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catalysts Review Looking for the “Dream Catalyst” for Hydrogen Peroxide Production from Hydrogen and Oxygen Federica Menegazzo 1,* , Michela Signoretto 1 , Elena Ghedini 1 and Giorgio Strukul 2,* 1 CATMAT Lab, Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice and INSTM RUVe, via Torino 155, 30172 Venezia Mestre, Italy; [email protected] (M.S.); [email protected] (E.G.) 2 Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, Via Torino 155, 30172 Venezia Mestre, Italy * Correspondence: [email protected] (F.M.); [email protected] (G.S.); Tel.: +39-041-234-8551 (F.M.); +39-41-234-8931 (G.S.) Received: 16 January 2019; Accepted: 4 March 2019; Published: 11 March 2019 Abstract: The reaction between hydrogen and oxygen is in principle the simplest method to form hydrogen peroxide, but it is still a “dream process”, thus needing a “dream catalyst”. The aim of this review is to analyze critically the different heterogeneous catalysts used for the direct synthesis of H2O2 trying to determine the features that the ideal or “dream catalyst” should possess. This analysis will refer specifically to the following points: (i) the choice of the metal; (ii) the metal promoters used to improve the activity and/or the selectivity; (iii) the role of different supports and their acidic properties; (iv) the addition of halide promoters to inhibit undesired side reactions; (v) the addition of other promoters; (vi) the effects of particle morphology; and (vii) the effects of different synthetic methods on catalyst morphology and performance. Keywords: hydrogen peroxide; hydrogen; oxygen; palladium catalysts; palladium alloys; acidic supports; green processes 1. Introduction Hydrogen peroxide (H2O2) is considered a green oxidant and has been listed as one of the 100 most important chemicals in the world [1]. However, the method by which it is manufactured is far from being considered a sustainable one. The process via anthraquinone autoxidation is now over 70 years old but still the only one commercially used to produce H2O2, even if it suffers from many drawbacks including the disposal of considerable amounts of toxic organic wastes and the need to be run in large plants to compensate for the high operating costs. Therefore, despite its advantages as a clean oxidant, H2O2 is not produced via an environmentally friendly process and is still rather costly (current price is about 500 US $ per ton for 50% hydrogen peroxide [2]), thus not economically competitive for the production of bulk chemicals or for a more widespread use in wastewater treatment. For these reasons, the development of a green process for the synthesis of H2O2 is of both environmental and economic importance. Among the various alternative approaches to produce H2O2, the direct reaction between hydrogen and oxygen to H2O2 is conceptually the most straightforward and therefore the most attractive one. However, despite a long history and significant R&D efforts by many academic groups and industrial companies, the process still has a long way to go before reaching the commercial scale. A widespread production of H2O2 from hydrogen and oxygen in small/medium scale plants, integrated with its direct use in catalytic selective oxidation, is a good example of the future direction of sustainable chemistry [3]: on-site H2O2 production would significantly boost the use of H2O2 in all fine chemical productions, with a relevant positive impact in terms of reduction of waste, improvement of eco-compatibility, safety problems and working conditions. Catalysts 2019, 9, 251; doi:10.3390/catal9030251 www.mdpi.com/journal/catalysts Catalysts 2018, 8, x FOR PEER REVIEW 2 of 33 Catalysts 2019, 9, 251 2 of 32 relevant positive impact in terms of reduction of waste, improvement of eco-compatibility, safety problems and working conditions. InIn 1914, 1914, Henkel Henkel and and Weber Weber [4] [ 4were] were the the first first to propose to propose for this for process this process the use the of use noble of metal noble catalysts.metal catalysts. However, However, the low theH2O low2 selectivity H2O2 selectivity and in parallel and in the parallel high selectivity the high selectivityfor water formation for water was,formation and still was, is, anda major still obstacle is, a major to obstacleits industrial to its feasibility. industrial H feasibility.2O2 formation H2O is2 formationinvolved in is a involved scheme ofin parallel a scheme and of consecutive parallel and reactions consecutive (Figure reactions 1) where (Figure the catalyst1) where leading the catalystto H2O2 (reaction leading to1) His also2O2 responsible(reaction 1) isfor also its responsibledecomposition. for its In decomposition. fact, H2O2 is unstable In fact, Hwith2O2 isrespect unstable to both with respecthydrogenation to both (reactionhydrogenation 2) and (reaction decomposition 2) and decomposition (reaction 3), (reaction while 3),water while waterformation formation is by is byfar far the the most thermodynamicallythermodynamically favored favored reaction reaction between hydr hydrogenogen and oxygen (reaction(reaction 4).4). Therefore, the challenge hashas alwaysalways been been to to find find a catalysta catalyst capable capable of maximizingof maximizing reaction reaction (1) while (1) while depressing depressing paths paths(2), (3) (2), and (3) (4): and (4): -211 KJ/mol H2 H2O2 2 H2O -136 KJ/mol 2 1 3 -106 KJ/mol H2 + O2 4 -242 KJ/mol H O + 1/2 O 2 2 FigureFigure 1. 1. TheThe different different reactions reactions involved involved wh whenen reacting hydrogen and oxygen. As a result, this process is a typical example in which the activity and selectivity of catalysts should be preciselyprecisely balancedbalanced in in a a framework framework of of high high efficiency efficiency [5 ].[5]. This This selectivity selectivity problem problem has has not not yet yetbeen been solved solved in a in satisfactory a satisfactory manner manner and stilland representsstill represents one ofone the of major the major barriers barriers to the to use the of use H2 Oof2 Hprepared2O2 prepared from hydrogenfrom hydrogen and oxygen. and oxygen. Furthermore,Furthermore, the direct contact between H2 andand O 22 constitutesconstitutes a a significan significantt hazard hazard [6,7] [6,7] because because H22/O/O2 2mixturesmixtures are are explosive explosive in in a a wide wide range range of of co compositions,mpositions, hence hence operating operating under intrinsically safe conditions conditions is is a a very very important important issue issue for for the the viability viability of ofthe the process. process. The The flammability flammability limits limits of H of2 inH 2Oin2 are O2 arebetween between 4% 4%and and 94%, 94%, implying implying that that intrinsically intrinsically safe safe mixtures mixtures are are very very diluted. diluted. Clearly, Clearly, operating underunder safe safe conditions conditions results results in a dropin a of drop the H 2ofO 2 theproductivity H2O2 productivity to industrially to unacceptableindustrially unacceptablevalues [8]. For values all these [8]. For reasons, all these the processreasons, hasthe notprocess yet beenhas not commercialized yet been commercialized [5,9–13], despite [5,9–13], the despiteefforts ofthe several efforts chemical of several companies chemical thatcompanies believe th theat developmentbelieve the development on an industrial on an scale industrial would scale be a wouldreal breakthrough be a real breakthrough in oxidation technologies.in oxidation Typicaltechnologies. reaction Typical conditions reaction are: conditions diluted H2 /Oare:2 mixturediluted ◦ Hfeed2/O2 gas, mixture supported feed gas, noble supported metal catalysts, noble metal acid catalysts, promoters, acid methanol promoters, as methanol solvent, near as solvent, 0 C reaction near 0 °Ctemperature, reaction temperature, atmospheric oratmospheric positive pressure or positi andve gas–liquid–solid pressure and three-phase gas–liquid–solid reaction. three-phase The safety reaction.can be improved The safety mainly can be by improved adding an mainly inert gas, by addi usingng alternative an inert gas, solvents using suchalternative as supercritical solvents such CO2, aslimiting supercritical the hydrogen CO2, limiting amount the to hydrogen less than 4%amount and using to less membrane than 4% and catalysts. using membrane catalysts. All in all, even if the direct synthesis is in principle the simplest method to form hydrogen peroxide, it it is is still still a a “dream “dream process” process” that that needs needs a “dream a “dream catalyst”. catalyst”. The The aim aim of this of this review review is to is see to whethersee whether this this“dream “dream catalyst” catalyst” is at is athand hand analyz analyzinging the the different different proposals proposals present present in in the the recent literatureliterature andand makingmaking evident evident the the role role of of the the following followin factors:g factors: (i) the(i) choicethe choice of the of metal; the metal; (ii) the (ii) metal the metalpromoters promoters and alloys and usedalloys to used improve to improve the activity the acti and/orvity theand/or selectivity; the selectivity; (iii) the role(iii) ofthe different role of differentsupports andsupports their acidicand their properties; acidic (iv)properties; the addition (iv) ofthe halide addition and otherof halide promoters and other to inhibit promoters undesired to inhibitside reactions; undesired (v) theside effects reactions; of particle
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