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Au/Ceo2 Catalysts: Structure and CO Oxidation Activity

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Au/Ceo2 Catalysts: Structure and CO Oxidation Activity catalysts Review Au/CeO2 Catalysts: Structure and CO Oxidation Activity Miguel Angel Centeno 1,*, Tomás Ramírez Reina 2, Svetlana Ivanova 1, Oscar Hernando Laguna 1 and José Antonio Odriozola 1 1 Departamento de Química Inorgánica, Universidad de Sevilla e Instituto de Ciencias de Materiales de Sevilla Centro mixto US-CSIC Avda. Américo Vespucio 49, Seville 41092, Spain; [email protected] (S.I.); [email protected] (O.H.L.); [email protected] (J.A.O.) 2 Department of Chemical and Process Engineering, University of Surrey Guildford, Guildford GU2 7XH, UK; [email protected] * Correspondence: [email protected]; Tel.: +34-95-4489543 Academic Editors: Leonarda F. Liotta and Salvatore Scirè Received: 6 September 2016; Accepted: 11 October 2016; Published: 18 October 2016 Abstract: In this comprehensive review, the main aspects of using Au/CeO2 catalysts in oxidation reactions are considered. The influence of the preparation methods and synthetic parameters, as well as the characteristics of the ceria support (presence of doping cations, oxygen vacancies concentration, surface area, redox properties, etc.) in the dispersion and chemical state of gold are revised. The proposed review provides a detailed analysis of the literature data concerning the state of the art and the applications of gold–ceria systems in oxidation reactions. Keywords: gold–ceria catalysts; oxidation reactions; preferential oxidation reactions (PROX); water gas shift (WGS) 1. Introduction Carbon monoxide (CO) is one of the extended pollutants of our environment, thus requiring prevention and control to insure the adequate protection of public health [1]. Carbon monoxide is hardly detectable, colorless, tasteless, odorless, non-irritating very toxic gas for humans and animals because of its high affinity to hemoglobin and the cell’s oxygen transport blocking. Exposure to CO must never exceed 25 ppm in 8 h or 50 ppm in 4 h because, above these limits, important detrimental effects are observed up to the lethal concentrations of around 650–700 ppm [2]. Moreover, carbon monoxide is also a precursor of ground-level ozone, which can generate serious respiratory problems. All of these environmental issues render CO oxidation reaction the preferred option for CO depletion in polluted atmospheres. In 1987, almost 30 years ago, the discovery of Haruta and co-workers revolutionized the concept of CO oxidation for environmental proposes [3]. They demonstrated that very small gold nanoparticles are extremely active for carbon monoxide oxidation at sub-ambient temperatures. This achievement provoked a great scientific interest converting gold in the most popular metal for the catalytic oxidation of CO [4–28]. Figure1 shows the growing trend in a number of publications per year involving CO oxidation using gold-based catalysts. A several fold increment in the last decade of the publications with search strings “CO oxidation” and “gold-based catalysts” was observed indicating the importance of this catalytic process in heterogeneous catalysis. A number of other possible applications for gold-based catalysts able to perform CO oxidation at room temperature appear such as carbon dioxide lasers, gas sensors, respirators for protecting firefighters, and miners from CO poisoning, air-cleaning devices, etc. [29]. Catalysts 2016, 6, 158; doi:10.3390/catal6100158 www.mdpi.com/journal/catalysts Catalysts 2016, 6, 158 2 of 30 Catalysts 2016, 6, 158 2 of 29 1800 1600 1400 1200 1000 800 600 Number of Number publications 400 200 0 1985 1990 1995 2000 2005 2010 2015 year Figure 1.1.Number Number of of published published papers papers from from 1985 1985 in the in Scopus the Scopus Scholar Scholar database database using “gold using catalysts” “gold andcatalysts” “CO oxidation” and “CO oxidation” as search strings as search (September strings (September 2016). 2016). Out of thethe environmentenvironment protection issue, the CO abatement is also a key reaction in many industrialindustrial processes. The water gas shiftshift reactionreaction (CO(CO ++ HH22O $↔ CO2 + H H22)) is is one one of of these these reactions. Despite the fact that the reaction was described at the end of the 18th century, itsits mainmain applicabilityapplicability waswas notnot foundfound before thethe 1920s, when the Haber–Bosh process for ammonia synthesis called for pure hydrogen production. From this moment on, the water gasgas shiftshift (WGS)(WGS) reactionreaction gainedgained importanceimportance and becamebecame an inseparableinseparable partpart ofof variousvarious industrialindustrial processes,processes, suchsuch asas ammoniaammonia andand methanolmethanol production,production, hydrogen hydrogen supplies supplies for thefor petrochemicalthe petrochemical industry, industry, Fisher–Tropsch Fisher–Tropsch reactions, reactions, and numerous and reformingnumerous reactions.reforming reactions. Presently, aa renewedrenewed interestinterest inin thethe WGSWGS reactionreaction emergesemerges associatedassociated toto thethe hydrogenhydrogen fuel cellcell (FC)(FC) technologies.technologies. Hydrogen as an energy carrier isis usuallyusually producedproduced fromfrom hydrogen-containinghydrogen‐containing molecules, such as hydrocarbons,hydrocarbons, ammonia,ammonia, or chemicalchemical hydrideshydrides [[30].30]. The bestbest short-termshort‐term optionoption seems toto bebe the the hydrogen hydrogen production production via via hydrocarbons hydrocarbons reforming. reforming. However, However, the the resulting resulting products products are aare mixture a mixture mainly mainly constituted constituted by hydrogen by hydrogen and carbon and carbon oxides [31oxides]. The [31]. presence The presence of CO in theof CO hydrogen in the flows,hydrogen even flows, in small even amounts in small at the amounts ppm level, at inactivatesthe ppm level, the fuel inactivates cell electrodes. the fuel Therefore, cell electrodes. to avoid fuelTherefore, cell anode to avoid poisoning, fuel cell several anode fuel poisoning, processing several steps, fuel including processing CO steps, removal including by means CO of removal a WGS andby means CO preferential of a WGS and oxidation CO preferential reactions (PROX)oxidation or reactions methanation (PROX) must or be methanation applied. The must development be applied. ofThe efficient development catalysts of efficient for these catalysts processes for isthese fundamental processes is to fundamental guarantee the to successguarantee of the the success so-called of “hydrogenthe so‐called economy”. “hydrogen economy.” Among all thethe reportedreported catalysts,catalysts, gold-basedgold‐based materials have been broadly employed for the WGSWGS reactionreaction showingshowing promisingpromising results inin manymany casescases [[32–34].32–34]. Some of thethe mainmain benefitsbenefits ofof usingusing gold-basedgold‐based catalystscatalysts include include good good activity activity in the low-temperaturein the low‐temperature range, the absencerange, ofthe pre-treatment, absence of andpre‐treatment, pyrophoricity. and Nevertheless, pyrophoricity. these Nevertheless, systems also these show drawbackssystems also including show drawbacks extreme sensibility including to theextreme preparation sensibility procedure to the (existence preparation or not procedure of gold nanoparticles), (existence or the not importance of gold nanoparticles), of a support nature the (especiallyimportance development of a support ofnature an active (especially gold–support development interface), of an andactive relatively gold–support low catalyst interface), stability and attributedrelatively low to support catalyst deactivation. stability attributed However, to support when these deactivation. parameters However, are carefully when controlled these parameters and the supportare carefully is prudently controlled selected, and the gold-based support is materials prudently are selected, outstanding gold‐ forbased the materials WGS reaction. are outstanding for theThe WGS production reaction. of hydrogen pure enough to feed polymer electrolyte membrane fuel cell (PEMFC) requiresThe furtherproduction clean-up of hydrogen processes pure after enough the WGS to unit feed in polymer order to reduceelectrolyte the COmembrane concentration fuel cell to compatible(PEMFC) requires levels, typically further 10–50 clean ppm‐up dependingprocesses after on the the cell WGS anode unit [10]. Thein order preferential to reduce CO oxidation the CO inconcentration rich-H2 streams to compatible (Scheme1) haslevels, been typically proposed 10–50 as cheap ppm and depending efficient alternative on the cell for anode the elimination [10]. The ofpreferential the final traces CO oxidation of CO [35 –in38 rich]. The‐H2 oxidation streams (Scheme is rapid and1) has responds been proposed quickly to as changes cheap and in operating efficient alternative for the elimination of the final traces of CO [35–38]. The oxidation is rapid and responds quickly to changes in operating conditions, but it is essential to choose catalysts and operating Catalysts 2016, 6, 158 3 of 30 Catalysts 2016, 6, 158 3 of 29 conditionsconditions, that but itminimize is essential the to oxidation choose catalysts of hydrogen. and operating In this regard, conditions control that of minimize temperature the oxidation is found toof be hydrogen. particularly In this important regard, control[10]. of temperature is found to be particularly important [10]. Scheme 1. Block diagram showing the possible paths for oxidizing CO–H2 mixture. Attending the above
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