Development and use of catalysts has greatly progressed over past decades and they are used for nearly 80% of all chemical production. It is envisioned that future development of existing catalysts or identification of novel materials for the next generation of catalysts will rely on rational design,which requires an understanding of the complete catalytic reaction system at the atomic scale [1]. A better understanding of the atomic-scale structure of a catalyst requires investigation of model catalysts with simple structures,such as single crystalline films or single crystal surfaces [2]. The mechanisms of real catalytic processes are highly complicated and it is difficult to identify the micro-steps in these reactions. Therefore,analysis of catalytic performance — including the study of elementary reactions steps,reaction intermediates and related topics — is typically accomplished via a combination of surface science and experimental methods. Such studies have led to an in-depth understanding of catalytic reactions under real-life conditions.
Many catalysts,such as noble metals and transition metal oxides,have been explored for gas-solid reactions. Recently,supported ruthenium (Ru)-based catalysts were employed in many gas-solid reactions because of their excellent activities under low temperature and pressure conditions.
This review describes properties of Ru,the oxidation of Ru metal and the theoretical research on Ru-based catalysts. In addition,we explain the gas-solid reactions catalyzed by Ru-based materials. As shown in Fig. 1,the catalytic applications of these materials include the catalytic oxidation of volatile organic compounds (VOCs) [3-5],preferential oxidation of CO (PROX) [6, 7],synthesis of ammonia [8],oxidation of HCl (the Deacon process) [9] and partial oxidation of CH4 [10-12]. Finally,this review summarizes existing problems as well as future developmental trends for Ru-based catalysts.
Ru is a rare transition metal belonging to the platinum group in the periodic table. The electronic structure of elemental Ru is 4d75s1 and Ru exhibits a rich variety of oxidation states,including +2,+3,+4,+6 and +8. Ru materials are mainly composed of metallic Ru,RuO2 and RuO4. The boiling points of these forms are 4100 °C [13, 14],1400 °C and 100 °C [15, 16],respectively. The crystal planes of RuO2 are illustrated in Fig. 2. In the bulk structure of RuO2 (rutile structure),the Ru atoms bind to six oxygen atoms,forming a slightly distorted RuO6 octahedron,while the oxygen atoms are coordinated to three Ru atoms in a planar configuration that is consistent with the sp2 hybridization of oxygen [1].
Volatility at high temperature is recognized as a serious limitation in applications of Ru-based catalysts. In Ru-based catalysts for gas-solid reactions,metallic Ru and/or RuO2 have been commonly identified as the active species and the reaction temperatures are usually below 400 °C,much lower than the boiling points of Ru and RuO2. Hence,volatility of Ru species is not observed in most gas-solid catalytic reactions and there have been no reports of this behavior for such reactions.
The transition from oxygen adsorption to oxide formation on Ru is structurally complex because of the various phases of oxygen on the surface [17]. However,the orientation of the RuO2 depends on the orientation of the Ru. H. Over et al. [18] demonstrated that the active part of this “O-rich” Ru phase is RuO2,which grows epitaxially with its (110) plane parallel to the Ru (0001) surface at high O2 partial pressure and elevated temperatures. When the Ru (0001) surface is exposed to molecular oxygen under UHV conditions,a (2×2)-O layer and a (2×1)-O overlayer are formed [19]. At much higher concentrations of oxygen,for example,more than 1 000 L at room temperature,the Ru (0001) surface stabilizes two additional phases of chemisorbed oxygen,namely,(2×2)-3O and (1×1)-O [20]. However,Ru (0001) is oxidized by an autocatalytic process,because of formation of oxide nuclei on the Ru(0001)-(1×1)O surface [17]. The autocatalytic reaction indicates that the surface produces its own ‘catalyst’,thus readily dissociating the oxygen molecules and resulting in self-acceleration of the oxidation process [21]. This complexity is characteristic of oxygen chemistry on many transition metal surfaces,as shown in Fig. 3. The oxidation of Ru occurs only under specific conditions and requires a minimum threshold temperature. Goodman et al. [22] discovered that formation of RuO2 on Ru (0001) required,at a minimum,an O2/CO pressure ratio of 18/10,a temperature of 360 °C,and a reaction time of 700 s. To generate RuO2,the oxygen must cover a volume of 4 ML and must have at least two layers of the tri-layer structure [23].
Relatively simple reaction structures are usually chosen to represent the atomic-scale structure of the catalyst for study. CO oxidation is one of the most extensively studied reactions because it is relatively simple and includes all basic steps of a heterogeneous process,that is,adsorption,diffusion,dissociation,reaction and desorption [24]. In addition,it is also the most important part of the PROX reaction.
Ru has long been known as an exception in the Pt group metals,in terms of its behavior towards the oxidation of CO. Under UHV conditions,Ru is an extremely poor catalyst for oxidation of CO [25],yet its activity becomes greater than that of the other Pt group metals under high pressure and oxidizing conditions [26-28]. The reaction proceeds through the Langmuir-Hinshelwood (L-H) mechanism which requires the chemisorption of reactants before their recombination [28]. Stampfl et al. [29] found that formation of (1×1)-O overlayer on a Ru (0001) single crystal surface caused the material to become nearly inactive. It was proposed that the “pressure gap” can be bridged by forming RuO2 on Ru catalysts at elevated reactant pressures [21]. Over et al. [2] discussed the complex structure-activity correlation in CO oxidation on Ru,based on results of in situ surface X-ray diffraction (SXRD) and on-line mass spectrometry measurements. They discovered that the activity of the catalyst increased when it underwent structural changes,either because it was reduced or because it formed RuO2 (110). According to density-functional theory (DFT) calculations (Fig. 4) reported by Over et al. [18],the cus (a coordinative unsaturated site) Ru atom has an exposed dangling bond at the surface,explaining the high reactivity of the RuO2 (110) surface. Gao et al. [30] expressed a different view that a chemisorbed oxygen surface is the most thermodynamically stable phase and active phase for CO oxidation. The core-shell structure — with an ultrathin RuO2 shell coating a metallic Ru core — was identified as the most active and stable state. In contrast,Qadir et al. [31] showed that smaller Ru nanoparticles could form a huge bulk of RuO2 on their surfaces,l eading to a lower catalytic activity. They reported that core-shell type RuO2 became more stable as the size of the nanoparticle increased.
VOCs emitted from industrial processes and transportation activities are regarded as major contributors to air pollution and as dangerous to human health. Approaches for removal of VOCs include thermal incineration,catalytic oxidation and adsorption-based techniques. Because of advantages such as high efficiency,low temperature and high selectivity for harmless products,catalytic oxidation is considered a promising pathway to reduce VOCs emissions and meet the standards of air pollution control regulations.
Various catalysts,such as noble metals (Pt,Pd,and Au) [32-34] and transition metal oxides (Co3O4,MnOx,and V2O5) [35-38],have been utilized for complete oxidation of VOCs. However,use of these catalysts often presents problems,including relatively low activity,rapid deactivation by chlorine poisoning or coking,high cost and formation of undesired polychlorinated benzenes. VOCs-oxidation catalyzed by Ru-based materials,compared with by other noble metals,has been much less studied. However,recently,Ru-promoted oxidation of VOCs was broadly reported because of its unique properties,with these reports summarized in Table 1.
Most recently,our group [39] prepared the Ru-based catalysts Ru/TiO2,Ru/SiO2,Ru/γ-Al2O3 and Ru/ZrO2 and evaluated their usefulness for CH3Br oxidation. We found that Ru/TiO2 performed better catalytically than the other materials. By transmission electron microscopy (TEM) characterization,the Ru species were primarily distributed on the rutile phase of P25 TiO2. Ru/TiO2 was also employed in the catalytic oxidation of CO,benzene,methyl acetate and multi-pollutants of a simulated PTA off-gas. Interestingly,when the temperature was below 200 °C,all pollutants had conversions below 10%,results far different from those of single-pollutant tests (Fig. 5). We tentatively propose that the oxygen-rich nature of methyl acetate was the key factor in its being oxidized far more rapidly when in competition with other compounds on the catalyst surface.
Using different types of TiO2 (anatase,P25 and rutile) as supports,our group [40] synthesized supported-Ru catalysts with homogeneously distributed Ru particles via wet impregnation. All of these catalysts were very active for trichloroethylene oxidation,but we found that Ru oxide species formed during the reaction were very unstable in the anatase phase of TiO2. In the presence of the rutile phase of TiO2,which had a similar structure to that of RuO2,the Ru species tended to distribute on the rutile TiO2. The Ru/P25 catalyst,with a larger particle size,showed apparently better catalytic performance than the other two samples. This was tentatively attributed to a size effect,which has been well demonstrated in Pt catalytic systems. The existence of additional water vapor greatly increased HCl formation and reduced the amount of tetrachloroethylene and pentachloroethane as the organic byproducts,whereby catalytic activity was also suppressed,as shown in Fig. 6.
Our group [41] reported Ru/Co3O4-metal-organic framework (MOF)- catalyzed toluene oxidation. In the preparation,the Co3O4-MOF support was synthesized through a template method for MOF-synthesis and the catalyst was then obtained by the impregnation method. For comparison,the Ru/Co3O4-B catalyst was also prepared by the traditional precipitation-impregnation method. We showed that Ru/Co3O4-MOF exhibited far better catalytic activity,CO2 selectivity,stability and anti-moisture properties than other materials. BET,XRD and XPS characterization revealed that Ru/Co3O4-MOF had a larger surface area than Ru/Co3O4-B,and that Ru species were well dispersed on the surface of Ru/Co3O4-MOF (Fig. 7).
Huang et al. [42] loaded Ru on CeO2 with different morphological structures,such as nanorods,nanocubes and nanoocto. Their characterization studies revealed that Ru/CeO2-r exhibited the most abundant Ru4+,oxygen vacancies and Ru-O-Ce bonds. The investigators concluded that a strong interaction existed between Ru and the CeO2-r support. Indeed,chlorobenzene oxidation tests showed that Ru/CeO2-r gave the highest catalytic activity,demonstrating that the activity of Ru/CeO2 for chlorobenzene was greatly influenced by the shape/crystal plane (Fig. 8).
Dai et al. [43] studied the mechanism of CB oxidation over Ru/CeO2 catalysts and proposed that the C-Cl cleavage would first occur at the Ce3+/Ce4+ active sites,forming Cl2 through the Deacon process. The partial C and H of the intermediates would then be oxidized into CO2 and H2O by the active oxygen and lattice oxygen on the catalyst surface and the Cl2 would finally be adsorbed on the active sites,leading to deactivation of the catalyst (Fig. 9).
In their next study,Dai et al. [44] doped TiO2 into 1%Ru/CeO2 to obtain 1%Ru/Ti-CeO2,reporting an increased activity of this catalyst for CB oxidation. They further reported that 1%Ru/5%Ti-CeO2 showed the highest activity. H2-TPR and TEM results showed that 1%Ru/Ti-CeO2 exhibited great reducibility at low temperatures because of its active oxygen-rich nature and that the strengthened interaction between Ru and CeO2 was an oxidation,ascribed to the high-energy lattice planes of CeO2 (110) and (100). Okal et al. [45] employed 4.9%Ru/γ-Al2O3 and a H2-reduced catalyst in propane. The T95 values of the former and latter catalysts were 195 and 205 °C,respectively. By TEM characterization,the former exhibited Ru particles with an average size of 1.6 nm,whereas those in the latter had an average size of 6.0 nm. In another study,Okal et al. [46] prepared Ru/ZnAl2O4 catalysts with different Ru contents (0.5%,1%,and 4.5%),and employed these catalysts for propane oxidation. The 4.5% Ru/ZnAl2O4 showed the highest catalytic efficiency. XRD and HRTEM/SAED results revealed that the Ru species had a small particle size and these were well dispersed on ZnAl2O4. The decreased ZnAl2O4 particle size facilitated the catalytic efficiency. Mitsui et al. [48] compared the catalytic performance of CeO2-supported noble metals,such as Ru,Pt,Pd and Rh,in the oxidation of ethyl acetate and found that the Ru-containing catalyst demonstrated the highest catalytic activity with a T90 = 190 °C. Okal et al. [49] reported the preparation of washed 4.6% Ru/γ-Al2O3 and their XPS spectra revealed a very low chlorine content. Catalytic oxidation of propane over unwashed and washed catalysts gave T50 = 315 and 175 °C,respectively. It was proposed that chlorine extensively occupied the active sites on the catalyst surface [50, 51]. Aouad et al. [52] prepared Ru/CeO2 with different Ru contents of (0.67%,1%,3% and 5%). Based on catalytic oxidation of propene,the catalytic efficiency was slightly changed when the Ru content was above 1%. Based on TPR results,the Ru species were saturated on the catalyst surface for 1%Ru/CeO2. Miranda et al. [53] employed 0.5%Ru/Al2O3 for trichloroethene oxidation. Compared with the blank test,they found that T50 was decreased from 700 to 326 °C. Hosokawa et al. [54] loaded Ru on different supports (CeO2,γ-Al2O3,and ZrO2) and employed these for propene oxidation. RuO2/CeO2 exhibited the highest catalytic activity,with T100 = 423 °C,and it was proposed that propene molecules were easily influenced by Ru=O.
Currently available studies were primarily focused on the catalytic oxidation of single VOC pollutant and catalytic evaluation and stability test were commonly reported. The detailed oxidation process has been explored far less frequently,though this would inform catalyst development. In addition,reaction conditions closer to those involving real waste gas should be employed in such studies. Hence,mechanistic and kinetic studies and catalytic oxidation of VOCs with multi-pollutants should be strongly encouraged for inclusion in future research.
In recent years,hydrogen-polymer electrolyte membrane fuel cells (H2-PEMFC) have attracted increasing interest as an environmentally compatible supply of electric energy [55, 56]. In these fuel processors,hydrocarbons are transformed into H2. PROX is an essential step in the fuel processing system for a low temperature H2-PEMFC and is employed to remove residual CO from an H2-rich stream. This is done because CO can degrade the electrochemical performance of the PEMFC.
In the PROX system,CO oxidation (Eq. (1)),H2 oxidation (Eq. (2)),CO methanation (Eq. (3)),CO2 methanation (Eq. (4)) and the water-gas shift reaction may occur concurrently because all of the reactants (CO,CO2,H2,O2,and H2O) coexist in the system. Consequently,undesired reactions,such as H2 oxidation,CO methanation and CO2 methanation,must be minimized during PROX. For practical applications,PROX catalysts should have a wide temperature window and be able to perform under low temperatures to facilitate complete CO removal. Thus,we concluded that the Ru-based catalysts are suitable for PROX systems,based on recent reports using these agents,as summarized in Table 2.
Kim et al. [57] obtained SiO2 with different surface areas by varying the calcination temperatures for supports and the Ru/SiO2 catalysts were synthesized through the incipient-wetness impregnation method. These investigators observed that the surface area decreased with increased calcination temperature and the Ru particle size increased with decreased surface area. When the calcination temperature for support was 830 °C,the Ru/SiO2 was more effective than other materials. In another study [58],a series of Ru-based catalysts,Ru Ru/x-Al2O3 (x = α,κ,γ,η,δ,and θ),were employed in PROX. Ru/α-Al2O3 was the most efficient catalyst,with an average Ru particle size of 2.6 nm and well dispersed Ru particles on the catalyst. H2-TPR experiments for Ru/α-Al2O3 showed that Ru species could be reduced at a low temperature of 100 °C. Kim et al. [59] loaded Ru on different supports (yttria-stabilized zirconia (YSZ),ZrO2,TiO2,SiO2,γ-Al2O3) via wetness-impregnation. In the PROX reaction,Ru/YSZ showed an apparently higher catalytic activity than other materials. On TEM images,Ru species were well dispersed on the YSZ support and the average Ru particle size was 1.2 nm. The outlet CO concentration was below 10 ppm,even when CO2 (17.4 vol%) and H2O (13.0 vol%) were introduced. Kim et al. [60] examined γ-Al2O3supported noble metal catalysts in which the metal content was (5% or 0.5%),at different reaction temperatures,as shown in Fig. 10. They found that the catalytic activity of 5% Ru/γ-Al2O3 was the highest,compared with those of other catalysts. In the absence of H2O and CO2,100% CO conversion could be achieved over 5% Ru/γ-Al2O3 over a wide range of reaction temperatures,from 60 to 200 °C. Though 0.5 wt% Ru/γ-Al2O3 and 5 wt% Ru/γ-Al2O3 had similar particle size distributions by TEM analysis,the latter catalyst,with a smaller amount of chemisorbed CO and O2 per Ru atom,showed much better PROX activity.
Hidenobu et al. [61] studied the influence of NH3 on Ru/Al2O3-catalyzed selective CO oxidation and found that the catalyst showed lower activity in the presence of NH3 at a high ratio of [O2]/[CO]. It was suggested that high O2-concentration facilitated oxidation of NH3 and generation of nitrosyl,which led to the deactivation of the catalyst. Similarly,the presence of NO or NO2 also resulted in nitrosyl-induced deactivation (Fig. 11). Chin et al. [62] examined the influence of Ru precursors,H2O and CO2 on the Ru/Al2O3-catalyzed PROX reaction. The activity of the catalyst made from nitrate precursors (Ru(NO)(NO3)3·xH2O) was apparently higher than those made from chloride hydroxyl precursors (RuCl3·xH2O,Ru3(CO)12). 99% CO conversion was achieved at 140 °C with the catalyst using Ru(NO)(NO3)3·xH2O as a precursor. The high turnover frequency (TOF) was ascribed to the homogeneous dispersal of Ru species,as seen from HRTEM images. The introduction of 10% H2O had a significant inhibitory effect when the reaction temperature was below 180 °C and a promotional effect was demonstrated clearly when the temperature was above 200 °C. Moreover,15% CO2 severely inhibited CO oxidation. Han et al. [64] employed Ru/γ-Al2O3 to remove CO from fuel gas and the CO conversion reached 96% at 75 °C. In this study,TEM images showed a Ru particle size of 2.5 nm.
Simply stated,the key to the application of PROX is to develop a highly active and selective catalyst that could be employed over a wide temperature range and that exhibits good resistance to CO2 and H2O. Introduction of the second transition metal component to form a bimetallic catalyst [6] or core-shell structure [31] may be a useful approach to enhance low temperature activity for PROX.
Ammonia and its processed products play significant roles in the pharmaceutical,metallurgy and environmental protection industries. However,ammonia synthesis is highly energy consuming. The primary difficulty in ammonia synthesis originates from the character of the N≡N bond,which is the strongest bond among diatomic molecules. The role of the catalyst is to lower the dissociation energy of the N≡N bond [65]. To decrease the energy required for this reaction,significant effort has focused on the search for efficient catalysts for ammonia synthesis at low temperature and low pressure.
Iron (Fe)-based materials are well established as firstgeneration ammonia synthesis catalysts. Yet,the requirements of high temperature and pressure directly lead to high costs and energy consumption. In the seventies,Aika et al. [66] found that Ru-K/AC was very efficient for ammonia synthesis,10 times more than was the Fe-based catalyst at 250 °C,under ordinary pressure. This report received considerable attention from researchers worldwide. Compared with the traditional Fe catalyst,the supported-Ru catalyst was more reactive at lower temperatures under lower pressures [67, 68]. Additionally,the Ru catalyst is insensitive to H2O,COx and NH3 concentrations and,thus,is regarded as the ideal second-generation ammonia synthesis catalyst. Many studies proved that the catalytic performance of Ru-based catalysts for ammonia synthesis depended on the nature of the supports. It was proposed that strong interactions between Ru and the support led to rapid electron transfer from the support to Ru particle surface. Doping of other elements might provide more oxygen vacancies and effectively prevent catalyst agglomeration from high temperatures calcination. The results of recent studies on Ru-catalyzed ammonia synthesis are summarized in Table 3.
Wang et al. [69] prepared Ru/BaTiO3 with the impregnation method and observed that the catalyst exhibited higher efficiency for ammonia synthesis than Ru/TiO2,Ru/MgO or Ru/CeO2. They proposed that strong interactions between Ru and the support led to rapid electron transfer from the support to the Ru particle surface. This would greatly promote the cleavage of N≡N and improve the catalytic activity for ammonia synthesis. Zhou et al. [70] modified mesoporous carbon (MC) by doping SiO2 to change its surface area and pore size and prepared Ba-Ru-K/MC,using modified MC as the support. They found that the particle size of SiO2 and the ratio of SiO2 to MC greatly affected the surface area and the pore size of the modified MC. The catalytic activity for ammonia synthesis increased with the improved surface area found in the modified MC. Lin et al. [71] employed K-Ru/Ba-ZrO2 for ammonia synthesis at high temperatures and found that formation of BaZrO3 and introduction of K+ improved electronic conductivity and basicity. Zhang et al. [72] loaded Ru on CeO2 doped with Sm (0,2,5,7 and 10 wt%) and showed that the resultant Ru/Sm2O3-CeO2 was most efficient when the Sm content was 7 wt%. The introduction of Sm3+ prevented sintering of the catalyst,preserved its morphology by generating more oxygen vacancies and facilitated CeO2 reduction. Pan et al. [73] investigated Fe-Ru/AC-catalyzed ammonia synthesis under various conditions (temperature,space velocity,pressure and H2/N2 ratio) and also conducted tests with Fe/AC and Ru/AC for comparison. Yang et al. [74] discovered that Ru/BaCeO3 yielded higher catalytic activity than Ru/γ-Al2O3,Ru/MgO,or Ru/CeO2. XRD,HRTEM,XPS,and H2-TPR data confirmed that strong interactions between Ru and the support facilitated the electron transfer process and that Ce3+ was crucial to the reduction of the Ru species. Luo et al. [75] applied Ru/CeO2-La2O3 to ammonia synthesis. At 188 °C and 10 MPa,with a La2O3 content of 10% and an NH3 concentration of 13.9%,the catalytic activity of Ru/CeO2-La2O3 was apparently higher than that of Ru/CeO2. These investigators concluded that introduction of La2O3 facilitated oxygen reduction on the catalyst surface and enhanced stability of the catalyst. Lin et al. [76] prepared Cl-depleted Ru/γ-Al2O3 through hydrazine reduction and observed that the chlorine content was lowest when the mole ratio of hydrazine to Ru was 3 and the corresponding catalyst had the highest activity. Saito et al. [77] synthesized a MgO-CeO2 support through a co-precipitation method and found that its surface area decreased with increasing CeO2 content. The modified Ru particle distribution was attributed to isolation of CeO2 by amorphous MgO,formed after loading Ru. The partially reduced CeO2-δ (0 ≤ δ ≤1) would facilitate the electron transfer to Ru and thus promote cleavage of N≡N,whereby the catalytic activity for ammonia synthesis was improved. For example,when CeO2 content was 50 mol%,the rate of ammonia synthesis in the presence of Ru/(MgO-CeO2) was 8 times higher than that in the presence of Ru/MgO. Xu et al. [78] prepared K-Ru/CNTs-K-Ru/MgO by mixing K-Ru/MgO and K-Ru/CNTs (carbon nanotubes) in acetone. They found that this catalyst did not have the drawbacks of MgO,including low basicity of CNTs and poor electron transfer ability,and the activity of the composite catalyst was substantially higher than those of catalysts with single supports (Fig. 12).
Recycling elemental chlorine is important to reduce HCl emissions and to meet the increasing demand for Cl2. An attractive route for chlorine recovery is the gas-solid catalytic oxidation of HCl with air or oxygen,the Deacon reaction. Supported CuCl2 catalysts are the preferred catalysts for the Deacon process,even though they still exhibit serious drawbacks when used at 430-500 °C. These include limited activity,rapid deactivation due to volatilization of CuCl2 above 400 °C and corrosion caused by unreacted HCl and the product H2O.
In 2003,Sumitomo Corporation [79, 80] reported novel Ru-based catalysts and applied them in industrial plants. In these catalysts,TiO2,ZrO2,Al2O3 or zeolite were used as supports and the TiO2 supported material showed the highest efficiency. Ru content was within the range of (2-6) wt%. In addition,some dopants such as Pd,Cu,Cr,V,rare earth compounds or alkali metal compounds were added to the catalysts to enhance their catalytic performance. At standard pressure,a space velocity of 20-1000 h-1 and a temperature of 200-380 °C,the HCl conversion reached 95.9% and the lifetime exceeded 16000 h. Nonetheless,the catalytic activity decreased with time and irreversible deactivation usually occurred as a result of the impurities in the HCl gas or because of errors in technical operation. BASF Corporation [81] used RuO2,RuCl3,or other Ru precursors to synthesize Ru catalysts loaded on SiO2,Al2O3,TiO2 or ZrO2. In HCl oxidation,these investigators observed that the per-pass conversion of HCl reached (40-90)%. López et al. [82] studied the mechanism of HCl oxidation over RuO2/TiO2 and their DFT studies revealed that the reaction mechanism was consistent with the Mars-van-Krevelen mechanism in 5 steps. These steps were hydrogen abstraction from HCl (6),recombination of atomic chlorine (7),hydroxyl recombination (8),water desorption (9) and dissociative oxygen adsorption (10).
It was also discovered that Cl2 conversion increased with an increased ratio of O2 to HCl. This change was attributed to the increased O concentration on the catalyst surface and the decreased binding energy of Cl. Seitsonen et al. [83] also studied the mechanism of HCl oxidation over RuO2(110)/TiO2(110) using DFT studies. As shown in Fig. 13,a calculated 1 ML (monolayer) s-RuO2 was loaded on TiO2,affording a well-defined s-RuO2@TiO2 catalyst model.
When 1/2O2 was introduced,Oot was formed at the 1f-cus (the one-fold under coordinated metal site) site. Some Oot sites adsorbed HCl,forming OotH and Clot. In addition,other surface HCls were adsorbed,forming OotH2 and 2Clot and,finally,H2O and Cl2 were desorbed and released. Recently,scientists from Tsinghua University [84] reported a series of monolithic catalysts,composed of FeCrAl alloy or cordierite as the skeleton,(7-12) wt% supports and (1-3) wt% active species RuO2 (110). In HCl oxidation,87.3% HCl conversion was achieved at a low temperature of 280 °C. Over [85] compared activities of several types of RuO2-based catalysts for HCl oxidation,reporting that the catalytic activity of RuO2-rutile-TiO2 was significantly higher than that of RuO2-anatase-TiO2 and other catalysts (Fig. 14).
In fact,the actual reaction process is more complicated because of differences in properties of the oxygen species on the surface and in oxygen and chlorine coverage. For instance,O in Eqs. (6) and (8) can be either Ocus or Ob and each should be simulated separately. The lattice oxygen Ob plays an extremely important role in the process of hydrogen abstraction from HCl and hydroxyl recombination in water formation. However,the structural oxygen in ruthenium oxide (Ob) cannot be removed as water (H2Ob). In contrast,Ocus is crucial to formation of hydroxyl groups on the surface and water molecules and the binding energy of the latter is sufficiently low under reaction conditions [82]. Therefore,the Ru-based catalyst must be able to supply enough oxygen species and oxygen vacancies under actual reaction conditions and to prevent catalyst deactivation because of the existence of the Cl element.
The conversion of methane to syngas is commonly classified into three methods: steam reforming,partial oxidation of CH4 and CO2 reforming. Among those approaches,partial oxidation of CH4 has been recognized as the most efficient method because of its low energy cost,high reaction rate and the convenient synthesis of methanol. The partial oxidation of CH4 is often performed on Ni-based catalysts. However,the reaction must be conducted at high temperatures to activate the metal nickel,which causes catalyst deactivation by sintering and carbon deposition [86, 87]. This is the motivation for developing stable and active catalysts that can operate at the low temperatures. Nishimoto et al. [88] prepared 0.5%Ru/Y2O3 and employed it for partial oxidation of CH4 at 600 °C,with CH4/O2 ratio = 5. The CH4 conversion was 27% with n(CO)/n(H2) = 0.5 and the selectivity was 75%. No carbon deposit was observed on the catalyst surface after 10 h of stream reaction. Figen et al. [89] found that complete oxidation was the dominant process within the temperature range of 600-650 °C and partial oxidation was dominant when the temperature was above 700 °C. Cordierite-supported Co,Co-Ni,Co-Ru,Co-Ni-Ru and Ni catalysts were prepared through the impregnation method and used for partial oxidation of CH4. TG-FTIR results illustrated that Co-Ni-Ru exhibited less carbon deposits than did other materials,primarily because of the catalytic efficiency and stability imparted by Ru. The conversion of CH4 was 93.96% at 800 °C and the selectivity to H2 and CO was 91.81% and 90.85%,respectively.
Ru-based catalysts have attracted considerable attention as models for surface science and computational studies. Ru-based catalysts have been well established in various gas-solid catalytic reactions,such as the catalytic oxidation of VOCs,PROX,ammonia synthesis,the Deacon process and partial oxidation of CH4,because of their advantages including high activity,great stability and low yield of byproducts. Catalytic performance is greatly influenced by the physicochemical properties of the catalysts. Morphology and size effects have been well demonstrated in catalytic oxidation of VOCs and PROX and in partial oxidation of CH4. For ammonia synthesis,it is crucial to ensure rapid electron transfer,and this is attributed to strong interactions between Ru and the support. In the Deacon process,active oxygen species and oxygen vacancies play important roles.
However,challenges still exist in the field of catalytic systems and novel catalysts and,also,their reaction mechanisms should be explored in future research. For example,more formidable catalysts are needed for oxidation of VOCs with multi-pollutants. Many studies have proven that the catalytic performance of Ru-based catalysts for ammonia synthesis is influenced by the nature of the supports. A promising support would show high conductivity and electron-donating ability. It is critical to avoid side reactions and byproducts in PROX and partial oxidation of CH4. Based on the needs we have summarized here,we believe that this review will be beneficial to those addressing future development and application of Ru-based catalysts.
2016,
Vol. 37 Issue (8): 1181-1192 
PDF (2692 KB)
2016,
Vol. 37 Issue (8): 1181-1192 PDF (2692 KB)