Boosting the Characterization of Heterogeneous Catalysts for H2O2 Direct Synthesis by Infrared Spectroscopy

catalysts

Review Boosting the Characterization of Heterogeneous Catalysts for H2O2 Direct Synthesis by Infrared Spectroscopy

Maela Manzoli

Department of Drug Science and Technology, University of Turin, Via Pietro Giuria 9, 10125 Turin, Italy; [email protected]; Tel.: +39-011-670-6663 Received: 30 November 2018; Accepted: 19 December 2018; Published: 2 January 2019

Abstract: Infrared (IR) spectroscopy is among the most powerful spectroscopic techniques available for the morphological and physico-chemical characterization of catalytic systems, since it provides information on (i) the surface sites at an atomic level, (ii) the nature and structure of the surface or adsorbed species, as well as (iii) the strength of the chemical bonds and (iv) the reaction mechanism. In this review, an overview of the main contributions that have been determined, starting from IR absorption spectroscopy studies of catalytic systems for H2O2 direct synthesis, is given. Which kind of information can be extracted from IR data? IR spectroscopy detects the vibrational transitions induced in a material by interaction with an electromagnetic field in the IR range. To be IR active, a change in the dipole moment of the species must occur, according to well-defined selection rules. The discussion will be focused on the advancing research in the use of probe molecules to identify (and possibly, quantify) specific catalytic sites. The experiments that will be presented and discussed have been carried out mainly in the mid-IR frequency range, between approximately 700 and 4000 cm−1, in which most of the molecular vibrations absorb light. Some challenging possibilities of utilizing IR spectroscopy for future characterization have also been envisaged.

Keywords: H2O2 direct synthesis; IR spectroscopy; FTIR characterization; Diffuse Reﬂectance Fourier Transform IR characterization; catalyst characterization; Pd catalysts; AuPd catalysts; bimetallic catalysts

1. The Direct Synthesis of Hydrogen Peroxide from Molecular Hydrogen and Oxygen Hydrogen peroxide is a benign and environmentally friendly oxidising reagent extensively employed in the production of both fine and bulk chemicals. Moreover, it finds application in water treatment, paper and pulp bleaching, textiles, and in large-scale selective oxidation processes, such as the epoxidation of olefins, the hydroxylation of aromatics, and the synthesis of cyclohexanone oxime, which is strategic for nylon-6 production (Figure1)[ 1]. These applications would greatly benefit from on-site, moderate-scale production facilities in order to reduce transport costs. However, H2O2 is produced worldwide on a multi-million ton scale annually by the energy intensive Riedl−Pfleiderer anthraquinone oxidation process [2]. Significant amounts of organic waste due to the anthraquinone, along with several energy consuming separation and concentration steps, and the use of benzene as the solvent are the main drawbacks of the process, which is economically feasible only in large-scale plants. In this frame, the research is devoted to the development of new economic scalable methods for the direct synthesis of H2O2 starting from hydrogen and oxygen and to the optimization of efficient oxidation catalysts able to work in the presence of H2O2 for sustainable production.

Catalysts 2019, 9, 30; doi:10.3390/catal9010030 www.mdpi.com/journal/catalysts Catalysts 2019, 9, 30 2 of 34

Catalysts 2018, 8, x FOR PEER REVIEW 2 of 36

44 Figure 1. Main applications of H2O2. Figure 1. Main applications of H2O2.

45 The direct synthesis reaction can be considered as a dream reaction involving molecular H2 and The46 direct O2, synthesisin which the reaction formationcan of H be2O2 consideredtakes place upon as aproton dream−electron reaction transfer involving to O2 and molecular*OOH H2 and 47 intermediates. Conversely, the formation of H2O implies the O–O bond cleavage of the surface O2, in which43 the formation of H2O2 takes place upon proton−electron transfer to O2 and *OOH 48 *OOH species [3]. As a matter of fact, H2O2 selectivity is ruled by the competitive O–H bond intermediates.4449 formation Conversely, and O–O the bond formation cleavage.Figure of 1. H Main2O applications implies theof H2 O–OO2. bond cleavage of the surface *OOH species [503]. As aH matter2O2 direct of fact,synthesis H2 Ocan2 selectivity potentially halve is ruled the bycost thewith competitive respect to the O–H anthraquinone bond formation and 45 The direct synthesis reaction can be considered as a dream reaction involving molecular H2 and O–O bond51 cleavage.commercial process, resulting in a much lower environmental impact, also due to the possibility to 4652 Oavoid2, in whichchlorine the for formation oxidation of chemistry H2O2 takes [1,4]. place In uponaddition, proton the−electron direct synthesistransfer toreaction O2 and has *OOH the H 47O directintermediates. synthesis Conversely, can potentially the formation halve of H the2O costimplies with the respectO–O bond to cleavage the anthraquinone of the surface commercial 2 532 potential to produce inexpensive H2O2 at the mid-scale in geographically distributed facilities, process,4854 resulting *OOHbecause inspecies such a much facilities[3]. As lower awould matter environmental have of fact,much H lower2O2 selectivity impact,capital and is also ruledoperating due by tothe costs thecompetitive than possibility Riedl-Pfleiderer O–H tobond avoid chlorine 49 formation and O–O bond cleavage. for oxidation55 plants chemistry with comparable [1,4]. In dimension addition, [5,6]. the Nevertheless, direct synthesis no alternative reaction process has for the H potential2O2 direct to produce 5056 synthesisH2O2 hasdirect yet synthesisbeen marketed, can potentially despite several halve published the cost patentswith respect [2,7–11] to and the literature anthraquinone papers inexpensive5157 commercial[10,12–25]. H2O2 at Indeed, process, the mid-scale selectivity resulting inisin stilla much geographically far fromlower bein environmentalg high distributed and mustimpact, be alsosignificantly facilities, due to the improved becausepossibility for to sucha facilities 52 would have58 avoid muchsuccessful chlorine lower industrial for capital oxidation exploitation. and chemistry operating Indeed, [1,4]. wate costsInr is addition, thethan most thethermodynamically Riedl-Pﬂeiderer direct synthesis favouredreaction plants producthas with the comparable 53 potential to produce inexpensive H2O2 at the mid-scale in geographically distributed facilities, 59 (as shown in 2) and the H2-O2 gas mixtures are explosive in a very broad range of compositions (4– dimension [5,6]. Nevertheless, no alternative process for H2O2 direct synthesis has yet been marketed, 5460 because96%), posing such severefacilities limitations would have to the much practi lowercability cap ofital the andprocess operating under safecosts conditions. than Riedl-Pfleiderer despite5561 several plants published with comparable patents dimension [2,7–11 ][5,6]. and Nevertheless, literature papersno alternative [10,12 process–25]. Indeed,for H2O2 direct selectivity is still far from56 being synthesis high and has yet must been be marketed, signiﬁcantly despite improvedseveral published for a patents successful [2,7–11] industrial and literature exploitation. papers Indeed, 57 [10,12–25]. Indeed, selectivity is still far from being high and must be significantly improved for a water is58 the mostsuccessful thermodynamically industrial exploitation. Indeed, favoured water product is the most (as thermodynamically shown in Figure favoured2) and product the H 2-O2 gas mixtures59 are (as explosive shown in in2) and a very the H broad2-O2 gas rangemixtures of are compositions explosive in a very (4–96%), broad range posing of compositions severe limitations (4– to the practicability60 96%), of the posing process severe underlimitations safe to the conditions. practicability of the process under safe conditions. 61

63 Figure 2. Reactions involved in the direct synthesis of H2O2.

64 Palladium, either alone [26–37] or associated with gold [17,22,25,38–44], with platinum [45–48], 65 with tin [49,50], with nickel [51], with tellurium [52], and with zinc [53,54], has been extensively 66 studied and it is considered as the best catalyst in terms of both activity and selectivity, the latter 62 67 being fundamental for commercial applications [19,55]. Rh and Ru additives have detrimental 63 Figure 2. Reactions involved in the direct synthesis of H2O2. 68 effects on theFigure H2O2 yields 2. Reactions because they involved promote in H the2O2 directdecomposition synthesis and/or of H enhance2O2. H2 to H2O 69 activity [19]. 64 Palladium, either alone [26–37] or associated with gold [17,22,25,38–44], with platinum [45–48], Palladium,65 with eithertin [49,50], alone with [nickel26–37 [51],] or with associated tellurium with[52], and gold with [ 17zinc,22 [53,54],,25,38 has–44 been], with extensively platinum [45–48], with tin66 [ 49,50studied ], with and it nickel is considered [51], as with the best tellurium catalyst in [ 52terms],and of both with activity zinc and [ 53selectivity,,54], has the beenlatter extensively studied67 and itbeing is considered fundamental asfor thecommercial best catalyst applications in terms [19,55]. of Rh both and activity Ru additives and have selectivity, detrimental the latter being 68 effects on the H2O2 yields because they promote H2O2 decomposition and/or enhance H2 to H2O fundamental69 activity for commercial [19]. applications [19,55]. Rh and Ru additives have detrimental effects on the H2O2 yields because they promote H2O2 decomposition and/or enhance H2 to H2O activity [19].

The effect of the nature of the support (ZrO2, Ga2O3, CeO2, SiO2, ThO2, CeO2–ZrO2, Al2O3, carbon etc.) on the catalytic activity and selectivity was extensively investigated [19,32,56]. Carbon porosity and surface structure play a key role to achieve high chemoselectivity [38]. Unluckily, the catalysts employed to perform the direct synthesis of H2O2 are also active for its decomposition. It was shown that an acid pretreatment of the carbon support of AuPd alloy catalysts turned off the H2O2 decomposition [21]. In particular, the acid treatment induced a decrease in the size of the bimetallic nanoparticles, which were supposed to decorate the sites, hence inhibiting the Catalysts 2019, 9, 30 3 of 34 decomposition reaction, and hydrogen selectivity >95% were achieved. It was also demonstrated that H2O2 can be produced over metallic Pd sites which have been geometrically and electronically modified by additives, such as Cl−, Br−, oxygen, and/or nitrogen atoms of the support [21,57–59]. In this frame, it has also been shown that the modification with ligands of Pd nanoparticles supported on carbon created a hybrid nanostructure with unique catalytic performance [60]. Acids and halides have been commonly added in order to improve the H2O2 selectivity [1,12,58,61,62]. Indeed, acid additives are able to impede the H2O2 decomposition, whereas halide additives hinder water formation [1,14,61]. Nevertheless, both corrosion of the reactor and dissolution of the catalytically-active metal occur in the presence of acid additives. Alternative acid 2− − supports, such as solid acid supports like SO3H- functionalized mesoporous silicas [24], SO4 -, Cl -, F−-, and Br−-doped zirconia [63], insoluble heteropolyacids [64–66], and insoluble heteropolyacids supported mesoporous silica [66,67], cesium-containing CsH3−PW12O40/MCF [68,69], and have been reported to act as acid sources. Cs-exchanged phosphotungstic acid was employed as an acid additive to an Au-Pd/TiO2 catalyst with significant improvement of the H2O2 synthesis rate and selectivity with respect to the promotional effect of common oxides and non-halo acids [70]. It has also been reported that acidity of the catalysts played a crucial role in determining the catalytic performance in the direct synthesis of hydrogen peroxide [66,67]. Approaches, such as the catalyst wet pre-treatment method (CWPM), involving the post-modification of commercially available Pd catalysts with aqueous solutions of Br− with different concentrations, have been shown to enhance the selectivity [30].

2. Infrared Absorption Spectroscopy as a Tool for Catalyst Characterization Infrared (IR) absorption spectroscopy is one of the most popular characterization tools to describe heterogeneous catalysts [71]. In FTIR analysis, a molecule is excited to a higher vibrational energy state upon absorption of the IR radiation. Only the molecules that undergo to a variation of the dipole moment during the vibrational transition are IR active [72]. It should be, therefore, remembered that both O2 and H2 molecules, i.e., the reactants involved in the direct synthesis of hydrogen peroxide, owing to their symmetry, are IR inactive (no change of the dipole moment). The energy associated to the excited states is related to the molecular bond vibrations, such as stretching, bending, rocking, twisting, wagging, as well as out-of-plane deformations, which take place at different frequencies or wavenumbers (cm−1) in the IR spectroscopic region [72]. The position of a band is deﬁned by Equation (1), which describes the behaviour of an anharmonic oscillator:

ν = 1/2πc·(k/µ)1/2 (1) where ν is the frequency, c is the light speed, k is the force constant related to the bond between the two atoms which constitute the oscillator, and µ represents the reduced mass of the oscillator. As a consequence, the position of an IR band depends either on the strength of the bond or on the mass (isomeric effect). This implies that the specific physicochemical properties of the molecule or adsorbate (i.e., when performing a FTIR experiment of adsorbed probe molecules), i.e., the bonds involved and the local environment, affect the position of the IR absorbance peak, making the technique diagnostic, since the corresponding IR spectrum represents a fingerprint of that particular functional group, such as C–H, O–H, C=O, C≡O, N=N, and so on. Moreover, similar vibrational modes possess similar energetics, meaning that the interpretation of the IR spectra of unknown species is useful to identify the specific moieties that constitute the sample. In particular, the position of the absorption bands of the adsorbed species gives a measure of the strength of the bond between the adsorbate and the adsorbing site [73]. Moreover, the specific frequencies of individual vibrational modes can probe the chemical nature of the surrounding catalytic environment. Catalysts 2019, 9, 30 4 of 34

The energy related to most of the molecular vibrations falls in the mid-infrared (MIR) region of the electromagnetic spectrum, and this is the reason for which the largest amount published papers reports spectra in this region, which ranges between 4000 and 400 cm−1. Another point is that the absorbance related to each molecular vibration is proportional to the abundance of the corresponding functional groups, hence the concentration of the species can be derived by the Beer-Lambert Law, Equation (2):

A = −log10 I/I0 = ε·l·c (2) where A stands for the absorbance (dimensionless), and I and I0 represent the intensity of the transmitted and incident light, respectively. The molar absorptivity ε parameter is expressed as L·mol−1·cm−1), the thickness of the sample l is reported in cm, and the molar concentration c is in mol·cm−1.

2.1. FTIR Spectroscopy of Adsorbed Probe Molecules The surface sites in catalysts can be efficiently characterized by FTIR spectroscopy of probe molecules. FTIR spectroscopy is ideal for this approach, due to the specificity of the spectra for each adsorbate and to the high sensitivity to the kind of surface bonding to the local environment. The choice of the most appropriate probe molecule is therefore strategic: some probes are able to adsorb on specific sites, which can be detected and quantified, based on the intensity of the absorption. Other probes display different bonding on different sites, which can be identified by the corresponding changes in the IR spectra. Generally, the use of opportune molecular probes combined with FTIR absorption spectroscopy can provide information on: (i) Acidic sites, in particular Brønsted sites, such as OH surface groups, and Lewis cationic sites. In this case, basic probe molecules as for example pyridine or other amines, NH3, and acetonitrile are typically employed [74,75]. Moreover, the acid strength can also be determined by the shift observed in the O–H stretching region upon interaction by hydrogen bonding with weak basic probes, such as CO and NO [76,77]. (ii) Basic sites, normally surface oxygen atoms in oxides, CO2 is usually employed, along with other probes less commonly used such as methanol [78], and also CO. CO2 adsorption gives rise to the formation of mono- and bidentate carbonate species, hydrogen carbonates, and linearly-coordinated 2− x+ 2− x+ CO2, providing detailed knowledge on the basic O sites and M –O pairs (M = metallic cation) [74].

Carbon Monoxide A separate paragraph is dedicated to carbon monoxide because this molecular probe can be considered as the most versatile and clever probe [79]. Only vibrational modes with high cross-sections can be readily detected by FTIR spectroscopy, and this is among the reasons for which a large fraction of spectroscopic investigations concerns the adsorption of carbon monoxide (or other molecules containing C–O bonds). Indeed, besides providing information on the acidic and basic sites, the use of the CO probe offers knowledge on the nature of the metal catalytic sites, their oxidation state, and coordination mode [77,80,81]. In addition, CO can also be employed to determine the amount of sites with coordinative vacancies and available for catalysis. The CO molar absorptivity is commonly high, giving rise to strong IR signals for the C–O stretching mode in a frequency range almost without interference due to other species. The bands due to linear bonded CO can be observed in the 2000–2170 cm−1 range, whereas the position of those related to doubly bridged species is in the 1880–2000 cm−1 region, and multiply-bridged CO species give rise to bands below 1880 cm−1. Moreover, the C–O stretching frequency is strongly dependant from the nature of the bond with the surface, i.e., from the electronic properties of the adsorbent site. Due to the small size, the CO molecule adsorption is ruled by steric hindrance effects. The frequency of the free molecule (2143 cm−1) changes Catalysts 2018, 8, x FOR PEER REVIEW 5 of 36

166 size, the CO molecule adsorption is ruled by steric hindrance effects. The frequency of the free 167 molecule (2143 cm–1) changes when CO is adsorbed and such a shift is explained by the Bhyholder 168 Model [82], in which the bond between the adsorbent site (a metal site) and CO is the result coming Catalysts1692019 , 9from, 30 two main contributions, which are shown in Figure 3. The first contribution is the  donation 5 of 34 170 that arises from the overlapping of the 5 full orbital of the carbon atom (C, with weak anti-bonding 171 nature) and of the empty d orbital of the metal site (M), which possesses dz2 symmetry. The result of 172 when CO isthis adsorbed overlapping and is suchan electron a shift density is explained transfer from by the the CO Bhyholder molecule (lone Model pair on [C82 atom)], in to which M the bond 173 with an increase of the C–O bond strength. As a consequence, according to Equation (1), the CO blue between174 theshift adsorbents with respect site to (athe metal position site) of the and free molecule. CO is the result coming from two main contributions, which175 are shownThe in second Figure contribution3. The ﬁrst is the contribution  back donation is (d the→pσ) withdonation bonding that character, arises resulting from from the overlapping of the176 5σ fullthe orbital overlapping of the among carbon two atomfull d orbitals (C, with of M weak and the anti-bonding anti-bonding 2 nature)* degenerate and CO of molecular the empty d orbital 177 orbitals. Such overlapping brings electron density in the anti-bonding CO orbital and the bond 2 of the178 metal strength site (M), is decreased. which possesses Therefore, the dz positionsymmetry. of the CO The band result red-shift ofs this with overlapping respect to the free is an electron Catalysts 2018, 8, x FOR PEER REVIEW 5 of 36 density179 transfer molecule. from These the two CO factors molecule can predo (loneminate pair on oneach C other atom) based to Mon the with nature an increaseof the adsorbent: of the C–O bond 180 generally,  donation is prevalent for metal ions with high oxidation state, on the contrary  back strength.166 As asize, consequence, the CO molecule according adsorption to is Equationruled by ster (1),ic hindrance the νCO effects.blue shiftsThe frequency with respect of the free to the position 181 donation control the adsorption by neutral or partially negatively-charged metal atoms. of the free167 molecule.molecule (2143 cm–1) changes when CO is adsorbed and such a shift is explained by the Bhyholder 182168 Model [82], in which the bond between the adsorbent site (a metal site) and CO is the result coming 169 from two main contributions, which are shown in Figure 3. The first contribution is the σ donation 170 that arises from the overlapping M  C bond of the 5σ full orbital of the carbon atom (C, with weak anti-bonding _ _ _ 2 171 nature) and of the empty dM orbital+ of the+ metal+ siteC_O (M),: whichM possessesC_ dzO: symmetry. The result of 172 this overlapping is an electron density transfer from the CO molecule (lone pair on C atom) to M 173 with an increase of the C–O bond strength. As a consequence, according to Equation (1), the νCO blue 174 shifts with respect to the Mposition → C bond of the free molecule. 175 The second contribution is the π back donation (dπ→pπ) with bonding character, resulting from + _ + _ + + _ _ _ π 176 the overlapping among twoM full d+ orbitalsC _O of: M and the anti-bondingM : _2 :* degenerate CO molecular _ _ C O 177 orbitals. Such overlapping_ brings+ electron density+ in the anti-bonding_ CO+ orbital and the bond 183178 strength is decreased. Therefore, the position of the CO band red-shifts with respect to the free 179 molecule. These two factors can predominate on each other based on the nature of the adsorbent: 184 Figure 3. MetalFigure atomic3. Metal atomic (M) orbitals (M) orbitals and and CO CO molecular orbitals orbitals involved involved in the adsorption. in the adsorption. 180 generally, σ donation is prevalent for metal ions with high oxidation state, on the contrary π back 185181 2donation.2. Experimental control Setup the adsorption by neutral or partially negatively-charged metal atoms. The182 second contribution is the π back donation (dπ→pπ) with bonding character, resulting from 186 The experiments that are presented and discussed in this review have beenπ carried out mainly the overlapping187 in the among mid IR frequency two full range, d orbitals between of approximately M and the 700 anti-bonding and 4000 cm–1 2, in* which degenerate most of the CO molecular orbitals.188 Suchmolecular overlapping vibrations absorbbrings light. electron Currently, density Fourier intransform the anti-bonding infrared spectrometers CO orbital (FTIR) are and the bond strength189 is decreased.employed, and Therefore,such instruments the are position fairly cheap of and the widespread CO band in red-shiftsmost analytical with and respectresearch to the free 190 laboratories. In FTIR instruments a Michelson interferometer processes the collimated IR light before molecule.191 Thesefocusing two it onto factors the sample can and predominate further collects on and each detect others the resulting based beam on. the According nature to Figure of the adsorbent: generally,192 σ4donation, the IR beam is is prevalentsplit into two forequal metal beams, ions the former with ishigh refracted oxidation in the direction state, of ona fixed the mirror contrary, π back donation193 controlwhereas the the adsorption latter is transmitted by neutral to a movable or partially mirror. negatively-charged metal atoms.

fixed mirror 2.2. Experimental183 Setup 184 Figure 3. Metal atomic (M) orbitals and CO molecular orbitals involved in the adsorption. The experiments that are presented and discussed in thistranslating review have been carried out mainly mirror in the mid185 IR2.2. frequency Experimental Setup range, between approximately 700 and 4000 cm−1, in which most of the molecular186 vibrationsThe experiments absorb light.thatIR source are presented Currently, and Fourierdiscussed in transform this review have infrared been carried spectrometers out mainly (FTIR) are 187 in the mid IR frequency range, between approximately 700 and 4000 cm–1, in which most of the employed, and such instruments are fairlybeamsplitter cheap and widespread in most analytical and research 188 molecular vibrations absorb light. Currently, Fourier transform infrared spectrometers (FTIR) are laboratories.189 employed, In FTIR instrumentsand such instruments a Michelson are fairly cheap interferometersample and position widespread processes in most analytical the collimated and research IR light before focusing190 it ontolaboratories. the sample In FTIR and instruments further a Michelson collects andinterf detectserometer processes the resulting the collimated beam. IR According light before to Figure4, 191 the IR beamfocusing is split it into onto twothe sample equal and beams, further collects the former anddetector detects is refractedthe resulting inbeam. the According direction to Figure of a ﬁxed mirror, 194192 4, the IR beam is split into two equal beams, the former is refracted in the direction of a fixed mirror, whereas195193the latterwhereas is the transmitted latter isFigure transmitted 4 to. Schematic a movableto a movable representation mirror. mirror. of a Michelson interferometer.

194 195 Figure 4.FigureSchematic 4. Schematic representation representation of of a aMichelson Michelson interferometer. interferometer.

Then, these beams are recombined by the beam splitter and further directed toward the sample. The difference in optical path between the two beams is called retardation, and is varied over time by scanning the movable mirror. As a consequence, an interferogram is obtained by recording the signal Catalysts 2019, 9, 30 6 of 34 from the detector as a function of the retardation. The Fourier transformation of that signal provides the IR spectrum, which is reported as a function of the frequency. Modern FTIR instruments are usually equipped by glow-bar IR lamps as the source and by Catalysts 2018, 8, x FOR PEER REVIEW 6 of 36 mercury–cadmium–telluride (MCT) detectors for signal detection, due to the high sensitivity and to the196 capability Then, to detectthese beams most are recombined of the mid by IRthe beam frequency splitter and range. further Due directed to toward the opportunity the sample. to collect 197 simultaneouslyThe difference all wavelengths, in optical path higher between signal-to-noise the two beams is ratio called can retardation, be achieved and is varied at a givenover time scanning time. 198 by scanning the movable mirror. As a consequence, an interferogram is obtained by recording the Moreover,199 thesignal throughput from the detector is determined as a function onlyof the byretardation. the diameter The Fourier of thetransformation collimated of that IR signal beam. However, being200 a non-zero-background provides the IR spectrum, technique, which is reported FTIR as a instruments function of the frequency. have limited sensitivity and glow-bar 201 lamps should beModern replaced FTIR by instruments synchrotron are usu radiationally equipped or by by glow tunable-bar IR IR lamp laserss as the [71 source]. and by 202 mercury–cadmium–telluride (MCT) detectors for signal detection, due to the high sensitivity and to Nevertheless,203 the capability the widespread to detect most acceptance of the mid IR of frequency IR absorption range. Due spectroscopy to the opportunity to efficiently to collect characterize solid catalysts204 simultaneously is mainly all due wavelengths, to the availability higher signal-to of-noise several ratio can experimental be achieved at setups, a given scanning which render the 205 technique adaptabletime. Moreover, to a the number throughput of experiments is determined only that by can the be diameter tailored of the on collimated the nature IR beam of the. sample to 206 However, being a non-zero-background technique, FTIR instruments have limited sensitivity and be investigated.207 glow-bar lamps should be replaced by synchrotron radiation or by tunable IR lasers [71]. The208 most commonNevertheless, experimental the widespread setups acceptance employed of forIR absorptioncatalyst characterization spectroscopy to efficiently using IR absorption spectroscopy209 characterize are (i) transmission solid catalysts is FTIR mainly mode, due to (ii)the diffuseavailability reflectance of several experimental (DRIFTS) setups, mode, which (iii) attenuated 210 render the technique adaptable to a number of experiments that can be tailored on the nature of the total reflection211 sample (ATR) to be mode,investigated. in which the sample is placed in tight contact with a flat surface of the prism212 used to directThe most the IR common beam, experimental and (iv) reflection–absorption setups employed for catalyst (RAIRS) characterization mode, in using which IR the IR beam is recoiled213 fromabsorption a flat spectroscopy reflective surfaceare (i) transmission before collection. FTIR mode, (ii) This diffuse last reflectance setup is usually(DRIFTS) mode, employed (iii) for model 214 attenuated total reflection (ATR) mode, in which the sample is placed in tight contact with a flat systems215 in surface-sciencesurface of the prism studies. used to Indirect this the review, IR beam, we and will (iv) reflectionmainly– focusabsorption on the(RAIRS) transmission mode, in FTIR and diffuse216 reflectance which the (DRIFTS) IR beam is recoiled common from setups. a flat reflective surface before collection. This last setup is usually 217 employed for model systems in surface-science studies. In this review, we will mainly focus on the 2.2.1. Transmission218 transmission FTIR FTIR Mode and diffuse reflectance (DRIFTS) common setups. Most219 of2.2 the.1. Transmission spectroscopic FTIR studies Mode reported in the literature have been performed in transmission mode.220 Generally Most the of setup the sconsistspectroscopic in a studies self-sustaining reported in pellet the literature of the havepowered been catalyst performed orin of a mixture 221 with IR transparenttransmission KBr, mode placed. Generally inside the setup a cell consists allowing in a self to-sustaining work in pellet controlled of the powered atmosphere, catalyst or and collect in 222 of a mixture with IR transparent KBr, placed inside a cell allowing to work in controlled atmosphere, situ IR223 spectra and [39 collect,40 ].in Asitu schematic IR spectra [39, picture40]. A ofschematic an example picture of an transmission example of transmission mode is mode shown is in Figure5. There224 is also shown the possibility in Figure 5. There to follow is also the spectroscopically possibility to follow thespectroscopically catalytic reaction the catalytic and reaction to collect and operando 225 IR spectra. to collect operando IR spectra. 226

sample pellet IR beam

KBr windows 227 Figure228 5. TransmissionFigure 5. Transmission (TIR) mode,(TIR) mode, where where the the IRIR beam is isdirected directed through through a self-sustained a self-sustained sample sample 229 placed inside the catalytic reactor and collected after exiting for analysis. placed inside the catalytic reactor and collected after exiting for analysis. 230 Despite being quite popular, transmission mode setups undergo to the following constraints: Despite231 the being catalyst quite must be popular, transparent transmission to IR radiation in mode order setupsto obtain undergospectra withto reasonable the following intensity, constraints: the catalyst232 the must catalyst be transparenthas to give the possibility to IR radiation to make thin in self order-sustaining to obtain pellets, spectra and the intrinsic with reasonable nature of intensity, the catalyst has to give the possibility to make thin self-sustaining pellets, and the intrinsic nature of the catalyst and the ﬁnal sample form, such as pellets, depositions, and monoliths, can give serious problems for mass transport. Catalysts 2019, 9, 30 7 of 34

Catalysts 2018, 8, x FOR PEER REVIEW 7 of 36 2.2.2. Diffuse-Reflectance (DRIFTS) Mode 233 the catalyst and the final sample form, such as pellets, depositions, and monoliths, can give serious To234 successfully problems for overcome mass transport. the problems arising from poorly transparent catalysts, or from samples not adequate to prepare a pellet, the diffuse-reflectance (DRIFTS) mode can be alternatively adopted. 235 2.2.2. Diffuse-Reflectance (DRIFTS) Mode Here, the powdered sample is placed in a cell, where the interaction with the IR beam gives 236 raise to scatteredTo light,successfully which overcome is collected the problems with arising appropriate from poorly optics transparent (Figure catalysts6). With, or from this approach, 237 samples not adequate to prepare a pellet, the diffuse-reflectance (DRIFTS) mode can be alternatively the preparation238 adopted of. theHere, sample the powdered is simpler sample is than placed that in a withcell, where transmission the interaction FTIR. with the Due IR beam to this gives experimental setup,239 the intensityraise to scattered of DRIFT light, which signal is collected tends with to be appropriate low and optics collecting (Figure 6). good With this quality approach, spectra the becomes 240 difficult. However,preparation the of intensity the sample isof simpler the bands than that can with be transmissionquite more FTIR. enhanced Due to this when experimental working in DRIFT 241 setup, the intensity of DRIFT signal tends to be low and collecting good quality spectra becomes mode242 with respectdifficult. toHowever, the acquisition the intensity in of transmissionthe bands can be mode, quite more due enhanced to the lightwhen multipleworking in internalDRIFT reflection close to243 the samplemode with surface, respect toresulting the acquisition also in in improved transmission sensitivity mode, due to towards the light multiplethe surface internal species rather than towards244 reflection the gas close phase to the species. sample surface, On the resulting contrary, also in the improved major sensitivity drawbacks towards are the related surface to the poor 245 species rather than towards the gas phase species. On the contrary, the major drawbacks are related reproducibility246 to the due poor to reproducibility the variations due of to scattering the variations coefficients of scattering depending coefficients depending on the cellon the geometry, cell on the sample247 insertion geometry procedure, on the sample and insertion on the procedure occurrence and on the of occur temperaturerence of temperature gradient gradient within within the powdered sample248 with the respect powdered to thesample surface. with respect to the surface. 249

Mirror IR source

sample powder Detector

250 Figure251 6. DiffuseFigure reflectance6. Diffuse reflectance (DRIFTS) (DRIFTS) mode, mode, where where an an IR IR beam beam is focused is focused on the p onowder theedpowdered samples samples 252 and the scattered light is collected by a parabolic mirror. and the scattered light is collected by a parabolic mirror. 253 3. FTIR Spectroscopic Characterization: What Can Be Learned? 3. FTIR Spectroscopic Characterization: What Can Be Learned? 254 Usually, the use of FTIR spectroscopy is dedicated to the identification of the surface exposed Usually,255 sites the of usethe catalyst of FTIR to establish spectroscopy structure- isactivity dedicated relationships. to the Another identification application deals of the with surface the exposed 256 identification of the adsorbed species, and sometimes to their quantification and evolution during sites of257 the catalystthe reaction. to In establish this review structure-activity, we will try to center relationships.on some meaningful Another examples applicationthat shed light dealson with the identification258 open of the questions adsorbed and improved species, the and knowledge sometimes on the to theiractive quantification sites involved in and the H evolution2O2 direct during the reaction.259 Insynth thise review,sis and on we the willparameters try to that center have aon beneficial some effect meaningful on both conversion examples and selectivity. that shed light on open questions260 and3.1.improved Nature of the Active the knowledge Sites on Pd Cataly onsts the active sites involved in the H2O2 direct synthesis and on the261 parameters Many that studies have focused a beneficial on the identification effect on of both the nature conversion and the androle of selectivity. Pd sites in promoting 262 the direct synthesis of hydrogen peroxide from hydrogen and oxygen. In a very recent and 3.1. Nature263 ofinteresting the Active paper, Sites Han on et Pd al.Catalysts deduced that the active sites should not be properly represented by 264 metallic or oxidized Pd species and elegantly pointed out that the active sites for the reaction Many265 studiesoriginated focused from the oninterfaces the identification among Pd and PdO ofthe domains nature on Pd/TiO and the2 catalysts, role of as Pdshown sites in F inigure promoting the direct266 synthesis 7a [59] of. The hydrogen combined peroxide use of high from resolution hydrogen transmission and electron oxygen. microscopy In a very (HRTEM) recent, X and-ray interesting paper,267 Han Diffraction et al. deduced (XRD), in thatsitu diffuse the activereflectance sites Fourier should Transform not IR be spectroscopy properly (DRIFT) represented of adsorbed by metallic or 268 CO, X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES), and oxidized269 PdExtended species andX-ray elegantly Absorption pointed Fine Structure out (EXAFS) that the allowed active to sites demonstrate for the thatreaction the structure originated of from the interfaces among Pd and PdO domains on Pd/TiO2 catalysts, as shown in Figure7a [ 59]. The combined use of high resolution transmission electron microscopy (HRTEM), X-ray Diffraction (XRD), in situ diffuse reflectance Fourier Transform IR spectroscopy (DRIFT) of adsorbed CO, X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES), and Extended X-ray Absorption Fine Structure (EXAFS) allowed to demonstrate that the structure of such Pd ensembles can be finely tuned by varying the Pd loading, while keeping constant the particle size. Indeed, the surface configuration of Pd atoms changed significantly for 1.0–5.0 wt% Pd loadings, whereas the size and the crystallinity of Pd particles remains unchanged. In situ DRIFTS spectra of adsorbed CO were collected on the Pd/TiO2 catalysts with different Pd loadings to determine the surface structure and the results are shown Figure7b. Catalysts 2018, 8, x FOR PEER REVIEW 8 of 36

270 such Pd ensembles can be finely tuned by varying the Pd loading, while keeping constant the 271 particle size. Indeed, the surface configuration of Pd atoms changed significantly for 1.0–5.0 wt% Pd 272 loadings, whereas the size and the crystallinity of Pd particles remains unchanged. In situ DRIFTS 273 spectra of adsorbed CO were collected on the Pd/TiO2 catalysts with different Pd loadings to 274 determine the surface structure and the results are shown Figure 7b. 275 The sharp peak at 2094 cm–1 is due to CO linearly adsorbed on low coordinated metallic Pd 276 atoms located at the corners or at the edges of the nanoparticles. Moreover, the bands at 1978, 1932, 277 and 1876 cm–1 are related to CO adsorbed on bridge and hollow sites of the Pd ensembles [83]. Such 278 absorptions decreased in intensity by decreasing the Pd loading, however, the linear species/bridged 279 and multibonded species ratio increases regularly. The intensity of the CO band at 1876 cm–1 was 280 almost nil in the case of 1.0% Pd/TiO2, which means that the increase of the Pd content induced the 281 formation of continuous Pd ensembles exposing flat sites, thus lowering the amount of Pd corners or Catalysts 2019, 9, 30 8 of 34 282 edges. 283

a b

284

285 Figure 7. (a) Structural model of the Pd/TiO2 catalyst. (b) In situ DRIFTS of CO adsorption over Formatted: Not Highlight Figure 7. (a) Structural model of the Pd/TiO2 catalyst. (b) In situ DRIFTS of CO adsorption over 286 different catalysts at room temperature in an Ar flow (50 mL/min): A: 1.0% Pd, B: 2.0% Pd, C: 3.0% Pd, different287 catalystsand D: 5.0% at room Pd. Reprinted temperature from J. Catal in an., 321, Ar L. flow Ouyang, (50 P. mL/min): Tian, G. Da, A:X. Xu, 1.0% C. Ao, Pd, T. B:Chen, 2.0% R. Si, Pd, J. C: 3.0% Pd, and288 D: 5.0%Xu, Pd. Y. Reprinted Han , The origin from of J.active Catal., sites 321,for direct L.Ouyang, synthesis of P. H Tian,2O2 on G.Pd/TiO Da,2 catalysts: X. Xu, C.Interfaces Ao, T. of Chen, R. Si,Formatted: J. Subscript 289 Pd and PdO domains, 70–80, Copyright (2018), with permission from Elsevier. Formatted: Subscript Xu, Y. Han, The origin of active sites for direct synthesis of H2O2 on Pd/TiO2 catalysts: Interfaces of Pd and PdO domains, 70–80, Copyright (2018), with permission from Elsevier. Formatted: Subscript 290 A rate in TOF of 630 h–1 along with 61% selectivity to H2O2 were obtained for the 1.0 wt% Pd 291 catalyst at 283 K and 1 atm in a semibatch reactor. The reasons for the good catalytic performance −1 The292 sharpwere peak disentangled at 2094 with cm the contributionis due toof spectroscopic CO linearly experiments adsorbed focused on onlow the coordinated Pd surface metallic Pd atoms293 locatedsites’ reactivity at the that corners will be discussed or at in the Section edges 3.3. of the nanoparticles. Moreover, the bands at 294 The origin of −Pd1 particle size effects on H2O2 synthesis was rarely investigated [25,57]. It was 1978,295 1932, reported and 1876 that high cm metalare dispersion related of Pd to nanoparticles CO adsorbed supported on on bridge silica has and a detrimental hollow effect sites of the Pd ensembles296 [on83 ]. the Such catalytic absorptions activity and selectivity decreased towards in intensity H2O2 [31]. byMoreover, decreasing shape-dependent the Pd catalyticloading, however, the linear297 species/bridgedactivity of Pd nano andcubes multibondedwith {1 0 0} facets, species and octahedrons ratio increases with {1 1 regularly.1} facets was Thedemonstrated intensity of the CO 298 2 2 band at 1876[84 cm]. The−1 Pdwas octahedron almost shape nil in gave the rise case to higher of 1.0% H O Pd/TiO selectivity ,and which productivity means than that the thecatalyst increase of the 299 with Pd cubes, therefore, suggesting that the Pd {1 1 1} facet is more2 active than the Pd {1 0 0} one. In Pd content300 inducedanother paper, the formation{1 0 0} facet-enclosed of continuous Pd nanocubes Pd ensemblesof uniform shape, exposing size, and flat crystallinity sites, thus were lowering the amount301 of Pdprepared corners and or supported edges. on silica. It was found that the {1 0 0} Pd facet efficiently catalyzed H2O2 302 A rate information, TOF of however 630 h,− it1 promotedalong with side reactions, 61% selectivity thus decreasing to H theO selectivitywere obtained[85]. for the 1.0 wt% Pd 303 In this frame, a series of size-controlled Pd/hydroxyapatite (HAp)2 2 catalysts ranging from single catalyst304 at 283sites K (Pd and clusters) 1 atm to innanoparticles a semibatch (∼30 nm) reactor. were synthesized The reasons and tested for the in H good2O2 direct catalytic synthesis, performance were disentangled305 showing withup to 94 the% selectivity contribution toward of H spectroscopic2O2 formation when experiments the Pd particlefocused size is in the on range the Pd2.5– surface sites’ 306 reactivity that1.4 willnm [57] be. Extensive discussed characterization in Section by3.3 multi. -technique approach was performed with the aim to 307 obtain detailed information on the role played by crystal phase, morphology, surface electronic The308 originstates of, and Pd coordination particle sizenumber effects of Pd particles on H2O (from2 synthesis atomic level was to nano rarelymeters) investigated and to establish [25 a ,57]. It was reported that high metal dispersion of Pd nanoparticles supported on silica has a detrimental effect on the catalytic activity and selectivity towards H2O2 [31]. Moreover, shape-dependent catalytic activity of Pd nanocubes with {1 0 0} facets, and octahedrons with {1 1 1} facets was demonstrated [84]. The Pd octahedron shape gave rise to higher H2O2 selectivity and productivity than the catalyst with Pd cubes, therefore, suggesting that the Pd {1 1 1} facet is more active than the Pd {1 0 0} one. In another paper, {1 0 0} facet-enclosed Pd nanocubes of uniform shape, size, and crystallinity were prepared and supported on silica. It was found that the {1 0 0} Pd facet efficiently catalyzed H2O2 formation, however, it promoted side reactions, thus decreasing the selectivity [85]. In this frame, a series of size-controlled Pd/hydroxyapatite (HAp) catalysts ranging from single sites (Pd clusters) to nanoparticles (∼30 nm) were synthesized and tested in H2O2 direct synthesis, showing up to 94% selectivity toward H2O2 formation when the Pd particle size is in the range 2.5–1.4 nm [57]. Extensive characterization by multi-technique approach was performed with the aim to obtain detailed information on the role played by crystal phase, morphology, surface electronic states, and coordination number of Pd particles (from atomic level to nanometers) and to establish a proper size range to obtain the best catalytic performances in the H2O2 direct synthesis. In particular, in situ CO DRIFT measurements were carried out to define the electronic structure of the surface Pd atoms and the spectra are shown in Figure8a. Firstly, no vibrational band related to the HAp support was observed. The intensity of the peak at 2091 cm−1 related to CO linearly adsorbed on atop sites increased with decreasing the Pd particle size. Conversely, a gradual decrease in intensity of the absorption bands associated with CO adsorbed on bridged (1988–1964 cm−1) and three-fold (1913–1873 cm−1) sites was detected [83]. More in detail, the presence of the band at 2091 cm−1, attributed to linear adsorbed CO, signalled the presence of highly dispersed Pd atoms and clusters Catalysts 2018, 8, x FOR PEER REVIEW 9 of 36

309 proper size range to obtain the best catalytic performances in the H2O2 direct synthesis. In particular, 310 in situ CO DRIFT measurements were carried out to define the electronic structure of the surface Pd 311 atoms and the spectra are shown in Figure 8a. Firstly, no vibrational band related to the HAp Catalysts 2019, 9, 30 9 of 34 312 support was observed. The intensity of the peak at 2091 cm−1 related to CO linearly adsorbed on atop 313 sites increased with decreasing the Pd particle size. Conversely, a gradual decrease in intensity of the 314 absorption bands associated with CO adsorbed on bridged (1988–1964 cm−1) and three-fold (1913– (0D and315 2D 1873 particles) cm−1) sites on was the detected 0.5–2.0 [83] wt%. More Pdin detail, catalyst. the presence In addition, of the band suchat 2091 acm band−1, attributed did not to change in intensity316by increasinglinear adsorbed the CO, Pd signalled content. the Thepresence authors of highly inferred dispersed that Pd theatoms surface and clusters local (0D environment and 2D of the 317 HAp supportparticles) strongly on inﬂuencedthe 0.5–2.0 wt% the Pd formation catalyst. In ofaddition, these such particles. a band Bandsdid not relatedchange in to intensity CO bridged by species, 318 increasing the Pd content. The authors inferred that the surface local environment of the HAp growing319 in intensitysupport strongly withthe influenced Pd content, the formation were detectedof these particles. on the Bands 0.5 wt% related Pd to catalyst.CO bridged These species, spectroscopic features,320 i.e., growing the bands in intensity observed with in the the Pd 1913–1873 content, were cm detected−1 range, on the due 0.5 to wt% CO Pd adsorbed catalyst. Theseon Pd to hollow 321 −1 sites, pointedspectro out thescopic presence features, ofi.e.,Pd the clusters,bands observed composed in the 1913 of– high-density1873 cm range, due sites, to CO or largeradsorbed 3D on Pd particles. 322 Pd to hollow sites, pointed out the presence of Pd clusters, composed of high-density sites, or larger In addition,323 3D the Pd band particles due. In toaddition, linear the carbonyls band due to totally linear carbonyls disappeared totally disappeared in the case in the of case the of 3.0 the and 5.0 wt% Pd catalysts,324 3.0 suggesting and 5.0 wt% thatPd catalysts neither, suggest 0D noring that 2D neither particles 0D nor are 2D particles present are at present the surface at the surface of these of catalysts, due to325 Pd coalescencethese catalysts, phenomena due to Pd coalescence during phenomena the preparation. during the preparation. 326

a b

327 328 Figure 8. (a) In situ DRIFTS of CO adsorption over different catalysts at 283 K in an Ar flow (50 Formatted: Not Highlight Figure 8. (a) In situ DRIFTS of CO adsorption over different catalysts at 283 K in an Ar ﬂow 329 mL/min): a: HAp, b: 0.5 wt% Pd, c: 1.0 wt% Pd, d: 2.0 wt% Pd, e: 3.0 wt% Pd, and f: 5.0 wt% Pd. (b) (50330 mL/min): Size a: dependence HAp, b: 0.5of H wt%2O2 selectivity Pd, c: 1.0over wt% Pd/HAp Pd, catalysts: d: 2.0 a wt%–e: catalysts Pd, e: calcined 3.0 wt% at 673 Pd, K andwith Pd f: 5.0 wt% Pd. (b331) Size dependenceloadings of of 0.5, H 1.0,2O 2.0,2 selectivity 3.0, and 5.0 wt% over, respectively; Pd/HAp f: catalysts:3.0 wt% Pd calcined a–e: catalysts at 773 K; g) calcined 3.0 wt% Pd at 673 K with Pd332 loadings calcined of 0.5, 1.0, at 873 2.0, K; 3.0,(c) Structures and 5.0 ofwt%, catalysts respectively; corresponding f: 3.0 to three wt% scales Pd calcined of particle at sizes 773 and K; g) 3.0 wt% 333 proposed mechanism for H2O2 synthesis. Blue, red, white, purple, and green spheres are palladium, Pd334 calcined oxygen, at 873 hydrogen, K; (c) Structures phosphorus, of and catalysts calcium atoms, corresponding respectively. Reprinted to three from scales J. Catal., of particle 349, P. sizes and proposed335 mechanismTian, L. Ouyang, for HX .2 Xu,O2 C.synthesis. Ao, X. Xu, R. Blue, Si, X. Shen, red, M. white, Lin, J. purple,Xu, Y.-F. Han, and The green origin spheres of palladium are palladium, oxygen,hydrogen, phosphorus, and calcium atoms, respectively. Reprinted from J. Catal., 349, P. Tian, L. Ouyang, X. Xu, C. Ao, X. Xu, R. Si, X. Shen, M. Lin, J. Xu, Y.-F. Han, The origin of palladium particle

size effects in the direct synthesis of H2O2: is smaller better? 30–40, Copyright (2017), with permission from Elsevier.

Based on the experimental findings in combination with DFT calculations, a structure–activity relationship by varying the Pd particle size was established (Figure8b). Particularly, based on the Pd particle sizes and on the catalytic performance, three types of catalysts were identified: the first type is the single site catalyst (Pd(IE)), which is composed of atomically dispersed Pd species, and is almost inactive due to the lack of active sites (Figure8c). The catalysts with 0.5–2.0 wt% Pd belong to type II, and are mainly made up by 2D and 3D Pd sub-nanoparticles with average size ranging from 1.4 to 2.5 nm (Figure8c). According to the experimental results and the DFT calculations, these species are active in the H2O2 synthesis (Figure8b), proving that the surface arrangement of Pd atoms plays a decisive role in the catalytic activity [86]. The 3.0–5.0 wt% Pd catalysts can be classified as the third type (Figure8b,c), which is composed by large nanoparticles with size >2.5 nm, exposing mainly the Catalysts 2018, 8, x FOR PEER REVIEW 10 of 36

336 particle size effects in the direct synthesis of H2O2: is smaller better?, 30–40, Copyright (2017), with Formatted: Subscript 337 permission from Elsevier. Formatted: Subscript

Catalysts 2019338, 9, 30 Based on the experimental findings in combination with DFT calculations, a structure–activity 10 of 34 339 relationship by varying the Pd particle size was established (Figure 8b). Particularly, based on the Pd 340 particle sizes and on the catalytic performance, three types of catalysts were identified: the first type 341 is the single site catalyst (Pd(IE)), which is composed of atomically dispersed Pd species, and is Pd(1 1 1)342 facets. almost The inactive H2O due2 formation to the lack of is active inhibited sites (Figure on 8c these). The sites,catalysts due with to 0.5 the–2.0 wt% activity Pd belong of metallicto Pd towards343 O2 dissociation,type II, and are thus mainly decreasing made up by the2D and selectivity. 3D Pd sub-nanoparticles with average size ranging from The344 HAp 1.4 support to 2.5 nm helped (Figure 8 inc). According keeping to the the size experimental distribution results and of the the DFT Pd calculations, particles these ina deﬁned 345 species are active in the H2O2 synthesis (Figure 8b), proving that the surface arrangement of Pd range, resulting346 atoms in theplays formation a decisive role of in Pdthe catalytic particles activity with [86 different]. The 3.0–5.0 geometric wt% Pd catalysts and can electronic be classified structures. In particular,347 as the the interactionthird type (Figure between 8b,c), which Pd is composed and OH by groups large nanoparticles of HAp with favoured size >2.5 nm, the exposing generation of δ Formatted: Not Highlight sub-nanometer348 mainly Pd speciesthe Pd(1 1 with 1) facets. a high The H Pd2O2 formation+/Pd0 ratio. is inhibited This on thesewas sites, ascribed due to tothe be activity responsible of of 349 metallic Pd towards O2 dissociation, thus decreasing the selectivity. δ+ the remarkable350 improvementThe HAp support of helped H2O in2 keepingselectivity the size because distribution the of the presence Pd particles of in Pd a definedspecies range, lower the dissociative351 O2resultingactivation in the along formation with of Pd the particles H2O2 withdecomposition different geometric [59 ]. and Hence, electronic the structures. Pd particles In with sub-nanometer352 particular, size are the strongly interaction interacting between Pdwith and the OH support groups ofand HAp act favoured as primary the g activeeneration sites. of 353 sub-nanometer Pd species with a high Pdδ+/Pd0 ratio. This was ascribed to be responsible of the 354 2 2 δ+ 3.2. Site Isolationremarkable and Electronic improvement Effects of H O selectivity because the presence of Pd species lower the 355 dissociative O2 activation along with the H2O2 decomposition [59]. Hence, the Pd particles with High356 dispersion sub-nanometer of Pd size sites are strongly as well interacting as incremented with the support Pd 0andcontent act as primary can boost active sites the. hydrogenation rate of hydrogen357 3.2. Site peroxide, Isolation and however Electronic Effects lowering H2O2 selectivity. Theoretical studies revealed the beneﬁcial358 effect ofH Pdigh dispersion monomers, of Pd whichsites as well favour as incremented H2O2 formation Pd0 content can and, boost at the the hydrogenation same time, rate avoid the O–O bond359 dissociation of hydrogen [ 87peroxide]. Ouyang, however et lowering al. studied H2O2 bothselectivity. H2O Theoretical2 direct synthesisstudies revealed and the side beneficial reactions over 360 effect of Pd monomers, which favour H2O2 formation and, at the same time, avoid the O–O bond different Pd–Au/TiO2 catalysts and discovered that the productivity of hydrogen peroxide increased 361 dissociation [87]. Ouyang et al. studied both H2O2 direct synthesis and side reactions over different by decreasing the Pd/Au ratio [86]. The in situ DRIFT spectra of CO adsorbed on Pd and Pd–Au 362 Pd–Au/TiO2 catalysts and discovered that the productivity of hydrogen peroxide increased by catalysts,363 are reporteddecreasing in the Figure Pd/Au 9 ratioa. Different[86]. The in metallic situ DRIFT Pd spectra sites of were CO adsorbed observed on onPd and the Pd monometallic–Au catalyst,364 and assignedcatalysts, are to reported linear COin Figure adsorbed 9a. Different on low metallic coordination Pd sites were Pdobserved atoms, on the presumably monometallic corners or 365 catalyst, and assigned to linear CO adsorbed− on1 low coordination Pd atoms, presumably corners or edges of366 the nanoparticlesedges of the nanoparticles (the band (the at band 2096 at cm2096 cm)[–1)88 [88],],and and to to bridged bridged and andmultibonded multibonded CO on Pd CO on Pd −1 ensembles367 formed ensembles by two formed or by three two or adjacent three adjacent atoms atoms (the (the bands bands at 1977, 1977, 1910 1910,, and and1851 cm 1851–1) [8 cm3]. )[83]. 368 a b

369 370 Figure 9. (a) In situ DRIFTS of CO adsorption over different catalysts at room temperature in streams Figure 9. (a) In situ DRIFTS of CO adsorption over different catalysts at room temperature in streams of 371 of Ar with a flow rate of 50 ml/min. A: Pd, B: Pd2.5Au0.5, C: Pd2.0Au1.0, D: Pd1.5Au1.5, E: Ar with372 a ﬂowPd1.0Au2.0, rate of 50 ml/min.F: Pd0.5Au2.5. A: Pd,(b) Mechanism B: Pd2.5Au0.5, of H2O2C: synthesis Pd2.0Au1.0, directly D:from Pd1.5Au1.5, H2 and O2 on E: a Pd1.0Au2.0,Pd F: Pd0.5Au2.5.373 monomer (b) Mechanism catalyst. Reprinted of H2O from2 synthesis J. Catal., 311, directly L. Ouyang, from G. H Da2 and , P. Tian, O2 on T. Chen, a Pd G. monomer Liang, J. catalyst. Reprinted374 fromXu,Y. J. Catal.,-F. Han, 311, Insight L. Ouyang, into active G. sites Da, of P. Pd Tian,–Au/TiO T.2 Chen, catalysts G. in Liang, hydrogen J. Xu, peroxide Y.-F. Han, synthesis Insight into Formatted: Subscript 375 directly from H2 and O2, 129–136, Copyright (2014), with permission from Elsevier. active sites of Pd–Au/TiO2 catalysts in hydrogen peroxide synthesis directly from H2 and O2, 129–136,

Moreover, the intensity of these bands diminished dramatically by decreasing the Pd/Au ratio. Conversely, the ratio of the linear CO bands vs. bridged- and multi-bonded CO bands grew from 0.4 (monometallic Pd catalyst) to 1.0 (bimetallic Pd1.0Au2.0 catalyst). Based on these observations, the amount of surface Pd ensembles was lowered upon Au addition, whereas isolated Pd sites were increased (geometric effect occurring between Pd and Au), due to a dilution by gold. A small red-shift of the band related to the linear carbonyls, and a shift of the absorption of bridged carbonyls from 1977 to 1969 cm−1 were observed after Au addition. These shifts are possibly a consequence of the charge-transfer from gold to palladium, which enhanced the electron density of the involved d orbitals, resulting in a stronger back-donation to the 2π CO molecular orbitals. The authors suggested that molecularly adsorbed O2 is activated without suppressing the O–O bond on Pd monomers embedded in low-coordinated Au sites to form H2O2 (Figure9b). These Pd isolated sites are present at the surface of the homogeneous Pd–Au alloy observed by High Angle Annular Dark Field (HAADF)-TEM analyses. Conversely, the Pd ensembles favoured H2O2 hydrogenation. It was also proposed that the Catalysts 2018, 8, x FOR PEER REVIEW 11 of 36

376 Moreover, the intensity of these bands diminished dramatically by decreasing the Pd/Au ratio. 377 Conversely, the ratio of the linear CO bands vs. bridged- and multi-bonded CO bands grew from 0.4 378 (monometallic Pd catalyst) to 1.0 (bimetallic Pd1.0Au2.0 catalyst). Based on these observations, the 379 amount of surface Pd ensembles was lowered upon Au addition, whereas isolated Pd sites were 380 increased (geometric effect occurring between Pd and Au), due to a dilution by gold. A small 381 Catalysts 2019, 9,red 30-shift of the band related to the linear carbonyls, and a shift of the absorption of bridged 11 of 34 382 carbonyls from 1977 to 1969 cm–1 were observed after Au addition. These shifts are possibly a 383 consequence of the charge-transfer from gold to palladium, which enhanced the electron density of 384 the involved d orbitals, resulting in a stronger back-donation to the 2 CO molecular orbitals. The sites at385 the interfaceauthors suggested between that Au molecularly and TiO adsorbed2 are active O2 is activated in the without H2O formation suppressing directlythe O–O bond from on H2 and O2, because386 of thePd lowest monomers selectivity embedded andin low the-coordinated absence Au of sites contiguous to form H2 PdO2 (Figure ensembles. 9b). These Pd isolated 387 sites are present at the surface of the homogeneous Pd–Au alloy observed by High Angle Annular The promotional effect of gold was previously reported for Pd–Au/SiO2 catalysts by XPS 388 Dark Field (HAADF)-TEM analyses. Conversely, the Pd ensembles favoured H2O2 hydrogenation. It measurements389 was combined also proposed with that the in sites situ at DRIFTthe interface spectroscopy between Au and of TiO adsorbed2 are active in CO. the H These2O formation studies proved the occurrence390 directly of surface from H2 and and electronic O2, because modiﬁcations of the lowest selectivity of Pd and when the alloyed absence of with contiguous Au atoms, Pd leading 391 ensembles. to enhanced reactivity and selectivity for the H2O2 direct synthesis [89]. Very recently, Flaherty et al. 392 The promotional effect of gold was previously reported for Pd–Au/SiO2 catalysts by XPS investigated393 measurements Pd and Au xcombinedPd1 nanoparticles with in situ DRIFT with spectroscopy similar of size adsorbed (7–11 CO. nm) These to studies understand proved the the effect of alloying394 Pd withoccurrence Au [of90 surface]. In particular,and electronic themodifications aim was of toPd ﬁndwhen betteralloyed explanationswith Au atoms, leading for the to high H2O2 395 enhanced reactivity and selectivity for the H2O2 direct synthesis [89]. Very recently, Flaherty et al. selectivity of these bimetallic catalysts by measuring the steady-state formation rates of both H2O2 and 396 investigated Pd and AuxPd1 nanoparticles with similar size (7–11 nm) to understand the effect of H2O with397 respectalloying to Pd the with pressure, Au [90]. theIn particular, temperature, the aim and was theto find nature better ofexplanations the solvent. for the The high authors H2O2 indicated that H2398O2 is producedselectivity of bythese sequential bimetallic catalysts surface by proton-electron measuring the steady transfers.-state formation Interestingly, rates of both an H increase2O2 of the Au:Pd399 ratio correspondsand H2O with respect to different to the pressure, and simultaneous the temperature, increasesand the nature in theof the activation solvent. The enthalpies authors related 400 indicated that H2O2 is produced by sequential surface proton-electron transfers. Interestingly, an to the H4012O 2 increaseand H 2ofO the production. Au:Pd ratio co Theserresponds changes to different are and due simultaneous to the occurrence increases in the of electronicactivation changes induced402 by theenthalpies addition related of gold to the to H palladium2O2 and H2O andproduction. are responsible These changes for are the due improved to the occurrence selectivity of towards 403 electronic changes induced by the addition of gold to palladium and are responsible for the H2O2 reported for AuPd bimetallic catalysts. The Pd and AuxPd1 were characterized also by FTIR 404 improved selectivity towards H2O2 reported for AuPd bimetallic catalysts. The Pd and AuxPd1 were spectroscopy,405 characterized and the results also by areFTIR reported spectroscopy, in and Figure the results 10a. are reported in Figure 10a. 406 a b

407

408 Figure 10. (a) Infrared spectra of CO adsorbed on Pd (black; 0.5 wt% Pd), Au1Pd1 (red; 0.5 wt% Pd), Figure 10. (a) Infrared spectra of CO adsorbed on Pd (black; 0.5 wt% Pd), Au1Pd1 (red; 0.5 wt% Pd), 409 and Au7Pd1 (blue; 0.014 wt% Pd) obtained after saturating surfaces under flowing CO (~15 min in 5 and410 Au 7Pd1 (blue;kPa CO, 0.01496 kPa wt%He, 303 Pd) K) obtainedand purging after gas-phase saturating CO with surfacesHe (101 kPa, under 47.5 cm3 ﬂowing min–1, 303 CO K). (~15 min in − 5 kPa411 CO, 96Spectra kPa He, are the 303 average K) and of 128 purging scans with gas-phase a 4 cm–1 resolution. CO with Peak He heights (101 were kPa, normalized 47.5 cm 3bymin the 1, 303 K). Spectra412 are theatop average bound CO of feature 128 scansat 2090– with2000 cm a– 41. ( cmb) Steady−1 resolution.-state H2O2 (black) Peak and heights H2O formation were normalizedrates (red) by the 413 and selectivity towards H2O2 (blue) on catalysts with different ratios of Au to Pd (55 kPa H2, 60 kPa atop bound CO feature at 2090–2000 cm−1.(b) Steady-state H O (black) and H O formation rates (red) 414 O2, 305 K, 30 cm3 min–1 20% v/v methanol). Dashed bars indicate2 2 that rates were2 undetectable. and415 selectivity Reprinted towards from H2 OJ. 2 Catal.,(blue) 357, on N. catalysts M. Wilson, with P. Priyadarshini, different ratios S. Kunz, of Au D. to W. Pd Flaherty, (55 kPa Direct H2 , 60 kPa O2, 305 K, 30 cm3 min−1 20% v/v methanol). Dashed bars indicate that rates were undetectable. Reprinted

from J. Catal., 357, N. M. Wilson, P. Priyadarshini, S. Kunz, D. W. Flaherty, Direct synthesis of H2O2 on Pd and AuxPd1 clusters: Understanding the effects of alloying Pd with Au, 163–175, Copyright (2018), with permission from Elsevier.

The absorption bands in the 2090–2000 cm−1 range are assigned to atop CO species adsorbed on Pd, whereas those between 1960 and 1940 cm−1 are due to the formation of bridge bounded CO on Pd [83]. Upon gold addition to Pd, the peaks related to the atop carbonyl species are signiﬁcantly red-shifted of about 90 cm−1 [91]. The ratio between the integrated areas of the bridge bounded/atop bands was taken as an indication of the relative amount of Pd ensembles with respect to Pd monomers [86], and decreased by adding Au to Pd, indicating the simultaneous presence of Pd and Au atoms within the same nanoparticle. The formation turnover rates of H2O2 and H2O are dependent on the Au content of the bimetallic −1 AuxPd1 nanoparticles (Figure7b). In particular, the formation rates of 0.11 mol H 2O2 (mol Pds s) −1 and 0.08 mol H2O (mol Pds s) over the Au12Pd1 catalyst are lower than those obtained for the −1 −1 Pd catalyst (0.40 mol H2O2 (mol Pds s) and 1.4 mol H2O (mol Pds s) . Bare Au nanoparticles catalysed neither H2O2 nor H2O formation, meaning that Pd is needed for the H2O2 direct synthesis and that Pd atoms are probably the active sites. Moreover, an enhancement of the selectivity towards H2O2 from 23% (bare Pd) up to 60% (Au12Pd1) was observed, since the decrease in H2O formation at Catalysts 2018, 8, x FOR PEER REVIEW 12 of 36

416 synthesis of H2O2 on Pd and AuxPd1 clusters: Understanding the effects of alloying Pd with Au, 163– Formatted: Subscript 417 175, Copyright (2018), with permission from Elsevier. Formatted: Subscript Formatted: Subscript 418 The absorption bands in the 2090–2000 cm–1 range are assigned to atop CO species adsorbed on Subscript 419 Pd, whereas those between 1960 and 1940 cm–1 are due to the formation of bridge bounded CO on Pd Formatted: 420 [83]. Upon gold addition to Pd, the peaks related to the atop carbonyl species are significantly 421 red-shifted of about 90 cm–1 [91]. The ratio between the integrated areas of the bridge bounded/atop 422 bands was taken as an indication of the relative amount of Pd ensembles with respect to Pd Catalysts4232019 , 9,monomers 30 [86], and decreased by adding Au to Pd, indicating the simultaneous presence of Pd and 12 of 34 424 Au atoms within the same nanoparticle. 425 The formation turnover rates of H2O2 and H2O are dependent on the Au content of the 426 bimetallic AuxPd1 nanoparticles (Figure 7b). In particular, the formation rates of 0.11 mol H2O2 (mol increasing Au amount was larger than that of H2O2 at the experimental conditions reported in the 427 Pds s)–1 and 0.08 mol H2O (mol Pds s)–1 over the Au12Pd1 catalyst are lower than those obtained for the paper (55 kPa H , 60 kPa O , 305 K). These observations were in agreement with the observed decrease 428 Pd catalyst2 (0.40 mol2 H2O2 (mol Pds s)–1 and 1.4 mol H2O (mol Pds s)–1. Bare Au nanoparticles of the bridge/atop429 catalysed CO neither ratio H2 (FigureO2 nor H72Oa), formation, and with meaning literature that Pd ﬁndings is needed for on the AuPd H2O2 nanoparticlesdirect synthesis supported 430 and that Pd atoms are probably the active sites. Moreover, an enhancement of the selectivity towards on carbon [92], TiO2 [27], and SiO2 [89]. The increase in selectivity at increasing Au amounts was 431 H2O2 from 23% (bare Pd) up to 60% (Au12Pd1) was observed, since the decrease in H2O formation at associated with greater activation enthalpies for H O and H O production on isolated Pd atoms than 432 increasing Au amount was larger than that of H22O2 2at the experimental2 conditions reported in the on ensembles433 paper of Pd (55 atoms kPa H2, [ 8760 ].kPa O2, 305 K). These observations were in agreement with the observed 434 decrease of the bridge/atop CO ratio (Figure 7a), and with literature findings on AuPd nanoparticles With the aim of obtaining catalysts enriched with surface isolated Pd active sites for H2O2 435 supported on carbon [92], TiO2 [27], and SiO2 [89]. The increase in selectivity at increasing Au synthesis,436 Zhangamounts et was al. associated [54] synthesized with greater Pd-Znactivation bimetallic enthalpies for catalysts H2O2 and H supported2O production onon isolated alumina and with −1 −1 different437 Pd/ZnPd atoms metal than ratios.on ensembles The of Pd 1Pd5Zn atoms [87]. bimetallic catalyst reached 25,431 mol kgPd h of 438 With the aim of obtaining catalysts enriched with surface isolated Pd active sites for H2O2 produced H2O2, which was superior than the productivity achieved in the presence of monometallic 439 synthesis, Zhang et− al.1 [54]− 1synthesized Pd-Zn bimetallic catalysts supported on alumina and with 1Pd catalyst440 (8533different mol Pd/Zn kg Pdmetal hratios.). The 1Pd5Zn bimetallic catalyst reached 25,431 mol kgPd−1 h−1 of The441 authors produced bestowed H2O2, which the was marked superior improvement than the productivity of catalyticachieved inactivity the presence to of geometric monometallic and electronic effects442 induced 1Pd bycatalyst Zn (8533 addition. mol kgPd XPS−1 h−1).results indicated that at increasing Zn loadings the number of 443 The authors bestowed the marked improvement of catalytic activity to geometric and electronic surface444 contiguous effects induced Pd sites by Zn decreased addition. XPS and, results at the indicated same that time, at increasing an increase Zn loadings of the the number number of of isolated Pd sites was445 observed.surface contiguous Indeed, Pd Zn sites is decreased able to and, disassemble at the same time, the an surface increase Pdof the agglomerates, number of isolated thus Pd enhancing the amount446 ofsites Pd was islands observed. and Indeed, single Zn is atomsable to disass [53].emble The the catalysts surface Pd wereagglomerates, reduced thus atenhancing 300 ◦C by using a 447 the amount of Pd islands and single atoms [53]. The catalysts were reduced at 300 °C by using a 10% ◦ 10% H2448/N 2 mixtureH2/N2 mixture for 60for min,60 min, and and then cooled cooled in ain N2a atmosphere N2 atmosphere before CO before interaction CO at interaction 30 °C for 15 at 30 C for 15 min.449 The DRIFTmin. The spectraDRIFT spectra collected collected on on monometallic monometallic Pd and Pd on and bimetallic on bimetallic PdZn catalysts PdZn are shown catalysts in are shown in Figure450 11 . Figure 11. 451

452

Figure 11. In situ DRIFTS of CO adsorption over different catalysts. Reprinted from Appl. Catal. A: General, 531, S. Wang, K. Gao, W. Li, J. Zhang, Effect of Zn addition on the direct synthesis of hydrogen peroxide over supported palladium catalysts, 89–95, Copyright (2017), with permission from Elsevier.

Bands at 2089 (linear CO adsorbed on isolated surface Pd sites [51]), 1994 (bridged CO species on Pd edges [93]), and 1941 cm−1 (bridged CO species on Pd atoms exposed at the surface of either (100) or (111) facets [39]) due to CO on different Pd sites were observed on the monometallic 1Pd catalyst [88]. Upon Zn addition, a red-shift of these peaks along with a gradual decrease in intensity of the peak at 1941 cm−1, related to bridged carbonyls on Pd atoms on either (100) or (111) facets, was observed (red and blue curves), which almost completely disappeared in the case of the 1Pd5Zn catalyst (pink curve). These spectroscopic features put in evidence a strong Pd surface modiﬁcation induced by the presence of Zn. In agreement with Ouyang et al. [86], the authors ascribed the observed red-shift to an increase of the electron density of the d-orbital due to the charge transfer from Zn to Pd species [86], resulting in an increase of the isolated Pd0 sites amount. Indeed, the Pd-Zn electronic interaction positively promoted both H2 conversion and H2O2 productivity. Changes in surface composition and structure of bimetallic PdZn catalysts supported on silica were also already elegantly evaluated by Miller et al. using DRIFTS of CO adsorption combined with electron microscopy, X-ray absorption spectroscopy, CO chemisorption, and calorimetric analysis [53]. They observed the surface changes induced by varying the pre-reduction temperature from 225 up to 500 ◦C and reported the linear-to-bridge-bound ratio obtained by comparing the areas of the Catalysts 2019, 9, 30 13 of 34 peaks observed on two PdZn samples with different Pd:Zn molar ratios. The results are reported in Table1. A signiﬁcant increase in linear-bound CO was observed upon Zn addition, moreover the addition of a Zn excess zinc (2%Pd–10%Zn catalyst) resulted in a contained enhancement of the linear-to-bridge-bound CO ratio (2.5:1) with respect to the 3%Pd–1.8%Zn catalyst (2.1:1).

Table 1. Initial heat of CO adsorption values (±5 kJ/mol) in the presence of chemisorbed hydrogen and linear-to-bridge ratios from DRIFTS. Reprinted from J. Catal., 318, D. J. Childers, N. M. Schweitzer, S. Mehdi Kamali Shahari, R. M. Rioux, J. T. Miller, R. J. Meyer, Modifying structure-sensitive reactions by addition of Zn to Pd, 75–84, Copyright (2014), with permission from Elsevier.

CO (Initial) Heat of Reduction Sample 1 Adsorption with Chemisorbed Linear-to-Bridge Ratio Temperature (◦C) H (kJ/mol CO) 2 3 2Pd/SiO2 225 92 0.2:1 2Pd–10Zn/SiO2 300 102 2.5:1 3Pd–1.8Zn/SiO2 300 99 2.1:1 550 93 1.8:1 1 Catalysts were initially reduced under 5.11% H2/Ar (both gases, 99.999% UHP) for 2 h at the specified reduction 2 temperature,Catalysts followed 2018, 8, byx FOR subsequent PEER REVIEW cooling in H2. Initial heat of CO adsorption determined by microcalorimetry14 of 36 on a reduced Pd or PdZn surface containing chemisorbed hydrogen. 3 Monometallic Pd catalyst from previous work495 studied usingBased theon modifiedthe characterization procedure. results, the authors proposed simplified models of the catalyst 496 structure, which are shown in Figure 12b. In particular, the bimetallic nanoparticles of the 3%Pd– 497 1.8%Zn catalyst have a Pd core with surface Zn atoms upon reduction at 275 °C, and the surface The498 comparison PdZn film possesses of the DRIFT an intermetallic spectra alloy of COstructure, adsorbed based on on the the high 3%Pd–1.8%Zn intensity of the band catalyst related pre-reduced at 300499 and 550to linear-bound◦C (Figure CO 12 bands.a) put An increase in evidence in the surface a shift PdZn from intermetallic 2075 toalloy, 2070 with cm few− residual1 of the Pd peak due to 500 linear CO peakensembles, by increasing occurs at 550 the °C, reductionwhile the nanoparticle temperature core is Pd-rich. at 500 This◦C, is indicating due to the limited a small amount increase of the 501 of Zn atoms interacting with Pd, resulting in the impossibility to form a fully intermetallic PdZn population502 ofalloy linear nanoparticle carbonyls at such located temperature at the [94]. Pd edge sites. Analogously, the absorption band related to bridge-bounded503 Conversely, CO species in the case showed of the 3%Pd–1.8%Zn a broadening catalyst, and a ashell shift made from up by 1975 PdZn to alloy, 1965 totally cm −1 upon the 504 covering the Pd-rich core was proposed already at 275 °C. For both catalysts, the PdZn metallic increase505 of the reduction temperature. However, the linear-to-bridge ratio did not change significantly: surface◦ consists of an ordered◦ alloy structure, and a random distribution of metallic Pd and Zn from 2.1:1506 atsurface 275 C atoms to 1.8:1 is very at unlikely. 550 C. At 550 °C, there is almost complete formation of the PdZn alloy shell.

507 Figure508 12. (a)Figure Normalized 12. (a) Normalized DRIFT DRIFT spectra spectra of the of the 3%Pd–1.8%Zn 3%Pd–1.8%Zn catalyst catalyst pre-reduced pre-reduced at 300 °C at (red) 300 ◦C (red) and 509 ◦ and at 550 °C (blue). (b) Cross-section of Pd and PdZn catalysts showing structure change as at510 550 C (blue).reduction (b) temperature Cross-section increased. of Pd Reprinted and PdZn from J. catalystsCatal., 318, showingD. J. Childers, structure N. M. Schweitzer, change S. as reduction temperature511 increased.Mehdi Kamali Reprinted Shahari, R. M. from Rioux, J. J. Catal., T. Miller, 318, R. J. D. Meyer, J. Childers, Modifying N.structure-sensitive M. Schweitzer, reactions S. Mehdi Kamali Shahari,512 R. M.byRioux, addition J.of T.Zn Miller, to Pd, 75–84, R. J. Copyrigh Meyer,t Modifying(2014), with permission structure-sensitive from Elsevier. reactions by addition of Zn to513 Pd, 75–84,However, Copyright isolated (2014), Pd withatoms permissionwere responsible from for Elsevier. the enhancement of the catalytic activity and 514 the authors ascribed this improvement to geometric effects rather than an electronic effect due to Based515 onalloy the formation. characterization results, the authors proposed simpliﬁed models of the catalyst structure,516 whichAn areexcellent shown Pd–Te/TiO in Figure2 catalyst with12b. almost In 100% particular, H2O2 selectivity the under bimetallic mild conditions nanoparticles (283 of the 517 K, 0.1 MPa, using a semi-batch continuous flow reactor) was recently investigated [52]. Upon ◦ 3%Pd–1.8%Zn518 addition catalyst of tellurium, have Pd a Pdnanoparticles core with with surface smaller size Zn were atoms obtained, upon and reduction such Te-modified at 275 Pd C, and the surface519 PdZn surfaces ﬁlm possesses hindered the anO2 dissociative intermetallic activation, alloy favouring structure, at the based same time on the the non-dissociative high intensity O2 of the band related520 to linear-boundhydrogenation. CO In bands.order to Anfind increasestructure–activity in the surfacerelationships, PdZn DRIFT intermetallic spectroscopy alloy, of adsorbed with few residual 521 CO was employed in combination◦ with STEM-EDS, EXAFS, O2-TPD, XPS, and DFT calculations to Pd ensembles,522 shed occurs light on atthe 550electronicC, structure while theof the nanoparticle surface Pd atoms. core is Pd-rich. This is due to the limited amount523 of Zn atomsAs shown interacting in Figure 13a, with the band Pd, ascribed resulting to CO in linearly the impossibility adsorbed on Pd sites to formred-shifted a fully from intermetallic –1 PdZn alloy524 nanoparticle2079 to 2051 cm at upon such addition temperature of Te. Moreover, [94]. according to DFT calculations, the binding energy 525 of CO adsorbed on low-coordinated Pd sites exposed at the surface of smaller particles was 526 noticeably higher than that related to CO adsorbed on the Pd terrace sites of extended surfaces. 527 Particularly, the amount of smaller Pd nanoparticles increased at increasing Te loading, resulting in 528 an increase of the low-coordinated Pd corners and edges, as indicated by the enhancement of the 529 intensity of the band related to linearly carbonyls and by the gradual decrease of the absorption 530 centred at 1982–1902 cm–1 ascribed to CO on bridge and hollow sites [83]. Indeed, only the band 531 related to CO linearly bounded was present in the case of the Pd17Te4 and Pd3Te2 catalysts, 532 confirming the presence of highly dispersed clusters or isolated Pd atoms. 533 A mechanism explaining the role of Te in improving the catalytic activity was proposed based 534 on STEM-EDS, EXAFS, and CO-DRIFTS results. In particular, the reason for the noticeable catalytic

Catalysts 2019, 9, 30 14 of 34

Conversely, in the case of the 3%Pd–1.8%Zn catalyst, a shell made up by PdZn alloy, totally covering the Pd-rich core was proposed already at 275 ◦C. For both catalysts, the PdZn metallic surface consists of an ordered alloy structure, and a random distribution of metallic Pd and Zn surface atoms is very unlikely. At 550 ◦C, there is almost complete formation of the PdZn alloy shell. However, isolated Pd atoms were responsible for the enhancement of the catalytic activity and the authors ascribed this improvement to geometric effects rather than an electronic effect due to alloy formation. An excellent Pd–Te/TiO2 catalyst with almost 100% H2O2 selectivity under mild conditions (283 K, 0.1 MPa, using a semi-batch continuous flow reactor) was recently investigated [52]. Upon addition of tellurium, Pd nanoparticles with smaller size were obtained, and such Te-modified Pd surfaces hindered the O2 dissociative activation, favouring at the same time the non-dissociative O2 hydrogenation. In order to find structure–activity relationships, DRIFT spectroscopy of adsorbed CO was employed in combination with STEM-EDS, EXAFS, O2-TPD, XPS, and DFT calculations to shed light on the electronic structure of the surface Pd atoms. As shown in Figure 13a, the band ascribed to CO linearly adsorbed on Pd sites red-shifted from 2079 to 2051 cm−1 upon addition of Te. Moreover, according to DFT calculations, the binding energy of CO adsorbed on low-coordinated Pd sites exposed at the surface of smaller particles was noticeably higher than that related to CO adsorbed on the Pd terrace sites of extended surfaces. Particularly, Catalysts 2018, 8, x FOR PEER REVIEW 15 of 36 the amount of smaller Pd nanoparticles increased at increasing Te loading, resulting in an increase of the low-coordinated535 performance Pd corners was ascribed and to edges, the presence as indicated of selectively by active the enhancementsites bimetallic Pd–Te, of the in which intensity the Te of the band 536 2 2 inertness toward O adsorption significantly inhibited the dissociative O activation as revealed by −1 related to537 linearly XPS carbonylsand O2-TPD andanalyses. by theAs agradual matter of decreasefact, the Pd of150Te the1 catalyst absorption containing centred poor Te at amount 1982–1902 cm ascribed538 to COshowed on bridge higher and selectivity hollow than sites the [ 83monometallic]. Indeed, Pd only catalyst. the band Side reactions related towere CO noticeably linearly bounded 539 inhibited and a selectivity of almost 100% was achieved by increasing the Te/Pd atomic ratio to 1:100, was present in the case of the Pd17Te4 and Pd3Te2 catalysts, confirming the presence of highly dispersed 540 because of the increased number of surface Pd–Te sites. Indeed, a much contained increase of the clusters or541 isolated Te/Pd Pdratio atoms. to 1:50 and 1:17 resulted to lower selectivity (78.8% and 67.3%, respectively). 542

543

Figure544 13. (a) InFigure situ 13. DRIFTS (a) In situ measurement DRIFTS measurement of CO of adsorptionCO adsorption over over the the Pd–Te/TiO Pd–Te/TiO2 catalysts2 catalysts at 283 K at 283 K in 545 in an Ar flow. (b) Proposed mechanism for the direct synthesis of H2O2 over Pd–Te/TiO2 catalysts. an Ar flow. (b) Proposed mechanism for the direct synthesis of H2O2 over Pd–Te/TiO2 catalysts. Blue 546 Blue and brown spheres are Pd and Te atoms, respectively. Reprinted from P. Tian, X. Xu, C. Ao, D. and547 brown spheresDing, areW. Li, Pd R. andSi, W. Te Tu, atoms, J. Xu, Y.-F. respectively. Han, ChemSusChem, Reprinted 10, from3342–3346, P. Tian, Copyright X. Xu, (2017), C.Ao, John D. Ding, W. Li, R.548 Si, W. Tu,Wiley J. Xu, and Y.-F. Sons. Han, ChemSusChem, 10, 3342–3346, Copyright (2017), John Wiley and Sons. 549 At the same time, as indicated by XRD, TEM, and CO-DRIFTS findings, the Pd particle size was A mechanism550 diminished explaining at increasing the Te/Pd role ratios, of Te and in improvinglow-coordinated the active catalytic edges and activity corners were was formed. proposed based on STEM-EDS,551 Such EXAFS, low-coordinated and CO-DRIFTS sites are much results. more active In particular,to bind and dissociate the reason molecular for theO2, with noticeable respect catalytic 552 to terrace sites [95]. As a result, the selectivity to H2O2 of the Pd50Te1 and Pd17Te1 catalysts is lower performance was ascribed to the presence of selectively active sites bimetallic Pd–Te, in which the 553 than that observed in the case of Pd100Te1. Thus, a high amount of surface Te sites would inhibit both Te inertness554 towardH2 and O O2 2reactantadsorption molecules significantly adsorption, resulting inhibited in inert thePd17Te dissociative4 and Pd3Te2 materials. O2 activation Indeed, the as revealed 555 surface Pd atoms of these catalysts are isolated by Te atoms, as revealed by EXAFS and CO-DRIFTS. by XPS and O2-TPD analyses. As a matter of fact, the Pd150Te1 catalyst containing poor Te amount 556 The results indicate that only by finely tuning the Pd/Te ratio it is possible to control the amount of showed higher557 terrace selectivity Pd-Te sites, than as thewell monometallicas the particle size Pdto obtain catalyst. highly-performing Side reactions catalysts. were noticeably inhibited and a selectivity558 ofVery almost recently 100% Flaherty was et achievedal. reported that by increasingthe steady-state the selectivity Te/Pd to atomic H2O2 of AgPt ratio octahedra to 1:100, because of the increased559 supported number on ofsilica surface (50%) is Pd–Te 10-fold sites.greater Indeed,than observed a much with containedPt/SiO2 nanoparticles increase with of similar the Te/Pd ratio 560 size (6%) [45]. In addition, the abundance and location of Pt atoms on the AgPt octahedra surface to 1:50 and 1:17 resulted to lower selectivity (78.8% and 67.3%, respectively). 561 play a role on the initial formation rates selectivity to H2O2. The population of these sites can be At the562 samemodulated time, aseither indicated by CO exposure by XRD, at TEM,373 K to and obtain CO-DRIFTS a Pt-rich surface findings, (with 16% the initial Pd particle H2O2 size was diminished563 atselectivity) increasing or by Te/Pd inert gases ratios, exposure and at low-coordinated the same temperature activeto achieve edges a Pt-poor and surface corners (withwere 36% formed. 564 initial H2O2 selectivity). Such low-coordinated565 FTIR experiments sites are muchof adsorbed more 12CO active were then to bindperformed and on dissociate the monometallic molecular Pt catalyst O2 and, with respect 566 on the AgPt catalyst pretreated in 12CO or He (Figure 14a). Firstly, it has to be recalled that 12CO does 567 not adsorb on the Ag sites under the adopted experimental conditions. The bands of adsorbed 12CO 568 on AgPtCO (black curve) and AgPtHe (red curve) octahedra differ significantly in both position and 569 intensity, due to modifications of surface structure of the AgPt octahedral, because the FTIR spectra 570 were obtained by submitting the same sample pellet to in situ CO and He pre-treatments. 571 Conversely, the band at 2098 cm–1 was assigned linear carbonyl species adsorbed on single Pt atoms 572 exposed at the surface of the nanoparticles [96] and on Pt(111) [81]. The positions of the bands 573 related to linearly adsorbed 12CO are significantly red-shifted at 2051 cm–1 for AgPtCO and at 2033 cm– 574 1 in the case of AgPtHe, revealing different extents of electron exchange between the 12CO probe. Such 575 a feature can be ascribed to lower dipole–dipole coupling between the 12CO molecules due to the

Catalysts 2019, 9, 30 15 of 34

to terrace sites [95]. As a result, the selectivity to H2O2 of the Pd50Te1 and Pd17Te1 catalysts is lower than that observed in the case of Pd100Te1. Thus, a high amount of surface Te sites would inhibit both H2 and O2 reactant molecules adsorption, resulting in inert Pd17Te4 and Pd3Te2 materials. Indeed, the surface Pd atoms of these catalysts are isolated by Te atoms, as revealed by EXAFS and CO-DRIFTS. The results indicate that only by finely tuning the Pd/Te ratio it is possible to control the amount of terrace Pd-Te sites, as well as the particle size to obtain highly-performing catalysts. Very recently Flaherty et al. reported that the steady-state selectivity to H2O2 of AgPt octahedra supported on silica (50%) is 10-fold greater than observed with Pt/SiO2 nanoparticles with similar size (6%) [45]. In addition, the abundance and location of Pt atoms on the AgPt octahedra surface play a role on the initial formation rates selectivity to H2O2. The population of these sites can be modulated either by CO exposure at 373 K to obtain a Pt-rich surface (with 16% initial H2O2 selectivity) or by inert gases exposure at the same temperature to achieve a Pt-poor surface (with 36% initial H2O2 selectivity). FTIR experiments of adsorbed 12CO were then performed on the monometallic Pt catalyst and on the AgPt catalyst pretreated in 12CO or He (Figure 14a). Firstly, it has to be recalled that 12CO does not adsorb on the Ag sites under the adopted experimental conditions. The bands of adsorbed 12CO on AgPtCO (black curve) and AgPtHe (red curve) octahedra differ significantly in both position and intensity, due to modifications of surface structure of the AgPt octahedral, because the FTIR spectra were obtained by submitting the same sample pellet to in situ CO and He pre-treatments. Conversely, the band at 2098 cm−1 was assigned linear carbonyl species adsorbed on single Pt atoms exposed at the surface of the nanoparticles [96] and on Pt(111) [81]. The positions of the bands related to linearly 12 −1 −1 adsorbed CO are significantly red-shifted at 2051 cm for AgPtCO and at 2033 cm in the case of 12 AgPtHe, revealing different extents of electron exchange between the CO probe. Such a feature can be ascribed to lower dipole–dipole coupling between the 12CO molecules due to the lower coverage of linear carbonyls, which results in a red-shift of the 12CO band [81]. In addition, the intensity of the peaks related to the AgPt octahedra pre-treated with CO was about six times the intensity observed 12 for AgPtHe, which pointed out that the CO pre-treatment was able to extract a much bigger amount of exposed Pt sites at the surface of the AgPt octahedra with respect to the treatment in He. Moreover, −1 the difference in position of about 20 cm for AgPtCO and AgPtHe was in agreement with the literature on CO adsorbed on well-coordinated Pt terraces and on under-coordinated Pt edges [96]. In order to verify the occurrence of different electronic effects on the catalysts, the singleton frequency can be evaluated by using 13CO–12CO isotopic mixture to dilute the adsorbed 12CO molecules and by evaluating of exchange rates between adsorbed 13CO and 12CO. The changes in the singleton frequency of adsorbed 13CO clearly pointed out the electronic changes in Pt atoms as shown in section b of Figure 14. In particular, a red-shift of the band related to adsorbed 13CO was 13 13 observed by decreasing the CO coverage (θ CO), signifying the occurrence of dipole–dipole coupling. The 13CO singleton frequency was calculated by extrapolation of the spectra (shown in section a) to a zero coverage, and obtaining 2016 ± 6 cm−1 for the monometallic Pt catalyst, 1974 ± 8 cm−1 for −1 AgPtCO, and 1979 ± 12 cm for AgPtHe. The last two values were much lower than on the Pt catalyst, which indicates a greater extent of electron back-donation from the Pt atoms to the CO 2π* orbitals on the AgPt octahedra. The above findings were unexpected, since the AgPt octahedra gave higher selectivity to H2O2 than Pt nanoparticles. 13 12 The isosteric CO desorption under CO flow on AgPtHe, AgPtCO and on Pt nanoparticles are reported in Figure 14, section c. It can be observed that the 13CO desorption and exchange with 12 13 CO take 400 s on Pt, whereas only about 50% and 15% CO exchange occurs in 800 s on AgPtHe 13 and AgPtCO, respectively. The CO desorption rates were described by Equation (3), a first-order desorption process for surfaces containing n adsorbing sites:

n dθ13CO − = ∑ ki·θ13CO,i (3) dt i Catalysts 2018, 8, x FOR PEER REVIEW 16 of 36

576 lower coverage of linear carbonyls, which results in a red-shift of the 12CO band [81]. In addition, the 577 intensity of the peaks related to the AgPt octahedra pre-treated with CO was about six times the 578 intensity observed for AgPtHe, which pointed out that the 12CO pre-treatment was able to extract a 579 much bigger amount of exposed Pt sites at the surface of the AgPt octahedra with respect to the 580 treatment in He. Moreover, the difference in position of about 20 cm–1 for AgPtCO and AgPtHe was in 581 agreement with the literature on CO adsorbed on well-coordinated Pt terraces and on Catalysts 2019, 9, 30 16 of 34 582 under-coordinated Pt edges [96]. 583 In order to verify the occurrence of different electronic effects on the catalysts, the singleton 584 frequency can be evaluated by using 13CO–12CO isotopic mixture to dilute the adsorbed 12CO −1 where 585ki is themolecules rate constant and by evaluating for CO of desorption exchange rates from between site adsorbedi expressed 13CO and in 12CO. s The. The changes sum in ofthe all coverages 13 586 singleton frequency of adsorbed 13CO clearly pointed out the electronic13 changes in Pt atoms as θ CO,I = 1 at the exchange onset. The desorption rates for >95% of CO molecules from the surfaces 587 shown in section b of Figure 14. In particular, a red-shift of the band related to adsorbed 13CO was of the Pt nanoparticles were accurately delineated by a model that describes a uniform surface for 588 observed by decreasing the 13CO coverage (θ13CO), signifying the occurrence of dipole–dipole which 589n = 1.coupling. This was The not13CO truesingleton in the frequency case ofwas AgPt calculatedHe and by extrapolation AgPtCO octahedral, of the spectra because(shown in two distinct sites have590 tosection be considered a) to a zero coverage, to explain and obtaining the desorption 2016 ± 6 cm kinetics–1 for the monometallic and the signiﬁcant Pt catalyst, 197413CO ± 8 amount that 591 cm–1 for AgPtCO, and 1979 ± 12 cm–1 for AgPtHe. The last two values were much lower than on the Pt remain592 adsorbed catalyst, during which indicates the experiment. a greater extent H of2 Oelectron2 and back-donation H2O form from on distinctthe Pt atoms active to the CO sites 2π* and the ratio between593 those orbitals on whichon the AgPt H2 Ooctahedra.2 is formed The above and findings those were on whichunexpected, H2 Osince formation the AgPt octahedra occurs gave increased upon Ag addition.594 higher selectivity to H2O2 than Pt nanoparticles. 595

596 597 Figure 14. (a) infrared spectra of adsorbed 12CO following the purge of gas-phase 12CO with He (101 Figure 14. (a) infrared spectra of adsorbed 12CO following the purge of gas-phase 12CO with He 598 kPa, 30 cm3 min–1, 303 K) on Pt (blue), AgPtCO (black), and AgPtHe (red). Spectra are the average of 128 3 −1 (101599 kPa, 30 cmscansmin with a 4, cm 303–1 resolution. K) on Pt Dotted (blue), lines AgPt are intendedCO (black), to show and the peak AgPt positionHe (red). for each Spectra spectrum. are the average −1 13 13 13 of600 128 scans with(b and ac) 4Peak cm positionresolution. of ν(C=O) for DottedCO as a function lines are of the intended coverage of to adsorbed show theCO peak(θ CO) position(total for each 601 of 1 kPa CO, 100 kPa He, 100 cm3 min–1, 298 K) and change in the coverage of θ13CO as a function of spectrum. (b,c) Peak position of ν for 13CO as a function of the coverage of adsorbed 13CO (θ13 ) 602 time during exchange with (C=O)12CO on Pt (blue triangles) and CO-treated (black squares) and He-treated CO 3 −1 13 (total603 of 1 kPa(red CO, circles) 100 AgPt kPa octahedral He, 100 particles cm (0.2min kPa 12CO,, 298 100 cm K)3 min and–1, change298 K; surfaces in the pre-saturated coverage with of θ CO as a function604 of time13CO). duringSolid lines exchange in (b) represent with a linear12CO fit to on the Pt data, (blue and da triangles)shed lines in and panel CO-treated c represent the (black fit squares) 605 of θ13CO as a function of time. Adapted with permission from ACS Catalysis, N. M. Wilson, Y.-T. Pan, and He-treated (red circles) AgPt octahedral particles (0.2 kPa 12CO, 100 cm3 min−1, 298 K; surfaces 606 Y.-T. Shao, J.-M. Zuo, H. Yang, D. W. Flaherty, Direct Synthesis of H2O2 on AgPt Octahedra: The Formatted: Subscript pre-saturated with 13CO). Solid lines in (b) represent a linear fit to the data, and dashed lines in panel c 607 Importance of Ag–Pt Coordination for High H2O2 Selectivity, 8, 2880–2889. Copyright (2018) Formatted: Subscript 13 represent608 theAmerican fit of θ ChemicalCO as Society. a function of time. Adapted with permission from ACS Catalysis, N. M. Wilson, Y.-T. Pan, Y.-T. Shao, J.-M. Zuo, H. Yang, D. W. Flaherty, Direct Synthesis of H2O2 on AgPt 609 The isosteric 13CO desorption under 12CO flow on AgPtHe, AgPtCO and on Pt nanoparticles are Octahedra:610 reported The Importancein Figure 14, section of Ag–Pt c. It Coordinationcan be observed forthat Highthe 13CO H2 desorptionO2 Selectivity, and exchange 8, 2880–2889. with 12CO Copyright (2018)611 Americantake 400 s Chemical on Pt, whereas Society. only about 50% and 15% 13CO exchange occurs in 800 s on AgPtHe and 612 AgPtCO, respectively. The 13CO desorption rates were described by Equation (3), a first-order 3.3. Reactivity613 desorption of the Exposed process for Metal surfaces Sites containing n adsorbing sites: Only few studies proposed in situ approaches to investigate by FTIR spectroscopy of adsorbed probes the reactivity of the metallic active sites upon interactions with the reagents, i.e., molecular hydrogen and oxygen or mixtures of the two. It is worth noting that the reagents involved in the H2O2 direct synthesis are IR inactive, because there is no change in the dipole moment of the molecules upon interaction with the IR radiation. One very recent example on this approach is reported in the study by Han et al. [58], which has been already discussed in Section 3.1 as for the nature of the Pd sites on Pd/TiO2 catalysts. In addition to the common investigation carried out by adsorbing the CO probe, the authors characterized the catalysts submitted to subsequent adsorption of molecular oxygen and hydrogen, before the inlet of the CO probe. Indeed, the surface of the freshly reduced catalyst dramatically changed −1 upon O2 preadsorption: the narrow peak at 2094 cm due to the linear carbonyls (see Figure7b) was transformed into two weak peaks at 2090 and 2064 cm−1 (Figure 15). Moreover, bridged and multibonded CO species on Pd ensembles red-shifted to 1986, 1914, and 1874 cm−1, and also the relative intensities were modified, due to the geometric and electronic modification of Pd atoms induced by oxygen adsorption. Such changes were accompanied by an increase in intensity of the band at 1646 cm−1 due to the formation of carbonate species. Catalysts 2018, 8, x FOR PEER REVIEW 17 of 36

d 𝑘  (3) d𝑡 , 614 615 where ki is the rate constant for CO desorption from site i expressed in s–1. The sum of all coverages 616 θ13CO,I = 1 at the exchange onset. The desorption rates for >95% of 13CO molecules from the surfaces of Formatted: Not Highlight 617 the Pt nanoparticles were accurately delineated by a model that describes a uniform surface for 618 which n = 1. This was not true in the case of AgPtHe and AgPtCO octahedral, because two distinct sites 619 have to be considered to explain the desorption kinetics and the significant 13CO amount that remain 620 adsorbed during the experiment. H2O2 and H2O form on distinct active sites and the ratio between 621 those on which H2O2 is formed and those on which H2O formation occurs increased upon Ag 622 addition.

623 3.3. Reactivity of the Exposed Metal Sites 624 Only few studies proposed in situ approaches to investigate by FTIR spectroscopy of adsorbed 625 probes the reactivity of the metallic active sites upon interactions with the reagents, i.e., molecular 626 hydrogen and oxygen or mixtures of the two. It is worth noting that the reagents involved in the 627 H2O2 direct synthesis are IR inactive, because there is no change in the dipole moment of the 628 molecules upon interaction with the IR radiation. 629 One very recent example on this approach is reported in the study by Han et al. [58], which has 630 been already discussed in Section 3.1 as for the nature of the Pd sites on Pd/TiO2 catalysts. In 631 addition to the common investigation carried out by adsorbing the CO probe, the authors 632 characterized the catalysts submitted to subsequent adsorption of molecular oxygen and hydrogen, 633 before the inlet of the CO probe. Indeed, the surface of the freshly reduced catalyst dramatically 634 changed upon O2 preadsorption: the narrow peak at 2094 cm–1 due to the linear carbonyls (see Figure 635 7b) was transformed into two weak peaks at 2090 and 2064 cm–1 (Figure 15). Moreover, bridged and 636 multibonded CO species on Pd ensembles red-shifted to 1986, 1914, and 1874 cm–1, and also the 637 relative intensities were modified, due to the geometric and electronic modification of Pd atoms 638 induced by oxygen adsorption. Such changes were accompanied by an increase in intensity of the Catalysts 2019, 9, 30 17 of 34 639 band at 1646 cm–1 due to the formation of carbonate species. 640

641 642 Figure 15. In situ DRIFTS of CO adsorption over the reduced 1.0% Pd catalyst: A: pretreatment with Figure 15. In situ DRIFTS of CO adsorption over the reduced 1.0% Pd catalyst: A: pretreatment with 643 O2, adsorption of CO and purged with Ar; B: H2 treatment of A, readsorption of CO and purged with O2644, adsorption Ar; ofC: O CO2 retreatment and purged of B, readsorption with Ar; B:of CO H2 andtreatment purged with of Ar. A, All readsorption the experiments of were CO carried and purged with Ar;645 C: O2 retreatmentout at 283 K. of Reprinted B, readsorption from J. Catal., of 321, CO L. andOuyang, purged P. Tian, with G. Da, Ar. X. Xu, All C. the Ao, experimentsT. Chen, R. Si, J. were carried out at 283 K. Reprinted from J. Catal., 321, L. Ouyang, P. Tian, G. Da, X. Xu, C. Ao, T. Chen, R. Si, J. Xu,

Y. Han, The origin of active sites for direct synthesis of H2O2 on Pd/TiO2 catalysts: Interfaces of Pd and PdO domains, 70–80, Copyright (2018), with permission from Elsevier.

At the same time a new band at 2126 cm−1, probably due to CO on positively-charged Pd formed after oxygen pre-adsorption and the broad and complex absorption at 1800–2000 cm−1 remained −1 unchanged. Upon H2 contact for 5 min the band at 1646 cm decreased in intensity and the peak at 2126 cm−1 was totally depleted, pointing out the reduction of the positively-charged Pd sites. The intensity of these absorptions was completely recovered after further O2 inlet for 5 min and re-adsorption of CO, which is an indication of partial oxidation of the surface Pd atoms. This behaviour suggested that the electronic structure of the surface Pd atoms undergoes to dynamical changes during the reaction as a consequence of the interaction and adsorption of molecular oxygen and hydrogen reactants. Moreover, the comparison with the CO bands observed in the presence or in the absence of oxygen (Figure7a), allowed understanding that oxygen is preferentially adsorbed on the Pd corners or edges sites, according to the marked decrease in intensity of the bands related to linearly adsorbed CO. At the same time, oxygen is also adsorbed on the flat sites of the Pd particles, as revealed by the change occurred on the bands assigned to CO adsorbed on Pd continuous ensembles. Therefore, the Pd corner or edge sites, which are highly reactive towards oxygen dissociation are modified by adsorbed oxygen species or are partially oxidized to PdO, which weakens the O2–Pd interaction. Such modification positively influenced the molecular O2 activation and the further H2O2 synthesis, which is assumed to occur at the interface of the Pd and PdO domains. The characterization results showed that a fraction of the surface Pd atoms can be easily oxidized at 283 K (47.6% for the 1.0 wt% Pd catalyst and 35.2% for the 5.0 wt% Pd one), and form Pd–PdO ensembles. The electronic structure of the surface Pd atoms has proved to change dynamically in different atmospheres. The experimental results presented in the paper clearly demonstrated that the structure of supported Pd active sites can be tuned by choosing opportunely the metal loading while the size of the metal particles is unchanged. Therefore, the variation of the metal loading does not correspond to a simple increase in the number of active sites, but it is an effective approach to tune the structure of the active sites. These insights into the Pd–PdO–TiO2 interface greatly contributed to understand the active phase geometric and electronic structure and the mechanism for direct H2O2 synthesis. Strukul et al. previously observed that surface oxidized monometallic Pd and bimetallic Pd-Au catalysts pretreated with H2 and O2 displayed higher activity and selectivity than the untreated samples [97]. Moreover, even though gold itself is not active, both productivity and selectivity were improved by Au addition. The catalysts with different supports have been tested the H2O2 direct synthesis under very mild and non-explosive conditions (1 bar, 20 ◦C). The catalysts supported on Catalysts 2018, 8, x FOR PEER REVIEW 19 of 36

Catalysts6982019 , 9,dispersion 30 can modify the interaction with oxygen, strongly affecting the selectivity. It was reported 18 of 34 699 that oxygen cannot dissociate on the very small metallic clusters, due to the repulsive effects 700 between the two negatively-charged O atoms[99]. Upon H2 pre-treatment, the band at 1946 cm−1 701 observed for both mono- and bimetallic catalysts (fine curves), and assigned to CO adsorbed on sulphated702 zirconiabridge-bonded (ZS) CO gave on Pd the terrace best sites performance of (111) facets (1270[100], increased mmol in H intensity.2O2/gPd Conversely, h after this three hours of reaction,703 with band 61% was selectivity) almost completely in the depleted presence after interaction of H2/O with2 mixtures the H2–O2 with mixture a largeon ZS–Pd excess (Figure of 16a, oxygen (4/96). HRTEM704 dashed measurements curve), and slightly indicated decreased that in intensity gold and on ZS–PdAu palladium (Figure were 16b, dashed in close curve). contact This and FTIR 705 behaviour indicates that on ZS–PdAu a large fraction of the terrace sites was preserved after spectroscopy706 oxidation, was employed resulting in toa less (i) defe monitorctive and the less nature energetic and surface. the Moreover, electronic in the properties case of the ZS– of the Pd sites upon Au707 addition, PdAu catalyst, and (ii) the the intensity reactivity of the of absorption these sites at after1770 cm pretreatment−1 was maximum, in Odue2 and/or to a larger H2 atmosphere. CO was708 adsorbed back-donation at room from temperaturethe metal to the onCO theprobes, ZS-Pd as a result and ZS-PdAuof an electronic catalysts effect of andgold theon results are 709 palladium. compared710 in FigureThe presence 16a,b, of respectively. gold in close proximity The FTIR of palladium spectra was were therefore collected proposed on thebased untreated on the catalysts (bold curves),711 changes on those in the submitted electron density to H induced2 pre-treatment on Pd sites evidenced at room by temperature FTIR and in the (ﬁne morphology curves), and on the 712 observed by HRTEM. catalysts pre-treated with H2 and O2 (dashed curves). 713

714 715 Figure 16. FTIR spectra of ZS–Pd (a) and ZS–PdAu (b) after CO adsorption at r.t on the as received Figure 16. FTIR spectra of ZS–Pd (a) and ZS–PdAu (b) after CO adsorption at r.t on the as received 716 samples (bold curves), on the samples pretreated at room temperature with hydrogen (fine curves) samples717 (boldand curves), pretreated on with the hydrogen samples and oxygen pretreated (dashed at curves). room The temperature spectra have been with normalized hydrogen to the (ﬁne curves) and718 pretreated same with Pd content. hydrogen Reprinted and from oxygen J. Catal., (dashed 257, F. Menegazzo, curves). P. The Burti, spectra M. Signoretto, have M. been Manzoli, normalized S. to the same719 Pd content.Vankova, Reprinted F. Boccuzzi, from F. Pinna, J. Catal., G. Strukul, 257, F.Effe Menegazzo,ct of the addition P. Burti,of Au M.in zirconia Signoretto, and ceria M. Manzoli, S. 720 supported Pd catalysts for the direct synthesis of hydrogen peroxide, 369–381, Copyright (2008), with Vankova,721 F. Boccuzzi,permission from F. Pinna, Elsevier. G. Strukul, Effect of the addition of Au in zirconia and ceria supported Pd catalysts for the direct synthesis of hydrogen peroxide, 369–381, Copyright (2008), with permission 722 2 2 from Elsevier.The authors investigated the effect of the H –O pre-treatment at room temperature on the 723 surface oxidation state of the bimetallic catalysts by FTIR of CO absorption (Figure 17). Firstly, the 724 intensity of the CO bands observed for the AuPd catalyst supported−1 on ceria (CeS–PdAu, orange −1 Different725 curve) bands was quite in the weak, range analogously of linear to what (2200–2000 observed on the cm same )sample and simply bridged outgassed (2000–1700 at room cm ) CO carbonyl726 species temperature wereobserved (data not shown). upon COHowever, adsorption in this case have the been on top detected CO linear [ 98species/bridged]. The ZS-Pd CO and ZS-PdAu catalysts727 possessspecies veryintensity similar ratio obtained spectroscopic without pre-treatment features: and all after the H2–O produced2 interaction bandswas quite were similar, related to Pd carbonyl species, no bands due to CO on Au sites were observed for the bimetallic catalysts, and the intensity of the CO bands produced on the ZS-PdAu sample was much higher than that related to the bands observed for the monometallic catalyst, independently from their pretreatments. FTIR spectroscopy pointed out that in the bimetallic catalysts the Pd dispersion was higher, in agreement with HRTEM results. In addition to the increased number of active sites, a high metal dispersion can modify the interaction with oxygen, strongly affecting the selectivity. It was reported that oxygen cannot dissociate on the very small metallic clusters, due to the repulsive effects between −1 the two negatively-charged O atoms [99]. Upon H2 pre-treatment, the band at 1946 cm observed for both mono- and bimetallic catalysts (ﬁne curves), and assigned to CO adsorbed on bridge-bonded CO on Pd terrace sites of (111) facets [100], increased in intensity. Conversely, this band was almost completely depleted after interaction with the H2–O2 mixture on ZS–Pd (Figure 16a, dashed curve), and slightly decreased in intensity on ZS–PdAu (Figure 16b, dashed curve). This behaviour indicates that on ZS–PdAu a large fraction of the terrace sites was preserved after oxidation, resulting in a less defective and less energetic surface. Moreover, in the case of the ZS–PdAu catalyst, the intensity of Catalysts 2019, 9, 30 19 of 34

Catalysts 2018, 8, x FOR PEER REVIEW 20 of 36 the absorption728 which at 1770indicates cm a −contained1 was maximum,decrease of the dueexposed to sites a larger and no back-donation changes in their nature from and the relative metal to the CO probes,729 as a resultabundance. of an electronic effect of gold on palladium. 730 Conversely, the overall intensity of the bands observed for the ZS–PdAu catalyst (blue curve) The731 presencewas much of stronger. gold inIn closeaddition, proximity the linear/bridged of palladium intensity ratio was was thereforemuch higher proposed for ZS–PdAu based on the changes732 in thewith electronrespect to densityCeS-PdAu. inducedThis spectroscopic on Pd feat sitesure is evidenced due to the different by FTIR nature and of inthe thetwo morphology observed733 bysupports. HRTEM. Ceria can be quite easily reduced at the interface with the metallic particles under mild 734 conditions, which results in the production of oxygen vacancies, and migration of oxygen on the The735 authors small investigatedmetallic particles the [84]. effect Moreover, of the on this H2 cataly–O2 pre-treatmentst a large fraction of at the room Pd sites temperature was present as on the surface oxidation736 statePdO, of on the which bimetallic CO does not catalysts adsorb, upon by H FTIR2–O2 treatment of CO absorption[101]. PdO species (Figure formation 17). takes Firstly, place the intensity of the CO737 bandspreferably observed at the particle/support for the AuPd interface, catalyst wherea supporteds the Pd atoms on located ceria on (CeS–PdAu, top of the particle, orange far curve) was 738 from the interface between the metal and the oxide keep on being metallic when the particle size quite weak,739 analogouslyis >3 nm [102]. toIn this what frame, observed the nature on of the the samesupport sample strongly simplyaffected both outgassed the formation at room and temperature (data not740 shown).stabilization However, of oxidized in particles. this case the on top CO linear species/bridged CO species intensity 741 ratio obtained withoutSuch process pre-treatment was promoted and on after the ceria H –O supportinteraction because of was its quiteoxygen similar,storage-release which indicates a 742 properties along with the smaller size of the meta2 llic2 particles detected on both the Pd and PdAu contained743 decreasecatalysts supported of the exposed on CeS catalysts, sites and according no changes to the HRTEM in their observations. nature and relative abundance.

0.25 A

1946

2126 1982 1905 2164 1995 2099 1950 1769 1889

2300 2200 2100 2000 1900 1800 1700 -1 744 Wavenumbers (cm ) 745 Figure 17. FTIR spectra of CO adsorbed at r.t. on the ZS–PdAu (blue curve) and CeS–PdAu (orange Figure 17. FTIR spectra of CO adsorbed at r.t. on the ZS–PdAu (blue curve) and CeS–PdAu (orange 746 curve) samples previously contacted with a H2–O2 mixture at room temperature. The spectra have curve)747 samplesbeenpreviously normalized to contactedthe same Pd withcontent. a Reprinted H2–O2 mixturefrom J. Catal at., room257, F. Menegazzo, temperature. P. Burti, The M. spectra have been748 normalizedSignoretto, to theM. Manzoli, same PdS. Vankova, content. F. Boccuzzi, Reprinted F. Pinna, from G. Strukul, J. Catal., Effect 257, of the F. addition Menegazzo, of Au in P. Burti, M. Signoretto,749 M.zirconia Manzoli, and ceria S. supported Vankova, Pd F.catalysts Boccuzzi, for the F.direct Pinna, synthesis G. Strukul,of hydrogen Effect peroxide, of the369–381, addition of Au 750 Copyright (2008), with permission from Elsevier. in zirconia and ceria supported Pd catalysts for the direct synthesis of hydrogen peroxide, 369–381, Copyright751 (2008),Based withon the permission characterization from results, Elsevier. a reaction mechanism for the Pd and Pd-Au bimetallic 752 catalysts was proposed depending on the nature of the active metallic sites, i.e., less energetic and Conversely,753 more theenergetic overall sites intensity[103]. As illustrated of the in bands Figure observed18, in the former for thecase, ZS–PdAuH2O2 direct synthesis catalyst occurs (blue curve) was 754 by: (i) oxygen activation without dissociation on non-defective (less energetic) Pd sites, followed by much stronger.755 (ii) protonation In addition, external the H+ linear/bridged, and (iii) reaction with intensity molecular H ratio2 to form was H2 muchO2 with H higher+ restoration. for ZS–PdAuOn with respect756 to CeS-PdAu.the contrary, O This2 is chemisorbed spectroscopic by dissociative feature way is on due defect, to edge, the differentand corner sites nature (more of energetic the two supports. Ceria can757 besites), quite which easily are able reduced to re-adsorb at the H2O interface2. Thus, due with to the thepresence metallic of chemisorbed particles H2, under water can mild be conditions, 758 formed by reaction with oxygen atoms or with the OH fragments coming from H2O2. which759 results in the production of oxygen vacancies, and migration of oxygen on the small metallic particles [84]. Moreover, on this catalyst a large fraction of the Pd sites was present as PdO, on which CO does not adsorb, upon H2–O2 treatment [101]. PdO species formation takes place preferably at the particle/support interface, whereas the Pd atoms located on top of the particle, far from the interface between the metal and the oxide keep on being metallic when the particle size is >3 nm [102]. In this frame, the nature of the support strongly affected both the formation and stabilization of oxidized particles. Such process was promoted on the ceria support because of its oxygen storage-release properties along with the smaller size of the metallic particles detected on both the Pd and PdAu catalysts supported on CeS catalysts, according to the HRTEM observations. Based on the characterization results, a reaction mechanism for the Pd and Pd-Au bimetallic catalysts was proposed depending on the nature of the active metallic sites, i.e., less energetic and more energetic sites [103]. As illustrated in Figure 18, in the former case, H2O2 direct synthesis occurs by: (i) oxygen activation without dissociation on non-defective (less energetic) Pd sites, followed by + + (ii) protonation external H , and (iii) reaction with molecular H2 to form H2O2 with H restoration. On the contrary, O2 is chemisorbed by dissociative way on defect, edge, and corner sites (more energetic sites), which are able to re-adsorb H2O2. Thus, due to the presence of chemisorbed H2, water can be formed by reaction with oxygen atoms or with the OH fragments coming from H2O2. Catalysts 2019, 9, 30 20 of 34

Catalysts 2018, 8, x FOR PEER REVIEW 21 of 36

760

761 Figure 18. Possible pathway for the synthesis of H2O2 and H2O on mono- and bimetallic Pd–Au Figure 18. Possible pathway for the synthesis of H2O2 and H2O on mono- and bimetallic Pd–Au 762 catalysts. Reprinted from J. Catal., 257, F. Menegazzo, P. Burti, M. Signoretto, M. Manzoli, S. catalysts.763 ReprintedVankova, fromF. Boccuzzi, J. Catal., F. Pinna, 257, F.G. Menegazzo,Strukul, Effect P.of Burti,the addition M. Signoretto, of Au in zirconia M. Manzoli, and ceria S. Vankova, F. Boccuzzi,764 F. Pinna,supported G. Pd Strukul, catalysts Effectfor the direct of the synthesis addition of hydrogen of Au peroxide, in zirconia 369–381, and Copyright ceria supported (2008), with Pd catalysts

for765 the direct synthesispermission from of hydrogen Elsevier. peroxide, 369–381, Copyright (2008), with permission from Elsevier.

766 Therefore, upon gold addition of gold, the amount of terrace sites (less energetic sites) increased Therefore,767 along upon with goldthe selectivity addition to H of2O2 gold,. However, the as amount pointed out of terraceby FTIR measurements, sites (less energetic the amount sites) of increased along with768 themore selectivity energetic active to H sites2O did2. However,not rise, as expected. as pointed The active out phase by FTIR of Pd–Au measurements, bimetallic catalysts the amount of 769 more energeticwas active confirmed sites to did be notmade rise, up by as a expected. core-shell morphology The active with phase an ofAu-rich Pd–Au core/Pd-rich bimetallic shell catalysts was 770 morphology and a Pd-rich surface according to the studies by Hutchings et al. [26,27,41,104–106]. confirmed771 toThe be occurrence made up of by an aelectron core-shell donation morphology from the Au core with to the an Pd Au-rich shell, which core/Pd-rich results in less reactive shell morphology and a Pd-rich772 sites surface toward according O2 dissociation, to was the demonstrated studies by by Hutchings the presence et of al.the [band26,27 at, 411770,104 cm–−1106. Gold]. was The occurrence 773 of an electronproven donation to have from a complex the Aurole, coresince it to improves the Pd the shell, Pd catalytic which activity results by indecreasing less reactive the size, sites toward 774 morphology and electron density of the Pd particles. −1 O2 dissociation,775 In was a following demonstrated paper by the by same the presenceauthors, the ofeffect the ofband the morphological at 1770 cm and .composition Gold was proven to have a776 complex properties role, sinceof differently it improves prepared the Pd–Au Pd catalytic catalysts activitysupported by ZrO decreasing2 on the direct the synthesis size, morphology of and 777 electron densityhydrogen of the peroxide Pd particles. was investigated [39]. It was shown that the consecutive deposition of gold by 778 deposition-precipitation and then of palladium by incipient-wetness impregnation allowed to In779 a followingprepare catalysts paper by(1Au2Pd) the samewith a authors, H2O2 productivity the effect at ofpressure the morphologicalcorresponding to and18,000composition 780 mmolH2O2/gPdh with 59% selectivity, unusually higher than that observed at atmospheric pressure. properties of differently prepared Pd–Au catalysts supported ZrO2 on the direct synthesis of 781 hydrogen peroxideMoreover, was large investigated roundish bimetallic [39 ].particles It was with shown an Au/Pd that ratio the ranging consecutive from 0.3 up deposition to 1.5, as of gold by 782 determined by EDS analysis, were mainly observed on the 1Au2Pd catalyst. To obtain more insight deposition-precipitation783 on the surface sites, and CO then adsorption of palladium was perfor bymed incipient-wetness at 180 K in order to observe impregnation also the Pdδ+ carbonyl allowed to prepare 784 species, which do not chemisorb the probe at room temperature. Indeed, CO bands at 2160 cm−1 and catalysts (1Au2Pd) with a H2O2 productivity at pressure corresponding to 18,000 mmolH2O2 /gPdh 785 2135 cm−1 signalled the presence of Pd2+ ions stabilized by chloride ions coming from the metal with 59% selectivity, unusually higher than that observed at atmospheric pressure. Moreover, large 786 precursor [107] and of Pdδ+ at the surface of oxidized Pd0 particles [108] on the catalysts simply roundish787 bimetallicoutgassed particles for 1 hour at with room antemperature Au/Pd (data ratio not ranging shown). The from FTIR 0.3 spectra up of to CO 1.5, adsorbed as determined on Pd by EDS analysis,788 were (Figure mainly 19a), observed1Au2Pd (Figure on the19b), 1Au2Pd1Pd2Au (Figure catalyst. 19c), and To obtainPdAu (Figure more 19d) insight submitted on to the H2surface sites, 789 pre-treatment at room temperature (fine curves) and to subsequent interactionδ +with H2, then O2 CO adsorption790 atmosphere was performed at the same temperature at 180 K (bol ind order curves) to are observe compared alsoin Figure the 19. Pd carbonyl species, which −1 −1 do not791 chemisorb the probe at room temperature. Indeed, CO bands at 2160 cm and 2135 cm signalled the presence of Pd2+ ions stabilized by chloride ions coming from the metal precursor [107] and of Pdδ+ at the surface of oxidized Pd0 particles [108] on the catalysts simply outgassed for 1 h at room temperature (data not shown). The FTIR spectra of CO adsorbed on Pd (Figure 19a), 1Au2Pd (Figure 19b), 1Pd2Au (Figure 19c), and PdAu (Figure 19d) submitted to H2 pre-treatment at room temperature (fine curves) and to subsequent interaction with H2, then O2 atmosphere at the same temperature (bold curves) are compared in Figure 19. Generally, the spectra display similar band positions, which are in the range of on top, two-fold bridged or three-fold bridged Pd0 carbonyls. A band at 2169 cm−1, related to CO adsorbed on Zr4+ sites of the support, was detected for the 1Pd2Au catalyst (Figure 19c). Moreover, the spectra differed as for the overall intensity, and those related to the 1Au2Pd catalyst had the lowest intensity, which suggests the presence of large particles, in agreement with the results of the HRTEM analysis. The pre-treatment in H2 and O2 atmosphere (bold curves) induced a marked decrease in intensity of the spectra for all samples, pointing out that a surface oxide layer, exposing sites unable to adsorb the probe, was formed [108,109]. In particular, a 95% intensity band decrease of all carbonyls was observed on the 1Au2Pd sample (Figure 19b), and ascribed to electronic and chemical effects induced by Au on the Pd sites. A curve fitting procedure of the experimental spectra collected after H2 and O2 pre-treatment was carried out and the obtained deconvolutions are shown in Figure 20. Linear CO species adsorbed on Pd0 sites of (1 1 1) facets (band at 2108 cm−1), bridged CO on Pd0 edge sites (1999 cm−1), bridged CO species on Pd0 sites of (1 0 0) or (1 1 1) facets (band observed at 1941 cm−1), Catalysts 2019, 9, 30 21 of 34 and CO species on different threefold hollow Pd sites (bands at 1880 and 1809 cm−1) were observed for the Pd monometallic catalyst [110]. Catalysts 2018, 8, x FOR PEER REVIEW 22 of 36

792 Figure793 19. FTIRFigure spectra 19. FTIR ofspectra CO of adsorbed CO adsorbed at at 180 180 KK on Pd Pd (a), ( a1Au2Pd), 1Au2Pd (b), 1Pd2Au (b), 1Pd2Au(c), and PdAu (c), (d and) PdAu (d) 794 after pre-treatment in H2 at room temperature (fine curves) and after pre-treatment in H2 and O2 at after795 pre-treatmentr.t. (bold curves). in H2 atReprinted room from temperature J. Catal., 268, (fineF. Menegazzo, curves) M. and Signoretto, after M. pre-treatment Manzoli, F. Boccuzzi, in H 2 and O2 at r.t.796 (bold curves).G. Cruciani, Reprinted F. Pinna, from G. Strukul, J. Catal., Influence 268, of F. the Menegazzo, preparation method M. Signoretto, on the morphological M. Manzoli, and F. Boccuzzi, G.797 Cruciani, composition F. Pinna, properties G. Strukul, of Pd–Au/ZrO Influence2 catalysts of the andpreparation their effect on the method direct synthesis on the of hydrogen morphological andFormatted: Subscript 798 peroxide from hydrogen and oxygen, 122–130, Copyright (2009), with permission from Elsevier. composition properties of Pd–Au/ZrO2 catalysts and their effect on the direct synthesis of hydrogen peroxide799 fromGenerally, hydrogen the andspectra oxygen, display 122–130,similar band Copyright positions, which (2009), are with in the permission range of on top, from two-fold Elsevier. 800 bridged or three-fold bridged Pd0 carbonyls. A band at 2169 cm−1, related to CO adsorbed on Zr4+ Upon801 thesites sameof the support, hydrogen–oxygen was detected for pre-treatment, the 1Pd2Au catalyst carbonyls (Figure 19c). on Moreover, Pd2+ andthe spectra Pdδ+ (2160 and 802 −1 differed as for the overall intensity, and those related to the 1Au2Pd catalyst had the lowest intensity, 2134 cm803 ) andwhich COsuggests adsorbed the presence on of differentlarge particles, Pd in sitesagreement (weak with componentsthe results of the HRTEM at 2104, analysis. 1985, 1941, and −1 1879 cm804 ) wereThe pre-treatment detected for in H the2 and most O2 atmosphere performing (bold curves) 1Au2Pd induced catalyst a marked (Figure decrease 20 b).in intensity The low of intensity of 805 the spectra for all samples, pointing out that a surface oxide layer, exposing sites unable to adsorb such absorption pointed out that the O2 reactant molecule can easily oxidize the Pd sites present in this 806 the probe, was formed [108,109]. In particular, a 95% intensity band decrease of all carbonyls was catalyst807 due toobserved the electronic on the 1Au2Pd effect sample induced (Figure by gold19b), onand theascribed Pd atoms, to electronic due toand the chemical higher effects electronegativity of gold808 (2.54) induced with respectby Au on the to Pd that sites. of A palladium curve fitting pr (2.2)ocedure [111 of]. the Moreover, experimental the spectra red-shift collected observedafter for the bands809 at 2104 H2 (small and O2 pre-treatment shift) and 1985was carried cm− out1 (large and the shift) obtained with deconvolutions respect to are the shown same in Figure carbonyls 20. of the Pd 810 Linear CO species adsorbed on Pd0 sites of (1 1 1) facets (band at 2108 cm−1), bridged CO on Pd0 edge Formatted: Not Highlight catalyst811 was ascribedsites (1999 cm to− lateral1), bridged interaction CO species on effects, Pd0 sites suggesting of (1 0 0) or (1 the 1 1) presence facets (band of observed more isolated at 1941 sitesFormatted: on the Not Highlight 1Au2Pd812 catalyst. cm−1), Thisand CO was species a consequence on different threefold of the hollow alloying Pd sites of the (bands two atmetals 1880 and and 1809 not cm−1 of) were the smaller size of the particles.813 observed Thus, for thethe Pd band monometallic at 1985 catalyst cm−1 [110].was assigned to bridge bonded carbonyl species on Pd 814 couples on the edges of the metal particles, changed by the interaction with neighbouring Au atoms. These couples were oxidised in a lower extent (25% decrease) by the subsequent interactions with H2 and O2 and were involved in hydrogen dissociation during the reaction. Quite different spectra were collected in the case of the catalyst in which firstly Pd and then gold were deposited (1Pd2Au, Figure 20c). No carbonyls on Pd2+ sites (band at 2160 cm−1) were detected, because the chlorine atoms were removed during Au deposition in basic conditions. Au0 sites (CO band at 2101 cm−1) decorating the Pd edge couples (the band related to these CO bridge bonded species is totally missing) were observed. Conversely, the spectra collected on the PdAu catalyst (in which Pd and Au were simultaneously deposited) were quite similar to those obtained for 1Au2Pd sample (Figure 20d,b, respectively), with the exception of the carbonyls on Pd0 edges (band at 2003 cm−1), whose band was not affected by lateral interaction phenomena, indicating site isolation and the presence of small particles, according to HRTEM analysis. Catalysts 2019, 9, 30 22 of 34

Catalysts 2018, 8, x FOR PEER REVIEW 23 of 36

815 816 Figure 20. FTIR spectra of CO adsorbed at 180 K on Pd (a), 1Au2Pd (b), 1Pd2Au (c), and PdAu (d) Figure 20. FTIR spectra of CO adsorbed at 180 K on Pd (a), 1Au2Pd (b), 1Pd2Au (c), and PdAu (d) after 817 after pre-treatment in H2 and O2 at r.t. (bold curves) and relative deconvolutions (fine curves). pre-treatment818 Reprinted in H2 andfrom OJ. Catal.,2 at r.t. 268, (bold F. Menegazzo, curves) M. and Signoretto, relative M. Manzoli, deconvolutions F. Boccuzzi, (fineG. Cruciani, curves). F. Reprinted from819 J. Catal.,Pinna, 268, G. F. Strukul, Menegazzo, Influence M.of the Signoretto, preparation M.method Manzoli, on the morphological F. Boccuzzi, and G. composition Cruciani, F. Pinna, G. Strukul,820 Influenceproperties of of the Pd–Au/ZrO preparation2 catalysts method and their on effect the on morphological the direct synthesis and of hydrogen composition peroxide properties Formatted: of Subscript 821 from hydrogen and oxygen, 122–130, Copyright (2009), with permission from Elsevier. Pd–Au/ZrO2 catalysts and their effect on the direct synthesis of hydrogen peroxide from hydrogen and822 oxygen, Upon 122–130, the same Copyright hydrogen–oxygen (2009), with pre-treatment, permission carbonyls from Elsevier. on Pd2+ and Pdδ+ (2160 and 2134 823 cm−1) and CO adsorbed on different Pd sites (weak components at 2104, 1985, 1941, and 1879 cm−1) 824 were detected for the most performing 1Au2Pd catalyst (Figure 20b). The low intensity of such The authors suggested that the co-presence of the oxide layer activates molecular O2 without 825 absorption pointed out that the O2 reactant molecule can easily oxidize the Pd sites present in this 0 dissociation,826 catalyst whereas due theto the Pd electroniccouples effect located induced onby thegold edgeson the dissociatePd atoms, due hydrogen, to the higher explaining the improved827 productivityelectronegativity and of gold selectivity (2.54) with towards respect to H that2O of2. palladium (2.2) [111]. Moreover, the red-shift 828 observed for the bands at 2104 (small shift) and 1985 cm−1 (large shift) with respect to the same 3.4. Insights829 oncarbonyls the Exposed of the Pd Sites catalyst and was on theascribed Properties to latera ofl interaction the Support effects, suggesting the presence of 830 more isolated sites on the 1Au2Pd catalyst. This was a consequence of the alloying of the two metals Very831 recently,and not Chung of the smaller et al. size developed of the particles. the selective Thus, theadsorption band at 1985 depositioncm−1 was assigned method to bridge to prepare Pd/C 832 bonded carbonyl species on Pd couples on the edges of the metal particles, changed by the catalysts833 with interaction improved with catalytic neighbouring performance Au atoms. inThese the couples H2O2 directwere oxidised synthesis in a [lower112]. extent The method(25% consisted 2+ of (i) the834 selectivedecrease) adsorptionby the subsequent of interactions a cationic with Pd H2 precursor,and O2 and were [Pd(NH involved3 in)4 ]hydrogen, on adissociation negatively-charged activated835 carbonduring surface, the reaction. and (ii) conversion to Pd hydroxide in homogeneous phase in which the 836 Quite different spectra were collected in the case of the catalyst in which firstly Pd and then hydroxide837 ionsgold were were graduallydeposited (1Pd2Au, produced Figure upon 20c). No urea carbonyls decomposition. on Pd2+ sites Following(band at 2160 such cm−1) procedure,were it was possible838 to obtaindetected, ultra-small, because the chlorine monodispersed atoms were removed Pd nanoparticles during Au deposition with in unprecedented basic conditions. Au and0 excellent 839 sites (CO band at 2101 cm−1) decorating the Pd edge couples (the band related to these CO bridge catalytic activity. Indeed, the Pd/C#C1 catalyst gave a high initial H2O2 productivity of 8606 mmol 840 bonded species is totally missing) were observed. Conversely, the spectra collected on the PdAu H2O2/(g841Pd ·h)catalyst and H(in2 whichselectivity Pd and ofAu 95.1% were simultaneously after 30 min deposited) reaction were under quite safe similar and to non-corrosivethose obtained conditions. These results842 for were 1Au2Pd 12 andsample 7.1 (Figure times 20d,b, higher, respectively respectively,), with the than exception those of obtainedthe carbonyls for on thePd0 catalystedges prepared by conventional deposition-precipitation (Pd/C#A1). DRIFT characterization, showed in Figure 21a, was carried out on the activated carbon treated with 1 wt% HNO3 (green curve), 10 wt% HNO3 (blue curve), and 10 wt% HNO3/10 wt% H2O2 (red curve). The results firstly pointed out that the distribution of the oxygen groups on the activated carbon surface depends on the acid treatment conditions, and that large amounts of carboxyl groups were inserted on the surface of the activated carbon, according to titration, XPS, and TPD-mass experiments. More in detail, the C=O stretching band at 1780 cm−1 was assigned to carboxylic acid or to its anhydride form. Moreover, the peak around 1600 cm−1 was ascribed to the overlap of the polyaromatic C=C stretching bands of the highly conjugated carbonyl and/or quinone groups. Conversely, the broad band in the 1100–1450 cm−1 range was possibly due to the C=O stretching mode of carboxylic acid and of the lactone groups and/or to the O–H bending of phenol groups [113,114]. Catalysts 2018, 8, x FOR PEER REVIEW 24 of 36

843 (band at 2003 cm−1), whose band was not affected by lateral interaction phenomena, indicating site 844 isolation and the presence of small particles, according to HRTEM analysis. 845 The authors suggested that the co-presence of the oxide layer activates molecular O2 without 846 dissociation, whereas the Pd0 couples located on the edges dissociate hydrogen, explaining the 847 improved productivity and selectivity towards H2O2.

848 3.4. Insights on the Exposed Sites and on the Properties of the Support 849 Very recently, Chung et al. developed the selective adsorption deposition method to prepare 850 Pd/C catalysts with improved catalytic performance in the H2O2 direct synthesis [112]. The method 851 consisted of (i) the selective adsorption of a cationic Pd precursor, [Pd(NH3)4]2+, on a 852 negatively-charged activated carbon surface, and (ii) conversion to Pd hydroxide in homogeneous 853 phase in which the hydroxide ions were gradually produced upon urea decomposition. Following 854 such procedure, it was possible to obtain ultra-small, monodispersed Pd nanoparticles with 855 unprecedented and excellent catalytic activity. Indeed, the Pd/C#C1 catalyst gave a high initial H2O2 856 productivity of 8606 mmol H2O2/(gPd·h) and H2 selectivity of 95.1% after 30 min reaction under safe 857 and non-corrosive conditions. These results were 12 and 7.1 times higher, respectively, than those 858 obtained for the catalyst prepared by conventional deposition-precipitation (Pd/C#A1). 859 DRIFT characterization, showed in Figure 21a, was carried out on the activated carbon treated 860 with 1 wt% HNO3 (green curve), 10 wt% HNO3 (blue curve), and 10 wt% HNO3/10 wt% H2O2 (red 861 curve). The results firstly pointed out that the distribution of the oxygen groups on the activated 862 carbon surface depends on the acid treatment conditions, and that large amounts of carboxyl groups 863 were inserted on the surface of the activated carbon, according to titration, XPS, and TPD-mass 864 experiments. More in detail, the C=O stretching band at 1780 cm−1 was assigned to carboxylic acid or 865 to its anhydride form. Moreover, the peak around 1600 cm−1 was ascribed to the overlap of the 866 polyaromatic C=C stretching bands of the highly conjugated carbonyl and/or quinone groups. 867 Conversely, the broad band in the 1100–1450 cm−1 range was possibly due to the C=O stretching 868 mode of carboxylic acid and of the lactone groups and/or to the O–H bending of phenol groups 869 [113,114]. 870 However, it was found that the carboxyl groups can negatively influence the catalytic activity, 871 regardless of the preparation method. Indeed, the increase of the amount of carboxyl groups 872 promoted H2O2 decomposition by hindering or poisoning the active sites. The authors suggested 873 that despite H2O2 decomposition can be overcome in the presence of a strong electrolyte as H2SO4, Catalysts8742019 , 9carboxylic, 30 acid is a weak electrolyte, thus it does not efficiently suppress H–OOH dissociation, but 23 of 34 875 preferentially foster H2O2 hydrolysis by forming hydrogen bonds, as described in Figure 21b.

876 Figure877 21. (Figurea) DRIFTS-IR 21. (a) DRIFTS-IR spectra spectra or theor the surface-modiﬁed surface-modified activated activated carbon. carbon.(b) Illustration (b) of Illustration the of the 878 unfavourable effect of carboxyl groups in accelerating H2O2 hydrolysis as well as hindering or unfavourable879 poisoning effect the of active carboxyl metal sites. groups Reprinted in from accelerating J. Catal., 365, H S.2 OLee,2 H.hydrolysis Jeong, Y.-M. asChung, well Direct as hindering or poisoning880 thesynthesis active of metalhydrogen sites. peroxide Reprinted over Pd/C from catalyst J. Catal., prepared 365, by selective S. Lee, adsorption H. Jeong, deposition Y.-M. Chung, Direct synthesis881 ofmethod, hydrogen 125–137, peroxide Copyright over (2018), Pd/C with catalyst permission prepared from Elsevier. by selective adsorption deposition method, 125–137, Copyright (2018), with permission from Elsevier.

However, it was found that the carboxyl groups can negatively influence the catalytic activity, regardless of the preparation method. Indeed, the increase of the amount of carboxyl groups promoted H2O2 decomposition by hindering or poisoning the active sites. The authors suggested that despite H2O2 decomposition can be overcome in the presence of a strong electrolyte as H2SO4, carboxylic acid is a weak electrolyte, thus it does not efficiently suppress H–OOH dissociation, but preferentially foster H2O2 hydrolysis by forming hydrogen bonds, as described in Figure 21b. A series of palladium catalysts supported on HZSM-5 with different Si/Al molar ratio (X = 15, 30, 75, 100, and 150) were prepared and tested in the hydrogen peroxide direct synthesis [67]. It was found that the electivity to H2O2 displayed a volcano-shaped curve with respect to the Si/Al molar ratio, whilst the hydrogen conversion showed no great difference. Thus, also the H2O2 yield showed a volcano-shaped curve and strictly depended on the Brønsted acidity/Lewis acidity (B/L) ratio of the Pd/HZSM-5-X catalysts. In order to investigate the nature of the acid sites, in situ FT-IR spectroscopy of adsorbed pyridine was carried out, and the spectra collected on the Pd/HZSM-5-X catalysts are shown in Figure 22. The broad band at 1640 cm−1, observed in all spectra was due to the bending vibration of O-H groups of H2O adsorbed on the windows of the cell [115]. Upon pyridine adsorption, bands at 1440 and 1595 cm−1, related to the presence of hydrogen bonded pyridine, at 1450, 1490, and 1580 cm−1 due to Lewis acid bound pyridine, and at 1490 and 1545 cm−1, assigned to Brønsted acid bound pyridinium ion were observed on the Pd/HZSM-5- X catalysts [115–117]. Both Brønsted and Lewis acidity were quantified according to the procedure reported in the literature [118] by using the integrated molar extinction coefficients of 1.67 and 2.22 cm/µmol for Brønsted and Lewis acid sites, respectively. In particular, the quantities of Brønsted and Lewis acid sites were obtained from the integrated areas of the bands at 1545 and at 1450 cm−1, according to Equations (4) and (5): IA(B) × π × R2 Bronsted acid site amount = (4) 1.67 × W IA(L) × π × R2 Lewis acid site amount = (5) 2.22 × W where IA(B) represents the integrated absorbance of Brønsted acid site (expressed in cm−1), IA(L) is the integrated absorbance of Lewis acid site (expressed in cm−1), R is the inner radius of the sample cup (cm) and W is the weight of the catalyst (mg). The obtained B/L ratio displayed a volcano-shaped trend with respect to the Si/Al molar ratio, and the Pd/HZSM-5-30 catalyst had the highest B/L ratio. In particular, Brønsted acid sites in HZSM-5 are due to the presence of bridging hydroxyl group in Si–OH–Al moieties [119,120]. Conversely, the Lewis acid sites are the Al in the hydroxoaluminum complex [119]. Hence, the authors come to the conclusion that the B/L ratio was decreased by Catalysts 2018, 8, x FOR PEER REVIEW 25 of 36

882 A series of palladium catalysts supported on HZSM-5 with different Si/Al molar ratio (X = 15, 883 30, 75, 100, and 150) were prepared and tested in the hydrogen peroxide direct synthesis [67]. 884 It was found that the electivity to H2O2 displayed a volcano-shaped curve with respect to the 885 Si/Al molar ratio, whilst the hydrogen conversion showed no great difference. Thus, also the H2O2 886 yield showed a volcano-shaped curve and strictly depended on the Brønsted acidity/Lewis acidity 887 (B/L) ratio of the Pd/HZSM-5-X catalysts. 888 In order to investigate the nature of the acid sites, in situ FT-IR spectroscopy of adsorbed 889 pyridine was carried out, and the spectra collected on the Pd/HZSM-5-X catalysts are shown in 890 Figure 22. The broad band at 1640 cm–1, observed in all spectra was due to the bending vibration of 891 O-H groups of H2O adsorbed on the windows of the cell [115]. Upon pyridine adsorption, bands at 892 1440 and 1595 cm−1, related to the presence of hydrogen bonded pyridine, at 1450, 1490, and 1580 893 cm−1 due to Lewis acid bound pyridine, and at 1490 and 1545 cm−1, assigned to Brønsted acid bound 894 pyridinium ion were observed on the Pd/HZSM-5- X catalysts [115–117]. 895 Both Brønsted and Lewis acidity were quantified according to the procedure reported in the 896 literature [118] by using the integrated molar extinction coefficients of 1.67 and 2.22 cm/μmol for 897 Brønsted and Lewis acid sites, respectively. In particular, the quantities of Brønsted and Lewis acid 898 sites were obtained from the integrated areas of the bands at 1545 and at 1450 cm−1, according to 899 Equations (4) and (5): 𝐼𝐴𝐵 𝜋 𝑅 𝐵𝑟𝑜𝑛𝑠𝑡𝑒𝑑 𝑎𝑐𝑖𝑑 𝑠𝑖𝑡𝑒 𝑎𝑚𝑜𝑢𝑛𝑡 (4) 1.67 𝑊 𝐼𝐴𝐿 𝜋 𝑅 𝐿𝑒𝑤𝑖𝑠 𝑎𝑐𝑖𝑑 𝑠𝑖𝑡𝑒 𝑎𝑚𝑜𝑢𝑛𝑡 (5) 2.22 𝑊 900 where IA(B) represents the integrated absorbance of Brønsted acid site (expressed in cm−1), IA(L) is 901 the integrated absorbance of Lewis acid site (expressed in cm−1), R is the inner radius of the sample 902 cup (cm) and W is the weight of the catalyst (mg). The obtained B/L ratio displayed a 903 volcano-shaped trend with respect to the Si/Al molar ratio, and the Pd/HZSM-5-30 catalyst had the Catalysts 2019, 9, 30 24 of 34 904 highest B/L ratio. In particular, Brønsted acid sites in HZSM-5 are due to the presence of bridging 905 hydroxyl group in Si–OH–Al moieties [119,120]. Conversely, the Lewis acid sites are the Al in the 906 hydroxoaluminum complex [119]. Hence, the authors come to the conclusion that the B/L ratio was increasing907 thedecreased totality by ofincreasing Lewis the acid totality sites, of inLewis consequence acid sites, in ofconsequence the increase of the ofincrease the populationof the of the hydroxoaluminum908 population complex of the hydroxoaluminum upon incorporation complex ofupon a largeincorporation Al amount of a large into Al the amount HZSM-5 into the framework. 909 HZSM-5 framework.

910

Figure 22. FT-IR spectra of pyridine adsorbed on Pd/HZSM-5-X (X = 15, 30, 75, 100, and 150) catalysts. H: hydrogen bonded pyridine; L: Lewis acid bound pyridine; B: Brønsted acid bound pyridinium ion. ReprintedCatalysts 2018 from, 8, x J.FOR Molecul. PEER REVIEW Catal. A: Chemical, 363, S. Park, J. Lee, J. H. Song, T.26 J.of Kim,36 Y.-M. Chung,911 S.-H.Figure Oh, 22. I. K.FT-IR Song, spectra Direct of pyridine synthesis adsorbed of on hydrogen Pd/HZSM-5-X peroxide (X = 15, 30, from 75, 100, hydrogen and 150) and oxygen over912 Pd/HZSM-5 catalysts. catalysts: H: hydrogen Effect bonded of pyridine; Brønsted L: Lewis acidity, acid 230–236,bound pyridine; Copyright B: Brønsted (2012), acid bound with permission 913 pyridinium ion. Reprinted from J. Molecul. Catal. A: Chemical, 363, S. Park, J. Lee, J. H. Song, T. J. from Elsevier. 914 Kim, Y.-M. Chung, S.-H. Oh, I. K. Song, Direct synthesis of hydrogen peroxide from hydrogen and 915 oxygen over Pd/HZSM-5 catalysts: Effect of Brønsted acidity, 230–236, Copyright (2012), with Interestingly,916 permission it was from found Elsevier. that the H2O2 yield increased by augmenting the B/L ratio, which points out the key role played by Brønsted acid sites of the Pd/HZSM-5-X catalysts. Indeed, the catalyst 917 Interestingly, it was found that the H2O2 yield increased by augmenting the B/L ratio, which with the918 highest points Brønstedout the key acidityrole played (Pd/HZSM-5-30) by Brønsted acid sites gave of the the Pd/HZSM-5-X largest yield. catalysts. A Indeed, correlation the between the H2919O2 yieldcatalyst and with the the B/Lhighest ratio Brønsted of theacidity Pd/HZSM-5-X (Pd/HZSM-5-30) gave catalysts the largest was yield. shown A correlation (see Figure 23a), unambiguously920 between showing the H2O2that yield theand the yield B/L ratio was of intimately the Pd/HZSM- linked5-X catalysts to the was B/L shown ratio. (see Figure Moreover, 23a), the yield 921 unambiguously showing that the yield was intimately linked to the B/L ratio. Moreover, the yield increased922 withincreased the Brønsted with the Brø aciditynsted acidity measured measured by by ammonia temperature-programmed temperature-programmed desorption desorption (TPD) measurements923 (TPD) measurements (Figure (Figure23b). These23b). These results results were were consistent consistent with with the increase the increase in H2O2 selectivity in H2O 2 selectivity observed924 by theobserved addition by the addition of acid of additives acid additives [1 ,[1,61].61]. 925

926 927 Figure 23. (a) Correlation between yield for hydrogen peroxide over Pd/HZSM-5-X (X = 15, 30, 75, Figure 23. (a) Correlation between yield for hydrogen peroxide over Pd/HZSM-5-X (X = 15, 30, 75,Formatted: Not Highlight 928 100, and 150) catalysts and B/L ratio of the catalysts. (b) Correlation between yield for hydrogen 100,929 and 150)peroxide catalysts over andPd/HZSM-5- B/L ratioX (X= of15, the30, 75, catalysts. 100, and 150) (b) catalysts Correlation and Brø betweennsted acidity yield of the for hydrogen peroxide930 overcatalysts. Pd/HZSM-5- Adapted fromX J.(X Molecul.= 15, Catal. 30, A: 75, Chemical, 100, and 363, S. 150) Park, catalysts J. Lee, J. H. Song, and T. Brønsted J. Kim, Y.-M. acidity of the 931 Chung, S.-H. Oh, I. K. Song, Direct synthesis of hydrogen peroxide from hydrogen and oxygen over catalysts. Adapted from J. Molecul. Catal. A: Chemical, 363, S. Park, J. Lee, J. H. Song, T. J. Kim, 932 Pd/HZSM-5 catalysts: Effect of Brønsted acidity, 230–236, Copyright (2012), with permission from Y.-M.933 Chung,Elsevier. S.-H. Oh, I. K. Song, Direct synthesis of hydrogen peroxide from hydrogen and oxygen over Pd/HZSM-5 catalysts: Effect of Brønsted acidity, 230–236, Copyright (2012), with permission 934 2 2 from Elsevier.Au-Pd bimetallic catalysts are very efficient in the H O direct synthesis process, in particular 935 when supported on acidic supports. Indeed, the use of acidic supports promotes the hydrogen 936 peroxide stabilization and can also inhibit the hydrogenation and decomposition reactions. The Au-Pd937 bimetallicpreparation and catalysts testing of areAu-Pd very catalysts efﬁcient supported inthe on insoluble H2O2 directheteropolyacids synthesis were process,reported by in particular when supported938 Hutchings on acidicet al. [65]. supports. Ions such Indeed,as Cs+, Rb the+, K+ use, and ofAg acidic+ counterbalance supports the promotescharge of the the hydrogen 939 heteropolyacids supports, and the catalyst were prepared by ion exchange and impregnation 940 methods. The FTIR spectra collected on the bare heteropolyacids containing different counter ions 941 are reported in Figure 24a. The typical features of the Keggin structure of Cs substituted 942 phosphotungstic acid were observed in all spectra [121]. 943

Catalysts 2019, 9, 30 25 of 34 peroxide stabilization and can also inhibit the hydrogenation and decomposition reactions. The preparation and testing of Au-Pd catalysts supported on insoluble heteropolyacids were reported by Hutchings et al. [65]. Ions such as Cs+, Rb+,K+, and Ag+ counterbalance the charge of the heteropolyacids supports, and the catalyst were prepared by ion exchange and impregnation methods. The FTIR spectra collected on the bare heteropolyacids containing different counter ions are reported in Figure 24a. The typical features of the Keggin structure of Cs substituted phosphotungstic acid were observed in all spectra [121]. Catalysts 2018, 8, x FOR PEER REVIEW 27 of 36

944 Figure945 24. (aFigure) FT-IR 24. ( spectraa) FT-IR spectra of heteropolyacid of heteropolyacid supportsupport materials materials made madeby incorporating by incorporating various various 946 metal ions. (b) FT-IR spectra of heteropolyacid support materials made by incorporating various metal947 ions. (aboutb) FT-IR of Cs ions. spectra Reprinted of heteropolyacid from Catal. Today, support248, S. J. Freakley, materials R. J. Lewis, made D. by J. Morgan, incorporating J. K. various about948 of CsEdwards, ions. Reprinted G. J. Hutchings, from Direct Catal. synthesis Today, of hydrogen 248, S. peroxide J. Freakley, using Au–Pd R. J. supported Lewis, D.and J. Morgan, J. K.949 Edwards, ion-exchanged G. J. Hutchings, heteropolyacids Direct precipitated synthesis with of hydrogenvarious metal peroxideions, 10–17, Copyright using Au–Pd (2015), with supported and 950 permission from Elsevier. ion-exchanged heteropolyacids precipitated with various metal ions, 10–17, Copyright (2015), with permission951 fromMore Elsevier. in detail, the vibrational bands for the Keggin structure were detected at 1090 cm−1 that is 952 due to νas(P-O) vibration, at 1016 cm−1, assigned to terminal νas(W=O vibration, and 912 and 863 cm−1, More953 indue detail, to νas the(W-O-W) vibrational vibrations. bands In some for cases, the bands Keggin at 1710 structure cm−1 distinctive were of detected protonated at H5 1090O2+ cm−1 that 954 clusters were observed, but the spectra−1 were collected in air and therefore these (O-H) vibrations is due955 to ν as(P-O)cannot be vibration, solely ascribed at 1016to the cmheteropolyacid, assigned structure to terminaland the intensityνas(W of this = O absorption vibration, band and 912 and −1 −1 863 cm956, duecannot to ν beas (W-O-W)taken as a measure vibrations. of the acidity In some of the cases, catalysts. bands Moreover, at 1710 the P-O cm stretchingdistinctive mode was of protonated + 957 observed in all samples upon Cs+ substitution, pointing out that the substitution of Cs+ for H+ did not H5O2 clusters were observed, but the spectra were collected in air and therefore these (O-H) vibrations 958 induce strain on the P-O4 tetrahedra within the structure. The incorporation of Au and Pd by cannot959 be solelyimpregnation ascribed or by to ion the exchange heteropolyacid did not originate structure any change and or new the band intensity in the spectra, of this possibly absorption band cannot960 be takenbecause as aof measurethe very small of amounts the acidity of Au ofand the Pd that catalysts. were introduced Moreover, gave rise the to P-Osignals stretching under the mode was observed961 in alldetection samples limit. uponThe Keggin Cs+ structuresubstitution, was maintained pointing after calcination out that of the the substitution catalysts and also of after Cs+ for H+ did 962 the incorporation of different amounts of Cs+ (Figure 24b), according to the XRD results. not induce963 strainThe on H the2O2 productivity P-O4 tetrahedra reached withinby Pd0.075 theAu0.05 structure.Cs2.5H0.5W12O40 The was incorporationmuch higher than of that Au and Pd by impregnation964 obtained or by ionby the exchange most active did conventional not originate catalyst any2.5% changeAu/2.5% Pd/C or new (2% bandHNO3), inbut the the spectra,H2O2 possibly because965 of thedegradation very small rate amountswas comparable. of Au Despite and Pd the that notably were high introduced degradation gave rates, rise the tocatalysts signals under the 966 containing Cs+ and Rb+ (Au/2.5% Pd/Rb2.5H0.5PW12O40 and 2.5% Au/2.5% Pd/Cs2.5H0.5PW12O40) and detection967 limit.prepared The Kegginby ion exchange structure were wasremarkably maintained more active after (an calcinationorder of magnitude of the higher) catalysts than the and also after the incorporation968 conventional of different catalyst amountsunder reaction of Csconditions+ (Figure prescribed 24b), accordingby potential applications to the XRD (water results. as a The969 H Osolventproductivity and ambient reachedtemperature). by Based Pd onAu environmentalCs H andW economicO was considerations, much higher the than that 970 2 authors2 suggested that these catalysts could0.075 be the0.05 starting2.5 point0.5 for12 the 40design of more efficient obtained971 by thecatalysts most for the active H2O2 conventionaldirect synthesis. catalyst 2.5% Au/2.5% Pd/C (2% HNO3), but the H2O2 degradation972 rateMonometallic was comparable. Pd and Despitebimetallic the PdAu notably catalysts high supported degradation on SBA15 rates, and the SiO catalysts2 were containing 973 2 2 Cs+ and Rb+ (Au/2.5%synthesized and Pd/Rb tested inH the HPWO directO synthesisand 2.5% by using Au/2.5% a batch autoclave Pd/Cs (atH 10 °CPW and 17.5O bar)) and prepared 974 without the addition2.5 of halides0.5 12and 40acids [122]. SBA15-supported catalysts2.5 0.5displayed12 better40 by ion975 exchange performance were remarkablywith respect to more those activesupported (an on order SiO2 (Si), of magnitudedue to the possibility higher) to thanregulate the the conventional catalyst976 under dispersion reaction in conditionsthe 5-7 nm range, prescribed as well as by the potential stability of applications the nanoparticles. (water Interestingly, as a solvent despite and ambient 977 2 2 temperature).bromine Based is a on well-known environmental a promoter andof the economic H O direct synthesis, considerations, a decrease in the both authors productivity suggested that 978 and selectivity was observed upon bromopropylsilane addition during the grafting process, as a these catalysts979 consequence could be of larger the starting size poor pointstability forof the the metal design nanoparticles, of more as revealed efﬁcient by catalystsTEM images. for the H2O2 direct synthesis.980 However, a synergistic effect between Pd and Au was highlighted either in the presence or in the 981 absence of bromopropylsilane grafting on the catalyst surface. Moreover, SBA15 was modified by Monometallic Pd and bimetallic PdAu catalysts supported on SBA15 and SiO2 were synthesized 982 the addition of Al, CeO2, and Ti to investigate the effect of the surface properties of the support on ◦ and tested in the H2O2 direct synthesis by using a batch autoclave (at 10 C and 17.5 bar) without the addition of halides and acids [122]. SBA15-supported catalysts displayed better performance with respect to those supported on SiO2 (Si), due to the possibility to regulate the dispersion in the 5–7 nm range, as well as the stability of the nanoparticles. Interestingly, despite bromine is a well-known a Catalysts 2019, 9, 30 26 of 34

promoter of the H2O2 direct synthesis, a decrease in both productivity and selectivity was observed upon bromopropylsilane addition during the grafting process, as a consequence of larger size poor stability of the metal nanoparticles, as revealed by TEM images. However, a synergistic effect between Pd and Au was highlighted either in the presence or in the absence of bromopropylsilane grafting on the catalyst surface. Moreover, SBA15 was modiﬁed by the addition of Al, CeO2, and Ti to investigate the effect of the surface properties of the support on the dispersion of the metal phase and on the catalytic activity. According to Table2, the catalyst in which Al was incorporated (PdAu/Al-SBA15) displayed higher productivity and selectivity. On the contrary, no improvement of the H2O2 yields was observed by adding Ti or CeO2.

Table 2. H2O2 synthesis with SBA15 supported catalysts: acidity, H2 conversion after 15 min of reaction, productivity and selectivity (after 15 min of reaction and at complete H2 conversion). Reprinted by permission from Springer Nature: Springer Nature, Topics in Catalysis, 56, 540–549, Reactivity Aspects

of SBA15-Based Doped Supported Catalysts: H2O2 Direct Synthesis and Disproportionation Reactions, N. Gemo, P. Biasi, P. Canu, F. Menegazzo, F. Pinna, A. Samikannu, K. Kordás, T. O. Salmi, J.-P. Mikkola, Copyright (2013).

H Complete Productivity H O 2 2 2 Selectivity (%) Conversion H Acidity (µmol/g) (mol H O /mol h) 2 2 2 metal @ 15 min Conversion Catalyst @ @ Complete Complete @ 15 min H @ 15 min (%) (min) Lewis Brønsted 2 H Conversion 2 Conversion Pd/SBA15 359 311 20 20 99 20 - - PdAu/SBA15 634 326 28 24 65 41 - - Br-Pd/SBA15 39 ------Br-PdAu/SBA15 192 - 24 - 13 - - - PdAu-Br/SBA15 0 0 0 0 - - - - Pd/Si 850 234 17 15 51 85 - - PdAu/Si 183 77 8 8 77 57 - - PdAu/CeO2-SBA15 399 314 16 14 48 34 39 0 PdAu/Ti-SBA15 538 463 13 12 77 18 16 0 PdAu/Al-SBA15 870 422 30 24 67 37 22 8 Br-PdAu/Al-SBA15 76 - 20 - 11 - 9 0 Conditions: T = 10 ◦C, P = 17.5 bar.

The improved performance of PdAu/Al–SBA15 was ascribed to an increased amount of Brønsted acid sites as reveled by FT-IR spectroscopy of adsorbed pyridine. In addition, the presence of Brønsted and/or Lewis acid sites was revealed in the modified SBA15 supports. The spectroscopic experiments were carried out according to the reported procedure [118]. The spectra were collected at different temperatures (250, 350, and 450 ◦C) and the bands related to adsorbed pyridine were detected only at 250 ◦C. Measured acid sites concentrations are reported in Table1 (entries 8–11). The amounts of Lewis and Brønsted acid sites were estimated on areas of the bands observed at 1450 and 1545 cm−1, by assuming 1.67 cm/µmol (Brønsted sites) and 2.22 cm/µmol (Lewis sites) integrated molar extinction coefficients. In addition, these analyses pointed out the presence of Brønsted and/or Lewis acid sites in the modified SBA15 supports. The authors showed that the H2O2 hydrogenation was overcome when the peroxide is surrounded by protons, i.e., the Brønsted acid sites [61,68]. Therefore, the H2O2 productivity can be enhanced by increasing the Brønsted acidity by Al insertion.

4. Final Remarks and Open Issues FTIR spectroscopy, either in transmission or in DRIFT mode, was shown to be a flexible technique which can be easily adapted to investigate the large variety of catalytic systems described here in order to establish catalyst structure-activity relationships in direct H2O2 synthesis. Despite being one of the first spectroscopic techniques employed for catalyst characterization, it can be considered as one of the most effective to obtain detailed chemical information on systems with continuously increasing complexity. Indeed, FTIR spectroscopy allowed, with no particular Catalysts 2019, 9, 30 27 of 34 effort (and with acceptable instrumentation costs), to shed light on the nature and on the properties of the active sites exposed at the surface of the Pd nanoparticles, which are the most active, as well as the most commonly investigated, catalyst for hydrogen peroxide direct synthesis. The selectivity Catalysts 2018, 8, x FOR PEER REVIEW 29 of 36 of such systems is improved by the addition of a second metal in order to inhibit the sites that are active1012 for dissociativeeffort (and with oxygen acceptable activation. instrumentation Studies costs), dealing to shed light with on the the nature addition and on ofthe severalpropertiesheteroatoms 1013 of the active sites exposed at the surface of the Pd nanoparticles, which are the most active, as well as to the1014 active the palladium most commonly phase investigated, have demonstrated catalyst for hydrogen that peroxide accurate direct analysis synthesis. of The the selectivity spectra of of adsorbed probes1015 (mainly such CO) systems collected is improved in by well-controlled the addition of a second experimental metal in order conditions, to inhibit the sites allows that are for active obtaining deep and precious1016 for insights dissociative on Pdoxygen site activation. isolation Studies and dealing electronic with the effects. addition of several heteroatoms to the 1017 active palladium phase have demonstrated that accurate analysis of the spectra of adsorbed probes Many1018 papers(mainly investigated,CO) collected in well-controlled in detail, the experiment natureal of conditions, exposed allows active for obtaining metalsites, deep and however, only a minority1019 triedprecious to insights monitor on Pd by site IR isolation spectroscopy and electronic the effects. reactivity of the exposed metal sites towards the molecular1020 hydrogenMany papers and investigated, oxygen in reactants,detail, the nature and of toexposed obtain active information metal sites, however, regarding only a the relative 1021 minority tried to monitor by IR spectroscopy the reactivity of the exposed metal sites towards the coverages1022 ofmolecular the reactive hydrogen species and presentoxygen reactants, on the and metal to obtain nanoparticles information duringregarding catalysis. the relative Additionally, FTIR characterization1023 coverages ofprovided the reactive insightsspecies present on theon the exposed metal nanoparticles sites of the during employed catalysis. Additionally, supports and on their properties.1024 OpportuneFTIR characterization probe molecules provided insights were on used the ex toposed measure sites of the the employed acidity supports and to and quantify on their the Brønsted 1025 properties. Opportune probe molecules were used to measure the acidity and to quantify the and the1026 Lewis Brønsted sites. and the Lewis sites. Nevertheless,1027 Nevertheless, FTIR absorption FTIR absorption spectroscopy spectroscopy needsneeds to to be becombined combined with many with other many techniques, other techniques, 1028 such as XAS, especially with in situ or operando studies during direct H2O2 synthesis, to correlate such as XAS, especially with in situ or operando studies during direct H2O2 synthesis, to correlate the 1029 the electronic and structural states of Pd with the catalytic performance, as exemplified in Figure 25. electronic1030 and structural states of Pd with the catalytic performance, as exemplified in Figure 25.

1031 1032 Figure 25.FigureRole 25. of Role the ofFTIR the FTIR spectroscopy spectroscopy in incatalyst catalyst characterisation. characterisation. 1033 Therefore, FTIRTherefore, characterization FTIR characterization contributed contributed signiﬁcantly significantly toto the development development of a ofrational a rational design 1034 design for new catalysts and of the optimization of process parameters for H2O2 direct synthesis. for new1035 catalysts However, and there of theare many optimization paths to be covered of process in the parametersuse of FTIR absorption for H2 spectroscopyO2 direct synthesis.for future However, there are1036 many studies paths in H to2O2 bedirect covered synthesis. in Some the possible use of directions FTIR absorption can be tentatively spectroscopy proposed here. for future studies in 1037 H O direct synthesis.In situ studies Some performed possible at directions low temperature can becould tentatively potentially proposedbe useful to here.investigate the 2 2 1038 saturation of the metal active sites with O2* and OOH* species during the reaction in alcohol In1039 situ studiessolvents [3,90]. performed In this frame, at low IR spectroscopy temperature can coulddetect peroxides potentially or superoxides be useful that towould investigate the saturation1040 ofsupport the metal the effective active presence sites of withthese intermediates. O2* and OOH* species during the reaction in alcohol 1041 The addition halides to prevent O2 dissociation can result in the presence of dense adlayers [62], solvents [3,90]. In this frame, IR spectroscopy can detect peroxides or superoxides that would support 1042 the effects of the presence of halides on the metal sites have not been investigated yet by IR the effective1043 presencespectroscopy. of these intermediates. 1044 Restructuring (and degradation [13]) phenomena occurring on the metal nanoparticles under The addition halides to prevent O2 dissociation can result in the presence of dense adlayers [62], 1045 reaction conditions could be monitored using a combined XAFS/DRIFTS or FTIR transmission the effects1046 of theapproach, presence as proposed of halides by Gibson on theet al. metal for AuPd sites bimetallic have notcatalysts been during investigated CO oxidation yet [123]. by IR spectroscopy. Restructuring1047 At present, (and degradationisotopic studies [are13 underestimated]) phenomena despite occurring their ability on to the identify metal distinct nanoparticles active under reaction1048 conditions sites and to could probe the be kinetics monitored and reversibility using of a specific combined steps [45]. XAFS/DRIFTS or FTIR transmission approach,1049 asAcknowledgments: proposed by Gibson This work etwas al. supported for AuPd by the Universi bimetallicty of Turin catalysts (Progetti duringRicerca Locale CO 2018). oxidation [123].

At present, isotopic studies are underestimated despite their ability to identify distinct active sites and to probe the kinetics and reversibility of speciﬁc steps [45].

Acknowledgments: This work was supported by the University of Turin (Progetti Ricerca Locale 2018). Conﬂicts of Interest: Maela Manzoli declares no conﬂict of interest. Catalysts 2019, 9, 30 28 of 34

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