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• Flame Sprayed Pyrolysis (FSP) cerium-zirconium oxides show high catalytic activities. • Coprecipitated (CP) and FSP mixed oxides exhibits different surface properties. • FSP enables a surface enrichment in cerium oxide at the top layer. • Cerium enrichment gives higher concentration of adsorbed methanol at the surface. The direct synthesis of Dimethyl carbonate (DMC) from methanol and CO 2 is a green process which allows CO 2 valorization. Among efficient catalysts, ceria, zirconia and cerium-zirconium mixed oxides are often reported as the most active catalysts. In a recent report, we discovered that cerium-zirconium mixed oxides prepared by Flame Spray Pyrolysis (FSP) show greater catalytic activities than those prepared by precipitation, although both exhibit very similar surface area and bulk features. The objective of this study was to find out the origins of the superior catalytic activities obtained by flame spray pyrolysis synthesis method by a deeper analysis of bulk and surface properties. We have opted to focus on mixed ceria-zirconia of equimolar composition (Ce 0.5 Zr 0.5 O 2) as it exhibits maximum catalytic activity for both synthesis methods. Combining bulk and surface characterization as well as surface adsorption measurements using probe molecules, we propose that flame spray pyrolysis enables a surface enrichment in cerium oxide still in interaction with zirconium oxide that leads to a high concentration of adsorbed methanol at the surface, which might explain the greater activity of the catalysts prepared using this method. Beyond the application of DMC synthesis, we can anticipate that Flame Spray Pyrolysis synthesis should generate relative high surface area mixed oxides with different catalytic performances with respect to mixed oxides prepared at lower temperature owing their metastable nature. [ABSTRACT FROM AUTHOR]

Abstract: Perovskites are promising catalysts for oxidation reactions. Good NO selectivity is reported for the oxidation of ammonia into nitric oxide over LaCoO3. More interestingly over this catalyst very little N2O is produced, which makes it a potential candidate for industrial ammonia oxidation. In order to further develop perovskite catalysts, an understanding of the reaction mechanism is necessary. Steady-state, TAP and oxygen exchange experiments over LaCoO3 have been carried out. A reaction mechanism that describes the product distribution (NO, N2O and N2) as a function of the oxidation degree of the catalyst has been proposed. The reaction proceeds by a Mars and Van Krevelen mechanism. NO and N2O are formed through parallel routes from ammonia via surface nitroxyl (HNO) species. Formation of nitrogen occurs through at least three routes. Decomposition of both reaction products NO and N2O leads to N2, the latter decomposition being the fastest. The third route to N2 consists of the reaction of adsorbed ammonia with short-lived oxygen surface species, such as peroxide or superoxide species. [Copyright &y& Elsevier]

[Display omitted] • The surface of cobalt was monitored by operando IR under CO 2 -CO-H 2 mixtures. • The selectivity to methane decreased monotonously with CO surface coverage. • The state of the cobalt metallic surface remained unaffected by the CO/CO2 ratio. An operando IR study was carried out to investigate the loss of selectivity to higher hydrocarbons observed over cobalt-based Fischer-Tropsch catalysts when CO is replaced with CO 2. Our data unambiguously show that the structure and oxidation state of the surface of metallic cobalt remains the same whether CO 2 or CO are cofed with H 2. The selectivity to CH 4 appears to increase monotonously with decreasing surface coverage of adsorbed CO. The switch from Fischer-Tropsch synthesis to methanation using CO 2 -rich feeds over metallic cobalt can therefore be assigned to surface coverage effects only, assuming that most, if not all, hydrocarbons products are formed on metallic cobalt. [ABSTRACT FROM AUTHOR]

Identifying the causes of the segregation of Pt-based alloy nanoparticles is crucial for the design and operation of the corresponding catalysts and electrodes. This article describes a yet unreported mode of alloy segregation upon exposure to CO. In situ IR studies indicated that Pt-Sn alloy nanoparticles were not stable under (i) H 2 -free CO at any temperature or (ii) CO/H 2 mixtures at temperatures below ca. 450 K. This was rationalized by the ability of Pt to dissociate CO. The oxygen adatoms readily reacted with metallic Sn to form a SnOx species, leading to Pt-Sn segregation, alongside carbon deposition. XPS and XANES analyses confirmed Sn reoxidation. While a contamination by traces of O 2 at the sub-ppm level cannot be excluded, the data reported indicate that the structural modifications undergone by the Pt-Sn nanoparticles are more consistent with a reaction involving CO rather than one involving O 2 . In particular, the XPS analysis after CO exposure revealed an increased fraction of graphitic carbon, while that of oxidized carbon decreased. Thermodynamic calculations indicate that the oxidation of tin by CO, with concomitant carbon formation (Sn + CO → SnO + C) is as favourable as Boudouard reaction (2 CO → CO 2  + C) at low temperatures. Alloy stability in the presence of CO must therefore be a concern when CO dissociation is possible and one of the alloyed metal is oxophilic. [ABSTRACT FROM AUTHOR]

Highlights: [•] We investigated reactions between methanol and alkenes catalyzed by zeolite H-ZSM-22. [•] We used a temporal analysis of products reactor to investigate kinetics. [•] Micro-kinetic modeling of two reaction pathways was based on DFT calculations. [•] We conclude that alkenes react with methoxy-species on the zeolite surface. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we report a quantitative structure-activity relationship of functionalized ZIF-type materials (SIM, Substituted Imidazolate Material). Keeping the catalytic center unmodified, post-synthetic modification allows an efficient control of the hydrophilic–hydrophobic balance which drives the competitive adsorption of reactants and products. Hence, we attribute the increase in the reaction rate of Knoevenagel condensation (up to seven times) to the modification of the environment of catalytic sites through the creation of hydrophobic environment surrounding the catalytic sites. Surface tension properties are monitored by measuring the contact angle with a water droplet on a SIM thin film grown on a flat support. We show that the contact angle is directly correlated to the catalytic activity of Knoevenagel condensation. Hence, we propose that the contact angle of a catalyst thin layer with water can be a quantitative descriptor of direct relevance for catalytic processes which produces water and/or for catalysts that are poisoned with water. Finally, we measured the intrinsic reaction rate free of external and internal diffusion limitations by controlling the thickness of the SIM shell on SIM/alumina beads. Insights on the localization of the active sites (surface vs. bulk) are provided. [Copyright &y& Elsevier]

[Display omitted] • Polymeric carbon was deposited over cobalt catalysts by an ethylene treatment at 230 °C and 260 °C. • The deposited species were found to be similar to those formed during FTS. • Ethylene treatments induced severe catalyst deactivation of the catalysts. • An important deselectivation was observed at similar conversion levels. Polymeric carbon was deposited over Siralox supported cobalt catalysts by an ethylene treatment at 230 °C and 260 °C. Cobalt catalysts with cobalt particle sizes of 10 and 14 nm were studied. Temperature programmed hydrogenation and Raman spectroscopy analyses of the deposited species were found to be similar to those formed during Fischer-Tropsch synthesis. The cobalt catalysts were tested in high-throughput 16 parallel reactors at 20 bars and 220 °C, using a H 2 /CO ratio of 2.12. Data were collected at different space times to compare activities and selectivities at iso -conversion levels. Ethylene treated catalysts showed lower activity that corresponded roughly to the loss of CO adsorption sites. Additionally, a decrease of the heavy products selectivity, an increase in the methane selectivity and a decrease of the olefin to paraffin ratio was observed after the ethylene treatments. Unlike the loss in activity, the deselectivation level was found to depend on the amount of deposited carbon. This phenomenon can be explained by a steric effect of the deposit, but an electronic effect cannot be excluded, but is challenging to prove. [ABSTRACT FROM AUTHOR]

[Display omitted] • Carburization of cobalt catalysts was performed using CO treatments. • No significant carbon deposition was observed. • CO treatments induced severe catalyst deactivation of the catalysts. • Catalyst deactivation is Co particle dependent. • An important deselectivation was observed. Fischer-Tropsch cobalt catalyst deselectivation was studied using accelerated aging treatments dedicated to the carburization phenomenon. Cobalt carbide was successfully obtained by CO treatments, with limited simultaneously carbon deposition nor sintering. Particle sizes were found to influence the catalyst sensitivity to carburization, likely because of a possible core–shell mechanism. An important loss of both activity and selectivity to heavy products was observed after the treatments, making carburization one of the potential mechanisms responsible for deselectivation in Fischer-Tropsch synthesis. Side products were also impacted by cobalt carbide formation, with an increase of CO 2 production and a decrease of the selectivity to olefins. The extent of carburization was found to directly dictate the level of the selectivity shift. [ABSTRACT FROM AUTHOR]

Abstract: A series of Ir/CeO2 catalysts of different oxide and metallic phase dispersions was investigated by XRD, TPR, HRTEM, CO2 TPD/calorimetry, in situ DRIFT spectroscopy, and oxygen isotopic exchange techniques in order to elucidate the specific influence of catalyst morphology and structure on the oxidative steam reforming (OSR) of ethanol. Structure/texture sensitivity is demonstrated on the basis of the OSR mechanism, which involves, in order: (i) ethanol adsorption on the ceria surface, (ii) partial oxidation to acetate and surface migration of the C2 intermediates along the ceria support toward the Ir particles, (iii) cracking and further oxidation into Ir carbonyls and ceria carbonates, and (iv) hydrogen and carbon monoxide desorption from the Ir particles. Structure sensitivity is established by considering two types of sites: the ceria surface sites, which are essentially pairs of oxygen vacancies and basic OH groups, as well as interfacial sites between Ir and ceria phases, involving pairs of Ir coordinately unsaturated sites (CUSs) and the aforementioned peripheral ceria sites. This structure sensitivity is revealed by TOF calculations based on these two types of sites, which correspond to the two main rate controlling steps of the ethanol OSR mechanism, namely steps (ii) and (iii). The materials considered display activity, selectivity, and resistance to aging (manifested by coke deposition and sintering) that are closely related to their initial structure and texture. Two domains in terms of iridium and ceria particle size are identified, leading either to stable and selective catalysts or to unstable and unselective catalysts. Such an original and quantified structure sensitivity analysis should prove useful for further process development. [Copyright &y& Elsevier]