Irina Borbáth

Formation of multilayered tin organometallic surface species. Preparation of new type of supported Sn–Pt catalysts

Abstract In this study, a new aspect of anchoring of tin organic moieties onto platinum is described and discussed. The new approach resulted in substantial increase of the Sn/Pt ratio in tin modified Pt/SiO2 catalysts. In the earlier approach, adsorbed hydrogen was exclusively used for tin anchoring, resulting in a monolayer of tin organic moieties at the top of platinum. In the new approach, a large excess of either tin tetraethyl or hydrogen was used in the tin-anchoring reaction. The presence of coadsorbents, such as oxygen, also led to a substantial increase of the amount of tin anchored. When the primary formed –SnR3 surface entities were partially decomposed, the coordinatively formed unsaturated –SnR2 and –SnR surface species provided additional anchoring sites for the next layer of SnR4. In the presence of adsorbed oxygen, additional new types of landing sites were created to anchor SnR4 in the neighborhood of platinum. The above approach resulted in Sn–Pt/SiO2 catalysts with exclusive tin–platinum interaction and an Sn/Pt ratio ca. 2. Results obtained in this study also reveal that the formation of surface organometallic moieties takes place in a stepwise way, e.g. the buildup of tin organic moieties occurs layer-by-layer. The supported Sn–Pt bimetallic entities formed showed both high activity and selectivity in the hydrogenation of crotonaldehyde to crotylalcohol.

Preparation of new type of Sn-Pt/SiO2 catalysts for carbonyl activation

Abstract The paper is aimed to demonstrate how the activity and selectivity of a Pt/SiO2 catalyst can be tailored by introduction of tin either to hydrogenate the aldehyde group in α,β-unsaturated aldehydes (UA) or oxidize carbon monoxide at room temperature. In this paper general aspects of catalyst design based on the use of tin tetraalkyls are described in detail. Results summarized in this work show that upon using new approaches for anchoring tin into platinum high Snanch/Pts can be achieved. The experimental evidences indicate also that the hydrogenation of the aldehyde group in α,β-unsaturated aldehydes and oxidation of CO requires in situ formed “Sn4+–Pt ensemble” sites to activate either the carbonyl group or the CO molecule.

Low temperature oxidation of CO over tin-modified Pt/SiO2 catalysts

Abstract Low temperature oxidation of CO over alloy type Sn–Pt/SiO 2 catalysts with different Sn/Pt ratios has been investigated at different CO partial pressure using thermal programmed oxidation (TPO) technique and time on stream (TOS) experiments. The introduction of tin into platinum strongly increased the activity of the catalyst. The activity had a maximum, which depended on both the Sn/Pt (at./at.) ratio and the CO partial pressure. TOS experiments revealed the aging of the Sn–Pt/SiO 2 catalysts. FTIR and Mossbauer spectroscopy has been used to follow compositional and structural changes of Sn–Pt/SiO 2 catalysts during the catalytic run. The results show that the in situ formed, highly mobile “Sn n + –Pt” ensemble sites are responsible for high activity, while formation of relatively stable SnO x type surface species are involved in the catalyst deactivation.

New approaches to prepare supported Sn-Pt bimetallic catalysts

General principles of modification of supported Pt catalysts with tin tetraalkyls are summarized and a new approach to prepare supported Sn−Pt catalysts is described. The new approach leads to the formation of Multilayered Organometallic Complexes (MLOC) anchored onto supported platinum. The formed MLOC can be decomposed either in reductive or oxidative atmosphere with the formation of new type of supported Sn−Pt catalysts with high Sn/Pts ratios. The decomposition in a reductive atmosphere leads to the formation of alloy-type bimetallic phases, while the decomposition in the presence of oxygen provides Lewis-acid type surface species anchored to supported platinum.

Zeolite supported Sn-Pt catalysts prepared by surface reactions

Abstract A Pt/H–mordenite (Pt/H–MOR) catalyst has been modified with tin using the controlled surface reaction between tin tetraalklyls and hydrogen adsorbed on platinum. Experimental evidences show that upon modification of Pt/H-MOR catalyst with tin tetraalkyls, the surface chemistry established for Pt/SiO 2 and Pt/Al 2 O 3 cannot be maintained, i.e., the formation of multilayered organometallic complexes (MLOC) is hindered. Consequently, the results indicate that the introduction of tin into platinum is possible if the Sn/Pt s ratio is below 0.5. In this case, zeolite supported alloy type Sn–Pt nanoparticles are formed. At higher Sn/Pt s ratios, in addition to the formation of alloy type Sn–Pt nanoparticles, ionic forms of tin anchored onto the zeolite are also formed. The reaction between tin tetraalkyls and surface OH groups of the zeolite is involved in the formation of ionic forms of tin stabilized on the zeolite surface. Upon applying Mossbauer spectroscopy, different tin containing surface species were identified including two SnPt alloy phases. Tin introduced in this way slightly decreases the H/Pt and CO/Pt ratios measured by chemisorption and changes the activity and selectivity of these catalysts in n -hexane isomerization at 275°C.

Reaction-Induced Transformations in Pt-Sn/SiO2 Catalysts: In Situ 119Sn Mössbauer Study

Reaction-induced separation of tin-rich surface layers and tin-depleted inner region was observed in metallic particles of Pt–Sn/SiO2 catalysts in two reactions: (i) dechlorination of 1,2-dichloroethane at 473 K (modeling catalytic removal of chlorine from hazardous chlorocarbons) and (ii) oxidation of carbon monoxide at room temperature. In the former, a Pt : Sn catalyst (1 : 2 atomic ratio, 1 wt% metal content), prepared via co-impregnation, showed high selectivity (>80%) toward ethylene (at the expense of ethane), but only after a prolonged (ca. 24 h) period. In situ Mössbauer studies revealed stabilization of a homogeneous Pt–Sn alloy and SnCl2 after activation in hydrogen; whereas tin-depleted and tin-rich components were separated after a 24-h period. Hence, inhibition of the hydrogenation activity of Pt, by surface tin enrichment and Cl deposition favors high ethylene selectivity. For the oxidation of CO at room temperature, a catalyst with a Pt : Sn atomic ratio of 3 : 2 (3 wt% Pt) was prepared by an organometallic (CSR) method using 119Sn(CH3)4. Platinum-rich PtSn(1) and tin-rich PtSn(2) components were separated in the Mössbauer spectra of catalyst activated at 570 K. The PtSn(2) component is primarily involved in surface reactions. Both in CO oxidation and the subsequent re-activation in hydrogen at room temperature a reversible PtSn(2) ↔ Sn4+ interconversion occurred. d ln (A77/A300)/dT data indicate the surface location of the involved components.

Hydrogenation of benzonitrile on Sn-Pt/SiO2 Catalysts prepared by introducing SnEt4 to Pt/SiO2: Role of TIN

Liquid phase hydrogenation of benzonitrile was studied over Sn-Pt/SiO2 catalysts prepared by introducing tetraethyl tin onto the 3 wt.% Pt/SiO2 catalyst. Tin content of the catalysts ranged from 0.05 to 0.63 wt.%, whereas Sn/Pt surface atomic ratios determined by chemisorption measurements were between 0.1 to 3.5. Dibenzylamine selectivity influenced to a small extent by the level of conversion and the Sn/Pt ratio wasca. 75 %. The addition of tin to Pt in the range of (Sn/Pt)surface = 0.50–1.25 led to an increase in the turnover frequency (TOF) by a factor of 2. TOF showed a maximum at a surface atomic ratio of Sn/Pt = 1. The enhancement of catalyst activity upon the addition of tin is explained by the formation of Snδ+-Pt ensemble sites on the surface of bimetallic nanoclusters. It is suggested that highly dispersed positively charged tin species, by polarizing the triple bond, enhance the reactivity of the -CN group. Calcination at 300°C followed by re-reduction of the catalysts resulted in a monotonic decrease of specific activity with increasing Sn/Pt ratio.

Time dependence of tin anchoring to supported platinum

In this paper new kinetic results obtained in the two step tin anchoring process are summarized. In this tin anchoring process the key step is the controlled surface reaction (CSR) between tin tetraalkyl and hydrogen adsorbed over platinum. In this study the focus was laid on the influence of the duration of tin anchoring on the type of surface organometallic species formed. The results of temperature programmed decomposition (TPD) of surface organometallic species in a hydrogen atmosphere indicated that at the beginning of tin anchoring, i.e. at low tin coverage, one of the main surface species is Sn(C2H5)4 strongly adsorbed into platinum. As the surface reaction proceeded the amount of tin anchored increased and the strongly adsorbed form of Sn(C2H5)4 was transformed into surface species with general formula of -Sn(C2H5)(4−x), and -{Sn(C2H5)(4−x)[Sn(C2H5)4]}. This behavior has been well-demonstrated on different supported platinum catalysts at relatively low [Sn]o/Pts ratios ([Sn]o/Pts<2). Under this condition monolayer tin coverage can be achieved, i.e. the ratio of Snanch/Pts in the formed alloy type supported Sn-Pt catalysts is around 0.4–0.5.

Preparation of new type of supported Sn-Pt bimetallic catalysts containing Lewis acid sites anchored to the platinum

Abstract Methods of Surface Organometallic Chemistry (SOC) were used to prepare new type of supported Sn-Pt catalysts. In the new approach the focus was laid on the stabilization of tin in the form of SnOxanchored directly to the platinum. The new surface species have been characterized by different methods. The results indicate the platinum nano-clusters are decorated by SnOx surface entities with Lewis-acid character. The new Sn-Pt/A12O3 catalysts were tested in hydrocarbon reactions above 500 °C. Due to the presence of new acid sites in the atomic closeness to the platinum the new Sn-Pt catalysts showed decreased aromatization and increased isomerization selectivities.

Control of the Particle Size of Gold on Tin Modified Silica Support

In this study we describe a two-step synthesis method for the preparation of gold nanoparticles supported on tin modified silica. The key step of the preparation procedure is the formation of surface grafted organotin complexes. The reduction of these surface species in a hydrogen atmosphere resulted in both metallic (Sno) and ionic (Sn2+) forms of tin. These forms of tin are capable to reduce gold from chloroauric acid solution forming gold nanoparticles with high dispersion on the surface of silica. It has been found that the particle size of gold strongly depends on the pH value of the gold solution and the presence or absence of ammonia.