extensive research efforts have been dedicated towards the exploration of gold chemistry and catalysis. The first report of supported gold-catalyzed olefin hydrogenation by Bond et al.[98] was followed by the demonstration of gold’s exceptional ability to oxidize carbon monoxide at low temperatures by Haruta et al.[96]and Hutching’s[94] report on gold catalyzed hydrochlorination of alkynes to vinyl chloride. Now, gold-based catalysts are common place owing to their versatility and selectivity in catalyzing oxidation as well as hydrogenation-type reactions and it is only a matter of time before they make a foray into industry.[99–102]
One of the first fundamental studies of formic acid decomposition on gold was conducted by Outka and Madix who explored the adsorption of the acid on clean and oxidized Au(110) surface.[51,103] They found that while weak molecular adsorption took place on the clean Au(110) surface, on the oxidized surface, oxygen acted as a Brønsted base in abstracting the proton from formic acid. This was followed by hydrogen transfer from the adsorbed formate to gold which constituted the RDS. The weak Au-H bond strength necessitated the transfer of hydrogen to other surface species like hydroxyls to form water rather than recombination with another hydrogen atom to form hydrogen.[103] Isotopic studies confirmed that the oxygen in carbon dioxide exclusively arose from the dosed formic acid and not from the oxygen in the gas phase.
Figure 1.5 Proposed mechanism for hydrogen evolution from decomposition of formic acid/amine mixtures on Au/ZrO2. Adapted with permission from Reference[66]. Copyright (2012) American Chemical Society.
Recently, many studies have demonstrated the excellent potential of supported gold catalysts for selective formic acid decomposition to hydrogen and carbon dioxide.[63,66,89,91] Ojeda and Iglesia reported that TEM-invisible gold clusters supported on alumina were responsible for the unprecedentedly high activity for formic acid decomposition, while the TEM visible clusters catalyzed carbon monoxide oxidation. Furthermore, using H/D kinetic isotope effects, the C-H bond cleavage of formate was identified as the RDS. Along these lines, Au/ZrO2 was reported to
Introduction
13
show high activity for formic acid dehydrogenation particularly when formic acid/amine mixtures were used.[66] The amine was proposed to facilitate the formation of the Au-formate complex, the relevant reaction intermediate, by acting as a proton scavenger in facilitating the O-H bond breakage (Figure 1.5). The resulting alkyl ammonium ion (+HNR3) promoted further dehydrogenation of the formate to produce hydrogen and carbon dioxide via aβ-elimination pathway.[83,104,105]
In parallel to the gas phase studies, adsorbed formate is reported to be the relevant surface-bonded intermediate during electrochemical oxidation of formic acid on gold.[106] Using surface-enhanced Raman spectroscopy, the RDS was demonstrated to involve formate dehydrogenation to carbon dioxide on gold electrodes with the aid of water. Consistent with these studies, DFT studies on gold surfaces reported that gold exclusively catalyzed formic acid dehydrogenation and that the mechanism was mediated by a formate intermediate.[107] With the purported mechanistic links between formic acid decomposition and WGS, it is no surprise that gold-based catalysts have also shown high WGS activity.[108–113] Gold’s inability to activate water, one of the most important steps of reaction is said to be overcome by the formation of oxidic gold species (Au-O-MOx) upon strong-metal-support-interaction with metal oxides such as titania or ceria.[114,115] An extensive kinetic and mechanistic study by Behm and coworkers on low temperature WGS on Au/CeO2 catalysts revealed that the ionic gold species (Auδ+) at the metal-support interface are the likely active sites that catalyze the reaction of adsorbed carbon monoxide and hydroxyl to form surface formates and their subsequent decomposition to carbon dioxide and hydrogen.
With oxidic gold species proposed as the active centers for formate decomposition, it is essential to understand the oxidative chemistry on gold. Oxygen activation on gold has been a highly debated subject in literature spanning from a Mars van-Krevelen type pathway[116]
involving lattice oxygen and its replenishment by gaseous oxygen to a Langmuir-Hinshelwood type pathway[117] involving molecularly or dissociatively adsorbed oxygen. Since oxygen and water are both ubiquitously present in the simulated exhaust feed employed in this study, it is highly interesting to consider the implications of water on oxygen activation. Several works point towards the direct participation of water as a promoter in opening up new, energetically more favorable pathways for oxygen activation.[118–120] Furthermore, strong indications of the temperature-sensitive role of moisture in oxidation reactions exist in literature.[121] At high temperatures, oxygen molecules were activated directly over the Au/TiO2 (110) surface, whereas moisture participated in the activation at low temperatures. Hydroperoxy species (HOO*) that are facilely produced in aqueous environments by proton shift equilibrium between adsorbed oxygen (O2*) and water (H2O*) are proposed as the active oxygen species on gold
(Eq. 1.14).[119,122,123] Such species have often been invoked in oxidation mechanisms necessitating C-H bond activation of alkoxy groups.[123–129]
H2O* + O2* → HOO* +HO* (1.14) 1.4 Rationale of the work
The aim of this thesis is to develop mechanistic understanding of formic acid decomposition on titania-supported gold catalysts under SCR-relevant conditions and to use the gained insights towards rational design of better catalyst systems. Formic acid decomposition which is commonly studied under stoichiometric conditions was explored in a wholly different context where significant influence of gas phase oxygen and water exists. The mechanistic insights gained from this study are important in furthering the knowledge on gold catalysis and formic acid decomposition chemistry as well as in practice for the rational design of dedicated hydrolysis catalysts for the decomposition of formate-based ammonia precursors in the SCR process.
Chapter 2 describes the experimental methodologies employed in this work which includes the details of the synthesis, characterization and the testing of the catalysts. Chapter 3 presents the first findings which demonstrate the high activity and selectivity of Au/TiO2 for the decomposition of ammonium formate without oxidizing the co-evolved ammonia. Chapter 4 reports the promotional effect of gas phase ammonia on formic acid decomposition. Chapter 5 describes the realization of the aforementioned gas phase effect as a catalytic effect by modification of Au/TiO2 with a basic metal oxide. Chapter 6 is devoted to the kinetic and mechanistic investigation of gold-catalyzed formic acid decomposition in the presence of oxygen and water.
A kinetically-consistent hydroperoxyl-mediated mechanism is proposed for the oxidative dehydrogenation of formic acid to carbon dioxide and water. Chapter 7 presents the validity of the proposed mechanism as tested by numerically modelling. Chapter 8 shows the optimization of the lanthanum effect to achieve the highest activity and selectivity for formic acid decomposition to carbon dioxide. Chapter 9 presents the conclusions and discusses the outlook of the work.