η‐Cu₃Si percentage and particle size in various rochow contact masses

Abstract

Various Rochow contact masses have been characterized by catalytic tests as well as by X-ray diffraction with respect to q-Cu,Si percentage and particle size. The presence of q-CuSi in the contact masses is neither a necessary nor a sufficient precondition of catalytic activity, but a certain contribution of dispersed q-Cu3Si to the activity cannot be excluded. The results can be explained consistently by assuming highly dispersed or even two-dimensional Cu-Si species, not detectable by X-rays, as catalytically active species. The promoters zinc, antimony and tin do not significantly change the percentage and particle size of q-Cu3Si.

SEM-EDX and SAM-AES Investigations on Rochow Contact Masses

“…The small bright particles could not be resolved by EDX. In our preceding paper 4 we were able to show by XRD that, in all CuCl 2 -containing contact masses, practically all of the copper added exists as the Cu-Si phaseCu 3 Si. Hence, the bottoms of the pits are partially covered by -Cu 3 Si particles, as has already been observed or discussed by others.…”

“…In most cases, only one relatively large copper-rich grain is to be seen within each of these pits. According to our recent XRD results on CuCl 2 containing contact masses, 4 these particles must consist of -Cu 3 Si. It is a key observation that these particles within the reactive areas are surrounded by a region which is characterized by a very low copper concentration, hardly higher than outside the reactive pits, i.e.…”

“…4), is reminiscent of the analogous observation with contact masses containing copper oxalate as copper source. 4 The mechanism of this apparent -Cu 3 Si crystallite splitting is not yet clear. Possibly, the reaction itself is involved.…”

“…Frequently, the -phase Cu 3 Si has been considered as catalytically active, [1][2][3]16 but there have also been opposing views. 17,18 In a recent paper in this journal 4 we briefly reviewed relevant papers, and we reflected on the -phase hypothesis on the basis of catalytic and X-ray experiments on various contact masses. We concluded that the presence of -Cu 3 Si is neither a sufficient nor a necessary precondition of the Rochow reaction, and that -Cu 3 Si, if potentially active at all, needs appropriate modification in order to become active.…”

“…The method chosen must be able to differentiate between several copper species, differing in the physical and chemical state, and it must be very sensitive, because only a rather low proportion of the total copper content of the contact mass seems to be catalytically active. 4 Moreover, a high spatial resolution power is required, because the catalytic activity seems to be restricted to distinct micrometer-sized areas on the silicon surface. Several authors have applied modern surface methods to surfaces of massive silicon or massive copper silicides, [5][6][7][8][9] which were expected to model the surface of real powder-like contact masses.…”