Salvatore Abate

Dynamics of Palladium on Nanocarbon in the Direct Synthesis of H2O2

This work aims to clarify the nanostructural transformation accompanying the loss of activity and selectivity for the hydrogen peroxide synthesis of palladium and gold-palladium nanoparticles supported on N-functionalized carbon nanotubes. High-resolution X-ray photoemission spectroscopy (XPS) allows the discrimination of metallic palladium, electronically modified metallic palladium hosting impurities, and cationic palladium. This is paralleled by the morphological heterogeneity observed by high-resolution TEM, in which nanoparticles with an average size of 2 nm coexisted with very small palladium clusters. The morphological distribution of palladium is modified after reaction through sintering and dissolution/redeposition pathways. The loss of selectivity is correlated to the extent to which these processes occur as a result of the instability of the particle at the carbon surface. We assign beneficial activity in the selective hydrogenation of oxygen to palladium clusters with a modified electronic structure compared with palladium metal or palladium oxides. These beneficial species are formed and stabilized on carbons modified with nitrogen atoms in substitutional positions. The formation of larger metallic palladium particles not only reduces the number of active sites for the synthesis, but also enhances the activity for deep hydrogenation to water. The structural instability of the active species is thus detrimental in a dual way. Minimizing the chance of sintering of palladium clusters by all means is thus the key to better performing catalysts.

On the Nature of Selective Palladium-Based Nanoparticles on Nitrogen-Doped Carbon Nanotubes for the Direct Synthesis of H2O2

Catalysts based on Pd and Pd–Au nanoparticles supported on N‐doped carbon nanotubes (N‐CNTs) are studied in the direct synthesis of H2O2. The initial selectivity in H2O2 formation is rather high (>95 %); however, there is a fast initial decrease during the first hour of time on stream. This was due to the initial presence of an organic capping agent (polyvinyl alcohol, which is used in the catalyst synthesis to obtain a high dispersion of metal particles). The removal of this capping agent during the reaction leads to a high mobility of metal nanoparticles. The high initial selectivity, when the capping agent is present, is due to small Pd terraces fully covered with chemisorbed O2 and limited H2 chemisorbed sites that consecutively hydrogenate the formed H2O2. The alloying of Pd with Au decreases the intrinsic reaction rate (per mg of Pd) and increases the selectivity in H2O2 formation, whereas Au alone is inactive. Au also has a minor effect on the consecutive conversion of H2O2 in both the decomposition and hydrogenolysis (in the presence of H2 only) reactions. These results suggest that Au does not block the unselective sites of H2O2 conversion but mainly creates isolated small terraces of Pd that can limit H2 chemisorption sites, which thus leads to higher selectivity to H2O2 under given reaction conditions.

New Sustainable Model of Biorefineries: Biofactories and Challenges of Integrating Bio‐ and Solar Refineries

The new scenario for sustainable (low-carbon) chemical and energy production drives the development of new biorefinery concepts (indicated as biofactories) with chemical production at the core, but flexible and small-scale production. An important element is also the integration of solar energy and CO2 use within biobased production. This concept paper, after shortly introducing the motivation and recent trends in this area, particularly at the industrial scale, and some of the possible models (olefin and intermediate/high-added-value chemicals production), discusses the opportunities and needs for research to address the challenge of integrating bio- and solar refineries. Aspects discussed regard the use of microalgae and CO2 valorization in biorefineries/biofactories by chemo- or biocatalysis, including possibilities for their synergetic cooperation and symbiosis, as well as integration within the agroenergy value chain.

Disruptive catalysis by zeolites

The analysis of the new scenario for the industrial production of energy vectors and chemicals evidences the need to foster research in the field of catalysis by zeolites towards a novel, potentially disruptive, type of applications. To stimulate research in this direction, this perspective paper analyses a series of emerging concepts in catalysis by zeolites: i) the role of confinement, ii) the use of the Lewis acidity of zeolites, iii) the new possibilities to extend the concept of confined reactivity, iv) the role of defect sites, and iv) the organo-catalysis by guest species in zeolite cages. Then, two areas of novel possibilities for catalysis by zeolites are discussed more specifically: i) metallo-zeolites for methane conversion and ii) functionalized zeolites for reaction with CO2.

Chemical Energy Conversion as Enabling Factor to Move to a Renewable Energy Economy

Abstract The role of chemical energy storage and solar fuels as key elements for the sustainable chemical and energy production is discussed in this concept paper. It is shown how chemical energy storage, with the development of drop-in carbon-based solar fuels, will play a central role in the future low-carbon economy, but it is necessary to consider its out-of-the-grid use, rather than being limited to be a tool for smart grids. Related aspects discussed are the possibility to: (i) enable a system of trading renewable energy on a world scale (out-of-the-grid), including the possibility to exploit actually unused remote resources, (ii) develop a solar-driven and low-carbon chemical production, which reduces the use of fossil fuels and (iii) create a distributed energy production, going beyond the actual limitations and dependence on the grid.

Catalytic partial oxidation and membrane separation to optimize the conversion of natural gas to syngas and hydrogen.

The multistep integration of hydrogen-selective membranes into catalytic partial oxidation (CPO) technology to convert natural gas into syngas and hydrogen is reported. An open architecture for the membrane reactor is presented, in which coupling of the reaction and hydrogen separation is achieved independently and the required feed conversion is reached through a set of three CPO reactors working at 750, 750 and 920 °C, compared to 1030 °C for conventional CPO technology. Obtaining the same feed conversion at milder operating conditions translates into less natural gas consumption (and CO(2) emissions) and a reduction of variable operative costs of around 10 %. It is also discussed how this energy-efficient process architecture, which is suited particularly to small-to-medium applications, may improve the sustainability of other endothermic, reversible reactions to form hydrogen.

Role of Feed Composition on the Performances of Pd-Based Catalysts for the Direct Synthesis of H2O2

The role of feed composition, in particular the O2 to H2 ratio and the concentration of CO2, has been investigated on two Pd/N-CNT and PdAu/N-CNT catalysts. There is a significant influence on the catalytic behavior, but the effect is complex, and cannot be analyzed in terms of conventional kinetic approaches, due to the presence also of a change in the catalyst characteristics as a function of time on stream (in the fresh samples). Depending on the O2 to H2 feed and catalyst nature, the trend of productivity and selectivity varies differently with the time on stream. A simplified kinetic model has been developed which well accounts for the observed behavior. The model is based on the concepts that (i) the selective sites are associated to small Pd terraces covered by chemisorbed O2 with limited sites for H2 chemisorption, and (ii) during the reaction, due to catalyst modifications, a change of available chemisorbed H2 for unselective parallel formation of H2O and H2O2 hydrogenolysis occurs. The changes during time on stream are related to (i) the removal of PVA capping agent, with an initial increase of available Pd surface area leading also to an increase of the H2 chemisorption sites for unselective parallel conversion and consecutive H2O2 hydrogenolysis, and (ii) for the longer times on stream aggregation of some Pd nanoparticles leading to some decrease in the available Pd surface area. The isolation of Pd ensembles by Au (PdAu/N-CNT) influences this trend of productivity and selectivity to H2O2 as a function of the O2 to H2 ratio in the feed. The change is consistent with the model indicated above.

Enhanced Hydrogen Transport over Palladium Ultrathin Films through Surface Nanostructure Engineering.

Palladium ultrathin films (around 2 μm) with different surface nanostructures are characterized by TEM, SEM, AFM, and temperature programmed reduction (TPR), and evaluated in terms of H2 permeability and H2-N2 separation. A change in the characteristics of Pd seeds by controlled oxidation-reduction treatments produces films with the same thickness, but different surface and bulk nanostructure. In particular, the films have finer and more homogeneous Pd grains, which results in lower surface roughness. Although all samples show high permeo-selectivity to H2 , the samples with finer grains exhibit enhanced permeance and lower activation energy for H2 transport. The analysis of the data suggests that grain boundaries between the Pd grains at the surface favor H2 transfer from surface to subsurface. Thus, the surface nanostructure plays a relevant role in enhancing the transport of H2 over the Pd ultrathin film, which is an important aspect to develop improved membranes that function at low temperatures and toward new integrated process architectures in H2 and syngas production with enhanced sustainability.

Direct synthesis of H2O2 on Pd based catalysts: modelling the particle size effects and the promoting role of polyvinyl alcohol

Sol immobilization is a relevant preparation method, particularly for the preparation of Pd‐based catalysts for the direct synthesis of H2O2. In this preparation, polyvinyl alcohol (PVA) acts as capping agent for Pd particles. Modelling the role of PVA on the Pd nanoparticle size and its promoting effect on the selectivity is important to understand better these catalysts. Here, Pd based catalysts prepared by sol immobilization have been tested in a semi‐batch reactor with an H2/O2 ratio ≈1, analysing the influence of the Pd nanoparticle size and the effect of PVA on the rate constants of the reaction network and their change during the reaction. A stepwise testing protocol coupled with transmission electron microscopy characterization as a function of reaction time has been used. These catalysts were compared with other catalysts prepared by hydrazine reduction and impregnation‐decomposition, where PVA is absent. The results are analysed in terms of rate constants for the various rates in the reaction network in relation with the Pd average particle size and related sites distribution, as a function of the changes occurring during extended catalytic tests. The SI‐series of catalysts (prepared by sol‐immobilization) shows enhanced properties, attributed to the effect of PVA capping agent in forming less defective Pd nanoparticles. This induces both a decrease of direct combustion and secondary hydrogenolysis reactions, and an enhancement of the direct synthesis route. The PVA layer limits H2O2 back‐diffusion with a negative influence on the productivity and selectivity, with its removal leading to initial enhanced performances, but in long‐term to a worsening due to sintering of Pd nanoparticles in extended operations.

Hydrotalcite based Ni–Fe/(Mg, Al)Ox catalysts for CO2 methanation – tailoring Fe content for improved CO dissociation, basicity, and particle size

It is essential for mankind to address greenhouse gas emission, depletion of fossil resources and development of efficient storage and transportation of renewable energy. By using the atmospheric gas CO2 as raw material for the value-added chain, an overall CO2 neutral circular economy is imaginable. The power to gas (PtG) concept tackles all three topics by producing methane as energy carrier using renewable hydrogen and capture of CO2. Methanation has been known for decades; however, the preparation of noble metal free catalysts possessing high activity, selectivity and stability remains challenging. Bimetallic FexNix−1 alloy catalysts have shown synergistic effects for methanation as they can be tailored to the CO dissociation energy. Multicomponent hydrotalcite precursors allow close proximity of the individual metals enabling high dispersion and continuous variation of the composition. Herein, we studied Ni–Fe/(Mg, Al)Ox bimetallic catalysts derived via mixed hydrotalcite precursors. Iron has a significant influence on both activity and selectivity due to small particle sizes, facilitated CO dissociation, and tailored surface basicity. For the best catalyst (Fe/Ni = 0.1), the CO2 conversion rate reaches about 6.96 mmol CO2 molFe+Ni−1 s−1 at 335 °C with a selectivity of 99.3% to CH4, remaining constant for at least 24 h time on stream.