Characterization of [Cu]-MCM-41 by XPS and CO or NO adsorption heat measurements

We report the characterization of copper doped MCM-41 prepared by original direct synthesis by XPS and adsorption calorimetry of CO and NO, which are selective molecular probes for Cu(I) and Cu(II) respectively. Investigation of the nature of the copper ions in this particular calcined Cu-MCM-41 by NO adsorption calorimetry shows that two types of energetically distinct adsorption sites exist, meaning the presence of two populations of Cu(II) species differently coordinated to the silica surface in quasi-equimolar concentration, as ascertained by XPS data. A small amount of Cu(I) was also detected, probably stemming from a partial reduction upon the successive vacuum treatments. The respective proportion of Cu(II) and Cu(I) were estimated from the amounts of NO and CO chemisorbed on the surface and by deconvolution of the XPS Cu 2p3/2 peak. Despite the same Cu(II)/Cu(I) ratios are obtained by both XPS and calorimetry, the major part of copper is inaccessible to the molecular probes, probably because copper is well dispersed within the siliceous pore walls. But, even if the percentage of copper detected is quite low, the adsorption calorimetry of NO and CO proved a very powerful technique for the characterization of the accessible catalytically copper species.