Surface characterization of copper(II) oxide-zinc oxide methanol-synthesis catalysts by x-ray photoelectron spectroscopy. 2. Reduced catalysts

Abstract

The surface state of the coprecipitated CuO-ZnO catalysts reduced at 250 "C with Hz was investigated by X-ray photoelectron spectroscopy (XPS). It was found that a monovalent copper species was present in the surface of the catalysts with low copper contents. It was revealed that two distinct types of copper metal species, both of them being negatively charged by electron transfer from ZnO, were produced in the catalyst surface upon the reduction. In the high copper content catalysts (>25 wt % CuO), the predominant Cu metal species is described by well-dispersed metal particles, whereas in the low copper content catalysts (<IO w t % CuO), the major copper metal species is best characterized by a two-dimensional epitaxial copper layer over ZnO. It is concluded that both the two-dimensional copper metal and Cu+ are formed from Cu2+ dissolved in a ZnO lattice, while the copper metal particles are originating mainly from crystalline and amorphous copper oxide phases. The two-dimensional copper metal species was found to be preferentially reoxidized to Cu+ when exposed to air at room temperature. The two-dimensional Cuo-Cu+ species are suggested to be catalytically active for the methanol synthesis at low temperature and pressure. The reduction of ZnO was observed only when reduced at 500 "C and in the presence of copper. In the case of the impregnated catalysts, no appreciable interactions were found between Cu metal and ZnO, in contrast to the coprecipitated catalysts.Chem., preceding article in this issue.

Direct dimethyl ether synthesis from syngas on copper–zeolite hybrid catalysts with a wide range of zeolite particle sizes

“…The intensity of the signal at 50 ppm is 17 weaker on ZSM-5@65 and ZSM-5@80, indicating their lower framework Al content. Closer 18 inspection of the corresponding 27 Al NMR spectra reveals also the presence of a peak at 19 29 ppm that could be attributed either to perturbed tetrahedrally coordinated or penta-20 coordinated aluminum in these two samples. Therefore 27 Al NMR results indicate, however, 21 lower amount of tetrahedral Al in very small zeolite nanoparticles (Table 1 in Supporting 22 Material) in ZSM-5@65 and ZSM-5@80 zeolites.…”

“…The influence of zeolite particle sizes on the catalytic performance seems even 18 more significant than the effect of total concentration of zeolite Brönsted acid sites. 19 The catalysts showed noticeable deactivation with time on stream. The deactivation 20 rate was correlated with the concentration of Brönsted acid sites on the external surface of 21 zeolite crystallites.…”

“…In binfunctional copper zeolite catalysts the DME yield could be a 18 function of the rates of both methanol synthesis and methanol dehydration. The concentration 19 and distribution of zeolite acid sites is therefore important for DME productivity. 20 The results obtained in the present work show that in addition to the zeolite acidity, the 21 zeolite crystallite size and crystallite aggregation can also strongly affect the rate of direct 22 DME synthesis.…”

Production of hydrogen from methanol over binary Cu/ZnO catalysts

“…CuO displays two components at 933.4 and 935.0 eV in sample CP-60, while MET/10-34 exhibits only one symmetrical peak at 933.5 eV. After exposure to hydrogen at 250 • C, both samples display a peak shift to 932.5-932.7 eV, indicative of Cu 0 formation [14,38,39,41,[43][44][45][46]. A composite peak at 934.3-934.6 eV is also visible in both samples, indicating that Cu is not completely reduced.…”

Production of hydrogen from methanol over Cu/ZnO catalysts promoted by ZrO2 and Al2O3

“…In the calcined catalyst, CuO displays the characteristic peak at 934.3 eV [22,37,45,[48][49][50][51][52][53][54]. After exposure to H 2 , the peak shifts to 932.4 eV, indicative of Cu 0 formation [22,37,48,50,[52][53][54][55]]. The reduced sample was then exposed to O 2 /CH 3 OH.…”