Water-based synthesis of TiO₂/CeO₂ composites supported on plasma-treated montmorillonite for parathion methyl degradation

Highlight

• TiO₂/CeO₂-Montmorillonite composites were synthesized by water-based method.

• Prepared composites showed good ability to degrade parathion methyl.

• Pillaring of the MMT layers reflected in a higher degradation activity.

• Prior plasma treatment of the MMT caused further improvement of the samples activity.

Abstract

The undemanding water-based synthesis was employed for the preparation of TiO₂/CeO₂ composites supported on Montmorillonite (MT). The prior plasma treatment of Montmorillonite led to significantly faster dispergation in water during the synthesis. The composites were used as reactive adsorbents against toxic organophosphorus pesticide parathion methyl. Efficient pillaring of the Montmorillonite layers by surface deposition of TiO₂/CeO₂ composites reflected in a substantially improved degradation activity towards parathion methyl in non-polar (heptane) and polar solvents (acetonitrile) investigated by HPLC/DAD. Moreover, the use of plasma treated Montmorillonite in the composites resulted in a higher degradation rate than with use of pristine MT. The formation of composites and their physicochemical properties were studied by XRD, XPS, FTIR, nitrogen physisorption, SEM and HRTEM/EDS mapping. Prior plasma treatment of the MT in a simple arrangement led to an easier formation of the composites and caused further improvement of the samples activity.

Introduction

The principal use of cerium dioxide is for glass polishing, nevertheless, its distinctive properties - especially reversible conversion to a nonstoichiometric oxide, open up the scope to countless other applications such as catalytic oxidation of automotive exhaust (Kaspar et al., 1999), petroleum cracking (Dejhosseini et al., 2013), organic chemicals synthesis (Tamura and Tomishige, 2015), energy storage (Arul et al., 2015), high-temperature and corrosion coatings (Ardelean et al., 2008, Sreedhar et al., 2012), or sensors (Izu et al., 2002, Zhai et al., 2009). The list of applications is being constantly updated by strenuous research efforts in the field of fuels cells (Yu and Xi, 2012), photocatalytic degradation of organic pollutants (Arul et al., 2015, Yang et al., 2014), thermochemical water splitting (Al-Shankiti et al., 2013), or even in nanobiology and regenerative medicine (Das et al., 2013). In several recent studies, Janoš et al. demonstrated CeO₂ can also be exploited in environmental protection as cheap, non-toxic, earth-abundant, and efficient reactive adsorbent for remediation of highly toxic organophosphorus compounds, i.e. civil warfare agents (CWA) and pesticides (Janos et al., 2016, Janos et al., 2014, Janos et al., 2015). For example, parathion methyl is a widely used effective organophosphorus pesticide and insecticide that, however, possess extremely high toxicity. Therefore, numerous studies have been devoted to its toxicology (Abdollahi and Karami-Mohajeri, 2012), analytical detection (Gong et al., 2012, Yang et al., 2011), mapping of its fate (Sapbamrer and Hongsibsong, 2014, Villaverde et al., 2008) and chiefly to its removal from the environment; several eco-friendly approaches were proposed, e.g. using bacteria biodegradation (Pino and Penuela, 2011), Fenton's reaction (Diagne et al., 2007), or photocatalytic oxidation (Sud and Kaur, 2012). The surface reactive adsorption should be included among these methods since it is safe, rapid, and does not require energy supply. Furthermore, relatively cheap and available materials such as nanoscale ceria, whose unusual activity towards organophosphates is attributed to the ability of lattice oxygen to participate in the reaction (Mitchell et al., 2003), can be used.

The new attractive properties of single metal oxides can be unfolded by coupling them with other metal oxides, organic frameworks, or various substrates in mixed solid solutions and composites. Improvement of the ceria redox properties, oxygen mobility, modification of surface acidity and surface area is often achieved in combination with ZrO₂ (Neto and Schmal, 2013, Si et al., 2013), TiO₂ (Li et al., 2012a, Li et al., 2012b), CuO (Wang et al., 2013), MnOx (Li et al., 2012a, Li et al., 2012b), NiO (Si et al., 2013) and other oxides. Furthermore, ceria is liable to form interesting structures with extraordinary properties using metal organic frameworks (Kim et al., 2013, Maiti et al., 2014), 0D, 1D and 2D nanomaterials such as graphene (Rajendran et al., 2014, Wang et al., 2011), C₃N₄ (Wang et al., 2015), or MoS₂ (Gong et al., 2016), and layered aluminosilicates. Clay minerals are regarded as green materials as they are readily available in nature, and can be used with minimum processing. They are composed of ultra-thin crystalline layers superimposed on interlayer of hydrated ions (Kloprogge et al., 1999). Thanks to their structure, clay minerals possess unique physicochemical properties such as large surface areas, swelling, ion exchange, and active broken-edge MO bonds. The main disadvantage of the clay minerals is, however, lack of permanent porosity. They swell upon hydration, but after severe dehydration (e.g. heating) the layers collapse and their interlayer surface become inaccessible for chemical processes (Pavlidou and Papaspyrides, 2008). The introduction of stable pillars can solve this problem and leads to convenient improvement of the properties of formed composites that are superior to those of their individual components (Kloprogge et al., 2005). Ramaswamy et al. (2008) and Cecilia et al. (2015) investigated Montmorillonite (MT) as a high-surface-area support for CuO-CeO₂-based catalysts. CeO₂/TiO₂-pillared clay composite prepared by impregnation method was also used for elemental mercury removal from flue gas (He et al., 2016). In this case, TiO₂-pillared clay served as a support for CeO₂ catalyst but their synergistic effect is not discussed in detail. Lin et al. prepared the Laponite pillared by the CeO₂/TiO₂ by microwave intercalation and tested it for photocatalytic decoloration of methylene blue (Lin et al., 2010). The coupling of TiO₂ with CeO₂ improved the utilization efficiency of UV light to produce more trap centers of electron and hole, and inhibit the electron-hole recombination, while Laponite increased the effective surface area for MB adsorption. However, the synthesis utilize relatively expensive organometallic Ti compounds and ethanol as a solvent. In the recent publication, MT supported with various single metal oxide nanoparticles were prepared by ion exchange, precipitation, and successive calcination and subsequently tested towards CWA sulfur mustard (Kumar et al., 2015).

The modification of clay minerals by various species (neutral polar molecules, cations, complex cations) is based mainly on their surface anchoring or intercalation predominantly via thermal and chemical treatment. Only a very few studies are focused on plasma modification of clay minerals, see reference (Capkova et al., 2016). Our group successfully modified MT in fluidized bed reactor with Argon gas feed at low pressure about 560 Pa by RF plasma in wide range of power from 10 W up to 100 W. The modification to a state typical for long time heat annealing, hours at 450 °C, was reached by the plasma treatments in seconds (Capkova et al., 2016).

Recently, our group have demonstrated that homogeneous precipitation with urea is a convenient method for preparation of TiO₂/CeO₂ composites used for degradation of toxic organophosphates (OP) DMMP and Parathion methyl (PM) (Henych et al., 2016), while plasma treatment of the MT is a suitable and very fast method for its dehydration and dehydroxylation (Capkova et al., 2016). Dehydroxylation result in a creation of active sites in the clay mineral structure that can improve its catalytic activity and can serve as bonding sites for guest species in a design of functional nanostructures. The purpose of this work was to demonstrate use of cheap, earth-abundant materials and facile methods to construct ternary composites based on TiO₂/CeO₂-pillared MT and study their synergistic effect on the degradation of toxic organophosphorus chemicals. We employed very easy low-temperature water-based co-precipitation method for preparation of ternary composites that possess significantly higher degradation activity towards OP than pure ceria and titania and TiO₂/CeO₂ composites. The use of as-received and plasma-treated MT was also investigated.