Matveer, K., Bukhtoyarov, I., Shultz, N., Yanova. O., Kinet. Katal., 5, 649 L i t e r a t u r e Cited (1964). Dozono, .t.%Shiba, T., Bull Jpn. Pet. lnst., 15,7 (1963). J., Hafner, W., Jirra, F., Sedlmeier, J., Sieber, R., Runinger, R., Kojer, ~T,, /nd, ~ Chem,, ~ prd, ~ R ~i ~ Smidt, ,, Fujimoto, K., Negarni, y.,Takahashi, T,, ~ H., Angew. Chem., 71, 176 (1959). Dev., 11, 303 (1972). Fujinioto, K., Takeda, H., Kunugi, T., lnd. fng. Chem., Prod. Res. Dev., 13, 237 (1974). Henry, P: M., J. Am. Chem. SOC.,86, 3246 (1964). Received for review M a r c h 29,1976 Kusunoki. Y., Katsuno. R.,Hasegawa, N., Kurematsu, S., Nagao, K., ishi-i, K., Tsutsumi, S., Bull. Chem. SOC.Jpn., 39,2021 (1966). Accepted July 11, 1976
Sulfur Storage on Automotive Catalysts Kathleen C. Taylor Physical Chemistry Department, Research Laboratories, General Motors Corporation, Warren, Michigan 48090
A study has been made of the factors which influence the catalytic oxidation of sulfur dioxide over auto exhaust catalysts. These include such catalyst properties as noble metal composition, catalyst support characteristics, and catalyst pretreatment. Operating parameters such as gas hourly space velocity, temperature, and air/fuel ratio were also examined. The storage of sulfur in the catalyst, primarily via reaction with the SO3 which formed, was a dominant feature of these pelleted catalysts. In fact, sulfur storage tended to override other influences considered here. Results of laboratory experiments are discussed in relation to the control of automotive sulfate emissions.
Introduction The property of alumina-supported oxidation catalysts to store sulfur has been described by Hunter (1972) and by Hammerle et al. (1975). Sulfur storage during catalyst testing by the Federal Test Procedure (FTP) has been reported by Beltzer et al. (1974) and Begeman et al. (1974). Several other studies have also appeared which were aimed at measuring and/or characterizing the sulfur emissions from catalystequipped vehicles (Pierson et al., 1974; Beltzer et al., 1975; Pierson, 1975). The retention of exhaust sulfur compounds on the catalyst potentially suppresses automotive sulfate eniissions. Because of the potential benefit of sulfur storage to the control of exhaust sulfate emissions, we were interested to determine the interplay of sulfur storage, catalyst operating conditions, and catalyst composition on sulfate emissions. Sulfur storage complicates measurements of SO2 conversion since a mass balance between the SO2 inlet and SO2 and SO3 outlet is not achieved. This necessitates measurement of SO3 (or H 2 S 0 4 )rather than just SO2 disappearance. In this paper we report on laboratory studies of the catalytic conversion of SO2 over supported noble metal catalysts. The dependence of product distribution on catalyst pretreatment with SO2 was determined. Moreover, the effect of operating temperature, gas hourly space velocity, oxygen concentration, and catalyst composition on H2S04 formation were examined. Experimental Section Catalysts. The platinum-palladium catalyst used in this work is supported on Vs-in. alumina pellets (Rhone Progil, SCF-79) and contains a 5:2 Pt/Pd weight ratio and a total precious metal loading of 0.05 wt %. The other catalysts were prepared in our laboratory. The platinum catalysts were prepared by the impregnation of either alumina spheres (Kaiser, surface area = 200 m2/g, bed density = 0.55 g/cm3) or silica-alumina pellets (Strem, # 14-174) with an aqueous solution of chloroplatinic acid (10% solution H2PtClvGH20, 264
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15,No. 4, 1976
Matheson Coleman and Bell). The palladium catalyst and rhodium catalyst were prepared by impregnation of the Kaiser alumina with aqueous solutions of palladium chloride (5% solution, Matheson) and rhodium chloride (RhC1~4H20, ROC/RIC), respectively. Following impregnation, the catalysts were dried in air at ambient temperature and then at 100 "C. The catalysts were calcined a t 500 OC for 4 h in flowing air. The precious metal dispersions were determined by hydrogen or carbon monoxide chemisorption. The chemisorption of hydrogen by a flow technique was used for Pt and Rh. Static chemisorption of carbon monoxide was used for Pd. Apparatus. The catalytic reactor used to study catalyst activity for sulfur dioxide oxidation (Figure 1)was a 2.8-cm 0.d. silica tube which was situated in an electrically heated furnace. A thermocouple situated in the center of the catalyst bed monitored and controlled catalyst temperature. Silicon carbide was placed in the tube in front and behind the catalyst to ensure adequate heating of the inlet gases and a uniform shape for the horizontal bed. The reactor was terminated a t the end of the furnace by a fritted glass T 13 cm in length which also joined a Goksdyr-Ross coil (Gksdyr and Ross, 1962) for SO3 and H2S04collection. The gases leaving the coil were passed through a pulsed fluorescence SO2 analyzer (Therm0 Electron Corp., Series 40) which provided a continuous measurement of SO2 concentration. Beltzer et al. (1975) reported on the problem of interference from other exhaust gas constituents when using this technique to monitor Son. While oxygen might potentially interfere with the SO2 measurements made here, the problem was handled by comparing inlet and outlet SO2 with the background gases present. The volume of gas passing through the system was measured using a wet test meter at the back end of the SO2 analyzer. The amount of SOa or H2S04 formed was determined by titration of the condensate collected in the coil with a 0.1 N solution of sodium hydroxide with the end point of the titra-
7
Figure I. Diagram of experimental apparatus used for measurement of sufuric acid tormation over catalj sta
tion det,ermined using bruoiophenol blue indicator. Acid--base titration is appropriate for determination of HYS04 in the present st,udy but may not be so in general if other acids are present. The sampling time for H2SOd was generally 1h. For some experiments, t,he total sulfur content of the catalysts was determined using an igiiition technique. The laboratory studies were carried out using bottled gases. T h e typical feetistreani iised t o monitor SO? coiiversion contained 50 ppm of SO? and %% 0 2 in a Nz atmosphere. A gas flow rate of 2.36 I./inin and catalyst volume of 15 cn?? gave a gas hourly space velocit:. (GHSV) of 9500. Water vapor was not generally inc1:hided in the feed; however, similar results were obtained with and without wat,er vapor. The experiments of interest were carried out above 501, "C where water vapor would not tie expected to have a n effect,. Our experimental technique does nut disrrimiiiate between product SO:SO:! indicated on the continuous SO:! analyzer. Release or H~SOJ measured after the catalyst, we chose to examine the of SO- stored in excess the saturation amount under the effect of operating paranieters and catalyst characterization react i, 111 rolidiiioiis Wacs complete in 30 min. A mass balance 011 SO? conversion fur fresh catalysts as well as sulfur-prehetween the SO2 inlet and the sum of the SO2 and H2S01 treated catalysts. outlet rising the preconditioned catalysts confirmed that t lie Temperature. The temperature dependence of SO:! concatalysts liad h e n saturated with sulfur by this procedure. version and simultaneous sulfur storage was examined in experiments using both fresh catalyst samples and samples Results and Discussion preconditioned in SO?. T h e preconditioned samples were S u l f u r Storage. Experiments were carried out, t o detersaturated with sulfur and the sum of the outlet S0:j and SO2 mine the relation betsveen sulfur storage and SO:i or H ~ S O I was equal to the inlet S O i . Figure 3 shows that a t 19 (300 h--' production for a typical pellet,ed Pt-Pd/Al?Oy catalyst. space velocity the SO:
