Adsorption and Reaction Pathways of 1H-Pyrazole on Cu(100) and O/Cu(100)

Temperature-programmed reaction/desorption (TPR/D), reflection-absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) have been employed, with the aid of density functional theory calculations, to investigate the adsorption and reaction mechanisms of 1H-pyrazole on Cu(100) and oxygen-precovered Cu(100) (O/Cu(100)). On Cu(100), the adsorbed 1H-pyrazole molecules interact with each other through hydrogen bonding, exhibiting broad infrared absorptions between ∼2750 and ∼3300 cm^-1, but without the N-H stretching peak detected. Near a monolayer coverage, heating the surface to ∼200 K induces a change in the adsorption layer structure and generates upright or near upright 1H-pyrazole molecules attaching to the surface through the imine nitrogens. The 1H-pyrazole undergoes N-H bond cleavage first to evolve H₂ (∼230 K) and leaves pyrazolate on the surface. This intermediate is proposed to be adsorbed perpendicularly via the two nitrogen atoms, which are close to two surface atop sites. The pyrazolate decomposes by simultaneous C-H bond scission and ring opening with preferential bond breaking steps at ∼550 K. The C-H dissociation, particularly at the ⁴C-H, is the main origin for the H₂ formation from the pyrazolate. In the ring-opening process of the pyrazolate, cleavage of the two C-N bonds produces N₂, while concomitant dissociation of the N-N and ³C-⁴C bonds leads to the formation of HCN and CH₃CN. A small amount of the pyrazolate may recombine with hydrogen to evolve 1H-pyrazole. As 1H-pyrazole is adsorbed on O/Cu(100) at 120 K, it dissociates into adsorbed H₂O and dehydrogenated 1H-pyrazole species. Further reaction of the dehydrogenated species at 480 K produces adsorbed NCO and pyrazolate and gaseous H₂, H₂O, HCN, CO, and CO₂, with other products of CH₂â•CHNH₂ at 515 K and NH₃ at 535 K. The remaining pyrazolate decomposes at 550 K to generate H₂, N₂, HCN, and CH₃CN.