A Mini Review of in situ Near-Ambient Pressure XPS Studies on non-Noble, Late Transition Metal Catalysts

Due to the increasing price of noble metals and the progress in materials synthesis, non-noble transition metal catalysts have gained intensive interest in heterogeneous catalytic processes. Transition metals can have multiple oxidation states, which can be easily exchanged depending of the conditions (gas atmosphere, pressure and temperature). Understanding the interaction of catalytic surfaces with reactive gaseous or vapor atmospheres is crucial for the design of new transition metal catalysts and the optimization of those already in use. X-ray photoelectron spectroscopy (XPS) is one of the most powerful spectroscopic tools to obtain chemical information about solid surfaces. However, conventional XPS instruments operate in ultra-high vacuum conditions (< 10-8 mbar) limiting the characterization of catalysts, before and after reaction. Recently, a new type of XPS apparatus has emerged allowing performing measurements in gaseous environments in the mbar pressure range. These 'environmental XPS' systems, commonly referred to as near ambient pressure XPS (NAP-XPS), have open up new opportunities to study dynamic modifications at surfaces and correlate them with the catalytic performance. In this review we discuss NAP-XPS studies of non-noble, late transition metal catalysts (Fe, Co, Ni and Cu) under process conditions. The aim is to describe the rich chemistry of transition metal surfaces during catalytic reaction and provide crucial elements for sophisticated design of next generation catalysts.