Influence of surface morphology on the performance of nanostructured ZnO-loaded ceramic honeycomb for syngas desulfurization

A facile seeding-growth protocol was employed to immobilize nanostructured ZnO with nanorod and nanosheet morphologies (ZnO-nR and ZnO-nS, respectively) on cordierite-mullite honeycomb support. By varying the hexamethylenetetramine (HMTA) concentration, Zn precursor, and number of growth cycles during synthesis, different nanorod sizes, nanosheets textures and ZnO layers were obtained. The ZnO-loaded honeycombs were characterized using FESEM, EDX and XRD indicating that the immobilized layer of nanostructured ZnO was highly-crystalline with a thickness of ∼1 µm. The synthesized nanostructured ZnO-loaded honeycombs and a commercial ZnO sorbent were applied for removal of sulfur compounds (H2S and COS) from syngas at 400 °C. The ZnO-nS showed significantly longer breakthrough time (BTTS) and higher total sulfur sorption capacity (48.7 mg g−1 ZnO, BTTS = 75.4 min) than the ZnO-nR (9–12 mg g−1 ZnO, BTTS = 23–25 min) and commercial ZnO sorbent (4.6 mg g−1 ZnO, BTTS = 6.8 min). The superior sorption capacity of ZnO-nS was attributed to the significantly better surface coverage and higher crystallinity of ZnO nanosheets on the honeycomb. The introduction of additional ZnO nanosheets layers (up to 3 layers) through repeated growth process increased the ZnO loading to ∼1.5 ± 0.1 mg mm−1 (from ∼0.9 ± 0.1 mg mm−1 in the single layer) but resulted in poorer performance (11.6 mg g−1 ZnO, BTTS = 24.6 min) compared to ZnO-nS. This was due to the increased internal mass transfer resistance and decreased density of the effective reactive sites. The mechanism of ZnO-nS formation is also proposed to provide further insights. Overall, the ZnO-nS showed better regenerability, lower mass transfer resistance, and higher sorption capacity compared to the commercial ZnO and ZnO-nR sorbents indicating that it has a promising potential for syngas desulfurization.