Photoinduced reactivity of titanium dioxide

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

Cerium dioxide (CeO2, ceria) is becoming an ubiquitous constituent in catalytic systems for a variety of applications. 2016 sees the 40th anniversary since ceria was first employed by Ford Motor Company as an oxygen storage component in car converters, to become in the years since its inception an irreplaceable component in three-way catalysts (TWCs). Apart from this well-established use, ceria is looming as a catalyst component for a wide range of catalytic applications. For some of these, such as fuel cells, CeO2-based materials have almost reached the market stage, while for some other catalytic reactions, such as reforming processes, photocatalysis, water-gas shift reaction, thermochemical water splitting, and organic reactions, ceria is emerging as a unique material, holding great promise for future market breakthroughs. While much knowledge about the fundamental characteristics of CeO2-based materials has already been acquired, new characterization techniques and powerful theoretical methods are dee...

Fundamentals and Catalytic Applications of CeO2-Based Materials

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

The NOx storage process over Ba/Al2O3 and Pt–Ba/Al2O3 NSR catalysts has been analyzed in this study by performing experiments at 350 °C with NO2 and NO/O2 mixtures using different complementary techniques (Transient Response Method, in situ FT–IR and DRIFT spectroscopies).The collected data suggest that over the Pt–Ba/Al2O3 catalyst the NOx storage process from NO/O2 mixtures occurs forming at first nitrite species, which progressively evolve to nitrates. In addition, a parallel nitrate formation via disproportionation of NO2 (formed upon NO oxidation) cannot be excluded.

NOx adsorption study over Pt-Ba/alumina catalysts: FT-IR and reactivity study

Silver- and ruthenium-based materials are herein investigated for the simultaneous removal of particulate matter (soot) and NOx and their behaviour is compared with that of a model Pt–Ba/Al2O3 catalyst. The catalytic activity for diesel soot combustion, in loose contact conditions, is studied by means of TPO (temperature programmed oxidation) while NOx removal is investigated through NOx Storage-Reduction cycles (SRC), performed both in the presence and in the absence of soot. Both the Ag- and Ru-based formulations result active in the soot oxidation, more than the traditional Pt-containing DPNR catalyst. Also, the Ru-based sample shows remarkable performances in the DeNOx-DeSoot activity, although its activity in the reduction step of the stored NOx still needs further improvements.

Simultaneous Removal of Soot and NOx Over Silver and Ruthenium-Based Catalysts

In this study, Pt–K/TiO2 LNT catalysts having different K loadings (2, 5, 10, 15, 20 wt%) have been synthesized, characterized and tested, along with a Pt/K2Ti6O13 reference material. The effects of K loading and thermal aging on NOx storage performance have been addressed, and the formation/decomposition of stored NOx species over Pt-free samples has been investigated by FT-IR, TPD and XRD techniques. NOx storage-reduction tests of Pt–K/TiO2 catalysts indicate that 10 wt% K-loaded samples show the highest NOx storage capacity, registered at 300 °C. Both the temperature at which the maximum NOx storage capacity is attained and the NOx uptake yields are a function of the K-loading. A low K utilization in the NOx storage has been observed. Especially at high K contents (K = 20 wt%) this result can be attributed, in part, to the low surface area of the support material, and to the depletion of the K storage phase via reaction between K and TiO2. In fact XRD analyses demonstrate that K and TiO2 react already during the catalyst synthesis calcination process by forming a “K2Ti6O13-like” potassium titanate at temperatures as low as 550 °C. This reaction is promoted by increasing the K content and calcination temperatures. Activity, XRD and FT-IR measurements have demonstrated that poorly crystalline potassium titanates have appreciable storage properties. Upon aging treatments, K incorporates into the TiO2 structure, leading to the formation of potassium titanates: this increase in the stability of K, on the other hand decreases the NOx storage capacity. This raises questions concerning the capability of such systems to meet the durability requirements for vehicle emission control applications.

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Performance and properties of K and TiO2 based LNT catalysts

Synthesis of alcohols from carbon oxides and hydrogen: VI. Zn and Ti Oxides: Preparation and Catalytic Activity

The overall NSR operation was tested over a bimetallic Pt/Rh-BaO lean NO (x) trap (LNT) catalyst in the range of 473-673 K with simulated diesel exhausts and compared to monometallic 1 wt% Pt/BaO/gamma-Al2O3 and 0.5 wt% Rh/BaO/gamma-Al2O3 samples. The results showed the beneficial effect of the simultaneous presence of 0.5 wt% Pt and 0.25 wt% Rh on the catalytic performance under lean-burn conditions at low temperatures. It was observed that both Pt/BaO/gamma-Al2O3 and Rh/BaO/gamma-Al2O3, which both were mildly aged, have limited NO (x) reduction capacity at 473 K. However, combining Pt and Rh in the NO (x) storage catalyst assisted the NO (x) reduction process to occur at lower temperatures (473 K). One possible reason could be that the combined Pt and Rh sample was more resistant to aging. In addition, the NO2-TPD data showed that the presence of Rh into the Pt/BaO/gamma-Al2O3 system has a considerable effect on the spill-over process of NO (x) , accelerating the release of NO (x) at lower temperatures. These results were in a good agreement with the observed higher rate of oxygen release of the bimetallic Pt/Rh catalyst, leaving a significant number of noble metal sites available for adsorption at lower temperatures than that of the monometallic Pt sample. The superior NSR performance of the bimetallic Pt/Rh/BaO/gamma-Al2O3 catalyst under lean-burn conditions suggested the existence of synergetic promotion effect between the Pt and Rh components, increasing the NO (x) reduction efficiency in comparison with that of the monometallic Pt and Rh-BaO LNT catalysts.

Luca Lietti

Papers

NOx adsorption study over Pt-Ba/alumina catalysts: FT-IR and reactivity study

Simultaneous Removal of Soot and NOx Over Silver and Ruthenium-Based Catalysts

Performance and properties of K and TiO2 based LNT catalysts

Synthesis of alcohols from carbon oxides and hydrogen: VI. Zn and Ti Oxides: Preparation and Catalytic Activity

Enhanced Low Temperature NOx Reduction Performance Over Bimetallic Pt/Rh–BaO Lean NOx Trap Catalysts