Reduction and oxidation of rhodium/aluminum dioxide and rhodium/titanium dioxide catalysts as studied by temperature- programmed reduction and oxidation Citation for published version (APA): Vis, J. C., Blik, van 't, H. F. J., Huizinga, T., Grondelle, van, J., & Prins, R. (1984). Reduction and oxidation of rhodium/aluminum dioxide and rhodium/titanium dioxide catalysts as studied by temperature-programmed reduction and oxidation. Journal of Molecular Catalysis, 25(1-3), 367-378. https://doi.org/10.1016/0304- 5102(84)80059-0 DOI: 10.1016/0304-5102(84)80059-0 Document status and date: Published: 01/01/1984 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. 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If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policy If you believe that this document breaches copyright please contact us at: [email protected] providing details and we will investigate your claim. Download date: 02. Oct. 2021 Journal ofMolecular Catalysis, 25 (1984) 367 - 378 367 REDUCTION AND OXIDATION OF Rh/Al,Os AND Rh/TiOa CATALYSTS AS STUDIED BY TEMPERATURE-PROGRAMMED REDUCTION AND OXIDATION J. C. VIS, H. F. J. VAN’T BLIK, T. HUIZINGA, J. VAN GRONDELLE and R. PRINS* Eindhoven University of Technology, Laboratory for Inorganic Chemistry, P.O. Box 513, 5600 MB Eindhoven (The Netherlands) Summary Careful preparation of Rh/A120s catalysts leads to ultradisperse systems (H/Rh > 1.0). Temperature-programmed reduction (TPR) shows that these catalysts are almost completely oxidized during passivation. Identical prepa- ration of Rh/TiOz catalysts leads to less disperse systems (H/Rh = 0.3), which exhibit two reduction peaks in TPR. These peaks are due to the reduction of small, well-dispersed Rh,Os particles and of large, bulk-like RhZ03 particles. In all cases reduction of Rh,Oj is complete above 450 K. TiO? is partly reduced by a metal-catalysed process above 500 K. Introduction Over the past years it has become clear that rhodium catalysts occupy a special position in the field of supported transition metal catalysts, because they are able to produce hydrocarbons as well as oxygenated products (alcohols, aldehydes, acids) from synthesis gas [ 1 - lo]. Various workers have tried to influence the selectivity and activity of the rhodium catalysts via special preparation [ 1, 21, additives [ 3 - 51 or mixed oxides [8 - lo], and where this succeeded at least two groups gave as the reason the presence of stabilized Rh+ on the surface [8, lo]. Some authors claimed the presence of isolated Rh+ sites in mono- metallic Rh catalysts on the basis of IR evidence (Worley et al. [ll - 131, Primet [14]), while others using electron microscopy found the Rh to be present as metallic rafts (Yates et al. [15]). In all cases it seems obvious that the support plays an important role in either bringing or keeping the metal in a certain state of (non-)reactivity. A special example of such a metal-support interaction was discovered by Tauster et al. [16,17] and is now known as Strong Metal-Support Inter- action (SMSI): Supported metals such as Pt and Rh lose their capability for chemisorption of Hz, CO and NO if they have been reduced at high *Author to whom correspondence should be addressed. 0304-5102/84/$3.00 0 EIsevier Sequoia/Printed in The Netherlands 368 temperatures (e.g. 773 K) on supports such as TiOz and V203. Normal chemisorption behaviour can be restored by oxidation at elevated tempera- tures, followed by low-temperature reduction, at for instance 473 K [16]. SMSI has been related to the occurrence of lower oxides of the support [l&19], but the exact nature of the interaction still remains unclear. Many of the above-mentioned phenomena involve one common prop- erty: The oxidation-reduction behaviour of supported Rh catalysts. We decided to study two systems, representative of many of those referred to above: 2.3 wt.% Rh/Al*O, and 3.2 wt.% Rh/TiO,. Via sintering (see Experi- mental) we have induced a variation in particle size (dispersion) in order to examine the following questions: (i) How is oxidation-reduction influenced by particle size? (ii) Does particle size show any effect upon SMSI? Before discussing the experimental techniques, we must introduce one last item: passivation. Since it is obvious that reduced systems cannot simply be stored in air, we passivate and stabilize them by applying a layer of oxy- gen on the metal particles in a controlled manner (see Experimental). Some authors have already examined the state of the catalysts after storage in air. Burwell Jr. et al. have used Wide Angle X-ray Scattering, Extended X-ray Absorption Fine Structure (EXAFS), hydrogen chemisorption and hydro- gen-oxygen titration to characterize their supported Pt and Pd catalysts [20 - 231. We will show that good insight in these matters can be gained via Temperature Programmed Reduction and Oxidation (TPR and TPO), sup- ported by chemisorption measurements. TPR as a characterization technique was presented by Jenkins et al. in 1975 [24, 251 and has been used exten- sively in the past few years, as is seen from a recent review by Hurst et al. [26]. The technique allows one to obtain (semi)quantitative information on the rate and ease of reduction of all kinds of systems; once the apparatus has been built, analyses are fast and relatively cheap. We used an apparatus as described by Boer et al. [27 1, which enabled us to extend the analyses to TPO, and thus gather information about the rate and ease of oxidation as well. Experimental TiOz (anatase, Tioxide Ltd, CLDD 1367, surface area 20 m* g-i, pore volume 0.5 cm3 g-i) and r-Al,O, (Ketjen, OOO-1.5E, surface area 200 m2 g -l, pore volume 0.6 cm3 g-i) were impregnated with an aqueous solution of RhCls-xH20 via the incipient wetness technique to prepare a 2.3 wt.% Rh/AI,O, and a 3.2 wt.% Rh/TiOz catalyst, as was established spectro- photometrically. The catalysts were dried in air at 355, 375 and 395 K for 2 h successively, followed by direct (pre)-reduction in flowing H, at 473, 773 or 973 K for 1 h. Prior to removal from the reduction reactor, the catalysts were passivated at room temperature by replacing the H2 flow 369 Fig. 1. Schematic representation of TPR-TPO apparatus. with NZ, and subsequently slowly adding up to 20% Oz. The catalysts were then taken out of the reactor and stored for further use. The TRP/TPO apparatus used is schematic~ly represented in Fig. 1: a 5% H&r-Ar or a 5% O&n-He flow (300 ml h-r) can be directed through a microreactor. The temperature of the reactor can be raised or lowered via linear programming H, or O2 consumption is monitored continuously by means of a thermal conductivity detector (TCD). A typical reactor sequence is as follows (a similar sequence is fol- lowed during TPO) : - the passivated or oxidized sample is flushed under Ar flow at 223 K; - Ar is replaced by the Ar/H2 mixture, causing at least an apparent H, consumption (first switch peak); - the sample is heated under Ar/H, flow at a rate of 5 K min-’ to 8’73 K; - after 15 min at 873 K, the sample is cooled down at 10 K mine1 to 233 K; - the reduced sample is flushed with Ar; - Ar flow is replaced by the Ar/H2 mixture once more, now causing only an apparent E-I2consumption (second switch peak). The switch peak procedure deserves some closer at~ntion. The strong signal we call the first switch peak is mainly due to the displacement of Ar by Ar/H, in the reactor, but in some cases real hydrogen consumption takes place, even at 223 K. Therefore we repeat the whole procedure after the TPR has been performed, when the catalyst has been reduced and cooled down to 223 K, and as a consequence is covered by hydrogen. Then we replace the Ar/H, by pure Ar, resulting in a negative TCD signal. Subse- quently we switch back to Ar/Hz. Since we do not expect any hydrogen consumption from the reduced hydrogen-hovered sample at this time, the resulting second switch peak will be due solely to the displacement of Ar by Ar/H,.