Mechanistic implications of lanthana modification

In Chapter 5, gold supported on commercial lanthana-modified titania was found to exhibit a selective enhancement in formic acid decomposition to carbon dioxide. Base-modification by lanthana addition resulted in enhanced formation of bidentate formates (Figure 8.6 a) which are the kinetically relevant precursors for carbon dioxide formation (see Chapter 6). Another consequence of base-modification weakening of the C-H bond owing to the increased electron density of the catalyst, observable from the red-shifting of the (CH) band of formate (Figure 8.6 b).^[289,290] Hence, the enhancement in the activity of the base-modified catalysts can be traced back to the acceleration of the rate limiting step involving C-H bond cleavage of the formate.

Catalysts prepared by wet-impregnation were less active than their co-precipitated analogues.

The steadily increasing population of lanthanum on the surface (Tables 8.1 and 8.2) has mechanistic implications that can explain this behaviour. According to the proposed single-site mechanism for formic acid decomposition under the investigated conditions (Chapter 6), hydroperoxy species (OOH*) formed from the proton shift equilibrium between adsorbed oxygen and water are responsible for the decomposition of the abundantly present formates in the rate-determining-step (RDS) forming carbon dioxide. In the case of the wet-impregnated catalysts, a drastic increase in basicity triggered by high surface concentrations of lanthanum manifests itself in notably higher formate coverages compared to the coprecipitated catalysts (Figure 8.6).

This entails an extensive blockage of the active sites by formates which in turn diminish their availability for the formation of the hydroperoxy species. This explanation is in line with the more negative orders in formic acid and more positive orders in oxygen.

0

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Besides inducing a rapid increase in the surface basicity (Figure 8.6) and a decrease in the surface area (Figure 8.4), an increase in the surface lanthanum coverage (Tables 8.1 and 8.2) in the case of the wet-impregnated catalysts can additionally impede the catalytic rates by reducing the interaction between gold and titania. While gold is crucial for the formation of the hydroperoxy species, tetrahedrally coordinated Ti⁴⁺ sites have been postulated to play a pivotal role in binding these species before transferring them to the active site (See Chapter 9).^[125,227]

Hence, the blanketing of titania particles by lanthana (inset of Au15LT-WI in Figure 8.5) can adversely affect the Au-Ti synergy that is responsible for the formation of hydroperoxy species.

Figure 8.14 In situ DRIFTS spectra of formic acid adsorption in nitrogen on gold catalysts of different lanthana loadings showing the inhibition of the band at 1670 cm^-1.

In the proposed mechanism (Chapter 6), the dissociative chemisorption of formic acid as formate (HCOO*) is an exothermic process while the formation of the active oxygen species (OOH*) is an activated process. Hence, the formation of the former intermediate is favored at lower temperatures and the generation of the hydroperoxy species is accelerated at higher temperatures. In Chapters 5 and 6, the formic acid orders were found to change as a function of temperature and gas phase formic acid concentration, becoming less negative at lower temperature and lower formic acid concentration regime. This is a clear sign of the measured kinetic parameters being apparent and the activation energy bearing non-trivial contributions from the heats of adsorption of the reactants. In this study, the compensation phenomenon where an increase in the measured (apparent) activation energy, Eapp is offset by an increase in the pre-exponential factor (A_app) originates from a change in the relative surface coverages of formates and hydroperoxy species and thus, inherently linked to the differences in their heats of adsorption with increasing lanthana concentrations^[202,291]. An increased basicity can be speculated to enhance the stability of adsorbed oxygen species required for the formation of active hydroperoxy species.^[128,229] Bond et al. rigorously maintained that ∆E_app should ideally be at least 50% of the smallest Eapp measured.^[202] In this study, this is ~52%, thus adding credence

1800 1600 1400 1200

Au5.0LT-CP

Absorbance, a.u

Wavenumber, cm^-1

ads. in N₂ AuT Au2.3LT-CP 260 C

1670

to the observed compensation phenomenon. The compensation phenomenon can be described by the Constable–Cremer relation (Eq. 8.1).

Ln A_app= mE_app+ c (8.1)

Besides unifying the mechanisms operating on the Au/TiO2 catalysts of different lanthana loadings, the observed obedience to the linearity of the Constable–Cremer relation can be mechanistically interpreted as the increase in the adsorption enthalpy of active-oxygen species with increasing lanthanum content, which in turn entails higher energy (barrier) for destabilization.^[171] This is in line with the negative orders in formic acid and positive orders in oxygen. The decrease in the C-H bond strength does not cause significant reduction in the activation energy.^[292,293] The pre-exponential factor represents the number as well as the strength of the active sites. Overall, the observed optimum in the basicity-induced rate enhancement can be attributed to a favorable combination of factors including decreased C-H bond strength of formates, increased probability of formation of hydroperoxy species arising from increased stability of adsorbed oxygen and increased number of active sites owing to the decreased gold particle size. The highly negative formic acid orders in the case of unmodified as well-as base-modified catalysts ascertain that the classical effect of basicity associated with an increased degree of formic acid deprotonation to formate does not entail higher activity. Thus, this study coupled with the mechanistic investigation performed in Chapter 6 offers clarity on the more realistic effect of basicity on the catalytic activity for formic acid decomposition.

Modification by lanthana also favorably suppressed the carbon monoxide production. The introduction of only 1 atomic % of lanthanum to the surface of titania (Au15LT-CP, Table 8.2) led to ~85% drop in formic acid conversion to carbon monoxide (Figure 8.10). With increasing lanthana content, the tendency to form monodentate formates which are the intermediates relevant for carbon monoxide formation (Chapter 6) was substantially decreased (Figure 8.14).

Additionally, the increased density of surface hydroxyls (Figure 8.2) that are engendered on the surface of lanthanum-modified titania can disfavor the RDS involving the decomposition of monodentate formate to carbon monoxide and a hydroxyl at a bridging oxygen anion.^[42] These results are consistent with reports on decreased formic acid dehydration activity with increasing basicity.^[28,29]

8.4 Conclusions

In this chapter, the basicity of titania supported gold catalysts was tailored by incremental addition of lanthana via wet-impregnation and coprecipitation. Irrespective of the synthesis method, introduction of lanthana to Au/TiO₂ favored smaller gold particle and anatase crystallite sizes and steered the selectivity towards higher carbon dioxide production from formic acid

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decomposition. The rate-determining-step (RDS) involving the hydroperoxyl-mediated C-H bond cleavage of formate to form carbon dioxide is speculated to be accelerated as a consequence of progressive weakening of the C-H bond as well as an increased availability of hydroperoxy species with increasing lanthana content. However, very high surface basicity reduced the availability of active surface oxygen species owing to an extensive blockage of the active sites by formates, thus leading to poor performance of highly basic catalysts. The systematic changes in the relative coverages of formates and the hydroperoxy species leads to kinetic compensation between Eapp and Ln(Aapp). At an optimum surface lanthanum concentration of ~1.3 atomic %, gold supported on coprecipitated lanthanum-modified titania catalyst exhibited close to three-fold enhancement in carbon dioxide production while carbon monoxide selectivity and ammonia oxidation activity were restricted to negligible levels. Another effect of increasing basicity was the suppression of monodentate formates which are the precursors to carbon monoxide.

Chapter 9