不同Pt前体制备Pt/CeO_2催化剂对其结构及性能的影响

Pt与载体间的相互作用会影响到本征Pt纳米粒子的催化活性,不同Pt前体制备Pt/CeO_2催化剂会使其表现出完全不同的催化性能。分别采用金属胶体粒子原位沉积法、浸渍法以及浸渍还原的方式制备了Pt/CeO_2催化剂,通过X射线衍射、程序升温还原、X射线光电子能谱以及高分辨透射电镜对催化剂进行表征,在CO氧化以及甲苯燃烧反应中评价催化剂活性。结果表明,胶体粒子原位沉积法制备Pt/CeO_2催化剂,能够将优先合成好的Pt纳米粒子直接以金属态Pt~0的形式负载到载体表面,且保证其高度均匀分散,丰富的表面Pt~0很好地充当了CO、甲苯反应时的活化位点,催化剂表现出优异的性能;浸渍还原法中,Pt纳米粒子之间会发生团聚现象,同时部分Pt又以Pt~(2+)的形式与CeO_2之间形成了Pt-O-Ce相互作用,载体表面暴露Pt~0含量的下降是催化剂表现出较弱活性的主要原因;浸渍法中,以Pt离子对Pt进行负载,Pt完全以Pt~(2+)的形式参与到Pt-O-Ce键成键中,表面Pt~0缺失,催化剂表现出明显的失活现象。Pt/CeO_2催化剂中,起主要活性作用的是金属态Pt~0,胶体粒子原位沉积法能够实现Pt~0的直接负载,对于提高Pt基催化剂中Pt的利用率,降低Pt资源消耗都具有重要意义。     

磷酸改性CeO2纳米棒负载Pt催化剂催化丙烷燃烧性能的研究

机动车和石油化工等污染源排放造成的环境污染日益严重，其中以丙烷为代表的低碳烷烃结构稳定，难以实现低温完全氧化，亟需开发高效低碳烷烃低温催化氧化催化剂。以氧化铈纳米棒为载体，通过酸改性合成不同量磷酸改性的1%Pt/CeO₂-y PO_x催化剂，发现磷酸改性之后丙烷催化燃烧T₅₀降低了60℃。通过XRD、TEM、EDS mapping等表征发现Pt和P均匀分布在氧化铈表面，且在磷酸改性和Pt负载过程后，氧化铈结构保持稳定。XPS和原位CO-DRIFTs表征结果表明磷酸改性之后，Pt在催化剂表面颗粒大小随磷酸改性程度增加而逐渐变大，并存在最优的Pt²⁺和Pt⁴⁺比，有利于丙烷在催化剂表面吸附和反应。H₂-TPR和NH₃-TPD表征结果表明，磷酸改性之后，催化剂表面产生了大量的酸性位，而磷酸改性并未大幅降低催化剂的氧化还原能力，从而提高了铈基催化剂的丙烷燃烧活性。     

Pt/CrCeAlO催化剂对二氯甲烷的催化氧化性能

催化氧化是消除挥发性有机废气的有效手段,而二氯甲烷是含氯有机废气的代表性化合物。采用沉淀法制备了不同CrO_x含量的CrCeAlO催化剂,并用浸渍法制备了Pt/CrCeAlO催化剂,将其用于二氯甲烷催化氧化。结果表明,催化剂均表现出较好的活性,Cr_0.03Ce_0.05Al_0.95O₂催化剂在390 ℃时即可完全氧化二氯甲烷。而负载Pt后的催化剂活性明显提高,2.0Pt/Cr_0.03Ce_0.05Al₀.₉₅O₂催化剂表现出最好的活性,在340 ℃条件下,转化率即达100%。采用XRD、SEM、TEM、H₂-TPR和NH3-TPD对催化剂进行表征,表明催化剂的活性主要受其表面酸性和氧化还原性的影响,表面酸性位提供二氯甲烷化学吸附位,而催化剂表面氧化还原性则有利于反应中氧物种的活化。催化剂中添加Pt后,由于Pt、CeO₂ 和 CrO_x物种间的相互作用而增强了催化剂的氧化还原性,从而进一步促进了反应活性的提高。     

过渡金属改性氮掺杂多孔碳负载Pt催化甘油氧化制备甘油酸

将不同过渡金属掺杂到金属有机骨架化合物中后,通过高温煅烧得到过渡金属改性氮掺杂多孔碳材料(M@NHC),然后通过胶体沉积法生成Pt/M@NHC催化剂,在碱性条件下用作催化剂将甘油氧化成甘油酸。研究表明：过渡金属的种类对甘油的转化率和甘油酸的选择性有较大影响,其中Ni掺杂的Pt/Ni@NHC催化剂的催化活性最佳。结合N₂物理吸附、X射线衍射、X射线光电子能谱、CO₂程序升温脱附等表征发现：Ni的加入既影响Pt表面电子结构,还能凭借Ni-Pt金属之间的协同作用增强Pt的抗氧化能力;表面N原子的掺杂会增加Pt和吸附氧表面的电子使分子氧快速活化并且产生更多的活性位点。此外,Pt/Ni@NHC催化剂中Pt纳米颗粒在所有催化剂中颗粒最小,有利于催化性能的提高。对Pt/Ni@NHC在不同制备条件下所得催化剂的催化性能进行考察发现：当Ni负载量为3%,在载体煅烧温度800℃,反应压力1 MPa,反应时间6 h的条件下制备的Pt/Ni@NHC具有最佳的催化活性,此时甘油的转化率为63%,甘油酸选择性为75%。     

碱处理法制备Pt/ZSM-5催化剂用于肉桂醛选择加氢反应

负载型催化剂中贵金属的负载方法常常会影响催化剂的结构以及催化反应性能。本研究采用碱处理法制备了ZSM-5负载Pt纳米粒子催化剂，并用透射电镜(TEM)、X射线衍射(XRD)、傅里叶变换红外光谱(FTIR)、N₂物理吸脱附、氨气程序升温脱附(NH₃-TPD)等方法对催化剂物理化学性质进行了表征，同时将肉桂醛选择加氢反应作为模型反应，考察了合成催化剂的催化性能。结果表明：对比传统浸渍法制备的Pt/ZSM-5催化剂，用碱处理制备的催化剂能够显著地提高肉桂醛的转化率，其转化频率(TOF)值明显高于传统方法制备的催化剂，并且通过改变压力和反应温度可以进一步提高其催化活性。表征结果发现：碱处理法制备的Pt/ZSM-5催化剂具有更小粒径的Pt纳米粒子和更多的表面酸性位，有利于肉桂醛在催化剂表面的吸附和活化，使其具有更高的催化活性。     

不同晶粒尺寸HZSM-5载Pt催化剂上氢气选择催化还原氮氧化物的研究

将微晶和纳晶HZSM-5分子筛作为催化剂载体应用于富氧条件下氢气选择催化还原氮氧化物(H₂-SCR)反应。研究发现,微晶HZSM-5载Pt催化剂表现出较好的H₂-SCR反应活性,100℃时NO_x转化率高达90%,N₂选择性也明显高于纳晶HZSM-5载Pt催化剂。利用XRD、SEM、NH₃-TPD、XPS和IR对纳晶和微晶HZSM-5及其载Pt催化剂晶型结构、表面形态、表面酸性以及原位反应表面物种进行表征。结果表明,Pt在纳晶和微晶HZSM-5载体上均高度分散,微晶HZSM-5载体上的总酸量多、酸强度强,有利于催化剂上Pt保持活性金属位,使得催化剂上活性金属Pt位数目较多,进而促进了还原剂H₂的活化及重要活性中间物种氨物种的生成和稳定,从而提高了NO_x转化率和N₂选择性。     

Pt/TiO2/ZSM-5催化剂的制备及其催化转化正丁烷

采用溶胶-凝胶法和等体积浸渍法,分别对ZSM-5分子筛进行TiO₂改性和Pt负载,获得了具有脱氢-裂解双功能的Pt/TiO₂/ZSM-5催化剂,采用XRD、N₂吸附-脱附、TEM、XPS和NH₃-TPD对样品的晶体结构,孔结构、形貌、活性金属价态和酸性质等进行了表征,并研究了正丁烷在此催化剂上催化转化制备低碳烯烃的反应规律。研究结果表明,TiO₂的引入,一方面使得改性后的ZSM-5分子筛获得了额外的酸性中心,特别是强酸性位含量的增加,有助于促进正丁烷的活化;另一方面Pt与TiO₂之间存在“金属-载体”强相互作用（SMSI）,在H₂还原气氛下,Pt能够促进TiO₂的还原,生成Ti³⁺物种,而Ti³⁺的存在增加了Pt周围的电荷密度,降低了Pt对低碳烯烃（C₂⁼～C₃⁼）的吸附能力,抑制了深度脱氢和生焦反应,从而提高双功能催化剂对烯烃的选择性。当H₂还原温度为450℃时,Pt/10TiO₂/ZSM-5催化剂在625℃下的正丁烷转化率为76.1%,低碳烯烃（C₂⁼～C₃⁼）收率为50.9%,分别比Pt/ZSM-5催化剂提高了16.7%和12.6%。     

C5/C6烷烃异构化Pt/Al2O3-Cl催化剂的性能

采用浸渍法制备了Pt/Al₂O₃,在300℃、CCl₄氯化1h,制备出Pt/Al₂O₃-Cl催化剂。采用FT-IR、XRD、TEM、CO-IR、Py-IR和TPD等方法表征了催化剂,并与中温型RISO催化剂的催化性能进行比较。结果表明,在氯化处理过程中氯取代了氧化铝的表面羟基,导致3000～3800cm^-1红外吸收峰强度大幅度减小,但催化剂的晶相不发生改变;氯化使Pt粒子的平均粒径增大,粒径分布变宽,金属分散度降低;氯化后金属Pt主要以+2价的PtCl₂的形式出现,其中一部分生成了易升华的PtCl₂·2AlCl₃,从而导致Pt含量降低;氯化后的催化剂上只有L酸,评价后既有L酸,又有B酸;氯化后的催化剂热稳定不高,随着温度升高,3种类型的氯化物相继脱出;Pt/Al₂O₃-Cl相对于中温型RISO催化剂表现出较好的异构化性能,正己烷转化率达88.17%,2,2-二甲基丁烷选择性达29.68%,裂化和氢解几乎没有发生。     

WO3对Pt/α-Al2O3催化萘深度加氢的促进作用

采用无酸性的α-Al₂O₃为载体,预先沉积WO₃然后浸渍法负载Pt物种,合成了系列Pt-WO₃/α-Al₂O₃催化剂用于萘深度加氢反应,系统地研究了氧化钨物种在萘加氢反应中的作用。通过XRD、Raman、HRTEM、XPS和H₂-TPR技术表征了Pt和WO₃物种在载体表面的分散情况和状态,并利用Py-IR研究了载体负载氧化钨和Pt前后的酸性质变化。在温和的反应条件下（70℃、3 MPa、1 h）Pt-WO₃/α-Al₂O₃催化剂表现出优异的萘深度加氢活性,萘的转化率和十氢萘的选择性均达到100%。结果表明,预先在载体表面引入的WO₃和Pt产生了强相互作用,WO₃提高了Pt物种的分散程度,催化剂的酸性来源于氧化钨物种的引入且和负载量成正比关系。催化剂较强的酸性和较高的Pt分散程度是Pt-WO₃/α-Al₂O₃在低温条件下能够使萘深度加氢的关键因素,对于十氢萘作为储氢介质工艺具有重要的意义。     

低温等离子体协同CeO2/13X催化降解甲苯

低温等离子体协同催化剂技术（NTP-CAT）由于操作方便、能耗低等特点,特别适合用于工业非连续或连续消除低浓度VOCs过程。本研究发现NTP-CAT体系中CeO₂基催化剂更适合负载于13X载体以降解甲苯,并进一步考察CeO₂负载量对VOCs消除效果的影响。结果发现,NTP-CAT 体系中30% CeO₂/13X表现出最优性能,其可降解约85%的甲苯,CO₂产物选择性可达55%。表征结果也表明,Ce组分在30% CeO₂/13X表面仍可较好分散,而且表面的Ce³⁺物种含量最高。O₂-TPD实验结果证实表面Ce³⁺物种来源于Ce⁴⁺物种的等离子体处理。而且,表面Ce³⁺含量越高,有利于产生更多的氧物种,随后将与其周边13X吸附活化的甲苯反应。因此,甲苯降解在NTP-CAT体系中应存在分工协同机制。     

助剂Ba对Pd/Al_2O_3和Pt/Al_2O_3催化剂的C1-C3烷烃催化燃烧性能的影响

通过浸渍法制备了Al_2O_3负载的Pd和Pt催化剂,考察催化剂的甲烷、乙烷和丙烷催化燃烧活性,以及助剂Ba对催化性能的影响。对于Pd/Al_2O_3催化剂,加入Ba使活性物种PdO颗粒变大和还原温度升高,形成更稳定的PdO活性物种,是Pd-Ba/Al_2O_3催化剂活性提升的主要原因。对于Pt/Al_2O_3催化剂,加入Ba助剂使活性物种Pt0含量降低,PtO_x与Al_2O_3载体相互作用增强,使PtO_x物种更难被还原为Pt~0,导致Pt-Ba/Al_2O_3催化剂活性降低。Pd和Pt催化剂催化烷烃氧化反应活性规律一致:丙烷乙烷甲烷。Pd/Al_2O_3催化剂有利于C—H键活化,Pt/Al_2O_3催化剂有利于C—C键活化。Pt/Al_2O_3催化剂对C1-C3烷烃氧化活性的差别明显大于Pd/Al_2O_3催化剂。Pt/Al_2O_3催化剂对碳比例高的烷烃活性更高。     

The size effect and high activity of nanosized platinum supported catalysts for low temperature oxidation of volatile organic compounds

Pt/Al₂O₃ catalysts with smaller size of Pt nanoparticles were prepared by ethylene glycol reduction method in two different way and their oxidation activities for three typical VOCs (volatile organic compounds) were evaluated. The catalyst prepared by first adsorption and then reduction procedure is denoted as L-Pt/Al₂O₃ while the catalyst prepared by first reduction and then loading procedure is defined as R-Pt/Al₂O₃. The results show that L-Pt/Al₂O₃ with the stronger interaction between Pt species and Al₂O₃ exhibit smaller size of Pt nanoparticles and favorable thermal stability compared with R-Pt/Al₂O₃. L-Pt/Al₂O₃ is favor of the formation of more adsorbed oxygen species and more Pt²⁺ species, resulting in high catalytic activity for benzene and ethyl acetate oxidation. However, R-Pt/Al₂O₃ catalysts with higher proportion of Pt⁰/Pt²⁺ and bigger size of Pt particles exhibits higher catalytic activity for n-hexane oxidation. Pt particles in R-Pt/Al₂O₃ were aggregated much more serious than that in L-Pt/Al₂O₃ at the same calcination temperature. The Pt particles supported on Al₂O₃ with~10 nm show the best catalytic activity for n-hexane oxidation.     

Catalytic oxidation of low concentration formaldehyde over Pt/TiO2 catalyst

Formaldehyde(HCHO) is an important indoor pollutant.Catalytic oxidize low concentration HCHO is an effective way to eliminate indoor pollution.In this study,a series of Pt/TiO_2 catalysts are prepared by impregnation and reduced by NaBH_4.The effects of loading amount of Pt and cry stal type of TiO_2 on the physical and chemical properties and the catalytic performance in HCHO oxidation reaction are investigated.The results show that the quantity of active site and the oxygen vacancy of catalysts increa sed with increasing Pt content,which is beneficial to promote the further performance of catalysts.Nevertheless,with the further rises of Pt content,the specific surface area further decreases,and the proportion of Pt~(2+) species on the catalyst surface which is significant to catalytic properties also decreases,causing catalytic performance decreases.Compared with the catalyst supporting on rutile,the Pt/α-TiO_2 catalyst supporting on anatase has larger specific surface area,more Pt~(2+) phase and easier to form oxygen vacancy in the support,which cause better catalytic performance.The catalyst with Pt content of0.1 wt% and supported by anatase TiO_2 has the best catalytic performance.The HCHO conversion efficiency reaches 98% and 100% at 50℃ and 100 ℃, and the stabilization time is longer than 140 h.     

MnOx对Pt/CeZrO2催化剂在水汽和CO2环境下CO氧化的影响

以CeZrO₂固溶体为载体,发现MnO_x的添加能促进Pt/CeZrO₂催化剂的CO氧化性能,并研究了MnO_x含量对催化剂CO氧化活性及抗H₂O和CO₂性能的影响。结果表明,随着MnO_x含量增加,催化剂活性呈现先升高后降低的趋势,在MnO_x含量为0.5%（质量分数）时活性最佳。MnO_x的添加降低了Pt颗粒尺寸并影响催化剂还原性能从而促进反应活性。水汽和CO₂对Pt/CeZrO₂催化剂的CO氧化活性有抑制作用,而MnO_x的加入能显著提高催化剂的抗水汽和CO₂的能力。反应动力学结果表明,在Pt/CeZrO₂催化剂上,反应气中引入H₂O和CO₂后,CO的反应级数有明显升高,说明H₂O和CO₂在催化剂表面与CO竞争吸附,导致CO反应活性下降;而在Pt/MnO_x/CeZrO₂催化剂上,CO的反应级数略有升高,说明MnO_x的添加能有效抑制H₂O和CO₂与CO的竞争吸附,从而改善了催化剂的抗H₂O和CO₂性能。     

Low-temperature complete oxidation of BTX on Pt/activated carbon catalysts

The catalytic destruction of volatile organic compound (VOC) benefits from a low oxidation temperature due to less energy consumption. In this study, activated carbon-supported Pt catalysts were prepared for benzene, toluene and xylene (BTX) deep oxidation at below 200°C. Activated carbon can serve as a media for concentrating VOC. The carbon supports were heated to 400 or 800°C under N₂ flow and washed with HF acid to remove surface impurities and/or minerals. The 0.3 wt.% Pt/activated carbon catalysts were prepared by the incipient wetness method, followed by H₂ reduction at 300°C for 2 h. The catalytic oxidation was conducted with a BTX concentration ranging from 640 to 2000 ppmv in air at volume hour space velocity (VHSV) of approximately 21 000 h⁻¹. The light-off curves were very steep and the light-off temperatures ranged between 130 and 150°C, well below those of the Pt/Al₂O₃ catalyst. The oxidation activity was promoted because of a higher surface BTX concentration due to the adsorption capability of activated carbons. Moisture reduces the activity only slightly due to the hydrophobicity of activated carbon. Generally, the Pt catalysts with thermally-treated activated carbon had lower ignition temperatures. Experimental results indicated that high-temperature pretreatment of activated carbon could effectively increase the catalyst activity. Meanwhile, X-ray photoelectron spectroscopy (XPS)/secondary ion mass spectroscopy (SIMS) investigation revealed that the graphitized surface might play a role in catalytic activity. Finally, this work suggested a reaction mechanism based on the adsorption-migration of hydrocarbons to reveal the enhanced activity of activated carbon support.     

Study on catalytic performance of CeO2/CuO catalysts for preferential CO oxidation

采用水热法合成纳米尺寸的CuO,然后采用微乳液法或浸渍法将CeO₂负载在CuO上制备逆负载的CeO₂/CuO催化剂。通过X射线衍射（XRD）、程序升温还原（TPR）、比表面分析（BET）和富氢气中CO优先氧化活性测试等研究手段对催化剂进行了表征。研究发现,CeO₂/CuO催化剂的活性和选择性与CeO₂和CuO颗粒的尺寸密切相关,大颗粒的CuO载体有利于提高催化剂的选择性;小颗粒的氧化铈负载在大颗粒的氧化铜上,可以产生更多两相接触界面,有助于提高催化剂的活性。     

MnOx/Pt/Al2O3 catalysts for CO oxidation in H2-rich streams

The catalytic activity of Pt on alumina catalysts, with and without MnO_x incorporated to the catalyst formulation, for CO oxidation in H₂-free as well as in H₂-rich stream (PROX) has been studied in the temperature range of 25–250 °C. The effect of catalyst preparation (by successive impregnation or by co-impregnation of Mn and Pt) and Mn content in the catalyst performance has been studied. A low Mn content (2 wt.%) has been found not to improve the catalyst activity compared to the base catalyst. However, catalysts prepared by successive impregnation with 8 and 15 wt.% Mn have shown a lower operation temperature for maximum CO conversion than the base catalyst with an enhanced catalyst activity at low temperatures with respect to Pt/Al₂O₃. A maximum CO conversion of 89.8%, with selectivity of 44.9% and CO yield of 40.3% could be reached over a catalyst with 15 wt.% Mn operating at 139 °C and λ = 2. The effect of the presence of 5 vol.% CO₂ and 5 vol.% H₂O in the feedstream on catalysts performance has also been studied and discussed. The presence of CO₂ in the feedstream enhances the catalytic performance of all the studied catalysts at high temperature, whereas the presence of steam inhibits catalysts with higher MnO_x content.     

Pt/MnOx–CeO2 catalysts for the complete oxidation of formaldehyde at ambient temperature

MnO_x–CeO₂ mixed oxides with a Mn/(Mn + Ce) molar ratios of 0–1 were prepared by a modified coprecipitation method and investigated for the complete oxidation of formaldehyde. The MnO_x–CeO₂ with Mn/(Mn + Ce) molar ratio of 0.5 exhibited the highest catalytic activity among the MnO_x–CeO₂ mixed oxides. Structure analysis by X-ray powder diffraction and temperature-programmed reduction of hydrogen revealed that the formation of MnO_x–CeO₂ solid solution greatly improved the low-temperature reducibility, resulting in a higher catalytic activity for the oxidation of formaldehyde. Promoting effect of Pt on the MnO_x–CeO₂ mixed oxide indicated that both the Pt precursors and the reduction temperature greatly affected the catalytic performance. Pt/MnO_x–CeO₂ catalyst prepared from chlorine-free precursor showed extremely high activity and stability after pretreatment with hydrogen at 473 K. 100% conversion of formaldehyde was achieved at ambient temperature and no deactivation was observed for 120 h time-on-stream. The promoting effect of Pt was ascribed to enhance the effective activation of oxygen molecule on the MnO_x–CeO₂ support.     

Potential rare-earth modified CeO2 catalysts for soot oxidation part II: Characterisation and catalytic activity with NO + O2

Ceria (CeO₂) and rare-earth modified ceria (CeReO_x with Re = La³⁺, Pr^3+/4+, Sm³⁺, Y³⁺) supports and Pt impregnated supports are studied for the soot oxidation under a loose contact with the catalyst with the feed gas, containing NO + O₂. The catalysts are characterised by XRD, H₂-TPR, DRIFT and Raman spectroscopy. Among the single component oxides, CeO₂ is significantly more active compared with the other lanthanide oxides used in this study. Doping CeO₂ with Pr^3+/4+ and La³⁺ improved, however, the soot oxidation activity of the resulting solid solutions. This improvement is correlated with the surface area in the case of CeLaO_x and to the surface area and redox properties of CePrO_x catalyst. The NO conversion to NO₂ over these catalysts is responsible for the soot oxidation activity. If the activity per unit surface area is compared CePrO_x is the most active one. This indicates that though La³⁺ can stabilise the surface area of the catalyst in fact it decreases the soot oxidation activity of Ce⁴⁺. The lattice oxygen participates in NO conversion to NO₂ and the rate of this lattice oxygen transfer is much faster on CePrO_x. In general, the improvement of the soot oxidation is observed over the Pt impregnated CeO₂ and CeReO_x catalysts, and can be correlated to the presence of Pt°. The surface reduction of the supports in the presence of Pt occurred below 100 °C. The surface redox properties of the support in the Pt catalysts do not have a significant role in the NO to NO₂ conversion. In spite of the lower surface area, the Pt/CeYO_x and Pt/CeO₂ catalysts are found to be more active due to larger Pt crystal sizes. The presence of Pt also improved the CO conversion to CO₂ over these catalysts. The activation energy for the soot oxidation with NO + O₂ is found to be around 50 kJ/mol.     

Catalytic combustion of toluene on platinum-containing monolithic carbon aerogels

Monolithic organic aerogels were prepared by the sol–gel procedure from the polymerisation reaction of resorcinol and formaldehyde in water. The organic aerogels were heat treated in inert atmosphere at either 500 or 1000 °C to obtain the carbon aerogels. The catalysts were prepared by impregnation with an aqueous solution of [Pt(NH₃)₄]Cl₂ or by dissolving this salt in the initial aerogel mixture. Supported catalysts were pretreated in He at 400 °C or H₂ at 300 °C before their characterization by H₂ chemisorption, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy or before testing their catalytic activity. Catalyst activities in toluene combustion were evaluated by conversion versus temperature (light-off curves) and conversion versus time catalytic tests. In the case of catalysts prepared by impregnation, the light-off curves for the total combustion of toluene were shifted to lower temperatures with increasing Pt particle size. This suggests that the reaction was sensitive to the Pt structure within the dispersion range of these catalysts. However, the reverse occurred with catalysts prepared by mixing the precursor in the initial aerogel mixture. Results found could be due to the different surface Pt content of these catalysts as revealed by X-ray photoelectron spectroscopy. This difference was related to the growth of large three-dimensional Pt particles on the surface of the less dispersed catalyst. This means that there is a critical Pt particle size above which the toluene combustion activity decreases with increasing Pt particle size, due to the reduction in active surface sites available for the combustion reaction. Other effect that might influence the activity of these last catalysts is the encapsulation of some Pt particles by the carbon matrix.