the relationship between metal modification, selection of support and preparation method, morphology and structure design and SO2/H2O resistance. Given the current problems, this review points out the future research focus of Mn-based catalysts and also puts forward corresponding countermeasures to solve the existing problems.

A review of Mn-based catalysts for low-temperature NH3-SCR: NOx removal and H2O/SO2 resistance.

The origin of the beneficial effects of post-treatment processing with HNO3 and NH4NO3 solutions on the catalytic performance of copper-amine templated zeolites used in the NH3-SCR reaction was investigated. By careful design and optimization of the dual-treatment procedure (HNO3-NH4NO3), Cu-SSZ-13 catalysts were obtained using Cu-TEPA as a template that exhibited NOx conversion above 90% in a wide temperature window from 250 to 450 ℃ with GHSV of ∼ 400,000 h−1, even after hydrothermal aging at 750 and 800 ℃. The results of XRD and H2-TPR indicated that the HNO3 post-treatment adjusts the zeolite crystallinity and optimizes the copper species distribution in Cu-SSZ-13 catalysts. Further treatment with NH4NO3 reduces the Cu/Al ratios effectively and avoids the accumulation of Cu2+ ions to form CuOx clusters during hydrothermal aging. We investigated the influence of Cu2+ on the hydrothermal stability of Cu-SSZ-13, and two opposing effects were found. Cu2+ ions inhibit dealumination of the SSZ-13 zeolite structure, while excessive quantities of them easily accumulate to form CuOx clusters, leading to collapse of long-range order in the zeolite structure during hydrothermal aging. Before the zeolite structure collapses, Cu-SSZ-13 catalysts with high Cu loading exhibit higher NH3-SCR activity due to the preservation of more active Cu2+. However, Cu-SSZ-13 catalysts totally lose deNOx activity in NH3-SCR once the zeolite structure collapses. To guarantee both the activity for NOx reduction and hydrothermal tolerance, the optimal Cu loading (Cu/Al = 0.22∼0.31, here) and structural stability of in-situ synthesized Cu-SSZ-13 should be carefully tuned by HNO3 and NH4NO3 treatment.

Precise control of post-treatment significantly increases hydrothermal stability of in-situ synthesized cu-zeolites for NH3-SCR reaction

Fresh and hydrothermally aged Cu-SSZ-13 catalysts were tested for the standard and fast selective catalytic reduction of NOx with NH3 (NH3-SCR). Interestingly, NO2 exhibited distinctly different effects on these two catalysts. NOx conversion over fresh Cu-SSZ-13 was clearly inhibited by NO2, while hydrothermal aging could reverse this inhibition effect. It was found that the NOx conversion decreased with progressive hydrothermal aging under standard SCR (SSCR) conditions but increased under fast SCR (FSCR) conditions. The N2 adsorption/desorption isotherms indicated the formation of mesopores induced by hydrothermal aging, which is favorable for gas diffusion. Temperature programmed desorption (TPD) and in situ DRIFTS, combined with other auxiliary measurements, demonstrated that L -NH3 (NH3 adsorbed onto Lewis acid sites) is more active toward both NO+O2 and NO2 for the formation of N2, while B-NH3 (NH3 adsorbed onto Bronsted acid sites) is mainly responsible for the formation of NH4NO3. The amount and stability of NH4NO3 formed under FSCR conditions decreased with progressive hydrothermal aging due to the loss of Bronsted acid sites.

Jinpeng Du

Papers

A comparative study of the activity and hydrothermal stability of Al-rich Cu-SSZ-39 and Cu-SSZ-13

Selective catalytic reduction of NOx with NH3: opportunities and challenges of Cu-based small-pore zeolites

Precise control of post-treatment significantly increases hydrothermal stability of in-situ synthesized cu-zeolites for NH3-SCR reaction

Hydrothermal aging alleviates the inhibition effects of NO2 on Cu-SSZ-13 for NH3-SCR

Effects of alkali and alkaline earth metals on Cu-SSZ-39 catalyst for the selective catalytic reduction of NOx with NH3