A review on photo-thermal catalytic conversion of carbon dioxide

Accepted Manuscript A review on photo-thermal catalytic conversion of carbon dioxide Ee Teng Kho, Tze Hao Tan, Emma Lovell, Roong Jien Wong, Jason Scott, Rose Amal PII:

S2468-0257(17)30064-X

DOI:

10.1016/j.gee.2017.06.003

Reference:

GEE 73

To appear in:

Green Energy and Environment

Received Date: 3 April 2017 Revised Date:

5 June 2017

Accepted Date: 5 June 2017

Please cite this article as: E.T. Kho, T.H. Tan, E. Lovell, R.J. Wong, J. Scott, R. Amal, A review on photo-thermal catalytic conversion of carbon dioxide, Green Energy & Environment (2017), doi: 10.1016/ j.gee.2017.06.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Energy-harvesting from the sun via concentrated solar irradiation or localised plasmonic

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heating for catalytic conversion of carbon dioxide

ACCEPTED MANUSCRIPT Article Type: Mini Review

Title:

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A review on photo-thermal catalytic conversion of carbon dioxide Ee Teng Kho, Tze Hao Tan, Emma Lovell, Roong Jien Wong, Jason Scott*, and Rose Amal*

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Particles and Catalysis Research Group School of Chemical Engineering

Sydney 2052 New South Wales, Australia. Email: [email protected]

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[email protected]

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The University of New South Wales

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Abstract The conversion of carbon dioxide into value-added products is of great industrial and environmental interest. However, as carbon dioxide is relatively stable, the input energy required for this conversion is a significant limiting factor in the system’s performance. By

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utilising energy from the sun, through a range of key routes, this limitation can be overcome. In this review, we present a comprehensive and critical overview of the potential routes to harvest the sun’s energy, primarily through solar-thermal technologies and plasmonic

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resonance effects. Focusing on the localised heating approach, this review shortlists and compares viable catalysts for the photo-thermal catalytic conversion of carbon dioxide.

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Further, the pathways and potential products of different carbon dioxide conversion routes are outlined with the reverse water gas shift, methanation, and methanol synthesis being of key interest. Finally, the challenges in implementing such systems and the outlook to the

Keywords

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future are detailed.

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Carbon dioxide conversion; photo-thermal; plasmonic catalysis; solar thermal

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Table of Contents Abstract ...................................................................................................................................... 1 Keywords ................................................................................................................................... 1 Table of Contents ....................................................................................................................... 2 Introduction ........................................................................................................................ 3

2.

Overview on photo-thermal catalysis ................................................................................ 4

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1.

Motivation for photo-thermal catalysis ....................................................................... 4

2.2.

Heat-harvesting for photo-thermal catalysis ............................................................... 6

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2.1.

Solar thermal technologies ................................................................................... 6

2.2.2.

Plasmonic resonance effect .................................................................................. 9

2.2.3.

Evaluating the relative strengths and weaknesses of solar thermal vs. plasmonic

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2.2.1.

technologies for photo-thermal CO2 conversion.............................................................. 13 Overview of common CO2 conversion products and pathways ...................................... 14 3.1.

Reverse water-gas shift reaction ............................................................................... 14

3.2.

Methanation ............................................................................................................... 16

3.3.

Methanol synthesis .................................................................................................... 18

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3.

Available candidates and opportunities in catalyst materials selection ........................... 19

5.

Challenges in the execution of catalytic photo-thermal CO2 reduction ........................... 25

6.

Conclusion and outlook ................................................................................................... 28

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4.

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References ................................................................................................................................ 30

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1. Introduction It is undeniable that the global energy demand has an ever-growing thirst, driven primarily by our incessant craving for a better quality of life and the consequent urbanisation and economic growth. Despite rising environmental awareness and establishment of policies that

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have facilitated the diversification of energy sources to include more renewable alternatives, fossil fuels will continue to dominate as a primary source of energy [1]. One of the key consequences in having such a dependence on carbon-based energy sources is increasing

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carbon dioxide (CO2) emissions. Increasing emissions of this greenhouse gas and its subsequent accumulation in the atmosphere, due to its relative inertness, has led to great

caused to local ecosystems.

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concerns regarding the ensuing irregularities in global weather patterns and devastation

An appealing solution to the persistent fuel demand and the inevitable CO2 emissions is to

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complete the carbon cycle via the recycling of CO2 into synthetic fuels. This is possible through the reduction of CO2. Nevertheless, one of the main hindrances to such a process is its intense energy consumption. Despite the overall exothermicity of reduction reactions, such

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as methanation and methanol synthesis, the relative stability of a CO2 molecule necessitates energy provision to initiate its conversion. The conventional industrial practice of deriving

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process heat from fossil fuel incineration to fulfil the energy demand is not ideal as it further contributes to CO2 release. Therefore, in addition to studying effective pathways for CO2-tofuel conversions, it is also crucial that a renewable energy feed source, which is effective and suitable for such purpose, is accessible. As the Earth’s most abundant energy resource, the utilisation of sunlight as an alternative energy supply has become a popular field of scientific study. Several opportunities exist in its application to provide for the demand of CO2 reduction processes. While clean, solar energy-

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ACCEPTED MANUSCRIPT powered chemical processes are more commonly associated with photocatalysis, the achievable conversion efficiencies at current stages of development are often unfeasible for large-scale operations as compared to the more industrially established thermal catalysis.

thermal catalytic CO2 reduction thus seem favourable.

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With solar irradiation comprising primarily both heat and light energy, prospects for photo-

In the ensuing sections, the possibility of harnessing heat and light from solar energy through solar thermal technologies and plasmonic resonance effects is outlined. Additionally, the

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application of such technologies for effective catalytic recycling of CO2, with particular

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reference to the strength and limitations of the differing routes, is reviewed. In addition to discussing the potentials of different candidate materials for simultaneously achieving successful solar absorption and CO2 reduction, the foreseen challenges ahead in the realisation of such a process are also assessed.

2.1.

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2. Overview on photo-thermal catalysis Motivation for photo-thermal catalysis

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Thermal catalysis is one of the most common forms of catalysis, with conventional industrial processes such as the ammonia synthesis and reforming utilising heat input to compensate for

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the endothermicity of these reactions. One of the key reasons for industrial dependence on thermally driven catalysis is the high efficiency and applicability for large scale processes. Nevertheless, the large energy demand is often provided for via the incineration of fossil fuels [2]. With growing concerns regarding the depletion of non-renewable fuel resources and rising awareness regarding environmental footprints, harnessing solar energy to drive energetically demanding catalytic processes is desirable. Aside from the photovoltaic transformation of sunlight into electricity to facilitate conversion, the direct activation of

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ACCEPTED MANUSCRIPT catalytic materials through sunlight absorption is also possible. The work by Honda and Fujishima [3] on the use of a titanium dioxide semiconductor for water-splitting instigated a rapid expansion in the field of photocatalytic research. However, despite the energetically ideal concept of the direct translation of light into chemical energy, the practical application

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of photocatalytic technologies is greatly hindered by the lack of effective conversions, especially for industrially-scaled operations [4], with the difference in efficiencies often several orders of magnitude below the capabilities of thermal catalysis.

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This subsequently inspired the concept of light-for-heat in catalysis as an alternative to

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conventionally fossil fuel-powered systems. What distinguishes such photo-thermal catalysis from traditional thermal- or photo-catalysis is the utilisation of both the light and heat components of solar radiation, whereby heat is either obtained directly from or initiated by the light absorption. This thus enables the utilisation of a sustainable energy source (as with

thermal catalysis.

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photocatalysis) all the while attaining effective catalytic conversions typically obtained for

The possible pathways to realising the concept are visualised in Figure 1. The ‘direct’

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procurement of heat from solar radiation is possible via the traditional concept of solar thermal heating. Solar thermal heating operates based on the principle of utilising a light-

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absorbing material or infrastructure to capture heat from the visible or infrared spectrum of solar radiation. The trapped heat will then result in an elevation in temperature, the extent to which depends on the properties of the light-absorbing material and infrastructure design. Another pathway from which heat energy can be obtained from sunlight is through the plasmon resonance phenomenon. Although both these ‘direct’ and ‘indirect’ routes essentially rely on vibration at a subatomic scale, the surface plasmon resonance distinguishes itself from the conventional perception of heating. In addition to the

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ACCEPTED MANUSCRIPT conventional heat energy production upon light absorption, this phenomenon allows the formation of intense, localised hotspots due to the resonance of surface plasmons of

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plasmonic materials upon exposure to radiation of certain frequencies.

2.2.

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Figure 1. Sourcing heat from the sun for a photo-thermal catalytic system. Heating of the catalyst bed can be achieved either through directing concentrated solar irradiation onto the catalyst material or relying primarily on the localised plasmonic heating effects through surface plasmon excitation.

Heat-harvesting for photo-thermal catalysis

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There exists a range of routes to harvest heat from the sun for photo-thermal catalytic

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conversions. These routes are examined in this Section, specifically in the context of solar thermal technologies and the plasmonic resonance effect. Finally, a critical evaluation of the application of such heat-harvesting approaches for carbon dioxide is presented.

2.2.1. Solar thermal technologies The sensation of heat felt upon exposure to sunlight crudely explains the concept of solar thermal heating. From residential rooftop water heating systems to solar furnaces capable of temperatures beyond 2300 K, records of solar thermal technology applications can be traced back to prehistoric times [5]. In heat exchanging applications for domestic purposes, whereby 6

ACCEPTED MANUSCRIPT temperature requirements are not as stringent (e.g. water-heating), the basic infrastructure comprises a collector upon which solar radiation is ‘gathered’ and reflected to a working fluid (either gaseous, liquid or molten salts) to conduct and/or circulate the absorbed energy. For higher temperature systems, the collector usually is designed to allow for light-concentrating

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capabilities for heat intensity amplification (Figure 2). Depending on the solar concentrating systems utilised, concentration ratios between the range of 30 – 100 (for a parabolic trough system to achieve 500 – 700 K temperatures, Figure 2a) and up to 5000 – 10000 (in a double-

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concentration system consisting of a heliostat field, a reflective tower and a ground receiver

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capable of temperatures in excess of 1500 K, Figure 2d) are attainable [6].

Figure 2. Main high temperature solar concentrator system categories: (a) parabolic trough, (b) central power tower, (c) parabolic dish, and (d) double concentration. Adapted from [6].

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To achieve effective heat trapping, two key qualities are required by the absorbing material: high solar absorption and low thermal emissivity. Materials with high solar absorption enable more effective heat generation whereas low thermal emissivity is essential to minimise subsequent heat loss through radiation. Therefore, in addition to the intuitive choice of using a dark-coloured heat absorbing material, much effort has been invested in the understanding of optical and thermal properties of materials for constructing solar absorber coatings such as paints, enamel, carbides, and cermets [7, 8].

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ACCEPTED MANUSCRIPT One of the main appeals of the technique of heat harvesting is the extremely high temperatures attainable through the concentration of solar energy. As mentioned previously, temperatures beyond 1000 K are possible depending on the solar concentrator configuration. Such capability is subsequently taken advantage of for thermochemical conversions. These

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processes involve a redox cycle whereby firstly, high temperature (> 1500 K) treatment of a metal oxide (e.g. CeO2 [9-11] and ZnO [12-14]) is performed to obtain its reduced form. The ensuing step is then conducted at a slightly lower temperature whereby the reduced metal

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oxide is exposed to reactants such as CO2 (or H2O) to promote their reduction. Solar-aided reforming is another common conversion process that has been tested in a solar reactor

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configuration [4, 15-17]. A catalyst is usually utilised in order to lower the temperature requirements for conversion (≤ 1000 K). In this case, the catalyst can either be located in a reactor decoupled from the receiver or be included as part of the receiver where it is also

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exposed to irradiation (Figure 3).

Figure 3. Different configurations for a solar reactor: (left) reactor decoupled from solar receiver system, (middle) integrated tubular reactor/receiver, and (right) catalytically active solar absorber. Adapted from [4].

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ACCEPTED MANUSCRIPT 2.2.2. Plasmonic resonance effect As opposed to focusing on infrastructure design, as is the case for solar thermal technologies, the plasmonic route places greater attention on developing the light-absorbing material itself. While both traditional photocatalysis and plasmonic catalysis rely on light-source activation,

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their similarities stop here. Conventional photocatalysis utilises light energy to overcome the bandgap energy difference of a semiconductor, subsequently generating active electrons and holes that are capable of a myriad of redox reactions depending on the reducing/oxidising

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potentials of the resulting charge carriers (i.e. the positions of the conduction/valence bands). Plasmonic catalysis, on the other hand, relies on the resonance of the oscillations of free

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electrons (i.e. plasmons) upon the absorption of incident electromagnetic radiation. Such resonance occurs when the delocalised electrons are supplemented with an energy wave, whose frequency corresponds to the characteristic resonant frequency of the metal

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nanoparticle (Figure 4).

Figure 4. Schematic of plasmon oscillation for a sphere, showing the displacement of the conduction electron charge cloud relative to the nuclei. Reprinted with permission from ref. [18]. Copyright (2003) American Chemical Society.

The radiation frequency required for such excitation depends on factors such as composition, shape, and size [19-24] as illustrated in Figure 5. The dependence of plasmonic response on particle composition and morphology can be explained by Mie theory, which has been readily exploited for theoretical analyses of particle optical properties [25]. Mie theory states that the 9

ACCEPTED MANUSCRIPT total extinction cross-section of a particle depends on its spherical volume, the dielectric constant of its surrounding environment, and the dielectric function of the particle. Factors such as composition, shape, and size influence the dielectric function of the particle. In polyhedral studies (Figure 5a), blue plasmonic shifts have been observed for particles with

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increasing number of facets, with spherical particles having the smallest plasmonic peak position [22]. In these studies, it was also noted that polyhedral particles may exhibit multiple surface plasmon resonances, leading to band broadening and the emergence of new discrete

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peaks [20]. Similarly, changing the composition of the particles through alloying can promote the formation of plasmonic peaks, notably as a mixture of the individual plasmonic

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components (Figure 5b) [23]. The dependency of plasmonic response on particle composition and morphology permits the flexible tuning of absorption wavelength based on the intended application.

In terms of material choices, gold, silver and copper are amongst the most commonly studied

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metals for plasmonic applications due to their strong plasmonic responses [26]. Whilst nanosized gold and silver are popular candidates, nickel and copper have also garnered

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significant attention due to their relatively cheaper costs. These metals also have the advantage of having their plasmonic frequencies correspond to electromagnetic radiation

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residing within the visible-near infrared region, which is in greater abundance within the solar spectrum as compared to the ultraviolet light, upon which most effective semiconductor photocatalysts are reliant on.

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Figure 5. Influence of: (a) dimensions of silver nanoparticles over their peak absorption wavelength [23]; and (b) composition of silver-gold alloys over the spectral absorption profiles of the nanoparticles [24]. Reprinted with permission from ref. [24]. Copyright (2000) American Chemical Society.

For the purpose of photo-thermal catalysis, plasmonic resonance presents itself as an appealing alternative to conventional heating for reactions that require elevated temperatures. While the earlier studies into plasmonic-assisted reactions were mainly dedicated to their photochemical aspects, it was not until 2007 when Cao et al. [27] first looked into utilising thermoplasmonics for nanoscale material fabrication. Ni pigments have long been used for solar thermal heating despite not being previously deemed as plasmonic nanoparticles [28,

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ACCEPTED MANUSCRIPT 29]. In contrast to conventional heating, where heat is provided externally, thermoplasmonics enables rapid and localised heating within the vicinity of the plasmonic nanoparticle [30]. In addition to the thermal energy supplement, reactant molecules in the vicinity of a

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plasmonic nanoparticle can experience an enhanced incident photon rate and/or photogenerated hot electron formation (Figure 6b-c [31]). Not to be confused with thermoplasmonics, photogenerated hot electrons have an athermal charge charrier distribution which cannot be described using a Fermi-Dirac distribution. Instead, their behaviours are

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more similar to electron-hole pairs in semiconductors [32]. The injection of hot electrons may

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occur directly from catalyst to reactant or through a semiconductor support interface, similar to that of a dye-sensitised photocatalyst [33]. By acting as both the optical absorption and catalytic sites, plasmonic photocatalysts have advantages over semiconductors due to their higher electron density and affinity to reactants. Consequently, in contrast to conventional photocatalysis, which typically provides yields in the micromole (per gram catalyst per hour)

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range, plasmonic catalysts are often capable of yields greater by orders of magnitude [34-36].

Figure 6. Possible mechanisms for plasmonic catalysis: (a) thermoplasmonic temperature increase for heat transfer to reactant within vicinity; (b) nearby reactant experiencing an apparent increase in incident photon rate (i.e. near field optical enhancement); (c) hot electron injection into nearby reactant; (d) plasmonic heat generation resulting in enhanced electronhole generation within a semiconductor for reactant molecule activation; (e) near field optical enhancement experienced by semiconductor, leading to enhanced electron-hole generation; and (f) hot electron injection from a plasmonic nanoparticle into the neighbouring semiconductor, which is then transferred into a reactant molecule nearby. Reproduced from ref. [31] with permission from The Royal Society of Chemistry.

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ACCEPTED MANUSCRIPT 2.2.3. Evaluating the relative strengths and weaknesses of solar thermal vs. plasmonic technologies for photo-thermal CO2 conversion Aside from the capabilities and potential of the respective solar energy-harvesting techniques discussed above, it is important to acknowledge the relative limitations of the technologies

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for the design of a successful photo-thermal CO2 conversion system that is practical for real world applications. While laboratory-scale experiments often place a primary focus on aspects such as conversions and yields, industrial considerations such as scalability, cost

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effectiveness, and robustness of the system also have to be taken into account for successfully

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transferring novel technologies to commercial operations.

Solar thermal catalysis has the advantage of technical maturity with tests carried out in commercial scale-receiver/reactors since the 1980’s [2, 16, 37, 38]. While the tests have delivered promising results, the robustness of catalyst materials under the typically high

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operating temperatures of solar thermal catalytic reactors is frequently cited to be the shortcoming of such systems with catalyst disintegration via sintering and/or the encapsulation of active phases being amongst the most commonly reported issues [16, 17, 39-

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41]. These occurrences largely stem from the fact that the high temperature reaction conditions place severe stress on the lifetime of a catalyst. Moreover, the need for a thermally

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resistant reactor with high levels of technical precision, in addition to the large footprint requirements, leads to exorbitant costs associated with constructing solar thermal infrastructure.

On this basis, plasmonic catalysis appears to offer the advantage as the intense heating occurs locally, within the vicinity of the active nanoparticles. As a result, the demand for external heat supply to the system and overall thermal stress on the infrastructure is much less for plasmonic-based catalytic systems. In the case where heat is sourced from within the

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the available thermoplasmonic studies, strict requirements on the nanoparticles size (<5 nm), high incident light intensity, and/or extreme isolation of nanoparticles are currently the challenges faced for plasmonic catalysis [42]. At such a relative stage of infancy, most

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investigations remain invested in the development and understanding of materials and are yet

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to venture into feasibility studies for industrially scaled catalytic processes.

3. Overview of common CO2 conversion products and pathways Owing to the low CO2 conversion efficiency in CO2 reduction reactions via plasmonic catalysis, understanding CO2 chemistry is critical for improving the conversion efficiency.

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CO2 is an apolar molecule due to the linear configuration of its two polar C=O bonds. Due to its relative inertness, the resulting highly positive Gibbs free energy means that its conversion

activate the CO2.

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is often not thermodynamically favourable. Consequently, a catalyst is often required to

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Transforming CO2 into synthetic fuels can occur through different pathways into a myriad of C-based products. In this review, the focus will be placed upon its catalytic hydrogenation into CO, CH4, and CH3OH, which are often quintessential building blocks for the downstream production of fuels and chemical commodities.

3.1.

Reverse water-gas shift reaction

The reduction of carbon dioxide into carbon monoxide (Equation 1) is one of the more popular routes for CO2 conversion studies mainly due to its low hydrogen consumption (as

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Equation 1

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CO2 + H2 ⇌ CO + H2O

The mechanisms suggested for reverse water-gas shift (RWGS) reaction fall into two main categories: (i) the redox pathway [46, 47] and (ii) the formate decomposition mechanism [48, 49]. The redox mechanism is most often described over a Cu-based catalyst, whereby the

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metallic Cu0 atoms are partially oxidised to form Cu2O and carbon monoxide (Equation 2)

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and the oxidised copper is later reduced by the hydrogen present in the system, reforming it to its metallic state accompanied by the formation of a water molecule (Equation 3). CO2(g) + 2Cu0(s) ⇌ CO(g) + Cu2O(s)

Equation 2

H2(g) + Cu2O(s) ⇌ H2O(g) + 2Cu0(s)

Equation 3

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In contrast to the redox mechanism, whereby the hydrogen molecule does not actively interact with the carbon dioxide molecule, the formate decomposition pathway suggests that the hydrogen molecule first hydrogenates the carbon dioxide into a formate intermediate. It is

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then from this intermediate that the cleaving of the carbon-oxygen double bond occurs to release a CO molecule [50]. The formate-mediated route is also reported to occur for the

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methanation pathway (discussed in Section 3.2), but has often been debated regarding the extent of its contribution toward product formation. An investigation by Goguet et al. [51] is one such example. Using a Pt/CeO2 catalyst for their in operando spectrokinetic investigation, they proposed that the Pt metal maintains the active (i.e. reduced) ceria surface via H2-spillover. The resulting vacancies on the ceria surface then bond with CO2 molecules to form surface carbonates, which are the main intermediates that facilitate the final formation of CO species. 15

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3.2.

Methanation

The production of methane from CO2 (Equation 4) is of great interest in certain geographical locations due to the ease in incorporating its distribution into existing infrastructure and

with the added bonus of regulating CO2 emissions. CO2 + 4·H2 ⇌ CH4 + 2·H2O

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networks [52]. Furthermore, it serves as an attractive solution for renewable energy storage

∆H = -165 kJ/mol

Equation 4

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In terms of the mechanistic pathway involved, claims from both experimental and theoretical studies again falls into two main domains: (i) direct hydrogenation of CO2 or (ii) CH4

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formation via a CO intermediate [53], with majority of the findings suggesting the latter process is the more probable methanation route.

First proposed by Bahr, CH4 formation via the CO pathway was supported by Peebles and Goodman in an investigation, which compared the methanation of CO and CO2 over a Ni(100)

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surface [54]. They found the activation energy and reaction rate values for CH4 formation from CO2 and CH4 formation from CO were similar under identical reaction conditions. Later studies utilising spectroscopic methods, such as diffuse reflectance infrared spectroscopy

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(DRIFTS), and Fourier-transformed infrared spectroscopy (FTIR) coupled with mass spectroscopy, have observed both formate and CO intermediates on the catalyst surface [55-

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57]. The ensuing interpretations, however, have led to different conclusions. Marwood et al. [55] proposed the formation of formate through a carbonate species, which was subsequently hydrogenated into an adsorbed CO species (COads) (illustrated in Figure 7), which has been suggested to be the rate determining step [56]. In contrast, an in situ DRIFTS study (using Rh/ γ-Al2O3) by Jacquemin et al. [57] reported instead that COads is a product of the direct dissociation of CO2 without mentioning the detection (and role) of formate species.

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Figure 7. Proposed CO2 methanation mechanism involving the formation of formate through a carbonate species. S = support, M = metal, I = metal-support interface [55].

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Experimental measurements coupled with computational methods have also been used to probe the role of intermediate species during CO2 hydrogenation on nickel. While it was acknowledged that both formate and CO are present under the reaction conditions commonly applied in the mechanistic studies of CO2 methanation, Vesselli et al. [58] demonstrated that

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formate was purely a spectator species and that the reaction proceeded via the direct hydrogenation of the C-O bond of CO2. This conclusion arose mainly from the findings that formate has high reaction barriers for further hydrogenation for its transformation into CO.

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This agrees with the findings by Marwood et al. [55] who, despite proposing a different

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methanation pathway, also reported the relative stability of the formed formate species, and further lends credibility to the argument of Goguet et al. [51] (discussed above) on the formation of CO not being dominated by the formate route. Ren et al. [59] performed density functional theory (DFT) calculations for the three mechanisms listed in Figure 8 on the Ni(111) surface with the energy involved in each step of the three paths being illustrated in Figure 9. A conclusion similar to Vesseli et al.’s was reached whereby a comparison of the energy barriers involved in the rate determining steps of each route revealed the mechanism

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plausible.

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Figure 8. CO2 methanation mechanism: (a) via CO involving the formate species; (b) via CO without the participation of the formate species; and (c) direct hydrogenation of CO2 without the participation of a CO intermediate [59].

3.3.

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Figure 9. Potential energy diagram for different CO2 methanation mechanisms. Path 1, 2, and 3 correspond to the pathways listed in Figure 8a, b, and c, respectively [59]. TS = transition state.

Methanol synthesis

One of the most apparent advantages methanol has over products such as CO and CH4 is its liquid state at normal atmospheric temperature and pressure. This is desirable as it provides a practical alternative for energy storage and transport. Furthermore, much like CO, it can also serve as an important starting constituent for olefins and aromatics [60-63]. One of the major challenges for the methanol synthesis process (Equation 5) is it is usually less selective than 18

ACCEPTED MANUSCRIPT methane and CO under low-pressure conditions. High pressures (50 – 100 bars) are often required to suppress CO formation via the RWGS pathway (Equation 1) [64]. ∆H = -49 kJ/mol

Equation 5

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CO2 +3·H2 ⇌ CH3OH + H2O

Similar to the Sabatier case, arguments on the CO2 to methanol reaction pathway are dominated by the formate route and the CO route factions. The pathway suggested for the latter case proposes that CO2 first undergoes RWGS-like hydrogenation before the CO

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formed is sequentially hydrogenated into formyl (HCO), formaldehyde (H2CO), methoxy

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(H3CO), and finally methanol [65]. The study by Grabow and Mavrikakis [63] suggested the formate pathway being more likely, whereby the hydrogenation of CO2 into methanol occurs via the following sequence: CO2* → HCOO* → HCOOH* → CH3O2* → CH2O* → CH3O*→CH3OH* where the symbol * indicates an adsorbed species.

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A mechanistic study by Studt et al. [66], which compared CO2 to methanol pathways on different Cu-based catalysts, shed further light on the debate regarding the importance of differences in catalyst composition impacting on the resulting mechanism experienced by the

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CO2 molecules. In addition to the known effects of variation in reaction conditions (e.g. feed gas composition, temperature, and pressure), their calculations demonstrated that the

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promoting effect of Zn in a Cu-ZnO containing catalyst was not only kinetically-based, but also stemmed from a deviation in the reaction mechanism due to the different sites at which the key reactions occur.

4. Available candidates and opportunities in catalyst materials selection Materials that have been studied for CO2 hydrogenation is listed in Table 1. One of the most common families of materials used for general catalysis is the metal family. For CO2 conversion, transition metals such as platinum [67, 68], copper [63, 65, 66, 69, 70], cobalt [71, 19

ACCEPTED MANUSCRIPT 72], ruthenium [56, 73, 74], rhodium [75-79], and nickel [53, 79-85] are amongst those found to be active for CO2 hydrogenation. Aside from being applied singly, the alloying of different metals to form a bimetallic catalyst has also been exploited to manipulate a catalyst’s electronic properties [86-91]. One of the primary aims of doing so is to affect product

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selectivity. For example, on combining platinum and cobalt a greater tendency toward CO production was reported [89] whereas varying the composition of a cobalt-iron bimetallic catalyst influenced the resulting alkane/alcohol selectivity [91]. More recently, a nickel-

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gallium catalyst [90] was developed and shown to be capable of higher selectivity toward

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methanol production.

Due to the effect of particle size on catalyst activity, nano-scaled metal catalyst size is often obtained and/or maintained by its dispersion upon a support material. Rather than acting simply as an inert host, such supports are often capable of exerting significant influence over the resulting surface chemistry of a catalyst. For the purpose of CO2 conversion, one of the

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most useful traits of a support is its ability to encourage the adsorption and even the activation of the CO2 reactant. Such observations have been reported for conventionally

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popular oxide-type supports. While ZnO is one of the most studied oxide supports for (thermal) CO2 catalysis, due to its synergistic effects with copper catalysts [51] [92-94],

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investigations on surface defect-rich oxides such as TiO2 [43, 95-97], CeO2 [86, 98, 99], Ga2O3 [100, 101], In2O3 [102-105], and their combinations [65, 106-108] have also been gaining momentum. Similar CO2 activation capabilities were found for carbide materials such as titanium carbide [109, 110] and molybdenum carbides [111-113], where CO2 conversions were also obtained despite the absence of metals. Nevertheless, when coupled with metals such as copper, nickel, or gold, a significant enhancement in activity (with varying results in product selectivity) have been observed [109]. Such synergism is often attributed to the interaction between the metal catalyst and the oxide or carbide surface via occurrences such 20

ACCEPTED MANUSCRIPT as creation of active centres at the metal-support interface [43, 108, 114], promoted CO2 adsorption upon the oxide or carbide surface [106, 115] and charge redistribution at the metal-support interface.

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Aside from the conversion aspects, there is also an opportunity to adapt solar energy absorbing functionalities to these active catalyst materials. For example, amongst the metals active for CO2 reduction, Cu, Ni, and Au nanoparticles exhibit plasmonic absorption that lies within the solar spectrum [30, 116-121]. The concept of plasmonic CO2 catalysis has, in fact,

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been demonstrated in the works of Navarrete et al. [122] and Wang et al. [123]; the former

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presenting a proof-of-concept Cu/ZnO-impregnated aerogel-containing microreactor for selfheating CO2 reduction and the latter detailing a study on the effectiveness of laser-induced heating on Au/ZnO for a similar application.

Aside from the dependence on the metal component, there also exist prospects for the non-

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metallic (support) materials to contribute to the harvesting of solar energy. Transition metal oxides, such as those mentioned above, are semiconductors that are capable of light energy absorption due to their unique band structures. Therefore, as described by Baffou and

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Quidant [31], in addition to thermoplasmonic effects, the combination of an active, plasmonic metal catalyst with a semiconducting material can result in enhanced charge carrier

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generation (due to thermoplasmonics or optical near-field enhancement) or hot electron injection into the semiconductor (Figure 5d-f). Such interactions upon light illumination have been evidenced in various works on both catalytic and mechanistic studies [124-130]. Additionally, it is not uncommon for metal/metal oxide-type materials to be used as selective absorber coatings for solar thermal collectors. The coupling of aluminium oxide with metals such as cobalt [131, 132], silver [133], platinum [134], molybdenum [135, 136], and most prevalently, nickel [137-141] has been previously reported. The popularity of the nickel-

21

ACCEPTED MANUSCRIPT alumina composite stems from its solar absorption properties – it possesses the ideal combination of high absorbance (for effective energy absorption) and low thermal emissivity (to minimise losses through thermal radiation) [138]. The possibility of utilising the spectrum-selective characteristics of such a combination of materials for solar thermal

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catalysis has been revealed to be attainable through the CAESAR (CAtalytically Enhanced Solar Absorption Receiver) project [142]. CAESAR was applied to the CO2 reforming of methane using a catalyst material (in the form of a catalyst foam), which acted as a receiver

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upon which the concentrated solar radiation was reflected to. Carbide materials also exhibit great potential for such an application. Carbides that have been tested for selective solar

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absorbance purposes include hafnium, tantalum, and titanium carbides [143-145]. Although studies specifically on carbides active for CO2 reduction (e.g. Ti- and Mo-carbides) are less common than that of alumina, the carbide family of refractory materials has the distinctive advantage of high thermal resistance [146]. This is an important attribute for fabricating a

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catalyst/absorber foam with greater durability as the loss of structural integrity is frequently cited as the mode of failure for alumina-based catalyst foams [16, 17].

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Table 1. Catalyst materials reported for CO2 hydrogenation reactions and their respective performance. Catalyst Reaction Products Performance Ref conditions 1 wt% Pt/TiO2 [43] • Fixed bed • CO (~50 %) • 2.03 - 6.48 × 10−3mol s-1 flow gcat-1 (573 K) • Remaining • 573 – 873 K composition • TOF = 0.423 – 2.716 mol s1 not (573 K) explicitly mentioned • Fixed bed • CO – major • TOF = 0.44 – 2.716 s-1 (573 [95] flow product K) • 576 – 873 K • < 10 % CH4 production • Other products present but not explicitly mentioned

22

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Ni/Ni(Al)Ox

• • • • • •

NiO/SBA-15

2 wt% Pt/CeO2

Co/CeO2 promoted)

(K- • • •

Pt, Ni, Pd, Co on • CeO2

Fixed bed 673 – 1173 K Fixed bed 573 – 723 K Fixed bed In operando DRIFT-MS system Packed-bed 673 – 873 K Atmospheric pressure

• •

Ni /CZ**

•

423 – 673 K

EP • •

• •

Fixed bed 423 – 673 K

• • • Ce2O3-promoted • In2O3 • CeO2-promoted Ga- •

Packed bed 473 – 923 K

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Fixed bed Atmospheric pressure 673 – 753 K Fixed bed 423 – 673 K

Ni/Ce0.72Zr0.28O2

Ni-Rh/CZ**

In2O3

• 1.62 – 4.18x10-3 min-1 gcat-1

[86]

• •

CH4 CO

• 0 – 80 % conversion

[81]

• • • • • • • • • •

CH4 CO CH4 Trace CO Trace C2H6 CO CH4 CH4 CO Pure CO products

• 0 – 75 %

[82]

• Approx. 90 % CO2 conversion

[83]

• 0 – 50 % CO2 conversion • TOF = 0 – 12 min-1 • 80 – 100 % CO2 conversion

[147]

Fixed bed 523 – 773 K Fixed bed

• -

[148] [51]

• •

CO CH4

• 13 – 38 % CO2 conversion

[98]

• •

CO CH4

• 2.88 – 12.51x10-3 min-1 gcat-

[86]

• •

CO CH4

[44]

• • • • • •

CH44 CO C2H6 CH4 CO C2H6

• 0.5 – 1.1 mmol gcat-1 min-1 (673 K) • 5.8- 20 mmol gcat-1 min-1 (823 K) • 70 – 80 % CO2 conversion • 2.16 – 2.41 mol CO2 gNi-1 s• Best activity at 76 % CO2 conversion

[149]

•

CH4

• 0.71 – 2.07 mol CO2 gNi-1 s-

[85]

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NixCe0.75Zr0.25−xO2

Batch reactor with FTIR attachment 673 – 823 K 1 atm

CO CH4

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• • • •

• •

SC

• •

Ni/ Al2O3

Batch reactor with FTIR attachment Fixed bed 473 – 773 K, atmospheric pressure Fixed bed 523 – 773 K 523 – 773 K 5 – 20 bar

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Pt, Ni, Pd, Co on γ- • alumina

• • • •

CH4 CO C2H6 CO

•

CO

•

CO

1

[84]

1

1

• 41.1 – 85.2% CO2 conversion • 78 % CO2 conversion • 2.37 mol CO2 gNi-1 s-1

[84]

• 6 – 35 % CO2 conversion

[102]

• 0.39 – 19.98 % CO2 conversion • 2.58 – 10.99 % CO2

[107] [106]

23

Pt, Pd, Ni, Fe, or Cu • on La-ZrO2 • •

La1−xSrxCoO3-δ

• •

La0.75Sr0.25Co(1− y)FeYO3 TiC

• • • • •

ZrC

•

NbC

•

TaC

• • •

Mo2C

• • •

WC

•

•

CO Other products not mentioned CO ( 96 % selectivity) CH4

•

CO

•

• 30 – 35 % CO2 conversion

[151]

• 18.5 – 40.6 % CO2 conversion

[152]

[153]

• 110.8 µmol min-1 g-1 average CO generation rate

CO

• 40 – 80 µmol min-1 g-1

[154]

•

CO

• 0.59 % CO2 conversion • TOF = 2.1 min-1

[155]

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• •

• •

[150]

•

EP

• •

Electric-field assisted thermal reaction Fixed bed 423 K with 3 mA input current RWGS-CL system* 1123 K (during CO2 feed) RWGS-CL system* 823 K Fixed bed reactor 573 K Atmospheric pressure Fixed bed reactor 573 K Atmospheric pressure Fixed bed reactor 573 K Atmospheric pressure Fixed bed reactor 573 K Atmospheric pressure Fixed bed reactor 573 K Atmospheric pressure Fixed bed reactor

CO (97 % selectivity)

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• •

•

conversion • 37.5 % (for BaZr0.8Y0.16Zn0.04O3)

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FeO nanoparticles

523 – 773 K Fixed bed 873 K Atmospheric pressure Fixed bed 873 K

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22O3 • BaZrO3 (Y, Zn, Ce- • doped) • •

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ACCEPTED MANUSCRIPT

•

CO

• 0.46 % CO2 conversion • TOF = 8.9 min-1

[155]

• •

CO CH4

• 2.09 % CO2 conversion • TOF = 61.1 min-1

[155]

•

CO

• 1.73 % CO2 conversion • TOF = 52.8 min-1

[155]

• • •

CH4 CO

• 4.67 % CO2 • TOF = 66.5 min-1

[155]

• •

CO CH4

• 3.30 % CO2 conversion • TOF = 59.1 min-1

[155]

24

ACCEPTED MANUSCRIPT

CO CH3OH C2H5OH CH3OCH3 CH4 C2H6 C3H8 and others CO (major product) CH4 CH3OH

Batch reactor • attached to UHV chamber • • 0.5 atm • CO2:4.5 atm H2 * RWGS-CL: Reverse water-gas shift chemical looping ** CZ: ceria-zirconia

• 4 – 31 % conversion

• 10 – 14x1015 produced molecules cm-2 s-1

[112]

[109]

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Au/TiC

•

• • • • • • •

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Cu, Ni, or Co on • Mo2C •

573 K Atmospheric pressure 473 – 575 K 2 MPa

SC

• •

5. Challenges in the execution of catalytic photo-thermal CO2 reduction An ideal catalyst possesses the following qualities: high activity, selectivity, and stability. To design a photo-thermal catalyst whereby heat generation is supplemented, partially if not

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completely, from its solar absorption capabilities, additional considerations have to be taken into account for the material’s heat generation efficiencies.

EP

For example, the issue of material stability becomes even more substantial in the case of plasmonic catalysis as plasmonic absorption and the resulting effects are greatly affected by

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both the size and shape of the nanoparticles [20, 21, 156]. It is also known that nanoparticles tend to succumb to events such as sintering and loss of nanostructured morphology upon extensive exposure to heat. Therefore, the control of such properties of a material becomes one of the key steps to ensure the practical applicability and robustness of the phenomenon. The design of photo-thermal materials often calls for a composite component to facilitate optimisation of the solar heat generation performance and catalytic conversion capabilities. In this case, the effect each component has on the other also must be taken into account. For

25

ACCEPTED MANUSCRIPT instance, in the case of Au/TiO2 whereby both the metal and support are capable of lightactivation, Tan et al. [157] used in situ spectroscopic studies to illustrate a difference in the preferred pathway (for ethanol oxidation) upon introducing a light source to a thermal reaction. In addition, on utilising light sources with different wavelength ranges (ultraviolet

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and visible), variations in the origins of the active electrons and their subsequent transfer routes were observed, which consequently altered the reaction outcome. The study highlighted the importance of understanding the pathways associated with photo-thermal CO2

SC

reactions to gain better insights into catalyst performance especially in terms of product

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selectivity.

Complementary to material design and fabrication, the experimental setup for photo-thermal catalysis requires careful consideration, as conventional systems used for gas-solid heterogeneous catalysis (e.g. enclosed plug-flow tube reactors) are often not suited to lightbased applications. Existing solar thermal reactors, such as the one designed by Palumbo et al.

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(Figure 10), can be modified for photo-thermal catalyses under concentrated solar irradiation [158]. In their study, Meng et al. [159] retrofitted a water splitting rig for tandem hydrogen

EP

generation and photo-thermal CO2 reduction using a xenon arc lamp (20 mW/cm2) as the illumination source, Nonetheless, the main drawback of these designs is the lack of a

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secondary heating system for deconvoluting photothermal heating and photo-induced charge transfer effects. To achieve this, both Westrich et al. and Tan et al. utilised external heating elements to heat up the catalyst bed from the base while exposing the top to continuous illumination [160, 161]. Alternatively, Upadhye et al. [162] adapted a commercially available reactor (Harrick Scientific, HVC-MRA-5), which was originally designed for in-situ spectroscopy analyses, to study the impact of visible light illumination on CO2 reduction by oxide-supported Au catalysts. In these studies, the secondary heating systems were capable of generating temperatures up to 400 – 600 oC, which allows for the study of non-illuminated 26

ACCEPTED MANUSCRIPT (i.e. dark) thermal catalysis, while thermal heating from the irradiation sources was

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SC

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minimised through the use of specific bandwidth sources and/or filters.

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Figure 10 Potential reactor configurations for coupling and/or decoupling of heat and light in photo-thermal catalytic studies: (a) Palumbo et al. [158]; (b) Meng et al. [159]; (c) Westrich et al. (Adapted with permission from ref. [160]. Copyright (2011) American Chemical Society.); (d) Tan et al. (Adapted with permission from ref. [161]. Copyright (2016) American Chemical Society.); and (e) Upadhye et al. (Reproduced from ref. [162] with permission from The Royal Society of Chemistry.).

While the focus thus far has been placed on the reduction of CO2 to fuels, it must be kept in

EP

mind that hydrogen plays an equally important role in the reaction. This then brings us to the

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issue of a suitable hydrogen source. Despite the increasing abundance of CO2, hydrogen is not as readily available and requires additional steps for its production. In terms of current approaches, the steam reforming of methane remains as the primary route for its large-scale production. However, despite its effectiveness, steam reforming is an energy intensive process often powered by fossil fuels. This will then further contribute to CO2 emissions and defeat the original intention of utilising hydrogen for CO2 recycling. Aside from natural gas, water is another appealing hydrogen source. In order to overcome the energy intensive water splitting process for effective mass hydrogen (and oxygen) production, 27

ACCEPTED MANUSCRIPT three potential solutions are frequently proposed: photocatalytic water splitting [163, 164], photoelectrochemical water splitting [165] and solar powered electrocatalytic water splitting [166]. Hydrogen can be produced by photocatalysis via the direct utilisation of solar energy. Reviews detailing the studies and applications of sustainable photocatalysts to achieve cost-

RI PT

effective photocatalytic water splitting are readily available [164, 167]. Recent developments in photocatalysis include the introduction of carbon-based nanostructures coupled with existing semiconductors or as standalone photocatalysts [163, 164, 168-170]. While

SC

exhibiting potential, separation of the hydrogen and oxygen gas products remains a major

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bottleneck for coupling this technology with photo-thermal CO2 recycling. Another potentially sustainable hydrogen generation technology is photoelectrochemical water splitting which is essentially a short-circuited solar-powered electrochemical cell. Photoelectrochemical water splitting utilises semiconductor-based electrodes, which are capable of splitting water into hydrogen and oxygen upon photoexcitation. Nevertheless, the

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viability of this solution for large-scale application is often challenged by its low solar energy to electricity conversion. To date, electrocatalysis coupled with matured photovoltaic

EP

technology is thought to be the most feasible system for mass producing hydrogen as a feed for CO2 recycling. The conversion record is currently held at 40 % efficiency with the use of

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a triple junction solar cell [171]. The feasibility of such solar-powered electrocatalytic water splitting has, in fact, been evidenced by Jia et al. [166], whose work demonstrated a solar-tohydrogen conversion capability of more than 30 %.

6. Conclusion and outlook Solar-powered fuel production that consumes CO2 as feedstock is an ideal concept that shows great potential for simultaneous CO2 mitigation and renewable fuel production. While the thermal catalytic reduction of CO2 has been relatively well-studied, the effects of coupling 28

ACCEPTED MANUSCRIPT the catalytic process with light requires further work and understanding in order to gain a better comprehension of the process chemistry and mechanisms involved. Although solar thermal technologies and plasmonic catalysis have mostly been discussed

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individually, they do not have to be mutually exclusive. Concurrent use of both technologies may, in fact, complement the shortcomings of each technique. For example, the utilisation of plasmonic materials in a solar thermal reactor may be used to reduce the amount of solar concentration required from the collector, thus reducing the severe requirements on the solar

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generation to achieve a sufficient temperature.

SC

reactor infrastructure while reducing the sole dependence on thermoplasmonic heat

To accomplish the realisation of photo-thermal CO2 catalysis, one of the key factors is successful integration of multiple disciplines of study: solar absorbing infrastructure design; materials development; the sourcing of renewable hydrogen; effective CO2 capture; and an

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understanding of CO2 conversion pathways. Furthermore, surveys on influential factors, such as solar irradiation consistency, space availability, and the integration of synthetic fuel production with the existing distribution network and infrastructure will also be necessary to

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locations.

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assess the suitability of photo-thermal CO2 reduction processes for different geographical

29

ACCEPTED MANUSCRIPT

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