CeO2 and TiO2 support material effects on NH3 decomposition pathway mechanism over Cu–Zn catalysts

Abstract

The decomposition of NH3 over Cu–Zn catalysts, supported on ceria and titania, was investigated, producing carbon (di)oxide-free H2. The rate of the H2 formation at the exposed metallic Cu atoms (cumulative turnover frequency) for dispersed Cu–Zn depended strongly on the type of metal oxide support, indicating that the pathway active sites of Cu–Zn nano-catalysts were not identical in quantity. The investigation of the associative desorption process of atomic nitrogen moieties formed by the NH3 scission over Cu–Zn alloy revealed that N2 interface evolution temperature was affected by the nature of substrates. The characterization of the nanomaterials before the catalyzed splitting reactions of NH3 was carried out by XRD, N2 physisorption, N2O chemisorption, H2 TPD, CO2 TPD, NH3 TPD and H2 TPR. TEM and XPS were conducted on both fresh and used composites. Cu–Zn dispersion increased in the presence of TiO2. These spinel forms exhibited a strong Cu0.5Zn0.3Ti0.2O2–x (x = 0–1) structure interaction. Cu–Zn/CeO2 showed a 79% conversion of ammonia at 600 °C, whereas Cu–Zn/TiO2 demonstrated 63% at comparative operating conditions. A high catalytic activity, reflected in almost total consumption of ammonia at 700 °C with the hydrogen reactive production of 33.2 mmol g−1 min−1, was achieved over Cu–Zn/TiO2. No reactivity loss considering the NH3 depletion under applied reactor runs was detected, implying that materials had excellent time-on-stream stability. By scanning and transmission electron microscopy, bimetallic Cu–Zn was also observed, displaying a uniform TiO2 particle distribution on a structured sample surface. Impregnated zinc and copper species were uniformly distributed over Cu–Zn/TiO2. In the case of higher precursor content, a slight Cu domain agglomeration was noted. A heterogeneous mixed phase of Cu and Cu2O was present in all examined powders, as exposed by Cu L-edge spectra ratios.