CNT supported Mo_xC catalysts: Impact of loading and carburization parameters

MoxC/CNT catalysts were prepared through carburization of an oxidic molybdenum precursor impregnated on multiwalled carbon nanotubes (CNTs). The effects of different carburization atmospheres, heating rates, and molybdenum loadings were tested. The catalysts were characterized by using CO temperature-programmed desorption, XRD, N2 physisorption, SEM, and TEM. The catalytic performance in the steam reforming of methanol was used as a sensitive probe to indicate changes in the catalyst surface during the catalytic action. Contrary to the bulk MoxC catalysts, the heating rate during carburization has no effect on the catalysts. Instead, molybdenum loading and carburization atmosphere are the key factors for catalyst structure and performance. The molybdenum-based activity decreases at loadings >10 wt % at a constant product selectivity. The CO2/CH4 product ratio indicates changes in the catalyst properties at the loadings 4/H2 yields 2 nm sized crystallites of cubic α-MoC. Carburization in pure H2 and He yields hexagonal β-Mo2C with a larger particle size. Both phases show different catalytic performances in terms of activity and CO2/CH4 selectivity. Thus, a multiparameter toolbox for fine-tuning of catalyst properties is presented

Modification of the carbide microstructure by N- and S-functionalization of the support in MoxC/CNT catalysts

A series of catalysts based on molybdenum carbide nanoparticles supported on carbon were prepared by carburization of an oxidic Mo precursor impregnated on differently treated multi-walled carbon nanotubes (CNTs) and reference carbons, respectively. The effects of surface defects and decoration of the support with heteroatoms (O, N, and S), as analyzed by IR and Raman spectroscopy as well as by TPD, were investigated. The catalysts were characterized by XRD, N2 physisorption, and electron microscopy. The catalytic performance in steam reforming of methanol was used as a probe to indicate changes in the catalyst surface during catalytic action. The surface chemistry of the carbon supports influences the process of carburization and the nature of resulting supported MoxC (nano) particles. This includes crystal phase composition (α- and β-MoxC) and crystallite as well as particle diameter. However, if the surface decoration of the support is limited to oxygen groups, these differences are not reflected in the catalytic action, which is almost identical for oxygen functionalized carriers. A significant modification of the catalytic performance can only be achieved by surface modification of a CNT support with S- or N-containing functionalities, which causes changes in the lattice constant of the resulting carbide compared to reference systems. These changes are sensitivily reflected in activity and CO2/CH4 product ratio in steam reforming of methanol

Operando spectroscopy study of the carbon dioxide electro-reduction by iron species on nitrogen-doped carbon

The carbon–carbon coupling via electrochemical reduction of carbon dioxide represents the biggest challenge for using this route as platform for chemicals synthesis. Here we show that nanostructured iron (III) oxyhydroxide on nitrogen-doped carbon enables high Faraday efficiency (97.4%) and selectivity to acetic acid (61%) at very-low potential (−0.5 V vs silver/silver chloride). Using a combination of electron microscopy, operando X-ray spectroscopy techniques and density functional theory simulations, we correlate the activity to acetic acid at this potential to the formation of nitrogen-coordinated iron (II) sites as single atoms or polyatomic species at the interface between iron oxyhydroxide and the nitrogen-doped carbon. The evolution of hydrogen is correlated to the formation of metallic iron and observed as dominant reaction path over iron oxyhydroxide on oxygen-doped carbon in the overall range of negative potential investigated, whereas over iron oxyhydroxide on nitrogen-doped carbon it becomes important only at more negative potentials

Ammonia Decomposition and Synthesis over Multinary Magnesioferrites: Promotional Effect of Ga on Fe Catalysts for the Decomposition Reaction

Magnesioferrite (MgFe2O4)-derived Mesoporous spinels of the type MgFeM3+O4 with M3+=Fe, Al, and Ga obtained upon calcination of hydrotalcite-like compounds were investigated in the ammonia decomposition reaction at 1 bar and the synthesis of ammonia at 90 bar. The corresponding precursors were synthesized by co-precipitation at 50 °C and constant pH of 10.5. N2 physisorption, PXRD, HR-TEM, H2-TPR, and NH3-TPD were applied in order to obtain information about the textural, (micro-)structural, solid-state kinetics in reducing atmosphere, and adsorption properties of the samples. While phase-pure layered double hydroxides (LDHs) were obtained for Al and Ga, magnesioferrite as the desired oxide phase and a low fraction of magnetite were formed besides the targeted precursor phase during co-precipitation in the presence of Fe2+ and Fe3+ species. Reduction of the binary and ternary magnesioferrites occurs via two consecutive reactions. Only the second stage is shifted towards higher temperatures after incorporation of Al and Ga. The latter element boosts the catalytic decomposition of ammonia, yielding a 2-fold and 5-fold higher conversion at 500 °C compared to the samples containing Fe3+ and Al3+ species, respectively. In situ XRD measurements showed that this unprecedented promotional effect is related to the generation of (Fe, Ga)Fe3N. This phase, however, is detrimental for the synthesis of ammonia at elevated pressures in which the binary system outperforms the ternary spinels, yielding 30 % of the activity obtained with a highly promoted Fe-based industrial catalyst

Topotactic Synthesis of Porous Cobalt Ferrite Platelets from a Layered Double Hydroxide Precursor and Their Application in Oxidation Catalysis

Monocrystalline, yet porous mosaic platelets of cobalt ferrite, CoFe2O4, can be synthesized from a layered double hydroxide (LDH) precursor by thermal decomposition. Using an equimolar mixture of Fe2+, Co2+, and Fe3+ during co-precipitation, a mixture of LDH, (FeIICoII)2/3FeIII1/3(OH)2(CO3)1/6·mH2O, and the target spinel CoFe2O4 can be obtained in the precursor. During calcination, the remaining FeII fraction of the LDH is oxidized to FeIII leading to an overall Co2+:Fe3+ ratio of 1:2 as required for spinel crystallization. This pre-adjustment of the spinel composition in the LDH precursor suggests a topotactic crystallization of cobalt ferrite and yields phase pure spinel in unusual anisotropic platelet morphology. The preferred topotactic relationship in most particles is [111]Spinel‖ [001]LDH. Due to the anion decomposition, holes are formed throughout the quasi monocrystalline platelets. This synthesis approach can be used for different ferrites and the unique microstructure leads to unusual chemical properties as shown by the application of the ex-LDH cobalt ferrite as catalyst in the selective oxidation of 2-propanol. Compared to commercial cobalt ferrite, which mainly catalyzes the oxidative dehydrogenation to acetone, the main reaction over the novel ex-LDH cobalt is dehydration to propene. Moreover, the oxygen evolution reaction (OER) activity of the ex-LDH catalyst was markedly higher compared to the commercial material

Role of Nanoscale Inhomogeneities in Co2FeO4 Catalysts during the Oxygen Evolution Reaction

Spinel type catalysts are promising anode materials for the alkaline oxygen evolution reaction OER , exhibiting low overpotentials and providing long term stability. In this study, we compared two structurally equal Co2FeO4 spinels with nominally identical stoichiometry and substantially different OER activities. In particular, one of the samples, characterized by a metastable precatalyst state, was found to quickly achieve its steady state optimum operation, while the other, which was initially closer to the ideal crystallographic spinel structure, never reached such a state and required 168 mV higher potential to achieve 1 mA cm2. In addition, the enhanced OER activity was accompanied by a larger resistance to corrosion. More specifically, using various ex situ, quasi in situ, and operando methods, we could identify a correlation between the catalytic activity and compositional inhomogeneities resulting in an X ray amorphous Co2 rich minority phase linking the crystalline spinel domains in the as prepared state. Operando X ray absorption spectroscopy revealed that these Co2 rich domains transform during OER to structurally different Co3 rich domains. These domains appear to be crucial for enhancing OER kinetics while exhibiting distinctly different redox properties. Our work emphasizes the necessity of the operando methodology to gain fundamental insight into the activity determining properties of OER catalysts and presents a promising catalyst concept in which a stable, crystalline structure hosts the disordered and active catalyst phas