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 onesones [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]) solid offer somecatalysts practical [31]) advantagesoffer 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 methods by methods that that are are able able to to control control and and tailor tailor the the physico physico-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 surface concerned., is Notwithstandingconcerned. Notwithstanding the plethora ofthe works plethora concerning of works CO2 reduction,concerning a thoroughCO2 reduction understanding, a thorough of the mechanismsunderstanding is stillof the lacking mechanisms and/or debatedis still lacking [18]. As and/ faror as debated heterogeneous [18]. As catalysts far as heter are 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