Editorial

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Open AccessEditorial

by Jae‐Soon Choi and Petr Kočí

Catalysts 2016, 6(10), 155; https://doi.org/10.3390/catal6100155 - 06 Oct 2016

Cited by 6 | Viewed by 5204

Emission control catalysts constitute an essential part of today’s vehicles powered by internal combustion engines, mitigating the harmful effects of pollutants in the exhaust such as carbon monoxide, hydrocarbons, nitrogen oxides, and particulate matter [1].[...] Full article

(This article belongs to the Special Issue Automotive Emission Control Catalysts)

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Adsorption and Oxidation Investigations over Pt/Al₂O₃ Catalyst: A Microcalorimetric Study

by Oana Mihai, Derek Creaser and Louise Olsson

Catalysts 2016, 6(5), 73; https://doi.org/10.3390/catal6050073 - 17 May 2016

Cited by 7 | Viewed by 5231

Abstract

The differential adsorption heats of oxygen and NO, as well as catalytic oxidation behavior during NO oxidation and NO₂ dissociation reactions over supported Pt-catalysts, were investigated by microcalorimetric measurements. The average heat of adsorption (∆H) of oxygen ranged from 310 kJ/mol at [...] Read more.

The differential adsorption heats of oxygen and NO, as well as catalytic oxidation behavior during NO oxidation and NO₂ dissociation reactions over supported Pt-catalysts, were investigated by microcalorimetric measurements. The average heat of adsorption (∆H) of oxygen ranged from 310 kJ/mol at 200 °C to 289 kJ/mol at 400 °C. Over this temperature range formation of platinum oxides and coverage dependence caused variations in the apparent heat of adsorption. NO heat of adsorption from 50 to 150 °C was near constant with an average value of 202 kJ/mol over the temperature range. Full article

(This article belongs to the Special Issue Automotive Emission Control Catalysts)

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Impact of Lubricant Additives on thePhysicochemical Properties and Activity of Three‐Way Catalysts

by Chao Xie, Todd J. Toops, Michael J. Lance, Jun Qu, Michael B. Viola, Samuel A. Lewis, Donovan N. Leonard and Edward W. Hagaman

Catalysts 2016, 6(4), 54; https://doi.org/10.3390/catal6040054 - 04 Apr 2016

Cited by 22 | Viewed by 6010

Abstract

As alternative lubricant anti‐wear additives are sought to reduce friction and improve overall fuel economy, it is important that these additives are also compatible with current emissions control catalysts. In the present work, an oil‐miscible phosphorous‐containing ionic liquid (IL), trihexyltetradecylphosphonium bis(2‐ethylhexyl) phosphate ([P [...] Read more.

As alternative lubricant anti‐wear additives are sought to reduce friction and improve overall fuel economy, it is important that these additives are also compatible with current emissions control catalysts. In the present work, an oil‐miscible phosphorous‐containing ionic liquid (IL), trihexyltetradecylphosphonium bis(2‐ethylhexyl) phosphate ([P₆₆₆₁₄][DEHP]), is evaluated for its impact on three‐way catalysts (TWC) and benchmarked against the industry standard zinc‐dialkyl‐dithio‐phosphate (ZDDP). The TWCs are aged in different scenarios: neat gasoline (no‐additive, or NA), gasoline+ZDDP, and gasoline+IL. The aged samples, along with the as‐received TWC, are characterized through various analytical techniques including catalyst reactivity evaluation in a bench‐flow reactor. The temperatures of 50% conversion (T50) for the ZDDP‐aged TWCs increased by 30, 24, and 25 °C for NO, CO, and C3H6, respectively, compared to the no‐additive case. Although the IL‐aged TWC also increased in T50 for CO and C3H6, it was notably less than ZDDP, 7 and 9 °C, respectively. Additionally, the IL‐aged samples had higher water‐gas‐shift reactivity and oxygen storage capacity than the ZDDP‐aged TWC. Characterization of the aged samples indicated the predominant presence of CePO₄ in the ZDDP‐aged TWC aged by ZDDP, while its formation was retarded in the case of IL where higher levels of AlPO₄ is observed. Thus, results in this work indicate that the phosphonium‐phosphate IL potentially has less adverse impact on TWC than ZDDP. Full article

(This article belongs to the Special Issue Automotive Emission Control Catalysts)

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Mechanistic Investigation of the Reduction of NO_x over Pt- and Rh-Based LNT Catalysts

by Lukasz Kubiak, Lidia Castoldi, Luca Lietti, Stanislava Andonova and Louise Olsson

Catalysts 2016, 6(3), 46; https://doi.org/10.3390/catal6030046 - 15 Mar 2016

Cited by 14 | Viewed by 4853

Abstract

The influence of the noble metals (Pt vs. Rh) on the NO_x storage reduction performances of lean NO_x trap catalysts is here investigated by transient micro-reactor flow experiments. The study indicates a different behavior during the storage in that the Rh-based [...] Read more.

The influence of the noble metals (Pt vs. Rh) on the NO_x storage reduction performances of lean NO_x trap catalysts is here investigated by transient micro-reactor flow experiments. The study indicates a different behavior during the storage in that the Rh-based catalyst showed higher storage capacity at high temperature as compared to the Pt-containing sample, while the opposite is seen at low temperatures. It is suggested that the higher storage capacity of the Rh-containing sample at high temperature is related to the higher dispersion of Rh as compared to Pt, while the lower storage capacity of Rh-Ba/Al₂O₃ at low temperature is related to its poor oxidizing properties. The noble metals also affect the catalyst behavior upon reduction of the stored NO_x, by decreasing the threshold temperature for the reduction of the stored NO_x. The Pt-based catalyst promotes the reduction of the adsorbed NO_x at lower temperatures if compared to the Rh-containing sample, due to its superior reducibility. However, Rh-based material shows higher reactivity in the NH₃ decomposition significantly enhancing N₂ selectivity. Moreover, formation of small amounts of N₂O is observed on both Pt- and Rh-based catalyst samples only during the reduction of highly reactive NO_x stored at 150 °C, where NO_x is likely in the form of nitrites. Full article

(This article belongs to the Special Issue Automotive Emission Control Catalysts)

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On the Effect of Preparation Methods of PdCe-MOR Catalysts as NO_x CH₄-SCR System for Natural Gas Vehicles Application

by Acácio Nobre Mendes, Maria Filipa Ribeiro, Carlos Henriques and Patrick Da Costa

Catalysts 2015, 5(4), 1815-1830; https://doi.org/10.3390/catal5041815 - 27 Oct 2015

Cited by 9 | Viewed by 5570

Abstract

In the present work, the effect of several parameters involved in the preparation of PdCe-HMOR catalysts active for NO_x selective catalytic reduction with methane (NO_x CH₄-SCR) was studied. Results show that the catalytic performance of Pd-HMOR is better when [...] Read more.

In the present work, the effect of several parameters involved in the preparation of PdCe-HMOR catalysts active for NO_x selective catalytic reduction with methane (NO_x CH₄-SCR) was studied. Results show that the catalytic performance of Pd-HMOR is better when palladium is introduced by ion-exchange, namely at room temperature. It was also shown that Pd loading does not influence the formation of cerium species, namely surface Ce⁴⁺ (CeO₂) species and CeO₂ species in interaction with Pd. However, when Ce is introduced before Pd, more surface CeO₂ species are stabilized in the support and less CeO₂ become in interaction with Pd, which results in a worse NO_x CH₄-SCR catalytic performance. Full article

(This article belongs to the Special Issue Automotive Emission Control Catalysts)

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Part II: Oxidative Thermal Aging of Pd/Al₂O₃ and Pd/Ce_xO_y-ZrO₂ in Automotive Three Way Catalysts: The Effects of Fuel Shutoff and Attempted Fuel Rich Regeneration

by Qinghe Zheng, Robert Farrauto and Michel Deeba

Catalysts 2015, 5(4), 1797-1814; https://doi.org/10.3390/catal5041797 - 23 Oct 2015

Cited by 44 | Viewed by 8605

Abstract

The Pd component in the automotive three way catalyst (TWC) experiences deactivation during fuel shutoff, a process employed by automobile companies for enhancing fuel economy when the vehicle is coasting downhill. The process exposes the TWC to a severe oxidative aging environment with [...] Read more.

The Pd component in the automotive three way catalyst (TWC) experiences deactivation during fuel shutoff, a process employed by automobile companies for enhancing fuel economy when the vehicle is coasting downhill. The process exposes the TWC to a severe oxidative aging environment with the flow of hot (800 °C–1050 °C) air. Simulated fuel shutoff aging at 1050 °C leads to Pd metal sintering, the main cause of irreversible deactivation of 3% Pd/Al₂O₃ and 3% Pd/Ce_xO_y-ZrO₂ (CZO) as model catalysts. The effect on the Rh component was presented in our companion paper Part I. Moderate support sintering and Pd-Ce_xO_y interactions were also experienced upon aging, but had a minimal effect on the catalyst activity losses. Cooling in air, following aging, was not able to reverse the metallic Pd sintering by re-dispersing to PdO. Unlike the aged Rh-TWCs (Part I), reduction via in situ steam reforming (SR) of exhaust HCs was not effective in reversing the deactivation of aged Pd/Al₂O₃, but did show a slight recovery of the Pd activity when CZO was the carrier. The Pd⁺/Pd⁰ and Ce³⁺/Ce⁴⁺ couples in Pd/CZO are reported to promote the catalytic SR by improving the redox efficiency during the regeneration, while no such promoting effect was observed for Pd/Al₂O₃. A suggestion is made for improving the catalyst performance. Full article

(This article belongs to the Special Issue Automotive Emission Control Catalysts)

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Part I: A Comparative Thermal Aging Study on the Regenerability of Rh/Al₂O₃ and Rh/Ce_xO_y-ZrO₂ as Model Catalysts for Automotive Three Way Catalysts

by Qinghe Zheng, Robert Farrauto, Michel Deeba and Ioannis Valsamakis

Catalysts 2015, 5(4), 1770-1796; https://doi.org/10.3390/catal5041770 - 23 Oct 2015

Cited by 43 | Viewed by 10753

Abstract

The rhodium (Rh) component in automotive three way catalysts (TWC) experiences severe thermal deactivation during fuel shutoff, an engine mode (e.g., at downhill coasting) used for enhancing fuel economy. In a subsequent switch to a slightly fuel rich condition, in situ catalyst regeneration [...] Read more.

The rhodium (Rh) component in automotive three way catalysts (TWC) experiences severe thermal deactivation during fuel shutoff, an engine mode (e.g., at downhill coasting) used for enhancing fuel economy. In a subsequent switch to a slightly fuel rich condition, in situ catalyst regeneration is accomplished by reduction with H₂ generated through steam reforming catalyzed by Rh⁰ sites. The present work reports the effects of the two processes on the activity and properties of 0.5% Rh/Al₂O₃ and 0.5% Rh/Ce_xO_y-ZrO₂ (CZO) as model catalysts for Rh-TWC. A very brief introduction of three way catalysts and system considerations is also given. During simulated fuel shutoff, catalyst deactivation is accelerated with increasing aging temperature from 800 °C to 1050 °C. Rh on a CZO support experiences less deactivation and faster regeneration than Rh on Al₂O₃. Catalyst characterization techniques including BET surface area, CO chemisorption, TPR, and XPS measurements were applied to examine the roles of metal-support interactions in each catalyst system. For Rh/Al₂O₃, strong metal-support interactions with the formation of stable rhodium aluminate (Rh(AlO₂)_y) complex dominates in fuel shutoff, leading to more difficult catalyst regeneration. For Rh/CZO, Rh sites were partially oxidized to Rh₂O₃ and were relatively easy to be reduced to active Rh⁰ during regeneration. Full article

(This article belongs to the Special Issue Automotive Emission Control Catalysts)

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385 KiB

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VO_x Surface Coverage Optimization of V₂O₅/WO₃-TiO₂ SCR Catalysts by Variation of the V Loading and by Aging

by Adrian Marberger, Martin Elsener, Davide Ferri and Oliver Kröcher

Catalysts 2015, 5(4), 1704-1720; https://doi.org/10.3390/catal5041704 - 14 Oct 2015

Cited by 82 | Viewed by 8625

Abstract

V₂O₅/WO₃-TiO₂ selective catalytic reduction (SCR) catalysts with a V₂O₅ loading of 1.7, 2.0, 2.3, 2.6, 2.9, 3.2 and 3.5 wt. % were investigated in the fresh state and after hydrothermal aging at 600 [...] Read more.

V₂O₅/WO₃-TiO₂ selective catalytic reduction (SCR) catalysts with a V₂O₅ loading of 1.7, 2.0, 2.3, 2.6, 2.9, 3.2 and 3.5 wt. % were investigated in the fresh state and after hydrothermal aging at 600 °C for 16 h. The catalysts were characterized by means of nitrogen physisorption, X-ray diffraction and X-ray absorption spectroscopy. In the fresh state, the SCR activity increased with increasing V loading. Upon aging, the catalysts with up to 2.3 wt. % V₂O₅exhibited higher NO_x reduction activity than in the fresh state, while the catalysts with more than 2.6 wt. % V₂O₅ showed increasing deactivation tendencies. The observed activation and deactivation were correlated with the change of the VO_x and WO_x surface coverages. Only catalysts with a VO_x coverage below 50% in the aged state did not show deactivation tendencies. With respect to tungsten, above one monolayer of WO_x, WO₃ particles were formed leading to loss of surface acidity, sintering, catalyst deactivation and early NH₃ slip. An optimal compromise between activity and hydrothermal aging resistance could be obtained only with V₂O₅ between 2.0 and 2.6 wt. %. Full article

(This article belongs to the Special Issue Automotive Emission Control Catalysts)

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Use of a µ-Scale Synthetic Gas Bench for Direct Comparison of Urea-SCR and NH₃-SCR Reactions over an Oxide Based Powdered Catalyst

by Mickael Seneque, Fabien Can, Daniel Duprez and Xavier Courtois

Catalysts 2015, 5(3), 1535-1553; https://doi.org/10.3390/catal5031535 - 04 Sep 2015

Cited by 10 | Viewed by 6635

Abstract

The selective catalytic reduction (SCR) of NO_x by NH₃ has been extensively studied in the literature, mainly because of its high potential to remediate the pollution of diesel exhaust gases. The implementation of the NH₃-SCR process into passenger cars [...] Read more.

The selective catalytic reduction (SCR) of NO_x by NH₃ has been extensively studied in the literature, mainly because of its high potential to remediate the pollution of diesel exhaust gases. The implementation of the NH₃-SCR process into passenger cars requires the use of an ammonia precursor, provided by a urea aqueous solution in the conventional process. Although the thermal decomposition and hydrolysis mechanisms of urea are well documented in the literature, the influence of the direct use of urea on the NO_x reduction over SCR catalysts may be problematic. With the aim to evaluate prototype powdered catalysts, a specific synthetic gas bench adjusted to powdered material was developed, allowing the use of NH₃ or urea as reductant for direct comparison. The design of the experimental setup allows vaporization of liquid urea at 200 °C under 10 bar using an HPLC pump and a micro injector of 50 μm diameter. This work presents the experimental setup of the catalytic test and some remarkable catalytic results towards further development of new catalytic formulations specifically dedicated to urea-SCR. Indeed, a possible divergence in terms of DeNO_x efficiency is evidenced depending on the nature of the reductant, NH₃ or urea solution. Particularly, the evaluated catalyst may not allow an optimal NO_x conversion because of a lack in ammonia availability when the urea residence time is shortened. This is attributed to insufficient activity of isocyanic acid (HNCO) hydrolysis, which can be improved by addition upstream of an active solid for the hydrolysis reaction such as ZrO₂. Thus, this µ-scale synthetic gas bench adjusted to powdered materials enables the specific behaviour of urea use for NO_x reduction to be demonstrated. Full article

(This article belongs to the Special Issue Automotive Emission Control Catalysts)

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