Hydrotalcite-type anionic clays: Preparation, properties and applications.

Owing to the increasing emissions of carbon dioxide (CO2), human life and the ecological environment have been affected by global warming and climate changes. To mitigate the concentration of CO2 in the atmosphere various strategies have been implemented such as separation, storage, and utilization of CO2. Although it has been explored for many years, hydrogenation reaction, an important representative among chemical conversions of CO2, offers challenging opportunities for sustainable development in energy and the environment. Indeed, the hydrogenation of CO2 not only reduces the increasing CO2 buildup but also produces fuels and chemicals. In this critical review we discuss recent developments in this area, with emphases on catalytic reactivity, reactor innovation, and reaction mechanism. We also provide an overview regarding the challenges and opportunities for future research in the field (319 references).

Recent advances in catalytic hydrogenation of carbon dioxide

The open literature concerning chemical and mechanistic aspects of the selective catalytic reduction of NO by ammonia (SCR process) on metal oxide catalysts is reviewed. Catalytic systems based on supported V2O5 (including the industrial TiO2-supported V2O5–WO3 and/or V2O5–MoO3 catalysts) and catalysts containing Fe2O3, CuO, MnOx and CrOx are considered. The results of spectroscopic studies of the adsorbed surface species, adsorption–desorption measurements, flow reactor and kinetic experiments are analyzed. The proposed reaction mechanisms are described and critically discussed. Points of convergence and of disagreement are underlined.

Chemical and mechanistic aspects of the selective catalytic reduction of NOx by ammonia over oxide catalysts: A review

The infrared spectral performance of the N x O y species observed on oxide surfaces [N2O, NO−, NO, (NO)2, N2O3, NO+, NO2 − (different nitro and nitrito anions), NO2, N2O4, N2O5, NO2, and NO3 − (bridged, bidentate, and monodentate nitrates)] is considered. The spectra of related compounds (N2, H-, and C-containing nitrogen oxo species, C─N species, NH x species) are also briefly discussed. Some guidelines for spectral identification of N x O y adspecies are proposed and the transformation of the nitrogen oxo species on catalyst surfaces are regarded.

Identification of Neutral and Charged N x O y Surface Species by IR Spectroscopy

CO2 conversion covers a wide range of possible application areas from fuels to bulk and commodity chemicals and even to specialty products with biological activity such as pharmaceuticals. In the present review, we discuss selected examples in these areas in a combined analysis of the state-of-the-art of synthetic methodologies and processes with their life cycle assessment. Thereby, we attempted to assess the potential to reduce the environmental footprint in these application fields relative to the current petrochemical value chain. This analysis and discussion differs significantly from a viewpoint on CO2 utilization as a measure for global CO2 mitigation. Whereas the latter focuses on reducing the end-of-pipe problem “CO2 emissions” from todays’ industries, the approach taken here tries to identify opportunities by exploiting a novel feedstock that avoids the utilization of fossil resource in transition toward more sustainable future production. Thus, the motivation to develop CO2-based chemistry does...

Sustainable Conversion of Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle Assessment

Fourier transform infrared study of the adsorption and coadsorption of nitric oxide, nitrogen dioxide and ammonia on TiO2 anatase

A high surface area K-promoted iron-based catalyst is prepared by thermal decomposition of ammonium glycolate complexes, followed by impregnation with an aqueous solution of potassium carbonate, drying and calcination. It is found that the duration of the calcination has a dramatic effect on the textural and structural properties of the obtained material, as well as on the iron oxidation state. Fast calcinations allow to synthetize materials isostructural with Fe3O4/γ-Fe2O3 that, after a carburization treatment with CO/H2 mixtures, are highly active in the CO2 hydrogenation to lower (C2–C4) olefins at mild process conditions (300 °C and 5 barg). Interestingly, the decrease of the operating pressure leads to a shift from the “hydrocarbon synthesis” regime to reverse water gas shift regime. The selectivity to lower olefins is instead maximised at mild process conditions due to the moderate chain growth probability and the slow rate of olefin secondary hydrogenations, which dominate at higher pressure and temperatures, respectively. The former process is activated by type II catalytic sites (iron carbides), while the latter occurs on type III catalytic sites (Fe0). Type I catalytic sites also exist (Fe3O4) and are responsible for the RWGS activity of the catalyst. Collected data suggest that CO is the primary product of CO2 hydrogenation, while hydrocarbons are formed via CO hydrogenation, following a Fischer-Tropsch type mechanism. Eventually, the performances of the prepared catalyst are compared with those of K-promoted reference model samples based on commercial α-Fe2O3 and Fe3O4 powders. It is found that, due to a better carburization and the higher surface area, the catalyst prepared by fast decomposition of ammonium glycolate complexes is more active than the reference materials in terms of both CO2 conversion and C2–C4 olefins selectivity.

CO2 hydrogenation to lower olefins on a high surface area K-promoted bulk Fe-catalyst

The mechanism of the selective oxidation of methanol on two V-Ti oxide catalyst samples, prepared by impregnation and coprecipitation techniques, respectively is investigated. The interaction of methanol and its oxidation products (i.e., formaldehyde, dimethoxymethane, formic acid, and methyl formate) is studied by FR-IR spectroscopy and compares with the results of flow reactor measurements performed at different temperatures, contact times, and methanol/oxygen molar feed ratios. The data are interpreted on the basis of a reaction mechanism which involved the following steps: (i) condensation of methanol with surface VOH groups; (ii) H abstraction from methoxy groups leading to coordinated formaldehyde; (iii) formation of dioxymethylene species by interaction of adsorbed formaldehyde with nucelophilic sites; (iv) reaction of dioxymethylene species with methanol to give dimethoxymethane; (v) successive oxidation of dioxymethylene groups to formate ions; (vi) reaction of these ions either with methanol to produce methyl formate or with water to give formic acid; (vii) decomposition of formate species to produce carbon monoxide; (viii) parallel oxidation of methanol to carbon dioxide. The behaviors of the surface species are compared with those monitored on other systems and the catalyst requirements for the title reaction are discussed.

Mechanism of selective methanol oxidation over vanadium oxide-titanium oxide catalysts: a FT-IR and flow reactor study

Reduction of Pd° and decomposition of palladium oxide supported on γ-alumina were studied at atmospheric pressure under different atmospheres (H(2), CH(4), He) over a 4 wt% Pd/Al(2)O(3) catalyst (mean palladium particle size: 5 nm with 50% of small particles of size below 5 nm). During temperature programmed tests (reduction, decomposition and oxidation) the crystal domain behaviour of the PdO/Pd° phase was evaluated by in situ Raman spectroscopy and in situ XRD analysis. Under H(2)/N(2), the reduction of small PdO particles ( 5 nm) starts at 100 °C and is achieved at 150 °C. Subsequent oxidation in O(2)/N(2) leads to reoxidation of small crystal domain at ambient temperature while oxidation of large particles starts at 300 °C. Under CH(4)/N(2), the small particle reduction occurs between 240 and 250 °C while large particle reduction is fast and occurs between 280 and 290 °C. Subsequent reoxidation of the catalyst reduced in CH(4)/N(2) shows that small and large particle oxidation of Pd° starts also at 300 °C. Under He, no small particle decomposition is observed probably due to strong interactions between particles and support whereas large particle reduction occurs between 700 and 750 °C. After thermal decomposition under He, the oxidation starts at 300 °C. Thus, the reduction phenomenon (small and large crystal domain) depends on the nature of the reducing agent (H(2), CH(4), He). However, whatever the reduction or decomposition treatment or the crystal domain, Pd° oxidation starts at 300 °C and is completed only at temperatures higher than 550 °C. Under lean conditions, with or without water, the palladium consists of reduced sites of palladium (Pd°, Pd(δ+) with δ < 2 or PdO(x) with x < 1) randomly distributed on palladium particles.

Pio Forzatti

Papers

Fourier transform infrared study of the adsorption and coadsorption of nitric oxide, nitrogen dioxide and ammonia on TiO2 anatase

CO2 hydrogenation to lower olefins on a high surface area K-promoted bulk Fe-catalyst

Mechanism of selective methanol oxidation over vanadium oxide-titanium oxide catalysts: a FT-IR and flow reactor study

In situ Raman and in situ XRD analysis of PdO reduction and Pd° oxidation supported on γ-Al2O3 catalyst under different atmospheres.

Characterization of tungsta-titania catalysts