CO₂ photoreduction with water: Catalyst and process investigation

Highlights

• Evaluation of photocatalytic activity in CO₂ reduction with H₂O in CH₄ production.

• Optimization of a synthesis route of N-doped titania photocatalysts with both CuO as co-catalyst.

• Study of the influence of the amount of metal dopants on photoactivity and selectivity.

• The co-presence of Cu and N in the TiO₂ systems enhance the methane production.

• The best catalytic performances have been obtained with a small copper amount (0.2 wt.%).

Abstract

Economic development should not be separated from the concept of sustainability. The goal can be pursued by means of technologically advanced materials and processes that enable environmental protection. Carbon dioxide photoreduction using water as reducing agent could be a green and effective way to pursue this aim and titania is a good photocatalyst for this reaction. In this work the performances of N doped CuO–TiO₂ photocatalysts in gas phase CO₂ reduction have been studied. We have focused the attention on both the catalysts design and the process optimization. We have investigated, in particular, the effect of the presence of nitrogen and copper amount on the final catalysts performances. In order to learn high control of the catalytic process and to manage productivity and selectivity, by operating in very mild reaction conditions, the last part of the work has been centered on tuning the process parameters (CO₂/H₂O ratio). It has been observed that the CH₄ formation is sensitive to copper amount and that exists a close correlation between the catalytic behavior and the reagents ratio.

Introduction

It is well known that climate has rapidly changed in the last decades in terms of average temperatures and severity of weather phenomena [1], [2]. Though climate variability is common and natural in geological history, nowadays it definitely has an anthropogenic boost [3]. The main driving force in climate change is the increasing concentration of greenhouse gases (GHGs) [4]. Among them, carbon dioxide emissions are the most abundant [5]: the International Environmental Agency (IEA) stated that in 2012 CO₂ emissions reached 31.6 Gt [6]. Since the beginning of the industrial era, average CO₂ concentration in atmosphere increased from 270 ppm to 400 ppm in May 2013 [7]. This trends are dependent on fossil fuels utilization and our economic system strongly relies on carbon-based energy technologies [8]. Therefore the two most challenging issues of the 21st century, that are CO₂ increasing emissions and fossil fuels depletion, are strictly connected one to another. So it is extremely important to transform this pollutant into products whose market have worldwide dimensions, i.e. fuels [9]. The inspiration for this approach comes from nature: [10] planet Earth balanced its atmosphere composition by itself for thousands, if not millions of years by means of photosynthesis. With this natural process, plants transform water and carbon dioxide into the high energy molecules they need for living. A similar approach is believed to be a winning solution to the environmental problem. In literature, many works focus on the so-called “artificial photosynthesis” [11], [12]. The similarity is that in both cases light is the primary energy source and high energy products are formed; on the other hand, typical artificial systems are completely different from natural ones [11]. Common products are gaseous and liquid fuels, as reported in Eq. (1).

x CO₂ + y H₂O + hν → solar fuels + z O₂ (1)

They can be readily used in stationary and mobile applications, since they are equivalent to the fossil ones. Differently from traditional fuels, carbon dioxide from solar hydrocarbons combustion can be recycled, closing carbon circle by means of a non-biological process [11]. Moreover, solar fuels distribution is compatible with already present infrastructures for fossil hydrocarbons [13], [14]. For all these reasons, photocatalysis appears to be an attractive technology to pursue carbon dioxide photoreduction [15], [16], [17]. In this work the attention has been addressed to photoreduction of carbon dioxide using water as reducing agent [18], [19], [20], [21] (Eq. (2)).

CO₂ + 2 H₂O → CH₄ + 2 O₂ ΔH° = +802 kJ/mol (2)

As a catalyst, titania has been chosen. This material is nontoxic and inexpensive, and it has good photocatalytic properties [22], [23], [24], [25], [26], [27]. It has been already applied to several environmental and energetic applications, like water splitting [28], VOC [29], [30] and NOx abatement [31], [32], [33]. Inoue et al. [34] first reported that titania is able to catalyse CO₂ photoreduction with water. In fact, when irradiated, a charge separation is generated and the electron-hole pair is formed [35]: in the valence band, oxygen from water is oxidized to molecular oxygen while the excited electron reduces carbon from CO₂ releasing oxygen as well [36], [37]. Then protons and reduced carbon react together generating the desired product [38].

Other stable compounds (such as CO, formic acid, formaldehyde and methanol) might be produced in this process but reaction pathway and process selectivity are highly dependent on light absorption, creation of charge carriers and their use in the process [39], [40].

Unfortunately, the relatively fast recombination rate of photoinduced electron-hole pairs and a low quantum yield for oxygen production as a result of the UV photons absorption restrict the potential photo-application of TiO₂. To overcome these drawbacks, many efforts have been carried out including deposition of noble metals, dye sensitization, metal cation doping, carbon and nitrogen doping [24] and morphology and structure control [41]. Positive effects on titania photoactivity can be pursued by doping with non-metal elements like boron [42], carbon [43], nitrogen [44], fluorine [45] and iodine [46]. These dopants can either substitute oxygen in titanium dioxide lattice or occupy interstitial sites [47]. In both cases, a red shift in titania light adsorption is observed due to a decrease in band gap energy. Among non-metal dopants, nitrogen is one of the most investigated [48]. The incorporation of this element in titanium dioxide lattice causes several electronic modifications that are responsible for enhanced photoactivity: in particular, doping introduces intra band gap electronic states that cause a decrease in required energy for photoexcitation [49]. In addition to that, doping with several noble metals were tested like platinum [50], silver [51] and gold [52], [53] that stabilize the separation of photoexcited charge carriers. Generally, metal loading is very low, usually less than 1 wt.% [54]: high metal fractions are detrimental to titanium dioxide photoactivity [55]. Greater attention has been put on less precious transition metals as Fe and Cu. The incorporation of transition metal ions (e.g., Cu²⁺, Cu⁺, Fe³⁺, etc.) can lead to the formation of electron trapping sites and promote charge transfer from TiO₂ to metal ions, thus resulting in the enhanced photoreaction of surface adsorbed species. Among the transition metal ions, Cu is an appealing dopant due to low cost, availability and enhancement of photoactivity, especially in CO₂ photoreduction. The formation of pn junction between Cu and TiO₂ is considered as the major reason for the improvement [56], [57]. For heterojunctions with mixed semiconductors, the difference between band edges is the major driving force to improve the charge transfer and subsequently the photocatalytic performance. There have been some studies carried out on the Cu₂O/TiO₂ systems, which have confirmed that this system could enhance the charge separation efficiency and lead to a high photocatalytic activity. Lalitha et al. reported that Cu₂O/TiO₂ nanocomposites with size about 20–40 nm exhibited higher activity than pure TiO₂ in the H₂ evolution [58]; Chu et al. synthesized a Cu₂O/TiO₂ catalyst that showed an excellent photocatalytic activity in 4-nitrophenol degradation [59]. Xu et al. [60] suggested that the Cu (identified as Cu⁺) species deposited on TiO₂, forming Ti–O–Cu surface bonds, served as acceptors of electrons that were transferred from the TiO₂ conduction band. Doping with Au or Ag nanoparticles or clusters the sensitivity of the photocatalyst to the visible spectrum can also extend due to their localized surface plasmonic resonance properties [61]. Nevertheless, Cu-loaded TiO₂ catalyst compared to Ag–TiO₂ photocatalysts show higher photoactivity since Cu particles act as electron trapping sites while still maintaining the mobility of photoelectrons [62]. Catalytic reduction of CO₂ with H₂O in the gaseous phase is further investigated by using Cu–Fe/TiO₂ catalyst coated on optical fibres [63]. The synergistic presence of Fe as a co-dopant in Cu/TiO₂ catalyst is has been evidenced in reduction of CO₂ with H₂O to ethylene at the quantum yield and total energy efficiency of 0.024% and 0.016%, respectively. While metal ion modifications of TiO₂ lead to enhancements in charge separation, their effects in altering the optical properties of TiO₂ are limited. In order to overcome this limit, co-doped titania photocatalysts have been studied [64]: in this way electron-hole lifetime is lengthened and visible light adsorption is shifted toward visible light. The used metals are usually noble ones (platinum, gold and silver [65]) while nitrogen is preferred among the non-metallic promoters [66]. So it would be a definite breakthrough to couple a non metal dopant like nitrogen to a less noble metal, like copper.

Catalyst tailoring aside, reaction design conditions must be carefully considered. Fluidized bed reactor is the most common photoreactor [67]. It is employed for batch processes in two-phase heterogeneous systems and generally the catalyst is suspended in an aqueous medium. For this reason the main drawback is CO₂’s poor solubility in water [68]. Otherwise, many studies employ photoelectrochemical (PEC) reactors for gas phase reduction but an external electrical energy must be supplied [67]. Moreover, water adsorption on titania is generally more likely to happen compared to CO₂ [69], though both have to interact on the catalytic surface for CO₂ photoreduction to take place. In fact, on titania surface, water splitting reaction might also occur [70]. Other limits of this system are the low surface area and the complicated separation process required to isolate the catalyst grains. To overcome these drawbacks a valid alternative could be the use of a fixed bed reactor using gaseous reactants. In literature only few examples of gaseous CO₂ photoreduction are reported and the reaction is carried out by using hard conditions in order to implement the final productivity. As a matter of fact, high temperature (up to 100 °C), pressures and irradiance (up to 500 W m⁻²) are suitable to improve photoactivity [63], [71], [72], though this choice makes the process more expensive and less sustainable. Moreover, the majority of the reported works employs batch processes in liquid phase. The novelty of this work is the use of N doped CuO-TiO₂ photocatalysts for CO₂ photoreduction in gas phase and under the mildest conditions (i.e. room temperature and atmospheric pressure and low irradiance) in order to overcome all the issues rising from the utilization of liquid phase systems. Therefore, in this work a green, innovative and effective technology for photocatalytic CO₂ abatement and transformation into fuels has been reported. Particular interest has been centred on the catalyst design considering the formulation of several N doped CuO–TiO₂ photocatalysts. At the best of our knowledge, only a few works focus the attention to co-promoted photocatalysts for CO₂ photoreduction; in particular, literature lacks information about N doped CuO–TiO₂ photocatalysts for this process in mild conditions, though these materials might be suitable to this purpose. The physico-chemical features of the catalysts and the process parameters (CO₂/H₂O ratio) will be analyzed and discussed in depth by correlating synthesis, characterizations and reactivity tests.