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•Pd/ZnO is a poor methanol synthesis catalyst showing RWGS.•Reduction produces PdZn alloy nanoparticles.•PdZn shows high selectivity to methanol.•Sol immobilisation produces smaller, ...more stable particles.
The direct hydrogenation of CO2 into methanol is crucial for providing a means of CO2 fixation and a way to store cleanly produced hydrogen in a more energy-dense and transportable form. Here we have prepared two series of Pd/ZnO catalysts, both by immobilisation of PVA-protected Pd colloids and by Pd impregnation of PdCl2 to investigate structure activity relationships for direct CO2 hydrogenation. Very different performances were found for the different preparation methods, and the Pd loading and pre-reduction of the catalysts were shown to be important factors for optimising methanol yield. The crucial factor for high methanol yield is the formation of a Pd–Zn alloy, either during the reaction itself, or better by high temperature pre-reduction. The formation of the alloy greatly reduces CO production by the reverse water gas shift reaction. The catalysts prepared by sol-immobilisation were relatively stable to thermal treatment. In contrast, the impregnated catalysts were much less thermally stable, due to the presence of remnant chloride on the surface of the catalyst, which was absent for the case of sol immobilisation preparation. The results illustrate the importance of controlling the PdZn particle size and its surface structure for the catalysts to achieve high methanol selectivity (60%, the rest being CO) and conversion (11%) at 250°C and 20bar. Selectivity for sol-immobilised catalysts decreases from 60% at 3nm average diameter, to 20% at 7nm.
Glycerol is an important byproduct of biodiesel production, and it is produced in significant amounts by transesterification of triglycerides with methanol. Due to the highly functionalized nature of ...glycerol, it is an important biochemical that can be utilized as a platform chemical for the production of high-added-value products. At present, research groups in academia and industry are exploring potential direct processes for the synthesis of useful potential chemicals using catalytic processes. Over the last 10 years, there has been huge development of potential catalytic processes using glycerol as the platform chemical. One of the most common processes investigated so far is the catalytic oxidation of glycerol at mild conditions for the formation of valuable oxygenated compounds used in the chemical and pharmaceutical industry. The major challenges associated with the selective oxidation of glycerol are (i) the control of selectivity to the desired products, (ii) high activity and resistance to poisoning, and (iii) minimizing the usage of alkaline conditions. To address these challenges, the most common catalysts used for the oxidation of glycerol are based on supported metal nanoparticles. The first significant breakthrough was the successful utilization of supported gold nanoparticles for improving the selectivity to specific products, and the second was the utilization of supported bimetallic nanoparticles based on gold, palladium, and platinum for improving activity and controlling the selectivity to the desired products. Moreover, the utilization of base-free reaction conditions for the catalytic oxidation of glycerol has unlocked new pathways for the production of free-base products, which facilitates potential industrial application. The advantages of using gold-based catalysts are the improvement of the catalyst lifetime, stability, and reusability, which are key factors for potential commercialization. In this Account, we discuss the advantages of the using supported gold-based nanoparticles, preparation methods for achieving highly active gold-based catalysts, and parameters such as particle size, morphology of the bimetallic particle, and metal–support interactions, which can influence activity and selectivity to the desired products.
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•Variety of ultrasonic reactor designs for the achievement of efficient cavitational activity depending on the application.•Detailed components design insights on the optimisation of ...cavitational activity.•Adjustment in frequency, temperature and pressure for optimal outcomes in sonochemical applications.•Challenges on scaling up reactor systems and suitable reactor type.
Sonochemistry contributes to green science as it uses less hazardous solvents and methods to carry out a reaction. In this review, different reactor designs are discussed in detail providing the necessary knowledge for implementing various processes. The main characteristics of ultrasonic batch systems are their low cost and enhanced mixing; however, they still have immense drawbacks such as their scalability. Continuous flow reactors offer enhanced production yields as the limited cognition which governs the design of these sonoreactors, renders them unusable in industry. In addition, microstructured sonoreactors show improved heat and mass transfer phenomena due to their small size but suffer though from clogging. The optimisation of various conditions of regulations, such as temperature, frequency of ultrasound, intensity of irradiation, sonication time, pressure amplitude and reactor design, it is also discussed to maximise the production rates and yields of reactions taking place in sonoreactors. The optimisation of operating parameters and the selection of the reactor system must be considered to each application’s requirements. A plethora of different applications that ultrasound waves can be implemented are in the biochemical and petrochemical engineering, the chemical synthesis of materials, the crystallisation of organic and inorganic substances, the wastewater treatment, the extraction processes and in medicine. Sonochemistry must overcome challenges that consider the scalability of processes and its embodiment into commercial applications, through extensive studies for understanding the designs and the development of computational tools to implement timesaving and efficient theoretical studies.
Safe and efficient hydrogen generation and storage has received much attention in recent years. Herein, a commercial 5 wt% Pd/C catalyst has been investigated for the catalytic, additive-free ...decomposition of formic acid at mild conditions, and the experimental parameters affecting the process systematically have been investigated and optimised. The 5 wt% Pd/C catalyst exhibited a remarkable 99.9% H
2
selectivity and a high catalytic activity (TOF = 1136 h
−1
) at 30 °C toward the selective dehydrogenation of formic acid to H
2
and CO
2
. The present commercial catalyst demonstrates to be a promising candidate for the efficient in-situ hydrogen generation at mild conditions possibiliting practical applications of formic acid systems on fuel cells. Finally DFT studies have been carried out to provide insights into the reactivity and decomposition of formic acid along with the two-reaction pathways on the Pd (111) surface.
Noble metal nanoparticles (Au, Pd, Au–Pd alloys) with a narrow size distribution supported on nanocrystalline TiO2 (M/TiO2) have been synthesized via a sol-immobilization route. The effect of metal ...identity and size on the photocatalytic performance of M/TiO2 has been systematically investigated using phenol as a probe molecule. A different phenol degradation pathway was observed when using M/TiO2 catalysts as compared to pristine TiO2. We propose a mechanism to illustrate how the noble metal nanoparticles enhance the efficiency of phenol decomposition based on photoreduction of p-benzoquinone under anaerobic conditions. Our results suggest that the metal nanoparticles not only play a role in capturing photogenerated electrons, but are strongly involved in the photocatalytic reaction mechanism. The analysis of the reaction intermediates allows us to conclude that on M/TiO2 undesired redox reactions that consume photogenerated radicals are effectively suppressed. The analysis of the final products shows that the reusability performance of the catalyst is largely dependent on the pretreatment of the catalyst and the identity of the metal nanoparticle. Interestingly, the as-prepared Pd and Au–Pd decorated TiO2 materials exhibit excellent long-term photoactivity, in which ∼90% of the phenol can be fully decomposed to CO2 in each cycle.
Photocatalytic hydrogen evolution may provide one of the solutions to the shift to a sustainable energy society, but the quantum efficiency of the process still needs to be improved. Precise control ...of the composition and structure of the metal nanoparticle cocatalysts is essential, and we show that fine-tuning the Au–Pd nanoparticle structure modifies the electronic properties of the cocatalyst significantly. Specifically, Pdshell–Aucore nanoparticles immobilized on TiO2 exhibit extremely high quantum efficiencies for H2 production using a wide range of alcohols, implying that chemical byproducts from the biorefinery industry can be used as feedstocks. In addition, the excellent recyclability of our photocatalyst material indicates a high potential in industrial applications. We demonstrate that this particular elemental segregation provides optimal positioning of the unoccupied d-orbital states, which results in an enhanced utilization of the photoexcited electrons in redox reactions. We consider that the enhanced activity observed on TiO2 is generic in nature and can be transferred to other narrow band gap semiconductor supports for visible light photocatalysis.
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•Pt-based carbonyl clusters are suitable precursor for small Pt NPs.•Cluster derived materials are effective in base-free HMF oxidation.•Sn addition enhances catalyst activity and ...stability.
In the present work, Pt and Pt/Sn nanoparticles (NPs), synthesized from carbonyl cluster precursors were deposited on TiO2 and the resulting materials were tested as catalysts in selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA). The work was mainly focused on the study of the formation of bimetallic or mixed oxide-metal nanoparticles on TiO2 starting with Pt/Sn carbonyl clusters and on the structure-activity relationship in the reaction of HMF oxidation in base-free conditions. The developed synthesis procedure allowed to obtain very small mono and bimetallic particles characterized by a narrow particle size distribution. Promising results in base-free FDCA production have been achieved using the prepared samples. In particular, the introduction of Sn in an equimolar amount with Pt improved the catalyst activity as well as its time stability upon operation, demonstrating that the modification of Pt electronic configuration by Sn is a key factor for the mastering of functional performances.
Selective oxidation of primary carbon-hydrogen bonds with oxygen is of crucial importance for the sustainable exploitation of available feedstocks. To date, heterogeneous catalysts have either shown ...low activity and/or selectivity or have required activated oxygen donors. We report here that supported gold-palladium (Au-Pd) nanoparticles on carbon or TiO₂ are active for the oxidation of the primary carbon-hydrogen bonds in toluene and related molecules, giving high selectivities to benzyl benzoate under mild solvent-free conditions. Differences between the catalytic activity of the Au-Pd nanoparticles on carbon and TiO₂ supports are rationalized in terms of the particle/support wetting behavior and the availability of exposed corner/edge sites.
Au-Based catalysts supported on nanosized NiO (nNiO) were synthesized and were investigated in the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) under base-free ...conditions using molecular oxygen as the oxidant, at 90 °C. By choosing the optimal composition of Au-Pd nanoparticles (6 : 4 Au/Pd atomic ratio), we report an efficient and stable nNiO-supported Au-Pd alloy catalyst. The presence of nNiO and Au-Pd nanoparticles on the surface was essential to achieve high conversion (95%) and high activity, high yield of FDCA (70%) and good level of stability. Significant synergistic effects were observed between Au and Pd in the alloy as well as on NiO. The present work provides mechanistic insights into the alloying effect and support-metal interaction in terms of understanding better the role of the alloy and support in affecting specific reaction pathways. Finally, the outcome of this knowledge can help develop efficient catalysts for the aerobic oxidation of biomass-derived molecules under base-free conditions in water and under mild reaction conditions.
AuPd nanoparticles supported on NiO exhibit high activity and stability in the base free oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA).
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