Selective Catalytic Reduction Conditions Research Articles

Comparison of Industrial and Lab-Scale Ion Exchange for the DeNOx-SCR Performance of Cu Chabazites: A Case Study

The efficiency and robustness of selective catalytic reduction (SCR) by NH3 catalysts for exhaust gas purification, especially of heavy-duty diesel engines, will continue to play a major role, despite the increasing electrification of powertrains. With that in mind, the effect of the synthesis scale on commercially available Cu-exchanged chabazite catalysts for SCR was investigated through physicochemical characterizations and catalytic tests. During hydrothermal aging, both industrial and lab-scale prepared catalysts underwent structural dealumination of the zeolite framework and redistribution of the Al sites. Although both catalysts demonstrated similar NO conversion activity under SCR conditions, the lab-scale catalyst showed higher selectivity and lower activity in NH3 oxidation. Variations in N2O formation and NH3 oxidation rate were found to correlate with the formation of different copper species, and the compositions become less controllable in industrial-scale process. This case study focused on routes of ion exchange, and the results provide new insights into catalytic performance of the industrially-produced zeolites.

Operando diffuse reflectance infrared detection of cyanide intermediate species during the reaction of formaldehyde with ammonia over V2O5/WO3-TiO2

Hydrogen cyanide (HCN) is a highly poisonous gas that can form through the reaction between formaldehyde and ammonia (NH3) in the exhaust. While this detrimental side reaction has been the subject of recent catalytic studies under selective catalytic reduction conditions, the molecular mechanism leading to the release of HCN remains far from elucidated. Here, we report the detection of the cyanide intermediate species by operando diffuse reflectance infrared spectroscopy, thereby providing unambiguous evidence that the HCN-forming reaction occurs on the catalyst surface. Formaldehyde likely reacts from the gas phase with pre-adsorbed NH3 to form HCN. Although formate species were formed abundantly upon formaldehyde introduction, they were not responsible for the high level of HCN emissions observed. TiO2 alone is sufficient to catalyze the side reaction, but VOx and/or WOx sites increase HCN production by preventing the formation of inactive polyamide species. These mechanistic insights should serve as a basis for understanding the chemistry responsible for the side reaction so that mitigation measures can be put forward.

HC-SCR system combining Ag/Al2O3 and Pd/Al2O3 catalysts with resistance to hydrothermal aging for simultaneous removal of NO, HC, and CO

A combined Ag/Al2O3 and Pd/Al2O3 catalyst was investigated for the selective catalytic reduction (SCR) of NOx by H2-assisted C3H6 in the presence of excessive O2. The coexistent H2 significantly lowered the active temperature window of the de-NOx reaction because of the enhanced partial oxidation of C3H6 and NO oxidation to NO2. A low Ag loading of 1 wt% exhibited better de-NOx performance than that attained using 2 wt% Ag. Although Ag/Al2O3 exhibited a high resistance to hydrothermal aging, a high reaction temperature (>500 °C) was required to oxidize CO and C3H6 completely. In contrast to Ag/Al2O3, Pd/Al2O3 suppressed C3H6 and CO slip at relatively low temperatures, and a complete oxidation of C3H6 and CO was achieved at 195–275 °C and 240–275 °C, respectively, under varying SCR conditions and Pd contents (0.5–1.5 wt%); however, 30% de-NOx performance in a narrow temperature window of 210–240 °C was attained. The best configuration was found to be a sequential arrangement of 1Ag/Al2O3 followed by 1Pd/Al2O3, which enabled the reduction of NOx, C3H6, CO, and H2 over wide reaction temperatures, unlike the stand-alone configuration. In this configuration, the reacted NOx was reduced to N2 with negligible formation of by-products, even after hydrothermal aging.

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>

Reduction of oxidized mercury over NOx selective catalytic reduction catalysts: A review

• The Hg 2+ reduction process over SCR catalysts were reviewed for the first time. • The effects of typical flue gases on Hg 2+ reduction were presented. • The involved reaction mechanisms for Hg 2+ reduction were discussed in details. • Suggestions to alleviate the reduction of Hg 2+ over SCR catalysts were proposed. The control of mercury emission from flue gas has long been a serious task for the global environment and human health. Utilization of selective catalytic reduction (SCR) catalysts to oxidize Hg 0 to form Hg 2+ and subsequent capture the Hg 2+ through a wet flue gas desulfurization (WFGD) scrubber is an efficient and economical technology for Hg 0 removal in coal-fired power plants. Extensive studies have been conducted with regard to Hg 0 oxidation over SCR catalysts. However, recent studies found that the reduction of Hg 2+ , an inverse process to Hg 0 oxidation, also occurred even predominated under some SCR conditions. This new observation overturns the general knowledge that SCR catalysts always enhance Hg 0 oxidation. To avoid the neglect of Hg 2+ reduction and overcome the shortages of existing studies, a critical review of Hg 2+ reduction over SCR catalysts was undertaken. The Hg 2+ reduction behaviors under various SCR conditions were summarized and the effects of typical flue gas components like hydrogen chloride (HCl), sulfur dioxide (SO 2 ), water vapor (H 2 O), volatile organic compounds (VOCs), and carbon monoxide (CO) on Hg 2+ reduction were also presented. The involved reaction mechanisms for Hg 2+ reduction were discussed in details. Moreover, feasible measures to suppress the reduction of Hg 2+ in the SCR system were proposed. We expect to clearly explore the mechanism of the Hg 0 redox process on SCR catalysts in this review to avoid the neglect of Hg 2+ reduction, hence providing guidance for the design of SCR catalysts to achieve an efficient Hg 0 oxidation.

Effects of insulation on exhaust temperature and subsequent SCR efficiency of a heavy-duty diesel engine

Ammonia/urea selective catalytic reduction (SCR) is an efficient technology to control NOx emission from diesel engines. One of the critical challenges of SCR systems is its lack of catalytic activities in low temperature conditions such as cold start or warm-up periods. In this study, the effect of pipe insulation on the reduction of exhaust temperature drop was experimentally investigated based on dynamometer tests with a heavy-duty non-road diesel engine. The temperature drop of exhaust gas was measured with and without applying pipe insulation. Measurement results revealed that the effect of insulation on the mitigation of temperature loss becomes significant as engine brake power decreases. Thus, it is expected that the need for pipe insulation would be increased to keep SCR deNOx efficiency high, especially at low temperature conditions. Also, this study carried out numerical simulations to quantify the impact of pipe insulation on NOx conversion efficiency of a commercial SCR catalyst over a scheduled non-road transient cycle (NRTC) mode. Under the current heavy-duty non-road engine and SCR conditions, the application of pipe insulation would reduce 19.5 % of total cumulative tail-out NOx emission over NRTC hot mode. These results led to the cycle-averaged SCR efficiency of 98.2 % for ‘with insulation’, while it was predicted to be lowered to 97.6 % for ‘without insulation’.

Modeling Ammonia and Water Co-Adsorption in CuI-SSZ-13 Zeolite Using DFT Calculations

Cu-SSZ-13 efficiently catalyzes the selective catalytic reduction (SCR) of NO by NH3 but the structure of the active site and, particularly, the redox state of the copper (+I or +II) is still debated. This paper focuses on the possible contribution of CuI species using quantum chemistry of adsorption and co-adsorption of NH3 and H2O on CuI species. The calculations show that CuI clearly migrates upon adsorption of NH3 and H2O. All the CuI complexes sit in the cage containing the 8 MR and interact with the zeolite framework through several H-bonds. In the experimental temperature and pressure domain of SCR conditions, calculated phase diagrams show that a coordination number of two is predicted for the co-adsorption of NH3 and H2O on CuI. Finally, the calculated phase diagrams of CuI-SSZ-13 are discussed together with those of CuII-SSZ-13 and recent experimental characterizations, providing a wider picture of the real catalyst in SCR conditions.

Low-temperature activity and mechanism of WO3-modified CeO2-TiO2 catalyst under NH3-NO/NO2 SCR conditions

The CeO2-TiO2 (CeTi) and CeO2/WO3-TiO2 (CeWTi) catalysts were prepared by a sol-gel precipitation method and their NH3-NO/NO2 selective catalytic reduction (SCR) performance was studied. N2O formation and effect of oxygen concentration on SCR performance over CeWTi catalyst were also investigated while varying the NO2/NOx ratio. Results indicate that fast SCR behavior of CeWTi catalyst has the best NH3-NO/NO2 SCR performance due to the catalyst reoxidation rate by NO2 higher than by O2. Compared with CeTi catalyst, CeWTi catalyst exhibits higher de-NOx performance under NH3-NO/NO2 SCR conditions. As the CeTi and CeWTi catalysts exhibit similar redox property, addition of WO3 provides more acid sites which accelerate the reaction between NH4NO3 and NO to get a superior low-temperature activity. Amount of N2O formation shows a peak at 250 °C mainly derived from NH4NO3 decomposition.

In situ, multiparameter optical sensor for monitoring the selective catalytic reduction process of diesel engines

Selective catalytic reduction (SCR) is an effective technique to reduce nitrogen oxide (NOX) emission in high-temperature combustion processes. SCR requires strict reaction conditions, including proper temperature and ammonia (NH3) injection volume. To satisfy SCR conditions, we present a low-cost, in situ optical sensor for multiparameter measurement during SCR processes. The sensor simultaneously detected NH3 concentration, water (H2O) concentration, temperature, and pressure by using a single near-infrared diode laser. To ensure the high-quality multiparameter measurement, we realized two novel ideas: (1) wavelength modulation spectroscopy (WMS) with varied modulation amplitude and (2) an optimized multispectral fitting algorithm. We demonstrated the method by theory analysis, model simulation, laboratory verification, and field-testing with a diesel engine. The relative measurement uncertainty of NH3 concentration, H2O concentration, temperature, and pressure were 8.4%, 8.0%, 5.3% and 7.0%, respectively. The results of the field test were consistent with those obtained by thermocouples and electrochemical sensors. The presented sensor is quite suitable for SCR processes and other harsh-environment applications because of its low cost and robustness. The proposed multispectral detection method also can be used for other WMS applications.

A skeletal reaction scheme for selective catalytic reduction of NOx with NH3 over CeO2/TiO2 catalyst

CeO2/TiO2 has been regarded as the promising catalyst used for the selective catalytic reduction (SCR) of NOx with NH3. The complete NH3-SCR reaction mechanism of NO over CeO2/TiO2 catalyst was systemically investigated using density functional theory (DFT) calculations. Based on the DFT-derived energetics, a skeletal reaction scheme including 20 elementary reaction steps was proposed for NH3-SCR reaction of NO over CeO2/TiO2 catalyst. The results indicate that NH3, NO, O2, and NO2 adsorption on CeO2/TiO2(110) surface is dominated by the chemisorption mechanism. The successive dehydrogenation process of NH3 is cleaved by NH2 dehydrogenation step. NH2 derived from NH3 dehydrogenation step can stably exist on CeO2/TiO2 catalyst surface and serve as a key intermediate in NH3-SCR reaction. The activation energy barrier of N2O formation is higher than that of N2 formation, which is responsible for the excellent N2 selectivity of CeO2/TiO2 catalyst. NO2 formation under the typical low-temperature SCR conditions can make a contribution to NO reduction over CeO2/TiO2 catalyst. Reaction pathway analysis illustrates that the dominant reaction pathway of NO reduction by NH3 over CeO2/TiO2 catalyst is a three-step process: (1) NH2 formation from NH3 dehydrogenation, (2) N-N coupling reaction, and (3) NH2NO decomposition. During NO reduction, NH2 formation from NH3 dehydrogenation is the rate-determining step of N2 formation.

Surface Species and Metal Oxidation State during H2-Assisted NH3-SCR of NOx over Alumina-Supported Silver and Indium

Alumina-supported silver and indium catalysts are investigated for the hydrogen-assisted selective catalytic reduction (SCR) of NOx with ammonia. Particularly, we focus on the active phase of the catalyst and the formation of surface species, as a function of the gas environment. Diffuse reflectance ultraviolet-visible (UV-vis) spectroscopy was used to follow the oxidation state of the silver and indium phases, and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to elucidate the formation of surface species during SCR conditions. In addition, the NOx reduction efficiency of the materials was evaluated using H2-assisted NH3-SCR. The DRIFTS results show that the Ag/Al2O3 sample forms NO-containing surface species during SCR conditions to a higher extent compared to the In/Al2O3 sample. The silver sample also appears to be more reduced by H2 than the indium sample, as revealed by UV-vis spectroscopic experiments. Addition of H2, however, may promote the formation of highly dispersed In2O3 clusters, which previously have been suggested to be important for the SCR reaction. The affinity to adsorb NH3 is confirmed by both temperature programmed desorption (NH3-TPD) and in situ DRIFTS to be higher for the In/Al2O3 sample compared to Ag/Al2O3. The strong adsorption of NH3 may inhibit (self-poison) the NH3 activation, thereby hindering further reaction over this catalyst, which is also shown by the lower SCR activity compared to Ag/Al2O3.

Improvement of Nb Doping on SO2 Resistance of VOx/CeO2 Catalyst for the Selective Catalytic Reduction of NOx with NH3

The influence of sulfation treatment on Nb–VOx/CeO2 and VOx/CeO2 catalysts for the selective catalytic reduction (SCR) of NOx with NH3 was fully investigated. The Nb–VOx/CeO2 catalyst showed higher catalytic activity and stronger resistance to SO2 than VOx/CeO2. The formation of sulfates, small specific surface area, and reduction in the number of active sites were all responsible for the low catalytic activity over VOx/CeO2 after sulfation under SCR conditions. On the contrary, Nb–VOx/CeO2 adsorbed much more nitrate than sulfate when sulfated under SCR conditions and showed much higher NH3-SCR activity than VOx/CeO2 after the same treatment. After sulfation by SO2 + O2 only, instead of sulfation under SCR conditions, both of the samples exhibited decreased NH3-SCR activity, mainly due to the formation of sulfates and the blockage of the Langmuir–Hinshelwood reaction pathway.

Improvement in activity and alkali resistance of a novel V-Ce(SO4)2/Ti catalyst for selective catalytic reduction of NO with NH3

A series of V-Ce(SO4)2/Ti catalysts for selective catalytic reduction (SCR) of NO with ammonia are prepared by impregnation method. Low temperature SCR activity and alkali resistance of the optimal V-0.5Ce(SO4)2/Ti sample are found to be better than on the commercial V-W/Ti catalyst. Also, V-0.5Ce(SO4)2/Ti shows an excellent durability in the presence of SO2 and H2O, indicating to have prospects for the industrial application.Based on catalysts characterization and in-situ DRIFTS studies, a higher proportion of surface active oxygen generated by the introduction of Ce and a much faster H2 reduction point out the improved redox properties of V-0.5Ce(SO4)2/Ti, which results in a stronger NO oxidative activation and is confirmed by a more abundant formation of surface NO+ and NO3− species. When exposed to SCR conditions where both NH3 and NO are present, this enhanced NO activation can produce more reactive nitrite and/or NO+ intermediates which then readily react with adsorbed NH3 and decompose to N2 and H2O, accounting for the improved SCR activity of V-0.5Ce(SO4)2/Ti at low temperatures.The addition of Ce(SO4)2 also provides abundant reactive acid sites and adsorbed NH3 species thus increase. Even after Na poisoning, adequate surface acidity and redox properties still remain. Furthermore, relatively higher contents of V-OH are preserved owing to the interaction between Na and OSO, acting as a protection for the active sites. These promotional effects contribute to the better alkali resistance of V-0.5Ce(SO4)2/Ti. Therefore, all the results suggest that V-0.5Ce(SO4)2/Ti is a promising candidate as a catalyst for NH3-SCR in coal-fired power plants, especially under high Na-content conditions.

Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites.

The relationships among the macroscopic compositional parameters of a Cu-exchanged SSZ-13 zeolite catalyst, the types and numbers of Cu active sites, and activity for the selective catalytic reduction (SCR) of NOx with NH3 are established through experimental interrogation and computational analysis of materials across the catalyst composition space. Density functional theory, stochastic models, and experimental characterizations demonstrate that within the synthesis protocols applied here and across Si:Al ratios, the volumetric density of six-membered-rings (6MR) containing two Al (2Al sites) is consistent with a random Al siting in the SSZ-13 lattice subject to Löwenstein's rule. Further, exchanged Cu(II) ions first populate these 2Al sites before populating remaining unpaired, or 1Al, sites as Cu(II)OH. These sites are distinguished and enumerated ex situ through vibrational and X-ray absorption spectroscopies (XAS) and chemical titrations. In situ and operando XAS follow Cu oxidation state and coordination environment as a function of environmental conditions including low-temperature (473 K) SCR catalysis and are rationalized through first-principles thermodynamics and ab initio molecular dynamics. Experiment and theory together reveal that the Cu sites respond sensitively to exposure conditions, and in particular that Cu species are solvated and mobilized by NH3 under SCR conditions. While Cu sites are spectroscopically and chemically distinct away from these conditions, they exhibit similar turnover rates, apparent activation energies and apparent reaction orders at the SCR conditions, even on zeolite frameworks other than SSZ13.

Ab initio X-ray absorption modeling of Cu-SAPO-34: Characterization of Cu exchange sites under different conditions

Copper-exchanged SAPO-34 (Cu-SAPO-34) provides excellent catalytic activity and hydrothermal stability in the selective catalytic reduction (SCR) of NOx by using NH3 as a reductant. We find that the 6-membered ring (6MR) site is the most energetically favorable for a Cu+ ion while the 8-membered ring (8MR) sites are less favorable by about 0.5eV with respect to the 6MR site in Cu-SAPO-34. Upon adsorption of molecular species (H2O, O, OH, O2), the energy differences between Cu in the 8MR and 6MR sites decreases and almost disappears. Further, a thermodynamic phase diagram study shows that a Cu+ ion bound to a single H2O molecule is the most stable species at low oxygen potential values while a Cu2+ ion bound to 2 OH species is more stable when the oxygen chemical potential is sufficiently high. By comparing Cu K-edge XANES between Cu-SSZ-13 and Cu-SAPO-34 with Cu in different oxidation states, we conclude that it is difficult to differentiate the simulated XANES of Cu in these structures at a given oxidation state. By studying the Cu K-edge XANES of several favorable structures in Cu-SAPO-34 in the presence of adspecies, the simulated XANES results capture the real trend of the edge shift with oxidation state and gives new insights into the experimentally determined XANES of Cu-SAPO-34 obtained under standard SCR conditions.

Sulfated Temperature Effects on the Catalytic Activity of CeO2 in NH3-Selective Catalytic Reduction Conditions

In this work, the influence of sulfated temperature on the selective catalytic reduction of NO by NH3 over the sulfated CeO2 was investigated. The NO conversion of the sulfated CeO2 samples decreased with the increase of sulfated temperature. The fresh CeO2 and the CeO2 sulfated at different temperatures were characterized by X-ray diffraction (XRD), laser Raman spectroscopy (LRS), in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), thermogravimetry and differential thermal analysis (TG-DTA), H2-temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS). The obtained results indicate that the sulfation process was gradually deepened, and the existence states of sulfate species over CeO2 were changed from surface sulfates to bulk-like sulfates and to bulk sulfates with raising the sulfated temperature. Meanwhile, the formed bulk-like and bulk sulfates were the main reason to result in the decrease of catalytic activity. In addition, the correspon...

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.

Isolation of the copper redox steps in the standard selective catalytic reduction on Cu-SSZ-13.

Operando X-ray absorption experiments and density functional theory (DFT) calculations are reported that elucidate the role of copper redox chemistry in the selective catalytic reduction (SCR) of NO over Cu-exchanged SSZ-13. Catalysts prepared to contain only isolated, exchanged Cu(II) ions evidence both Cu(II) and Cu(I) ions under standard SCR conditions at 473 K. Reactant cutoff experiments show that NO and NH3 together are necessary for Cu(II) reduction to Cu(I). DFT calculations show that NO-assisted NH3 dissociation is both energetically favorable and accounts for the observed Cu(II) reduction. The calculations predict in situ generation of Brønsted sites proximal to Cu(I) upon reduction, which we quantify in separate titration experiments. Both NO and O2 are necessary for oxidation of Cu(I) to Cu(II), which DFT suggests to occur by a NO2 intermediate. Reaction of Cu-bound NO2 with proximal NH4(+) completes the catalytic cycle. N2 is produced in both reduction and oxidation half-cycles.

Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu‐SSZ‐13

AbstractOperando X‐ray absorption experiments and density functional theory (DFT) calculations are reported that elucidate the role of copper redox chemistry in the selective catalytic reduction (SCR) of NO over Cu‐exchanged SSZ‐13. Catalysts prepared to contain only isolated, exchanged CuII ions evidence both CuII and CuI ions under standard SCR conditions at 473 K. Reactant cutoff experiments show that NO and NH3 together are necessary for CuII reduction to CuI. DFT calculations show that NO‐assisted NH3 dissociation is both energetically favorable and accounts for the observed CuII reduction. The calculations predict in situ generation of Brønsted sites proximal to CuI upon reduction, which we quantify in separate titration experiments. Both NO and O2 are necessary for oxidation of CuI to CuII, which DFT suggests to occur by a NO2 intermediate. Reaction of Cu‐bound NO2 with proximal NH4+ completes the catalytic cycle. N2 is produced in both reduction and oxidation half‐cycles.

Methods for NH3 titration of Brønsted acid sites in Cu-zeolites that catalyze the selective catalytic reduction of NOx with NH3

Three methods were developed to use ammonia as a selective titrant of residual Brønsted acid sites in partially Cu-exchanged MFI (ZSM-5) and CHA (SSZ-13) zeolites that mediate the selective catalytic reduction (SCR) of NO with NH3. These methods involved saturation with gas-phase NH3 at 433K followed by purging in flowing helium, saturation with gas-phase NH3 at 323K followed by washing with deionized water, or saturation with aqueous NH4NO3 solution (353K) followed by washing with deionized water. H-ZSM-5 (Si:Altot=17–89) and Cu-ZSM-5 (Cu:Altot=0–0.27 at Si:Altot=17) samples saturated with NH3 using these three methods, and with n-propylamine titrants that react selectively with Brønsted acid sites to form C3H6 and NH3, evolved identical amounts of NH3 in temperature-programmed desorption experiments. The consistency among these four methods indicates that NH3 can be used as a selective titrant of H+ sites in the presence of Lewis acid sites on Cu-zeolites. The number of H+ sites on MFI and CHA zeolites measured by direct chemical titration was often less than the number of framework Al (Alf) atoms determined from 27Al MAS NMR spectra (H+:Alf=0.7–1.0), yet another reminder that Alf structures can be imprecise surrogates for active H+ sites on zeolites. Although n-propylamine (at 323K) titrated all H+ sites in Cu-ZSM-5, it titrated in Cu-SSZ-13 (Cu:Altot=0–0.20 at Si:Altot=4.5) only a small fraction (0–0.26) of the H+ sites that were accessible to NH3, which is the reductant in NO SCR reactions with NH3 (standard SCR). Standard SCR turnover rates on Cu-SSZ-13 samples, after normalization by the number of active Cu(II) sites, showed an apparent zero-order dependence on the number of residual H+ sites measured by NH3 titration. These data suggest that reactive NH4+ intermediates are present in excess, relative to metal-bound NOx species, under the SCR conditions studied. The methods reported herein, which titrate Brønsted acid sites selectively in the presence of Lewis acid sites, are useful in investigating the mechanistic role of H+ sites in standard SCR on small-pore Cu-zeolites and in other reactions catalyzed by metal-exchanged zeolites.