J. Chem. SOC., Furaduy Trans. I , 1987,83, 1347-1353 Nature of Oxide-supported Copper@) Ions and Copper derived from Copper@) Chloride Paul A. Sermon,* Keith Rollins, Patricio N. Reyes, Stephen A. Lawrence, Maria A. Martin Luengo and Michael J. Davies Department of Chemistry, Brunel University, Uxbridge, Middlesex UB8 3PH Cu2+ derived from aqueous CuC1, solutions and supported upon silica and anatase, but not alumina, shows asymmetric e.s.r. peaks and an absence of X.P.S. shake-up satellite peaks. Consideration is given to whether this may be attributed in part to its higher dispersion and different constrained symmetry. Divalent copper is stabilised by these oxide supports but in two different forms : one highly dispersed (with properties detectable by X.P.S. and e.s.r.) and the other poorly dispersed [as Cu,(OH),CI-type species on alumina and CuCl, on silica and anatase] characterisable by X.r.d.In respect to its effect upon the X.P.S. and reduction (but not e.s.r.) properties of supported Cu2+ species, rutile is different from anatase, a phenomenon which may be of some value in optimising activity of heterogeneous catalysts. The results cast doubt upon X.P.S. diagnosis of divalent copper. Resistivity measurements for titania-supported copper after reduction at 700 K suggest that very little of the copper may have been intercalated into the anatase support, but that on rutile such an intercalation may have been significant. This may be relevant to SMSI effects, especially at even higher reduction temperatures. -~ Studies' of many Cuz+ compounds have observed intense X.P.S.shake-up satellite peaks at kinetic energies 8-9 eV lower than the Cu 2p,,, and Cu 2p,,, core level peaks; this was not the case for Cu+ compounds and doubt2 was cast upon sample purities in earlier observations3 of copper(1) shake-up satellites. Subsequently, the intensity of X.P.S. shake-up satellites, caused by simultaneous 2p and 3d excitation,* has been deemed a useful analytical tool for differentiating Cu2+ and Cu+ species in solid^.^ The properties of Cu2+ derived from aqueous solutions of copper(I1) chloride upon several oxide supports has now been considered, together with the properties of the zero-valent Cu produced therefrom by reduction. Experimental Materials CuCl, - 2H20 (F'isons, purity 98 % ) was used unsupported and also after impregnation from aqueous solution onto y-alumina, boehmite, silica, anatase and rutile.The predried supports defined in table 1 were wetted with aqueous CuCl, solution of sufficient volume to just fill the support pore volume and were then dried in air at 393 K. Samples of supported Cu2+ so prepared are listed in table 1. Methods Temperature-programmed bulk reduction (t.p.b.r.) was carried out on pre-dried samples (0.2-0.3 g) using 6% H, in N, (BOC) as described previously.s X.P.S., e m . and X.r.d. were carried out using Kratos ES300 and ES200 spectrometers with incident 1347 45-21348 Oxide-supported Copper(r1) Ions Table 1. Samples of Cu2+ derived from CuCl, t.p.b.r. results mol H,/mol Cu; (T/K) sample wt % Cu supporta a P CUCI,.~H,O - CUl'/S- 1 3.8 Cu"/S-2 5.0 Cu"/A- 1 4.2 Cu"/A-2 5.0 CuI'/TA- 1 4.2 Cu"/TA-2 5.0 Cu1'/TA-3 5.0 Cu"/TR- 1 5.0 0.51 (705) 923 0.50 (583) 200 0.43 (598) 6373 0.48 (535) C 0.47 (588) P25 0.52 (533) - P25 0.44 (53.8) A 0.29(513j R 0.33 (573) 0.57 (838) 0.55 (641) 0.48 (733) 0.47 (678) 0.56 (753) 0.54 (623) 0.44 (643) 0.84 (603) 0.38 (633) a 923 and 200 denote Davison 923 and Degussa aerosil200 silica supports.6373 and C denote Norton SA6373 boehmite AlOOH and Degussa Alumin Oxid C y-alumina. P25, A and R denote Degussa P25 predominantly anatase titania, Tioxide anatase and Tioxide rutile supports. radiation at 1253.6 eV and 1486.6 eV and calibration with C 1s peaks, Varian E4 and Bruker ER-200-SRC e.s.r. spectrometers with samples in air, vacuum or nitrogen, and Philips diffractometers were used.Resistivity measurements on some samples of titania- supported copper were measured as described. Results T.p.b.r. results in table 1 suggest that all of the samples prepared as described above and shown in this table contain copper essentially in a divalent state, but that the precise temperature and contribution of the a and /? reduction steps vary with the nature of the support. Given the fact that all unreduced samples must, therefore, contain Cu2+, it is surprising that fig. 1 shows that, unlike unsupported CuC12-2H,0 and Cuz+ upon alumina (y-alumina or boehmite), divalent copper upon silica (CuII/S- 1) and anatase (Cu*I/TA-l) does not show significant shake-up satellites adjacent to the 2p core-level peaks. However, it is clear that Cu binding energies are lower upon silica and anatase.Nevertheless, this may arise from ligand effects as seen previously rather than a real reduction. E.s.r. responses of Cu2+ are often broad isotropic symmetrical peaks at ca. g = 2.18 [attributed to a mobile species with a distorted octahedral configuration, such as Cu(H,O);+] or asymmetric peaks with gll > g , (attributed to Cu2+ species with greater octahedral distortion), although in addition very broad signals for Cuz+ in tetrahedral states and asymmetric resonances for the trigonal-bipyramidal state have also been reported. E.s.r. spectra for these same samples are shown in fig. 2. First, these e.s.r. signals confirm the presence of divalent d9 (t&:eg) Cu2+. Secondly, the e.s.r.spectrum for Cu2+ upon y-alumina is different from thqt upon silica and anatase, with the former being similar to symmetrical peaks for mobile hydrated Cu2+, while the latter are asymmetric. This correlates with differences seen in X.p. spectra; it might suggest a greater distortion of Cu2+ octahedral symmetry on anatase and silica than upon y-alumina. Thirdly, the e.s.r. responses do not seem to be dependent upon the extent of vacuum treatment. Furthermore, fig. 3 reveals that the e.s.r. spectrum for one sampleP. A . Sermon et al. 1349 A 1 binding energy Fig. 1. Normalised X-ray photoelectron spectra of (a) unsupported CuO, (b) unsupported CuC1,. 2H,O, (c) Cu"/A-l, (d) Cu"/TA-l, and (e) CuI1/S-1 plotted as intensity us. binding energy relevant to the Cu 2p,,, core-level peak.A1 radiation at 1486.6 eV was used. Absolute binding energies for Cu 2p,,, peaks (936.1 eV for CuCI2.2H,O; 935.4 eV for Cu'I/A-l; 932.9 eV for Cu"/TA-l and 933.4eV for Cu'I/S-l) were lower upon silica and titania supports than on alumina, but for the Cu (Auger) peaks (328 eV for CuC1,.2H2O and 337.8 eV for CuII/A-l; 338.3 eV for CuI1/TA-l and 338.4 eV for CuI1/S-l) were higher for Cu2+ upon silica and titania supports. of alumina-supported Cu2+ before t.p.b.r. is unaffected significantly by programmed reduction to the point between a and B t.p.b.r. peaks. This suggests that the two t.p.b.r. peaks do not reflect reduction of the Cu2+ to Cu+ and Cu+ to Cuo, respectively, but to reduction of Cu2+ in two different states to essentially the zero-valent metal.Discussion Motschis suggested recently that the interaction of hydrated Cu2+ ions in aqueous solution with silica and titania surfaces involves the formation of a square-pyramidal surface complex, while the interaction with alumina surfaces produces an octahedrally coordinated surface complex. The differences might be judged to arise from the higher metal-oxygen bond strength in alumina or the greater solubility of alumina in the acidic aqueous CuC1, solution (with the formation of a Cu-A1 complex in solution which then1350 Oxide-supported Copper(q) Ions 7 Fig. 2. E.s.r. results obtained for Cu2+ samples [(a) CuT1/S-2, (b) Cu11/A-2, (c) Cu"/TA-2] derived from aqueous solutions of CuC1, measured in air at atmospheric pressure (-), in vacuum (----) and in air at atmospheric pressure after release of vacuum ( - .a ) at ambient temperature and constant e.s.r. conditions (0.1 G modulation amplitude; 0.5 s time constant; 500 s scanning time; 3150 G field; 2500 G scan; 13 dB power; 9.78-9.77 GHz frequency) apart from variable gain (e.g. for Cu11/S-2; 10000 in air, 63000 in uacuo, and 6300 in air after). E.s.r. responses were unchanged in shape by vacuum treatment, but the intensity of the derivative peaks for silica- and titania-supported CuII were increased by a factor of 1.6 by vacuum treatment. This increase was not subsequently lost on release of vacuum to air at ambient temperature. Width of spectra = 2500 G. absorbs). However, were this to be so, Cu2+ would be expected to absorb in the same state upon anatase and rutile.Fig. 4 shows that this is by no means the case. Thus, with respect to its effect upon X.P.S. responses of supported Cu2+, anatase appears to behave like silica, but rutile appears to behave like alumina. The surface geometries of anatase and rutile might dictate different spatial geometries for Cu2+ supported thereon which change the location of the three e, electrons by greater octahedral distortion (although CuCl, is itself greatly distorted Cu-Cl, = 298 pm; Cu-C1, = 231 pm) or might induce a tetrahedral configuration with a resultant 3d-electron rearrangement, prohibiting intense shake-up satellites. Certainly studies with CuCr,Fe,-,O, (x < 2) suggest that the shake-up satellite intensity increases as the percentage of Cu2+ in octahedral sitesP.A . Sermon et al. 1351 Fig. 3. E.s.r. results for alumina-supported Cu2+ in CulI/A-l before reduction (-), after reduction in t.p.b.r. to a temperature between its u and p peaks (----) and after reduction in t.p.b.r. to a temperature above its fi peak ( * - a ) . Results were obtained with a Varian E3 instrument operating at a sample temperature of 298 K, scan range 2500 G and microwave power 10 mW. Width of spectra = 3650 G. increases and the percentage in tetrahedral sites decreases. However, fig. 4(b) shows distorted e.s.r. responses for Cu2+ upon both anatase and rutile. The validity of this geometric model therefore remains somewhat uncertain. Nevertheless, it is clear from fig. 4(c) that anatase and rutile constrain Cu2+ in states which exhibit different reducibilities, thus the temperatures and areas of the a and B peaks are very different upon these titania supports.Conclusions First, the use of X.p.s. shake-up satellite intensity as a diagnostic tool for differentiating divalerit (from mono- and zero-valent copper) may not be always valid. Secondly, although SMSIlO may be thought of as a contamination effect with titania (and other reducible supports), it appears that there are strong support-transition-metal ion interactions in the precursor stages before reduction which may be relevant to the preparation of heterogeneous catalysts. Thirdly, an absence of X.P.S. shake-up satellite peaks for Cu2+ supported upon silica and anatase may be associated with the spatial geometry of hydroxy groups on oxide surfaces rather than the support solubility, acidity or reducibility.Such effects could only be induced in highly dispersed supported Cu2+ species. It must be noted that X-ray diffraction also revealed rather poorly dispersed copper species in all unreduced samples (e.g. average particle sizes of 69.0 nm in CuI1/A-l and 68.6 nm in CuI1/TA-l) so the molecular description of X.p.s. and e.s.r. results presented here can only apply to a fraction of unreduced supported copper in a highly dispersed form. It may be that the two t.p.b.r. peaks relate to the reduction of a monodispersed positive oxidation state copper and the reduction of a poorly dispersed copper species X.r.d. suggests that on silica and titania (both anatase and rutile) this may be closer to CuCl, but closer to Cu,(OH),Cl on aluminas.Othersll have also suggested the strongest interaction of CuCl, phases is with alumina. Impregnation is likely to produce both highly and poorly dispersed Cu2+ species simultaneously.1352 0 xide-suppo r ted Copper (11) Ions Fig. 4. (a) Normalised X-ray photoelectron spectra, (b) e.s.r. spectra and (c) t.p.b.1. of CuZf supported upon titania as anatase A and rutile R by impregnation of CuCl, from aqueous solution giving 5 wt% Cu on drying. On anatase, unlike rutile, Cu2+ shows no shake-up satellites of measurable intensity. However, e.s.r. responses reveal Cu2+ on both titania supports, which are both asymmetric as a result of distortion of the octahedral state. The areas, ratios and temperatures of a and p t.p.b.r. peaks for Cu2+ on anatase and rutile are different, suggesting a different ease of reduction of copper thereon. Samples upon anatase and rutile are Cu*'/TA-3 and Cu"/TR-l, respectively. Width of e.s.r.spectra = 5860 G.P. A . Sermon et al. 1353 Finally, it is important to note that resistivity measurements on Cu11/TA-3 and Cu"/TR-l before reduction in t.p.b.r. to 700 K and then afterwards revealed no major increase in conductivity for the the Cu-anatase system (i.e. p was 34.0 MR cm before reduction and 29.0 M a cm after reduction), but for the Cu-rutile system the conductivity of the support was increased substantially by reduction (i.e. p was above 1 GR cm before reduction and decreased to 38.1 MR cm after reduction). Titania is normally an n-type semiconductor and it is interesting to speculate whether the reduction of Cu2+ to Cuo on rutile at 70 K allows partial electron transfer (producing Cud+ with a change in the Ti3+:Ti4+ ratio and simultaneous intercalation of the copper to form the bronze Cu,TiO,).Certainly, Cu and Ag vanadium bronzes X,V,O, are known, as are other bronzes of Ti.12 This throws a new light on titania-supported catalysts and their ability to hold metals in a partially monodispersed and positive oxidation state. It also highlights a further profound difference between Cu-anatase and Cu-rutile interactions. Reduction or heating in an inert atmosphere at higher temperatures may have increased these differences still further. 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Sermon, Spectrochim. Acta, Part B, submitted for publication. Paper 61126; Received 18th January, 1986