Some of the structural parameters of the oxidized carbons are compiled in Table 1 and high resolution TEM images are depicted in Fig. 1. The measured BET surface areas were 78 m2/g for CNP, 178 m2/g for CNF and 247 m2/g for CNT. The increase in BET surface area is ascribed to a decrease in fiber diameter (Table 1) for the different carbon materials (CNP: 40–200 nm, CNF: 25–40 nm and CNT: 10–25 nm), which will result in a larger surface-to-volume ratio.
The pore volume established from nitrogen physisorption was significantly higher for CNT (1.01 mL/g) than for CNP (0.24 mL/g) and CNF (0.28 mL/g). No micropores were found in the used materials. The substantially larger pore volume observed for CNT compared to CNF and CNP is ascribed to the fact that CNT has a lower bulk density combined with a small particle size. Therefore, a large intratubular pore volume is created. Moreover, the individual fibers in CNP and CNF seem to be more entangled as compared to the thinner CNT. This may result in a higher pore volume for CNT .
ICP-OES showed that platinum loadings for the catalysts were 1.7–1.8 wt% (Table 2). The amount of acidic, oxygen surface groups was determined using acid–base titrations. CNT displayed 0.50 mmol strong acidic sites per gram CNT, while 0.10 mmol strong acidic sites per gram were found for CNP and 0.17 mmol strong acidic sites per gram for CNF. At pH 7.5 0.84 mmol acidic sites per gram CNT was titrated, while for CNF and CNP the results were similar: 0.23 mmol acidic sites per gram material. For CNT the oxygen containing groups must be located on defects in the graphene sheets , which are either present in the as prepared materials or can be formed during the nitric acid treatment used to introduce the oxygen surface groups . Also after platinum deposition, titrations were performed to determine the amount of acidic, oxygen surface groups. At pH 5 this resulted in 0.02 mmol acidic sites per gram for Pt/CNF and Pt/CNP and in 0.06 mmol acidic sites per gram for Pt/CNT and Pt/CNT-red503. For Pt/CNF and Pt/CNP 0.07 mmol acidic sites per gram were found while for Pt/CNT and Pt/CNT-red503 0.14 and 0.13 mmol acidic sites per gram were present respectively at pH 7.5.
The TPR profiles of the materials after synthesis and drying are depicted in Fig. 2. The peak between 450 and 575 K represents the platinum reduction, while the peak at higher temperature is ascribed to gasification of the supports. The onset temperature for reduction was determined to be 461 K for Pt/CNF, 466 K for Pt/CNP and 487 K for Pt/CNT. Thus the reduction for Pt/CNT is retarded compared to Pt/CNF and Pt/CNP. The amount of platinum per gram material is approximately the same for all catalysts (i.e. 0.09 mmol Pt/g material; see also Table 2). Titrations showed that CNT have the highest amount of acidic sites i.e. 0.50 mmol acidic sites/g at pH 5 and 0.84 mmol/g at pH 7.5 which is a 5–9 time excess compared to the amount of platinum deposited on this material. It has been shown earlier by Toebes et al.  that acidic oxygen surface groups are required to anchor platinum on CNF. Even after metal deposition still CNT have the highest amount of acidic sites. Since it is known that acidic oxygen surface groups are required to anchor platinum, it is speculated that the high amount of acidic oxygen surface groups on CNT resulted in a strong binding and stabilization of the cationic platinum. The formation of metallic platinum particles during reduction might therefore be hindered, which in turn results in a higher reduction temperature for this sample compared to Pt/CNF and Pt/CNP (see Fig. 3). It has been described in literature that the reduction temperature of palladium on nanostructured carbon is affected by and related to the presence of stable acidic surface groups . This is in good agreement with our results.
It must be noted here that during metal deposition and reduction more oxygen surface groups are removed on CNT (i.e. 0.84–0.14 = 0.7 mmol/g) compared to Pt/CNF and Pt/CNP (0.23–0.07 = 0.16 mmol/g). These groups can decompose and being removed due to e.g. hydrogen spill-over during reduction . The substantial removal of oxygen surface groups for CNT is ascribed to the fact that this material has the highest concentration of oxygen surface groups present after oxidation of this material. As a consequence, more oxygen surface groups are in the vicinity of platinum resulting in a relative high removal of these groups.
The first peak in the TPR profile for Pt/CNF is clearly the smallest and is related to platinum reduction. It is calculated that for this peak the theoretical H2/Pt ratio is around 6, which is above the expected value of 1. This is probably the result of the formation of other gases during the reduction/anion decomposition such as nitrogen oxides, ammonia, CO and CO2 (the latter two gases originate from support gasification [22, 23]), which will affect the TCD signal making quantitative statements obsolete.
H/Pt ratios of the catalysts were calculated using hydrogen chemisorption and resulted for Pt/CNF in 0.65, Pt/CNP in 0.64, Pt/CNT in 0.17 and Pt/CNT-red503 in 0.35. Based on these data, average particle sizes were calculated and the results are given in Table 2. Platinum particle sizes were also analyzed using TEM (see Fig. 4). Pt/CNF and Pt/CNP showed platinum particles of 1–3 nm, while for Pt/CNT, TEM did not show platinum particles. For the latter catalyst, the presence of platinum was confirmed using TEM–EDX and ICP-OES. For Pt/CNT-red503 large platinum particles, ranging from 2 to 11 nm were observed. Histograms of the observed particle size distribution of the different catalysts are compiled in Fig. 5. Based on TEM histograms the average particle sizes were calculated for all catalysts (see Table 2). As mentioned before, TEM did not show unambiguously the presence of platinum particles for Pt/CNT, though ICP-OES and TEM–EDX confirmed the presence of platinum. Also hydrogen chemisorption resulted in a low H/Pt ratio, which shows that at least some platinum is present in the metallic state. These results are inconsistent with the observations using TEM, unless the sample is not well-reduced. The TPR profile of Pt/CNT suggests that this might be the case and therefore a reduction of Pt/CNT at 503 K was performed. The measured H/Pt ratio of this sample was indeed larger compared to the one reduced at 473 K, which shows that a higher reduction degree is obtained. Still, the H/Pt ratio is lower compared to Pt/CNF and Pt/CNP. TEM analysis of this sample showed relatively large particles, which leads to the conclusion that sintering had occurred. The average particle size for Pt/CNT-red503 determined using TEM (4.2 nm) and hydrogen chemisorption (3.2 nm) are relatively close to each other and therefore, it is concluded that reduction at 503 K resulted in metallic platinum. Thus to summarize, the high amount of acidic oxygen surface groups on CNT results in a high platinum-precursor dispersion. These platinum species are more difficult to reduce and need a higher reduction temperature compared to platinum deposited on CNF and CNP. Once reduced at higher temperature, TEM and hydrogen chemisorption revealed that the platinum particles on CNT were sintered.
The catalysts were tested for the cinnamaldehyde (CALD) hydrogenation (see Fig. 6 for the used abbreviations) and the results are depicted in Fig. 7. Both Pt/CNP and Pt/CNF were active (60% and 25% CALD conversion after 300 min respectively), though for the latter catalyst a clear deactivation is observed. For these test reactions by-products (only propylbenzene and β-methylstyrene) were observed: for Pt/CNP this was around 2% and for Pt/CNF this was around 1%. Pt/CNT showed only in the initial state some hydrogenation activity, but deactivated rapidly resulting in no further conversion at all. This also indicates that the standard chosen reduction temperature of 473 K is not high enough to create an active metal surface. Indeed when the catalyst is reduced at a higher temperature (i.e. Pt/CNT-red503), hydrogenation activity is observed (Fig. 7) resulting in 13% conversion after 300 min of reaction time and about 0.5% of by-product was observed. This conversion is still substantially lower compared to Pt/CNF and Pt/CNP (25% and 60% respectively).
The initial turn-over frequencies (TOF) are calculated based on H/Pt ratios and are summarized in Table 2. The initial TOF is the same for all catalysts, but as mentioned before deactivation quickly starts to affect the catalytic activity for several catalysts. Since the initial TOF is similar for all catalysts, the low weight based catalytic activity of Pt/CNT-red503 must be the result of the lower metal dispersion of that catalyst.
For Pt/CNF, Pt/CNP and Pt/CNT-red503 the conversion of CALD is plotted versus the selectivity to cinnamyl alcohol (CALC) (Fig. 8). The selectivity to CALC for Pt/CNP is significantly higher compared to that with Pt/CNF, while the platinum particle size, amount of oxygen surface groups and TOF is the same for these samples. It is therefore concluded that the selectivity for Pt/CNP and Pt/CNF changes as a function of the graphene sheet orientation. It is tempting to ascribe this to an electronic effect which has also been used to explain catalytic differences between CNF and CNT materials [5, 24], however that requires more research. For Pt/CNT-red503 the platinum particle sizes, amount of acidic sites as well as the graphene sheet orientation are different compared to Pt/CNF and Pt/CNP. Since it is known that for example variable platinum particle sizes have a significant influence on the catalytic activity , it is not possible to establish an intrinsic influence of the CNT graphene sheet orientation with respect to catalytic selectivity.