Synthesis of Metastable Ternary Pd-W and Pd-Mo Transition Metal Carbide Nanomaterials

Research and catalytic testing of platinum group transition metal carbides have been extremely limited due to a lack of reliable, simple synthetic approaches. Powder samples have been reported to phase separately above 1%, and only thin-film samples have been reported to have appreciable amounts of precious metal doping. Herein, we demonstrated, through the simple co-precipitation of Pd and W or Mo precursors and their subsequent annealing, the possibility to readily form ternary carbide powders. During the investigation of the Pd-W ternary system, we discovered a new hexagonal phase, (PdW)2C, which represents the first non-cubic Pd ternary carbide. Additionally, the solubility of Pd in the Pd-W-C and Pd-Mo-C systems was increased to 24 and 32%, respectively. As a potential application, these new materials show an enhanced activity for the methanol oxidation reaction (MOR) compared to industrial Pd/C.


Introduction
Platinum group metals and their compounds have a widespread use in both technological and industrial applications. Both Pt and Pd are highly active for various electrocatalytic processes including hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and methanol oxidation reaction (MOR) [1][2][3]. However, there are several limitations to Pt group catalysts like poisoning, aggregation, and stability in harsh environments [4]. Previous researchers have alloyed Pt and Pd with other metals to improve reactivity and stability, but there is still room for improvement. One area of research that has been particularly overlooked is Pt group metal carbides. This is primarily due to limited synthetic approaches leading to their formation. In fact, very few binary carbides exist for group 7-10 metals, and most of those reported phases have a limited stability in water and acid [5]. The heat of formation for these binary carbide compounds becomes less negative moving from left to right along the d-block, with TiC being the most stable carbide [6]. TiC has a ∆H • f of −92.9 kJ/mole, while V 4 C 3 increases to −40.3 kJ/mole. For late transition metal carbides like Fe 3 C, the heat of formation flips signs and goes positive with a value of 4.7 kJ/mole for Fe 3 C. Even though there is a limited stability with these compounds, they provide another avenue for electronic tuning because the metal-carbon bond strength decreases when the number of d-electrons increases in late transition metals, as confirmed by ab initio calculations [7,8].
Although no stable carbides exist for platinum group metals, it has been demonstrated that such phases do exist as metastable compounds [9]. For example, the formation of a palladium carbide-like phase has been demonstrated during the hydrogenation of 1-pentyne over a palladium catalyst [10]. In this system, the carbide phase is selective for alkene production while the hydride phase favors alkanes. Further evidence of the formation of surface PdC has been seen during the catalytic combustion of lean methane/air mixtures over a Pd catalyst, during dehydrogenation reactions, and during methane production [11][12][13][14][15]. Perhaps the most important discovery came from pioneering work by

Results and Discussion
It is valuable, for greater clarity, to briefly discuss the various phases of molybdenum and tungsten carbide present within this article. Both the Mo-C and W-C systems have phases with MC and M 2 C stoichiometry. α-MoC 1−x and γ-WC 1−x have a face-centered cubic structure (fcc) with ABCABC stacking. β-Mo 2 C and W 2 C are in a hexagonal closepacked arrangement (hcp) with ABAB stacking. In the case of W-C, there is also an α-WC phase (also known as WC type) with a simple hexagonal structure and AA stacking [32]. While other phases of Mo-C do exist, they are not pertinent to this publication [33]. All of the crystal information can be found in Tables S1 and S2 of the Supporting Information. The following compounds, Pd x W 1−x C, (Pd x W 1−x ) 2 C, and Pd x Mo 1−x C, were all made using the amine metal oxide composite method. A detailed description of the synthesis can be found in Section 3. In short, precursors of Pd (K 2 PdCl 6 ), W (ammonium metatungstate) or Mo (ammonium heptamolybdate), and C (o-phenylenediamine) were added to an aqueous solution while stirring at 45 • C. The mixture was then co-precipitated by careful addition of 3 M HCl to a pH below 3. The precipitate was then annealed at a variety of temperatures to study the composition and phase formation. Figure 1 shows XRD data demonstrating the presence of Pd x W 1−x C and (Pd x W 1−x ) 2 C. This sample was made by combining a 2:1 ratio of W and Pd. The XRD data reveals a number of transitions starting with the AMOC precursor at room temperature, whose diffraction pattern does not match any known patterns in the PDF database. At 500 • C, metallic Pd peaks first emerge, followed by Pd x W 1−x C at 850 • C, which forms in the cubic γ-WC 1−x structure. At 850 • C, the Pd peaks are no longer present, and the diffraction peaks have all shifted to lower angles, with the formation of the carbide indicating that the Pd has dissolved into the lattice of the WC phase. The lattice constants for the synthesized materials and peak positions are listed in Table S2. The lattice constant for the cubic Pd-W-C phase is 4.252 Å, which correlates well with Gregoire et al. who reported~4.2 Å for compounds with 0-25% Pd and with other reports of WC and WC 1−x [31]. At 975 • C, a unique diffraction pattern is observed, which can be indexed to a hexagonal unit cell with a similar structure to the β-W 2 C phase. This compound, (Pd x W 1−x ) 2 C, has never been cited in the literature and is the first synthetic example of Pd bound to carbon in a noncubic Molecules 2021, 26, 6650 3 of 10 structure [34,35]. All previous publications of Pd in WC and MoC showed the rock-salt phase as the preferred crystal structure, including both powder and thin-film samples. However, with this synthetic approach, the hexagonal structure of (PdW) 2 C can be isolated as a stable phase. Finally, at 1200 • C, the metastable ternary carbide phase separates into hexagonal δ-WC and metallic Pd, along with remnants of the β-W 2 C phase. This series of data helps identify the location of Pd during the various transition temperatures. Pd is reduced to a metallic state from the precursor material at low temperatures, dissolves into the carbide lattice in two phases of tungsten carbide, and finally separates into an elemental state again at high temperatures, where the δ-WC phase also appears.
Molecules 2021, 26, x FOR PEER REVIEW 3 of 10 a similar structure to the β-W2C phase. This compound, (PdxW1−x)2C, has never been cited in the literature and is the first synthetic example of Pd bound to carbon in a noncubic structure [34,35]. All previous publications of Pd in WC and MoC showed the rock-salt phase as the preferred crystal structure, including both powder and thin-film samples. However, with this synthetic approach, the hexagonal structure of (PdW)2C can be isolated as a stable phase. Finally, at 1200 °C, the metastable ternary carbide phase separates into hexagonal δ-WC and metallic Pd, along with remnants of the β-W2C phase. This series of data helps identify the location of Pd during the various transition temperatures. Pd is reduced to a metallic state from the precursor material at low temperatures, dissolves into the carbide lattice in two phases of tungsten carbide, and finally separates into an elemental state again at high temperatures, where the δ-WC phase also appears. The XRD data can be correlated with the thermogravimetric analysis (TGA) data from Figure S1, which is similar to previous studies on carbide formation [36]. From 25-225 °C, there is a mass loss that corresponds to the loss of water and volatile species present in the precursor. From 225-300 °C, another event occurs as free amine leaves the system. From 300-425 °C , there is another mass loss, which gives rise to the formation of crystalline Pd, as seen in the XRD from Figure 1. This mass loss is likely due to the decomposition of palladium chloride into Pd metal and HCl gas. Finally, the mass loss at 800 °C corresponds to the reduction of W and subsequent formation of carbide. While this synthesis method is still under investigation, it was observed that the acidity of the reaction mixture had a significant effect on the solubility of Pd. As seen in Figure S2, pHs below the pKa1 of o-phenylenediamine, 0.80, result in metallic Pd being present as a distinct phase in the XRD. Phase pure ternary carbides were only obtained at pHs above 0.8.
To further demonstrate the inclusion of Pd into the WC lattice, SEM images with energy dispersive X-ray spectroscopy (EDS) mapping were studied. Figure 2 shows the cubic Pd-W-C ternary carbide with 24% Pd. Other compositions including 15% and 7% were also synthesized, with the NaCl structure indicating a significant degree of composition control. The hexagonal carbide was found at 15% and 24% Pd. The EDS mapping shows homogeneity with both Pd and W found in the same locations and compositions throughout the samples, indicating a uniform distribution within the carbide. Figure S3 shows the XRD patterns and SEM/EDS for samples with varying compositions. Table S3 shows the % Pd content based on EDS measurements, further confirming composition a similar structure to the β-W2C phase. This compound, (PdxW1−x)2C, has never been cited in the literature and is the first synthetic example of Pd bound to carbon in a noncubic structure [34,35]. All previous publications of Pd in WC and MoC showed the rock-salt phase as the preferred crystal structure, including both powder and thin-film samples. However, with this synthetic approach, the hexagonal structure of (PdW)2C can be isolated as a stable phase. Finally, at 1200 °C, the metastable ternary carbide phase separates into hexagonal δ-WC and metallic Pd, along with remnants of the β-W2C phase. This series of data helps identify the location of Pd during the various transition temperatures. Pd is reduced to a metallic state from the precursor material at low temperatures, dissolves into the carbide lattice in two phases of tungsten carbide, and finally separates into an elemental state again at high temperatures, where the δ-WC phase also appears.

 ✖ 
The XRD data can be correlated with the thermogravimetric analysis (TGA) data from Figure S1, which is similar to previous studies on carbide formation [36]. From 25-225 °C, there is a mass loss that corresponds to the loss of water and volatile species present in the precursor. From 225-300 °C, another event occurs as free amine leaves the system. From 300-425 °C, there is another mass loss, which gives rise to the formation of crystalline Pd, as seen in the XRD from Figure 1. This mass loss is likely due to the decomposition of palladium chloride into Pd metal and HCl gas. Finally, the mass loss at 800 °C corresponds to the reduction of W and subsequent formation of carbide. While this synthesis method is still under investigation, it was observed that the acidity of the reaction mixture had a significant effect on the solubility of Pd. As seen in Figure S2, pHs below the pKa1 of o-phenylenediamine, 0.80, result in metallic Pd being present as a distinct phase in the XRD. Phase pure ternary carbides were only obtained at pHs above 0.8.
To further demonstrate the inclusion of Pd into the WC lattice, SEM images with energy dispersive X-ray spectroscopy (EDS) mapping were studied. Figure 2 shows the cubic Pd-W-C ternary carbide with 24% Pd. Other compositions including 15% and 7% = Pd metal 46-1043, a similar structure to the β-W2C phase. This compound, (PdxW1−x)2C, has in the literature and is the first synthetic example of Pd bound to carb structure [34,35]. All previous publications of Pd in WC and MoC sho phase as the preferred crystal structure, including both powder and t However, with this synthetic approach, the hexagonal structure of (Pd lated as a stable phase. Finally, at 1200 °C, the metastable ternary carbid into hexagonal δ-WC and metallic Pd, along with remnants of the β-W2C of data helps identify the location of Pd during the various transition te reduced to a metallic state from the precursor material at low temperatu the carbide lattice in two phases of tungsten carbide, and finally sepa mental state again at high temperatures, where the δ-WC phase also ap


The XRD data can be correlated with the thermogravimetric ana from Figure S1, which is similar to previous studies on carbide formati 225 °C, there is a mass loss that corresponds to the loss of water and vo sent in the precursor. From 225-300 °C, another event occurs as free ami tem. From 300-425 °C, there is another mass loss, which gives rise to crystalline Pd, as seen in the XRD from Figure 1. This mass loss is likely d position of palladium chloride into Pd metal and HCl gas. Finally, the m corresponds to the reduction of W and subsequent formation of carbid thesis method is still under investigation, it was observed that the acid mixture had a significant effect on the solubility of Pd. As seen in Figu the pKa1 of o-phenylenediamine, 0.80, result in metallic Pd being pre phase in the XRD. Phase pure ternary carbides were only obtained at pH To further demonstrate the inclusion of Pd into the WC lattice, S energy dispersive X-ray spectroscopy (EDS) mapping were studied. Fi cubic Pd-W-C ternary carbide with 24% Pd. Other compositions inclu = W metal 00-004-0806, a similar structure to the β-W2C phase. This co in the literature and is the first synthetic exam structure [34,35]. All previous publications of phase as the preferred crystal structure, inclu However, with this synthetic approach, the h lated as a stable phase. Finally, at 1200 °C, the into hexagonal δ-WC and metallic Pd, along wi of data helps identify the location of Pd during reduced to a metallic state from the precursor m the carbide lattice in two phases of tungsten mental state again at high temperatures, wher The XRD data can be correlated with th from Figure S1, which is similar to previous s 225 °C, there is a mass loss that corresponds to sent in the precursor. From 225-300 °C, anothe tem. From 300-425 °C, there is another mass crystalline Pd, as seen in the XRD from Figure position of palladium chloride into Pd metal a corresponds to the reduction of W and subseq thesis method is still under investigation, it w mixture had a significant effect on the solubil the pKa1 of o-phenylenediamine, 0.80, result phase in the XRD. Phase pure ternary carbides To further demonstrate the inclusion of energy dispersive X-ray spectroscopy (EDS) m cubic Pd-W-C ternary carbide with 24% Pd. O = PdWC, a similar structure to the β-W2C phase. This in the literature and is the first synthetic ex structure [34,35]. All previous publications phase as the preferred crystal structure, in However, with this synthetic approach, the lated as a stable phase. Finally, at 1200 °C, th into hexagonal δ-WC and metallic Pd, along of data helps identify the location of Pd dur reduced to a metallic state from the precurso the carbide lattice in two phases of tungst mental state again at high temperatures, wh The XRD data can be correlated with from Figure S1, which is similar to previou 225 °C, there is a mass loss that correspond sent in the precursor. From 225-300 °C, ano tem. From 300-425 °C, there is another ma crystalline Pd, as seen in the XRD from Figu position of palladium chloride into Pd meta corresponds to the reduction of W and sub thesis method is still under investigation, it mixture had a significant effect on the solu the pKa1 of o-phenylenediamine, 0.80, resu phase in the XRD. Phase pure ternary carbid To further demonstrate the inclusion energy dispersive X-ray spectroscopy (EDS cubic Pd-W-C ternary carbide with 24% Pd = (PdW) 2 C, a similar structure to the β-W2C pha in the literature and is the first syn structure [34,35]. All previous publi phase as the preferred crystal struc However, with this synthetic appro lated as a stable phase. Finally, at 12 into hexagonal δ-WC and metallic Pd of data helps identify the location of reduced to a metallic state from the p the carbide lattice in two phases of mental state again at high temperatu

✖
The XRD data can be correlate from Figure S1, which is similar to p 225 °C, there is a mass loss that corr sent in the precursor. From 225-300 tem. From 300-425 °C, there is ano crystalline Pd, as seen in the XRD fro position of palladium chloride into P corresponds to the reduction of W a thesis method is still under investig mixture had a significant effect on t the pKa1 of o-phenylenediamine, 0. phase in the XRD. Phase pure ternar To further demonstrate the inc energy dispersive X-ray spectroscop cubic Pd-W-C ternary carbide with = WC-hex 00-025-1047).
The XRD data can be correlated with the thermogravimetric analysis (TGA) data from Figure S1, which is similar to previous studies on carbide formation [36]. From 25-225 • C, there is a mass loss that corresponds to the loss of water and volatile species present in the precursor. From 225-300 • C, another event occurs as free amine leaves the system. From 300-425 • C, there is another mass loss, which gives rise to the formation of crystalline Pd, as seen in the XRD from Figure 1. This mass loss is likely due to the decomposition of palladium chloride into Pd metal and HCl gas. Finally, the mass loss at 800 • C corresponds to the reduction of W and subsequent formation of carbide. While this synthesis method is still under investigation, it was observed that the acidity of the reaction mixture had a significant effect on the solubility of Pd. As seen in Figure S2, pHs below the pKa 1 of o-phenylenediamine, 0.80, result in metallic Pd being present as a distinct phase in the XRD. Phase pure ternary carbides were only obtained at pHs above 0.8.
To further demonstrate the inclusion of Pd into the WC lattice, SEM images with energy dispersive X-ray spectroscopy (EDS) mapping were studied. Figure 2 shows the cubic Pd-W-C ternary carbide with 24% Pd. Other compositions including 15% and 7% were also synthesized, with the NaCl structure indicating a significant degree of composition control. The hexagonal carbide was found at 15% and 24% Pd. The EDS mapping shows homogeneity with both Pd and W found in the same locations and compositions throughout the samples, indicating a uniform distribution within the carbide. Figure S3 shows the XRD patterns and SEM/EDS for samples with varying compositions. Table S3 shows the % Pd content based on EDS measurements, further confirming composition control in these phases. There is no interference between the Pd and W peaks in the EDS spectra, allowing for a confident analysis of their compositions (see Figure S4). control in these phases. There is no interference between the Pd and W peaks in the E spectra, allowing for a confident analysis of their compositions (see Figure S4). The particle size, lattice constant measurements, and selected area electron diff tion (SAED) were analyzed using high-resolution TEM, as shown in Figure 3. The parti appear to be made of 5-20 nm spheres that aggregate to form roughly 100 nm clust Lattice fringes were measured to be 0.22 nm, in close agreement with the (011) plane (PdW)2C. SAED in Figure 3 also confirms the presence of (PdW)2C. EDS from the T samples demonstrates the homogeneous presence of both Pd and W on the nanome scale, as seen in Figure S5. Molybdenum carbide was also reported to have a preferential phase growth w 0.2% Pd doping, causing the material to adopt the cubic phase MoC rather than the co mon hexagonal Mo2C. In the present investigation, rock-salt MoC was also found, with Pd doping now reaching 32%. From the TGA data in Figure S1, free amine was at 300 °C followed by a Pd reduction at ~400 °C. However, carbide formation does occur until 1000 °C or higher for the Mo carbide system. At 500 °C, the XRD reveals pe belonging to Pd metal, as seen in  The particle size, lattice constant measurements, and selected area electron diffraction (SAED) were analyzed using high-resolution TEM, as shown in Figure 3. The particles appear to be made of 5-20 nm spheres that aggregate to form roughly 100 nm clusters. Lattice fringes were measured to be 0.22 nm, in close agreement with the (011) plane of (PdW) 2 C. SAED in Figure 3 also confirms the presence of (PdW) 2 C. EDS from the TEM samples demonstrates the homogeneous presence of both Pd and W on the nanometer scale, as seen in Figure S5.
control in these phases. There is no interference between the Pd and W peaks in th spectra, allowing for a confident analysis of their compositions (see Figure S4). The particle size, lattice constant measurements, and selected area electron d tion (SAED) were analyzed using high-resolution TEM, as shown in Figure 3. The pa appear to be made of 5-20 nm spheres that aggregate to form roughly 100 nm cl Lattice fringes were measured to be 0.22 nm, in close agreement with the (011) pl (PdW)2C. SAED in Figure 3 also confirms the presence of (PdW)2C. EDS from th samples demonstrates the homogeneous presence of both Pd and W on the nano scale, as seen in Figure S5. Molybdenum carbide was also reported to have a preferential phase growt 0.2% Pd doping, causing the material to adopt the cubic phase MoC rather than th mon hexagonal Mo2C. In the present investigation, rock-salt MoC was also foun with Pd doping now reaching 32%. From the TGA data in Figure S1, free amine w at 300 °C followed by a Pd reduction at ~400 °C. However, carbide formation do occur until 1000 °C or higher for the Mo carbide system. At 500 °C, the XRD reveals belonging to Pd metal, as seen in  Molybdenum carbide was also reported to have a preferential phase growth with 0.2% Pd doping, causing the material to adopt the cubic phase MoC rather than the common hexagonal Mo 2 C. In the present investigation, rock-salt MoC was also found, but with Pd doping now reaching 32%. From the TGA data in Figure S1, free amine was lost at 300 • C followed by a Pd reduction at~400 • C. However, carbide formation does not occur until 1000 • C or higher for the Mo carbide system. At 500 • C, the XRD reveals peaks belonging to Pd metal, as seen in  The SEM of Pd0.11Mo0.89C annealed to 1200 °C is shown in Figure 5. The EDS mappin shows the carbide to be homogenous, with both Pd and Mo present in the same location of the SEM images, again indicating a uniform elemental dispersion within the carbid Table S3 shows the % Pd content based on EDS measurements, demonstrating a goo composition control including 32%, 22%, and 11% Pd. The particles appear mostly sphe ical in the SEM, with some aggregates appearing slightly plate-like.   The symbols for ( the carbide lattice in two phases of tungsten carbide, and finally separates into an elemental state again at high temperatures, where the δ-WC phase also appears.

 ✖ 
The XRD data can be correlated with the thermogravimetric analysis (TGA) data from Figure S1, which is similar to previous studies on carbide formation [36]. From 25-225 °C, there is a mass loss that corresponds to the loss of water and volatile species present in the precursor. From 225-300 °C, another event occurs as free amine leaves the system. From 300-425 °C, there is another mass loss, which gives rise to the formation of crystalline Pd, as seen in the XRD from Figure 1. This mass loss is likely due to the decomposition of palladium chloride into Pd metal and HCl gas. Finally, the mass loss at 800 °C corresponds to the reduction of W and subsequent formation of carbide. While this synthesis method is still under investigation, it was observed that the acidity of the reaction mixture had a significant effect on the solubility of Pd. As seen in Figure S2, pHs below the pKa1 of o-phenylenediamine, 0.80, result in metallic Pd being present as a distinct phase in the XRD. Phase pure ternary carbides were only obtained at pHs above 0.8.
To further demonstrate the inclusion of Pd into the WC lattice, SEM images with energy dispersive X-ray spectroscopy (EDS) mapping were studied. Figure 2 shows the cubic Pd-W-C ternary carbide with 24% Pd. Other compositions including 15% and 7% ) Pd in the figure are based on JCPDS files Pd46-1043, and PdMoC ( of data helps identify the reduced to a metallic state the carbide lattice in two mental state again at high

 ✖ 
The XRD data can b from Figure S1, which is 225 °C, there is a mass lo sent in the precursor. Fro tem. From 300-425 °C, th crystalline Pd, as seen in t position of palladium chl corresponds to the reduc thesis method is still und mixture had a significant the pKa1 of o-phenylened phase in the XRD. Phase To further demonstr energy dispersive X-ray s cubic Pd-W-C ternary ca ) does not have a reported structure.
The SEM of Pd 0.11 Mo 0.89 C annealed to 1200 • C is shown in Figure 5. The EDS mapping shows the carbide to be homogenous, with both Pd and Mo present in the same locations of the SEM images, again indicating a uniform elemental dispersion within the carbide. Table S3 shows the % Pd content based on EDS measurements, demonstrating a good composition control including 32%, 22%, and 11% Pd. The particles appear mostly spherical in the SEM, with some aggregates appearing slightly plate-like.  The SEM of Pd0.11Mo0.89C annealed to 1200 °C is shown in Figure 5. The EDS mappin shows the carbide to be homogenous, with both Pd and Mo present in the same location of the SEM images, again indicating a uniform elemental dispersion within the carbid Table S3 shows the % Pd content based on EDS measurements, demonstrating a goo composition control including 32%, 22%, and 11% Pd. The particles appear mostly spher ical in the SEM, with some aggregates appearing slightly plate-like.    To investigate these materials' potential application as electrocatalysts, their activit for the methanol oxidation reaction was tested. Figure Table S5. This increase in electro chemical activity can be attributed to the favorable interaction between Pd in a carbid lattice as compared to when it is deposited on a carbon support. In addition, the X-ra Photoelectron Spectroscopy (XPS) of these samples showed that the Pd 3d peaks ha shifted to 335.9eV, similar to previous reports of Pd-C, as seen in the supporting info i Figures S8 and S9 [10]. Carbides have in general been shown to be superior catalysts for variety of reactions including HER, ORR, and MOR, and the addition of Pd further en hanced their activity [37].  To investigate these materials' potential application as electrocatalysts, their activity for the methanol oxidation reaction was tested. Figure Table S5. This increase in electrochemical activity can be attributed to the favorable interaction between Pd in a carbide lattice as compared to when it is deposited on a carbon support. In addition, the X-ray Photoelectron Spectroscopy (XPS) of these samples showed that the Pd 3d peaks had shifted to 335.9 eV, similar to previous reports of Pd-C, as seen in the supporting info in Figures S8 and S9 [10]. Carbides have in general been shown to be superior catalysts for a variety of reactions including HER, ORR, and MOR, and the addition of Pd further enhanced their activity [37]. To investigate these materials' potential application as electrocatalysts, their activity for the methanol oxidation reaction was tested. Figure Table S5. This increase in electrochemical activity can be attributed to the favorable interaction between Pd in a carbide lattice as compared to when it is deposited on a carbon support. In addition, the X-ray Photoelectron Spectroscopy (XPS) of these samples showed that the Pd 3d peaks had shifted to 335.9eV, similar to previous reports of Pd-C, as seen in the supporting info in Figures S8 and S9 [10]. Carbides have in general been shown to be superior catalysts for a variety of reactions including HER, ORR, and MOR, and the addition of Pd further enhanced their activity [37].

Materials and Methods
Pd-W-C synthesis. In a typical experiment, a ratio of 1:25 (moles of metal:moles of carbon) is weighed out. For a sample of 2.85:1 (W:Pd), this results in 0.0623 g K 2 PdCl 6 being The contents of the beaker are dissolved in 60 mL of D.I. water and placed on a hot plate at 45 • C while stirring for 20 min. After the K 2 PdCl 6 has finished sonicating, its contents are added to the reaction beaker. From here, a pH probe is used to monitor the addition of 1:3 conc. HCl:D.I. H 2 O. The pH is subsequently dropped to just below 3. The sample is stirred vigorously at 45 • C for 2 h. At this time, a precipitate will have formed. The reaction is then cooled to room temperature. Stirring is turned off, and the reaction beaker is placed on a lab bench for 10 min or until all of the precipitate has sunk to the bottom. From here, the supernatant is decanted off into a waste container, the stir bar is removed, and the remaining contents are placed in a drying oven at 60 • C to dry for 1 h. The precursor powder is then collected and taken to an X-ray diffractometer (Rigaku Smart Lab) for data collection. Finally, the sample is ready for annealing in a thermogravimetric analyzer The contents of the beaker are dissolved in 60 mL of D.I. water and placed on a hot plate at 45 • C while stirring for 20 min. After the K 2 PdCl 6 has finished sonicating, its contents are added to the reaction beaker. From here, a pH probe is used to monitor the addition of 1:3 conc. HCl:D.I. H 2 O. The pH is subsequently dropped to just below 3. The sample is stirred vigorously at 45 • C for 2 h. At this time, a precipitate will have formed. The reaction is then cooled to room temperature. Stirring is turned off, and the reaction beaker is placed on a lab bench for 10 min or until all of the precipitate has sunk to the bottom. From here, the supernatant is decanted off into a waste container, the stir bar is removed, and the remaining contents are placed in a drying oven at 60 • C to dry for 1 hr. The precursor powder is then collected and taken to an X-ray diffractometer for data collection. Finally, the sample is ready for TGA annealing under an Ar flow of 100 mL/min to the desired reaction temperature at a ramping rate of 20 • C/min. All samples were rapidly cooled to room temperature.

TEM Grid Preparation
TEM grids were prepped with~1 mg of sample sonicated in 3 mL of ethanol for 45 min. The dispersed sample, light grey in color, was then drop-cast (0.2 mL) onto a Cu or Ni grid. In order to reduce drift under TEM, the sample was allowed to dry for 12 h before being analyzed.

X-ray Photoelectron Spectroscopy
Measurements were performed on a Physical Electronics 5800 instrument. High resolution spectra were shifted to a C1s hydro carbon peak at 284.8 eV. High resolution spectra were collected at a pass energy of 23.5 eV in 0.1 eV increments. A low energy electron gun was used to neutralize any charging.

Electrochemical Analysis
The were allowed to dry for 12 h before use. Once the electrodes were prepared, 50 CVs were performed in 1.0 M MeOH/1.0 M KOH. The 50th scan has been reported for comparison.

Conclusions
In conclusion, this low temperature synthesis route was shown to greatly enhance the solubility of Pd in both Mo-C and W-C carbide systems. Because all three elements are coordinated in the precursors, they combine to form ternary (or pseudo-binary) metal carbide nanomaterials at low temperatures. Through this investigation, a new compound, (PdW) 2 C, was synthesized. Furthermore, the solubility of Pd in these Pd-W-C and Pd-Mo-C systems was greatly increased to 24 and 32%, respectively. The composition control was demonstrated for both systems, which allows yet another avenue for tuning the catalytic activity. Finally, these materials showed an enhanced catalytic activity for methanol oxidation and will likely show a catalytic activity for many other processes. Given the promise of Gregoire's solid solutions in thin films, it is possible that the solubility can be further increased, and it is likely that other late transition metals will find an increased solubility in these metal carbide systems.

 ✖ 
The XRD data can be correlated with the from Figure S1, which is similar to previous stu 225 °C, there is a mass loss that corresponds to sent in the precursor. From 225-300 °C, another tem. From 300-425 °C, there is another mass l crystalline Pd, as seen in the XRD from


The XRD d from Figure S1, 225 °C, there is sent in the precu tem. From 300crystalline Pd, a position of palla corresponds to thesis method i mixture had a s the pKa1 of o-p phase in the XR To further energy dispersi cubic Pd-W-C t ), and (PdW) 2 C ( the carbide lattice in two phases of tungsten carbide, and finally separates into an elemental state again at high temperatures, where the δ-WC phase also appears. The XRD data can be correlated with the thermogravimetric analysis (TGA) data from Figure S1, which is similar to previous studies on carbide formation [36]. From 25-225 °C, there is a mass loss that corresponds to the loss of water and volatile species present in the precursor. From 225-300 °C, another event occurs as free amine leaves the system. From 300-425 °C, there is another mass loss, which gives rise to the formation of crystalline Pd, as seen in the XRD from Figure 1. This mass loss is likely due to the decomposition of palladium chloride into Pd metal and HCl gas. Finally, the mass loss at 800 °C corresponds to the reduction of W and subsequent formation of carbide. While this synthesis method is still under investigation, it was observed that the acidity of the reaction mixture had a significant effect on the solubility of Pd. As seen in Figure S2, pHs below the pKa1 of o-phenylenediamine, 0.80, result in metallic Pd being present as a distinct phase in the XRD. Phase pure ternary carbides were only obtained at pHs above 0.8.
To further demonstrate the inclusion of Pd into the WC lattice, SEM images with energy dispersive X-ray spectroscopy (EDS) mapping were studied. Figure 2 shows the cubic Pd-W-C ternary carbide with 24% Pd. Other compositions including 15% and 7% ). (b) XRD of PdMoC synthesized at 1000 • C (pH = 0.5 (bottom) and pH = 2.41 (top)) Pd ( reduced to a metallic state from the precursor material at low temperatures, dissolves into the carbide lattice in two phases of tungsten carbide, and finally separates into an elemental state again at high temperatures, where the δ-WC phase also appears.

 ✖ 
The XRD data can be correlated with the thermogravimetric analysis (TGA) data from Figure S1, which is similar to previous studies on carbide formation [36]. From 25-225 °C, there is a mass loss that corresponds to the loss of water and volatile species present in the precursor. From 225-300 °C, another event occurs as free amine leaves the system. From 300-425 °C, there is another mass loss, which gives rise to the formation of crystalline Pd, as seen in the XRD from Figure 1. This mass loss is likely due to the decomposition of palladium chloride into Pd metal and HCl gas. Finally, the mass loss at 800 °C corresponds to the reduction of W and subsequent formation of carbide. While this synthesis method is still under investigation, it was observed that the acidity of the reaction mixture had a significant effect on the solubility of Pd. As seen in Figure S2, pHs below the pKa1 of o-phenylenediamine, 0.80, result in metallic Pd being present as a distinct phase in the XRD. Phase pure ternary carbides were only obtained at pHs above 0.8.
To further demonstrate the inclusion of Pd into the WC lattice, SEM images with energy dispersive X-ray spectroscopy (EDS) mapping were studied. Figure 2 shows the cubic Pd-W-C ternary carbide with 24% Pd. Other compositions including 15% and 7% ), PdMoC ( into hexagonal δ-WC and metallic Pd, along with remnants of the β-W2C phase. This series of data helps identify the location of Pd during the various transition temperatures. Pd is reduced to a metallic state from the precursor material at low temperatures, dissolves into the carbide lattice in two phases of tungsten carbide, and finally separates into an elemental state again at high temperatures, where the δ-WC phase also appears.

 ✖ 
The XRD data can be correlated with the thermogravimetric analysis (TGA) data from Figure S1, which is similar to previous studies on carbide formation [36]. From 25-225 °C, there is a mass loss that corresponds to the loss of water and volatile species present in the precursor. From 225-300 °C, another event occurs as free amine leaves the system. From 300-425 °C, there is another mass loss, which gives rise to the formation of crystalline Pd, as seen in the XRD from Figure 1. This mass loss is likely due to the decomposition of palladium chloride into Pd metal and HCl gas. Finally, the mass loss at 800 °C corresponds to the reduction of W and subsequent formation of carbide. While this synthesis method is still under investigation, it was observed that the acidity of the reaction mixture had a significant effect on the solubility of Pd. As seen in Figure S2, pHs below the pKa1 of o-phenylenediamine, 0.80, result in metallic Pd being present as a distinct phase in the XRD. Phase pure ternary carbides were only obtained at pHs above 0.8.
To further demonstrate the inclusion of Pd into the WC lattice, SEM images with energy dispersive X-ray spectroscopy (EDS) mapping were studied. Figure 2 shows the cubic Pd-W-C ternary carbide with 24% Pd. Other compositions including 15% and 7% ) and Mo ( into hexagonal δ-WC and metallic Pd, along with remnants of the β-W2C p of data helps identify the location of Pd during the various transition tem reduced to a metallic state from the precursor material at low temperature the carbide lattice in two phases of tungsten carbide, and finally separa mental state again at high temperatures, where the δ-WC phase also appe


The XRD data can be correlated with the thermogravimetric analy from Figure S1, which is similar to previous studies on carbide formatio 225 °C, there is a mass loss that corresponds to the loss of water and vola sent in the precursor. From 225-300 °C, another event occurs as free amin tem. From 300-425 °C, there is another mass loss, which gives rise to t crystalline Pd, as seen in the XRD from Figure 1. This mass loss is likely du position of palladium chloride into Pd metal and HCl gas. Finally, the ma corresponds to the reduction of W and subsequent formation of carbide. thesis method is still under investigation, it was observed that the acidit mixture had a significant effect on the solubility of Pd. As seen in Figure the pKa1 of o-phenylenediamine, 0.80, result in metallic Pd being prese phase in the XRD. Phase pure ternary carbides were only obtained at pH To further demonstrate the inclusion of Pd into the WC lattice, SE energy dispersive X-ray spectroscopy (EDS) mapping were studied.  Figure S3: Variable loading of PdWC, (PdW) 2 C, and PdMoC. XRD shows different amounts (%) of Pd found in the sample by EDS via SEM. Figure S4: Example of EDS spectra of Pd x W 1−x C. Figure S5: Dark-field STEM and corresponding EDS mapping for (PdW) 2 C. Figure S6: Example of EDS spectra of Pd x Mo 1−x C. Figure S7: Bright-field TEM image of Pd-Mo cubic ternary carbide (32% addition) with lattice fringe analysis and SAED. Figure S8: XPS Data for PdWC with (a) survey scan, (b) W4f region, and (c) Pd3d region. Figure S9: XPS Data for (PdW)2C with (a) survey scan, (b) W4f region, and (c) Pd3d region. Table S1: Structural information of Pd-M-Cs with literature comparisons. Table S2: Lattice constants and peak positions for carbides with and without Pd insertion. Table S3: W loadings measured before and after synthesis with corresponding pH, temp., and resulting phase. Table S4: Mo loadings measured before and after synthesis with corresponding pH, temp., and resulting phase. Table S5: ICP-OES of 10.5% Pd/C, 9.5% (PdW) 2 C, and 5.5% PdMoC for electrochemical analysis.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to pending patent applications.