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Mn-promoted bulk iron catalysts with a high specific surface area (82–211 m2·g−1) were synthesized by coprecipitation followed by drying under supercritical conditions. The catalysts were tested in the CO2 hydrogenation to valuable C2-C5 hydrocarbons. The Mn-promoted iron catalysts exhibited better textural properties than the bare Fe2O3 catalyst, allowing better dispersion of the active phase, easier reduction and carburization of iron oxides and, consequently, resulting in higher catalytic activity than the bare Fe2O3 catalyst. The best activity results were obtained by catalyst promotion with a very low amount of Mn (Mn/Fe atomic ratio of 0.05). Upon steady state conditions (T = 340 °C, total pressure of 20 bar and H2/CO2 = 3), this catalyst exhibited high CO2 conversion (44.2%) and selectivity to C2-C4 hydrocarbons (68%, olefin to paraffin ratio of 0.54), while the selectivity to C5+ hydrocarbons, CH4 and CO was about 3.2, 38.5 and 5%, respectively. A close correlation was found between catalyst textural properties and CO2 conversion. The most active MnFe-0.05 catalyst exhibited high stability during 72 h of reaction related to a low amount of soft coke formation and catalyst activation through the formation of the χ-Fe5C2 phase during the on-stream reaction.

The CO2 methanation catalyzed by supported metals has been widely investigated. In spite of its apparent simplicity, there are still open questions regarding its mechanism, with apparently contradictory proposals that rely mainly on infrared (IR) spectra of functioning catalysts. Here we review the various mechanistic proposals for the reaction. Special focus is made on the following questions: (a) what is the actual physical evidence of adsorbed species present on the catalysts?, (b) how can reaction intermediates be distinguished from spectators?, (c) what is the identity of the active site(s) for the reaction, and (d) could multiple reaction routes occur simultaneously on the catalysts? Kinetic models of the reaction and a survey of the apparent contradictions in the interpretation of IR spectra characterizing functioning catalysts are also presented.

El descubrimiento de catalizadores sólidos generalmente se realiza a través de una metodología que incorpora elementos de prueba y error. En el mejor de los casos, esta estrategia se apoya en información obtenida de técnicas de caracterización. Sin embargo, como la estructura de los catalizadores sólidos es generalmente complicada, dicha información es insuficiente para tener una descripción precisa y rigurosa del modo en que funcionan los catalizadores. Además, la estrategia tampoco permite establecer si los catalizadores encontrados serán los mejores posibles. Como contraparte de la prueba y error está el diseño de catalizadores, con el cual se pretende desarrollar materiales a partir de su comprensión a nivel fundamental (i.e., detalles específicos y rigurosos de la naturaleza del sitio activo). En principio, el diseño permitiría predecir la estructura necesaria en un material para que este sea catalíticamente activo. En este trabajo se contrastan las estrategias que típicamente se emplean para estudiar catalizadores sólidos, poniendo énfasis especial en la posibilidad de diseñar materiales. Se discute qué significa comprender el funcionamiento de los catalizadores a nivel fundamental y se hace un análisis crítico de posibles oportunidades que ofrece el diseño para abordar algunos problemas relevantes en el estudio de catalizadores en la actualidad.

On the occasion of his 80th birthday, we take stock of the remarkable and ongoing career of Professor Bruce C. Gates. This Account highlights his key scientific achievements in three areas of significance: supported metal clusters and atomically dispersed metal complexes, hydroprocessing reaction networks and catalysts, and strong acid catalysis. His contributions in all three areas are noteworthy and have been sustained over 50 years. The work of his group evolved from mainly catalyst synthesis and reaction studies to also incorporate sophisticated characterization techniques based on X-ray absorption spectroscopies and improved scanning transmission electron microscopic examination of catalytic surfaces. In the course of this work, he has addressed, and contributed to the solutions of some of the most significant and challenging problems in catalysis of the last half century., NA

The production of methane from the reaction between CO2 and H2 (CO2 methanation) has gained increasing attention in recent years. The rational design of novel catalytic materials for this reaction will depend on the fundamental description of the active sites and the identification of surface reaction intermediates. Currently, there is a debate regarding the mechanism for the CO2 methanation on supported metals, with apparently contradictory proposals suggesting that various surface species could be either reaction intermediates or spectators. Similarly, there is a discussion regarding the nature of the surface sites on the catalysts that activate the CO2 molecule during the reaction. Specifically, it has been suggested that different reaction routes could occur on different metalsupport combinations and on various surface structures. In this manuscript, we critically review the literature on CO2 methanation and discuss the physical evidence that has been presented to propose reaction mechanisms on various supported metals. The relevance of the presence of nanosized metal particles in the catalysts is also discussed.

In this work, a di-n-alkyldichlorosilane (di-n-octyldichlorosilane) was used as an agent to improve the integration of titanium dioxide particles into isotactic polypropylene (iPP) matrix. The thermal stability of the silane coating was studied to confirm the applicability of these particles under extrusion temperature; we found by infrared spectrometry and mass spectroscopy that silanization remains stable at temperatures around 400 °C. The non-isothermal analysis of the crystallization process, conducted by differential scanning calorimetry, showed a decrement in the energetic requirements to complete the process when we used the silanized particles and filled the iPP at 0.5% in mass. Also, these silanized particles demonstrate the capability to be a selective α nucleating agent in comparison with the neat TiO2 particles that showed a slight portion of β crystals according to the wide-angle X-ray diffraction analysis.

ZrO2-supported rhodium nanoparticles prepared by impregnation of RhCl3 are active for CO2 methanation at temperatures above 180 oC. Infrared (IR) spectra recorded during catalysis allowed identification of Rh carbonyls and formate species bonded to the support. To verify their individual involvement in the catalysis, their transformations were investigated by IR spectra measured as a sample of ZrO2-supported Rh was treated with CO and isotopically labelled formic acid. The data indicate two coexisting reaction routes: a dissociative route in which CO2 reacts on the Rh sample to give Rh carbonyls, and an associative route in which the CO2 molecule is activated on the support, in the form of bicarbonate species that are hydrogenated to give formate species prior to methane formation. Our data show that labelled formates are transformed into Rh–13CO, thus connecting the dissociative and associative catalytic routes. The results indicate the existence of a dual mechanism for CO2 methanation.

Graphene oxide (GO) was functionalized with pimelic acid (PA) on its surface to produce a β-crystal nucleating agent. The functionalization was characterized by Fourier transform infrared spectroscopy, showing the presence of new bands at 1570 and 1406 cm−1 indicating a bridging bidentate link of the PA on the GO surface through the Ca used during the functionalization. These observations were concomitant with the Raman spectroscopy results where the D–G intensity ratio increased, suggesting that the disorder in GO structure increased when the PA molecules are attached on the GO surface. The modified graphene oxide (GO-PA) showed high thermal stability, making it attractive for melt blending composites. These results were observed by thermal differential scanning calorimetry (DSC) and thermogravimetric analysis. The DSC and wide-angle X-ray diffraction (WAXD) results demonstrated that the modified GO is an excellent β-nucleating promoter even at low percentages (0.05 GO-PA % w/w) in contrast to the unmodified GO. The content of β-crystals under isothermal crystallization conditions reached a value as high as 92% by DSC while the extrudes prepared by compression and analyzed by WAXD showed a kβ = 0.78 by Turner-Jones method at the same percentage of modified filler. The storage modulus for the iPP composites (GO/iPP and GO-PA/iPP) was in all cases higher than that for the raw iPP sample because of the presence of GO in the polymer matrix. However, a decrease in the storage modulus can be observed for the GO-PA/iPP composites because of the presence of high percentages of β-crystal.

Graphene oxide is widely-used for many applications because of its surface area and the variety of oxygen functional groups that possess. This latter feature allows it to be a good candidate for polar compounds adsorption such as furfural. Furfural is a toxic compound that has been detected in a wide variety of foods of daily consumption. Frequent exposure in high doses can represent a risk to human health. In the present work, it is proposed an easy procedure to obtain reduced graphene oxide based coatings through dip-coating for Solid Phase Micro-Extraction (SPME) of furfural. The coatings were made with graphene oxide and carboxylated graphene oxide. These coatings were thermally reduced and then tested using a furfural standard. The thermally reduced graphene oxide coatings were characterized using FT-IR, Raman spectroscopy and SEM. SEM micrographs revealed that graphene oxide based fibers had thickness values between 1.5 and 2.5 μm approximately. Subsequently the results of furfural extraction were compared with those obtained from the Divinylbenzene/Carboxen/Polydimethylsiloxane (DVB/CAR/PDMS) commercial fibers.

ZrO2-supported Ni samples were prepared by an impregnation method and tested as catalysts for CO2 methanation. Infrared spectra recorded during catalysis show that CO2 was initially adsorbed in the form of carbonate and bicarbonate species bonded to sites of the support. The data indicate that bicarbonates were hydrogenated to give surface formate species, which reacted with H2 at increasing temperature and led to the formation of methane, as evidenced by changes in the mass spectral signal of methane in the effluent gases from a flow reactor/DRIFT cell. No surface carbonyls were observed during the reaction. Separate experiments in which the catalysts were tested for the hydrogenation of methanol and formic acid also show their activity towards methane formation. In both cases, surface methoxy species bonded to Lewis acid sites of ZrO2 were identified. Our results indicate a bifunctional character of the ZrO2-supported Ni catalysts for CO2 methanation, with the CO2 being activated on sites of the support and the role of Ni consisting on providing sites for hydrogen adsorption and dissociation.

En este trabajo se presenta de manera general la investigacion que lleva a cabo el Grupo de Catalisis Heterogenea del Departamento de Ingenieria Quimica del Instituto Tecnologico de Celaya (ITC). Se describen brevemente algunos proyectos realizados recientemente y se hace enfasis en la aproximacion que el grupo utiliza para avanzar hacia la comprension del funcionamiento de catalizadores de metales soportados. Se discuten tambien las perspectivas del grupo y su intencion de aprovechar las oportunidades de colaboracion dentro y fuera del Departamento de Ingenieria Quimica del ITC.

Cleavage of the β C H bond of ethanol during its dehydrogenation on TiO 2 -supported gold samples was investigated by the combined use of infrared (IR) spectroscopy and mass spectrometry. IR spectra characterizing the samples during ethanol adsorption shows that the alcohol was adsorbed both molecularly and dissociatively in the form of ethoxy species bonded to Ti 4+ sites. When a supported gold sample was exposed to consecutive pulses of ethanol 1,1-D 2 (CH 3 CD 2 OH) at 220 °C, mass spectra of the products indicated the formation of acetaldehyde 1-D 1 (CH 3 CDO) and three forms of hydrogen, namely H 2 , HD and D 2 . Because the formation of H 2 was much higher than those of HD and D 2 , it was inferred that cleavage of the β C H bond did not necessarily occur on the gold particles, but on sites of the support. Bolstering this idea, IR spectra measured under reaction conditions indicate the appearance of a band at 2712 cm −1 , assigned to the υ OD vibration mode of deuteroxyl groups on TiO 2 . The data are consistent with the abstraction of deuterium atoms from the ethoxy species by basic sites of the support. It is proposed that the presence of gold particles enhances the diffusion of hydrogen atoms on the sample, as evidenced by marked changes in the absorbance of IR and UV–vis radiation during the reaction, which indicate the presence of electrons trapped at defect sites on the metal oxide near the bottom of the conduction band. Those electrons might have been formed during the reverse spillover of hydrogen from the support to the gold particles, on which they were readily recombined and desorbed as H 2 .

The adsorption and reactions of diethylamine on the surfaces of TiO2 and TiO2-supported gold samples were investigated by infrared (IR) spectroscopy and mass spectrometry. IR spectra measured as the samples were treated in flowing diethylamine at room temperature indicate that the amine was preferentially adsorbed molecularly on surface Ti4 + sites. Thermal treatment of the samples with flowing diethylamine led to the formation of ethylene and acetonitrile as dehydrogenation products. The data show that the reactions occurred at lower temperatures in the presence of supported gold samples than on TiO2, and IR spectra recorded under reaction conditions show evidence of amine-derived surface species bonded to gold nanoparticles that could be regarded as reaction intermediates. The results indicate that the gold nanoparticles provide sites for subtraction and recombination of hydrogen atoms from the amine, ensuing its dehydrogenation.

Few reports exist on the use of Diffuse Reflectance Infrared Fourier Transform Spectrometry coupled with Mass Spectrometry (DRIFTS-MS) in situ to monitor the dehydroxylation of kaolinitic clays. The use of DRIFTS-MS in situ allows study of the effect of heat treatment on the dehydroxylation, identifying intensities and temperatures at which the hydroxyl groups are released, forming metakaolinite and meta-alunite. The effluent gases from the infrared cell were analysed by mass spectrometry. The decrease in intensity of the bands at 3694, 3669, 3650 and 3621 cm−1 associated with the −OH stretching vibration modes of AlVI−OH−AlVI of kaolinite began at 450°C. Two additional bands at 3513 and 3485 cm−1 are associated with the vibration of AlVI−OH of alunite that also began to disappear during thermal treatment. Monitoring of the fractions m/e 17 and 18 using a mass spectrometer revealed that the intensity of these fractions increased starting at 450°C. Therefore, it is possible to study the dehydroxylation process of clays during thermal treatment.Chemical and mineralogical characterization of a kaolinitic clay (KN) fromMexico showed that the clay consists of 64.8% kaolinite, 11.0% alunite and 24.4% quartz based on PXRD, EDS, TG/DTA, TEM and FTIR results, and suggested that the material might have potential for use in the manufacture of ceramics, refractory bricks or geopolymers.

TiO 2 –supported gold samples prepared by deposition-precipitation were tested as catalysts for methanol carbonylation to give methyl acetate at atmospheric pressure in the presence of methyl iodide as cocatalyst. Infrared (IR) spectroscopy was used to investigate the reactions of methanol, CO and CH 3 I on the surface of the samples. The results show the formation of methoxy species bonded to Ti 4+ sites upon adsorption of methanol on the supported gold samples. Admission of CO to the flow reactor/IR cell led to the formation of gold carbonyls, which reacted with the surface methoxy species only when CH 3 I was present to give methyl acetate. The data indicate that CH 3 I is necessary to form gold–acetyl species, which are attacked by neighboring methoxy species to give the carbonylation product. Our results show that the methanol carbonylation catalyzed by TiO 2 –supported gold in the presence of CH 3 I occurs by a reaction mechanism that is similar to that occurring on supported Rh catalysts.

The activity and deactivation of Au/CeO2 catalysts in the PROX reaction were measured during long-term experiments (TOS = 160 h) by in situ DRS-UV–vis spectroscopy. During the reaction, the support showed a decrease in band gap energy. The activity and selectivity to CO2 were closely linked to the reduction of Aun+ species, the sintering of the reduced Au species, and to changes in the Ce3+/Ce4+ ratio. This ratio increased during long-term PROX experiments, showing a decrement of surface oxygen atoms from the CeO2 phase which participated in the reaction, after which the total oxygen fed to the reaction was wholly consumed. Also, almost all of the Au3+ species initially present in the catalysts were reduced to Au+ and Au0. Results suggest that the activity loss is connected to the fast reduction of Au species and is also linked to changes in the redox properties of ceria, breaking the cycle that provides oxygen to the reaction. Sintering of the Au nanoparticles occurs in a longer time scale. On the other ...

Supported rhodium carbonyls were synthesized by deposition of Rh6(CO)16 on TiO2. The samples were active for gas-phase dimethyl ether carbonylation in the presence of methyl iodide to give methyl acetate. Infrared spectra recorded under reaction conditions indicate that CO was activated on Rh sites in the form of Rh dicarbonyls, while dimethyl ether was dissociatively adsorbed on the support in the form of methoxy species bonded linearly to Ti4+ sites. The results also indicate the presence of Rh–acetyl and acetate complexes, as evidenced by the appearance of νCO bands at 1727 and 1668 cm−1 during catalysis. These surface species could be regarded as reaction intermediates. The data demonstrate the bifunctional character of metal oxide-supported Rh catalysts and suggest that the carbonylation reactions of dimethyl ether and methanol catalysed by supported Rh occur by analogous mechanisms.

The adsorption and reactions of ethanol on the surfaces of TiO2 and TiO2-supported gold samples were investigated by infrared (IR) spectroscopy and mass spectrometry. IR spectra measured as the samples were treated in flowing ethanol at room temperature indicate that the alcohol was adsorbed both molecularly and dissociatively in the form of ethoxy species bonded to Ti4+ sites. Our results suggest that the presence of neighboring gold particles facilitates cleavage of the β-C H bond of the ethoxy species at increasing temperature, leading to the formation of acetaldehyde linearly bonded to Ti4+ sites, which is readily desorbed. In the process, hydrogen adatoms are recombined on gold particles to give H2. In absence of gold, Ti4+ sites abstract hydrogen atoms from the ethoxy species to give titanium hydride species, which react with hydroxyl groups to produce water and leave oxygen vacancies on the TiO2 surface. Acetaldehyde is formed in the process and it is adsorbed in a bidentate configuration on the oxygen vacancies, where it undergoes reductive coupling to give butene. Our data show that the ability of gold particles to abstract and recombine hydrogen atoms prevents the formation of oxygen vacancies on the support and, in consequence, hinders the subsequent reactions of the produced acetaldehyde.

Iron oxide-supported gold samples were prepared by co-precipitation from HAuCl(4) and Fe(NO(3))(3). The activities of the samples as CO oxidation catalysts were tested without thermal treatment and following treatments in flows of He and O(2) at various temperatures. It was found that the untreated samples and those treated in a flow of He at 150 °C were more active than samples that had been treated at 400 °C in either a flow of O(2) or of He. Infrared spectra recorded during CO oxidation catalysis indicate the presence of bonded CO molecules to cationic gold on all samples, whereas spectra of the least active catalysts indicate a predominant presence of Fe(2+) carbonyls, which were highly stable under the conditions of our experiments. Our results indicate that in the least active samples the Fe(2+)-bound CO blocks sites that would otherwise be available for oxygen activation.

The adsorption and reactions of 2-propanol on γ-Al2O3 and γ-Al2O3-supported gold samples were investigated by infrared (IR) spectroscopy, modulated differential scanning calorimetry (MDSC) and mass spectrometry. Adsorption of the alcohol on the samples at room temperature led to formation of molecularly adsorbed 2-propanol and 2-propoxide species bonded to Al3+ sites. Treatment of γ-Al2O3 after alcohol adsorption in flowing He from 25 to 300 °C led to 2-propanol desorption, without evidence of surface reactions. In contrast, when supported gold samples were exposed to the same thermal treatment, formation of acetone and H2 was observed by mass spectra of the effluent gases from the flow reactor. Concomitantly, IR spectra of the samples showed the appearance of a band at 1698 cm− 1, assigned to νCO vibrations of adsorbed acetone. The formation of acetone occurred by the dehydrogenation of 2-propoxide species bonded to Al3+ sites, as evidenced by (a) the decrease in the intensities of their IR bands and (b) the presence of a MDSC peak at approximately the same temperature as that at which acetone was formed and the 2-propoxide species were consumed. It is proposed that gold particles on the γ-Al2O3 surface facilitate breaking of the β-C H bond of neighboring surface 2-propoxide species to give acetone. Our results emphasize the bifunctional character of supported gold catalysts for the dehydrogenation of alcohols.

TiO2-supported rhodium samples synthesized from the Rh6(CO)16 complex were catalytically active and selective for the gas-phase methanol carbonylation at 140 °C and atmospheric pressure in the presence of methyl iodide as promoter. Infrared (IR) spectra recorded during catalysis allowed the identification of molecularly adsorbed methanol, together with linear, doubly, and triply-bridged surface methoxy species on Ti4+ sites of the support. IR bands characteristic of rhodium complexes that might be regarded as reaction intermediates were also observed, in addition to bands assigned to surface acetate species attributed to the formation of methyl acetate. Our results reveal that only linear methoxy species on Ti4+ sites react with flowing CO in the presence of CH3I to give methyl acetate, whereas bridged methoxy species and molecularly adsorbed methanol are only spectators in methanol carbonylation.

Iron oxide-supported gold samples were charac-terized by X-ray absorption near edge structure (XANES) spectroscopy during treatments in flowing H2 at increasing temperature. Spectra were recorded at the Au LIII and Fe K edges to monitor the reduction of both metals and to determine the influence of gold on the reducibility of the support. The results show that reduction of Fe3+ to Fe2+ on the support occurs at lower temperatures in samples containing gold than on samples of the bare support, with the reduction temperature being dependent on the gold content. X-ray diffraction patterns characterizing samples after H2 treatments at various temperatures complement the XANES data and indicate that the presence of gold favors the crystallization of the support to give Fe3O4. Our data emphasize the power of XANES spectroscopy in following changes in the oxidation states of both gold and iron and suggest that the role that gold might have in promoting the reduction and crystallization of iron oxide support is to...

Samples of gold supported on γ-Al2O3 were prepared by deposition–precipitation and tested as catalysts for the aerobic oxidation of 2-propanol to give acetone. IR spectra recorded during catalysis show that the alcohol is first dissociatively adsorbed on the catalysts to form predominantly 2-propoxide species bonded to coordinatively unsaturated Al3+ sites. It is proposed that gold particles in the proximity of the 2-propoxide species are capable of subtracting hydrogen from the β-C–H bond to give acetone bonded to Al3+ sites, as evidenced by the appearance of a band at 1704 cm−1 at a similar temperature as that at which the onset of formation of acetone occurred. Other possible surface reaction pathways are suggested, indicating the interactions of the alcohol with support and metal sites. These results (with a model reaction) might be general and explain the activity of supported gold for the oxidation of other alcohols.

Au(CH3)2(C5H7O2) was used as a precursor to synthesize mononuclear gold complexes in faujasites, NaY zeolite, and dealuminated Y zeolite. Treatment of these supported complexes in flowing helium as the temperature was increased led to the gradual reduction of the gold and formation of gold nanoclusters in each zeolite, which were characterized by X-ray absorption spectroscopy. After treatment at the highest temperature, 573 K, the average diameters of the gold clusters were approximately 10 and 8 A in NaY zeolite and dealuminated Y zeolite, respectively; these clusters are smaller than the diameters of the α-cages of the zeolite. The results indicate that the supported clusters were trapped within these cages and that the cages limited their sizes; in contrast, clusters formed similarly on the surfaces of metal oxides are significantly larger than those formed in the zeolites.

The state of gold on a Au/NaY zeolite sample prepared from Au(CH3)2(C5H7O2) and subjected to various treatments was monitored by infrared spectroscopy of adsorbed CO as a probe molecule. Spectra of...

Recent intense interest in catalysis by supported gold has focused mainly on the oxidation of CO at low temperatures, which offers the potential for many important applications, including the purification of hydrogen for automotive fuel cells. Numerous, contradictory proposals have been made concerning the nature of the active sites in supported gold catalysts. We now present evidence from a set of complementary experimental methods characterizing Au/α-Fe2O3 catalysts, demonstrating that cationic gold plays a crucial role in catalyzing CO oxidation at 298 K, as well as in the hydrogenation of crotonaldehyde. A series of catalysts based on a co-precipitated 5-wt% Au/Fe2O3 precursor heat treated in different ways were compared for CO oxidation at ambient temperature. The catalysts were structurally and chemically analyzed by HREM and STEM-XEDS. A combination of EXAFS, in situ XANES, XPS, and Mossbauer effect spectroscopy demonstrates the important role of cationic gold in the activity of these iron oxide-supported gold catalysts.

This article is a review of the physical characterization of well-defined site-isolated molecular metal complexes and metal clusters supported on metal oxides and zeolites. These surface species are of interest primarily as catalysts; as a consequence of their relatively uniform structures, they can be characterized much more precisely than traditional supported catalysts. The properties discussed in this review include metal nuclearity, oxidation state, and ligand environment, as well as metal-support interactions. These properties are determined by complementary techniques, including transmission electron microscopy; X-ray absorption, infrared, Raman, and NMR spectroscopies; and density functional theory. The strengths and limitations of these techniques are assessed in the context of results characterizing samples that have been investigated thoroughly and with multiple techniques. The depth of understanding of well-defined metal complexes and metal clusters on supports is approaching that attainable for molecular analogues in solution. The results provide a foundation for understanding the more complex materials that are typical of industrial catalysts.

Mononuclear gold complexes in zeolite NaY were synthesized from initially physisorbed Au(CH3)2(C5H7O2) and characterized by X-ray absorption and IR spectra recorded as the samples were exposed to flowing CO. X-ray absorption spectra demonstrate the formation of zero-valent gold nanoparticles during the CO treatment. Three new nCO bands grew in during this treatment, at 2070, 2033, and 2000 cm-1, characteristic of carbonyls of Au0. Because the relative intensities of these bands decreased monotonically when the flow of CO was replaced by flowing He, it is inferred that they correspond to a single Au0(CO)3 species, on low-coordinated Au atoms. This is the first example of an Au0(CO)3 species. [on SciFinder (R)]

Gold supported on partially dehydroxylated γ-Al2O3 was prepared from Au(CH3)2(C5H7O2). The activities of the materials as CO oxidation catalysts were investigated following pretreatment in flowing O2 and, alternatively, in flowing CO. The catalytic activity data are complemented with extended X-ray absorption fine structure and X-ray absorption near edge structure spectra recorded as the samples in flow reactors were treated in O2, in CO, and in CO + O2 while functioning as CO oxidation catalysts. Mass spectrometry was used to identify the gases evolved during the treatments. The data show that samples pretreated in O2 contained predominantly cationic gold and were more active as catalysts than samples pretreated with CO, which contained gold predominantly as zerovalent clusters. These results reinforce recent reports of the role of cationic gold in the catalytic sites for CO oxidation.

Mononuclear gold complexes bonded to TiO(2) were synthesized from Au(CH(3))(2)(C(5)H(7)O(2)), and their decomposition and conversion into gold nanoclusters on the TiO(2) surface were characterized by time-resolved X-ray absorption and infrared spectroscopies as the temperature of the sample in flowing helium was ramped up. Mass spectra of the evolved gases were also measured during this process. The results show (a) the onset of formation of CH(4) as a decomposition product, (b) the reduction of Au(III) to Au(0), and (c) the formation of Au-Au bonds, all occurring in approximately the same temperature range (about 335-353 K), indicating that the reduction and aggregation of the supported gold are simultaneous processes facilitated by the removal of methyl ligands initially bonded to the gold. IR spectra recorded during the treatment indicate that water on the TiO(2) surface may be involved in the process by reacting with methyl groups bonded to Au(III) to give CH(4).

Mononuclear gold complexes in zeolite NaY were synthesized from initially physisorbed Au(CH3)2(C5H7O2) and characterized in reactive atmospheres by X-ray absorption and infrared spectroscopies. Und...

Gold nanoclusters on TiO2 powder were prepared from adsorbed AuIII(CH3)2(C5H7O2) (dimethyl acetylacetonate gold(III)) and characterized by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies. The samples were tested as catalysts for CO oxidation at 298 K and atmospheric pressure and characterized by EXAFS and XANES with the catalysts in the working state. The XANES results identify Au(III) in the initially prepared sample, and the EXAFS data indicate mononuclear gold complexes as the predominant surface gold species in this sample, consistent with the lack of Au–Au contributions in the EXAFS spectrum. The mononuclear gold complex is bonded to two oxygen atoms of the TiO2 surface at an Au–O distance of 2.16 A. Treatment of this complex in He or in H2 at increasing temperatures led to formation of metallic gold clusters of increasing size, ultimately those with an average diameter of about 15 A. The data demonstrate the presence of metallic gold clusters in the working catalysts and also show these clusters alone are not responsible for the catalytic activity.

Mononuclear gold complexes synthesized from AuIII(CH3)2(acac) in zeolite NaY were characterized by time-resolved X-ray absorption spectroscopy and infrared spectroscopy as they catalyzed CO oxidation at 298 K and 760 Torr in flow systems. Initial contact with a CO + O2 mixture led to the rapid formation of cationic gold complexes in which Au was bonded to approximately two zeolite O atoms, on average. Further contact with CO + O2 led to breaking of an Au-surface oxygen bond, giving a gold carbonyl anchored to approximately one O atom. The process was reversed in the absence of CO and O2.

There are many examples of catalysis in solution by cationic complexes of gold, and recent results, reviewed here in this critical review, demonstrate that cationic gold species on oxide and zeolite supports are also catalytically active, for reactions including ethylene hydrogenation and CO oxidation. The catalytically active gold species on supports are evidently not restricted to isolated mononuclear gold complexes, but include gold clusters, which for at least some reactions are more active than the mononuclear complexes and for some reactions less active. Fundamental questions remain about the nature of cationic gold in supported catalysts, such as the nature of the cationic gold clusters and the nature of gold atoms at metal–support interfaces (88 references).

CO oxidation catalyzed by supported gold has emerged as a vigorous research field in the preceding few years, driven by the recognition that this unreactive metal, when highly dispersed, is remarkably active and selective as a catalyst, offering good prospects for application. It also offers good opportunities for fundamental understanding, because some of the reactants converted by supported gold are small molecules that are informative probes of the catalyst surface. Thus, CO oxidation has been used extensively to characterize supported gold catalysts. Nonetheless, no consensus has yet emerged regarding the catalytically active species, and it seems likely that catalytic activity is not restricted to a single kind of species. In the following review, we focused specifically on one class of catalytically active gold species for CO oxidation, cationic gold. The review includes summaries of (a) evidence of gold in various oxidation states in supported CO oxidation catalysts, including theoretical and experimental results, especially spectroscopic evidence of functioning catalysts; (b) evidence demonstrating that cationic gold plays a role in the catalytic sites in supported catalysts that contain both zerovalent and cationic gold; and (c) evidence that cationic gold plays such a role in supported catalysts that lack detectable zerovalent gold.

This paper is a brief review of suggested structures of catalytic sites and reaction mechanisms in CO oxidation catalyzed by highly dispersed supported gold. The review is focused on evidence of the sites, which has been determined primarily by infrared, X-ray absorption, and other spectroscopies, often combined with transmission electron microscopy; theory has also been applied to characterize supported gold catalysts. The literature gives strong evidence of the reaction of oxygen with CO adsorbed on zerovalent gold at low temperature, consistent with the role of such adsorbed CO in the catalytic reaction. There is also strong evidence of the involvement of cationic gold, even isolated single sites, in the catalysis at higher temperatures (e.g., room temperature). Because CO is only weakly adsorbed on zerovalent gold and more strongly adsorbed on cationic gold, it is suggested that the cationic gold sites may be kinetically more significant than the zerovalent gold sites under potentially practical reaction conditions.

Adsorption of CO on gold supported in zeolite NaY at 85 K led to the formation of (i) various carbonyls and isocarbonyls typical of the zeolite and (ii) carbonyls formed at cationic gold sites (observed in the 2186-2171 cm(-1) region). Analysis of the behavior of the bands allows their assignment to carbonyls of Au(3+) ions. At temperatures higher than 220 K, CO adsorption led to the formation of a new type of Au(3+)-CO species (2207 cm(-1)). Once formed, these complexes could be transformed into the dicarbonyls Au(3+)(CO)(2) when the sample was cooled to 85 K in the presence of CO. The results are explained by migration of Au(3+) ions to more accessible positions within the zeolite at increasing temperatures. When a CO molecule is already adsorbed, it stabilizes the Au(3+) ion in the new position, and a second CO molecule can be coordinated, thus forming a geminal species. These results are the first evidence of Au(3+)(CO)(2) complexes.

Mononuclear La2O3-supported AuIII complexes synthesised from AuIII(CH3)2(C5H7O2) and characterised by X-ray absorption spectroscopy are highly active, stable CO oxidation catalysts at room temperature, demonstrating the importance of the support in stabilizing catalytically active gold species, which need not include zerovalent gold for high activity.

There are many examples of catalysis in solution by cationic complexes of gold, and recent results, reviewed here in this critical review, demonstrate that cationic gold species on oxide and zeolite supports are also catalytically active, for reactions including ethylene hydrogenation and CO oxidation. The catalytically active gold species on supports are evidently not restricted to isolated mononuclear gold complexes, but include gold clusters, which for at least some reactions are more active than the mononuclear complexes and for some reactions less active. Fundamental questions remain about the nature of cationic gold in supported catalysts, such as the nature of the cationic gold clusters and the nature of gold atoms at metal-support interfaces (88 references).