Designing Pd-based supported bimetallic catalysts for environmental applications

Supported bimetallic nanoparticulate catalysts are an important class of heterogeneous catalysts for many reactions including selective oxidation, hydrogenation/hydrogenolysis, reforming, biomass conversion reactions, and many more. The activity, selectivity, and stability of these catalysts depend on their structural features including particle size, composition, and morphology. In this review, we present important structural features relevant to supported bimetallic catalysts focusing on Pd-based bimetallic systems and recently reported strategies to control them through different synthesis methodologies. Further, we focus on a few reactions that are relevant to environmental catalysis, i.e. CO oxidation, hydrocarbon oxidation, hydrodechlorination, and NO x decomposition, where Pd-based catalysts are often used successfully. In spite of much progress in these areas, still there is a need for more advanced catalytic technologies to address the grand challenges like environmental remediation. Some of the recent advances in the design of bimetallic catalysts were made because of the combined efforts of material scientists, spectroscopists, microscopists, catalysis chemists, and engineers through state-of-the-art characterization methodologies, mechanistic investigations, and structure-activity correlations. This review is aimed at inspiring scientists to rationally design catalysts for a green and sustainable future

Effect of support acidity during selective hydrogenolysis of glycerol over supported palladium-ruthenium catalysts

We report the role of the acidity of support during the selectivity hydrogenolysis of glycerol over supported bimetallic palladium–ruthenium (PdRu) catalysts. The PdRu nanoparticles were supported on a series of metal oxides and zeolitic supports via the modified impregnation method and tested for the liquid-phase hydrogenolysis of glycerol using gaseous hydrogen. The relative acid site densities of selected catalysts were determined by ammonia temperature-programmed desorption and pyridine desorption experiments. Based on these studies, we report a direct correlation between the catalytic activity (conversion and 1,2 propane diol yield) and two different acid sites (strong acid sites and very strong acid sites). Besides zeolite-supported catalysts, TiO2 supported PdRu nanoparticles exhibit moderate catalytic activity; however, this catalyst shows high selectivity for the desired C–O bond cleavage to produce C3 products over the undesired C–C bond cleavage to produce < C3 products

Second-generation biofuels: exploring imaginaries via deliberative workshops with farmers

Second generation biofuels derived from agricultural lignocellulosic waste represent what is hoped to be a significant technological, but also socio-economic advance beyond the shortcomings of first generation biofuels (chiefly bioethanol). The development of advanced catalytic techniques is a central part of making such technologies viable. However, assessing the potential socio-economic significance of the socio-technical arrangements necessary to translate such fundamental techniques into mature technologies is also a central part of shaping the development of second generation technologies in a way that both avoids the shortcomings of first generation fuels and ensures that future developments are genuinely responsive to social needs. A pilot project is described in which a deliberative workshop with farmers in Wales is used to explore the potential societal impacts of novel nanocatalysis methods for the production of lignocellulosic biofuels developed by members of the research team. Using risk- and benefit-ranking/issue mapping methodologies, the workshop examined the potential future role of bioeconomies of different scales, in which second generation biofuels play a significant part, in transforming rural communities. Grounded scepticism from workshop participants delineated key socio-technical issues that will be highly consequential for the development of second-generation technologies, thus laying the ground for subsequent planned work on responsible innovation and nanocatalytic methods of biofuel production

The effect of polymer addition on base catalysed glycerol oxidation using gold and gold-palladium bimetallic catalysts

The oxidation of glycerol represents both a viable route to catalytic upgrading of biomass and has become a model reaction for catalytic polyol oxidation. Gold and gold–palladium nanoparticle catalysts prepared by colloidal methods involving polymer additives have been extensively studied. However, the effect of residual polymer at the catalyst surface on reaction pathways has not been decoupled from particle size effects. We show that when using catalysts prepared without polymer stabilisers the addition of either polyvinyl alcohol or polyvinylpyrrolidone to the reaction changes the reaction rate and results in a change in reaction selectivity. We conclude that the polymer additive has a significant effect on the reaction pathway and that these systems should be considered as a metal surface–polymer interface catalytic systems and properties should not be rationalised solely based on nanoparticle size

Plasmonic oxidation of glycerol using AuPd/TiO₂ catalysts

AuPd nanoparticles supported on P25 TiO2 (AuPd/TiO2) were prepared by a facile sol-immobilisation method and investigated for surface plasmon-assisted glycerol oxidation under base-free conditions. The AuPd/TiO2 samples were characterized by UV-vis spectroscopy and transmission electron microscopy. The sol-immobilisation method readily permitted the Au : Pd molar ratio to be changed over a wide range whilst keeping the mean particle size of the AuPd nanoparticles at 3 nm. Visible light irradiation during the reaction has a beneficial effect on the conversion of glycerol with the most marked effect being observed with gold-rich catalysts and the increase of conversion on light irradiation increases linearly with the gold content of the nanoparticles. The reaction selectivity is also affected by the plasmon-assisted oxidation and glycolic acid, not observed during the dark reactions, was observed for all illuminated reactions due to the enhanced activity of these catalysts

Plasmonic oxidation of glycerol using AuPd/TiO₂ catalysts

AuPd nanoparticles supported on P25 TiO2 (AuPd/TiO2) were prepared by a facile sol-immobilisation method and investigated for surface plasmon-assisted glycerol oxidation under base-free conditions. The AuPd/TiO2 samples were characterized by UV-vis spectroscopy and transmission electron microscopy. The sol-immobilisation method readily permitted the Au : Pd molar ratio to be changed over a wide range whilst keeping the mean particle size of the AuPd nanoparticles at 3 nm. Visible light irradiation during the reaction has a beneficial effect on the conversion of glycerol with the most marked effect being observed with gold-rich catalysts and the increase of conversion on light irradiation increases linearly with the gold content of the nanoparticles. The reaction selectivity is also affected by the plasmon-assisted oxidation and glycolic acid, not observed during the dark reactions, was observed for all illuminated reactions due to the enhanced activity of these catalysts.</p

Sustainable synthesis of dimethyl- and diethyl carbonate from CO2 in batch and continuous flow─lessons from thermodynamics and the importance of catalyst stability

Equilibrium conversions for the direct condensation of MeOH and EtOH with CO2 to give dimethyl- and diethyl carbonate, respectively, have been calculated over a range of experimentally relevant conditions. The validity of these calculations has been verified in both batch and continuous flow experiments over a heterogeneous CeO2 catalyst. Operating under optimized conditions of 140 °C and 200 bar CO2, record productivities of 235 mmol/L·h DMC and 241 mmol/L·h DEC have been achieved using neat alcohol dissolved in a continuous flow of supercritical CO2. Using our thermodynamic model, we show that to achieve maximum product yield, both dialkyl carbonates and water should be continuously removed from the reactor instead of the conventionally used strategy of removing water alone, which is much less efficient. Catalyst stability rather than activity emerges as the prime limiting factor and should thus become the focus of future catalyst development

Transesterification of salmon oil to produce biofuel:Experimental design and process simulation

Waste oils are considered a promising and sustainable source of raw materials in the biofuel industry. In recent years, research has been focused on the conversion of free fatty acids (FFAs) in fish oil to fatty acid methyl esters (FAMEs) mainly through the transesterification reaction. The biodiesel produced from salmon wastes provides an alternative source of raw material for sustainable energy production, meeting the growing energy demand without damaging the environment. The aim of this work is to investigate biodiesel production from salmon oil through a complete experimental design of the transesterification process. The biodiesel production was carried out by a homogeneous alkaline transesterification process using methanol and potassium methylate as a catalyst. The salmon oil was transesterified and then the FAMEs content was estimated using the peak areas of several FAMEs peaks observed in the chromatogram using GC-MS/FID analysis. The experimental results were modelled through Aspen Plus software, where the salmon oil was reacted with methanol in the presence of a catalyst and the results were validated against the experimental data. This study demonstrated that the oil produced from salmon wastes showed suitable characteristics and is a promising source for sustainable biofuel production

Deactivation studies of bimetallic AuPd nanoparticles supported on MgO during selective aerobic oxidation of alcohols

Here we report the synthesis and characterisation of high surface area MgO prepared via the thermal decomposition of various magnesium precursors (MgCO3, Mg(OH)2 and MgC2O4). Bimetallic gold-palladium nanoalloy particles were supported on these MgO materials and were tested as catalysts for the solvent-free selective aerobic oxidation of benzyl alcohol to benzaldehyde. All these catalysts were found to be active and very selective (>97%) to the desired product (benzaldehyde). However, MgO prepared via the thermal decomposition of magnesium oxalate displayed the highest activity among all the magnesium oxide supports tested. Attempts were made to unravel the reasons for the deactivation of these catalysts using different characterisation techniques namely in situ XRD, XPS, ICP-MS, TEM, and TGA-MS. From the data obtained, it is clear that MgO undergoes phase changes from MgO to Mg(OH)2 and MgCO3 during catalyst preparation as well as during the catalytic reaction. Besides phase changes, strong adsorption of reactants and products on the catalyst surface, during the reaction, were also observed and washing the catalyst with organic solvents did not completely remove them. The phase change and catalyst poisoning were reversed through high temperature heat treatments. However, these processes led to the sintering of the metal nanoparticles. Moreover, substantial leaching of the support material (MgO) was also observed during the reaction. These latter two processes led to the irreversible deactivation of AuPd nanoparticles supported on MgO catalyst during the solvent-free selective aerobic oxidation of alcohols. Among the three different MgO supports studied in this article, an inverse correlation between the catalytic activity and Mg leaching has been observed. This article reports a deeper understanding of the mode of deactivation observed in metal nanoparticles supported on MgO during liquid phase reactions

Cinnamyl alcohol oxidation using supported bimetallic Au-Pd nanoparticles: An optimization of metal ratio and investigation of the deactivation mechanism under autoxidation conditions

The aerobic oxidation of cinnamyl alcohol in toluene under autoxidation conditions has been studied using a range of 1 wt% Au–Pd/TiO2 catalysts. The catalysts have been studied to determine the effect of preparation method (impregnation and sol immobilisation) and metal ratio on the conversion of cinnamyl alcohol and the selectivity to cinnamaldehyde. The catalysts prepared by sol-immobilisation demonstrate higher selectivity to the desired aldehyde than the analogous impregnation materials. The most active catalyst was found to be 0.75 wt% Au–0.25 wt% Pd/TiO2 prepared by sol-immobilisation and this demonstrates the importance of metal ratio optimisation in this catalytic process. Furthermore, this metal ratio was found to be most stable under the reactions conditions with little change observed over multiple uses