Introduction

As one of the three ferromagnetic metals in the periodic table of elements, cobalt is widely used in magnetic alloys with the advantage of heat resistance. Cobalt-based materials manifest a wide range of applications including permanent magnets1, information storage2 and aerospace manufacturing3. Since air and water affect the lifetime of these materials and the retention of their properties4,5, corrosion is an ever-present concern in the world of metals6,7,8. It is important to fully understand the interface interactions and reaction mechanism, which can guide the rational design of anticorrosion strategy for practical applications. The related metal–water interactions are also an important theme of research in chemistry and biology as well as energy source and environment9,10.

On the other hand, the low-cost and high-efficiency hydrogen production by water dissociation via electrolysis and photocatalysis is a long-term research topic11,12,13,14, for which cobalt nanocatalysts have attracted extensive interest, although cobalt usually does not react with water at ambient conditions15,16,17,18,19,20,21. Catalytic O–H dissociation and HAT is vital to water dehydrogenation and O-O bond formation22,23,24; however, catalytic oxidative dehydrogenation often exhibits limited activity and poor selectivity, despite decades of research efforts in this field. Small metal clusters possess distinct catalysis in contrast to their bulk analogues due to the quantum size effect and unique electronic structures25. For instance, dehydrogenation of water on some Aln− clusters was observed at room temperature26,27,28, leading to the establishment of a complementary active site (CAS) mechanism27. Dehydrogenation of H2O molecules by reacting with gas-phase vanadium clusters was also noted29, showing diverse VnO+, VnO2+ and VnO3+ products by rapid reactions of Vn≥3+ with water in a fishing mode30. In contrast to aluminium and vanadium, however, cobalt does not support water dehydrogenation according to the activity series of metals31. There comes a pending question if subnanometer cobalt clusters can support spontaneous water dehydrogenation and oxidation, which leads to a better understanding of the corrosion of cobalt nanosurfaces.

Based on this motivation, herein we report a comprehensive study of the gas-phase reactions of Con±/0 clusters with water and oxygen. Well-resolved Con±/0 (c.a., n = 1–30) clusters are prepared, and their reactions with water are studied by using our self-developed ultrafast deep ultraviolet laser ionisation mass spectrometer (DUV-LIMS, Supplementary Fig. S1)32,33. As a result, we found all these Con±/0 clusters react with oxygen to form diverse oxides. However, the Con+ clusters readily react with water giving rise to diverse adsorption products, which contrasts with the anionic Con– clusters which allow for dehydrogenation in reacting with water. Combined with density functional theory (DFT) calculations, we illustrated the reaction dynamics and unveiled the altered binding mode of water on the small Con– cluster anions (Con–···H–OH) compared with their cationic and neutral analogues (Con+/0·OH2). Apart from the charge effect, we also elucidated the spin effect and cooperative multi-site effect that promote water dissociation and dehydrogenation on the Con– clusters, showing enhanced activity of such cobalt clusters without being restricted by the principles of activity series of metals.

Results and discussion

Anionic Con – clusters reacting with water

The reactions of Con±/0 clusters with oxygen have been addressed in our previous study34, showing a tendency to form diverse oxides (Supplementary Fig. S2) but with a stable cluster Co13O8 showing up in the presence of sufficient oxygen reactant. Here we emphasize on the reactions with water. Figure 1A presents a typical mass spectrum of anionic Con– (n = 5–59) clusters in the absence and presence of water carried by bubbling of He buffer gas. The prepared Con– clusters display a regular Gaussian/Rayleigh distribution centred at Co25–. Isotope-labelled water, H218O, was used to exclude the interference of trace amount of oxygen contamination (also, deuterium water D2O was also used to unambiguously identify the dehydrogenation products, Supplementary Fig. S3). Figure 1B displays an enlarged area to visualise the products of Con– (n = 10–20) clusters reaction with H218O. The observation of a series of products [Con18O]–, [Con18O2]– and [Con (18OH)2]– suggests that the Con– clusters undergo dehydrogenation with one and two water molecules.

Fig. 1: Typical mass spectrum of Con– reacting with H218O.
figure 1

A Typical mass spectra of the Con– clusters produced by the homemade LaVa source, within a 35 mm nozzle and 10.0 atm He buffer gas, and the reaction products with H218O being introduced into the flow tube, controlled by a pulsed valve with a pulse width at 260 µs. B Enlarged area for the Con– (n = 10–20) clusters after the reaction with H218O.

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We monitored the reaction of Con– clusters with water at varying doses of the water controlled by the pulsed valve (Fig. 2 and Supplementary Fig. S4). An estimation of reaction rates is given in Supplementary Fig. S5. When the cobalt clusters reacted with small amounts of water, a series of [Con18O]– products (accompanied by minor ConO– contamination) were observed in the mass spectra. As the dose of water was gradually increased, the [Con (18OH)2]– and [Con18O2]– products appeared in the mass spectra (and the nascent ConO– contamination peaks disappeared). When a large amount of water was involved in the reaction, a series of [Con(18OH)2]– products dominated the mass spectra. Interestingly, the adsorption products [ConH218O]– and [Con(H218O)2]– were absent in the mass spectrometry observation, indicating that the HAT and dehydrogenation proceeded rapidly. This was also verified by the experiments based on deuterium water (D2O, Supplementary Fig. S3). This experimental observation challenges the previously established principles that cobalt reacts with protonic acid (but not H2O)31 to form H2. The dehydrogenation reactions of anionic cobalt clusters with water can be written as,

$${{{{{{\rm{Co}}}}}}}_{n}^{{{-}}}+{{{{{{\rm{H}}}}}}}_{2}{{{{{\rm{O}}}}}}\to {{{{{{\rm{Co}}}}}}}_{n}{{{{{{\rm{O}}}}}}}^{{{-}}}+{{{{{{\rm{H}}}}}}}_{2}$$
(1)
$${{{{{{\rm{Co}}}}}}}_{n}^{{{-}}}+{2{{{{{\rm{H}}}}}}}_{2}{{{{{\rm{O}}}}}}\to {{{{{{\rm{Co}}}}}}}_{n}{({{{{{\rm{OH}}}}}})}_{2}^{{{-}}}+{{{{{{\rm{H}}}}}}}_{2}$$
(2)
$${{{{{{\rm{Co}}}}}}}_{n}^{{{-}}}+2{{{{{{\rm{H}}}}}}}_{2}{{{{{\rm{O}}}}}}\to {{{{{{\rm{Co}}}}}}}_{n}{{{{{{\rm{O}}}}}}}_{2}^{{{-}}}+2{{{{{{\rm{H}}}}}}}_{2}$$
(3)
Fig. 2: Mass spectrum of Con– reacting with H218O of different doses.
figure 2

a Mass spectra of the Con– clusters. b–d Mass spectra of Con– clusters after reactions with different amounts of H218O, controlled by a pulsed valve with varying pulse widths at 180 µs, 240 µs, and 260 µs, respectively, corresponding to the original Supplementary Fig. S4. The peaks marked with stars (*) correspond to oxygen attachment due to the trace amount of contamination.

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The reactions of neutral and cationic Co clusters

To compare the reaction behaviour of cobalt cluster anions, neutrals and cations, Fig. 3 displays the mass spectra of the cobalt cluster cations and neutrals before and after reacting with water, respectively. To distinguish the likely hydrogenation products, the isotope chemical D2O was used. As a result, the cationic Con+ clusters were found to adsorb multiple D2O molecules showing diverse Con+(D2O)m complexes; however, almost no dehydrogenation products were observed except for Co3+. The observation of strong water adsorption on Con+ clusters is consistent with the previous studies of Rhn+ clusters reacting with water, as well as the Con+ clusters reacting with NH3 (ref. 35). Natural bond orbital (NBO) analysis shows maximal donor–acceptor orbital overlap interaction energy between the cationic clusters and H2O molecule (Supplementary Fig. S12), which is also in agreement with the results of charge decomposition analysis and potential scan for a H2O molecule in approaching a cobalt cluster (Supplementary Figs. S16 and S17). There is a similar case for the neutral cobalt clusters which also exhibit weak reactivity with water, with a few water-adsorption products being observed, such as Co9-18D2O (Fig. 3B). The different reactions of Con±/0 clusters with water embody the charge dependence of metal cluster reactivity, as revealed in the previous studies on the reactivities of Al, Nb and Rh clusters36,37,38,39.

Fig. 3: Con+,0 reacting with D2O.
figure 3

A Typical mass spectra of the cationic Con+ (n = 2–15) clusters before and after reacting with D2O in the flow tube. B Typical mass spectra of the neutral Con (n = 2–30) clusters before and after reacting with D2O.

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Reaction dynamics and charge effect

We have conducted DFT calculations to elucidate the charge effect and H2 release mechanism of cobalt clusters in reacting with water molecules. The structures of Con±/0 and [ConH2O]±/0 (n = 2–13) clusters with different spin multiplicities are optimised at the PBE-D3/def2-TZVP level of theory (Supplementary Figs. S7–11 and Table S1). Interestingly, the lowest-energy structures of water adsorption on the cationic and neutral clusters prefer Co–O coordination (i.e., forming [ConOH2]+/0), with slight fluctuation of the bond lengths (Supplementary Fig. S14); however, water adsorption on the Con− (n = 1, 2, 3, 5, 6) clusters results in Co–H bonding (Con–···H–OH). This is consistent with the previous study of M(H2O)– (M = Cu, Ag, Au)40. In addition, the binding energies (Ead) of H2O onto the Con±/0 clusters show significant charge dependence. The Ead values of the cations are larger than those of the neutral and anionic clusters (Supplementary Fig. S13), in line with the experimental observation that the cations can adsorb multiple water molecules while the reaction products of the anions are relatively small, although they support H2 release.

We carried out DFT calculations on the thermodynamic energies and reaction kinetics typically for one and two H2O molecules to react with Co6±/0 which has an octahedral structure. As shown in Fig. 4, both Co6 and Co6+ clusters suffer from unsurmountable energy barriers of the H-atom transfer (TS1_1 at 0.52 eV and 0.44 eV higher than the reactants); in contrast, HAT on the Co6− cluster is thermodynamically favourable and kinetically favourable with a small energy barrier. While for the reaction of “Co6±/0 + 2 H2O”, the reaction coordinates indicate that three reactions are exothermic, but the energy for the H-atom transfer step (TS1_2) still differs from each other, and the neutral and cationic clusters take on larger single-step energy barriers. In addition, the reaction pathways for the cationic and anionic Co6± clusters obey spin conservation, but the energy barrier of the rate-determining step for the Co6− cluster (0.43 eV) is much lower than that of the Co6+ (1.88 eV). Also, for the reaction of neutral Co6 with two H2O molecules, the pathway of spin conservation (red-curve) suffers from a higher energy barrier of TS2_2 (1.52 eV); in comparison, the blue-curve pathway begins with a lower spin adsorption state (13Co6) and undergoes a relatively lower energy barrier (1.00 eV) of the rate-determining step. Although the spin crossing causes a reduced energy barrier, the dehydrogenation on neutral Co6 is still not favourable compared with the anionic Co6−. It can be concluded that both spin states and charge effect play a dramatic role in the catalytic dehydrogenation on such nanometals41. Notably, the cool He buffer gas could take away part of the energy during the reaction, rendering the cationic and neutral clusters not having enough energy for dehydrogenation (Supplementary Tables S2 and S3), especially for those having large energy barriers of the transition states.

Fig. 4: Reaction coordinates of Co6±/0.
figure 4

a–c The energy diagram for cationic 16Co6+, anionic 14Co6−, and neutral 15Co6 clusters in reacting with one and two water molecules. Energies are given in eV. The inset shows electrostatic potentials (EPS, kcal mol−1) for a H2O molecule in approaching the Co6− cluster with spontaneously regulated orientation.

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Size effect and multi-site cooperation

According to our DFT calculations, the reactions of Co3− and Co11− are also initiated by hydrogen-metal bonding adsorption (Fig. 5), similar to the aforementioned Co6− in reacting with two H2O molecules. Notably, the first H-atom transfer of a single H2O on the Co3− cluster is thermodynamically unfavourable. This is different from the previous finding of Vn≥3− clusters in reacting with a single water molecule to release H2, which is associated with the nature of the metal activity sequence. Nevertheless, Co3− reacts with two H2O molecules to release H2, shedding light on the importance of synergetic active sites and multiple molecule cooperation.

Fig. 5: A comparison of Co3− with Co11−.
figure 5

a Reaction energy diagram of “Co3− + 2 H2O → Co3(OH)2− + H2”. b Reaction energy diagram of “Co11− + H2O → Co11O− + H2” and “Co11− + 2 H2O → Co11(OH)2− + H2”. Energies are given in eV. The insets show the corresponding structures. The inset on the left bottom shows the HOMO and LUMO patterns of [Co11H2O]−.

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In comparison, the reaction of Co11− with a single H2O finds a relatively larger energy gain of the adsorption and smaller energy barrier for the first H-atom transfer; nevertheless, the final transition state of H-H recombination for H2 evolution displays comparable single-step energy barrier as the Co3−. Notably, the reaction of “Co11− + 2 H2O → Co11(OH)2− + H2” shows a much smaller energy barrier (0.26 eV) for the H-atom transfer (TS1_2) compared with the rate-determining step for a complete dehydrogenation (1.34 eV for TS3_2, Supplementary Fig. S18). This coincides with the experimental observation of a larger mass abundance of Con(OH)2− than ConO2−.

The reactions of both water and oxygen

Considering that the corrosion of metals is essentially related to their chemical reaction with oxygen and water to form oxidation and dehydrogenation products, we further studied the reactions of the anionic Con− clusters by introducing both water and oxygen as reactants into the flow tube. The results are given in Fig. 6. It is seen that oxygen reacts with the Con− clusters to form ConO2x− clusters without exception but with slightly lower reaction rates at Co5− and Co6−, likely due to their structural stability5. Meanwhile, partial dehydrogenation products, including a series of [CoxOy·18O]−, [CoxOy·18O2]− and [CoxOy(18OH)2]− were observed, indicating that oxygen undergoes competitive adsorption but does not hinder dehydrogenation. Nevertheless, from the diverse products of oxidation and dehydrogenation, it can be inferred that nanoscale cobalt suffers from inevitable corrosion, although this reactivity could not be so fast as iron. By referring to the corrosion equation of iron42, the reactivity of cobalt clusters with water and oxygen could be summarised by an integrated reaction channel,

$${{{{{\rm{C}}}}}}{{{{{{\rm{o}}}}}}}_{n}^{{{-}}} +\, x{{{{{{\rm{H}}}}}}}_{2}{{{{{\rm{O}}}}}}+y{{{{{{\rm{O}}}}}}}_{2}\to {{{{{{\rm{Co}}}}}}}_{n}{{{{{{\rm{O}}}}}}}_{m}{({{{{{\rm{OH}}}}}})}_{u}{({{{{{\rm{H}}}}}}_{2}{{{{{\rm{O}}}}}})}_{v}^{{{-}}}\\ + \, (x-v-u/2){{{{{{\rm{H}}}}}}}_{2},x+2y=m+u+v$$
(4)
Fig. 6: Con− reacting with O2 and D2O.
figure 6

a Typical mass spectrum of the anionic Con− (n = 3–23) clusters. b The mass spectrum after their reactions with oxygen (1% in He). c The mass spectrum after reactions with both O2 and H218O in the flow tube, controlled by two pulse valves. The enlarged mass ranges for Co10-12− are given on the right side, respectively.

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We would like to put forward more discussion on the corrosion mechanism of nano-cobalt. Under ambient moist and oxygen-rich atmosphere, most metals suffer from spontaneous oxidation and likely thermodynamical dehydrogenation with few exceptions (gold and platinum)43. Some metals such as aluminium, chromium, magnesium and nickel can be well protected by a layer of impenetrable oxide coatings that prevents further destruction of the surface. So does the stainless steel which usually involves chromium and nickel to attain dense protection, thus avoiding unwanted corrosion. Whether or not a tight protective film, it is vital to avoid the formation of hydrated metal oxide and prevent the metal rusts from continually flaking off, without an exposure of fresh metal surfaces to oxygen and water. According to our results, it could be helpful to avoid negative charge accumulation; in other words, it would be important to check surface static charge regularly and keep neutral surfaces. In addition, a previous study found that Co13O8 is a highly stable cluster oxide34, which could also breed a strategy of tight protective film like Al2O3.

Conclusions

In summary, we report a joint experimental and theoretical study of cobalt clusters Con±/0 in reacting with water and oxygen. All the Con±/0 clusters were found to react with oxygen regardless of the presence of water or not. However, water dissociation is observed only for the anionic Con– (n = 5–59) clusters, but the cationic Con+ and neutral Con clusters do not support the observation of dehydrogenation except for Co3+. Combined with the DFT results, we unveil the bonding mechanisms of the Con±/0 clusters and illustrate the reaction kinetics of typical Con– clusters toward water in forming ConO–, Con(OH)2–, and ConO2– products. Notably, the cobalt catalysis for dehydrogenation processes is not inhibited in the presence of oxygen; instead, a series of products of oxidation and partial dehydrogenation embody the corrosion of nano-cobalt surfaces.

Methods

Experimental methods

The instrumentation used in this study is based on a customised reflection time-of-flight mass spectrometer (Re-TOFMS). Detailed descriptions can be found in our previous publications33,35,44. In brief, the Re-TOFMS is equipped with a flow tube reactor which is connected with dual pulse valves enabling reactions with two reactant gases (e.g., O2 and water). Isotopic chemicals of both D2O and H218O were used to help identify the reaction products. The Con± clusters were prepared by ablating a clean cobalt disk (\(\varPhi\) = 16 mm, 99.95%) with a pulsed laser (10 Hz 532 nm Nd: YAG) in the presence of helium buffer gas (99.999%, 10.0 atm). The Con± clusters were prepared and ejected out of a nozzle (\(\varPhi\) = 2 mm, L = 35 mm) during a supersonic expansion process controlled by the pulsed valve (Series 9, General Valve). For reactions between the cobalt clusters and water (D2O and H218O), water vapour was injected into the flow tube reactor (\(\varPhi\) = 6 mm, L = 60 mm) by the He (99.999%, 1 atm) bubbling method. Oxygen reactant was diluted (1% in helium) and introduced from the other pulse valve connected to the same flow tube reactor. The reactants were controlled by varying the on-time pulse width. All metal clusters and their reaction products were detected and analysed by the Re-TOFMS. For the neutral Con clusters reacting with D2O, we used an all-solid-state deep ultraviolet (DUV) laser (177.3 nm wavelength, 15.5 ps pulse width, 10 Hz repletion rate, and ∼15 μJ energy per pulse) with a head-to-head mode in the ionisation zone.

DFT calculation methods

The DFT calculations were performed with the PBE-D3 corrected functional45 using the Gaussian 16 programme46. The geometric optimisation and reaction coordinate research were carried out using the balanced triple-zeta def2-TZVP basis set47 for Co, O and H atoms. Vibrational frequency calculations were carried out to ensure that the lowest-energy structures of reaction products have no imaginary frequencies and the transition states (TSs) have only one imaginary frequency. All energies were corrected with zero-point vibrations and the intrinsic reaction coordinate (IRC) scan was employed to ensure a connection with both intermediates in the reaction pathway. The natural bond orbital (NBO), electrostatic potential (ESP), and charge decomposition analysis were analysed by Multiwfn software48. Orbitals and ESP patterns were drawn by the visual molecular dynamics (VMD) software49.