CNF-Functionalization as Versatile Tool for Tuning Activity in Cellulose-Derived Product Hydrogenation.

Carbon nanofibers (CNFs) have been functionalized by introducing O, N, and P containing groups in order to investigate the effect of support functionalization in Ru catalysed hydroxymethyl furfural (HMF) and levulinic acid (LA) hydrogenation. In the case of HMF, despite the fact that no effect on selectivity was observed (all the catalysts produced selectively gamma-valerolactone (GVL)), the functionalization strongly affected the activity of the reaction. O-containing and N-containing supports presented a higher activity compared to the bare support. On the contrary, in HMF hydrogenation, functionalization of the support did not have a beneficial effect on the activity of a Ru-catalysed reaction with respect to bare support and only CNFs-O behaved similarly to bare CNFs. In fact, when CNFs-N or CNFs-P were used as the supports, a lower activity was observed, as well as a change in selectivity in which the production of ethers (from the reaction with the solvent) greatly increased.

Ruthenium on Carbonaceous Materials for the Selective Hydrogenation of HMF.

We report the use of Ru catalysts supported in the activated carbon (AC) and carbon nanofibers (CNFs) for the selective production of liquid fuel dimethylfuran (DMF) and fuel additives alkoxymethyl furfurals (AMF). Parameters such as the reaction temperature and hydrogen pressure were firstly investigated in order to optimise the synthesis of the desired products. Simply by using a different support, the selectivity of the reaction drastically changed. DMF was produced with AC as support, while a high amount of AMF was produced when CNFs were employed. Moreover, the reusability of the catalysts was tested and deactivation phenomena were identified and properly addressed. Further studies need to be performed in order to optimise the stability of the catalysts.

Characterisation of gold catalysts.

Au-based catalysts have established a new important field of catalysis, revealing specific properties in terms of both high activity and selectivity for many reactions. However, the correlation between the morphology and the activity of the catalyst is not always clear although much effort has been addressed to this task. To some extent the problem relates to the complexity of the characterisation techniques that can be applied to Au catalyst and the broad range of ways in which they can be prepared. Indeed, in many reports only a few characterization techniques have been used to investigate the potential nature of the active sites. The aim of this review is to provide a critical description of the techniques that are most commonly used as well as the more advanced characterization techniques available for this task. The techniques that we discuss are (i) transmission electron microscopy methods, (ii) X-ray spectroscopy techniques, (iii) vibrational spectroscopy techniques and (iv) chemisorption methods. The description is coupled with developing an understanding of a number of preparation methods. In the final section the example of the supported AuPd alloy catalyst is discussed to show how the techniques can gain an understanding of an active oxidation catalyst.

Glycerol oxidation using gold-containing catalysts.

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.

N-Modified Carbon-Based Materials: Nanoscience for Catalysis.

Carbon-based materials constitute a large family of materials characterized by some peculiarities such as resistance to both acidic and basic environments, flexibility of structure, and surface chemical groups. Moreover, they can be deeply modified by simple organic reactions (acid-base or redox) to acquire different properties. In particular, the introduction of N-containing groups, achieved by post-treatments or during preparation of the material, enhances the basic properties. Moreover, it has been revealed that the position and chemical nature of the N-containing groups is important in determining the interaction with metal nanoparticles, and thus, their reactivity. The modified activity was addressed to a different metal dispersion. Moreover, experiments on catalysts, showing the same metal dispersion, demonstrated that the best results were obtained when N was embedded into the carbon structure and not very close to the metal active site.

Au-Based Catalysts: Electrochemical Characterization for Structural Insights.

Au-based catalysts are widely used in important processes because of their peculiar characteristics. The catalyst performance depends strongly on the nature and structure of the metal nanoparticles, especially in the case of bimetallic catalysts where synergistic effects between the two metals can be occasionally seen. In this paper, it is shown that electrochemical characterisation (cyclovoltammetry CV and electrochemical impedance spectroscopy EIS) of AuPd systems can be used to determine the presence of an electronic interaction between the two metals, thus providing a strong support in the determination of the nature of the synergy between Au and Pd in the liquid phase oxidation of alcohols. However, it seems likely that the strong difference in the catalytic behavior between the single metals and the bimetallic system is connected not only to the redox behaviour, but also to the energetic balance between the different elementary steps of the reaction.

Gold colloids: from quasi-homogeneous to heterogeneous catalytic systems.

Ruby red colloids of gold have been used for thousands of years and in the past have attracted much attention due to their optical properties. Surface plasmon resonance (SPR) bands are responsible for gold colloid colors and typically appear for nanometer-sized gold nanoparticles (GNPs). These lie in the visible range and their position (and intensity) depends on the size, distribution of size, and shape of GNPs but also on their interaction with other materials (i.e., support). Scientists consider colloids as quasi-homogeneous systems, but because of their intrinsic thermodynamic instability, they need different capping agents providing sufficient stability. The strength and the nature of the interaction between the protective (or functionalizing) molecule and the GNP surface is of utmost importance. It can determine the catalytic properties of the nanoparticles, as they mainly interact with the active sites, thus interfering with reactant. Therefore, the protective layer should contribute to the colloid stability, but at the same time, it should not be irreversibly adsorbed on the active site of the GNP surface providing convenient accessibility to reactant. From a catalytic point of view, the milder the interaction is between the particle surface and the capping agent, the more the activity increases. Unfortunately, the reaction conditions often do not allow the required stability of GNPs, which constitutes a fundamental prerequisite for stable catalytic activity. Anchoring GNPs on suitable supports can circumvent the problem, and this technique is now considered a valuable alternative to classical methods to produce highly dispersed gold catalysts. In this Account, we describe the advantages in using this technique to produce gold heterogeneous catalysts of high metal dispersion on a large variety of supports with the possibility of tuning to a large extent the size and (even partially) the shape of GNPs. We also review our recent progress on the sol-immobilization technique. Specifically, we highlight how, depending on its nature, the protective agent not only mediates the activity of GNPs in alcohol oxidation process but also actively participates in the anchoring process and to the stability of GNPs depending on the support surface. We can also use the modification of the metal surface operated by the capping agent to prepare bimetallic species and influence the surface potential, which modifies the intrinsic activity of the GNP. In conclusion, this technique implies many contributions (sometimes not yet clarified factors) that are not simply concerning dimension and dispersion of GNPs or type of support. Chemists should make careful selection of the protective agent and reaction parameters depending on which support is used in which reaction.

AuRu/AC as an effective catalyst for hydrogenation reactions.

AuRu bimetallic catalysts have been prepared by sequential deposition of Au on Ru or vice versa obtaining different nanostructures: when Ru has been deposited on Au, a Au(core)-Ru(shell) has been observed, whereas the deposition of Au on Ru leads to a bimetallic phase with Ru enrichment on the surface. In the latter case, the unexpected Ru enrichment could be attributed to the weak adhesion of Ru on the carbon support, thus allowing Ru particles to diffuse on Au particles. Both structures result very active in catalysing the liquid phase hydrogenolysis of glycerol and levulinic acid but the activity, the selectivity and the stability depend on the structure of the bimetallic nanoparticles. Ru@Au/AC core-shell structure mostly behaved as the monometallic Ru, whereas the presence of bimetallic AuRu phase in Au@Ru/AC provides a great beneficial effect on both activity and stability.

Gold on carbon: one billion catalysts under a single label.

Despite the wide use of carbon materials as supports for heterogeneous catalysis, generic labels are often used to describe the catalysts, i.e. Au/C, making comparisons between different Au/C catalysts difficult even for the same application. A variety of structures and chemically modified surfaces are in fact available, making gold nanoparticles supported on carbonaceous supports extremely versatile catalysts.

Tailoring the morphology of Pd nanoparticles on CNTs by nitrogen and oxygen functionalization.

The influence of N and O functionalization of CNT on the morphology of supported Pd-PVA nanoparticles is studied with respect to the catalytic activity in the liquid phase oxidation of benzyl alcohol to benzaldehyde. The impact of specific N and O sites on the carbon surface induced by the high temperature N-functionalization in the temperature range 673-873 K was observed by HRTEM as increased nanoparticles dispersion and enhanced metal wetting at the carbon surface. Those small nanoparticles that stabilized at the N-CNTs surface are beneficial for improving catalytic performance. The interaction of O(2) with the metal surface was studied by microcalorimetry. At 353 K, the PVA shell hinders the dissociative oxygen chemisorption at the surface of the fresh catalyst. Differently, a very high (maximum for Pd/N-CNT873K 750 kJ mol(-1)) and oscillating exothermic differential heat is registered for the washed samples. Such high differential heat on the "washed" sample is due to the sum of oxygen chemisorption and PVA oxidation. Thereby, it is demonstrated that the PVA overlayer suppresses the total combustion reaction pathway. This contribution has highlighted the impact of the dynamic change of morphology of these Pd nanoparticles under the reaction conditions on the catalytic performance and how this is modulated by the nature of the support as well as the PVA. The support with its varying ability to strongly bind Pd regulates the morphology of the nanoparticles on which the sub-surface penetration of O, H, C from the reactants depends, all modulating the electronic structure and thus the reactivity.

An Insight into the Role of Reactant Structure Effect in Pd/C Catalysed Aldehyde Hydrogenation.

The different activity of a 1% Pd/carbon catalyst towards aromatic and aliphatic aldehydes hydrogenation has been explored by 13C NMR relaxation. The ratio between T1 relaxation times of adsorbed (ads) and free diffusing (bulk) molecules (T1ads/T1bulk) can be used as an indicator of the relative strength of interaction between the reactant and the catalytic surface, where the lower the T1ads/T1bulk, the higher the adsorption strength. It can be seen that 1% Pd/carbon showed a reverse catalytic behaviour towards benzaldehyde and octanal hydrogenation, which can be explained by analysing the T1 relaxation times related to each substrate in the presence of the catalyst. Comparing and correlating the different T1ads/T1bulk values, we were able to prove that the different catalytic results mainly depend on the contrasting adsorption behaviour of substrates on the catalyst. Moreover, the role of the solvent has been disclosed, as NMR results revealed that the adsorption of the reactants was strongly affected by the choice of solvent, which is revealed to be critical in modulating catalytic activity. As a consequence, T1ads/T1bulk measurements can provide a guide to the selection of appropriate reaction conditions for improving catalytic activity.

Effect of Metal Cocatalysts and Operating Conditions on the Product Distribution and the Productivity of the CO2 Photoreduction.

The CO2 photoreduction is a promising way to convert one of the most abundant greenhouse gases to valuable chemicals. The photoreduction in the liquid phase is limited by the low solubility of CO2 in water, but this point is overcome here by using an innovative photoreactor, which allows one to work up to pressures of 20 bar, improving the overall productivity. The photoreduction was performed in the presence of Na2SO3 and using in primis commercial titanium dioxide (P25) and a set of titania catalysts functionalized by surface deposition of either monometallic or bimetallic cocatalysts. The gaseous products were hydrogen and traces of CO, while, in the liquid phase, formic acid/formate, formaldehyde and methanol were quantitatively detected. The pH was observed to shift the products distribution. A neutral environment led mainly to hydrogen and methanol, while, at pH 14, formate was the most abundant compound. The trend for monometallic cocatalysts showed enhanced productivity when using noble metals (i.e., gold and platinum). In order to limit the cost of the catalytic material, bimetallic cocatalysts were explored, adding titania with Au+Ag or Au+Pt. This may open to the possibility of performing the reaction with a smaller amount of the most expensive metals. In the end, we have expressed some conclusions on the cost of the photocatalysts here employed, to support the overall feasibility assessment of the process.

The Effect of Sibunit Carbon Surface Modification with Diazonium Tosylate Salts of Pd and Pd-Au Catalysts on Furfural Hydrogenation.

Herein, we investigated the effect of the support modification (Sibunit carbon) with diazonium salts of Pd and Pd-Au catalysts on furfural hydrogenation under 5 bars of H2 and 50 °C. To this end, the surface of Sibunit (Cp) was modified with butyl (Cp-Butyl), carboxyl (Cp-COOH) and amino groups (Cp-NH2) using corresponding diazonium salts. The catalysts were synthesized by the sol immobilization method. The catalysts as well as the corresponding supports were characterized by Fourier transform infrared spectroscopy, N2 adsorption-desorption, inductively coupled plasma atomic emission spectroscopy, high resolution transmission electron microscopy, energy dispersive spectroscopy, X-ray diffraction, Hammet indicator method and X-ray photoelectron spectroscopy. The analysis of the results allowed us to determine the crucial influence of surface chemistry on the catalytic behavior of the studied catalysts, especially regarding selectivity. At the same time, the structural, textural, electronic and acid-base properties of the catalysts were practically unaffected. Thus, it can be assumed that the modification of Sibunit with various functional groups leads to changes in the hydrophobic/hydrophilic and/or electrostatic properties of the surface, which influenced the selectivity of the process.

Triazine-based polymers as nanostructured supports for the liquid-phase oxidation of alcohols.

A covalent triazine framework (CTF) was used as support for palladium nanoparticles (NPs) and Pd/CTF was applied as the catalyst in the selective oxidation of benzyl alcohol. N groups in the CTF appeared more efficient than those created on carbon nanotubes (CNTs) by NH(3) /high-temperature treatment in stabilizing Pd NPs against growth during the immobilization step. This assured a high metal dispersion, which led to a highly active and stable catalyst in the alcohol oxidation reaction. Indeed, Pd on the CTF was more stable in recycling than Pd on activated carbon (AC) and on nitrogen-doped CNTs, particularly avoiding leaching of Pd NPs. Moreover, Pd on the CTF was less sensitive than Pd on AC to the decrease of reactant concentration. This in turn led to a higher selectivity to benzaldehyde (98 %) with a considerable activity (turnover frequency 1453 h(-1) ).

Covalent triazine framework as catalytic support for liquid phase reaction.

An important goal in the preparation of highly active supported metal particles is the enhancement of the metal support interaction, providing a more stable catalyst, especially for liquid phase reactions as the leaching and reconstruction of the active phase causes deactivation. In this work, a covalent triazine framework (CTF) as support for Pd nanoparticles is compared to activated carbon (AC), the typical support used in liquid phase reactions. The results indicate that the presence of the N-heterocyclic moieties on the surface of the frameworks is beneficial for improving the stability of Pd nanoparticles during the liquid phase glycerol oxidation. Pd/CTF showed better activity and in particular better stability when compared to Pd supported on activated carbon (AC).

Ruling Factors in Cinnamaldehyde Hydrogenation: Activity and Selectivity of Pt-Mo Catalysts.

To obtain selective hydrogenation catalysts with low noble metal content, two carbon-supported Mo-Pt bimetallic catalysts have been synthesized from two different molybdenum precursors, i.e., Na2MoO4 and (NH4)6Mo7O24. The results obtained by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) combined with the presence and strength of acid sites clarified the different catalytic behavior toward cinnamaldehyde hydrogenation. After impregnating the carbon support with Mo precursors, each sample was used either as is or treated at 400 °C in N2 flow, as support for Pt nanoparticles (NPs). The heating treatment before Pt deposition had a positive effect on the catalytic performance. Indeed, TEM analyses showed very homogeneously dispersed Pt NPs only when they were deposited on the heat-treated Mo/C supports, and XPS analyses revealed an increase in both the exposure and reduction of Pt, which was probably tuned by different MoO3/MoO2 ratios. Moreover, the different acid properties of the catalysts resulted in different selectivity.

In situ formation of Au-Pd bimetallic active sites promoting the physically mixed monometallic catalysts in the liquid-phase oxidation of alcohols.

The catalytic oxidation of alcohols with molecular oxygen on supported nanometallic catalysts represents one of the green methods in a crucial process for the synthesis of fine chemicals. We have designed an experiment using physically mixed Au/AC and Pd/AC (AC=activated carbon) as the catalyst in the liquid-phase oxidation of benzyl alcohol by aerobic oxygen. The evolution of the physically mixed catalyst structures at different stages in the catalytic reaction was investigated by aberration-corrected high-resolution transmission electron microscopy and spatially resolved element mapping techniques at the nanometre scale, and they were also compared with the structure of the bimetallic alloy. For the first time we show the formation of surface Au-Pd bimetallic sites by reprecipitation of Pd onto Au nanoparticles. Negligible Au leaching was observed. The in situ structural evolution can be directly correlated to the great enhancement of the catalyst activity. Moreover, we distinguish the different behaviours of Au and Pd, thus suggesting an oxygen differentiating mechanism for Au and Pd sites. The findings are of great importance to both the understanding of the structure-activity correlation and the design of highly active catalysts in green chemistry.

Using supported Au nanoparticles as starting material for preparing uniform Au/Pd bimetallic catalysts.

One of the best methods for producing bulk homogeneous (composition) supported bimetallic AuPd clusters involves the immobilization of a protected Au seed followed by the addition of Pd. This paper investigates the importance of this gold seed in controlling the resulting bimetallic AuPd clusters structures, sizes and catalytic activities by investigating three different gold seeds. Uniform Au-Pd alloy were obtained when a steric/electrostatic protecting group, poly(vinyl alcohol) (PVA), was used to form the gold clusters on activated carbon (AC). In contrast Au/AC precursors prepared using Au nanoparticles with only electrostatic stabilization (tetrakis(hydroxypropyl)phosphonium chloride (THPC)), or no stabilization (magnetron sputtering) produced inhomogeneous alloys and segregation of the gold and palladium. The uniform alloyed catalyst (Pd@Au(PVA)/AC) is the most active and selective catalyst, while the inhomogenous catalysts are less active and selective. Further study of the PVA protected Au clusters revealed that the amount of PVA used is also critical for the preparation of uniform alloyed catalyst, their stability, and their catalytic activity.

Effect of Carbon Support, Capping Agent Amount, and Pd NPs Size for Bio-Adipic Acid Production from Muconic Acid and Sodium Muconate.

The effect of support, stabilizing agent, and Pd nanoparticles (NPs) size was studied for sodium muconate and t,t-muconic acid hydrogenation to bio-adipic acid. Three different activated carbons (AC) were used (Norit, KB, and G60) and carbon morphology did not affect the substrate conversion, but it greatly influenced the adipic acid yield. 1% Pd/KB Darco catalyst, which has the highest surface area and Pd surface exposure, and the smallest NPs size displayed the highest activity. Furthermore, the effect of the amount of the protective agent was studied varying metal/protective agent weight ratios in the range of 1/0.00-1/1.20, using KB as the chosen support. For sodium muconate reduction 1% Pd/KB_1.2 catalyst gave the best results in terms of activity (0.73 s-1), conversion, and adipic acid yield (94.8%), while for t,t-muconic acid hydrogenation the best activity result (0.85 s-1) was obtained with 1% Pd/KB_0.0 catalyst. Correlating the results obtained from XPS and TEM analyses with catalytic results, we found that the amount of PVA (polyvinyl alcohol) influences mean Pd NPs size, Pd(0)/Pd(II) ratio, and Pd surface exposure. Pd(0)/Pd(II) ratio and Pd NPs size affected adipic acid yield and activity during sodium muconate hydrogenation, respectively, while adipic acid yield was related by exposed Pd amount during t,t-muconic acid hydrogenation. The synthesized catalysts showed higher activity than commercial 5% Pd/AC.