Morphology Changes of Cu₂O Catalysts During Nitrate Electroreduction to Ammonia**

This manuscript reports the electrosynthesis of ammonia from nitrate catalysed Cu derived from Cu2O materials. Cu2O (111) and (100) preferential grain orientations were prepared through electrodeposition. Cu derived from Cu2O (111) is more active and selective for ammonia formation than Cu2O (100) derived Cu. The highest faradaic efficiency (FE) was achieved for both catalysts at −0.3 V vs RHE, with Cu derived from Cu2O (111) reaching up to 80 %. Additional measurements with quasi-in situ X-ray photoelectron spectroscopy and in situ Raman spectroscopy revealed that Cu0 is the active phase during the reaction. The stability of the catalysts was examined by ex situ methods such as SEM, XRD and ICP elemental analysis. The catalysts underwent severe morphological changes as a function of the applied potential and the reaction time, most likely due to the dissolution and redeposition of Cu. After 3 hours of reaction, the entire surface of the catalysts was reconstructed into nanoneedles. The FE after 3 hours remained higher for the Cu derived from Cu2O (111), suggesting that the activity is dependent on the initial structure and the different rates of dissolution and re-deposition.</p

Flame Synthesis of Cu/ZnO-CeO2Catalysts: Synergistic Metal-Support Interactions Promote CH3OH Selectivity in CO2Hydrogenation

The hydrogenation of CO2 to CH3OH is an important reaction for future renewable energy scenarios. Herein, we compare Cu/ZnO, Cu/CeO2, and Cu/ZnO-CeO2 catalysts prepared by flame spray pyrolysis. The Cu loading and support composition were varied to understand the role of Cu-ZnO and Cu-CeO2 interactions. CeO2 addition improves Cu dispersion with respect to ZnO, owing to stronger Cu-CeO2 interactions. The ternary Cu/ZnO-CeO2 catalysts displayed a substantially higher CH3OH selectivity than binary Cu/CeO2 and Cu/ZnO catalysts. The high CH3OH selectivity in comparison with a commercial Cu-ZnO catalyst is also confirmed for Cu/ZnO-CeO2 catalyst prepared with high Cu loading (∼40 wt %). In situ IR spectroscopy was used to probe metal-support interactions in the reduced catalysts and to gain insight into CO2 hydrogenation over the Cu-Zn-Ce oxide catalysts. The higher CH3OH selectivity can be explained by synergistic Cu-CeO2 and Cu-ZnO interactions. Cu-ZnO interactions promote CO2 hydrogenation to CH3OH by Zn-decorated Cu active sites. Cu-CeO2 interactions inhibit the reverse water-gas shift reaction due to a high formate coverage of Cu and a high rate of hydrogenation of the CO intermediate to CH3OH. These insights emphasize the potential of fine-tuning metal-support interactions to develop improved Cu-based catalysts for CO2 hydrogenation to CH3OH

Highly dispersed cobalt oxides nanoparticles on activated carbon fibres as efficient structured catalysts for the transfer hydrogenation of m-nitrostyrene

The facile preparation of structured catalysts featuring highly-dispersed non-precious metal (Fe, Ni, Co) oxide nanoparticles stabilized within a super-microporous network of activated carbon fibres (ACF) is described. The catalyst morphology was controlled over multiple levels from the nano-designed active phase up to the macro-structure of the ACF support. A range of physicochemical techniques was used to characterize the catalyst and rationalize the catalytic data during chemoselective transfer hydrogenation of nitroarenes. The challenging reduction of m-nitrostyrene containing an easily reducible vinyl-group was obtained under mild conditions with a near-quantitative yield of m-vinylaniline using hydrazine hydrate as the reducing agent over CoOx/ACF as a structured catalyst. (C) 2016 Elsevier B.V. All rights reserved

Ceria-Zirconia encapsulated Ni nanoparticles for CO2 methanation

We prepared uniformly-sized Ni nanoparticles on ceria-zirconia (CZ) as model catalysts for CO2 methanation in the context of renewable energy storage. CZ was synthesized via a sol-gel method with Ni being introduced either via incipient-wetness impregnation on calcined CZ or as a colloidal Ni nanoparticle (NiNP) dispersion during sol-gel synthesis. Catalysts were characterized with XRD, N2 physisorption, H2-TPR, H2 chemisorption, XPS and HAADF-STEM with EDX mapping. The Ni/CZ (IWI) catalyst contained large Ni particles after reduction, whereas co-gelation led to NiNP encapsulated in CZ, the particles retaining their initial size of 4.5 nm obtained by the earlier colloidal synthesis. Encapsulated Ni@CZ exhibited superior catalytic activity and stability for CO2 methanation over Ni/CZ. A comparison of Ni@CZ with (encapsulated) Ni@SiO2 prepared from the same colloidal NiNP dispersion showed that CZ shows a strong synergy with Ni in CO2 methanation and results in an order of magnitude higher activity compared to SiO

Reversible nature of coke formation on Mo/ZSM-5 methane dehydroaromatization catalysts

Non-oxidative dehydroaromatization of methane over Mo/ZSM-5 zeolite catalysts is a promising reaction for the direct conversion of abundant natural gas into liquid aromatics. Rapid coking deactivation hinders the practical implementation of this technology. Herein, we show that catalyst productivity can be improved by nearly an order of magnitude by raising the reaction pressure to 15 bar. The beneficial effect of pressure was found for different Mo/ZSM-5 catalysts and a wide range of reaction temperatures and space velocities. High-pressure operando X-ray absorption spectroscopy demonstrated that the structure of the active Mo-phase was not affected by operation at elevated pressure. Isotope labeling experiments, supported by mass-spectrometry and 13 C nuclear magnetic resonance spectroscopy, indicated the reversible nature of coke formation. The improved performance can be attributed to faster coke hydrogenation at increased pressure, overall resulting in a lower coke selectivity and better utilization of the zeolite micropore space

Operando Spectroscopy Unveils the Catalytic Role of Different Palladium Oxidation States in CO Oxidation on Pd/CeO2_2 Catalysts

Establishing structure–activity relationships for practical catalysts requires a multi-technique operando approach. Using advanced spectroscopic tools and kinetic methods the catalytic role of different Pd oxidation states in CO oxidation was unveiled for Pd/CeO$_2$ catalysts. Single-atom Pd–oxo species and Pd−O−Ce interface are responsible for low-temperature activity, whereas metallic Pd sites significantly contribute only at elevated temperature

Nitrate electrochemical reduction to ammonia on Cu2O catalysts

This manuscript reports the electrosynthesis of ammonia from nitrate catalysed by Cu2O. Cu2O (111) and (100) preferential orientations were prepared through electrodeposition to investigate the effect of surface structure. Cu2O (111) is more active and selective for ammonia formation than Cu2O (100). The highest faradaic efficiency (FE) was achieved for both catalysts at -0.3 V vs RHE, with Cu2O (111) reaching up to 80%. Additional measurements with quasi-in situ X-ray photoelectron spectroscopy and in-situ Raman spectroscopy revealed that Cu0 is the active phase during the reaction. The stability of the catalysts was examined by ex-situ methods such as scanning electron microscopy, X-ray diffraction and inductively coupled plasma-optical emission spectrometry. The catalysts undergo severe morphological changes as a function of the applied potential and the reaction time, most likely due to the dissolution and redeposition of Cu. After three hours of reaction, the entire surface of the catalysts was reconstructed into nanoneedles. The final FE was still higher for the original Cu2O (111) electrode

Ni-Mn catalysts on silica-modified alumina for CO2 methanation

The viability of the Power-to-Gas (PtG) concept is strongly dependent on the development of highly active and stable methanation catalysts obtained from cheap and abundant elements. In this paper, the promotional effect of MnO on Ni catalysts supported on silica-modified γ-Al2O3 (SA) was investigated in CO2 and CO methanation on catalysts with Mn/Ni atomic ratios between 0 and 0.25. Significantly higher methanation rates and CH4 selectivities were obtained for Mn-promoted compositions compared to Ni-only catalysts. The optimal NiMn/SA (Mn/Ni = 0.25) catalyst exhibited improved stability compared with unpromoted Ni/SA at 20 bar. The nature of the catalyst precursor and active catalyst was studied with STEM-EDX, XPS, and X-ray absorption spectroscopy (XAS). Evidence of a mixed Ni-Mn oxide in the catalyst precursor was obtained by EXAFS. EXAFS measurements revealed that the reduced catalyst consisted of metallic Ni particles and small oxidic Mn2+ species. Moreover, Mn addition improved the Ni dispersion and enhanced the Ni2+ reducibility by weakening the interaction between the Ni-oxide precursor and the support. A mechanistic study involving IR spectroscopy and steady-state isotopic (13CO2) transient kinetic analysis (SSITKA) showed that the presence of Mn enhanced CO2 adsorption and activation

Operando Spectroscopy Unveils the Catalytic Role of Different Palladium Oxidation States in CO Oxidation on Pd/CeO2 Catalysts

Aiming at knowledge-driven design of novel metal–ceria catalysts for automotive exhaust abatement, current efforts mostly pertain to the synthesis and understanding of well-defined systems. In contrast, technical catalysts are often heterogeneous in their metal speciation. Here, we unveiled rich structural dynamics of a conventional impregnated Pd/CeO2 catalyst during CO oxidation. In situ X-ray photoelectron spectroscopy and operando X-ray absorption spectroscopy revealed the presence of metallic and oxidic Pd states during the reaction. Using transient operando infrared spectroscopy, we probed the nature and reactivity of the surface intermediates involved in CO oxidation. We found that while low-temperature activity is associated with sub-oxidized and interfacial Pd sites, the reaction at elevated temperatures involves metallic Pd. These results highlight the utility of the multi-technique operando approach for establishing structure–activity relationships of technical catalysts