3.1. Characterization of the Supports
BET surface area values for the catalyst supports are shown in Table 1
. They vary in a wide range from 8 m2
/g to 348 m2
/g for the g-C3
and Sibunit support, respectively (Table 1
). Unfortunately, the used synthesis procedure of the g-C3
material did not provide as high surface area as that reported by Tahir et al. [26
] (290 m2
/g) for unknown reasons. We used different heating modes for the intermediate, but it did not allow increasing the surface area of the obtained g-C3
material. For example, heating of the sample at 673 K for 2 h gave even a lower surface area (6.5 m2
Doping of the Sibunit surface with substances containing pyridinic nitrogen leads to a decrease of the surface area owing to occupation of the pores by the nitrogen precursor. The decrease is the strongest when melamine is used. However, the surface areas of the melamine-based supports were still high as compared to that of g-C3N4 and reached 86.5 and 42 m2/g.
X-ray diffraction patterns of the g-C3
, Mel/C, Sibunit supports and the intermediate for the carbon nitride synthesis (Inter-Cx
) are shown in Figure 2
. The pattern for the intermediate shows a lot of sharp peaks, differs from the patterns presented in literature for melamine [26
] and melem [32
], but is very close to the pattern of melaminium nitrate [30
]. Hence, we assign the Inter-Cx
intermediate to melaminium nitrate. The multiple peaks of this compound disappear after pyrolysis at 673 K. The pattern for the obtained g-C3
sample shows only one intensive peak at 27.1° in the studied region. This peak is characteristic interplanar stacking peak for g-C3
(JCPDS 87-1526) [33
]. It indicates the presence of the ordered structure of layers of heptazine which is a monomer of g-C3
(002 plane). Interesting that the formation of g-C3
from melaminium nitrate takes place at the temperature lower than those from melamine reported in literature (823 K) [31
Diffraction patterns of the Sibunit containing samples show diffraction peaks at 25.5° and 43.1° for the (002) and (100) planes of graphite (JCPDS 41-1487), respectively. The structure of the obtained g-C3N4 is denser, as the stacking distance for the carbon nitride is by 0.2 Å smaller than that for the Sibunit carbon.
Comparing the XRD patterns for the N-containing samples with the original Sibunit sample, we conclude that g-C3
cannot be determined using XRD because of the low carbon nitride content (12–14%) in the Mel/C sample and close positions of the g-C3
and Sibunit characteristic peaks ((002) plane). However, an important result is that multiple characteristic peaks of the melamine, melem or possible intermediates are absent (Figure 2
). Evidently, that melamine or intermediates completely converted to supported carbon nitride or to another type of a CN containing material.
atomic ratio was determined for our g-C3
support by CHN-analysis and was found to be equal to 0.63. It indicates higher nitrogen content in the sample as compared to stoichiometric g-C3
(0.75). Such low ratios were reported for carbon nitrides earlier [31
] and could be explained by the relatively low pyrolysis temperature, which we used.
3.2. Catalytic Activity and H2 Selectivity
To elucidate the support effect, using the obtained dependences between the formic acid conversion and temperature, we calculated the reaction rates at low conversions and plotted them as the Arrhenius plots (Figure 3
). It is seen that both Pd/Mel/C samples convert formic acid better than the other samples (Pd/C and Pd/g-C3
). The sample synthesized in the muffle furnace is slightly more active than the sample prepared using the microwave synthesis. However, it is worth noting that the synthesis time in the muffle furnace was 2 h, while that for the microwave synthesis was only 140 s. The difference in the rates between the melamine based Pd samples and the Pd/C sample is significant. The rates differ by a factor of 4 at about 373 K. In contrast, the Pd/g-C3
sample showed the activity even lower than that for the Pd/C catalyst.
Analyzing the data presented in Figure 4
, we can estimate the N-precursor effect on the catalytic activity. The catalysts prepared with bipyridine and phenanthroline show lower activities than that for the melamine-based catalyst, and they are not higher than the one of the Pd/C catalysts. These results are consistent with the data for the apparent activation energies (Ea
, Table 1
). The most active Pd/Mel/C catalysts show the smallest activation energy (32 kJ/mol), while the less active catalysts demonstrate higher apparent activation energies (42–46 kJ/mol). The Ea
value for the Pd/g-C3
catalyst is intermediate (39 kJ/mol). The difference in the apparent activation energies is significant and should be related to different properties of active sites of the catalyst. The obtained Ea
values for the Pd/Mel/C catalyst are among the smallest values for formic acid decomposition over Pd catalysts in liquid and gas phase.
Important features necessary for efficient hydrogen production from formic acid are selectivity and stability. The Pd/Mel/C (muf) catalyst showed the highest selectivity (>98%) at 50% conversion (528 K) while the phenanthroline based samples showed the lowest selectivities (90–92%).
The stability of the most active Pd/Mel/C (muf) catalyst was tested for 5 h (Figure S2
). In this experiment, the catalyst was not stabilized by pretreatment in the formic acid flow at 573 K as was done before other experiments. However, no significant changes of the formic acid conversion on time-on-stream was observed, and it is possible to conclude that this catalyst possesses sufficient stability to produce hydrogen from gas phase formic acid.
Hence, among the studied samples, the melamine based Pd catalysts showed the highest activity. We compared the activities of these samples with the activities of 1 wt% Ru, Pt and Au samples on the N-doped and N-free carbon supports (Table S1
). This comparison showed that the metal mass-based activity of the Pd/Mel/C (muf) catalyst is slightly lower than that of the Pt/N-C catalyst, but it is higher than the activities of the other catalysts. The apparent activation energy for the melamine based Pd sample was the lowest.
3.3. Characterization of the Pd Catalysts
To understand the reasons of the differences in the activities of the catalysts (Figure 3
and Figure 4
) the Pd samples after the reaction were studied using HRTEM and X-ray Photoelectron spectroscopy (XPS). Figure 5
shows HRTEM images. Particle size distributions are presented in Figure S3
. They are quite narrow for all studied samples indicating that the particles are rather uniform. Mean particle size data are presented in Table 1
. The most active Pd/Mel/C (muf) catalyst demonstrated the smallest mean particle size (2.0 nm). An XRD study of this sample was not sensitive enough to determine Pd in this sample confirming its low content and high dispersion. The highest mean particle size was obtained for the Pd/g-C3
sample, which corresponds to 2.6 nm. This could be related to the lowest BET surface area of the support used (Table 1
). However, the mean Pd particle sizes for other two catalysts did not differ much. Generally, as it is seen in Table 1
, the deposition of nitrogen precursors over the carbon surface did not provide a significant change in Pd dispersion.
A C1s XP spectrum for the Pd/g-C3
catalyst after the reaction is shown in Figure S4
. It consists of two main peaks at 288.3 and 284.8 eV assigned to pyridinic (C-N=C) and adventitious carbon, respectively. The presence of the first peak confirms the formation of g-C3
, as it is typical for this compound [26
]. The C1s spectra for other catalysts did not differ much from each other and, hence, are not demonstrated.
N1s XP spectra for all catalysts after the reaction are shown in Figure 6
. The main lines are observed at 398.8 eV assigned to pyridinic nitrogen (C-N=C) and at 400.2 eV assigned to tertiary nitrogen (N(C)3
) like those in g-C3
]. A shift of the N1s pyridinic line towards higher binding energies by 0.3 eV is observed for the Pd/Mel/C (MW) catalyst. This may indicate a stronger interaction of these species in the surface layer of this sample than in the case of the Pd/Mel/C (muf) and Pd/g-C3
shows the total surface N concentration determined by XPS (Ntot
). The highest concentration was obtained for the Pd/g-C3
catalyst (46%) followed by the Pd/Mel/C (MW) (8.9 at %) and Pd/Mel/C (muf) (4.5 at %) catalysts. The spectra of the melamine-based samples demonstrate the presence of both N1s lines, but the ratio of pyridinic species is decreased as compared to the Pd/g-C3
sample (Table 2
). The content of pyridinic N species in the Pd/Phen/C sample is negligible and Pd/Bpy/C and (Pd+Phen)/C samples do not contain pyridinic nitrogen at all. This shows that the pyridinic species have been converted either to tertiary N species or to gas products during the pyrolysis. The difference in the spectra of the melamine based and phenanthroline/bipyridine based samples can be explained at least partly by a much lower content of nitrogen in the bipyridine and phenanthroline precursors as compared to melamine (Figure 1
). Generally, the state of the deposited N-containing layer in the supported samples differs from that in the g-C3
sample. This can affect the state of supported Pd and provide the observed difference in catalytic properties (Figure 3
and Figure 4
). Therefore, the electronic state of Pd was also studied by XPS.
Pd3d XP spectra are shown in Figure 7
. The spectra consist of two components: one with Pd3d5
at 335.0–336.4 eV (Pd3d3
at 340.3–341.7 eV) and another with Pd3d5
at 336.9–338.5 eV (Pd3d3
at 342.2–343.7 eV). Taking into account the HRTEM data (Figure 5
) similar for all samples, we attribute the first component to metallic Pd (Pd0
), and the second component—to oxidized Pd (Pd2+
). The latter can exist as a surface oxide over Pd particles. It is formed due to oxidation of the metallic Pd surface by atmospheric oxygen. Alternatively, some Pd2+
species may exist in atomically dispersed state strongly interacting with nitrogen, oxygen or carbon defects of the support [13
]. This Pd could not be observed by the HRTEM equipment used in this work.
line at 335.0 eV is observed for the Pd/g-C3
catalyst, which is close to the Pd state in the unsupported bulk metal, for example, in Pd foil. The closeness of the electronic state of Pd in the sample to the Pd metal may indicate that there is no strong interaction of this Pd with the support. This can be due to small support surface area and low concentration of support sites, which can strongly interact with Pd. The lowest value of the Pd3d5
binding energy among the Sibunit-containing catalysts is observed for the Pd/C catalyst and corresponds to 335.8 eV. This Pd state is usually attributed to small metal particles interacting with the carbon support [13
]. The Pd3d5
lines for the Pd/Mel/C (muf) and Pd/Mel/C (MW) catalysts are strongly shifted to higher binding energies (336.4 and 336.3 eV, respectively) as compared to that for the Pd/C sample. This cannot be assigned to sample charging and indicates an electron-deficient state of Pd in these samples due to a strong interaction with the support. These catalysts showed the highest activity in the formic acid decomposition reaction and the lowest values of apparent activation energies (Table 1
). For the phenanthroline and bipyridine-based samples, the Pd3d5
line positions (336.0 eV) are close to that for the Pd/C sample. The catalytic properties of these systems are also close (Figure 4
It is an interesting question whether the Pd3d5
line in the 336.3–336.4 eV region for the melamine-based samples can be assigned to Pd oxide (PdO). We believe that it cannot, because the Pd3d5
binding energy for the PdO is normally located at a higher binding energy (336.8 eV) [38
] like those observed for the Pd/g-C3
and Pd/C samples. However, we will perform soon the necessary experiments allowing to discriminate whether the high binding energies for the Pd3d5/2
line (336.3–336.4 eV) should be assigned to Pd metal or to Pd oxide by performing XPS measurements after the pretreatment of the samples in H2
in the pretreatment chamber of the XP spectrometer, as it was done in some of our previous works [10
]. This pretreatment easily transforms the Pd oxide into metallic Pd.
Hence, utilization of HRTEM and XPS for the catalysts allowed explaining the effects observed for the catalytic reaction. The obtained data demonstrated that when pyridinic nitrogen exists on the surface, Pd becomes electron-deficient and the highest catalytic activity was obtained. At the same time, the presence of only tertiary nitrogen species does not positively affect the reaction.