Impacto en la determinación del valor

Fig 4.23 shows a series of X-ray excited photoemission spectra in Ru 3d core level region for Ru modified SnOz -0 .2%Sb sample decomposed in dry air and subsequently exposed to 50% relative humidity in air. The spectrum o f the sample decomposed in dry air

appeared to be very similar to that observed for the reduced sample after its exposure to moisture. Surface Ru species are characterized by a broad feature in the XPS spectra with maximum around 282.2eV. The peak intensity, however appeared to be considerably smaller than that observed for the reduced samples. On subsequent exposure to moist air (50% RH) no significant change in either peak position or intensity was observed.

5 5 0 0 f = r 4 5 0 0 4 0 0 0 — CO ^ 3 5 0 ( 2 5 0 0 — 200(

I'M

Binding Energy (eV)

F ig u re 4.23 XP sp ectra of the Ru 3d region fo r SnO2-0.2% Sb- supported Ru recorded following decom position in d ry a ir (b), and subsequent exposure to a ir of 50% relative hum idity (c).

The XPS data for the Ru surface content o f the studied sample is given in Table 4.8.

Table 4.8 Surface Ru content o f the SnOz-supported Ru sample decomposed in dry air, followed by moist air exposure, as found by XPS.

Sample [Ru]/[Sn+Ru+Sb]%

Ru modified, undecomposed

2.82

Decomposed in air 1.11 Exposed to moist air 1.42

The apparent surface Ru content o f the air decomposed sample was decreased to half that before decomposition.

4.2.3.4 EXAFS.

Samples were subjected to prolonged exposure to ambient air prior to measurements.

(i) R educed R u modified sam ple

Fig. 4.24 (a) and (b) shows the Ru K-edge EXAFS oscillations for the reduced supported Ru species (after exposure to ambient air) and its Fourier transform, respectively. N o metallic Ru-Ru bonding was found. The best curve fitting results were obtained based on a standard Ru02 model as shown in Fig 4.25 (a). Only one peak corresponding to a shell o f nearest neighbor R u -0 bonding was observed in the Fourier transform. N o statistically significant second Ru—Ru shell could be modeled and no additional features could be seen in the Fourier transform above the background. This implies that we do not have a bulk

E X A F S ( e x p e r I m e n t ) * k * * 3 C u r v e d Wave 5 5 Th eo r y - k * =t <3 4 - - i5 .5 6 .5 ,7 .5 - 2 - - 4 - 6 -T- - 1 0 --- 1---1--- 1---i--- FOURI ER TRAN5F0RM ( e x p e r i m e n t ) ' . 0 - - F Ü U R I E R TRANSFORM ( t h e o r y ) 0.9- - 0 . 3 - - 0.7- - 0 . 6 - - 0 . 5 — 0 . 4 - - 0 . 5 0 . 2 - - 0 . 1 - 0 . 0 5 7 a 9 1 0 2 6

Figure 4.24 Ru K-edge EXAFS spectra (a), and associated Fourier transform (b) , of reduced Ru modified SnO2-0.2%Sb. ( ___ ) Experimental spectrum, (—) theoretical best fit.

(ii) R u m odified sam ple decom posed in air.

Fig 4.25 (a) shows the EXAFS spectra for the supported Ru species after decomposition in air and exposure to moisture. Good fitting results were obtained on the basis o f RUO2

standard model. The intense peaks corresponding to the first R u-0 and second Ru—Ru bonding could be observed in the Fourier transform Fig. 4.25 (b). Evidently, bulk RuOz species were formed as a result of decomposition in air.

. E X A F S t e x p e r I m e n t ) C u rv e d Wave 5 5 T h e o r y * k # * 3 FOURIER TRANSFORM (e x p e r im e n t ) FOURIER TRANSFORM ( t h e o r y ) rv 2 5 10

F igure 4.25 Ru K-edge EXAFS spectra (a), and associated F o u rier tran sfo rm ( b ) , of a ir decomposed Ru modified SnO2-0,2%Sb. ( ___ ) Experim ental spectrum , (— ) theoretical best fit.

Table 4.9 summarizes the bond lengths, r, and coordination numbers, N, for the two studied samples, determined by EXAFS. The data for a RuOi standard are also given.

Table 4.9 Curve-fitting results for Ru K-edge EXAFS data.

Material Atom type N r / Â 2G "/A "

RuOi 0 6 1.989 0.008 Ru 12 3.436 0.010 Ru^Sn02 0 4.3 2.047 0.008 reduced Ru --- --- --- Ru^SnOz 0 6 1.925 0.006 air decomposed Ru 2 3.079 0.008 8 3.515 0.008

It is apparent that the structure o f the supported Ru species for the sample decomposed in air is that ofR uO :. N ote however that the reduced Ru species as well as having only one nearest neighbor coordination, are characterized by lower coordination number and longer R u -0 bonding than the former, signifying a very small oxidized species.

4.3 Discussion

(i) Surface am o u n t of Ru.

It is interesting to estimate the fraction o f the SnO2-0.2%Sb surface hydroxyl groups which reacted with the Ru complex. The SnC^ (110) surface was used as a model for this purpose. The unit cell parameters were obtained from Camargo

et

3.0Â. On this surface, if every exposed oxygen was hydroxylated, the number o f surface hydroxyl groups would be 1*10^^ cm'^. Assuming that three atomic layers were probed by XPS, a depth o f about 10A, the total number o f lattice oxygen atoms probed by the measurement would be 6*10^^ cm'^ .

Therefore the surface hydroxyl contribution to the total oxygen signal would be about 14%, which is in fact slightly lower than that measured by XPS (23%). Therefore, given the approximation in the estimation, it is reasonable to assume a full surface hydroxylation.

The amount o f Ru reacted with 200mg o f SnO2-0.2%Sb support was measured as

5*10'^mol or 30*10^^ atoms. The surface area o f the support was about 20*10* cm^g'^ (see B ET surface area measurements) or 4*10* cm^ per 200 mg. Thus, the surface loading o f Ru was about 8*10^^ Ru atoms cm'^ , which means that about 10% o f the total surface OH groups had reacted with the Ru complex.

We can also check the correspondence between XPS measurements and the estimate o f Ru coverage obtained from the amount o f complex reacted. So, the number o f Sn atoms probed by XPS, based on the (110) model surface, would be 1*10^^ cm‘^ per layer. Assuming a measurement depth o f three atomic layers implies 3*10^^ cm'^. Based on the amount o f complex reacted, the fraction o f the total surface metal atoms which is Ru therefore would be about 3%, which is roughly the same as measured by XPS . Therefore, substantially all o f the Ru atoms were visible to the XPS probe implying that none o f the Ru was present in agglomerates more than about 3 atomic layers thick.

(ii) Surface state of the Ru and speculations on structure.