Conversion of CO2 and small alkanes to platform chemicals over Mo2C-based catalysts.

The performance of Mo2C-based catalysts in CO2 assisted oxidative dehydrogenation (CO2-ODH) of ethane was evaluated. Mo2C on SiO2 was synthesized via three different techniques: wet impregnation (WI), hybrid nanocrystal technique (HNC) and sol-gel method (SG) and exposed to the same carburization conditions. In terms of characteristic properties, the allotrope composition was the most affected, with the SG sample containing MoOxCy and the WI and HNC samples containing ß-Mo2C. The two different allotropes were suggested to follow different reaction pathways, leading to small differences in the catalytic performance. However, overall, all three catalysts showed a decrease in activity (below 6%) and an increase in C2H4 selectivity (from 60 to 80 C%) with time on stream (TOS). The deactivation mechanism was suggested to be mainly due to oxidation of the carbide to MoOx and carbon deposition. Mo2C was also supported on various metal oxide materials via the wet impregnation technique. Mo2C supported on Al2O3 and ZrO2 increased initial activity (about 8% C2H6 conversion) but a faster deactivation with TOS was observed. Mo2C/Ga2O3 favoured the direct dehydrogenation reaction achieving high C2H4 selectivities (above 80 C%), but deactivation with TOS due to carbon deposition was significant. Mo2C supported on CeO2 and TiO2 had lower activity (about 3% C2H6 conversion). Oxidation to MoO2 and carbon deposition is again suggested to be the main deactivation mechanism. H2 co-feeding, on Mo2C/SiO2 and Mo2C/ZrO2, increased the stability of the catalysts but C2H4 yield was affected (from 5 to 2%). At 17 vol% H2 co-feeding, Mo2C/ZrO2 showed promising catalyst stability over a 20 h period, paralleled by a stable C2H4 yield.

Supported FexNiy catalysts for the co-activation of CO2 and small alkanes.

The effect of both the Fe : Ni ratio (5 to 1 : 1) and the relative Lewis acidity of a metal oxide support on catalytic activity, selectivity and stability was investigated in the CO2 mediated oxidative dehydrogenation of ethane (CO2-ODH). To avoid effects of varying pore sizes, shapes and volumes of the supports, chromia and zirconia overlayers were coated onto a common γ-Al2O3 carrier (CrOx@Al2O3 and ZrOx@Al2O3). Separately, oxidic FexNiy alloy precursor nanoparticles were prepared using a nonaqueous surfactant-free method and deposited by sonication onto the carrier. In comparison to previous studies in the field, this synthesis technique yields closely associated iron and nickel increasing the chances for alloy formation. During reduction, a mixture of a bcc and a fcc alloy phase was formed, with the content of bcc increasing with increasing iron content as predicted by the bulk phase diagram. Upon exposure to carbon dioxide at elevated temperatures, the bcc metallic phase is selectively oxidised to an inverse spinel structure via the dissociation of CO2. When exposed to CO2-ODH conditions, the bare ZrOx@Al2O3 support shows no activity. The presence of FeNi phases increases the conversion of ethane and CO2 marginally (<2%) but forms ethylene at high selectivity (SC2H4 > 80%). The CrOx@Al2O3 support shows some initial activity (XC2H6 < 5%) at very high ethylene selectivity (SC2H4 > 90%) but deactivates with time on stream. Comparison of the ethane and carbon dioxide conversions suggests that direct dehydrogenation rather than the oxidative pathway is taking place. When FeNi particles with the highest Fe content are added, the ethane conversion behavior hardly changes, but the CO2 conversion is increased now supporting the stoichiometric CO2-ODH reaction (SC2H4 > 95%). It is therefore evident that a tandem catalyst system between a reducible oxide carrier and the FeNi species is required. Increasing the Ni content results in an increase in activity and stability while changing the dominant reaction pathway to a combination of dry reforming, CO2-ODH and possibly the reverse Boudouard reaction, with the latter countering catalyst deactivation through carbon deposition.

Synthesis, characterisation and water-gas shift activity of nano-particulate mixed-metal (Al, Ti) cobalt oxides.

The formation of mixed-metal cobalt oxides, representing potential metal-support compounds for cobalt-based catalysts, has been observed at high conversion levels in the Fischer-Tropsch synthesis over metal oxide-supported cobalt catalysts. An often observed increase in the carbon dioxide selectivity at Fischer-Tropsch conversion levels above 80% has been suggested to be associated to the formation of water-gas shift active oxidic cobalt species. Mixed-metal cobalt oxides, namely cobalt aluminate and cobalt titanate, were therefore synthesised and tested for potential catalytic activity in the water-gas shift reaction. We present a preparation route for amorphous mixed-metal oxides via thermal treatment of metal precursors in benzyl alcohol. Calcination of the as prepared nanoparticles results in highly crystalline phases. The nano-particulate mixed-metal cobalt oxides were thoroughly analysed by means of X-ray diffraction, Raman spectroscopy, temperature-programmed reduction, X-ray absorption near edge structure spectroscopy, extended X-ray absorption fine structure, and high-resolution scanning transmission electron microscopy. This complementary characterisation of the synthesised materials allows for a distinct identification of the phases and their properties. The cobalt aluminate prepared has a cobalt-rich composition (Co1+xAl2-xO4) with a homogeneous atomic distribution throughout the nano-particulate structures, while the perovskite-type cobalt titanate (CoTiO3) features cobalt-lean smaller particles associated with larger ones with an increased concentration of cobalt. The cobalt aluminate prepared showed no water-gas shift activity in the medium-shift temperature range, while the cobalt titanate sample catalysed the conversion of water and carbon monoxide to hydrogen and carbon dioxide after an extended activation period. However, this perovskite underwent vast restructuring forming metallic cobalt, a known catalyst for the water-gas shift reaction at temperatures exceeding typical conditions for the cobalt-based Fischer-Tropsch synthesis, and anatase-TiO2. The partial reduction of the mixed-metal oxide and segregation was identified by means of post-run characterisation using X-ray diffraction, Raman spectroscopy, and transmission electron microscopy energy-dispersive spectrometry.