Hydrogen on Cobalt Phosphide | Journal of the American Chemical

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Hydrogen on Cobalt Phosphide Murielle F. Delley, Zishan Wu, M. Elizabeth Mundy, David Ung, Brandi M. Cossairt, Hailiang Wang, and James M. Mayer J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07986 • Publication Date (Web): 03 Sep 2019 Downloaded from pubs.acs.org on September 3, 2019

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Journal of the American Chemical Society

Hydrogen on Cobalt Phosphide Murielle F. Delley,*,† Zishan Wu,† M. Elizabeth Mundy,‡ David Ung,‡ Brandi M. Cossairt,‡ Hailiang Wang,† James M. Mayer*,† †Department ‡ Department

of Chemistry, Yale University, New Haven, Connecticut 06520-8107, USA

of Chemistry, University of Washington, Seattle, Washington 98195-1700, USA [email protected]; [email protected]

Abstract Cobalt phosphide (CoP) is one of the most promising earth-abundant replacements for noble metal catalysts for the hydrogen evolution reaction (HER). Critical to HER is the binding of Hatoms. While theoretical studies have computed preferred sites and energetics of hydrogen bound to transition metal phosphide surfaces, direct experimental studies are scarce. Herein, we describe measurements of stoichiometry and thermochemistry for hydrogen bound to CoP. We studied both mesoscale CoP particles, exhibiting phosphide surfaces after an acidic pretreatment, and colloidal CoP nanoparticles. Treatment with H2 introduced large amounts of reactive hydrogen to CoP, ca. 0.2 H per CoP unit, and on the order of one H per Co or P surface atom. This was quantified using alkyne hydrogenation and H-atom transfer reactions with phenoxy radicals. Reactive H-atoms were even present on the as-prepared materials. Based on the reactivity of CoP with various molecular hydrogen donating and accepting reagents, the distribution of binding free energies for H-atoms on CoP was estimated to be roughly 51 - 66 kcal mol-1 (∆G°H  0 to -0.5 eV vs. H2). Operando X-ray absorption spectroscopy gave preliminary indications about the structure of hydrogenated CoP, showing a slight lattice expansion and no significant change of the effective nuclear charge of Co under H2-flow. These results provide a new picture of catalytically active CoP, with a substantial amount of reactive Hatoms. This is likely of fundamental relevance for its catalytic and electrocatalytic properties. Additionally, the approach developed here provides a road map to examine hydrogen on other materials.

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Hydrogen on Cobalt Phosphide

Page 2 of 27 Delley, Mayer et al.

Introduction The best-known catalysts for the hydrogen evolution reaction (HER) are based on expensive and rare noble metals, such as platinum. Transition metal phosphides are promising candidates to replace noble metal catalysts, due to their high activities and acid-compatibility in hydrogen evolution and in other applications.1 One of the most promising candidates among transition metal phosphides for HER electrocatalysis is cobalt phosphide, CoP.2 This has been ascribed to the moderate binding of hydrogen to the surface of CoP, following the Sabatier principle. This states that an optimal catalyst will bind reaction intermediates neither too strongly nor too weakly. The binding and loss of hydrogen from the surface (electron + proton) are crucial elementary steps in HER and many other electrocatalytic reactions. Theoretical considerations lead Parsons and Gerischer independently to similar conclusions and to the first qualitative ‘volcano plot’ in electrocatalysis indicating that the best catalysts have a free energy for addition of H-atoms from H2 that is close to zero (ΔGH ≈ 0 eV).3 Experimental measures of exchange current density (catalytic activity) have been correlated with calculated hydrogen binding energies to give a ‘volcano’ relationship for metals,4 and more recently for transition metal phosphides5 (though there is some debate about such plots6). Transition metal phosphide electrocatalysts can be prepared directly by electrodeposition,2b,7 by in-situ phosphidation of an oxide or metal precursor on a conducting support,2a,8 or by deposition of transition metal phosphide nanoparticles.9 Due to the growing sophistication in nano-synthesis of recent years, these nanoparticles can be prepared with size- and phasecontrol,10 and in some cases even with atomic precision.11 For deposited nanoparticle-electrodes, the nanoparticle ligands are typically removed by annealing or by ligand stripping,1b,9,12 which has been shown to improve electrocatalysis.9a,12b,12c,13 The complexity of such surfaces, often being non-stoichiometric and with defects, oxidized phosphorus species, and ligands, make directly relevant calculations extremely challenging.14 While theoretical studies for hydrogen binding energies are very valuable and have shaped our understanding, the actual surfaces under catalytic conditions likely do not closely resemble the typically modeled pristine crystalline facets. The various and complex surface structures can also change under reaction conditions.15 For instance, as-prepared cobalt phosphide usually has an oxidized surface layer. This is removed under HER conditions to reveal a phosphide surface thought to be the active material.2b,7a,8b,9d Theoretical studies have identified hydrogen in Co-Co bridging sites and P-top sites as likely candidates to be the active sites, and have estimated their H-binding energies.16 Given the complexity of the actual surface/solution interfaces, it is critical to obtain experimental information on the hydrogen binding to phosphides. Very few spectroscopic observations of hydrogen on phosphides have been reported,17 and experimental data on their thermochemistry and stoichiometry is largely lacking. Such data are needed for the rational development of better catalysts. Herein, we use molecular hydrogen atom transfer reagents to study the thermochemistry and stoichiometry of hydrogen binding to cobalt phosphide. This novel approach allows us to obtain estimates for the thermodynamics of reactive hydrogen-accepting and -donating sites on 2 ACS Paragon Plus Environment

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Journal of the American Chemical Society Hydrogen on Cobalt Phosphide

Delley, Mayer et al.

CoP. In solution, the bond dissociation free energy (BDFE) of a hydrogen atom transfer reagent A–H, is given by the ∆G° for the reaction A–H  A + H.18 Formation of a CoP surface–H bond from A–H is exoergic only if the CoP–H bond dissociation free energy (BDFE) is larger than the BDFE(A–H). This BDFE description, referenced to the free energy of an H atom in solution,19 is common for molecular systems.18 It is equivalent to the ΔGH description, referenced to H2(g), which is common for materials. The difference between these values is 55 ± 2 kcal mol-1.18 We report studies of both mesoscale, ligandless cobalt phosphide, CoP-M, and colloidal oleylamine-ligated cobalt phosphide nanoparticles, CoP-NP, for complementary information. As a bulk material without ligands, CoP-M has attributes similar to typical electrocatalysts, while the colloidal CoP-NP system enabled molecular-like chemistry to more precisely determine reaction stoichiometries. The number of reactive hydrogen atoms on the material was probed by chemical reactions with alkynes or phenoxy radical H-acceptors and quantifying the hydrogenated products. Additionally, we obtained first insights into the structure of H-sites on cobalt phosphide by operando X-ray absorption spectroscopy under H2-flow.

Results I. Preparation and pre-treatments of mesoscale and nanoscale cobalt phosphide Two kinds of cobalt phosphide were used in this study: an insoluble mesoscale material prepared from cobalt oxide and PH3, termed CoP-M,8b and colloidal nanoscale nanoparticles, CoP-NP,9a prepared from CoCl2 and an aminophosphine, with capping ligands. A. Mesoscale CoP-M. Mesoscale CoP-Mox was prepared by reacting hydrothermally synthesized Co3O4 nanoparticles at 300 °C with in-situ generated PH3(g) from NaH2PO2•H2O decomposition, as previously reported.8b Powder XRD measurements confirmed the formation of CoP (SI). XPS measurements of the obtained as-prepared CoP-Mox confirmed that the surface is partially oxidized,8b as evidenced by Co 2p components at 799.0 and 782.0 eV and a P 2p component at 134.5 eV corresponding to oxidized species8b,2b,20 (Figure 1a and b, Co-Ox and P-Ox). This is expected as phosphides oxidize in air. The as-prepared CoP-Mox particles were therefore immersed in degassed 0.5 M H2SO4(aq) for 30 min. This acidic pre-treatment removed the oxidized surface layer and exposed fresh CoP (CoP-Mox after acid-treatment, termed CoP-M), as shown by XPS and XANES analysis of the material, and by UV/Vis and 31P NMR analysis of the decanted solution, discussed below. The XPS spectrum of CoP-M after H2SO4-treatment measured under inert atmosphere showed major 2p Co and 2p P spin-orbit-split signals consistent with CoP2b,20-21 (Figure 1a and b, Co-P at 793.5 and 778.5 eV, and P-Co at 130.1 and 129.3 eV). The XPS Co 2p and P 2p signals associated with oxidized species were significantly decreased compared to the XPS spectrum of as-prepared CoP-Mox. The P 2p oxidized component also shifted to lower binding energy, from 134.5 eV to 133.5 eV, indicating a decrease in the effective atomic charge of oxidized P. The lack of any XPS signal with binding energies typical for sulfur (ca. 168 eV) in the spectrum of 3 ACS Paragon Plus Environment




CoP-M after acidic pretreatment and washing suggested that no sulfate, nor any other sulfur containing group, remained on the surface. The Co K-edge X-ray Absorption Near-edge (XANES) spectrum of as-prepared CoP-Mox exhibited a pre-edge feature at 7711 eV, and nearedge features at 7718, and 7724 eV (Figure 1c). In the corresponding XANES of acid-treated CoP-M the near-edge feature at 7724 eV decreased in intensity. By comparison to XANES spectra of Co(OH)2, Co(acac)2, and Co(acac)3, this feature is likely attributed to Co2+ coordinated by oxygen being removed by the acidic wash (SI). XPS and XANES showed that acidic treatment of as-prepared CoP-Mox removed most of the oxidized species on the surface and exposed fresh CoP. This conclusion is in line with previous reports using X-ray techniques to examine cobalt phosphide electrode surfaces in acidic electrolyte under hydrogen evolution reaction conditions.2b,7a,8b

Figure 1. XPS spectra of as-prepared CoP-Mox (red) and H2SO4-treated CoP-M (blue) for binding energies corresponding to a) 2 p Co, and b) 2 p P regions. c) Normalized Co K-edge XANES spectra of as-prepared mesoscale CoP-Mox (red), H2SO4-treated CoP-M (blue), and colloidal nanoscale CoP-NP (green). d) SEM picture of the nanostructured surface of H2SO4treated CoP-M. e) TEM of CoP-NP with diameters of 6±1 nm. Insert: particle size histogram. 4 ACS Paragon Plus Environment

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In parallel with the X-ray-based characterization of acid-treated CoP-M, analysis of the acidic washing solution showed that this procedure removed both cobalt and phosphate. UV/Vis spectra of the pink solution overlapped well with the known optical absorption of Co2+(aq) (SI). The formation of Co2+ was expected because of the strongly oxidizing nature of Co3+ species bound to only oxygen ligands.22 A broad resonance at ca. 2 ppm in the 31P NMR spectrum of the acidic washing solution was assigned to phosphate (SI). The extinction coefficient of Co2+(aq) and 31P NMR integration allowed quantification of the dissolved species. The ratio of released Co2+ and phosphate in solution varied from 1:1 to 2:1 for different CoP-M batches. Quantitative XPS analysis, using known relative sensitivity factors23 showed that for each batch, the particle top layer had a ca. 1:1 ratio of Co:P after acid pretreatment (XPS samples ca. the outermost 3-10 nm). Similar results have been reported for a cobalt phosphide HER electrocatalyst under acidic reaction conditions.2b All experiments described in the following sections used H2SO4-treated CoP-M and were performed with exclusion of air to prevent re-oxidation of the surface. The morphology and size distribution of the mesoscale CoP-M was assessed by microscopy. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of CoP-M after H2SO4-treatment showed that the CoP-M powder consisted of particles with irregular shapes and a wide particle size distribution with diameters ranging from hundreds of nm to hundreds of μm (Figure 1d and SI). The surface of these particles contained micro-pores and clusters of micro-flakes, and was nanostructured. B. Colloidal nanoscale nanoparticles (CoP-NP). Colloidal cobalt phosphide nanoparticles, CoP-NP, were prepared by thermolysis of CoCl2 with tris-diethylaminophosphine in oleylamine as previously reported.9a The prepared spherical CoPNP had average diameters of 6 ± 1 nm according to TEM (Figure 1e), similarly to previously reported batches.9a The XANES spectrum of the CoP-NP exhibited a pre-edge feature at 7711 eV, and a near-edge feature at 7718 eV (Figure 1d), consistent with the previous report.9a This XANES spectrum is similar to the corresponding spectrum of mesoscale CoP-M after acidtreatment (Figure 1c), showing that the two cobalt phosphide systems have comparable bulk structures. ICP-MS analysis showed 5.9 mol Co per mg material. Assuming a 1:1 ratio of Co:P and that the rest of the material is coordinated oleylamine, this implies ca. 1400 oleylamine ligands per NP or ~2 oleylamine ligands per surface Co atom (see below for the number of surface Co atoms). The oleylamine-capped CoP-NP were soluble in THF and toluene, and insoluble in MeCN. Previous work has demonstrated that treatment of electrodes made from such CoP-NP with Meerwein’s reagent, Et3O+BF4–, substantially increased the HER activity by removal of some of the capping oleylamine ligands.9a Studies below were done both with as-prepared CoPNP and with CoP-NP that had been reacted with a 0.1 M solution of Et3OBF4 in MeCN for 1 h. Analysis of the decanted solution by NMR spectroscopy showed that ca. 100 oleylamine molecules per CoP-NP were removed by this method (SI). 31P NMR spectroscopy also showed removal of some phosphorus containing species that have yet to be identified (SI).

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II. Reactivity of mesoscale CoP, CoP-M The general methodology of this study was reacting CoP-M with molecular H-atom transfer reagents while following the reaction by 1H NMR spectroscopy and GC-MS. Our approach to studying the H-atom transfer reactivity of materials contrasts with typical electrochemical studies, where electrons are provided by the circuit and protons from the electrolyte. This strategy allows insight into the thermochemistry and stoichiometry of hydrogen on cobalt phosphide. It will first be demonstrated that CoP-M exhibits hydrogen-atom transfer reactivity (Section IIA). In a second step, this reactivity is connected with stoichiometry by quantification of reaction products (Section IIB), and with thermochemistry via the bond dissociation free energies (BDFEs) of the molecular reagents (see discussion). A. Demonstration of H-atom transfer reactivity. Treatment of CoP-M with diphenylhydrazine in toluene at RT and monitoring of the reaction by 1H NMR spectroscopy indicated the formation of azobenzene and aniline (Scheme 1a, SI). The reaction occurred faster at 50 °C. In the absence of CoP-M, disproportionation of diphenylhydrazine did not occur at these temperatures (consistent with prior literature24). Thus, CoP-M acted as a catalyst for diphenylhydrazine disproportionation. CoP-M hence effected a net migration of H-atoms (as well as N–N bond cleavage). The mechanism of the CoP-M-catalyzed diphenylhydrazine disproportionation likely involves the transfer of H-atoms to and from CoP-M. A strong indication for the intermediate transfer of H-atoms to CoP-M can be obtained from reactions involving an intermolecular transfer of H-atoms, e.g. from the simplest H-atom donor molecule, H2, to a substrate. The activation of H2 by CoP-M was probed by examining catalytic hydrogenations of unsaturated hydrocarbons. Reactions of CoP-M with excess phenylacetylene in THF under a H2 atmosphere (2 bar) at 85 °C yielded styrene and ethylbenzene over hours and days, as shown by 1H NMR spectroscopy (Scheme 1b, SI). No hydrogenation of PhCCH was observed in the absence of CoP-M, or with the CoP-M-precursor Co3O4, or with Co(OH)2, or CoII ions (SI). In one typical experiment with CoP-M, 6.1 μmol styrene and 1.3 μmol of ethylbenzene were formed per mg of CoP-M after 2 days at 85 °C. These amounts correspond to 0.55 and 0.11 equiv product per CoP unit, but only a small fraction of the Co and P are at the surface and likely to be catalytically active. These results show that CoP-M is a catalyst for alkyne and alkene hydrogenation, and that H2 can be activated by CoP-M and transferred to a substrate. The activation of H2 implies that other A-H bonds may be activated by CoP-M as well. In the following the reactivity of various H-atom transfer reagents with CoP-M was probed.


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Scheme 1. a) CoP-M-catalyzed diphenylhydrazine disproportionation to form azobenzene and aniline. b) CoP-M-catalyzed hydrogenation of phenylacetylene to form styrene and ethylbenzene. c) Reaction of CoP-M with dihydrophenazine to form phenazine and H2.

Dihydrophenazine was chosen as a substrate for CoP-M because it has weak N–H bonds (see discussion) and cannot undergo disproportionation. Upon treatment of CoP-M with dihydrophenazine in d3-MeCN, 1H NMR analysis of the reaction mixture showed line broadening of dihydrophenazine resonances and showed peaks consistent with the formation of phenazine (Scheme 1c, SI). The H-atoms lost from the dihydrophenazine could have been transferred to CoP-M, or could have formed H2. H2 was identified in one experiment by its NMR resonance at 4.57 ppm in MeCN,25 but this was only observed transiently and was not reproducible. Likely only a small amount of H2 was formed and it diffused into the headspace of the J-Young NMR tube (also see discussion and SI for thermochemical considerations). The integrated amount of H2 (correcting for the unobserved para-H2) in this experiment accounted only for ¾ of the observed phenazine, which suggests that some H-atoms could be bound to CoP-M (see Section IIB). The reverse reaction was also studied: CoP-M with phenazine and excess H2 in MeCN at RT (Scheme 1c). Monitoring by 1H NMR spectroscopy showed consumption of phenazine and formation of dihydrophenazine within 24 h (SI). In the absence of CoP-M, no hydrogenation of phenazine was observed. To further investigate the reversibility of these reactions, CoP-M was reacted with dihydrophenazine in the presence of D2 (SI). By 1H NMR spectroscopy, more H2 and HD were observed compared to a control experiment without CoP-M (accounting for H2 and HD traces in commercial D2). GC-MS of the reaction mixture showed the formation of d2dihydrophenazine (none was observed in the control experiment without CoP-M, see SI). CoPM thus catalyzed the N-H/D scrambling of dihydrophenazine with D2. The sum of these experiments indicate that CoP-M catalyzed the interconversion of dihydrophenazine with phenazine and H2, and some H-atoms are likely bound to CoP-M (see Section IIB).





The reactivity of CoP-M with H2 and dihydrophenazine discussed above demonstrates that CoP-M activates A-H bonds and some H-atoms may be transferred to CoP-M. To explore this reactivity with a class of closely inter-related molecular H-atom transfer reagents, reactions of CoP-M with hydroquinones in THF were also studied (Figure 2). 1H NMR spectra of reaction mixtures of CoP-M with 9,10-anthrahydroquinone (H2AQ) or 1,8-dichloro-9,10-anthrahydroquinone (H2ClAQ) showed the formation of the corresponding quinones at RT (see SI for data and controls). After the reaction with H2AQ or H2ClAQ for 1 d at RT, the CoP-M was removed, washed with THF, and tBu3ArO• was added. 1H NMR spectra then showed the formation of tBu ArOH, roughly in the amount expected based on the amount of quinone formed (SI). This is 3 good evidence for the presence of reactive H-atoms on or in CoP-M, as discussed in more detail below (see Section IIB, and SI). Treatment of CoP-M with the poorer H-atom donors 2,6-di-tertbutylhydroquinone (H2tBuQ) or 9,10-phenanthrenediol (H2PQ) on the other hand did not lead to the formation of the corresponding quinones (SI).

Figure 2. a) Reaction of CoP-M with hydroquinones. Tested hydroquinones and their reactivity with CoP-M are given below. Right: XPS spectrum of CoP-M after reaction with H2ClAQ at binding energies for the (b) Co 2p and (c) P 2p components. XPS analysis provided additional information about the CoP-M surface after reaction with H2ClAQ. After removal of CoP-M and washing with THF, an XPS spectrum was measured (Figure 2b,c). The Co 2p component is nearly identical to what was observed for H2SO4-treated CoP-M (Figure 1a,b). The small P 2p component at 133.5 eV corresponding to oxidized phosphorus in H2SO4-treated CoP-M has shifted to slightly lower binding energies of 132.5 eV. This is consistent with P having a less positive effective atomic charge than in phosphate (< ca. +5),26 but higher than in CoP (effective charge of P is ca. -1).21


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In summary, probing reactions of CoP-M with various molecular H-atom transfer reagents demonstrated the H-atom transfer reactivity of CoP-M. CoP-M activates A-H bonds, including molecular hydrogen. H-atoms are transferred from H2 and other molecular reagents to CoP-M, and can be transferred to a substrate. The H-atom stoichiometry on CoP-M is discussed in the next section. B. Quantification of reactive H-atoms on CoP-M-Hx. In the following, the number of H-atoms on CoP-M after reaction with H2 or other hydrogen-atom transfer reagents was probed by subsequent reactions with two different H-atom abstractors, phenylacetylene or tBu3ArO•, and quantification of the reaction products by 1H NMR spectroscopy. Quantification of H-atoms added to CoP-M by H2. To measure the number of H-atoms on CoP-M upon hydrogenation, we examined the catalytic hydrogenation in two separate steps: treating CoP-M with H2, then removing the H2 and reacting the hydrogenated material (CoP-MHx) with phenylacetylene (Figure 3a). D-phenylacetylene (PhCCD, 99% D) was used as the substrate to allow unambiguous identification of the hydrogenation products. This was necessary because of the presence of styrene as an impurity in phenylacetylene. PhCCD also provided a probe for possible hydrogen scrambling pathways. In a typical procedure, CoP-M was immersed in d8-THF in a J-Young tube and put under a H2 atmosphere of 2 bar at RT for 1 d. The H2 atmosphere and any dissolved H2 were then removed by at least five freeze-pump-thaw cycles, or until no more H2 could be observed by 1H NMR spectroscopy. Addition of excess PhCCD and heating to 85 °C led to the formation of β-d1-styrene (2-d-vinylbenzene) over days as evidenced by 1H NMR spectroscopy and GC-MS analysis (Figure 3b-d). Fitting and integration of the observed characteristic resonances for cis- and trans-β-d1-styrene vs. a TMB internal standard quantified the reaction products, and therefore the number of H-atoms transferred to the acetylene. These reactive H-atoms must have come from the solid CoP. The number of transferred H-atoms per ‘CoP’ unit was calculated from the mass of CoP-M, assuming that it was pure cobalt phosphide. This analysis gives 0.11 transferred H-atoms per ‘CoP’ unit in the whole solid.





Figure 3. a) Scheme for reacting CoP-M with H2 giving CoP-M-Hx, and quantification of reactive H-atoms on CoP-M-Hx by reaction with excess PhCCD. Below the scheme, 1H NMR spectra of the reaction mixtures of CoP-M-Hx and PhCCD, before (bottom, blue) and after heating at 85 °C (top, red) showing the vinylic resonances of d0-styrene (impurity in PhCCD, denoted with °) and the formation of resonances associated with (b) cis- and (c) trans-β-d1styrene, (denoted with *). d) GC-MS data of the formed styrene in H-quantification experiments using PhCCD (red) is given as the ratio of MS intensity of d1-styrene (105 Da), or d2-styrene (106 Da) vs. d0-styrene (104 Da) to highlight the observed isotopic enrichment. The data was averaged from three different experiments with three GC-MS measurements each (red, see SI for individual experiments), and is compared to a control experiment in the absence of CoP-M (blue). The NMR and GC-MS analyses show multiple products from the hydrogenation of PhCCD. Roughly 80% of the β-d1-styrenes are formed by cis addition of H2-addition, vs. 20% addition in a trans-fashion. 1H NMR spectra also showed the formation of non-deuterated styrene, beyond that present as an impurity in the PhCCD (by integration vs. the TMB standard) (SI). Non-deuterated styrene accounted for the transfer of 0.11 equiv H-atoms per ‘CoP’ unit, in addition to the 0.11 equiv H-atoms per ‘CoP’ unit from the β-d1-styrenes. GC-MS also showed the formation of d2-styrene, though in small amounts (Figure 3d, also see SI for a discussion of the variability in these data). The formation of d0-styrene and d2-styrene indicate that PhCC-D/H scrambling occurred with the H-atoms of the CoP-M-Hx. Scrambling was also indicated in a similar experiment, in which CoP-M was reacted with D2, forming CoP-M-Dx, followed by quantification with PhCCH at 85 °C. GC-MS and 1H NMR analyses showed formation of both deuterated and non-deuterated styrenes (SI). The deuterated styrene was mostly mono-deuterated. Slightly more d2-styrene relative to d1-styrene was formed in this 10 ACS Paragon Plus Environment

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experiment compared to the experiment performed with H2. 1H NMR integration indicated the transfer of 0.18 equiv 1H-atoms per ‘CoP’ unit to give PhCH=CH2 and 0.036 equiv 1H or 2Hatoms to give PhCH=CHD (2:1 cis- to trans-). Finally, the presence of reactive H-atoms on CoPM after H2 treatment is supported by the reaction of CoP-M-Hx with D2: 1H NMR spectra showed the formation of HD over hours at 85°C (after correction for the H2 and HD impurities in our commercial D2). H2/D2 scrambling does not occur in the absence of CoP-M under these conditions. tBu

3ArO

•

was used as an alternative reactant for H-atoms on or in CoP-M after loading the material with H2, similar to the procedure used above with a hydroquinone as the H-donor (Section IIA). H2-treated CoP-M (CoP-M-Hx) was reacted with an aliquot of a tBu3ArO• solution in d8-THF at RT. Monitoring the reaction by 1H NMR showed formation of tBu3ArOH over a period of hours (SI). The amount of tBu3ArOH formed accounted for a total of 0.18 equiv of reactive H-atoms per CoP-M (Figure 4). This is quite close to the value of 0.22 obtained in the quantification experiments with PhCCD addition described above. Together, these experiments show the presence of substantial hydrogen on or in the CoPM-Hx material. Roughly 0.2 H per ‘CoP’ unit were transferred to phenylacetylene or tBu3ArO•. In other words, the formula of the hydrogenated solid is CoP-M-H0.2 (or CoP-M-D0.2). We emphasize that the number of H-atoms per surface CoP unit is much larger, as discussed below, because only a small fraction of the cobalt and phosphorus are at the surface of this mesoscale material (estimates for an upper bound indicate that < 17% of all atoms are on the surface, see SI). Quantification of H-atoms added to CoP-M by other H-atom transfer reagents. The phenylacetylene and tBu3ArO• H-abstraction protocols discussed above for H2 as H-atom donor have also been applied to the reactions of CoP-M with other H-atom transfer reagents described in Section IIA. CoP-M was reacted with dihydrophenazine in MeCN, the CoP-M–Hx was isolated by decantation and was washed with MeCN, and with THF, and then excess dphenylacetylene was added at 85 °C to quantify the number of H-atoms in CoP-M–Hx. 1H NMR spectroscopy and GC-MS showed formation of 0.04 equiv β-d1-styrene per ‘CoP’ (5:1 cis- to trans-, Figure 4, and SI). In one experiment, CoP-M was reacted with H2ClAQ in THF and in a similar procedure the CoP-M–Hx was isolated, washed with THF, and d-phenylacetylene was added at 85 °C for an H-atom quantification. 1H NMR spectroscopy and GC-MS showed formation of 0.04 equiv β-d1-styrene per ‘CoP’ (3:1 cis- to trans-, see SI). The presence of reactive H-atoms in or on CoP-M after reaction with H2ClAQ was also confirmed by the formation of 0.10 equiv tBu3ArOH per ‘CoP’ unit when tBu3ArO• was added to CoP-M-Hx (Figure 4, and SI). In these reactions, the total amount of 0.08 equiv reactive H-atoms per ‘CoP’ from reaction with dihydrophenazine or H2ClAQ, respectively was inferred from the observed d1styrene, while no d0-styrene formation was observed. Compared to the experiments using H2 as H-atom donor, followed by PhCCD reaction, there was only slightly less d1-styrene per ‘CoP’ observed (Figure 4). Overall, there were less reactive H-atoms on CoP-M from reaction with dihydrophenazine or H2ClAQ by roughly a factor 2 than obtained from treatment with H2. The 11 ACS Paragon Plus Environment




rigorous washing procedure necessary to remove dihydrophenazine and H2ClAQ reactants may have led to partial loss of CoP-M-Hx, however. The NMR quantitation can therefore be taken as lower bounds giving stoichiometries of CoP-M-H≥0.08, and CoP-M-H≥0.08-0.1 for H-transfer from dihydrophenazine and H2ClAQ, respectively.

Figure 4. a) Scheme and table listing the molecular reagent (AH) used to add H-atoms to CoPM, and the molecular reagents (B) used to remove H from CoP-M to assess the CoP-M-Hx stoichiometry. Numbers for x should be viewed as approximate values (also see discussion). b) GC-MS data of the formed styrene in H-quantification experiments using PhCCD from CoPM-Hx, previously reacted with H2 (red, copied from Figure 3 for comparison), H2ClAQ (green), or dihydrophenazine (light blue), or from un-treated CoP-M (orange). The data is given as the ratio of MS intensity of d1-styrene (105 Da), or d2-styrene (106 Da) vs. d0-styrene (104 Da) to highlight the observed isotopic enrichment, and was averaged from three GC-MS measurements each. A control experiment in the absence of CoP-M is shown in blue (copied from Figure 3). C. Hydrogen on acid-treated mesoscale CoP-M. To assess whether any reactive H-atoms are present on acid-treated CoP-M, without H2treatment, similar quantification experiments as described in Section IIB were performed with dphenylacetylene at 85 °C. The characteristic 1H NMR resonances for d1- and d0-styrene indicated the presence of 0.08 equiv reactive H-atoms per ‘CoP’ unit. In this case 60% of the styrene was non-deuterated, showing the prevalence of H/D scrambling with the PhCCD. A similar amount of reactive H, 0.08 equiv, was observed by tBu3ArO• titration of a separate CoP-M sample. Thus, reactive H-atoms are already present in or on CoP-M before hydrogenation. These ‘inherent’ reactive H-atoms could stem from the acidic pretreatment of the CoP-M particles. To assess this possibility, as-prepared CoP-M was pre-treated with D2SO4 in D2O before reaction with excess PhCCH. 1H NMR and GC-MS analyses showed the formation of d1-styrene and d2-styrene, accounting for 0.029 equiv reactive H/D-atoms per ‘CoP’ unit. Therefore the ‘inherent’ reactive H(D)-atoms stem at least in part from the acidic pre-treatment. 1H NMR spectroscopy also evidenced the formation of d -styrene (accounting for 0.098 equiv 0


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per ‘CoP’ unit), presumably from PhCC-H/D scrambling in the presence of excess phenylacetylene, or from the CoP-M synthesis, which used PH3. Experiments with smaller amounts of PhCCH, with the styrene impurity below the 1H NMR detection limit, unambiguously confirmed the formation of mostly d0-styrene in addition to the β-d1- and β-d2styrenes (SI). In sum, acid-treated CoP-M has a stoichiometry of CoP-M-H0.08 and treatment with H2 increases the amount of hydrogen to CoP-M-H0.2. Treatments with dihydrophenazine, or H2ClAQ introduced intermediate amounts of hydrogen of CoP-M-H≥0.08-0.1. To our knowledge, this is the first report that acid treatment of CoP materials would generate significant amounts of hydrogen on or in the material.

III. Reactivity and H-quantification of colloidal cobalt phosphide (CoP-NP) The colloidal, oleylamine-capped cobalt phosphide nanoparticles (CoP-NP) provided complementary insights to the mesoporous material studied in the sections above. CoP-NP have better defined morphologies and reactions were conducted in homogeneous suspensions. In contrast to the reaction mixtures containing solid CoP-M particles used in the previous sections, homogeneous suspensions of colloidal CoP-NP can be split for parallel experiment/control experiment protocols using exactly the same reaction mixture. Hence, for such an experiment pair it is possible to directly compare the reaction stoichiometries obtained from H2-treatment vs no treatment. Suspensions of CoP-NP in THF catalyze the hydrogenation of phenylacetylene to styrene, at 2 bar of H2 at 85 °C over days, as monitored by 1H NMR spectroscopy (SI). This shows a direct parallel between the reactivity of the mesoscale CoP-M (Section IIA) and these oleylamine-coordinated CoP-NP. Following a similar procedure as described in Section II, colloidal CoP-NP were loaded with H2 at RT for one day, the headspace- and dissolved H2 was removed, and the remaining suspension treated with excess PhCCD. 1H NMR and GC-MS analyses of the reaction mixture showed the formation of cis- and trans-β-d1-styrene (SI). Quantification of the amount of β-d1-styrene after 13 d at 85 °C indicated the transfer of 200 equiv H-atoms per NP. A parallel experiment with the same CoP-NP mixture was conducted to probe for any reactive H-atoms present prior to treatment with H2. Reaction of as-prepared CoPNP with excess PhCCD at 85 °C still formed d-styrene, but about seven times less. These data implied the transfer of 30 equiv H-atoms per as-prepared particle. There are roughly 5,000 Co atoms per 6 nm CoP-NP, using the lattice parameters for the MnP-type orthorhombic structure of CoP.27 Assuming a likely simplified orthorhombic structure with only (011) facets exposed, the number of Co and P atoms at the surface can be estimated as ca. 700 per NP. These values implied the transfer of 0.28 H-atoms per surface CoP unit for H2treated CoP-NP (assuming that all the H's are at the surface). For the CoP-NP without H2 treatment, there are roughly 0.04 H-atoms per surface CoP. As with the mesoscale CoP-M, hydrogen is present in the as-synthesized material, and reaction with H2 adds substantially more reactive H-atoms. 13 ACS Paragon Plus Environment




For these hydrogen per CoP values we estimate a ca. 30% uncertainty for a single experiment. This is based on propagation of the uncertainties in the NMR integrations (20%), in weighing the small amount of CoP-NP (±0.2 mg) and in the number of CoP surface units (±100). However, there is a larger variation in these values from batch to batch of NPs and from experiment to experiment even for the same batch. For instance, another experiment gave values for equiv H-atoms per surface CoP unit of 2.1(5) for H2-treated NPs and 0.22(5) without H2 exposure (SI). The origin(s) of this variability could involve differences in the bound and free ligand concentrations, the presence of acid or other impurities, partial instability of CoP-NP at 85 °C, and perhaps the crystallinity of the materials; this is a subject of continuing study. Still, overall, our data clearly and consistently show that H2-treatment of the particles introduced a substantial amount of H-atoms to the CoP-NP, and that even the as-prepared (untreated) CoPNP contained reactive H-atoms. Previous work has demonstrated that electrodes containing these CoP-NP had increased HER activity after treatment with Meerwein’s reagent.9a We hypothesized that such pretreatment could increase the number of bound H-atoms upon H2-treatment and that could be a contributor to the higher HER activity. Reaction of a Meerwein’s-treated CoP-NP suspension with H2, removal of excess H2, and reaction with excess PhCCD showed the formation of dstyrene. One experiment showed the transfer of 800 equiv H-atoms per particle, corresponding to 1.2(3) equiv H-atoms per surface CoP (Figure 5). The same batch of Meerwein’s-treated CoPNP without H2-pretreatment showed the transfer of ca. 100 equiv H-atoms per particle or 0.18(4) equiv H-atoms per surface CoP (Figure 5). The variability noted above has been found in these experiments as well: a second experiment indicated 0.4(1) equiv H-atoms per surface CoP (after H2) and 0.17(4) equiv H-atoms per surface CoP for the same solution not pre-treated with H2. Thus, compared to as-prepared CoP-NP, there is no clear trend whether pre-treatment with Et3OBF4 leads to more H-atoms present on the NP. This is perhaps not surprising because only about 10% of the oleylamine capping ligands were removed in the pre-treatment step (by 1H NMR). In sum, while there is some variability in the data, clear conclusions can be drawn. The as-prepared and Meerwein-treated CoP-NP contained some reactive H-atoms, even without H2treatment. After exposure to 2 bar H2 for a day, substantially more reactive hydrogen atoms are present. Assuming that all of the hydrogen is on the surface, the H2-treated CoP-NP have ca. 0.3 to 2 H-atoms per surface CoP, i.e., on the order of a ‘monolayer’ of H-atoms.


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Figure 5. a) 1H NMR spectra of the reaction mixtures of Et3OBF4-treated CoP-NP and PhCCD, before (bottom, black), after heating at 85 °C (middle, blue), and a similar reaction mixture for Et3OBF4-treated CoP-NP after H2-loading. The spectra show the vinylic resonances associated with cis- and trans-β-d1-styrene, and with d0-styrene (impurity in PhCCD, °). # denotes olefinic resonances associated with oleylamine. b) Time-course of the number of Hatoms transferred per surface CoP for CoP-NP after treatment with Meerwein’s reagent and H2 (red) or with just Meerwein’s reagent (blue). The number of H-atoms was inferred from 1H NMR quantification of d1-styrene and the number of surface atoms was calculated using the particle size and structure. The absolute numbers vary from experiment to experiment of the NPs (see SI). Error bars in this figure were obtained by error propagation as described in the main text.

IV. Operando X-ray Absorption Spectroscopy studies of cobalt phosphide hydrogenation. To obtain structural information on the species formed on cobalt phosphide by reaction with H2, we performed operando X-ray absorption spectroscopy (XAS) experiments at the Co K-edge for both CoP-M and CoP-NP under a H2 flow (3.5 v/v-% H2 in He, see SI for experimental details). The XANES regions in the energy-space of the measured spectra before and during H2flow were nearly identical (SI), indicating that the average effective nuclear charge of the Co atoms did not change significantly. In the EXAFS regions scattering from hydrogen atoms are not expected to be observable by XAS techniques, because the lack of core electrons make Hatoms very weak backscattering atoms. The R-space spectra of CoP-M and CoP-NP show absorption signals at 1.76 Å (Figure 6). Due to a phase shift, the real distance to scatterers is ~0.5 Å larger than is apparent in the Fourier-transformed data,7a,28 giving real interatomic distances of ~2.3 Å. In line with previous reports for Co-P distances in cobalt phosphide and related materials,7a,27,29 these absorptions were mainly attributed to P scatterers in the first coordination sphere of Co. Upon contact with H2 these absorption signals shifted to slightly higher radial distances by 0.04 or 0.01 Å, respectively. Relative interatomic distances can be resolved to 0.005





Å by EXAFS,30 indicating that this shift measured on the exact same sample pellet is likely significant. We propose that this apparent increase in Co–P distance may be due to a slight lattice expansion of cobalt phosphide upon reaction with H2. Larger shifts in XAS distances have previously been observed upon absorption of hydrogen into Pd nanoparticles (in this case forming a Pd hydride phase) and have been attributed to a lattice expansion.31 Shifted EXAFS signals to higher/lower apparent distances have also been observed for CoP used as Li-battery anodes upon (dis)charge (Li+ and e– addition/removal),32 and we have shown, at least for ZnO reduction, that the addition of Li+ and e– is similar to the addition of H+ and e– (together a Hatom).33

Figure 6. Operando Co K-edge XAS spectra in R-space of cobalt phosphide before (blue) and during H2-flow (red). Spectra are for (a) acid-treated CoP-M and (b) CoP-NP pre-treated with Meerwein’s reagent. The inserts show enlarged regions at the absorption maxima of the first coordination sphere P scatterer, with the grey lines indicating the small increase in the interatomic Co-P distances upon contact with H2.

Discussion We have examined the hydrogen-atom transfer reactivity of two different forms of cobalt phosphide: a mesoscale, ligandless, insoluble material (CoP-M) and colloidal nanoparticles (CoP-NP) with oleylamine ligands. A protocol of an acidic treatment removed the native oxidized layer on as-prepared mesoscale CoP. Strictly anaerobic material handling of this material allowed us to examine the reactivity of a predominantly unoxidized, ligandless cobalt phosphide surface with a ratio of Co:P of 1:1. Studies of electrocatalytic hydrogen evolution using cobalt phosphide have suggested that a catalytically active phosphide surface is formed in situ under acidic conditions.2b,7a,8b,9d The acid-washed bulk material, CoP-M, has similar attributes to cobalt phosphide HER electrocatalysts, highlighting the relevance of CoP-M in this context. The studied 6 nm-diameter CoP-NP with oleylamine ligands can be used for molecularlike chemistry to establish reaction stoichiometries per particle or per surface atom in direct comparison to control experiments. The similar XANES spectra of the larger CoP-M particles and the CoP-NP and their similar catalytic reactivity in alkyne hydrogenation and HER8b,9a show that the two cobalt phosphide systems are related and can give complementary insights due to


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their respective experimental advantages. The features that are common to CoP-M and CoP-NP seem likely to be typical of CoP materials in general. The reported activity of cobalt phosphide in HER electrocatalysis implies the transfer of protons from the acidic electrolyte and electrons from the circuit to form hydrogen atoms on the surface. Here we used an alternative strategy to investigate the H-atom reactivity of CoP-M using a variety of molecular hydrogen atom transfer reagents. Disproportionation of diphenylhydrazine to aniline and azobenzene, or N-H/D scrambling of dihydrophenazine clearly indicated that CoP-M effected net H-atom transfers. The reactivity of CoP-M with other H-atom donors and acceptors indicates the generality of this H-atom or proton-coupled electron transfer (PCET) reaction chemistry. Dihydrophenazine formed phenazine and likely some H2 in the presence of CoP-M, which could be considered a molecular mimic of electrocatalytic hydrogen evolution. I. Thermochemistry: The CoP–H Bond Dissociation Free Energy (BDFE). The Sabatier principle states that an active catalyst should bind substrates and intermediates neither too strongly nor too weakly. This is a conceptual basis of scaling relations and volcano plots in heterogeneous catalysis and electrocatalysis, which often use the H-atom binding energy as the scaling parameter or ‘descriptor’.4-5 Qualitatively, if a surface can abstract H-atoms from a strong A-H bond in a molecule, that implies a stronger surface–H bond and hence likely a bad catalyst. The work reported here studied the reactivity of CoP-M with hydrogen atom transfer reagents containing A-H bonds of varied bond strengths. This provides the first experimental measures of the strength of H-binding to cobalt phosphide. Since CoP is an active HER catalyst, it should therefore only be able to break weak A-H bonds in a molecule and not strong ones. Specifically, the CoP-H bonds formed under catalytic conditions should have bond dissociation free energies (BDFEs) that are close to half of the H2 BDFE. When these are equal, the equilibrium CoP–H CoP + ½ H2 is isoergic. The BDFE of H2 in MeCN solution is 102.3 kcal mol-1, or 51.2 kcal mol-1 per H-atom.18 Theoretical investigations suggest that the binding energies of hydrogen on cobalt phosphide are coverageand facet- dependent, among other factors.5a,16 The computed surface-H BDFEs have been in the range of ~52-69 kcal mol-1. These values were reported as ΔG°H = -0.6 eV to +0.1 eV vs. the Gibbs free energy of gas-phase molecular H2, which is equivalent to our preferred moleculartype definition of BDFE(AH) = ∆G° for AH  A + H in a solvent: BDFE (kcal mol-1) = 23.06ΔG°H (eV) + ∆G°f(H•) in the solvent.18-19 The reactions of CoP-M with various H-atom donors and acceptors, summarized in Scheme 2, provide experimental measures of the CoP-M-H BDFE. We approximate the BDFEs of molecular reagents from the reported standard reduction potentials under aqueous conditions,18,34 via average BDFE(AH2) = 23.06[E°(2H+/2e–)] + 57.6 kcal mol-1).18 Solution BDFEs in most cases vary little with solvent, with variations typically within ±2 kcal mol-1. Molecular hydrogen was observed to add substantial H to the surface, based on the subsequent quantification with PhCCD. This indicates that the CoP-M-H BDFE is stronger than 51 kcal mol-1, half of the BDFE(H2). Hydrogen atoms can also be added to CoP-M with molecular





hydrogen atom transfer reagents with BDFEs < ~64 kcal mol-1, such as dihydrophenazine (BDFE ~63 kcal mol-1) and 1,8-dichloro-9,10-anthrahydroquinone (BDFE ~60 kcal mol-1). Based on the subsequent quantification experiments these reagents introduced fewer H-atoms to CoP-M than H2. This suggests that there is a distribution of different CoP-M-H BDFEs. In contrast, the molecular hydrogen atom transfer reagents with BDFEs > ~66 kcal mol-1, 2,6-di-tertbutylhydroquinone (~71 kcal mol-1) and 9,10-phenanthrenediol (~67 kcal mol-1), did not react with CoP-M. This gives an upper limit for the bond strengths of hydrogen in CoP-M-Hx of < 67 kcal mol-1. Thus the derived range for the CoP-M–H BDFEs is ~51 to 66 kcal mol-1, probably exhibiting a distribution of BDFEs within this range. Remarkably, this is very close to the range of computed values mentioned above.5a,16 Very weakly bound hydrogen on CoP-M may also be present, which could form H2 spontaneously and can therefore not be probed by our quantification experiments. We note that after loading CoP-M with H2, a surprisingly large number of freeze-pump cycles were necessary to remove all of the headspace and dissolved H2 (up to 10 times). It is possible that some of that H2 came from hydrogen on CoP-M with weak bonds (≤~51 kcal mol-1), which are only stable under an H2 pressure and not after headspace-H2 removal. Scheme 2. Hydrogen atom transfer reactions of CoP-M with molecular reagents.

a

E°(2 H+/ 2 e–) for hydroquinones and dihydrophenazine under aqueous conditions from previous literature reports.18,34-35 b Average A-H BDFE values were calculated from reported E° values under aqueous conditions. Reactions of substrates with BDFEs between 60 and 67 kcal mol-1 provide a somewhat more nuanced view. For example, 9,10-anthrahydroquinone (H2AQ) has an average O–H BDFE of ~60 kcal mol-1 and transfers some H-atoms to CoP-M. On the other hand, 9,10phenanthrenediol (H2PQ, BDFE ~67 kcal mol-1) is unreactive. As a cis-diol, H2PQ is expected to be able to chelate and bind more strongly to CoP-M than the related trans-diol H2AQ. The observation of quinone formation only in case of H2AQ under otherwise comparable conditions 18 ACS Paragon Plus Environment

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suggests that the O–H BDFE and not substrate binding are determining whether a reaction occurred in that case. These reactivity data with molecular hydrogen atom transfer reagents are in line with the Sabatier principle and provide an alternative perspective on this well-known principle. The reaction of CoP-M with dihydrophenazine (BDFE ~63 kcal mol-1) forming phenazine and H2 (BDFE 51 kcal mol-1 per H) indicates that additional thermochemical contributions besides pure BDFE-based factors can be significant to CoP-M reactivity. The formation of H2 is endoergic if only the BDFEs are considered. Based on the broadening of the 1H NMR resonances of phenazine and dihydrophenazine, it appears that these species can bind to CoP-M. The binding of phenazine is presumably part of the driving force for CoP-M to dehydrogenate dihydrophenazine. In sum, the observed reactivity of CoP-M can largely be understood on the basis of reactant/product BDFEs and in some cases additional thermochemical factors. These results show, that there is a distribution of H-atom sites possible on CoP with a range of BDFEs and kinetic properties (minutes to days, see SI). In addition, some H-atoms likely come off CoP spontaneously as H2. Our chemical method adding hydrogen to CoP using and then removing residual H2 implies that the large majority of the hydrogen quantified here is stable to H2 loss. In an electrochemical context, such hydrogen would be termed ‘underpotential deposited (UPD) hydrogen.’ This suggests that under HER conditions – producing H2 on the seconds timescale in the presence of acid and reducing potentials (such as in linear sweep voltammogram experiments) – there are additional H’s on the surface, beyond the extensive amount observed here. Whether the H-atoms on CoP-M probed herein are involved in its HER reactivity,8b perhaps by dynamical exchange between different sites, is subject of ongoing investigations. II. Stoichiometry of Hydrogen Atoms on Cobalt Phosphide. The H-atom quantification experiments described above show that a substantial number of H-atoms can be added and stabilized on CoP-M and CoP-NP. While there is some variability in the data, the number of H-atoms added per surface CoP for CoP-NP was roughly on the order of one monolayer. Analogous estimates of surface coverage for CoP-M are not possible because its complex surface and particle shape and size distribution preclude an estimation of the surface area (see SI). Still, the observation of 0.2 reactive hydrogen atoms per total CoP in the material is remarkably high. This means there is an H for every fifth CoP unit in the total material. In such a mesoscale solid, only a small fraction of the material is at the surface. An upper limit for the fraction of surface Co on CoP-M can be obtained from the amount of Co2+ released during the acidic pre-treatment (see SI). This indicated ≤17% Co atoms are on the surface and therefore, there is at least 1 H per surface Co. The stoichiometry of CoP-M-H0.2 hence suggests that hydrogen atoms are present in the subsurface and perhaps in the bulk of the material, as well as on the surface. The possible incorporation of H in the bulk of CoP-M is supported by the XAS spectra, which indicate a slight lattice expansion of the bulk cobalt phosphide lattice upon treatment with H2. We emphasize that the chemical quantification experiments measure only the reactive hydrogen atoms, so any subsurface H must be mobile because they can be abstracted by PhCCH or tBu3ArO•. 19 ACS Paragon Plus Environment




The H-atom quantification experiments suggested that significant numbers of reactive Hatoms are also associated with CoP-M, without any treatment with H2 or other hydrogen donor. For CoP-M, isotopic labelling experiments indicated that these reactive H-atoms stem in part from the acidic pre-treatment. Interestingly, the H-atoms added to CoP-M via acidic pretreatment seem to behave similarly to those added with H2 or other reagents. Reactive Hatoms are also present in the as-prepared CoP-NP. For CoP-NP no acidic pretreatment was performed, which showed that reactive H-atoms on cobalt phosphide can also have alternative origins. This previously unrecognized feature of cobalt phosphide should be incorporated into discussions of its catalytic activity. With the inherent inhomogeneities of the mesoscale CoP-M and CoP-NP, the batch to batch variabilities, and the challenges in quantifying small amounts of products, the specific number of hydrogen atoms determined from these quantification experiments have significant uncertainty. Even with this variability, there is general consistency of the results from these two very different cobalt phosphide systems. The as-prepared materials contain hydrogen, and treatment with H2 gives between two and ten times more hydrogen. After hydrogen treatment, the total amount of H approaches and can likely exceed a monolayer of H-atoms on the material. The possibility of stabilizing H-atoms on cobalt phosphide is, as noted above, in line with computational studies.5a,16 Hydrogen surface coverage up to a monolayer has been evaluated theoretically, but, to our knowledge, such high H-concentrations have not been previously observed for cobalt phosphide. These experiments indicate that cobalt phosphide has significant similarities with noble metals such as Pt and Pd, with substantial underpotential deposited hydrogen (from H2) and the potential for H incorporation in the bulk. For Pd, Pt, Ni and some metal-nitrides, it has been shown that sub-surface hydrogen is capable of hydrogenating surface adsorbates.36 III. Novel Reaction Chemistry of Cobalt Phosphide. The hydrogenation and hydrogen-atom transfer reactions described above are, to our knowledge, previously unprecedented for cobalt phosphide materials. Alkyne hydrogenation has previously been observed for nickel phosphide particles,37 but not for cobalt phosphide. CoP-M also catalyzed scrambling of H2/D2 and formed scrambled hydrogenation products. The scrambling mechanism is not known, but could occur via exchange of acetylenic hydrogen atoms. This H/D scrambling was faster than alkyne hydrogenation. It is particularly significant that CoP-M can manipulate C–H bonds. Metal-oxide materials, for instance, readily form and cleave O–H and N–H bonds, but are typically much less reactive with C–H bonds. The breadth of reactivity reported here suggests that a much more extensive reaction chemistry of cobalt phosphide surfaces remains to be discovered. IV. Questions About Structure and their Implications. Our results indicate that CoP-M and CoP-NP inherently contain reactive H-atoms, and more H-atoms can reversibly be transferred to these materials from H-donors with low BDFEs. This gives insight into the number and thermodynamics of these hydrogen atoms, but little is known about the structure of the corresponding sites. Spectroscopic evidence for hydrogen on 20 ACS Paragon Plus Environment

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phosphides or other higher group V materials, like arsenides, is scarce in the literature.17,38 For mesoscale CoP-M, the reactive H-atoms seem likely to be present both on the surface and in the bulk. Reactive H-atoms on/in cobalt phosphide could be associated with Co, or with P atoms, or both. DFT calculations for cobalt phosphide HER catalysts suggested that Co-Co bridging sites and P top sites could be involved in H2 evolution.16 The phosphorus could be reduced P or residual oxidized P, as indicated by the change in P binding energies after treatment of CoP-M with H2ClAQ (see Section IIA). For colloidal CoP-NP an additional possibility is that reactive H-atoms are associated with the oleylamine ligands. Previous work suggested that ligands prevent access to surface sites decreasing the catalyst’s HER activity,9a,13 but this does not exclude that reactive sites could involve polar headgroups of the ligands. To our knowledge, there is no experimental evidence for the structure of H-sites on CoP. The operando X-ray absorption spectra provide some insights on the structure of the hydrogenated cobalt phosphide. Co K-edge XAS measurements of ligandless CoP-M particles and CoP-NP under H2-flow showed similar XAS signatures in energy-space and R-space. These measurements suggested that the Co atoms in both materials largely retained their effective nuclear charge in presence of H2. In molecular organometallic chemistry, the charge on a metal undergoes little change upon formal oxidative addition of H2, because the M-H bonds are not very polar.39 In contrast, addition of H to reducible oxide semiconductors such as TiO2 forms O– H bonds with concomitant reduction of the metal ions (proton-coupled electron transfer).40 Tentatively, the XAS spectra also indicated a slight lattice expansion of the CoP lattice structure. This indicates that some H-atoms may be dissolved in bulk because XAS samples the entire material, not just the surface. This would explain how large numbers of hydrogen can be bound to these materials. Further spectroscopic studies to elucidate the structure of the reactive H-sites on CoP-M and CoP-NP are underway. The large density of hydrogen on these diverse samples makes it likely that there is a distribution of different CoP-M-H species with different binding energies. This is supported by the observation that the number of reactive hydrogen atoms introduced to CoP-M varied with the type of reagent (as well as with different cobalt phosphide samples). For instance, the ability of CoP-M to dehydrogenate dihydrophenazine, even though its N–H bonds are at the very high end of the estimated range, suggests that there are some sites with relatively high CoP-M–H BDFEs. Theoretical investigations of materials such as cobalt phosphide often assume well-ordered facets in their models of catalytic HER activity.16,41 This is not our vision. Acidic pre-treatment of the oxidized CoP-M material likely leaves a complex, heterogeneous material, with a small amount of oxidized phosphorus remaining, with hydrogen present, and perhaps some oxide/hydroxide/water ligands. For CoP-NPs, the number and type of binding of the oleylamine ligands can vary. Even for much simpler noble metal surfaces, defects, step edges and other sites play important roles in the catalytic reactivity. It seems likely that such imperfections will prove to be even more important for materials such as cobalt phosphide, which has the likelihood of local non-stoichiometry as well as defects, edges, etc.





Conclusions We present a novel approach to probing the hydrogen-atom reactivity of cobalt phosphide using molecular reagents. This provided the first experimental estimates of the thermochemistry and stoichiometry of hydrogen binding on cobalt phosphide. Both mesoscale CoP-M and colloidal CoP-NP with oleylamine ligands were studied. Reactive H-atoms were found on the as-prepared materials, some arising from the acid pre-treatment of CoP-M to remove the native oxidized layers. Reactions with H2 and other molecular H-atom transfer reagents added 2-10 times more hydrogen to these materials. Reactive H-atoms were quantified by alkyne hydrogenation or by reaction with a phenoxy radical. For CoP-NP, the amount of hydrogen added was as much as one monolayer. For mesoscale CoP-M, a remarkable bulk stoichiometry of CoP-M-H~0.2 was determined, suggesting that some of the reactive H-atoms may be in the bulk or sub-surface. This was also tentatively indicated by the slight lattice expansion of both CoP-M and CoP-NP upon contact with H2 (by operando X-ray absorption spectroscopy). Based on the observed reactivity, the [CoP]–H bond dissociation free energies (BDFEs) for hydrogen atoms on the cobalt phosphide materials probed in this study are bracketed between ~51 and 66 kcal mol-1. For instance, H-atoms were added to CoP-M only by reagents with BDFE(A–H) < 67 kcal mol-1). These are the first experimental estimates of the CoP-M–H BDFEs for cobalt phosphide under solution conditions. Such BDFEs are very common descriptors of reactivity in the Bell-Evans-Polanyi / Sabatier principle / volcano plot approach to heterogeneous catalysis and electrocatalysis. The experiments suggest that the observed range of [CoP]–H BDFEs reflects a distribution of binding energies and binding sites. These studies present a novel perspective on cobalt phosphide and its catalytic activities. The observation of common features between quite different mesoscale and nanoscale cobalt phosphide systems suggests that these results will prove general for this material, and perhaps others. The materials have substantial amounts of hydrogen that has interesting reactivity, for instance for hydrogenating phenylacetylene. The H-atoms are likely a key feature of the chemistry of cobalt phosphide and play a major role in its catalytic and electrocatalytic activities.

Associated Content Supporting Information. Experimental procedures, material characterization by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray absorption spectroscopy, analysis of pre-treatment solutions by UV/Vis spectroscopy, and 1H and 31P NMR spectroscopy, 1H NMR spectroscopy and GC-MS data for reactivity studies. Acknowledgments This research was supported as part of the Center for Molecular Electrocatalysis (CME), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. This research used resources of the Advanced Photon Source


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