Synthesis

The method for synthesizing Co@Si x is summarized in Fig. 1. To construct the Co‒O‒SiO n linkage, the hydrolysis of tetraethoxysilane (TEOS) was performed in a basic liquor containing Co(NO 3 ) 2 , followed by calcination of the resultant solid at 500 °C to form a product containing predominantly Co 3 O 4 , as shown by X-ray diffraction (XRD) crystallography (Co 3 O 4 @Si x , Supplementary Fig. 1). The final product was obtained by reduction with hydrogen at 600 °C. The composition was adjusted by changing the amount of TEOS in the starting solution, giving Co@Si x , where x is the molar ratio of silica to cobalt (Supplementary Tables 1 and 2). For comparison, a conventional catalyst consisting of cobalt nanoparticles supported on silica (Co/SiO 2 ) was synthesized by a deposition method (details in the SI), the cobalt loading was 43 wt%.

Fig. 1: Synthesis and catalysis strategies of Co@Si x catalysts. The procedures with cobalt phyllosilicates as intermediates for synthesizing Co@Si x . Within the highlighted square, the CO 2 -to-methanol transformation on Co@Si x catalysts. Full size image

Catalysis in CO 2 hydrogenation

Fig. 2 shows the performance of a set of cobalt catalysts in CO 2 hydrogenation with a feed gas at a pressure of 2.0 MPa containing CO 2 and H 2 (H 2 /CO 2 = 3:1, molar). The products, besides methanol, were CO and methane, formed respectively by the reverse water‒gas shift and methanation reactions. A cobalt catalyst without silica (CoO x ) was characterized under our conditions by a CO 2 conversion of 6.7%, with CO and methane as the dominant products, and a slight amount of methanol (Fig. 2a). Significantly, the inclusion of silica in the cobalt catalyst improved both the CO 2 conversion and methanol selectivity. For example, the Co@Si 0.52 catalyst gave CO 2 conversion and methanol selectivity of 9.0% and 47.9%, respectively. The methanol selectivity was further optimized by changing the cobalt/silica ratio, with the methanol selectivity of 70.5% at a CO 2 conversion of 8.6% for Co@Si 0.95 (Supplementary Table 1, Fig. 2b and c). In the catalytic reaction experiment, methanol was the sole carbon-containing liquid product (condensed in a cold trap downstream of the reactor) without any C 2+ by-products, which are usually formed in conversions with cobalt-containing catalysts5,7, revealing a potentially valuable methanol production process.

Fig. 2: Performance of Co@Si x catalysts in CO 2 hydrogenation. a Performance of catalysts, standard reaction conditions for Co@Si 0.95 : 0.2 g of catalyst, pressure = 2.0 MPa; H 2 /CO 2 feed ratio = 3:1, molar; temperature = 320 °C; GHSV = 6000 mL/g h. The mass of the other Co@Si x catalysts was chosen to give the same cobalt content in the reactor. b Dependences of methanol yield on temperature. Each reaction was performed three times, and the corresponding data points are provided in the bar charts. Error bounds for the conversion and selectivity are ± 0.3% and ± 0.5%, respectively. c CO 2 conversion and CH 3 OH selectivity of Co@Si x catalysts at 260‒320 °C. d Stability test of Co@Si 0.95 catalyst operated for 100 h in a flow reactor. Full size image

In contrast, more silica in the catalyst led to decreased CO 2 conversions and lower methanol selectivity, illustrated by data characterizing the performance of Co@Si 1.48 and Co@Si 1.87 , which might be due to changes in the state of cobalt and/or blocking of cobalt active sites by silica. In contrast, the conventional cobalt catalyst (Co/SiO 2 ) gave a CO 2 conversion of 7.3% and a methanol selectivity at 16.6%, with CO being the dominant product under the equivalent reaction conditions. These data confirm the unusual catalytic performance of Co@Si 0.95 in the CO 2 hydrogenation.

As expected, increased operating temperatures of the Co@Si 0.95 catalyst (Supplementary Fig. 4) gave higher conversions, with the methanol selectivity being >70% at 260‒320 °C but decreasing at temperatures >320 °C. Similar trends were observed with the other Co@Si x catalysts (Supplementary Figs. 2‒7). In these cases, the Co@Si x catalysts exhibited a marked decrease in selectivity to the undesired methane compared with the conventional cobalt catalysts (Supplementary Fig. 8). The conventional Co/SiO 2 was characterized by methanol selectivity generally <25% at temperatures in the range of 260‒380 °C (Supplementary Figs. 9 and 10), where the C 2+ hydrocarbons were also detected with selectivity of 4.0%‒8.5% at 260‒380 °C. As shown in Fig. 2b, Co@Si 0.95 catalyst gave methanol productivity of 3.0 mmol g cat −1 h−1, outperforming Co/SiO 2 and even the other supported copper and noble-metal catalysts that have been reported to be excellent for the CO 2 -to-methanol transformation (Supplementary Table 3)19,25,28. For example, the methanol productivity of Co@Si 0.95 was found to be comparable to that of Cu/SiO 2 under comparable conditions39.

The conventional supported metal nanoparticle catalysts generally suffer from the poor stability29,30. For example, the standard commercial Cu/ZnO/Al 2 O 3 catalyst (Supplementary Figs. 11–13) for synthesis of methanol from CO 2 hydrogenation, evaluated in a wide temperature range (200‒380 °C, Supplementary Fig. 12), gave the performances that are sensitive to the reaction temperatures. The best methanol yield appeared at 240 °C, giving productivity of 3.5 mmol g cat −1 h−1 with CO 2 conversion of 15.2% and methanol selectivity of 47.6%, which is higher than that of the Co@Si 0.95 catalyst (3.0 mmol g cat −1 h−1). However, the Cu/ZnO/Al 2 O 3 was characterized by a markedly inferior performance in the reaction life test, losing almost half of the methanol yield after reaction at 240 °C for 50 h (Supplementary Fig. 13). This result is in agreement with the knowledge of the Cu/ZnO/Al 2 O 3 catalyst, whereby the Cu nanoparticles easily sinter into larger ones and cause deactivation29,30. Significantly, Co@Si 0.95 underwent almost negligible decay in the CO 2 conversion and methanol selectivity in 100 h of onstream operation (70 h at 320 °C and 30 h at 380 °C, Fig. 2d).

To the best of our knowledge, this excellent performance of Co@Si 0.95 catalyst in the CO 2 hydrogenation to methanol is unmatched. We are led to hypothesize that the silica support plays a key role, because the comparable silica-supported catalyst, Co/SiO 2 , did not show this behavior. We were thus motivated to investigate the catalysts in depth and to determine catalytic structure‒performance relationships.

Catalyst structure study

A transmission electron microscopy (TEM) image of Co@Si 0.95 (Fig. 3a) shows a lamellar structure of cobalt phyllosilicates. A high-angle annular dark field scanning transmission electron microscopy image (HAADF-STEM, Fig. 3b) and EDX elemental maps (Fig. 3c–e) demonstrate uniform dispersions of cobalt and silicon. The TEM image of Fig. 3f shows cobalt nanoparticles with an average diameter of 3.9 nm supported on the silica. A high-resolution TEM (HRTEM) image reveals the co-existence of metallic Co and CoO phases on the cobalt nanoparticles present in Co@Si 0.95 (Fig. 3g), which is further confirmed by the fast Fourier-transform (FFT) analysis (Fig. 3h) and XRD patterns. The cobalt nanoparticles on a series of Co@Si x samples have similar diameters, as evidenced by the HRTEM characterization. In contrast, the CoO x and Co/SiO 2 catalysts incorporate metallic Co as the dominant phase (Supplementary Figs. 14–18). These data indicate a role of silica controlling the dispersion and the oxidation state of cobalt.

Fig. 3: Structural characterization of Co@Si 0.95 catalyst. a TEM image of Co@Si 0.95 . b–e, b HAADF-STEM image and EDX elemental maps of c Co, d O, and e Si of Co@Si 0.95 . f, g HRTEM images of Co@Si 0.95 . The orange circles highlight the Co nanoparticles (inset of f: size distribution of Co nanoparticles.) h FFT image of the Co nanoparticles corresponding to the HRTEM image in g. Scale bars: 100 nm in a–e, 20 nm in f, and 5 nm in g. i FT-IR spectra of Co@Si 0.95 samples. j, k Co K-edge X-ray absorption spectra. j XANES spectra (inset: enlarged pre-edge region) and k EXAFS spectra with k2-weighted data (solid line) and fit corresponding to recommended model (dashed line) of Co@Si x and Co/SiO 2 samples. l In situ Co 2p XPS spectra of Co 3 O 4 @Si 0.95 under 0.1 mbar of H 2 at various temperatures. Full size image

The cobalt‒silica interaction on Co@Si x samples was investigated with FT-IR spectroscopy, with the bands at 665 and 1025 cm−1, assigned to the Co‒O‒SiO n linkage (Fig. 3i and Supplementary Fig. 19)40. In contrast, these bands are undetectable in the FT-IR spectrum of Co/SiO 2 , consistent with the lack of substantial interactions between cobalt and silica. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were recorded to characterize the oxidation states and coordination environments of Co in the Co@Si x samples. The Co K-edge XANES spectra of Co@Si x samples exhibit pre-edge features of the Co 1s‒3d absorption transition at 7709.5 eV, with absorption edge positions of 7721.6 ± 0.2 eV (Fig. 3j and Supplementary Fig. 20)—these features are characteristic of cobalt oxides41. In contrast, the Co K-edge XANES of Co/SiO 2 is represented by an edge position of 7709.0 eV, assigned to metallic cobalt. These results point to the presence of cationic cobalt bonded to the silica, with Co‒O‒SiO n linkages at the Co‒SiO 2 interfaces stabilizing the dispersed cobalt species in Co@Si x . X-ray absorption spectra (XAS) recorded at the O K-edges of Co@Si x provide evidence confirming the hypothesis: the spectra include peaks assigned to Co‒O bonds, at 532.5 and 539.8 eV42, whereas the Co/SiO 2 exhibits an extremely weak Co‒O signal because of its metallic feature (Supplementary Fig. 21).

In contrast, EXAFS spectra of Co/SiO 2 , recorded at the Co K-edge (Fig. 3k), include a Co‒Co shell with a distance determined in the fitting to be 2.50 5Å, with a coordination number of 9.3, indicating the dominant presence of metallic cobalt (Supplementary Table 4). Consistent with our interpretation, the Co‒Co contributions characteristic of metallic cobalt are extremely weak in the spectra of Co@Si x samples. The EXAFS spectra indicate Co‒O and Co‒Co shells at distances of 2.05 and 3.02 Å, respectively, for Co@Si 0.95 , with coordination numbers of 4.2 and 10.7, consistent with the presence of nonmetallic cobalt bonded to silica.

To further characterize these dispersed cobalt species, we did in situ Co 2p XPS experiments with the samples undergoing reductive treatments (Fig. 3l and Supplementary Figs. 22–25). The as-synthesized Co 3 O 4 @Si 0.95 sample was characterized by a dominant signal assigned to Co3+ (782.4 eV, Supplementary Fig. 26), which was resistant to reduction and unchanged even after exposure to H 2 at 500 °C. Reduction at 600 and 650 °C gave spectra indicating the predominant presence of Co2+ (781.5 eV) with some Co0 (777.8 eV), indicating that the surface of Co@Si 0.95 incorporated predominantly cobalt oxide species and a small amount of metallic cobalt after vigorous reduction. Although the in situ XPS was performed using hydrogen with a lower pressure than that of the practical reduction treatment because of the XPS technique limitation43, it is sufficient to reduce the cobalt species. For example, the Co3+ on Co 3 O 4 /SiO 2 was easily reduced to Co2+ at a temperature of only 300‒400 °C, and Co0 was the only cobalt species detected after reduction at 500 and 600 °C (Supplementary Figs. 27 and 28). This result is in good agreement with the H 2 -TPR measurement of the cobalt oxide sample (Supplementary Fig. 29). These results all support the conclusion that the cobalt species in Co@Si x are strongly resistant to reduction.

In order to provide more evidence, we treated the Co@Si 0.95 sample with relatively high-pressure H 2 at 600 °C for 2 h (10% H 2 in Ar, 2 MPa), which should provide enough hydrogen for reducing the cobalt species. Significantly, the treated Co@Si 0.95 still contained cobalt oxide as the dominant phase with a small amount of metallic cobalt, as confirmed by the XRD (36.4°, 42.5° and 61.5° assigned to CoO phase) and XPS characterizations (781.5 eV assigned to Co2+ and 777.8 eV assigned to Co0) (Supplementary Fig. 30). The conclusion is further confirmed by in situ Raman spectra (Supplementary Fig. 31). By increasing reduction temperature to 600 °C, Co@Si x samples still showed a typical Raman signal of Co‒O species, which was undetectable on the reduced Co/SiO 2 . Even after reaction for 100 h under the practical CO 2 hydrogenation conditions (Fig. 2d), the Co@Si 0.95 sample still exhibited the dominant CoO phase with a relatively small amount of metallic Co (XRD and XPS in Supplementary Fig. 32), confirming the difficult-to-reduce cobalt species on the Co@Si 0.95 catalyst, in good agreement with the in situ XPS investigation.

On the basis of these results, we propose that the silica influences the cobalt oxidation state, resulting in structures that are active and selective catalysts for methanol formation and not for methane and CO formation35. The relationships between the methanol yield in CO 2 hydrogenation and Co0/Co2+ ratio for various catalysts are presented in Supplementary Fig. 24f. Compared with CoO x catalyst, the Co@Si 0.52 and Co@Si 0.95 with Co2+ species exhibited enhanced methanol yields. Further decreasing the Co0/Co2+ ratio reduced the methanol yields over Co@Si 1.48 and Co@Si 1.87 catalysts. These data confirm the balanced metallic Co and CoO phases on the catalysts are important for the methanol production. More Co0 species cause the formation of a large amount of methane with poor methanol selectivity. Consistent with this picture, the hydrogen dissociation ability was evaluated by the catalysis in HD production by the reaction of H 2 with D 2 (a measurement of activity for activation of dihydrogen) over the Co/SiO 2 and Co@Si 0.95 catalysts. The product of the former contained 85% HD and only 27% HD for the latter (Supplementary Fig. 33), suggesting the H 2 dissociation ability of Co@Si 0.95 was weakened because such ability was strongly related to the metallic Co. The surprising finding is that the Co@Si 0.95 with lower H 2 dissociation ability even exhibits higher CO 2 conversions than the Co/SiO 2 catalysts with high activity in H 2 activation. The sole CoO phase is known to have poor activity for the hydrogenation. Therefore, the Co@Si 0.95 catalyst with balanced phases exhibited the best performance among these samples (Supplementary Figs. 34–36). Apart from influencing the cobalt oxidation state, more silica species might block more surface sites of the Co@Si x catalysts, which would also influence the catalytic performance. These data might explain why the various Co@Si x catalysts with similar cobalt nanoparticle sizes have markedly different catalytic performance.

We conclud that the silica acts as an effective support for turning the cobalt nanoparticles from catalysts for methanation/CO formation into catalysts for methanol production, exhibiting simultaneously high activity, selectivity, and durability for the CO 2 -to-methanol transformation. Such different catalytic features compared with the conventionally supported cobalt catalysts are associated with the Co‒O‒SiO n linkage. It is reasonable to understand this linkage stabilizes the cobalt nanoparticles and hinders the sintering during the calcination/reduction/reaction under harsh conditions. For example, after reaction for 100 h, the used Co@Si 0.95 catalyst still incorporated the cobalt nanoparticles with an average diameter of 3.9 nm, which is almost unchanged compared with the as-synthesized catalyst (Supplementary Fig. 37). The Co 2 C/CoC species are undetectable on Co@Si 0.95 as confirmed by the XRD pattern and HRTEM images (Supplementary Figs. 32 and 37). In contrast, the used Co/SiO 2 contained predominantly metallic Co accompanied by Co 2 C species (Supplementary Figs. 38 and 39) after the equivalent test for 100 h, in good agreement with expectation44,45. The remarkably different phenomena of Co@Si 0.95 compared with the conventional cobalt catalysts are attributed to the Co‒O‒Si linkage on the Co@Si 0.95 catalyst, which hindered the carbonization of cobalt species46,47.

Mechanism study

In order to gain insight into how the silica modification influences the reaction pathways, we characterized the samples using IR spectroscopy in CO 2 adsorption and hydrogenation. Supplementary Fig. 40 shows the spectra of various catalysts after exposure to CO 2 , with the CoO x characterized by bands at 1260, 1530, 2850, 2945, and 3015 cm−1, assigned to carboxylate (CO 2 δ-, 1260 cm−1), formate (*HCOO, 1530, 2850, and 2945 cm−1), and *CH x species (3015 cm−1), respectively28,48,49,50. The CO 2 δ- is from the chemisorbed CO 2 species on the cobalt sites, and the *HCOO and *CH x are from the interaction of chemisorbed CO 2 with hydrogen adatom on cobalt sites resulted from the H 2 pretreatment. The *CH x species, which are known intermediates in methane formation, confirm that deep hydrogenation occurs on the CoO x catalyst50. It is significant that the *CH x band (3015 cm−1) was almost undetectable in the spectra of the Co@Si x catalysts, consistent with the suppression of deep hydrogenation of CO 2 which requires metallic sites35. The spectra further show that more silica species in Co@Si x correspond to lower intensity of *HCOO (2850 and 2945 cm−1), also being correlated with those of the chemisorbed CO 2 (CO 2 δ-, 1244‒1276 cm−1).

To identify reaction intermediates, we collected in situ DRIFTS spectra (Fig. 4a and Supplementary Figs. 41–43), bringing the catalysts in contact with feed gases having varied CO 2 and H 2 concentration at 350 °C. Exposure of Co@Si 0.95 to CO 2 without H 2 gave rise to bands, mainly including those of CO 2 δ- (1246, 1592 cm−1), CO 3 2- (1435 cm−1), and *HCOO (1337, 2850, 2945 cm−1)28. When H 2 was present (CO 2 :H 2 , molar ratio = 3), the bands of CO 2 δ- (1246, 1592 cm−1) were markedly weakened and those of *HCOO (1360, 1560 cm−1) enhanced. Simultaneously, new bands appeared at 1048, 1462, 2830, and 2928 cm−1, assigned to *CH 3 O species. Continuous feeding of H 2 (switch off CO 2 ) markedly increased the *HCOO and *CH 3 O band intensities (Fig. 4b, 0–12 min). After 12 min, the *HCOO signal was constant, but the *CH 3 O signal continued to increase. In contrast with the spectra of Co@Si 0.95 , the comparable spectra Co/SiO 2 and CoO x give evidence of only trace of *CH 3 O (in contrast to the stronger bands of *HCOO and/or *CH x species). The *CH 3 O species are readily converted into methanol by hydrogenation22,24, and the high methanol selectivity of Co@Si 0.95 is ascribed to these species as reaction intermediates. The trace of *CH 3 O signal on Co/SiO 2 and CoO x is consistent with their low methanol selectivity.

Fig. 4: Mechanism of CO 2 hydrogenation on Co@Si 0.95 catalyst. a In situ DRIFTS spectra of Co@Si 0.95 catalyst at 350 °C in contact with CO 2 + H 2 . b Time-dependent DRIFTS band intensities characterizing surface *HCOO and *CH 3 O species during the CO 2 + H 2 reaction on Co@Si 0.95 at 350 °C. c In situ C 1s XPS spectra of Co@Si 0.95 in contact with 1.2 mbar of CO 2 + H 2 atmosphere with controlled ratios at 250 °C. Full size image

Further investigation of the reaction intermediates on Co@Si 0.95 was performed with ambient-pressure (AP) XPS. Although the CO 2 and H 2 pressure was much lower than that in the practical tests, it is sufficient to react with the catalyst surface. Changes in the surface and reaction intermediates were shown by X-ray photoelectron spectra16,27,50. CO 2 is readily adsorbed on this catalyst, giving rise to C 1s bands at 293.0, 290.6, 289.2, 288.4, and 287.2 eV, assigned to gaseous CO 2 , CO 3 2-, *HCOO, CO 2 δ-, and HCO 3 - species, respectively (Fig. 4c)51. When the sample was exposed to H 2 (CO 2 :H 2 , molar ratio = 3), signals characteristic of CO 2 δ- and HCO 3 - were reduced and that of *HCOO enhanced. Concomitantly, a signal appeared at 286.3 eV and became dominant, indicating the formation of abundant *CH 3 O species. More H 2 in the feed gas (CO 2 /H 2 ratio = 1/3, molar) markedly reduced the bands of chemisorbed CO 2 (CO 2 δ- and CO 3 2-), which were quickly transformed to *CH 3 O species by feeding sufficient hydrogen, whereas the signal of *HCOO remained almost unchanged. When the feed gas was switched to pure H 2 without CO 2 , the *CH 3 O signal disappeared immediately—this species was evidently further hydrogenated to form methanol. However, the *HCOO signal remained essentially constant, as this species was resistant to hydrogenation on the catalyst (Supplementary Figs. 44 and 45). In contrast, the Co/SiO 2 catalyst was also characterized by chemisorbed CO 2 , but with extremely weak *CH 3 O bands under the equivalent conditions (Supplementary Fig. 46), in good agreement with the DRIFTS spectra. These data confirm the importance of the silica-supported species containing cationic cobalt for *CH 3 O formation and stabilization, even when the reaction atmosphere contains only little H 2 (CO 2 /H 2 , molar ratio = 10:1, Supplementary Figs. 47 and 48).

The easily detected abundant *CH 3 O signals in the in situ DRIFTS and XPS characterization confirm the fast formation and slow further transformation of *CH 3 O on the Co@Si x catalyst (Supplementary Figs. 49 and 50). Apart from the hydrogenation to *CH 3 OH, the *CH 3 O species might also undergo C‒O cleavage and the subsequent hydrogenation to CH 4 24,28,52, as well as the dehydrogenation to CO. With regard to the *CH 3 O transformation, multiple reaction pathways have been proposed in the formation of *CH x intermediates, that are ready to proceed the methanation50. In this route, the C‒O cleavage is always regarded to be the rate controlling step24,28,52. Reported density functional theoretical calculations have revealed that the cleavage of the C‒O bond in *CH 3 O requires the metallic Co surface or the CoO surface with abundant oxygen vacancies. The CoO(100) surface saturated with oxygen leads to a high energy barrier for the *CH 3 O dissociation at 2.71 eV [1.45 eV for Co(111) surface and 1.01 eV for the oxygen vacancy-rich CoO(100)]35. The Co@Si 0.95 catalyst with not-easy-to-reduce oxygen species provided an ideal catalyst surface for hindering the C‒O cleavage. In addition, the C‒O bond cleavage is known to be assisted by hydrogen53, and the relatively lower activity of Co@Si 0.95 for dihydrogen activation (Supplementary Fig. 33) might also contribute to stabilization of *CH 3 O intermediates to avoid C‒O cleavage. Apart from the *CH 3 O decomposition, another possible route for methane or other higher hydrocarbons formation is via the direct CO dissociation into *C species, which has been experimentally and theoretically studied in the cobalt-catalyzed Fischer‒Tropsch synthesis31,54. Metallic cobalt and cobalt carbide were found to the efficient for CO dissociation, but the oxidized cobalt surface is known to be less active, which is also confirmed by the poor activity of Co@Si 0.95 in the CO hydrogenation (CO conversion of 0.7% and methanol selectivity of 22.7%) under the employed reaction conditions (360 °C, 2.0 MPa, Supplementary Fig. 51).