Nanostructured Catalysts for Environmental Applications

In the last decade, many efforts have been dedicated to the research of metal oxide catalysts that could be able to replace the noble metals presently used in the catalytic combustion of volatile organic compounds (VOC). Among them, the spinel oxides, such as ferrospinels (AFe₂O₄), manganites (AMn₂O₄), and cobaltites (ACo₂O₄), attracted most interest due to their unique structure and peculiar bulk and surface properties which are expected to have a positive effect on their catalytic behavior in the complete oxidation reaction. Therefore, this chapter focuses on Fe-, Mn-, and Co-based spinel oxides catalysts for the complete oxidation of VOC, with an emphasis on the research published in the last decade. Correlations between the synthesis method used and their physicochemical characteristics, on one hand, and their catalytic performance in VOC total oxidation, on the other hand, are highlighted.

A set of transition metal oxides was prepared via a sol-gel synthesis using different metal (Mn, Cu, and Fe) nitrates to ensure a proper manganese to metal ratio in the final product. Specifically, three pure metal oxides (Mn₂O₃, CuO, and Fe₂O₃) and mixed oxides (MnCu₁₅, MnFe₁₅, and MnCu_7.5Fe_7.5) were synthesized and characterized by means of XRD, N₂-physisorption at −196 °C, H₂-TPR, FESEM, and XPS techniques. The catalysts were tested for the catalytic oxidation of volatile organic compounds (VOCs) using two probe molecules, namely ethylene and propylene. As a result, the best reaction rates were observed for the MnCu_7.5Fe_7.5 powder sample and were attributed to the synergistic interactions occurring between the Mn, Cu, and Fe species in the crystalline structure. Similarly, Mn₂O₃ showed a good catalytic performance. The excellent catalytic activity of the oxides was correlated with the high amount of reactive chemisorbed oxygen species located on the surface, since these species are useful for the oxidation of VOCs. As well, the improvement of the catalytic activity corresponded to the enhanced reducibility of the catalysts at lower temperatures (i.e., better lattice-oxygen mobility) observed during the temperature-programmed reduction studies.

Preferential oxidation of carbon monoxide in hydrogen-rich streams needs a suitable catalyst that selectively oxidizes CO avoiding H₂ oxidation. Among the proposed catalysts, copper oxide supported on ceria (CuO/CeO₂) received wide interest due to its intrinsic activity and selectivity and low cost with respect to noble metals.

In particular, it has been shown that the performances are significantly affected by optimizing the copper-ceria interaction, and then the copper dispersion. In this light, reducing to nanoscale levels has been proven to be the solution.

In this chapter, results of the effect of nano and subnano structures of CuO/CeO₂ catalysts on the CO-PROX performance are reviewed and critically discussed.

At nanosize, Cu dispersion and oxygen mobility are both enhanced. Furthermore, the copper reduction to the metallic Cu (H₂ oxidation sites) is limited and CO₂ desorption is activated at lower temperatures.

The role of dopants and/or supports as graphene and carbon nanotubes in improving the intrinsic activity and the resistance to the inhibiting effect of carbon dioxide and water vapor are also discussed, highlighting the effect of dopants on the modification of the redox properties by increasing bulk and/or surface oxygen vacancies.

Mesoporous materials are interesting supports for the dispersion and the size control of metallic particles used as active phases of heterogeneous catalysts. The large porosity of such materials provides some confinement effect and physical barriers against sintering, which are expected to enhance the catalytic properties. In this work, a systematic study of the influence of the pore size on the performances of catalysts designed for the Dry Reforming of Methane reaction has been carried out. To do so, three siliceous SBA-15 supports (S4, S5, S7) with similar grain sizes (0.4–0.6 μm) but different mean pore diameters (4, 5, and 7 nm, respectively) were impregnated by an aqueous solution of nickel(II) nitrate in presence of pentane (“Two-solvents method”, 5 wt.% of Ni). After calcination and reduction with H₂, the smallest Ni particles were observed in the S7-based Ni catalyst (D_Ni = 5.4 nm), in contrast to the S4-based one (D_Ni = 7 nm). This is explained by an improved accessibility of the larger pores during the nickel impregnation step. As a consequence of this better metal dispersion, increasing the pores size of the siliceous support also led to an enhancement of the catalytic properties in terms of both activity and stability. In particular, the CH₄ conversion increased from 63% (Ni/S4) to 75% (Ni/S7) at 650 °C under a GHSV of 72 L g⁻¹ h⁻¹.

This chapter is focused on the sustainable valorization of lignin and its derived molecules, through the application of the transfer hydrogenolysis technology, by using nanostructured bimetallic Pd-based catalysts, in order to achieve high added-value products. In particular, nanostructured bimetallic co-precipitated Pd-based catalysts (Pd-M systems), such as Pd/Fe₃O₄, Pd/Co and Pd/Ni, were used and their textural and structural properties have been deeply elucidated through several characterization techniques (XRD, TEM, SEM, H₂-TPR, XPS and EXAFS) in order to highlight the key factors that influence the peculiar catalytic activity in the reductive upgrading of lignin-derived aromatic ethers. Hydrogenolysis and transfer hydrogenolysis processes were focused on three model molecules of lignin: Benzyl Phenyl Ether (BPE), Phenethyl Phenyl Ether (PPE) and Diphenyl Ether (DPE) that mimic typical C-O lignin linkages, such as α-O-4, β-O-4 and 4-O-5 bonds. A comparison between the performance of bimetallic Pd-M catalysts and that of the commercial Pd/C is also included.

Catalytic (i.e., catalyst-coated) diesel particulate filters (DPFs) represent the best option for removing particulate matter, which is mostly composed of soot, from diesel engine exhaust. In this chapter, the main critical issue for regeneration of catalytic DPFs, i.e., the contact in the solid-solid reaction between soot and catalyst, is addressed ranging from powdered soot-catalyst mixtures to real catalytic filter. The main factors affecting the soot-catalyst contact are discussed in the light of the most important literature results, with a particular emphasis on nanostructured catalysts. The improvement in catalytic properties at the soot-catalyst interface occurring at the nanoscale is analyzed. Challenges to be addressed in the near future are also highlighted.

In this work, different TiO₂-based systems were synthesized. Specifically, phosphorous was considered as nonmetal dopant into TiO₂ structure of the photocatalysts. The doped samples were herein labeled as TiO₂-P_0.6, TiO₂-P_0.7, and TiO₂-P₃, where 0.6, 0.7, and 3 indicate the average atomic phosphorus content into each sample.

The physico-chemical properties of the samples were investigated by complementary techniques, including XRD, N₂ physisorption at −196 °C, FESEM, EDX, XPS, and (DR)UV-Vis spectroscopies. Then, the samples were tested for the total oxidation of ethylene under two different sources: UVB (wavelength = 312 nm, intensity = 12 W m⁻²) and UVA (wavelength = 365 nm, intensity = 8 W m⁻²).

The results under UVB source have shown that the most promising catalyst is TiO₂-P₃ (TOF = 7.5 μmol h⁻¹ g⁻¹, TOS = 160 min) and a positive reactivity trend was observed: the higher the P-content, the higher the reactivity. On the other hand, under the UVA source, the most promising catalyst is TiO₂-P_0.6 (TOF = 21.3 μmol h⁻¹ g⁻¹, TOS = 160 min). In fact, the samples with higher P-contents decrease their performances at longer TOS, likely due to the surface deposition of carbon-like molecules.

Light-driven reactions for solar fuels have been receiving tremendous interest, leading of the possibility to store solar energy, our biggest and cleanest renewable energy source. Efficient solar to fuel conversion needs photosynthetic materials with strong absorption and high photocatalytic properties. Colloidal semiconductor nanocrystals are cutting-edge materials for this application, thanks to their tunable optical and electronic properties through size, composition, morphology, and assembly. In this chapter, some insights on the challenges to improve photocatalytic performance are reported, followed by an overview of different parameters that can be controlled to cope with these limitations. Finally, some devices at the forefront are illustrated.

The scale-up of photochemical or photocatalytic processes is a hard task, requiring the correct definition of light distribution across the device. After a collection of examples of different photoreactor layouts adopted for water treatment, the main modelling issues are reviewed.

Alternative radiation modelling approaches are compared. The reaction rate expressions are presented, considering the dependence on light, catalyst and reactant distribution, including possible mass transfer limitations.

Environmental catalysis is a promising technology to be integrated into the present water management systems to address several emerging challenges, such as rising pressures on water availability and the impact of micropollutants.

Increasing understanding (and capabilities for development) of nanotechnologies has created opportunities to enhance the performance of various catalytic systems. In particular, the characteristics of carbon nanomaterials have been shown to potentially improve the performance of traditional carbon materials as catalysts or catalyst supports in water treatment.

This chapter details how manipulation of carbon nanomaterials from the nano- to the macroscale is used to create catalysts tailored to the requirements of the target applications. The relationships between the design at the nanoscale, including surface chemistry and textural modifications, and the deployment of the catalysts at the macroscale are illustrated through a number of relevant examples. Several cases from recent literature are used to delineate the current state-of-the-art.

A brief outlook for the future of carbon nanomaterials as catalyst and catalyst support in water treatment is offered. The challenges in integrating these solutions in real applications are discussed, and a pathway to the future is suggested.

Adsorption is an affordable and feasible strategy for water purification. In recent decades, graphene-based adsorbents have emerged as the next-generation sorbents due to their intriguing physicochemical properties like large specific surface area, excellent chemical stability, and two-dimensional structure. In this chapter, recent progresses on the synthesis protocols of graphene-based sorbents are compiled critically. Their environmental applications, especially for the adsorptive removal of dyes from water, are reviewed based on different working mechanisms. At last, current challenges and future prospects are identified.

Carbon nanomaterials have demonstrated their potential as adsorbents, self-catalysts, and catalyst supports, to effectively remove pollutants from air and water. These materials possess unique properties, such as inertness, stability in acidic and basic media, and mainly the ability to tune their porosity and surface chemistry. Thus, the hydrophobic nature of carbons allows the interaction with nonpolar pollutants, but the creation of heteroatom and chemical functionalities of the carbon surface improves their affinity to polar pollutants. Furthermore, thermal/activation methods are applied to tailor the porosity and surface area. This book chapter provides an overview on the properties and performance of carbon nanomaterials for air and water remediation. A special attention is given to the removal of typical pollutants from air (e.g., CO₂, NO_x, SO_x, and volatile organic compounds). The use of nanostructured carbons, alone or combined with metal oxides, is also reviewed for water remediation using advanced oxidation processes (AOPs) with special emphasis on photocatalysis, although other AOPs such as ozonation, catalytic wet air or peroxide oxidation, and Fenton-based processes over mostly graphene-based materials are also addressed.

In this chapter, the application of natural zeolites as sustainable materials for environmental protection is described. In particular, the clinoptilolite was studied for capturing carbon dioxide (CO₂) emitted from industrial processes at moderate outlet temperatures and for wastewater remediation. Specifically, for CO₂ capture and storage, the clinoptilolite was used as an adsorbent solid and it was tested at 20 and 65 °C. The wastewater treatment was achieved via Fenton-type reactions with a solution of azo-dyes acid orange 7 (AO7), as target molecule for azo dyes. The physico-chemical properties of the clinoptilolite, along with the ion-exchanged materials, were analyzed by means of N₂ physisorption at −196 °C, X-Ray Diffraction (XRD), Field Emission Electron Microscopy (FESEM), and Energy Dispersive X-ray spectroscopy analysis (EDX). The results evidenced that the clinoptilolite can be a sustainable material for capturing CO₂, because of the interesting adsorption capacity at moderate temperature. On the other hand, remarkable results for the AO7 degradation were obtained with the Fe-clinoptilolite catalyst in the presence of both ascorbic acid and H₂O₂.

Enzymes are biological catalysts capable of recognizing a substrate and catalyze reactions of hydrolysis and synthesis. The most significant property of enzymes is their high specificity toward their substrates since they are able to recognize and act upon a molecule from a pool of similar compounds.

Enzymes are labile catalysts at certain operative conditions that may severely affect their stability. However, the attachment of enzymes to solid supports has proven to be a good solution to stabilize them and, thus, to preserve their catalytic performances.

The fundamentals of enzyme biocatalysis in sustainable processes are summarized in this chapter. The advantages of immobilized enzymes in environmental applications and sustainable processes will be addressed considering the most suitable materials and the most common immobilization methods. The use of biocatalysts in bioremediation, biofuel production, and in the valorization of waste streams is reviewed.