Postulate of a Mechanism for Temperature Programmed Reaction Spectroscopy (TPRS) Which Accounts for the Coincident Evolution of H2 and CO2 in the TPRS of Succinates and Formates Adsorbed on Cu: Description of a Novel Method for the Identification of the Surface Morphology of Cu on Supported Cu Catalysts

Abstract

The combination of temperature programmed reaction spectroscopy, temperature programmed oxidation (TPO) and N2O reactive frontal chromatography (N2O-RFC) has shown that the adsorbate produced by dosing dimethylsuccinate on to a Cu/Al2O3 catalyst is a succinate species with 8 out of 10 surface Cu atoms having a succinate species adsorbed on them. The succinate species are bonded end-on and unidentate to the Cu atoms. This configuration constitutes a self-assembled monolayer. Temperature programming causes vicinal strands of the adsorbed succinate to interact, resulting in simultaneous dehydrogenation and decarboxylation, producing coincident evolution of H2 and CO2 in peaks at 668 and 793 K and leaving the C of the succinate chain on the Cu surface This C is removed by TPO, producing CO2 at a peak maximum temperature of 593 K. Prior to the oxidation of the C to CO2, TPO oxidises the Cu metal to CuO in the temperature range 323–493 K. Reduction of this CuO produces Cu metal whose metal surface area as measured by N2O-RFC is 9.8 m2 g−1, a value which is identical to that of the fresh Cu/Al2O3 catalyst. Therefore oxidation of the Cu metal to bulk CuO and re-reduction does not cause sintering of the Cu. However, the surface morphology of the Cu metal produced by reduction of the CuO has changed as evidenced by temperature programmed reduction (TPR) of the N2O surface oxidised Cu in H2/He (5 % H2 101 kPa). TPR of the N2O surface oxidised fresh Cu/Al2O3 has two peaks in H2 consumption at 395 and 410 K corresponding to Cu–O surface bond energies of 341 and 358 kJ mol−1. TPR of the N2O surface oxidised Cu metal, resulting from reduction of the CuO, has three peaks in the consumption of H2 at 410, 422 and 434 K corresponding to surface Cu–O bond energies of 379, 381 and 388 kJ mol−1. Hydrogen temperature programmed reduction of N2O surface oxidised Cu (and other metals) constitutes a particularly apt technique for the determination of surface Cu–O bond strengths and hence of the morphology of the Cu surface. Dosing dimethylsuccinate on to Cu/Al2O3 produces a self-assembled monolayer of succinate species bonded to 80 % of the Cu atoms. Temperature programming causes the individual strands of succinate to interact, producing CO2 and H2 coincidently at 668 and 793 K. Hydrogen temperature programmed reduction of N2O oxidised Cu determines the surface morphology of the Cu.