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F. Paris, Supplementary information S1: impact of CO adsorption on surface acidic O-H vibrators

, weak bands related to terminated PO-H contributions [1] are observed in the V-modified samples (x = 0, 3, 5.2), as exemplified in Figure S1a in the case of x = 0. In the case of vanadium-rich samples (x = 5.22 and 6), additional contributions of weak intensity are observed at 3627 and 3496 cm -1 (Figure S1b for x = 6) that might be attributed to ?VO-H vibrators. [2] As illustrated for the samples with x = 0 and 3 in Figure S1c, CO adsorption at 100 K results in the perturbation of the surface PO-H groups. This clearly shows that, even in presence of vanadium in the bulk composition, some BrØnsted acidic PO-H groups remain accessible on the top surface. For the V-HAp sample with x =5.22, due to the much lower extinction coefficients of the ?PO-H contributions compared to those of the ?CO contributions, the perturbation of the residual ?PO-H contribution of very low intensity is more difficult to assess

, the interaction of the surface with H2 (30 mL min -1 , 5% H2 in Ar) upon heating up to 723 K. The obtained difference DRIFT spectrum is reported in Log(1/R') with the relative reflectance R' = IHAp? / IHAp ? (spectrum recorded at 423 K during the cooling step / spectrum recorded at 423 K during the temperature raising step, respectively)

, Given the thermal limitations of the experimental set-up in the presence of H2 in the flow, exposure of the Ca10(VO4)6(OH)2 sample to hydrogen was perfomed up to 723 K only. The difference spectrum recorded at 423 K after exposure to H2 at 723 K shows a new contribution at 2174 cm -1 (Figure S3b)

, cm -1 being reported for hydride species stabilized on 3-fold Mg 2+ cations sites upon H2 dissociation). [3c] By analogy, ?Ca-H contributions are thus expected to be observed in the 1000-1200 cm -1 region. In the present study, due to the saturation of the structural ?V-O bands on Ca10(VO4)6(OH)2 (1000-1100 cm -1 ), this region can not be probed. The contribution observed at 2174 cm -1 on the difference spectrum, Earlier studies have reported on Mg-H contributions in the 1030 to 1125 cm -1 range, 1125.

, This result strongly supports the occurrence of the dissociation of H2 over the considered surface

, Figure S3b: difference DRIFT spectrum of Ca10(VO4)6(OH)2 sample recorded at 423 K after exposure to H2/Ar flow, pp.30-31

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