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Multimetallic aerogels have emerged as promising unsupported and high-surface-area metal materials for different applications in heterogeneous catalysis and electrochemistry, which are fabricated using a gelation process characterized by the controlled aggregation of metallic nanoparticles to form a macroscopic network structure in aqueous solution. However, the achievement of the structural homogeneity of the multimetallic aerogels in terms of the diameter of the nanochains and the chemical composition at the nano- and the macro-scale is still a great challenge. In this paper, we investigated two Pd-Pt aerogels prepared by the two-step gelation method. The structural homogeneity and chemical distribution of both metals inside the aerogels were analyzed by using high-resolution (scanning) transmission microscopy, energy-dispersive X-ray spectroscopy, extended X-ray absorption fine structure spectroscopy and cyclic voltammetry. The Pd-Pt aerogels show the presence of Pd/Pt-rich domains inside the long-range framework. It is evident that the initial monometallic features dominate over alloying during the gelation process. Although the same synthetic approach for Pd-Pt aerogels with different atomic ratios was used, we observed that the sizes of these monometallic domains varied strongly between the Pd-rich and Pt-rich aerogels. The presence of such metal clusters influenced the electrochemical robustness of the Pd-Pt aerogels dramatically. Electrochemical durability investigations revealed that the aerogels with a high content of Pd are less stable because of the gradual dissolution of the less noble metal particularly inside the Pd-rich domains. A chemical and structural homogeneity might improve the lifetime of the Pd-Pt aerogels under electrochemical conditions. In this work, we provide a better understanding of the structure and chemical distribution of the bimetallic aerogel framework prepared by the two-step gelation process.

Catalytic processes that employ Ru catalysts in supercritical water are capable of converting organics, such as wood waste or biosolids, into synthetic natural gas (CH4) with high efficiencies at relatively moderate temperatures of around 400 °C. However, Ru catalysts are prone to S poisoning and are quickly deactivated. As S is ubiquitous in raw biomass and technologies to remove S from hydrothermal biomass feeds are lacking, regeneration protocols that efficiently reactivate S-poisoned catalysts are required to realize efficient conversion processes and long catalyst lifetimes. In this work, we developed a method to remove S from a S-poisoned Ru catalyst under hydrothermal conditions through an oxidative treatment in the aqueous phase. By using in situ X-ray absorption spectroscopy under the reaction conditions, we show that Ru is oxidized by dilute H2O2 at low temperatures, which leads to the removal of adsorbed S species from the catalyst surface. By optimizing the regeneration conditions, it was possible to prevent oxidation of the catalyst carbon support, as revealed by ex situ TEM. This treatment led to a reactivation of the Ru catalyst with a significant increase in carbon-to-gas conversion and methane selectivity.

The removal of sulfur species from the biomass-derived producer gas after gasification is required to protect downstream catalysts. Finding suitable materials for high-temperature desulfurization is one of the main challenges for the improvement in the efficiency of catalytic biomass conversion. The biomass-derived producer gas usually contains not only H2S but also organic sulfur species such as thiophene (C4H4S), which cannot be removed by most sorbent materials. Herein, we explored Al2O3-supported molybdenum catalysts as bifunctional materials for the removal of H2S and catalytic conversion of C4H4S at high temperatures. By using X-ray absorption spectroscopy under reaction conditions, we show that H2S is removed through the sulfidation of MoO3. C4H4S is catalytically converted over MoO2 to MoS2 and hydrocarbon species. The subsequent oxidation of MoS2 to MoO3 and SO2 regenerates the material and allows the sequestration of sulfur from the gas stream. Furthermore, the negative effect of steam on sulfur removal is shown to be caused by competitive adsorption with sulfur species. These findings show the possibility of the high-temperature desulfurization of biomass-derived gas for catalytic conversion, for instance, to synthetic natural gas.

Catalytic processes that employ Ru catalysts in supercritical water have been shown to be capable of converting organics, such as wood waste, into synthetic natural gas (CH4) with high efficiencies at relatively moderate temperatures of around 400 °C. However, the exact roles of the catalyst and the descriptors that would enable the search for other catalysts with high conversions have not been determined. In the current work, we use electronic structure calculations coupled with batch experiments to understand the interaction of methane (CH4) and water (H2O) with a common catalyst material, ruthenium, to understand the final steps of the methanation reaction. The calculations predict that when CH4 and H2O react with the Ru surface, the species will undergo rapid scrambling; interchanging most of the hydrogens with the surface before escaping as CH4 and H2O once again. We conducted experiments using CH4 as a feedstock in supercritical D2O (deuterated water) in the presence of a commercially available carbon-supported Ru catalyst, and found this mechanism to be confirmed: nearly all reacted CH4 was converted to the fully substituted CD4 or the 3/4-substituted CHD3 isotopomers, with less significant production of the 1/4- or 1/2-substituted species CH3D and CH2D2. The experiment was repeated with an in-house impregnated RuO2-on-carbon catalyst, with similar results. Although other criteria such as the ability to cleave CC and CO bonds and resistance to poisoning will also prove important, this study suggests that a characteristic of an effective catalyst for supercritical water gasification to methane is its ability to promote rapid equilibria through scrambling mechanisms.