Title : Carbon nanotubes with tunable properties towards the design of efficient water denitration catalyst
Abstract:
Since their discovery in 1991, the unique tubular morphology of carbon nanotubes (CNTs) represents a great research challenge for many scientists over the world. The possibility of using different types of CNTs as catalyst support is still under extensive exploration. The process of water denitration is of great environmental significance. The catalytic reduction of nitrate to N2 represents the efficient water remediation technique in terms of the achieved nitrate removal, but still with a main drawback – the production of ammonia as the undesired product. Thus, efforts are undertaken to solve the problem, in terms of both reactor and catalyst design. The application of chemically unmodified tubes for catalyst design has not received sufficient attention regardless of their initially different structural properties. The CNT defects originating from their preparation using various catalysts can be considered as a suitable substituent for subsequent deliberately formed anchoring sites on CNT surface by the functionalization process. Thus, the step in the catalyst preparation procedure that involves the use of strong mineral acids can be avoided, leading to simpler, cheaper and environmentally friendly way to synthesize the catalyst. In this regard, the aim of this work was to synthesize various Pd-Cu/CNT denitration catalysts using multiwalled carbon nanotubes (MWCNTs) of different structural quality, evaluate the influence of their surface chemistry on the metal active phase characteristics and, consequently, test their performances in water denitration reaction. Water denitration is a structure sensitive reaction related to Pd particle size, and this effect is further emphasized in the decoration of the particle by the incorporation of Cu atoms into Pd particle corners and edges. The extent of lattice defects in CNTs was found to dictate the size and composition of the Pd-Cu active phase, playing an important role in nitrate reduction. Catalyst samples characterized by close proximity of Pd and Cu, as in an alloy, displayed the highest degree of nitrate conversion. At the same time, the size of the Pd particles, alone, or decorated to different extents with Cu atoms, was found to be critical for the second step of denitration reaction, directing the formed nitrite to either N2 or NH3.
