Ultra-durable Ni-Ir/MgAl 2 O 4 catalysts for dry reforming of methane enabled by dynamic balance between carbon deposition and elimination

The highly dispersed Ni-Ir/MgAl 2 O 4 alloy catalyst has high activity and long-period stability (600 h) with zero carbon deposition for dry reforming of methane at industrially relevant temperature (650 (cid:1) C). This special catalytic system follows a balance mechanism between the active sites and the carbon deposits-elimina-tions, where Ni played the role of activating CH 4 and MgAl 2 O 4 adsorbed CO 2 to form carbonate species that can be effectively used by Ir to eliminate carbon species generated from CH 4 activation on Ni. SUMMARY Carbon deposition is the main cause for the catalyst deactivation of methane dry reforming, and researchers are committed to exploring effective catalyst systems with zero carbon deposition in order to achieve a practically long lifetime. In this work, we propose an equilibrium theory with matched rates of CH 4 dissociation and CO 2 activation to establish a balance between carbon deposition and carbon elimination and construct highly dispersed Ni-Ir/MgAl 2 O 4 alloy catalysts accordingly, where Ni activated CH 4 , MgAl 2 O 4 adsorbed CO 2 to form surface carbonates, and Ir effectively used the carbonates to eliminate carbon species generated by CH 4 dissociation. Theoretical assessment further unveiled that the preferred CO 2 activation on Ir over Ni is derived from its stronger oxophilicity. With an optimal Ni/ Ir atomic ratio of 1/2, high activity and long-period stability (600 h) with zero carbon deposition were obtained concurrently for dry reforming of methane at industrially relevant temperature (650 (cid:1) C). surface. 31 The redox recycling on Ni-Fe/Mg x Al y O z catalysts, where metallic Fe was oxidized by CO 2 to form FeO x and the adsorbed C atom obtained from the dissociation of CH 4 reduced FeO x back to Fe, can effectively eliminate carbon deposits. 32 Ni-Mo nanoparticles were stabilized on the edge of a single MgO crystal to inhibit the particle sintering at high temperatures and effectively prevent the generation of carbon deposits over a long reaction period (850 h). 33 of the activation barrier CO dissociation compared with the Ni 8 cluster, the generation of O* species on the Ir 8 cluster was much more efﬁcient, rendering a fast elimination of carbon deposits and thus a high catalyst stability during DRM. For the two Ni 6 Ir 2 /MgAl 2 O 4 models, CO 2 was dissociated on Ni 6 Ir 2 /MgAl 2 O 4 -a with the formed CO* species bound to a Ni site via a C-Ni average activity of the reaction, indicating an extremely high stability of the catalytic system.


INTRODUCTION
Dry reforming of methane (DRM) is a primary method for producing syngas (a mixture of CO and H 2 ) that acts as a key platform for the industrial production of fuels and chemicals. [1][2][3][4][5] Among group VIII metals, 6-10 low-cost Ni has drawn the most attention for DRM because of its strong CH 4 dissociation ability, which endows Ni with an excellent catalytic performance of DRM. [11][12][13][14][15] However, Ni-based catalysts suffer from serious carbon deposition and metal sintering issues under the high-temperature condition of DRM (700-1000 C), which limits their large-scale application. [16][17][18][19][20] The main causes of carbon deposition include the disproportionation of carbon monoxide (2 CO(g) # C(s) + CO 2 (g); DH = À171 kJ/mol) and the deep dissociation of methane (CH 4 (g) # C(s) + 2H 2 (g); DH = 75 kJ/mol), while the latter prevails at temperatures above 600 C. 21 Various methods have been attempted to suppress the carbon deposits in DRM. By using supports rich in oxygen vacancies such as CeO 2 22-24 or alkaline oxide supports like MgO, 25,26 CO 2 activation can be strengthened to accelerate the elimination of carbon deposits. This enhances the redox property of the catalyst and inhibits the development of carbon deposition. 27,28 The accumulation and sintering of metallic Ni at high temperatures (700-1,000 C) can result in the formation of larger Ni particles, which promotes the carbon deposition. 29 The strong metal-support interaction between metallic Ni and the hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 ] (HAP) support 30 was found to enhance the dispersion of Ni particles and alleviate sintering and carbon deposits concurrently. In addition, it was reported that decorating the Ni surface with Co prevented the continuous formation of carbon nanotubes on the Ni The bigger picture Dry reforming of methane (DRM) can effectively use the greenhouse gas CO 2 and CH 4 to produce syngas (a mixture of CO and H 2 ), which acts as a key platform for the industrial production of fuels and chemicals. Carbon deposition is the main cause for the catalyst deactivation of DRM, so core of the stable DRM reaction is the equilibrium between efficient CH 4 dissociation and CO 2 activation. Here, we design an ultra-stable Ni-Ir/MgAl 2 O 4 alloy system for DRM, which has high activity and long-period stability (600 h) with zero carbon deposition obtained concurrently for DRM. In this system, Ni plays the role of activating CH 4 , and MgAl 2 O 4 adsorbs CO 2 to form carbonate species that can be effectively used by Ir to eliminate carbon species generated from CH 4 activation on Ni. We proposed a balance mechanism between the active sites and the carbon deposits-eliminations. Our results would inspire new ideas on the development of novel catalysts for activation of inert C 1 molecules.
surface. 31 The redox recycling on Ni-Fe/Mg x Al y O z catalysts, where metallic Fe was oxidized by CO 2 to form FeO x and the adsorbed C atom obtained from the dissociation of CH 4 reduced FeO x back to Fe, can effectively eliminate carbon deposits. 32 Ni-Mo nanoparticles were stabilized on the edge of a single MgO crystal to inhibit the particle sintering at high temperatures and effectively prevent the generation of carbon deposits over a long reaction period (850 h). 33 The core issue for achieving practically durable DRM catalysts with minimal carbon deposition is to establish an efficient equilibrium between CH 4 dissociation and CO 2 activation on the catalyst surface. In this work, we show that a highly efficient and stable DRM system can be built on bimetallic Ni-Ir/MgAl 2 O 4 catalysts, brought forth by a synergy between the CH 4 dissociation on Ni sites and MgAl 2 O 4 -enhanced CO 2 adsorption and activation on Ir sites that can scavenge surface carbon species generated from the CH 4 dissociation step. Through in situ spectroscopic characterization of the generation and elimination of carbon species and theoretical calculations on the CO 2 activation, we propose an equilibrium mechanism of carbon generation and elimination for designing the effective and durable catalysts for the DRM process.  Figure S1). The surface areas of MgAl 2 O 4 -supported Ni, Ni 3 Ir 1 , and Ir catalysts were 126.0, 133.9, and 136.1 m 2 /g, respectively, as determined by nitrogen physical adsorption ( Figure S2). X-ray diffraction (XRD) results ( Figure 1A) showed that the diffraction peaks of metallic Ni for Ni 3 Figure S3), consistent with the smaller atomic radius of Ni than Ir.

RESULTS AND DISCUSSION
As shown in XPS profiles of Figure 1B, the Ni 0 2p 3/2 (855.2 eV) and Ni 2+ 2p 3/2 (858.2 eV) signals were observed for the Ni/MgAl 2 O 4 catalyst. The presence of Ni 2+ cations indicates the formation of NiAl 2 O 4 species on the support surface during the synthesis process. As shown in the temperature programmed reduction (TPR) of Figure S4, the reduction peak of Ir was at around 220-260 C, and the initial reduction temperature of Ni was at around 600 C.  Figure S5). It is clearly indicated that the formation of the Ni-Ir alloy improved the dispersion of Ni, consistent with the XRD results shown above (Figure 1A). In addition, the EDS analysis ( Figure 1E) showed that the small particles in the  its initial activity was high. For the Ir catalyst, the initial conversions of CH 4 and CO 2 were only 50.2% and 63.6%, respectively, but the catalyst activity did not decrease significantly during the 100-h test. For the Ni 3 Ir 1 /MgAl 2 O 4 catalysts, the conversions of CH 4 and CO 2 did not drop significantly and remained around 61.0% and 73.0% during the 100-h test, respectively, indicating the Ni-Ir alloy catalyst possessed both of high activity and improved stability. As shown in Table S1, we summarize the typical catalyst systems for DRM, and the Ni-Ir/MgAl 2 O 4 system developed in our study had superior conversion rates of CH 4 and CO 2 than the state-of-the-art catalysts. 9,[34][35][36][37][38][39][40][41] For the DRM test with different temperatures (600-800 C, Figure S8), the initial conversion of CH 4 on the Ni, Ni 3 Ir 1 , and Ir catalysts increased with increasing temperature from 42.1%, 46.8%, and 20.9% at 600 C to 89.8%, 90.4%, and 90.2% at 800 C, respectively. Although the three catalysts showed significant differences in activity at the lower temperature, the gap decreased as the temperature increased until the activity was almost identical at 800 C. The ratio of H 2 /CO in the syngas showed a similar phenomenon, increasing from 0.67, 0.80, and 0.60 at 600 C to 0.96, 0.97, and 0.94 at 800 C. It is suggested that DRM is preferred over the reverse watergas shift reaction (RWGS: CO 2 + H 2 / CO + H 2 O) at temperatures above 600 C, which leads to an increase in the H 2 /CO ratio and gradually approaches unity as the temperature rises.
The different performances of the three catalysts in DRM may be directly related to their intrinsic abilities for the adsorption and activation of the CH 4   reaction order values were À0.13 and À0.57, respectively. These data indicate the adsorption strength of CO 2 on the three catalysts decreased with an order of Ir/ MgAl 2 O 4 , Ni 3 Ir 1 /MgAl 2 O 4 , and Ni/MgAl 2 O 4 . As shown below, DFT calculations unveil that the higher oxophilicity of Ir than Ni accounts for the stronger CO 2 adsorption on Ir/MgAl 2 O 4 .
The measured reaction orders for CH 4 over the Ni, Ni 3 Ir 1 , and Ir catalysts were À0.38, À0.50, and À1.14, respectively, indicating the dissociative adsorption of CH 4 on Ir/MgAl 2 O 4 was also the strongest among the three catalysts. This outstanding adsorption ability of Ir made it difficult for the reactive species to desorb from the catalyst surface, which limited the catalytic activity at medium to low temperatures. As the temperature increased, the desorption of the reactive species became easier and the catalytic activity of Ir/MgAl 2 O 4 increased faster compared with the other two catalysts, as evidenced by the changes of CH 4 conversion with increasing temperature ( Figure S8).

Characterization of spent Ni-Ir catalysts
It is generally accepted that carbon deposition is one of the main causes of catalyst deactivation in DRM. 16,18 The SEM images ( Figures 2B-2D) of the spent catalysts clearly showed the appearance of carbon deposits after the DRM reaction, which were interlaced and attached to the catalyst surface, blocking contact between the active metal site and the reactants and thus inhibiting the catalytic activity. Specifically, Ni/MgAl 2 O 4 had more carbon deposits than Ni 3 Ir 1 /MgAl 2 O 4 , and the carbon chains on Ni/MgAl 2 O 4 were longer and thicker than the latter, with average diameters of about 27.9 versus 21.5 nm, due to the different amount of carbon species provided for the carbon chain growth for each catalyst. It is worth noting that the carbon deposition was nearly negligible on the Ir catalyst ( Figure 2D), consistent with the high stability of this catalyst for DRM.
XRD patterns of the spent Ni/MgAl 2 O 4 catalyst showed an obvious graphite-2H signal (2q = 25.9 ), which is typical of amorphous carbon species produced in DRM ( Figure 2G). The intensity of the graphite-2H signal for Ni 3 Ir 1 was lower than that for Ni, while this signal was not detectable on the Ir catalyst. Raman spectra of these spent catalysts ( Figure 2H) showed peaks at 1,334 and 1,598 cm À1 for both Ni and Ni 3 Ir 1 , which were assigned to the D-band and G-band signals of carbon species. Although the peak intensity of Ni was higher, the peak intensity ratios of the D-band and G-band signals were similar between the two catalysts, indicating no significant differences in the types of carbon deposits formed. For the spent Ir catalyst, no corresponding peaks were observed, consistent with the XRD characterization. To quantify the amount of carbon deposits formed on the spent catalysts, thermogravimetric (TGA) analysis was conducted ( Figure 2I). No significant weight loss was observed for the Ir catalyst during the TGA process, while the Ni and Ni 3 Ir 1 catalysts showed weight losses of 16.7% and 10.2%, respectively. These data unambiguously reflect that the addition of Ir suppressed the formation of carbon deposits in DRM.
TEM characterization was further applied to analyze the structure and morphology of the spent catalysts after long-time reaction ( Figure S9).  Figure S10A). TGA was further performed to assess the CO 2 adsorption capacity of MgAl 2 O 4 ( Figure S10B), which was approximately 1.0 wt % at 650 C. As the temperature decreased, the adsorption capacity of MgAl 2 O 4 continued to increase, and reached about 2.6 wt % at 50 C. Although the adsorption capacity was not significant at high temperatures, the adsorption and desorption of CO 2 occurred simultaneously during the DRM process and could dynamically provide CO 2 for the active metal sites, beneficial for the capture and utilization of gaseous CO 2 .
To further ascertain the specific impact of carbonates on MgAl 2 O 4 for DRM, in situ DRIFTS was used to analyze the activation process through the step-by-step introduction of CO 2 -CH 4 -CO 2 atmosphere. Within the first minute of introducing CO 2 , all three catalysts showed vibrational peaks at 1,643 and 1,542 cm À1 (attributed to monodentate carbonate species 42,43 ), indicating all the catalysts can adsorb CO 2 to form carbonate ( Figure S11). Subsequently, the IR reactor cell was purged with Ar, and CH 4 was introduced in the second stage. Two distinct reaction pathways are proposed to explain the observed difference during the in situ DRIFTS experiments described above. The Ni/MgAl 2 O 4 catalyst followed reaction path A ( Figure 3G). Namely, when CO 2 was first introduced, the MgAl 2 O 4 support adsorbed gaseous CO 2 and formed monodentate carbonate species (CO 2 (g) / CO 3

2À
, ads ), thus allowing the carbonate species to reach saturation. When CH 4 was introduced in the second stage, it was dissociated on the active metal sites, forming surface CH x * (x = 0-3) species (CH 4 (g) / H* + CH x *), which continued to accumulate and form carbon deposits. The carbonate species on MgAl 2 O 4 cannot be used by the active Ni sites directly, while it can decompose into gaseous CO 2 (CO 3 2À ads / CO 2 (g)) again, which then adsorbed on the active Ni sites and further reacted with CH x * to form CO* (CO 2 * + CH x * / 2CO (g) + x/2H 2 ). CO* was finally released from the catalyst surface to form gaseous CO. Therefore, when CH 4 was introduced during the second stage, CO was not produced until sufficient carbonate was consumed to generate CO 2 . Only easily eliminated carbon species were activated and removed by the reaction with CO 2 released by the support in the second stage, leaving the catalyst surface covered by the recalcitrant carbon chains produced by overgrowth. Since Ni could not effectively activate and use CO 2 to eliminate the carbon chains, when CO 2 was reintroduced in the third stage, only the accumulation of carbonate species on the support occurred without the formation of CO.
In contrast to Ni/MgAl 2 O 4 , the Ir/MgAl 2 O 4 catalyst followed reaction path B (Figure 3H). The difference was that the Ir active sites could directly use and activate the carbonate species on the support, eliminating the CH x * and forming gaseous CO (CO 3

2À
, ads + CH x * / CO (g)). The carbonate species did not have to decompose into CO 2 (g) to be reused, therefore there was a large amount of CO formed without obvious CO 2 formation. The direct utilization of carbonate on the Ir-based catalysts increased the utilization rate of carbonate species, resulting in a slower rate of consumption compared with route A. Compared with Ni, Ir had an insufficient ability to activate CH 4 , therefore the carbonate species that adsorbed on the surface of MgAl 2 O 4 in the first stage was sufficient to eliminate the CH x * generated during the second stage, which resulted in no CO formation in the third stage. We also increased the time for CH 4 introduction in the second stage of the Ir/MgAl 2 O 4 catalyst to ensure that enough CH x * was generated. After CO 2 was introduced again, gaseous CO signals appeared indicating that the Ir/MgAl 2 O 4 catalyst could reduce the carbon species and CO 2 to CO ( Figure S12).
The Ni 3 Ir 1 /MgAl 2 O 4 catalyst also followed reaction path B. Due to the presence of Ni, the dissociation ability of CH 4 was better than the Ir/MgAl 2 O 4 catalyst, therefore CH x * species were not completely eliminated and grew into carbon chains in the second stage. In the third stage, gaseous CO formed in addition to the carbonate but was slower because the carbonate was not saturated during the initial stage of CO 2 introduction. With saturation of the carbonate species, the formation rate of gaseous CO increased accordingly, which further confirmed that carbonate was used as a bridge to adsorb and convert CO 2 rather than directly use gaseous CO 2 . Both the Ni 3 Ir 1 /MgAl 2 O 4 and Ni/MgAl 2 O 4 catalysts generated carbon species, which grew into carbon chains in the second stage. However, in the third stage, only the Ni 3 Ir 1 /MgAl 2 O 4 catalyst following path B eliminated the carbon species completely and formed gaseous CO. This suggests that path B was more beneficial to the activation of CO 2 and the elimination of carbon species.
To verify the unique roles of MgAl 2 O 4 in the DRM, an Ir/Al 2 O 3 catalyst was further characterized by the in situ DRIFTS experiment ( Figure S13). When CO 2 was introduced in the first stage, no carbonate species formed and only obvious gaseous CO 2 signal peaks appeared. In the second stage, no gaseous CO signal peak appeared during the introduction of CH 4 , because the carbon species were eliminated in a CH 4 atmosphere without carbonates formed on the support surface. The third stage was the same as the second stage, with no obvious gaseous CO signal peaks. Although carbon species were produced by the activation of CH 4 (temperature programmed surface reaction-mass spectrometry [TPSR-MS], in situ DRIFTS) in the second stage, Ir weakly used gaseous CO 2 to eliminate carbon species, and only a small Article amount of carbon species that had not continued to grow was further activated, forming a small amount of CO. Therefore, only the Ir-CO signal (2000 cm À1 ) was observed on the Ir catalyst, and the amount of gaseous CO did not reach the detection limit. This proves that the carbonate formed on the MgAl 2 O 4 support played a key role in the effective removal of carbon by Ir during the DRM.
In order to further explore the adsorption and activation ability of CO 2 on different catalysts, the catalysts were characterized with temperature programmed desorption of CO 2 (CO 2 -TPD) and showed a signal peak appeared only at 614 C on MgAl 2 O 4 , and the peak position of Ni/MgAl 2 O 4 was similar to that of MgAl 2 O 4 , but with a lower intensity ( Figure S14). For Ni 3 Ir 1 /MgAl 2 O 4 , in addition to a peak that appeared at 621 C (similar to the support), there were peaks at 778 and 873 C, and the peaks at 621 C and 809 C were present for Ir/MgAl 2 O 4 . This was because Ir had better dispersion and CO 2 could be effectively adsorbed and used at the Ir-MgAl 2 O 4 interface, while the Ni-MgAl 2 O 4 interface has no effect on CO 2 adsorption. This further supports the findings from the kinetics studies and in situ DRIFTS, which Ir sites impacted adsorption and activation of CO 2 and restricted carbon deposits during the DRM.

TPSR-MS analysis of the activation of methane and the elimination of carbon deposits
The activation and dissociation of CH 4 is the main cause of carbon deposition during DRM, therefore, the mass with the TPSR-MS was used to explore it. First, CH 4 was introduced during the temperature-programming process ( Figure 4A). The Ni/ MgAl 2 O 4 , Ni 3 Ir 1 /MgAl 2 O 4 , and Ir/MgAl 2 O 4 catalysts exhibited CH 4 activation abilities at 289, 328, and 347 C, respectively. The carbon species generated by the dissociation of CH 4 blocked the active sites, restricting the activation of CH 4 . This caused the CH 4 activation ability to decrease at 490, 610, and 590 C, respectively. This indicates that Ni was better than Ir for dissociation of CH 4 , and the dissociation rate increased with raising temperature.
Subsequently, TPSR-MS was performed on the catalysts after CH 4 treatment to explore how the different catalytic systems eliminate carbon deposits in a CO 2 atmosphere ( Figure 4B)  temperatures, which led active sites difficult to activate CH 4 , and the amount of carbon deposits and the resulting peak area were both relatively small. The above results show that Ni 3 Ir 1 /MgAl 2 O 4 and Ir/MgAl 2 O 4 , which followed path B, had advantages in the process of activating CO 2 and removing carbon species.
Raman spectra of the catalysts treated with CH 4 showed that all of them had D-band and G-band carbon peaks ( Figure 4C) and the carbon content followed Ni-Ir > Ir > Ni, which was consistent with the TPSR-MS-CO 2 results. In addition, the ratio of the D-band to the G-band indicated that the catalysts formed the same type of carbon species upon the dissociation of CH 4 . The different temperatures for CO 2 removal reflect the difference in the activation mechanism of CO 2 as described in the previous section.
In addition, we carried out TPSR-MS-DRM experiments ( Figure S15) in order to observe the behaviors of the three catalysts by temperature-programming under the DRM reaction atmosphere. In terms of CH 4 activation, the Ni 3 Ir 1 /MgAl 2 O 4 catalyst exhibited similar behavior to Ni/MgAl 2 O 4 , whose initial temperature of CH 4 activation was around 340 C. Ir/MgAl 2 O 4 catalyst needed to be above 480 C to activate CH 4 , which was higher than the former. Therefore, it can be indicated that in catalyzing CH 4 conversion, Ni species play a major role, and the electron transfer between Ni and Ir did not weaken the role of nickel in activating methane species.
Theoretical assessment of CO 2 activation on supported metal catalyst As shown above, the activation of CO 2 over metal surface is critical for the efficient removal of carbon deposits during DRM. DFT calculations were further performed to understand the difference between Ir and Ni on CO 2 activation at a molecular level. Herein, an Ni 8 cluster, an Ir 8 cluster, and two Ni 6 Ir 2 clusters with distinct configurations (denoted as Ni 6 Figure 5A). In particular, the Ni 6 Ir 2 /MgAl 2 O 4 -a and Ni 6 Ir 2 /MgAl 2 O 4 -b models have an atomic Ni/Ir ratio of 3/1 identical to that for the Ni 3 Ir/MgAl 2 O 4 catalyst used in the experiments. Previous studies indicate that such M 8 (M = metal) clusters were sufficient to reflect the characteristic of supported metal nanoparticles. 44,45 Considering that the elimination of carbon deposits during DRM mainly depends on the active O* species generated from CO 2 dissociation, 46-50 we therefore focused on the adsorption and dissociation processes of CO 2 at the metal-support interface. As shown in Figure 5B, CO 2 adsorption was much stronger on Ir 8 /MgAl 2 O 4 than Ni 8 / MgAl 2 O 4 (À2.14 eV versus À1.14 eV), consistent with the in situ DRIFTS experiments. For Ni 6 Ir 2 /MgAl 2 O 4 , the adsorption energy of CO 2 was between those for the Ir 8 and Ni 8 clusters, no matter the C atom of the adsorbed CO 2 on the Ni 6 Ir 2 clusters was bound to the Ni site (i.e., Ni 6 Ir 2 /MgAl 2 O 4 -a; À1.33 eV) or the Ir site (i.e., Ni 6 Ir 2 / MgAl 2 O 4 -b; À1.77 eV).
DFT calculations showed that the CO 2 dissociation (CO 2 * / CO* + O*, Figure 5B Figure 5B), further reflecting different stabilities of the incipiently formed CO* and O* species at the transition state (TS) of CO 2 dissociation on the Ir and Ni sites (TS structures shown in Figure S16).
In order to unveil the determining factors on the activity of CO 2 dissociation, adsorption energies for O atom and CO on the supported M 8 clusters were calculated independently to compare the stabilities of these two species on different metal sites. As shown in Figure 5C, the CO 2 dissociation barrier presented a nearly linear correlation with the O adsorption energy, while no apparent correlation was observed ll between the CO 2 dissociation barrier and the CO adsorption energy. These data clearly imply that the activity of CO 2 dissociation is mainly determined by the stabilization of the O* species at the TS, which prefers the Ir site over the Ni site.
The projected density of states (DOS) distributions of adsorbed O and M 8 clusters were further analyzed ( Figure 5D) to obtain a deeper understanding of the higher oxygen affinity of Ir. It was found that the antibonding states of O 2p became more populated in the trend of Ir 8  Mechanism of carbon deposits-elimination balance in DRM and its application With respect to the Ni 3 Ir 1 /MgAl 2 O 4 catalyst, Ni mainly played the role of dissociating CH 4 , while MgAl 2 O 4 not only acted as the support of the active metal sites, but also adsorbed CO 2 , forming carbonate species to enrich CO 2 . These carbonate species could be effectively activated by Ir to eliminate carbon species and inhibit carbon deposition. Since the main activation sites of CH 4 and CO 2 were different (Ni for CH 4 ; Ir for CO 2 ), the ratios of Ni and Ir atoms can greatly modulate the activation rates of CH 4 and CO 2 .
During the DRM process, although the Ni-Ir/MgAl 2 O 4 system had more carbon deposits, it maintained high stability. Interestingly, the formed carbon deposits seemed to have no effect on the catalytic activity. Therefore, carbon deposits formed on the catalysts, along with the reaction, were examined ( Figure 6A). We found that Ni/MgAl 2 O 4 had a faster rate of carbon deposition at the beginning and steadily increased. Further, as the carbon deposits increased, carbon chains blocked the contact between the active metal site and CH 4 . The reduction in the amount of active sites for CH 4 dissociation led to a decrease in the rate of carbon deposition and DRM activity. In contrast, the Ir/MgAl 2 O 4 catalyst did not form carbon deposits during the 100-h test, while carbon deposits increased within the first 20 h for Ni 3 Ir 1 /MgAl 2 O 4 , which was then stabilized with no significant additional increase until 100 h.
Accordingly, we proposed a carbon deposits-elimination balance mechanism (Figure 6B). With excessive Ni, the activation rate of CH 4 would be faster than that for CO 2 , and the dissociated carbon species would not be eliminated in time, leading to the generation of carbon deposits. This type of carbon deposition occurred on Ni, the site activated by CH 4 , which would block some of the Ni active sites. With a decrease in the Ni active sites, the activation rate of CH 4 decreased, resulting in a balance in the activation rate of CH 4 and CO 2 . Similarly, excessive carbon deposits led to higher activation rates for CO 2 than CH 4 , leading to the elimination of carbon deposits and exposing the Ni active sites previously covered. This caused the activation rate of CH 4 to increase until it matched that for CO 2 activation.
In the long-term test, this system gradually approached equilibrium until n(CH 4 ) z n(CO 2 ) (n(CH 4 ): the activation rate of CH 4 ; n(CO 2 ): the activation rate of CO 2 ). This was also why numerous carbon deposits were observed in the Ni-Ir/MgAl 2 O 4 catalytic system, but the effect on the catalytic activity was minimal. As n(CH 4 ) approached n(CO 2 ), the carbon deposits would no longer increase, but the existing carbon deposits would not be eliminated. In other words, the system achieved a balance between carbon deposition and elimination, and the overall reactivity was determined by the activation of CO 2 . Over the Ni/MgAl 2 O 4 catalyst, the dissociation of CH 4 and the activation of CO 2 occurred at the same active site, which did not follow this mechanism; thus, carbon deposits increased throughout the test. For the Ni 3 Ir 1 /MgAl 2 O 4 catalyst, the Ni sites were reduced sufficiently such that the reaction system reached n(CH 4 ) z n(CO 2 ) equilibrium after 20 h.
Based on these results, Ni-Ir/MgAl 2 O 4 catalysts were prepared with different Ni/Ir ratios for a 100-h test to assess carbon deposition ( Figures 6C, 6D, S17, and S18). With an increase in the Ir content from Ni/Ir = 12/1 to 1/3, the initial CH 4 conversion of the catalyst increased from 60.7% to 62.5%, and the average conversion of CH 4 increased from 59.1% to 62.4%. With increasing the Ir content, the activity and stability both increased. When the Ir/Ni ratio was 2, both of the initial and average conversions reached the highest level, and there was no further impact with an additional increase of the Ir content. In addition, the carbon deposits decreased from 14.2 wt % (Ni/Ir = 12/1) to 0 wt % (Ir/Ni = 2), and no carbon deposition occurred with further increasing the Ir content. This was because the increased Ir content brought more catalytic sites for CO 2 activation, allowing the system to reach the equilibrium between carbon deposition and elimination faster. Therefore, while maintaining high activity, the amount of carbon deposits was regularly reduced during the test. As the number of CO 2 activation sites increased, the activity of the ratedetermining step in the reaction increased, and the system exhibited higher activity. This result was consistent with the balance theory, as when Ir/Ni = 2, n(CO 2 ) matched n(CH 4 ) during the initial stage of the test. Therefore, the carbon deposition was almost negligible, with almost no differences between the initial activity and the average activity of the reaction, indicating an extremely high stability of the catalytic system.
We anticipated that a match between n(CO 2 ) and n(CH 4 ) in the initial state of the reaction by tuning the Ni-Ir relative contents would achieve a balance between the generation and elimination of carbon deposits, leading to a DRM catalytic system with zero carbon deposits. Because Ir is a precious metal, it was necessary to reduce the Ir content as much as possible. As shown in Figures 6C and 6D, the Ir/Ni ratio of 2 was found as the best catalyst ratio. We then conducted a 600-h long-time test under the conditions of 650 C, GHSV = 40,000 mL g À1 h À1 , and 1 bar ( Figure 6E). The initial conversion rates of CH 4 and CO 2 were 62.67% and 72.94%, respectively, with no significant change in activity after 600 h. Raman spectra of the spent catalyst showed no obvious signal peaks for amorphous carbon at 1,334 cm À1 or 1,598 cm À1 ( Figure 6F). In addition, no significant weight loss was observed from TGA ( Figure 6G). These results confirmed that no obvious carbon deposition occurred on the catalyst, consistent with the results of the catalytic system optimized based on our proposed theory.

Conclusions
DRM to generate syngas can serve as a bridge for the high-volume utilization of greenhouse gases and synthesis of industrial platform molecules. The core of the stable DRM reaction is the equilibrium between efficient CH 4 dissociation and CO 2 activation. We designed and constructed an ultra-stable Ni-Ir/MgAl 2 O 4 alloy system for DRM showing high activity, where Ni played the role of activating CH 4 and MgAl 2 O 4 adsorbed CO 2 to form carbonate species that can be effectively used by Ir to eliminate carbon species generated from CH 4 activation on Ni. Based on the feature of Ni, Ir, and MgAl 2 O 4 , we proposed a balance mechanism between the active sites and the carbon deposits-eliminations over Ni 3 Ir 6 /MgAl 2 O 4 catalytic system, which achieved equilibrium of n(CH 4 ) z n(CO 2 ) at the beginning of the reaction, and effectively inhibited the generation of carbon deposits while maintaining high activity and stability. This deeper understanding of the relationship among the activity of CH 4 dissociation, CO 2 activation, and carbon deposition-elimination would inspire new ideas on the rational design and development of novel catalysts for activation of inert C 1 molecules.

EXPERIMENTAL PROCEDURES
Resource availability Lead contact Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Dr. Chen Zhao (czhao@chem.ecnu.edu.cn).

Materials availability
All other data supporting the findings of this study are available within the article and the supplemental information.

Data and code availability
The complete experimental procedures are provided in the supplemental information.