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Reaction selectivity as a test for catalysis on exposed metal
Applied Catalysis, 48 (1989) 235-239 Elsevier
Science
Publishers
235
B.V., Amsterdam
-
Printed
in The Netherlands
Reaction Selectivity as a Test for Catalysis on Exposed Metal JEFFREY
M. COGEN
and WILHELM
F. MAIER*.”
Department of Chemistry, University of Califorrua, Berkeiey, CA 94720 jL!.S.A.) (Received
1 March
1988, revised manuscript
received
10 December
1988)
ABSTRACT The metal sensitive
selectivities
of hydrogenation
reactions
are employed
to differentiate
be-
tween metal catalysis and spillover catalysis. Two cataiysr metal specific reactions. dihydrogen addition to 2-hexyne and 1,6-dimethylcyclohexene, are studied on silicon supported palladium and platinum films covered with silica overlayers. At temperatures near 200’ C selectivities observed are characteristic of the metal in the underlayer. Since such films have been shown not to contain transition metal in the silica layer, the results suggest that cracks in the silica are present during reaction. In contrast, the selectivity observed at temperatures below 100’ C is not that of the metal underlayer resents invoking
and is therefore
consistent
with a spillover
phenomenon.
a general test which may also prove useful to ruie out transition a hydrogen
spillover
The method
metal catalysis
rep-
in studies
mechanism.
INTRODUCTION
We recently reported that silica-covered platinum films (see Scheme 1) are active catalysts for hydrogen/deuterium (H/D ) exchange in alkanes [ 1 ] and for conversion of cyclohexene to benzene and cyclohexane [ 2 1. Secondary ion mass spectrometry (SIMS), Rutherford back scat.tering spectrometry (RBS) and Auger electron spectrometry (AES) of the front and back side of these catalysts combined with various control experiments established that the observed activity was not due to transition metal impurities (sensitivity limit 10” Pt-atoms/cm2) at or near the surface of the silica overlayers or the back side of the catalysts [ 11.This was confirmed by a lack of catalytic activity with surface impregnated SiOJSi films with platinum concentrations from 1081014Pt-atoms/cm2, which were readily detected by subsequent RBS and SIMS analyses [ 1,3]. No indications for the presence of cracks or pores could be obtained by any of the applied analytical techniques and control experiments. “Present address: Prof. Dr. W.F. Maier, 103 764, D-4300 Essen, F.R.G.
0166-9834/89/$03.50
Fachbereich
0 1989 Elsevier
Science
Chemie,
Publishers
Universit&
B.V.
GHS Essen,
Postfach
236
30, Pt
or Pd
Si
Scheme 1. I
=
w+k
n0pcc.a Scheme 2.
To explain the activity of these films, it was proposed that dihydrogen diffuses through the overlayer to become activated at the underlayer. This activated (dissociated) hydrogen then migrates back to the outer silica surface to cause the observed catalysis (spillover mechanism). Although this mechanism is consistent with all data collected, it does not consider the temperature difference between the experimental conditions and analytical data collection (room temperature j . One alternative explanation of the observed catalysis is an accessibility of the metal underlayer through temperature dependent crack opening based on the different expansion coefficients of the overlayer (SiO,), underlayer (Pt or Pd), and substrate (Si). In order to address this question a method was developed which distinguishes between catalysis by hydrogen spillover and catalysis by transition metal in the underlayer. The method is quite general and should be useful in any system, where hydrogen spillover from platinum or palladium particles is believed to be the carrier of catalytic activity. In the case of our silica covered platinum films the spillover mechanism predicts that silica-covered palladium should show the same selectivity as silica covered platinum, since spillover hydrogen on silica would be the active site or the originator of the active site on both catalysts. The crack mechanism on the other hand, with accessible transition metal at the crack bottom, should lead to selectivities which depend on the specific metal in the underlayer for reactions sensitive to the nature of the cataiyst metal. Our original reactions, cyclohexene hydrogenation/dehydrogenation [ 2 ] and alkane H/D-exchange [ 11, are not suitable for this test, since product distributions can not differentiate between platinum and palladium catalysts. We therefore selected two reactions sensitive to the nature of the catalyst metal (Scheme 2). Dihydrogen addition to alkynes (I) leads to highly selective cis-alkene formation on palladium (cis/cis + tram ratio > 80% ), while platinum results in a rather unselective cis/cis + trans ratio of < 75% [ 41.
237
In contrast, dihydrogen addition to 1,6-dimethylcyclohexene (II) is more ratio > 50% ), selective on platinum (ci.s/cis + trans-1,Zdimethylcyclohexane while palladium produces a ratio < 50% [ 51. EXPERIMENTAL
Films were prepared by evaporation of palladium (resistive heating) or platinum (electron beam) at lo-’ Torr (1 Torr = 133.3 Pa) onto a silicon single crystal wafer which had a 2-mm mask around the edges. The mask was removed and silica was deposited over the entire film by electron beam evaporation of quartz at lop7 Torr. The preparationprocedure and analysisof the resulting films is described elsewhere (film thickness reproducibility + 1.5 nm) [l-3]. The films were then moved to a flow reactor where the following general procedure was used to determine selectivities. The film was reduced at 250°C in dihydrogen for 15 min and cooled to reaction temperature under dihydrogen. With a hydrogen flow of 10 ml/min 2-hexyne (freshly distilled) or 1,6-dimethylcyclohexene (freshly purified by preparative GC) was added by syringe pump at 16 pl/min and 3.8@/min, respectively. Product samples were analyzed by capillary GC. Retention times were verified by injection of standard samples. RESULTS AND DISCUSSION
In gas phase flow experiments [1,2], 2-hexyne was passed over Pt/Si and Pd/Si catalysts with and without silica overlayers with dihydrogen as carrier gas. The results are summarized in Table 1. The uncovered palladium catalysts showed the expected high selectivities [cis-/ (cis- + trans-2-hexene) ] of 94,94 and 86% respectively at 6O’C, 100” C and 250” C respectively. At 60” C the selectivity of the silica covered palladium film drops significantly to 68% supporting the spillover mechanism. At higher temperatures, however, the silica covered Pd/Si films show increasing palladium selectivity supporting increasing metal accessibility. The results suggest that at 60 c C spillover may be domTABLE 1 Selectivities”of P-hexyne hydrogenation on various catalysts at the indicated reaction temperatures. Catalyst
60°C
100°C
Pd/Si SiO,/Pd/Si Pt/Si SiO,/Pt/Si
94 68
94 79
“(% cis/% ck+trarw)
+3%
136’C
250°C
76 81
86 90 73 70
inating the product distribution, at 100 r^ C the selectivity may be composed of contributions from spillover catalysis and crack-opening, while at 250°C there is either crack opening in the silica overlayers or deterioration of the palladium-silica interface and the observed catalysis is dominated by direct metalsubstrate contact. This is also supported by an increase in selectivity of the silica covered Pd/Si catalyst with extended use, which was accompanied by a visible deterioration of the silica-palladium interface. As control experiments for the high selectivity attributed to palladium exposure serve hydrogenations on the intrinsically inselective platinum films on silicon. There is no significant difference between the selectivities of silica-covered and uncovered platinum films at 136°C and 250” C (low overall activity excluded reliable measurements at the lower temperatures j and the low selectivity of the silicacovered palladium film at 60 ’C. The other test reaction, the dihydrogen addition to 1,6-dimethylcyclohexene, has a low turnover frequency and could therefore only be studied at elevated temperatures (see Table 2 ) due to our experimental detection limits. At the reaction temperature of 100°C and 205 ‘C all catalysts displayed selectivities characteristic of the transition metal present, even in the presence of the silica overlayer, supporting that the hydrogenation reaction is dominated by direct metal-reactant interaction through cracks in the silica overlayer. It is concluded that at lower temperatures hydrogenation reactions on silica covered Pd/Si and Pt/Si film catalysts may be due to the spillover mechanism, while at elevated temperatures direct metal-alkene interaction dominates the product distribution due to increasing metal accessibility. Metal sensitive reaction selectivities as demonstrated in this study, are the most sensitive tool for the detection of transition metal accessibility through overlayers and should be used as control reactions in future overlayer studies. Such metal sensitive reactions might also prove useful as supplemental tests in other studies invoking hydrogen spillover mechanisms [ 61, since they are independent of the actual metal concentration (detectable or undetectable) and merely a function of measurable catalytic activity. TABLE 2 Selectivities” of l,&dimethylcyclohexene peratures indicated Catalyst
100°C
205°C
Pd/Si SiO,/Pd/Si Pt/Si SiO,/Pt/Si
38 32 66 65
29 35 46 49
“(% cis/% cis+trans)
+3%
hydrogenation on various catalysts at the reaction tem-
239
ACKNOWLEDGEMENT
This study was supported by the National Institutes of Health, Grant Nr. GM-33386- 01.
REFERENCES 1 A.B. McEwen, W.F. Maier, R.H. Fleming and S.M. Baumann, Nature (London), 329 (198’7) 531. 2 J.M. Cogen, K. Ezaz-Nikpay, R.H. Fleming and SM. Baumann and W.F. Maier, Angew. Chem., 99 (1987) 1222; Int. Ed., 26 (1987) 1182. 3 A.B. McEwen, Ph.D. Thesis, University of California, Berkeley, CA, 1987. 4 A. Molnar, G.V. Smith and M. Bartdk, J. Catal., 101 (1986) 67. 5 M. Bartbk, Stereochemistry of Heterogeneous Metal Catalysis, Wiley, New York, 1985, Ch. III. 6 W.C. Conner, G.M. Pajonk and S.J. Teichner, Adv. Catal., 34 (1986) 1.
Science
Publishers
235
B.V., Amsterdam
-
Printed
in The Netherlands
Reaction Selectivity as a Test for Catalysis on Exposed Metal JEFFREY
M. COGEN
and WILHELM
F. MAIER*.”
Department of Chemistry, University of Califorrua, Berkeiey, CA 94720 jL!.S.A.) (Received
1 March
1988, revised manuscript
received
10 December
1988)
ABSTRACT The metal sensitive
selectivities
of hydrogenation
reactions
are employed
to differentiate
be-
tween metal catalysis and spillover catalysis. Two cataiysr metal specific reactions. dihydrogen addition to 2-hexyne and 1,6-dimethylcyclohexene, are studied on silicon supported palladium and platinum films covered with silica overlayers. At temperatures near 200’ C selectivities observed are characteristic of the metal in the underlayer. Since such films have been shown not to contain transition metal in the silica layer, the results suggest that cracks in the silica are present during reaction. In contrast, the selectivity observed at temperatures below 100’ C is not that of the metal underlayer resents invoking
and is therefore
consistent
with a spillover
phenomenon.
a general test which may also prove useful to ruie out transition a hydrogen
spillover
The method
metal catalysis
rep-
in studies
mechanism.
INTRODUCTION
We recently reported that silica-covered platinum films (see Scheme 1) are active catalysts for hydrogen/deuterium (H/D ) exchange in alkanes [ 1 ] and for conversion of cyclohexene to benzene and cyclohexane [ 2 1. Secondary ion mass spectrometry (SIMS), Rutherford back scat.tering spectrometry (RBS) and Auger electron spectrometry (AES) of the front and back side of these catalysts combined with various control experiments established that the observed activity was not due to transition metal impurities (sensitivity limit 10” Pt-atoms/cm2) at or near the surface of the silica overlayers or the back side of the catalysts [ 11.This was confirmed by a lack of catalytic activity with surface impregnated SiOJSi films with platinum concentrations from 1081014Pt-atoms/cm2, which were readily detected by subsequent RBS and SIMS analyses [ 1,3]. No indications for the presence of cracks or pores could be obtained by any of the applied analytical techniques and control experiments. “Present address: Prof. Dr. W.F. Maier, 103 764, D-4300 Essen, F.R.G.
0166-9834/89/$03.50
Fachbereich
0 1989 Elsevier
Science
Chemie,
Publishers
Universit&
B.V.
GHS Essen,
Postfach
236
30, Pt
or Pd
Si
Scheme 1. I
=
w+k
n0pcc.a Scheme 2.
To explain the activity of these films, it was proposed that dihydrogen diffuses through the overlayer to become activated at the underlayer. This activated (dissociated) hydrogen then migrates back to the outer silica surface to cause the observed catalysis (spillover mechanism). Although this mechanism is consistent with all data collected, it does not consider the temperature difference between the experimental conditions and analytical data collection (room temperature j . One alternative explanation of the observed catalysis is an accessibility of the metal underlayer through temperature dependent crack opening based on the different expansion coefficients of the overlayer (SiO,), underlayer (Pt or Pd), and substrate (Si). In order to address this question a method was developed which distinguishes between catalysis by hydrogen spillover and catalysis by transition metal in the underlayer. The method is quite general and should be useful in any system, where hydrogen spillover from platinum or palladium particles is believed to be the carrier of catalytic activity. In the case of our silica covered platinum films the spillover mechanism predicts that silica-covered palladium should show the same selectivity as silica covered platinum, since spillover hydrogen on silica would be the active site or the originator of the active site on both catalysts. The crack mechanism on the other hand, with accessible transition metal at the crack bottom, should lead to selectivities which depend on the specific metal in the underlayer for reactions sensitive to the nature of the cataiyst metal. Our original reactions, cyclohexene hydrogenation/dehydrogenation [ 2 ] and alkane H/D-exchange [ 11, are not suitable for this test, since product distributions can not differentiate between platinum and palladium catalysts. We therefore selected two reactions sensitive to the nature of the catalyst metal (Scheme 2). Dihydrogen addition to alkynes (I) leads to highly selective cis-alkene formation on palladium (cis/cis + tram ratio > 80% ), while platinum results in a rather unselective cis/cis + trans ratio of < 75% [ 41.
237
In contrast, dihydrogen addition to 1,6-dimethylcyclohexene (II) is more ratio > 50% ), selective on platinum (ci.s/cis + trans-1,Zdimethylcyclohexane while palladium produces a ratio < 50% [ 51. EXPERIMENTAL
Films were prepared by evaporation of palladium (resistive heating) or platinum (electron beam) at lo-’ Torr (1 Torr = 133.3 Pa) onto a silicon single crystal wafer which had a 2-mm mask around the edges. The mask was removed and silica was deposited over the entire film by electron beam evaporation of quartz at lop7 Torr. The preparationprocedure and analysisof the resulting films is described elsewhere (film thickness reproducibility + 1.5 nm) [l-3]. The films were then moved to a flow reactor where the following general procedure was used to determine selectivities. The film was reduced at 250°C in dihydrogen for 15 min and cooled to reaction temperature under dihydrogen. With a hydrogen flow of 10 ml/min 2-hexyne (freshly distilled) or 1,6-dimethylcyclohexene (freshly purified by preparative GC) was added by syringe pump at 16 pl/min and 3.8@/min, respectively. Product samples were analyzed by capillary GC. Retention times were verified by injection of standard samples. RESULTS AND DISCUSSION
In gas phase flow experiments [1,2], 2-hexyne was passed over Pt/Si and Pd/Si catalysts with and without silica overlayers with dihydrogen as carrier gas. The results are summarized in Table 1. The uncovered palladium catalysts showed the expected high selectivities [cis-/ (cis- + trans-2-hexene) ] of 94,94 and 86% respectively at 6O’C, 100” C and 250” C respectively. At 60” C the selectivity of the silica covered palladium film drops significantly to 68% supporting the spillover mechanism. At higher temperatures, however, the silica covered Pd/Si films show increasing palladium selectivity supporting increasing metal accessibility. The results suggest that at 60 c C spillover may be domTABLE 1 Selectivities”of P-hexyne hydrogenation on various catalysts at the indicated reaction temperatures. Catalyst
60°C
100°C
Pd/Si SiO,/Pd/Si Pt/Si SiO,/Pt/Si
94 68
94 79
“(% cis/% ck+trarw)
+3%
136’C
250°C
76 81
86 90 73 70
inating the product distribution, at 100 r^ C the selectivity may be composed of contributions from spillover catalysis and crack-opening, while at 250°C there is either crack opening in the silica overlayers or deterioration of the palladium-silica interface and the observed catalysis is dominated by direct metalsubstrate contact. This is also supported by an increase in selectivity of the silica covered Pd/Si catalyst with extended use, which was accompanied by a visible deterioration of the silica-palladium interface. As control experiments for the high selectivity attributed to palladium exposure serve hydrogenations on the intrinsically inselective platinum films on silicon. There is no significant difference between the selectivities of silica-covered and uncovered platinum films at 136°C and 250” C (low overall activity excluded reliable measurements at the lower temperatures j and the low selectivity of the silicacovered palladium film at 60 ’C. The other test reaction, the dihydrogen addition to 1,6-dimethylcyclohexene, has a low turnover frequency and could therefore only be studied at elevated temperatures (see Table 2 ) due to our experimental detection limits. At the reaction temperature of 100°C and 205 ‘C all catalysts displayed selectivities characteristic of the transition metal present, even in the presence of the silica overlayer, supporting that the hydrogenation reaction is dominated by direct metal-reactant interaction through cracks in the silica overlayer. It is concluded that at lower temperatures hydrogenation reactions on silica covered Pd/Si and Pt/Si film catalysts may be due to the spillover mechanism, while at elevated temperatures direct metal-alkene interaction dominates the product distribution due to increasing metal accessibility. Metal sensitive reaction selectivities as demonstrated in this study, are the most sensitive tool for the detection of transition metal accessibility through overlayers and should be used as control reactions in future overlayer studies. Such metal sensitive reactions might also prove useful as supplemental tests in other studies invoking hydrogen spillover mechanisms [ 61, since they are independent of the actual metal concentration (detectable or undetectable) and merely a function of measurable catalytic activity. TABLE 2 Selectivities” of l,&dimethylcyclohexene peratures indicated Catalyst
100°C
205°C
Pd/Si SiO,/Pd/Si Pt/Si SiO,/Pt/Si
38 32 66 65
29 35 46 49
“(% cis/% cis+trans)
+3%
hydrogenation on various catalysts at the reaction tem-
239
ACKNOWLEDGEMENT
This study was supported by the National Institutes of Health, Grant Nr. GM-33386- 01.
REFERENCES 1 A.B. McEwen, W.F. Maier, R.H. Fleming and S.M. Baumann, Nature (London), 329 (198’7) 531. 2 J.M. Cogen, K. Ezaz-Nikpay, R.H. Fleming and SM. Baumann and W.F. Maier, Angew. Chem., 99 (1987) 1222; Int. Ed., 26 (1987) 1182. 3 A.B. McEwen, Ph.D. Thesis, University of California, Berkeley, CA, 1987. 4 A. Molnar, G.V. Smith and M. Bartdk, J. Catal., 101 (1986) 67. 5 M. Bartbk, Stereochemistry of Heterogeneous Metal Catalysis, Wiley, New York, 1985, Ch. III. 6 W.C. Conner, G.M. Pajonk and S.J. Teichner, Adv. Catal., 34 (1986) 1.