Methanol synthesis over Cu–ZnO aggregates supported on carbon nanotubes

Surface Alloy or Metal–Cation Interaction‐The State of Zn Promoting the Active Cu Sites in Methanol Synthesis Catalysts

“…The Zn K EXAFS spectra resemble those taken from reduced Cu‐ZnO/CNT samples and from SiO 2 ‐encaged Cu‐ZnO in earlier work, where the new signal appeared at a distance smaller than Zn in Zn metal and was sometimes even more intense due to lower Zn/Cu ratios. Its assignment to a Zn‐Cu pair was derived from the absence of Zn 2+ reduction suggested by the XANES, which was measured at lower resolution, however, at that time.…”

“…In our work, attention was focused on the comparison of structural and catalytic properties after “standard reduction” (2 % H 2 /He, 513 K, 1 h) and “severe reduction” (10 % CO/H 2 , 673 K, 30 min), the latter intended to mimic the above‐mentioned activation treatment in the limited time available at a synchrotron. As reported elsewhere, we detected alloy formation after severe reduction of our CNT‐based samples by XRD, but EXAFS showed this to be a minority phenomenon in most of them. The contradiction arises from the insensitivity of XRD to the highly disperse matter predominating in these catalysts.…”

“…The low‐energy shoulder in the first derivative after severe reduction (Figure ) and during catalysis (Figures , S3) indeed indicates reduction of ZnO x moieties. Comparison with the reference data (Zn metal, X‐ray amorphous CuZn alloy nanoparticles in CNTs) suggests that this reduction does not result in significant formation of Zn 0 : the onset of the signal (1st derivative >0) occurs at energies where the XANES of Zn 0 species in bulk Zn or in the nano‐alloy is already near to its first inflection point (maximum of 1st derivative). It is, therefore, more likely that the changes in the XANES reflect an increasing oxygen deficiency of the zinc oxide moieties.…”

“…Cu K XAFS spectra are compared with those of reference samples in Figure S2. The XANES shows the Cu in the +2 state initially (Figure S2 a) unlike the case of the CNT‐based analogues where Cu I could be clearly detected along with Cu II . After standard reduction, the fcc Cu metal signature appeared in both EXAFS and XANES.…”

“…The contradiction arises from the insensitivity of XRD to the highly disperse matter predominating in these catalysts. Notably, in the Zn K EXAFS, a metallic‐like Zn‐X (X=Cu) shell appeared also in samples where significant reduction of Zn 2+ was excluded by the Zn K X‐ray absorption near edge structure (XANES). This was interpreted to indicate a direct Zn 2+ ‐Cu interaction at the surface of Cu particles.…”