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The influence of the zeolite framework structure on the formation of N2O during ammonia-SCR of NOx was studied for three different copper-functionalized zeolite samples, namely Cu-SSZ-13 (CHA), Cu-ZSM-5 (MFI), and Cu-BEA (BEA).

The efficient conversion of CO2 to hydrocarbons offers a way to replace the dependency on fossil fuels and mitigate the accumulation of surplus CO2 in the atmosphere that causes global warming. Therefore, various efforts have been made in recent years to convert CO2 to fuels and value-added chemicals. In this review, the direct and indirect hydrogenation of CO2 to hydrocarbons via methanol as an intermediate is spotlighted. We discuss the most recent approaches in the direct hydrogenation of CO2 into hydrocarbons via the methanol route wherein catalyst design, catalyst performance, and the reaction mechanism of CO2 hydrogenation are discussed in detail. As a comparison, various studies related to CO2 to methanol on transition metals and metal oxide-based catalysts and methanol to hydrocarbons are also provided, and the performance of various zeolite catalysts in H2, CO2, and H2O rich environments is discussed during the conversion of methanol to hydrocarbons. In addition, a detailed analysis of the performance and mechanisms of the CO2 hydrogenation reactions is summarized based on different kinetic modeling studies. The challenges remaining in this field are analyzed and future directions associated with direct synthesis of hydrocarbons from CO2 are outlined.

The effect of phosphorus poisoning on the catalytic behavior of diesel oxidation catalysts was investigated over model and supplier monolith catalysts, i.e., Pd–Pt/Al2O3. The results of ICP and XPS from the vapor-phase poisoning over model catalysts suggested that the temperature of phosphorus poisoning affects both the overall content of phosphorus and the dispersion of phosphorus (i.e., inlet/outlet and surface/bulk). Phosphorus oxide (P2O5), metaphosphate (PO3−), and phosphate (PO43−) were identified in the poisoned model and supplier catalysts. The distribution of these species on poisoned model catalysts was highly dependent on the poisoning temperature, i.e., a higher temperature resulted in a higher concentration of PO43−. The outlets of the monoliths contained more PO43− and less P2O5 than the inlets. Both active sites and surface OH groups on model and supplier catalysts were contaminated upon phosphorus poisoning. It is found that PO43− had a stronger influence on the active sites than P2O5. One significant finding in this study is that the vapor-phase phosphorus poisoning could be a practical and cost efficient approach to simulate an accelerated aging/poisoning process.

The Cu/SAPO-34 catalysts, used for NH3 selective catalytic reduction (SCR), are systemically studied with various characterization techniques before and after low temperature water deactivation and regeneration using techniques such as XRD, BET, ICP-SFMS, 27Al MAS NMR, 29Si MAS NMR, and H2-TPR. Analysis of results suggests that, during low-temperature water deactivation, hydrolysis of Si–O–Al occurs resulting in Si condensation and formation of Si clusters. It is proposed that these formed Si clusters are mainly responsible for the deactivation of Cu/SAPO-34 catalysts since they suppress the mobility of [Cu–(NH3)]+ and hinder the formation of the transient [CuI(NH3)2]+–O2–[CuI(NH3)2]+ intermediate, which is considered to be the rate-limiting step for NH3-SCR reaction. The regeneration of the deactivated Cu/SAPO-34 catalysts with acid can be explained by the ability of the acid to convert the Si clusters back to Si–O–H, which is able to revert to the SAPO-34 framework via reverse hydrolysis as the temperature increases.

The performance failure of Cu/SAPO-34 material used as an NH3-SCR catalyst at low temperature in the presence of water has caused a gradual withdrawal of its usage from the market. There is an urgent need to clearly understand its deactivation mechanism and to find a way to regenerate the Cu/SAPO-34 catalyst material. Interestingly and surprisingly, we have discovered that, under certain conditions, 10% H2O can regenerate a previously deactivated Cu/SAPO-34(MO), as long as its zeolite structure is maintained. By using experimental observations of NH3-SCR reaction, solid state NMR, NO-DRIFTS, and in situ H2-TPR, a mechanism is proposed which can explain both the deactivation and regeneration of Cu/SAPO-34(MO). We propose that the transformation of Si(4OAl) and Si(3OAl) to (2Al)Si(2OH) and (3Al)Si(OH) and the Si condensation in the pores of the framework, which result from H2O exposure, are responsible for the deactivation of Cu/SAPO-34(MO). We suggest that the formation of condensed Si which results in Si clusters, hinders the mobility of the linear complex, [Cu–(NH3)2]+, which is the active species during the low temperature NH3-SCR reaction. Moreover, we propose a regeneration mechanism when Cu/SAPO-34(MO) is exposed to 10% H2O, where the Si clusters are transformed back to Si–O–H bonds, and thereafter transferred back to the framework. This can explain the regeneration of deactivated Cu/SAPO-34(MO).

The low temperature hydrothermal stability of Cu/SAPO-34 catalysts for the NH3-SCR reaction, prepared by three different structure directing agents (SDAs), i.e., morpholine (MO), triethylamine (TEA), and tetraethylammonium hydroxide (TEAOH), was investigated by exposing them to various amounts of water vapor. XRD and BET studies indicate that there was no sign of Cu/SAPO-34 catalyst's chabazite (CHA) structural collapse due to water vapor exposure up to 55 h regardless of SDA choice. However, a multinuclear solid-state magic angle spinning (SS-MAS) NMR study of Cu/SAPO-34(MO, TEA, TEAOH) suggests that the water vapor exposure had significantly altered the coordination environment of Al, P, and Si, the extent of which depends on the choice of SDA along with water vapor exposure time. NO-DRIFTS and H2-TPR studies suggest different mobility for Cu ions between the 6MR and 8MR of the CHA structure in Cu/SAPO-34(MO, TEA, TEAOH) as the result of water vapor exposure and during the NH3-SCR reaction. The mechanisms for low temperature deactivation of Cu/SAPO-34 were proposed as follows: 1) irreversible Si condensation in the support and 2) Cu migration to less accessible sites and/or formation of CuOx clusters depending on Cu mobility.

In the combined lean NOx trap (LNT) and selective catalytic reduction (SCR) concept, the SCR catalyst can be exposed to rich conditions during deSOx of the LNT. Aging of Cu/SSZ-13 SCR catalysts, deposited on a cordierite monolith, was therefore studied in rich, lean and cycling lean/rich operations at 800 °C (lean condition: 500 ppm NO, 8% O2, 10% H2O and 10% CO2; rich condition: 500 ppm NO, 1% H2, 10% H2O and 10% CO2). The structure of the catalyst was investigated by X-ray diffraction (XRD), surface area measurements and scanning transmission electron microscopy (STEM). In general, aging decreased the SCR activity and NH3 oxidation. However, rich conditions showed a very rapid and intense deactivation, while lean aging led to only a small low-temperature activity decrease. The XRD results showed no sign of structure collapse, but the number of active sites, as titrated by NH3 temperature-programed desorption (NH3-TPD) and in situ DRIFTS, revealed an important loss of acid sites. NH3 storage was significantly more depleted after rich aging than after lean aging. The Lewis sites, corresponding to exchange Cu2+, were preserved to some extent in lean conditions. Lean aging also decreased the enthalpy of NH3 adsorption from −158 kJ mol−1 to −136 kJ mol−1. Moreover, a comparison of aging in lean-rich cycling conditions with aging only in rich conditions revealed that adding lean events did not hinder or reverse the deactivation, and it was mainly the time in rich conditions that determined the extent of the deactivation. The STEM images coupled with elemental analysis revealed the formation of large Cu particles during rich aging. Conversely, Cu remained well dispersed after lean aging. These results suggest that the copper migration and agglomeration in large extra-framework particles, accelerated by the action of hydrogen, caused the observed severe deactivation.

Cu/SAPO-34 and Cu/SSZ-13 having chabazite structure (CHA) have attracted significant attention because of their high activity and N2 selectivity during SCR reaction as well as superior resistance to hydrocarbon poisoning. Cu/SAPO-34 has shown better hydrothermal durability than Cu/SSZ-13 at high temperature. However, we have observed earlier that Cu/SAPO-34 prepared using morpholine as a structure directing agent (SDA) deteriorated under water exposure at low temperature, where the NOx conversion activity decreased from 87% to 6% after 9 h of low temperature exposure. In this study, Cu/SAPO-34 catalysts prepared using three different SDAs, i.e., morpholine (MO), triethylamine (TEA), and tetraethylammonium hydroxide (TEAOH), were prepared by the incipient wetness impregnation (IWI) method. A commercially purchased SAPO-34 (SAPO-34(ACS)) was also used for comparison purposes. After low temperature water deactivation, Cu/SAPO-34(TEA) and (TEAOH) mostly recovered their activities while Cu/SAPO-34(MO) and (ACS) only regained part of their activities after regeneration tests under a series of experimental conditions for the NH3-SCR reaction. Solid-state MAS NMR was employed to study the impact of SDAs on the coordination of Al, P, and Si in the SAPO-34 supports and Cu/SAPO-34 catalysts. CO-DRIFTS, NO-DRIFTS, and H2-TPR employed in this study collectively propose the presence of two different Cu locations in Cu/SAPO-34(MO, TEA, TEAOH, and ACS). It is suggested that the concentrations of Cu in two distinct locations within Cu/SAPO-34 catalysts characterized by CO-DRIFTS, NO-DRIFTS, and H2-TPR studies are significantly influenced by the choice of SDA, which will be important for understanding the deactivation mechanism of Cu/SAPO-34 catalysts during low temperature NH3-SCR reaction.

In this study, the effect of oxygen concentration and the presence of water on methane oxidation were examined over a Pd/Al2O3 catalyst. The physicochemical properties of the catalyst were investigated in detail using BET, XRD, STEM, O2-TPO and CH4-TPR. Ramping experiments from 150 to 700 °C were conducted using rich, stoichiometric and lean gas mixtures in the absence and presence of water. It was found that increasing the oxygen concentration in a dry atmosphere resulted in higher methane oxidation activity, which can be connected to the facilitation of palladium oxide formation. The TPO data showed that only minor amounts of PdO up to 700 °C were decomposed; however, in the stoichiometric and rich reaction mixture, PdO was still decomposed because of the oxygen limitation. This fact resulted in a “negative activation” during cooling, with increased activity because of palladium re-oxidation. Moreover, methane steam reforming and water gas shift reactions were important reactions under rich conditions over the metallic palladium sites. A significant inhibiting effect of water on the Pd-catalyst with loss of methane activity was found. Interestingly, the inhibition effect was much greater using high oxygen concentration in the gas mixture (500 ppm CH4, 8% O2, 5% H2O) than that at lower oxygen levels (800–1200 ppm) and we propose that the hydroxyl species formation, which blocks the active sites, are facilitated by a large oxygen excess. In addition, the re-oxidation of palladium occurring during the cooling ramp in dry feed using rich and stoichiometric gas mixtures was also significantly suppressed in the presence of a large amount of water. Thus, water impedes the oxidation of palladium, which significantly deactivates the Pd catalyst.

A deactivation study of Cu-SSZ-13 has been conducted using SO2 exposure under SCR conditions and examining its effect on different reactions involving NH3-SCR. Several reactions, including NH3 storage/TPD, NO/NH3 oxidation, standard SCR, fast SCR and SCR with 75% NO2, as well as NH3–NO2 storage/TPD, were investigated at a temperature range of 100–400 °C after exposing the catalyst to 30 ppm SO2 under SCR conditions at 300 °C for 90 min. The catalyst was characterized using XRD, BET, ICP-SFMS and H2-TPR. The BET surface area and pore volume decreased after the sulfur treatment presumably due to blocking by sulfur and/or ammonium–sulfur species. It was found that sulfur was not uniformly deposited along the monolith channel. The deposition occurred from the inlet towards the outlet, as evident from ICP-SFMS measurements. Part of the sulfur was removed after an SCR experiment up to 400 °C. However, this removal was observed only in the inlet half of the sample and not in the outlet. Ammonia TPD experiments revealed that the sulfur poisoning resulted in additional sites that were capable of adsorbing ammonia, resulting in increased ammonia storage. Moreover, standard SCR was significantly deactivated by SO2 poisoning under SCR conditions. Due to the site-blocking effect of the ammonium–sulfur species, fewer copper sites are likely available for the redox SCR cycle. Furthermore, the effect of sulfur poisoning on NH3 oxidation and NO2-SCR as well as N2O production in various SCR reactions were observed. Finally, it was found that the conditions for the sulfur poisoning were critical in which SO2 deactivation under SCR conditions (NH3 + NO + O2 + H2O) was more severe compared to SO2 poisoning in O2 + H2O alone.