Aqueous Phase Reforming over Platinum Catalysts on Doped Carbon Supports: Exploring Platinum-Heteroatom Interactions

A series of platinum catalysts supported on carbon nanofibers with various heteroatom dopings were synthesized to investigate the effect of the local platinum environment on the catalytic activity and selectivity in aqueous phase reforming (APR) of ethylene glycol (EG). Typical carbon dopants such as oxygen, nitrogen, sulfur, phosphorus, and boron were chosen based on their ability to bring acidic or basic functional groups to the carbon surface. In situ X-ray absorption spectroscopy (XAS) was used to identify the platinum oxidation state and platinum species formed during APR of EG through multivariate curve resolution alternating least-squares analysis, observing differences in activity, selectivity, and platinum local environment among the catalysts. The platinum-based catalyst on the nitrogen-doped carbon support demonstrated the most favorable properties for H2 production due to high Pt dispersion and basicity (H2 site time yield 22.7 h-1). Direct Pt-N-O coordination was identified by XAS in this catalyst. The sulfur-doped catalyst presented Pt-S contributions with the lowest EG conversion rate and minimal production of the gas phase components. Boron and phosphorus-doped catalysts showed moderate activity, which was affected by low platinum dispersion on the carbon support. The phosphorus-doped catalyst showed preferential selectivity to alcohols in the liquid phase, associated with the presence of acid sites and Pt-P contributions observed under APR conditions.

Molecular Ruthenium Cyclopentadienone Bifunctional Catalysts for the Conversion of Sugar Platforms to Hydrogen

Molecular ruthenium cyclopentadienone complexes were employed for the first time as pre-catalysts in the homogeneously catalysed Aqueous Phase Reforming (APR) of glucose. Shvo's complex resulted the best pre-catalyst (loading 2 mol %) with H2 yields up to 28.9 % at 150 °C. Studies of the final mixture allowed to identify the catalyst's resting state as a mononuclear dicarbonyl complex in the extracted organic fraction. In situ NMR experiments and HPLC analyses on the aqueous fraction gave awareness of the presence of sorbitol, fructose, 5-hydroxymethylfurfural and furfural as final fate or intermediates in the transformations under APR conditions. These results were summarized in a proposed mechanism, with particular emphasis on the steps where hydrogen was obtained as the product. Benzoquinone positively affected the catalyst activation when employed as an equimolar additive.

Catalytic Reforming of the Aqueous Phase Derived from Diluted Hydrogen Peroxide Oxidation of Waste Polyethylene for Hydrogen Production

The thermal degradation and conversion of waste polyethylene (PE) using a two-step process including hydrothermal oxidation (HO) and aqueous phase reforming (APR) were investigated. The objective of this study was to achieve efficient disposal of waste PE and generate H₂ in a mild and green way. The effects of various HO conditions on both HO and APR processes were studied. A high H₂ O₂ concentration caused overoxidation of PE resulting in more CO₂ . Decreasing the H₂ O₂ concentration weakened the overoxidation. The process using diluted H₂ O₂ exhibited the highest selectivity for acetic acid among the produced carboxylic acids. When the HO temperature exceeded 200 °C, there was an increase in the CO₂ yield during the HO process and a decrease in the H₂ yield during the APR process. In addition, the effects of various monometallic and bimetallic catalysts on the reforming of the aqueous phase from the HO of PE were discussed. The highest H₂ mole fraction (51.52 %) in gaseous products from the APR process was obtained with Ru/mesoporous carbon. Nevertheless, Ru-Ni exhibited a higher stability than the monometallic Ru catalyst.

Microemulsion Derived Titania Nanospheres: An Improved Pt Supported Catalyst for Glycerol Aqueous Phase Reforming

Glycerol aqueous phase reforming (APR) produces hydrogen and interesting compounds at relatively mild temperatures. Among APR catalysts investigated in literature, little attention has been given to Pt supported on TiO₂. Therefore, herein we propose an innovative titania support which can be obtained through an optimized microemulsion technique. This procedure provided high surface area titania nanospheres, with a peculiar high density of weak acidic sites. The material was tested in the catalytic glycerol APR after Pt deposition. A mechanism hypothesis was drawn, which evidenced the pathways giving the main products. When compared with a commercial TiO₂ support, the synthetized titania provided higher hydrogen selectivity and glycerol conversion thanks to improved catalytic activity and ability to prompt consecutive dehydrogenation reactions. This was correlated to an enhanced cooperation between Pt nanoparticles and the acid sites of the support.

Influence of the Catalyst Particle Size on the Aqueous Phase Reforming of n-Butanol Over Rh/ZrO2

Butanol is a by-product obtained from biomass that can be valorized through aqueous phase reforming. Rh/ZrO₂ catalysts were prepared and characterized, varying the size of the support particles. The results showed a relatively mild effect of internal mass transport on butanol conversion. However, the influence of internal transport limitations on the product distribution was much stronger, promoting consecutive reactions, i.e., dehydrogenation, hydrogenolysis, and reforming of propane and ethane. Hydrogen consuming reactions, i.e., hydrogenolysis, were more strongly enhanced than hydrogen producing reactions due to internal concentration gradients. Large support particles deactivated faster, attributed to high concentrations of butyraldehyde inside the catalyst particles, enhancing deposit formation via aldol condensation reactions. Consequently, also the local butyric acid concentration was high, decreasing the local pH, enhancing Rh leaching. The influence of internal transfer limitation on product distribution and stability is discussed based on a reaction scheme with three main stages, i.e., (1) formation of liquid intermediates via dehydrogenation, (2) formation of gas via decarbonylation/decarboxylation reactions, and (3) hydrocarbon hydrogenolysis/reforming/dehydrogenation.