Solar Fuels by Heterogeneous Photocatalysis: From Understanding Chemical Bases to Process Development

Abstract

The development of sustainable yet efficient technologies to store solar light into high energy molecules, such as hydrocarbons and hydrogen, is a pivotal challenge in 21st century society. In the field of photocatalysis, a wide variety of chemical routes can be pursued to obtain solar fuels but the two most promising are carbon dioxide photoreduction and photoreforming of biomass-derived substrates. Despite their great potentialities, these technologies still need to be improved to represent a reliable alternative to traditional fuels, in terms of both catalyst design and photoreactor engineering. This review highlights the chemical fundamentals of different photocatalytic reactions for solar fuels production and provides a mechanistic insight on proposed reaction pathways. Also, possible cutting-edge strategies to obtain solar fuels are reported, focusing on how the chemical bases of the investigated reaction affect experimental choices.

1. Why Do We Need Renewable Energy? Prospects and Challenges in Solar Fuels Production

Modern society is intrinsically dependent on reliable and efficient energy sources to sustain its activities. Since the beginning of the industrial revolution at the end of eighteenth century, fossil fuels by combustion have proved to be more consistent energy sources than biomass [1,2].

Among the scientific community there is still a debate on how long fossil fuels will be available, due to the intrinsic uncertainty of reserve estimations [3]. According to Shafiee and Topal in 2009, oil, coal and gas reservoirs will be finished in 35, 107 and 37 years respectively [4]. In recent years this value has not decreased much: in fact, fossil fuel utilisation has been accompanied by technological improvements in the exploitation of shale gas, shale oil, tar sands and hydrocarbon hydrates [5,6]. Despite this, the durability of fossil fuels is a significant concern, and the most critical issue is their intrinsic unsustainability. Fossil fuels do not satisfy the definition of sustainability provided by the World Commission on Environment and Development [7] since their utilisation rate is much higher than their formation, reducing their availability for future generations. However, their worldwide distribution network, efficiency, and most importantly, lower cost (27 €/MWh for coal and 39 €/MWh for natural gas [8]) compared to alternative energy sources (75 €/MWh for biomass [8]) encourages their continued utilisation.

On the contrary, the cost of fossil fuel combustion on the environment is extremely high. Due to the presence of nitrogen and sulphur impurities, and in the case of coal, metal traces, fossil fuel combustion for heating and electricity generation yields products other than water and carbon dioxide (CO₂), such as methane (CH₄), nitrogen oxides (NO_x) and sulphur oxides (SO_x), as well as particulate matter (PM), with detrimental effects for both humans and the environment [9,10]. Most importantly, fossil fuels represent the main cause of CO₂ emissions in the atmosphere: their global related CO₂ emissions grew approximately 25% in the period between 1990 and 2004 [11] and in December 2017, CO₂ concentration reached 408 ppm, the highest observed value in human history. This value will grow to by 2.8 ppm/year in the foreseeable future [12].

Increasing CO₂ emissions represent the most important threat to the atmosphere and environment since it has been established that there is a clear connection between anomalous global warming and CO₂ emissions [13,14]. Due to the anthropogenic origin of these huge environmental phenomena, the word Anthropocene was coined to describe this particular historical moment we are living in. According to Crutzen, the Anthropocene started in the 1960s, when the effects of human activities on climate and environment became relevant [15]. In the late 1990s, policymakers’ search for solid solutions to the environmental threat of CO₂ led to the Kyoto Protocol of 1997 [16] and their efforts were recently renewed by the Paris Agreement in December 2015, a treaty endorsed by 194 countries aimed at keeping world average temperature increase below 2 °C [17]. There are several strategies to pursue this aim [18]:

• to increase efficiency in energy production and consumption processes;
• to improve the ability to capture and sequester CO₂ from the atmosphere and its utilisation;
• to decrease the carbon intensity of the economic system.

While the first strategy relies on optimising already existing technologies for the production and utilisation of traditional fuels, the other two rely on finding either new strategies to obtain alternative energy products or new energy sources. In other words, they require the replacement of fossil fuels, which the world depends on for 80% of energy production [19]. This would mean a complete change in the paradigm of energy and transportation technologies, and thus a transition from Anthropocene to Sustainocene, an era in which economic and social wealth meets wise and responsible use of environmental resources [20]. Considering these last two strategies more closely, one of them relies on CO₂ as an alternative and sustainable source for the same C-based molecules already used as fuels. Such an approach to CO₂ utilisation could be a significant opportunity for innovative industrial processes and also fit Circular Economy dictamens, providing a concrete integration of economic activities and social and environmental wellbeing in a sustainable way [21].

Conversely, reduction of carbon intensity in the economy represents a more radical approach, which pursues the use of alternative C-free energy vectors and intrinsically unties energy production from carbon dioxide emissions. Nowadays, several C-free energy technologies are available (such as hydropower, geothermal, wind, nuclear, ocean energy and photovoltaics [22]), however, they suffer from being efficient for electricity only, limiting their application in the automotive industry. Recently, among the alternative non-hydrocarbon fuels, hydrogen is an emerging fuel for the future for several reasons: no harmful combustion products are produced; it can be obtained through several technologies and from several sources, and its high diffusivity and high heat capacity make hydrogen generally safer than other fuels [23,24]. Moreover, it can be directly used in highly efficient devices, i.e., fuel cells [25,26,27].

Nonetheless, several issues make hydrogen uncompetitive with traditional fuels such as hydrocarbons: insufficient supply for a worldwide market, severe and energy demanding production processes, and last but not least, lack of transportation networks and fuel cells’ cost [25,28]. Due to the issues related to direct utilisation of hydrogen as a fuel, it can be used indirectly to produce new attractive fuels from different sources, especially those that are bio-derived [29,30,31,32]. Nowadays hydrogen is produced mainly from fossil fuels which account for 96% of its production capacity [33,34], but an appealing method to couple sustainable hydrogen production and renewable biomass valorisation is oxygenate compounds reforming [35,36,37,38], which can be used as a hydrogen-storage medium converting into hydrogen on-board through reforming processes, then fed into a fuel cell [39].

This kind of energy source is readily available: Sims et al. assessed that 100 EJ/year of raw lignocellulose could be available worldwide at 2–3 $/GJ [40], while Long et al. estimated 251–1272 EJ/year of biomass from surplus cultivated land [41].

As reviewed by Li et al., several sources and technologies are available to produce bio-derived hydrogen [42]. Steam-reforming requires high temperatures (300–800 °C) [43,44], aqueous phase reforming can reduce operative temperatures (220–270 °C) but requires high pressure (25–30 MPa) while plasma reforming, despite its efficiency, requires electric energy [45]. Photocatalytic reforming, known as photoreforming (PR), has the advantages of mild operational conditions [46] and exploits solar light, the most abundant and widespread renewable energy [47].

Both CO₂ reduction and hydrogen formation are uphill processes, since the products’ energy is higher compared to reagents. In fact, if we consider Gibbs free energy of formation for most simple compounds, CO₂ and water are characterised by the lowest value of all C-based molecules (−394.0 and −228.4 kJ∙mol⁻¹), indicating that any transformation requires energy [48].

Required energy to perform this reaction can be provided in different ways, generally either as heat or electricity. Promising results were obtained both in CO₂ reduction by methanation and in steam reforming of biomass-derived alcohols [49,50,51]; however, these processes require temperatures higher than 400 °C, increasing the final cost of the alternative fuel [52,53,54,55].

A promising alternative energy source is sunlight. The big advantage of sunlight exploitation is that it is available almost anywhere on the Earth’s surface, it can be considered renewable on a human time scale. and most importantly, it is inexpensive [56,57,58]. In the last couple of decades, the exploitation of sunlight for energy production has gained even more attention. At the moment, the only available technologies in the open market are photovoltaics (PV), which are used for electricity production [59,60]. For this reason, the scientific community is actively looking for innovative, sustainable yet efficient processes to convert solar energy into high energy molecules, which are generically called solar fuels. Moreover, Lewis and Nocera have explained why photocatalysis for solar fuels production, can also be thought of as a reliable solution to sunlight intermittency which is an issue in solar energy utilization [61]. In recent years, a great number of papers have been published on these topics, providing either an insight on physicochemical properties of these reactions or technologically advanced materials and photoreactor designs, although the connection between the two topics has not yet been sufficiently highlighted.

For this reason, this review is aimed at summarizing the advancement of the research on the two most promising solar fuel production technologies (i.e., CO₂ photoreduction and photoreforming), providing a comprehensive overview on the most innovative solutions to overcome physicochemical limitations by process design.

2. CO₂ Photoreduction with Water

2.1. Proposed Reaction Pathways

The first study on photoelectrocatalytic carbon dioxide reduction using semiconductors was reported by Inoue and co-workers in 1979 [62]. Irradiation was used to activate a semiconductor anode which was coupled with a platinum cathode, allowing water oxidation and carbon dioxide reduction. It took only a year from this pioneering result for a published paper on purely photocatalytic CO₂ reduction with water on common semiconductors (such as WO₃, SrTiO₃ and TiO₂) [63]. However, the latter approach only became of interest from the late 1990s onwards, when many studies were published on the topic. In all cases, the design of an effective process requires a deep physicochemical knowledge of the reaction.

As with any heterogeneous catalytic process, CO₂ photoreduction also consists of three different steps:

• Reactants’ adsorption and photons absorption on the photocatalyst;
• Heterogeneously catalysed chemical reaction; and
• Products’ desorption.

The first step is fundamental because it allows both reactants to interact with each other with a suitable orientation for the redox reaction to happen.

When the reaction is performed in water, both as a reductant and a reaction medium, CO₂ solubility in the aqueous phase system must be considered as a possible limiting step for the reaction in the liquid phase whereas in the gas phase, mixability is not an issue to be considered. Gaseous CO₂ adsorption can occur with different geometries, affecting overall process efficiency. In the literature, it is well reported that CO₂ adsorption on titanium oxide is considerably weaker than vapour [64,65,66]. In fact, CO₂ adsorption on TiO₂ follows a Freundlich model, which is generally used for non-ideal sorption processes [67]:

$$q=k_f{p}^{1/n}$$

(1)

where q is adsorbed gas (mmol of gas/g of adsorbent), p is pressure ad equilibrium, while k_f and n are Freundlich constants. The calculated 1/n value, 0.4, indicates that the predominant CO₂ adsorption mechanism is chemical adsorption rather than physical adsorption.

Unfortunately, Krischok et al. observed that when CO₂ and water competitive adsorption occurs, CO₂ adsorption was blocked by the presence of pre-adsorbed water on the titanium dioxide surface while weakly adsorbed CO₂ was displaced by post-dosed H₂O, and there was little evidence of bicarbonate formation in either case [68]. Tan and co-workers calculated adsorption constants for CO₂ and H₂O on graphene oxide modified TiO₂ according to a Langmuir-Hinshelwood model and calculated a value of 0.0193 and 8.070 bar⁻¹ respectively, confirming findings from previous research [69].

As observed by Henderson and co-workers for TiO₂ based materials [70,71], oxygen from water coordinates with surface titanium sites, whilst protons interact with bridging oxygen, yielding surface hydroxyl groups, which act as adsorption sites for molecular water [72]. He et al. described several possible configurations of CO₂ and TiO₂ interactions, calculating binding energies by computational models [73]. CO₂ can be linearly adsorbed by Ti-O bonds (Figure 1a) or bidentate chelate-like structures can be observed by the interaction of oxygen atoms in CO₂ with adjacent Ti centres (Figure 1b) or carbonate-like structures can be found when C and O atoms from CO₂ interact with surface Ti and O sites (Figure 1c). According to their calculations, the last two configurations, which involve the weakening of C=O bond, seem to be the most likely intermediate in the reduction route.

Very recently, Olivo et al., by means of CO₂ adsorption studies using FTIR analyses, observed that all three species are present on metal modified TiO₂, but samples with the highest photocatalytic activity proved to be characterised by the highest share of adsorbed CO₂ in bidentate structures [74].

From all provided evidence, it is clear that CO₂ adsorption on commonly used semiconductors, and titanium dioxide in particular, represents one of the most challenging issues in the development of a photocatalytic process for CO₂ reduction.

Considering the sequent step, i.e., photocatalytic electron transfer, without any catalyst or photocatalyst, reduction cannot happen due to the high energy barrier in the gas phase (>200 kJ∙mol⁻¹) [75]. As reported by Nguyen and Ha from theoretical and spectroscopic studies, CO₂ can interact with water vapour [76]; oxygen lone pairs in H₂O can interact with anti-bonding 2π_u at central carbon atoms in CO₂, leading to the formation of van der Waals complexes [77].

Despite the great efforts in understanding CO₂ photoreduction by using conventional catalysis tools, the effect of photons has not been successfully described, and due to substantial differences in experimental procedures and reaction regimes, it is difficult to compare data and assumptions from different teams. UV light is more commonly used than visible light. In fact, bare titanium dioxide, the most common catalyst, requires in the case of anatase phase, a radiation wavelength equal to or lower than 388 nm [78]. On the Earth’s surface, the average irradiance provided by the sun is 1000 W∙m⁻², but only ca. 5% is UV irradiation [79]. In most reported CO₂ photoreduction tests, energy input is considerably higher that available solar energy. In fact, irradiance value is usually in the range of 1000–3000 W∙m⁻², which is much higher than the UV light fraction in sunlight [80,81,82,83]. Results from tests performed at lower irradiances are reported in only a few papers; for instance, Woolerton et al. performed CO₂ photoreduction tests in the aqueous phase under 450 W∙m⁻² irradiance [84], whilst more recently Tahir et al. reported tests conducted at 200 W∙m⁻² in the vapour phase conditions [85]. Nonetheless, at the moment, papers reporting CO₂ photoreduction tests using an irradiance below this value are very rare [86,87].

Reactions involved in all semiconductors’ light harvesting indicate that a high photons input, from a kinetic point of view, enhances the formation of photocatalytically active sites (Equation (2)) [88].

$${\mathrm{TiO}}_2+\mathrm{h}\mathsf{\nu }\rightleftharpoons {\mathrm{h}}_{\mathrm{VB}}^{+}+{\mathrm{e}}_{\mathrm{CB}}^{-}$$

(2)

However, the reverse reaction can also happen, and according to Liu and Li [89], the recombination rate of e⁻–h⁺ pairs is nearly two or three orders of magnitude faster than the rate of charge separation/transport and chemical reaction itself. Therefore, at high irradiance conditions, where the surface is saturated with photons, the rate of charge carrier recombination becomes higher than both light adsorption and surface reaction rates [90]. Despite these general considerations, there are no specific guidelines for photons’ absorption for CO₂ photoreduction. In fact, only Tan et al. have reported the effect of irradiance on methane formation [69]. In their paper, they reported that between 650 W∙m⁻² and 1800 W∙m⁻², which is a much higher range than the one considered in this part of the work, methane production increased with irradiance. However, the growth was not linear and the subject was not further investigated.

Electronic transfer is of great importance in CO₂ photoreduction, since desired product formation requires more than two electrons transfer. In addition, according to Rasko and Solymosi, this step determines overall kinetic rates of the process [91]. On the contrary, from a thermodynamic point of view, considering reduction potentials, reported in

Table 1. Electrons required to reduce CO₂ to C₁ hydrocarbons [92,93].

Products	Electrons to Obtain Product from CO2 Reduction	Redox Potential (eV)
CO	2	−0.53
HCOOH	2	−0.61
HCHO	4	−0.48
CH3OH	6	−0.38
CH4	8	−0.24

, methane formation is the most favoured among the C₁ products due to its less negative potential [92].

However, kinetic insights on this process are not unanimous; different reaction mechanisms have been proposed for this reaction according to different experimental parameters, such as developed catalyst, irradiation source and reaction medium [64,88,89,94]. All proposed mechanisms describe similarly what occurs in oxidative sites on the photocatalyst surface; valence band’s (VB) electron deficiency, induced by sufficiently energetic incident irradiation, is responsible for water oxidation to molecular oxygen [95,96]. On the contrary, the phenomena involved in the reduction process in the conduction band (CB) have not been clarified yet.

In 1995, Anpo et al. for the first time proposed a reaction mechanism for CO₂ photoreduction: XANES, EXAFS and photocatalytic tests indicated that CO₂ reduction occurs on (Ti³⁺–O⁻)* transient species via C=O cleavage and O₂ desorption and subsequent hydrogenation of remaining C surface species with H and OH surface species, without further details on the reducing species. Later, Dimitrijevic et al. used EPR studies to confirm the formation of the common intermediate for any product is CO₂⁻ radical anion, which is bound on the TiO₂ surface [97], as previously observed by Tanaka and co-workers on MgO [98,99]. In the literature, several authors confirmed that the first step is the formation of peroxocarbonate species, which are reduced to formic acid, formaldehyde, and methanol afterwards [89,100,101]. Nonetheless, over the last decade, due to the growth in the processes and catalysts developed, a single mechanism cannot be assumed because a variety of reaction pathways can occur according to reaction conditions and catalysts [89,95].

According to CO₂ photoreduction tests performed in aqueous phase, as reported by Liu and Li [89], the presence of peroxide species in water were hypothesised; but, being characterised by a high redox potential, they are extremely unstable, and thus, unreliably detectable. Therefore, peroxocarbonate species are the first intermediates, then, they are directly converted into formate ions, formic acid, and further reduction products. However, formaldehyde is the first product formed when basic aqueous solutions are adopted, and further reduction to methanol or oxidation to formic acid may occur.

According to Galli et al., wide product distribution in liquid phase tests indicates that CO₂ photoreduction occurs by consecutive reaction steps at neutral pH [102] (Equations (3)–(8)).

$$\mathrm{VB}:2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{h}}^{+}\to 4{\mathrm{H}}^{+}+{\mathrm{O}}_2$$

(3)

$$\mathrm{CB}:{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to ·\mathrm{H}$$

(4)

$${\mathrm{CO}}_2+2·\mathrm{H}\to \mathrm{HCOOH}$$

(5)

$$\mathrm{HCOOH}+2·\mathrm{H}\to \mathrm{HCHO}+{\mathrm{H}}_2\mathrm{O}$$

(6)

$$\mathrm{HCHO}+2·\mathrm{H}\to {\mathrm{CH}}_3\mathrm{OH}$$

(7)

$${\mathrm{CH}}_3\mathrm{OH}+2·\mathrm{H}\to {\mathrm{CH}}_4+{\mathrm{H}}_2\mathrm{O}$$

(8)

Due to the high H₂O/CO₂ ratio in liquid phase systems, CO₂ undergoes hydrogenation faster than deoxygenation, leading to all oxygenated products found in reaction medium. However, it was observed that TiO₂ catalyst modification with metal electron traps increases hydrogenation rate and improving methane yield by enhancing the stability of electron-hole couples [103,104].

Furthermore, the accumulation of organic compounds in the liquid phase can be followed by hydrogen formation by PR; produced organic compounds act as hole scavengers themselves in a PR reaction (Equation (9)). This reaction is very likely to happen with methanol due to its reactivity compared to other C₁ compounds [105,106].

$${\mathrm{CH}}_3\mathrm{OH}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{CO}}_2+3{\mathrm{H}}_2$$

(9)

On the contrary, CO₂ photoreduction can also be performed in the gas phase, feeding water as vapour, leading to slightly different results in observed mechanisms. For example, Karamian and co-workers [88] reported that in most cases, in gaseous systems CO is the first intermediate product of CO₂ photoreduction by water vapour [74].

$${\mathrm{CO}}_2\to \mathrm{CO}+{\mathrm{O}}_2$$

(10)

Generally speaking, in the gas phase deoxygenation is faster than hydrogenation and it leads to ·C species that are reduced afterwards, whereas in the liquid phase, peroxo species undergo hydrogenation preferentially [107], yielding to possible intermediate products. However, reaction conditions, and in particular, temperature, irradiance, and reaction time modify the reaction pathway and product distribution as a consequence. In detail, if CO₂ deoxygenation is faster than dehydrogenation, methane production is favoured with respect to oxygenated compounds [94]. This is the case for the vapour phase reaction, characterised by CO₂ excess. This mechanism involves the formation of ·C radicals that recombine with ∙H originated from water [108].

From all the evidence reported in the literature, briefly summarized in

Table 2. Summary of proposed mechanisms found in the literature.

Ref.	Catalyst	Reaction Conditions	Products Formation	Notes
[64]	Anatase TiO2	- CO2 0.04–0.15 mmol - H2O 0.04–0.25 mmol - UVA lamp - 0–50 °C, 1 atm	CH4 0.17 μmol∙g−1∙h−1 H2 8.33 μmol∙g−1∙h−1	- C=O cleavage to form C species, which interact with H atoms and OH radicals
[110]	TiO2 (P25)	- CO2 2.8 MPa - 0.1 M i-PrOH in H2O as hole scavenger - UVA lamp - 20 °C, 2.8 MPa	CH4 1.2 μmol∙g−1∙(Ti) energy efficiency 0.0065%	- i-PrOH improved methane production by enhancing H+ formation
[111]	Pt/CuAlGaO4 Pt/SrTiO3 WO3	- 2 mM FeCl3/FeCl2 with Nafion membrane - CO2 dissolved - 300 W Ne lamp 270 mW∙cm−2 - pH 2.6, r.t. and 1 atm	CH3OH 473.3 μmol∙h−1	- dividing oxygen and hydrogen improves methanol yield
[112]	TiO2	- 0.08 M NaHCO3 - 500 W Xe lamp - r.t., 1 atm	CH3OH 0.59 μmol∙g−1	- CO32−/HCO3− reduction improved by catalyst charge separation on the catalyst
[102]	TiO2 (P25)	- 1.7 g∙L−1 Na2SO3 + 11 g∙L−1 NaOH - pressurised CO2 - 104.2 W∙m−2 - pH 14, 80 °C, 7 bar	CO 0.72 μmol∙g−1∙h−1 HCOOH 1859 μmol∙g−1∙h−1 HCHO 16,537 μmol∙g−1∙h−1 CH3OH 351 μmol∙g−1∙h−1	- pressure improves CO2 solubility in reaction medium - two parallel reaction pathways are hypothesized
[113]	Cu/TiO2	- CO2 + H2O gaseous mixture 2 mL∙min−1 - 150 W lamp 90 mW∙cm−2 - r.t., 1.5 bar	CO 25 μmol∙g−1 CH4 4 μmol∙g−1	- The first step is CO2 splitting into CO and O2
[81]	Cu/TiO2	- CO2 + H2O gaseous mixture - optical fibres lamp (λ = 365 nm) 1–16 W∙cm−2 - 75 °C, 1.05–1.40 bar	CH3OH 0.4 μmol∙g−1∙h−1	- Kinetic studies indicate that the redox reaction is the rate determining step of the process (not adsorption nor desorption)
[114]	Montmorillonite/TiO2 monolith	- CO2 + H2O gaseous mixture (1.4 bar) - 200 W Hg lamp - r.t., 1.4 bar	CH4 139 μmol∙g−1∙h−1	- Reaction occurs via multiple single-electron transfers - Bimolecular Lagmuir-Hinshelwood model
[115]	SO42−/TiO2	- CO2 + H2O + H2 + N2 gaseous mixture - low pressure Hg lamp (λ = 365 nm) 2.0 mW∙cm−2 - 35–85 °C, 101.3 kPa	CO 0.85 μmol∙g−1∙h−1 CH4 0.14 μmol∙g−1∙h−1	- Oxygen formation retards CO2 reduction
[69]	Graphene oxide/TiO2	- CO2 + H2O + N2 gaseous mixture (25–101 kPa) - Xe lamp (AM 1.5) 65–177 mW∙cm−2 - 25 °C, 1 bar	CO 14.91 μmol∙g−1 CH4 3.98 μmol∙g−1	- adsorption of water is 10 times faster than carbon dioxide - proposed mechanism include only CO and CH4 as products

, it is possible to assume that CO₂ reduction can undergo two different pathways, as shown in Figure 2.

Once reagents are turned into products, they need to be desorbed from the catalytic surface so as to leave catalytic sites available for new reagents’ molecules to be adsorbed.

2.2. Photoreactor Design

The choice of the reactor design and process conditions heavily affect substrate and photon delivery to the photocatalytic surface. While mass transfer is commonly considered in conventional catalysis processes, in photocatalysis, photon transfer must be addressed too. Due to the novelty of this technology and the increase in publications on this topic, different rig configurations are reported in the literature. In fact, if we consider reported photoactivity data for a worldwide benchmark photocatalyst, such as Evonik’s P25, with the same conditions of temperature, pressure and light intensity, performances might change either in terms of activity and product distribution [92,116,117]. In the first studies, three-phase photoreactors (solid catalyst, liquid reaction medium, gaseous CO₂) are generally reported [62]. In this case, the solvent, which is water, acts as a reductant as well [118], therefore, several phenomena should be considered for mass transfer such as solubility, diffusion, convection and migration. CO₂ solubility in an aqueous phase system must be considered as a possible limiting step for the reaction in the liquid phase.

From an historical point of view, the first developed photoreactors for CO₂ reduction were batch reactors where the catalyst was suspended in a liquid medium, usually water, and carbon dioxide was bubbled through reaction medium and light reached the reaction medium through quartz windows [62,119,120,121]. This choice of three-phases slurry reactors was, and still is, widely used due to many advantages. These experimental rigs are similar to those used for carbon dioxide photoelectrocatalytic reduction [122,123], and most importantly, they have been widely used in photooxidation processes, which in some cases, are available on a commercial scale [124,125].

In addition, the catalyst does not require any casting or shaping procedure before its use so it can be used as a powder without further modification. Mass transfer is achieved by mixing, either magnetically or mechanically, to provide homogeneity in the reaction medium.

However, liquid phase setups are affected by experimental drawbacks that need to be overcome. The two main drawbacks to be overcome are extremely low carbon dioxide solubility in water (0.03 M at 25 °C and 1 atm) [126], and inefficient light harvesting by medium scattering [127,128]. The most common strategy to solve the first problem is the introduction of a base to improve CO₂ solubility. Many authors use basic solutions as reaction media, which leads to the formation of more soluble and stable carbonates and bicarbonate species in the solution [129]. Generally, the most used base is sodium hydroxide [130,131], but use of other bases is reported, such as sodium bicarbonate [132], potassium biphosphate [133] and triethanolamine (TEA) [134].

Quin and co-workers did not use water as a reaction medium, but used methanol, whose CO₂ solubility is much higher [135], and they observed that the main product was methyl formate which was obtained by esterification of the solvent and formic acid from CO₂ reduction. Alternatively, Liu and co-workers used isopropanol as a solvent, increasing CO₂ solubility and leading to similar results [136]. However, it has been observed that these solvents can act as sacrificial agent and be oxidised in TiO₂’s valence band instead of water [111]. This means minor sustainability of the process because in these photocatalytic systems, the reductant is more expensive and industrially derived from fossil sources (mainly natural gas or coal gasification).

Rossetti et al. reported that the use of a pressurised photoreactor increased CO₂ dissolution in aqueous media, yielding higher catalytic activity in methane production avoiding chemical absorption as carbonates in high pH conditions [87], which might lead to reactor walls corrosion. Indeed, the increase in pressure enhances carbon dioxide dissolution in reaction medium to increase reduction products and overall process productivity, but it was observed that reaction thermodynamics hindered photocatalytic performances for values higher than 10 bars [137]. Even tuning temperature represents a challenge, since its increase positively influences kinetics and mass transfer, but it reduces CO₂ solubility and favours electron-hole recombination. Kaneco and co-workers performed photoreduction in liquid CO₂, overcoming solubility problems [138] at 20 °C, but, compared to common photocatalytic reactions, pressure is extremely high (6.5 MPa) in order to maintain CO₂ in the liquid phase.

The second issue to face in three-phase batch reactors is light transfer. In fact, photons are required to travel from light source through reaction medium to the photocatalyst surface, where they are absorbed and activate the photocatalyst. At 20 °C and with 361 nm light wavelength, the water refractive index is 1.34795, whereas the refractive index value is 1.000464 for CO₂ and 1.000256 for water vapour [126,139]. This means that light transfer in aqueous solutions is much more difficult in liquid media compared to gas phase systems.

For light harvesting, a variety of photoreactors geometries aimed at maximising catalyst photoactivation, were reported. In the liquid phase, the most commonly-used systems feature a quartz window that allows light to enter [64,140,141]. More recently, some papers in the literature have reported the use of an annular design that allows a homogeneous light transfer to the catalyst, mainly in the radial direction [142]. For example, Matějová and co-workers used a batch reactor with this geometry: the UV lamp is cast at the centre of the reactor to maximise light harvesting efficiency and it irradiates the aqueous catalyst suspension and CO₂ is bubbled through it to reach saturation, as depicted in Figure 3 [107].

Finally, the use of a fine suspended catalyst might lead to fouling of radiation source, lower active surface contact area and higher separation cost for product collection. Despite technological advances, some of these issues such as high refractive index and poor solubility cannot be completely eliminated, reducing the potentialities of these systems.

Over the years, gas-solid photoreactors have become popular in the literature. This solution finally provides a solution for issues related to CO₂ solubility in water. In fact, as any gas, water vapour and CO₂ are perfectly mixable and water vapour diffusion in CO₂ constant is relatively high (0.138 cm²∙s⁻¹); thus, it is possible to tune reactants’ ratio and perform photoreduction in CO₂ excess while avoiding water splitting. Moreover, separation is easier since reagents and products are in gas phase while the catalyst is solid.

Mixing the reagents can be performed using different physical phenomena and technological solutions, which differ in their control of the CO₂/H₂O ratio. Bazzo and Urawaka reported the use of moist quartz wool to generate water vapour in situ [143]; vapour feed control was not precise leading to errors in CO₂/H₂O ratio calculations. Collado and co-workers used a controlled evaporator mixture to maintain reactants ratio [144] whilst other researchers, such as Tahir and Amin [145] or Cybula et al. [146] employed a water bubbler to saturate flowing CO₂ with water vapour, as reported in Figure 4. In this way, it is possible to tune the water amount controlling temperature, pressure and carbon dioxide flow. However, in the reported papers temperature is not controlled, though it affects water vapour pressure and thus CO₂/H₂O ratio, decreasing the tests reproducibility.

Gas-solid systems also allow a great choice of catalyst introduction techniques. Packed bed reactor design is the easiest technological solution, since low pressure drops can be achieved, controlling catalyst particle size and providing promising results [147]. However, it suffers from a low irradiated surface area to volume ratio that negatively affects photon harvesting, and thus, light absorption and scattering [148]. According to Kočí and co-workers [149], an annular reactor, where the catalyst is embedded within two concentric cylinders and the radiation source is in the centre, improved irradiation homogeneity, despite issues with gases mixing due to small cross-section.

To overcome inhomogeneous irradiance, several solutions have been developed. The use of optical fibres instead of a single light source provides high transmission and irradiation uniformity [150]. This kind of irradiation is generally coupled with the use of honeycomb monoliths, which also minimise pressure drops. The catalyst is layered within monolith inner walls by wash-coating or, as reported by Ola and Maroto-Valer [151], it is synthesised in situ by a sol-gel method. However, in these systems, mass transfer of reagents and products to and from the catalytic sites might be too critical and channel opacity might decrease light harvesting efficiency [152].

Finally, another possibility for gas-solid reactors is thin film reactors. In this configuration, the photocatalyst is not immobilised onto beads, fibres or monoliths, but it is deposited on a plate, or even better, on the surface of the photoreactor. In this case, irradiation and light distribution are influenced by geometry and light source with significant effects on global photoactivity [153]. Several geometries were reported in the literature, for example, Pathak et al. used Nafion membranes to support photocatalyst film [154] whereas Tan and co-workers used a quartz rod within the photoreactor [69].

Ideally, a solar light driven photoreactor must have [129]:

• high coverage area and homogeneous catalyst distribution with good exposure to light;
• high CO₂ velocity and high mass transfer;
• intimate contact between reagents, catalyst and photons;
• efficient light harvesting.

Comparing both photoreactors (whose features are summarised in

Table 3. Summary of features to consider in CO₂ photoreactor design.

Type of Photoreactor	Issue	Approach
Three-phase photoreactors	CO2 solubility	Basic reaction medium Alternative solvent High Pressure
	Water splitting	Sacrificial Agent
	Light scattering	Efficient stirring Wise reactor geometry
	Fouling	Preformed Catalyst
	Separation	Preformed Catalyst
Gas-Solid photoreactors	Variable CO2/H2O ratio	Control of reactants feed
	High contact time	Bath reactor
	Irradiation inhomogeneity	Geometry Optic fibres Catalyst immobilisation

), gas-solid systems are the most promising option for the design of a photoreactor due to their flexibility in process development and fewer limitations due to mass-related physical phenomena (i.e., CO₂ adsorption and light scattering through reaction medium).

Considering reactants feed, bubbling gaseous CO₂ seems the easiest most efficient way to obtain CO₂-rich reactant mixtures, avoiding the use of sacrificial agents, although it is possible to obtain a stable and constant CO₂/H₂O ratio with careful control of CO₂ flow and bubbler temperature.

3. Photoreforming of Biomass-Derived Substrates

3.1. Proposed Reaction Pathway

Reforming processes and in particular PR, as shown in the introduction, could be a very promising answer for both low-cost hydrogen production and storage. Nonetheless, hydrogen can also be produced from pure water through so-called photocatalytic water splitting [155]. As pointed out by Ma et al. [156], a lot of papers make claims about “water splitting” while using a so-called hole scavenger [157]; but since no oxygen is evolved in this case, it is more proper to term these reactions as photoreforming or partial water splitting [156]. Despite its attractiveness, this reaction is more difficult than PR, both from a thermodynamic and kinetic point of view, leading to lower hydrogen productivity [158].

The effect of these hole scavengers, or sacrificial reagents, as boosting agents in photocatalytic hydrogen production from water was first observed by Kawai and Sakata in 1980 [159]. Today several compounds are known to be effective in PR with methanol [160], ethanol [161] and glycerol [162] being the most used.

The general mechanism of photocatalysis has been widely described by several authors [46,156]. Briefly, upon absorption of a sufficiently energetic photon, charge carriers, unless they recombine, can be transferred to a suitable electron acceptor (for electrons) or donor (for holes) [163]. CB energy should be higher than the acceptor while VB energy should be lower than the donor, so that reaction can occur [46]. In a PR reaction, the acceptor is a hydrogen ion, yielding molecular hydrogen (H₂), while the donor, previously labelled as a hole-scavenger, is the reducing agent, either inorganic [164] or organic [165]. For the oxidative reaction pathway that consumes the hole scavenger, two main pathways have been proposed, depending on the water/substrate ratio [166]:

• Direct path, consuming the substrate directly by holes;
• Indirect path, a hydroxyl radical-mediated mechanism, where these radicals are produced by interaction of holes with adsorbed water or surface hydroxyl moiety.

Beside these two main pathways, a deeper understanding of degradation mechanism is useful. A general mechanism scheme is reported in Figure 5, as proposed by Puga [46].

It has been proposed that alcoholic moiety, one of the most abundant oxygen-containing functional groups in biomass, undergoes consecutive oxidation to carboxylic acid, eventually decomposing to an alkyl fragment or radical and carbon dioxide (decarboxylation) [167]. Nonetheless, this mechanism seems to only be logical for polyols; ethanol is known to yield acetaldehyde as its main co-product [168,169,170], suggesting a key role played by the substrate’s structure in enhancing C-C bond cleavage. Besides this mechanism, an alternative pathway was proposed by Bahruji et al. on Pd/TiO₂ where alcohol was adsorbed as alkoxide, then decarbonylation yielded an alkyl fragment and CO, which further oxidized to CO₂ by holes [171]. Concerning the as-formed alkyl fragments, several pathways have been proposed, for example, recombination with either hydrogen atoms or other alkyl fragments yields hydrocarbons [172], while coupling with hydroxyl radicals gives alcohols [173], that can be further oxidised until complete mineralization of the carbon skeleton to CO₂. This suggests a higher availability of these radicals improves substrate degradation. Finally, unexpected by-products suggest other side-pathways such as etherification and esterification [174], ketonisation [175], radical coupling [176] and hemiacetalisation [168].

Thus, the reaction mechanism is a complex phenomenon, strongly substrate and catalyst-dependent. Nonetheless, the key point of reaction pathway knowledge is to understand how by-products are produced and how these side processes can be correlated to catalyst and reaction conditions, as will be explained in the following sections.

3.2. Catalyst Formulation

The ideal photocatalyst should be selective to complete organics mineralisation, stable and as cheap as possible in order to be applied on an industrial scale. The greatest challenge for a photocatalyst is strong visible light absorption and high carrier mobility (i.e., reduced recombination) [177] in order to harvest solar light efficiently. Moreover, visible light absorption allows lower energy losses since the energy required to excite the photocatalyst exceeds the thermodynamic requirements of reforming [178]. Concerning photocatalytic material properties, little attention has been focused on important parameters such as band edges, optical absorbance and carrier mobility [177]. Knowledge of these properties is important for efficient light harvesting and utilisation.

TiO₂ is certainly the most studied and well-known material in photocatalysis due to its inexpensiveness, safety, chemical stability and photostability [156,179]. TiO₂ aside, different materials have been reported such as zinc oxide (ZnO) [180], copper oxides (CuO and Cu₂O) [181], strontium titanate (SrTiO₃) [182], sodium tantalate (NaTaO₃) [183], indium tantalate (InTaO₄) [184], cadmium sulphide (CdS) [185], tantalum nitride (Ta₃N₅) [186], tantalum oxynitride (TaON) [187], graphene [188] and carbon nitride (C₃N₄) [189]. Besides pure materials, solid solutions have also been used to tune the band structure and improve visible light absorption. Examples include bismuth-yttrium vanadate (Bi_xY_1−xVO₄) [190] and zinc-cadmium sulphide (Zn_0.5Cd_0.5S) [191]. In order to improve the catalytic activity of pristine material, several strategies have been widely reviewed in the literature [57,156], targeting visible light harvesting, reducing electron-hole recombination and improved reaction kinetics. Such approaches include nanostructure architecture [180,192,193,194], doping [193,195,196], heterojunction [197,198,199,200,201,202,203], use of plasmonic metals [186,204,205], as well as co-catalysts [164,206,207]. Concerning nano-composites (joining of two or more materials at nanoscale), an interesting concept is the so-called Z-scheme. Bard suggested the first mechanism proposal of this system in 1979 [208] and the first all-solid-state Z-scheme system for overall water splitting, was developed by Sasaki et al. [209]. As pointed out by Zhou et al., it involves the transfer of an electron from the lowest-lying VB to the highest-lying CB of the composite, thus gaining in reactivity for both electron and holes compared to heterojunctions [210]. Few Z-scheme systems have been stated for PR [211,212], but very appealing systems are reported in the literature, such as CdS/Au/WO₃ [194] and g-C₃N₄/WO₃ [213]. These nano-composites are able to fully harvest visible light, while exploiting high-reactive charge carriers for photocatalytic reactions, as pictured in Figure 6. Due to great potentiality of Z-scheme systems and the wide variety of available semiconductors [177,210], a lot of work can be done on assessing diverse materials on their nano-composites’ photocatalytic properties.

Among the other approaches mentioned above for activity enhancement, the use of co-catalysts is an appealing method, in particular, by improving reaction kinetics [214]. An increase in hydrogen evolution up to 2–3 magnitude order compared to pristine material has been reported [215,216]. The role of co-catalysts is to trap excited electrons [217] or, less commonly, holes [218], and act as a reaction site [214]. Reduction co-catalysts are the most used to increase catalytic activity and usually rely on noble metals such as gold (Au) [168,219], platinum (Pt) [193,220] and palladium (Pd) [171]. Chiarello et al. observed increased reactivity upon co-catalysts loading, in order, Ag < Au < Pt, explained by their increasing work function, and thus electron-trap capability [174]. Al-Azri et al. found an increase in activity, in order, Au < Pt < Pd, suggesting the higher reactivity of Pd is due to higher Fermi level and density of states despite it having a lower work function than Pt [178]. Besides expensive noble metals, non-noble ones have been evaluated too. Copper (Cu) [170] and nickel (Ni) [221,222] have proved to be as active as noble metals in improving photocatalysts’ activity. Metal oxides such as copper(II) oxide (CuO) [216,223], nickel oxide (NiO) [224] and cobalt oxide (CoO_x) [164] have also been reported to be suitable co-catalysts, although NiO has been proved to be converted in situ in metallic Ni, the truly active phase [222]. Ni and Cu-based co-catalyst are surely the most reported [46], but cobalt-based materials also show interesting activity in hydrogen evolution [225] even though they are scarcely reported in literature. Moreover, few comparisons among these non-noble metal co-catalysts exist. Fujita et al. reported an increased reactivity, in order, CoO_x < NiO < CuO on TiO₂ [226], while Sun et al. observed an improved hydrogen evolution, in order, RuO_x < NiO < CuO < CoO_x < Pt on a barium titanate [158], suggesting that interactions between the co-catalyst and the main semiconductor play a crucial role too. Alloying has been proved to boost the catalytic activity [227,228,229], probably due to synergistic effects among the two components as suggested by Jung et al. [230]; unfortunately, these reported alloys contain at least one noble metal, increasing the cost of the catalyst. Further work on purely non-noble metal alloys is necessary to gain a comprehensive knowledge of alloyed co-catalysts. Interesting alternatives to metal and metal oxides are metal sulphides, such as molybdenum disulphide (MoS₂) [231,232] and tungsten disulphide (WS₂) [233,234]; Zong et al. demonstrated a higher activity of MoS₂ compared to noble metals on CdS photocatalyst [235]. Oxidation co-catalysts are widely studied for overall water splitting [155], and compared to oxide co-catalyst (RuO_x, IrO_x, CoO_x), cobalt phosphate (Co-Pi) has been reported to obtain higher activity in oxygen evolution [236] and to also exhibit self-healing properties in the presence of phosphate ions in solution, thus improving its stability [237]. Recently, Di et al. reported an enhancing of activity in PR with Co-Pi co-catalyst, thus boosting the oxidative side of the reaction mechanism [238]. Beyond the choice of the material, the loading is also important. Usually a very low amount of co-catalyst is able to boost the activity, for instance Kondarides et al. assessed 0.1–0.5% as the optimal Pt loading on TiO₂ [162] while Chen et al. estimated that 1.25% of CuO [223] and 0.5% of Ni [222] gave the best result on TiO₂. Increasing the loading certainly increases the number of available active sites, but above a certain upper value the activity decreases due to enhanced charge carrier recombination, light-shielding effect or the decrease of active sites at the metal-semiconductor interface [162]. Besides activity enhancement, the co-catalyst plays a significant role in selectivity [174,239], moreover, its size has been proved to have a remarkable role in enhancing both activity and selectivity of PR reactions [240]. Finally, as reported by Kawai and Sakata in 1980 [159], the use of two co-catalysts (for both reduction and oxidation half-reactions) could remarkably improve photocatalytic performance, thus, suggesting further development on this side.

In

Table 4. Summary of some used photocatalysts for biomass PR.

Ref.	Catalyst	Co-Catalyst	Reaction Conditions	Products Formation	Notes
[168]	TiO2	1.0% Au	- 50:50 v/v EtOH/H2O - 1 g/L of catalyst - UV light (125 W) - 20 °C and 1.4 bar - 2 h of reaction	H2 11,242 μmol·g‒1·h‒1 CH4 88 μmol·g‒1·h‒1 C2H4 110 μmol·g‒1·h‒1 C2H6 7 μmol·g‒1·h‒1 CO 36 μmol·g‒1·h‒1 CO2 52 μmol·g‒1·h‒1 CH3CHO 8258 μmol·g‒1·h‒1	-
[220]	TiO2	0.3% Pt	- Olive mill wastewater (OMW) (3.3% v/v) - pH 3.4 - 4 h of reaction - 2 g/L of catalysts - UVA light; 5.8 × 10−7 mol photons·s−1	H2 183 molH2·molPt‒1	- uncomplete mineralization of OMW - catalyst regeneration by photooxidation - improved H2 yield by removing lipids from OMW
[171]	TiO2	0.5% Pd	- 0.1% v/v alcohols/H2O - 2 g/L of catalyst - simulated solar light - 3 h of reaction	- 12 mL H2 (glycerol) - 8 mL H2 (glucose) - 4 mL H2 (sucrose) - 2 mL H2 (cyclohexanol)	- improved H2 yield with number of α-H in alcohol molecule - observed alkanes as side-product
[170]	TiO2	0.5% Cu0.5% Au	- 1.5% EtOH and 19% H2O on inert gas stream - simulated solar light, 1000 W/m2 - 60 °C - 100 mg of catalyst immobilized on 10 cm2 of conducting glass substrate	0.5% Au/TiO2 H2 212 μmol·h‒1 CH4 8.4 μmol·h‒1 CO 11.8 μmol·h‒1 CO2 7.8 μmol·h‒1 CH3CHO 181 μmol·h‒1 0.5% Cu/TiO2 H2 186 μmol·h‒1 CH4 6.9 μmol·h‒1 CO 9.8 μmol·h‒1 CO2 5.5 μmol·h‒1 CH3CHO 162 μmol·h‒1	- co-catalysts introduced by photodeposition - reaction in dynamic conditions (5 mL/min gas flow)
[216]	TiO2	0–9% CuO	- 0.1 M glycerol in H2O - UVA LED (365 nm); 800 W/m2 - 1 g/L of catalyst	H2- - from 15.9 μmol·g‒1·h‒1 (0% CuO) to 2061 μmol·g‒1·h‒1 (1.3% CuO)	- linearly decrease of organic in solution (8 h of irradiation)
[222]	TiO2	0–4% Ni	- 10:90 and 80:20 v/v EtOH/H2O - UVA light; 65 W/m2 - 0.325 g/L of catalyst	10:90 EtOH/H2O H2 11.6 μmol·g‒1·h‒1 (0.5% Ni/TiO2) 80:20 EtOH/H2O H2 20.7 μmol·g‒1·h‒1 (0.5% Ni/TiO2)	- improved H2 yield by reduction of NiO to Ni
[164]	CdS/TiO2	0–2.8% CoOx	- 0.125 M Na2S and 0.175 M Na2SO3 in H2O - visible light (λ > 400 nm) - 0.5 g/L of catalyst	H2 660 μmol·g‒1·h‒1 (2.1% CoOx/CdS/TiO2)	-
[238]	CdS	0–20% Co-Pi	- Lactic acid in H2O - visible light (λ > 420 nm) - 0.625 g/L of catalyst	H2 - from 5.2 mmol·g‒1·h‒1 (0% Co-Pi) to 13.3 mmol·g‒1·h‒1 (10% Co-Pi)	- improved catalyst stability with loading Co-Pi
[194]	CdS-Au-WO3	-	- 0.2 M Na2S and 0.2 M Na2SO3 in H2O - visible light (λ > 420 nm) - 0.2 g/L of catalyst	H2 - 34.6 μmol·h‒1 (CdS-Au-WO3) - 16.5 μmol·h‒1 (CdS-Au) - 6.3 μmol·h‒1 (CdS- WO3) - 4 μmol·h‒1 (CdS)	- WO3 photonic crystal; Z-scheme system
[213]	g-C3N4-WO3	1% Pt	- 10% v/v TEOA/H2O - simulated solar light - 0.625 g/L of catalyst	H2 - 0.44 mmol·g‒1·h‒1 (Pt/g-C3N4) - 3.12 mmol·g‒1·h‒1 (Pt/g-C3N4/WO3)	- Z-scheme system

some of the most interesting used photocatalysts are summarized, reporting reaction conditions and product formation.

The key point of this section is that coupling two different materials is necessary in order to fulfil the requirement discussed at the beginning of this section. In this context, co-catalysts play a crucial role in catalytic activity and selectivity enhancement.

3.3. Reaction Conditions

Photocatalyst improvement is just part of the story; reaction conditions play an important role in efficiency enhancement of the overall process. Herein, we focus attention on liquid vs gas phase reaction conditions, since, even though some remarkable differences exist, they have scarcely been highlighted in the literature for PR reaction.

Similarly to CO₂ photoreduction, PR has been extensively carried out in the liquid phase [161,162,168,191,216,241]. Gas phase conditions have been barely reported and only volatile compounds such as methane [242,243], methanol [244,245] and ethanol [170,175,246] have been used. Nevertheless, as reported by Ampelli et al., gas phase conditions have several advantages over liquid conditions [170], such as:

• Low light-scattering losses;
• Easier product recovery;
• Avoiding metal-leaching issues;
• Good catalyst exposure to light.

Several other reaction parameters can affect both the activity and selectivity of PR reactions; these will be briefly summarised here, along with the underlying issues and key points.

Since photocatalysis is a light-driven process, the obvious parameters are light wavelength and intensity in both the liquid and gas phase. Concerning the former, it is known that the lower the incoming photon wavelength, the higher the hydrogen evolution rate [247] and the apparent quantum yield (AQY) [243]. This experimental behaviour is unexpected since the as-formed charge carrier experienced a fast thermalisation (a few hundred femtoseconds) [248], losing their extra-energy at the bottom of CB. However, Xu et al. proposed two explanations for faster methanol dissociation on TiO₂ (110) surface: direct injection of electrons in methanol’s lowest unoccupied molecular orbital (LUMO), or a phonon-assisted methanol dissociation using energy from thermalisation [249]. Speaking of the latter (light intensity or irradiance), as previously explained in Section 2.1, lower irradiances improve photon utilization and boost AQY. Unfortunately, few reports exists about the effect of light intensity on PR activity [46]; despite this, a wide variety of light intensities have been used from 2 W∙m⁻² [175] to 250 W∙m⁻² [250]. It is worth highlighting that light wavelength and intensity affect the photon utilization efficiency, rather phenomena involving reagents, products or reactions intermediates such as adsorption, desorption or change in activation energy.

Although the energy source in photocatalysis is light, thermal energy can play a significant role within a narrow temperature range (20–90 °C) [46]. It is widely known that a higher temperature increases hydrogen yield for both the gas [175] and liquid phase [162,251] as reaction medium. Concerning the former, Taboada et al. suggested such evidence is due to improved desorption of reaction intermediates [175]. Nonetheless, Caravaca et al. observed that on Pt/TiO₂, methanol reforming occurs without irradiation at temperatures higher than 160 °C, thus suggesting a shift to a pure thermocatalytic process [245].

Substrates and their concentrations strongly influence both activity and selectivity. Several compounds can be used as hole scavengers in PR, but the most interesting are certainly oxygenates organic compounds because they have higher reactivity than hydrocarbons and potentially lower environmental impact (biomass or biomass-derived compounds) [42]. Hydrocarbons and fossil fuels have been used too [173]. Yoshida et al. reported photocatalytic methane conversion that, despite requiring noble metal co-catalyst and high methane-water ratio, successfully yielded hydrogen and CO₂ [242,243]. We have cited this process since it could be an appealing way to move from traditional thermocatalytic reforming processes to milder and less energy-demanding PR processes on well-established fossil fuel-based technologies.

Concerning oxygenates, methanol, ethanol and glycerol are certainly the most used as mentioned in Section 3.1. Focusing on the relationship between substrate structure and hydrogen evolution activity, Chen et al. observed, using a Pd/TiO₂ catalyst, increased reactivity with an increasing number of hydroxyl moiety and α-hydrogen on the substrate, and reported the following order of reactivity: glycerol > 1,2-ethanediol > 1,2-propanediol > methanol > ethanol > 2-propanol > tert-butanol [178]. Bahruji et al. also reported a similar effect on alcoholic substrates (methanol, ethanol, 2-propanol, 1-propanol, 1-butanol and tert-butanol) [171]. Regarding selectivity, it is known that methanol [252] and glycerol [162] afford a complete conversion to CO₂ (mineralisation), while ethanol gives acetaldehyde as the main co-product [168], suggesting a key role of hydroxyl moiety in activation of C-C bond cleavage. It is also known that side pathways yield alkanes from the alkyl chain of alcohols [171], thus lowering hydrogen yield. It is worth noting that these alcohols can be obtained from biomass through chemical [253,254,255] or biochemical [256] processes, while glycerol is an abundant by-product of the biodiesel industry [257]. Other organic compounds have been also used, such as carboxylic acids and aldehydes, since they constitute bio-oil [258] and wastewater [259]. Wu et al. compared formaldehyde and formic acid, recognised as intermediates in methanol PR, with methanol itself, and found an increase in reactivity as follows: methanol < formaldehyde < formic acid [240]. Likewise, acetic acid was found to be more active than acetaldehyde, while formic acid has showed higher hydrogen yield [252], thus confirming higher reactivity of carboxylic acids and lower reactivity for compounds with longer alkyl chains. Carbohydrates have been extensively used in PR reactions too and are of particular interest as the main components of biomass [260]. Kawai and Sakata firstly reported the use of these compounds in photocatalytic hydrogen evolution, observing a H₂/CO₂ ratio close to stoichiometric value, meaning complete mineralisation of sugar occurred, and a decrease in reactivity moving from simple sugars to polysaccharides [159]. Kondarides et al. studied the reactivity for simple sugars (lactose, cellobiose, maltose), and like Kawai and Sakata, they observed a decrease in hydrogen evolution for starch and cellulose due to their more complex structure [261]. Further study by Caravaca et al. assessed that the size of cellulose also plays a significant role, and moving toward a real lignocellulosic substrate, observed lower yield in hydrogen, although it was higher than pure water [221]. Finally, Speltini et al. have reported some interesting results on waste PR, in particular on olive mill wastewater (OMW) [220] and swine sewage [262], although no complete conversion to CO₂ was observed. PR reaction has also been carried out in a pilot plant using wastewater as substrate and yielding hydrogen, although no complete mineralisation to CO₂ was observed in this case [241].

Another important parameter is reagent concentration. Langmuir’s equation has often been observed to fit experimental data on PR reactions in the liquid phase with different substrate and photocatalysts [167,191,263]:

$$r_{{\mathrm{H}}_2}=\frac{k_{{\mathrm{H}}_2}K{C}_0}{1+K{C}_0}$$

(11)

where r_H2 is the reaction rate, C₀ is the initial substrate concentration, k_H2 is the rate constant for hydrogen evolution and K is the adsorption constant. This means that the phenomenon is controlled by substrate adsorption on the catalyst’s surface [167]. It has been observed that the addition of little amounts of a hole scavenger (substrate) can increase hydrogen evolution compared to pure water [162], making this process suitable for simultaneous pollutant removal and hydrogen production from very diluted aqueous medium. As the concentration of hole scavenger increased, the hydrogen production was improved too, in both liquid [264] and gas phase [168] systems. Nevertheless, besides a certain upper water-substrate ratio, a decrease in activity or a constant value has been observed in both gas [174] and liquid phase [191,262] conditions, meaning that water becomes the limiting reagent. Moreover, Chiarello et al. suggested that water plays a key role in gas phase conditions, not only as oxidant but also as a proton conductor, allowing a facile transfer from oxidation to reduction sites [166]. Effect on the activity aside, the water-substrate ratio also affects selectivity improving, for higher ratios, complete conversion to CO₂ and reduction of side-products [174].

In the liquid phase, medium acidity and catalyst concentration also have a significant role. The effect of pH is strongly substrate-dependent: methanol [240], acetic acid [172] and cellulose [265] PR on TiO₂ was favoured at neutral pH; OMW, a phenolic-rich mixture, gave better results in acidic media [220] while ethanol [264], glycerol [163], glucose [167] and sewage [262] showed best performances in alkaline solutions. The acidity also plays a crucial role in selectivity. Sakata et al. observed a remarkable decrease in methane formation due to side reaction, in acetic acid PR at higher pH, despite the lower hydrogen yield [172]. As observed by Simon et al., using photoluminescence and transient absorption spectroscopy, alkali-enhanced activity on ethanol PR is due to improved formation of hydroxyl radicals from hydroxyl ion-rich medium, that boost organics decomposition [266]. Finally, one should keep in mind that pH also affects substrate adsorption on catalyst’s surface [191,220], catalyst’s particle dispersion [240] and pH-dependence on CB and VB [172,266].

Concerning the concentration of catalyst in the suspension reaction medium, it is known that too low an amount of catalyst hinders the activity while too much powder in suspension reduces light penetration [162,262].

3.4. Photoreactors Design

Hence, we want to consider reactor configurations, which is hardly reported and reviewed for PR. Liquid phase reactions are usually carried out in slurry-type reactors, in which the powdered photocatalyst is suspended in the aqueous medium, allowing a good irradiation pattern of the catalyst. The major drawback of this configuration is the catalyst recovery, as experienced in water photoremediation. Immobilisation on a solid support avoids complex separation procedures, though it lowers catalytic activity due to mass-transfer limitations [267]. Concerning gas phase reactions, several different reactor configurations have been reported, such as packed bed [244], plate thin-film [170] and coated honeycomb [175], although no direct comparison among them has been done. It is worth observing that reactor design, as reported in Section 2.2, has a crucial role in photocatalysis, since as we recently report for CO₂ photoreduction, a proper irradiation pattern creates higher productivity with lower amounts of catalyst [74]. Nonetheless, there is plenty of work that can be done on PR reactors’ design.

Eventually, the purification of hydrogen in gaseous streams from CO₂ and other by-products should be addressed as issues, in particular, for scaling-up this technology. This problem is actually handled by traditional hydrogen production processes (e.g., methane steam reforming, MSR) [268]. CO₂ removal, the sole H₂ co-product in stoichiometric PR reaction, can be gained by absorption in liquid alkaline solution, as in post-combustion CO₂ capture technologies [269]. The resultant CO₂-rich solution can be used for CO₂ photoreduction in the liquid phase as previously shown in Section 2.2; the combination of these two processes should fulfil both H₂ purification (from CO₂) and avoids CO₂ emission into the atmosphere, while maintaining photocatalysis advantages of mild conditions and solar light utilization. Nonetheless, up until now, none of these processes have been coupled to a PR reaction rig in both gas and liquid phase medium equipment. This additional knowledge can be useful for deeper study on reactor design, for scaling-up PR technology.

In conclusion, the choice of reaction conditions has a crucial impact (

Table 5. Summary of features to consider on PR reaction improvement.

Issue	Approach	Aim
Light utilisation efficiency	Photon energy close to photocatalyst’s bandgap	Reduced photon energy losses
	Low light intensity	Increased AQY (photon utilization)
	Reactor design	Good irradiation pattern of the photocatalyst
Activity enhancement	Increased temperature	Improved product desorption
	Substrate chemical structure	Increased reactivity by increasing the number of hydroxyl moiety
	Increased substrate concentration	Avoiding limiting reactants issues
	pH (substrate-dependent)	Improved decomposition in radical-rich medium (alkaline) or improved substrate adsorption
Selectivity enhancement	Substrate chemical structure	Decrease alkane formation by side reaction by shorter alkyl chains moiety
	Increased water concentration	Improved mineralization by higher water content
	pH (substrate-dependent)	Improved mineralization by enhanced radical formation in alkaline medium

) on process energy efficiency and on enhancing both activity and selectivity. Currently a lack of knowledge about the effect of some parameters such as light irradiance and reactor configuration, as well as no reported works on H₂ purification directly from gaseous PR streams, are critical issues, particularly for scaling-up PR processes and light utilisation efficiency assessment. Moreover, the incomplete conversion to CO₂, i.e., the selectivity of the process, is a crucial issue, since it reduces hydrogen yields, requiring further processing steps in order to “clean” the effluents. This additional knowledge could be useful for evaluating the industrial reliability of this technology as a cheap H₂ source, particularly if one wants to compare PR with other available H₂ production processes.

4. Conclusions

CO₂ photoreduction and biomass PR are very challenging processes that could withstand the transition from Anthropocene to Sustainocene, as cited in the introduction. The main advantage of photocatalysis is the direct use of sunlight, a very abundant and cheap primary energy source. Nonetheless, several issues hinder the application on an industrial scale; through this review the main concerns were described. It is clear that upgrading photocatalysis processes is a comprehensive challenge that requires a multidisciplinary approach. Reaction conditions are usually less investigated than photocatalyst formulation, but significantly affect the activity enhancement and further scale-up of these processes. Finally, reactor design is surely a key issue in scaling-up, since both mass-transfer and light-transfer phenomena heavily affect overall efficiency of the process. Though further work is needed, huge efforts have been made on improving photocatalysts, whilst process conditions and reactor design still need to be implemented to achieve promising results, particularly for PR. Thus, there is plenty of work that should be done to improve knowledge and applicability of photocatalysis to solar fuels production.