Perspective pubs.acs.org/JPCL
Catalytic Reduction of CO2 by Renewable Organohydrides Chern-Hooi Lim,† Aaron M. Holder,†,‡,§ James T. Hynes,‡,∥ and Charles B. Musgrave*,†,‡ †
Department of Chemical and Biological Engineering and ‡Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States § National Renewable Energy Laboratory, Golden, Colorado 80401, United States ∥ Chemistry Department, Ecole Normale Supérieure-PSL Research University, Sorbonne Universités-UPMC University Paris 06, CNRS UMR 8640 Pasteur, 24 rue Lhomond, 75005 Paris, France ABSTRACT: Dihydropyridines are renewable organohydride reducing agents for the catalytic reduction of CO2 to MeOH. Here we discuss various aspects of this important reduction. A centerpiece, which illustrates various general principles, is our theoretical catalytic mechanism for CO2 reduction by successive hydride transfers (HTs) and proton transfers (PTs) from the dihydropyridine PyH2 obtained by 1H+/1e−/1H+/1e− reductions of pyridine. The Py/PyH2 redox couple is analogous to NADP+/NADPH in that both are driven to effect HTs by rearomatization. High-energy radical intermediates and their associated high barriers/overpotentials are avoided because HT involves a 2e− reduction. A HT−PT sequence dictates that the reduced intermediates be protonated prior to further reduction for ultimate MeOH formation; these protonations are aided by biased cathodes that significantly lower the local pH. In contrast, cathodes that efficiently reduce H+ such as Pt and Pd produce H2 and create a high interfacial pH, both obstructing dihydropyridine production and formate protonation and thus ultimately CO2 reduction by HTPTs. The role of water molecule proton relays is discussed. Finally, we suggest future CO2 reduction strategies by organic (photo)catalysts.
T
The conversion of CO2 to MeOH is a six-electron reduction described by the overall reaction in eq 1. When this reduction is carried out as a series of six one-electron transfers (ETs) and six proton transfers (PTs), every odd reduction necessarily produces a high-energy radical (open-shell) intermediate. Consequently, the three odd ETs generally result in slow kinetics and low selectivities unless these radicals are stabilized, for example, by conjugation to an aromatic π-system or by orbital mixing with delocalized states of a metal surface.16 The issue of the difficulty of creating high-energy intermediates by the odd electron reductions is exemplified by the oneelectron reduction of CO2 to CO2−•, which involves a very unfavorable reduction potential E0 of −2.14 V versus SCE.17 One approach to circumvent this obstacle is to avoid radical intermediates in favor of closed-shell, stable intermediates by performing reductions two electrons at a time as hydride (H−) transfers (HTs), effectively 2e−/H+ transfers.18 Thus, we can combine the six ETs with three PTs of eq 1 to produce three HTs; this converts the general one-electron reduction route to a two-electron route through HTs, and we can rewrite eq 1 as
he efficient chemical reduction of CO2 to fuels has been of interest to scientists for decades, with growing concerns about the impact of CO2 on climate and future global energy demands motivating increasing efforts to meet this challenge.1−4 One principal strategy here is to mimic nature’s carbon economy, which photochemically reduces vast quantities of CO2 to store solar energy and sequester carbon in natural products that serve as materials and fuels.5,6 However, the molecular structures of the light-harvesting and chemical reduction systems of photosynthesis are intricate, and their detailed mechanisms are not fully understood; imitating their abilities has posed a difficult challenge. Even attaining the specific goal of developing catalysts that efficiently transform CO2 into valuable products proves to be enormously difficult.7−13
One conversion of specific interest, the reduction of CO2 to methanol (MeOH), is the focus of this Perspective. This conversion has been promoted by Olah as the basis of a MeOH economy.14,15 Arguments here involve MeOH’s utility as a practical C1 source for chemical synthesis and its attractive properties as a fuel, not demanding the massive changes to the transportation fuels infrastructure required for a hydrogen economy. The partial reduction of CO2 to methanol is generally preferred over its complete reduction to methane; the former is a more valuable product and is easier to handle and transport as a liquid fuel, which is more compatible with existing transportation fuel technology. © 2015 American Chemical Society
We begin this contribution by examining CO2 reduction “catalyzed”19 by ammonia borane (AB; NH3BH3), a compound Received: August 20, 2015 Accepted: November 24, 2015 Published: November 24, 2015 5078
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possessing both hydridic and protic hydrogens;20 despite its disadvantage as a stoichiometric rather than catalytic reagent, examination of AB assists conceptual understanding of the key HT and PT steps in the reduction. We then examine organic dihydropyridines (DHPs) as catalytic, rather than sacrificial, hydride donors in chemical, electrochemical, photochemical, and photoelectrochemical systems.21,22 One highlight here is the detailed theoretical mechanism that we have proposed for catalytic conversion of CO2 to MeOH with aromatization as the principal driving force for catalytic HT reactions from DHPs and related hydrides.23 We further argue for the PT step’s importance in intimate connection with HT. This HT can be significantly assisted by a proton relay chain mediated by water molecules. Finally, we offer our perspective on future CO2 reduction strategies including, but not limited to, achieving CO2 reduction using organic photocatalysts. AB: A Species Containing Both Hydridic and Protic Hydrogens. Various nonbiological hydride donors have been used to reduce CO2; these include metal hydrides in the form of transitionmetal complexes,24,25 hydrosilanes,26 hydroboranes,27,28 and AB.29,30 Of these donor types, certain of the first type are catalytic. Indeed, various transition-metal complexes have proven competent in catalyzing CO2 reduction. There are however some drawbacks; they generally involve expensive metals, require high overpotentials to electrochemically produce a hydride, and in some cases require a sacrificial Lewis base.24 As for the other hydride-donor classes, even though hydrosilanes, hydroboranes, and AB are not catalytic, they provide important clues for which features might lead to effective HT catalysts. In particular, consideration of their driving force for reduction via HT can be fruitful. Hydrosilanes and hydroboranes derive this driving force by forming strong Si−O and B−O bonds upon HT; for example, their immediate products of HT to CO2 are formatosilane and formatoborate.26,27 In contrast, AB contains both nucleophilic hydridic (red) and electrophilic protic (blue) hydrogens capable of reducing and, in effect, hydrogenating CO2 to MeOH (eq 3).31,32 AB’s driving force for hydride donation has been recently analyzed.32
proton.32 Here, the PT and HT each stabilize the other, thus lowering the reaction barrier; while CO2 is neither a good hydride nor proton acceptor, the transferring proton activates CO2 for HT while the transferring hydride activates CO2 for PT. Further, the conversion of the N−B dative bond to a covalent bond that drives HT necessitates dissociation of both the NH and BH bonds; CO2’s reduction by AB involves HT and PT that are not only coupled but also are simultaneous. We note however that activation of CO2, for example, by a Lewis acid, can result instead in HT preceding PT.31,33 To derive the mechanism just described, one of us used highlevel CCSD(T) coupled cluster theory to predict the HT and PT from AB to CO2.32 This reduction via HT involved a relatively low free-energy barrier (∼24 kcal/mol) due to the driving force described above. Although our predicted reduction of CO2 by AB is not catalytic, Lee and co-workers demonstrated that AB does in fact reduce CO2,29 likely by the mechanism that we originally proposed. This motivated us to suggest that this mechanism could inspire development of catalysts that operate similarly. Unfortunately, because the conversion of the N−B dative bond to a covalent bond also produces a lone pair on N and an empty orbital on B (see Figure 1), the NH2BH2 produced on AB dehydrogenation oligomerizes,34 rendering this reaction irreversible. Nevertheless, AB provides an example of CO2 reduction by HTs and PTs from metal-free species. It also motivates posing the question of whether metal-free catalysts might possess effective driving forces that drive reduction but, in contrast to AB, produce an oxidized catalyst that can be reconverted into a hydride donor to complete the catalytic cycle. It seemed to us that the examination of natural photosynthesis could offer insights into designing HT catalysts.
This driving force derives from the conversion of AB’s relatively weak N−B dative bond into a strong covalent bond upon transfer of both a hydridic H from its borane and a protic H from its amine to form NH2BH2 (see Figure 1). We have
DHPs as Catalytic Hydride Donors. Nature provides a remarkable example of catalytic HT in the NADPH/NADP+ redox couple of the Calvin cycle of photosynthetic CO2 reduction.35,36 Indeed, NADPH’s HT ability to effect reductions was recognized soon after its discovery in the 1930s.21 Figure 2 shows the essential pyridinic and dihydropyridinic structures of NADP+ and NADPH. As we discuss below, the reduction abilities of NADPH, involving HT to a carbonyl to produce a C−H bond, ultimately originate from the aromatic pyridine (Py) core of NADP+, which is converted to a nonaromatic DHP species in its active form, NADPH. This role of the aromatic heterocycle of this couple’s NADP+ member could provide inspiration for the design of new catalysts mimicking NADPH’s catalytic abilities. In fact, we argue that such abilities have already been demonstrated for some existing catalysts, although perhaps not always with full appreciation for the detailed mechanisms or the essential principles involved. NADPH is a DHP37 produced by the formal 2e−/1H+ transfer to NADP+, driven by and coupled to light-dependent reactions in Photosystems I and II. NADP+ is aromatic, as
We argue that the abilities to mimic NADPH have already been demonstrated for some existing catalysts, although perhaps not always with full appreciation for the detailed mechanisms or the essential principles involved.
Figure 1. Conversion of the N−B dative bond to a N−B covalent bond in AB drives simultaneous HT and PT to CO2 to form formic acid and NH2BH2.
proposed that these two transfers synergistically complement the proton- and hydride-accepting abilities of CO2, whose electropositive carbon can accept a nucleophilic hydride while one of its electronegative oxygens accepts an electrophilic 5079
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by reacting their corresponding Py bases or alkylated salts with strong reducing agents.40
Figure 2. NADPH/NADP+ redox couple involved in the Calvin cycle of photosynthesis. (Modified from ref 23. Copyright 2014 American Chemical Society.).
mentioned above, with six electrons occupying its Py ring’s π-space. Upon 2e −/1H + reduction, the ring loses its aromaticity; this results in the nonaromatic DHP NADPH in which the two para position hydrogen atoms are hydridic. It is these hydridic hydrogens that transfer to reduce carbonyls in the Calvin cycle converting CO2 to sugars.38 The key principle that drives HT from NADPH is the recovery of aromaticity (or aromatization) of NADPH’s pyridinic ring upon HT, returning it to its originally low-energy aromatic state in NADP+ that it lost upon DHP formation. Thus, this aromatization provides a significant driving force for 2e−/1H+ donation via a hydride. Below we ask: might other catalysts be discovered or already exist that share the underlying attributes that make NADPH an effective reducing agent? Recognition of NADPH’s HT abilities soon after its discovery has inspired numerous subsequent studies of NADPH-like DHPs.21 In the following, we highlight examples of DHPs produced via chemical, electrochemical, photochemical, and photoelectrochemical pathways. The latter three pathways utilize electricity or photons to regenerate the DHP after HT, enabling promising avenues to catalyze CO2 to MeOH conversion. Our summary aims to illustrate essential principles of HT chemistry by DHPs to reduce CO2 and is not intended as a comprehensive review; for excellent reviews, we refer the interested reader to refs 21, 22, and 37. Chemical Pathways: One notable DHP example is Hantzsch’s ester (Figure 3a), discovered in 1882. One-pot
DHPs are dearomatized species and thus unstable and prone to oxidation (but are relatively stable under inert conditions); however, their instability is precisely what enables them to drive reductions of stable species such as CO2 to high-energy species such as MeOH. When introduced to substrates containing CC, CN, and CO groups, DHPs effect direct HTs in solution. For example, Zhou and co-workers demonstrated that dihydrophenanthridine transfers both its hydride and proton to benzoxazinone while regenerating phenanthridine (eq 4a); this hydrogenation reaction is catalytic because the [Ru(p-cymene)l2]2 complex regenerates dihydrophenanthridine with H2.41 Fukuzumi et al. showed that 10-methyl-9,10-dihydroacridine and HClO4 homogeneously perform HT and PT, respectively, to benzaldehyde to produce benzyl alcohol (eq 4b).42 In another instance, Shinkai et al. similarly demonstrated HT and PT to trifluoroacetophenone by 1-benzyl-3-carbamoyl-1,4-dihydroquinoline and acetic acid (eq 4c).43 These observations that involve both HT and PT are consistent with a large number of reported experimental observations demonstrating that DHPs derived from Py analogues are capable hydride donors.21
One approach to circumvent the 1e− reduction of CO2 to CO2−• is to avoid radical intermediates in favor of closed-shell, stable intermediates by performing reductions two electrons at a time as hydride transfers. Recently, we reported the first detailed theoretical mechanism that predicted that PyH2 (Figure 3b) is a competent renewable organohydride donor capable of converting CO2 to MeOH in aqueous solution.23 Keith and Carter (KC) alternatively suggested that PyH2 adsorbed on GaP and formed by HT from a surface hydride to adsorbed Py could reduce CO2 heterogeneously (vide infra).44 Here, we simply note that in contrast to the detailed mechanism that we calculated for MeOH production from CO2,23 no corresponding reduction mechanism involving an adsorbed PyH2 has yet been reported. In ref 23, we predicted that the PyH2 catalyst can be formed, in addition to the chemical synthetic route discussed above, via sequential PT−ET−PT−ET to Py in electrochemical, photochemical, and photoelectrochemical systems.
Figure 3. DHPs synthesized chemically. (a) Hantzsch’s ester, (b) PyH2, (c) dihydrophenanthridine, (d) 10-methyl-9,10-dihydroacridine, and (e) 1-benzyl-3-carbamoyl-1,4-dihydroquinoline.
synthesis of various Hantzsch’s esters has been achieved in aqueous solvent using aldehydes, acetoacetate esters, and ammonium acetate as reactants.39 DHPs such as 1,2-dihydropyridine (PyH2) (Figure 3b), dihydrophenanthridine (Figure 3c), 10-methyl-9,10-dihydroacridine (Figure 3d), and 1-benzyl-3carbamoyl-1,4-dihydroquinoline (Figure 3e) can be synthesized 5080
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In an effort to quantify PyH2’s hydride-donating capabilities, we calculated its hydride nucleophilicity and hydricity.23 The hydride nucleophilicity or N value, popularized by Mayr,45 quantifies the hydride’s strength based on the HT rate to wellcharacterized hydride acceptors; a larger N value corresponds to a better donor. We predicted the kinetic N value of PyH2 to be 11.4, making it a better hydride donor than Hantzsch’s ester (N = 9.0; see Figure 3a) but not as capable as NaBH4 (N = 14.7). We also predicted PyH2’s thermodynamic hydricity parameter to be 41.5 kcal/mol, which is smaller than formate’s 43 kcal/mol value (in acetonitrile solvent),46,47 indicating that HT from PyH2 to CO2 to form formate and pyridinium is thermodynamically downhill. In aqueous solvent, the same reactions to form the ionic formate and pyridinium products are expected to be even more favorable because water molecules can hydrogen-bond to further stabilize them. For example, CO2 reduction to formate by the Ru hydride complex [Ru(η6-C6Me6)(bpy)H]+ is endergonic by ∼11 kcal/mol in acetonitrile, while in aqueous solvent, it is exergonic by ∼3 kcal/mol.48 A further important feature of PyH2 is that it contains both hydridic (ortho C−H bond in 1,2-PyH2) and protic (N−H bond) hydrogens; this contrasts with, for example, 10-methyl9,10-dihydroacridine (Figure 3d), which does not possess a protic hydrogen. Thus, PyH2 is analogous to AB in that both involve a protic hydrogen on N neighboring hydridic hydrogens, on the ortho-C of PyH2 and on the B of AB. In Figure 4, using CHELPG-derived charges,49 we show the
Our calculated HTPT mechanism whereby PyH2 reduces CO2 commences with eq 5a, where HT from PyH2 to CO2 produces a stable formate anion (HCOO−) and a pyridinium cation (PyH+). A subsequent PT step from PyH+ (pKa = 5.2) to formate to form formic acid (pKa = 3.8) is unlikely because the equilibrium favors PyH+. However, assistance of the protonation of HCOO− to formic acid (HCOOH) can be provided in the case of electroreduction by cathode-assisted local pH lowering (see the Proton Transfer Aspects discussion). HCOO−’s weak basic character results in the overall HTPT step from PyH2 to CO2 to form HCOOH, involving sequential and uncoupled HT and PT substeps; the HT and PT each involve its own separate TS. In the next step, eq 5b, the produced HCOOH is found to react with PyH2 via a coupled HTPT step, with the HT and PT slightly asynchronous but with a single TS in the free-energy profile. PyH2 first transfers its hydride to the C of HCOOH to form an unstable hydroxymethanolate anion moeity [(HCOOH)H−] and PyH+; the TS exhibits predominantly HT character with essentially no contribution from PT. Somewhat past this TS, the proton from PyH+ transfers to (HCOOH)H− via a proton relay chain, composed of water molecules, to form the methanediol (CH2(OH)2) product. The proton relay aspect of PT will be further considered in the Proton Transfer Aspects discussion.
Figure 4. Hydridic and protic hydrogens on AB and PyH2. Partial charges on hydridic and protic hydrogens are calculated using the CHELPG formalism.
expected protic and hydridic character of AB’s hydrogens (δ = +0.25; δ = −0.27) and PyH2 (δ = +0.39; δ = −0.12). Despite these similarities, AB and PyH2 possess different HT and PT driving forces, as now discussed. As we discussed above, the driving force for HT in AB necessitates dissociating both an N−H and a B−H bond to convert the N−B bond into a covalent bond; the HT and PT are highly cooperative and thus strongly coupled. Therefore, the “sequence” of HT and PT is of critical importance here; it is concurrent in order to maximize this cooperativity. However, for the nonaromatic PyH2, the driving force for HT is different. It derives from the aromatization that results from removing two electrons (and one H+) from the π-system, lowering its occupancy from 8 electrons in PyH2 to 6 electrons in PyH+.23 Consequently, the oxidized pyridinium (PyH+) product now satisfies the Hückel aromaticity criterion,50 and it can then act as an acid to perform PT to recover the aromatic Py species; thus, HT will precede PT. We now summarize the HT and PT (HTPT) steps involved in CO2 to MeOH conversion by PyH2.23 Our goal is to describe key aspects of our recent computational results for this modeled process in aqueous solution; various important details are discussed subsequently.
The final portion of the reduction is shown in eq 5c. For further reduction, the methanediol intermediate must first be dehydrated to generate formaldehyde (OCH2). We found that this species reacts with PyH2 via a coupled HTPT step (mediated by a proton relay) to form the desired MeOH product via an asynchronous single TS analogous to that of formic acid reduction. Our calculations predict that the processes in eqs 5a−5c are both kinetically (ΔG‡HT < 20 kcal/mol) and thermodynamically viable (ΔG°rxn < 0 kcal/mol) in aqueous solution.23 Our theoretical results, schematized in eqs 5a−5c, portray PyH2 as a competent reducing agent capable of performing successive HTPT steps to reduce CO2 and its intermediary products formic acid and formaldehyde to ultimately form the desired MeOH. These results underpinned our conclusion that aromatization is the key driving force for HT by PyH2. However, to achieve the goal of sustaining a catalytic HTPT cycle, Py must be converted back to PyH2. In the next several subsections, we detail the possibility of converting Py and other related Pys to their corresponding DHPs via electrochemical, photochemical, and photoelectrochemical pathways; these pathways are fundamental to achieving catalytic conversion of 5081
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nucleophilic attack on the C of CO2; this initiates an innersphere ET process that forms a transient PyH+·CO2−• complex at the TS. This complex is stabilized by delocalization of the radical electron over PyH+ and CO2’s conjugated π-system, thus avoiding the high energetic cost of forming the CO2−• anion radical. Further, ET from PyH0 is stabilized by forming the aromatic species pyridinium, PyH+. PT from PyH+·CO2−• to first form Py·CO2−• and a transitory H3O+ and then PyCOOH0 occurs along the exit channel. The PT producing the latter is mediated by a proton relay; this relay aspect of PT will be further discussed in the Proton Transfer Aspects discussion. Thus, we predicted that like PyH2, PyH0’s ability to reduce CO2 to PyCOOH0 is derived from rearomatization and follows the sequence ET then PT. This particular sequence is critical. If the reverse sequence were applied, PyH0 would be ruled out55 as a catalyst for reducing CO2 due to its high pKa of ∼31. However, PT occurs from PyH+ (oxidized PyH0 in the PyH+· CO2−• complex) and not PyH0, and thus, the latter’s pKa is irrelevant. The facts that the TS exhibits predominantly ET character, that the barrier is low, and that PT occurs along the exit channel well below the TS all support PyH0’s ability to reduce CO2 to PyCOOH0. Consequently, KC’s claim55 that PyH0 cannot reduce CO2 due to PyH0’s high pKa is not correct. Indeed, as we describe in more detail in the Perspectives discussion, the sequence of ET and PT steps is critical in correctly analyzing these catalytic processes.
CO2 to MeOH by DHPs powered by renewable sources of electricity or photons. Electrochemical Pathways - Sequential PT and ET Steps to Pyridines to Form DHPs: The electrochemical reduction of Py’s to DHPs has been reported for various cases on carbon and mercury electrodes.51−54 In one example (eq 6a), nicotinamide undergoes 2H+/2e− reduction to form the DHP species 1,6-dihydronicotinamide on a Hg electrode.53 The first PT and ET to nicotinamide were proposed to be simultaneous under the employed experimental conditions, forming the nicotinamide neutral radical; this radical was proposed to then undergo another simultaneous PT and ET to produce 1,6-dihydronicotinamide. We recently examined the prospect of reducing Py to PyH2 via a more general sequential PT−ET−PT−ET process,23 although a coupled PT and ET process can certainly operate under certain pH conditions. In this sequential process shown in eq 6b, the protonation of Py forms PyH+ (pKa = 5.2); this cation is then reduced electrochemically to form the pyridinium radical (pyridinyl, PyH0). We calculated the standard reduction potential (E0) for this one-electron reduction as ∼−1.3 V versus SCE.16,55,56 This potential is quite negative because the reduction dearomatizes PyH+ by adding a seventh electron to the π-system. We predicted that the subsequent PyH0 protonation in eq 6b occurs at the C2 position (pKa,calc = 4.1), which is more facile than at the C3 (pKa,calc = 0.2) or C4 (pKa,calc = 2.4) carbons. The resulting cation radical PyH2+• (E0 = 0.11 V versus SCE) can now undergo reduction to produce the desired catalyst PyH2.
Electrochemical Pathways - Competing Pathways through High-Energy Intermediate Pyridinic Neutral Radicals: We now discuss pathways that can compete with DHP formation, especially those originating from the high-energy intermediate neutral radical PyH0 derived from Py or its substituted pyridinic analogues. The neutral radicals produced by PT and ET to Py’s are reactive intermediates that can also lead to side products. For example, in addition to producing 1,6-dihydronicotinamide in eq 6a, the nicotinamide neutral radical can self-quench to give the 6,6′ dimeric product, shown in eq 7a.53 Equation 7b shows the proposed similar self-quenching of PyH0 to form a 4,4′ dimeric product57 and possible production of H2.58 Another interesting feature shown there is that the disproportionation of two PyH0 molecules can also lead to the desired PyH2, a process observed for the related acridine neutral radical.59 An alternative pathway is illustrated in eq 7b in which PyH0 acts as a potent one-electron reducing agent that reacts with CO2 to form a Py-carbamate complex, Py·COOH0. This complex was proposed by Bocarsly and co-workers as an important intermediate in MeOH formation when Py is used as an electrocatalyst.60 In an earlier report, our high-level ab initio CCSD(T) calculations supported this complex’s formation.16 Several of us showed that the N of PyH0 first executes a
Our predictions are also corroborated by a number of experimental observations,61,62 particularly in a recent report where, upon irradiation by 254 nm light, PyH+ is photoreduced to PyH0 by a sacrificial donor (2-PrOH) in aqueous solution. Reaction between PyH0 and CO2 produces solution-phase PyCOOH0 (later quenched by a H atom), characterized with mass spectroscopy; a 9 kcal/mol activation barrier was reported,62 compatible with our predictions.16 Even though PyCOOH0 formation has been shown to be possible, any conversion of this species to formic acid and ultimately to MeOH remains to be elucidated. We propose that instead, the dominant route is transformation of PyH0 into the closed-shell DHP PyH2 via the mechanism that we have discussed (outlined in eq 6b); PyH2 then catalyzes a series of HT and PT steps that converts CO2 to MeOH (shown in eqs 5a−5c). Electrochemical Pathways - Surface Effects in Electrochemical Reduction Involving Pyridines: The experimental situation for this subsection’s topic is not completely clear for active cathodes; we define these as cathodes that chemically interact with key species in the reaction, including those that catalyze processes competing with CO2 reduction, for example, Pt and Pd, which catalyze proton reduction. This complication has led to some controversy. Other cathode results are discussed in the 5082
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homogeneous CO2 catalytic reduction nor prove that catalytic CO2 reductions are invariably affected by the cathode. Computational and experimental mechanistic studies55,66,70 on the Pt/Py system seem to suggest that the production of PyH2 from solution-phase PyH0 via our proposed mechanism in eq 6b is either nonoperative or only a minority process for this system.23 At the experimental bulk pH of 5.2, ∼50% of Py exists in the protonated form PyH+. Bocarsly and Saveant both obtained quasi-reversible CV waves with E0 = −0.58 V versus SCE and attributed it to the 1e− reduction of PyH+ to form surface H atoms;66,70,71 the reduction potentials shift to more negative potentials upon changing from Pt to Au, Ni, Hg, and glassy carbon electrodes (making H+ reduction progressively more difficult) and occur in an order similar to that of the wellknown hydrogen overvoltage.58,72 A number of weak acids exhibit similar quasi-reversible 1e− reduction waves as PyH+ (to form surface H atoms), where these acids’ pKa’s scale linearly (slope = −61 mV) with the reduction potentials.66,70 These surface H atoms then combine to form H2, the main faradaic loss product.58 Batista and co-workers suggested that in addition to homolytically quenching to form H2, surface H atoms on Pt undergo a second ET to form a surface hydride, which together with a PyH+ in the solution executes a proton-coupled HT (PCHT) to CO2 to form formic acid (HCOOH); no further reduction was discussed. In this view, PyH+ serves as a Brønsted acid, delivering protons to the Pt surface and the formate formed by HT from the surface-bound hydride.71 However, this interesting proposed mechanism does not explain why other Brønsted acids with similar pKa’s could not play this proton delivery role equally well. Further, as noted at this subsection’s beginning, efficient electrochemical reduction of protons leads to a precipitous drop in [H+] at the cathode surface, which would limit the participation of protons in any protonation, including PCHT. An aspect apparently generally overlooked concerning active surfaces such as Pt is the potentially important role of chemisorption of PyH+ on such surfaces. PyH+ chemisorbs on Pt, especially strongly when oriented horizontally (“flat”), which maximizes the interaction between pyridinium’s π-space and the d orbitals of Pt.73,74 Strong chemisorption of PyH+ is further indicated by the observed absence of exchange between adsorbed, isotopically enriched PyH+, and an unlabeled solution of PyH+.73 A similar PyH+ adsorption on a Au electrode was observed in a recent electrochemical study.74 Several of us calculated strong chemisorption of PyH+ on various metals, including Pt.16 Figure 5 (top) shows the solvated and adsorbed PyH+ systems on the Pt (111) surface used to predict an adsorption energy of ∼1.0 eV/molecule, and the projected density of states plot in Figure 5 (bottom) shows the mixing, broadening, and stabilization of the frontier orbitals of solvated PyH+ upon adsorption.16 The strong chemisorption affects PyH+’s redox properties, especially its reduction potential. We predicted significant mixing between PyH+’s LUMO and Pt’s surface states and that 0.56 e− transfers from the Pt surface to PyH+; this shows that PyH+ is already partially reduced by Pt prior to the application of electrical bias. Chemisorption allows Pt surface states to significantly stabilize the adsorbed PyH+’s LUMO, facilitating its reduction. We suggest that the effect of chemisorption of PyH+ on active cathodes should be considered in future mechanistic studies of Py-catalyzed CO2 reduction in these systems.
Photoelectrochemical Pathways discussion, but a few preliminary remarks are important concerning the p-GaP semiconductor cathode. p-GaP has been shown to photoelectrochemically reduce CO2 to MeOH using Py as the catalyst.63 Our view is that p-GaP provides photoexcited electrons that are sufficiently reducing to reduce PyH+ to PyH0 prior to PyH2 formation (eq 6b); reduction by PyH2 involving HT and PT steps then converts CO2 to MeOH, as outlined in eqs 5a−5c. While homogeneous Py-catalyzed CO2 reduction was demonstrated by recent photochemical experiments (Photochemical Pathways discussion) and argued by us to also be active in the p-GaP/Py system, heterogeneous aspects of active cathodes, such as proton reduction in the cases of Pt and Pd, obscure its nature. Py catalyzes the reduction of CO2 to MeOH, albeit at moderate yield, using active cathodes such as Pd and Pt.60,64 For example, on a Pt electrode in an acidified solution of pH 5.2, Bocarsly found that Py catalyzes conversion of CO2 to MeOH with a faradaic efficiency of 22%.60 Sariciftci and coworkers obtained similar results for Py-catalyzed MeOH formation, with a faradaic efficiency of 14%.65 On the other hand, Saveant and co-workers concluded that the Pt/Py system does not catalyze CO2 reduction; their observed conversion of
