“Smart” Catalysts for CO2 Hydrogenation

catalysts

Review Microwave, Ultrasound, and Mechanochemistry: Unconventional Tools that Are Used to Obtain “Smart” Catalysts for CO2 Hydrogenation

Maela Manzoli 1 ID and Barbara Bonelli 2,* ID 1 Department of Drug Science and Technology, NIS and INSTM reference Centres, Università degli Studi di Torino, Via Pietro Giuria 9, 10125 Torino, Italy; [email protected] 2 Department of Applied Science and Technology and INSTM-Unit of Torino Politecnico, Corso Duca degli Abruzzi 24, 10129 Torino, Italy * Correspondence: [email protected]; Tel.: +39-011-090-4719

Received: 6 April 2018; Accepted: 21 June 2018; Published: 28 June 2018

Abstract: The most recent progress obtained through the precise use of enabling technologies, namely microwave, ultrasound, and mechanochemistry, described in the literature for obtaining improved performance catalysts (and photocatalysts) for CO2 hydrogenation, are reviewed. In particular, the main advantages (and drawbacks) found in using the proposed methodologies will be discussed and compared by focusing on catalyst design and optimization of clean and efﬁcient (green) synthetic processes. The role of microwaves as a possible activation tool used to improve the reaction yield will also be considered.

Keywords: CO2 hydrogenation; microwaves; ultrasound; mechanochemistry; catalyst preparation

1. Introduction

Atmospheric CO2 concentration is growing continuously: in 2017, a “record concentration” of 413 ppm was registered (according to https://www.co2.earth/daily-co2) and a concentration of ca. 570 ppm is expected to be reached by the end of this century [1]. The mitigation of CO2 (the most popular among atmospheric greenhouse gases) is currently a scientific, technological, social, and economic issue [1]. Several strategies have been proposed to limit/decrease CO2 concentration in the atmosphere: besides lowering CO2 emissions with the help of low-C fuels (and of appropriate policies from developed countries), there is a great spur of studies and technologies aimed at optimizing several processes of CO2 capture & storage [2–10] and reutilization [11,12]. The first two steps, currently fairly developed, are indeed functional to CO2 reutilization processes, which mostly need a concentrated and rather pure CO2 stream apt to further chemical transformations. The chemical processes for CO2 reutilization are catalytic [13–15], photocatalytic [16–20], electrocatalytic [21–25], or photoelectrocatalytic [18,26]. As a whole, the chemical reduction of CO2 is a challenge from both the industrial and the catalytic point of view [27], but it is also a hot scientific topic, since CO2 is an ideal C1 feedstock for producing different types of chemicals and, ultimately, fuels. Unfortunately, its high thermodynamic stability limits the number of processes in which CO2 can be utilized as a chemical feedstock (e.g., the synthesis of urea), and usually electrocatalytic processes are needed [28]. Most of the catalytic processes (like those reported in Scheme1) imply the use of H 2, which is per se a costly reagent due to its scarce natural abundance (from which arises the issue of its production), along with safety and transportation issues. All this notwithstanding, the products of CO2 reduction are ultimately fuels, raw materials, and/or chemical intermediates [1].

Catalysts 2018, 8, 262; doi:10.3390/catal8070262 www.mdpi.com/journal/catalysts Catalysts 2018, 8, 262 2 of 35 Catalysts 2018, 8, x FOR PEER REVIEW 2 of 35

Scheme 1. Some products of COCO22 hydrogenation. Reproduced from Ref. [1] [1] with permission of The Royal Society of Chemistry.

Though some homogenoushomogenous catalysts are highly active and selectiveselective inin severalseveral COCO22 reduction processes [29] [29],, they they are are hardly hardly applicable applicable on an on industrial an industrial scale scale,, and therefore and therefore there is there a great is interest a great interesttowards towards the development the development of efficient of efficient heterogeneous heterogeneous catalysts catalysts and/or and/or the the immobilization immobilization of homogeneous ones ones [30 ].[30] For. theFor industrial the industrial application application of CO2 hydrogenation of CO2 hydrogenation processes, heterogeneous processes, catalystsheterogeneous (e.g., Fe-, catalysts Cu-, and (e.g., Ni-based Fe-, Cu solid-, and catalysts Ni-based [31]) solidoffer some catalysts practical [31]) advantages offer some compared practical toadvantages homogeneous compared ones: to solids homogeneous with large ones: specific solids surface with area, large small specific particles surface of area, active small phase, particles and fair of metalactive dispersionphase, and arefair usually metal dispersion more active are and usually selective, more and active also and more selective, stable andand characterizedalso more stable by longerand characterized life. However, by longer they often life. provideHowever, low they yields often and provide poor selectivity, low yields mainly and poor due selectivity to fast kinetics, mainly of C–Hdue to bond fast formationkinetics of [C1].–H bond formation [1]. In order toto allowallow industrialindustrial applicationsapplications of heterogeneous catalysis, green processes should be developed, whichwhich oftenoften require require efficient efficient (nano)catalysts (nano)catalysts that that are are able able to selectively to selectively transform transform CO2 COinto2 addedinto added value value molecules molecules [27]. Depending[27]. Depending on the on reaction the reaction adopted adopted for CO for2 reduction, CO2 reduction, different different active phasesactive phases (mostly (mostly Ni, but Ni, also but more also noble more metals noble [ 31metals]) have [31] to) behave supported to be supported on different on oxidesdifferent (γ -Aloxides2O3 being(γ-Al2O the3 being most usedthe most one used [31]) thatone have[31]) that to be have stable to atbe high stable temperatures, at high temperature provide as, certainprovide amount a certain of acid/baseamount of sites,acid/base and stabilizesites, and the stabilize catalytically the catalytically active metal active particles, metal etc. particles, etc. Whatever the type of COCO22 hydrogenation process, active, selective,selective, and stable heterogeneousheterogeneous catalysts are preferable preferable in in view view of of an an actua actuall industrial industrial application application [1] [1. ].They They should should also also be bebased based on onearth earth-abundant-abundant and and non non-toxic-toxic elements elements and andobtained obtained by green by green synthesis synthesis methods methods:: in one in word one word,, they theyshould should be “smart” be “smart”.. Though Though the output the outputss of CO of2 CO reduction2 reduction are arediverse diverse [1], [“1smart], “smart”” catalysts catalysts have have to tomeet meet some some requirements requirements that that can can be be obtained obtained by by optimiz optimizinging their their preparation preparation strategy strategy,, for for instance instance.. TheThe physico-chemicalphysico-chemical properties of a heterogeneous catalyst are determined and controlled by its preparation method: therefore, catalysts prepared by differentdifferent means may have markedly different features and,and, thus, different catalyticcatalytic performancesperformances [[32,33]32,33].. In order order to to allow allow an an actual actual development development of feasible of feasible industrial industrial CO2 CO hydrogenation2 hydrogenation processes, processes, new newcatalysts catalysts should should be obtained be obtained by methodsby methods that that are are able able to to control control and and tailor tailor the the physicophysico-chemical-chemical properties in order to improveimprove thethe catalystcatalyst performanceperformance [[32]32].. Moreover, aa deeperdeeper understandingunderstanding of the reaction mechanisms with the help of both experiments and molecular simulations simulations is is needed needed [1] [1],, especially asas far far as as the the first first step step of CO of2 reduction, CO2 reduction, i.e., its i.e. adsorption, its adsorption at the catalyst at the surface, catalyst is surfaceconcerned., is Notwithstandingconcerned. Notwithstanding the plethora of the works plethora concerning of works CO2 reduction, concerning a thorough CO2 reduction understanding, a thorough of the mechanismsunderstanding is still of the lacking mechanisms and/or debated is still lacking [18]. As and/ faror as debated heterogeneous [18]. As catalysts far as heterare concerned,ogeneous therecatalysts is a are general concerned, consensus there aboutis a general the idea consensus that the about active the site idea is provided that the active by the site primary is provided catalyst by (e.g.,the primary the metal catalyst particle) (e.g. acting, the synergistically metal particle) with acting the supportsynergisti (and/orcally with the promoter)the support [32 (and/or]. Evidence the ofpromoter) such synergy [32]. Evidence is provided of such in several synergy works, is provided but the actualin several mechanism works, but is stillthe unclear,actual mechanism even for the is synthesisstill unclear, of methanoiceven for the acid synthesis (HCOOH), of methanoic which is only acidthe (HCOOH) first step, which of CO 2ishydrogenation only the first step [1]. of CO2 hydrogenationFrom both [1] industrial. and scientific points of view, the great interest in heterogeneous catalysts basedFrom on abundant both industrial and low and cost scientific metals likepoint Ni,s of Cu, view, and Fethe is great frustrated interest by in their heterogeneous poor activity catalysts in mild based on abundant and low cost metals like Ni, Cu, and Fe is frustrated by their poor activity in mild reaction conditions, in that an extreme dispersion of metal particles and a strong interaction with the support should be attained [31].

Catalysts 2018, 8, 262 3 of 35 reaction conditions, in that an extreme dispersion of metal particles and a strong interaction with the Catalysts 2018, 8, x FOR PEER REVIEW 3 of 35 support should be attained [31]. As mentionedAs mentioned before, before from, from an environmental an environmental point point of view,of view, catalysts catalyst shoulds should be be obtained obtained by by green synthesisgreen methods synthesis [27 methods]. In our [27] opinion,. In theour use opinion, of microwaves, the use of ultrasound, microwaves, or mechanochemistryultrasound, or representsmechanochemistry promising andrepresent simples promising approaches, and which simple are approach alternativeses, which to conventionally are alternatives adopted to syntheticconventionally protocols, adopted to obtain synthetic novel catalystsprotocols, withto obtain improved novel catalysts activity with and improved selectivity, activity in contrast and to selectivity, in contrast to conventional methods, which usually encompass multi-step processes, conventional methods, which usually encompass multi-step processes, heating, and/or the addition heating, and/or the addition of expensive and/or hazardous chemicals. The development of such of expensive and/or hazardous chemicals. The development of such techniques has increased techniques has increased significantly in recent years, with most reports published over the past 5– signiﬁcantly10 years in(vide recent infra) years,. with most reports published over the past 5–10 years (vide infra). The aimThe ofaim the of presentthe present review review is is to to supply supply a a briefbrief (but(but exhaustive exhaustive)) description description of the of themost most recent recent advancesadvances in the in synthesis the synthesis of solid of solid catalysts catalysts (and (and photocatalysts) photocatalysts) by by microwave microwave (MW), (MW), ultrasound ultrasound (US), and mechanochemistry(US), and mechanochemistry (MC) to (MC) be compared to be compared to conventional to conventional methods. methods.

2. Microwaves2. Microwave ass a as Tool a Tool to Improveto Improve Catalytic Catalytic CO CO22 HydrogenationHydrogenation

2.1. General2.1. General Principles Principles of MW-Assisted of MW-Assisted Synthesis Synthesis of of (Nano)Materials (Nano)Materials MW-assistedMW-assisted synthesis synthesissimply simply requires requires the thepresence presence of a MW of reactor a MW, simultaneously reactor, simultaneously ensuring ensuringshorter shorter reaction reaction time, notime, contact no contact between between the radiation the radiation source and source the reacting and the mixture reacting, and mixture, a homogeneous heating of the latter [34,35]. and a homogeneous heating of the latter [34,35]. Homogeneous heating is a crucial point, as thermal gradients affect both nucleation and growth Homogeneous heating is a crucial point, as thermal gradients affect both nucleation and growth steps, which finally determine the physico-chemical properties of the resulting nanomaterials. For steps,this which reason, finally MW determine-assisted synthesis the physico-chemical was initially developed properties to prepare of the resultingceramic materials, nanomaterials. for which For it this reason,is crucial MW-assisted to avoid synthesistemperature was gradients initially that developed may induce to formation prepare ceramicof fractures materials, in the final for product which it is crucial[36] to. avoid temperature gradients that may induce formation of fractures in the final product [36]. MW irradiationMW irradiation favors favors fast fast nucleation nucleation and and crystalline crystalline growth, growth, wh whichich can can accelerat acceleratee the formation the formation of hierarchicalof hierarchical structures, structures like, like the the bismuth bismuth oxyiodideoxyiodide (BiOI) (BiOI) architectures architectures shown shown in Figure in Figure 1 [37]1[. 37]. HierarchicalHierarchical porous porous solids solids can can bebe produced produced by by MW MW-assisted-assisted synthesis synthesis in non- inaqueous non-aqueous solvents, since solvents, sinceo organicrganic solvents solvents with with high high dielectric dielectric loss lossfactor factor are required are required to ensure to ensure efficient efficient MW absorption MW absorption and rapid heating [37]. and rapid heating [37].

− FigureFigur 1. SEMe 1. SEM (a,b ()a and,b) and TEM TEM (c (,cd,d)) images images of BiOI BiOI flower ﬂower-like-like spheres spheres (dimensions (dimensions in the in500 the nm 500−1 µm nm 1 µm range)range) mademade up up by by nanosheets nanosheets as bu asilding building blocks. blocks. Reprinted Reprinted from Chem. from Eng.Chem. J., 235 Eng.Ai, L.; J.Zeng,, 235 Y.; Ai, L.; Zeng,Jiang, Y.; Jiang, J., HierarchicalJ., Hierarchical Porous Porous BiOI BiOI Architectures: Architectures: facile facile Microwave Microwave Nonaqueous Nonaqueous Synthesis, Synthesis, Characterization,Characterization and, and Application Application in in the the Removal Removal of Congo R Reded from from Aqueous Aqueous Solution, Solution, 331– 331–339.339. CopyrightCopyright (2014) (2014 with) with permission permission from from Elsevier. Elsevier.

Catalysts 2018, 8, 262 4 of 35

Additionally, MW heating can be easily coupled with several chemical methods including combustion synthesis (CS), hydrothermal (HT) synthesis, and sol-gel (SG). For instance, powders obtained by MW-assisted CS are more chemically and thermally stable than those obtained by “conventional” CS; moreover, smaller particles with a lower tendency to aggregation are obtained [36]. This occurs because MW irradiation uniformly heats the reagents mixture, whereas an electric oven heats a mixture from the exterior to the interior, finally creating a temperature gradient and leading to heterogeneous powders. Moreover, MW interacts with polar reagents, inducing dipole oscillations: the oscillating species collide more frequently and at higher energy, facilitating bond breakage and new bond formation. Consequently, the heat needed for the reaction is generated within the mixture with no need of external heat supply [34,35]. MW-assisted CS, for instance, allows obtaining sizeable amounts of phases (mainly oxides, nitrides, and carbides) that are stable at high temperatures and have nanometer size, highly specific surface area and defectivity, and mechanically stable pore structure [36]. MW is also known to favor the formation of given crystalline phases and can be efficiently used to drive the synthesis towards the desired products. All the above parameters are indeed important for catalytic applications.

2.2. MW-Assisted Syntheses of Heterogeneous Catalysts for CO2 Reduction

Cai et al. [38] supported some Cu-based catalysts on ZnO and on Ga2O3 and tested them in the CO2 hydrogenation to CH3OH:

CO2 + 3H2 CH3OH + H2O (1)

The Cu-based catalysts were prepared by co-precipitation and impregnation, and the effect of MW on both preparation methods was studied: as a whole, the catalysts prepared by MW-assisted synthesis showed better Cu dispersion and enhanced catalytic activity in terms of both CO2 conversion and CH3OH yield, especially when a single-step MW-assisted method was adopted [38]. The catalysts were characterized by X-Ray Diffraction (XRD), H2 temperature programmed reduction (TPR), ◦ N2 adsorption/desorption isotherms at −196 C, CO2 temperature-programmed desorption (TPD), adsorption calorimetry, and X-ray Photoemission Spectroscopy (XPS). Since all the samples are calcined at 350 ◦C, no significant changes in their surface area were observed, although the sample obtained by a single-step, MW-assisted method showed slightly larger values for both surface area and pore volume. The XRD patterns of the same sample revealed the likely presence of small crystallites of Cu2O (whereas CuO mostly formed in the other samples) and the smallest size of ZnO particles. TPR showed the occurrence of a strong Cu/support interaction in the sample obtained by single-step, MW-assisted method, which, according to CO2 TPD and CO2 adsorption calorimetry, adsorbed −1 a considerable amount of CO2 with the largest average adsorption heat value (ca. −20 kJ mol ). This latter result showed that the use of MW seemed to improve the support basicity, a feature that helps CO2 adsorption, a crucial step during the catalytic process. The joint presence of well-dispersed Cu particles and strong basic sites at the support surface resulted in strong Cu/support synergy, leading to better catalytic performance. Besides strong Cu/support interactions, which took place with the catalysts prepared under MW irradiation, the authors observed that Cu particles were particularly active, probably because their surface was partially oxidized due to migration of Zn species from the support to the active metal (Cu) surface, which finally improved CO2 adsorption. On the contrary, such synergistic effects were not observed with same composition catalysts that were prepared by conventional methods. Huang et al. [39] studied the effect of MW heating during HT synthesis of CuO-ZnO-ZrO2 catalysts for CO2 hydrogenation to CH3OH (Equation (1)), for which CuO-ZnO is usually considered the best catalyst and where ZrO2 has the role of imparting high thermal and chemical stability under redox conditions. This type of catalyst is usually prepared by HT method, requiring long reaction times: the (expected) effect of MW irradiation was indeed the shortening of preparation time. Different Catalysts 2018, 8, 262 5 of 35 temperatures (i.e., 80, 100, 120, and 150 ◦C) were adopted during the MW-assisted HT method to Catalysts 2018, 8, x FOR PEER REVIEW 5 of 35 obtain the catalysts named CZZ80, CZZ100, CZZ120, and CZZ150 [39]. ◦ TheThe catalyst catalyst obtained obtained at 120at 120C °C (CZZ120) (CZZ120) showed showed the the best best catalytic catalytic propertiesproperties in terms of of both both CO2 conversionCO2 conversion and selectivity and selectivity to CH3 OH,to CH becoming3OH, becoming rather rather stable stable after 100after h 100 reaction. h reaction XRD. XRD analysis analysis allowed the calculationallowed the ofcalculation the size ofof boththe size CuO of both and CuO Cu crystallites: and Cu crystallites: the CZZ120 the CZZ120 catalyst catalyst showed showed the smallestthe CuOsmallest and Cu CuO crystallites and Cu crystallites before and before after and reduction, after reduction, respectively, respectively, though though Cu crystallitesCu crystallites (ca. (ca. 18 nm) were18 larger nm) were than larger parent than CuO parent crystallites CuO crystallites (ca. 7.6 (ca. nm). 7.6 nm) The. The same same catalyst catalyst was was also characterized characterized by by the largest Cu area (SCu); the largest specific surface area and by a higher amount of CO2 molecules the largest Cu area (SCu); the largest specific surface area and by a higher amount of CO2 molecules adsorbed by relatively weak basic sites. CuO crystallites in CZZ120 were easily reduced, as the adsorbed by relatively weak basic sites. CuO crystallites in CZZ120 were easily reduced, as the samples samples showed the lowest reduction temperature during H2 TPR analysis. The CH3OH yield was showed the lowest reduction temperature during H TPR analysis. The CH OH yield was directly directly related to the surface of Cu particles (SCu), as shown2 in Figure 2. On the contrary,3 XPS analysis relateddid tonot the show surface significant of Cu differences particles in (S theCu), type as shownof surface in Cu Figure species2. Onin catalysts the contrary, obtained XPS at different analysis did not showtemperatures. significant As adifferences whole, an inoptim theum type temperature of surface exist Cued species for the in synthesis catalysts of obtained CuO-ZnO at-ZrO different2 temperatures.catalysts [39] As: Cu a particle whole, size, anoptimum surface area temperature, and basicity existedexhibited for a volcano the synthesis trend with of CuO-ZnO-ZrOa maximum 2 catalystsat 120 [ 39°C.]: Cu particle size, surface area, and basicity exhibited a volcano trend with a maximum at 120 ◦C.

FigureFigure 2. 2.Linear Linear relation between between the theCH3 CHOH 3%OH yield % and yield the surface and the of Cu surface particles of (S CuCu). particlesReprinted (SCu). Reprintedfrom Huang, from Huang,C.; Mao, D.; C.; Guo, Mao, X.; D.; Yu, Guo, J., Microwave X.; Yu, J.,-Assisted Microwave-Assisted Hydrothermal Synthesis Hydrothermal of CuO-ZnO Synthesis- ZrO2 as Catalyst for Direct Synthesis of Methanol by Carbon Dioxide Hydrogenation. Energy Technol. of CuO-ZnO-ZrO2 as Catalyst for Direct Synthesis of Methanol by Carbon Dioxide Hydrogenation. EnergyCopyright Technol. (2017Copyright) with permission (2017) with from permission Wiley. from Wiley.

CO2 methanation (Equation (2)) is a process that allows transformation of CO2 in a fuel through anCO exothermic2 methanation reaction (Equation (∆H0 = −165 (2)) kJ is mol a process−1), for which that allows Ni/Al2O transformation3 is the commercial ofCO catalyst2 in a [40] fuel: through 0 −1 an exothermic reaction (∆H = −165 kJ mol ), for which Ni/Al2O3 is the commercial catalyst [40]: CO2 + 4H2 ⇌ CH4 + 2H2O (2)

Besides Al2O3, other supports canCO 2be+ ( 4Hmostly2 ) CHSiO24 and+ 2H TiO2O2, ZrO2 or CeO2. When other (more (2) noble) metals like Ru, Pt, Pd, or Au are used, selectivity is lower with respect to Ni, as they do not produceBesides exclusively Al2O3, other CH4, supports but also CH can3OH be and (mostly) CO (by Reversed SiO2 and Water TiO2 Gas, ZrO Shift,2 or RWGS CeO)2: . When other

(more noble) metals like Ru, Pt, Pd, or AuCO are2 + used,H2 ⇌ CO selectivity + H2O is lower with respect to Ni, as(3 they) do not produce exclusively CH4, but also CH3OH and CO (by Reversed Water Gas Shift, RWGS): As a whole, Ni dispersion is crucial in the catalytic activity towards CO2 methanation [41–43], and therefore the catalyst synthesis method plays a prominent role [44–46]: according to Stangeland CO + H CO + H O (3) et al. [40], catalysts should be developed2 with2 improved activity2 at lower temperatures and high activity/stability up to ca. 500 °C. Conventional co-precipitation and impregnation methods may As a whole, Ni dispersion is crucial in the catalytic activity towards CO methanation [41–43], require several hours in order to obtain a fair Ni dispersion, whereas under MW2 irradiation the andpreparation therefore time the may catalyst decrease synthesis from hours method to 15 min plays (this a is prominentone of the greatest role [advantages44–46]: accordingof using to StangelandMW irradiation et al. [40 during], catalysts co-precipitation should be and/or developed impregnation with improved). activity at lower temperatures and high activity/stability up to ca. 500 ◦C. Conventional co-precipitation and impregnation methods may require several hours in order to obtain a fair Ni dispersion, whereas under MW irradiation the Catalysts 2018, 8, 262 6 of 35 preparation time may decrease from hours to 15 min (this is one of the greatest advantages of using Catalysts 2018, 8, x FOR PEER REVIEW 6 of 35 MW irradiation during co-precipitation and/or impregnation).

Moreover,Moreover, MW-assisted MW-assisted synthesis synthesis of of Ni/Al Ni/Al2OO33 catalystscatalysts for for CO CO2 2methanationmethanation [47] [47 resulted] resulted in in higherhigher Ni Ni dispersion, dispersion as, as well well as as higher higher basicity basicity of of the the support support withwith respect to Ni/Al2O2O3 catalysts3 catalysts obtainedobtained by by impregnation. impregnation. Physico-chemical Physico-chemical characterizationcharacterization of of the the catalysts catalysts showed showed some some relevant relevant differences:differences: for for instance, instance, Raman Raman spectroscopy spectroscopy revealed a a higher higher amount amount of of NiO NiO oxide oxide in inthe the sample sample preparedprepared by by impregnation, impregnation, which which also also showed showed lower lower surface surface area area ( =∼(≅143143 mm22 g−1 1againstagainst 163 163 m m2 g2 −g1).− 1). TheThe most most important important property property was, was, however, however, thethe NiNi dispersion:dispersion: it it was was likely likely favored favored by by the the fact fact that that MWMW helps help thes the dispersion dispersion of inorganic of inorganic salts salts onthe on support, the support as it, can as it be can clearly be clearly seen in seen Figure in 3 Figure, showing 3, showing the TEM analysis of two Ni/Al2O3 catalysts produced with and without MW assistance. The the TEM analysis of two Ni/Al2O3 catalysts produced with and without MW assistance. The average sizeaverage of Ni particlessize of Ni obtained particles byobtained MW assisted by MW synthesis assisted synthesis was 10 nm wa ands 10 16nm nm and in 16 the nm sample in the sample prepared prepared by impregnation. The sample obtained by MW-assisted synthesis also consumed ca. 25% by impregnation. The sample obtained by MW-assisted synthesis also consumed ca. 25% more H2 more H2 during TPD analysis. during TPD analysis.

FigureFigure 3. 3TEM,. TEM, HRTEM, HRTEM and, and SAED SAED patterns patterns ofof Ni/AlNi/Al2OO33 preparedprepared under under MW MW radiation radiation (a– (ac)– cand) and withoutwithout MW MW irradiation irradiation ( d(d–f–).f). Reprinted Reprinted fromfrom Int. J. J. Hydrogen Hydrogen Energy Energy, 42,, 42, Song, Song, F.; F.; Zhong, Zhong, Q.; Q.; Yu, Yu, Y.; Y.; Shi,Shi, M.; M.; Wu, Wu, Y.; Y.; Hu, Hu, J.; J.; Song, Song, Y., Y., Obtaining Obtaining Well-Dispersed Well-Dispersed Ni/Al Ni/Al2OO33 CCatalystatalyst for for CO CO2 2MMethanationethanation withwith a Microwave-Assisted a Microwave-Assisted Method, Method, 4174–4183. 4174–4183. CopyrightCopyright ( (2017)2017) wit withh permission permission from from Elsevier. Elsevier.

TheThe better better Ni dispersion Ni dispersion obtained obtained by MW by joint MW with joint a with stronger a stronger basicity ofbasicity support of (as support determined (as determined by CO2 TPD) likely led to enhanced CO2 methanation activity at lower temperature. The by CO2 TPD) likely led to enhanced CO2 methanation activity at lower temperature. The corresponding corresponding CH4 selectivity was also maintained over the whole temperature range (200◦ –400 °C), CH4 selectivity was also maintained over the whole temperature range (200–400 C), and the and the catalyst was rather stable under prolonged (72 h) catalytic tests without appreciable catalyst was rather stable under prolonged (72 h) catalytic tests without appreciable agglomeration of agglomeration of Ni species [47]. Ni species [47]. MW can be used during catalyst preparation in different manners: for instance, the effect of MW MW can be used during catalyst preparation in different manners: for instance, the effect of MW drying vs. conventional drying in the preparation of Ni-exchanged zeolites for CO2 methanation (2) dryinghas been vs. conventionalrecently studied drying [48]. in The the catalysts preparation were of prepared Ni-exchanged by incipient zeolites wetness for CO impregnation2 methanation of (2) hasUS beenY (Ultra recently Stabilized studied Y) [zeo48].lite The with catalysts 5 wt % wereNi. The prepared effects of by drying incipient method wetness after impregnationimpregnation, of USYcalcination (Ultra Stabilized temperature Y),zeolite and pre with-reduct 5 wtion %temperature Ni. The effects on the ofcatalyst drying performance method after were impregnation, evaluated. The drying method deeply affected the catalyst performance: in particular, MW-assisted drying

Catalysts 2018, 8, 262 7 of 35 calcination temperature, and pre-reduction temperature on the catalyst performance were evaluated. The drying method deeply affected the catalyst performance: in particular, MW-assisted drying induced remarkable changes on the type, location, and reducibility of Ni species within the zeolite Catalysts 2018, 8, x FOR PEER REVIEW 7 of 35 structure and, simultaneously, led to effects on the support structural and textural properties and on the averageinduced size remarkable of Ni particles. changes on the type, location, and reducibility of Ni species within the zeolite DR-UV-Visstructure and spectroscopy, simultaneously showed, led to that effects after on conventionalthe support structural drying, and most textural NiO species properties occurred and on at the outerthe surface average of zeolitesize of Ni particles, particles whereas. with MW-assisted drying, octahedral Ni2+ species formed if drying wasDR performed-UV-Vis spectroscopy in a closed showed container that after (when conventional MW-assisted drying, drying most wasNiO performedspecies occurred in an at open container,the outer similar surface Ni species of zeolite were particles, obtained whereas as they with were MW during-assisted conventional drying, octahedral drying). Ni2+ According species to the authors,formed duringif drying MW was heatingperformed in ain close a closed container, container higher (when temperature MW-assisted and drying pressure was performed led to formation in an open container, similar Ni species were obtained as they were during conventional drying). of Ni2+ species in exchange position within the zeolite cavities, whereas if the container was left open According to the authors, during MW heating in a close container, higher temperature and pressure during drying, NiO species mostly occurred at the outer surface of zeolite particles. led to formation of Ni2+ species in exchange position within the zeolite cavities, whereas if the MW-assistedcontainer was left drying open during also deeplydrying, NiO affected species zeolite mostly occurred crystallinity: at the outer in asurface closed of zeolite container, someparticles amorphous. phase formed (as detected by X-Ray powders diffraction), likely due to the high temperatureMW reached-assisted within drying thealso container, deeply affected whereas zeolite with crystallinity MW-assisted: in dryinga closed in container, an open container,some the zeoliteamorphous crystallinity phase wasformed preserved (as detected (as for by conventional X-Ray powder drying).s diffraction Finally,), likely MW-assisted due to thedrying high led to less reducibletemperature Ni reached species, within especially the container, when octahedral whereas with Ni2+ MWspecies-assisted occurred. drying in As an observed open container with, other Ni-basedthe zeolite catalysts, crystallinity MW-assisted was preserved drying led (as tofor formation conventional ofsmaller drying). Ni Finally, particles MW (in-assisted the 17–27 drying nm led range). 2+ Unfortunately,to less reducible the positive Ni species effects, especially brought when about octahedral by MW-assisted Ni species occurred drying,. As like observed better Niwith dispersion, other Ni-based catalysts, MW-assisted drying led to formation of smaller Ni particles (in the 17–27 nm were counterbalanced by the loss in zeolite crystallinity and, as a result, no appreciable improvement range). Unfortunately, the positive effects brought about by MW-assisted drying, like better Ni in thedispersion catalytic, activity were counterbalanced was obtained inby termsthe loss of in both zeolite CO crystallinity2 conversion and, and as selectivity a result, no to appreciable CH4. This was likelyimprovement due to the fact in the that catalytic the support activity was was aobtained zeolite, in for terms which of both the crystallinityCO2 conversion of and porous selectivity cavities is a crucialto CH aspect4. This [ 48wa].s likely due to the fact that the support was a zeolite, for which the crystallinity of Theporous material cavities thermal is a crucial response aspect [48] to. MW heating is a crucial aspect, as shown by Dong et al. [49], with Re promotedThe material Ni-Mn/Al thermal 2responseO3 catalysts to MW for heating partial is methanation a crucial aspect, (Scheme as shown2), aby process Dong et in al. which [49], the − amountwith of Re (noxious) promoted CO Ni- isMn/Al reduced2O3 catalysts by coupling for partial CO methanationmethanation (Scheme (∆H0 = 2)−, 206a process kJ mol in which1): the amount of (noxious) CO is reduced by coupling CO methanation (∆H0 = −206 kJ mol−1):

COCO + 3H+ 3H2 2 ⇌ CHCH4 ++ H H2O2O (4) (4)

0 −1 to waterto water gas shiftgas shift (WGS, (WGS∆H, ∆0H= =− −4141 kJkJ mol−)1 ) CO + H2O ⇌ CO2 + H2 (5) CO + H2O CO2 + H2 (5)

SchemeScheme 2. Scheme 2. Scheme of theof the processes processes occurring occurring atat the surface surface of of Re Re-promoted-promoted Ni- Ni-MnMn bimetallic bimetallic particles particles

on Alon2O Al3 support.2O3 support. Reprinted Reprinted from from Appl. Catal. Catal. A Gen. A Gen., 552,, 552, Dong, Dong, X.; Jin, X.; B.; Jin, Sun, B.; Y.; Sun, Shi, Y.;K.; Yu, Shi, L., K.; Re Yu,- L., Re-PromotedPromoted Ni-Mn Ni-Mn BifunctionalBifunctional C Catalystsatalysts P Preparedrepared by by M Microwaveicrowave Heating Heating for forPartial Partial Methanation Methanation Coupling with Water Gas Shift under Low H2/CO Conditions. Copyright (2018) with permission from Coupling with Water Gas Shift under Low H2/CO Conditions. Copyright (2018) with permission Elsevier. from Elsevier. Figure 4 reports heating curves under MW irradiation (1.0 kW output power) of the different reagents used to produce the catalysts: Al2O3 is a poor MW absorber, but when Re was firstly incorporated in γ-Al2O3 (Re-Al2O3), the materials showed heating curves with higher slope (curves 3

Catalysts 2018, 8, 262 8 of 35

Figure4 reports heating curves under MW irradiation (1.0 kW output power) of the different reagents used to produce the catalysts: Al2O3 is a poor MW absorber, but when Re was ﬁrstly incorporatedCatalysts in 2018γ-Al, 8, x2 FORO3 PEER(Re-Al REVIEW2O 3 ), the materials showed heating curves with higher8 of slope 35 (curves 3 and 4 in Figure4). Due to the presence of Re, both the Re-Al 2O3 and Ni-Mn/Re-Al2O3 materials and 4 in Figure 4). Due to the presence of Re, both the Re-Al2O3 and Ni-Mn/Re-Al2O3 materials absorbed more MW with respect to pure γ-Al O . This had a deep impact on the reaction time, in that absorbed more MW with respect to pure γ-Al22O33. This had a deep impact on the reaction time, in that ◦ the synthesisthe synthesis temperature temperature (ca. 400(ca. 400C) °C) was was reached reached shortly shortly when when Re-Al Re-Al2O3 wa2sO used3 was for usedthe deposition for the deposition of the bimetallicof the bimetallic Ni-Mn Ni catalyst-Mn catalyst starting starting from from the respective respective nitrates nitrates..

Figure 4. MW heating curves of different catalyst precursors: (1) Ni-Mn/Al2O3, (2) Ni-Mn-Re/Al2O3, Figure 4. MW heating curves of different catalyst precursors: (1) Ni-Mn/Al2O3,(2) Ni-Mn-Re/Al2O3, (3) Re-Al2O3, and (4) Ni-Mn/Re-Al2O3. Reprinted from Appl. Catal. A Gen., 552, Dong, X.; Jin, B.; Sun, (3) Re-Al2O3, and (4) Ni-Mn/Re-Al2O3. Reprinted from Appl. Catal. A Gen., 552, Dong, X.; Jin, B.; Y.; Shi, K.; Yu, L., Re-Promoted Ni-Mn Bifunctional Catalysts Prepared by Microwave Heating for Sun, Y.; Shi, K.; Yu, L., Re-Promoted Ni-Mn Bifunctional Catalysts Prepared by Microwave Heating for Partial Methanation Coupling with Water Gas Shift under Low H2/CO Conditions. Copyright (2018) Partial Methanationwith permission Coupling from Elsev withier. Water Gas Shift under Low H2/CO Conditions. Copyright (2018) with permission from Elsevier. Other phenomena occurring during MW heating can be inferred by a close inspection of the curves in Figure 4: when Re is absent, the temperature plateau reached at ca. 150 °C is ascribed to Otherthermal phenomena balance between occurring (Ni and during Mn) nitrate MWs decomposition heating can and be MW inferred irradiation by. The a close phenomenon inspection of the curves indoes Figure not 4 occur: when in the Re heating is absent, curves the of Re temperature-added samples, plateau likely because reached of atthe ca. enhancement 150 ◦C is of ascribed to thermal balancethermal decomposition between (Ni of and nitrates Mn) due nitrates to the presence decomposition of Re. Simultaneously, and MW irradiation.MW heating favors The the phenomenon formation of hot spots that induce an effective surface dispersion of the metal. A side-effect of fast does notMW occur heating in the was heatingobserved, curvesi.e., formation of Re-added of AlO(OH) samples, phase (as detected likely becauseby X-ray diffraction). of the enhancement MW of thermal decompositionheating, however, offavored nitrates the dispersion due to the of the presence NiO phase of, Re.which Simultaneously, showed broader X- MWray diffraction heating favors the formationpeaks of hot with spots respect that to inducethe catalyst an effectiveobtained by surface conventional dispersion heating. of A the better metal. dispersion A side-effect of the Ni of fast MW heating wasactive observed, phase was i.e., also formation observed after of AlO(OH) reduction, phase confirming (as detected the overall by positive X-ray effect diffraction). of MW. At MW heating, variance with what was observed with Ni-USY zeolite, MW heated samples were more easily however,reducible, favored theas the dispersion onset of NiO of the reduction NiO phase,shifted to which lower showed temperatures. broader Another X-ray important diffraction point peaks with respect toobserved the catalyst by the authors obtained was bythat conventional MW-heated catalysts heating. led to Alower better formation dispersion of carbon of deposit the Nis, an active phase was also observedundesired phenomenon after reduction, that can conﬁrming lead to catalyst the deactivation overall positive [49]. effect of MW. At variance with what was observedThe with presence Ni-USY of Re, zeolite, however, MW also affected heated some samples important were structural more easilyand electronic reducible, properties as the onset of of the catalysts, as shown by the characterization of their physico-chemical properties. For instance, NiO reductionH2 TPR shiftedanalysis showed to lower a higher temperatures. reducibility Anotherof β-type NiO important (a type of pointNiO that observed has medium by strong the authors was that MW-heatedinteraction catalysts with γ-Al2 ledO3), toindicating lower the formation facile formation of carbon of catalytically deposits, active an Ni undesired atoms. XPS phenomenonanalysis that can lead topointed catalyst out also deactivation a decrease of [ 49the]. binding energy of the Ni 2p line when Re was present, ascribed to Thethe presence electron oftransfer Re, however,from ReOx species also affected to Ni atoms. some The importantincreased electron structural cloud density and electronicof Ni atoms properties could indeed favor the breakage of the C-O bond, positively affecting the catalytic activity. of the catalysts,CO2 asreforming shown of by methane the characterization (Equation (6)) is an of endothermic their physico-chemical reaction (∆H0 = +247 properties. kJ mol−1, also For instance, H2 TPR analysisknown as showeddry reforming) a higher to produce reducibility syngas mixtures of β-type with NiO low H (a2:CO type ratio of NiOthat could that be has used medium for strong further Fischer-Tropsch synthesis (FTS) to produce light olefins: interaction with γ-Al2O3), indicating the facile formation of catalytically active Ni atoms. XPS analysis pointed out also a decrease of the bindingCO energy2 + CH4 ⇌ of 2CO the + 2H Ni2 2p line when Re was present,(6) ascribed to the electron transferThe reaction from has ReO not yetx species been industrially to Ni atoms. implemented The increased, since there electron are no commercial cloud density catalysts of Ni atoms could indeedthat can favor operate the without breakage undergoing of the C-O deactivation bond, positivelydue to carbon affecting deposition the [50] catalytic. An alternative activity. to Ni-based catalysts (and to noble metals like Ru, Rh, Pd, Ir, and Pt) is provided by Ni-containing0 −1 CO2 reforming of methane (Equation (6)) is an endothermic reaction (∆H = +247 kJ mol , also known as dry reforming) to produce syngas mixtures with low H2:CO ratio that could be used for further Fischer-Tropsch synthesis (FTS) to produce light oleﬁns:

CO2 + CH4 2CO + 2H2 (6) Catalysts 2018, 8, 262 9 of 35

The reaction has not yet been industrially implemented, since there are no commercial catalysts that can operate without undergoing deactivation due to carbon deposition [50]. An alternative to Ni-based catalysts (and to noble metals like Ru, Rh, Pd, Ir, and Pt) is provided by Ni-containing oxides, i.e., either Ni perovskites with defined structure or solid solutions with La, Al, or Mg (spinel, fluorite, and perovskite) [51]. Ni perovskites with defined structure are promising catalysts, with respect Catalysts 2018, 8, x FOR PEER REVIEW 9 of 35 to noble metals, due to their low cost, thermodynamic stability at relatively high temperatures, and catalyticoxides activity., i.e., either Barros Ni perovskites et al. [51] with prepared defined Lastructure2NiO4 or/ αsolid-Al 2solutionsO3 catalysts with La, for Al CO, or 2Mgreforming (spinel, of CH4 by MW-assistedfluorite,CS, and byperovskite) using glycine [51]. Ni orperovskites urea as with fuel defined and metal structure nitrates are promising as oxidizers. catalysts, For with comparison, respect to noble metals, due to their low cost, thermodynamic stability at relatively high a catalyst with similar chemical composition was prepared by impregnation of α-Al2O3 prepared temperatures, and catalytic activity. Barros et al. [51] prepared La2NiO4/α-Al2O3 catalysts for CO2 by MW. reforming of CH4 by MW-assisted CS, by using glycine or urea as fuel and metal nitrates as oxidizers. XRD diffractionFor comparison, showed a catalyst the with formation similar chemical of La3 compositionNi2O6.92, Lawas2NiO prepared4, and by LaAlimpregnationxNix−1 Oof 3αcrystalline- structuresAl independently2O3 prepared by MW. of the preparation method. The sample prepared by impregnation was XRD diffraction showed the formation of La3Ni2O6.92, La2NiO4, and LaAlxNix−1O3 crystalline very active, but the crystalline structure was practically destroyed, with significant carbon deposition. structures independently of the preparation method. The sample prepared by impregnation was very ◦ The catalystsactive, prepared but the crystalline by MW-assisted structure was CS practically showedCH destroyed,4 conversion with significant above carbon 80% at deposition. 700 C, good The stability, and a lowcatalysts coke formation prepared by after MW about-assisted 60 CS h showed on stream. CH4 conversion The catalyst above prepared 80% at 700 by °C,impregnation good stability, showed and a low coke formation after about 60 h on stream. The catalyst prepared by impregnation showed instead a decrease of CH4 conversion after 60 h, likely due to deactivation by coke deposition, according instead a decrease of CH4 conversion after 60 h, likely due to deactivation by coke deposition, to TPO (Temperature Programmed Oxidation) analysis and TEM inspection, which showed the according to TPO (Temperature Programmed Oxidation) analysis and TEM inspection, which occurrenceshowed of carbon the occurrence nanotubes, of carbon not nanotubes observed, not when observed the when catalysts the catalysts were were obtained obtained by MW MW-assisted- CS (Figureassisted5). CS (Figure 5).

Figure 5. TEMFigure pictures 5. TEM pictures of the of catalysts the catalysts prepared prepared byby MW MW-assisted-assisted CS ( CSa) and (a) impregnation and impregnation (b) after (b) after reaction. Reprinted from Appl. Catal. A Gen., 378, Barros, B. S.; Melo, D. M. A.; Libs, S.; Kiennemann, reaction. Reprinted from Appl. Catal. A Gen., 378, Barros, B. S.; Melo, D. M. A.; Libs, S.; Kiennemann, A., A., CO2 Reforming of Methane Over La2NiO4/α-Al2O3 Prepared by Microwave-Assisted Self- CO2 ReformingCombustion of Methane Method, Over 69–75. La Copyright2NiO4/ (α2010-Al) 2withO3 Preparedpermission byfrom Microwave-Assisted Elsevier. Self-Combustion Method, 69–75. Copyright (2010) with permission from Elsevier. Moreover, the crystalline structure of the catalysts prepared by MW-assisted CS was preserved after CO2 reforming of CH4, whereas a prominent loss of crystallinity occurred in the catalyst obtained Moreover,by impregnation the crystalline (which structurewas also deactivated) of the catalysts [51]; the preparedauthors ascribed by MW-assisted the stability achieved CS was by preserved after CO2 reformingMW-assisted of CS CH to the4, whereas strong interaction a prominent of Ni active loss sites of crystallinity with the perovskite occurred structure. in the catalyst obtained by impregnationCO2 selective (which r waseduction also to light deactivated) olefins is a process [51]; the that authorsrecently has ascribed progressed the due stability to the urgent achieved by need of converting CO2 into building-block chemicals [13,14,52]. To raise selectivity towards light MW-assisted CS to the strong interaction of Ni active sites with the perovskite structure. olefins, catalysts that are more effective are needed, since CO2 is very stable, and FTS usually leads CO2 selectiveto mixtures reduction of products. to K- lightpromoted olefins Fe catalyst is a process is very selective that recently towards has CO2 progressed hydrogenation due to light to the urgent need of convertingolefins; additionally CO2 into to building-blockK, Zn is also used chemicals as a promoter [13 that,14 ,52 accelerates]. To raise FTS, selectivityincreases catalyst towards light stability, enhances light olefins selectivity, improves CO2 adsorption, promotes H2 dissociation, and olefins, catalysts that are more effective are needed, since CO2 is very stable, and FTS usually leads is highly active in both WGS and RWGS reactions [52]. The effect of MW was studied on the synthesis to mixturesof HT of products.precipitation K-promotedfollowed by incipient Fe catalyst wet impregnation is very (to selective add K) of towards a series of COK-promoted2 hydrogenation Fe- to light olefins;Zn additionallycatalysts with different to K, Zn Fe/Zn is also molar used ratios. as Physico a promoter-chemical that characterization accelerates FTS, of the increases catalysts catalyst stability, enhancesshowed they light were olefins characterized selectivity, by rather improves uniform COparticle2 adsorption, size (ca. 100 promotesnm). XRD analysis H2 dissociation, showed and is highly activethe inZn bothaddition WGS to the and K-promoted RWGS reactions Fe2O3 matrix [52 led]. Theto formation effect of of MW ZnFe was2O4 spinel studied phase on and the ZnO synthesis of phase, and also brought about an increase of the surface area. Moreover, the Fe-Zn interaction HT precipitation followed by incipient wet impregnation (to add K) of a series of K-promoted Fe-Zn affected the adsorption and reduction of CO2 adsorption. catalysts with different Fe/Zn molar ratios. Physico-chemical characterization of the catalysts showed they were characterized by rather uniform particle size (ca. 100 nm). XRD analysis showed the Zn addition to the K-promoted Fe2O3 matrix led to formation of ZnFe2O4 spinel phase and ZnO phase, Catalysts 2018, 8, 262 10 of 35 and also brought about an increase of the surface area. Moreover, the Fe-Zn interaction affected the adsorption and reduction of CO2 adsorption. The catalysts exhibited excellent activity for CO2 hydrogenation and high selectivity toward C2–C4 olefins owing to the promoting effect of Zn, due to the fact that particles with uniform shape were obtained, and a fair dispersion of active components occurred with respect to conventional co-precipitation. Additionally, the formed ZnFe2O4 phase imparted stability to the catalyst, although Catalysts 2018, 8, x FOR PEER REVIEW 10 of 35 lowering its activity due to segregation of a fraction of Fe (the active phase).

The catalysts exhibited excellent activity for CO2 hydrogenation and high selectivity toward C2– 2.3. MW-AssistedC4 olefins Synthesis owing to the of promoting Photocatalysts effect of for Zn, CO due2 Reductionto the fact that particles with uniform shape were obtained, and a fair dispersion of active components occurred with respect to conventional co- Photocatalytic CO reduction processes (i.e., biomimetic artificial photosynthesis) are very precipitation. Additionally,2 the formed ZnFe2O4 phase imparted stability to the catalyst, although challenginglowering due toits activity intrinsic due limitationsto segregation of athe fraction used of Fe semiconductors, (the active phase). like poor absorption in the visible range and fast electron/holes recombination rate (e.g., for TiO2) and stability (e.g., for CdS). 2.3. MW-Assisted Synthesis of Photocatalysts for CO2 Reduction “Plasmonic” photocatalysis is a plasmonic metal-enhanced photocatalytic process occurring when Photocatalytic CO2 reduction processes (i.e., biomimetic artificial photosynthesis) are very noble metal nanoparticles (NPs) decorate the surface of a semiconductor (e.g., TiO2) to improve its challenging due to intrinsic limitations of the used semiconductors, like poor absorption in the visible photocatalytic efficiency through the localized surface plasmon resonance (LSPR) effect of NPs [53]. range and fast electron/holes recombination rate (e.g., for TiO2) and stability (e.g., for CdS). Most“Plasmonic” of the studies photocatalysis on the subjectis a plasmonic concern metal either-enhanced Au photocataly or Ag NPstic process supported occurring on TiOwhen2, but their actual usenoble is hampered metal nanoparticles by the cost (NPs and) decorate the limited the surface abundance of a semiconductor of the noble (e.g., metals. TiO2) to Recently,improve its it has been shown thatphotocatalytic also Cu NPs efficiency can have through LSPR the lo effect,calized indicating surface plasmon that resonance Cu could (LSPR) replace effect noble of NPs metals. [53]. Most of the studies on the subject concern either Au or Ag NPs supported on TiO2, but their Cu NPs-decoratedactual use is hampered TiO 2bynanoflower the cost and the films limited (Cu/TiO abundance2 NFFs)of the noble were metal prepareds. Recently, using it has both HT method andbeen MW-assisted shown that also reductionCu NPs can process.have LSPR Polyvinylpyrrolidoneeffect, indicating that Cu could was replace used noble as capping metals. agent and NaH2PO2*H2OCu as NPs reducing-decorated agent TiO2 nanoflower in diethylglycole films (Cu/TiO solution2 NFFs) under were prepared MW irradiation. using both HT The method catalysts were and MW-assisted reduction process. Polyvinylpyrrolidone was used as capping agent and tested in the photocatalytic reduction of CO2 to CH3OH [54]. NaH2PO2*H2O as reducing agent in diethylglycole solution under MW irradiation. The catalysts were Physico-chemical characterization of the films showed that both Cu NPs and TiO films were tested in the photocatalytic reduction of CO2 to CH3OH [54]. 2 characterizedPhysico by a- fairchemical dispersion characterization of Cu of NPsthe films on showed the TiO that2 NFFsboth Cu (Figure NPs and6 )TiO and2 films by were visible light harvestingcharacterized properties, by due a fair to dispersion LSPRof of Cu Cu NPs on and the theTiO2 peculiarNFFs (Figure nanostructure 6) and by visible of thelight TiO2 film. Moreover,harvesting the presence properties, of Cu due NPs to allowedLSPR of Cu fluorescence NPs and the quenching peculiar nanostructure due to the of suppressionthe TiO2 film. of charge Moreover, the presence of Cu NPs allowed fluorescence quenching due to the suppression of charge carriers. The CH OH production rate reached 1.8 mmol cm−2 h−1 (with an energy efficiency of 0.8%) carriers. 3The CH3OH production rate reached 1.8 mmol cm−2 h−1 (with an energy efficiency of 0.8%) with a 0.5with wt %a 0.5 Cu/TiO wt % Cu/TiO2 NFFs2 NFFs under under UV UV and and visiblevisible light light irradiation: irradiation: with Cu with NPs, Cu the NPs, production the production rate was ca.rate 6.0 was times ca. 6.0 higher times higher than than with with pure pure TiO TiO22 filmfilm (Figure (Figure 7).7 ).

Figure 6. SEM picture of pure TiO2 film (a) and Back Scattered Electrons images of TiO2 NFFs Figure 6. SEM picture of pure TiO2 ﬁlm (a) and Back Scattered Electrons images of TiO2 NFFs containing increasing amounts of Cu from (b–d). Reprinted from Mater. Res. Bull., 68, Liu, E.; Qi, L.; containing increasing amounts of Cu from (b–d). Reprinted from Mater. Res. Bull., 68, Liu, E.; Qi, L.; Bian, J.; Chen, Y.; Hu, X.; Fan, J.; Liu, H.; Zhu, C.; Wang, Q., A Facile Strategy to Fabricate Plasmonic,

Bian, J.; Chen,Cu-M Y.;odified Hu, TiO X.;2 Fan,Nano- J.;flower Liu, F H.;ilmsZhu, for Photocatalytic C.; Wang, Q.,Reduction A Facile of CO Strategy2 to Methanol, to Fabricate 203–209 Plasmonic,. Cu-ModiﬁedCopyright TiO2 (2015)Nano-ﬂower with permission Films from for Elsevier. Photocatalytic Reduction of CO2 to Methanol, 203–209. Copyright (2015) with permission from Elsevier.

Catalysts 2018, 8, x FOR PEER REVIEW 11 of 35

Catalysts 2018, 8, x FOR PEER REVIEW 11 of 35 Catalysts 2018, 8, 262 11 of 35

Figure 7. CH3OH production rate by photocatalytic reduction of CO2 as obtained under UV light with both pure TiO2 film and a Cu/NFF obtained by starting from 0.5 mM Cu2+ solution (0.5 Cu/TiO2 film). Reprinted from Mater. Res. Bull., 68, Liu, E.; Qi, L.; Bian, J.; Chen, Y.; Hu, X.; Fan, J.; Liu, H.; Zhu, C.; FigureFigure 7. 7.CHCH3OH3OH production production rate rate by by photocatalytic photocatalytic reduction reduction of of CO CO2 as2 as obtained obtained under under UV UV light light with with Wang, Q., A Facile Strategy to Fabricate Plasmonic, Cu-Modified TiO22+ Nano-Flower Films for bothboth pure pure TiO TiO2 film2 film and and a Cu/NFF a Cu/NFF obtained obtained by starting by starting from from0.5 mM 0.5 Cu mM2+ solution Cu solution (0.5 Cu/ (0.5TiO Cu/TiO2 film). 2 Photocatalytic Reduction of CO2 to Methanol, 203–209. Copyright (2015) with permission from Reprintedfilm). Reprinted from Mater. from Res.Mater. Bull. Res., 68, Liu, Bull. ,E.; 68, Qi, Liu, L.; E.;Bian, Qi, J.; L.; Chen, Bian, Y.; J.; Hu, Chen, X.; Y.;Fan, Hu, J.; X.;Liu, Fan, H.; J.;Zhu, Liu, C.; H.; Elsevier. Wang,Zhu, C.; Q., Wang, A Facile Q., AS Faciletrategy Strategy to Fabricate to Fabricate Plasmonic Plasmonic,, Cu-M Cu-Modifiedodified TiO TiO2 N2anoNano-Flower-Flower Films Films for for

PhotocatalyticPhotocatalytic R Reductioneduction of CO CO22to to Methanol, Methanol, 203–209. 203–209 Copyright. Copyright (2015) (2015) with permissionwith permission from Elsevier. from The positive photocatalytic performance towards CO2 reduction to CH3OH observed when Cu Elsevier. NPs were present (Figure 7) was ascribed to a synergistic mechanism of Cu NPs at the surface of TiO2 The positive photocatalytic performance towards CO reduction to CH OH observed when Cu (Scheme 3): Cu NPs obtained by MW-assisted reduction method2 likely suppressed3 the recombination NPsThe were positive present photocatalytic (Figure7) was performance ascribed to a towards synergistic CO mechanism2 reduction ofto CuCH NPs3OH atobserved the surface when of TiOCu of charge carriers and improved the charge transfer efficiency. 2 NPs(Scheme were 3present): Cu NPs (Figure obtained 7) was by ascribed MW-assisted to a synergi reductionstic mechanism method likely of suppressedCu NPs at the the surface recombination of TiO2 (Schemeof charge 3): carriersCu NPs andobtained improved by MW the-assisted charge transferreduction efﬁciency. method likely suppressed the recombination of charge carriers and improved the charge transfer efficiency.

Scheme 3. The synergistic mechanism proposed for the photocatalytic reduction of CO2 to CH3OH Scheme 3. The synergistic mechanism proposed for the photocatalytic reduction of CO2 to CH3OH with Cu/TiO2 NFFs. Reprinted from Mater. Res. Bull., 68, Liu, E.; Qi, L.; Bian, J.; Chen, Y.; Hu, X.; Fan, J.; with Cu/TiO2 NFFs. Reprinted from Mater. Res. Bull., 68, Liu, E.; Qi, L.; Bian, J.; Chen, Y.; Hu, X.; Fan, Liu, H.; Zhu, C.; Wang, Q., A Facile Strategy to Fabricate Plasmonic, Cu-Modiﬁed TiO2 Nano-Flower J.; Liu, H.; Zhu, C.; Wang, Q., A Facile Strategy to Fabricate Plasmonic, Cu-Modified TiO2 Nano- SchemeFilms for3. The Photocatalytic synergistic Reductionmechanism of proposed CO2 to Methanol, for the photocatalytic 203–209. Copyright reduction (2015) of CO with2 to permission CH3OH Flower Films for Photocatalytic Reduction of CO2 to Methanol, 203–209. Copyright (2015) with withfrom Cu/ Elsevier.TiO2 NFFs. Reprinted from Mater. Res. Bull., 68, Liu, E.; Qi, L.; Bian, J.; Chen, Y.; Hu, X.; Fan, permission from Elsevier. J.; Liu, H.; Zhu, C.; Wang, Q., A Facile Strategy to Fabricate Plasmonic, Cu-Modified TiO2 Nano- Flower Films for Photocatalytic Reduction of CO2 to Methanol, 203–209. Copyright (2015) with InIn anotheranother study,study, Cu-Graphene Cu-Graphene OxideOxide (Cu/GO) (Cu/GO) compositecomposite photocatalystsphotocatalysts werewere obtainedobtained usingusing permission from Elsevier. aa simplesimple andand rapidrapid one-potone-pot MWMW processprocess [[55]55],, shownshown inin FigureFigure8 a.8a. In another study, Cu-Graphene Oxide (Cu/GO) composite photocatalysts were obtained using a simple and rapid one-pot MW process [55], shown in Figure 8a.

Catalysts 2018, 8, 262 12 of 35 Catalysts 2018, 8, x FOR PEER REVIEW 12 of 35

Figure 8.8. SSchemecheme of of the the MW assisted synthesis synthesis process process adopted adopted to to decorate decorate GO GO with with Cu Cu NPs NPs ( (aa)).. Sections (b−dd)):: TEM TEM images images of of Cu/GO Cu/GO hybrids hybrids with with 5, 5, 10 10,, and and 15 15 wt wt % % Cu, Cu, respectively. respectively. Inset: Inset: HRTEM HRTEM image of single Cu Cu-NP-NP of of the the respective respective Cu/GO Cu/GO hybrid. hybrid. Sections Sections (e (–eg–)g: ):size size distribution distribution of ofCu Cu-NPs-NPs in inCu/GO Cu/GO hybrids hybrids with with 5, 10 5,, and 10, 15 and wt 15 % wtCu, % respectively. Cu, respectively. Representative Representative TEM image TEM of image pure ofGO pure (h). GOReprinted (h). Reprinted with permission with permission from Shown, from I.; Shown, Hsu, H. I.;-C.; Hsu, Chang, H.-C.; Y.- Chang,C.; Lin, Y.-C.;C.-H.; Lin,Roy, C.-H.; P. K.; Roy,Ganguly, P.K.; Ganguly,A.; Wang, A.; C. Wang,-H.; Chang, C.-H.; J. Chang,-K.; Wu, J.-K.; C.-I.; Wu, Chen, C.-I.; L. Chen,-C.; Chen, L.-C.; K. Chen,-H., Highly K.-H., Highly Efficient Efﬁcient Visible Visible Light Photocatalytic Reduction of CO2 to Hydrocarbon Fuels by Cu-Nanoparticle Decorated Graphene Light Photocatalytic Reduction of CO2 to Hydrocarbon Fuels by Cu-Nanoparticle Decorated Graphene Oxide. Nano Lett. 14, 6097 6097–6103.–6103. Copyright ( (2014)2014) A Americanmerican C Chemicalhemical S Society.ociety.

By using Cu(NO3)2 as metal precursor, short MW heating (ca. 3 min at 400 W) was performed in By using Cu(NO ) as metal precursor, short MW heating (ca. 3 min at 400 W) was performed in ethylene glycol solution3 2 (Figure 8a), leading to a fair dispersion of crystalline Cu NPs in the GO. ethylene glycol solution (Figure8a), leading to a fair dispersion of crystalline Cu NPs in the GO. The obtained Cu NPs (with a size of ca. 4–5 nm, Figure 8) were strongly interacting with GO: the The obtained Cu NPs (with a size of ca. 4–5 nm, Figure8) were strongly interacting with GO: obtained composite was particularly active under solar irradiation in the production of both CH3OH the obtained composite was particularly active under solar irradiation in the production of both and acetaldehyde. Cu NPs were shown to enhance the GO photocatalytic activity of GO, essentially CH OH and acetaldehyde. Cu NPs were shown to enhance the GO photocatalytic activity of GO, by suppressing3 electron-hole recombination, and also by reducing the GO bandgap and by modifying essentially by suppressing electron-hole recombination, and also by reducing the GO bandgap and by its work function (Scheme 4). The presence of Cu NPs greatly enhanced absorption of GO in the modifying its work function (Scheme4). The presence of Cu NPs greatly enhanced absorption of GO visible range, though the shift in the band gap by effect of Cu NPs was not well understood. XPS in the visible range, though the shift in the band gap by effect of Cu NPs was not well understood. analysis also showed that oxygenated groups of GO (i.e., -COOH and C-O-C groups) were almost XPS analysis also showed that oxygenated groups of GO (i.e., -COOH and C-O-C groups) were almost completely removed by the MW treatment. No other Cu oxides were detected, indicating that the completely removed by the MW treatment. No other Cu oxides were detected, indicating that the MW MW heating led to the reduction of all the Cu ions. heating led to the reduction of all the Cu ions.

Catalysts 2018, 8, 262 13 of 35 CatalystsCatalysts 2018 2018, 8,, x8 ,FOR x FOR PEER PEER REVIEW REVIEW 13 13of 35of 35

SchSchemeSchemeeme 4. 4.Section4. Section (a ) (:a work):): work functions functions of ofpure purepure GO GOGO and andand of ofofCu/GO Cu/GOCu/GO hybrids hybrids hybrids with with with 5, 105, ,10 10, and, and 15 15wt wt% %Cu Cu (Cu/GO(Cu/GO-1,(Cu/GO-1, -Cu/GO1, Cu/GO Cu/GO-2,-2, -2and, and and Cu/GO Cu/GO Cu/GO-3,-3, -respectively).3, respectively). respectively). Section Section Section (b )(:b( bband):): band band-edge-edge-edge positions positions positions of ofpure of pure pure GO GO and and

Cu/GOCu/GO hybrids hybrids hybrids in incomparison in comparison comparison with with with CO CO CO2/CH22/CH/CH3OH3OH3 andOH and CO and CO2/CH CO2/CH23/CHCHO3CHO3 CHOformation formation formation potentials. potentials. potentials. Section Section Section (c) (: c): proposed(proposedc): proposed photocatalytic photocatalytic photocatalytic reaction reaction reaction mechanism mechanism mechanism under under solar under solar irradiation. solar irradiation. irradiation. Reprinted Reprinted Reprinted with with permission withpermission permission from from Shown,fromShown, Shown,I.; I.;Hsu, Hsu, I.;H. Hsu,-H.C.;-C.; Chang, H.-C.; Chang, Chang, Y. -Y.C.;-C.; Lin, Y.-C.; Lin, C. Lin,-C.H.;-H.; C.-H.;Roy, Roy, P. Roy, P.K.; K.; P.K.;Ganguly, Ganguly, Ganguly, A.; A.; Wang, A.; Wang, Wang, C. -C.H.; C.-H.;-H.; Chang, Chang, Chang, J.- K.;J. J.-K.;-K.;

Wu,Wu, C. - C.-I.;C.I.;- I.;Chen, Chen, L. -L.C.; L.-C.;-C.; Chen, Chen, Chen, K. -K.H. K.-H.- H.Highly Highly Highly Efficient Efficient Efﬁcient Visible Visible Visible Light Light Light Photocatalytic Photocatalytic Photocatalytic Reduction Reduction Reduction of ofCO ofCO2 COto2 to2 HydrocarbontoHydrocarbon Hydrocarbon Fuels Fuels Fuels by by Cu by Cu-Nanoparticle Cu-Nanoparticle-Nanoparticle Decorated Decorated Decorated Graphene Graphene Graphene Oxide. Oxide. Oxide. Nano NanoNano Lett. Lett. Lett. 14, 14, 14, 6097 60976097–6103.–6103–6103. . CopyrightCopyright (2014) (2014) American American Chemical Chemical Society. Society.Society.

XPSXPS analysis analysis analysis [55] [55] [ 55showed ]showed showed the the likely the likely likely oc occurrencecurrence occurrence of ofa chargea of charge a charge transfer transfer transfer from from GO fromGO to toCu GO Cu NPs toNPs, Cu and, and NPs, a a correlationandcorrelation a correlation was was found found was between found between between the the Cu Cu thecontent content Cu content and and the the and rate rate the of ofrate formation formation of formation and and selectivity and selectivity selectivity of of the the of products.theproducts. products. The The Cu/GO The Cu/GO Cu/GO catalyst catalyst catalyst with with 10 with 10 wt 10wt % wt %Cu %Cu was Cu was wasmore more more than than than 60 60 times 60 times times more more more active active active than than than pure pure pure GO GO andand ca. ca. 240 240 times timestimes more more active active than than commercial commercial TiO TiOTiO2 (2Degussa2 (Degussa(Degussa P25) P25) photocatalyst photocatalyst under under visible visiblevisible light. light.light. AlsoAlso,Also, Cu, CuCu2O22 Ocan can be beused used for forfor CO COCO2 photoconversion:22 photoconversion: An AnAn et etetal. al.al. [56] [[56]56 produced] producedproduced Cu CuCu2O/RGO22O/RGOO/RGO (Reduced (Reduced GrapheneGraphene Oxide) Oxide) composites composites using using a facile a facile,facile, one, one-step,one-step-step, MW, MW-assistedMW-assisted-assisted HT HTHT synthesis. synthesis.synthesis. X - X-rayXray-ray diffraction diffractiondiffraction showedshowed that that the the Cu Cu2O2 Owas was likely likely the the only only Cu Cu-containingCu-containing-containing phase phase in inthe the composites composites,composites, and, and MW MW-assistedMW-assisted-assisted synthesissynthesis allowed allowed the thethe formation formation of ofofcomposites composites where wherewhere intimate intimateintimate contact contactcontact between betweenbetween RGO RGORGO sheets sheetssheets and andand Cu CuCu2O22 O microspheresmicrospheres (a (are sult resultresult of ofthe the strong strongstrong affinity afﬁnityaffinity between betweenbetween the thethe metal metalmetal oxide oxideoxide and andand the thethe abundant abundantabundant functional functionalfunctional groupsgroups of ofGO) GO) was was enhanced. enhanced.enhanced. Such Such strong strong interaction interaction between between Cu CuCu2O22 Oand and RGO RGO was was attributed attributed to tothe the adoptedadopted MW MW-assistedMW-assisted-assisted synthesis synthesis:synthesis: such: such interaction interaction was was very very effective effective in inthat that RGO RGO po positivelypositivelysitively affect affectedaffecteded thethe photocatalytic photocatalyticphotocatalytic activity, activity, improving improvingimproving both bothboth activity activityactivity and andand stability stabilitystability of ofofCu CuCu2O,22O, due due to toan an efficient efﬁcientefficient charge charge separationseparation and and transfer transfer to totoRGO RGORGO (Scheme (Scheme(Scheme 5).5 5). ).

SchemeScheme 5. 5.5Schematic. SchematicSchematic picture picturepicture of of of the the the charge chargecharge transfer transfer transfer process processprocess in in inCu Cu Cu2O/RGO2O/RGO2O/RGO composites composites composites during duringduring CO COCO2 22 photoreductionphotoreduction to totoCO COCO in ininwater. water.water. Reprinted ReprintedReprinted from fromfrom Ref. Ref.Ref. [56] [[56]56. Copyright].. CopyrightCopyright 2014 20142014 with withwith permission permissionpermission of ofofWiley. Wiley.Wiley.

Catalysts 2018, 8, 262 14 of 35 Catalysts 2018, 8, x FOR PEER REVIEW 14 of 35 Catalysts 2018, 8, x FOR PEER REVIEW 14 of 35

Cu-decoratedCu-decorated graphitic graphitic carbon carbon nitride nitride (g-C 3 (gN-C4)3N nanosheets4) nanosheets were were obtained obtained by a procedure by a procedure depicted Cu-decorated graphitic carbon nitride (g-C3N4) nanosheets were obtained by a procedure in Schemedepicted6 ,in in Scheme which the6, in ﬁnal which step the involved final step the involved MW-assisted the MW HT-assis synthesisted HT synthesis of Cu NPs of Cu [57 ].NPs [57]. depicted in Scheme 6, in which the final step involved the MW-assisted HT synthesis of Cu NPs [57].

Scheme 6. Schematic picture of the synthesis of Cu decorated g-C3N4 nanosheets. Reprinted from Scheme 6. Schematic picture of the synthesis of Cu decorated g-C3N4 nanosheets. Reprinted from Appl.Scheme Surf. 6 Sci.. Sch, 427,ematic Shi, picture G.; Yang, of theL.; Liu,synthesis Z.; Chen, of Cu X.; decorated Zhou, J.; Yu,g-C Y.,3N 4Photocatalytic nanosheets. Reprinted Reduction from of Appl. Surf. Sci., 427, Shi, G.; Yang, L.; Liu, Z.; Chen, X.; Zhou, J.; Yu, Y., Photocatalytic Reduction of CO2 COAppl.2 to Surf.CO OSci.ver, 427,Copper Shi,- DG.;ecorated Yang, L.; g- CLiu,3N4 Z.;Nanosheets Chen, X.; withZhou, E nhancedJ.; Yu, Y., Y Photocatalyticield and Selectivity, Reduction 1165 –of to CO Over Copper-Decorated g-C3N4 Nanosheets with Enhanced Yield and Selectivity, 1165–1173. CO2 to CO Over Copper-Decorated g-C3N4 Nanosheets with Enhanced Yield and Selectivity, 1165– Copyright1173. Copyright (2014) with (2014 permission) with permission from Elsevier. from Elsevier. 1173. Copyright (2014) with permission from Elsevier.

The composites were tested for the photocatalytic reduction of CO2 to CO. During MW-assisted HTThe synthesis,The composites composites crystalline were were testedCu tested NPs for were for the the obtained: photocatalytic photocatalytic some of reduction reduction them were ofof deposited COCO22 to CO CO. at. During Duringthe outer MW MW-assisted surface-assisted of HTnanosheets,HT synthesis, synthesis, crystallineand crystalline other were Cu Cu NPswrapped NPs were were within obtained: obtained: them, some as shown of of them them by were wereTEM deposited depositedpictures atin attheFigure the outer outer 9. surfaceSuch surface an of ofintimate nanosheets,nanosheets, contact and and betweenother other were were the wrapped two wrapped phases within withinlikely them, enhance them, as shownd as the shown by transfer TEM by pictures of TEM electrons picturesin Figure and in positivel9. FigureSuch any9 . Suchaffectintimate aned intimate the contact photocatalytic contact between between activity. the the two two phases phases likely likely enhance enhancedd the the transfer transfer of of electrons electrons and and positivel positivelyy affectedaffect theed the photocatalytic photocatalytic activity. activity.

Figure 9. TEM images of g-C3N4 at low (a) and high (b) magnification and TEM image of Cu decorated 3 4 FiguregFigure-C3N 9.4 TEMnanosheets 9. TEM images images (c of). Reprinted g-Cof g3-CN4Nat atfrom low low (aAppl. ()a and) and Surf. high high Sci. ( (bb,) )427, magniﬁcationmagnification Shi, G.; Yang, and L.;TEM Liu, image image Z.; Chen, of of Cu Cu X.; decorated decorated Zhou, 3 4 g-CJ.;3g N-Yu,C4 Nnanosheets Y.,nanosheets Photocatalytic (c ().c). ReprintedReprinted Reduction from from of CO Appl.Appl.2 to Surf.CO Surf. OSci.ver Sci., 427,C,opper 427, Shi,- Shi,DG.;ecorated Yang, G.; Yang, L.; g- CLiu,3N L.; 4Z.; N Liu, anosheetsChen, Z.; X.; Chen, Zhou,with X.; Zhou,EJ.;nhanced Yu, J.; Yu, Y., Y Y.,Photocatalyticield Photocatalytic and Selectivity, Reduction Reduction 1165 –of1173 CO of. 2Copyright CO to 2COto COOver (2014) Over Copper with Copper-Decorated- Dpermissionecorated g from-C3N g-C 4Elsevier. N3anosheetsN4 Nanosheets with withE Enhancednhanced Yield Yield and and Selectivity, Selectivity, 1165 1165–1173.–1173. Copyright Copyright (2014) (2014) with with permission permission from fromElsevier. Elsevier. The presence of Cu NPs also affected the ability of absorbing visible light, as shown by DR UV- The presence of Cu NPs also affected the ability of absorbing visible light, as shown by DR UV- VisThe spectr presenceoscopy of: Cu Cu NPs NPs likely also facilitate affected the the light ability harvesting of absorbing process visible; moreover, light, the as shownabsorption by DR edge UV- Vis spectroscopy: Cu NPs likely facilitate the light harvesting process; moreover, the absorption edge Visof spectroscopy: the composites Cu NPs shift likelyed towards facilitate longer the light wavelength, harvesting process; indicating moreover, that a the highe absorptionr number edge of of of the composites shifted towards longer wavelength, indicating that a higher number of theelectrons composites/holes shifted could towards be generated. longer wavelength, indicating that a higher number of electrons/holes electrons/holes could be generated. could bePhotocatalytic generated. tests showed that Cu had a positive effect of the CO yield. An optimal amount of Cu wasPhotocatalytic found, correspond tests showeding to 6 that wt %Cu Cu. had That a positive content effect was ofa kindthe CO of yieldtradeoff. An between optimal 3amount wt % (a of Photocatalytic tests showed that Cu had a positive effect of the CO yield. An optimal amount tooCu lowwas Cufound, content correspond) and 9 ing wt to%, 6 inwt which % Cu. aThat higher content number was a of kind Cu NPsof tradeoff can bec betweename centers 3 wt %for (a of Cu was found, corresponding to 6 wt % Cu. That content was a kind of tradeoff between 3 wt % electrons/holestoo low Cu content recombination.) and 9 wt Scheme %, in 7 which reports a the higher proposed number mechanism, of Cu NPs in canwhich bec photogeneratedame centers for (a too low Cu content) and 9 wt %, in which a higher number of Cu NPs can became centers for electronselectrons/holes excited recombination. in g-C3N4 migrate Scheme to the 7 reports surface the of proposed Cu NPs, mechanism,acting both asin whichtrapping photogenerated centers and electrons/holeselectrons excited recombination. in g-C3N4 migrate Scheme to7 reportsthe surface the proposedof Cu NPs, mechanism, acting both in as which trapping photogenerated centers and electrons excited in g-C3N4 migrate to the surface of Cu NPs, acting both as trapping centers and

Catalysts 2018, 8, 262 15 of 35

Catalysts 2018, 8, x FOR PEER REVIEW 15 of 35 catalytic sites, for the photocatalytic CO2 reduction to CO. Simultaneously, photogenerated holes Catalystscatalytic 2018 sites,, 8, x FOR for PEER the photocatalyticREVIEW CO2 reduction to CO. Simultaneously, photogenerated15 holes of 35 migrated to the surface of the photocatalyst for water oxidation to O . migrated to the surface of the photocatalyst for water oxidation to O2. 2 catalytic sites, for the photocatalytic CO2 reduction to CO. Simultaneously, photogenerated holes migrated to the surface of the photocatalyst for water oxidation to O2.

Scheme 7. Scheme of the photocatalytic reduction of CO2 to CO and simultaneous H2O oxidation to Scheme 7. Scheme of the photocatalytic reduction of CO2 to CO and simultaneous H2O oxidation O2 in the presence of Cu-decorated g-C3N4 nanosheets. Reprinted from Appl. Surf. Sci., 427, Shi, G.; to O2 in the presence of Cu-decorated g-C3N4 nanosheets. Reprinted from Appl. Surf. Sci., 427, SchemeYang, L.; 7. Liu,Scheme Z.; Chen,of the photocatalyticX.; Zhou, J.; Yu, reduction Y., Photocatalytic of CO2 to COReduction and simultaneous of CO2 to COH2O O oxidationver Copper to - Shi, G.; Yang, L.; Liu, Z.; Chen, X.; Zhou, J.; Yu, Y., Photocatalytic Reduction of CO2 to CO Over OD2 ecoratedin the presence g-C3N4 Nofanosheets Cu-decorated with Egnhanced-C3N4 nanosheets. Yield and Reprinted Selectivity, from 1165 Appl.–1173 .Surf. Copyright Sci., 427, (2014) Shi, with G.; Copper-DecoratedYang,permission L.; Liu, from Z.; g-C Elsevier.Chen,3N 4X.;Nanosheets Zhou, J.; Yu, with Y., Photocatalytic Enhanced Yield Reduction and Selectivity, of CO2 to 1165–1173.CO Over Copper Copyright- (2014)Decorated with permission g-C3N4 Nanosheets from Elsevier. with Enhanced Yield and Selectivity, 1165–1173. Copyright (2014) with permissionCdS is one from of the Elsevier. most promising candidate semiconductors as photocatalysts active in the Vis CdSrange: is Yu one et ofal. the[58] mostprepared promising RGO–CdS candidate nanorods semiconductors composites in ethanolamine as photocatalysts–water solution active using in the Vis a oneCdS-step is MWone of-assisted the most HT promising method. Thecandidate composites semiconductors exhibited a as high photocatalysts activity in the active photocatalytic in the Vis range: Yu et al. [58] prepared RGO–CdS nanorods composites in ethanolamine–water solution using range:reduction Yu et of al. CO [58]2 to prepared CH4, even RGO without–CdS noblenanorods metal composi catalysttes ( i.e.in ,ethanolamine Pt, which is usually–water solutionadopted usingas co - a one-step MW-assisted HT method. The composites exhibited a high activity in the photocatalytic acatalyst one-step). MWMW--assistedassisted HT synthesismethod. The requires composites shorter exhibited time and a highless energyactivity than in the conventional photocatalytic HT reductionreductionsynthesis of CO.of Furthermore,CO2 to2 to CH CH4,4, even evenparticles without without with noble noble uniform metal metal catalystsize catalyst are (i.e. usually, (i.e.,Pt, which Pt, obtained which is usually. is Raman usuallyadopted and adoptedas XPSco- as co-catalyst).catalystspectroscopies). MW MW-assisted-assisted confirmed HT HT the synthesis synthesisreduction requires of requires parent shorter GO shorter usingtime and timethe MWless and - energyassisted less energy thanmethod. conventional than TEM conventional analysis HT HT synthesis.synthesis(Figure 10). Furthermore, showed Furthermore, an intimate particles particles connection with with uniform between uniform size thesize twoare materials. areusually usually obtained obtained.. Raman Raman and XPS and XPS spectroscopiesspectroscopies conﬁrmed confirmed the the reduction reduction ofof parent GO GO using using the the MW MW-assisted-assisted method. method. TEM TEManalysis analysis (Figure(Figure 10) showed10) showed an an intimate intimate connection connection betweenbetween the the two two materials. materials.

Figure 10. TEM images of pure GO (a) and pure CdS nanorods (b). TEM (c) and HRTEM (d) images

of RGO-CdS nanorods composites. Reproduced from Ref. [58]. Copyright 2014 with permission from FigureFigureThe 10. Royal TEM10. TEM Society images images of ofChemistry of pure pure GO GO. (a ()a) and and pure pure CdSCdS nanorods ( (bb).). TEM TEM (c ()c and) and HRTEM HRTEM (d) ( imagesd) images of of RGO-CdS nanorods composites. Reproduced from Ref. [58]. Copyright 2014 with permission from RGO-CdS nanorods composites. Reproduced from Ref. [58]. Copyright 2014 with permission from The TheAn Royaloptimal Society RGO of contentChemistry of. 0.5 wt % was found, leading to the highest CH4 yield. Higher RGO Royal Society of Chemistry. content led to lower CH4 yields; the authors ascribed this result to RGO shielding light absorption by CdSAn nanorods. optimal RGO RGO acted content as coof- 0.5catalyst, wt % enhancingwas found, the leading photocatalytic to the high reductionest CH4 yieldof CO. 2Higher to CH 4RGO, with contentAn optimal led to lower RGO CH content4 yields of; the 0.5 authors wt % ascribed was found, this result leading to RGO to theshielding highest light CH absorp4 yield.tion by Higher RGOCdS content nanorods. led RGO to lower acted as CH co4-catalyst,yields; enhancing the authors the photocatalytic ascribed this reduction result to of RGOCO2 to shieldingCH4, with light absorption by CdS nanorods. RGO acted as co-catalyst, enhancing the photocatalytic reduction Catalysts 2018, 8, 262 16 of 35

Catalysts 2018, 8, x FOR PEER REVIEW 16 of 35 Catalysts 2018, 8, x FOR PEER REVIEW 16 of 35 of CO2 to CH4, with the best performance attained with a RGO content of 0.5 wt %; the corresponding the best performance attained with a RGO−1 −content1 of 0.5 wt %; the corresponding CH4 production CH4 production rate was 2.51 mmol h g , which exceeded that of pure CdS nanorods by more ratethe bestwas performance2.51 mmol h attained−1 g−1, which with exceeda RGOed content that of of pur0.5e wt CdS %; nanorodsthe corresponding by more CH than4 production 10 times. thanrate 10 times.was 2.51 According mmol h−1 g to−1, the which authors, exceeded RGO that was of pur ablee CdS to adsorb nanorods more by more CO2 thanmolecules 10 times. and to According to the authors, RGO was able to adsorb more CO2 molecules and to activate them very activate them very efﬁciently, simultaneously generating a higher2 number of photogenerated electrons efficiently,According simultaneouslyto the authors, RGOgenerat wasing able a higher to adsorb number more of COphoto moleculesgenerated and electrons to activate on its them surface, very efficiently, simultaneously generating a higher number of photogenerated electrons on its surface, on itsfinally surface, leading ﬁnally to an enh leadingancement to an of enhancementCO2 reduction perfor of COmance2 reduction of the RGO performance–CdS composite of the (Scheme RGO–CdS composite8)finally. (Schemeleading to8 an). enhancement of CO2 reduction performance of the RGO–CdS composite (Scheme 8).

SchemeScheme 8. Scheme8. Scheme of of the the mechanism mechanism proposed for for the the photocatalytic photocatalytic reduction reduction of CO of2 COto CH2 to4 by CH 4 by Scheme 8. Scheme of the mechanism proposed for the photocatalytic reduction of CO2 to CH4 by RGO-CdSRGO-CdS nanorods nanorods composites. composites. ReprintedReprinted from from Ref. Ref. [58] [58. ].Copyright Copyright 2014 2014 with with permission permission from The from The RGO-CdS nanorods composites. Reprinted from Ref. [58]. Copyright 2014 with permission from The RoyalRoyal Society Society of Chemistry.of Chemistry. Royal Society of Chemistry. In another work, MW were found to positively affect the immobilization of homogeneous Co(III) In another work, MW were found to positively affect the immobilization of homogeneous Co(III) cyclamIn another complex work, (cyclam MW were is 1,4,8,11 found-tetraazacyclotetradecane to positively affect the, immobilizationnamely: [Co(cyclam)Cl of homogeneous2]Cl) at the Co(III) cyclam complex (cyclam is 1,4,8,11-tetraazacyclotetradecane, namely: [Co(cyclam)Cl2]Cl) at the cyclamsurface complex of SBA (cyclam-15 silica. is The 1,4,8,11-tetraazacyclotetradecane, photocatalytic activity towards CO namely:2 reduction [Co(cyclam)Cl to CO was studied2]Cl) at in the the surface surface of SBA-15 silica. The photocatalytic activity towards CO2 reduction to CO was studied in the of SBA-15presence silica. of p The-terphenyl photocatalytic as a photosensitizer activity towards [59] CO. Usually,2 reduction Co(III) to CO cyclam was studiedimmobilization in the presence is presence of p-terphenyl as a photosensitizer [59]. Usually, Co(III) cyclam immobilization is of p-terphenylperformed as by areflux photosensitizer method, but [59 MW]. Usually,-assisted Co(III) immobilization cyclam immobilization of the Co(III) is cyclam performed complex by reflux performed by reflux method, but MW-assisted immobilization of the Co(III) cyclam complex method,occur butred more MW-assisted rapidly and immobilization led to more uniform of the Co(III) Co(III) surface cyclam species complex. Moreover, occurred a smaller more amount rapidly and occurred more rapidly and led to more uniform Co(III) surface species. Moreover, a smaller amount led toof Co(III) more uniformcomplex was Co(III) needed surface with respect species. to Moreover,reflux method a. smaller This latter amount aspect ofled Co(III)to well- complexdefined, was molecularlyof Co(III) complex dispersed was Co(III)needed siteswith withinrespect SBAto reflux-15 mesopores method. This. IR latter spectroscopy aspect led showed to well - thatdefined, the needed with respect to reflux method. This latter aspect led to well-defined, molecularly dispersed complexmolecularly interacts dispersed with theCo(III) silica sites surface within mainly SBA -by15 chemisorptionmesopores. IR on spectroscopy silanol (≡SiOH) showed groups that. The the Co(III) sites within SBA-15 mesopores. IR spectroscopy showed that the complex interacts with the photocatalystcomplex interacts prepared with bythe MW silica-assisted surface synthesis mainly bywas chemisorption more active and on selectivesilanol (≡ thanSiOH) that groups prepared. The silicabyphotocatalyst surface reflux method, mainly prepared byin the chemisorption by pre MWsence-assisted of a on synthesis molecular silanol was ( photosensitizer≡SiOH) more active groups. and and The selective sacrificial photocatalyst than electron that prepared prepareddonors by MW-assisted(Figureby reflux 11) synthesis.method, in was the premoresence active of a and molecular selective photosensitizer than that prepared and sacrificial by reflux electron method, donors in the presence(Figure of 11) a molecular. photosensitizer and sacrificial electron donors (Figure 11).

Figure 11. CO production by CO2 photochemical reduction after 4 h reaction with 2.0 mM p-terphenyl FigurewithFigure 11. Co(III) CO11. CO productioncyclam production deposited by by CO CO by2 2 photochemicalMW photochemical-assisted synthesis reduction reduction and after afterconventional 4 h 4 reaction h reaction reflux with with 2.0method. mM 2.0 mMp Reprinted-terphenyl p-terphenyl withfromwith Co(III) Ref.Co(III) cyclam[59] cyclam. Copyright deposited deposited 2017 by withby MW-assisted MW permission-assisted from synthesis The Royaland and conventional conventionalSociety of Chemistry. reflux reﬂux method. method. Reprinted Reprinted fromfrom Ref. Ref. [59]. [59] Copyright. Copyright 2017 2017 with with permission permission from The The Royal Royal Society Society of Chemistry. of Chemistry.

Catalysts 2018, 8, 262 17 of 35

3. Ultrasound as Unconventional Activation Method

3.1. General Principles of US-Assisted Synthesis of (Nano)Materials US is essentially a mechanical longitudinal wave with frequency equal to or higher than 20 kHz, which is the upper limit of human hearing range. US propagation gives sinusoidal variation in the static pressure in the liquid medium inducing cavitation, a phenomenon controlled by external parameters, i.e., US frequency and intensity and type solvent. Cavitation is based on radical and mechanical effects. The radical effect occurs either at the bubble interface or in the interior cavity and is ascribed to the occurrence of the sonolysis of molecules. The mechanical effect, occurring after the cavity collapse, arises from shear forces, microjets, and shock waves outside the bubble, therefore producing severe physical changes in the presence of solids or metals. The transient implosive collapse of the bubble is an adiabatic phenomenon, during which inside the bubble very high temperature and pressure occur (~4600 ◦C and ~50 MPa). The phenomenon affects the reaction system [60] by generating both an intense local convection in the reaction medium (ﬁnally enhancing the mass transfer processes) and highly reactive radical species within the bubbles, due to high local temperature and pressure. Upon fragmentation of the bubble, the radicals generated therein get released into the medium, where they can induce/accelerate chemical reactions [61]. On the one hand, US has emerged as a potential green technique for the intensiﬁcation of different physical, chemical, and biological processes. Compared with conventional heating, the use of US irradiation reduced the reaction time from hours to minutes. On the other hand, US can be effectively employed in the synthesis of NPs with a high production yield, without using expensive instruments and/or extreme conditions [62,63], as, for example, carbon encapsulated Fe NPs with superior catalytic activity and reusability for hydrogenation reaction in the liquid phase [64]. US allow obtaining of highly reactive surfaces, enhancing mixing and increasing mass transport. Therefore, such unconventional method can be particularly helpful for preparing and activating catalytic materials [65].

3.2. US-Assisted Synthesis of Cu–ZnO Catalysts and of Their Improved Formulations

Under typical industrial conditions, CH3OH is formed mostly via CO2 hydrogenation, with CO acting as CO2 source and as oxygen scavenger from water, which inhibits the active metal sites [66]. A synergism between Cu0 and Cu+ sites is the most adopted explanation for the activity of Cu/ZnO-promoted catalysts [38,61,67,68]; however, Cu0 sites [69,70] and/or Cu–Zn alloy [71–73] have been proposed to have a catalytic role as well. Many unconventional preparation routes have been adopted to obtain Cu–ZnO catalysts with improved activity, selectivity, resistance to poisons, and enhanced lifetime [38,74–78]. In this frame, the SG method was largely adopted in order to increase the metal surface area along with the total surface area [79,80]. Moreover, the excellent activity of oxalate co-precipitated catalysts was also demonstrated [81], and Cu–Zn catalysts prepared by “spark-erosion” showed superior performance in terms of selectivity [82], with noticeable TOF in the CO2 hydrogenation reaction that was ascribed to a close contact among the phases in Cu/ZnO/ZrO2 catalyst produced by combustion. To further improve the physico-chemical properties of a Cu–ZnO/ZrO2 catalyst (Znat/Cuat, 0–3; ZrO2, 42–44 wt %), a new synthetic procedure consisting in the US-assisted reverse co-precipitation was proposed [83]. The US irradiation guaranteed highly increased total surface area (BET, 120–180 m2 g−1), although the Zn/Cu atomic ratio strongly affected either the speciﬁc surface area or the volume of the pores. Indeed, the materials with low (≤0.1) and high (>0.7) Zn/Cu ratios have lower surface area than those with a Zn/Cu ratio of 0.3–0.4, corresponding to a maximum of 175 m2/g. An analogous volcano trend was observed for the pore volume and pore diameter, in which the catalyst with Zn/Cu ratio equal to 0.4 contains mainly mesopores with diameter in the 70–120 Å range and, as a consequence, an enhanced cumulative volume (0.6 cm3 g−1). Moreover, the authors found that the US irradiation 2 −1 led to very high dispersion (3–58%) and exposure of Cu (SCu = 9–63 m g ). The catalytic activity in ◦ the CH3OH production by CO2 hydrogenation was explored in a range of temperatures (160–260 C) Catalysts 2018, 8, 262 18 of 35

and pressures (1.0–3.0 MPa), and compared with a commercial Cu–ZnO/Al2O3 catalyst with optimum resultsCatalysts ascribed 2018, 8, x FOR to the PEER presence REVIEW of highly dispersed Cu particles. 18 of 35 The direct synthesis of dimethyl ether starting from syngas involves two consecutive reactions, temperatures (160–260 °C) and pressures (1.0–3.0 MPa), and compared with a commercial Cu– i.e., CH OH synthesis and CH OH dehydration [84]. Alternatively, dimethyl ether can be directly ZnO/Al3 2O3 catalyst with optimum3 results ascribed to the presence of highly dispersed Cu particles. obtainedThe from direct syngas synthesis or COof dimethyl2 hydrogenation. ether starting from syngas involves two consecutive reactions, i.e.The, CH3 useOH ofsynthesis novel and bi-functional CH3OH dehydration catalysts [84] based. Alternatively, on metal oxidesdimethyl (namely, ether can CuO, be directly ZnO, Al2O3, andobtained possibly from some syngas precious or CO2 metalhydrogenation. oxides, such as Cr2O3) to synthesize CH3OH and on acid solids (namely,The HZSM-5 use of novel and HYbi-functional zeolites or catalysts SAPOs) based to convert on metal it into oxides dimethyl (namely ether, CuO, can ZnO, signiﬁcantly Al2O3, and enhance possibly some precious metal oxides, such as Cr2O3) to synthesize CH3OH and on acid solids (namely, the yield with respect to the two-step process, because CH3OH dehydration can interfere with the equilibriumHZSM-5 and of theHY zeolites synthesis or reactionSAPOs) to [85 convert]. it into dimethyl ether can significantly enhance the yield with respect to the two-step process, because CH3OH dehydration can interfere with the In this contest, Cu-ZnO-Al O , as well as Al O and HZSM-5 catalysts, have been equilibrium of the synthesis reaction2 [85]3 . 2 3 widely investigated [86–88]. In particular, a previous study reported on the preparation of In this contest, Cu-ZnO-Al2O3, as well as Al2O3 and HZSM-5 catalysts, have been widely CuO-ZnO-Alinvestigated2 O[863–/HZSM-588]. In particular, hybrid a catalysts previous (CZA/HZSM-5) study reported on using the preparation different techniques, of CuO-ZnO- such as impregnation,Al2O3/HZSM-5 co-precipitation-physicalhybrid catalysts (CZA/HZSM- mixing5) using different method, techniques, and a such US-assisted as impregnation, co-precipitation co- procedureprecipitation [89-].physical It was mixing observed method, that and the a US CZA/HZSM-5-assisted co-precipitation catalysts procedure prepared [89] by. It US-assisted was co-precipitationobserved that the displayed CZA/HZSM higher-5 catalysts activity prepared than the by systems US-assiste obtainedd co-precipitation by conventional displayed physical higher mixing andactivity impregnation. than the systems Further, obt variousained by catalysts conventional with different physical mixing CZA and and HZSM-5 impregnation. wt. ratio Further, (namely, 2, 3, andvarious 4) werecatalysts synthesized with different by theCZA US-assisted and HZSM-5 co-precipitation wt. ratio (namely, procedure2, 3, and 4) were summarized synthesize ind by Figure 12 inthe order US- toassisted establish co-precipi a correlationtation procedure between summarized the catalyst in Figure composition 12 in order and to establish the displayed a correlation activity [90]. between the catalyst composition and the displayed activity [90]. The experimental conditions for US The experimental conditions for US irradiation in Ar atmosphere were 90 W for 45 min, with 10 min irradiation in Ar atmosphere were 90 W for 45 min, with 10 min current pulse time and 0.5 min rest currenttime, by pulse means time of andthe experimental 0.5 min rest apparatus time, by meansshown ofin Figure the experimental 13. apparatus shown in Figure 13.

Figure 12. Schematic flow chart for preparation steps of CZA/HZSM-5 catalysts synthesized via Figure 12. Schematic ﬂow chart for preparation steps of CZA/HZSM-5 catalysts synthesized via combined co-precipitation-ultrasound method with different active phase to support ratios. combined co-precipitation-ultrasound method with different active phase to support ratios. Reprinted Reprinted from Ref. [90]. Copyright 2014 with permission of The Royal Society of Chemistry. from Ref. [90]. Copyright 2014 with permission of The Royal Society of Chemistry.

Catalysts 2018, 8, 262 19 of 35 Catalysts 2018, 8, x FOR PEER REVIEW 19 of 35

FigureFigure 13. 13Experimental. Experimental setup setup for for dispersiondispersion of CZA over over HZSM HZSM-5-5 using using US US energy. energy. Re Reprintedprinted from from Ref. [90]. Copyright 2014 with permission of The Royal Society of Chemistry. Ref. [90]. Copyright 2014 with permission of The Royal Society of Chemistry.

It is worth noting that the zeolite preserved its crystal structure during the preparation. Moreover,It is worth the noting crystallinity that the was zeolite significantly preserved influenced its crystal by structure varying duringof the CZA the preparation. content. XRD Moreover, peaks therelated crystallinity to monoclinic was significantly CuO, and hexagonal influenced ZnO by phases varying were of observed. the CZA FESEM content. analy XRDses peaks indicated related the to monoclinicpresence CuO, of a uniform and hexagonal coating ZnO of the phases HZSM were-5 surface observed. by CZA FESEM upon analyses US irradi indicatedation during the presence the ofsynthesis. a uniform In coatingaddition, of the the catalysts HZSM-5 consisted surface of bymany CZA small upon particles US irradiation with almost during spherical the shape. synthesis. At Invariance addition, with the catalystsreducibility, consisted the surface of many area smallwas not particles dependent with almoston the CZA spherical content. shape. The At HZSM variance-5 withstructure reducibility, was not the damaged surface upon area wasloading not CZ dependentA, as demonstrated on the CZA by content.FTIR spectroscopy The HZSM-5 results structure. The wasoptimal not damaged composition upon of loadingthe CZA/HZSM CZA, as-5 demonstratedcatalysts for theby conversion FTIR spectroscopy of syngas to results.dimethylether The optimal was composition4:1 at 275 °C of and the 40 CZA/HZSM-5 bar. catalysts for the conversion of syngas to dimethylether was 4:1 at 275 ◦CInterestingly, and 40 bar. it was reported that the sonication time significantly affected both morphology andInterestingly, structure of the it was CZA reported/HZSM-5 that catalyst the sonication[91]. In particular, time significantly larger CuO affectedcrystals formed both morphology at longer andirradiation structure times, of the and CZA/HZSM-5 at the same time catalyst the [particle91]. In particular,aggregates largerchanged CuO into crystals smaller formed size and at more longer uniform shape, resulting in an increase of the surface area. The catalyst synthesized under long US irradiation times, and at the same time the particle aggregates changed into smaller size and more irradiation showed very good activity during time on stream test, whilst the non-sonicated sample uniform shape, resulting in an increase of the surface area. The catalyst synthesized under long US lost its activity. irradiation showed very good activity during time on stream test, whilst the non-sonicated sample lost The effect of US irradiation power was evaluated on different CZA/HZSM-5 catalysts prepared its activity. by US-assisted co-precipitation method [92]. The results of a thorough physico-chemical The effect of US irradiation power was evaluated on different CZA/HZSM-5 catalysts prepared by characterization evidenced that the nature of the precursors and the time of irradiation strongly US-assistedaffected the co-precipitation morphology, method the surface [92 ]. area, The resultsthe functional of a thorough groups physico-chemical, and the structure characterization of the final evidencedmaterials. that In theparticular, nature ofas theshown precursors in Figure and 14, the highly time irradiated of irradiation acetate strongly-based affected catalysts the ha morphology,d uniform themorphology, surface area, as the well functional as smaller groups, CuO particles and the structurewith higher of thesurface final area, materials. due to Inhigh particular, nucleation as shownrate inand Figure high 14 cooling, highly rates, irradiated resulting acetate-based in a stronger catalysts interaction had among uniform the morphology,components in as comparison well as smaller to CuOthe particles catalysts withprepared higher by surface starting area, from due nitrate to high precursors. nucleation rate and high cooling rates, resulting in a strongerIt is interactionworth noting among that an the increa componentsse of the irradiation in comparison power to gave the catalysts rise to smaller prepared particles, by starting resulting from nitratein an precursors. improved dispersion and surface area of the catalysts. Moreover, CuO particles with lower crystallinityIt is worth were noting generated that an increaseat high power, of the irradiationresulting in power an enhanced gave rise interaction to smaller among particles, the resulting metal inoxide an improved particles. dispersion In addition, and such surface ultrasound area of-assisted the catalysts. co-precipitation Moreover, method CuO allowed particles to withproduce lower crystallinityCZA/HZSM were-5 catalysts generated that at were high more power, stable resulting with in respect an enhanced to those interaction prepared by among conventional the metal oxideprecipitation, particles. for In addition,which 18% such CO conversion ultrasound-assisted and 58% dimethyl co-precipitation ether yield method decreases allowed in 24 h to time produce on CZA/HZSM-5stream test were catalysts observed. that were more stable with respect to those prepared by conventional precipitation, for which 18% CO conversion and 58% dimethyl ether yield decreases in 24 h time on stream test were observed.

Catalysts 2018, 8, 262 20 of 35 Catalysts 2018, 8, x FOR PEER REVIEW 20 of 35

Figure 14. Surface particle size distribution histogram of synthesized CZA/HZSM-5 nanocatalyst Figure 14. Surface particle size distribution histogram of synthesized CZA/HZSM-5 nanocatalyst using acetate precursor and ultrasound irradiation power of 150 W (CZAZ-AU150). Reprinted from using acetate precursor and ultrasound irradiation power of 150 W (CZAZ-AU150). Reprinted from Ultrason. Sonochem., 21, Allahyari, S.; Haghighi, M.; Ebadi, A.; Hosseinzadeh, S., Ultrasound-Assisted Ultrason. Sonochem., 21, Allahyari, S.; Haghighi, M.; Ebadi, A.; Hosseinzadeh, S., Ultrasound-Assisted Co-Precipitation of Nanostructured CuO-ZnO-Al2O3 Over HZSM-5: Effect of Precursor and Co-Precipitation of Nanostructured CuO-ZnO-Al O Over HZSM-5: Effect of Precursor and Irradiation Irradiation Power on Nanocatalyst Properties and2 3Catalytic Performance for Direct Syngas to DME, Power on Nanocatalyst Properties and Catalytic Performance for Direct Syngas to DME, 663–673. 663–673. Copyright (2014) with permission from Elsevier. Copyright (2014) with permission from Elsevier. The authors proposed a pathway for the ZnO synthesis under ultrasound irradiation in CZA thatThe is authorsillustrated proposed in Figure a pathway15. Such pathway for the ZnO can synthesis be extended under to other ultrasound precursors irradiation, as well in as CZA to CuO that is illustratedand Al2O in3. Figure More 15specifically. Such pathway, the metallic can be extended precursors to underwent other precursors, decomposition as well as and to CuO hydratation and Al2O 3. Moreupon specifically, ultrasound the irradiation, metallic precursors and H· and underwent OH· radicals decomposition formed by water and sonolysis hydratation [93] upon. The ultrasoundfree OH· irradiation,radicals formed and H· and in the OH cavities· radicals or formedat interface by water can sonolysis react with [93 the]. The metal free cations OH· radicals forming formed metal in thehydroxides cavities or atthat interface are converted can react with to the the metalcorresponding cations forming metal metaloxides hydroxides upon thermal that are treatment. converted toSimultaneously the corresponding, by co metal-precipitation oxides upon of the thermal metal precursor, treatment. the Simultaneously,metal carbonates by(that co-precipitation will then be ofconvert the metaled in precursor,to the metal the oxides metal) are carbonates produced. (that Therefore, will then a strong be converted synergistic into action the of metal US during oxides) areco produced.-precipitation Therefore, was advised. a strong In particular, synergistic the OH action· radicals of US coming during from co-precipitation the water sonolysis was are advised. able to convert a fraction of the metal salt to the corresponding metal oxide without any help from the In particular, the OH· radicals coming from the water sonolysis are able to convert a fraction of precipitant. Moreover, a significant increase of the nucleation rate under peculiar temperature and the metal salt to the corresponding metal oxide without any help from the precipitant. Moreover, pressure conditions upon cavity collapse gives rise to the formation of small and well-dispersed NPs. a significant increase of the nucleation rate under peculiar temperature and pressure conditions In particular, the irradiation cycles impede the growth of the nucleated particles, resulting in the upon cavity collapse gives rise to the formation of small and well-dispersed NPs. In particular, production of small catalyst particles. Finally, particle agglomeration is prevented, indeed, the thepopulations irradiation of cycles OH· impederadicals the and growth of the nucleation of the nucleated sites are particles, enhanced resulting at high US in thepower production, and the of smallconversion catalyst to particles. metal oxides Finally, is therefore particle improved. agglomeration is prevented, indeed, the populations of OH· radicals and of the nucleation sites are enhanced at high US power, and the conversion to metal oxides is therefore improved. An US-assisted co-precipitation method was employed for the preparation of CZA/HZSM-5 catalysts supported on multi-walled carbon nanotubes pretreated with H2SO4/HNO3 mixture [94], in which the acid pretreatment had the aim of activating and functionalizing the surface of the nanotubes. The acid-pretreated carbon nanotubes were dispersed in deionized water by sonication more easily than the pristine ones, because the presence of the surface functional groups increased the capability to form hydrogen bonds, converting the surface of the multi-walled carbon nanotubes from hydrophobic to hydrophilic, therefore resulting in improved dispersion and suspension stability. The CZA NPs of about 10–20 nm were quite homogeneously distributed and densely deposited at the surface of the acid-pretreated, multi-walled carbon nanotubes (as shown in Figure 16). The catalysts were tested in the hydrogenation of CO2 to produce dimethyl ether in a fixed-bed reactor. It was shown that the presence of the multi-walled carbon nanotubes has a promoting effect on the catalytic activity and on the dimethyl ether. The CO2 conversion was 46.2%, with dimethyl ether ◦ yield and selectivity of 20.9% and 45.2%, respectively, at 262 C, 3.0 MPa, volume ratio H2/CO2 = 3, −1 −1 and space velocity = 1800 mL gcat h .

Catalysts 2018, 8, x FOR PEER REVIEW 20 of 35

Figure 14. Surface particle size distribution histogram of synthesized CZA/HZSM-5 nanocatalyst using acetate precursor and ultrasound irradiation power of 150 W (CZAZ-AU150). Reprinted from Ultrason. Sonochem., 21, Allahyari, S.; Haghighi, M.; Ebadi, A.; Hosseinzadeh, S., Ultrasound-Assisted

Co-Precipitation of Nanostructured CuO-ZnO-Al2O3 Over HZSM-5: Effect of Precursor and Irradiation Power on Nanocatalyst Properties and Catalytic Performance for Direct Syngas to DME, 663–673. Copyright (2014) with permission from Elsevier.

The authors proposed a pathway for the ZnO synthesis under ultrasound irradiation in CZA that is illustrated in Figure 15. Such pathway can be extended to other precursors, as well as to CuO and Al2O3. More specifically, the metallic precursors underwent decomposition and hydratation upon ultrasound irradiation, and H· and OH· radicals formed by water sonolysis [93]. The free OH· radicals formed in the cavities or at interface can react with the metal cations forming metal hydroxides that are converted to the corresponding metal oxides upon thermal treatment. Simultaneously, by co-precipitation of the metal precursor, the metal carbonates (that will then be converted into the metal oxides) are produced. Therefore, a strong synergistic action of US during co-precipitation was advised. In particular, the OH· radicals coming from the water sonolysis are able to convert a fraction of the metal salt to the corresponding metal oxide without any help from the precipitant. Moreover, a significant increase of the nucleation rate under peculiar temperature and pressure conditions upon cavity collapse gives rise to the formation of small and well-dispersed NPs. In particular, the irradiation cycles impede the growth of the nucleated particles, resulting in the production of small catalyst particles. Finally, particle agglomeration is prevented, indeed, the Catalystspopulations2018, 8, 262 of OH· radicals and of the nucleation sites are enhanced at high US power, and the21 of 35 conversion to metal oxides is therefore improved.

Catalysts 2018, 8, x FOR PEER REVIEW 21 of 35

Figure 15. Reaction pathway for synthesis of ZnO in CZA nanocatalyst by ultrasound-assisted co- precipitation of zinc acetate. Reprinted from Ultrason. Sonochem., 21, Allahyari, S.; Haghighi, M.; Ebadi, A.; Hosseinzadeh, S., Ultrasound assisted co-precipitation of nanostructured CuO-ZnO-Al2O3 over HZSM-5: Effect of Precursor and Irradiation Power on Nanocatalyst Properties and Catalytic Performance for Direct Syngas to Dimethyl Ether, 663–673. Copyright (2014) with permission from Elsevier.

An US-assisted co-precipitation method was employed for the preparation of CZA/HZSM-5 catalysts supported on multi-walled carbon nanotubes pretreated with H2SO4/HNO3 mixture [94], in which the acid pretreatment had the aim of activating and functionalizing the surface of the Figurenanotubes. 15. TheReaction acid-pretreated pathway forcarbon synthesis nanotubes of ZnO were in dispersed CZA nanocatalyst in deionized by water ultrasound-assisted by sonication more easily than the pristine ones, because the presence of the surface functional groups increased co-precipitation of zinc acetate. Reprinted from Ultrason. Sonochem., 21, Allahyari, S.; Haghighi, M.; the capability to form hydrogen bonds, converting the surface of the multi-walled carbon nanotubes Ebadi, A.; Hosseinzadeh, S., Ultrasound assisted co-precipitation of nanostructured CuO-ZnO-Al O from hydrophobic to hydrophilic, therefore resulting in improved dispersion and suspension2 3 over HZSM-5: Effect of Precursor and Irradiation Power on Nanocatalyst Properties and Catalytic stability. The CZA NPs of about 10–20 nm were quite homogeneously distributed and densely Performance for Direct Syngas to Dimethyl Ether, 663–673. Copyright (2014) with permission deposited at the surface of the acid-pretreated, multi-walled carbon nanotubes (as shown in Figure from Elsevier. 16).

Figure 16. TEM images of CZA supported on multi-walled carbon nanotubes pretreated with a Figure 16. TEM images of CZA supported on multi-walled carbon nanotubes pretreated with H2SO4/HNO3 mixture. Reprinted from Appl. Surf. Sci., 285, Zha, F.; Tian, H.; Yan, J.; Chang, Y., Multi- a H2SO4/HNO3 mixture. Reprinted from Appl. Surf. Sci., 285, Zha, F.; Tian, H.; Yan, J.; Chang, Y., Walled Carbon Nanotubes as Catalyst Promoter for Dimethyl Ether Synthesis from CO2 Multi-Walled Carbon Nanotubes as Catalyst Promoter for Dimethyl Ether Synthesis from CO Hydrogenation, 945–951. Copyright (2013) with permission from Elsevier. 2 Hydrogenation, 945–951. Copyright (2013) with permission from Elsevier.

The catalysts were tested in the hydrogenation of CO2 to produce dimethyl ether in a fixed-bed 3.3. US-Assistedreactor. It was Synthesis shown that of Catalysts the presence with of Ferromagnetic the multi-walled Properties carbon nanotubes has a promoting effect on the catalytic activity and on the dimethyl ether. The CO2 conversion was 46.2%, with dimethyl The synthesis of nanostructured materials based on ferromagnetic elements is of great interest ether yield and selectivity of 20.9% and 45.2%, respectively, at 262 °C, 3.0 MPa, volume ratio H2/CO2 due to= 3 the, and possibility space velocity to tune = 1800 their mL magnetic gcat−1h−1. properties [95–97], as well as to promote physico-chemical surface processes [98,99] by controlling the shape and size of the obtained NPs. In addition, magnetic catalysts3.3. US can-Assisted be easily Synthesis positioned of Catalysts and recoveredwith Ferromagnetic by using Properties an external magnetic ﬁeld. In aThe recent synthesis paper, of Vargas nanostructured et al. reported materials a facilebased on two-step ferromagnetic UP procedure elements tois of synthesize great interest Ni NPs and thendue to support the possibility them on to m tune-ZrO the2irto magnetic obtain low-cost properties and[95– active97], as catalystswell as to for promote CO2 methanation physico- (Equationchemical (2)) surface [100]. processes More in detail,[98,99] by Ni controlling NPs were the ﬁrstly shape produced and size of by the chemical obtained reductionNPs. In addition, of a metal precursormagnetic (nickel catalysts chloride) can be with easily either positioned hydrazine and hydraterecovered (N by2H using4·H2 O)an external or sodium magnetic borohydride field. (NaBH4) In a recent paper, Vargas et al. reported a facile two-step UP procedure to synthesize Ni NPs and then support them on m-ZrO2 to obtain low-cost and active catalysts for CO2 methanation (Equation (2)) [100]. More in detail, Ni NPs were firstly produced by chemical reduction of a metal

Catalysts 2018, 8, 262 22 of 35

Catalysts 2018, 8, x FOR PEER REVIEW 22 of 35 in water,precursor in the (nickel presence chloride) of citric with acid either and sodium hydrazine citrate hydrate as protecting (N2H4·H2 agents.O) or sodium The chemical borohydride reduction was carried(NaBH4) outin water, under in the US presence irradiation of citric (40 acid kHz, and 300sodium W) citrate without as protecting any additional agents. The stirring chemical sources. Then,reduction the black was colored carried solution, out under indicating US irradiation the presence (40 kHz, of 300 Ni W)0 NPs, without was any added additional to other stirring aqueous 0 suspensionssources. ofThen commercial, the black monocliniccolored solution, ZrO 2indicating. In order the to obtainpresence homogenous of Ni NPs, was materials, added theto other obtained mixturesaqueous were suspensions maintained of overnightcommercial in monoclinic a conventional ZrO2. ultrasonicIn order to obtain bath at homogenous ambient conditions. materials, Thethe two samplesobtained were mixtures centrifuged, were washed maintained several overnight times inwith a conventionala mixture of ultrasonic deionized bath water at and ambient ethanol, and driedconditions. under The vacuum. two samples The catalysts were centrifuged, had the samewashed metal several loading times (1with wt a % mixture Ni), but of different deionized metal water and ethanol, and dried under vacuum. The catalysts had the same metal loading (1 wt % Ni), particle sizes. It was found that the ﬁnal particle size was strongly related to the employed reducing but different metal particle sizes. It was found that the final particle size was strongly related to the ± agent.employed Particularly, reducing homogeneous agent. Particularly, Ni particles homogeneous with globular Ni particles shape with and globular average shape size and of average 40 1 nm formedsize in of the 40 ± presence 1 nm formed of hydrazine, in the presence as shown of hydrazine, in Figure as shown17A. in Figure 17A.

Figure 17. TEM images of nickel nanoparticles developed using hydrazine (A), high magnification Figure 17. TEM images of nickel nanoparticles developed using hydrazine (A), high magniﬁcation image (inset), and sodium borohydride (B). Hydrodynamic diameter (C), colloidal systems images image (inset), and sodium borohydride (B). Hydrodynamic diameter (C), colloidal systems images (inset), and XRD patterns (D) of samples. Reproduced from [100] with permission of The Royal (inset), and XRD patterns (D) of samples. Reproduced from [100] with permission of The Royal Society Society of Chemistry. of Chemistry.

On the contrary, when NaBH4 was employed, smaller and well isolated Ni NPs with average diameter equal to 2.7 ± 0.4 nm were observed (Figure 17B). Such differences can be explained by On the contrary, when NaBH4 was employed, smaller and well isolated Ni NPs with average 2+ diameterconsidering equal tothat 2.7 in the± 0.4 case nm of N were2H4*H observed2O, the formation (Figure of 17 theB). [Ni(N Such2H differences4)3] complexcan implied be explained a slower by kinetics, hence inhibiting the formation of nuclei and favoring particle growth. Instead, the use of considering that in the case of N H *H O, the formation of the [Ni(N H ) ]2+ complex implied sodium borohydride gave rise to 2a larger4 2 number of Ni nuclei and, consequently,2 4 to3 smaller particles. a slower kinetics, hence inhibiting the formation of nuclei and favoring particle growth. Instead, Hydrodynamic diameters larger than those measured by TEM were detected in both samples (Figure the use17C of) in sodium agreement borohydride with the presence gave rise of citrate to a larger at the numbersurface of of NPs Ni and nuclei of the and, occurrence consequently, of dynamics to smaller particles.dipolar Hydrodynamic magnetic interactions. diameters Peaks larger at 44.7°, than those52.1°, and measured 76.7° 2 byTheta TEM degrees were detectedwere observed in both in samplesthe (Figuredif fraction17C) in pattern agreement of the with catalyst the with presence large Ni of citrateparticles at (Ni(40) the surface in Figure of NPs18c), andassigned of the to occurrencethe (111), of dynamics(200), dipolarand (220) magnetic reflections interactions. of the cubic Peaks Ni phase at 44.7 (Figure◦, 52.1 17◦,D and). As 76.7 for◦ the2 Theta catalyst degrees with small were NP observeds, in the(Ni(3) diffraction in Figure pattern 18c), of only the thecatalyst (111) with peak large was Ni dete particlescted. The (Ni(40) catalysts in Figurewere tested 18c), inassigned the CO to2 the (111),methanation (200), and (220)reaction reﬂections, revealing ofhigh the activity cubic (70%) Ni phase and excellent (Figure 17selectivityD). As forto CH the4 (90%) catalyst at 720 with K. small NPs, (Ni(3) in Figure 18c), only the (111) peak was detected. The catalysts were tested in the CO2 methanation reaction, revealing high activity (70%) and excellent selectivity to CH4 (90%) at 720 K. Catalysts 2018, 8, 262 23 of 35 Catalysts 2018, 8, x FOR PEER REVIEW 23 of 35

Figure 18. XRD patterns of CZAx catalysts (x = 200 (a), 300 (b), and 400 (c)). Reprinted from Wu, W.; Figure 18. XRD patterns of CZAx catalysts (x = 200 (a), 300 (b), and 400 (c)). Reprinted from Wu, W.; Xie, K.; Sun, D.; Li, X.; Fang, F. CuO/ZnO/Al2O3 Catalyst Prepared by Mechanical-Force-Driven Solid- Xie, K.; Sun, D.; Li, X.; Fang, F. CuO/ZnO/Al O Catalyst Prepared by Mechanical-Force-Driven State Ion Exchange and Its Excellent Catalytic Activity2 3 under Internal Cooling Condition. Ind. Eng. Solid-StateChem. Res. Ion Exchange56, 8216–8223. and Copyright Its Excellent (2017) Catalytic American Activity Chemical under Society. Internal Cooling Condition. Ind. Eng. Chem. Res. 56, 8216–8223. Copyright (2017) American Chemical Society.

Ni/ZrO2 catalysts modified with rare earth elements, in particular Ni/(Zr-Sm)Ox catalysts, showed improved activity in the CO2 methanation (Equation (2)) [101,102]. Conventional wet Ni/ZrO2 catalysts modified with rare earth elements, in particular Ni/(Zr-Sm)Ox catalysts, impregnation method or preparation starting from amorphous Ni-Sm alloys were the preparation showed improved activity in the CO methanation (Equation (2)) [101,102]. Conventional wet procedures usually adopted for these2 catalysts. In particular, the activity of these systems was impregnation method or preparation starting from amorphous Ni-Sm alloys were the preparation correlated with the presence of (Zr–Sm)Ox units with the tetragonal ZrO2 structure. Moreover, the procedurescatalytic usually performance adopted was for also these influenced catalysts. by preparation In particular, method, the surfaceactivity area of, these and pore systems size was correlateddistribution with of the the presence catalysts [101,103] of (Zr–Sm)Ox. units with the tetragonal ZrO2 structure. Moreover, the catalyticA Ni performance-based (40 at % was Ni) alsocatalyst influenced supported by on preparationmesoporous yttria method,-stabilized surface-zirconia area, composite and pore size distributionwas prepared of the by catalysts a US method [101,103 using]. sodium dodecyl sulfate as a templating agent to obtain high Asurface Ni-based area (193 (40 atm2 %/g) Ni) [104] catalyst. The application supported of onthemesoporous US method resulted yttria-stabilized-zirconia in short reaction time composite (6 h) was preparedand no requirement by a US methodfor the glycolation using sodium of nickel, dodecyl yttrium, sulfate and zirconium as a templating ions. In agentanother to paper obtain by high surfacethearea same (193 research m2/g) grou [104p,]. a The simple application one-step ofUS the-assisted US method synthesis resulted (in which in short the reactioncorresponding time (6 h) inorganic salts were used as precursors) was applied to prepare 20–40 at % Ni supported on and no requirement for the glycolation of nickel, yttrium, and zirconium ions. In another paper by the mesoporous ZrO2 [105]. In these catalysts, Ce and Sm rare earth ions were introduced in the support same research group, a simple one-step US-assisted synthesis (in which the corresponding inorganic to further enhance its activity towards CO2 methanation. The obtained catalysts possessed in situ saltsgenerated were used mesopores, as precursors) and the wasNi oxide applied (or hydroxide to prepare) that 20–40 were about at % 10 Ni nm supported were incorporated on mesoporous into ZrO2tetragonal[105]. In theseZrO2 modified catalysts, by Ce the and rare Sm earth rare metals. earth In ions particular, were introduced the catalyst incontaining the support 30 at to% Ni further enhanceloading its activityhad maximum towards porous CO 2volumemethanation. and size, Theand at obtained the same catalysts time displayed possessed the highest in situ activity, generated mesopores,which was and further the Ni enhanced oxide (or upon hydroxide) proper thatoxidation were– aboutreduction 10 nmactivation were incorporatedtreatment, during into which tetragonal ZrO2additionalmodified active by the centers rare earth were metals.produced. In particular, the catalyst containing 30 at % Ni loading had maximumCO porous2 methanation volume was and also size, performed and at the over same Ni/Al time2O3 displayedcatalysts prepared the highest by a activity,US-assisted which co- was furtherprecipitation enhanced [106] upon. The proper authors oxidation–reduction observed that an increase activation in the Ni treatment, loading from during 5 wt which% up to additional25 wt % produced a surface area enhancement, resulting in better catalytic performances, and active centers were produced. simultaneously decreased the crystallinity and improved the catalyst reducibility. A further increase CO methanation was also performed over Ni/Al O catalysts prepared by a US-assisted of the2 metal loading had a negative effect on the surface area2 , as3 well as on the catalytic activity, due co-precipitation [106]. The authors observed that an increase in the Ni loading from 5 wt % up to 25 wt % to the drop of the Ni dispersion. The 25 wt % Ni/Al2O3 catalyst attained high CO2 conversion (74%) producedand almost a surface complete area enhancement,CH4 selectivity (99%) resulting at 350 in °C. better The catalyst catalytic demonstrated performances, also andhigh simultaneously stability at decreasedthe same the temperature crystallinity after and 10 improvedh time-on-stream. the catalyst The catalytic reducibility. performance A further was observed increase to decrease of the metal loadingat higher had a Gas negative Hourly effect Space on Velocity, the surface due to area, both as reduced well as contact on the time catalytic between activity, the reactants due to and the the drop of the Nicatalyst dispersion. surface The, as well 25 wt as %the Ni/Al amount2O 3ofcatalyst adsorbed attained reactants. high Conversely, CO2 conversion a positive (74%) effect andon the almost catalyst activity was observed when◦ higher H2/CO molar ratio was employed. Furthermore, the complete CH4 selectivity (99%) at 350 C. The catalyst demonstrated also high stability at the same temperatureperformance after of 10 catalysts h time-on-stream. calcined at higher The catalytic temperature performance was observed was to observed decrease. to decrease at higher Gas Hourly Space Velocity, due to both reduced contact time between the reactants and the catalyst surface, as well as the amount of adsorbed reactants. Conversely, a positive effect on the catalyst activity was observed when higher H2/CO molar ratio was employed. Furthermore, the performance of catalysts calcined at higher temperature was observed to decrease. Catalysts 2018, 8, 262 24 of 35

In another recent study, Co/SBA-15 and Co/SBA-16 catalysts with 20 wt % Co loading were prepared by conventional incipient wetness impregnation method [107]. Additional vacuum and US treatments were performed after the synthesis. N2 physisorption, small angle XRD (SAXRD), wide angle XRD, SEM and energy dispersive X-ray spectroscopy (SEM/EDX), TEM, and TPR were employed to characterize the catalysts. It was observed that Co dispersion on the SBA-15 support was enhanced upon such post-synthesis treatments, due to the decrease of the metal crystallite size. The presence of Co3O4 and of different various CoxOy species was detected. Both post-impregnation treatments affected the metal-support interaction, increasing the activity in the CO2 hydrogenation. It was observed that the vacuum treatment gave rise to a material in which crystalline cobalt oxide was located in the inner pores, whilst cobalt oxide agglomeration at the outer surface took place upon the US treatment. A series of 0.5–5.0% Ru catalysts supported on Al2O3 for the selective methanation of CO in H2-containing streams were investigated with the aim to clarify the influence of process parameters, such as the metal loading, the temperature of calcination, the chlorine ions content, and the space velocity, on the catalytic performances [108]. Careful characterization by XRD, Thermal Gravim- etry/Differential Thermal Gravimetry (TG/DTG), and SEM was performed. A number of laboratory experiments were performed to clarify the influence of process parameters (Ru loading, calcinations temperature, space velocity, and chlorine ions content) on the activities of catalysts. It was observed that by carrying out the synthesis of the catalysts by US-assisted impregnation, the catalytic activity was significantly improved, and the impregnation time was significantly reduced. In particular, agglomeration accompanied by non-homogeneous distribution of Ru deposits on Al2O3 were observed when the catalysts were prepared by wetness impregnation method. Conversely, US impregnation strongly decreased the particle size and improved the distribution of Ru during the impregnation. ◦ The best results were achieved by the catalyst containing 2 wt % Ru on Al2O3 calcined at 400 C. Long-term tests demonstrated high activity and stability under realistic reaction conditions.

4. The Role of Mechanochemical Activation in the Synthesis of Nanostructured Catalysts

4.1. General Principles of MC MC induces chemical transformations by mechanical action (compression, shear or friction) that allows breaking intramolecular bonds and further chemical reactions. More in detail, radical species are mechanochemically produced due to the breaking of weak bonds and to the extreme surface plasma conditions generated by the mechanical impact. Mechanochemical activation processes have a long history, and their importance does not decrease with time, because many solid-phase reactions (with stoichiometric amount of reactants) can be quantitatively and rapidly promoted under solvent-free conditions [109]. Therefore, the mechanochemical mixing process can be an environmentally advantageous alternative to conventional chemical syntheses, in which solvents often play a key role in energy dispersion and chemicals solvation and transportation [110]. The preparation of nanomaterials for advanced applications by mechanochemical synthesis is among the most suitable alternative routes because of its simplicity and high reproducibility. Indeed, such methodology is also environmentally advantageous in comparison to conventional preparation procedures, since it is often solvent-free, and it is carried out under mild reaction conditions and involving short reaction times.

4.2. MC as a Tool for the Synthesis of Catalysts The MC synthesis of heterogeneous catalysts strongly affects their catalytic properties. Indeed, the excess of potential energy produced during grinding, together with shear and friction forces, can deeply modify the materials by producing a large variety of defects, hence improving or modifying the reactivity [109,111]. Catalysts 2018, 8, 262 25 of 35

A variety of heterogeneous catalysts, ranging from supported NPs to nanocomposites has been obtained by using MC synthesis, as summarised in Scheme9[109]. Catalysts 2018, 8, x FOR PEER REVIEW 25 of 35

Scheme 9. Pictorial representation of different types of MC synthesized (nano)materials. From Scheme 9. Pictorial representation of different types of MC synthesized (nano)materials. supported metal NPs, composite nanomaterials to metal oxide NPs and metal organic frameworks From supported metal NPs, composite nanomaterials to metal oxide NPs and metal organic frameworks (MOFs) including covalent organic frameworks (COF). Examples of relevant materials include zinc- (MOFs) including covalent organic frameworks (COF). Examples of relevant materials include based ZIF-8 structures (BIT-11), Cu(INA)2, and Cu-containing HKUST-1, as well as supported noble zinc-basedmetals on ZIF-8 aluminosilicates, structures (BIT-11), graphene, Cu(INA) etc. Reprinted2, and from Cu-containing Ref. [109]. Copyright HKUST-1, 2015 as with well permission as supported nobleof metals the Royal on Society aluminosilicates, of Chemistry. graphene, etc. Reprinted from Ref. [109]. Copyright 2015 with permission of the Royal Society of Chemistry. MC catalyst activation has been successfully applied for many years. Mori et al. [112] investigated the influence of the mechanical milling of MgO-mixed catalysts of Ru, Ni, Fe, or their MC catalyst activation has been successfully applied for many years. Mori et al. [112] investigated combinations at temperatures ranging from 80 to 150 °C under initial pressures of 100 Torr CO2 and the influence of the mechanical milling of MgO-mixed catalysts of Ru, Ni, Fe, or their combinations 500 Torr H2 on the hydrogenation of CO◦ 2 to CH4. They found that the CH4 yield was related to the at temperaturesnature of the ranging catalyst from (Ru/MgO 80 to gave150 C the under best yield, initial whilst pressures Ni/Fe of showed 100 Torr the CO worst2 and activity). 500 Torr H2 on theInterestingly, hydrogenation the MC of COactivation2 to CH produced4. They found catalysts that with the CHimproved4 yield CH was4 production related to therate nature and of the catalystsignificantly (Ru/MgO lowered gave the reaction the best activation yield, whilst energy. Ni/Fe Indeed, showed an apparent the worst activation activity). energy Interestingly, of 39 kJ the MCmol activation−1 and kJ mol produced−1 was accomplished catalysts with for Ni improved-Fe-MgO CHand4 Ruproduction-MgO catalysts, rate andrespectively, significantly upon loweredthe the reactionmilling activation. activation Such energy. values Indeed, were analmost apparent halved activation with respect energy to the of same 39 kJ systems mol−1 producedand kJ mol −1 was accomplishedwithout mechanical for Ni-Fe-MgOactivation. Interestingly, and Ru-MgO the authors catalysts, found respectively, that the catalysts upon submitted the milling to further activation. Suchmilling values activation were almost pre-treatment halved withdid not respect shown toany the improvement same systems in the produced activity, suggesting without that mechanical the milling during reaction played the major role. SEM revealed an increased number of larger particles activation. Interestingly, the authors found that the catalysts submitted to further milling activation at increasing milling time, due to small particle agglomeration during milling. However, no pre-treatment did not shown any improvement in the activity, suggesting that the milling during significant increase of surface area was observed. Electron probe microanalysis (EPMA) results reactionrevealed played uniform the major distribution role. SEM and revealed aggregation an increased of Ni, Fe, number and MgO of larger particles particles during at mil increasingling, millingindicating time, due that to the small activ particleity had agglomeration to be ascribed to during changes milling. in the However, surface structure no significant rather than increase to of surfaceincreased area was surface observed. area. Electron probe microanalysis (EPMA) results revealed uniform distribution and aggregationIn the same of Ni, years, Fe, and another MgO study particles reported during on the milling, influence indicating of the MC that activation the activity of several had to be ascribedtraditional to changes catalysts in the such surface as Ni, structure Zr, Zr-Ni rather-containing than to materials, increased Zr surface hydride area., and different Zr-Ni Inhydrides the same employed years, in another the hydrogenation study reported of CO and on of the CO influence2 [113]. Inactive of the NiO MC and activation ZrO2 were of also several traditionalinvestigated catalysts for comparison such as Ni, purposes. Zr, Zr-Ni-containing In particular, the study materials, underlined Zr hydride, the difference and between different the Zr-Ni traditional catalytic hydrogenation and the mechanically induced hydrogenation to CH4. The study hydrides employed in the hydrogenation of CO and of CO [113]. Inactive NiO and ZrO were also was carried out by employing a flow-milling vial as a catalytic2 reactor. The authors found that 2Zr and investigated for comparison purposes. In particular, the study underlined the difference between the Zr-Ni hydrides were the most active in CH4 production, despite the fact the hydrogen present in the traditional catalytic hydrogenation and the mechanically induced hydrogenation to CH . The study mixture had a negative effect, since it overcame the CH4 formation. On the contrary, Ni4 -based was carriedcatalysts out were by unable employing to produce a flow-milling CH4, but were vial active as a catalytic in CO disproportionation. reactor. The authors Zirconium found hydride that Zr and Zr-Niwas hydrides the only were one that the mostshowed active activity in CHfor 4COproduction,2 hydrogenation. despite Unexpectedly, the fact the the hydrogen production present of CH in4 the mixturewas had observed a negative on NiO effect, and since ZrO it2 overcame. The differences the CH 4 information. activity were On the ascribed contrary, to Ni-basedthe structural catalysts weretransformations unable to produce of the CH metal4, but and were hydride active catalysts in CO disproportionation. under milling, i.e., Zirconiumto the presenc hydridee of low was- the coordinate centers upon MC treatment that were able to efficiently activate CO. only one that showed activity for CO2 hydrogenation. Unexpectedly, the production of CH4 was

Catalysts 2018, 8, 262 26 of 35

observed on NiO and ZrO2. The differences in activity were ascribed to the structural transformations of the metal and hydride catalysts under milling, i.e., to the presence of low-coordinate centers upon MC treatment that were able to efficiently activate CO. MC synthesis was also proven to be effective for obtaining nanophase carbides by facile ball milling of elemental iron and carbon powders at room temperature. Such catalysts were tested in the hydrogenation of CO2 and showed superior activity and selectivity with respect to Fe/C mixtures and coarse-grained conventionally synthesized carbides [114]. It was proposed that nanocrystalline iron carbides with sizes ranging between 8 and 16 nm were the catalytically active species. The presence of defects and/or highly active sites, strained regions, and grain boundaries formed during the grinding guaranteed effective hydrocarbons chain propagation. Interestingly, also a commercial cementite powder displayed some activity upon the ball-milling activation. Re–Co/Al2O3 catalysts were synthesized via ball milling by using tungsten carbide balls in a tungsten carbide container [115]. In contrast to the preparation of catalysts containing noble metal NPs, in which an oxide precursor or a metal salt are usually employed, the preparation of catalysts with transition metal involves the direct milling of the metal precursors. The obtained bimetallic Re–Co-containing catalysts were more active in the CH4 conversion than in the CO hydrogenation, whilst the activity of the catalysts prepared by incipient wetness impregnation was exactly the opposite. Such difference was explained by assuming that crystallinity in MC prepared material was introduced upon the activation heat treatment at 650 ◦C. Indeed, X-ray powder diffraction indicated the presence of disordered Re–Co phases before the heat treatment. Similar systems, such as Co–Fe and Ni catalysts supported on ZrO2 and TiO2, were also prepared by ball milling and showed enhanced catalytic activity in CO hydrogenation [116]. It is worth noting that upon mechanical treatment of TiO2 in the anatase form, the authors observed the transition to brookite and rutile already at room temperature. The activity was ascribed to Co50Fe50 and Ni domains with average size of about 30 and 50 nm, distributed on the supports. XRD measurements confirmed that the mechanochemical dispersion of the Co50Fe50 and Ni phases on TiO2 and ZrO2 prevented the Co, Fe, and Ni oxidation by reaction with the supports. Oxidation of such species usually takes place during calcination during preparation of the catalysts. Among the most effective and economic materials able to hydrogenate CO2 to CH3OH, Cu–Zn-based catalysts have been extensively studied [117–121]. Usually, commercial Cu–Zn catalysts are prepared by co-precipitation on Al2O3 support: such method implies several steps (precipitation, aging, filtration, drying, calcination, and reduction). Along with specific surface area and CuO and ZnO distribution, such steps have a strong influence on the activity of the final catalyst [122–125]. In particular, the distribution of CuO and ZnO has pivotal relevance during the aging, 2+ 2+ because the ion exchange between Cu in malachite (Cu2CO3(OH)2) and Zn in hydrozincite (Zn5(OH)6(CO3)2) occurs, and the formation of zincian malachite ((Cu,Zn)2CO3(OH)2) and/or aurichalcite ((Cu,Zn)5(OH)6(CO3)2) hydroxy carbonate precursors are extremely sensitive to pH, duration, temperature, and stirring [126–128]. To overcome such problem, very recently a new mechanical-force-driven, solid-state ion-exchange procedure has been refined [129]. It was found that the mutual substitution between Cu2+ and Zn2+ was improved with increasing mechanical ball-milling speed from 200 to 400 rpm, leading to significant enhancement of the catalytic activity. The XRD characterization shown in Figure 18 provided evidence of the presence of peaks related either to the CuO phase or the ZnO phase. Such peaks become broader and at the same time decrease in intensity upon the increase of the milling speed, indicating a decrease in crystallinity and crystal size. The absence of the peaks related to the presence of the Al2O3 phase was explained by the alumina change into an amorphous state after ball-milling [126,129]. The results of TEM measurements performed on the CZA400 catalyst reported in Figure 19 indicated a close cross-distribution of CuO and ZnO due to the occurrence of the solid-state ion exchange, in which ZnO NPs act as spacers, thereby avoiding the CuO agglomeration (as shown in Figure 19b). Catalysts 2018, 8, 262 27 of 35 Catalysts 2018, 8, x FOR PEER REVIEW 27 of 35 Catalysts 2018, 8, x FOR PEER REVIEW 27 of 35

Figure 19. TEM (a) and HRTEM (b) images of CuO/ZnO/Al2O3 catalyst prepared at 400 rpm. FigureFigure 19.19. TEM ( (aa)) and and HRTEM HRTEM (b (b)) images images of of CuO/ZnO/Al CuO/ZnO/Al2OO33 catalystcatalyst prepared prepared at 400 rpm. Reprinted from Wu, W.; Xie, K.; Sun, D.; Li, X.; Fang, F. CuO/ZnO/Al2O3 Catalyst Prepared by ReprintedReprinted from from Wu, Wu, W.; W.; Xie, Xie, K.; K.; Sun, Sun, D.; D.; Li, Li, X.; X.; Fang, Fang, F. F. CuO/ZnO/Al CuO/ZnO/Al22OO33 CatalystCatalyst Prepared Prepared by by Mechanical-Force-Driven Solid-State Ion Exchange and Its Excellent Catalytic Activity under Internal Mechanical-Force-DrivenMechanical-Force-Driven Solid-StateSolid-State IonIon ExchangeExchange andand ItsIts ExcellentExcellent CatalyticCatalytic ActivityActivity underunder InternalInternal Cooling Condition. Ind. Eng. Chem. Res. 56, 8216–8223. Copyright (2017) American Chemical Society. CoolingCooling Condition.Condition. Ind.Ind. Eng.Eng. Chem.Chem. Res. 56,56, 8216–8223.8216–8223. Copyright (2017) Ame Americanrican Chemical Society. Such alternation of CuO and ZnO NPs represents the result of the complete ion exchange SuchSuch alternation alternation of of CuO CuO and and ZnO ZnO NPs NP representss represents the result the resultof the complete of the complete ion exchange ion exchange between 2+ 2+ 2 between2+ Cu 2+2+ and Zn 2+ in the precursors. For the CZA400 catalyst, impressive 59.5% CO conversion Cubetweenand Cu Zn andin theZn precursors. in the precursors. For the For CZA400 the CZA400 catalyst, catalyst impressive, impressive 59.5% 59.5% CO2 conversion CO2 conversion and and 73.4% CH3OH selectivity were obtained at◦ 240 °C and 4 MPa, resulting in a noticeable yield to 73.4%and 73.4% CH3OH CH selectivity3OH selectivity were obtainedwere obtained at 240 atC 24 and0 °C 4 MPa, and 4 resulting MPa, resulting in a noticeable in a noticeable yield to CHyield3OH to CH3OH of 43.7%. Therefore, this mechanical-force-driven, solid-state ion-exchange method proved ofCH 43.7%.3OH of Therefore, 43.7%. Therefore, this mechanical-force-driven, this mechanical-force-driven solid-state, solid ion-exchange-state ion-exchange method method proved proved to be to be a valuable alternative to synthesizing CZA catalysts with improved practical application ato valuable be a valuable alternative alternative to synthesizing to synthesiz CZA catalystsing CZA with catalysts improved with practical improved application practical potential application for potential for chemical hydrogenation of CO2 to CH3OH. chemicalpotential hydrogenationfor chemical hydrogenation of CO to CH ofOH. CO2 to CH3OH. In another very recent study,2 Au clusters3 were directly deposited onto a functionalized support InIn anotheranother very very recent recent study, study, Au Au clusters clusters were were directly directly deposited deposited onto onto a functionalized a functionalized support support by by newly developed, milling‐mediated solid reduction procedure [130]. The proposed method newlyby newly developed, developed milling-mediated, milling‐mediated solid reduction solid reduction procedure procedure [130]. The [130] proposed. The proposed method involved method involved the use of HAuCl4 as Au precursor, which was pre‐adsorbed on the Schiff base modified theinvolved use of HAuClthe use4 ofas HAuCl Au precursor,4 as Au whichprecursor, was pre-adsorbedwhich was pre‐adsorbed on the Schiff on base the modiﬁed Schiff base silica modified surface silica surface by electrostatic interaction in water. After grinding the dried carrier with solid NaBH4, bysilica electrostatic surface by interaction electrostatic in interaction water. After in grinding water. After thedried grinding carrier the withdried solid carrier NaBH wit4h, solid according NaBH to4, according to Scheme 10, the Au precursor can be reduced in situ by atomic H coming from the NaBH4 Schemeaccording 10 ,to the Scheme Au precursor 10, the Au can precursor be reduced can inbe situreduced by atomic in situ H by coming atomic fromH coming the NaBH from thereducing NaBH4 reducing agent in the solid state. Depending on the gold loading, highly dispersed clusters4 and agentreducing in the agent solid in state. the solid Depending state. Depending on the gold on loading, the gold highly loading, dispersed highly clusters dispersed and clusters isolated and Au isolated Au atoms can be obtained by solid grinding. atomsisolated can Au be atoms obtained can bybe solidobtained grinding. by solid grinding.

Scheme 10. Illustration of the in situ construction of ultrafine Au clusters on a nitrogenous carrier Scheme 10. Illustration of the in situ construction of ultrafine Au clusters on a nitrogenous carrier Schemeusing the 10. solidIllustration reduction of method. the in situ Reprinted construction from of Ref. ultraﬁne [130]. Au Copyright clusters 2018 on a withnitrogenous permission carrier of using the solid reduction method. Reprinted from Ref. [130]. Copyright 2018 with permission of usingWiley. the solid reduction method. Reprinted from Ref. [130]. Copyright 2018 with permission of Wiley. Wiley. Following such such activation, activation, it was it was found found that the that Au the species Au were species dispersed were asdispersed 1.0 nmnanoclusters as 1.0 nm Following such activation, it was found that the Au species were dispersed as 1.0 nm (Figurenanoclusters 20a), as (Figure determined 20a), byas thedetermined aberration-corrected, by the aberration‐corrected HAADF-STEM (high-angle, HAADF‐STEM annular (high‐angle dark-ﬁeld nanoclusters (Figure 20a), as determined by the aberration‐corrected, HAADF‐STEM (high‐angle scanningannular dark‐field transmission scanning electron transmission microscopy) electron analysis. microscopy) At the same analysis. time, aAt lot the of sam isolatede time, Au a atomslot of annular dark‐field scanning transmission electron microscopy) analysis. At the same time, a lot of wereisolated detected, Au atoms even were if as a detected minor fraction, even if of as the a total minor metal fraction content. of the It was total proposed metal content. that such It was Au isolated Au atoms were detected, even if as a minor fraction of the total metal content. It was subnanoclustersproposed that such were Au effectivelysubnanoclusters stabilized were by effectively the Schiff stabilized base groups by the present Schiff onbase the groups silica present surface proposed that such Au subnanoclusters were effectively stabilized by the Schiff base groups present byon the providing silica surface strong by electronic providing interaction. strong electronic Differently, interaction. a variety Diffe ofrently, Au shapesa variety and of sizesAu shapes were on the silica surface by providing strong electronic interaction. Differently, a variety of Au shapes and sizes were observed on the support (Figure 20b) when an Au-containing solution was reduced and sizes were observed on the support (Figure 20b) when an Au-containing solution was reduced by NaBH4. The obtained statistical particle size distributions were in agreement with the L3‐edge by NaBH4. The obtained statistical particle size distributions were in agreement with the L3‐edge

Catalysts 2018, 8, 262 28 of 35

observedCatalysts 2018 on, 8,the x FOR support PEER REVIEW (Figure 20b) when an Au-containing solution was reduced by NaBH28 of 435. The obtained statistical particle size distributions were in agreement with the L3-edge extended X-rayextended absorption X‐ray absorption ﬁne structure fine (EXAFS,structure Figure (EXAFS, 20c), Figure revealing 20c), anrevealing increase an in increase intensity in ofintensity the peak of relatedthe peak to therelated ﬁrst to shell, the asfirst well shell as, in as the well Au as coordination in the Au coordination number when number the conventional when the conventional wet method waswet employed.method was Inemployed. addition, In a addition, decrease a in decrease the white in the line white intensity line intensity compared compared with the wi Auth foilthe Au in thefoil X-rayin the absorption X‐ray absorption near-edge near‐edge structure structure (XANES) (XANES) spectra spectra obtained obtained for the for Au the NPs Au (FigureNPs (Figure 20d) is20 and) indication is an indication that the that functionalization the functionalization of the silica of the surface silica with surface nitrogen with allowednitrogen one allowed to optimize one to theoptimize Au d-electron the Au d‐electron density [131 density]. [131].

FigureFigure 20. 20. Aberration-correctedAberration‐corrected HAADF-STEM HAADF‐STEM images images of of Au/SiO Au/SiO22‐Schiff-Schiff prepared by by ( (aa)) solid solid reductionreduction method method and and (b ) ( wetb) wet chemistry chemistry method, method, respectively. respectively. The NPs, The subnanoclusters, NPs, subnanoclusters and isolated, and Auisolated atoms Au were atoms indicated were indicated by white circles,by white triangles, circles, andtriangles squares,, and respectively. squares, respectively. (c) Fourier transform(c) Fourier 3 oftransform k -weighted of k3‐weighted EXAFS spectra EXAFS and spectra (d) normalized and (d) normalized Au L3-edge Au XANESL3‐edge XANES spectra ofspectra Au/SiO of Au/SiO2-Schiff2‐ catalystsSchiff catalysts prepared prepared by wet chemistryby wet chemistry and solid and reduction solid reduction method, respectively.method, respectively. Reprinted Reprinted from Ref. [ 130from]. CopyrightRef. [130]. 2018Copyright with permission 2018 with permission of Wiley. of Wiley.

TheseThese features features put put in in evidence evidence that that the the opportunity opportunity to to use use the the solvent-free solvent‐free solid solid reduction reduction synthesissynthesis allowsallows the preventi preventionon of of Au Au growth growth;; indeed indeed,, crystal crystal nucleation nucleation wa wass hampered hampered,, giving giving rise riseto either to either subnanoclusters subnanoclusters or singl or single-atoms,e‐atoms, depending depending on onthe the Au Au content. content. The The catalysts catalysts were were tested tested in inthe the hydrogenation hydrogenation of of CO CO2 2toto formate, formate, and and a a marked marked dependence dependence on on the the Au Au size, due toto thethe differentdifferent electronicelectronic structure structure of ofthe the metal metal species, species, was observed. was observed. The authors The authors claimed claimed that the milling-mediated that the milling‐ solidmediated reduction solid procedurereduction couldprocedure be considered could be asconsidered a general as methodology a general methodology to achieve basic to achieve knowledge basic ionknowledge the catalysis ion the by catalysis Au, providing by Au new, providing insights new for insights supported for catalystsupported design catalyst for reactionsdesign for involving reactions theinvolving CO2 transformation. the CO2 transformation.

5.5. FinalFinal RemarksRemarks TheThe mainmain purpose purpose of of this this review review was was to to point point out out the the possibilities possibilities offered offered by by microwave, microwave, ultrasound,ultrasound, and and mechanochemical mechanochemical protocols protocols in materials in materials engineering engineering at a nanoscale at a nanoscale level for level catalytic for catalyticapplications. applications. Summarising,Summarising, concerningconcerning thethe synthesissynthesis ofof COCO22 reductionreduction catalysts,catalysts, assistance assistance of of microwave, microwave, ultrasound,ultrasound,and and mechanochemistrymechanochemistry allowsallowsone one toto obtainobtain highlyhighly disperseddispersed activeactive sitessites andand controlledcontrolled particle dimension, and in most cases reinforces the active site/support interaction, giving rise to synergistic effects that help CO2 reduction and improve catalyst stability. The procedures reported are usually simple, and require relatively short reaction times (minutes) without implying many steps. Moreover, stronger and/or more abundant basic sites occurring at the support surface favour

Catalysts 2018, 8, 262 29 of 35 particle dimension, and in most cases reinforces the active site/support interaction, giving rise to synergistic effects that help CO2 reduction and improve catalyst stability. The procedures reported are usually simple, and require relatively short reaction times (minutes) without implying many steps. Moreover, stronger and/or more abundant basic sites occurring at the support surface favour CO2 adsorption, a key step for its further reduction. Complex systems (hierarchical structures, composites, bi-functional catalysts, etc.) may be obtained by relatively simple methods. Finally, a further crucial aspect concerns the effect of microwave irradiation during reaction, whilst carbon deposition usually leads to catalyst deactivation, under microwave the same carbon deposits may act as hot spots per se favouring the reaction.

Funding: This research was funded by the Università degli Studi di Torino (Progetto Ricerca Locale 2016), and Politecnico di Torino (Progetto Ricerca di base 2017). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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