Photoelectrocatalytic degradation of ethylene by a combination of TiO₂ and activated carbon felts

Abstract

Postharvest loss of quality is an important problem in the food and horticultural product industry. One of the major factors contributing to loss of quality is the uncontrolled exposure of the products to small amounts of ethylene gas during storage. In this study we investigated the photoelectrocatalytic (PEC) degradation of ethylene gas at a temperature of 3 ± 1 °C and relative humidity of 90 ± 3% on an activated carbon felts (ACF)-supported photocatalyst titanium dioxide photoelectrode [TiO₂/ACF] or on a photoelectrode which had been modified by coating the ACF support with platinum [TiO₂/ACF-Pt]. The apparent pseudo-first-order kinetic model was used to describe the PEC degradation of ethylene. The key designing parameters for a PEC reactor affecting the degradation efficiency in terms of the rate constant of this model were studied, including the bias voltage and the light intensity. Degradation of ethylene by applying a bias voltage to the [TiO₂/ACF] |Nafion|[TiO₂/ACF] electrode-membrane assembly or to the [TiO₂/ACF-Pt] |Nafion|[TiO₂/ACF-Pt] electrode-membrane assembly enhanced the efficiency of photocatalytic (PC) degradation. The combination of the ACF support modified with platinum and the applied bias voltage were found to have an additive enhancement effect on the rate constant compared to PEC degradation carried out using the unmodified ACF support. With respect to the [TiO₂/ACF-Pt] |Nafion|[TiO₂/ACF-Pt] electrode-membrane assembly, a kinetic model was established using response surface methodology to describe the relationship between the rate constant and the affecting parameters. Optimized parameters were found to be a light intensity of 3.1 mW cm⁻² with a bias voltage of 47.5 V.

Introduction

Ethylene gas (C₂H₄) is released by fruit and vegetable and acts as a plant hormone that controls many plant responses [1]. Ethylene gas is beneficially used in many instances such as the promotion of uniform ripening in bananas, and de-greening of fresh citrus. However, the effects of ethylene gas are not all positive. Even small amounts of ethylene gas in the atmosphere in the storage facility can induce undesirable reactions in most fresh produce, such as development of senescence, bitter flavors, chlorophyll loss, disease susceptibility and physiological disorders. Storage of produce is of economic importance to the food and horticultural industries. Storage allows producers, handlers and sellers to spread availability over periods of high and low demand, maintaining supply and stabilizing costs. Within the industry in Guangdong Province, PRC, it is estimated that 30% of losses are directly related to ethylene exposure. Removal of ethylene from the storage environment retards spoilage, reduces loss and increases profit. While conventional methods such as venting, potassium permanganate oxidation, adsorption onto brominated carbon, catalytic oxidizers, and hypobaric storage help to control and remove ethylene from storage facilities, each has limitations in controlling ethylene levels in the presence of high ethylene-yielding produce and can result in the alteration of temperature and humidity conditions in the storage facilities [2].

Photocatalysis (PC) using titanium dioxide (TiO₂) as the catalyst is seen as a promising alternative to the oxidation processes used to degrade ethylene in most conventional methods and has attracted considerable attention over the last 10 or more years [3], [4], [5], [6]. Maneerat et al. [7] reported that the photocatalytic degradation of ethylene using TiO₂ can be carried out under high humidity at both room temperature and low temperature. The major advantage of this technology is the complete mineralization of undesirable organic contaminants in gas phases to CO₂ and H₂O in addition to the chemical stability of the catalyst, which is also non-toxic and inexpensive. When TiO₂ is irradiated with ultraviolet (UV) light whose photon energy exceeds 3.2 eV, holes (H⁺) and electrons (e⁻) are created on the surface of the TiO₂, the former being powerful oxidizing agents and the latter reducing agents_. However, recombination of photogenerated holes and electrons (H⁺/e⁻) occurs at the same time. The high level of recombination between holes and electrons is a major factor contributing to the reduction in the catalytic ability of TiO₂ and to the control of the PC efficiency [8], [9]. In this regard, various attempts have been made to enhance the PC efficiency of TiO₂, such as the photoelectrocatalytic (PEC) oxidation process. In the case of PEC, the PC reaction is further accelerated by applying an external potential which moves the conduction band electrons away from the TiO₂ photo-anode towards a counter electrode [10]. This is an efficient way to prevent the recombination of H⁺/e⁻ pairs and results in extending the life of the active holes [11], [12], [13]. There are some reports relating to the PEC process [14], [15], [16], [17], but in these studies, the organic contaminants in water or wastewater were used to investigate the PEC degradation.

TiO₂ is mostly frequently applied to a carrier or support before use [18]. The development of a TiO₂ photocatalyst anchored on supporting materials with a large surface area on which ethylene could be condensed would be of great importance in the indoor environment of storage facilities where the ethylene concentration is dilute under conditions of high relative humidity and low temperature. Activated carbon felts (ACF) are especially interesting as potential support materials due to their excellent characteristics and properties. The surface area of these activated carbon felts is very high, and the porous network is mainly formed by deep pores in a narrow range of sizes, especially micropores [19]. This form of activated carbon has important advantages over granular activated carbon, such as, the uniform distribution of microporosity, a superior rate of adsorption and desorption, and a more rapid attainment of equilibrium [20], [21]. ACF has been introduced as the support material for TiO₂ in some photocatalysis studies, and the enhancement of gas phase toluene condensation as a result of the combined adsorption of ACF with photocatalysis TiO₂ has been reported [22], [23]. Recent studies have reported the modification of ACF by metal doping which has led to the improved removal of specific contaminants in the gas as a result of changes to the physical and chemical properties of the surface of the carbon materials [24], [25]. Although the photoelectrocatalytic (PEC) degradation of organic pollutants in liquid phases has been carried out using a TiO₂ film, the PEC degradation of ethylene gas under conditions of high relative humidity and low temperature on an ACF-supported TiO₂ photoelectrode [TiO₂/ACF] or on a photoelectrode consisting of the photocatalyst TiO₂ supported on an ACF support modified with a noble metal [TiO₂/ACF-M] has rarely been reported until now.

Taking into account these findings, we have carried out preliminary work on the photoelectrocatalytic (PEC) degradation of ethylene gas at a temperature of 3 ± 1 °C and relative humidity 90 ± 3% using an ACF-supported photocatalyst TiO₂ photoelectrode [TiO₂/ACF] or a photoelectrode consisting of the photocatalyst TiO₂ supported on an ACF support modified by the deposition of Platinum [TiO₂/ACF-Pt]. The key designing parameters for a PEC reactor affecting the degradation efficiency in terms of the rate constant were studied, including the bias voltage and the light intensity. A systematic experimental design based on response surface methodology (RSM) was used for modeling and optimization of the designing parameters of PEC reactor. In addition, the surface morphology and elements of electrode have been characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS).