Solar fuel from photo-thermal catalytic reactionswith spectrum-selectivity: a review

Abstract

Solar fuel is one of the ideal energy sources in the future.The synergy of photo and thermal effects leads to a new approach tohigher solar fuel production under relatively mild conditions. Thispaper reviews different approaches for solar fuel production fromspectrum-selective photo-thermal synergetic catalysis. The reviewbegins with the meaning of synergetic effects, and the mechanismsof spectrum-selectivity and photo-thermal catalysis. Then, from atechnical perspective, a number of experimental or theoretical worksare sorted by the chemical reactions and the sacrificial reagentsapplied. In addition, these works are summarized and tabulated basedon the operating conditions, spectrum-selectivity, materials, andproductivity. A discussion is finally presented concerning futuredevelopment of photo-thermal catalytic reactions with spectrum-selectivity.

Keywords

photo-thermal catalysis / spectrum-selectivity / solar fuel / full-spectrum

Introduction

Energy crisis and growing environmentalconcerns have raised interests toward the utilization of renewableenergy. Solar energy is one of the ideal renewable sources for electricityand fuels in the future. Compared to electricity, solar fuels areof high energy density and easier to store and transport. There aregenerally two ways to produce solar fuels: by solar thermocatalyticreactions and by solar photocatalytic reactions. To conduct solarthermocatalytic reactions for mass-production of fuels, high temperatureor other specific conditions are needed, which requires costly solarconcentrators and mirror fields. On the other hand, most photocatalyticreactions operate in relatively mild conditions. However, the fuelproduction rate is limited. As a result, researchers have come upwith photo-thermal catalysis to achieve a complement of the two approachesto utilize their synergetic effects.

Photo-thermalcatalysis is interpretedas the integration of the thermal acceleration of photocatalytic reactionand photo enhancement of thermocatalytic reactions. In thermal accelerationof photocatalytic reaction, the visible and most of infrared partsof solar spectrum may be ineligible for excitation of hole/electrons.However, their thermal effect could be utilized in photocatalysis.In photo enhancement of thermocatalytic reactions, the light-assistedthermocatalysis often operates at a lower temperature and pressurethan their heat-driven counterparts.

Research on thermal accelerationof photocatalysis began in 1980s with oxidative degradation of a varietyof organics, such as formic acid, isopropanol [1], phenol [2], methyl orange [3], acetone [4], and ammonia[5], over TiO₂ photocatalysts. Besides, temperature dependence ofphotocatalytic hydrogen production was experimentally investigated,such as the dehydrogenation of various alcohols over Pt/TiO₂ [6], thehydrogen production from methanol-water mixtures over Rh/TiO₂ [7], andthe water splitting over Pt-RuO₂/TiO₂ [8]. Localizedheat generated from infrared light by carbon nanodots on TiO₂ nanotube increased the rate of pollutant decompositionto 1.5 times [9]. Theheat from infrared light was also found to double the activity ofPt/TiO₂ in 10 vol% methanol aqueous solution[10]. Plasmonic heatingwas utilized to benefit carbonic anhydrase driven photocatalytic hydrogengeneration in CO₂ saturated water [11]. The photo and thermal chemicaleffects of a state-of-art photo-thermal catalyst, Au/TiO₂, was summarized [12]. Surface chemical properties such as reactant adsorption, plasmonresonance (LSPR), and interactions of Au/TiO₂ interface was discussed in detail.

The term “photo-assisted”was coined in a report on thermal decomposition of methanol and isopropanolunder high-flux irradiation from a Xenon lamp [13]. Effects of light in thermal processeswere used in Fischer-Tropsch synthesis [14], decomposition of pollutants [15], purification of exhaust gas [16], etc. For example, thermal decompositionof methanol was known to occur at 200°C–300°C, whichoccurred at 35°C–60°C over Ti/TiO₂ photo-thermal synergetic catalysis [17]. Under irradiation, Ru loaded layered double hydroxideswas heated to 50°C–350°C, where CO₂ reduction was accelerated at elevated temperature [18]. From the perspective of reactionmechanism, thermal catalytic reduction of CO₂ into fuel was reviewed and compared with photo reduction [19] for their combinations. The abovecontent points out that photo-thermal catalysis is a synergy of boththermal and photo effects.

Spectrum-selectivity exists in avariety of materials from Cathedral glasses to heat transfer oil.In solar energy systems, spectrum splitters, in another word, filters,are applied to optimize efficiency in individual wavelength regions.For example, optical filters with selective coating splits lightswith a wavelength of 800–1100 nm to PV arrays while others topower cycle. Though spectrum splitting has been regarded to be a promisingtechnology for all-spectrum sunlight harvesting, the optical lossescaused by reflection and absorption cannot be retrieved in traditionalfilters. To reduce these losses, volumetrically-absorptive spectrumsplitters are proposed, such as semiconductor-doped glass, ethyleneglycol, heat transfer oil, and a variety of nanofluids. Photocatalystis also spectrum-selective due to its semiconductor nature. If properlydesigned, the photo-thermal catalysis system may work as spectrumfilters with a high fuel-production efficiency, which is called photo-thermalcatalytic reactions with spectrum-selectivity (see Fig. 1).

This review is focused on photo-thermalcatalytic reactions with spectrum-selectivity. The structure is basedon the progress in spectrum-selectivity of photocatalysis. To beginwith, the basic mechanisms of optical properties and photo-thermalcatalysis are introduced. Then, observations of photo-thermal catalysisare listed and explained. Next, a summary of experimental researchesis given. And finally, the challenges and potential applications arediscussed.

Unlike reviews in material or chemicalsciences, this review focuses on the technical applications of photo-thermalcatalysis.Therefore, the works reviewed are organized by the reaction type.

Mechanisms of photo-thermal catalytic reactions with spectrum-selectivity

Mechanism of spectrum-selectivity

Photocatalysts are mostly semiconductormetal oxides. When irradiated, semiconductor generates electrons andholes on the surface, where reactants adsorbed are reduced or oxidizedto form products. In this process, light energy was absorbed and storedas chemical energy in the products. The absorption edge is the thresholdfor eligible light to be used in photocatalysis [20]. It determines the longest wavelengthof light that a certain semiconductor accepts. For example, TiO₂ absorbs light with a wavelength shorter than 413.3nm while CdS has an absorption edge of 729.4 nm. Doping or defectsmay expand the absorption spectrum to visible region, such as WO₃ [21], TiO₂ [22], andCeO₂ [23].

Another way of spectrum-selectivityis localized surface plasmon resonance (LSPR). The LSPR is light-excitedcollective oscillation of electron charge in metallic nanoparticles(NPs). Such oscillation causes high absorption of lights with certainwavelengths. Metals, such as Au, Ag, Pt, and Pd, have absorption peakswithin the visible range and steep at absorption edges. During thepreparation of photocatalysts, LSPR effect can be controlled by alteringthe material and morphology of metallic NPs. Such characteristicsprovide convenience in design and applications of spectrum-selectivephotocatalysts.

In LSPR effect, the local electromagneticfield in the vicinity of the particle surface is amplified by electroniccharge oscillation. This strong field activates semiconductor catalystsby facilitating transportation of charge carriers [24]. Moreover, metal NPs are also co-catalystsin a wide variety of reactions.

Plasmonic heating from LSPR alsoenhances the reaction when the illumination intensity is sufficientlyhigh (2.5×10⁵ W/m² at 532 nm [25]). However,due to heat dissipation, this enhancement is barely observable incommon solar concentrators.

Reactants also have spectrum-selectivityand can be utilized to absorb the visible and infrared (Vis-IR) lightsthat can hardly excite carriers in photocatalysts. It is reportedthat Vis-IR light is absorbed by water, CO₂, ethylene glycol, propylene glycol, and various oils [26].The light absorbed provides heatsource for photo-thermal catalysis instead of being dissipated innormal photocatalysis.

Optical property of reaction system

In a photo-thermal catalytic reaction,the catalyst bed is either fluidized or fixed. The fluidized bed isthe nanocatalysts suspended in a nanofluid containing certain reactants.The fixed bed is fabricated by depositing nanocatalysts onto solidsupports. Theories or simulation methods for the optical propertyof one nanoparticle include the Mie theory, DDA and FDTD methods.The Mie theory provides the analytical solutions of absorption, scatteringand extinction cross sections for a nanosphere. DDA is an approximationof continuum nanostructure by a finite array of polarizable points.Incident light is expressed as electric field to calculate polarizationsat every point, then the absorption can be obtained [27]. Finite difference time domain(FDTD) method is an explicit time marching algorithm to calculateMaxwell’s curl equations on discrete spatial grids [28], which takes interparticle couplinginto account. Optical property of a nanofluid is calculated from theproperty of one particle and the base fluid [29]. The results acquired in theoriesand simulations help to design certain photo thermal catalysts [30].

The experimental method for determiningthe absorption of nanocatalyst is the Uv-Vis-NIR diffusive reflectancespectroscopy (DRS) test. The samples are nanocatalysts fixed on asolid support (in case of fixed bed) or nanofluids contained in aquartz colorimetric utensil (in case of fluidized bed).The data acquiredin the test can be converted into absorption spectrum through Kubelka-Munkfunction [31].

Mechanism and models of photo-thermal catalysis

A variety of mechanisms have beenreported on the kinetics of photocatalysis. The Mars-Van Krevelenmechanism states that in the thermocatalytic oxidation of moleculesadsorbed on a metal oxide catalyst (e.g. CeO₂, MnO₂, TiO₂), thesurface of catalyst acts as a redox mediator. The reactant adsorbedis oxidized by the surface oxygen on the metal oxide. Then, the metaloxide reduced (e.g. CeO_2−x) is re-oxidized by gaseousoxygen. This redox cycle is accelerated by the photo-thermal synergeticeffects [32]. The Eley-Ridealmechanism states that a heterogeneous reaction occurs between stronglychemisorbed atoms and physically adsorbed molecules. The latter isattached on the surface of the catalyst by weak van der Waals forces.The Langmuir-Hinshelwood mechanism, which is widely applied in kineticand mechanism studies of photo-thermal catalysis, states that thereaction occurs between two reactants adsorbed on the surface [33].

In terms of thermodynamics, the Gibbsfree energy is the available energy of an ensemble of thermalizedexcited states, which exists under the condition of constant temperatureand pressure [34]. Hence,the Gibbs free energy determines the maximum thermodynamic drivingforce for electrons and holes to induce the photocatalytic reactions[35]. However, the theoryof thermal effects on photocatalysis remains complicated [36] and optimal operating temperatureis usually determined through experiments.

Photo-thermal catalysis in solar all-spectrum utilization systems

Spectrum-selective photo-thermalcatalytic reactions are integrated with PV, ordinary photocatalysis,or power cycles to construct a full-spectrum utilization system ofconcentrated sunlight. Such integration ensures that each energy conversionprocess operates at its optimal working spectrum. For example, SiPV cells have the highest efficiency with wavelengths of 700–1100nm, while most spectra out of this region are dissipated into heat.For full-spectrum usage, a solar water purification and renewableelectricity generation (SOLWAT) system was designed [37–39]. It included a hybrid solar receiver consisted ofphoto-degradation and PV modules. In this system, photo-degradationselectively absorbed UV and IR radiations while transmitting visiblelight to PV.

Hybrid systems with photo-thermalcatalysis, PV, and other techniques are emerging concepts. Assessmentsof such systems are not yet widely discussed. Zamfirescu and Dincer[40] developed a modelof a 500 MW solar tower based on a spectrum splitting system integratingPV arrays, photocatalysis and volumetric absorbent Rankine cycles.A case study was presented for an oil sands exploitation area wheresulfurous aqueous wastes and hydrogen demand existed. This study offersinstructions because this system is also spectrum-selective and poly-generative,similar to hybrid photo-thermal chemical/PV/other systems.

When it comes to performance comparisonsbetween different poly-generative solar energy systems, solar-to-fuel(STF) efficiency is not globally applicable, because STF efficiencyis a criterion for a chemical process, where the only input poweris sunlight and the only output power is hydrogen derived from solar-drivenwater splitting. To provide a unified comparison for the system, theelectricity consumed by an electrolysis water-splitting needs to becalculated if identical amount of hydrogen is produced by photocatalysis[41]. As a detailed analysisof significant component, exergy comparisons of hydrogen productionmethods from renewable energy were summarized [42].

Progress in photo-thermal catalytic fuel production

Photocatalytic fuel production normallyincludes water splitting and CO₂ reduction.High fuel generation efficiency is difficult to achieve in pure wateror CO₂ because of the rapid recombination ofphoto-generated electrons and holes. Remedies are proposed such asnoble metal loading, addition of sacrificial reagent, and sensitization[43]. Among them, sacrificialreagent works as a hole consumer and enhances electron/hole separation,resulting in a higher quantum efficiency. A wide variety of biofuelor sewage components, such as alcohols, organic acids, alkanes, andorganic pollutants, are eligible as sacrificial reagents. To showprospects of photo-thermal catalysis, experiments and observationsare classified due to sacrificial reagents used. Remarkable experimentson LSPR effects in photo-thermal catalytic reactions are shown inSub-section 3.1.

LSPR effects in photo-thermal catalytic reactions

On photo-thermal catalysis with LSPReffects, the plasmon-assisted catalysis was referred to by Adlemanandcoworkers in 2009. They observed that Au NPs in glass microchannelperform steam reforming of methanol under irradiation of 532 nm laserbeam [44]. Under an irradiationflux of ~10⁸ W/m², LSPR heating provides the necessary heat of reaction while thereactant fluids remain under ambient conditions.

The LSPR effects are applied by depositingmetals on semiconductor NPs. Christopher et al. [45] synthesized Ag nanocubes (edgelength being 60 nm) supported a-Al₂O₃ surface.Due to plasmonic resonance of Ag, the absorption spectrum was expandedbeyond 400 nm. It was observed that the reaction rate for ethyleneepoxidation at 433 K/2.5 suns was comparable to that at 473 K in darkcondition. They continued to increase the selectivity of propyleneepoxidation with Cu/TiO₂ catalysts at 200°C,maintaining the reduced state of Cu NPs with LSPR [46].

Oxidation reactions of alcohols arewidely used in understanding the mechanisms of photo-thermal catalysishydrogen production. The mechanisms were examined over Au/TiO₂ film [47] between 100°C and 200°C under UV-Vis irradiation. A strongLSPR absorption of around 550 nm was achieved for the catalyst films,which exhibited blue color in visible light (Fig. 2). They clarifiedthe contributions of plasmonic and band gap excitation effects inphoto-thermal catalysis by probing the impact of different excitationwavelengths on the thermal-catalytic activity of Au/TiO₂.

Alcohols or organic acids as sacrificial reagents

The dependence of reaction rate ontemperature from 5°C to 30°C was initially observed in hydrogenproduction from Rh/TiO₂ catalyzed aqueous alcoholsolutions [6], such asmethanol, ethanol, 1-propanol, and 2-propanol.Then, the hydrogen yieldenhancement between 30°C and 100°C at different partial pressureswas observed in methanol-water vapor over Rh/TiO₂ and Pt/TiO₂ photocatalyst [7].

SiO₂/Ag@TiO₂ nanocomposites synthesized by Gao et al. [48] were designed to produce hydrogenand fresh water from seawater. Due to the LSPR effect and its core-shellstructure, this nanoparticle absorbs nearly full-spectrum sunlight.Sacrificial reagents, such as methanol, ethylene glycol, glucose,and glycerol were tested in laboratory conditions. On the other hand,parabolic trough collector (PTC) is the most commercialized equipmentto achieve a higher concentration ratio [49,50], whichfurther leads to a higher hydrogen production. A PTC-type reactorwas also designed and operated under the sun to demonstrate the feasibility(see Fig. 3).The maximum hydrogen generation rate from simulated seawater-glycerolsolution was 13.3 mmol/(g_cat·h) at thehighest reaction temperature (100°C). The results provide evidenceof photo-thermal synergetic mechanisms in natural sunlight.

Song et al. [51] investigated UV LED irradiatednon-plasmonic Pt/TiO₂ NPs. The loading of Ptexpanded the absorption to visible light region (Fig. 4(a)). Organicsacrificial reagents, such as methanol, trielthanolamne, formic acid,and glucose, were studied at different volume concentrations and 70°C–90°C.At 90°C and with formic acid as sacrificial agent, a 714.3 μmol/(g_cat·h) of hydrogen yield was acquired, which was8.1 or 4.2 times higher than that in photo or thermal conditions,respectively. The performances of photo-thermal catalysis over thermaland photo catalysis are shown in Fig. 4(b). They also further investigatedthe synergy of photo and thermal effects on Pt/TiO₂ catalyzed formic acid reforming [52].

For catalysts other than TiO₂, Pt/SrTiO₃ photocatalyst [53] was fabricated for hydrogen productionfrom various alcohol or Na₂SO₃ solutions. A temperature range from 15°C to 45°C was applied,and the highest rate is 500 μmol/(g_cat·h)with a Pt loading of 0.5 wt% under UV light. Though Na₂SO₃ appeared to have the lowest activity (150μmol/(g_cat·h)), it is a cheap materialfrom mines or seawater and has a great potential in application.

A recent research of photo-thermalsynergetic reaction for hydrogen evolution was implemented with Pt/TiO₂ photocatalysts dispersed in aqueous solutions of ethyleneglycol [36]. At a lightintensity of 6500 W/m², the reaction temperaturewas controlled from 38°C to 60°C to test the sensibility ofhydrogen production to heat. The results show that hydrogen productionin 4 h peaked at 15.18 mmol/g_cat at 55°C.Explanation of the peak was derived from thermodynamic deductions,although the optimal temperature remains complicated.

Alkanesor hydrogen as sacrificial reagents

Photocatalytic steam reforming of alkanes (PSRM)

Yoshida and coworkers showed thatwith UV lights (before 400 nm) and Pt/TiO₂,SRM is enabled to proceed around room temperature [54], instead of over 1073 K. They studiedthe effects of Pt loading, Pt size, light intensity, and CH₄ concentration. They continued loading Pt on other eligiblephotocatalysts, such as NaTaO₃:M_BM [55],where M_BM was La, Gd, and Ybor Ba. The bestobserved photo-thermal activity was from Pt/NaTaO₃:La (2%), which was more than twice that of Pt/TiO₂.The thermal effects slightly increased the activity of Pt(0.03)/NaTaO₃:La_BM (2%) and plateaued afterthe temperature reached 350 K. Later in 2011, they found that thereaction rate over Pt/Ga₂O₃ [56] for PSRM was promotedby stepwise increasing temperature from 318 K to 344 K. The thermalactivation energy was found to decrease with increasing light intensity.For example, the hydrogen yield at 318 K rose linearly as light intensitygrew from 50 W/m² to 300 W/m².

CO₂ reforming of alkane (CRM)

CH₄ and CO₂ are both greenhouse gases inducing a growing environmentalconcern. CO₂ reforming of methane (CRM) canconvert CH₄ and CO₂ mixtureinto solar fuels. In thermocatalysis, successive CRM requires a temperatureof 800°C–1000°C to overcome the high reaction barrierassociated. For solar fuels, this requires a large mirror field anddurable reactors. Photo-thermal catalysis is expected to break thethermodynamic barrier of endothermic reaction, e.g. CRM, to occurat a lower temperature.

Photoreduction of CO₂ to CO by methane was reported to occur on ZrO₂ and MgO at room temperature [57,58]. However, H₂ was scarcely obtained from methane. Other products,such as H₂O and surface formate species, wereacquired instead.

Yoshida’s group did CRM overcommercial Ga₂O₃ pretreatedat 800°C in 13.3 kPa oxygen [59]. Such catalyst can operate CRM reactions at mild temperatures between200°C and 400°C. The results show that the heating of catalystmay increase the activity under light irradiation. However, reactionsdid not occur in absence of light. It is speculated that the thermaleffects would help the thermal steps with low activation energy inthe photocatalytic CRM, e.g. desorption of products or migration ofphoto excited electron, while the main activation step on this CRMwould be promoted by photo energy.

For LSPR enhancements on photo-thermalCRM, Au NPs were added to Rh/SBA-15 (made of SiO₂) catalysts [60]. Allof the three catalysts have UV-Vis absorption in DRS spectra (Fig.5(a)). The operation temperature was kept at 500°C by visiblelight irradiation in all reaction sets. The highest fuel yield overAu-Rh/SBA-15 is 6900 μmol/(g_cat·s)for H₂ and 6800 μmol/(g_cat·s) for CO₂, which is 1.7 times tothat over Rh/SBA-15. As shown in Fig. 5(b) and (c), Au-Rh/SBA-15 hasa fuel yield of nearly 1.5 times of the sum of yields from Rh/SBA-15and Au/SBA-15 in the same condition, indicating the photo-thermalsynergetic effect. However, with increasing light intensity, the H₂ yield peaked at 2800 W/m² and then began to decrease. This may result from the Au meltingand aggregation induced by LSPR localized heat, since the Tammanntemperature of Au NPs is as low as 395°C. Carbon deposition inCH₄ cleavage may deactivate catalyst with prolongedreaction time.

Black TiO₂ has a smaller bang gap than ordinary TiO₂ due to a generated donor level (Ti³⁺).Such energy level meets the requirements of CRM over 150°C becauseof thermally-induced changes in CO₂/CH₄ redox potential. As an active co-catalyst, Pt was loadedon the black of TiO₂ to form photo-thermalcatalysts [61]. In visiblelight from an AM 1.5G solar simulator at 1 sun, reaction started overPt-black TiO₂ at 350°C, which was 200 Klower than thermocatalysis. The temperature dependence of quantumefficiency was studied at 550°C, 650°C, and 700°C anda peak of 64.9% was acquired at 650°C. This may result from abeneficial shift in redox potential at 650°C and a turnover tothermocatalysis above 650°C.

Methanation of CO or CO₂

Decomposition or reforming of alcoholsor alkanes is useful in hydrogen generation. However, removal of COor CO₂ from gaseous products consumes electricity.As an attractive strategy for the removal of CO in H₂-rich stream, Fu’s group studied photothermal catalytic COmethanation over Ru/TiO₂ [62] and Ni/TiO₂ catalysts [63] in UVlight from a Xenon lamp. They continued with CO₂ methanation over Ru/TiO_(2−x)N_x catalysts assisted with visible light [22]. Figure 6(a) depicts the absorptionspectra of Ru/TiO_(2−x)N_x in comparisons withTiO_(2−x)N_x.

For reverse water-gas shift reactions(RWGS),Hoch and coworkers investigated UV-Vis responsive In₂O_3−x(OH)_y calcinated at 250°C–450°C and operatedat the optimal temperature of 150°C–190°C [64–65]. Then, they synthesized In₂O_3−x(OH)_y NPs coated silicon nanowires (SiNWs) [66], realizing the dual function ofutilizing both light and heat energy provided by the broad-band solarirradiance. To enhance the understanding of this reaction, the mechanismin photothermal hydrogenation of CO₂ gas wasinvestigated on Pd@Nb₂O₅ nanocrystals [67].

Plasmonic Au NPs with an averagediameter of 3.5 nm on various oxides were tested for their enhancementon activity [68]. Depositionof Au NPs expanded absorption of TiO₂, CeO₂, and Al₂O₃ to the visible light region, as shown respectively in Fig. 7(a)–(c).The temperature of the reaction was varied from 100°C to 400°Cat constant pressure to test the CO₂ conversionrate. The results in Fig. 7(d) showed that light-induced LSPR enhancedthermal chemical performances, but the enhancements decreased withincreasing temperature. The LSPR effect was suggested to change theenergetics of the reaction because of an observed decrease in apparentactivation energy (Fig. 7(e)). It was proposed that the changes werecaused by either hot electron generation or adsorbate polarization.

Tahir et al. [69] analyzed nanostructured NiO–In₂O₃/TiO₂ catalyst for RWGS reaction. The effect of reaction temperaturesof 100°C, 120°C, and 140°C was investigated to determinethe effects of heating. They found that temperature rise graduallyincreased CH₄ production but decreased CO yieldat elevated temperature (140°C).

Lights assistance in Sabatier reactionswere studied over RuNPs supported on SiNW [70]. The as-synthesized catalyst absorbedVis-NIR lights (Fig. 8(a)) and turned 4:1 mixture of H₂ and CO₂ to methane. Ina Xenon lamp lightintensity of 3.2 sun and a temperature of 150°C, a maximum CO₂ conversion rate of 0.99 mmol/(g_cat·h) was achieved. After experiments at various wavelengthsin Fig. 8(c), a photo-thermal synergetic mechanism was proposed. Sub-bandgap photons generated heat and activated the thermal catalysis. Photoswith an energy greater than band-gap induced electron-hole pairs inthe SiNW support, then the pairs accelerated the reaction by activatingadsorbed hydrogen atoms. The results also showed that thermal catalysiswas in majority because the reaction rates were higher in lights withlonger wavelengths (Fig. 8(d)).

Fuel production without sacrificial reagents

Direct water splitting

In 1995, the dependences of lightintensity, Ph and temperature on water splitting were experimentallystudied over Pt-RuO₂/TiO₂ to derive a kinetic model [8]. The intrinsic rate was found to be linearly dependent on the intensityof incident light P, exponentiallydependent on the pH of reaction solution and on reciprocal temperature T. The mathematical expression is r_i=K_p{K}_{\mathrm{p}\mathrm{H}}{K}_T=k_{p}P\cdot k_{\mathrm{p}\mathrm{H}}{e}^{\alpha \mathrm{p}\mathrm{H}}\cdot k_T{e}^{-E/RT}=k_{\mathrm{0}}{e}^{\alpha \mathrm{p}\mathrm{H}}{e}^{-E/RT}P, where α, k₀, E areconstants. The value of α is 0.19, while the value of activation energy E is 27 kJ/mol, 18.5 kJ/mol, and 21 kJ/mol for anatase,rutile and P25 TiO₂. The value of k₀ is derived fromthe experimental data.

Hisatomi et al. [71] examined the reaction activityof water splitting over Rh_2−yCr_yO₃ loaded(Ga_1−xZn_x)(N_1−xO_x) when the reaction was affected by variationsin co-catalyst loading, light intensity, hydrogen/deuterium isotopes,and reaction temperature. They also studied the dependence of H₂ yield on temperature over Rh_2−yCr_yO₃ loaded Ga₂O₃: Zn [72].The water splitting rates monotonicallyincreased from 300 μmol/(h·g_cat)to 387 μmol/(g_cat·h) with increasingreaction temperature in the range from 5°C to 50°C.

The manganese/semiconductor catalystis highly sensitive to temperature and irradiation shifts [73]. An elevated temperature between25°C and 85°C can cause decomposition of the Mn(III/IV)-oxodimer [74] and increaseabsorption, resulting in increased absorption in three regions of280 nm, 310 nm–340 nm, and 400–440 nm. Nanostructuresand mechanisms for manganese-containing compounds to mimic naturalphotosynthesis were summarized in details [75].

CO₂ reduction into fuels

Different from RWGS or chemical cycles,CO₂ reduction without consumption of sacrificialreagents was listed here. Hydrogen treated mesoporous WO₃ [76] wasapplied at relatively low temperatures (<300°C) to convertCO₂ and water to CH₄ or CH₃OH invisible light (>420 nm).The catalystsexhibit a selectivity toward CH₄ evolutionin only visible light irradiation. Under photo-thermal conditions,the concentration of oxygen vacancies significantly influenced theperformance. In chemical engineering, rational reactor design is beneficialfor efficient CO₂ photo-thermal coupled reduction.A three-dimensional numerical optimization was applied on a solarparabolic trough receiver reactor (SPTRR) filled with catalysts [77]. The results showed that a moreuniform distribution of outer surface temperature and a higher chemicalenergy conversion per unit pump power were acquired. A modified concentratedsolar reactor was developed to test the performance of CO₂ reduction experimentally [78].

For solar photo-thermochemical alkanereverse combustion (SPARC) processes to produce C1 to C13 hydrocarbons,a one-step, gas-phase reaction with Co(5%)/TiO₂ catalysts [79] was conductedat 180°C–200°C under UV irradiation. A parametric studyof pressure and partial pressure ratio revealed that temperaturesin excess of160°C were needed to obtain a higher Cn products inquantity and that the product distribution shifted toward higher Cnproducts with increasing pressure. However, solar-to-fuel efficiencyof this process was not commercially applicable and needed to be increased.

Light-assisted high temperature solar fuel production cycles

T-Raissi et al. [80] analyzed light assisted high temperatureCO₂/CO cycle and SO₂/sulfuric acid cycles. The CO₂/CO cycle isbased on the premise that CO₂ becomes susceptibleto near-UV and even visible radiation at high temperatures (greaterthan 1300 K). Depicted in Fig. 9(a), SO₂/sulfuricis a modification of Westinghouse hybrid cycle, wherein the electrochemicalstep is replaced by a photocatalytic step. The results showed thatSO₂/sulfuric required no electricity inputand a maximum temperature lower than 1170 K, compared to the traditionalWestinghouse hybrid cycle. A solar energy system was constructed basedon the SO₂/sulfuric acid cycle (see Fig. 9(b)).

Thermally-driving chemical-loopingis widely studied such as ZnO/Zn,SnO₂/SnO,and Fe₃O₄/FeO cyclesfor solar fuels. However, two-step water-splitting chemical-loopingwas also enhanced by light irradiation. Under 1 sun, a Cu-loaded TiO₂ enabled the oxygen expulsion at room temperature andH₂ release from steam to take place at 140°C[81]. Absorption of TiO₂ was extended to 450 nm, but visible and infrared lightwere also utilized due to photo-thermal effects. Optimized loadingof Cu was 1%, and H₂ and O₂ yields decrease by 11%–13% after 25 cycles. In a 6 cm² reactor containing 0.25 g catalysts, the hydrogenyield of 4 μmol/(cm²·h) was acquired.The conversion efficiency was 0.25 at 1 sun and had the potentialto be further optimized.

A CO₂ reductioncycle was experimentally studied and modeled using the density functionaltheory to probe the efficacy of MnO_x nanoclusters surface modification of rutileand anatase [82]. TheMnO cycled between Mn³⁺ and Mn²⁺with inputs of thermal and photo energy using CO₂ to produce CO and heal oxygen vacancies.

Forms of photocatalysts

Photo-thermal catalysts are usuallyin the scale of millimeter or nanometer to acquire a high specificsurface area. When applied in liquid phase reactions, catalyst NPs(see Fig. 10(a)) are dispersed to form a stable mixture, providinga high contact area between the catalyst and reactants. A stable liquidmixture also works as a homogeneous selective filter of induced light,which is important to eliminate “hot spots” in underlyingPV arrays or thermal receivers.

In circumstances of elevated temperaturesor gas phase reactions, dramatic movements and relatively high surfaceenergy of particles may cause agglomeration of catalysts and reducereaction activity as well as light selectivity. To ensure stability,catalysts are deposited upon substrates like SiO₂ chips, films, or nanowires, like Fig. 10(b). Requirements for substratesare made on light absorption, specific surface area, and thermal endurance.Morphologies of deposited catalysts like arrays of nanosheets, nanotubes,and nanorods are developed for efficient light trapping. However,light absorption here is on the surface of deposited catalysts ratherthan volumetric in liquids, thus a rational design of reaction bedand reactor is still required.

Summary

The works reviewed from Sub-section3.1 to 3.3 are listed in Table 1. The works reviewed in Sub-section3.4 are listed in Table 2.The representative data highlighted by theauthors are presented. The short dash stands for the data that arenot given in literature.

Potential and challenges

The photo-thermal catalytic reactionswith spectrum-selectivity are emerging methods of solar fuel production.Photo-thermal catalysis is the synergetic effect rather than the superpositionof photo and thermal catalyzes. Performances of photo-thermal catalysisare usually reported to be higher than photo or thermal catalysis,while the underlying mechanism remains unclear and needs further exploration.Due to its spectrum selective nature, the photo-thermal catalysisreaction is more applicable in full-spectrum solar energy systemswhen coupled with PV arrays or power cycles. According to Section3, such reactions produce solar fuels from a wide variety of reactants,such as biomass, fossil fuels, sewages, water and CO₂, which suggests the feasibility satisfying the industrial requirementsin nowadays and the future.

As a newly-proposed type of reaction,spectrum-selective photo-thermal catalytic reaction yet demands formore academic attentions. Research directions may be as follows:

1) Optimizations on materials andstructure of catalysts are needed to achieve a higher conversion efficiencyand selectivity of products.

2) Consider the integration withother energy conversions, simple and cheap ways to alter catalystlight absorption should be developed.

3) Deeper understanding of the reactionmechanism, especially the photo-thermal synergy effects, is expectedto be explored.

4) Elevated temperature is likelyto induce deactivation of catalysts. Therefore, high-temperature durabilityis expected.

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