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The economic viability of the electrochemical chlorate process depends on toxic chromate to induce cathodic selectivity to hydrogen and mitigate reduction of hypochlorite or chlorate. In this study, it is shown that performance of a pilot plant for chlorate production can be sustained when a 1000-fold reduction in chromate concentration is compensated by addition of molybdate. Laboratory measurements employing a Quartz Crystal Microbalance suggest growth of a nanometre-thick hybrid Mo–Cr-oxide film to induce cathodic selectivity. An optimized energy efficiency for pilot plant operation was obtained using 0.8 mM molybdate and 27 μM chromate, balancing formation of an effective oxide layer and undesired Mo-induced decomposition of hypochlorite to oxygen in solution. Refinement at the pilot scale level is expected to further optimize the energy consumption, thereby increasing safety aspects and the economic viability of chlorate production. Graphical Abstract

The appearance of catalyst–reactant interactions observed in on-line ATR FT-IR spectra of Nafion/silica catalysed esterification and etherification reactions of 1-octanol was investigated. It was assessed by variation of catalyst and solvent that the catalyst–reactant band is a result of the reaction of silica with 1-octanol, yields Si–O–R functionalities. Based on off-line TPD-MS and TGA characterisation of the used solid catalyst powder, the formation of Si–O–R linkages on the SiO2 surface was confirmed. This demonstrates conclusively that in specific liquid-phase reactions, on-line analysis of intermediate species adsorbed on heterogeneous catalysts is possible. Using on-line particle analyzers, it was assessed that the absorptions were not the result of severe attrition of the catalyst particles in the course of the reaction, but apparently the result of apolar interactions of the particles with the diamond crystal.

Titanium–silica hollow spheres (TSHS) with a mesoporous shell were synthesized using an inverse multiple oil–water–oil (O/W/O) emulsion. These novel materials show interesting potential for catalysis. Excellent catalytic performance was found in the epoxidation of cyclohexene with tert-butylhydroperoxide. The investigation of the titanium environment by UV–vis, Raman and FTIR spectroscopy showed that at low Ti loading only isolated species are present and catalytic measurements demonstrated that these species are the active sites for this reaction. At higher loading Ti–O–Ti microdomains are present and they do not exhibit any significant catalytic activity.

Various Fe-BEA catalysts have been synthesized by liquid ion-exchange, varying the iron precursor (nitrate, sulfate or chloride), the iron content (0.2–1.5 wt% Fe), and the conditions of activation, i.e. atmosphere (air or inert) and heating rate (5 or 20 K min−1). The catalysts were tested in direct N2O decomposition in the temperature range of 625–825 K. An optimal iron loading of 0.8 wt% Fe was found, with no significant influence of the used Fe-precursor. Activation of the ion-exchanged catalysts in inert gas yields significantly better performance than activation in air. NO adsorption combined with infrared analysis was used to characterize the various Fe2+ species present in the differently prepared Fe-BEA zeolites. A correlation exists between the absorption intensity of NO at 1874 cm−1 and the activity of the catalysts in N2O decomposition. This relationship, which can be used as a reliable and fast assessment of catalyst performance, suggests an important role of ferrous ions in the activity of these catalysts. Based on these results and previous mechanistic studies using transient techniques, the NO absorption band at 1874 cm−1 is tentatively assigned to oligonuclear oxocations in the zeolite channels, with general formula FexOy.

Carbon-coated monolithic catalysts were prepared by dipcoating a cordierite substrate in a Furan resin, after which the ruthenium was incorporated by impregnation with a [RuCl5]2- complex. Immobilization of the precursor proceeded particularly via weak physisorption. Since the carbon-coated monolithic supports have a broad pore-size distribution, redistribution of the ruthenium precursor occurred upon drying due to capillary forces, resulting in low dispersion. A significant amount of ruthenium is present on the surface of the carbon inclusions located deep in the cordierite walls, making the diffusion path of the reactants to the active ruthenium sites considerably larger than the average carbon-coating thickness. For successful application of the carbon-coated monolithic catalysts in gas–liquid reactions, the deposition of carbon in the walls of the monolith should be prevented and maldistribution of ruthenium upon drying should be solved.

The decomposition of N2O is strongly promoted by NO over steam-activated FeZSM-5. The promoting effect of NO is catalytic, and in addition to NO2, О2 is formed much more extensively at lower temperatures than in the absence of NO. The promotion effect only requires low NO concentrations in the feed, with no significant improvements at molar NO/N2O feed ratios higher than 0.25. No inhibition by NO was identified even at a molar NO/N2O feed ratio of 10, suggesting different sites for NO adsorption and oxygen deposition by N2O. The latter sites seem to be remote from each other. Transient experiments using in situ FT-IR/MS and Multitrack over FeZSM-5 further elucidate the mechanism of NO promotion. The release of oxygen from the catalyst surface during direct N2O decomposition is a rate-determining step due to the slow oxygen recombination, which is favored by high reaction temperatures. NO addition promotes this oxygen desorption, acting as an oxygen transfer agent, probably via NO2 species. Adsorbed NO may facilitate the migration of atomic oxygen to enhance their recombination. Less than 0.9% of Fe seems to participate in this promotion. A model is proposed to explain the phenomena observed in NO-assisted N2O decomposition, including NO2 decomposition.

Fe-ZSM-5 catalysts were prepared by subliming FeCl3 into H-ZSM-5. The method used allowed Fe-ZSM-5 catalyst preparation by FeCl3 exchange at a desired sublimation temperature and was found to be more precise. The sublimation of FeCl3 into H-ZSM-5 was carried out at 320 and 700 °C. Fe-ZSM-5 prepared by sublimation of FeCl3 at 320 °C followed by rapid heating to 700 °C and the catalyst prepared by subliming FeCl3 at 700 °C were found to be more active for NO reduction with NH3 in the presence of simulated exhaust gases containing water vapor than catalysts prepared by subliming FeCl3 at 320 °C. To determine the active sites, the catalysts were characterized by H2-TPR, in situ DRIFTS of NO adsorption, NH3-TPD, XRD and chemical analysis methods. The observed NO conversion differences in selective catalytic reduction using NH3 could be correlated to the iron cation species present at different locations determined from diffuse reflectance infrared spectroscopy. Enhanced NO reduction activity was obtained when γ positions in Fe-ZSM-5, corresponding to Fe2+(NO) band at 1877 cm-1 in DRIFTS, were preferentially occupied.

IR spectra of NO adsorbed on isomorphously substituted [Fe,Al]MFI, [Fe,Ga]MFI and [Fe]MFI after steaming at 873 K in 30 vol% H2O are presented. On ex-[Fe,Al]MFI, NO adsorption leads to bands at 2133 cm-1 and a doublet at 1886 and 1874 cm-1. The 2133 cm-1 band is assigned to NO+ occupying cationic positions in the zeolite structure. Of the doublet, the 1874 cm-1 band is much more susceptible to reaction with O2 than the 1886 cm-1 band, yielding adsorbed NO2 with an absorption frequency of 1635 cm-1. After evaluation of the constitution of the catalyst and (sometimes contradictory) literature assignments, the 1886 cm-1 band is assigned to NO adsorbed on Fe ions located in isolated positions, and/or (FeO)n clusters inside the zeolite channels, whereas the 1874 cm-1 band is proposed to be induced by 2 nm FeAlOx nano-particles. The ex-[Fe,Ga]MFI catalyst showed a similar absorption pattern (doublet), which is shifted to lower wavenumbers (1881 and 1867 cm-1), suggesting that both frequencies are affected by the vicinity of Ga (or Al) to the Fe site involved. The absence of bands at 1765 and 1835 cm-1 suggests that the isolated sites causing these absorptions are in the FeIII state in ex-[Fe,Al]MFI and ex-[Fe,Ga]MFI. For the ex-[Fe]MFI sample, which did not contain any 2 nm FeOx nano-particles, an NO absorption band at 1854 cm-1 is assigned to mono-nitrosyl on extra-framework oligonuclear (FeIIO)n species in the zeolite channels.

Solid state electrochemistry is described as a method to characterize iron species in the different stages of the preparation of ex-framework FeZSM-5. The ex-framework method comprises the hydrothermal synthesis of isomorphously substituted FeZSM-5, followed by calcination at 823 K and steam treatment (300 mbar H2O in N2) at 873 K. Incorporation of FeZSM-5 samples in graphite-polyester composite (GPC) electrodes and identification of the electrochemical response provides information on the structural environment and oxidation state of electroactive iron species in the zeolite. 57Fe Mossbauer spectroscopy and TEM analysis were additionally used to interpret the electrochemical responses. Tetrahedral iron atoms in framework positions were the only species observed in the as-synthesized material. After calcination, also isolated extra-framework iron ions and oligonuclear iron--oxo complexes were identified, as well as a minor amount of FeOx nano-particles. The steaming procedure leads to extensive formation of the FeOx nano-particles of 1-2 nm in size, which were also identified by TEM. Both oxidative as well as reductive dissolution processes have been observed for the nano-particles, suggesting the presence of both Fe(II) and Fe(III) oxidation states in the steamed FeZSM-5, which is confirmed by Mossbauer spectroscopy. Tetrahedrally coordinated iron atoms in framework positions, as well as the isolated extra-framework iron ions and oligonuclear iron-oxo complexes in octahedral positions, yield electron-transfer processes that approach that of strongly electrode-attached species with distinctive variations of the electrochemical parameters on the pH and the potential scan rate.

The decomposition of N2O over an ex-framework FeZSM-5 catalyst is strongly promoted by NO. Activity data show that the promoting effect of NO is catalytic, and that besides NO2, O2 is formed much more extensively in the presence, than in the absence of NO. Transient in situ FT-IR/MS measurements indicate that NO is strongly adsorbed on the catalyst surface up to at least 650 K, showing absorption frequencies at 1884 and 1876 cm−1. A change in gas phase composition from NO to N2O results in the formation of adsorbed NO2, identified by a sharp IR band at 1635 cm−1. Switching back to the original NO gas phase induces a rapid desorption of NO2, restoring the original NO absorption frequencies. During the IR measurements, bands typical of nitro- or nitrate groups were not observed. Multi-Track (a TAP-like technique) experiments show that the presence of NO or NO2 on the catalyst surface significantly enhances the rate of oxygen desorption at the time of N2O exposure to the catalyst. The spectral changes and transient experiments are discussed and catalytic cycles are proposed, to explain the formation of NO2 and the (enhanced) formation of oxygen. The latter can be either explained by an indirect effect (electronic, steric) of NO adsorbed on sites neighboring the active sites, or by a direct effect involving reaction of adsorbed NO2 groups with neighboring oxidized sites yielding O2.

Selective catalytic reduction of NOx by propene has been investigated on Pt-USY and compared to other Pt-catalysts. The catalyst was characterized by XRD, Ar adsorption at 87 K, TEM, and CO chemisorption, and tested in a gas mixture system in excess oxygen. Pt-USY shows an excellent activity in the reaction, with a molar NOx conversion of 90% at 475 K. Stability during time-on-stream and resistance to SO2 and H2O in the feed stream has also been investigated. Pt-USY performs better under lean-burn conditions than other Pt-catalysts on ZSM-5, Al2O3, or SiO2. The selectivity to N2 was similar for all the catalysts (∼30%), the other major product being N2O.