K+-doped Bi0.02Co was investigated as catalyst for N2O decomposition. It was found that the catalytic performance of the Bi0.02Co catalyst, which was prepared by coprecipitation method, can be effectively modified by potassium cations via impregnation. The optimized K0.01Bi0.02Co catalyst exhibited much higher activity compared with Bi0.02Co and K0.01Co for the reaction in feed gas 0.2% N2O/Ar, irrespective of the presence or absence of impurity gas(volume fraction) 5%O2, 2%H2O, 0.12%NO and 10%CO2. Characterization of the catalysts with H2 temperature programmed reduction(H2-TPR) and O2 temperature programmed desorption(O2-TPD) indicate that the Co―O bond in Bi0.02Co was weakened by the K+ doping, and hence the K0.01Bi0.02Co catalyst has much higher turnover frequency(TOF) than Co3O4 spinel and Bi0.02Co for the reaction.

•Nanofibrous SUZ-4 zeolite is firstly used as catalyst support.•The nanofibrous SUZ-4 is characterized to be along its 6-, 10-ring channels.•The nanofibrous HSUZ-4 loading Pt is much more active than Pt/HZSM-5 for H2-SCR.•The activity of Pt/HSUZ-4 catalyst is well associated with the HSUZ-4 morphology.•A characterization method for identifying Pt ions in the SUZ-4 channel is proposed.Nanofibrous SUZ-4 zeolite (NF) was synthesized for preparing the Pt/HSUZ-4 catalyst used in the selective catalytic reduction of NOx by hydrogen (H2-SCR). The seed dissolution during the crystallization played an important role for the nanofibrous SUZ-4 formation. It was found that the zeolite NF is oriented along the 10-ring channels, and it has much more Brönsted acid sites on its external surface compared with general SUZ-4 zeolite. Consequently, Pt primarily existed as the well-dispersed particles (∼5 nm) on the NF external surface for the Pt/NF catalyst prepared by impregnation. In comparison with 0.1 wt.% Pt/HZSM-5 as well as general 0.1 wt.% Pt/HSUZ-4, the 0.1 wt.% Pt/NF catalyst gave much higher activity and N2 selectivity in the H2-SCR. For identifying the Pt ions in SUZ-4 zeolite channels, the band at 1652 cm−1 ascribed to nitrous species adsorbed on the ions was proposed.

Highlights•A strategy for the green synthesis of SUZ-4 with controllable morphology and Si/Al was proposed.•High-silica SUZ-4 zeolite with Si/Al ratio as high as 9 was obtained for the first time.•The crystallization time for obtaining the 100% SUZ-4 at 150 °C was shortened to 12 h from 96 h.•The crystallization temperature for obtaining SUZ-4 zeolite could be lowered to 90 °C from 150 °C.The influence of chemical composition of initial gel on the crystallization kinetic, product yield, particularly, on the morphology and SiO2/Al2O3 ratio of zeolite was investigated for the SUZ-4 zeolite synthesis with the assistance of seed slurry. It was found that the SiO2/Al2O3 ratio and morphology of the SUZ-4 zeolite can be controlled by manipulating the H2O content, SiO2/Al2O3 ratio, and K+ content of the initial gel. The crystallization time for obtaining the 100% SUZ-4 zeolite was shortened to 12 h from 96 h at 150 °C for the first time by reducing the H2O content in the gel. It was found that coexistence of Na+ with K+ in the gel does not influence the SUZ-4 zeolite synthesis, while only K+ can incorporate into the SUZ-4 framework. The SUZ-4 zeolite with the SiO2/Al2O3 ratio as high as 18.0 was obtained by decreasing K+ content and increasing SiO2/Al2O3 ratio of the gel with the assistance of seed slurry. Well-crystalline SUZ-4 was obtained in the synthesis recycling 100% mother liquid. As the mother liquid produced from each batch of synthesis was not changed in amount, a possible green process of SUZ-4 synthesis in industry was proposed.Graphical abstract

Uniform SUZ-4 zeolite nanofiber (about 12 × 1500 nm) was obtained for the first time. The SUZ-4 zeolite was synthesized by hydrothermal treatment of the initial aluminosilicate gel at 150 °C with the assistance of definite amount of special seed slurry while without additional use of template. It was found that morphology of the SUZ-4 zeolite synthesized can be simply controlled by the character of seed slurry that is mainly determined by the crystallization period in preparation. The seed slurry containing plenty of less stable SUZ-4 crystallites obtained by crystallizing the aluminosilicate gel (7.9 KOH:1.0 Al2O3:2.6 TEAOH:21.2 SiO2:498.6 H2O) under rotation at 150 °C for 16 h leads to the synthesis of nanofibrous SUZ-4, while that primarily containing stable SUZ-4 crystallites obtained by prolonged crystallization period leads to the synthesis of rod-like zeolite.Graphical abstractHighlights► SUZ-4 zeolite with uniform nanofiber morphology was synthesized for the first time. ► Pure SUZ-4 zeolite was synthesized without using additional template. ► The morphology was successfully controlled in the SUZ-4 synthesis. ► The mechanism concerning the formation of SUZ-4 zeolite nanofiber was proposed.

The specific behavior of HY in some steps that possibly occurred in the selective catalytic reduction of NO by hydrocarbon (HC-SCR) was investigated. Experimental results indicated that the activity of HY for NO oxidation to NO2 was much lower than those of the pentasil zeolites (HZSM-5, HFER and HMOR). In addition, FTIR measurements showed that both nitrosonium ions (NO+) and nitrate species were remarkably produced at 300 °C over the pentasil zeolites and they were highly active towards hydrocarbons at the temperature. On the contrary, no NO+ species could be detected over HY and the nitrate species produced over the zeolite were almost inactive towards reduction at the same reaction condition. It is proposed that the low amount of strong Brönsted acids of HY accounts for the above inferior founctions of the zeolite required by HC-SCR, leading to the low NO reduction activity.Specific behavior of HY in some processes that are possible reaction steps of the selective catalytic reduction of NO by hydrocarbon (HC-SCR) was investigated comparing to those of the pentasil zeolites. It indicated that the following functions of HY weaker than those of the pentasil zeolites should be strengthen for designing practical HC-SCR catalysts based on the zeolite.

SAPO-34 modified with lanthanum and yttrium exhibited higher selectivity to light olefins, lower methane formation, and longer lifetime (prolonged by 20%) than the parent SAPO-34 in the process of methanol conversion to olefins. The modified catalytic performance could be ascribed to the incorporation of La3+ and Y3+ into the framework of SAPO-34.

The performance of 6% W/HZSM-5 catalyst in the SCR of NO by C2H2, C3H6 and CH4 was compared. Contrary to the low selectivity of C3H6 in NO reduction and the high temperature required by CH4 for activation, high and steady NO conversion to N2 (89%) was achieved at 350 °C when C2H2 was used as reductant with a feed of 1600 ppm of NO, 800 ppm of C2H2, 10% of O2 in He. It was found that tungsten dispersed on HZSM-5 has a synergistic effect with protons in the C2H2-SCR of NO. Tungsten incorporation into HZSM-5 significantly accelerated the NO oxidation to NO2 and enlarged strong adsorption of NOx on the catalyst surface, and thus considerably enhanced the aimed reaction. For the C2H2-SCR of NO carried out over the 6% W/HZSM-5 catalyst, carbonous species containing a carbonyl group were active species responsible for the aimed reaction, which involves the reduction of bidentate and bridging nitrate species formed on the catalyst under the reaction conditions.

Influence of sodium in ferrierite (HFER) zeolite on selective catalytic reduction of NO by acetylene (C2H2-SCR) was investigated. NOx-TPD and FT-IR indicated that small amount of sodium exchanged into the proton-form zeolite with an exchange level of about 11.8% is beneficial for the title reaction by accelerating active nitrate species formation on catalyst surface from NO2 and by suppressing the reductant combustion. Nevertheless, no further improved catalytic performance in C2H2-SCR could be observed by a larger amount of sodium exchanged into HFER due to some inactive nitrate species formed on the zeolite. Instead, activity of the zeolite for C2H2-SCR was drastically reduced, since the capacity of the zeolite for catalyzing NO oxidation and accelerating active NO+ species formation was remarkably depressed.

Nitrate species is crucial intermediate of selective catalytic reduction of NO by acetylene (C2H2-SCR) over HZSM-5. It is proposed that formation of nitrate species on the zeolite is rate-determining step of C2H2-SCR. The proposition is supported by the experimental results: (1) No nitrate species could be detected by in situ FTIR in steady C2H2-SCR over HZSM-5 at 350 °C, while the bands due to nitrate species were clearly observed on the zeolite both in situ in NO + O2 and after a brief evacuation at the temperature; (2) Yttrium incorporation into HZSM-5 zeolite considerably enhanced nitrate species formation on the catalyst and correspondingly the activity of the catalyst for C2H2-SCR was significantly increased. Both the promotional effect of yttrium on C2H2-SCR and the rate-determining step of C2H2-SCR being nitrate species formation over the catalyst were first reported herein.

The influence of particle size of HZSM-5 zeolite on selective catalytic reduction of NO by acetylene (C2H2-SCR) was investigated. The zeolite with nano-particle behaved considerable higher activity than the micro-particle one for the reaction. It was revealed that the large difference in the activity for the C2H2-SCR of NO arising from the particle size of zeolite was not caused by limited intracrystalline diffusion of the reductant, but that of NO2, which was strongly supported by the adsorption results obtained over the zeolites.

Ce-Al2O3 catalysts prepared by co-precipitation are investigated both in NO oxidation by O2 and in selective catalytic reduction of NO by C2H2 (C2H2-SCR). It is found that C2H2-SCR is initiated and controlled by NO oxidation to NO2 over Al2O3. Ce loading on Al2O3 is almost inactive for NO oxidation below 350°C, since NO2 strongly adsorbs on cerium oxide, leading to the active sites being blocked, which was characterized by temperature-programmed desorption of NO and NO2 and Fourier transform infrared spectroscopy after NO+O2 coadsorption over the samples. However, in the case of C2H2-SCR, Ce loading on Al2O3 significantly improves the reaction by accelerating the NO oxidation step in the temperature range of 250–450°C, since the nitrate species produced by NO2 adsorption is an active intermediate required by C2H2-SCR.

Catalytic performance of W/HZSM-5 in selective catalytic reduction of NO by acetylene was investigated in a reaction system with 1600 ppm of NO, 800 ppm of C2H2, and 9.95% of O2 in He. It was found that promotional effect of tungsten on the reaction is strongly affected by catalyst preparation conditions and Si/Al ratio of the parent zeolite. A better dispersion of tungsten on HZSM-5 and relatively more monomeric tungsten species were found on 8%W/HZSM-5 prepared by impregnation of the zeolite with lower SiO2/Al2O3 ratio (25) in ammonic ammonium tungstate solution and calcination of the resulting material at higher temperature (550°C). The highest NO conversion to N2 of 86.3% in the reaction system was obtained at 350°C over the catalyst thus prepared. The mechanism of monomeric tungsten species improving the C2H2-SCR can be attributed to accelerating the formation of active nitrate species.

Selective catalytic reduction of NO with acetylene (C2H2-SCR) over mordenite-based catalysts (HMOR, 0.5% Mo/HMOR and NaMOR) was investigated by in situ Fourier transform infrared spectroscopy. A possible mechanism was proposed to explain catalytic performance of the mordenite-based catalysts in the C2H2-SCR: Nitrosonium ions (NO+) and bidentate nitrate are reactive nitric species towards acetylene at 250 °C. Isocyanate species thus formed are then hydrolyzed to acid amide species that are crucial intermediate of the C2H2-SCR. Bridging nitrate species become reactive towards the reductant when reaction temperature increased to 300 °C. Molybdenum loading on HMOR zeolite considerably increased the population of bridging nitrate species and therefore enhanced the title reaction above 300 °C.

Intermediate species formed on ZSM-5 zeolites during the selective catalytic reduction of NO by acetylene (C2H2-SCR) were investigated by transmittance FTIR and temperature-programmed desorption, to elucidate the different catalytic performance of the proton and sodium forms of ZSM-5. Bidentate nitrates formed exclusively on HZSM-5 were very active toward the reductant, whereas the nitrates associated with the Na+ ions on NaZSM-5 were inert. Although the saturation adsorption amount of acetylene on NaZSM-5 was about five times higher than that on HZSM-5 at 80 °C, most of the acetylene on NaZSM-5 was desorbed below 200 °C, whereas a considerable amount of acetylene was detected above 250 °C on HZSM-5 in TPD. The strongly adsorbed species formed on acetylene adsorption was vinyl alcohol (CH2CHOH) bound to the Brønsted acid sites that could be converted to acetate species at elevated temperatures on HZSM-5. Based on our results, a mechanism of the C2H2-SCR on HZSM-5 was proposed in which the bidentate nitrates and acetate species formed exclusively on HZSM-5 react with each other, leading to NOx elimination, which explains why the behavior of HZSM-5 is quite different from that of NaZSM-5 in the C2H2-SCR.