Ying Huang

Yechuan Chen

Mingjie Xu

Alvin Ly

Albert Gili

Eamonn Murphy

Tristan Asset

Yuanchao Liu

Vincent De Andrade

Carlo U Segre

Alex L Deriy

Francesco De Carlo

Martin Kunz

Aleksander Gurlo

Xiaoqing Pan

Plamen Atanassov
email: plamen.atanassov@uci.edu

Iryna V Zenyuk
email: iryna.zenyuk@uci.edu

Catalysts by pyrolysis: Transforming metal-organic frameworks (MOFs) precursors into metal-nitrogen-carbon (M-N-C) materials

Keywords: Pyrolysis, Oxygen reduction reaction, Transition metal-nitrogen-carbon electrocatalysts, Metal-organic frameworks, Catalysts synthesis

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Introduction

Hydrogen technology is central for the decarbonization of heavy-duty transportation sector, difficult to directly electrify manufacturing as well as enable long-duration energy storage. When converted to electricity in polymer electrolyte fuel cells (PEFCs) no greenhouse gas emissions are generated and the only byproduct is water. For widespread adoption of PEFCs, their stack cost needs to be reduced. In the frame of large-scale development, Pt catalyst is considered as the largest portion of the stack cost [START_REF] James | Fuel Cell Systems Analysis In 2021 DOE Hydrogen and Fuel Cells Program Review Presentation[END_REF].

Most of the Pt loading is on the cathode side, where oxygen reduction reaction (ORR) takes place.

Substantial efforts have been made to develop PGM-free catalysts, among which the pyrolyzed metalnitrogen-carbon (M-N-C) being one of the most promising alternatives to the PGM materials [START_REF] Lefèvre | [END_REF][3][4][5][6][7][8].

Metal-nitrogen moieties (M-Nx) embedded in the carbon matrix of M-N-C materials have been recognized as the most essential active sites in ORR [9][10][11][12]. Significant efforts have been made to atomically disperse the metal species in the carbon matrix in order to obtain a higher density of active sites, as overall catalyst activity is proportional to electrochemically accessible site density. Besides the M-Nx sites, rationally designed carbonaceous materials are also necessary for the M-N-C catalysts, as their porous space ensures gaseous permeability and liquid-phase accessibility and the solid-state framework provides electronic conductivity and, through its surface properties, proton conductivity as well. The most common approach is obtaining M-N-C catalysts with sufficient M-Nx sites and specialty carbonaceous materials is pyrolyzing the mixture of metal-, nitrogen-and carbon-containing precursors.

Typical materials reported as precursors are macrocyclic compounds [13,14], polymers [15][16][17], metalorganic frameworks (MOFs) [4,18] and the mixture of the above with templating agents [3,5]. Among these candidates, MOF-derived catalysts show great potential for high catalytic activity, which comes from the porous structure with high surface areas and periodic metal-nitrogen-carbon bonds. Zeolitic imidazolate framework (ZIF) materials are usually the selected MOF precursors to synthesize M-N-C catalysts. Previous studies showed that the half-wave potential for the ORR of the catalysts synthesized from iron-doped ZIF-8 with a surface area of 969 m 2 g -1 reached 0.82 V in acidic media [19]. The catalysts derived from Co-based MOF with a surface area of 1359 m 2 g -1 exhibit a half-wave potential of around 0.85 V in alkaline media [20]. Recently, a great deal of effort has been made to disperse the metal on ZIF-be transported over ZIF-8 derived Zn-N-C materials, transmetalating the Zn sites with Fe, yielding a Fe-N-C catalyst with well-dispersed Fe-N4 sites [21]. CVD was also conducted to deposit N-doped carbon on the prepared Fe-N-C catalysts, yielding highly stable pyridinic N-coordinated Fe-N4 sites, significantly improving the durability of the catalyst [22].

General interest in pyrolyzing MOFs to synthesize catalysts is based on the hypothesis that the favorable porous structure of MOFs can be sustained during heat treatment. The high porosity and the surface area of the structure facilitates molecular transport and provides the possibility for the active sites to be highly dispersed. However, the above hypothesis might not always be correct. Some synthesized M-N-C catalysts derived from MOF-based materials and their corresponding morphological information, as well as the ORR activity in the acid electrolyte, are summarized in Supplementary Material (Table S1). From the (scanning) transmission electron microscopy images in previous studies [20,23], the hexagonal shape of the MOFs observed by microscopy before the pyrolysis did not persist after the pyrolysis.

Despite a large number of original and review publications, there is no specific study of chemical and morphological changes of MOF pyrolytic transformations. Moreover, critical details of such synthesis are seldom omitted in reporting or even misrepresented. There is widespread confusion as many claim the "chemical templating nature of MOFs" while others report very active MOF-derived catalysts with no structural resemblance to the precursor. The technical reason why pyrolyzed precursors only partially maintain their original MOF structure remains unclear. The pyrolysis process is currently a "Pandora's box"the structure of the product is only revealed after the synthesis process. Recently, several studies started to dedicate efforts to reveal the relationship between the pyrolysis temperature and catalysts' properties and their performance. It was found that the porphyrin-like ZnN4 site could form when temperature reached 600 °C [24] and high temperature would lead the ZIF-8 from crystalline structure to porous carbon structure [25]. However, the extent to which the morphological properties affect the electrochemical performance of MOF-derived catalysts remains ambiguous. These factors hinder the rational design of MOF-based catalysts. Herein, we observed the pyrolysis of the MOF materials by a combination of in situ and ex situ techniques to reveal the transformation from the morphological and spectroscopic perspective. Two commercial ZIFs with the same structures (ZIF-8 named Basolite ® Z1200 from Sigma-Aldrich © and ZIF-67 from PlasmaChem © ) were selected as MOF precursors in the study. ZIF-8, coordinated with zinc ions, and ZIF-67, coordinated with cobalt ions, share the same tetrahedral imidazole ligand coordination and sodalite topology. ZIF-8 (Figure 1a) has been combined with other precursors to form good transition metal-N-C catalysts [23,24,[26][27][28]. ZIF-67 (Figure 1d) can be used as a sole precursor since it contains cobalt, carbon and nitrogen in the same source. Ideally, cobalt is expected to bond with nitrogen and form Co-Nx active sites after pyrolysis [20,29,30]. With commercial chemicals, fewer interfering factors, like unknown contaminations and unexpected operation errors, are involved. Thus, the pathway of MOF leading to the carbonaceous materials via pyrolysis can be better characterized. The direct observation and thorough analysis of "the birth of catalysts" would be the foundation for the rational design of M-N-C.

Results and Discussion

As verified by X-ray diffraction (XRD), ZIF-8 (Figure 1b) and ZIF-67 (Figure 1e) are both pure crystalline products. The pattern of the ZIF-8 correlates rigorously with the structure reported by Yaghi's group [31]. In the pattern of the ZIF-67, extra peaks were observed at 2θ8 keV ≈ 37.8°, 39.6°, and 46.1° (Xray energy was converted to 8 keV from 25 keV based on Bragg's law) compared to the standard simulated XRD pattern structure [32]. These peaks might correspond to Co(OH)2 (PDF# 00-030-0443) and CoN (PDF# 01-083-8039), which was probably in coexistence with the cobalt source materials used to synthesize the ZIF-67 and will be further discussed in the in situ XRD results. Thermogravimetric analysis (TGA) was performed to examine the sample mass changes during pyrolysis. The derivative thermogravimetry (DTG) profile reveals transforming temperature points, which were later used as observation points for in situ experiments. For ZIF-8, the DTG curve shows a sharp peak at 625 °C, which should be due to the ZIF-8 melting and decomposition [33]. We also observed the loss of Zn component under the STEM (discussed later); thus, the mass loss can also be caused by the Zn component evaporation. The sample mass of the ZIF-8 kept decreasing after 625 °C and did not reach a plateau. The DTG curve of ZIF-67 exhibits continuous peaks from around 490 °C to 615 °C. This should correspond to ZIF-67 melting and decomposition [34,35].

The pyrolysis step is critical for converting MOFs into M-N-C catalysts, and often leads to severe morphological transformations. In situ scanning transmission electron microscopy (STEM) experiments that were performed under a vacuum environment reveal the change of the precursor morphology within a selected field of view. In the pristine images of the ZIF-8, hexagonal facets were clearly observed (Figure 2a and Figure S1). The corresponding energy-dispersive X-ray spectroscopy (EDS) indicates that Zn, N, and C are all distributed uniformly (Figure S2). When the temperature was elevated, the overall geometry of the materials did not show apparent changes but the size shrinkage (Figure 2a and Figure S3). With image thresholding, the percentage change of the area of ZIF-8 shown in the STEM from all temperatures can be quantified. The captured area of the materials in the region of interest (ROI) decreased by 34.7% at 650 °C and then by 45.2% at 850 °C compared to the pristine (Table S2). Only half of the area of the material remained in the ROI after the pyrolysis process. The EDS mapping shown in Figure S8 reveals very little of the Zn existence after pyrolysis. A zoom-in ROI at different temperatures showing a hexagonal facet of the ZIF-8 is also provided (Figure S4-S7). The structure became loose and more porous when the temperature was elevated to 650 °C. At 850 °C, the structure transformed into a more fibrous and disordered state, and turbostratic carbon as well as some clusters appeared as Figure S6 shows. After the sample was pyrolyzed to 1050 °C and after cooling down, the structure became more hollow together with numerous bright "dots" indicating the formation of metal clusters over the carbon substrate. These clusters and bright dots are most likely zinc species (according to EDS results in Figure S8-S9). The changes of the materials in three-dimensional views are further revealed via in situ nano-CT experiments under nitrogen environment. No noticeable transformation was observed before 650 °C.

When comparing the volume of an ROI calculated based on the segmentation results, the materials shrank by 23.8% (Table S3 and Figure S10). This is consistent with the 600 °C results gained from thresholding STEM images as provided in Table S2a. More representative cross-section images from the in situ nano-CT experiments are found in the SI (Figure S11-S12).

Crystal structure transformations were observed by performing in situ XRD experiment during the pyrolysis. The evolution patterns of ZIF-8 were collected at the rate of 10 °C min -1 and under a flowing gas stream composed of 5% H2 and 95% Ar (Figure 2c and Figure S13). The atmosphere contains hydrogen to remove oxygen and avoid oxidation, essentially providing a reducing environment. Based on catalytic performance of the samples pyrolyzed in different atmospheres reported in the previous publication [5,36,37], the hydrogen fraction should not provide enough chemical potential to significantly change the pyrolysis process when comparing the samples pyrolyzed in pure Ar/N2. Note that all intensity peaks at elevated temperatures are shifted toward lower 2θ compared to the reference patterns due to thermal expansion and corresponding increase of d-spacing. All diffraction peaks related to pristine MOF structure started disappearing at around 520 °C, indicating decomposition or amorphization. With the temperature elevated to 640 °C, the signal disappeared, and only flat patterns remained (as shown in Figure S14 for zoomed 24°-30° 2θ region). Interestingly, there was a diffraction peak at 2θ ≈ 21.5° appearing at 970 °C, which later disappeared during the sample cooling down. This diffraction pattern belongs to α-cristobalite (P41212, PDF# 00-039-1425), belonging to the sample-containing SiO2 capillary, and thus is an artifact from sample preparation. This artifact was confirmed by comparison with the ex situ data of the pyrolyzed ZIF-8 (Figure S15), where this pattern was not observed, indicating that the SiO2 is not inside the samples. This artifact should not affect the conclusion since the results from the XRD were consistent with the observation from other characterizations. No other new diffraction appeared when comparing the final pattern with the pristine one, confirming that no extra crystalline structure but totally amorphous product was formed during the pyrolysis. The process of amorphization at the lower temperature can be better demonstrated by the relative crystallinity changes as a function of temperature in ZIF-8 (Figure S16). The relative crystallinity of ZIF-8 was reduced to 53% around 600 °C. This number further decreased to zero at around 640 °C, indicating that the sample was fully amorphized [38]. The decrease in the relative crystallinity can be attributed to the local Zn-N4 tetrahedron deformation, meaning that the long-range-order structure has been transformed into the short-range-order one.

To obtain more information about the structural changes, ex situ XAS at the Zn K-edge was measured on the ZIF-8 pyrolyzed to 650 °C, 850 °C, 980 °C, and 1050 °C under Ar atmosphere. The pronounced peaks at 1.55Å in Zn K-edge R space plots (Figure 2d) can be assigned to the Zn-N shell. It revealed that some Zn remained in the sample after pyrolysis, and they are bonded with N to form Zn-Nx sites (coordination number x=4.5±0.5, detailed fitting information in Table S4, Figure S17). The XANES data (Figure 2e and Figure S18) of pristine ZIF-8 shows features at 9666.1 eV and 9673.7 eV, which should be associated with tetrahedral Zn-N4. The former feature appears due to the Zn 1s → Zn 4p transitions, while the latter feature reflects the multiple scattering of the mid-range order structure [12,39,40]. After pyrolysis, a peak at ca. 9669 eV showed up as the only feature, which indicates the existence of Zn-N4 (D4h symmetry). This feature shifted to lower energy when comparing the sample at 650 °C to those at higher temperatures. This change was caused by the variation in the degree of distortion of the D4h symmetry.

Given the fact that square planar Zn-N4 should cause a peak at around 9667 eV, the above phenomenon indicates that Zn 2+ were sitting closer to the porphyrin-like N4 plane when the temperature increased [41].

Ex situ Raman spectroscopy was performed to understand the evolution of MOFs to carbonaceous materials. The ZIF-8 samples pyrolyzed to 650 °C, 850 °C, 980 °C, and 1050 °C under Ar atmosphere were measured (Figure 2f and Figure S20, Table S5). The sample pyrolyzed to 650 °C has not fully transformed into carbonaceous material; thus, the corresponding Raman spectrum is complicated by the bands assigned to methyl group vibration, D band, and G+D' band [42,43]. The G band comes from the vibration of the E2g symmetry of the ideal graphitic lattice while the D' band comes from the vibration of the E2g symmetry of disordered graphitic lattice. D' band always appeared as a shoulder of G band and here they were fitted together. When the samples were pyrolyzed to 850 °C, 980 °C, and 1050 °C, the corresponding Raman signal can then be convolved into four peaks associated with G+D' band (~1593 cm -1 ), distorted carbon (~1504 cm -1 ), D band (~1346 cm -1 ) and the sp 2 carbon outside of the graphene network (~1220 cm -1 ) (Figure S19) [43,44]. The increased peak intensity at ~1220 cm -1 suggests that the carbon matrix of the sample pyrolyzed to 980 °C contains higher out-of-the-plane carbon compared to the sample pyrolyzed to a lower temperature. Once the temperature reached 980 °C, further elevating the pyrolysis temperature did not have much effect on the carbon matrix. The ratio of the intensities of the D band and G+D' band are both around 1.35 of the ZIF-8 pyrolyzed to 980 °C and 1050 °C (Figure S19d).

From the morphological and structural characterizations of the ZIF-8 and its pyrolyzed products, the pyrolyzed ZIF-8 can be an excellent soft template for the M-N-C catalysts. It overall retains the favorable porous structure of MOF (which would be confirmed with N2 absorption in later discussion), contains carbon mainly with an open structure, and its metal element did not agglomerate but spread out uniformly on the matrix. The ZIF-8 pyrolyzed to 980 °C and 1050 °C should have similar performance in terms of being an N-C precursor for metal deposition for ORR, if there were no further catalytic graphitization triggered by extra mixture components. This performance could be probably attributed to the relatively inert behavior of Zn.

Compared to zinc, cobalt is a livelier transition metal element. Similar characterizations have been carried out to study the transformation of ZIF-67 during pyrolysis (Figure 3). In situ STEM experiments reveal the morphological transformation of the ZIF-67 during pyrolysis at the nanoscale, showing a thoroughly different behavior with Zn as described previously. Hexagonal facets were clearly shown in the 250 °C images (Figure 3a and Figure S20). Corresponding EDS indicates that Co, N, and C are all distributed uniformly (Figure S21-22). However, when the temperature reached 450 °C, metallic species started agglomerating violently and forming clusters of ~20 nm (Figure 3a and Figure S23-24). These metal nanoclusters further grew into large clusters of ~100 nm during the pyrolysis (Figure S25). The EDS taken after the pyrolysis confirmed that the clusters mainly consist of cobalt (Figure S26). The melting point and the vaporization point of the cobalt are over 1500 °C when the vapor pressure is higher than 1×10 -7 atm [45]. The Co evaporation temperature would increase if there was a gas atmosphere existing and the pressure was lower. Based on the empirical equation log 𝑝 = -2.5(10 4 𝑇 ⁄ ) + 9.5, the vapor pressure around 1000 K could be simply estimated as 3×10 -16 atm discarding phase change. This value is several orders smaller than the ultra-high vacuum (1×10 -7 Pa or 1×10 -12 atm) environment inside STEM setup [45]. Thus, after ZIF-67 melted and decomposed, the cobalt stayed in the solid state in our experiment. At more elevated temperatures, cobalt clusters further migrated due to the heating. Several clusters aggregated to decrease surface energy and become stable, leading to cluster size growth [46]. The cobalt clusters started catalyzing the carbon graphitization once the temperature reached over 1000 °C [47,48]. The porous structure thus collapsed due to the carbon graphitization, which should result in the surface area decrease (discussed later with N2 absorption results). From the STEM, the carbon phase in the pyrolyzed product was combined with the graphitic carbon and the amorphous carbon (Figure S25). Some of the graphitic carbon wrapped the metal particle as the heterogeneous catalysis occurred on the interface between cobalt and carbon, making the cobalt particle difficult to be removed by the acid etching, and causing decreased porosity in the final product. The STEM images showed the precursor material area after pyrolysis inside an ROI decreased by 58.1% compared to the one pyrolyzed up to 250 °C (Table S6). In situ nano-CT experiments under nitrogen environment further revealed the changes of the materials in three dimensions. The volume of the same ROI decreased from around 148.4 µm 3 to 45.7 µm 3 , which corresponds to percentage volume change of 69.2% after the temperature increased from 200 °C to 980 °C (Figure S27 and Table S7). More corresponding cross-section images from the in situ nano-CT experiments are shown in the SI (Figure S28-29).

The in situ XRD experiment of ZIF-67 was performed with a heating rate of 10 °C min -1 and under flowing 5% H2 and 95% Ar atmosphere (Figure 3c and Fig. S30-S34). Note that all diffractions are shifted toward lower 2θ compared to the reference patterns due to thermal expansion. All 2θ were reported based on the X-ray energy of 8 keV (converted from 25 keV based on Bragg's law). The main diffraction of ZIF-67 (011) at 2θ8 keV ≈ 7.30° disappeared at 446 °C, which is consistent with the aforementioned melting and decomposition point from the TGA result (Fig. 1f). The peaks at 2θ8 keV ≈ S32)

and probably transformed to the cubic structure at higher temperature [49,50]. CoN (PDF# 01-083-8039), which appeared in the precursor's spectrum, was still present after pyrolysis. The diffraction at 2θ8 keV ≈ 21.8°, which also appeared in the ZIF-8 spectrum, can be attributed to α-cristobalite (P41212, PDF# 00-039-1425) belonging to the quartz capillary for sample holding. The ex situ measurement of the ZIF-67 pyrolyzed at 1050 °C did not detect this feature (Figure S35), suggesting that the sample did not include SiO2. The signal at 2θ8 keV ≈ 26.4° that appeared at approximately 992 °C with a crystallite size of around 0.36 nm (FWHM=0.3820) corresponds to the graphite (002) diffraction (PDF# 00-041-1487) (Figure S36). After the pyrolysis, the crystal structure of ZIF-67 transformed to mainly metallic Co (48.2 wt.%) and some cobalt nitrite (15.5 wt.%) existed according to the fitting results (Figure S33 and S34). The metallic Co catalyzed the graphitization process, causing the carbon matrix to be destroyed. This phenomenon has also been observed in a previous study [48]. Besides Co, Fe [51] and other metals [52] can also catalyze the graphitization during pyrolysis process. Even though graphitic carbon can enhance the electrical conductivity of the final product, microporosity of the materials is also important for an ORR catalyst to reach a good performance [53]. The graphitization caused the collapse of the porous structure and decreased surface proton conductivity. One of the main reasons to use MOF as precursor materials is MOF holds a favorable porous structure, and the heavy graphitization process weakens the advantage.

Ex situ XAS at the Co K-edge was measured on the ZIF-67 pyrolyzed to 450 °C, 650 °C, 850 °C, and 1050 °C under Ar atmosphere. The most pronounced peak at 1.55Å in the FT-EXAFS of the pristine ZIF-67 is associated with the Co-N bonding, as shown in Figure 3d. After the sample was pyrolyzed, the features at 2.21Å related to the Co-Co shell became dominant (Figure 3d, detailed fitting information in Figure S37, and Table S8). This correlates with the appearance of the metal particles in the STEM images and XRD patterns. The XANES data (Figure 3e and Figure S38) of pristine ZIF-67 demonstrates features at 7710 eV and 7728 eV. The former one is associated with the Co 1s → 3d transition, while the latter one is associated with the Co 1s → 4p transition [54]. The data of pristine ZIF-67 included a sharper peak arising from the white line compared to the data of the sample pyrolyzed to 450 °C. This indicates that the Co was in the oxidation state of +2. This could be from the ZIF-67 itself [55]. Combining the results from XRD, this can also be from the Co(OH)2 existing in the pristine ZIF-67 [56,57]. When the sample was pyrolyzed to 650 °C and above, the XANES spectra demonstrated the same features revealing the metallic Co particles in the samples [58].

Ex situ Raman spectroscopy was utilized to understand the evolution of MOF to carbonaceous materials (Figure 3f and Figure S39, and D/G ratio information in Table S9). Similar to that of low-temperature treated ZIF-8, the spectrum of the sample pyrolyzed to 450 °C demonstrates the features corresponding to the methyl group vibration, as well as the D band and G+D' band. The samples pyrolyzed to 650 °C and above can be considered carbonaceous materials. The intensity of the G+D' band increased as the temperature raised. The ratio of the intensities of the D band and G+D' band in the sample pyrolyzed to 1050 °C is less than one, indicating that more graphitic carbon exists than the defected or disordered carbon in the materials. These results agree with the conclusion from in situ STEM and XRD experiments. It is also worth noticing that the spectrum of the sample pyrolyzed to 1050 °C contains a sharp D' band (~ 1615 cm -1 ) (Figure S39). This means more defected-induced graphitic features (armchair or zigzag edge in the graphitic carbon structure [59]) existed in the samples. The appearance of the sharp D' band indicates that a significant amount of graphitic carbon exists in the samples pyrolyzed to 1050 °C.

From the morphological and structural characterizations of the ZIF-67 and its pyrolyzed products, the pyrolyzed ZIF-67 lost the favorable porous structure of MOF. The reason that the structure collapsed was the carbon graphitization, which was catalyzed by the Co cluster that formed during the pyrolysis. In our case, the final product of pyrolysis of ZIF-8 had no metallic particle and thus catalyzed graphitization was not observed. This phenomenon indicates that other transition metals which were reported to have catalytic graphitization availability, including iron, nickel, etc., might cause the same issue, if they remain in the matrix and agglomerate. The cobalt-catalyzed graphitization process is due to heteroepitaxial growth. The carbon solubility is 3.41% atom in Co at 1000 °C [60]. This means that carbon easily diffused into the bulk Co. As a consequence, Co precipitated a significant amount of carbon onto its surface, and thus the graphitic carbon formed [61]. Also, Co has a strong interaction with graphene [62],

meaning that the graphitic carbon layers (which are multiple layers of graphene) could be attached to the Co particle surface firmly. On the other hand, due to the existence of the cobalt nanoparticles, the pyrolyzed product of ZIF-67 might have a high peroxide yield during ORR, which is confirmed later with the electrochemical testing results.

To learn more about the evolution of the surface chemistry and to confirm the porous network structures evolution envisaged from the previous STEM results, the samples pyrolyzed to 975 °C were acid-etched to remove the particles (scheme was described in Figure 4a-b). The samples were not pyrolyzed above 1000 °C to avoid the formation of excess graphitic carbon, which might cause difficulties in removing particles (Figure S40 shows the example where Co particles remained inside the graphite layer after acid etching). XPS was performed to understand the surface chemistry of the precursors (ZIF-8 and ZIF-67), the samples pyrolyzed to 975 °C, and the samples before and after acid etching (Figure 4c-d, Table S10-S11). Ar-etching technique was applied to reveal the chemistry of the content beneath the surface. The favorable for the first and second steps of the two-electron pathway [63]. The ratio of these above sites is related to the efficiency of this pathway. The ratio was close to one for the acid-etched pyrolyzed ZIF-8.

However, the ratio of the pyrolyzed ZIF-67 before and after acid-etching was 0.87 and 0.78, respectively.

Inductively coupled plasma-mass spectrometry (ICP-MS) was also performed to assess the quantity of the metal element inside the final catalysts before and after acid etching. The zinc in pyrolyzed ZIF-8 was 0.52 wt.% while the number decreased to 0.27 wt.% after acid etching. The cobalt in pyrolyzed ZIF-67 was 64.98 wt.% while the number dramatically was reduced to 1.80 wt.% after acid etching.

N2 absorption was applied to probe the changes in the pyrolyzed product before and after the acid etching step. The isotherms of the ZIF-8 samples showed abrupt increases at low relative pressure (Figure 4e).

This indicates the existence of micropores. There was no significant difference in the surface area and the pore size distribution of the samples after acid etching (surface area: 1,171.8 m² g -1 ) compared to the one before (surface area: 1,219.9 m² g -1 ). Thus, the acid etching did not affect significantly the pyrolyzed ZIF-8 zinc samples. The isotherms of the ZIF-67 samples displayed typical type-IV isotherms with a type H4 hysteresis loop in the range of P/P0 > 0.4, indicating the presence of mesopores. The sample after acid etching showed a higher surface area (surface area: 158.0 m² g -1 ) and increased pore volume (Figure S47) compared to the one before etching (surface area: 51.8 m² g -1 ). This can be due to the removal of the cobalt nanoparticles.

The electrochemical activity of the samples pyrolyzed at different temperatures was assessed with a rotating ring disk electrode setup (summary in Figure 4f, detailed information in Figure S48-S55).

Overall, the cyclic voltammetry (CV) curves of the samples pyrolyzed to higher temperatures demonstrated a quasi-rectangular shape. The capacitance of the ZIF-8 pyrolyzed to 1050 °C, as shown in Figure S50, was lower than the ones pyrolyzed at 850 °C and 980 °C, which indicates an eventual collapse of the pristine porous structure. This phenomenon can also be attributed to the existence of more disordered graphitic carbon and the decreased volume of pores. The half-wave potential of the ZIF-8 pyrolyzed to 1050 °C, although very similar to the one of the samples pyrolyzed to 980 °C as shown in Figure S52, is the highest. This is probably due to the metal-N active sites formation at high temperatures.

For ZIF-67, the sample pyrolyzed to 450 °C had a very low capacitance, indicating that the carbon substrate had not formed at this stage. With the pyrolysis temperature increased, the capacitance first increased and then decreased. That is due to the existence of metallic cobalt, which blocked some of the pores in the sample. The half-wave potential of the pyrolyzed ZIF-8 is higher than the pyrolyzed ZIF-67, disregarding the high degree of graphitization of the pyrolyzed ZIF-67. This reflects that the degree of graphitization is not the governing factor of good performance. However, the half-wave potential of the pyrolyzed ZIF-67 increased from 0.48 V to 0.61 V after acid etching, while the half-wave potential of the pyrolyzed ZIF-8 remained roughly the same. This is due to the increase of the capacitance and of the surface area after the removal of the metal particles. The peroxide yield of the pyrolyzed ZIF-67 is much higher compared to the pyrolyzed ZIF-8 no matter whether it was acid etched or not. The reason is that the product of pyrolyzed ZIF-67 contained more inactive metallic cobalt (embedded in the carbon structure) and probably cobalt carbide. This led to the insufficient existence of the M-Nx sites in the sample, which can further reduce the peroxide to water. Thus, the pyrolyzed ZIF-67 could not efficiently reduce the peroxide, which generation was catalyzed by the N-H sites in the final catalysts. The same conclusion can be drawn from the experiments performed under both 1600 rpm and 900 rpm.

Combining all the characterization results, the transformation from the MOF precursors to the M-N-C catalysts was directly observed. The most pronounced difference between the pyrolyzed product of ZIF-8

and ZIF-67 was the formation of the metallic particles (Figure 5). The metallic particles, though can be active for the ORR, would not be dispersed well during the pyrolysis and thus the metal active site density was low. Furthermore, the Co particles catalyzed the carbon graphitization process easily during the pyrolysis process due to the high carbon solubilities in cobalt. As contrast, the most crucial advantage of ZIF-8 being a precursor is the well-dispersed Zn after the pyrolysis process. However, the Zn concentration inside the sample was low since this element could evaporate and sublimate (metallic Zn boils around 550 °C or sublimates around 300 °C at low pressure [64]) at the early stage of the pyrolysis.

This indicates that not many metal-related active sites exist in the final catalysts but retained porous carbon matrix with decent N-C moieties. This significantly affects the performance, resulting in a low 0.6

V half-wave potential in acid but high peroxide yield. Thus, ZIF-8 has the promise to be a great templating precursor for ORR, as it can serve as a template for deposition of transition metal species, such as Fe that are active towards ORR [21,22,65].

In conclusion, MOF has a favorable porous structure that benefits catalytic activity. To make it conductive and to form M-N-C catalysts, pyrolysis is an essential and convenient step. However, pyrolyzing transition metal-doped MOFs can cause the formation of severely agglomerated particles. This formation not only significantly decreases the concentration of the active sites but also catalyzes carbon graphitization at high temperatures. The graphitization process causes the favorable structure to collapse, and thus voids the meaning of applying MOF as precursors. Direct heat treatment utilization of highconcentration transition metal MOFs towards carbonaceous electrocatalyst synthesis would not be recommended unless specific rational designs are developed to inhibit potential agglomeration and graphitization. ZIF-8 is a more promising MOF precursor candidate since its doping metal, Zn, evaporates and sublimes during the pyrolysis, thus it did not trigger catalytic graphitization during heat treatment and retains the porous structure. However, its limited electrocatalytic performance requires the involvement of other metal elements and improvement strategies. The possible synergetic effects between Zn and other metals in multi-metal MOFs during synthesis for electrocatalysis still remain uncertain. It is critical to note that transition nano-particles formation during pyrolysis plays the role of switching the molecular templating of MOF structure towards solid-state templating by the nascent metal/metal carbide/ metal oxide nanophase.

Methods

Materials synthesis

ZIF-8 was purchased from Sigma-Aldrich © (Basolite ® Z1200), while ZIF-67 was purchased from PlasmaChem © . The ex situ samples were pyrolyzed with a ramping rate of 10 °C min -1 under Ar. The acid etched products were prepared by etching the pyrolyzed samples with 3M HCl overnight followed by being washed with deionized water until the pH reached neutral values.

Scanning transmission electron microscope (STEM)

The in situ heating STEM experiments were done in a vacuum environment. The Protochips Fusion

Select TM was used where there is one heating chip (E-FHDC, Protochips, Inc.) applied and the φ = 8 μm holes were covered by holey carbon thin films. The MOF materials were dispersed in the ethanol solvent with ultrasound bath. The resulting sample suspension was then dropped onto the chip by pipette.

After the sample had dried on the chip, an optical microscope was used to check whether the powder sample was on the chip windows.

The sample for ex situ STEM experiments was dispersed in ethanol solvent by ultrasound bath. The resulting suspension was then dropped on the holey carbon-coated copper grid. The sample was dried before transferring into the STEM.

STEM was performed on a JEOL JEM-ARM300CF microscope with probe correctors and a 300 kV cold field emission gun. All reported temperatures were calculated by the measured electrical resistance of the heater. Energy dispersive X-ray spectroscopy (EDS) was collected at the pristine state, 250 °C, and at the final cooling-down state, with 100 mm 2 dual silicon drift detectors.

In situ X-ray nano-computed tomography (nano-CT)

The in situ heating X-ray nano-CT experiments were performed with the transmission X-ray microscope from beamline 32-ID of the Advanced Photon Source (APS) at Argonne National Laboratory (ANL).

Monochromatic X-rays of 8 keV energy were applied. A 50 nm outermost zone width Fresnel zone plate was used as an objective lens and a phase-ring placed in the back focal plane of the zone plate was used to perform imaging in Zernike phase contrast mode. The resulting voxel size is 21.3 nm.

Samples were mounted on the graphite pin, which was mounted onto a high-accuracy air-bearing rotary stage. The top part of the pin was enveloped by an inductive furnace with X-ray transparent window. The N2 gas flew through the furnace during the whole experiment. Scans were performed for pyrolysis process at pristine state, 200 °C, 400 °C, 600 °C, 850 °C, 980 °C, 1050 °C and at the final state after cooling down to room temperature. However, the images taken at high temperatures were not reconstructed since the rotating center was changing during the scan due to the intense vibrations. The temperature stepping ramp rate was used with 10 °C s -1 and 15 min hold time at each temperature.

The data was reconstructed with TomoPy and ASTRA toolbox [66][67][68]69 ]. Image processing was done with Dragonfly 2022.1 Software (Object Research Systems (ORS) Inc, Montreal, Canada; software available at http://www.theobjects.com/dragonfly). The semantic segmentation is done with UNet++ architecture [START_REF] Zhou | UNet++: A Nested U-Net Architecture for Medical Image Segmentation[END_REF][START_REF] Zhou | [END_REF].

X-ray diffraction (XRD)

The ex situ XRD is performed on Rigaku SmartLab with Cu Kα source. High-temperature in situ powder XRD was collected at beamline 12.2.2 of the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (LBNL) in an environmental cell previously reported [72][73][74][75][76]. Around 1 mg of the powder sample was introduced into a 0.7 mm outer diameter quartz capillary provided by Hilgenberg GmbH, Germany. The gas was delivered by a set of mass flow controllers to the sample via a tungsten carbide injection capillary. The quartz capillary and the tungsten carbide capillary were both mounted and enclosed inside a silicon carbide cylinder with two holes allowing X-ray beam to pass. The cylinder was heated with two infrared lamps (Osram 64635HLX) and served as a furnace. A pattern with a LaB6 standard NIST 660b was acquired for setup calibration before the experiment. The setup temperature was controlled using a S-type thermocouple placed next to the sample. Prior to performing experiments with the MOF samples, a temperature calibration curve was done using Pt as standard material all along the temperature range. The reported temperature was the average temperature during a 30 s exposure time.

All experiments were performed with an incident X-ray beam of 25 keV and a spot size of 30 × 30 µm 2 .

The XRD data was collected when the precursor material was heated from room temperature to 1035 °C at the rate of 10 °C min -1 , held at 1035 °C for 30 min, and cooled to room temperature under 5% H2 and 95% Ar atmosphere. The gas was flowing at 10 NmL min -1 . The patterns containing Debye diffraction rings were processed on-the-fly using Dioptas software [77] to yield intensity as a function of 2 Theta. The relative crystallinity was calculated by integrating area under the peaks and compared the integration to the one calculated based on the pristine spectrum. PDXL software was also used to analyze the data.

Thermogravimetric analysis (TGA)

TGA characterization was performed on a simultaneous thermal analyzer (Netzsch STA 449 F3 Jupiter).

The precursor materials were mounted in ceramic crucibles. Blank experiments were performed for background corrections. All samples were heated under Ar from 80 °C to 1020 °C with the rate of 10 °C•min -1 . NETZSCH Proteus Thermal Analysis was used to analyze the data.

Raman spectroscopy

The Raman spectra were obtained on a Horiba LabRAM HR Evolution confocal Raman microscope with 633 nm laser and 600 g mm -1 grating. 100x objective was applied. Twenty-five individual 20 s spectra were accumulated. CasaXPS software was used to analyze the data. All data was fitted with four peaks based on publications [42,44].

X-ray Absorption Spectroscopy (XAS)

XAS data were collected at the 10-BM beamline of APS at ANL [START_REF] Kropf | The New MRCAT (Sector 10) Bending Magnet Beamline at the Advanced Photon Source[END_REF]. The sample was mixed with some binder and pressed as a pellet. The pellet was then sealed in a holder with Kapton tapes. The products of pyrolyzed ZIF-8 were measured at the Zn K-edge (9659 eV) in transmission mode. The products of pyrolyzed ZIF-67 were measured at Co K-edge (7709 eV) in fluorescence mode. The data was processed and modeled using the Demeter XAS software package [START_REF] Ravel | [END_REF]. The EXAFS amplitude reduction factor S0 2 was determined based on the fitting of standard reference material. The fits were performed in k-weight = 1,2,3 with a Fourier Transform range and fitting window indicated in Table S3 and S6.

X-ray Photoelectron Spectroscopy (XPS)

XPS was performed on a Kratos AXIS Supra spectrometer with a monochromatic Al Kα source. Ar ion cluster was used to etch the sample surface for 30 s with 500 eV Ar250+ cluster setup after the surface was characterized. CasaXPS software was used to analyze the data. The fitting was using 70% Gaussian / 30% Lorentzian setup based on previous calculations and publications [16,63,[80][81][82][83].

Inductively Coupled Plasma-Mass Spectrometry (ICP-MS)

ICP-MS was mearsured on the ThermoFisher iCAP RQ ICP-MS system. The powder samples were digested in aqua regia over several days, and then diluted to ppb concentration levels in 1 vol.% nitric acid in Milli-Q water. The standard calibration curves were prepared by diluting an elemental standard in 1 vol.% nitric acid in Milli-Q water and then performing a series of auto-dilutions in the system.

Nitrogen physisorption

Nitrogen physisorption was done in a Micromeritics 3Flex Surface and Catalyst Analyzer. The samples were vacuumed at 90 °C for 1 h and then at 200 °C for 6 h before the measurement. The surface area was calculated by the Barrett-Emmett-Teller (BET) method. The pore size distribution was obtained with the non-local density functional theory model (NLDFT).

Electrochemical measurement

The electrochemical experiments were done with a rotating ring disk electrode (RRDE) setup. The catalyst inks (9.88 µg•µL -1 ) were prepared by mixing samples, 5 wt.% Nafion solution (Sigma-Aldrich) and 1:1 MilliQ H2O (18.2 MΩ) / Isopropanol (Sigma-Aldrich) under sonication. 2 × 10 µL of the catalyst ink was drop cast on the working electrode (glassy carbon electrode, Sgeo = 0.247 cm 2 with a platinum ring) while the electrode was rotating slowly. The electrode was dried under heat-flux (and while maintaining rotation) between each deposition. A final layer of 800 µg cm -2 loadings was derived. The catalyst was activated in an N2-saturated electrolyte through 100 cycles at 500 mV s -1 between 0.05 and 1.23 V vs. RHE, followed by 3 cycles at 20 mV s -1 and linear sweep voltammetry between 0.05 and 1.05

V vs. RHE at 20 mV s -1 (as a baseline correction). Then, linear sweep voltammetry between 0.05 and 1.05 V vs. RHE at 20 mV s -1 was measured at 900 rpm and 1600 rpm at O2-saturated electrolyte. The data was corrected from the ohmic losses in solution and the baseline obtained in N2-saturated electrolyte. f. TGA profile of ZIF-67 under Ar atmosphere. Arrows indicate the temperature points during the process chosen to be studied in the in situ STEM experiment d. Atomic concentration of different elements gained from XPS of ZIF-67, pyrolyzed ZIF-67 before and after acid etching (HCl), as well as acid etching after Ar beam etching (HCl ArE).

e. Adsorption isotherm plots of N2 on the pyrolyzed ZIF-8 and ZIF-67 before and after acid etching (980 °C)

f. Half-wave potential (blue bar) and peroxide yield (orange bar) gained from RRDE experiment for the pyrolyzed sample before and after acid etching. Disk and ring current density were collected under 1600 rpm rotating speed. The sample loading on the electrode was of 800 µg cm -2 geo and the electrode surface was of 0.247 cm 2 . The electrolyte is O2-saturated 0.1 M HClO4.

  surface chemistry changes mainly occurred during the pyrolysis process. The atomic concentration of the C 1s on the ZIF-8 sample surface increased by 30.7% after pyrolysis, while the N 1s and the Zn 2p decreased by 76.1% and 92.1%, separately. The decrease of the Zn 2p atomic concentration confirms that the metal species evaporated during the heat treatment. After acid etching, the atomic concentration on the sample surfaces of Zn 2p further decreased and the O 1s increased slightly. This indicates that some of the metal species etched away, and the surface adsorbed some water during the etching. The atomic concentrations of C 1s, N 1s and Zn 2p did not change, meaning that the contents are uniform on the sample surface. For the ZIF-67, the atomic concentration of the C 1s has similar changes as the ZIF-8 sample after pyrolysis and acid etching. The atomic concentration of O 1s did not change significantly on ZIF-67 surface after pyrolysis and decreased a little bit after the acid-etching. However, there was a large decrease in the N 1s and Co 2p concentrations on the surface of the ZIF-67. Most of the nitrogen and Co might be beneath the sample surface in a collapsed structure. The high-resolution C 1s and N 1s XPS spectra with curve fitting reveal the carbon framework and N moiety composition (FigureS41-46). The typical M-N-C carbon framework appeared after the samples were pyrolyzed. Metal-N is the most favorable site since it prefers the four-electron pathway[63]. The atomic concentration of metal-N in the pyrolyzed ZIF-67 increased by 66.3% after Ar etching. This indicates that most of the cobalt is embedded inside the samples. Pyridine N and N-H (including pyrrolic N and hydrogenated pyridinic N) are

Figure 1 .

 1 Figure 1. Characterization of the ZIF-8 and ZIF-67. a. the crystal structure of ZIF-8 b. X-ray diffraction patterns of ZIF-8 and simulated pattern (CCDC-602542). The X-axis of the pristine ZIF-8 data has been converted from 25 keV (0.4959 Å) to Cu Kα (1.5406 Å). c. TGA profile of ZIF-8 under Ar atmosphere. Arrows show the temperature points during the process chosen to be studied in the in situ STEM experiment d. the crystal structure of ZIF-67 e. X-ray diffraction patterns of ZIF-67 and simulated pattern (CCDC-671073). The X-axis of the pristine ZIF-67 data has been converted from 25 keV (0.4959 Å) to Cu Kα (1.5406 Å).

Figure 2 .

 2 Figure 2. In-situ transformation of the ZIF-8 during pyrolysis. a. In situ STEM experiment under vacuum. HAADF-STEM images for ZIF-8 during pyrolysis at 250 °C, 1050 °C and after cold down. Scale bar: 500 nm. b. In situ nano-CT experiment under N2 environment. Grayscale images and the three-dimensional volume renderings of the identical location of ZIF-8 during pyrolysis at 200 °C and 650 °C. Scale bar: 1,000 nm. c. In situ XRD experiment under flowing 5% H2 + 95% Ar. XRD patterns of ZIF-8 recoded at selected temperatures. The X-axis has been converted from 25 keV (0.4959 Å) to Cu Kα (1.5406 Å). The arrow indicates the reaction progress. d. Ex situ EXAFS of the ZIF-8 and ZIF-8 pyrolyzed at 650 °C, 850 °C, 980 °C and 1050 °C under Ar environment. e. Ex situ XANES of the ZIF-8 and ZIF-8 pyrolyzed at 650 °C, 850 °C, 980 °C and 1050 °C under Ar environment. f. Ex situ Raman of the ZIF-8 pyrolyzed at 650 °C, 850 °C, 980 °C and 1050 °C under Ar environment.

Figure 3 .

 3 Figure 3. Transformation of the ZIF-67 during pyrolysis. a. In situ STEM experiment under vacuum. HAADF-STEM images for ZIF-67 during pyrolysis at 250 °C, 1050 °C and after cold down. Scale bar: 500 nm. b. In situ nano-CT experiment under N2 environment. Grayscale images and the three-dimensional volume renderings of identical location of the ZIF-8 during pyrolysis at 200 °C and 980 °C are shown. Scale bar: 1,000 nm. c. In situ XRD experiment under flowing 5% H2 + 95% Ar. XRD patterns of ZIF-67 recoded at selected temperatures. ♠ represents Co, • represents CoN. The X-axis has been converted from 25 keV (0.4959 Å) to Cu kα (1.5406 Å). The arrow indicates the reaction progress. d. Ex situ EXAFS of the ZIF-67 and ZIF-67 pyrolyzed at 450 °C, 650 °C, 850 °C and 1050 °C under Ar environment. e. Ex situ XANES of the ZIF-67 and ZIF-67 pyrolyzed at 450 °C, 650 °C, 850 °C and 1050 °C under Ar environment. f. Ex situ Raman of the ZIF-67 pyrolyzed 450 °C, 650 °C, 850 °C and 1050 °C under Ar environment.

Figure 4 .

 4 Figure 4. Comparison of pyrolyzed ZIF-8 and ZIF-67 before and after acid etching. Samples were pyrolyzed to 975 °C.

Figure 5 .

 5 Figure 5. Schematic figure on pyrolyzing ZIF-8 and ZIF-67 to over 1000 ºC.

Acknowledgment IVZ would like to acknowledge support from the National Science Foundation under CBET Award 1605159. This research used the resources of the Advanced Light Source (ALS). The ALS is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the U.S. DOE Office of Science by Argonne National Laboratory under contract No. DE-AC02-06CH11357. XAS was performed in the Materials Research Collaborative Access Team (MRCAT). MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. STEM, XRD (SmartLab), ICP-MS and TGA were performed at the UC Irvine Materials Research Institute (IMRI). XPS was performed at the UC Irvine Materials Research Institute (IMRI) using instrumentation funded in part by the National Science Foundation Major Research Instrumentation Program under grant no. CHE-1338173. Raman was conducted in UC Irvine's Horiba Institute for Mobility and Connectivity 2 (HIMAC 2 ).

Data Availability

The raw data is available to be shared upon request from the authors.


Laurent Delafontaine

Eamonn Murphy

Shengyuan Guo

Yuanchao Liu

Tristan Asset

Xiaoqing Pan

Plamen Atanassov

Synergistic electrocatalytic syngas production from carbon dioxide by bi-metallic atomically dispersed catalysts

Keywords: CO2 reduction, syngas, bi-metallic, atomically dispersed catalyst, electrocatalysis

The production of syngas by traditional processes such as steam methane reforming is energetically intensive and produces a large amount of CO2 emissions. In contrast, the electrochemical CO2 reduction reaction (CO2RR) enables the carbon neutral production of syngas at ambient conditions. Among non-precious metal catalysts, metal-nitrogencarbon electrocatalysts are inexpensive and highly selective towards syngas production.

This study examined the selectivity of mono-and bi-metallic (M-N-C, M = Fe, Mo or FeMo) electrocatalysts towards syngas production. The ratio of the CO:H2 in the syngas was tuned by modifying the ratio of metallic precursors in the bi-metallic FeMo-N-C catalysts, tailoring the catalysts' selectivity towards the CO2RR or the hydrogen evolution reaction (HER). The catalyst synthesis temperature(s) are considered as they influence the catalyst morphology and activity. Further, the dependence of the ratio of CO:H2 in the syngas as a function of the potential is explored for the different bi-metallic catalysts. This work showed that by tailoring both the ratio of Fe:Mo in the bi-metallic catalyst and optimizing the reductive potential, a CO:H2 ratio between 0.25 to 5 was achievable. This study demonstrated a novel approach in which the ratio of the product syngas composition can be tailored in a single reaction, without the need for further downstream processing to reach a desired composition.

Introduction

Electrochemical CO2 reduction is a promising method to mitigate anthropogenic CO2 emissions while simultaneously producing value-added chemicals. One of the most feasible methods for industrial applications is the implementation of an H2O/CO2 coelectrolysis process where the target products are H2 and CO. Synthesis gas (syngas) is generally a mixture of CO, H2, CO2, and smaller amounts of CH4 and other gases. [START_REF] Lu | Electrosynthesis of Syngas via the Co-Reduction of CO2 and H2O[END_REF] The market value for syngas is low compared to other possible products from CO2 reduction (e.g., market value for syngas = 0.06 $ kg -1 ; market value for CO = 0.6 $ kg -1 and for npropanol = 1.43 $ kg -1 ). [START_REF] Delafontaine | Metal-Nitrogen-Carbon Electrocatalysts for CO2 Reduction towards Syngas Generation[END_REF][START_REF] Jouny | General Techno-Economic Analysis of CO2 Electrolysis Systems[END_REF][START_REF] Sánchez | Recent Advances in Industrial CO 2 Electroreduction[END_REF] However, the product stream from a CO2 electrolyzer producing CO and H2 may be used with minimal processing/purification in a subsequent step producing higher value products (e.g., for production of Cx (x >= 1) compounds through the Fischer-Tropsch process, for catalytic methanation of syngas, or for biological or fermentation-based processes producing high-value alcohols). [START_REF] Delafontaine | Metal-Nitrogen-Carbon Electrocatalysts for CO2 Reduction towards Syngas Generation[END_REF][START_REF] Jouny | General Techno-Economic Analysis of CO2 Electrolysis Systems[END_REF][START_REF] Sánchez | Recent Advances in Industrial CO 2 Electroreduction[END_REF][START_REF] Tackett | Net Reduction of CO2 via Its Thermocatalytic and Electrocatalytic Transformation Reactions in Standard and Hybrid Processes[END_REF][START_REF] Verma | Co-Electrolysis of CO2 and Glycerol as a Pathway to Carbon Chemicals with Improved Technoeconomics Due to Low Electricity Consumption[END_REF] The primary catalysts which are conventionally used in a H2O/CO2 co-electrolysis process are Ag and Au-based catalysts. These catalysts have the highest selectivity for CO but are prohibitively expensive for large scale use. [START_REF] Kim | Achieving Selective and Efficient Electrocatalytic Activity for CO2 Reduction Using Immobilized Silver Nanoparticles[END_REF][START_REF] Jiang | Silver Nanoparticles with Surface-Bonded Oxygen for Highly Selective CO2 Reduction[END_REF][START_REF] Lu | Electrochemical CO2 Reduction: Electrocatalyst, Reaction Mechanism, and Process Engineering[END_REF] Therefore, earth-abundant non-precious metal catalysts are increasingly developed as cathode materials for CO2 conversion processes.

Among non-precious metal catalysts explored, atomically dispersed, transition metal-nitrogen-carbon (M-N-C) catalysts have proven to be highly selective for CO2 reduction. [START_REF] Asset | Investigating the Nature of the Active Sites for the CO2 Reduction Reaction on Carbon-Based Electrocatalysts[END_REF][START_REF] Huang | Metal-Nitrogen-Doped Carbon Single-Atom Electrocatalysts for CO2 Electroreduction[END_REF] Metal-free N-C catalysts also show substantial selectivity for CO2 reduction with lower activity compared to their metal counterparts. [START_REF] Hu | Selective CO2 Reduction to CO in Water Using Earth-Abundant Metal and Nitrogen-Doped Carbon Electrocatalysts[END_REF] While Fe-containing M-N-C catalysts are known to catalyze the reduction of CO2 to CO at low overpotential, Mocontaining M-N-C catalysts have active Mo-Nx centers which may catalyze the competing HER reaction. [START_REF] Asset | Investigating the Nature of the Active Sites for the CO2 Reduction Reaction on Carbon-Based Electrocatalysts[END_REF][START_REF] Huang | Metal-Nitrogen-Doped Carbon Single-Atom Electrocatalysts for CO2 Electroreduction[END_REF][START_REF] Varela | Metal-Doped Nitrogenated Carbon as an Efficient Catalyst for Direct CO2 Electroreduction to CO and Hydrocarbons[END_REF][START_REF] Amiinu | Multifunctional Mo-N/C@MoS2 Electrocatalysts for HER, OER, ORR, and Zn-Air Batteries[END_REF] The active sites of M-N-C catalysts are diverse and complex, containing both nitrogen coordinated metal sites (M-Nx), and a variety of metal-free (N-C) sites whose participation in the CO2RR vs. HER is highly dependent on the specific Nmoiety and the surrounding chemical environment. [START_REF] Asset | Investigating the Nature of the Active Sites for the CO2 Reduction Reaction on Carbon-Based Electrocatalysts[END_REF][START_REF] Huang | Metal-Nitrogen-Doped Carbon Single-Atom Electrocatalysts for CO2 Electroreduction[END_REF][START_REF] Varela | Metal-Doped Nitrogenated Carbon as an Efficient Catalyst for Direct CO2 Electroreduction to CO and Hydrocarbons[END_REF] Fe-N-C catalysts are highly selective for CO formation at low overpotential (E > -0.50 VRHE), while the Ni-N-C catalyst boasts the lowest Tafel slope for CO2 reduction to CO and generally the highest overall CO production rates at higher overpotential (E < -0.75 VRHE). [START_REF] Möller | Efficient CO 2 to CO Electrolysis on Solid Ni-N-C Catalysts at Industrial Current Densities[END_REF] Generally, mono-metallic M-N-C catalysts which have substantial selectivity to CO (e.g., M-N-C catalysts where M = Fe, Ni, Mn, Cr, and to a lesser extent Co) have a potential-dependent faradaic efficiency for CO formation. [START_REF] Varela | Metal-Doped Nitrogenated Carbon as an Efficient Catalyst for Direct CO2 Electroreduction to CO and Hydrocarbons[END_REF][START_REF] Varela | Electrochemical Reduction of CO2 on Metal-Nitrogen-Doped Carbon Catalysts[END_REF] As more reductive potentials are applied, the faradaic efficiency (FE) for CO formation will reach a maximum (e.g., > 90 % for Fe and Ni-containing M-N-C) and then decrease as HER activity increases. [START_REF] Varela | Electrochemical Reduction of CO2 on Metal-Nitrogen-Doped Carbon Catalysts[END_REF] This offers a dynamic control over the syngas or CO:H2 product ratio by changing the applied reductive potential. The selectivity of several M-N-C catalysts for syngas generation are summarized in Table S1. The ability to selectively tailor the ratio of CO:H2 in the produced syngas in a single reaction enables a desired syngas compostion (to fit a desired application) to be realized without the need for further downstream mixing or processing.

One approach to selectively tailor the ratio of CO:H2 in syngas from the CO2RR was presented by Huan et al. [START_REF] Huan | Electrochemical Reduction of CO2 Catalyzed by Fe-N-C Materials: A Structure-Selectivity Study[END_REF] In this work, they demonstrated that a structure-selectivity relationship could be used to tailor the syngas composition. Using a Fe-N-C catalyst, they found that atomically dispersed Fe-N4 sites are highly selective for CO production, reaching a FECO over 90% at low overpotentials. In contrast, it was shown that over Febased nanoparticles, proton reduction to H2 dominates. Therefore, by selectively tailoring the amount of atomically dispersed Fe-N4 sites to Fe-nanoparticles the ratio of CO:H2 could be tuned between 0 -4.

This study offers a unique pathway to selectively tailor the syngas compostion. By controlling the synthesis of an atomically dispersed mono-and bi-metallic Fe, Mo and FeMo-N-C catalysts, in which the Fe-Nx sites selectively reduce CO2 to CO, while the Mo-Nx sites reduce H + to H2. Over mono-metallic Fe-N-C, a FECO greater than 90% could be achieved. Similarly, over Mo-N-C, a FEH2 of greater than 90% was achieved. By synthesizing a bi-metallic FeMo-N-C catalyst and tailoring the ratio of the Fe-Nx to Mo-Nx sites, the product ratio of CO:H2 is easily tuned. Furthermore, by simultaneously utilizing the potential dependence on product selectivity, a tunable CO:H2 ratio between 0.25 -5 was realized. These results demonstrate a novel approach to selectively tailor the syngas over a wide range of compositions in a single reaction.

Results and Discussion

Atomically dispersed mono-and bi-metallic M-N-C samples were synthesized using the previously established sacrificial support method (SSM). [START_REF] Serov | Original Mechanochemical Synthesis of Non-Platinum Group Metals Oxygen Reduction Reaction Catalysts Assisted by Sacrificial Support Method[END_REF] Briefly, the SSM involves solution phase mixing and mechanical integration of a carbon-nitrogen containing source, nicarbazine, with a hard silica template, and metal salt precursors. The precursor mixture was then pyrolyzed in a reductive 7% H2 atmosphere, followed by a hydrofluoric acid wash which removed the silica template. A second pyrolysis under a reductive 10% NH3 atmosphere was performed which removes any fluorinated species, increases the graphitization content, etches micropores, and thereby enhances the catalytic activity. [START_REF] Chen | Catalysts by Pyrolysis: Direct Observation of Transformations during Re-Pyrolysis of Transition Metal-Nitrogen-Carbon Materials Leading to State-of-the-Art Platinum Group Metal-Free Electrocatalyst[END_REF] The synthesis temperature was adjusted for the bi-metallic FeMo-N-C samples to preserve atomically dispersed metallic sites. Atomically dispersed M-Nx active sites provide for the highest atom use efficiency and allow for a selectivity study without the influence from a separate metallic phase. [START_REF] Chen | Single-Atom Catalysts: Synthetic Strategies and Electrochemical Applications[END_REF] When a high temperature of 975 ˚C or 950 ˚C was used in either the 1 st or 2 nd pyrolysis for the FeMo-N-C samples, the presence of a metallic nanoparticle phase was observed by X-ray diffraction (XRD) shown in Fig. 1a and observed by scanning transmission electron microscopy (STEM) (Figure S1). The larger high contrast spots (Fig. S1) correspond to alloyed nanoparticles of Fe/Mo metal, which are only observed at higher pyrolysis temperatures for the bi-metallic samples. This effect is not observed for the mono-metallic (Fe/Mo-N-C) catalysts, which maintain atomically dispersed sites at the higher pyrolysis tempeatures (Figures S2 a &b). The notation used throughout this study for the synthesis temperature(s) in ˚C for monoand bi-metallic M-N-C catalysts is written as M-N-C (X/Y), where X = the first pyrolysis temperature, while Y = the second pyrolysis temperature (e.g., FeMo-N-C (975/950) is first treated at 975 ˚C and then 950 ˚C). If no temperature is specified, then a pyrolysis temperature of 650 ˚C was used for both pyrolysis steps. The XRD spectra for the mono-and bi-metallic catalysts synthesized at both the high and low pyrolysis temperatures are shown in Fig. 1a ( and Fig. S3 a & b). The monometallic Fe/Mo-N-C catalysts and bi-metallic FeMo-N-C synthesized at the lower temperature of 650 ˚C show only the characteristic (002) and (100) peaks for mixed graphitic/amorphous carbon, which suggests that no metallic crystalline phases are present. While the FeMo-N-C pyrolyzed at the higher temperatures (975/950) shows the presence of a metallic crystalline phase at 2θ ~ 39 ˚. The asymmetric peak shape suggests the presence of carbon lattice defects while the increased width of the (002) peak suggests microcrystallites separate from graphitic carbon. [START_REF] Workman | Fe-N-C Catalyst Graphitic Layer Structure and Fuel Cell Performance[END_REF][START_REF] Lee | Analysis of Activation Process of Carbon Black Based on Structural Parameters Obtained by XRD Analysis[END_REF] The low mag aberration corrected high angle annular dark field (AC-HAADF) image of the FeMo-N-C catalyst in Fig. 1b shows the well defined hierarchical porous structure characteristic of the etched silica template (as confirmed through SEM and BET in Fig. S4 and Fig. S5, respectivley). The high mag AC-HAADF image in Fig. 1c shows distinct bright spots indicating atomically dispersed Fe and Mo sites. Elemental mapping from energy dispersive X-ray spectroscopy (EDS) in Fig. 1d shows the presence and homogenous distribution of C, N, Fe and Mo thoughout the catalyst. The percent atomic concentration (at.%) as obtained by XPS is summarized for the mono-and bi-metallic samples in Table S2 and is found to be a function of the pyrolysis temperature. The C and N at.% was 94.7 % and 2.5 % respectively, for the samples synthesized at the high temperatures (975/950), regardless of the metal dopant(s). The C at. % decreased to 85.1 % and N at. % increased to 9.8 % for the lower pyrolysis temperature FeMo-N-C(650/650) samples. The deconvoluted N 1s X-ray photoelectron spectroscopy (XPS) spectra in Fig. 1e reveals the presence of several N-moieties, assigned to be pyridinic (398 eV), pyrrolic (401 eV), quaternary (401.5 eV) and graphitic N (403 eV). Furthermore, the formation of nitrogen coordinated metal sites, M-Nx (399.5 eV) is observed. Likewise, the Fe 2p spectra (Fig. 1f) shows the presence of N-coordinated Fe-Nx sites. The Mo 3d spectra further reveals the formation of N-coordinated Mo-Nx sites.

The N 1s (XPS) spectra show that the relative proportions of N-moieties do not depend on the metallic precursor ratio of iron to molybdenum. The four samples with varying precursor ratios of Fe:Mo from 0.25:1 to 1:0.25 show remarkably similar percentages of N-moieties. The N 1s spectra show a high proportion of low binding energy nitrogen moieties such as pyridinic nitrogen and metal-coordinated nitrogen (i.e., Fe-Nx or Mo-Nx) versus higher binding energy nitrogen moieties which is indicative of favorable catalytic activity. The total metal atom content of atomically dispersed catalysts is difficult to assess and usually underestimated by XPS, so inductively coupled plasma mass spectroscopy (ICP-MS) was utilized to assess the total metal atom percentage (Table S3).

Additionally, according to an opinion article by Artyushkova, the ratio of N 1s low binding energy (BE) to high BE moieties was considered as another metric to standardize. [START_REF] Artyushkova | Misconceptions in Interpretation of Nitrogen Chemistry from X-Ray Photoelectron Spectra[END_REF] A high ratio of low BE N-moieties less than 400 eV to high BE above 400 eV has been shown to be beneficial for the oxygen reduction reaction (ORR). Similarly, a high concentration of pyridinic nitrogen species has been shown to promote CO2 reduction, hypothesized to be through offering an abundance of chemisorption sites. [START_REF] Luo | Significant Improvements in CO 2 Capture by Pyridine-Containing Anion-Functionalized Ionic Liquids through Multiple-Site Cooperative Interactions[END_REF][START_REF] Wang | Typical Transition Metal Single-Atom Catalysts with a Metal-Pyridine N Structure for Efficient CO2 Electroreduction[END_REF] For the CO-specific catalysts, the series of FeMo-N-C catalysts shows an increase in this ratio compared to the mono-and bi-metallic catalysts synthesized at higher temperatures (975/950), which suggests a more favorable local chemical environment for CO2 reduction (Table S4).

The Raman spectra of graphitic carbon shows a characteristic E2g vibrational mode at ~1581 cm -1 (G-band) while disordered carbon shows a peak at ~1350 cm -1 (D-band).

There is a decreasing Id/Ig ratio, which represents the ratio of D-band to G-band Raman peaks, with an increase in pyrolysis temperature (Fig. S6). This reveals an increase in the relative graphitization at the higher pyrolysis temperature(s), which is known to be autocatalyzed by the presence of iron at high temperatures. Evidence of local graphitization can be seen by annular bright-field (ABF-STEM) imaging in Fig. S7, via the presence of graphitic sheets which are observed for pyrolysis temperatures of 950 ˚C and above. The level of graphitization can have a substantial impact on catalyst morphology and reactivity. [START_REF] Workman | Fe-N-C Catalyst Graphitic Layer Structure and Fuel Cell Performance[END_REF][START_REF] Artyushkova | Misconceptions in Interpretation of Nitrogen Chemistry from X-Ray Photoelectron Spectra[END_REF] Therefore, it is an important metric to standardize for when assessing the influence of precursor metallic ratio on catalytic activity.

Previous studies have shown that atomically dispersed M-Nx sites are often located in the micropores of carbon supports. [START_REF] Tang | Single Rhodium Atoms Anchored in Micropores for Efficient Transformation of Methane under Mild Conditions[END_REF][START_REF] Lefèvre | Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells[END_REF] Furthermore, prior investigation has also shown porosity to be a controlling factor in the CO2 reduction activity on metal-free N-C active sites. [START_REF] Hursán | Morphological Attributes Govern Carbon Dioxide Reduction on N-Doped Carbon Electrodes[END_REF] A bimodal pore size distribution with meso/microporosity (a characteristic feature of catalysts synthesized using the SSM) is observed here (Fig. S5) which has been shown to have beneficial electrocatalytic properties. [START_REF] Liang | Hierarchically Porous Carbons with Optimized Nitrogen Doping as Highly Active Electrocatalysts for Oxygen Reduction[END_REF] The pyrolysis temperature also will greatly influence the overall surface area and electrochemically active surface area. An increase in the electrochemical surface area (ECSA) will lead to a higher overall reaction rate. In order to study the correlation between the Brunauer-Emmett-Teller (BET) surface area and the electrochemically active surface area, the double layer (D-L) capacitance values are plotted against the BET surface area values.

A positive correlation is seen between the higher synthesis temperatures and noteable increases in BET surface area, as well as D-L capacitance (Figure S8), which results in an increase in the geometric current density (Figure S9). After obtaining a full physical characterization of the catalysts, It was hypothesized, given that Fe-N-C is highly selective for CO formation while Mo-N-C catalysts have high HER activity, that altering the ratio of iron to molybdenum in the catalyst could be utilized as a tunable parameter for varying syngas composition.

Firstly, the aqueous-phase CO2RR vs. HER activity was investigated for the monoand bi-metallic M-N-C samples. Linear sweep voltammetry (LSV) was performed at a scan rate of 1 mV s -1 between 0.0 and -0.8 VRHE (e.g., sweeping from 0.0 V to -0.8 V vs. RHE in the cathodic direction). Figure 2a shows the differences in current response between the samples in the presence or absence of CO2. A substantial increase in the current density for the FeMo-N-C under CO2-saturated vs. CO2-absent conditions is evidence of its activity for the CO2RR. It is well-known that the overall activity of aqueous-phase CO2 electroreduction in a standard two compartment H-cell is limited by the limited solubility of CO2. To address the limited solubility of CO2 in aqueous electrolytes, gas diffusion electrode (GDE)-based systems are often employed. GDE assemblies bypass this solubility challenge through the creation of a triple-phase boundary which allow for higher conversion rates that are more feasible for industrial applications. [START_REF] Delafontaine | Metal-Nitrogen-Carbon Electrocatalysts for CO2 Reduction towards Syngas Generation[END_REF][START_REF] Möller | Efficient CO 2 to CO Electrolysis on Solid Ni-N-C Catalysts at Industrial Current Densities[END_REF][START_REF] Jiang | Transition-Metal Single Atoms in a Graphene Shell as Active Centers for Highly Efficient Artificial Photosynthesis[END_REF] For this reason, a custom-built GDE flow cell system was employed to examine catalyst selectivity for the CO2RR vs. HER.

The molar precursor ratio of iron to molybdenum was varied in the FeMo-N-C (650/650) catalyst series to observe the compositional changes in the produced syngas.

The yield rate and faradaic efficiency for CO/H2 for the bi-metallic samples show there is a positive correlation between the ratio of iron to molybdenum and the faradaic efficiency for CO. An increase in the iron to molybdenum molar precursor ratio results in the generation of more Fe-Nx sites and therefore an increase in selectivity for CO2 reduction vs. HER. The expected selectivity for the CO2RR to CO (Fe-N-C) and the HER (Mo-N-C) for the mono-metallic samples was confirmed. Fe-N-C in Fig. 2c is highly selective for CO generation and achieves a FECO between 84 % to 94 % over the potential window from -0.3 V to -1.1 V vs. RHE (FECO, max=94.4 % at -0.7 V). This selectivity of the Fe-N-C catalyst surpasses that of Ag nanoparticles, known to be selective for CO production (Fig. S11). By contrast, in Mo-N-C (975/950), Fig. 2d shows a high selectivity for the HER over the CO2RR over the entire potential window from -0.3 V to -1.1 VRHE. For the series of bimetallic FeMo-N-C catalysts, we see a mixed selectivity for CO2RR/HER which is intermediate between Fe-N-C and Mo-N-C. The onset potential for CO generation for the iron-containing M-N-C catalysts is near the thermodynamic minimum while for the Mo-N-C catalyst, the onset potential for CO generation is between ca. -0.3 to -0.5 VRHE and FECO reaches a maximum of only ca. 22 % between -0.5 V and -0.7 VRHE (Figure S12).

In terms of CO2RR to CO activity, the bimetallic FeMo-N-C (650/650) with a ratio of Fe:Mo = 1 reaches a maximum CO partial current density of -7.1 mA cm -2 at -0.9 VRHE. While, as expected, the mono-metallic Fe-N-C (975/950) was the most CO-selective catalyst for FECO (Fig. 2c) and boasted the largest CO partial current density of -31.3 +/-1.3 mA cm-2 at a potential of -1.1VRHE (Fig. S13) with a corresponding FECO = 88.6 %.

The large difference in the maximum CO partial current densities between the catalysts synthesized at different temperatures is likely a result of changes in the catalyst morphology which results in Fe-N-C (975/950) having a higher BET surface area and double-layer capacitance than the FeMo-N-C (650/650) series of catalysts (Figures S8 andS14). These morphological changes are due to the limiting synthesis conditions required to maintain atomic dispersion in the bi-metallic samples, which is necessary for investigating the selectivity changes for CO:H2 resulting from changes in the quantity of Fe-Nx and Mo-Nx sites.

By comparing Fig. 3a and Fig. 3b, it is observed when the ratio of iron to molybdenum is varied there is a marked shift in the selectivity from the CO2RR to HER at both low and high overpotentials. Figure 3 shows several interesting trends when the ratio of iron to molybdenum is varied. As expected, when the ratio of iron to molybdenum is increased (generating more Fe-Nx sites), the catalyst becomes more selective for the CO2RR, resulting in decreased (H2:CO) syngas compositions. ICP-MS was used to confirm that changing the Fe molar precusor ratio of Fe:Mo, results in corresponding changes in the molar ratio of Fe:Mo in the final catalyst, (e.g., for a precursor molar ratio of Fe:Mo = 0.25 and 4, the final atomic weight percentage ratio of Fe:Mo was determined to be 0.28 and 0.97, respectively (Table S3)). When the molar precursor ratio of Fe:Mo is 0.25, creating significantly more Mo-Nx sites, the jCO, max = -5.3 mA cmgeo -2 (FEco = 36.1 % at -0.9 VRHE). When this ratio is increased to 4.0, creating significantly more Fe-Nx sites, the maximum CO partial current density and FECO increases to jCO, max = -12.3 mA cmgeo - 2 (FEco = 63.2 % at -0.9 VRHE). Fig. 3c and Fig. 3d show that the syngas ratio is relatively independent of potential and depends primarily on the ratio of Fe:Mo until ca. -0.9 VRHE.

Once the potential is made more negative than -0.9 VRHE, the syngas ratio depends on both the potential and the ratio of Fe:Mo as is also observed in Fig. S15, where the largest shifts in H2/CO are seen for the most cathodic potentials investigated (e.g., -0.9 VRHE and -1.1 VRHE). By utilizing the changes in the ratio of Fe:Mo and altering the reductive potential, it was shown (Fig. 3d) that the H2:CO ratio could be tuned between 0.2 -4 (resulting in a corresponding CO:H2 ratio of 0.25 -5). Importantly, the partial current density for the HER (jH2) is much less sensitive to variations in the ratio of iron to molybdenum (jH2, max = ca. -15 mA for all FeMo-N-C (650/650) samples) as can be seen when comparing Fig. 4c and Fig. 4d. The partial current density for the HER (jH2) (Fig. 4d) increases monotonically as the potential is made more reductive from ca. -0.5 mA at -0.3 VRHE to ca. -15 mA at -1.1 VRHE regardless of the precursor Fe:Mo ratio. Variations in the Fe:Mo precursor ratio does not result in a significant change in jH2. It was observed that during the catalyst synthesis, molybdenum has a much higher retention in the final sample (Table S3): varying the metallic precursor ratio causes a more significant weight percent (wt.%) change for iron than for molybdenum (e.g., increasing precursor Fe:Mo from 0.25 to 4.0 results in a much larger relative increase in iron (from 0.40 to 1.46 wt.% Fe) than a relative decrease for Molybdenum (from 1.46 to 0.99 wt.% Mo)). This suggests, that for reducing the final Mo at.% (Mo-Nx sites), larger reductions in the molar Mo precursor ratio need to be considered to achieve the corresponding molar Mo at.% in the final material. The different final atomic metal wt.% sensitivity to precursor ratio has significant implications in terms of the CO2RR activity of the resulting catalysts (Fig. 4a and Fig. 4b). By changing the Fe:Mo precursor ratio from 0.5 to 4, a large increase in the yield rate of CO (as more Fe-Nx sites are present) is seen when comparing Fig. 4a and Fig. 4b. In contrast, the yield rate of H2 remains almost unchanged despite changes in the Fe:Mo precursor ratio. This supports the ICP-MS observation that changes in the final Mo wt.% are less sensitive to changes in the Fe:Mo precursor ratio (e.g., a large quantity of Mo-Nx sites remain even for large Fe/Mo precursor ratios). A similar trend is seen for samples with other Fe:Mo ratios shown in Fig. 4c and Fig. S16. The corresponding jCO and jH2 partial current densities for the FeMo-N-C catalysts at Fe:Mo ratios of 0.25, 0.5, 2 and 4 are shown in Fig. 4 c &d.

The trends for faradaic efficiency (Fig. 3) and partial current density (Fig. 4) for the FeMo-N-C (650/650) samples demonstrate that the syngas composition can be tuned through either changes in the ratio of Fe:Mo, and/or changes in the reductive potential. In addition to factors intrinsic to the catalyst composition which influences selectivity/reactivity, the reaction conditions were also investigated to uncover their role in syngas selectivity. Several control experiments were conducted to assess the influence of reaction conditions on CO2RR vs. HER activity (Fig. 5). These controls investigate the role of the catalyst loading, electrolyte type/concentration, and inlet feed composition on CO2 reduction vs. HER activity. To confirm the CO2 supplied from the gas feed is the main contributor to the observed CO2RR products, a control was conducted using a N2 gas feed in a 0.5M KHCO3 electrolyte. It is known that in the absence of gas phase CO2, some CO2 can be supplied through the KHCO3 electrolyte resulting in limited CO production, as is observed in Control 1 in Fig 5 . A trace amount of CO is produced (-0.19 mA cmgeo -2 at -1.38 VRHE) using bare Sigracet 29 BC carbon GDL, 100 % CO2 feed, and 0.5M KHCO3 but no CO is detected for potentials less cathodic than -1.38 VRHE (Fig. S17). For control 2, the catalyst loading can have a significant effect, when increased from 0.1 to 1.0 mg/cm 2 (denoted by an *): there is an increase in the CO partial current density and an increase in the FECO from 40.4 % to 76.3 %. Control 3 demonstrated that the electrolyte concentration plays a significant role on the observed CO2RR activity, where the high concentration 0.5M KHCO3 electrolyte showed a significantly higher CO partial current density (at both high and low catalyst loadings) than the 0.1M KHCO3 electrolyte. Control 4 confirms the source of CO to be largely from the CO2 feed gas with a small contribution from the KHCO3 electrolyte. By eliminating both the CO2 gas feed and CO2 containing KHCO3 electrolyte and utilizing a N2 gas feed with a phosphate buffer solution (PBS), no CO was detected during the electrolysis.

Conclusions

In summary, a novel approach to tailor the CO:H2 composition of syngas produced through the electrochemical CO2RR was demonstrated by selectively tuning the ratio of atomically dispersed Fe-Nx and Mo-Nx sites in a bi-metallic FeMo-N-C catalyst. It was demonstrated that the ratio of Fe:Mo in the catalyst could be altered with minimal effect on the catalyst's local chemical environment, such as degree of graphitization, N-moiety content, and morphological characteristics. This enabled changes in the CO:H2 ratio to be attributed to changes in the quantities of Fe-Nx and Mo-Nx sites, rather than to physical changes in the cataysts. Among the series of bimetallic FeMo-N-C catalysts, the catalyst with the highest Fe:Mo ratio of 1:0.25 shows the highest CO production rate and faradaic efficiency. The catalyst with the lowest Fe:Mo ratio of 0.25:1 shows the lowest FECO and jCO. The possibility for a dynamic control of the syngas composition was shown by changing the reductive potential. As demonstrated for the FeMo-N-C catalyst, a reductive potential between -0.5 to -0.7 VRHE yielded CO as the major product, while operating at more reductive potentials yielded H2 as the major product. By employing changes in the quantity of Fe-Nx to Mo-Nx sites and changes in the applied reductive potential, syngas ratios of CO:H2 between 0.25 -5 were obtained. The unique approach shown here provides a pathway to selectively tailor the ratio of CO:H2 in syngas, produced in a single reaction, which eliminates the need for downstream mixing to achieve a desired composition.

Methods

Synthesis of atomically dispersed Fe/Mo/FeMo-N-C

The series of Fe, Mo and FeMo-N-C catalysts utilized in this work were synthesized following the sacrifical support method. For a typical synthesis (taking FeMo-N-C as an example), 6.25 g nicarbazin; 0.6 g iron nitrate nonahydrate; 0.26 g ammonium molybdate tetrahydrate (The amount of metallic precursor is calculated such that the total number of metal atoms in each catalyst is the same); 1.25 g LM-150; 1.25 g OX-50 and 0.5 g stöber spheres were dispersed in water, sonciated for 30 min and dried at 45 ˚C under constant stirring. The resulting powder is ball-milled at 45 Hz for 1 hour, and then is pyrolyzed under 7 % H2 for 45 min at 650 ˚C. The powder is ball-milled again and etched with 40 wt. % HF for 4 days. The mixture is washed until pH ≥ 5. After drying, the powder undergoes a second pyrolysis for 30 min at 650 ˚C under a 10 % NH3 followed by a final ball-milling step.

For the mono-metallic Fe-N-C and Mo-N-C, the synthesis is identical with only the pyrolysis temperatures being changed to 975 ˚C in the first pyrolysis and 950 ˚C in the second.

For comparison, unsupported Ag nanoparticles were purchased having particle size < 100 nm and a 99.5 % (trace metal basis) purity.

Physical Characterization

The formation of atomically dispersed metal sites and catalyst strcuture were analyzed by aberration-corrected scanning transmission electron microscopy and energy dispersive X-ray spectroscopy (EDS) using a JEOL ARM300CF at an accelerating voltage of 300 kV. The surface morphology of the catalyts was visualized by scanning electron microscopy using a FEI Magellan 400 XHR SEM. The surface chemical structure, valence state and composition was analyzed using X-ray photoelectron spectroscopy performed on a Kratos AXIS Supra spectrometer with a monochromatic Al Kɑ source (The raw N 1s and Mo 3d spectra are given in Fig. S18 and S19, respectivley). The crystalline structure of the catalysts was analyzed by powder X-ray diffraction using a Rigaku Powder X-ray diffractometer. Inductively coupled plasma mass spectrometry was used to quantify the metallic content of the catalysts. N2 physisorption was performed on a Micromeritics 3Flex Analyzer at 77K. The surface area and pore size distribution were calculated using the Brunauer-Emmett-Teller method and non-local density functional theory model (NLDFT), respectively. Raman spectra were recorded on a InVia, Renishaw Corp., UK system.

Preparation of the working electrode

A GDE was fabricated by hot pressing a Sterlitech PTFE membrane filter to a Sigracet 29 BC GDL carbon paper. For a 1.0 mg/cm 2 loading, the catalyst ink was made by sonicating 900 𝜇L IPA, 100 𝜇L DI water, 10 mg catalyst powder, and 60 𝜇L 5 wt. % nafion ionmor solution (in lower aliphatic alcohols). The ink was spray coated onto a heated carbon paper. Unsupported Ag nanoparticles (particle size < 100 nm) were purchased from Sigma-Aldrich which have a 99.5 % (trace metal basis) purity and were used in the same ink formulation for Ag-based cathode.

Electrochemical CO2 Reduction

The electrocatalytic activity was evaluated using a custom-built gas diffusion electrode flow cell in a three-electrode configuration. Constant potential electrolysis (CPE) was performed across the potential range from -0.3 V vs. RHE to -1.1 V vs. RHE. The reference electrode was a Gaskatel TM reversible hydrogen electrode (RHE) used to apply the selected cathode potential in potassium bicarbonate or phospate buffer solution (PBS) based electrolyte. The faradaic efficiency for the product(s) detected was calculated based on the charge transferred to obtain the selected product divided by the total charge transferred to obtain all products.

𝐹𝐸 𝑖 = 𝑁 𝑖 𝑛 𝑖 𝐹 ∑ 𝑁 𝑖 𝑛 𝑖 𝐹

Where, Ni represents the number of moles of product detected by either GC or 1 H NMR, ni represents the number of moles of e -transferred per mole of product produced (e.g., 2 for CO and H2), and is Faraday's contant (F = 96485 C mol -1 of electrons).

Product detection

The gaseous products generated during CO2 reduction electrolysis were sampled and injected into a dual-column gas chromatograph to detect CO/H2 along with any possible CH4 or hydrocarbons. [START_REF] Lu | Electrosynthesis of Syngas via the Co-Reduction of CO2 and H2O[END_REF] H NMR was performed to confirm the absence of any liquid

products. An Agilent HP-MOLESIEVE column connected to a thermal conductivity detector (TCD) using Helium carrier gas was used to quantify CO while the carrier gas was switched to N2 using the same HP-MOLESIEVE column and TCD detector for H2 detection. A GS-CarbonPlot column connected to a flame ionization detector was used to screen for any hydrocarbon production.

Figure 1 .

 1 Figure 1. Physical characterization of catalysts: (a) XRD patterns for the Fe-N-C, Mo-N-C, and FeMo-N-C catalysts at high and low pyrolysis temperature(s). (b) Low magnification AC-HAADF STEM image of the FeMo-N-C (650/650) catalyst. (c) High magnification AC-HAADF STEM image of FeMo-N-C (650/650); atomically dispersed Fe and Mo sites are indicated by the high contrast points. (d) AC-HAADF image and corresponding elemental mapping of FeMo-N-C (650/650). XPS spectra of the FeMo-N-C (650/650) catalyst (e) N 1s spectra, indicating the presence of M-Nx moieties. (f) Fe 2p spectra showing the formation of Fe-Nx sites. (g) Mo 3d spectra showing the formation of Mo-Nx sites.

Figure 2 .

 2 Figure 2. (a) LSV curves (scan rate of 1 mV/s) in N2-saturated 0.1M PBS (K2HPO4/KH2PO4) solution and CO2-saturated 0.1M KHCO3 solution using 0.3 mg cmgeo -2 catalyst loading for FeMo-N-C (650/650). (b) Current density in CO2-saturated 0.1M KHCO3 using 0.3 mg cmgeo -2 catalyst loading for mono-and bi-metallic samples. CO2RR yield rates (nmol s -1 cm -2 ) for (c) Fe-N-C and (d) Mo-N-C, using a flow cell system.

Figure 3 .

 3 Figure 3. Faradaic efficiency (%) for FeMo-N-C (650/650) where the ratio of (a) Fe:Mo = 1:0.5 and (b) Fe:Mo = 0.25:1, using a catalyst loading of 1.0 mg/cm 2 . (c) and (d) The syngas ratio (H2/CO) vs. cathodic potential for different molar precursor ratios of Fe:Mo and Fe-N-C.

Figure 4 .

 4 Figure 4. Electrochemical performance of the bi-metallic FeMo-N-C samples. Yield Rate (nmol s -1 cmgeo -2 ) for (a) Fe: Mo = 0.5:1 and (b) Fe: Mo = 1:0.2. The magnitude of the (c) CO partial current density and (d) H2 partial durrent density (V vs. RHE). Samples labeled 1, 2, 3, 4 correspond to a ratio of Fe:Mo = 0.25, 0.5, 2, 4, respectively.

Figure 5 .

 5 Figure 5. Partial current density for H2 or CO, under several control experiments, using the FeMo-N-C (650/650) (Fe:Mo = 1:0.5). Low catalyst loading (no asterisk) is 0.1 mg cmgeo -2 while high catalyst loading (asterisk *) is 1.0 mg cmgeo -2 . Control 1 (C1): N2 inlet, 0.5M KHCO3(aq); Control 2 (C2): CO2 inlet, 0.5M KHCO3(aq); Control 3 (C3): CO2 inlet, 0.1M KHCO3(aq); Control 4 (C4): N2 inlet, 0.5M potassium phosphate buffer (PBS) (pH =7.5) (aq).

Acknowledgements

The authors acknowledge the use of facilities and instrumentation at the UC Irvine Materials Research Institute (IMRI), which is supported in part by the National Science Foundation through the UC Irvine Materials Research Science and Engineering Center (DMR-2011967). XPS work was performed using instrumentation funded in part by the National Science Foundation Major Research Instrumentation Program under grant no. CHE-1338173. The authors would like to thank Ying Huang for her help in the acquisition and processing of XPS data.