Standard Selective Catalytic Reduction Reaction Research Articles

Strikingly distinctive NH3-SCR behavior over Cu-SSZ-13 in the presence of NO2

Commercial Cu-exchanged small-pore SSZ-13 (Cu-SSZ-13) zeolite catalysts are highly active for the standard selective catalytic reduction (SCR) of NO with NH3. However, their activity is unexpectedly inhibited in the presence of NO2 at low temperatures. This is strikingly distinct from the NO2-accelerated NOx conversion over other typical SCR catalyst systems. Here, we combine kinetic experiments, in situ X-ray absorption spectroscopy, and density functional theory (DFT) calculations to obtain direct evidence that under reaction conditions, strong oxidation by NO2 forces Cu ions to exist mainly as CuII species (fw-Cu2+ and NH3-solvated CuII with high CNs), which impedes the mobility of Cu species. The SCR reaction occurring at these CuII sites with weak mobility shows a higher energy barrier than that of the standard SCR reaction on dynamic binuclear sites. Moreover, the NO2-involved SCR reaction tends to occur at the Brønsted acid sites (BASs) rather than the CuII sites. This work clearly explains the strikingly distinctive selective catalytic behavior in this zeolite system.

Identification of Intrinsic Active Sites for the Selective Catalytic Reduction of Nitric Oxide on Metal-Free Carbon Catalysts via Selective Passivation

Although carbon catalysts have been commercialized for the low-temperature selective catalytic reduction (SCR) of NO with NH3 for decades, the nature of the active sites remains unclear. Herein we design a proof-of-concept study to directly evidence that the nucleophilic ketonic carbonyl groups are the intrinsic SCR active sites on metal-free carbon catalysts through ex situ and in situ selective passivation strategies. The turnover frequency of the ketonic carbonyl group is provided, and the structure–reactivity relationship is established for the first time. Density functional theory calculations combined with in situ spectroscopy reveal that the standard carbon-catalyzed SCR reaction involves a redox cycle of ketonic carbonyl/phenol (C═O/C–OH) pairs, during which the activation of NH3 is the rate-limiting step. This work identifies the intrinsic SCR active sites and advances the metal-free carbon catalysis. As a further step, the carbon catalysts demonstrate analogous mechanistic features with vanadyl catalysts, opening up the possibility to parallel the SCR mechanisms on metal-free carbon catalysts to those on transition-metal catalysts on the atomic scale.

Dynamics of low temperature N2O formation under SCR reaction conditions over a Cu-SSZ-13 catalyst

Small pore Cu-SSZ-13 zeolite can effectively catalyze selective catalytic reduction (SCR) for NOx (NO and NO2) using NH3 as a reductant. However, side reactions leading to the formation of N2O, a greenhouse gas, occur in parallel under SCR reaction conditions. This work investigates the fundamental characteristics of N2O formation during SCR reactions over a Cu-SSZ-13 catalyst under both steady-state and transient conditions during reactant transitions at 200 °C. The absence of nitrate bands from in situ DRIFTS experiments and the inconsistencies between high stability of NH4NO3 and N2O formation at low temperature exclude the formation of surface nitrate or NH4NO3 as a precursor of N2O formation as the major path under standard SCR reaction conditions at 200 °C. The analysis from the transient N2O formation behavior during reactant transitions and NO + NH3 titration experiments indicates that more oxidized Cu-SSZ-13 catalyst, containing higher amount of Cu-oxy species, produces higher amount of N2O.

Impact of Water Vapor on the Performance of a Cu-SSZ-13 Catalyst under Simulated Diesel Exhaust Conditions

<div class="section abstract"><div class="htmlview paragraph">Cu-SSZ-13 selective catalytic reduction (SCR) catalysts are broadly applied in diesel aftertreatment systems for the catalytic conversion of oxides of nitrogen (NO + NO<sub>2</sub>). Diesel exhaust contains a wide range of water vapor concentrations depending on the operating condition. In this study, we evaluate the impact of water vapor on the relevant SCR catalytic functions including NO<sub>x</sub> conversion, NO oxidation, NH<sub>3</sub> oxidation, and N<sub>2</sub>O formation under both standard and fast SCR conditions. Reactor-based experiments are conducted in the presence and absence of water vapor. Results indicate that water vapor can have both a positive and negative impact on low temperature NO<sub>x</sub> conversion for standard SCR reaction. At low inlet NO<sub>x</sub> concentrations, the presence of water vapor has a negative effect on NO<sub>x</sub> conversion, whereas, at high inlet NO concentrations, water vapor improves NO<sub>x</sub> conversion. At high temperatures, water vapor has a beneficial effect on NO<sub>x</sub> conversion at all NO<sub>x</sub> inlet concentrations evaluated. Experiments under fast SCR conditions reveal that water vapor improves low temperature NO<sub>x</sub> conversion mainly by minimizing the accumulation of ammonium nitrate, as evidenced with temperature programmed desorption (TPD) results. Furthermore, NO oxidation by ammonium nitrate formed under fast SCR conditions is shown to be significantly faster in the presence of water vapor.</div></div>

Nitrogen Oxide Removal by Coal-Based Activated Carbon for a Marine Diesel Engine

Vanadium-based catalysts are mainly used for marine diesel exhaust denitration. However, their poor catalytic ability at low temperature and poor sulfur tolerance, as well as high toxicity and cost, are big turnoffs. AC (Activated carbon) exhibits good adsorption capacity and catalytic ability in denitration because of its high specific surface area and chemical activity. In this paper, coal-based AC was used for simulating diesel exhaust denitration in different conditions. The results show that the NO removal ability of AC is poor in an NO/N2 system. The NO2 removal ability is excellent in an NO2/N2 system, where NO is desorbed. The NOx removal efficiency is 95% when the temperature is higher than 200 °C in an NO2/NH3/N2 system. When the temperature is lower than 100 °C, AC can catalytically oxidize NO to NO2 in an NO2/O2/N2 system. The near-stable catalytic efficiencies of AC for a slow SCR (Selective Catalytic Reduction) reaction, a standard SCR reaction, and a fast SCR reaction at 300 °C are 12.1%, 31.6%, and 70.8%, respectively. When ships use a high-sulfur fuel, AC can be used after wet scrubber desulfurization to catalytically oxidize NO to NO2 at a low temperature. When ships use a low-sulfur fuel, AC can be used as a denitration catalyst at high temperatures.

NO, NO2 and N2O emissions over a SCR using DOC and DPF systems with Pt reduction

A Pt-reduced diesel oxidation catalyst (DOC) and diesel particulate filter (DPF) combination was used to evaluate the influences of Pt reduction on the oxidation performance in a nonroad diesel engine. For the Pt-reduced combination, a mass fraction of 1.5% Pt in the DOC was lower than a DOC of a non-reduction combination. These DOCs were composed of Pt only. The DPF in the Pt-reduced combination was composed of Pt and Pd with a 2:1 ratio, i.e., Pd was substituted for Pt. Whereas the DPF in the non-reduction combination was Pt only. The oxidation performance in the DOC and DPF systems of the Pt-reduced combination was relatively consistent with the non-reduction combination. The behavior of the undesirable oxidation of nitric oxide (NO) to nitrogen dioxide (NO2) occurred in both combinations. The selective catalytic reduction (SCR) system was also installed for NO and NO2 abatement, and NO and NO2 were evaluated based on the conversion efficiency. The fast SCR reaction, which involves both NO and NO2, was dominantly activated at the SCR inlet temperature of 380 °C because the NO2/NOX ratio is approximately 60%. The fast SCR reaction competed with the standard SCR reaction above 380 °C, and then the standard SCR reaction became the prevailing reaction as the temperatures increased. Nitrous oxide (N2O), which has 298 times more global warming potential than carbon dioxide, was also investigated mainly over the SCR. N2O has increased dozens of times over the SCR, and a lower ratio of N2O emission after the SCR to the NOX at the SCR inlet was obtained in the Pt-reduced combination than in the non-reduction combination.

A Computational Investigation of Industrial Selective Catalytic Reduction Systems for NOx Control

The selective catalytic reduction (SCR) is a promising NOx (a mixture of NO and NO2) reduction technology for various applications. The SCR process entails the conversion of NOx by the use of a reducing agent such as ammonia and a suitable catalyst. Due to increasingly stricter NOx emission regulations, the SCR technology for NOx control needs continuous improvement. The improvement requires better understanding of complex processes occurring in the SCR system. The current study employs a mathematical model to elucidate the effect of key operating and geometric parameters on the performance of SCR systems. The model considers both standard and fast SCR reaction processes. The model was used to investigate the effects of NH3/NOx and NO2/NOx ratios in the exhaust on the SCR performance and the effect of using a dual layer SCR system. Furthermore, the effect of different operating parameters and the interdependence of parameters is analyzed by using a factorial approach. The results show that the SCR performance is very sensitive to NH3/NOx ratio. The SCR performance is also affected by the NO2/NOx ratio particularly at low temperatures. The optimal NOx conversion performance requires a combination of NH3/NOx ratio of 1.0, NO2/NOx ratio of 0.5, low space velocities, and high inlet temperature. The results depict that adding a second catalyzed layer results in increased reaction activity especially when the concentration is still high after the first layer.

Recent Progress in Atomic-Level Understanding of Cu/SSZ-13 Selective Catalytic Reduction Catalysts

Cu/SSZ-13 Selective Catalytic Reduction (SCR) catalysts have been extensively studied for the past five-plus years. New and exciting fundamental and applied science has appeared in the literature quite frequently over this time. In this short review, a few topics specifically focused on a molecular-level understanding of this catalyst are summarized: (1) The nature of the active sites and, in particular, their transformations under varying reaction conditions that include dehydration, the presence of the various SCR reactants and hydrothermal aging; (2) Discussions of standard and fast SCR reaction mechanisms. Considerable progress has been made, especially in the last couple of years, on standard SCR mechanisms. In contrast, mechanisms for fast SCR are much less understood. Possible reaction paths are hypothesized for this latter case to stimulate further investigations; (3) Discussions of rational catalyst design based on new knowledge obtained regarding catalyst stability, overall catalytic performance and mechanistic catalytic chemistry.

Investigation of Iron Vanadates for Simultaneous Carbon Soot Abatement and NH3-SCR

FeVO4 and Fe0.5Er0.5VO4 were prepared and loaded over standard Selective Catalytic Reduction (SCR) supports based on TiO2-WO3-SiO2 (TWS) and redox active supports like CeO2 and CeZrO2 with the aim of finding a suitable formulation for simultaneous soot abatement and NH3-SCR and to understand the level of interaction between the two reactions. A suitable bi-functional material was identified in the composition FeVO4/CeZrO2 where an SCR active component is added over a redox active support, to increase carbon oxidation properties. The influence of the presence of ammonia in soot oxidation and the effect of the presence of soot on SCR reaction have been addressed. It is found that the addition of NO and NO/NH3 mixtures decreases at different levels the oxidation temperature of carbon soot, while the presence of carbon adversely affects the NH3-SCR reaction by increasing the oxidation of NH3 to NO, thus lowering the NO removal efficiency.

New insights into the mechanism of NH3-SCR over Cu- and Fe-zeolite catalyst: Apparent negative activation energy at high temperature and catalyst unit design consequences

We demonstrate an unusual behavior of a practically-important reaction of selective catalytic reduction (SCR) of NOx with NH3 over a state-of-the-art Cu-SSZ-13 catalyst. In response to increasing temperature, the rate of SCR reaction increases initially, attains a maximum around 300 °C, and then declines, resulting in a negative apparent activation energy at higher temperatures. This behavior has never been previously reported because the side reaction of NH3 oxidation obfuscated the kinetic analysis of SCR reaction at high temperatures. We were able to discover this phenomenon by performing severe hydrothermal aging of the catalyst that suppressed the NH3 oxidation activity without a significant change in the SCR activity. Further, we show that the phenomenon is more general and also found in other Cu- and Fe-exchanged zeolite and V2O5-(WO3)/TiO2 catalysts. The behavior is explained based on the fact that the activation energy for NH3 desorption is higher than that for standard SCR reaction. Therefore, at higher temperatures, the increase in the SCR reaction rate constant with temperature gets outpaced by the decline in the NH3 coverage, resulting in the overall decline of the reaction rate.One of the implications of the finding is that the studied Cu-SSZ-13 SCR catalyst never operates in a purely external mass transfer limited regime, under the range of conditions relevant to the practical SCR applications. In other words, there is always a certain contribution of slower reaction rate to the performance of the SCR catalyst, even after increasing the reaction rates by increasing catalyst amount, using the “fast” SCR reaction, or operating at elevated temperatures. This has significant consequences for the design of practical SCR catalysts, in particular related to the choices of the active material loading and channel hydraulic diameter, as empirically and analytically demonstrated in this paper.

High Temperature and High Concentration SCR of NO with NH3 for the Oxyfuel Combustion Process: Fitting of Kinetics to Data from a Laboratory Reactor Experiment

This study shows the result of a simulation of a proposed kinetic reaction scheme of high-concentration (5000 ppm NO) and high-temperature (570–800 K) selective catalytic reduction of NO with NH3 on a commercial vanadia based catalyst at a system pressure of 2.2 bar. The simulations are performed with COMSOL Multiphysics ver. 5.1. Experimental data from a catalytic flow reactor can be nicely fitted to a system of kinetic expressions for the selective catalytic reduction (SCR) reaction and its side reactions. Especial attention is given to the formation of N2O. All reaction rate expressions contain an ammonia adsorption term. The value for its pre-exponential factor is 9e−9 and its activation energy is −139 kJ/mol. The fastest reaction is the standard SCR reaction with a pre-exponential factor of 5.28e7 s−1 and an activation energy of 85 kJ/mol. This reaction is affected by internal diffusion limitations. The oxidation of NH3 to NO is the second fastest reaction with parameters 5e12 mol/m3s and 200 kJ/mol. The NSCR of NH3 to N2O is the third fastest reaction with values of 7.5e5 s−1 and 110 kJ/mol. The oxidation of NH3 to N2O has the parameters 8e11 mol/m3/s and 190 kJ/mol. At 640 K inlet temperature the rate of the SCR reaction is about 1.3 mol/m3/s. The oxidation of NH3 to NO is 4.7e−4 mol/m3/s. The rate of the NSCR producing N2O is 1.63e−4 mol/m3/s. The rate of the direct oxidation of NH3 to N2O is 3.7e−5 mol/m3/s. Thus the rate of the SCR reaction is about 35,000 faster than the high temperature production of N2O at this temperature.

Effects of microporous TiO2 support on the catalytic and structural properties of V2O5/microporous TiO2 for the selective catalytic reduction of NO by NH3

Selective catalytic reduction (SCR) of NO by NH3 over vanadium-based catalyst is often accompanied with unwanted nitrous oxide (N2O) formation, which has 300 times higher global warming potential than CO2. In this work, VOx dispersed on microporous TiO2 catalysts calcined at various temperatures were applied to standard SCR reaction compared with VOx on commercial TiO2 ones. Both catalysts showed stable NOx reduction activity although they did completely different trend in N2O formation. Specifically, N2O was much less produced on VOx/microporous TiO2 catalysts regardless of calcination temperature with or without water in the reactant. Structural characterization of the catalysts using H2-TPR and Vanadium XANES revealed that the microporous TiO2 could suppress the formation of bulk-like V2O5 species, which are generally suggested as the main cause of N2O formation, in comparison of non-microporous commercial TiO2 support. Also, NH3-TPD and in situ DRIFTS studies showed that VOx on microporous TiO2 maintained strong Brønsted acidity so that it was capable of providing adsorbed NH3 species readily up to high temperature, which led to the stable DeNOx performance. We found that such two promoting effects seemed functionally analogous to those of tungsten oxide, which is conventionally used as a promoting material for the VOx/TiO2 catalyst. Our results demonstrated that vanadium oxides can be effectively stabilized up to high loading by structurally modifying TiO2 support, and also provided reasonable explanation about the promoting effects of microporous TiO2 support on the catalytic activity and structural properties of VOx/TiO2 catalyst.

Impact of Copper Loading on NH3-Selective Catalytic Reduction, Oxidation Reactions and N2O Formation over Cu/SAPO-34

We developed a procedure for aqueous ion exchange to obtain different Cu loadings of Cu/SAPO-34 (between 0 and 2.6 wt %.) The catalysts were washcoated on monoliths and characterised with respect to their activity and selectivity under standard selective catalytic reduction (SCR), fast SCR, NH3 oxidation and NO oxidation reactions. They were further characterised using X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), H2-temperature programmed reduction (H2-TPR), ultraviolet (UV)-vis spectroscopy and NH3 adsorption. As expected, activity of all reactions increased with copper loading, due to increased number of active sites. However, the N2O formation during standard and fast SCR yielded interesting mechanistic information. We observed that N2O formation at low temperature increased with copper loading for the standard SCR reaction, while it decreased for fast SCR. The low-temperature N2O formation during fast SCR thus occurs predominantly over Brønsted sites. Species responsible for N2O formation during standard SCR, on the other hand, are formed on the copper sites. We further found that the fast SCR reaction occurs to a significant extent even over the H/SAPO-34 form. The Brønsted sites in SAPO-34 are thus active for the fast SCR reaction.

Effects of Ce and Nb additives on the de-NOx performance of SCR/CDPF system based on Cu-beta zeolite for diesel vehicles

In this study, the de-NOx performance of the Cu-beta zeolite (BEA) selective catalytic reduction (SCR) catalyst with Ce and Nb as additives was investigated for the diesel catalytic reduction catalyzed diesel particulate filter (SCR/CDPF) system. The Cu-BEA catalyst with the Ce and Nb additives improved the NOx conversion, compared to the Cu-BEA catalyst without additives, under conditions of both standard and fast SCR reaction. This result is related to the strength of the intensity ratio as (Cu2p3 B peak)/(Cu2p3 A peak) for the Ce and Nb added catalysts. At the aging temperature of >650°C, the Cu-BEA-Nb catalyst showed better de-NOx performance than the other catalysts, because the Nb2O5 (Nb5+) of the aged Cu-BEA-Nb catalyst remained. The Cu-BEA-CeNb catalyst was most affected by HC, and the addition of Nb to the Cu-BEA catalyst improved the oxidation ability and inhibited the HC poison at a high temperature.

A complete reaction mechanism for standard and fast selective catalytic reduction of nitrogen oxides on low coverage VOx/TiO2(0 0 1) catalysts

We present a complete catalytic mechanism describing both the Standard and the Fast selective catalytic reduction (SCR) reactions in their correct stoichiometric form on a vanadia titania (anatase 001 facet) based catalyst model. It consists of two cycles, a NO-activation cycle and a Fast SCR cycle that share the same reduction part but use NO+O2 and NO2 respectively for the reoxidation. The stoichiometry of the Standard SCR reaction is obtained by coupling the two cycles and the stoichiometry of the Fast SCR reaction is represented by the Fast SCR cycle. We establish structures and energetics for each elementary reaction allowing us to calculate the rate for the two reactions by microkinetic modeling. We find at low temperatures the rate for the Standard SCR reaction is determined by H2O formation and desorption as neither NO nor O2 reacts exothermically with the reduced site prior to H2O desorption. On the contrary NO2 reacts directly with the reduced site resulting in higher rate for the Fast SCR reaction at low temperatures. The rate for the two reactions is the same at higher temperatures as the rate determining step is in the reduction part which is common to both reactions.

A Consistent Reaction Scheme for the Selective Catalytic Reduction of Nitrogen Oxides with Ammonia

For the first time, the standard and fast selective catalytic reduction (SCR) of NO by NH3 are described in a complete catalytic cycle that is able to produce the correct stoichiometry while allowing adsorption and desorption of stable molecules only. The standard SCR reaction is a coupling of the activation of NO by O2 with the fast SCR reaction, enabled by the release of NO2. According to the scheme, the SCR reaction can be divided into an oxidation of the catalyst by NO + O2 and a reduction by NO + NH3; these steps together constitute a complete catalytic cycle. Furthermore, both NO and NH3 are required in the reduction, and finally, oxidation by NO + O2 or NO2 leads to the same state of the catalyst. These points are shown experimentally for a Cu-CHA catalyst by combining in situ X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and Fourier transform infrared spectroscopy (FTIR). A consequence of the reaction scheme is that all intermediates in fast SCR are also part of the s...

Combined experimental and kinetic modeling study of the bi-modal NOx conversion profile on commercial Cu-SAPO-34 catalyst under standard SCR conditions

Bi-modal NOx conversion profile has been consistently reported for selective catalytic reduction (SCR) of NOx with NH3 on certain Cu-exchanged zeolite catalysts under the so-called standard SCR conditions, with two distinct NOx conversion peaks located around 250 and 450°C. However, the nature of this practically important phenomenon remained unexplained. In this work, we have used a combination of experimental studies and kinetic modeling to characterize and de-convolute several side reactions, allowing us to quantitatively reconstruct the overall bi-modal conversion profile. In particular, we have shown that the first peak can be represented as a competition between the standard SCR reaction and parasitic NH3 oxidation. The subsequent increase in the NOx conversion can be attributed to the onset of in-situ NO oxidation to NO2 that enhances NOx reduction via fast SCR reaction route. Specifically, our modeling study indicates that NO oxidation reaction accelerates by orders of magnitude under the standard SCR conditions and becomes the governing step for NO reduction at high temperature.

SO2 poisoning impact on the NH3-SCR reaction over a commercial Cu-SAPO-34 SCR catalyst

Sulfur poisoning is a durability issue for Cu/SAPO-34 selective catalytic reduction (SCR) catalysts. In this study, the impact of SO2 on the SCR performance, and the sulfur poisoning mechanism itself, was investigated. SO2 inhibited SCR activity at low temperature (<300°C), while at higher temperatures no evident effect was observed. Temperature programmed desorption data show significant SO2 desorption starting at 300°C, thus the lack of impact noted at high temperature. Low temperature deactivation is primarily caused by the formation of ammonium sulfate species, with some contribution from competitive adsorption between SO2 and NOx, which is considered a key step in the standard SCR reaction, possibly through the formation of metal sulfate species. However, the surface sulfur species decomposed under high temperature thermal treatment in an O2/N2 mixture, such that activity was reattained.

백금계 촉매상에서 산화질소(NO)의 산화반응속도에 관한 실험 및 모델링 연구

To improve the <TEX>$NO_X$</TEX> conversion over a SCR (selective catalytic reduction) catalyst, the DOC (diesel oxidation catalyst) is usually placed upstream of the SCR catalyst to enhance the fast SCR reaction (<TEX>$4NH_3+2NO+2NO_2{\rightarrow}4N_2+6H_2O$</TEX>) using equimolar amounts of NO and <TEX>$NO_2$</TEX>. Here, a ratio of <TEX>$NO_2/NO_X$</TEX> above 50% should be avoided, because the reaction with <TEX>$NO_2$</TEX> only (<TEX>$4NH_3+4NO+O_2{\rightarrow}4N_2+6H_2O$</TEX>) is slower than the standard SCR reaction (<TEX>$4NH_3+4NO+O_2{\rightarrow}4N_2+6H_2O$</TEX>). In order to accurately predict the performance characteristics of SCR catalysts, it is therefore desired to develop a more simple and reliable mathematical and kinetic models on the oxidation kinetics of nitric oxide over a DOC. In the present work, the prediction accuracy and limit of three different chemical reaction kinetics models are presented to describe the chemicophysical characteristics and conversion performance of DOCs. Steady-state experiments with DOCs mounted on a light-duty four-cylinder 2.0-L turbocharged diesel engine then are performed, using an engine-dynamometer system to calibrate the kinetic parameters such as activation energies and preexponential factors of heterogeneous reactions. The reaction kinetics for NO oxidation over Pt-based catalysts is determined in conjunction with a transient one-dimensional (1D) heterogeneous plug flow reactor (PFR) model with diesel exhaust gas temperatures in the range of 115~<TEX>$525^{\circ}C$</TEX> and space velocities in the range of <TEX>$(0.4{\sim}6.5){\times}10^5\;h^{-1}$</TEX>.

Selective catalytic reduction of NO with NH3 on iron zeolite monolithic catalysts: Steady-state and transient kinetics

An experimental study of steady-state and transient selective catalytic reduction (SCR) of NO with NH3 on both commercial and in-house synthesized Fe-ZSM-5 (MFI type) monolith catalysts is presented. The reaction system was studied in a bench-flow reactor spanning a range of temperature, space velocity and washcoat thickness, affording an assessment of activity and selectivity, reaction pathways, temperature window, mass transport and pore diffusion effects. Experiments included NH3 uptake and temperature-programmed desorption, steady-state and transient NO and NH3 oxidation and standard SCR (NO+NH3+O2) reaction studies. Differential kinetics studies were performed for the standard SCR reaction and NO oxidation reaction systems to determine reaction orders and activation energies. In the temperature range of 200–300°C the standard SCR reaction was found to be nearly first-order and half-order with respect to NO and O2, respectively, but moderately negative-order (−0.3) with respect to NH3. Agreement in the kinetics for both reaction systems suggests that the oxidation of NO is the rate determining step, in line with recent literature studies. Rate inhibition by ammonia appears to be the result of blockage of NO oxidation reaction sites; a necessary step for the standard SCR reaction. Water is shown to inhibit the oxidation of NO significantly and of NH3 to a moderate extent, while having only a negligible effect on the standard SCR reaction. A mechanistic-based kinetic model is proposed in which NH3 reacts with surface bound NO2 and nitrous acid via a NH4NO2 intermediate. A mechanism in which gaseous NO reacts with adsorbed oxygen (Eley Rideal like reaction) cannot be ruled out. In the absence of NH3 these surface species inhibit the adsorption of NO and/or O2 during NO oxidation. Both external mass transfer limitations and washcoat diffusion limitations were ruled out for moderate temperature conditions (≤300°C). However, experiments on monoliths with different washcoat thicknesses conclusively show the appearance of washcoat diffusion limitations at higher temperatures.