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In this work, several nanostructured ceria catalysts were prepared by means of a hydrothermal procedure, in which the synthesis conditions (i.e., temperature and pH values) were varied. CeO2 samples of different shapes and structural properties were obtained, namely cubes, rods, cube and nanorod mixtures, and other polyhedra. The prepared materials were tested using four probe catalytic reactions: CO oxidation, NO oxidation, NOx-free soot oxidation, and NOx-assisted soot oxidation. The physicochemical properties of the prepared catalysts were studied by means of complementary techniques (i.e., XRD, N2-physisorption at --196 °C, CO-TPR (temperature-programmed reduction), field emission scanning electron microscopy (FESEM), micro-Raman spectroscopy). The abundance of defects of the catalysts, measured through in-situ Raman spectroscopy at the typical temperatures of each catalytic process, was correlated to the CO and NO oxidation activity of the prepared catalysts, while the soot oxidation reaction (performed in loose conditions), which was hindered by a poor soot-catalyst contact, was found to be less sensitive to the observed structural defects. [ABSTRACT FROM AUTHOR]

In this work, we investigated a set of ceria-based catalysts prepared by the hydrothermal and solution combustion synthesis. For the first time to our knowledge, we synthesized nanocubes of ceria doped with zirconium and praseodymium. The catalysts were tested for the CO and soot oxidation reactions. These materials exhibited different surface reducibility, as measured by H2-TPR, CO-TPR and Soot-TPR, despite their comparable chemical compositions. As a whole, Soot-TPR appears a suitable characterization technique for the soot oxidation catalysts, whereas CO-TPR technique allows to better discriminate among the CO oxidation activities. Praseodymium contributes positively toward the soot oxidation. On the other hand, it has an adverse effect on the CO oxidation over the same catalysts, as compared to pure ceria. The incorporation of zirconium into the ceria lattice does not have a direct beneficial effect on the soot oxidation activity, although it increases the catalyst performances for CO oxidation. © 2016 American Institute of Chemical Engineers AIChE J, 63: 216-225, 2017 [ABSTRACT FROM AUTHOR]

A set of nanostructured ceria–zirconia catalysts with different Zr-contents and structural features has been prepared to study the effect of both the Zr-amount and its surface-dependent activity towards soot combustion under different experimental conditions (namely in “loose” and “tight” soot-catalyst contact). A ceria–zirconia sample has been synthesized by means of solution combustion synthesis (SCS) for comparison purposes. The physico-chemical properties of the catalysts have been investigated using complementary techniques. The best catalytic performances have been achieved for the Ce 0.9 Zr 0.1 O 2 –NP catalyst (where NP means nano-polyhedra and 0.9 indicates the atomic ratio of Ce/Ce + Zr), due to the higher mobility of the lattice oxygen within the solid, and its easier reducibility, compared to high-surface area catalysts with the same Ce/Zr ratio. Moreover, better activities, in terms of soot conversions, have been reached for Ce 0.9 Zr 0.1 O 2 –NP, than similar nano-polyhedra with higher Zr-amounts (denoted as Ce x Zr 1− x O 2 –NP, where x = 0.8 or 0.7). The substitution of some Ce 4+ for Zr 4+ ions favors the formation of defects (i.e., oxygen vacancies) in the ceria lattice, thus inducing a distortion of the oxygen sublattice. However, the amount of redox Ce species decreases as the Zr-content increases. It therefore seems that the incorporation of Zr 4+ into the ceria lattice does not have a direct beneficial effect on the oxidation activity for catalysts calcined at low/mild temperatures, since it decreases the population of surface redox-active centers, which depend directly on the surface density of the Ce 3+ –Ce 4+ species. On the other hand, lower soot conversion values have been reached for both mesoporous (Ce 0.9 Zr 0.1 O 2 –M) and microporous (50-Ce 0.9 Zr 0.1 O 2 –NP/FAU) samples with the same Ce/Zr ratio. Moreover, the comparative catalyst (Ce 0.9 Zr 0.1 O 2 –SCS) exhibited worse activities than the porous materials, thus confirming the key role of the textural properties for this oxidation reaction. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Silane-stabilized Pt nanoparticles were deposited on ceria and ceria-praseodymia. • NO 2 adsorption on equimolar ceria-praseodymia was observed during the reaction. • Equimolar ceria-praseodymia had comparable activity with Pt-ceria in NO oxidation. • Equimolar ceria-praseodymia outperformed Pt-ceria in NO x -assisted soot oxidation. • Peculiar metal–support interaction was observed for Pt–ceria-praseodymia. Abstract This work compares the catalytic activity of nanostructured ceria-praseodymia impregnated with Pt nanoparticles stabilized by n -octylsilane (Pt/Ce50Pr50-NP), with pure ceria nanoparticles (Ce-NP), ceria-praseodymia (Ce50Pr50-NP) and Pt on ceria (Pt/Ce-NP). The idea behind these structures stems from the fact that both Pt/ceria and Ce-Pr mixed oxide are effective towards CO, NO and soot oxidations, as well as for the NO x -assisted soot oxidation. The oxide supports have been prepared via a hydrothermal synthesis. Catalytic activity tests have shown the effectiveness of Ce50Pr50-NP towards the NO x -assisted soot oxidation. The intrinsic activity of this material is even higher than the Pt/Ce-NP counterpart. This finding seems related to the adsorption of NO 2 onto ceria-praseodymia. The addition of Pt on the Ce50Pr50-NP surface appears unnecessary as the effect of Pr on the catalytic activity prevails. The samples have also been thermally aged and their catalytic performances have been compared. A smaller decrease in activity has been observed for Ce50Pr50-NP, compared to Ce-NP, and it has been linked to the material's persistent adsorptive properties. The deposited Pt nanoparticles on the surface of Ce50Pr50-NP, however, have suffered from sintering after the thermal aging, and therefore both aged Pt/Ce50Pr50-NP and Ce50Pr50-NP have comparable catalytic performances. [ABSTRACT FROM AUTHOR]

The current work introduces Cu nanoparticles (Cu-NPs) stabilized by organosilane and deposited as CuO-NPs onto ceria with two different morphologies: spongy, microstructured ceria synthesized by Solution Combustion (CeO 2 -SCS) and nanostructured ceria nanocubes (CeO 2 -NC). Catalytic activity tests have demonstrated that combination of CuO-NPs and CeO 2 -SCS bring significance to CO and NO oxidations as it results in easier reducibility and better metal dispersion on the surface. However, CuO-NPs with CeO 2 -NC give the opposite effect on CO and NO oxidations: the increase of Cu loading lowers the catalytic activity. However, Cu/CeO 2 -NC combination, especially the one with low Cu loading, gives the best synergy for normal soot oxidation with oxygen, thanks to the structure-sensitivity of the reaction. In the presence of NO x , Cu/CeO 2 -SCS catalysts are active for soot oxidation at low temperature as the catalysts favor early NO 2 formation. Lastly, a series of repeated tests with NO x -assisted soot oxidation was carried out to confirm that CuO-NPs suffer easily from particle sintering, possibly via migration and coalescence. [ABSTRACT FROM AUTHOR]

Nanostructured ceria-praseodymia catalysts with different praseodymium contents have been prepared through hydrothermal synthesis to study the effect of Pr as a dopant and the effect of morphology towards soot combustion under “loose” and “tight” soot-catalyst conditions. Samples synthesized through solution combustion synthesis (SCS) have also been prepared as comparative materials. Studies in physicochemical properties of the catalysts have been carried out using complementary techniques. The present work also resorts to soot-TPR as an unconventional method of investigating the ability of solid catalysts to initiate soot oxidation in the absence of bulk oxygen. Ce50Pr50 catalyst (where 50 indicates the atomic percentage of cerium as well as of praseodymium) with mixed structures of nanorods and nanocubes has attained the best catalytic performances, thanks to the high lattice oxygen mobility and the easy reducibility. The insertion of Pr cations to the ceria framework enhances the number of redox sites on the surface, thus generating more oxygen vacancies. As a whole, activity tests in general have proven that despite having relatively low surface areas, ceria-praseodymia nanocubes and nanorods facilitated soot combustion reaction more actively than SCS-based ceria-praseodymia catalysts with larger surface areas. This evidences the beneficial effect of well-defined nanostructures in soot combustion, due to their possession of highly reactive low-index facets (1 0 0) and (1 1 0). Within SCS-based samples, however, the specific surface area overshadows the importance of praseodymium. This eventually marks the synergistic combination of well-defined nanostructures and praseodymium as a dopant. [ABSTRACT FROM AUTHOR]

In the present work, novel ceria-based nanocatalysts containing different quantities of La and Nd were prepared via hydrothermal synthesis. The effects of doping on the structural and physico-chemical properties of ceria were examined with several techniques, such as XRD, FESEM, TEM, Raman spectroscopy, XPS, H 2 -TPR and O 2 /NH 3 /CO 2 -TPD. The catalytic activity of doped ceria towards CO, NO and soot oxidation was evaluated in different conditions; in particular, the role of the catalyst-soot contact and the influence of NO x and water on the soot oxidation performances were investigated. La and Nd ions were well incorporated in ceria structure, but the final morphology was significantly altered. The introduction of trivalent cations was also associated with a higher abundance of defects and oxygen vacancies, but an excessive oxygen deficiency detrimentally affected the material reducibility and catalytic activity for CO and NO oxidation. Conversely, soot oxidation benefited from La and Nd addition. In particular, the Ce-La equimolar oxide exhibited outstanding performances in all the tested conditions, thanks to its optimal morphology and surface acidity. A detailed comparison with equimolar ceria-praseodymia allowed to investigate how acid-base sites and redox properties control the different catalytic mechanisms involved in standard and NO x -assisted soot oxidation. [Display omitted] • La and Nd ions can be well incorporated in the lattice of ceria-based nanocatalysts. • Excessive oxygen deficiency and defectiveness negatively affect ceria reducibility. • Equimolar ceria-lanthana shows an outstanding activity towards soot oxidation. • Acid-base sites strength is closely related to doped ceria catalytic performances. • Different NO x -assisted soot oxidation pathways are promoted by La or Pr doped ceria. [ABSTRACT FROM AUTHOR]

A set of Dual Function Materials (DFMs) was prepared to seize the CO 2 from a rich feed gas and to in-situ convert it to methane (synthetic natural gas). Specifically, ruthenium-ceria composite materials were synthesized through successive impregnation depositions on two high surface area supports, namely Al 2 O 3 and ZSM-5. Cerium oxide has both the roles of CO 2 adsorbent and promoter support for ruthenium, which represents the active component for methanation. Three different quantities of ceria (10, 20, and 30 wt%) were dispersed onto the solid supports, and the adsorption capacities of the ceria-based materials were studied at different temperatures (150, 200, and 250 °C) at atmospheric pressure. The samples exhibiting the best results in terms of CO 2 adsorption (30 wt% CeO 2 /Al 2 O 3 and 30 wt% CeO 2 /ZSM-5) were subsequently impregnated to obtain ruthenium-loaded catalysts (2 wt% Ru). These functionalized materials were characterized by XRD, N 2 physisorption at − 196 °C, TPDRO, ICP-MS, XPS, FESEM, HRTEM, and FT-IR. Then, cyclic experiments of CO 2 adsorption and methanation were performed, simulating a real use of the catalysts at 250 °C and atmospheric pressure. The deposition of ruthenium-ceria on a high surface area support was found to be crucial for maintaining the methanation activity of this catalytic system under cyclic CO 2 adsorption-hydrogenation conditions. The Al 2 O 3 -supported ruthenium-ceria catalyst adsorbed a lower amount of CO 2 (ca. 200 μmol g−1 per each cycle) with respect to the zeolite-supported sample (ca. 300 μmol g−1); nevertheless, the former material presented the best methanation performances, thanks to an intermediate ruthenium-ceria interaction, yielding a maximum of 51% of CO 2 converted and producing up to 111 μmol g−1 of CH 4. [Display omitted] • As-prepared Ce-based samples show good CO 2 adsorption capacity at high temperature. • Synergic action of zeolite support allows a further rise in the adsorption capacity. • Cyclic methanation tests were performed, simulating continuous work of the material. • The catalysts are selective towards CH 4 production and active at ambient pressure. • A great stability is exhibited by the catalysts during cyclic experiments. [ABSTRACT FROM AUTHOR]

In recent decades, climate change has become a major issue that needs to be addressed. Many efforts have been made on the reduction of CO 2 emissions and its conversion in energy carriers and high value-added products such as methane, methanol, dimethyl-ether, and hydrocarbons. The present study focuses on the development of catalysts for hydrogenating CO 2 to methanol, which is a useful chemical and an alternative liquid fuel. According to the literature, In 2 O 3 -based catalysts are particularly selective in the hydrogenation of CO 2 to methanol, reducing the production of CO even at high space velocities compared to the more common ternary catalysts such as Cu/ZnO/Al 2 O 3 or Cu/ZnO/ZrO 2. Therefore, the effects of CeO 2 and ZrO 2 on In 2 O 3 -based catalysts were investigated in the present study. The In x Ce 100−x and the In x Zr 100−x mixed oxides catalysts were synthesized via gel-oxalate coprecipitation by varying the atomic ratios between the elements. Subsequently, they were analysed with several characterisation techniques to rationalise the catalytic performances that were obtained by testing the samples in a fixed bed reactor under different reaction conditions. The addition of different amounts Ce or Zr modified the structure and morphology of the samples and promoted the adsorption of CO 2 from 1.8 mmol CO2 ⋅g cat −1 up to 10.6 mmol CO2 ⋅g cat −1. ZrO 2 stabilises the structure and the results suggests that the greater specific activity (168 mg CH3OH ⋅g In2O3 −1⋅h−1 at 300 °C and 2.5 MPa of In 40 Zr 60) could be ascribed to the electronic promotion of Zr. On the contrary, the addition of CeO 2 did not reveal a beneficial effect on the activity. Concerning the stability, In 2 O 3 -ZrO 2 binary oxides seemed to be affected mainly by sintering; whereas In 2 O 3 -CeO 2 were affected by at least three deactivating phenomena: sintering, reduction of In 2 O 3 to metallic indium and coking. Consequently, the deactivation rate of these binary oxides increased from 1.04 ⋅ 10−2 h−1 of the In 100 to 4.13 ⋅ 10−2 h−1 of the In 40 Ce 60. [Display omitted] • CeO 2 and ZrO 2 modify the morphology and boost the CO 2 adsorption rate. • The electronic promotion effect of Zr enhances the methanol synthesis from CO 2. • ZrO 2 stabilises the structure and mitigates the deactivation phenomena. • CeO 2 increases the reducibility of the samples without stabilising the structure. • In 2 O 3 -CeO 2 samples are affected by sintering, formation of metallic In and coking. [ABSTRACT FROM AUTHOR]

Ceria-based catalysts with different topological and textural properties have been prepared to study the role of the soot-catalyst contact on the soot oxidation reaction. The physico-chemical features of the catalysts have been investigated by means of complementary techniques, such as powder XRD, N 2 physisorption at −196 °C, optical microscopy at variable temperature, FESEM, TEM, and thermogravimetric analysis. As a whole, the best catalytic activity has been obtained with the CeO 2 -nanocubes (denoted to as “Ce-NC”) because of their higher intrinsic reactivity. On the other hand, high-surface area materials prepared by the cerium nitrate decomposition (denoted to as “Ce-ND”) or hydrothermal route (CeO 2 -stars, referred to as “Ce-SAS”) resulted less effective toward the soot combustion, confirming the surface-sensitivity for this reaction. Moreover, it has been proven a higher dependence of the oxidation activity on the catalyst-to-soot ratio (wt./wt.) for the nanostructured catalyst (Ce-NC) exhibiting the lowest BET specific surface area (S BET = 4 m 2 g −1 ). On the other hand, the accessible (real) surface area, at variance of the BET surface area, seems to play a relevant role for this solid–solid reaction mediated by gas-phase oxygen. [ABSTRACT FROM AUTHOR]

• CO oxidation was studied by in situ Raman spectroscopy on nanostructured ceria-based materials. • In situ Raman analyses during CO oxidation allowed to monitor defect sites in operando conditions. • Cycles of reduction (CO in N 2) and oxidation (pure O 2) at 400 °C allow to study the stability of defects. • Oxygen vacancies evolved from clusters to isolated ones according to the atmosphere and temperature. • The involvement of the defect sites in structural rearrangement is fostered by easily reducible dopants. Among the factors affecting ceria activity, the defectiveness plays a key role in the case of CO oxidation. In this study, its connection with the catalytic performance was investigated via in-situ Raman spectroscopy on nanostructured pure and Cu/Mn-doped ceria, monitoring the defect sites and surface species evolution during the reaction. The accumulation of polyene-like chains, formed through CO dissociative adsorption at the catalyst surface, was observed and their disappearance was related to the catalyst light-off temperature. Moreover, the doped samples exhibited a rise of the Raman bands associated to defects after the tests, consequence of the structural rearrangements occurring during CO oxidation. Indeed, in-situ Raman measurements during reduction (CO/N 2) and oxidation cycles at 400 °C evidenced the formation of oxygen vacancy clusters in reducing atmosphere, which could reorganize not only in O 2 but also upon a temperature decrease, forming isolated vacancies and then evolving in Frenkel and extrinsic oxidized dopant-containing sites when exposed to oxygen. [ABSTRACT FROM AUTHOR]