Special Issue "Recent Developments in Rh Catalysts"

A special issue of Catalysts (ISSN 2073-4344).

Deadline for manuscript submissions: closed (31 December 2020).

Special Issue Editors

Prof. Dr. János Kiss
E-Mail Website
Guest Editor
Department of Applied and Environmental Chemistry, University of Szeged,Reaction Kinetics and Surface Chemistry Research Group of the Hungarian Academy of Sciences at the University of Szeged, H-6720 Szeged, Rerrich Béla squer. 1,Hungary
Interests: heterogeneous catalytic reactions; surface photochemistry; surface science
Dr. Imre Kovács
E-Mail Website
Guest Editor
Department of Science and Environment, Technical Institute, University of Dunaújváros, Táncsics M. u. 1, H-2401 Dunaújváros, Hungary
Interests: heterogeneous catalytic reactions; surface science; electron spectroscopy; EXAFS; NEXAFS

Special Issue Information

Dear Colleagues,

This Special Issue of Catalysts is devoted to Rh and Rh-related catalysts, which play an outstanding role in many technologically important catalytic reactions. Recently, Rh has been receiving considerable attention because of its high catalytic potential for producing hydrogen from hydrogen-containing molecules to power fuel cells. Catalytic reactions of oxygenated hydrocarbons as end products and as reactants have been the focus of studies for decades, with publications dating back to the early seventies. Hydrogenation of CO and CO2 to form hydrocarbons and oxygenated products over supported and unsupported Rh has been the subject of extensive research of Rh and Rh-containing catalysts. Supported Rh catalysts are promising candidates in the thermal and photocatalytic reduction of CO2 with hydrogen and with different saturated and unsaturated hydrocarbons. The reactions of CO2, an interesting and attractive C1 building block, not only contribute to alleviating global climate change induced by the increasing CO2 emissions, but also open up new sustainable routes for synthesizing useful feedstock chemicals and fuels. Rh supported on oxide supports is an excellent catalyst for environmentally important technologies, such as CO oxidation and NOx reduction. This Issue would supply the catalytic community with the present status of Rh-related catalysts exhibited in many catalytic reactions. This volume involves studies relating to the catalytic effects of Rh in a wide reaction scale, the modification of Rh surfaces, and the interaction mechanism between Rh and support, including the strong metal interactions and its importance in the catalytic reactions. Special attention will be given to the promoting effect of a small amount of Rh in the catalytic behavior of supported metal catalysts, including the formation of alloy in bimetallic systems, on different supports. The study of morphological changes of Rh during the reaction is also the subject of this Special Issue.

Prof. Dr. János Kiss
Dr. Imre Kovács
Guest Editors

Manuscript Submission Information

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Keywords

  • Rh catalysts
  • catalytic effect of Rh
  • promoter effect of Rh
  • bimetallic catalysts
  • power fuel cell
  • hydrogen production
  • CO and NOx elimination
  • surface modification of Rh
  • strong metal support interaction
  • morphological changes of Rh

Published Papers (9 papers)

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Editorial

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Open AccessEditorial
Recent Developments in Rh Heterogeneous Catalysts
Catalysts 2021, 11(4), 416; https://doi.org/10.3390/catal11040416 - 25 Mar 2021
Viewed by 377
Abstract
Rh-based catalysts successfully catalyze bond making and bond breaking reactions in most cases [...] Full article
(This article belongs to the Special Issue Recent Developments in Rh Catalysts)

Research

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Open AccessArticle
Numerical Simulation of Methane and Propane Reforming Over a Porous Rh/Al2O3 Catalyst in Stagnation-Flows: Impact of Internal and External Mass Transfer Limitations on Species Profiles
Catalysts 2020, 10(8), 915; https://doi.org/10.3390/catal10080915 - 10 Aug 2020
Cited by 2 | Viewed by 763
Abstract
Hydrogen production by catalytic partial oxidation and steam reforming of methane and propane towards synthesis gas are numerically investigated in stagnation-flow over a disc coated with a porous Rh/Al2O3 layer. A one-dimensional flow field is coupled with three models for [...] Read more.
Hydrogen production by catalytic partial oxidation and steam reforming of methane and propane towards synthesis gas are numerically investigated in stagnation-flow over a disc coated with a porous Rh/Al2O3 layer. A one-dimensional flow field is coupled with three models for internal diffusion and with a 62-step surface reaction mechanism. Numerical simulations are conducted with the recently developed computer code DETCHEMSTAG. Dusty-Gas model, a reaction-diffusion model and a simple effectiveness factor model, are alternatively used in simulations to study the internal mass transfer inside the 100 µm thick washcoat layer. Numerically predicted species profiles in the external boundary layer agree well with the recently published experimental data. All three models for internal diffusion exhibit strong species concentration gradients in the catalyst layer. In partial oxidation conditions, a thin total oxidation zone occurs close to the gas-washcoat interface, followed by a zone of steam and dry reforming of methane. Increasing the reactor pressure and decreasing the inlet flow velocity increases/decreases the external/internal mass transfer limitations. The comparison of reaction-diffusion and Dusty-Gas model results reveal the insignificance of convective flow on species transport inside the washcoat. Simulations, which additionally solve a heat transport equation, do not show any temperature gradients inside the washcoat. Full article
(This article belongs to the Special Issue Recent Developments in Rh Catalysts)
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Open AccessFeature PaperArticle
The Potassium-Induced Decomposition Pathway of HCOOH on Rh(111)
Catalysts 2020, 10(6), 675; https://doi.org/10.3390/catal10060675 - 16 Jun 2020
Cited by 2 | Viewed by 825
Abstract
Formic acid (FA) can be considered both a CO and a H2 carrier via selective dehydration and dehydrogenation pathways, respectively. The two processes can be influenced by the modification of the active components of the catalysts used. In the present study the [...] Read more.
Formic acid (FA) can be considered both a CO and a H2 carrier via selective dehydration and dehydrogenation pathways, respectively. The two processes can be influenced by the modification of the active components of the catalysts used. In the present study the adsorption of FA and the decomposition of the formed formate intermediate were investigated on potassium promoted Rh(111) surfaces. The preadsorbed potassium markedly increased the uptake of FA at 300 K, and influenced the decomposition of formate depending on the potassium coverage. The work function (Δϕ) is increased by the adsorption of FA on K/Rh(111) at 300 K suggesting a large negative charge on the chemisorbed molecule, which could be probably due to the enhanced back-donation of electrons from the K-promoted Rh into an empty π orbital of HCOOH. The binding energy of the formate species is therefore increased resulting in a greater concentration of irreversibly adsorbed formate species. Decomposition of the formate species led to the formation of H2, CO2, H2O, and CO, which desorbed at significantly higher temperatures from the K-promoted surface than from the K-free one as it was proven by thermal desorption studies. Transformation of surface formate to carbonate (evidenced by UPS) and its decomposition and desorption is responsible for the high temperature CO and CO2 formation. Full article
(This article belongs to the Special Issue Recent Developments in Rh Catalysts)
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Open AccessFeature PaperArticle
Hydrogenation and Hydrodeoxygenation of Oxygen-Substituted Aromatics over Rh/silica: Catechol, Resorcinol and Hydroquinone
Catalysts 2020, 10(5), 584; https://doi.org/10.3390/catal10050584 - 22 May 2020
Cited by 2 | Viewed by 805
Abstract
The hydrogenation and hydrodeoxygenation (HDO) of dihydroxybenzene isomers, catechol (1,2-dihydroxybenzene), resorcinol (1,3-dihydroxybenzene) and hydroquinone (1,4-dihydroxybenzene) was studied in the liquid phase over a Rh/silica catalyst at 303–343 K and 3 barg hydrogen pressure. The following order of reactivity, resorcinol > catechol > hydroquinone [...] Read more.
The hydrogenation and hydrodeoxygenation (HDO) of dihydroxybenzene isomers, catechol (1,2-dihydroxybenzene), resorcinol (1,3-dihydroxybenzene) and hydroquinone (1,4-dihydroxybenzene) was studied in the liquid phase over a Rh/silica catalyst at 303–343 K and 3 barg hydrogen pressure. The following order of reactivity, resorcinol > catechol > hydroquinone (meta > ortho > para) was obtained. Kinetic analysis revealed that catechol had a negative order of reaction whereas both hydroquinone and resorcinol gave positive half-order suggesting that catechol is more strongly adsorbed. Activation energies of ~30 kJ·mol−1 were determined for catechol and hydroquinone, while resorcinol gave a value of 41 kJ·mol−1. Resorcinol, and similarly hydroquinone, gave higher yields of the hydrogenolysis products (cyclohexanol, cyclohexanone and cyclohexane) with a cumulative yield of ~40%. In contrast catechol favoured hydrogenation, specifically to cis-1,2-dihydroxycyclohexane. It is proposed that cis-isomers are formed from hydrogenation of dihydroxycyclohexenes and high selectivity to cis-1,2-dihydroxycyclohexane can be explained by the enhanced stability of 1,2-dihydroxycyclohex-1-ene relative to other cyclohexene intermediates of catechol, resorcinol or hydroquinone. Trans-isomers are not formed by isomerisation of the equivalent cis-dihydroxycyclohexane but by direct hydrogenation of 2/3/4-hydroxycyclohexanone. The higher selectivity to HDO for resorcinol and hydroquinone may relate to the reactive surface cyclohexenes that have a C=C double bond β-γ to a hydroxyl group aiding hydrogenolysis. Using deuterium instead of hydrogen revealed that each isomer had a unique kinetic isotope effect and that HDO to cyclohexane was dramatically affected. The delay in the production of cyclohexane suggest that deuterium acted as an inhibitor and may have blocked the specific HDO site that results in cyclohexane formation. Carbon deposition was detected by temperature programmed oxidation (TPO) and revealed three surface species. Full article
(This article belongs to the Special Issue Recent Developments in Rh Catalysts)
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Open AccessFeature PaperArticle
Unraveling Toluene Conversion during the Liquid Phase Hydrogenation of Cyclohexene (in Toluene) with Rh Hybrid Catalysts
Catalysts 2019, 9(12), 973; https://doi.org/10.3390/catal9120973 - 20 Nov 2019
Cited by 2 | Viewed by 806
Abstract
Monitoring hydrogen consumption has allowed studying the progress of the liquid phase hydrogenation of cyclohexene in toluene with Rh SILP (supported ionic liquid phase) catalysts prepared by the immobilization of the [{RhCl(cod)}2] complex on different carbon materials. An excess of hydrogen [...] Read more.
Monitoring hydrogen consumption has allowed studying the progress of the liquid phase hydrogenation of cyclohexene in toluene with Rh SILP (supported ionic liquid phase) catalysts prepared by the immobilization of the [{RhCl(cod)}2] complex on different carbon materials. An excess of hydrogen consumption with respect to the required amount for cyclohexene hydrogenation was registered and related with the solvent (toluene) hydrogenation. The study carried out led to unraveling the extent of toluene hydrogenation and to determining if the rate of this reaction is affected by the properties of the carbon material used as support. The results revealed that the Rh SILP catalysts we prepared showed acceptable toluene conversion, with 100% selectivity to the total hydrogenated product, and that the effect of the carbon support is the same as for cyclohexene hydrogenation. Full article
(This article belongs to the Special Issue Recent Developments in Rh Catalysts)
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Open AccessArticle
Investigation of Thermal Stability and Reactivity of Rh Nanoclusters on an Ultrathin Alumina Film
Catalysts 2019, 9(11), 971; https://doi.org/10.3390/catal9110971 - 18 Nov 2019
Cited by 2 | Viewed by 801
Abstract
We studied the structural and morphological evolution of Rh clusters on an ordered ultrathin alumina film grown on NiAl(100) in annealing processes, under ultrahigh vacuum conditions and with various surface probe techniques. The Rh clusters, prepared on vapor deposition of Rh onto the [...] Read more.
We studied the structural and morphological evolution of Rh clusters on an ordered ultrathin alumina film grown on NiAl(100) in annealing processes, under ultrahigh vacuum conditions and with various surface probe techniques. The Rh clusters, prepared on vapor deposition of Rh onto the alumina film at 300 K, had an fcc phase and grew in the (100) orientation; the annealing altered the cluster structure little—the lattice parameter decreased by a factor <2%—but the cluster morphology significantly. With elevated temperature, small clusters (diameter ≤1.5 nm) decreased little in size; in contrast, large clusters (diameter ≥2.0 nm) varied in a complex manner—their mean diameter decreased to about 1.5 nm on annealing to 450 K, despite their similar height, while it increased to above 2.0 nm at temperature ≥570 K. This atypical decrease in size was governed predominantly by energetics. Such a reduced size enhanced the total surface area as well as the reactivity of the clusters toward methanol decomposition, so increased the production of D2 (H2) and CO from decomposed methanol-d4 (or methanol). The result implies a higher temperature tolerance for Rh clusters on the alumina film and a practical approach to prepare small Rh clusters with high reactivity. Full article
(This article belongs to the Special Issue Recent Developments in Rh Catalysts)
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Open AccessFeature PaperArticle
Low Temperature Infrared Study of Carbon Monoxide Adsorption on Rh/CeO2
Catalysts 2019, 9(7), 598; https://doi.org/10.3390/catal9070598 - 11 Jul 2019
Cited by 4 | Viewed by 1027
Abstract
Fundamental studies of the interaction of adsorbates with metal oxides alone and on which a noble metal is deposited provide information needed for catalytic reactions. Rh/CeO2 is one of the textbook catalysts for many reactions including syngas conversion to ethanol, water gas [...] Read more.
Fundamental studies of the interaction of adsorbates with metal oxides alone and on which a noble metal is deposited provide information needed for catalytic reactions. Rh/CeO2 is one of the textbook catalysts for many reactions including syngas conversion to ethanol, water gas shift reaction (WGSR), and ethanol steam reforming. In this work, the adsorption of CO is studied by infrared (IR) spectroscopy, over CeO2 and 0.6 at. % Rh/CeO2 at a temperature range of 90 to 300 K. CeO2 is in the form of nanoparticles with sizes between 5 and 10 nm and exposing predominantly {111} surface termination in addition to non-negligible fraction of the {100} termination, determined from high resolution transmission electron microscopy (HRTEM). The as prepared Rh/CeO2 contained metallic Rh as well Rh cations in higher oxidation states. At 90 K two IR bands were observed at 2183–2186 and 2161–2163 cm−1, with the former saturating first. The 2163 cm−1 peak was more sensitive to CO pressure than the 2186 cm−1. Heating resulted in the depopulation of the 2163 cm−1 before the 2186 cm−1 peak. The desorption energy computed, assuming a first-order desorption kinetic, was found to be 0.35 eV for the 2186 cm−1 and 0.30 for the 2163 cm−1 IR peak (+/−0.05 eV). The equilibrium constant at 90 K was computed equal to 1.83 and 1.33 Torr−1 for the 2183 and 2161 cm−1, respectively. CO adsorption at 90 K on Rh/CeO2 resulted (in addition to the bands on CeO2) in the appearance of a broad band in the 2110–2130 cm-1 region that contained two components at 2116 and 2126 cm−1. The high frequency of this species is most likely due to adsorption on Rh clusters with very small sizes. The desorption energy of this species was found to be equal to 0.55 eV (+/−0.05 eV). Heating the CO covered Rh/CeO2 surface accelerated the disappearance of CO species over CeO2 and resulted in the appearance of CO2 bands (at about 150 K) followed by carbonate species. At 300 K, the surface was mainly composed of carbonates. Full article
(This article belongs to the Special Issue Recent Developments in Rh Catalysts)
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Review

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Open AccessFeature PaperReview
Rh-induced Support Transformation and Rh Incorporation in Titanate Structures and Their Influence on Catalytic Activity
Catalysts 2020, 10(2), 212; https://doi.org/10.3390/catal10020212 - 10 Feb 2020
Cited by 2 | Viewed by 740
Abstract
Rh is one of the most effective metals in several technologically important heterogeneous catalytic reactions, like the hydrogenation of CO2, and CO, the CO+H2O reaction, and methane and ethanol transformations. Titania and titanates are among the most frequently studied [...] Read more.
Rh is one of the most effective metals in several technologically important heterogeneous catalytic reactions, like the hydrogenation of CO2, and CO, the CO+H2O reaction, and methane and ethanol transformations. Titania and titanates are among the most frequently studied supports for Rh nanoparticles. The present study demonstrates that the nature of the support has a marked influence on the specific activity. For comparison, the catalytic activity of TiO2 P25 is also presented. It is pointed out that a certain amount of Rh can be stabilized as cation (Rh+) in ion-exchange positions (i.e., in atomic scale distribution) of the titanate framework. This ionic form does not exists on TiO2. We pay distinguished attention not only to the electronic interaction between Rh metal and the titania/titanate support, but also to the Rh-induced phase transitions of one-dimensional titanate nanowires (TiONW) and nanotubes (TiONT). Support transformation phenomena can be observed in Rh-loaded titanates. Rh decorated nanowires transform into the TiO2(B) phase, whereas their pristine counterparts recrystallize into anatase. The formation of anatase is dominant during the thermal annealing process in both acid-treated and Rh-decorated nanotubes; Rh catalysis this transformation. We demonstrate that the phase transformations and the formation of Rh nanoclusters and incorporated Rh ions affect the conversion and the selectivity of the reactions. The following initial activity order was found in the CO2 + H2, CO + H2O and C2H5OH decomposition reactions: Rh/TiO2 (Degussa P25) ≥ Rh/TiONW > Rh/TiONT. On the other hand it is remarkable that the hydrogen selectivity in ethanol decomposition was two times higher on Rh/TiONW and Rh/TiO(NT) catalysts than on Rh/TiO2 due to the presence of Rh+ cations incorporated into the framework of the titanate structures. Full article
(This article belongs to the Special Issue Recent Developments in Rh Catalysts)
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Open AccessReview
Hydrogenation of Carbon Dioxide on Supported Rh Catalysts
Catalysts 2020, 10(2), 155; https://doi.org/10.3390/catal10020155 - 29 Jan 2020
Cited by 8 | Viewed by 1056
Abstract
The constant increase in the CO2 concentration in the atmosphere requires us to look for opportunities to convert CO2 into more valuable compounds. In this review, the activity and selectivity of different supported metal catalysts were compared in the hydrogenation of [...] Read more.
The constant increase in the CO2 concentration in the atmosphere requires us to look for opportunities to convert CO2 into more valuable compounds. In this review, the activity and selectivity of different supported metal catalysts were compared in the hydrogenation of carbon dioxide, and found that Rh is one of the best samples. The possibility of the CO2 dissociation on clean metal and on supported Rh was discussed separately. The hydrogenation of CO2 produces mainly CH4 and CO, but the selectivity of the reaction is affected by the support, in some cases the reduction of the support, the particle size of Rh, and the different additives. At higher pressure methanol, ethanol, and acetic acid could be also formed. The activity of the various supported Rh catalysts was compared and the results obtained for TiO2-, SiO2-, and Al2O3-supported catalysts were discussed in a separate chapter. The compounds formed on the surface of the catalysts during the reaction are shown in detail; mostly, different CO species, adsorbed formate groups, and different carbonates were detected. In a separate chapter the mechanism of the reaction was also discussed. Full article
(This article belongs to the Special Issue Recent Developments in Rh Catalysts)
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