Additional Catalyst Characterization

XPS was used to provide insight into the oxidation state of the W in the Pt-WOx/C

catalyst. To ensure Pt-WOx direct contact and enhance the signal-to-noise ratio of the XPS

spectra, these studies made use of a model catalyst consisting of a Pt foil that had been subjected to a single ALD WOx deposition cycle. XPS spectra were collected both

immediately after ALD deposition of the W and after exposing the model WOx-Pt catalyst

to m-cresol HDO reaction conditions (H2 plus 2-mol% m-cresol in dodecane, 36 bar total

pressure, 573 K for 5 h (no measurable rates were obtained due to the extremely low surface area). Note that in both cases the samples were exposed to air prior to XPS analysis. Figure 6.8 displays the Pt(4f) and W(4f) spectra obtained from these samples. The W(4f5/2) and

W(4f7/2) peaks for the freshly prepared catalyst are located at 38.1 and 36.1 eV,

respectively, which is consistent with those reported in the literature [167] for W+6 indicating that the ALD procedure results in deposition of WO3 on the Pt surface. Except

for a reduction in peak intensity due to more carbon contamination, the W(4f) spectrum of the sample exposed to HDO reaction conditions was similar to that for the fresh sample, although some intensity is apparent at lower BE suggesting that some reduction of the WO3

deposits may have occurred. While one must take into account that this sample was exposed to air prior to XPS analysis, the fact that both the W(4f) and Pt(4f) spectra provide no evidence for zero-valent W or alloy formation supports the conclusion that the W remains in an oxidized form under reaction conditions.

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Figure 6.8 XPS spectra of the Pt(4f) and W(4f) energy regions of WOx-decorated Pt model

catalysts (a and c) before and (b and d) after exposing to m-cresol HDO reaction conditions

(H2 plus 2-mol% m-cresol in dodecane, 36 bar total pressure, 573 K) for 5 h.

Measurement of the Pt dispersion via selective CO chemisorption was also used to provide additional insight into the structure of the Pt-WOx/C catalyst. These results along

with those for Pt/C are reported in Table 6.1. As shown in this table, the Pt/C catalyst exhibited a relatively high Pt dispersion of 17%. In contrast, the apparent dispersion for the highly selective Pt-WOx/C catalyst with 6.3-wt% WOx loading was only 3%. Since I have

previously shown that the Pt particle size is not altered upon WOx deposition [145], this

low dispersion value indicates that the majority of the Pt was coated with highly dispersed WOx. Interestingly, the XRD patterns (Figure 6.9) for both pristine and consumed Pt-

WOx/C catalysts showed no features associated with WOx. It suggests that the WOx species

are either amorphous or small enough in crystal size that they cannot be detected by XRD. This does not mean, however, that some small isolated WOx species were also not present

on the support. To further examine if intimate contact between Pt and WOx is a prerequisite

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catalyst was prepared in which the WOx was added using ALD. It has been demonstrated

previously using STEM-EDS mapping that the W(CO)6 ALD precursor preferentially

reacts on the Pt and not on the carbon support [145], thus insuring intimate contact between the Pt and WOx. The WOx loading for this catalyst was only 1.6-wt% and it had an apparent

Pt dispersion of only 5%. In contrast, an impregnated Pt-WOx/C catalyst with the same 1.6-

wt% WOx loading had a 9% apparent dispersion, suggesting less intimate contact between

the Pt and WOx for this catalyst. The reactivities of these catalysts were also measured and

as shown in Table 6.1 the conversion and selectivity to TOL for the reaction of m-cresol over ALD Pt-WOx/C was 55% and 97%, respectively. Thus, direct interactions between

the Pt and WOx appear to play an important role in obtaining this unusually high selectivity

for the HDO of m-cresol to produce TOL. Additionally, CO chemisorption was also performed on a consumed Pt/C catalyst to help evaluate the degree of coking. The apparent Pt dispersion decreased from 17% prior to reaction to 8% after reaction. This result is qualitatively consistent with our supposition that coking of bare Pt is in part responsible for the induction period shown in Figure 6.4.

Table 6.1 Metal dispersions for carbon supported catalysts after 573K reduction, assuming CO/Pt = 1. Conversion and selectivity for high-pressure HDO reaction of m-cresol. Reaction

conditions: 573 K, 36 bar, 2-mol% m-cresol/dodecane solution, WHSV = 0.6 hr-1_.

Catalysts WOx Loading (wt%) Pt Dispersion (%) Conversion (%) TOL selectivity (%) Pt 0 17 8.3 62 Pt-WOx 6.3 3 61 98 (ALD) Pt-WOx 1.6 5 55 97

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Pt-WOx 1.6 9 48 98

Figure 6.9 X-ray diffraction (XRD) pattern of (a) pristine Pt/C, (b) pristine Pt-WOx/C and

(c) consumed Pt-WOx/C after exposing to m-cresol HDO reaction conditions (H2 plus 2-

mol% m-cresol in dodecane, 36 bar total pressure, 573 K) for 5 h. XRD patterns are normalized and referenced to graphite feature (■) at 26.5˚. Pt facets are labeled by ▲.