JOURNAL OF CATALYSIS 110, 410-412 (1988)

Characterization of Vanadium Oxide-Promoted Ru/A1203 Catalyst by Secondary Ion Mass Spectrometry (SIMS)

So that the catalyst performance is highly improved from the industrial viewpoint and the reaction mechanism is unambiguously understood, it is of primary importance to characterize precisely the surface structure of a catalyst. Recent development of the instruments for surface analysis has en- abled us to understand such surface struc- ture. Among the techniques used, X-ray photoelectron spectroscopy (XPS) has been successful in identifying surface com- positions, especially in a multicomponent catalyst, and also in clarifying the oxidation or the electronic state of the surface ele- ments (1). Despite difficulty in obtaining quantitative information because of the ma- trix effect, secondary ion mass spectrome- try (SIMS), especially static SIMS, can make important contributions to catalyst studies with its sampling depth of one to two atomic layers and its ability to probe the local atomic environment on the surface (2-5).

In CO hydrogenation on Ru/A1203 cata- lyst, vanadium oxide added was found to enhance the C-O bond dissociation and thereby to highly improve the selective pro- duction of liquid fuel (6, 7). Chemisorption measurements revealed that the consider- able part of vanadium oxide added was present on the Ru metal surface. On this basis, the promotion mechanism was pro- posed; i.e., the vanadium ion adjacent to the Ru atom on which CO is adsorbed takes part in the C-O bond dissociation so that, due to the high affinity of this ion for the oxygen atom, the dissociation is enhanced (7, 8). In the present study, the vanadium oxide-promoted Ru/A1203 catalyst was characterized by SIMS so that the surface

structure determined previously from che- misorption measurements was confirmed.

Ru/A1203 catalyst (Ru loading, 10 wt%) was prepared by impregnating the A1203 support (reference catalyst of the Catalysis Society of Japan, JRC-ALO-4; surface area, 174 m2 g-l) with an aqueous solution of RuC13 followed by drying at 388 K over- night and subsequent reduction at 723 K for 3 h. The vanadium oxide-promoted catalyst (V/Ru atomic ratio, 1.0) was prepared by impregnating the Ru/A1203 catalyst with an aqueous solution of NH4V03 followed by the same treatments as those described above. The Ru loading is rather high in the present catalysts so that chemical species evolved from the catalyst surface by Ar+ sputtering can be detected easily in the SIMS experiments. SIMS measurements were made using ESCA-lab. 5 (VG Scien- tific Co., UK), which has room for various in situ pretreatments. The Ar+ sputtering of the catalyst surface was carried out at beam energy 4.5 keV and beam current 100 nA. Charge compensation was accomplished using a flood gun; without this, no SIMS signals were obtained. Under these condi- tions, during measurements for more than 1 h, no remarkable change was observed in the SIMS spectra.

Since, in the previous studies (6-8), the promotion effect of vanadium oxide on the C-O bond dissociation was investigated us- ing a low Ru loading catalyst (0.5 wt% Ru/ A1203), it is relevant to confirm such a pro- motion effect for a high Ru loading catalyst. Figure 1 illustrates the rate constants on the unpromoted and promoted Ru/A1203 cata- lysts. Here, the rate constant representing the intrinsic catalytic activity was deter-

410 0021-9517/88 $3.00 Copyright 0 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.

NOTES 411

0.5r

0.01’ I I I ,\ I 2.0 281 2.2 2.3 2.4 2.5

T-l / kK-'

FIG. 1. Rate constants for the C-O bond dissocia- tion determined by PSRA from dynamic behaviors of adsorbed CO during the CO hydrogenation over un- promoted Ru/A1203 (open circle) and vanadium oxide- promoted Ru/A1203 (closed circle).

mined by pulse surface reaction rate analy- sis (PSRA) in a manner described else- where (9-12). As illustrated, the addition of vanadium oxide results in a remarkable in- crease in the rate constant for the C-O bond dissociation, indicating that the pro- motion effect of vanadium oxide on this step can also be observed on the high Ru loading catalyst.

After evacuation at 373 K, SIMS mea- surements were carried out for both prere- duced catalysts, and the results are shown in Fig. 2. As shown, various signals can be observed in each SIMS spectrum. The Al+ and Ru+ signals appear for both catalysts, while the promoted catalyst gives the addi- tional signals of V+ and VO+ (Fig. 2b), orig- inating from the vanadium oxide added as a promoter. The most remarkable feature is, however, that the RuO+ signal is clearly ob- served in the SIMS spectrum of the vana- dium oxide-promoted catalyst (Fig. 2b). Because neither the unpromoted catalyst nor the mechanical mixture of Ru/A1203 with vanadium oxide gives this signal, the possible source of the oxygen atom in the RuO+ species must be vanadium oxide added as a promoter. Furthermore, from the absence of the RuO+ signal for the me-

chanical mixture, the recombination of Ru+ and 0+ species separately evolved is ex- cluded as a possible source of RuO+ spe- cies. Therefore, the present SIMS measure- ments enable us to conclude that on Ar+ sputtering onto the catalyst surface, the ox- ygen atoms can be evolved from vanadium oxide together with the Ru atoms in the neighborhood of vanadium oxide. This con- clusion indicates that vanadium oxide con- tacts intimately with Ru metal particles.

Results of chemisorption measurements support the conclusion obtained from SIMS measurements. Vanadium oxide added to the Ru/A1203 decreased the chemisorption ability significantly; the amount of CO ad- sorbed, determined by using a pulse ad- sorption technique, was 239 pmol g-l for the unpromoted catalyst and 112 pmol g-’ for the promoted catalyst. Because vana- dium oxide does not adsorb CO, this marked decrease in the amount of CO ad- sorbed on the promoted catalyst results from the decrease in the number of surface

(b)

No+

Ar+

I (a)

RU’

A

I I I I I I I I 20 40 60 80 100 120 140 160

m/e

FIG. 2. SIMS spectra of the prereduced unpromoted (a) and vanadium oxide-promoted Ru/A1203 (b).

412 NOTES

Ru metal sites for adsorption. X-ray diffrac- tion and electron microscopic measure- ments gave no reliable evidence for the re- duction of the adsorption sites; electron micrograms showed the fairly homoge- neous dispersion of the Ru metal particles with their sizes of ca. 50 A in both cata- lysts. Therefore, the marked reduction of adsorption sites is ascribed to site blocking by the added vanadium oxide but not to the crystal growth of the Ru metal particles. This indicates that the considerable part of vanadium oxide added is present on the Ru metal surface for the present high Ru load- ing catalyst, leading to a structural model similar to that for the low Ru loading cata- lyst and also to the same conclusion ob- tained from SIMS measurements.

XPS measurements also supported the conclusion described above. Relative to the Al signal, the Ru signal considerably de- creased in its intensity upon addition of va- nadium oxide; the ratio of Ru 3d to Al 2p

was 0.252 for the unpromoted catalyst and 0.179 for the promoted catalyst. The de- crease in this intensity ratio should be as- cribable to considerable covering of the sur- face Ru atoms by the added vanadium oxide, which is again in accordance with the conclusions from both SIMS and CO chemisorption measurements.

Thus, it was found that the vanadium ox- ide-promoted Ru/A1203 catalyst was satis- factorily characterized by SIMS through the detection of chemical species evolved from the catalyst surface.

ACKNOWLEDGMENTS

This work was partially supported by a Grant-in-Aid for Energy Research from the Ministry of Education, Science and Culture, Japan (60040016). The authors are grateful to Mr. Kenzi Suzuki of Government Industrial Research Institute, Nagoya, and to Mr. Takamasa Hanaichi of Nagoya University for X-ray diffraction and electron microscopic measurements, respectively.

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NAOKI TAKAHASHI TOSHIAKI .MoRI*J AKIO FURUTA? SHIN’ICHI KOMAI AKIRA MIYAMOTOS TADASHI HATTORI YUICHI MURAKAMI

Department of Synthetic Chemistry

Faculty of Engineering Nagoya University Chikusa-ku, Nagoya 464, Japan

*Government Industrial Research Institute, Nagoya Hirate-cho, Kita-ku, Nagoya 462, Japan TKinuura Research Development JGC Corporation Sunosaki-cho, Handa, Aichi 475, Japan SDepartment of Hydrocarbon Chemistry

Faculty of Engineering Kyoto University Sakyo-ku, Kyoto 606, Japan

Received July 22, 1987; revised November II, 1987

t To whom correspondence should be addressed.