Direct Synthesis of H2O2 from H2 and O2 on Pd Catalysts: Current

Jan 24, 2018 - (4, 37) High purity methanol, without halides or acids promoters, appears to be the most suitable solvent for these benchmark measureme...
2 downloads 8 Views 1MB Size
Download PDF
Viewpoint Cite This: ACS Catal. 2018, 8, 1520−1527

pubs.acs.org/acscatalysis

Direct Synthesis of H2O2 from H2 and O2 on Pd Catalysts: Current Understanding, Outstanding Questions, and Research Needs David W. Flaherty* Department of Chemical and Biomolecular Engineering, University of Illinois, Urbana−Champaign, Urbana, Illinois 61801, United States

1. MOTIVATION AND CHALLENGES FOR DIRECT SYNTHESIS OF H2O2 Catalytic reactions that utilize H2O2 as a terminal oxidant possess high selectivities for industrially relevant oxidations that rival Cl2 mediated chemistry and reactions with organic peroxides. H2O2 does not present, however, the same risks to environmental and human health as those more commonly used oxidants.1 The direct synthesis of hydrogen peroxide (reaction 1) over transition metal nanoparticles offers an environmentally attractive method to enable a wide array of impactful selective oxidation reactions including epoxidations of alkenes, oxidations of thiols, thiophenes, and sulfides, and conversion of alkanes to alcohols. H 2 + O2 → H 2O2

2. SELECTIVITY IN THE DIRECT SYNTHESIS REACTION The direct synthesis reaction involves reactions among intermediates derived from H2 and O2 on transition metal nanoparticles in a liquid solvent at low temperatures (273−313 K). A series of elementary steps appear to add H atoms sequentially to O2 to form H2O2 while avoiding the production of H2O by irreversible O−O bond rupture in any intermediate which is followed by further reduction. Success requires kinetically trapping unstable surface intermediates such as OO* and OOH* as well as H2O2 to prevent formation of thermodynamically preferred intermediates (O*, OH*) that lead to H2O. The primary challenge, therefore, is suppressing O−O bond rupture on metal nanoparticles that must activate H2 under conditions where O2 dissociation seems facile (∼300 K, 0.5−10 MPa H2, 0.5−10 MPa O2). Data from nearly five decades of academic research show that H2O2 rates and selectivities on Pd depend strongly on a number of factors. The fact that H2 and O2 pressures affect reaction rates4−6 is not unexpected. Figure 1 portrays several

(1)

The appeal of the direct synthesis reaction stems from the potential of direct synthesis to create inexpensive H2O2 at midscale, geographically distributed facilities, because direct synthesis facilities have much lower capital and operating costs than comparably sized Riedl−Pfleiderer plants (the autooxidation of anthraquinones, the incumbent technology).1,2 Direct synthesis has not emerged as a practical technology due to several technological and scientific barriers. Process designs reveal a few barriers, such as process safety. For example, direct synthesis requires that both H2 and O2 gas are present in a single reactor at pressures sufficiently high to achieve acceptable rates. Yet these conditions require also the use of significant amounts of diluent gases (e.g., N2, CO2) to prevent the formation of explosive mixtures.1,3 The use of diluted reactant streams decreases the H2O2 productivity per unit volume of reactor for a given total pressure and also introduces the need for a sizable recycle stream to recover the diluent gas and minimize compression costs.3 Perhaps more importantly, scientific challenges and questions remain that relate to the design of stable catalytic systems that convert H2 to H2O2 selectively. Specifically, how and why H2O2 formation rates and selectivities on Pd nanoparticle catalysts depend on reactant pressures and the solvents (water, alcohols) and promoters (halide salts and acids) used are not understood. Further, the solvents and promoters used to achieve significant selectivities on Pd catalysts reduce their stability and useful lifetime, which presents one of the greatest challenges for direct synthesis. This Viewpoint introduces and describes current understanding of this deceptively complex chemistry as well as concepts important for developing selective, productive, and stable Pd catalysts. Outstanding questions related to this promising catalytic reaction and needs for future investigations are described. © XXXX American Chemical Society

Figure 1. Direct synthesis of H2O2 on Pd proceeds in complex mixtures of aqueous solvents that contain mineral acids, halide salts, alcohols, and their reaction byproducts. The reasons why these additives affect H2O2 formation are not well understood.

other attributes of the catalytic system that are, perhaps, surprising to researchers not familiar with this chemistry. These include the choice of solvent (e.g., water, methanol, or ethanol), the presence of trace amounts of halide salts (e.g., ∼ 5 ppm NaBr), the addition of mineral acids (e.g., HCl, H2SO4), and the identity of the diluent gas (e.g., N2 or CO2).1,4 These complex combinations of halide salts, acids, and alcohol solvents are required to achieve even modest H2O2 selectivities (>60%) on supported Pd nanoparticles (the most selective and Received: November 30, 2017

1520

DOI: 10.1021/acscatal.7b04107 ACS Catal. 2018, 8, 1520−1527

Viewpoint

ACS Catalysis

intermediates. Perhaps surprisingly, the way in which O2 reduces to form H2O2 is still unclear. 3.1. H2O2 Formation in Water. Within purely aqueous solvents (i.e., in the absence of alcohols), most studies measure H2O2 formation in acidic solutions of either HCl or H2SO4 that also contain NaBr or NaCl, because H2O2 will not form in measurable quantities otherwise.5,6,25,26 Under these conditions, Lunsford reported that H2O2 formation increase in proportion to both H2 and O2 pressures at lower pressures and become independent of reactant pressures at greater values (water formation rates were not reported).22 Complementary experiments demonstrated rates did not differ between perhydrogenated and perdeuterated combinations of aqueous solvents, acids, and hydrogen.6 These data were interpreted as evidence that the reaction rates were transport limited or that O2 binding was rate determining,6 although these explanations seem inconsistent with the measured rate dependencies. The addition of HCl or H2SO4 to aqueous solvents (or neat ethanol) increases rates significantly but the manner by which these proton donors promoted H2O2 formation was not clear in initial reports.6,22,27 Contemporaneous work by Stahl et al. illustrated that H2O2 forms upon reduction of bathocuproine Pd−O2 complexes28,29 and inspired Lunsford to propose that a similar catalytic cycle may form H2O2 during direct synthesis.6,30 Figure 2 shows steps that corrosively oxidize Pd to form soluble Pd-peroxo complexes that are protonated by HCl and

frequently studied monometallic catalyst). Slight differences in these attributes of the catalytic systems significantly change H2O2 production rates and selectivities (several comparisons are provided in Table 2 of ref 1), and in some cases, these changes suggest that these species participate directly in the reaction. Efforts to increase H2O2 selectivities fall largely into two categories; catalyst design and reaction engineering. One challenge is that the stability of O2* and OOH* surface species that may be reduced to H2O2 are strongly correlated with those of reactive intermediates that ultimately form H2O (O*, OH*) on extended metal surfaces.7−11 Therefore, a common principle of catalyst design for direct synthesis focuses on the formation of bimetallic catalysts (e.g., PdAu,12,13 PdSn,14 PdZn15) or the use of promoters and additives (e.g., Cl−, Br−)16−18 that may change or break scaling relationships among O-containing intermediates. Both bimetallics and promoted Pd catalysts give greater H2O2 selectivities perhaps by preventing the formation of binding configurations thermodynamically preferred on uniform metal surfaces (η2-O2 on Pd(111)) in favor of those that involve less electron exchange (η1-O2 on PdxAu(111)).19−21 Reaction engineering approaches attempt to capitalize on differences between mechanisms and coreactants for the kinetically relevant steps and transition states that form H2O2 (e.g., OOH* + H* → H2O2* + *) and those that form H2O (e.g., OOH* + * → OH* + O*). Among other factors, changes in H2 pressure, the identity of the solvent, and the addition of mineral acids to the liquid phase affect H2O2 selectivities. Several plausible explanations have been proposed for these effects within individual studies, but direct evidence to support or disprove many hypotheses is difficult to obtain. Aside from the potential technological impact of selective and stable catalysts for the direct synthesis of H2O2, this chemistry is an appealing probe reaction to develop greater understanding (and quantitative descriptions) of the manner by which distinct aspects of catalytic systems (e.g., solvent, surface promoters, catalyst composition, etc.) dictate chemistry at the liquid−solid interface. Many opportunities exist for experimental and particularly computational investigations of this reaction, because, we understand much less about the direct synthesis of H2O2 than many other reactions of small molecules. Future studies need to embrace the complexity of this system and include the critical “additives” that permit H2O2 to form in order to account for the complete set of species and interactions required for this chemistry. Such work would help to build a conceptual framework that describes this fascinating chemistry. To build such a framework, we need first to understand the mechanism for H2O2 formation.

Figure 2. Proposed catalytic cycle for H2O2 formation by Pd within acidified water. O2 binds to soluble PdII complexes and reduces by proton electron transfer as described by Lunsford et al.6

subsequently reduced by H2 gas to reform metallic Pd and HCl. Importantly, this work hypothesized that proton−electron transfer pathways were responsible for H2O2 formation on Pd sites under nominally thermocatalytic conditions.6,30 Kinetic dependencies on reactant pressures similar to these early studies (rH2O2 ∼ [H2]0−1[O2]0−1) were recently reported from experiments in continuous flow microreactors that also used water as a solvent (5−10 ppm NaBr, 0.05−0.5 M H2SO4).5,26,31 However, these data were interpreted as evidence for a two-site surface reaction in which H*-atoms and O2* adsorb noncompetitively and react on the surface to reduce O2 to H2O2.5 Neither of these mechanisms contains quantitative descriptions that show protons or acids are coreactants or cocatalysts for H2O2 formation, rather, the given interpretations state that these species participate indirectly. Protons are frequently described as modifying the electronic structure of Pd.32 Similarly, halides are denoted as site blockers, despite the fact that even trace quantities of H2O2 do not form in aqueous solvents in their absence.32 These conclusions seem speculative and specific to aqueous systems. As described next, rate dependencies and kinetic behavior for H2O2 formation differ significantly within alcohol solvents and also in aprotic solvents.

3. PROPOSED MECHANISMS FOR H2O2 FORMATION ON PD Detailed discussions of the mechanism for direct synthesis of H2O2 date to seminal publications by Lunsford and co-workers who reported several key observations for H2O2 production over Pd-SiO2 catalysts from reactions in semibatch reactors.6,22−24 Analysis of the oxygen isotope labels in H2O2 produced from reactant mixtures of 18O2, 16O2, and H2 demonstrated that O−O bonds cleave irreversibly and always lead to H2O formation.22 Consequently, achieving high selectivities to H2O2 requires catalysts and active sites that bind O2* but lack e− of sufficient energy to populate the 2π* orbitals and cleave the O−O bond in O2*, OOH*, or other 1521

DOI: 10.1021/acscatal.7b04107 ACS Catal. 2018, 8, 1520−1527

Viewpoint

ACS Catalysis

oxidation of H2 (i.e., H2 → 2H+ + 2e−) and shuttle via oxonium species during consecutive proton−electron transfer steps that reduce O2 to H2O2. This mechanism accounts for the need for H2 as a reductant (i.e., H2O2 did not form at detectable rates in methanol without external H2) and provides a means to maintain the solvent pH and the electronic charge on Pd nanoparticles.4 Pd-based bimetallic nanoparticles also appear to directly involve alcohol molecules in the formation of H2O2. Bimetallic AuPd nanoparticle catalysts exhibit similar dependencies of rates on H2 and O2 pressures and on the protic nature of the solvent, even though H2O2 selectivities are nearly 6-fold greater on the most Au-rich nanoparticles.37 These differences reflect changes in the electronic structure of the Pd active sites that increase activation enthalpies for O−O bond dissociation and the formation of H2O, consistent with theoretical predictions for close-packed surfaces.19,38−40 Kinetic data and solvent requirements for H2O2 formation on intermetallic βPdZn nanoparticles15 also match the mechanism shown in Figure 3. Despite these recent findings, the molecular mechanism responsible for H2O2 formation is by no means resolved. The results reviewed here for both aqueous and nonaqueous solvents (Sections 3.1and 3.2) are all consistent with proton− electron transfer mechanisms. Based upon current findings, such heterolytic reaction mechanisms seem most likely to be responsible for the direct synthesis of H2O2. However, the identity of the kinetically relevant step, the most abundant surface intermediate, and the details of how the protic solvent (or the derived surface fragments) shuttle protons and electrons to O2 may differ. Additional experimental and computational investigations are needed to develop detailed understanding of the steps that combine H atoms with O2 and the ways in which halides, acids, and solvent molecules participate. In situ spectroscopy and increasingly complex molecular simulations should be integrated into future studies to probe other ways in which solvent molecules may be involved.

These differences suggest that the solvent participates directly within the catalytic cycle as either a coreactant or a cocatalyst. Moreover, H2O2 can form even without halide “site blockers.” 3.2. H2O2 Formation in Non-Aqueous Solvents. Direct synthesis may become an economical method to produce H2O2 for epoxidation or oxidation catalysis over transition metal oxides.1 Water can significantly decrease rates in these systems, and therefore, H2O2 may be produced in alcohol solvents that are compatible with these chemistries.33 The significant differences in H2O2 formation rates and selectivities between aqueous and alcohol solvents are frequently attributed to greater solubility of H2 in alcohols compared to water (vide inf ra).34−36 A few specific observations suggest, however, that the mechanism for H2O2 in methanol and ethanol may differ fundamentally from the mechanism within water. Lunsford first reported that H2O2 formation rates increase in proportion to H2 pressure and do not depend on O2 pressure (rH2O2 ∼ [H2]1[O2]0) within ethanol (0.12 M H2SO4),23 but the report did not propose a molecular mechanism to explain these results. These results were quickly followed by reports for the effects of H2SO4, Cl−, and CH3COOH on H2O2 rates within ethanol that were interpreted as evidence for high coverages of acetate (formed by ethanol oxidation) and Cl− on Pd surfaces that block Pd ensembles needed to dissociate O−O bonds (vide inf ra).32 Again, halides and protons derived from mineral salts and acids were described as critical promoters for H2O2 that modify the reactivity of the Pd surface but are not involved directly. Our group recently reported mechanistic analysis of steady state H2O2 and H2O formation rates measured as functions of reactant pressures along with complementary experiments probing the influence of the solvent properties.4 In pure methanol (i.e., without water, NaBr or H2 SO 4 ), H 2 O 2 formation rates increase with H2 pressure but do not depend on O2 pressure (rH2O2 ∼ [H2]1/2−1[O2]0; similar to reports in ethanol23).4 H2O formation rates depend more weakly on H2 and are also independent of O2 (rH2O ∼ [H2]0−1/2[O2]0).4 These observations are inconsistent with competitive adsorption of H2 and O2 on a single active site. Independent experiments demonstrated that H2O2 forms only in the presence of protic solvents (methanol, water) and was undetectable in aprotic solvents (acetonitrile, dimethyl sulfoxide, and propylene carbonate).4 Together these observations provide strong evidence that protic solvents cocatalyze H2O2 formation. Figure 3 depicts one method by which water or methanol may participate directly in H2O2 formation by introducing low barrier pathways for O2* reduction. Protons form by heterolytic

4. SOLVENTS HAVE MULTIPLE ROLES FOR H2O2 FORMATION The solvent used for the direct synthesis of H2O2 has a significant effect on the rate of the reaction and the selectivity to H2O2. Solvents such as water, methanol, and ethanol may influence H2O2 yields by facilitating transport of the reactants to the active sites and by chemically binding to and modifying exposed metal atoms. These effects are shown by comparisons of catalytic rates performance to the solubility of O2 and H2 in pure solvents and multiple component solutions.29,34,35 Several reports suggest that the adsorption of organic molecules (e.g., CH3COO−) to catalyst surfaces increases H2O2 selectivities in several reports.23,32,41 Solvent molecules seem likely to influence H2O2 formation and decomposition pathways by less frequently discussed methods as well. H2O2 only forms within protic solvents,4,15,37 which strongly suggests that solvents participate directly in proton or hydride transfer processes. In addition, solvent molecules may stabilize critical intermediates via intermolecular interactions such as hydrogenbonding. Undoubtedly, the use of appropriate solvents to facilitate mass transport is important for implementing direct synthesis chemistry, particularly in industrial reactors that would operate near the limit of mass transfer rates. Yet, carefully designed laboratory experiments can eliminate interphase and inter- and

Figure 3. Catalytically coupled heterolytic hydrogen oxidation and two electron oxygen reduction reactions may form H2O2 on Pd nanoparticles. Adapted with permission from ref 4. Copyright 2016 American Chemical Society. 1522

DOI: 10.1021/acscatal.7b04107 ACS Catal. 2018, 8, 1520−1527

Viewpoint

ACS Catalysis

quantities of H2O2.4,15,49 Further, alcohols may provide greater rates and selectivities than water by acting as coreagents for catalytic transfer hydrogenation of O2 by hydride-proton transfer steps akin to the Meerwein-Ponndorf-Verley mechanism for reduction of carbonyl groups.33 Fluid-phase solvent (or adsorbed fragments) molecules can also influence catalysis by stabilizing kinetically relevant transition states via intermolecular interactions such as hydrogen-bonding or by solvating highly charged structures (e.g., those involved in proton−electron transfer processes).50,51 Classical models for these effects in catalysis are described in texts51,52 and can be quantified in the framework of thermodynamic activity coefficients for solvent-species pairs, which can include transition states.

intraparticle concentration gradients such that measured H2O2 and H2O formation rates reflect only the kinetics of the catalytic reaction.42,43 Under these conditions, solvents still change formation rates of H2O2 and H2O, which shows that the solvent influences this catalytic reaction in ways that are not related to transport processes or reactant solubility alone. The potential ability to use solvent molecules or other species (e.g., halides, acids) to chemically modify the surface of nanoparticle catalysts or to stabilize transition states directly are intriguing. Lunsford and co-workers suggested that ethanol and acetic acid (added directly or produced in situ) may be responsible for greater H2O2 selectivities in ethanol in comparison to water.23,32 The formation of chelating acetate species32 may inhibit O−O bond rupture by disrupting ensembles of Pd atoms required to bind η2-O2, η1-OOH, and the transition states for their reduction or dissociation, similar to proposals for bimetallic catalysts with improved H2O2 selectivities.38,44−46 Similarly, Nijhuis et al. demonstrated that H2O2 selectivities increase by a factor of two on Pd and AuPd nanoparticles with the addition of small amounts (≥2 vol %) of acetonitrile to an aqueous solvent (containing 0.05 M H2SO4, 9 ppm NaBr), which was attributed to acetonitrile reducing the coverage of O2 at Pd ensembles responsible for O−O bond rupture.34 This concept could explain also one role of alcohol solvents (methanol), surface ligands (e.g., long-chain ammonium dihydrogen phosphates depicted in Figure 4),47,48 and

5. HALIDES AND ACIDS AFFECT H2O2 RATES AND SELECTIVITIES The vast majority of publications that report the direct synthesis of H2O2 in water or in alcohol solvents also include catalytically significant quantities of halide salts (e.g., NaBr) or inorganic acids (e.g., HCl, H2SO4). These additives are often used with little discussion of their role in catalysis, which suggests that their use is based on phenomenological observations that they increase H2O2 selectivities. Pospelova et al. first demonstrated that H2O2 formed in immeasurable quantities on pure supported Pd catalysts in water, but the addition of millimolar quantities of acids and halides increased H2O2 yields to nearly 60%.53 More recently, H2O2 selectivities above 90% and nearing 100% were achieved through more complex combinations of H2SO4, HCl, and Br− promoters. Several publications have probed how these species influence H2O2 formation and decomposition pathways, but few investigations probe the intrinsic way in which these species influence surface catalysis. The additions of halides, and more specifically Cl− and Br−, to solvents or to catalyst supports increase H2O2 selectivities significantly. Adsorption and activation of O2, H2, and other surface intermediates are undoubtedly influenced by the presence of halide coadsorbates, and these species are frequently described as blocking specific sites responsible for cleaving O−O bonds.6,16−18,24,53 However, halides are likely to influence the density of states of greater numbers of metal atoms in their vicinity and may confer greater selectivity for H2O2 formation to multiple sites by limiting the extent of e− back-donation to 2π* orbitals of O2 via through-surface interactions.54 This explanation is consistent with greater work functions for Pd(111)55 and weaker O*-atoms binding energies on Pt(111)56 and Au(111)57 in the presence of Cl*adatoms. However, increasing concentrations of halides oxidize Pd to form soluble Pd2+-complexes (e.g., PdCl42−),49 which leads to greater rates of metal dissolution from the support and reduces the longevity (and utility) of the catalyst. More commonly, Br− is used to promote H2O2 formation on Pd, because Br− leads to comparably high H2O2 selectivities yet does not leach Pd from the catalyst as rapidly.27 The changes in selectivities, H2O2 formation rates, and Pd leaching rates do not depend significantly on the precursor used to add Br− to the solvent for the reaction58 but are sensitive to the concentration of acids used as copromoters.59 The role of acids in H2O2 formation is more difficult to understand, because their dissociation leads to two intermediates that may each modify surface reactivity. The anions derived from acids commonly used (e.g., HBr, HCl, and

Figure 4. Long-chain alkyl ammonium phosphates coadsorbed on Pd increase H2O2 selectivities and are predicted to increase barriers for O−O bond rupture during the direct synthesis of H2O2. In general, dense adlayers of solvent molecules or intentional ligands can modify binding modes of reactive intermediates and electron exchange with surfaces and can subsequently influence H2O2 selectivities. Reproduced with permission from ref 47. Copyright 2017 Wiley-VCH.

perhaps diluent CO2,13,14 which forms carbonic acid in situ.4 All of these species can form adlayers of chelating species that reduce the number of accessible metal sites and hinder adsorption modes of reactive species such that O−O bonds are more difficult to cleave. In situ vibrational spectroscopy would be useful to determine if the proposed carboxylate and ligand adlayers exist during catalysis and to correlate the coverage of these species to rates and activation energies for H2O2 and H2O formation. These comparisons would directly test how these adlayers influence the two reaction pathways. Apparent activation energies, selectivities, and rates for H2O2 and H2O formation depend on the solvent likely because solvent molecules participate directly in the catalytic cycle and assist by solvating reactive intermediates or transition states through intermolecular interactions. Methanol, water, and their protonated forms cocatalyze proton−electron transfer steps by providing low barrier pathways to shuttle protons, which can explain the need for protic solvents to form detectable 1523

DOI: 10.1021/acscatal.7b04107 ACS Catal. 2018, 8, 1520−1527

Viewpoint

ACS Catalysis

indirectly. PdO nanoparticles supported on silica do not form measurable amounts of H2O2 in semibatch reactors (5 kPa H2, 75 kPa O2, 283 K, ethanol, 2 h).61 However, H2O2 forms readily within the same system after sparging with 20 kPa H2 (in N2, 1 h) and reintroducing the H2 and O2 coreactants.61 Han et al. proposed that Pd-PdO sites located at the interface with a TiO2 support were active sites for H2O2 formation based upon comparisons of measured H2O2 selectivities to ex situ XAS and XPS assessments of Pd oxidation states.62 Our group has observed that PdO nanoparticles exhibit significant induction times (∼0.5 h) prior to forming H2O2, whereas, Pd nanoparticles form H2O2 immediately (9 kPa H2, 5 kPa O2, 298 K, methanol).63 Cumulatively, these observations indicate that at least a fraction of the nanoparticles must exist as reduced Pd in order to form H2O2, but it is not clear if the active sites that bind and selectively reduce O2 to H2O2 are reduced, partially, or fully oxidized Pd atoms.62 A few publication have applied in situ X-ray absorption spectroscopy (XAS) to probe the state of Pd during catalysis.41,64 Results from Centomo et al. suggested that Pd nanoparticles on a resin support contained a significant fraction of Pd0 (60%) in pure methanol, which increased slightly upon adding the reactant stream (H2 and O2 in CO2).41 On the other hand, more recent results reported by Hensen and Schouten indicated that Pd nanoparticles existed largely as PdH during catalysis in water.64 These differences likely reflect differences in reactant pressures and solvents used. Systematic comparisons of H2O2 formation rates and selectivities to the state of the Pd nanoparticles (determined by in situ XAS or Raman) across a wide-range of O2 and H2 pressures would be useful, because such data may shed some light on the state of Pd responsible for selective H2O2 formation.

H2SO4) may influence catalysis by their adsorption and interaction with nanoparticle surfaces. For example, halide anions may coordinate to the surface of Pd nanoparticles and either inhibit reaction at Pd ensembles or electronically modify Pd sites by withdrawing charge (vide supra).54−56 In addition, acids have been proposed to inhibit the decomposition of H2O2 by preventing its deprotonation to more reactive forms (e.g., OOH−) that bind strongly to metal surfaces and decompose.27 Thus, the addition of acids may shift the position of equilibria related to these deprotonated and more reactive forms. Finally, the changes in the concentrations of acids (and also halide salts) in a solvent will influence the ionic strength of the solution and affect the thermodynamic activity of charged intermediates and transition states within the catalytic cycle.51 Such changes may preferentially stabilize pathways for H2O2 formation over those for H2O. These potential effects have not been deconvoluted in previous experimental investigations, but these points may be addressed with computational studies.

6. THE STATE OF THE ACTIVE PD CATALYST IS UNKNOWN Despite ongoing work to determine the mechanism(s) by which H2O2 forms, it is clear that molecular O2 binds to the active site and reduces to form H2O2 without O−O bond rupture.22 Pristine Pd surfaces and nanoparticles readily dissociate O2 at cryogenic temperatures, which suggests that active sites for direct synthesis differ significantly from these more reactive surfaces. Pd nanoparticles can form PdO and PdH phases when exposed to O2 and H2 respectively, which coexist at significant pressures during H2O2 formation. These observations and the effects of ligand and halide additives indicate that the active state(s) of Pd responsible for selective formation of H2O2 is not a clean, metallic surface. Rather, H2O2 likely forms either on surfaces of metallic Pd nanoparticles covered by halides or reactive intermediates (e.g., O*, OH*), surfaces of PdO or PdH nanoparticles, or on soluble Pdcomplexes formed by dissolution of nanoparticles. Lunsford first showed that rates of H2O2 formation in semibatch reactors did not correlate to the amount of Pd supported on SiO2 supports in the presence of HCl.49 The Pd nanoparticles dissolved into vividly colored Pd2+ complexes (e.g., PdCl42−, observed in many batch reactor experiments) and redistributed as nanometer sized Pd colloids throughout the solvent and across the walls of the reactor (steps shown in Figure 2).24,49 These Pd colloids were proposed to be the active catalysts responsible for the majority (>97%) of H2O 2 produced, because, significant mass transfer limitations limited the productivity of the Pd-SiO2 catalyst particles (i.e., effectiveness factors (η) ≪ 1).24,49 While this work suggests that Pd colloids form H2O2, it is not clear if the experiments performed could rule out the possibility that soluble Pdcomplexes are also active sites for H2O2 formation. We are not aware of further studies that have provided evidence for or against the idea that liquid-phase Pd2+ species contribute to measured H2O2 formation rates during direct synthesis. However, barriers for O2 dissociation on these and other mononuclear Pd complexes28,60 are greater than on pristine Pd surfaces,19,21 and the populations of such complexes increase significantly with the addition of halide and acid promoters. The surfaces of metal nanoparticles are most commonly considered to expose the active sites for H2O2 formation. Yet knowledge of the chemical state and structure of these nanoparticles during direct synthesis is often obtained

7. RECOMMENDATIONS FOR FUTURE RESEARCH Many fundamental aspects of the direct synthesis of H2O2 remain uncertain, but the great potential of this chemistry to enable selective and environmentally benign oxidations provides considerable motivation to continue investigating this complex reaction. The comparisons of the mechanistic studies highlight several questions that should be addressed in future studies, and which will need contributions from both theoretical and experimental methods to unravel the more intricate aspects of H2O2 formation. Specific points for further investigation include: the mechanism for O2 reduction to H2O2 and the potentially direct participation of solvent molecules in this process; the interaction between coadsorbates (i.e., halides, solvent molecules, and reactive intermediates) that influence the reactions of dioxygen intermediates; and chemical or electronic characterization of the active sites that produce H2O2. 7.1. Need for Computational Work with Complete Models. Computational studies of H2O2 formation have the potential to provide atomistic insight that eludes even the most carefully designed experiments. To date, density functional theory (DFT) calculations have been used to probe the elementary steps that determine selectivity and differences between Pd, PdH and more selective Pd-based bimetallic catalysts (e.g., PdAu).8,38−40,47,65 Investigations of AuPd bimetallic surfaces demonstrated that intrinsic activation energies for O−O bonds in O 2 * or OOH* surface intermediates increase systematically with the number of Au atoms within the close-packed (111) surfaces,19,38,39 whereas, the activation energies for hydrogenation pathways (e.g., O2* + 1524

DOI: 10.1021/acscatal.7b04107 ACS Catal. 2018, 8, 1520−1527

Viewpoint

ACS Catalysis H* → OOH* + H*) increase to a somewhat smaller extent.38,39 These trends reproduce the greater selectivities toward H2O2 on AuPd materials.12,13,49,66 Analysis of the densities of states upon Pd atoms within these bimetallic surfaces shows that the addition of Au leads to systematic shifts of the d-band to lower energies (presumably due to rehybridization of Pd atomic orbitals as a result of changes in strain)38,39 and a concurrent reduction in the extent of electron back-donation to 2π* orbitals of O2*. Notably, most computational investigations into direct synthesis of H2O2 utilize periodic boundary conditions for two-dimensional unit cells (e.g., 3 × 3) and treat low coverages of reactants (