Catalysts

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Open AccessEditorial

Advances in Catalyst Deactivation and Regeneration

by Calvin H. Bartholomew and Morris D. Argyle

Catalysts 2015, 5(2), 949-954; https://doi.org/10.3390/catal5020949 - 11 Jun 2015

Cited by 40 | Viewed by 7896

Abstract

Catalyst deactivation, the loss over time of catalytic activity and/or selectivity, is a problem of great and continuing concern in the practice of industrial catalytic processes. Costs to industry for catalyst replacement and process shutdown total tens of billions of dollars per [...] Read more.

Catalyst deactivation, the loss over time of catalytic activity and/or selectivity, is a problem of great and continuing concern in the practice of industrial catalytic processes. Costs to industry for catalyst replacement and process shutdown total tens of billions of dollars per year. [...] Full article

(This article belongs to the Special Issue Advances in Catalyst Deactivation)

Research

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11807 KiB

Open AccessArticle

Effect of Ce and Zr Addition to Ni/SiO2 Catalysts for Hydrogen Production through Ethanol Steam Reforming

by Jose Antonio Calles, Alicia Carrero, Arturo Javier Vizcaíno and Montaña Lindo

Catalysts 2015, 5(1), 58-76; https://doi.org/10.3390/catal5010058 - 30 Jan 2015

Cited by 37 | Viewed by 10027

Abstract

A series of Ni/Ce\(_{x}\)Zr\(_{1-x}\)O\(_{2}\)/SiO\(_{2}\) catalysts with different Zr/Ce mass ratios were prepared by incipient wetness impregnation. Ni/SiO\(_{2}\), Ni/CeO\(_{2}\) and Ni/ZrO\(_{2}\) were also prepared as reference materials to compare. Catalysts' performances were tested in ethanol steam reforming for hydrogen production and characterized by XRD, [...] Read more.

A series of Ni/Ce\(_{x}\)Zr\(_{1-x}\)O\(_{2}\)/SiO\(_{2}\) catalysts with different Zr/Ce mass ratios were prepared by incipient wetness impregnation. Ni/SiO\(_{2}\), Ni/CeO\(_{2}\) and Ni/ZrO\(_{2}\) were also prepared as reference materials to compare. Catalysts' performances were tested in ethanol steam reforming for hydrogen production and characterized by XRD, H\(_{2}\)-temperature programmed reduction (TPR), NH\(_{3}\)-temperature programmed desorption (TPD), TEM, ICP-AES and N\(_{2}\)-sorption measurements. The Ni/SiO\(_{2}\) catalyst led to a higher hydrogen selectivity than Ni/CeO\(_{2}\) and Ni/ZrO\(_{2}\), but it could not maintain complete ethanol conversion due to deactivation. The incorporation of Ce or Zr prior to Ni on the silica support resulted in catalysts with better performance for steam reforming, keeping complete ethanol conversion over time. When both Zr and Ce were incorporated into the catalyst, Ce\(_{x}\)Zr\(_{1-x}\)O\(_{2}\) solid solution was formed, as confirmed by XRD analyses. TPR results revealed stronger Ni-support interaction in the Ce\(_{x}\)Zr\(_{1-x}\)O\(_{2}\)-modified catalysts than in Ni/SiO\(_{2}\) one, which can be attributed to an increase of the dispersion of Ni species. All of the Ni/Ce\(_{x}\)Zr\(_{1-x}\)O\(_{2}\)/SiO\(_{2}\) catalysts exhibited good catalytic activity and stability after 8 h of time on stream at 600°. The best catalytic performance in terms of hydrogen selectivity was achieved when the Zr/Ce mass ratio was three. Full article

(This article belongs to the Special Issue Advances in Catalyst Deactivation)

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1490 KiB

Open AccessArticle

Inhibition of a Gold-Based Catalyst in Benzyl Alcohol Oxidation: Understanding and Remediation

by Emmanuel Skupien, Rob J. Berger, Vera P. Santos, Jorge Gascon, Michiel Makkee, Michiel T. Kreutzer, Patricia J. Kooyman, Jacob A. Moulijn and Freek Kapteijn

Catalysts 2014, 4(2), 89-115; https://doi.org/10.3390/catal4020089 - 15 Apr 2014

Cited by 44 | Viewed by 13718

Abstract

Benzyl alcohol oxidation was carried out in toluene as solvent, in the presence of the potentially inhibiting oxidation products benzaldehyde and benzoic acid. Benzoic acid, or a product of benzoic acid, is identified to be the inhibiting species. The presence of a basic [...] Read more.

Benzyl alcohol oxidation was carried out in toluene as solvent, in the presence of the potentially inhibiting oxidation products benzaldehyde and benzoic acid. Benzoic acid, or a product of benzoic acid, is identified to be the inhibiting species. The presence of a basic potassium salt (K₂CO₃ or KF) suppresses this inhibition, but promotes the formation of benzyl benzoate from the alcohol and aldehyde. When a small amount of water is added together with the potassium salt, an even greater beneficial effect is observed, due to a synergistic effect with the base. A kinetic model, based on the three main reactions and four major reaction components, is presented to describe the concentration-time profiles and inhibition. The inhibition, as well as the effect of the base, was captured in the kinetic model, by combining strong benzoic acid adsorption and competitive adsorption with benzyl alcohol. The effect of the potassium salt is accounted for in terms of neutralization of benzoic acid. Full article

(This article belongs to the Special Issue Advances in Catalyst Deactivation)

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379 KiB

Open AccessArticle

Investigation of the Deactivation Phenomena Occurring in the Cyclohexane Photocatalytic Oxidative Dehydrogenation on MoO_x/TiO₂ through Gas Phase and in situ DRIFTS Analyses

by Vincenzo Vaiano, Diana Sannino, Ana Rita Almeida, Guido Mul and Paolo Ciambelli

Catalysts 2013, 3(4), 978-997; https://doi.org/10.3390/catal3040978 - 13 Dec 2013

Cited by 11 | Viewed by 7046

Abstract

In this work, the results of gas phase cyclohexane photocatalytic oxidative dehydrogenation on MoO_x/SO₄/TiO₂ catalysts with DRIFTS analysis are presented. Analysis of products in the gas-phase discharge of a fixed bed photoreactor was coupled with in situ monitoring [...] Read more.

In this work, the results of gas phase cyclohexane photocatalytic oxidative dehydrogenation on MoO_x/SO₄/TiO₂ catalysts with DRIFTS analysis are presented. Analysis of products in the gas-phase discharge of a fixed bed photoreactor was coupled with in situ monitoring of the photocatalyst surface during irradiation with an IR probe. An interaction between cyclohexane and surface sulfates was found by DRIFTS analysis in the absence of UV irradiation, showing evidence of the formation of an organo-sulfur compound. In particular, in the absence of irradiation, sulfate species initiate a redox reaction through hydrogen abstraction of cyclohexane and formation of sulfate (IV) species. In previous studies, it was concluded that reduction of the sulfate (IV) species via hydrogen abstraction during UV irradiation may produce gas phase SO₂ and thereby loss of surface sulfur species. Gas phase analysis showed that the presence of MoO_x species, at same sulfate loading, changes the selectivity of the photoreaction, promoting the formation of benzene. The amount of surface sulfate influenced benzene yield, which decreases when the sulfate coverage is lower. During irradiation, a strong deactivation was observed due to the poisoning of the surface by carbon deposits strongly adsorbed on catalyst surface. Full article

(This article belongs to the Special Issue Advances in Catalyst Deactivation)

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Review

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Open AccessReview

Deactivation of Pd Catalysts by Water during Low Temperature Methane Oxidation Relevant to Natural Gas Vehicle Converters

by Rahman Gholami, Mina Alyani and Kevin J. Smith

Catalysts 2015, 5(2), 561-594; https://doi.org/10.3390/catal5020561 - 31 Mar 2015

Cited by 186 | Viewed by 13349

Abstract

Effects of H₂O on the activity and deactivation of Pd catalysts used for the oxidation of unburned CH₄ present in the exhaust gas of natural-gas vehicles (NGVs) are reviewed. CH₄ oxidation in a catalytic converter is limited by low [...] Read more.

Effects of H₂O on the activity and deactivation of Pd catalysts used for the oxidation of unburned CH₄ present in the exhaust gas of natural-gas vehicles (NGVs) are reviewed. CH₄ oxidation in a catalytic converter is limited by low exhaust gas temperatures (500–550 °C) and low concentrations of CH₄ (400–1500 ppmv) that must be reacted in the presence of large quantities of H₂O (10–15%) and CO₂ (15%), under transient exhaust gas flows, temperatures, and compositions. Although Pd catalysts have the highest known activity for CH₄ oxidation, water-induced sintering and reaction inhibition by H₂O deactivate these catalysts. Recent studies have shown the reversible inhibition by H₂O adsorption causes a significant drop in catalyst activity at lower reaction temperatures (below 450 °C), but its effect decreases (water adsorption becomes more reversible) with increasing reaction temperature. Thus above 500 °C H₂O inhibition is negligible, while Pd sintering and occlusion by support species become more important. H₂O inhibition is postulated to occur by either formation of relatively stable Pd(OH)₂ and/or partial blocking by OH groups of the O exchange between the support and Pd active sites thereby suppressing catalytic activity. Evidence from FTIR and isotopic labeling favors the latter route. Pd catalyst design, including incorporation of a second noble metal (Rh or Pt) and supports high O mobility (e.g., CeO₂) are known to improve catalyst activity and stability. Kinetic studies of CH₄ oxidation at conditions relevant to natural gas vehicles have quantified the thermodynamics and kinetics of competitive H₂O adsorption and Pd(OH)₂ formation, but none have addressed effects of H₂O on O mobility. Full article

(This article belongs to the Special Issue Advances in Catalyst Deactivation)

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4697 KiB

Open AccessReview

Deactivation and Regeneration of Commercial Type Fischer-Tropsch Co-Catalysts—A Mini-Review

by Erling Rytter and Anders Holmen

Catalysts 2015, 5(2), 478-499; https://doi.org/10.3390/catal5020478 - 26 Mar 2015

Cited by 118 | Viewed by 14486

Abstract

Deactivation of commercially relevant cobalt catalysts for Low Temperature Fischer-Tropsch (LTFT) synthesis is discussed with a focus on the two main long-term deactivation mechanisms proposed: Carbon deposits covering the catalytic surface and re-oxidation of the cobalt metal. There is a great variety in [...] Read more.

Deactivation of commercially relevant cobalt catalysts for Low Temperature Fischer-Tropsch (LTFT) synthesis is discussed with a focus on the two main long-term deactivation mechanisms proposed: Carbon deposits covering the catalytic surface and re-oxidation of the cobalt metal. There is a great variety in commercial, demonstration or pilot LTFT operations in terms of reactor systems employed, catalyst formulations and process conditions. Lack of sufficient data makes it difficult to correlate the deactivation mechanism with the actual process and catalyst design. It is well known that long term catalyst deactivation is sensitive to the conditions the actual catalyst experiences in the reactor. Therefore, great care should be taken during start-up, shutdown and upsets to monitor and control process variables such as reactant concentrations, pressure and temperature which greatly affect deactivation mechanism and rate. Nevertheless, evidence so far shows that carbon deposition is the main long-term deactivation mechanism for most LTFT operations. It is intriguing that some reports indicate a low deactivation rate for multi-channel micro-reactors. In situ rejuvenation and regeneration of Co catalysts are economically necessary for extending their life to several years. The review covers information from open sources, but with a particular focus on patent literature. Full article

(This article belongs to the Special Issue Advances in Catalyst Deactivation)

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6228 KiB

Open AccessReview

Heterogeneous Catalyst Deactivation and Regeneration: A Review

by Morris D. Argyle and Calvin H. Bartholomew

Catalysts 2015, 5(1), 145-269; https://doi.org/10.3390/catal5010145 - 26 Feb 2015

Cited by 1327 | Viewed by 99186

Abstract

Deactivation of heterogeneous catalysts is a ubiquitous problem that causes loss of catalytic rate with time. This review on deactivation and regeneration of heterogeneous catalysts classifies deactivation by type (chemical, thermal, and mechanical) and by mechanism (poisoning, fouling, thermal degradation, vapor formation, vapor-solid [...] Read more.

Deactivation of heterogeneous catalysts is a ubiquitous problem that causes loss of catalytic rate with time. This review on deactivation and regeneration of heterogeneous catalysts classifies deactivation by type (chemical, thermal, and mechanical) and by mechanism (poisoning, fouling, thermal degradation, vapor formation, vapor-solid and solid-solid reactions, and attrition/crushing). The key features and considerations for each of these deactivation types is reviewed in detail with reference to the latest literature reports in these areas. Two case studies on the deactivation mechanisms of catalysts used for cobalt Fischer-Tropsch and selective catalytic reduction are considered to provide additional depth in the topics of sintering, coking, poisoning, and fouling. Regeneration considerations and options are also briefly discussed for each deactivation mechanism. Full article

(This article belongs to the Special Issue Advances in Catalyst Deactivation)

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2591 KiB

Open AccessReview

Deactivation Pattern of a “Model” Ni/MgO Catalyst in the Pre-Reforming of n-Hexane

by Giuseppe Trunfio and Francesco Arena

Catalysts 2014, 4(2), 196-214; https://doi.org/10.3390/catal4020196 - 19 Jun 2014

Cited by 8 | Viewed by 7033

Abstract

The deactivation pattern of a “model” Ni/MgO catalyst in the pre-reforming of n-hexane with steam (T, 450 °C; P, 5–15 bar) is reviewed. The influence of the steam-to-carbon ratio (S/C, 1.5–3.5) on the rate of catalyst fouling by coking [...] Read more.

The deactivation pattern of a “model” Ni/MgO catalyst in the pre-reforming of n-hexane with steam (T, 450 °C; P, 5–15 bar) is reviewed. The influence of the steam-to-carbon ratio (S/C, 1.5–3.5) on the rate of catalyst fouling by coking is ascertained. Catalyst fouling leads to an exponential decay in activity, denoting 1^st-order dependence of the coking process on active sites availability. Hydrogen hinders the coking process, though slight activity decay is due to sintering of the active Ni phase. Deactivation by thiophene causes a sharp, almost linear, drop to nearly zero activity within only 6 h; this deactivation is likely due to dissociative adsorption of thiophene with subsequent strong, irreversible chemical adsorption of S-atoms on active Ni sites, i.e., irreversible poisoning. Modeling of activity decay curves (α, a_t/a₀) by proper kinetic equations allows assessing the effects of temperature, pressure, S/C, H₂ and thiophene feed on the deactivation pattern of the model Ni/MgO catalyst by coking, sintering, and poisoning phenomena. Full article

(This article belongs to the Special Issue Advances in Catalyst Deactivation)

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1826 KiB

Open AccessReview

Influence of Reduction Promoters on Stability of Cobalt/g-Alumina Fischer-Tropsch Synthesis Catalysts

by Gary Jacobs, Wenping Ma and Burtron H. Davis

Catalysts 2014, 4(1), 49-76; https://doi.org/10.3390/catal4010049 - 11 Mar 2014

Cited by 51 | Viewed by 10771

Abstract

This focused review article underscores how metal reduction promoters can impact deactivation phenomena associated with cobalt Fischer-Tropsch synthesis catalysts. Promoters can exacerbate sintering if the additional cobalt metal clusters, formed as a result of the promoting effect, are in close proximity at the [...] Read more.

This focused review article underscores how metal reduction promoters can impact deactivation phenomena associated with cobalt Fischer-Tropsch synthesis catalysts. Promoters can exacerbate sintering if the additional cobalt metal clusters, formed as a result of the promoting effect, are in close proximity at the nanoscale to other cobalt particles on the surface. Recent efforts have shown that when promoters are used to facilitate the reduction of small crystallites with the aim of increasing surface Co⁰ site densities (e.g., in research catalysts), ultra-small crystallites (e.g., <2–4.4 nm) formed are more susceptible to oxidation at high conversion relative to larger ones. The choice of promoter is important, as certain metals (e.g., Au) that promote cobalt oxide reduction can separate from cobalt during oxidation-reduction (regeneration) cycles. Finally, some elements have been identified to promote reduction but either poison the surface of Co⁰ (e.g., Cu), or produce excessive light gas selectivity (e.g., Cu and Pd, or Au at high loading). Computational studies indicate that certain promoters may inhibit polymeric C formation by hindering C-C coupling. Full article

(This article belongs to the Special Issue Advances in Catalyst Deactivation)

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