Identification of Outer and Inner Nickel Particles in a Mesoporous Support: How the Channels Modify the Reducibility of Ni/SBA‐15 Catalysts

Abstract

Two different nickel supported on SBA‐15 catalytic systems have been prepared by means of impregnation (Ni/SBA‐15‐ImU) and deposition‐precipitation (Ni/SBA‐15‐DP) methodologies. Upon calcination, Ni/SBA‐15DP presents a well‐developed nickel phyllosilicate phase, which after reduction gives rise to a dispersed and homogeneous metallic phase, mainly located inside the 5 nm in diameter mesoporous structure of the support. On the contrary, as evidenced by XRD and a double temperature programmed reduction (TPR) peak, the Ni/SBA‐15‐ImU catalyst presents two different NiO phases, which after reduction in hydrogen generate nickel particles in a wide range of sizes. In situ XAS and XPS have unambiguously showed that the distinct TPR profiles obtained for each system are related with particles located in and out the mesoporous structure of the SBA‐15 channels. The particles inside the porous are more difficult to reduce, clearly showing a kind of confinement effect of the SBA‐15 mesostructure, modifying the reducibility of the NiO phase.

Understanding the differences in catalytic performance for hydrogen production of Ni and Co supported on mesoporous SBA-15

“…On the contrary, the TEM image of Co/SBA-15 ( Figure 5c) shows bigger heterogeneous particles with a mean diameter around 13-15 nm (Table 1) Figure 4 indicated that after this treatment most of metal should remain oxidized, the undetected oxidized nickel must be located inside the mesoporous channels in both catalysts, which are invisible for XPS, a surface sensitive technique. So, as found previously for different preparation methods [40], the deposition-precipitation methodology seems to favor the confinement of nickel inside the porous of SBA-15.…”

“…Despite this loss of surface area, the analysis by TEM (Figure 1) shows that the channeled structure of the SBA-15 support is preserved [35]. The calcined Ni x Co 1-x /SBA-15 catalysts present the characteristic "fibrous" structures of phyllosilicate phases, previously found in similar systems [36][37][38][39][40]. The ascription of these fibrous shapes to phyllosilicate is confirmed by the XRD diagrams included in Figure 2a.…”

“…Both nickel containing samples, Ni/SBA-15 ( Figure 5a) and NiCo/SBA-15 (Figure 5b) generate small and homogeneous nickel particles of about 4-6 nm (Table 1), slightly bigger for the bimetallic catalyst. From the visual inspection of these two TEM images, it is evident that many metal particles appear aligned with the mesoporous channels of support, once again suggesting that they could be located inside the mesoporous [40]. On the contrary, the TEM image of Co/SBA-15 ( Figure 5c) shows bigger heterogeneous particles with a mean diameter around 13-15 nm (Table 1) Figure 4 indicated that after this treatment most of metal should remain oxidized, the undetected oxidized nickel must be located inside the mesoporous channels in both catalysts, which are invisible for XPS, a surface sensitive technique.…”

“…The Ni/SBA-15 sample presents a main peak centered at 660 o C, previously assigned to a nickel phyllosilicate phase [40,41], with a small shoulder at lower temperature, around 400 o C. According to previous results obtained by our group, these two reduction features can be respectively ascribed to the reduction of oxidized nickel located inside and outside the mesoporous channels of SBA-15 support [40]. For its part, the bimetallic NiCo/SBA-15 catalyst presents a similar profile, but shifted to higher temperature (peaks maximum at 700 o C and 500 o C, respectively).…”

Bimetallic Ni-Co/SBA-15 catalysts for reforming of ethanol: How cobalt modifies the nickel metal phase and product distribution

“…Simultaneously, the average pore size increases from 6.7 to 9-12 nm. Despite this loss of surface area and porosity, the images of the calcined catalysts obtained by TEM ( Figure 1) shows that the channeled structure of the SBA-15 support is preserved, with the "fibrous" structures of phyllosilicate phases generated during this treatment being visible [29][30][31]. The formation of (NiCo) 3 Si 2 O 5 (OH) 4 and/or (NiCo) 3 Si 4 O 10 (OH) 2 phyllosilicate phases [32] can be confirmed in the XRD diagrams of Figure 2 (left) by the wide peaks appearing around 35 and 61 o [33], the only phases detected in the calcined samples.…”