What Makes a Good Catalyst for the Deacon Process?

Bichloride-based ionic liquids for the merged storage, processing, and electrolysis of hydrogen chloride

Hydrogen chloride is produced as a by-product in industrial processes on a million-ton scale. Since HCl is inherently dangerous, its storage and transport are avoided by, e.g., on-site electrolysis providing H2 and Cl2 which usually requires complex cell designs and PFAS-based membranes. Here we report a complementary approach to safely store 0.61 kilogram HCl per kilogram storage material [NEt3Me]Cl forming the bichloride [NEt3Me][Cl(HCl)n]. Although HCl release is possible from this ionic liquid by heat or vacuum, the bichloride can be used directly to produce base chemicals like vinyl chloride. Alternatively, [NEt3Me][Cl(HCl)n] is electrolyzed under anhydrous conditions using a membrane-free cell to generate H2 and the corresponding chlorination agent [NEt3Me][Cl(Cl2)n], enabling the combination of these ionic liquids for the production of base chemicals.

Selective Ru Adsorption on SnO2/CeO2 Mixed Oxides for Efficient Destruction of Multicomponent Volatile Organic Compounds: From Laboratory to Practical Possibility

Ru-based catalysts have been extensively employed for the catalytic destruction of chlorinated volatile organic compounds (VOCs), but their versatility for other routine VOCs' destruction has been less explored. Herein, we show that Ru-decorated SnO₂/CeO₂ mixed oxides can sustain H₂O and HCl poisonings and are endowed with extraordinary versatility for a wide range of VOCs' destruction. Selective adsorption of Ru on the cassiterite SnO₂ and CeO₂ nanorods through a Coulomb force can rationally tune the oxidation and dechlorination centers on decorated catalysts, where the epitaxial growth of RuO_x on top of SnO₂ is endowed with excellent dechlorination ability and that on CeO₂ is functional as an oxidation center; the latter could also activate H₂O to provide sufficient H protons for HCl formation. Our developed Ru/SnO₂/CeO₂ catalyst can steadily destruct mono-chlorobenzene, ortho-dichlorobenzene, trichloroethylene, dichloromethane, epichlorohydrin, N-hexane, ethyl acetate, toluene, and their mixtures at an optimum temperature of 300 °C, and its monolithic form is also functional at this temperature with few dioxins being detected in the off-gas. Our results imply that the Ru-decorated SnO₂/CeO₂ catalyst can meet the demands of regenerative catalytic oxidation for the treatment of a wide range of VOCs from industrial exhausts.

Bromination mechanisms of aromatic pollutants: formation of Br2 and bromine transfer from metallic oxybromides

Bromination mechanisms of aromatic pollutants assume a chief contribution in the observed yields and pattern's distribution of a wide array of dioxin-like toxicants. However, salient features of the governing pathways remain largely speculative. This study presents detail mechanistic insights into two commonly discussed routes; namely: surface-assisted conversion of HBr into Br₂ and direct bromine transfer from oxybromides into a benzene ring. Utilizing iron surfaces, as structural representative of the metallic content in electronic wastes, results from density functional theory calculations portray accessible reactions into the successive dissociative adsorption of HBr over the Fe(100) surface and the subsequent evolution of gas phase bromine molecules. Activation energies for HBr uptake by the plain iron surface reside in the range of 129-182 kJ/mol. Over an oxygen pre-covered surface, dissociative adsorption of HBr leading to bromine molecules requires significantly lower activation energies (45-78 kJ/mol). Likewise, bromination of a benzene ring into a monobromobenzene molecule over Fe(100)_O*Br* (i.e., an oxybromide) configuration ensues with an opening activation energy of ~ 165 kJ/mol. Adsorption of a phenyl radical over an iron-oxybromide forms a phenolate moiety that subsequently desorbs from the surface into a phenoxy radical. Reaction pathways presented herein shall be useful in the ongoing efforts to comprehend the formation and bromination routes of the notorious bromine-bearing pollutants in real scenarios, such as, these encountered in the open burning and primitive thermal recycling of electronic wastes.

Can Gold be an Effective Catalyst for the Deacon Reaction?

Abstract

The Deacon reaction is an important industrial process for the oxidation of hydrogen chloride, thereby enabling chlorine to be recycled. As gold is an efficient catalyst for reactions involving hydrogen chloride and oxygen, we have studied the use of gold as a potential catalyst for the Deacon reaction. Unfortunately, gold displays only limited activity; however, this is markedly increased if hydrogen is cofed as a reactant.

Graphic Abstract

Catalytic Stability Studies Employing Dedicated Model Catalysts

Long-term stability of heterogeneous catalysts is an omnipresent and pressing concern in industrial processes. Catalysts with high activity and selectivity can be searched for by high-throughput screening methods based maybe on educated guesses provided by ab initio thermodynamics or scaling relations. However, high-throughput screening is not feasible and is hardly able to identify long-term stable catalyst so that a rational and knowledge-driven approach is called for to identify potentially stable and active catalysts. Unfortunately, our current microscopic understanding on stability issues is quite poor. We propose that this gap in knowledge can be at least partly closed by investigating dedicated model catalyst materials with well-defined morphology that allow for a tight link to theory and the application of standard characterization methods. This topic is highly interdisciplinary, combining sophisticated inorganic synthesis with catalysis research, surface chemistry, and powerful theoretical modeling. In this Account, we focus on the stability issues of Deacon catalysts (RuO₂ and CeO₂-based materials) for recovering Cl₂ from HCl by aerobic oxidation and how to deepen our microscopic insight into the underlying processes. The main stability problems under harsh Deacon reaction conditions concomitant with a substantial loss in activity arise from deep chlorination of the catalyst, leaching of volatile chlorides and oxychlorides, and decrease in active surface area by particle sintering. In general, powder materials with undefined particle shape are not well suited for examining catalyst stability, because changes in the morphology are difficult to recognize, for instance, by electron microscopy. Rather, we focus here on model materials with well-defined starting morphologies, including electrospun nanofibers, shape-controlled nanoparticles, and well-defined ultrathin crystalline layers. CeO₂ is able to stabilize shape-controlled particles, exposing a single facet orientation so that comparing activity and stability studies can reveal structure sensitive properties. We develop a quasi-steady-state kinetic approach that allows us to model the catalyst chlorination as a function of temperature and gas feed composition. For the case of pure CeO₂ nanocubes, this simple approach predicts chlorination to be efficiently suppressed by addition of little amounts of water in the reaction feed or by keeping the catalyst at higher temperature. Both process parameters have great impact on the actual reactor design. Thermal stabilization of CeO₂ by intermixing Zr has been known in automotive exhaust catalysis for decades, but this does not necessarily imply also chemical stabilization of CeO₂ against bulk-chlorination since Zr can readily form volatile ZrCl₄ and may quickly lose its stabilizing effect. Nevertheless, with model experiments the stabilizing effect of Zr in the Deacon process over mixed Ce_xZr_1-xO₂ nanorods is clearly evidenced. Even higher stability can be accomplished with ultrathin CeO₂ coatings on preformed ZrO₂ particles, demonstrating the great promise of atomic layer deposition (ALD) in catalysis synthesis.

CeO2–MnOx composite loaded on Al2O3 as a catalyst for HCl oxidation

CeO₂(12)–MnO_x(13)/Al₂O₃ catalyst exhibited an evidently improved activity with HCl conversion (X_HCl) above 72%, accompanied with a satisfactory stability for at least 200 h in the reaction of HCl oxidation to Cl₂.

Thermal Processing of Chloride-Contaminated Plutonium Dioxide

Over 80 heat treatment experiments have been made on samples of chloride-contaminated plutonium dioxide retrieved from two packages in storage at Sellafield. These packages dated from 1974 and 1980 and were produced in a batch process by conversion of plutonium oxalate in a furnace at around 550 °C. The storage package contained a poly(vinyl chloride) (PVC) bag between the screw top inner and outer metal cans. Degradation of the PVC has led to adsorption of hydrogen chloride together with other atmospheric gases onto the PuO₂ surface. Analysis by caustic leaching and ion chromatography gave chloride contents of ∼2000 to >5000 ppm Cl (i.e., μgCl g^-1 of the original sample). Although there are some subtle differences, in general, there is surprisingly good agreement in results from heat treatment experiments for all the samples from both cans. Mass loss on heating (LOH) plateaus at nearly 3 wt % above 700 °C, although samples that were long stored under an air atmosphere or preexposed to 95% relative humidity atmospheres, gave higher LOH up to ∼4 wt %. The majority of the mass loss is due to adsorbed water and other atmospheric gases rather than chloride. Heating volatilizes chloride only above ∼400 °C implying that simple physisorption of HCl is not the main cause of contamination. Interestingly, above 700 °C, >100% of the initial leachable chloride can be volatilized. Surface (leachable) chloride decreases quickly with heat treatment temperatures up to ∼600 °C but only slowly above this temperature. Storage in air atmosphere post-heat treatment apparently leads to a reequilibration as leachable chloride increases. The presence of a "nonleachable" form of chloride was thus inferred and subsequently confirmed in PuO₂ samples (pre- and post-heat treatment) that were fully dissolved and analyzed for the total chloride inventory. Reheating samples in either air or argon at temperatures up to the first heat treatment temperature did not volatilize significant amounts of additional chloride. With regard to a thermal stabilization process, heat treatment in flowing air at 800 °C with cooling and packaging under dry argon appears optimal, particularly, if thinner powder beds can be maintained. From electron microscopy, heat treatment appeared to have the most effect on degrading the square platelet particles compared to those with the trapezoidal morphology.

Cl2 Production by Photocatalytic Oxidation of HCl over TiO2

We studied the photocatalytic aerobic oxidation of HCl over TiO₂ for producing Cl₂ . Steady-state Cl₂ production rates were determined with a photocatalytic fixed-bed gas-phase reactor equipped with UV light-emitting diodes (LEDs) using iodometric titration as online analytics. We found stable Cl₂ production rates of up to 16 mmol h^-1  m^-2 for commercial anatase TiO₂ Hombikat UV100. The rate increased linearly with temperature from 21 to 140 °C, indicating the acceleration of the limiting desorption rate of the coupled product water. Comparing different TiO₂ polymorphs revealed that anatase possesses higher activity than rutile. The adsorption of HCl was monitored in situ by IR spectroscopy. The IR spectra indicated that HCl chemisorption chlorinates the surface of TiO₂ under the reaction conditions, suggesting it to be the first step of the reaction mechanism. High stability opens up the opportunity of developing a promising photocatalytic process of HCl recycling at lower temperatures suitable for reaching full conversion.

Electrophilic Chlorination of Naphthalene in Combustion Flue Gas

Naphthalene chlorination is an important formation mechanism of polychlorinated naphthalenes (PCNs) in combustion flue gas. In this study, a total of 21 metal chlorides and oxides were screened for their activities in the electrophilic chlorination of naphthalene. Copper(II) chloride exhibited the highest activity at 200-350 °C, followed by copper(I) chloride. Copper(II) chloride primarily acted as a strong chlorinating agent to facilitate chlorine substitution on naphthalene. Iron (II and III) chlorides were only highly active at 200-250 °C. At 250 °C, the average naphthalene chlorination efficiency over CuCl₂·2H₂O was 7.5-fold, 30.2-fold and 34.7-fold higher than those over CuCl, FeCl₃·6H₂O and FeCl₂·4H₂O, respectively. The other metal chlorides were less active. Under heated conditions, copper(II) and iron(III) chlorides were transformed to copper(I) and iron(II) chlorides via dechlorination, and then transformed to oxychlorides and oxides, thereby forming dechlorination-oxychlorination cycles of copper and iron species, respectively. The results obtained suggest that electrophilic chlorination of naphthalene in combustion flue gas is primarily driven by dechlorination-oxychlorination cycles of copper and iron species, and the reaction produces a selective chlorination pattern at 1 and 4 positions of naphthalene.

Ru-Ti Oxide Based Catalysts for HCl Oxidation: The Favorable Oxygen Species and Influence of Ce Additive

Several Ru-Ti oxide-based catalysts were investigated for the catalytic oxidation of HCl to Cl2 in this work. The active component RuO2 was loaded on different titanium-containing supports by a facile wetness impregnation method. The Ru-Ti oxide based catalysts were characterized by XRD, N2 sorption, SEM, TEM, H2-TPR, XPS, and Raman, which is correlated with the catalytic tests. Rutile TiO2 was confirmed as the optimal support even though it has a low specific surface area. In addition to the interfacial epitaxial lattice matching and epitaxy, the extraordinary performance of Ru-Ti rutile oxide could also be attributed to the favorable oxygen species on Ru sites and specific active phase-support interactions. On the other hand, the influence of additive Ce on the RuO2/TiO2-rutile was studied. The incorporation of Ce by varied methods resulted in further oxidation of RuO2 into RuO2δ+ and a modification of the support structure. The amount of favorable oxygen species on the surface was decreased. As a result, the Deacon activity was lowered. It was demonstrated that the surface oxygen species and specific interactions of the Ru-Ti rutile oxide were critical to HCl oxidation.

Oxygen storage capacity versus catalytic activity of ceria–zirconia solid solutions in CO and HCl oxidation

Ce_xZr_1−xO₂ solid solutions were prepared to explore the relationship between oxygen storage capacity and activity of oxidation reactions.

Copper slag as a catalyst for mercury oxidation in coal combustion flue gas

Copper slag is a byproduct of the pyrometallurgical smelting of copper concentrate. It was used in this study to catalyze elemental mercury (Hg⁰) oxidation in simulated coal combustion flue gas. The copper slag exhibited excellent catalytic performance in Hg⁰ oxidation at temperatures between 200 °C and 300 °C. At the most optimal temperature of 250 °C, a Hg⁰ oxidation efficiency of 93.8% was achieved under simulated coal combustion flue gas with both a high Hg⁰ concentration and a high gas hourly space velocity of 128,000 h^-1. Hydrogen chloride (HCl) was the flue gas component responsible for Hg⁰ oxidation over the copper slag. The transition metal oxides, including iron oxides and copper oxide in the copper slag, exhibited significant catalytic activities in the surface-mediated oxidation of Hg⁰ in the presence of HCl. It is proposed that the Hg⁰ oxidation over the copper slag followed the Langmuir-Hinshelwood mechanism whereby reactive chlorine species that originated from HCl reacted with the physically adsorbed Hg⁰ to form oxidized mercury. This study demonstrated the possibility of reusing copper slag as a catalyst for Hg⁰ oxidation and revealed the mechanisms involved in the process and the key factors in the performance. This knowledge has fundamental importance in simultaneously reducing industrial waste and controlling mercury emissions from coal-fired power plants.

Electrospinning of CeO2 nanoparticle dispersions into mesoporous fibers: on the interplay of stability and activity in the HCl oxidation reaction

High-surface-area CeO₂ fibers are obtained from a specially developed nanoparticle dispersion and are used as catalysts in the HCl oxidation reaction.

Structural motifs of water on metal oxide surfaces

This review describes the state-of-the-art of the molecular-level understanding of water adsorption, dissociation and clustering on model surfaces of metal oxides.

Novel Effective Catalyst for Elemental Mercury Removal from Coal-Fired Flue Gas and the Mechanism Investigation

Mercury pollution from coal-fired power plants has drawn attention worldwide. To achieve efficient catalytic oxidation of Hg(0) at both high and low temperatures, we prepared and tested novel IrO2 modified Ce-Zr solid solution catalysts under various conditions. It was found that the IrO2/Ce0.6Zr0.4O2 catalyst, which was prepared using the polyvinylpyrrolidone-assisted sol-gel method, displayed significantly higher catalytic activity for Hg(0) oxidation. The mechanism of Hg(0) removal over IrO2/Ce0.6Zr0.4O2 was studied using various methods, and the Hg(0) oxidation reaction was found to follow two possible pathways. For the new chemisorption-regeneration mechanism proposed in this study, the adsorbed Hg(0) was first oxidized with surface chemisorbed oxygen species to form HgO; the HgO could desorb from the surface of catalysts by itself or react with adsorbed HCl to be release in the form of gaseous HgCl2. O2 is indispensable for the chemisorption process, and the doping of IrO2 could facilitate the chemisorption process. In addition, the Deacon reaction mechanism was also feasible for Hg(0) oxidation: this reaction would involve first oxidizing the adsorbed HCl to active Cl species, after which the Hg(0) could react with Cl to form HgCl2. Additionally, doping IrO2 could significantly improve the Cl yield process. In summary, the novel IrO2 modified catalyst displayed excellent catalytic activity for elemental mercury oxidation, and the proposed reaction mechanisms were determined reasonably.

Ball milling synthesized MnOx as highly active catalyst for gaseous POPs removal: significance of mechanochemically induced oxygen vacancies

A rapid (1.5 h) one-step ball milling (BM) method was developed not only to modify commercial MnO2 via top-down approaches (BM0), but also to bottom-up synthesize MnO(x) by cogrinding of KMnO4 and MnC4H6O4 (BM1) or KMnO4 and MnSO4 (BM2). Catalysts activity on gaseous POPs removal was tested using hexachlorobenzene (HCBz) as surrogate. Catalytic performance decreases in the order of BM2 ≈ BM1 (T90% = 180-200 °C) > BM0 (260 °C) > CMO ≈ cryptomelane MnO2 (>300 °C). Both adsorption and destruction contribute to HCBz removal at 180 °C while destruction prevails at 200-300 °C. Mechanism studies show that destruction activity is lineally correlated with the amount of surface reactive oxygen species (Oads); stability is determined by the removal of surface chloride, which is associated with the mobility of bulk lattice oxygen (Olat); adsorption capacities are linearly correlated with surface area and pore structure. With the aid of extensive characterizations the excellent performance of BM prepared samples can be explained as (1) abundant surface vacancies enhance the generation of Oads; (2) massive bulk vacancies promote the mobility of bulk Olat; (3) large surface area and uniform pore size distribution facilitate the physisorption of HCBz.