Thermo-photo degradation of 2-propanol using a composite ceria-titania catalyst: Physico-chemical interpretation from a kinetic model
Graphical abstract
Introduction
Heterogeneous photocatalysis is considered to be one modern technology for decontamination of Volatile Organic Compounds (VOCs) [1]. One of most relevant advantages is that it operates under mild temperature and pressure conditions, and using oxygen from the air as the oxidizing agent [2], [3], [4]. As far as the materials are concerned, TiO2 is the most used photocatalyst due to its good catalytic properties. It is also a relatively inexpensive and nontoxic material [2]. On the other hand, VOCs decontamination processes by photocatalysis show several disadvantages. Low quantum efficiency brought by recombination of photogenerated electrons and holes is usually reported [5], [6], [7], [8]. In addition, titania appears only efficient for the removal of low concentration VOCs and displays low sunlight absorption due to its wide band gap (∼3.2 eV for the anatase polymorph). For all these reasons it has a limited industrial application [2]. As a way to increase the profit from sunlight as well as to enhance quantum efficiency, surface sensitizers have been added to formulations based in titania. In particular the use of ceria has received significant attention. Ceria-titania (CeOx-TiO2) composite materials have shown good photodegradation performance against a significant number of pollutants under different operation conditions, concerning gas or liquid phase, oxygen and water vapor concentration and other important variables in terms of catalytic output [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25].
On the other hand, it is interesting to note that catalytic thermal oxidation has been actively used for VOCs elimination with satisfactory results. However, it is a high energy consuming technology and the search of good and stable catalysts appears as a key research activity in this field. Ceria is a common component in both classic and new formulations presenting promising catalytic activity [26], [27], [28], [29], [30].
Alternatively to both thermal or photon-based catalytic technologies, the combination of heat and light energy sources using the adequate catalytic material would drive to a convenient way to overcome most of the problems related to thermal or light alone based decontamination technologies. From a pure thermal perspective the reduction of the operation temperature with consequent lowering of the cost as well as reduction of catalyst deactivation processes can be mentioned [26], [27], [28], [29]. From a photocatalytic perspective, the most important point to overcome is the low reaction rates and consequently, low quantum efficiencies usually achieved [1], [2], [3], [4], [5], [6]. Within this context, CeOx-TiO2 based and related materials would appear promising catalytic systems in thermo-photo catalysis. With photo-catalysis or thermo-catalysis and as above detailed, it has been proved that high activity can be achieved using CeOx-TiO2 based catalysts. Therefore, these materials recently launched the interest in composite systems and the synergetic effect between photo-catalysis and thermo-catalysis. Recent reports for cyclohexane and benzene degradation, CO+O2 or NO+CO reaction are some examples of thermo-photo-catalytic applications tested and showing activity enhancement with respect to thermal- and/or photo-only processes [31], [32], [33], [34], [35], [36], [37], [38].
In this work, we describe a kinetic study concerning the temperature analysis of 2-propanol thermo-photo-degradation using TiO2, CeO2 and CeOx-TiO2 samples. 2-propanol is a volatile organic pollutant present at urban atmospheres and particularly at indoor environments. Among the most typical sources of this pollutant, we can enumerate construction materials, household products, waxes, varnishes and many others [39], [40], [41]. Therefore this pollutant appears a classical benchmark for gas-phase photo-oxidation processes. Using a kinetic approach we expect to contribute to the understanding of the physico-chemical basis of the 2-propanol thermo-photo-catalytic oxidation process. Although several works carried out studies concerning the performance CeOx-TiO2 catalysts, the understanding of the physical roots of such new decontamination process is still low. Moreover, none of the above mentioned works present a detailed kinetic study of the catalytic results [31], [32], [33], [34], [35]. To reach the primary objective of shed light into the thermo-photo-catalysis understanding, we developed a kinetic formulation containing an intrinsic (reaction rate) expression for the reaction occurring in a gas-phase thermo-photo-reactor employing thermal and UV light sources. The procedure solve the differential mass transfer equation, coupled with a nonlinear least-squares fitting algorithm to obtain kinetic parameter value estimations. Such kinetic study is based on the well-established initial steps of all thermo-photo-catalytic processes and included explicitly the radiation (light/thermal)-matter interaction. This allowed determining true kinetic constants providing general information, without any experimental bias concerning the experimental procedure and conditions used. The result of the study not only has physico-chemical significance in terms of quantifying the different “thermo” and “photo” contributions to the reaction but also renders information required for scaling and design reactors [42], [43], [44].
Section snippets
Catalysts preparation
TiO2, CeOx and CeOx-TiO2 catalysts were prepared using a microemulsion preparation method. N-heptane (Scharlau) as organic media, Triton X- 100 (Aldrich) as surfactant and hexanol (Aldrich) as cosurfactant were used. Ultra pure water (Milli-Q) was used as aqueous phase [45]. The TiO2 sample was obtained in a microemulson using titanium tetraiso-propoxide as precursor. This precursor was introduced into the microemulsion drop by drop from a solution with isopropanol (2:3 v/v) [46]. CeO2-TiO2, and
Characterization summary
Although this work focuses on the kinetic analysis of the thermo-photo-degradation of gas phase 2-propanol, it includes a brief section concerning the characterization details of the samples. XRD (X-ray diffraction), UV–vis (UV–vis spectroscopy), microscopy, and XPS (X-ray photoelectron spectroscopy) experimental results are present in the Supporting Information document (“characterization results” section; Figs. S1–S3; Table S1). Table 1 shows a summary of chemical, morphological and optical
Conclusions
The catalytic performance of a ceria-titania composite system in the degradation of 2-propanol was analyzed with respect to the titania and ceria (parent) components using heat and light as energy sources of the reaction. The composite system displays an improved performance under the combined (as well as the independent) use of the two energy sources. This enhancement was analyzed using a kinetic formalism which explicitly includes the effect of the photon absorption in the reaction rate. The
Acknowledgements
We are thankful to MINECO (Spain) for supporting the work carried out through the ENE2016-77798-C4-1-R grant.
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