An atom-level insight into the oxide support effect of Ni-based catalysts on the syngas production in methane reforming

Biomass stands as a pivotal sustainable carbon source that offers a renewable pathway to platform chemicals such as methane via gasification^[1]. The reforming of biomass-derived methane presents an efficient route to produce green syngas (CO + H₂), a fundamental feedstock for industrial chemical synthesis. Catalytic conversion by adopting Ni-based catalysts is the main route for methane reforming, and the support is essential for the optimized activity and stability^[2]. Concurrently, it is widely recognized that electronic structure modification by different supports plays a decisive role in determining catalytic activity and stability^[3−5], for instance, via the surface acidity/basicity^[6] and strong metal-support interaction (SMSI)^[7]. They can refine the adsorption of reactants and intermediates, while SMSI can also inhibit metal particle agglomeration to enhance catalyst stability^[8,9].

The predominant supports for Ni-based catalysts are oxide substrates, encompassing Al₂O₃, ZrO₂, MgO, and SiO₂^[10−13]. Among these, Al₂O₃ exhibits exceptional mechanical strength, and the intrinsic pore architecture facilitates effective dispersion of Ni particles^[14,15], while surface acidic sites promote methane adsorption and activation through enhanced C–H bond cleavage. Nevertheless, the phase transition can induce sintering of active sites, compromising catalytic performance. ZrO₂ demonstrates remarkable crystal phase stability at elevated temperatures (> 800 °C), effectively suppressing Ni particle aggregation while simultaneously inhibiting carbon deposition^[16,17]. The basic sites of MgO enhance chemical adsorption and activation of CO₂ during methane dry reforming. Furthermore, SMSI facilitates electron transfer from the support to the active components, thereby activating the active species and reducing the energy barrier for methane reforming^[18−20]. SiO₂ presents a well-developed pore structure coupled with relatively low chemical reactivity, primarily providing a chemical environment conducive to Ni component dispersion. Notably, mesoporous zeolites can encapsulate active Ni species, making them ideal supports suitable for harsh operating conditions. However, their inherent weak metal-support interactions may also compromise active site stability^[21,22].

Given that different supports can fundamentally alter catalytic activity, a comprehensive understanding of how various supports influence methane reforming is imperative. This study focuses on four common support surfaces—Al₂O₃(110), ZrO₂(111), MgO(100), and SiO₂(110)—that have been extensively observed in experiments. Employing density functional theory (DFT) calculations, we systematically investigated the influence of supports on the active Ni component and the adsorption behaviors of key reaction intermediates. The investigation provides microscopic insights into SMSI phenomena and establishes a theoretical foundation for the rational design of high-performance methane reforming catalysts.

The optimized lattice parameters for the investigated materials are as follows: α-Al₂O₃ exhibits hexagonal symmetry with a = b = 4.81 Å and c = 13.12 Å; monoclinic ZrO₂ (m-ZrO₂) displays lattice constants of a = 5.15 Å, b = 5.23 Å, c = 5.33 Å, and β = 99.2°; face-centered cubic MgO presents a lattice parameter of a = b = c = 4.19 Å; and trigonal SiO₂ shows lattice parameters of a = b = 4.91 Å, c = 5.43 Å, and γ = 120°. Based on experimental observations demonstrating their stability under methane reforming conditions^[23,24], the Al₂O₃(110), m-ZrO₂(111), MgO(100), and SiO₂(110) surfaces were selected for theoretical model construction. A Ni₄ cluster was employed as the active catalytic component, positioned on the respective support surfaces.

Spin-polarized DFT calculations were performed using the Vienna ab initio simulation package (VASP)^[25]. The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was employed in conjunction with projected-augmented wave (PAW) pseudopotentials, utilizing a plane-wave cutoff energy of 400 eV. The k-point mesh resolution value was set to 0.03 (2π/Å) for all calculations. The convergence accuracies of electron self-consistent field and force during geometry optimization were 1 × 10⁻⁶ eV and −0.04 eV/Å, respectively. The Grimme D3 correction was considered in all calculations^[26]. A vacuum layer of 15 Å was implemented to eliminate spurious interactions between periodic images. Structural visualization and computational output analysis were conducted using VESTA^[27] and VASPKIT^[28] packages, respectively. The adsorption energy was determined according to Eq. (1).

where, the E_ads, E_total, E_slab, and E_gas are the adsorption energy, the total energy of the adsorbed slab system, the energy of the pristine slab, and the energy of the isolated gaseous molecule.

The electron density difference (EDD, ∆ρ) is calculated as follows:

where, the ρ_a+b, ρ_a, and ρ_b are the electron density distributions of complex a and b, single a, and single b. Furthermore, density of states (DOS) and crystal orbital Hamilton population (COHP) analyses were utilized to elucidate the electronic interactions between adsorbate species and adsorbent surfaces.

The optimized structures and EDD diagrams for the Ni₄ cluster supported on Al₂O₃(110), ZrO₂(111), MgO(100), and SiO₂(110) are shown in Fig. 1, together with the corresponding binding energies between the active clusters and their respective substrates.

Figure 1. 

Theoretical models and the electron transfer between the Ni₄ cluster and substrates.

On the Al₂O₃(110) surface, the Ni₄ cluster adopts a tetrahedral configuration, wherein two Ni atoms coordinate with surface Al atoms and a third Ni atom binds to surface O atoms. EDD analysis reveals electron accumulation in the interfacial region, originating from both the Ni₄ cluster and adjacent surface atoms, which signifies a robust metal-support interaction corroborated by a substantial binding energy of −6.13 eV. Similar interaction patterns are observed for Ni₄ clusters on ZrO₂(111) and MgO(100) surfaces, exhibiting binding energies of −5.29 and −2.85 eV, respectively, with slight electron transfer between the Ni₄ cluster and the substrate. Bader charge analysis reveals distinct electron transfer behaviors between the Ni₄ cluster and oxide substrates. The calculated Bader charges of the Ni₄ fragment are −1.201 |e|, +0.078 |e|, and –0.368 |e| for the Ni₄/Al₂O₃(110), Ni₄/ZrO₂(111), and Ni₄/MgO(100) systems, respectively. These small charge perturbations indicate that the Ni₄ cluster retains its intrinsic metallic character across all three supports, with the main charge redistribution at the cluster-substrate interface. In contrast, the Ni₄/SiO₂(110) system displays a fundamentally different interaction mechanism: the four Ni atoms become individually dispersed and embedded within the support surface, with each Ni atom coordinating to three O atoms. This configuration yields an exceptionally high binding energy of −22.78 eV. Consistently, EDD analysis reveals substantial charge redistribution across the entire surface. The pronounced positive charge accumulation on the Ni₄ cluster (+4.021 |e|) indicates a markedly ionic character, arising from the individual coordination of all Ni atoms with surface O and Si sites. This substantial electron depletion elucidates the exceptionally strong interfacial binding between Ni₄ and the SiO₂(110) surface.

The adsorption of methane reforming-related species on Ni₄/MO_n

Guided by the mechanistic framework of methane steam and dry reforming reactions, we systematically investigated the adsorption behaviors of key surface species, including CH₄, CO₂, H₂O, CO, H₂, CH, C, O, OH, and H across various catalyst surfaces. Multiple adsorption sites were comprehensively examined, as illustrated in Fig. 1. For the Ni₄/Al₂O₃(110), Ni₄/ZrO₂(111), and Ni₄/MgO(100) systems, distinct Ni₄ cluster sites, interfacial sites, and support sites were identified and evaluated. In the case of Ni₄/SiO₂(110), a unique set of adsorption sites was specifically characterized, encompassing single Ni, Si, and O sites, as well as their bridge sites. The adsorption energies of all investigated species on these catalyst surfaces, as well as on the reference Ni(111) surface, are compiled in Supplementary Tables S1–S5 and Supplementary Figs S1–S4. Meanwhile, Fig. 2 and Supplementary Table S6 summarizes the highest adsorption energy of all reaction species on various catalysts, with detailed analysis in the subsequent sections.

Figure 2. 

The adsorption energies of all reaction species on different catalysts.

The adsorption of reactants (CH₄, CO₂, and H₂O)

The adsorption of the reactant CH₄ constitutes a critical initial step in the reforming process, and CO₂ and H₂O serve as the fundamental feedstocks for methane dry and steam reforming. All reactants exhibit distinct adsorption behaviors across various supported catalysts.

CH₄ preferentially adsorbs atop the Ni₄ cluster on Ni₄/Al₂O₃(110) (Fig. 3), with an E_ads of –0.34 eV. EDD analysis reveals electron depletion from the H atoms of CH₄ toward the interfacial region between the Ni cluster and C atom, corroborated by DOS mapping, which demonstrates significant orbital overlap between the C 2p orbitals of CH₄ and the Ni 3d orbitals (Supplementary Fig. S5). Similar adsorption behavior is observed for CH₄ on Ni₄/ZrO₂(111) and Ni₄/MgO(100), where CH₄ stably adsorbs on the Ni₄ cluster with adsorption energies of −0.41 and −0.27 eV, respectively. Notably, in the Ni₄/MgO(100) system, substrate adsorption is also energetically favorable, exhibiting a comparable adsorption energy of −0.28 eV. Consistent with the Al₂O₃ system, electron transfer occurs from the hydrogen atoms of CH₄ to the Ni₄ cluster, with the C 2p and Ni 3d orbitals primarily governing the interfacial interaction. In marked contrast, CH₄ adsorption on Ni₄/SiO₂(110) follows a fundamentally different mechanism, with CH₄ preferentially occupying the Ni site. EDD analysis indicates electron flow from both the Ni atom and the hydrogen atoms of CH₄ into the interfacial region between CH₄ and Ni. However, DOS and COHP analyses reveal negligible interaction between Ni and CH₄, with only weak bonding between the C 2p and Ni 3d orbitals, consistent with the substantially reduced E_ads of −0.10 eV.

Figure 3. 

The most stable adsorption configurations and EDD diagrams of CH₄.

The adsorption of CH₄ on Ni₄/ZrO₂(111) exhibits the most exothermic behavior among the investigated systems (Fig. 3), with an E_ads of −0.41 eV, surpassing those observed for Ni₄/Al₂O₃(110), Ni₄/MgO(100), and Ni₄/SiO₂(110) surfaces. Notably, this adsorption strength also exceeds that on the pristine Ni(111) surface (E_ads = −0.33 eV, Supplementary Table S5), indicating that the Ni₄ cluster configuration provides enhanced advantages for CH₄ adsorption processes. Conversely, the markedly weak adsorption of CH₄ on Ni₄/SiO₂(110) (E_ads = −0.10 eV) suggests that isolated single Ni atoms do not constitute optimal active sites for methane activation.

On Ni₄/Al₂O₃(110), CO₂ adopts a side-on configuration on the Ni₄ cluster with a substantial E_ads of −0.99 eV, wherein two oxygen atoms coordinate with two Ni atoms, resulting in a pronounced bending of the CO₂ molecule to 141°. EDD analysis reveals electron transfer from both the Ni₄ cluster and C atom of CO₂ into the interfacial region between O and Ni, indicative of Ni–O bond formation mediated by hybridization between O 2p and Ni 3d orbitals, as confirmed by DOS analysis (Supplementary Fig. S6). Analogous adsorption phenomena are observed for Ni₄/ZrO₂(111) and Ni₄/MgO(100) systems, where CO₂ similarly adsorbs on the Ni₄ clusters with adsorption energies of −0.93 and −1.59 eV, respectively. Notably, the Ni₄/MgO(100) system demonstrates the most robust CO₂ adsorption, reflected in the most severe CO₂ bending to 134°. EDD mapping corroborates electron redistribution from the Ni₄ cluster and CO₂ toward Ni–O bonds, while DOS and COHP analyses collectively confirm the significant contribution of O 2p and Ni 3d electrons to Ni–O bond formation. In marked contrast, CO₂ interaction with Ni₄/SiO₂(110) is characterized by weak physisorption (E_ads = −0.15 eV), wherein CO₂ remains spatially separated from the catalyst surface. Consistent with this minimal interaction, DOS and COHP analyses reveal negligible electronic coupling between Si and CO₂, confirming the absence of significant chemical bonding.

The adsorption behavior of CO₂ demonstrates remarkable consistency across Ni₄/Al₂O₃(110), Ni₄/ZrO₂(111), and Ni₄/MgO(100) systems, wherein CO₂ preferentially adsorbs on the Ni₄ cluster (Fig. 4). Among these systems, Ni₄/MgO(100) exhibits the most thermodynamically favorable adsorption configuration with an E_ads of −1.59 eV. Notably, CO₂ undergoes weak physisorption on Ni₄/SiO₂(110) (E_ads = −0.15 eV), highlighting the superior efficacy of Ni clusters over isolated Ni atoms for CO₂ adsorption. When compared with the pristine Ni(111) surface (E_ads = −0.36 eV), all Ni₄ cluster configurations demonstrate enhanced affinity for CO₂ adsorption. These findings collectively establish Ni clusters as the primary reactive sites for CO₂ adsorption and subsequent activation processes.

Figure 4. 

The most stable adsorption configurations and EDD diagrams of CO₂.

As illustrated in Fig. 5, on Ni₄/Al₂O₃(110), H₂O preferentially adsorbs atop the Ni₄ cluster with a moderate E_ads of −0.78 eV, establishing a Ni–O bond with an interatomic distance of 2.01 Å. EDD analysis reveals electron depletion from both H₂O and the Ni₄ cluster toward the Ni–O interfacial region, corroborated by DOS and COHP analyses, which demonstrate significant orbital hybridization between O 2p and Ni 3d states contributing to bond formation (Supplementary Fig. S7). In contrast, on Ni₄/ZrO₂(111) and Ni₄/MgO(100) systems, H₂O preferentially adsorbs on the substrate surfaces, forming robust Zr–O and Mg–O bonds with adsorption energies of −0.81 and −0.63 eV, respectively. Consistent with this substrate-mediated adsorption, electron density accumulation predominantly occurs around the Zr–O/Mg–O bond regions, arising from the hybridization of O 2p electrons with Zr 5s or Mg 3s electrons (Supplementary Fig. S7). Notably, Ni₄/SiO₂(110) exhibits the most thermodynamically favorable H₂O adsorption (E_ads = −1.10 eV), wherein H₂O coordinates with Ni atoms to form strong Ni–O bonds.

Figure 5. 

The most stable adsorption configurations and EDD diagrams of H₂O.

The adsorption energy hierarchy follows the order Ni₄/SiO₂(110) > Ni₄/Al₂O₃(110) > Ni(111) (E_ads = −0.66 eV), indicating a clear preference for H₂O adsorption on isolated atoms and small clusters rather than extended metallic surfaces. Furthermore, the substrate-mediated adsorption behavior observed for ZrO₂(111) and MgO(100) supports suggest a strategic advantage in mitigating H₂O-induced active center sintering, thereby potentially enhancing catalyst stability under steam reforming conditions.

The adsorption of products (CO and H₂)

CO and H₂ are the principal products of methane reforming processes, requiring rapid desorption for sustained catalytic activity to avoid active site poisoning^[29].

The adsorption behavior of CO across various catalysts reveals distinct electronic and structural characteristics. On Ni₄/Al₂O₃(110), Ni₄/ZrO₂(111), and Ni₄/MgO(100) surfaces, CO exhibits remarkably similar adsorption configurations, preferentially coordinating to the Ni₄ cluster (Fig. 6). EDD analysis demonstrates significant electron accumulation in the interfacial region between the Ni cluster and CO molecule, indicative of strong metal-adsorbate interactions. Complementary DOS and COHP analyses reveal substantial orbital overlap near the Fermi level between Ni and CO, with bonding contributions predominantly arising from the hybridization of C 2p orbitals with Ni 3d/4s orbitals (Supplementary Fig. S8). In contrast, CO adsorption on Ni₄/SiO₂(110) occurs preferentially at the Ni site, where the Ni–C bond formation is mediated through C 2p and Ni 3d electronic interactions.

Figure 6. 

The most stable adsorption configurations and EDD diagrams of CO.

The thermodynamic hierarchy of CO adsorption energies follows the order: Ni₄/MgO(100) (E_ads = −3.29 eV) > Ni₄/ZrO₂(111) (E_ads = −2.90 eV) > Ni₄/Al₂O₃(110) (E_ads = −2.83 eV) > Ni(111) (E_ads = −2.17 eV) > Ni₄/SiO₂(110) (E_ads = −2.13 eV). Notably, CO demonstrates a pronounced preference for adsorption on Ni₄ clusters rather than on extended metallic surfaces or isolated atomic sites. Given the exceptionally strong CO adsorption energies observed, particularly on Ni₄/MgO(100), there exists a significant risk of catalyst poisoning through CO over-adsorption, which could potentially compromise the catalytic activity and operational stability of these systems.

The adsorption behavior of H₂ exhibits remarkable uniformity across the investigated supported Ni₄ catalysts (Fig. 7), wherein H₂ preferentially adsorbs atop the Ni₄ cluster in Ni₄/Al₂O₃(110), Ni₄/ZrO₂(111), and Ni₄/MgO(100) configurations, while coordinating to isolated Ni sites in Ni₄/SiO₂(110). EDD analysis reveals significant electron accumulation in the interfacial region between H₂ and the Ni cluster, arising from the hybridization of H 1s and Ni 3d electronic orbitals (Supplementary Fig. S9). Notably, COHP analysis demonstrates that the H₂–Ni interaction in Ni₄/SiO₂(110) is substantially weaker compared with the other catalyst systems.

Figure 7. 

The most stable adsorption configurations and EDD diagrams of H₂.

Consistent with these electronic structure findings, the adsorption energy of H₂ on SiO₂(110) is markedly lower (E_ads = −0.14 eV), reflecting its weak interaction strength. For the remaining three supported catalysts, H₂ adsorption energies are comparable (E_ads = −0.57 to −0.63 eV) and significantly stronger than that observed on the pristine Ni(111) surface (E_ads = −0.25 eV). These findings collectively establish that H₂ exhibits a pronounced preference for adsorption on Ni clusters rather than on extended metallic surfaces or isolated atomic sites, highlighting the enhanced catalytic potential of cluster-based architectures.

The adsorption of key intermediates (CH, C, O, H, and OH)

CH and C intermediates represent critical cracking species in methane reforming, whose adsorption is also a key indicator for carbon deposition. In addition, O, H, and OH groups represent critical secondary surface intermediates throughout the reaction pathway.

Across Ni₄/Al₂O₃(110), Ni₄/ZrO₂(111), and Ni₄/MgO(100) systems, CH intermediates demonstrate preferential interaction with Ni₄ clusters rather than with the substrates (Fig. 8). EDD mapping reveals significant electron accumulation in the interfacial region between Ni clusters and the C atom, indicative of robust Ni–CH interactions (Supplementary Fig. S10). Complementary DOS and COHP analyses demonstrate substantial orbital overlap near the Fermi level between Ni₄ clusters and CH species, with bonding contributions predominantly arising from C 2p and Ni 3d orbital hybridization. In contrast, CH exhibits dual-site interaction with Ni₄/SiO₂(110), simultaneously coordinating with both the Ni atom and surface oxygen atoms. Electronic structure analysis indicates electron depletion from Ni and O atoms toward the Ni–CH interfacial region, mediated through Ni 3d and C 2p orbital overlap (Supplementary Fig. S10).

Figure 8. 

The most stable adsorption configurations and EDD diagrams of (a)–(d) CH, and (e)–(h) C.

Thermodynamic analysis of adsorption strengths reveals that the interaction between CH and Ni₄/Al₂O₃(110) exhibits the highest binding affinity (E_ads = −7.86 eV), consistent with the strong orbital interactions evidenced by COHP analysis. This adsorption energy substantially exceeds that of CH on the pristine Ni(111) surface (E_ads = −6.76 eV), while other catalyst systems demonstrate moderate binding strengths (E_ads = −5.76 to −6.62 eV). Notably, the adsorption energy of CH on Ni₄/SiO₂(110) is significantly diminished compared with other catalysts, highlighting the superior affinity of Ni clusters and bulk metallic surfaces for CH intermediates relative to isolated Ni sites.

The adsorption behavior of C atoms across different catalyst systems reveals distinct structural and electronic characteristics that govern catalyst–carbon interactions. On Ni₄/Al₂O₃(110), C adsorption induces significant reconstruction of the Ni₄ cluster, wherein the C atom simultaneously coordinates with four Ni atoms (Fig. 8). EDD analysis demonstrates substantial electron accumulation in the interfacial region between the C atom and Ni₄ cluster, while DOS and COHP analyses reveal pronounced orbital overlap between Ni 3d and C 2p orbitals, indicating a robust interaction (Supplementary Fig. S11). Analogous adsorption phenomena are observed for Ni₄/ZrO₂(111) and Ni₄/MgO(100) systems, where the C atom interacts with Ni₄ clusters that maintain their structural integrity without significant distortion. Similar electron accumulation patterns emerge in the Ni–C interfacial regions, mediated through C 2p and Ni 3d orbital hybridization. In contrast, the Ni₄/SiO₂(110) system exhibits distinctive adsorption behavior, wherein the C atom simultaneously coordinates with both surface Ni and O atoms, forming Ni–C and C–O bonds with interatomic distances of 1.25 and 1.65 Å, respectively. The Ni–C interaction predominates in this system, with electronic contributions arising from the C 2p and Ni 3d orbital overlap.

Thermodynamic analysis of adsorption strengths reveals a clear hierarchy: Ni₄/MgO(100) demonstrates the most stable carbon adsorption (E_ads = −8.16 eV), followed by Ni₄/Al₂O₃(110) (E_ads = −7.74 eV). Both systems exhibit substantially stronger C affinity compared with the pristine Ni(111) surface (E_ads = −7.11 eV). Conversely, Ni₄/ZrO₂(111) (E_ads = −6.62 eV) and Ni₄/SiO₂(110) (E_ads = −6.69 eV) demonstrate relatively weaker carbon binding capabilities, highlighting the critical role of support materials in modulating the catalyst–carbon interaction.

On Ni₄/Al₂O₃(110) and Ni₄/ZrO₂(111) surfaces, oxygen species preferentially adsorb on the substrate, forming robust Al–O or Zr–O bonds that induce significant surface electronic structure reconstruction (Fig. 9). DOS and COHP analyses elucidate that these interactions predominantly arise from orbital hybridization between O 2p and Al 3p or Zr 4d orbitals (Supplementary Fig. S12). In contrast, on Ni₄/MgO(100), O adsorption occurs at the sublayer of the Ni₄ cluster, with electrons flowing from the Ni₄ cluster into the interfacial region between O and the metallic cluster. Both DOS and COHP analyses confirm the substantial contribution of Ni 3d and O 2p orbitals to these metal–oxygen interactions. Distinctively, oxygen adsorption on Ni₄/SiO₂(110) results in the formation of a Ni–O–Ni bridge structure, wherein both Ni–O interactions are mediated through Ni 3d and O 2p orbital hybridization.

Figure 9. 

The most stable adsorption configurations and EDD diagrams of (a)–(d) O, (e)–(h) H, and (i)–(l) OH.

Thermodynamic analysis reveals that O adsorption on Ni₄/Al₂O₃(110) exhibits the strongest binding (E_ads = −8.85 eV), reflecting the pronounced oxygen affinity of aluminum species. The O adsorption on Ni₄/MgO(100) and Ni₄/ZrO₂(111) demonstrates moderate binding strengths with adsorption energies of −6.19 and −6.02 eV, respectively. Notably, all these systems display superior O affinity compared with the pristine Ni(111) surface (E_ads = −5.95 eV), with the exception of Ni₄/SiO₂(110), which exhibits significantly weaker O adsorption (E_ads = −3.75 eV). These findings collectively demonstrate that O adsorption is highly sensitive to the supports, with a clear tendency for oxygen to preferentially adsorb on hydrophilic substrates such as Al₂O₃ rather than on the metallic active sites.

On Ni₄/Al₂O₃(110), Ni₄/ZrO₂(111), and Ni₄/MgO(100), H preferentially adsorbs on the Ni₄ cluster driven by nickel's high hydrogen affinity (Fig. 9). EDD, DOS, and COHP analyses collectively reveal significant electron accumulation in the interfacial region between the H atom and the Ni₄ cluster, with interactions predominantly arising from H 1s and Ni 3d orbital hybridization (Fig. 9 and Supplementary Fig. S13). In contrast, on Ni₄/SiO₂(110), H preferentially adsorbs on substrate oxygen atoms, resulting in O–H bond formation with interactions primarily mediated through O 2p and H 1s orbital overlap.

Thermodynamic analysis demonstrates that H adsorption on Ni₄/SiO₂(110) exhibits the highest stability (E_ads = −3.13 eV), marginally exceeding that on the pristine Ni(111) surface (E_ads = −3.07 eV). Conversely, H adsorption on Ni₄/Al₂O₃(110), Ni₄/MgO(100), and Ni₄/ZrO₂(111) demonstrates reduced binding strength compared with Ni(111), with adsorption energies of −2.89, −2.96, and −2.76 eV, respectively. These findings indicate a slight preference for H interacting with bulk Ni(111) surfaces over dispersed clusters, albeit the energetic differences are minimal.

The adsorption behavior of OH also exhibits distinct substrate-dependent interactions. On Ni₄/Al₂O₃(110) and Ni₄/ZrO₂(111), OH species preferentially adsorb on the substrate surfaces, establishing interactions with metal atoms through electron transfer from OH and Al/Zr species into Al/Zr–O bonds (Fig. 9). These interactions are predominantly mediated through orbital hybridization between O 2p and Al 3p or Zr 4d states (Supplementary Fig. S14). In contrast, OH adsorption on Ni₄/MgO(100) occurs preferentially at the interfacial region, where significant electron accumulation is observed, and O 2p and Ni 3d orbitals contribute substantially to these metal-adsorbate interactions. Distinctively, on Ni₄/SiO₂(110), OH coordinates directly with Ni atoms, forming Ni–O bonds through the hybridization of O 2p and Ni 3d orbitals, as evidenced by EDD, DOS, and COHP analyses.

Thermodynamic analysis of adsorption strengths reveals that OH adsorption on Ni₄/MgO(100) exhibits the most exothermic (E_ads = −4.69 eV), reflecting the exceptionally strong interaction intensity between the interfacial site and OH. Concurrently, OH demonstrates robust adsorption on Al₂O₃(110) and ZrO₂(111), with adsorption energies of −4.60 and −4.42 eV, respectively; both substantially exceeding that observed on the pristine Ni(111) surface (E_ads = −3.74 eV). Exceptionally, OH interaction with the Ni atom in Ni₄/SiO₂(110) releases only 2.75 eV of energy, indicating the relatively weak affinity of isolated Ni atomic sites for OH species compared with cluster-based configurations.

Discussion

The reactivity of different supported catalysts

Different supports profoundly influence the catalytic performance of Ni₄ clusters for methane reforming. Different supports influence both adsorption sites and energies, and the adsorption strength order is summarized in Supplementary Table S7.

Carbon-containing species (CH₄, CO₂, CO, CH, and C) and H species (H/H₂) preferentially adsorb on the Ni₄ cluster of Ni₄/Al₂O₃(110), Ni₄/ZrO₂(111), and Ni₄/MgO(100), indicating that the complete methane dry reforming process (CH₄ + CO₂ → 2CO + 2H₂) can proceed entirely on the active Ni₄ cluster. Conversely, O-containing species (O, OH, and H₂O) exhibit a strong affinity for the substrate due to the superior oxygen-binding capability of support metals compared to Ni, enabling substrates to participate in steam reforming as H₂O adsorption sites. Ni₄/SiO₂(110) represents a distinctive case, where most reaction species adsorb on isolated Ni sites.

Compared to the Ni(111) surface, supported Ni₄ clusters on Al₂O₃(110), ZrO₂(111), and MgO(100) carriers exhibit significantly enhanced adsorption energies for all species. In particular, Ni₄/Al₂O₃(110) exhibits substantially increased adsorption energies for C, CH, and O species by about 0.6, 1.1, and 2.8 eV, respectively, leading to the promoted deep cracking activity. Ni₄/ZrO₂(111) also demonstrates generally elevated adsorption energies for all species, especially for reactants (CH₄, CO₂, and H₂O), indicating that ZrO₂ can facilitate the reaction by enhancing the interaction between the Ni₄ cluster and reactants. Similarly, Ni₄/MgO(100) displays markedly increased adsorption energies for most species, with OH, C, and CO₂ exhibiting increases exceeding 1 eV. In contrast, the Ni₄ cluster disperses into isolated single-atom sites on SiO₂(110), resulting in diminished adsorption energies, except for H₂O adsorption. Consequently, SiO₂ support fails to improve Ni activity and may promote H₂O-induced sintering due to the strengthened Ni–H₂O interactions^[30].

Quantitatively, the adsorption strength of CH₄, CO₂, and H₂O on Ni₄/Al₂O₃(110), Ni₄/ZrO₂(111), and Ni₄/MgO(100) is comparable (E_ads = −0.28 to −0.41 eV, and −0.93 to −1.59 eV), substantially higher than those on the Ni(111) surface (−0.33 eV and −0.36 eV) and Ni₄/SiO₂(110) (−0.10 eV and −0.15 eV). Notably, MgO exhibits exceptional CO₂ adsorption capabilities, which substantially enhances dry reforming activity. This promotional effect is corroborated by experimental evidence where Jin et al.^[31] reported a 26% increase in methane dry reforming conversion at 850 °C upon MgO doping into Ni/Al₂O₃ catalysts, underscoring the critical role of basic oxide supports in facilitating CO₂ activation. For deep CH₄ dissociation intermediates (C/CH), the sequence is MgO (−8.16/−6.62 eV) > Al₂O₃ (−7.74/−7.86 eV) > ZrO₂ (−6.62/−6.31 eV) > SiO₂ (−6.69/−5.76 eV). The exceptionally strong adsorption on MgO and Al₂O₃ facilitates complete methane cracking, consistent with prior studies where MgO doping enhanced dry reforming via strengthened SMSI^[32]. However, for the adsorption of CO, the sequence is MgO (−3.29 eV) > Al₂O₃ (−2.90 eV) > ZrO₂ (−2.83 eV) > SiO₂ (−2.13 eV). Notably, MgO exhibits the strongest affinity toward both CO and CO₂ among the investigated supports, which may also lead to active-site poisoning. This observation aligns with the Sabatier principle that excessively strong adsorption of reactants can impede product desorption, thereby compromising catalytic performance. This also highlights a critical trade-off: while stronger adsorption of reactants/intermediates is beneficial, excessive CO adsorption can lead to active site poisoning and reaction inhibition^[29].

Furthermore, reaction kinetics govern the overall catalytic performance. Reported literature identifies C–H bond activation as the rate-determining step for methane reforming, particularly under high-temperature conditions^[33]. Accordingly, we evaluated the activation barriers for CH₄ dissociation across the four supported Ni₄ clusters (Supplementary Fig. S15). The calculated energy barriers follow the sequence: Ni₄/Al₂O₃(110) (0.397 eV) ≈ Ni₄/MgO(100) (0.400 eV) < Ni₄/ZrO₂(111) (0.588 eV) < Ni₄/SiO₂(110) (1.170 eV). This kinetic hierarchy indicates superior catalytic activity for Al₂O₃- and MgO-supported systems, corroborating both the thermodynamic predictions and experimental observations that Ni/Al₂O₃ outperforms Ni/ZrO₂ and Ni/SiO₂^[34], while MgO doping further enhances catalytic turnover^[31].

In summary, Ni₄ supported on MgO, Al₂O₃, and ZrO₂ supports generally exhibits promoted intrinsic reactivity by strengthening the adsorption of reactants and cracking intermediates.

Carbon deposition and elimination

The Sabatier principle also provides a fundamental framework for understanding carbon deposition resistance. Carbon adsorption energy serves as the fundamental descriptor governing carbon deposition propensity on supported Ni catalysts. According to the foregoing analysis, atomic C exhibits exceptionally strong binding on Ni₄/MgO(100), with an adsorption energy of −8.16 eV. According to the Sabatier principle, such excessive adsorption strength implies a substantial kinetic barrier for carbon removal or further conversion to gaseous products. Hence, this pronounced binding renders deposited carbon thermodynamically and kinetically trapped. Consistent with this, Zhang et al.^[35] reported that deep cracking products of CH₄ on MgO-supported catalysts exhibit limited removability, indicating inferior resistance to carbon deposition. This suggests that, despite the promotional effect of MgO on CO₂ activation, its inadequate capability for carbon elimination may compromise long-term catalytic stability. The availability of active oxygen species represents another crucial factor governing carbon elimination through the C* + O* → CO* pathway. Although C is also strongly adsorbed on Al₂O₃(110) support surface (E_ads = −7.74 eV), Al₂O₃(110) also demonstrates remarkable capability to stably adsorb oxygen atoms with exceptionally high binding energy (E_ads = −8.85 eV), effectively creating a rich 'oxygen reservoir' that facilitates carbon removal. In contrast, ZrO₂(111) exhibits a more balanced profile with moderate carbon adsorption strength (E_ads = −6.62 eV) coupled with strong oxygen affinity (E_ads = −6.02 eV), resulting in superior resistance to carbon deposition, in agreement with experimental findings^[36−38]. Ni₄/SiO₂(110) presents an interesting case study, displaying carbon adsorption intensity comparable to ZrO₂(111) (E_ads = −6.69 eV), yet significantly weaker oxygen affinity (E_ads = −3.75 eV), exhibiting a stronger tendency toward carbon deposition. These theoretical predictions are corroborated by experimental studies. Xu et al.^[39] demonstrated the inferior stability of Ni/SiO₂ relative to Ni/Al₂O₃, attributing this to the enhanced carbon deposition propensity. Meanwhile, Pizzolitto et al.^[34] established a stability hierarchy of Ni/Al₂O₃ > Ni/ZrO₂/Ni/SiO₂ under reaction conditions. Furthermore, Zhang et al.^[40] revealed that carbonaceous deposits on Ni/ZrO₂ exhibit superior reactivity toward CO₂ oxidation at lower temperatures compared with those on Ni/SBA-15, indicating the enhanced carbon resistance of ZrO₂ supports. Collectively, these findings establish the following trend in anti-coking performance: Al₂O₃ ≈ ZrO₂ > SiO₂. This comparative analysis reveals the delicate balance between carbon binding strength and oxygen availability in determining overall catalyst stability.

From the perspective of carbon deposition resistance, ZrO₂ emerges as the optimal support, offering an ideal compromise between moderate carbon adsorption and sufficient oxygen supply for effective carbon removal. This balanced approach ensures sustained catalytic activity while minimizing carbon accumulation.

Summary

Figure 10 presents a comprehensive comparison of oxide support effects on Ni-catalyzed methane reforming. Ni₄/Al₂O₃(110) exhibits superior catalytic performance, attributable to its optimal adsorption of reaction intermediates, facile C–H bond activation, and strong oxygen affinity that promotes carbon removal via CO formation. Ni₄/ZrO₂(111) similarly demonstrates excellent activity, with balanced carbon and oxygen adsorption energetics that effectively suppress coking. In contrast, Ni₄/MgO(100) displays high intrinsic activity, but suffers from excessive binding of atomic C and CO, leading to severe carbon deposition and active site poisoning that compromises operational stability. Finally, Ni₄/SiO₂(110) exhibits both low catalytic activity and pronounced carbon deposition propensity.

Figure 10. 

The influence of different substrates on Ni-catalyzed methane reforming.

The present study elucidates the profound influence of different oxide supports on the catalytic performance of Ni-based catalysts in methane reforming, through systematic DFT calculations. By examining the adsorption behavior of reactants, key intermediates, and products across various supported catalyst systems, the fundamental structure-activity relationships that guide catalyst design optimization are established.

Comparative analysis reveals that supported Ni catalysts generally exhibit superior reactivity for methane reforming compared with the bulk Ni(111) surface. Cluster-based catalysts, including Ni₄/Al₂O₃(110), Ni₄/ZrO₂(111), and Ni₄/MgO(100), demonstrate highly consistent adsorption behavior for reactants (CH₄, CO₂, and H₂O), indicating universal enhancement mechanisms across different oxide supports.

Distinctively, Al₂O₃(110) and MgO(100) supports significantly enhance reaction thermodynamics and exhibit strong reaction activity, as evidenced by the increased adsorption energies of C/CH and CO species, as well as their low C–H activation energy barrier. However, the strong adsorption of carbon-containing species also increases the risk of carbon deposition and potential active site poisoning by CO, especially for MgO(100), representing a classic activity-stability dilemma. ZrO₂ emerges as a particularly promising support, by offering moderate enhancement of Ni₄ cluster activity while simultaneously reducing carbon deposition tendencies. This balanced approach provides an effective strategy for addressing the fundamental activity-stability trade-off that plagues conventional Ni-based catalysts. In addition, Ni₄/SiO₂(110) exhibits relatively weak reactivity, although it is important to note that this system cannot represent the full spectrum of Si-based supported catalysts.

The development of composite catalysts that precisely control the distribution of active components while optimizing the anti-carbon deposition properties of substrates remains a crucial research priority. Such rational design strategies hold the key to breaking the long-standing activity-stability trade-off and enabling next-generation methane reforming catalysts with unprecedented performance and durability.