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A detailed understanding of reaction mechanisms and kinetics is required in order to develop and optimize catalysts and catalytic processes. While steady-state investigations are known to give a global view of the catalytic system, transient studies are invaluable since they can provide more comprehensive insight into elementary steps. For almost forty years temporal analysis of products (TAP) has been successfully utilized for transient studies of gas phase heterogeneous reactions, and there have been a number of advances in instrumentation and numerical modeling methods in that time. Since TAP is a complex methodology it is often viewed as a niche specialty. With the purpose to make TAP more relevant and approachable to a wider segment of the catalytic research community, part of the intention of this work is to highlight the significant contributions TAP has made to elucidating mechanistic and kinetic aspects of complex, multistep heterogeneous reactions. With this in mind, an outlook is also disclosed for the technique in terms of what is needed to revitalize the field and make it more applicable to the recent advances in catalyst characterization (e.g. operando modes).

Nanoporous gold is a complex material comprised of a small amount of silver that is the residual from the dealloying process used in its formation. This material activates dioxygen and selectively self-couples methanol. The dissociative adsorption of O 2 and the subsequent 2 reaction of methanol with the adsorbed atomic oxygen are critical steps in this selective oxidation. The density of sites for O 2 dissociation was determined to be 0.1% of the total surface(3×10 12 per cm 2 ) using both transient and steady flow measurements. The activation energy for O 2 dissociation wasmeasured to be 5.0 kcal/mol and is similar in magnitude to that on metallic Ag and much lower than expected for Au surfaces. Thearea-averaged dissociation probability of O 2 at 423 K is ~1×10 -7 , commensurate with the active site density and the activation barrier to dissociation. The reactive oxygen is immobile under reaction conditions. The collisional reaction probability of methanol striking an adsorbed O atom is 10 -4 -10 -5 , which correspondswell with the measured turnover frequency for methanol conversion to form methylformate of ~160 s -1 at 423 K. Taken together, these results strongly indicate that Ag is an integral part of the active site for O 2 activation and the subsequent activation ofmethanol.

The involvement of lattice oxygen species is important toward oxidative coupling of the methane reaction (OCM) over supported Mn-Na 2 WO 4 /SiO 2 catalysts, but there is no consensus regarding the types, role, and origin of lattice oxygen species present in supported Mn-Na 2 WO 4 /SiO 2 catalysts, which hinders the understanding of the OCM reaction network. In the present study, by utilizing the temporal analysis of products technique, we show that supported Na 2 WO 4 /SiO 2 catalysts possess two different types of oxygen species, dissolved O 2 and atomic O, at an OCM-relevant temperature. The addition of Mn-oxide to this catalyst increases the total amount and release rate of dissolved O 2 species and improves C 2 selectivity of both dissolved O 2 and atomic lattice O species. KEYWORDS: Mn-Na 2 WO 4 /SiO 2 catalyst, oxidative coupling of methane (OCM), lattice oxygen, dissolved oxygen, molten salt, temporal analysis of products (TAP)

The complex structure of the catalytic active phase, and surface‐gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported Na2WO4/SiO2 catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H2‐TPR, TAP and steady‐state kinetics) experiments, that the long speculated crystalline Na2WO4 active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na‐WOx sites. Kinetic analysis via temporal analysis of products (TAP) and steady‐state OCM reaction studies demonstrate that (i) surface Na‐WOx sites are responsible for selectively activating CH4 to C2Hx and over‐oxidizing CHy to CO and (ii) molten Na2WO4 phase is mainly responsible for over‐oxidation of CH4 to CO2 and also assists in oxidative dehydrogenation of C2H6 to C2H4. These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na2WO4/SiO2 catalysts.

The activation of molecular O2 as well as the reactivity of adsorbed oxygen species is of central importance in aerobic selective oxidation chemistry on Au-based catalysts. Herein, we address the issue of O2 activation on unsupported nanoporous gold (npAu) catalysts by applying a transient pressure technique, a temporal analysis of products (TAP) reactor, to measure the saturation coverage of atomic oxygen, its collisional dissociation probability, the activation barrier for O2 dissociation, and the facility with which adsorbed O species activate methanol, the initial step in the catalytic cycle of esterification. The results from these experiments indicate that molecular O2 dissociation is associated with surface silver, that the density of reactive sites is quite low, that adsorbed oxygen atoms do not spill over from the sites of activation onto the surrounding surface, and that methanol reacts quite facilely with the adsorbed oxygen atoms. In addition, the O species from O2 dissociation exhibits reactivity for the selective oxidation of methanol but not for CO. The TAP experiments also revealed that the surface of the npAu catalyst is saturated with adsorbed O under steady state reaction conditions, at least for the pulse reaction.

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