Spectroscopic and chemical characterization of active and inactive Cu species in NO decomposition catalysts based on Cu-ZSM5

Abstract

The number and type of Cu 2+ species present on O 2 -treated Cu-ZSM5 catalysts (Si/Al ¼ 13.1-14.6) with varying Cu/Al ratios (0.12-0.60) were measured using temperature-programmed reduction in H 2 or CO and desorption of O 2 with He as the carrier. The effluent stream was monitored using mass spectrometry and the structure and oxidation state was determined in parallel by X-ray absorption spectroscopy. Isolated Cu 2+ monomers and oxygen-bridged Cu 2+ dimers interacting with Al-Al next nearest neighbor pairs were the predominant Cu species on these catalysts. The fraction of Cu present as dimers increased from 0.46 to 0.78 as Cu/Al ratios increased from 0.12 to 0.60, as expected from the decreasing average Cu-Cu distance with increasing Cu content. In contrast, monomers reached a plateau of $0.15 Cu 2+ /Al, suggesting that only some Al-Al pairs can interact with small Cu 2+ monomer structures, while a much larger fraction can bind with larger oxygen-bridged Cu 2+ dimers. The measured distribution of Cu dimers and monomers is consistent with the number and bond distances of Al-Al pairs for the Si/Al ratio in these ZSM5 samples. The distributions of Cu species obtained from the amount of CO 2 formed (from CO), the amount of H 2 O formed (from H 2 ), and the amount of CO adsorbed after reduction in CO are in excellent agreement. The number of oxygen atoms removed as O 2 was significantly smaller than that removed with H 2 or CO, suggesting that only proximate Cu dimers autoreduce via recombinative desorption steps. NO decomposition turnover rates (normalized per Cu dimer) were nearly independent of Cu content, except at the lowest Cu/Al ratio, consistent with the involvement of Cu dimers as the active Cu species in NO decomposition redox cycles on Cu-ZSM5. Multiple O 2 and CO 2 peaks during desorption and reduction in CO suggest the presence of Cu dimers with varying oxygen binding energy and reactivity. The Cu dimers initially formed at low Cu/Al contents during exchange are less reducible, consistent with their lower NO decomposition turnover rates.

Structural and Functional Characterization of Redox Mn and Co Sites in AlPO Materials and Their Role in Alkane Oxidation Catalysis

“…The cyclohexyl hydroperoxide response was calibrated using parallel iodometric titrations of liquid samples extracted from the reactor. 9 Typical H 2 -TPR kinetic data are shown in Figure 1. On MnAPO samples, a H 2 consumption peak appeared at ∼580-600 K; Co cations reduced at higher temperatures (∼750 K).…”

“…To some extent, the structural complexity arising from multiple cation types and from ubiquitous changes in cation valency during treatment and catalytic turnovers prevent any one spectroscopic method from providing a rigorous assessment of cation speciation and catalytic relevance. Quantitative chemical methods have proven essential to provide such information for cation-modified aluminosilicates, 9 but they have not been rigorously used for cation-modified aluminophosphate materials.…”

Influence of lattice stability on hydrothermal deactivation of Cu-ZSM-5 and Cu-IM-5 zeolites for selective catalytic reduction of NOx by NH3

“…NO The reducibility of the Cu in ZSM-5 and IM-5 has been studied with H 2 -TPR using 1 % H 2 , which 558 is shown in Figure 10. In the fresh Cu-ZSM-5 and Cu-IM-5, the Cu is reduced in two stages giving The reason why the first reduction is seen as only one peak and not as two, as would be expected 570 when Cu-dimers are present, is because the limiting step is the dissociation of H 2 and not the 571 reduction itself in the first step [55]. show reduction in this temperature range [56,57].…”

“…Cu-ZSM-5 and especially on the identity and possibility of dimer formation, which has been and is 644 still being discussed extensively [30,17,55,60,61,62]. For this reason the first paragraph will be 645 devoted to the subject of speciation followed by discussion of the deactivation mechanism wherein 646 an attempt to disentangle the contributions from dealumination and copper migration will be made.…”

An operando optical fiber UV–vis spectroscopic study of the catalytic decomposition of NO and N2O over Cu-ZSM-5

“…Some studies [27][28][29][30] have suggested that the ELO is associated with mononuclear copper and is of the structure Cu 2+ -O − or Cu 2+ -O 2− . Alternatively, others [21,22,33,[39][40][41] proposed that a single-O-bridged copper pair, i.e., [CuOCu] 2+ , is essential in the NO decomposition cycle. Recent computational contributions of Goodman et al [38,42] stressed the eventual role of double-O-bridged copper pairs, i.e., [CuO 2 Cu] 2+ .…”