Engineering the optical response of the titanium-MIL-125 metal-organic framework through ligand functionalization

Herein we discuss band gap modification of MIL-125, a TiO2/1,4-benzenedicarboxylate (bdc) metal-organic framework (MOF). Through a combination of synthesis and computation, we elucidated the electronic structure of MIL-125 with aminated linkers. The band gap decrease observed when the monoaminated bdc-NH2 linker was used arises from donation of the N 2p electrons to the aromatic linking unit, resulting in a red-shifted band above the valence-band edge of MIL-125. We further explored in silico MIL-125 with the diaminated linker bdc(NH2)(2) and other functional groups (-OH, -CH3, -Cl) as alternative substitutions to control the optical response. The bdc-(NH2)2 linking unit was predicted to lower the band gap of MIL-125 to 1.28 eV, and this was confirmed through the targeted synthesis of the bdc-(NH2)(2)-based MIL,-125. This study illustrates the possibility of tuning the optical response of MOFs through rational functionalization of the linking unit, and the strength of combined synthetic/computational approaches for targeting functionalized hybrid materials

CO2 methanation: deciphering the role of dopants (Mn, Co, and Cu) in Ni/SiO2 catalysts prepared by sol-gel chemistry

CO2 methanation is effectively catalyzed by Ni-based catalysts, and reactivity can be further tuned by the addition of promoters. Deciphering the relationship between the promoter in Ni-based catalysts and the corresponding catalytic performance in CO2 methanation mechanism is of great meaning for the development of highly active catalysts. Herein, a series of model bimetallic catalysts were prepared by sol-gel chemistry to address this fundamental challenge. Compared to Ni/SiO2 catalyst, the Mn-doped and Co-doped catalysts showed a higher methanation activity, with the former showing better performance below 250 °C and the latter showing better performance over 300 °C. On the contrary, the Cu-promoted catalyst showed a lower CO2 conversion with a lower CH4 selectivity in the whole temperature range. A comprehensive characterization study (TEM, XRD, XPS, H2-TPR, CO2-TPD, in situ DRIFTS, and TPSR analyses) suggests that the effect of promoters is not directly related to improvement of dispersion, reducibility, or basicity. Instead, we show that the promoters orient the reaction mechanism and favor the conversion of key intermediates. Mn addition has the highest promoting effect on the hydrogenation of formaldehyde intermediate (*OCH2) to methoxy intermediate (*OCH3), i.e. the rate determining step of the “RWGS+CO hydrogenation” pathway which is shown to predominate at low reaction temperature. Co addition facilitates the formation of formate species, i.e. the rate determining step of the formate pathway which is also active at high reaction temperature. Cu addition has a negative effect on the rate determining step of those two pathways, resulting a lower performance of Ni-Cu/SiO2

Effect of the size and distribution of supported Ru nanoparticles on their activity in ammonia synthesis under mild reaction conditions

International audienceRu/gamma-Al2O3 catalysts were prepared using three different methods: wet impregnation, colloidal method and microemulsion. Ru-supported nanoparticles with different average sizes and distribution of sizes were obtained. The catalysts were tested in ammonia synthesis under mild reaction conditions, namely low temperature (100 degrees C) and low pressure (4 bar), and characterized by N-2 adsorption, XRD, XPS, TEM and TPR techniques. The results indicate that a good catalytic performance can be achieved by Ru supported nanoparticles fulfilling two requirements: (i) a relatively high average size (despite the usual assertion that only small particles are required) and (ii) a broad distribution of sizes that ensures the presence of both small particles, containing highly active sites, and large nanoparticles, which are shown to promote the reaction on small particles. This promotion results from a cooperative effect between small and large nanoparticles in good contact, which also allows keeping a highly reduced surface of ruthenium. It is proposed that, under mild reaction conditions, large Ru nanoparticles promote the ammonia synthesis reaction by allowing a more effective activation and transfer of hydrogen atoms, able to hydrogenate strongly adsorbed nitrogen atoms, and thus to release active sites for the activation of N2. (c) 2013 Elsevier B.V. All rights reserved

Aerosol-Assisted Sol-Gel Synthesis of Mesoporous TiO2 Materials, and Their Use as Support for Ru-Based Methanation Catalysts

Mesoporous TiO2 materials have been prepared by an aerosol process, which leverages on the acetic acid-mediated sol-gel chemistry and on the evaporation-induced self-assembly phenomenon to obtain materials with high specific surface area and large mesoporous volume. The obtained spherical particles are calcined to release the porosity. It is shown that the mesoscopic order can be preserved when the calcination is carried out at relatively low temperature (375 °C and below). Harsher calcination conditions lead to the progressive destruction of the mesostructured, concomitant with a progressive drop of textural properties and with the crystallization of larger anatase domains. The mesoporous TiO2 material calcined at 350°C (specific surface area = 260 m².g-1; pore volume = 0.36 cm³.-1; mean pore diameter = 5.4 nm) was selected as a promising support for preformed RuO2 nanoparticles, and subsequently annealed in air. It is shown that the presence of RuO2 nanoparticles and subsequent annealing provoke further intense modification of the texture and crystallinity of the TiO2 materials. In addition to a drop in the textural parameters, a RuO2-mediated crystallization of rutile TiO2 is highlighted at temperature as low as 250°C. After an in situ reduction in H2, the catalysts containing TiO2 rutile and relatively small RuO2 crystals showed the highest activity in the methanation of CO2. </div

Total oxidation of propane with a nano-RuO2/TiO2 catalyst

International audienceAn aqueous colloidal method was used to prepare 2 nm ruthenia nanoparticles from RuCl3 and H2O2. The nanoparticles were subsequently deposited onto a commercial TiO2 support and the obtained nanoRuO(2)/TiO2 catalyst was tested in the total oxidation of propane. This catalyst is very active (T50 of 255 degrees C) and fully selective to CO2. However, much lower performance is achieved if the catalyst is heated above 300 degrees C-either ex situ during calcination or in situ in reaction conditions. We show that the changes in activity are strongly correlated with structural and chemical alteration of the catalyst during heating. These modifications are inspected by characterizing the catalyst after various heat treatments (N2physisorption, XPS, XRD, TEM, H2-TPR). At relatively early stages of heating or reaction (-''150-250 C) RuO2 nanoparticles migrate, leaving anatase TiO2 particles and accumulating on rutile TiO2 surface. At higher temperature, crystallization and sintering provoke irreversible alteration of the catalyst. We suggest that more active VOC total oxidation catalysts could be obtained by avoiding unnecessary calcination step

Mesoporous TiO2 Support Materials for Ru-Based CO2 Methanation Catalysts

Mesoporous TiO2 materials have been prepared by an aerosol process, which leverages on acetic acid mediated sol–gel chemistry and on the evaporation-induced self-assembly of surfactants to obtain materials with high specific surface area (SSA) and large mesoporous volume. The obtained spherical particles are calcined to release the porosity. It is shown that the mesoscopic order can be preserved when calcination is carried out at relatively low temperature (375 °C and below). Harsher calcination conditions provoke the progressive destruction of the mesostructure, concomitant with a progressive drop of the textural properties and with the crystallization of larger anatase domains. The mesoporous TiO2 material calcined at 350 °C (SSA = 260 m2·g–1; pore volume = 0.36 cm3·g–1; mean pore diameter = 5.4 nm) was selected as a promising support for preformed RuO2 nanoparticles. It is shown that the impregnation of RuO2 nanoparticles and subsequent annealing provoke intense modifications of the texture and crystallinity of the TiO2 materials. In addition to a drop in the textural parameters, RuO2-mediated crystallization of rutile TiO2 is highlighted at a temperature as low as 250 °C. After an in situ reduction in H2, the catalysts containing rutile TiO2 and maintaining relatively small RuO2 crystals showed the highest activity in the methanation of CO2 (up to 2.05 μmolCH4·gcat.–1·s–1)

CO2 methanation on Ru/TiO2 catalysts: on the effect of mixing anatase and rutile TiO2 supports

The high CO2 methanation activity of Ru/TiO2 catalysts prepared by mixing both anatase and rutile TiO2 as a support is described, focusing on mild reaction temperature (50–200 °C). The specific catalyst design elucidated the impact of the support mixing. Pre-synthesized, monodispersed 2 nm-RuO2 nanoparticles were used to serve as precursors for active metallic Ru responsible for the CO2 hydrogenation reaction. Pure TiO2 supports with different crystallinity (anatase and rutile) were either prepared in the laboratory or obtained from commercial providers, mixed, and used as supports in different ratios. The mixing was also done at different stages of the catalyst preparation, i.e. before RuO2 deposition, before annealing or after annealing. Our study uncovers that the interaction between the RuO2 nanoparticles and the anatase and rutile TiO2 phase during the annealing step dictates the performance of the Ru/TiO2 methanation catalysts. In particular, when beneficial effects of support mixing are obtained, they can be correlated with RuO2 migration and stabilization over rutile TiO2 through epitaxial lattice matching. Also, support mixing can help prevent the sintering of the support and the trapping of the active phase in the bulk of the sintered support. On thermally stable TiO2 supports, however, it appears clearly that the sole presence of rutile TiO2 support is sufficient to stabilize Ru in its most active form and to prepare a catalyst with high specific activity