ZnO_x overlayer confined on ZnCr₂O₄ spinel for direct syngas conversion to light olefins

Abstract

ZnCrO_x oxides coupled with zeolites (OXZEO) allow direct conversion of syngas into light olefins, while active sites in the composite oxides remain elusive. Herein, we find that ZnO particles physically mixed with ZnCr₂O₄ spinel particles can be well dispersed onto the spinel surfaces by treatment in syngas and through a reduction-evaporation-anchoring mechanism, forming monodispersed ZnO_x species with uniform thickness or dimension on ZnCr₂O₄ up to a dispersion threshold ZnO loading of 16.0 wt% (ZnCr₂O₄@ZnO_x). A linear correlation between CO conversion and surface ZnO loading clearly confirms that the ZnO_x overlayer on ZnCr₂O₄ acts as the active structure for the syngas conversion, which can efficiently activate both H₂ and CO. The obtained ZnCr₂O₄@ZnO_x catalyst combined with SAPO-34 zeolite achieves excellent catalytic performance with 64% CO conversion and 75% light olefins selectivity among all hydrocarbons. Moreover, the ZnO_x overlayer is effectively anchored on the ZnCr₂O₄ spinel, which inhibits Zn loss during the reaction and demonstrates high stability over 100 hours. Thus, a significant interface confinement effect is present between the spinel surface and the ZnO_x overlayer, which helps to stabilize ZnO_x active structure and enhance the catalytic performance.

Introduction

Syngas conversion to value-added chemicals stands as a central industrial process in the efficient utilization of coal, natural gas, and biomass resources^1,2,3,4. The oxide–zeolite (OXZEO) process, utilizing oxides and zeolites as bifunctional catalysts for direct syngas conversion, overcomes the limitations imposed by the Anderson–Schulz–Flory distribution inherent to the traditional Fischer–Tropsch synthesis process^5,6,7,8. By optimizing the oxide and zeolite components, it is possible to selectively obtain high value-added chemical feedstocks^9,10,11,12, including light olefins, gasoline, and aromatics¹⁰. Many oxide catalysts are reported for conversion of syngas to light olefins in the OXZEO process, including ZnO¹³, ZnCrO_x^5,14, ZnZrO_x⁷, ZnO/ZrO₂¹⁵, ZnAlO_x^16,17, MnO_x¹⁸, ZnGaO_x¹⁹, and MnGaO_x²⁰. Among them, ZnCrO_x exhibits excellent ability for the activation of CO/H₂ and is demonstrated as one of the most active oxides^10,21.

Although ZnCrO_x is widely considered crucial for controlling overall activity^6,22, the active phase remains a subject of ongoing debate^10,21,23. Using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM), Jiao et al. confirmed that the ZnCrO_x composite oxide possessed a typical spinel structure, which was regarded as the active phase⁶. Song et al. hypothesized that an amorphous ZnO overlayer on the ZnCr₂O₄ spinel surface acted as the active phase for syngas conversion to methanol²². Similarly, Gao et al. conjectured that Zn enrichment on the ZnCr₂O₄ spinel surface was key to syngas conversion to isobutanol^24,25. Recently, Wang et al. discovered that ZnO–ZnCr₂O₄ interface exhibited low activation energy for syngas conversion to light olefins and was beneficial for the high catalytic performance²⁶. Chen et al. conducted CO activation studies on ZnCr₂O₄ samples using scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS), finding that ZnCr₂O₄ coated with a ZnO thin layer favors direct CO dissociation²¹. Despite significant progresses in understanding surface active phases, precise construction of the active structure and well control of structural evolution during reaction remain core challenges.

Surface oxygen vacancy (O_v) has been proposed to be active for both CO and H₂ activation on ZnCrO_x catalysts^22,27. Jiao et al. found that the spinel ZnCr₂O₄(111) surface could be reduced by CO or H₂, and the adsorption of CO and H was significantly enhanced with the increase in O_v concentration⁶. Lai et al. reported that two-coordinate O vacancy site served as the active site for CO activation and hydrogenation reaction, leading to formation of CHO, CH₂O, CH₂ and CH₂CO intermediates²⁸. Fu et al. demonstrated that reduced Cr-terminated ZnCr₂O₄(111) with a 50% O_v concentration exhibited higher activity for ketene (CH₂CO) production compared to methanol and methane²⁹. Despite these intensive studies, the active site of CO hydrogenation and the role of O_v/surface ZnO species on ZnCrO_x catalysts still need further exploration, particularly for real catalysts.

The structural complexity of ZnCrO_x composite oxide makes it challenging to experimentally identify active surface structures³⁰, further obscuring the precise identification of CO and H₂ reaction sites. Therefore, it is crucial to design real catalysts with well-defined surface and interface structures and understand their roles in the reaction. Accordingly, precise construction of high density of surface active sites allows to improve the catalytic performance. Herein, we utilize a vapor phase migration process to disperse ZnO onto the ZnCr₂O₄ spinel surface, effectively creating monodispersed ZnO_x species on ZnCr₂O₄ surface. A reduction-evaporation-anchoring mechanism is established using various characterization methods. We have identified the monodispersed ZnO_x species confined on ZnCr₂O₄ as the active sites for syngas conversion to light olefins. The active monodispersed ZnO_x species can activate both H₂ and CO, and further demonstrate excellent chemical and catalytic stability through the interface confinement effect of the oxide support.

Results

Syngas-induced ZnO dispersion onto ZnCr₂O₄ surface

ZnCr₂O₄ was synthesized using traditional co-precipitation method, as detailed in the experimental procedures. XRD pattern and N₂ adsorption–desorption analysis (Fig. S1) confirm the formation of a pure spinel phase with a mesoporous structure for the obtained powders^31,32. The crystallite size calculated using the Scherrer equation based on ZnCr₂O₄(311) plane is 5.9 nm and the specific surface area determined by the Brunauer–Emmett–Teller (BET) method is 133.0 m²·g^–1 (Table S1). Commercial ZnO and prepared ZnCr₂O₄ particles were physically mixed to obtain a ZnCr₂O₄-ZnO sample, containing 8.0 wt% ZnO. Diffraction peaks associated with wurtzite ZnO are clearly observed in XRD pattern from the mixture, as shown in Fig. 1a. ZnCr₂O₄-ZnO was then treated with a syngas (CO: H₂ = 1: 2.5) flow at 450 °C and 0.1 MPa, which was investigated by in situ XRD (Fig. 1a). The results show that the ZnO(10$\bar{1}$0) diffraction peak gradually gets weakened during the process and eventually only the diffraction peaks of ZnCr₂O₄ spinel are evident. Inductively coupled plasma optical emission spectroscopy (ICP-OES) result (Table S2) reveals that Zn content does not change after the treatment. Thus, it is suggested that ZnO particles become highly dispersed after the syngas treatment.

Fig. 1: Structural change of ZnCr₂O₄-ZnO mixture induced by syngas treatment.

a In situ XRD of ZnCr₂O₄-ZnO containing 8.0 wt% ZnO during syngas treatment at 450 ^oC for various time. The bottom black pattern is from the sample before the treatment and the top red pattern from the treated one for 6 h. b Quasi in situ XPS spectra of ZnCr₂O₄-ZnO, ZnCr₂O₄@ZnO_x, fresh ZnCr₂O₄ (named ZnCr₂O₄-F), and treated ZnCr₂O₄ (named ZnCr₂O₄-T). c UV-Vis spectra of ZnCr₂O₄-ZnO, ZnCr₂O₄@ZnO_x, ZnCr₂O₄-F, and ZnCr₂O₄-T. EDS mapping images of ZnCr₂O₄-ZnO (d) and ZnCr₂O₄@ZnO_x (e). Scale bar: 400 nm.

Quasi in situ X-ray photoelectron spectroscopy (XPS) analysis (Fig. 1b) and Auger spectra of Zn LMM (Fig. S2) indicate that Zn species on the treated ZnCr₂O₄-ZnO sample surface remains at Zn²⁺ state^33,34 and even exhibits a significant increase in the Zn 2p peak intensity. This further proves that ZnO species are dispersed onto the ZnCr₂O₄ surface (denoted as ZnCr₂O₄@ZnO_x). Ultraviolet-visible (UV-Vis) spectra show that the typical absorption band at 370 nm from bulk ZnO structure^35,36 cannot be observed in the ZnCr₂O₄@ZnO_x sample (Fig. 1c), excluding the existence of three-dimensional ZnO aggregates. In contrast, no changes have been observed in the XPS (Fig. 1b) and UV-Vis (Fig. 1c) spectra after the syngas treatment of the pure ZnCr₂O₄ sample. Energy dispersive spectroscopy (EDS) mapping result (Fig. 1d) shows obvious separated ZnO aggregates in ZnCr₂O₄-ZnO. However, the spatial distribution of Zn and Cr elements in ZnCr₂O₄@ZnO_x is very uniform. All the above results convincingly confirm that ZnO species in the oxide mixture dynamically migrate and then highly disperse onto the ZnCr₂O₄ surface after the syngas treatment.

Migration-dispersion mechanism and dispersion threshold

Gray deposits identified as Zn metal by XRD form on the inner wall of the quartz tube after the same syngas treatment of pure ZnO, suggesting that ZnO can be reduced and then Zn evaporates away from ZnO particles. When ZnCr₂O₄ sample is placed beneath ZnO and separated by an inert layer of quartz wool (ZnO | |ZnCr₂O₄), no Zn metal deposits on the quartz tube wall are observed, indicating that ZnCr₂O₄ may capture the evaporated Zn metal species (Fig. S3). To further study the dispersion mechanism in this syngas treatment process, we placed ZnCr₂O₄ particles beneath Zn metal powder and separated them by an inert layer of quartz wool (Zn | |ZnCr₂O₄). For the Zn | |ZnCr₂O₄ sample, H₂ evolution is observed to occur predominantly above 300 °C when heated in an Ar flow, with a notable increase beyond 430 °C (Fig. 2a). However, no H₂ generation is observed upon heating ZnO | |ZnCr₂O₄ in Ar. This indicates that Zn vapor from the Zn metal can migrate in the heated Ar flow and further react with surface OH on ZnCr₂O₄ to produce gaseous H₂ and surface ZnO_x species³⁷. When the ZnO | |ZnCr₂O₄ sample is exposed to a CO flow, H₂ generation is also observed at 400 °C (Fig. S4). The control experiments suggest that ZnO needs to be first reduced in reductive atmospheres, which then allows the further evaporation of Zn into the gas phase. Fourier transform infrared (FTIR) result (Fig. 2b, the whole spectra are shown in Fig. S5) confirms that surface OH groups on the treated ZnCr₂O₄ are strongly consumed after the migration and dispersion of ZnO_x species³⁷.

Fig. 2: Dispersion mechanism and dispersion threshold of ZnO onto ZnCr₂O₄.

a H₂ and H₂O evolution upon temperature-programmed heating of ZnCr₂O₄ packed below Zn metal or ZnO, separated by an inert layer of quartz wool with the comparison of ZnCr₂O₄-ZnO (same as ZnO-ZnCr₂O₄) and ZnCr₂O₄. b FTIR spectra of OH groups on treated ZnCr₂O₄ and ZnCr₂O₄@ZnO_x samples with various ZnO loadings. c Schematic illustrating formation mechanism of ZnCr₂O₄@ZnO_x during syngas treatment. d Surface OH amount on ZnCr₂O₄@ZnO_x as a function of ZnO loading. e Dispersion threshold of ZnO on ZnCr₂O₄ determined by XRD and XPS.

The ZnCr₂O₄-ZnO samples have been treated in various gases including Ar, O₂, H₂, and CO. The similar dispersion of ZnO particles happens in H₂ and CO flows but not in O₂ and Ar flows (Fig. S6). Therefore, ZnO particles should be first reduced to Zn metal in reductive gases e.g. CO, H₂ and syngas, which then migrate through the gas phase and are further anchored by OH sites on ZnCr₂O₄ surface. Overall, a reduction-evaporation-anchoring mechanism is suggested for the dispersion of ZnO onto ZnCr₂O₄ upon the syngas treatment as shown in Fig. 2c.

D₂ temperature programmed reaction (D₂-TPR) experiments were conducted to determine surface OH³⁸ of ZnCr₂O₄@ZnO_x catalysts with different ZnO loadings (Fig. S7). The quantitative results shown in Fig. 2d indicate that the OH content gradually decreases with the increasing dispersed ZnO loading, which is consistent with the FTIR results (Fig. 2b). However, when ZnO loading exceeds 15% the OH content no longer changes with further increasing ZnO loading. Meanwhile, trace amounts of Zn are found to deposit onto the inner wall of the quartz tube (Fig. S8). It can be inferred that there is a limit to the amount of Zn which can be anchored and dispersed on the ZnCr₂O₄ surface.

ZnCr₂O₄@ZnO_x catalysts prepared with different ZnO loadings were systematically studied using XRD and XPS (Fig. S9). With ZnO loading below 15.0 wt%, no XRD peaks corresponding to any ZnO phases are observed, confirming that all ZnO species are well dispersed on ZnCr₂O₄. Concurrently, Zn/Cr atomic ratio calculated from Zn 2p_3/2 and Cr 2p XPS spectra increases linearly with the ZnO loading (Fig. 2e). Above 15.0 wt%, diffraction peaks of hexagonal ZnO appear (Fig. S9) and their intensity increases linearly with the ZnO loading. Meanwhile, ZnO particles are observed in EDS mapping images (Fig. S10). The plot of Zn/Cr atomic ratio versus ZnO loading shows a turning point at 16.0 wt%, which also corresponds to an intercept at x-axis of the fitted straight line of the plot of XRD A_ZnO/A_ZnCr2O4 (the peak area ratio of the ZnO(10$\bar{1}$0) vs. ZnCr₂O₄(400) planes) as a function of the ZnO loading. The XPS and XRD results suggest that the growth of ZnO_x overlayer on ZnCr₂O₄ driven by the syngas treatment follows a characteristic monolayer-dispersion behavior^39,40 at low ZnO loading and the dispersion threshold of ZnO is 16.0 wt% (1.9 mmol ZnO·100 m⁻² ZnCr₂O₄).

Surface structure of the ZnCr₂O₄@ZnO_x catalysts

HRTEM was used to identify dispersed ZnO species on the ZnCr₂O₄ spinel surface. As shown in Fig. 3a, ZnCr₂O₄@ZnO_x-15, meaning the ZnCr₂O₄@ZnO_x catalyst with a ZnO loading of 15.0 wt%, exhibits the main ZnCr₂O₄ spinel phase and 0.3 nm thick amorphous overlayer. The distribution of Zn and Cr elements in ZnCr₂O₄@ZnO_x-15 was further analyzed using EELS with mapping and line scanning functions (Fig. 3b, c). Both Zn and Cr elements are uniformly distributed in the bulk region but the outermost amorphous layer is mainly composed of Zn element (Fig. 3b). For ZnCr₂O₄@ZnO_x-30, ZnCr₂O₄ spinel particles are completely covered by ZnO_x overlayer, and meanwhile isolated ZnO particles are also observed (Fig. 3d). The image data demonstrate that the ZnO_x overlayer with a thickness close to that of a ZnO monolayer can fully cover the ZnCr₂O₄ spinel when the ZnO loading exceeds 16.0 wt% (Fig. 3d, e), which is consistent with the monolayer dispersion behavior of ZnO on the ZnCr₂O₄ spinel surface given in Fig. 2e. Accordingly, we propose that ZnO_x species are highly dispersed on the ZnCr₂O₄ spinel surface with uniform thickness or dimension when ZnO loading is below the dispersion threshold, which is described as monodispersed ZnO_x overlayer in this work. Above 16.0 wt% both a complete monodispersed ZnO_x layer and additional ZnO particles form on the spinel surface.

Fig. 3: Surface structure of ZnCr₂O₄@ZnO_x catalysts.

a HRTEM images of ZnCr₂O₄@ZnO_x−15. Scale bar: 2 nm. b STEM-EELS mapping images of ZnCr₂O₄@ZnO_x−15. Scale bar: 2 nm. c Elemental composition of Zn and Cr obtained from the line-scan along the yellow line in (b) by EELS. d STEM-EELS mapping images of ZnCr₂O₄@ZnO_x−30. Scale bar: 20 nm. e Elemental composition of Zn and Cr obtained from the line-scan along the yellow line in (d) by EELS. f HS-LEIS spectra of ZnCr₂O₄@ZnO_x catalysts.

High sensitivity-low energy ion scattering (HS-LEIS) technique, sensitive to the surface elements (escape depth of ~0.3 nm)^41,42,43, was employed to study the outermost structure of ZnCr₂O₄@ZnO_x. The results shown in Fig. 3f indicate that the surface of stoichiometric ZnCr₂O₄ spinel is almost entirely Cr exposed, which is consistent with the results of Wachs et al⁴². When a small amount of ZnO is dispersed onto ZnCr₂O₄, the surface Zn content on ZnCr₂O₄@ZnO_x-5 increases while the surface Cr content decreases. The surfaces of ZnCr₂O₄@ZnO_x-15 and ZnCr₂O₄@ZnO_x-30 are primarily composed of Zn elements. In addition, the Cr/Zn element ratio on these three ZnCr₂O₄@ZnO_x surfaces gradually increases with increasing He⁺ sputtering cycles (Fig. S11), particularly for ZnCr₂O₄@ZnO_x-15 and ZnCr₂O₄@ZnO_x-30. Apparently, the amorphous ZnO_x overlayer at the outmost surface can be gradually etched away, leading to the progressive exposure of Cr from the spinel support, which is consistent with the EELS line scanning results (Fig. 3c, e).

Catalytic performance and active sites of ZnCr₂O₄@ZnO_x catalysts

The ZnCr₂O₄@ZnO_x catalysts coupled with SAPO-34 were evaluated for syngas conversion to light olefins at 4.0 MPa and 400 °C. Figure 4a shows that CO conversion on stoichiometric ZnCr₂O₄ spinel is 45%, with a selectivity of 57% for light olefins. The surface Cr species in ZnCr₂O₄ tend to promote excessive hydrogenation of light olefins presenting lower selectivity. Notably, the addition of 5.0 wt% ZnO species boosts CO conversion to 51% and elevate light olefin selectivity to 70%, suggesting that the monodispersed ZnO_x species effectively enhance both selectivity and overall activity. In contrast, the same syngas treatment of pure ZnCr₂O₄ spinel does not affect its performance (Fig. S12). Further, the dispersed ZnO_x overlayers over Cr₂O₃ and γ-Al₂O₃ show quite poor catalytic performance for the syngas conversion to light olefins (Fig. S13), indicating that the performance enhancement of ZnCr₂O₄@ZnO_x is not due to any changes in ZnCr₂O₄ but rather results from the formation of the monodispersed ZnO_x overlayer over ZnCr₂O₄ spinel. Interestingly, CO conversion of ZnCr₂O₄@ZnO_x increases linearly with ZnO loading, reaching a maximum of 64% at a ZnO loading of 15.0 wt%. Beyond this optimal loading, there is a modest decline in CO conversion, which is predominantly attributed to the dilution effect caused by the presence of low-activity ZnO particles. The linear positive correlation between ZnO loading (<15.0 wt%) and CO conversion clearly indicates that the active site on ZnCr₂O₄@ZnO_x should be related to the content of surface ZnO_x species dispersed on the ZnCr₂O₄ spinel.

Fig. 4: Catalytic performance and surface chemistry of ZnCr₂O₄@ZnO_x catalysts.

a Catalytic performance of ZnCr₂O₄ and ZnCr₂O₄@ZnO_x catalysts coupled with SAPO-34. Reaction conditions: 0.20 g ZnCr₂O₄@ZnO_x + 0.10 g SAPO-34, 68% H₂ + 27% CO + 5% Ar, 4.0 MPa, 15 mL·min⁻¹, 40–60 mesh, 400 °C. CO₂ selectivity does not change with the ZnO loading and is in the range of 38–41% for each catalyst. b Relationship between surface O_v concentration, amount of adsorbed CO, and ZnO loading of ZnCr₂O₄@ZnO_x catalysts. c Relationship between HD formation rate and ZnO loading of ZnCr₂O₄@ZnO_x catalysts. The pretreated ZnCr₂O₄@ZnO_x catalysts were exposed to a flow of 10% D₂, 10% H₂, and 80% Ar, and the formation of HD species was detected at 400 °C. d Scheme of the active sites on ZnCr₂O₄@ZnO_x catalysts.

Oxygen vacancies on the catalyst surfaces were analyzed using O₂-pulse experiments^44,45. Specifically, the catalysts were treated in syngas at 450 °C and subsequently tested for oxygen adsorption capacity using pulse O₂ gas at 120 °C (Fig. S14). The ZnCr₂O₄ spinel exhibits a significant oxygen adsorption capacity of 0.21 mmol O₂ per gram of sample. Quasi in situ XPS results (Fig. S15) show no significant change in Cr 2p spectra of ZnCr₂O₄ following the O₂ pulse, suggesting that the O₂ consumption is mainly due to capture by O_v rather than oxidation of Cr³⁺ to Cr⁶⁺ on ZnCr₂O₄. Quantitative results show that the concentration of O_v on the ZnCr₂O₄ surface is 0.42 mmol·g^–1, and it significantly decreases with the addition of monodispersed ZnO_x species, demonstrating a negative correlation between surface O_v concentration and coverage of surface ZnO_x species. Furthermore, O_v concentration of ZnCr₂O₄@ZnO_x stays constant at 0.15 mmol·g^–1 when ZnO loading exceeds 16.0 wt%.

To determine the influence of ZnO loading on CO adsorption capacity, the ZnCr₂O₄@ZnO_x catalysts were investigated by CO temperature-programmed desorption (CO-TPD). The primary desorbed species, identified as CO and CO₂ (Fig. S16), were quantified, as presented in Fig. 4b. Additionally, carbonate species from the adsorption of generated CO₂ are observed in the infrared spectra^46,47 (Fig. S17). Notably, a positive linear relationship is observed between the adsorbed CO amount and ZnO loading with ZnO loading below 16.0 wt%. However, once ZnO loading exceeds 16.0 wt%, further increase in ZnO loading results in a slight decrease in the adsorbed CO amount. This indicates that CO adsorption capacity of ZnCr₂O₄@ZnO_x is closely related to the monodispersed ZnO_x overlayer rather than the surface O_v.

To investigate H₂ activation on ZnCr₂O₄ and ZnCr₂O₄@ZnO_x catalysts, we conducted H₂ − D₂ exchange experiments. HD formation rate quantitatively reflects H₂ activation ability of catalysts¹⁷. As shown in Fig. 4c stoichiometric ZnCr₂O₄ exhibits the highest HD formation rate, while the rate for ZnCr₂O₄@ZnO_x decreases gradually as the ZnO loading increases. Once the dispersion threshold of 16.0 wt% is exceeded, HD formation rate for ZnCr₂O₄@ZnO_x remains almost unchanged. This suggests that the exposed Cr species on the ZnCr₂O₄ surface have a stronger ability for HD formation under this reaction condition in comparison with monodispersed ZnO_x overlayer on ZnCr₂O₄@ZnO_x. Additionally, in contrast with heterolytic dissociation of D₂ on bulk ZnCr₂O₄ and ZnO^48,49,50, D₂ dissociation by the monodispersed ZnO_x overlayer tends to proceed via homolytic route, forming more stable Zn−D species at higher temperature (Fig. S18) and promoting hydrogenation of CO^51,52. The negative correlation between CO conversion and O_v concentration but the positive correlation between CO conversion and dispersed ZnO loading imply that monodispersed ZnO_x species rather than O_v from ZnCr₂O₄ spinel serve as the key active sites for the syngas conversion (Fig. 4d).

Interface confinement effect on the ZnO_x overlayer

It is known that ZnO as an amphoteric oxide can be dissolved in (NH₄)₂CO₃ solution⁵³. XRD and ICP-OES analysis experiments were conducted on ZnCr₂O₄@ZnO_x catalysts with ZnO loading above 16.0 wt% before and after leaching with an (NH₄)₂CO₃ solution. After the leaching process, no ZnO diffraction peaks are detected in all XRD patterns (Fig. 5a). Moreover, ICP-OES results reveal that the leached catalysts all retain a ZnO amount close to 16.0 wt% (Fig. 5b). The results suggest that the ZnO_x overlayer is strongly confined on the ZnCr₂O₄ spinel, which can withstand chemical leaching in the (NH₄)₂CO₃ solution. In contrast, the excess ZnO particles are all removed by the leaching. Notably, ICP-OES analysis of the leaching filtrate shows an absence of Cr element and thus (NH₄)₂CO₃ solution only selectively etches ZnO particles but not ZnCr₂O₄ support.

Fig. 5: Stability of confined ZnO_x overlayer and its effect on syngas conversion.

a XRD patterns of ZnCr₂O₄@ZnO_x-30 catalysts before and after leaching with (NH₄)₂CO₃ solution (named as ZnCr₂O₄@ZnO_x-30-L). b ICP-OES results of ZnO content in ZnCr₂O₄@ZnO_x before and after leaching with (NH₄)₂CO₃ solution. c Catalytic performance of ZnCr₂O₄@ZnO_x-30 and ZnCr₂O₄@ZnO_x-30-L catalysts coupled with SAPO-34. Reaction conditions: 0.20 g ZnCr₂O₄@ZnO_x + 0.10 g SAPO-34, 68% H₂ + 27% CO + 5% Ar, 4.0 MPa, 25 mL·min⁻¹, 40–60 mesh, and 400 °C. d Catalytic performance versus time on stream (TOS) of ZnCr₂O₄@ZnO_x-15 and ZnCr₂O₄@ZnO_x-30 catalysts coupled with SAPO-34. Reaction conditions: 0.20 g ZnCr₂O₄@ZnO_x + 0.10 g SAPO-34, 68% H₂ + 27% CO + 5% Ar, 4.0 MPa, 25 mL·min⁻¹, 60–80 mesh, and 400 °C.

These ZnCr₂O₄@ZnO_x catalysts, both post-leaching and untreated, were evaluated for syngas conversion to light olefins under conditions of 4.0 MPa and 400 °C (Fig. 5c). For the ZnCr₂O₄@ZnO_x-30-L catalyst with the leaching treatment, both CO conversion and selectivity of light olefins are only slightly reduced compared to the ZnCr₂O₄@ZnO_x-30 catalyst. This further substantiates that the active site on ZnCr₂O₄@ZnO_x surface is the confined ZnO_x overlayer but not those excess ZnO particles. Long-term reactions were conducted over ZnCr₂O₄@ZnO_x-15 and ZnCr₂O₄@ZnO_x-30 catalysts, as depicted in Fig. 5d. For both catalysts, CO conversion exhibits a slight decrease at the onset of the reaction and then remains stable (Fig. S19). For the ZnCr₂O₄@ZnO_x-15 catalyst the selectivity of light olefins decreases from 79% to 76% after 100-hour reaction. In contrast, the light olefins selectivity of the ZnCr₂O₄@ZnO_x-30 catalyst decreases from 77% to 67%. In this catalyst excess ZnO particles are negative for the long-term reaction since these unconfined bulk ZnO can migrate into the interior of SAPO-34 and lower the selectivity⁵⁴. Overall, the monodispersed ZnO_x species confined by the ZnCr₂O₄ surface exhibits high stability against chemical leaching in solution and Zn loss during syngas reaction, which catalyzes the syngas conversion effectively and presents confinement enhanced performance.

Discussion

Syngas treatment of the mixture of ZnO and ZrCr₂O₄ particles induces dispersion of ZnO and formation of ZnO_x overlayer onto the ZnCr₂O₄ spinel. In this process, ZnO is reduced to Zn metal, which then migrates through gas phase and is re-oxidized by surface OH groups of ZnCr₂O₄, forming monodispersed ZnO_x species with a dispersion threshold of 16.0 wt%. These ZnO species cover the ZnCr₂O₄ spinel surface in the form of ZnO_x monolayer with a thickness of 0.3 nm as the ZnO loading is close to 16.0 wt%. The catalyst with high density of surface active sites can be well constructed through this unique reduction-evaporation-anchoring process and the roles of these surface sites can be clearly understood, which holds significant promise for preparation of well-defined oxide catalysts supported on oxides.

The formed ZnCr₂O₄@ZnO_x combined with SAPO-34 zeolite achieves 64% CO conversion and 75% light olefins selectivity in syngas conversion to light olefins. A direct linear correlation between CO conversion and surface ZnO loading is clearly identified, confirming the active site of the monodispersed ZnO_x species trapped on the ZnCr₂O₄ surface. The ZnO_x overlayer is effectively confined on the ZnCr₂O₄ spinel, which inhibits excessive reduction of ZnO and loss of Zn during the reaction, demonstrating high stability over 100 hours. The mechanism of H₂/CO activation at the active sites has been elucidated on the oxide catalysts. The monodispersed ZnO_x overlayer contributes to the stabilization of H species and promotes the adsorption and activation of CO, thereby offering active sites for efficient syngas conversion. Our work emphasizes the importance of atomic level analysis of surface active sites in practical catalytic systems, identifying the reaction mechanism from a surface science study perspective. Moreover, the current understanding of active sites and reaction mechanisms can inspire further researches on syngas conversion over other composite oxide systems, offering crucial guidance for the rational design and optimization of efficient oxide catalysts.

Methods

Catalyst synthesis and treatment

ZnCr₂O₄ spinel oxide was synthesized by conventional co-precipitation method. Analytical reagent grade (99.7%) Zn(NO₃)₃·6H₂O and Cr(NO₃)₃·9H₂O were used as the precursors. 8.96 g Zn(NO₃)₃·6H₂O and 24.01 g Cr(NO₃)₃·9H₂O were dissolved into 60 mL distilled deionized water to get stable solutions. Afterward, an ammonium carbonate (4.6 mol·L^–1) aqueous solution was dripped slowly into the mixed liquid until the pH value reached 8 under the condition of continuous stirring at 70 °C. After stirring for 3 h, the obtained mixtures were cooled down to room temperature, and the navy precipitate was washed three times with distilled deionized water. The precipitate was dried via vacuum drying at 70 °C for 12 h, which was followed by calcination at 500 °C in air for 1 h with a heating rate of 2 °C·min^–1. The molar composition of ZnCr₂O₄ was determined by ICP-OES, and the Zn/Cr ratio of the obtained sample conformed to the stoichiometric ratio (Table S1). The wurtzite ZnO sample is commercially available (99.9%, Aladdin reagent (Shanghai) co. LTD) and used without further purification. The mixtures of ZnO and ZnCr₂O₄ with a specific content were put into the sample bottle and shaken for 6 h, and the obtained sample is noted as ZnCr₂O₄-ZnO-x, x means x wt% of ZnO in the mixture. The ZnCr₂O₄@ZnO_x-x sample was gained from a treatment of ZnCr₂O₄-ZnO-x by syngas (68% H₂ + 27% CO + 5% Ar) flow with a pressure of 0.1 MPa at 450 °C for 6 h.

SAPO-34 was synthesized by a hydrothermal method. In detail, 7.46 g pseudo boehmite (82 wt% Al₂O₃) was added into 54 mL H₂O and stirred for 0.5 h. Then, 1.80 g silica sol (30 wt% SiO₂), 13.84 g orthophosphoric acid (85 wt% H₃PO₄) and 18.22 g triethylamine were added to the mixture in turn and stirred separately for 0.5 h. Finally, 0.31 g SAPO-34 seeds (NKF-9, Nankai University Catalyst Co., Ltd.) were added, and the mixtures were placed in a Teflon-lined autoclave and heated in a rotating oven at 180 °C under autogenic pressure for 48 h. After crystallization, the samples were subjected to centrifugal separation, washing, and drying for over 12 h at 120 °C, and final calcination for 3 h at 600 °C. The sample was subjected to XRD, X‐ray fluorescence, N₂ adsorption–desorption analysis, and NH₃ temperature‐programmed desorption (Fig. S20 and Table S3). The results were consistent with the reported literature⁵⁴.

The ZnO-containing catalyst is selectively leached in ammonium carbonate solution. In detail, 1.0 g oxide catalyst was added to 40 ml ammonium carbonate solution (1 mol·L⁻¹) under the condition of continuous stirring at 40 °C. Next, the mixture was filtered and washed several times with pure water. The obtained precipitate was dried via vacuum drying at 70 °C for 12 h. The powder was treated with syngas under conditions before use, and noted as ZnCr₂O₄@ZnO_x-x-L.

Catalyst characterization and syngas conversion reaction evaluation

X-ray diffraction (XRD) patterns were collected on an Empyrean diffractometer (Empyrean-100) using Cu Kα (λ = 1.5406 Å) radiation source and a scanning rate of 10 °·min^–1. In situ XRD measurements were conducted on the same instrument, and the reaction cell was controlled using an XRK900 controller. The catalysts were heated in an Ar flow, and syngas (68% H₂ + 27% CO + 5% Ar) was introduced at a flow rate of 30 mL·min^–1 when the temperature was 450 °C. XRD patterns were recorded from 25 to 50 ° versus holding time.

The textural properties of the samples were measured by N₂ adsorption–desorption on an automated gas sorption analyzer (Autosorb iQ2 and Quadrasorb evo) at liquid nitrogen temperature (−196 °C) to analyze the SAPO-34 zeolite and oxide catalyst. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method, and the pore size distribution was derived from the adsorption isotherm using the Barrett-Joyner-Halenda (BJH) method. Prior to analysis, the samples were degassed under vacuum at 300 °C for 4 h.

The contents of Zn and/or Cr were determined by inductively coupled plasma optical emission spectrometry (ICP-OES).

Ex situ, quasi in situ X-ray photoelectron spectra (XPS) and X-ray-excited Auger spectra were acquired with a spectrometer equipped with a Mg Kα X-ray source operated at 200 W. The background pressure was in the range of 10⁻⁹ mbar. Ex situ treatments were performed in a high-pressure reactor (SPECS, HPC-20). All quasi in situ XPS spectra were collected under ultra-high vacuum (UHV, 10⁻⁹–10⁻¹⁰ mbar) at room temperature. To prevent contact with O₂ and H₂O in air, the catalysts treated with syngas were transferred to a glove box through an airtight reaction tube. Subsequently, the catalysts were transferred from the glovebox to the XPS system using a mobile transfer chamber. The sample was kept in Ar during the entire transfer process to ensure no exposure to air.

Ultraviolet-visible (UV-vis) spectra were recorded on a Lambda 950 (Perkin Elmer) operated in diffuse reflectance mode.

Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) element mapping images were acquired on a JSM-7900F instrument operated at 5 and 15 kV.

Ar temperature-programmed heating experiments were conducted on an AutoChem II 2920 equipped with a GSD 350 OmniStar mass spectrometer. The ZnCr₂O₄ particles were placed beneath Zn metal or ZnO powders, separated by an inert layer of quartz wool (Zn | |ZnCr₂O₄ or ZnO | |ZnCr₂O₄). Typically, the sample was pretreated with Ar at 300 °C for 1 h to remove H₂O, then cooled to room temperature. The experiment was initiated to monitor the changes in the signals of H₂ (m/z = 2) and H₂O (m/z = 18) as the temperature was increased from 30 to 500 °C at a rate of 10 °C·min^–1 in an Ar flow. Similar experiments were conducted for oxide samples (ZnO, ZnCr₂O₄, ZnO-ZnCr₂O₄, ZnO | |ZnCr₂O₄) in different reducing atmospheres, i.e., H₂/Ar and CO/Ar, to monitor the changes in H₂ (m/z = 2), H₂O (m/z = 18), CO (m/z = 28), and CO₂ (m/z = 44) during the temperature-programmed process.

D₂ temperature-programmed reduction (D₂-TPR) was conducted on a homemade reactor equipped with a GSD 350 OmniStar mass spectrometer. The sample (0.10 g) was loaded in a quartz tube and then pretreated with a 68% H₂/Ar flow at 450 °C for 6 h, and then switched to an Ar flow for 1 h. The pretreated sample was cooled to room temperature under an Ar flow and then reduced by a 10% D₂/Ar flow. The experiment was initiated to monitor changes in the signals of D₂ (m/z = 4) as the temperature was increased from 30 to 600 °C at a rate of 10 °C·min^–1.

High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images, annular bright field (ABF) images, and energy dispersive spectroscopy (EDS) elemental distribution of the catalysts were obtained using JEOL JEM-F200 and JEM-ARM300 microscopes. Electron energy loss spectroscopy (EELS) elemental distribution of the catalysts was acquired on a JEM-ARM300.

High-sensitivity low-energy ion scattering (HS-LEIS) spectra were obtained using an Ion-TOF Qtac100 instrument. To minimize the surface damage, helium was selected as the ion source with a kinetic energy of 3 keV, an ion flux of 6000 pA·m^–2, and a spot size of 2 mm × 2 mm.

O₂ pulse experiments were performed in a quartz reactor at 120 °C on an AutoChem II 2920. Typically, the sample was pretreated with a syngas flow at 450 °C for 6 h, followed by purging with Ar gas for 1 h. The sample was then cooled to 120 °C, and 5% O₂/Ar gas was pulsed 40 times into the sample bed via the automatic six-way valve, with the thermal conductivity detector (TCD) signal monitoring the process. The amount of O₂ consumption on the catalyst was quantified by measuring the molar amount of O₂ species present in the quantitative loop.

CO temperature-programmed desorption (CO-TPD) experiments were conducted on an AutoChem II 2920 equipped with a GSD 350 OmniStar mass spectrometer. The ZnCr₂O₄@ZnO_x sample was first pretreated under a syngas flow at 450 °C for 0.5 h to recover the reduced surface, followed by switching to Ar flow for another 0.5 h. After cooling to room temperature, it was then exposed to a 10% CO/Ar flow at 30 °C, followed by Ar flushing for 1 h, and the temperature was ramped from 30 to 700 °C at a rate of 10 °C·min^–1. The desorbed CO (m/z = 28) and CO₂ (m/z = 44) signals were detected as the temperature was increased.

H₂–D₂ exchange experiments were conducted on an AutoChem II 2920 equipped with a GSD 350 OmniStar mass spectrometer. The ZnCr₂O₄@ZnO_x sample (0.01 g) mixed with the inert silica (0.09 g) was first pretreated under flowing 10% H₂/Ar at 450 °C for 0.5 h, followed by an Ar flow for another 0.5 h and cooling to room temperature. After the pretreatment, a mixture flow of 10% D₂, 10% H₂ and 80% Ar was introduced. The experiment was initiated to monitor changes in the signals of HD (m/z = 3) as the temperature was increased from 30 to 400 °C, with the reaction maintained at 400 °C for 0.5 h.

Transmission Fourier transform infrared (FTIR) spectra were obtained using a transmission FTIR spectrometer (Bruker, VERTEX 70 v) under vacuum (base pressure <5 × 10^–8 mbar). The IR spectra were recorded with 64 scans at a resolution of 4 cm^–1 using a liquid-N₂-cooled mercury cadmium telluride (MCT) detector. The sample with a mass of 10 mg was pretreated under vacuum at 300 °C to analyze the hydroxyl groups on the surface. And the sample was pretreated under vacuum condition at 480 °C to remove the surface impurities during the D₂ activation experiments.

In situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) was measured on a Thermo Nicolet iS 10 equipped with an MCT detector at a spectral resolution of 4 cm⁻¹ and an accumulation of 32 scans. Before the test, the powder samples were pretreated at 400 °C in helium flow (30 mL·min^–1) for 30 min. Background spectra were recorded at different temperatures in a He flow (30 mL·min^–1). 5% CO/He was fed into the sample cell at 30 °C for 10 min. Thereafter, 5% CO/He was introduced into the sample cell at 50 °C. Then, the sample was heated from 50 to 400 °C at a temperature interval of 50 °C in 5% CO/He. Sample spectra were obtained with the corresponding background spectra subtracted during the entire process.

Elemental analysis was carried out using a Zetium X‐ray fluorescence (XRF) spectrometer.

NH₃ temperature‐programmed desorption (NH₃‐TPD) experiment was carried out on a Micromeritics AutoChem II 2920 equipped with a TCD. The SAPO-34 zeolite was pretreated in flowing Ar at 550 °C for 1.5 h, followed by exposure to a 5% NH₃/He mixture at 100 °C. After NH₃ exposure, the sample was flushed with He and the temperature was ramped from 100 to 700 °C at a rate of 10 °C·min^–1.

Catalytic reactions were performed in a fixed-bed stainless steel reactor furnished with a quartz lining with an inner diameter of 6 mm. Typically, a 0.30 g catalyst with oxide/SAPO-34 = 2/1 (mass ratio) was heated from room temperature to 400 °C in an Ar flow. Syngas containing CO and H₂ with 5% Ar as the internal standard for online gas chromatography (GC) analysis was used as the feed. Reaction conditions were H₂/CO = 2.5, 4.0 MPa, 400 °C, 40–60 mesh, and 15 mL·min^–1 unless otherwise stated. Data were collected after at least 6 h on stream. The products of syngas conversion to light olefins were analyzed by online GC (Agilent 8890B). It was equipped with a TCD and two flame ionization detectors (FID). HP-PLOT Q and 5 Å molecular sieve-packed columns were connected to the TCD, while HP-INNOwax and GS-Alumina capillary columns were connected to the FID. Oxygen-containing compounds and hydrocarbons up to C₁₇ were analyzed by the FID, while CO, CO₂, C₂H₄, and Ar were analyzed by the TCD. C₂H₄ was used as a reference bridge between the FID and TCD. CO conversion was calculated on a carbon atom basis using the following equation:

$$X({CO})=\frac{{[{CO}]}_{{in}}-{[{CO}]}_{{out}}}{{[{CO}]}_{{in}}}\times 100\%$$

(1)

where CO_in and CO_out represent the concentrations of CO at the inlet and outlet, respectively.

CO₂ selectivity (${\mbox{Se}}{{\mbox{l}}}_{{\mbox{C}}{{\mbox{O}}}_{2}}$) was calculated according to

$${Sel}({{CO}}_{2})=\frac{{[{{CO}}_{2}]}_{{out}}}{{[{CO}]}_{{in}}-{[{CO}]}_{{out}}}\times 100\%$$

(2)

where [CO₂]_out is the concentration of CO₂ in the reactor outlet.

The selectivity of individual hydrocarbon C_nH_m (${{\rm{Se}}}{{{\rm{l}}}}_{{{{\rm{C}}}}_{{{\rm{n}}}}{{{\rm{H}}}}_{{{\rm{m}}}}}$) among hydrocarbons was obtained according to

$${{{\rm{Sel}}}} ( {{{\rm{C}}}}_{{{\rm{n}}}} {{{\rm{H}}}}_{{{\rm{m}}}})=\frac{{{{\rm{n}}}} {[ {{{\rm{C}}}}_{{{\rm{n}}}} {{{\rm{H}}}}_{{{\rm{m}}}}]}_{{{\rm{out}}}} } { {\sum }_{1}^{{{\rm{n}}}} {{{\rm{n}}}} {[{{{\rm{C}}}}_{{{\rm{n}}}} {{{\rm{H}}}}_{{{\rm{m}}}}]_{{{\rm{out}}}}}} \times 100\%$$

(3)

The selectivity to oxygenate was below 0.5% C and therefore was not reported in the product selectivity. The carbon balance was around 100%.