SO 2 Differences*
3.3.5 Dry reforming tests
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chemisorption, which in turn resulted in a higher residual active metal surface (Table 3). In fact activity coefficients suggest that the drop in the rate of methane steam reforming is mainly connected to the loss of those active sites capable of strongly chemisorb CO rather than on the total exposed metal sites [52].
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H2
uptakea Dispersionb Diameter CO uptakec (weak+strong)
Strong (CO gem.)
Weak (CO linear)
Dispersion Diameter
[μmol/g] [%] [nm] [μmol/g] [%] [nm]
Rh-P/LA_T 87 44.8 2.4 57.9 47.2 10.7 39.2 2.8
Rh/LA_T 38 19.7 5.5 30.5 26.1 4.1 19.6 5.6
Rh-P/LAc - - - 89.1 80.4 8.7 51.3 2.1
Rh/LAc - - - 57.1 49.5 7.6 37.5 2.9
a) chemisorption measured at 35°C a) stoichiometry H/Rh=1
c) see section (3.3.2)
Table 6: Results of H2 and CO chemisorption measurements, percentage metal dispersion and average diameter of crystallites.
In table 6, results obtained with H2 and CO chemisorption are reported. The dispersion values have been calculated with a stoichiometry H/Rh=1, while in the case of CO adsorption the stoichiometry has been assumed equal to 2 and 1 respectively for the strong and weak adsorption components on Rh accordingly to DRIFT experiments reported in section 3.3.2.
As regards the samples prepared at TUM, H2 and CO chemisorption data show a good agreement, with a very slight difference for Rh-P/LA_T, while perfectly coincident for the undoped catalyst.
Through the very similar comparative data, obtained with techniques employing different adsorbate molecules, it is possible to confirm the reliability of dispersion calculations of Rh metal particles by means of CO distinguishing between strong and weak adsorption. It must be emphasized, that the efficiency of CO measurements is also supported by the utilization of higher amount of catalyst (up to 1 gram) than the H2 tests, that together to a higher stoichiometry CO/Rh (=1-2) than H2/Rh (0.5), allows to measure a significant uptake. Moreover, it is claimed that if H2 chemisorption is carried out at room temperature, hydrogen spillover processes on the support can easily occur in presence of a metallic phase [61]. Thus, the relative higher value of dispersion calculated for Rh-P/LA_T by H2 in comparison to that calculated by CO could be related to a higher H/Rh ratio than the real value, due to the spillover effect. Despite all, focusing on the dispersion results, it is clear that the P addition leads to a higher dispersion of Rh particles: in the lower case (i.e. CO measurements) RhP/LA_T has a double dispersion than the reference sample and an average diameter of the metal particles equal to 2.8 instead of 5.6 nm calculated for Rh/LA_T. In table 2, the value of dispersion and dimension of particles relative to Rh-P/LA and Rh/LA already described in section 3.3.2 are also recalled for the sake of comparison. Their dispersion values are slightly higher than those corresponding to the catalysts prepared later, thus resulting in lower dimension of crystallites;
although phosphorous results in an improved dispersion in both the cases, the difference is probably
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ascribed to the changes in the preparation routes, especially taking account the different precursor for phosphorous used to prepare Rh-P/LA (see section 2.1.1).
In order to have a further probe on the difference of dispersion between the doped catalyst and its counterpart, TEM images were taken. Unfortunately, the pictures collected ( not shown) for the two samples didn’t allow to estimate particles mean size and particles size distribution. Indeed, for high surface areas supports, the TEM observation becomes more difficult because of the low contrast between the small metallic particles and the support and it is not easy to obtain quantitative data on metal dispersion [61].
3.3.5.2 Dry reforming catalytic tests
Figure 16: Time-dependent conversions of CH4 (a) and of CO2 (b) over Rh-P/LA_Tand Rh/LA_T , P=1 atm, T furnace=850°C, P 1 atm, space velocity 1.2*105 ml/(h g).
The time-dependent activities of supported Rh catalysts are plotted in Fig. 16. The conversions of CH4 and CO2 remained unchanged during the period of study, 16 h. Especially the P doped sample, which conversion values are arranged in a straight line, didn’t show any sign of deactivation. Both methane and CO2 conversions approached the equilibrium prediction for Rh-P/LA_T, resulting in an H2/CO ratio equal to 1, as it is shown in fig. 17, while the activity of undoped catalyst slightly departed from the equilibrium lines.
a) b)
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Figure 17: Time-dependent H2/CO ratio over Rh-P/LA_Tand Rh/LA_T , P=1 atm, T furnace=850°C, P 1 atm, space velocity 1.2*105 ml/(h g).
Altough the good stability and high activity of both the samples, Rh-P/LA_T displayed a better overall performance, i.e. higher conversion than the reference Rh/LA_T. This occurrence is likely ascribed to the higher metal dispersion of P doped Rh catalyst. Indeed, Wang and Ruckenstein [62]
investigated the effect of particle size on the specific activity of Rh for the CO2-reforming of CH4 over the Rh/γAl2O3 catalyst. They changed the particle size by varying the Rh loading: the turnover frequency (TOF) was found strongly dependent on the particle diameter of Rh in the range of 1–7 nm. This result suggested to the authors that the reforming reaction is structure sensitive on γAl2O3 supported Rh catalyst. These findings agree with the results obtained in this work, as higher activity is achieved with the more dispersed Rh-P/LA which average particle diameter is half in comparison to the reference Rh/LA.
Equilibrium calculations (shown in fig.18) indicated the occurrence of very small amount of solid carbon (0.58% mol) at operative conditions used in this work. However, during reforming operations, coke formation is possible even when it is not predicted by thermodynamics. For example, Rostrup-Nielsen investigating the carbon deposition reactions over Ni catalysts [27] stated that the Boudouard and methane decomposition reactions are catalyzed by Ni, causing carbon to grow as a fibre/whisker. In the context of steam methane reforming, in which the primary carbon deposition route is methane decomposition, Rostrup-Nielsen described carbon formation related also to a kinetic allowance in spite of overall thermodynamics leading to methane decomposition into carbon (instead of reacting with steam).
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Figure 18: Equilibriun mole fraction versus temperature calculated at atmospheric pressure and CH4/CO2/N2=1/1/2.
Under dry reforming conditions, this situation may even get worse in comparison to steam reforming, since the absence of steam as reactant that can gasify deposits of solid carbon.
In this work, both catalysts investigated, showed an excellent stability without any decay in conversion of CH4 and CO2, whose occurrence is generally explained through coke formation during dry reforming operations. This outcome is especially relevant for Rh-P/LA indicating the maintenance of a strong interaction between Rh and P also under severe reaction conditions, avoiding losses of phosphorous at high temperature. In addition to the stability results, visual inspection of the alumina tube in which the catalysts were allocated, didn’t show any carbon deposition after testing runs. The non-occurrence of coke formation is of particular relevance for Rh-P/LA_T, since the previous studies of Chakrabarti et al [6] found a negative effect of phosphorous addition (1 atom of P for every 5 atoms of Rh) to the catalysts during steam reforming of methane, in terms of conversion and carbon deposition. As above mentioned, they argued that phosphorous decreased the dispersion of Rh-αAl2O3 spheres from a value of 10% to 3% (with Rh particles of ca30nm). Moreover, they demonstrated through SEM images of P doped systems, the formation of carbon filaments on the catalyst after performance testing. They observed the presence of whisker carbon species, tipically formed on Ni catalysts due to methane decomposition and Bodouard reaction, attributing this situation to the poison effect of Phosphorus on the surface–
carbon gasification reactions, leading to accumulation of carbon on surface of rhodium. The impressive discrepancy in the effect of P is likely related to the different dispersions of doped
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catalysts as well as to the absence of a H2 pretreatment at high temperature [6] which is required to obtain an effictive interaction between the two metals.