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Ag(3 wt%)-loaded gamma-Al2O3 (Ag/Al2O3) catalysts were prepared using four types of commercially available alumina powders (CTB, PUR, VGL, and CFF). Based on the support, the activity of these catalysts for the H-2-assisted selective catalytic reduction (SCR) of NO by NH3 or C3H6 decreased in the order CTB > PUR > VGL > CFF. After sintering treatment (H-2 reduction at 800 degrees C), the particle size of the Ag metal nanoparticles (NPs) changed and was found to be correlated with the catalytic activity (CTB < PUR < VGL < CFF). After re-oxidation of H-2-reduced Ag/Al2O3 at 500 degrees C, the in situ infrared (IR) spectra showed negative bands at 3762 cm(-1) due to the HO-mu(1)-Al-VI site, where the band intensity increased in the order CTB > PUR > VGL > CFF. IR study of pyridine adsorbed on Ag-free gamma-Al2O3 showed that the number of strong Lewis acid sites (unsaturated Al-IV(3+)) increased in the same order, CTB > PUR > VGL > CFF, and the number of strong Lewis acid sites decreased when Ag was loaded on the supports. In situ X-ray absorption near-edge structure (XANES) and UV-vis studies of Ag/Al2O3 sintered under NO + O-2 at 400 degrees C showed oxidative redispersion of the Ag metal NPs to regenerate atomic Ag(i) sites. The amount of redispersed Ag metal and the initial rates of redispersion estimated from the in situ UV-vis results changed in the following order: CTB > PUR > VGL > CFF. These results suggest that the HO-mu(1)-Al-VI site adjacent to the unsaturated Al-IV(3+) site on gamma-Al2O3 is the anchoring site of the atomic Ag species, and the sintering resistance of Ag/Al2O3 increases with the number of HO-mu(1)-Al-VI sites. During H-2-assisted SCR, where both H-2 and NO + O-2 were co-fed to the catalysts, the number of highly dispersed Ag species (active sites) increased with the number of HO-mu(1)-Al-VI sites; hence, NO conversion increased with the number of HO-mu(1)-Al-VI sites on the support. The present results provide molecular-level insights into the design of sintering-resistant Ag/Al2O3 catalysts for SCR.

The modification of Cu-exchanged zeolites with phosphorus represents a promising method to enhance their hydrothermal stability, which is of pivotal importance for catalysts that promote the selective catalytic reduction of NO with ammonia (NH3–SCR). In this study, we investigated the reaction mechanism of NH3–SCR catalyzed by a phosphorus-modified CHA zeolite that contains active Cu species (Cu-P-CHA), and assessed the effect of the phosphorus modification on the hydrothermal stability of the catalyst by employing in situ/operando spectroscopic methods. In their entirety, the combined results from in situ/operando IR, UV–vis, and XANES experiments revealed that the NH3–SCR process over Cu-P-CHA proceeds via the reduction of Cu(II) to Cu(I) followed by a reoxidation of Cu(I) to Cu(II). In addition, we found that the phosphorus modification improved the hydrothermal durability of the catalyst by retaining a higher amount of the redox-active Cu species and the zeolite framework.

In situ/operando spectroscopic experiments and DFT calculations were combined to study the selective catalytic reduction of NO with NH3 (NH3-SCR) over Cu-AFX zeolites. Transient experiments (in situ/operando XANES, IR, and UV-vis) on the reduction half cycle show that the NH3 coordinated to the Cu(II) center reacts with NO to produce N2 and H2O, and simultaneously Cu(II) is reduced to Cu(I). On the other hand, transient experiments on the oxidation half cycle indicate that the oxidation of Cu(I) to Cu(II) species occurs with O2 as the sole oxidant. DFT calculations suggest plausible pathways for the oxidative activation of Cu(I) by O2 as the only oxidant. It is proposed that the oxidation of Cu(I) by O2 as the exclusive oxidant rather than the NO-assisted oxidation plays the key role in the standard NH3-SCR reaction at low temperatures.

Cu–zeolite catalysts are promising for the selective catalytic reduction of nitrogen oxides using ammonia as the reducing agent (NH3-SCR) but rational catalyst design requires a better understanding of the effect of zeolite topology on NH3-SCR activity at low temperatures. Herein, the catalytic activities of Cu–zeolites with different topologies (MOR, MFI, *BEA, and CHA) and various Cu ion-exchange levels for low-temperature NH3-SCR (at 473 K) were investigated. To allow comparisons of catalytic activities among Cu–zeolites with different topologies, the “cation density in micropores” of zeolites was defined, and a similar cation density in micropores was achieved for each topology by varying the Si/Al ratio. The dependence of the turnover frequencies at 473 K on the Cu density in micropores was strongly influenced by the parent zeolite, although the widely accepted reaction mechanism was independent of the zeolite topology. MOR zeolites showed low turnover frequencies irrespective of the Cu density in micropores. In contrast, *BEA zeolites exhibited extremely high turnover frequencies at a low Cu density in micropores, but the turnover frequencies did not increase considerably at higher Cu densities in micropores. Time-resolved UV-vis measurements were used to calculate the rate of the reduction–reoxidation cycle for Cu ions during model transient reactions over different zeolites. It was clearly demonstrated that the turnover frequency for NH3-SCR was dominated by the Cu+ to Cu2+ reoxidation cycle, regardless of the zeolite topology and the Cu density in micropores. The reoxidation of Cu ions on *BEA zeolites was rapid, even at low Cu densities in micropores, and the effect of increasing the mean Cu density in micropores was small. It was also suggested that Cu2+ interacted with zeolite framework oxygen, even under NH3 flow. Thus, it is likely that the local structure and distribution of ion-exchange sites for divalent ions affect the reoxidation cycle during NH3-SCR. It is proposed that development of Cu–zeolite catalysts that have both high reaction rates for NH3-SCR and low Cu volumetric densities can be achieved by considering the zeolite topology.

The selective catalytic reduction of NO with ammonia (NH3-SCR) catalyzed by Cu-CHA zeolites is thoroughly investigated using in situ spectroscopic experiments combined with on-line mass spectroscopy (MS) under steady-state NH3-SCR conditions and transient conditions for Cu(II)/Cu(I) redox cycles. Quantitative analysis of the in situ XANES spectra of Cu-CHA under steady-state conditions of NH3-SCR show that NH3-coordinated Cu(II) species is the dominant Cu species at low temperatures (100-150 degrees C). At higher temperatures, Cu(II) species and [Cu(NH3)(2)](+) complex coexist, possibly because the rate of the Cu(II) -> Cu(I) reduction step is comparable to that of the Cu(I)-> Cu(II) oxidation step. In situ XANES, IR/MS, and UV-vis/MS experiments on the reduction half cycle demonstrate that the reduction of Cu(II) species occurs via the reaction of NH3-liganded Cu(II) with NO to yield N-2 and H2O. For the oxidation half cycle, in situ XANES experiments of Cu(I) oxidation in 10 % O-2 at 200 degrees C indicate that an increased density in CHA zeolite exhibits a higher oxidation rate. In situ UV-vis experiments of Cu(I) reoxidation using different mixtures of oxidant feed gas demonstrate the key role of O-2 in the oxidation cycle. It is suggested that the reoxidation of Cu(I) to Cu(II) species occurs with only O-2 as the oxidant, and a high Cu density in CHA zeolite promotes SCR activity by enhancing the oxidative activation of Cu(I) to Cu(II) during the catalytic cycle.

Operando infrared (IR) spectroscopy and density functional theory (DFT) calculations were combined to investigate the selective catalytic reduction (SCR) of NOx by NH3 over H-AFX zeolites. The stea...

Selective catalytic reduction of nitrogen oxides using ammonia (NH3-SCR) over Cu-exchanged zeolites proceeds via reduction of Cu(II) to Cu(I) and subsequent reoxidation of Cu(I) to Cu(II). Although the mechanism of reduction half cycle has been relatively well established, reoxidation pathways of Cu(I) to form the original Cu(II) species are highly complicated and remain unclear. Herein, oxidation mechanisms of Cu(I) to Cu(II) species in CHA zeolites during the NH3-SCR process were investigated by periodic DFT calculations. The NH3-solvated Cu(I) and Cu(II) species were considered for exploring the oxidative activation reaction pathways. The results show that, with O2 as the sole oxidant, Cu(I) can be effectively oxidized to Cu(II) via multinuclear Cu-oxo intermediates with moderate reaction barriers. The NO-assisted oxidation of Cu(I) was found to favor the formation of Cu nitrate/nitrite species, which seem to only act as off-cycle resting states. We propose that reoxidation of Cu(I) to Cu(II) with O2 as the sole oxidant plays a key role in the oxidation half cycle under standard NH3-SCR conditions.

Practical catalysts that work at a low temperature for selective catalytic reduction of NOx using NH3 (NH3–SCR) have been required to treat NOx at the outlet temperature in boiler systems (100–150 ...

NH3-SCR (selective catalytic reduction) is important process for removal of NOx. However, water vapor included in exhaust gases critically inhibits the reaction in a low temperature range. Here, we report bulk W-substituted vanadium oxide catalysts for NH3-SCR at a low temperature (100–150 °C) and in the presence of water (~20 vol%). The 3.5 mol% W-substituted vanadium oxide shows >99% (dry) and ~93% (wet, 5–20 vol% water) NO conversion at 150 °C (250 ppm NO, 250 ppm NH3, 4% O2, SV = 40000 mL h−1 gcat−1). Lewis acid sites of W-substituted vanadium oxide are converted to Brønsted acid sites under a wet condition while the distribution of Brønsted and Lewis acid sites does not change without tungsten. NH4+ species adsorbed on Brønsted acid sites react with NO accompanied by the reduction of V5+ sites at 150 °C. The high redox ability and reactivity of Brønsted acid sites are observed for bulk W-substituted vanadium oxide at a low temperature in the presence of water, and thus the catalytic cycle is less affected by water vapor., NH3 selective catalytic reduction is an important technique for NOx removal but water vapor critically inhibits the reaction at a low temperature. Here the authors show bulk W-substituted VOx exhibits higher NOx removal ability than the TiO2 supported vanadia catalyst in the presence of water.

Here, we developed an enzyme assay of manganese peroxidase (MnP) by capillary electrophoresis using an in-capillary reaction and applied it to a simultaneous assay of MnP and lignin peroxidase (LiP). The enzyme activity of MnP was determined from the peak area corresponding to Mn(III)-malonate produced by the plug-plug reaction between MnP and Mn(II) in a separation capillary. A background electrolyte containing 250 mM malonate buffer (pH 4.5) and 5 mM cetyltrimethylammonium bromide was employed for the separation of Mn(III)-malonate from MnP at -10 kV after a plug-plug reaction for 5 min. Although the assay permitted the determination of purified MnP, we found that both LiP and MnP have similar activities against their substrates, that is, LiP catalyzed the oxidation reaction of Mn(II) as well as MnP, whereas MnP catalyzed the oxidation reaction of veratryl alcohol which was the substrate used in the LiP assay developed previously. Thus, we proposed a method to discriminate MnP from LiP based on the difference in the activities of these enzymes to each substrate. Amounts of MnP and LiP in a mixture were successfully evaluated by the proposed method.