Essential Role of Ru-Anion Interaction in Ru-Based Ammonia Synthesis Catalysts

The metal-anion interaction (MAI) widely exists over supported metal catalysts, which has a significant effect on tuning metal sites and reaction performance. However, the effect of MAI on NH3 synthesis and the reaction mechanism is still elusive. Here, we report that the strength of Ru-anion interaction gradually increases from Ru-N to Ru-H and Ru-O when the anion of the support is changed from H (ZrH2) to N (ZrN) and O (ZrO2). Moreover, the Ru-anion interaction induced by different support compositions can affect the hydrogen spillover and N-2 activation route. Due to the weak Ru-N interaction, the aggregation of Ru particles is observed over Ru/ZrN. Although Ru particles are highly dispersed over ZrO2 owing to the strong Ru-O interaction, hydrogen poisoning is inevitable over Ru/ZrO2. Comparatively, hydrogen poisoning can be alleviated over Ru/ZrH2 via hydrogen spillover to its support. The results of NEXAFS, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and isotope-labeling experiments show that N-2 can be activated via both dissociative and associative routes over Ru/ZrH2. Consequently, the developed Ba-promoted Ru/ZrH2 catalyst displays a high NH3 synthesis rate of 27.5 mmol g(-1) h(-1) and robust stability during 300 h time-on-stream at 400 degrees C and 1 MPa.

Automated summary

The metal-anion interaction has a significant effect on NH3 synthesis and reaction mechanisms. The composition of the support can influence the Ru-anion interaction, thereby affecting hydrogen spillover and N-2 activation routes. This study observed particle aggregation over Ru/ZrN and hydrogen poisoning over Ru/ZrO2, but hydrogen poisoning could be alleviated over Ru/ZrH2 via hydrogen spillover.