Chapter 5: Results and Discussion: Catalyst Characterization
5.6. Temperature Programmed Reduction (TPR)
The TPR profiles for CuO/Al2O3, CuO/SiO2 and CuO/Cr2O3 are presented in Figures 5.17(a), (b) and (c) respectively. The data obtained from these profiles (temperature at maximum and degree of reducibility values) are tabulated in Table 5.5. Since the reasoning governing the temperature at which a reduction peak is observed is often related to the support and its influence on the CuO, it was decided to perform a TPR analysis on unsupported CuO (which is also referred to as bulk CuO) in order to fully ascertain the effect the support had on the reduction of the CuO. The TPR profile for the unsupported CuO (bulk) is shown in Figure 5.17 (d), whilst the data are summarized in Table 5.5 as well.
The TPR profile of unsupported CuO (as shown in Figure 5.17(d)) displays a reduction peak at 416 °C that corresponds to the reduction of bulk CuO. The broadness of the peak is mainly due to large clusters of CuO with varying sizes10 and is an indication of the difficulty associated with the reduction of unsupported CuO. The larger CuO clusters present a smaller reactive surface area for reduction to occur thus accounting for the difficulty associated with the reduction of unsupported CuO.
Figure 5.17: TPR Profile of (a) CuO/Al2O3; (b) CuO/SiO2; (c) CuO/Cr2O3 and (d) Unsupported CuO
Table 5.5: List of temperature at maximum (Tm), degree of reduction and average oxidation state of Cu for different catalysts as determined by H2-TPR
Catalyst Temperature at maximum (Tm)/°C
Degree of reducibility/%a
Average Oxidation state of Cu based on H2
consumedb Unsupported CuO
(Bulk) 416 -- --
CuO/Al2O3 231, 298 84.4 0.3
CuO/Cr2O3 308, 468, shoulder peaks at
240 and 541 85.8* 0.3
CuO/SiO2 300 75.1 0.5
aDetermined using the equation: moles H2 consumed / moles reducible Cu x 100
bDetermined based on the assumption that: moles H2 consumed = 2 electrons required for reduction to Cu0 and using the equation: Oxidation state of Cu before reduction – (moles H2 consumed x 2 electrons/moles reducible Cu)
* Based on first reduction peak only
(c)
4.95 5.95 6.95 7.95 8.95 9.95
0 100 200 300 400 500 600 700
TCD Concentration
Temperature/°C
The reduction peak seen for CuO/Al2O3 at 231 °C and the first shoulder peak for CuO/Cr2O3 seen at 240 °C corresponds to the reduction of highly dispersed CuO. The reduction peaks at 298 °C, 308 °C and 300 °C for CuO supported on alumina, chromia and silica, respectively, corresponds to the reduction of bulk CuO that interacts with the support.11-13 It is uncertain as to what species is being reduced to give the second reduction peak seen at 468 °C as well as the shoulder peak at 541 °C for CuO/Cr2O3. Further investigation involving H2 in situ XRD was used to determine the species being reduced at these temperatures. The diffractogram obtained from the H2 in situ XRD is presented in Figure A 3 (Appendix A). It is seen that the phase changes observed between 465 – 468 °C and 539 – 542 °C are due to the reduction of Cr2O3 and CuCr2O4 respectively.
It is uncertain as to what phase the Cr2O3 phase is being reduced to, since XRD does not show peaks corresponding to chromium in an oxidation state less than +3. The reduction of CuCr2O4 is to Cu0 (due to the appearance of Cu0 and Cr2O3 peaks at 542 °C) and due to its spinel structure, this reduction occurs at a high temperature.
It is observed that the Tm value obtained for unsupported CuO is much higher than those of the supported CuO. Regardless of the support, the supported catalysts investigated displayed reduction temperatures lower than that of the unsupported CuO. A possible explanation for the ‘enhanced’ reducibility of supported CuO is that there is an increased dispersion of the supported oxide, providing a larger reactive surface area and therefore a higher concentration of defects at which reduction can start.14
The degree of reducibility for CuO/Al2O3 and CuO/Cr2O3 is 84 % and 86 % respectively whilst that of CuO/SiO2 is 75 %. The degree of reducibility for CuO/Cr2O3 was based on the first reduction peak, since this was the only peak that represented the reduction of the CuO to Cu0. However, for the other catalysts the degree of reducibility was based on all peaks in the TPR profile, since these peaks corresponded to the reduction of CuO to Cu0. The degree of reducibility obtained for the catalysts indicates that there remains a percentage of CuO that is not reduced. It is possible that there is some CuO that may be surrounded by an environment that does not allow it to be available for reduction, thus accounting for the degree of reducibility being less than 100 %. For CuO supported on alumina and silica, this can be linked to the presence of the clusters of CuO as seen in the backscattered SEM (Figure 5.3 and 5.6 (a) – (b)) and EDS composition scanning map data (Figure 5.4 and 5.7 (b) – (c)). Once the ‘outer core’ of the CuO cluster is reduced, it
encapsulates the ‘inner core’ CuO. It therefore creates a ‘boundary wall’ through which the hydrogen atoms cannot pass, leaving the ‘inner core’ CuO inaccessible to the hydrogen and thus unreduced. The lower degree of reducibility of CuO/SiO2 is probably due to the presence of a greater amount of clusters than the alumina supported catalyst (as observed in the backscattered SEM images).
The average oxidation state of Cu after the H2-reduction is listed in Table 5.5. This value indicates that for CuO supported on alumina, silica and chromia, the exposed Cu2+ species present was reduced to the zero oxidation state (Cu0).15 The slight deviation in this value from the expected value of zero is attributed to the degree of reducibility being less than 100%. In comparing the degree of reducibility obtained for each catalyst (Table 5.5) to the oxidation states determined (Table 5.5), it is seen that the higher the degree of reducibility the closer to zero the oxidation state will be (as for CuO/Al2O3 and CuO/Cr2O3). Thus, both the degree of reducibility and the average oxidation state from the amount of H2 consumed can indicate whether the total amount of CuO present in the catalyst is reduced. The presence of the characteristic Cu0 phase peaks in the H2 in situ diffractograms (Figure A 2 – 4 in Appendix A) confirms the reduction of CuO to Cu0. However, the H2 in situ diffractograms for CuO/Cr2O3 (Figure A 3, Appendix A 1) also reveal the presence of characteristic Cu2O phases. This indicates that the reduction of Cu2+ is to the Cu+1 oxidation state as well. In addition, the appearance of some CuO phase peaks in the H2 in situ diffractograms of the three catalysts (Figure A 2 – 4 in Appendix A) indicates the incomplete reduction of the total amount of CuO available for reduction.