Steady-state Reaction Conditions Research Articles

Stabilization of the Active Ruthenium Oxycarbonate Phase for Low-Temperature CO2 Methanation.

Interstitial carbon-doped RuO2 catalyst with the newly reported ruthenium oxycarbonate phase is a key component for low-temperature CO2 methanation. However, a crucial factor is the stability of interstitial carbon atoms, which can cause catalyst deactivation when removed during the reaction. In this work, the stabilization mechanism of the ruthenium oxycarbonate active phase under reaction conditions is studied by combining advanced operando spectroscopic tools with catalytic studies. Three sequential processes: carbon diffusion, metal oxide reduction, and decomposition of the oxycarbonate phase and their influence by the reaction conditions, are discussed. We present how the reaction variables and catalyst composition can promote carbon diffusion, stabilizing the oxycarbonate catalytically active phase under steady-state reaction conditions and maintaining catalyst activity and stability over long operation times. In addition, insights into the reaction mechanism and a detailed analysis of the catalyst composition that identifies an adequate balance between the two phases, i.e., ruthenium oxycarbonate and ruthenium metal, are provided to ensure an optimum catalytic behavior.

Structural and Composition Evolution of Palladium Catalyst for CO Oxidation under Steady-State Reaction Conditions

Understanding how the composition and structure of heterogeneous catalysts change during in situ reaction is of critical importance but greatly challenging because of the complexity of multiple time–space scale. Herein, we propose a self-adaptive simulation strategy driven by the first-principles microkinetic modeling and genetic-algorithm-based global structural search accelerated by machine learning that allows the interplay of surface reaction and catalyst evolution, and uncover the structural/compositional evolution of a Pd(111) single-crystal catalyst under the reaction conditions for CO oxidation, which is a classic open problem lasting for decades. The possible active phases at the kinetically steady state are identified and their common nature is unraveled. We show that, in the presence of CO/O2 reaction mixtures, a dynamically stable partially oxidized nonstoichiometric palladium oxide (PdO0.44 layer) grown on Pd(111) is formed with the O adatoms inserted into the Pd(111) sublayer that drives the formation of the surface oxide. Remarkably, the first-principles microkinetic analyses demonstrate that this self-evolved PdO0.44 surface at the steady state exhibits higher catalytic activity for CO oxidation as compared with Pd(111) and overoxidized PdO catalyst, which may explain the long-standing puzzle of the “PdOx” active phase for Pd catalyst during CO oxidation.

A study of the mechanism of triglyceride hydrodeoxygenation over alumina-supported and phosphatized-alumina-supported Pd catalysts

The mechanism of catalytic hydrodeoxygenation (HDO) of fats, vegetable oils, and fatty acids was studied using alumina-supported Pd catalysts and tricaprylin and valeric acid as model reactants. The chemistry of fatty acid/catalyst interaction was studied by quasi-operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The Pd/γ–Al2O3 catalyst showed good activity in the hydrogenolysis reaction of the ester bonds to convert tricaprylin to caprylic acid, but they were of poor activity in the consecutive hydrodeoxygenation (HDO) of the acid to paraffin. The surface modification of the support alumina by phosphate groups significantly increased the HDO activity of the Pd catalyst and, consequently, the paraffin yield. The activity change was accounted partly for the partial replacement of the weak base Al–OH groups by weak acid P–OH groups but mainly for the partial elimination of Lewis acid (Al⊕) – Lewis base (O⊝) pair sites on the surface of the support. Both surface Al–OH and P–OH groups were shown to participate in the reaction with carboxylic acid and formed bidentate surface carboxylate species, which further reacted with hydrogen to give paraffin. Carboxylates of less basic surface sites were found to be more prone to HDO reaction than those of strong base sites. Monodentate carboxylates, formed on Al⊕ O⊝ pair sites were of low reactivity. Phosphatizing eliminated most of the Lewis type acid-base pair sites, therefore, reactive bidentate carboxylates represented the most abundant surface intermediate (MASI) during the HDO reaction of triglyceride. The hydroxyl coverage of the carboxylated surface was shown to become somewhat higher under steady-state reaction conditions. The increased hydroxyl coverage implies that C–O bond hydrogenolysis of the surface carboxylate proceeds, regenerating OH groups and forming aldehyde that could be intermediate of paraffin formation.

The degree of rate control of catalyst-bound intermediates in catalytic reaction mechanisms: Relationship to site coverage

The degree of rate control (DRC) quantifies how much the energy of each species in a reaction mechanism (e.g., catalyst-bound intermediates and transition states) affects the net reaction rate. It thus plays an important role in understanding catalyst activity and selectivity and in efforts to find better catalysts. We show here that under steady-state reaction conditions, the DRC for any catalyst-bound intermediate n (Xn) is proportional to its fractional population of catalyst sites (θn),Xn=-σ×θn,where the proportionality constant σ is given byσ=∑iXi×ni.Here, Xi is the DRC of the transition state in step i, ni is the number of catalyst sites of the same type required for the elementary step i, and the sum is over all transition states (or elementary steps) i. Since only a few transition states typically have non-negligible DRCs, this simple sum (or weighted average of elementary-step site requirements) includes only a few terms (and only one term when there is a single rate-determining step). We also show that the DRCs of reactants and catalyst depend upon the choice of zero-energy reference, but simplify to zero when their standard states are used as the zero-energy references.

Effects of oxygen coverage on rates and selectivity of propane-CO2 reactions on molybdenum carbide

Mo2C catalyzes propane dehydrogenation and hydrogenolysis at 823 K; carbon selectivity can be tuned to >95% propylene via dehydrogenation in absence of H2, >95% CH4 via hydrogenolysis with H2 co-feed, or >80% CO via reforming pathways with H2 and CO2 co-feed. The changes in selectivity are mediated by an evolution in the coverage of oxidized (O∗) and carbidic (∗) surface sites which results in an evolution of O∗O∗, O∗∗, and ∗∗ site pairs that catalyze propane dehydrogenation. The fraction of O∗ in relation to ∗ was assessed from measured CO2/CO ratios because reverse water gas shift equilibrium exists under H2/CO2 co-feed steady state reaction conditions. Kinetic models based on the two-site dehydrogenation mechanism could be used to quantitatively describe measured rates of propane dehydrogenation at steady state with or without H2 and/or CO2 co-feed and the transient evolution in dehydrogenation rates upon removing H2 or CO2 in the influent stream to note that O∗∗ site pairs exhibit the highest rate per gram. This model also provides a rationale for O∗ inhibition of H-activated hydrogenolysis pathways and for promotion of oxidative dehydrogenation rates with the introduction of hydrogen into CO2-propane influent streams. This study extends concepts developed for examining the catalytic effects of O∗ coverage on oxidative light alkane conversion from transition metal catalysts to also include carbidic formulations.

Impact of Edge Oxidation State on Red-Ox Barriers during Hydrocarbon Oxydehydrogenation over Carbon Nanotube Catalysts: A Theoretical Study

A computational investigation of the oxidative dehydrogenation (ODH) of hydrocarbons over model edge-oxidized carbon nanotubes (CNTs) providing different degrees of C═O/C–OH surface termination (i.e., reduction) was performed. The pool of gaseous species is assumed to contain the initial reagents (hydrocarbons, O2) and volatile intermediates (R•, HO2•, HO•, H2O2). The barriers of both the H-abstraction from the substrate molecules by surface carbonyl groups as well as of the reoxidation of hydroxyl groups sensitively respond to the red-ox state of the model cluster. Accordingly, the surface state of CNT catalysts under ODH steady-state reaction conditions represents an intermediate degree of catalyst reduction. This conclusion agrees with previously published kinetic and in situ XPS studies of carbon-catalyzed ODH.

Active site densities, oxygen activation and adsorbed reactive oxygen in alcohol activation on npAu 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.

Low-Temperature Dehydrogenation of Ethanol on Atomically Dispersed Gold Supported on ZnZrOx

Atomically dispersed gold supported on nanoscale ZnZrOx composite oxides was prepared and investigated in this work as a catalyst for the low-temperature ethanol dehydrogenation reactions. The composite ZnZrOx support disperses gold atomically and stabilizes it against growth much better than either of the neat oxides. Sequential ethanol conversion reactions to acetaldehyde and acetone take place on the Au/ZnZrOx catalysts within well-separated temperature windows over the range of tested temperatures (30–400 °C). ZnO modulates the acidity of the ZrO2 surface, and the extent of this was followed by isopropanol temperature-programmed desorption with online mass spectrometry (IPA-TPD/MS; and by diffuse reflectance UV−Vis−IR). Catalyst activity and selectivity were tested by temperature-programmed surface reaction (TPSR) and under steady-state reaction conditions. The work has demonstrated that ZnZrOx with optimized ZnO distribution preserves the active Au–Ox surface species under reaction conditions and sup...

Surface-Bound Intermediates in Low-Temperature Methanol Synthesis on Copper: Participants and Spectators

The reactivity of surface-adsorbed species present on copper catalysts during methanol synthesis at low temperatures was studied by simultaneous infrared spectroscopy (IR) and mass spectroscopy (MS) measurements during “titration” (transient surface reaction) experiments with isotopic tracing. The results show that adsorbed formate is a major bystander species present on the surface under steady-state methanol synthesis reaction conditions, but it cannot be converted to methanol by reaction with pure H2 or with H2 plus added water. Formate-containing surface adlayers for these experiments were produced during steady-state catalysis in (a) H2:CO2 (with substantial formate coverage) and (b) moist H2:CO (with no IR visible formate species). These reaction conditions both produce methanol at steady state with relatively high rates. Adlayers containing formate were also produced by (c) formic acid adsorption. Various “titration” gases were used to probe these adlayers at modest temperatures (T = 410–450 K) and...

Electrocatalytic O2-Reduction by Synthetic Cytochrome c Oxidase Mimics: Identification of a "Bridging Peroxo" Intermediate Involved in Facile 4e(-)/4H(+) O2-Reduction.

A synthetic heme-Cu CcO model complex shows selective and highly efficient electrocatalytic 4e(-)/4H(+) O2-reduction to H2O with a large catalytic rate (>10(5) M(-1) s(-1)). While the heme-Cu model (FeCu) shows almost exclusive 4e(-)/4H(+) reduction of O2 to H2O (detected using ring disk electrochemistry and rotating ring disk electrochemistry), when imidazole is bound to the heme (Fe(Im)Cu), this same selective O2-reduction to water occurs only under slow electron fluxes. Surface enhanced resonance Raman spectroscopy coupled to dynamic electrochemistry data suggests the formation of a bridging peroxide intermediate during O2-reduction by both complexes under steady state reaction conditions, indicating that O-O bond heterolysis is likely to be the rate-determining step (RDS) at the mass transfer limited region. The O-O vibrational frequencies at 819 cm(-1) in (16)O2 (759 cm(-1) in (18)O2) for the FeCu complex and at 847 cm(-1) (786 cm(-1)) for the Fe(Im)Cu complex, indicate the formation of side-on and end-on bridging Fe-peroxo-Cu intermediates, respectively, during O2-reduction in an aqueous environment. These data suggest that side-on bridging peroxide intermediates are involved in fast and selective O2-reduction in these synthetic complexes. The greater amount of H2O2 production by the imidazole bound complex under fast electron transfer is due to 1e(-)/1H(+) O2-reduction by the distal Cu where O2 binding to the water bound low spin Fe(II) complex is inhibited.

Mechanistic studies on the flavin-dependent N6-lysine monooxygenase MbsG reveal an unusual control for catalysis

The mechanism of Mycobacterium smegmatis G (MbsG), a flavin-dependent l-lysine monooxygenase, was investigated under steady-state and rapid reaction conditions using primary and solvent kinetic isotope effects, substrate analogs, pH and solvent viscosity effects as mechanistic probes. The results suggest that l-lysine binds before NAD(P)H, which leads to a decrease in the rate constant for flavin reduction. l-lysine binding has no effect on the rate of flavin oxidation, which occurs in a one-step process without the observation of a C4a-hydroperoxyflavin intermediate. Similar effects were determined with several substrate analogs. Flavin oxidation is pH independent while the kcat/Km and kred/KD pH profiles for NAD(P)H exhibit single pKa values of ∼6.0, with increasing activity as the pH decreases. At lower pH, the enzyme becomes more uncoupled, producing more hydrogen peroxide and superoxide. Hydride transfer is partially rate-limiting at neutral pH and becomes more rate-limiting at low pH. An inverse solvent viscosity effect on kcat/Km for NAD(P)H was observed at neutral pH whereas a normal solvent viscosity effect was observed at lower pH. Together, the results indicate a unique mechanism where a rate-limiting and pH-sensitive conformational change occurs in the reductive half-reaction, which affects the efficiency of lysine hydroxylation.

Mechanism of NO-SCR by methane over Co,H-ZSM-5 and Co,H-mordenite catalysts

Results of X-ray photoelectron spectroscopic (XPS) examination and temperature-programmed reduction measurements by H2 (H2-TPR) showed that the Co-zeolite catalysts, which were found most active in the selective catalytic reduction of NO by methane to N2 in the presence of excess O2 (NO-SCR), contain both Co2+/[Co-OH]+/H+ exchange cations, Co-oxo species and cobalt oxide clusters. Using operando Diffuse Reflectance Infrared Fourier Transform Spectroscopic method (DRIFTS method) the NO-SCR reaction was shown to proceed in consecutive steps via bifunctional mechanism over active sites (i) promoting the oxidation of NO by O2 to NO2 (NO-COX reaction), and sites (ii) whereon disproportionation and charge separation of 2NO2 generates activated surface intermediate NO3−/NO+ ion pair. Latter process was found to require Co2+ zeolite cations. The NO-COX reaction was shown to proceed over Co-oxo species and cobalt oxide, if present, and also over Brønsted acid sites but at a significantly lower rate. In the reaction of methane and the NO3−/NO+ ion pair CO2, H2O, and N2 was formed and the active Co2+ sites were recovered (CH4/NO-SCR reaction). The surface concentration of the NO3−/NO+ ion pair must have been controlled by the relative magnitude of the apparent rate constants of the consecutive NO-COX and CH4/NO-SCR reactions. Below about 700K reaction temperature latter reaction governed the rate of the consecutive NO reduction process. Above about 700K combustion became the main reaction of methane. Because of the low equilibrium NO2 concentration at these high temperatures the NO-COX reaction took over the control over the rate of the NO-SCR process. Under steady state reaction conditions a temperature-dependent fraction of the Co2+ active sites was always poisoned by adsorbed H2O formed in the CH4 oxidation reaction.

Higher Alcohol Synthesis Using K-Doped CoRhMoS2/MWCNT Catalysts: Influence of Pelletization, Particle Size and Incorporation of Binders

In this study, alkalinized MWCNT supported MoS2 catalysts have been doubly-promoted with Co and Rh. Catalysts were prepared by the conventional co-impregnation method and stabilized under argon atmosphere. Characterization of the oxidic samples by BET revealed that the mesoporosity of the pristine MWCNT support was not compromised after loading a combined total of 30 wt% metals (9 wt% K, 4.5 wt% Co, 1.5 wt% Rh, and 15 wt% Mo) on the support; however, a significant decrease in specific surface area was observed. Broad angle XRD analysis confirmed the homogenous dispersion of catalyst metals on the support. Two catalyst grain sizes were first investigated to elucidate the effect of particle size: a finely ground powder (88 μm) and a pelletized form (1,780 μm). Despite the total alcohol yield of 0.261 g/(gcat h) observed by conducting higher alcohol synthesis reaction at T = 330 °C, P = 8.3 MPa, H2/CO = 1.25, and GHSV = 3.6 m STP 3 /(kgcat h) for the fine powdered sample, the relatively higher pressure drop could be minimized by using the pelletized form of the catalyst. Finally, a systematic study of variety of selected binders was conducted to gain insight of catalyst’s applicability for industrial purposes. Three selected binders namely: bentonite clay, coal tar, and humic acid were thus investigated; taking into consideration significant factors such as melting point and binder requirement per catalyst support. The CO conversions evaluated for the two binder-free catalysts (88 and 1,700 μm) showed that the catalyst with fine particle sizes (88 μm) performed better than that in the pelletized form (binder-free, 1,700 μm); yielding a maximum ethanol selectivity of 38.5 % at steady-state reaction conditions.

NO decomposition over La-doped Cu-ZSM5 monolith under adsorption-reaction conditions

The decomposition of nitrogen oxide has been studied under non steady state reaction conditions over a La-stabilised Cu-ZSM5 structured catalyst after adsorbing NO on the zeolite. The three main steps of the experiments are the following: (i) NO adsorption at 50°C until zeolite saturation, (ii) reactor isolation and heating up to the reaction temperature (480 or 520°C), (iii) cooling down back to 50°C re-opening the reactor once the adsorption temperature has been reached. No NOx are emitted during all steps. TPD tests have been performed after one or more cycles (i)–(iii) to check the mass balance. A very high NO conversion, much higher than those obtained under flowing (steady state) reaction conditions, is reached in all cycles. The unreacted NO is re-adsorbed on the zeolite in the step (iii). After two/three cycles (steps i–iii) the experiments are repeatable.The effect of O2 (2.5vol.%) co-feeding during adsorption step has been also studied; the adsorption capacity during the first cycle is higher than that obtained under O2-free reaction conditions, while NO conversion decreased. After few cycles, adsorption capacity and NO conversion appeared unaffected by O2 feed.

Insight into the Role of Electropositive Promoters in Emission Control Catalysis: An In Situ DRIFTS Study of NO Reduction by C3H6 Over Na-Promoted Pt/Al2O3 Catalysts

The nature and the relative population of adsorbed species formed on the surface of unpromoted and Na-promoted Pt/γ-Al2O3 catalysts, during the NO reduction by propene, was investigated by means of in situ diffuse reflectance infrared Fourier transform spectroscopy, in an effort to elucidate the pronounced effect of electropositive promoters in emission control catalysis. It was found that under steady-state reaction conditions the surface of the unpromoted catalyst is mainly covered by carbon containing species, such as hydrocarbon fragments, carboxylates and cyanides. On the opposite, the surface of Na-promoted catalysts is predominantly covered by NOx ad-species, carbonyls and isocyanates, implying the significant effect of Na promoter on surface species formation. To further gain insight into the effect of Na-promoter on the nature and the reactivity of ad-species, the transient respond of IR spectra under the cycle: NO → He → C3H6 → NO, is also investigated. The results imply that over Na-free catalysts the interaction of propene with NOx pre-adsorbed species (mainly nitrates) leads to the formation of strongly bonded carboxylates and cyanides, which are inactive towards NO. In contrast, NO interaction with the surface of Na-promoted catalysts results in the formation of nitro/nitrite species as well as to nitrosyls, which are highly active towards propene, leading to active intermediates such as isocyanates (NCO). The present results demonstrate that the excellent catalytic performance of Na-promoted catalysts can be well interpreted on the basis of these significant modifications induced by electropositive promoters on catalysts surface chemistry.

Removal of methylene blue from aqueous solution using wine-processing waste sludge

Dye wastewaters usually contain toxins and high chroma, making them difficult to treat with biological methods. The adsorption process plays an important role in removing dyes from wastewaters. This study aimed to explore the methylene blue (MB) adsorption mechanism by wine-processing waste sludge (WPWS). The WPWS contains a high cation-exchange capacity (64.2 cmol(c) kg(-1)) and organic matter (52.8%). The parameters affecting MB adsorption included pH, initial concentration of MB, reaction temperature, particle size and dosage of WPWS. The WPWS adsorption isotherms of MB were only well described by Langmuir adsorption isotherm. The maximum adsorption capacity (Q(m)) of MB was 285.7 mg g(-1) at 25 °C. The activation energy determined by Arrhenius equation is 29.995 kJ mol(-1). Under steady-state reaction conditions, the Gibb free energy (ΔG°) ranged from -24.607 to -27.092 kJ mol(-1) and ΔH° was -8.926 kJ mol(-1), indicating that lower reaction temperature would favor MB adsorption. Therefore, MB adsorption by WPWS was a spontaneous, exothermic and physisorption reaction.

“First‐Principles” kinetic monte carlo simulations revisited: CO oxidation over RuO2(110)

First principles-based kinetic Monte Carlo (kMC) simulations are performed for the CO oxidation on RuO(2) (110) under steady-state reaction conditions. The simulations include a set of elementary reaction steps with activation energies taken from three different ab initio density functional theory studies. Critical comparison of the simulation results reveals that already small variations in the activation energies lead to distinctly different reaction scenarios on the surface, even to the point where the dominating elementary reaction step is substituted by another one. For a critical assessment of the chosen energy parameters, it is not sufficient to compare kMC simulations only to experimental turnover frequency (TOF) as a function of the reactant feed ratio. More appropriate benchmarks for kMC simulations are the actual distribution of reactants on the catalyst's surface during steady-state reaction, as determined by in situ infrared spectroscopy and in situ scanning tunneling microscopy, and the temperature dependence of TOF in the from of Arrhenius plots.

Experiment-Based Kinetic Monte Carlo Simulations: CO Oxidation over RuO2(110)

Kinetic Monte Carlo (kMC) simulations of the CO oxidation over RuO2(110) have been performed for a variety of different reaction conditions ranging from 10–7 to 10 mbar and temperatures from 300 to 600 K. The kMC simulations are based on reaction rates of elementary steps including diffusion, adsorption/desorption, and recombination of surface CO and O. The activation barriers for these elementary processes are mostly taken from temperature programmed reaction and desorption experiments of well-defined coadsorbate layers on RuO2(110) under ultrahigh vacuum conditions. We show that the experimental kinetic reaction data under steady state reaction conditions both in the 10–7 mbar range and in a batch reactor up to 10 mbar are reconciled within this experiment-based kMC approach. Experimental in situ reflection absorption infrared (RAIR) spectra in the frequency range of the CO stretch vibration depend sensitively on both the adsorption site and the local environment of the CO molecules, encoding thus the s...

Kinetics and mechanism of olefin methylation reactions on zeolites

Ethylene and propylene methylation rates increased linearly with olefin pressure but did not depend on dimethyl ether (DME) pressures on proton-form FER, MFI, MOR, and BEA zeolites at low conversions (<0.2%) and high DME/olefin ratios (30:1) in accordance with a mechanism that involves the zeolite surface being predominantly covered by DME-derived species reacting with olefins. Higher first-order reaction rate constants for both ethylene and propylene methylation were observed over H-BEA and H-MFI compared with H-FER and H-MOR, indicating that olefin methylation reaction cycles involved in the conversion of methanol-to-gasoline over zeolitic acids are propagated to varying extents by different framework materials. Systematically lower activation barriers and higher rate constants were observed for propylene methylation in comparison with ethylene methylation over all frameworks studied, reflecting the increased stability of reaction intermediates and activated complexes with increasing olefin substitution. A binomial distribution of d0/d3/d6 in unreacted DME upon introduction of equimolar protium- and deuterium-form DME under steady-state reaction conditions of ethylene methylation over H-MFI suggests the presence and facile formation of reactive surface-bound methoxide species and the absence of C–H bond cleavage.

Investigating the mechanism of the H 2-assisted selective catalytic reduction (SCR) of NOx with octane using fast cycling transient in situ DRIFTS-MS analysis

A mechanistic study of the H 2-assisted Selective Catalytic Reduction (SCR) of NOx with octane as reductant over a Ag/Al 2O 3 catalyst was carried out using a modified DRIFTS cell coupled to a mass spectrometer. Using fast transient cycling switching of H 2, with a time resolution of a few seconds, it was possible to differentiate potential reaction intermediates from other moieties that are clearly spectator species. Using such a periodic operation mode, effects were uncovered that are normally hidden in conventional transient studies which typically consist of a single transient. In experiments based on a single transient addition of H 2 to, or removal of H 2 from, the SCR feed, it was found that the changes in the concentrations of gaseous species (products and reactants) were not matched by changes at comparable timescales of the concentration of surface species observed by IR. This observation indicates that the majority of surface species observed by DRIFTS under steady-state reaction conditions are spectators. In contrast, under fast cycling experimental conditions, it was found that a surface isocyanate species had a temporal response that matched that of 15N 2. This suggests that some of the isocyanate species observed by infrared spectroscopy could be important intermediates in the hydrogen-assisted SCR reaction although it is emphasised that this may be dependent on the way in which the infrared spectra are obtained. It is concluded that the use of fast transient cycling switching techniques may provide useful mechanistic information under certain circumstances.